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5.3.20- 00 Atomic resolution of the graphite surface by STM

1. Shift Ctrl key mouse up down Topography Scan forward Tip current Scan forward Line fit 436pm Line fit 1 17nA Fig 11 3D representation of constant current data left and constant height data right eka ff WE excellence in science 4 PHYWE Systeme GmbH amp Co KG All rights reserved 09620 00 TESS SHYWE Quantum Mechanics by STM Tunneling TEP expert Effect and Charge Density Waves Related Topics Scanning Tunneling Microscopy and Spectroscopy Tunneling Effect Local Density of States Peierl s Theorem Peierl s Transition Charge Density Waves Commensurability Incommensurability Transition Metal Chalcogenide Band Structure Principle In addition to the tunneling effect measured by tunneling Caution spectroscopy another quantum mechanical effect the charge aution density waves are investigated for different samples Charge Setup your system on a very density waves are modulated electron waves due to static and steady table periodic lattice distortion and therefore mappable with Do your experiments ina scanning tunneling microscopy The lattice distortion is caused calm vibrational free environment by a lowering of the total energy of the system due to a Peierl s transisiton Nesting of Fermi surfaces Equipment Compact Scanning Tunneling Microscope complete set incl l l 09600 99 tools sample kit and consumables in aluminum case 1 TaS on sample suppo
2. a a E 5 T i z gt a 2 E i Ol Ha OD 2 ao E LEE E ny Oo E yO cat Hoe pe qe on i Fig 17 a CDW at 30mV tip voltage Fig 17 b CDW at 30mV tip voltage at same location For lower bias voltages the CDW can appear similar to 17 a or b or even just as an array of dark spots Conclusion All our measurements reflect the theory very good Mainly image deformations lead to deviations in the tomic lattice and CDW periods All measurements done on TaS can also be done analogously on other CDW forming materials such as WSez2 another transition metal chalcogenide P2535000 PHYWE Systeme GmbH amp Co KG All rights reserved 9 TESS YWE Nanoscale characteristics by expert Scanning Tunneling Spectroscopy Related Topics Tunneling effect Scanning Tunneling Microscopy STM Scanning Tunneling Spectroscopy STS Local Density of States LDOS Band structure Band Gap k Space Brioullin Zone Metal Semi Metal Semiconductor Principle The tunneling current between a very sharp metal tip and an electrically conductive sample is used to investigate the Caution current voltage characteristics at a nanoscopic scale The Set up your system on a very bandstructure of gold graphite HOPG and MoS2 are steady table investigated Do your experiments in a calm vibrational free environment Equipment Compact Scanning Tunneling Microscope complete set incl l i 09600 99 tools s
3. 1 b 3 FEF Y 2 sa eaa Fig 22 PHYWE 2D model ES of the graphite surface atoms anda an cen ing graphene sheet top view The second aid you can use to get along on the atomic scale is the 2D model of graphite Fig 22 Start by bringing the foil matching on top of the paper the marker should show two circles Then translate the foil diagonal Afterwards the markers should show three circles indicating that you translated correctly The atoms from the surface layer with a neighbor in the layer below appear darker as those without a di rect neighbor they appear brighter This view corresponds to the imaging data of your HOPG sample you collect with your STM The distance between two rows of atoms of the same type is 245 pm From one atom to its next neighbor this distance is 140 pm Fig 23 X 0 245 nm Fig 23 2D sketch of the graphite lattice structure top view Measurements on atomic arrangements Keep in mind that you can t see every atom of a Ce ring as a bright spot but every second Use your 2D model of the graphite surface to compare it with the images you took also see Fig 24 Every second atom has a neighbor in the layer beneath it Each atom from the upper layer loses electron density to the direct neighbor in the layer below it making them to appear darker in STM images Atoms without a direct neighbor in the layer beneath have the full electron density and appear as bright spots As mentione
4. Spectroscopy button in the navigator or by ae i apeccroscopY clicking PE in the Imaging bar In a spectroscopic measurement the tunneling current is measured as a function of either the z distance or the tip voltage To determine the work function se lect Z Axis as modulated output Pin down values of your measurement parameters accurately If you P2535000 PHYWE Systeme GmbH amp Co KG All rights reserved 3 TEP Quantum Mechanics by STM Tunneling TESS gt HYWE Effect and Charge Density Waves expert experience problems in measuring the desired behavior try to adjust your parameter values For exam ple you can start experimenting with the following set of parameters for HOPG Set point 0 5 nA P Gain 500 Gain 500 Tip voltage 0 1 V Start value 0 nm End Value 5 nm Modulation time 0 2 s Data points 128 Averages 16 By using this set of parameters the device will divide the distance from Onm to 5nm from the current tip position into 128 points and measure the tunnel current at each of them The time used for one series of measurements is 0 2 s This process will be repeated 16 times The resulting curve is the average of the 16 measurements Notice When measuring on gold you will need higher gain values than for HOPG Positive Start and End values will bring the tip closer to the surface 1 nm will change the tunnel cur rent about 1 order of magnitude High t
5. 2Me a 10 25nm eV A 2 Here is the effective work function This is an actual workfunction mainly influenced by surface adsor bates and the high electrical field density at the tip The extreme dependence on the distance makes it possible to measure the tip sample movement very precisely By keeping the tip voltage constant and measuring the tunnel current in dependency of d the effective work function can be determined either by fitting an exponential function to the measured tunnel current 1 or by applying the logarithm to 1 which leads to In A d In f U 2 This is a linear function of d so can be determined from the slope of the expected straight line by a linear fit By the combination of the high resolution of the tunnel microscope and the spectroscopy function it is possible to determine work functions at the nanoscale Always keep in mind to do measurements preferably at the same room temperature Temperature de pendencies create uncertainty and errors in the expected values of your measurements P2533000 PHYWE Systeme GmbH amp Co KG All rights reserved 3 Nanoscale work function measurement TESS SHYWE Scanning Tunneling Spectroscopy expert Task 2 Investigate the topography of the gold and HOPG sample on clean terraces and defects in constant current mode Finding atomic terraces on the sample surface It is recommended to start over by checking and optimizing the
6. Repeat your measurements to have the possibility to reduce errors and to get better values by averaging your data see above Using the second method to measure the distance over several rows here 5 d 20 5 pm reduces the value to 144 1 pm with an error of 2 If you are not satisfied with the image quality try to adjust the Tip current Section tip current and the relative height above the sample Decreas ing the tip current will bring the tip in a larger distance to the sample reducing the possibility that the tip hits obstacles on the sample surface increasing the relative height will have a similar effect without reducing the tip sample interaction Raw data 876pA Tip current range 1 O3nA4 m t i ial oF Tip current range 337 pA Section 20 nm Fig 30 The step size corresponds to a differ ence in the tunneling current of 578 6 pA or 334 8 pm one atomic layer PHY WE excellence in science 12 PHYWE Systeme GmbH amp Co KG All rights reserved P2532000 TESS SHYWE Atomic resolution of the graphite surface by STM expert Tip current Scan forward Tip current Section A gt 5 18nm Line ft 386pA eTe gang y y E E E 7 a J O i o D T OEE FUE T A A n r 5 eed ss SS aL Fig 32 Cross sectioning Distance between lines d 235 1 pm hill to hill hill to val l l l ley gives d 117 4 pm The z distance Figo Consian
7. 317nm JE Tanl nam innenaeh i I 5 43nm x 11 19m Fig 15 Length measurement spanning 13a Fig 16 Angle measurements of CDW maxima Both lattice period and angle measurement indicate that our scan is deformed From the lattice period we know that the scan is ok in horizontal orientation but not in vertical This could be a reasoned by a not perfectly shaped tip the thermical drift of the sample which plays an important role at atomic resolution creep or piezo inlinearity The reason for the last point is that piezo crystals only move slowly into an equilibrium position after their elongation has been changed Task 4 Investigate in charge density waves at different voltages and interpret the imaged states filled and empty In the theory the band structure of the two dimensional sample is split up into three different bands in stead of two as in the one dimensional case This has the effect that the DOS is not very symmetrical around Er Only for higher energies the DOS becomes nearly symmetrical and is mainly leading to a contrast inversion when the bias voltage is inverted as shown in fig 17 Here the positive tip voltage im ages the filled states whereas the negative tip voltage images the empty states rPH 1 WE excellence in science 8 PHYWE Systeme GmbH amp Co KG All rights reserved P2535000 TESS SY WE Quantum Mechanics by STM Tunneling Effect and Charge Density Waves expert E m
8. 400 200 0 200 400 600 800 1000 mV Fig 15 I U spectroscopy curve of HOPG The derivate of the I U curve is called specific resistiviy and is proportional to the LDOS To analyze the derivate it is recommended to smooth the curve first Therefore go to Analy sis Smooth Choose the strongest smoothing and overwrite the existent curve Repeat the smoothing until you receive an Source channel 1 KU Calculate Help Operation i differentiate C integrate progressive average value Destination channel f add new y channel C overwrite Title KU ai uyau f into new measurement Symbol laau T asverhannel Unit hAm P2533500 Fig 16 Channel modification window PHYWE Systeme GmbH amp Co KG All rights reserved appropriate curve as shown in the graphs Now go to Analysis Channel Modifica tion and choose differentiate Fig 16 Now you can see the corresponding curve propor tional to the LDOS Apply the spectroscopy mode on the other samples in the same way and compare the results The resulting curves are shown in Fig 17 20 Note The x axis are zoomed in a bit use mousewheel on x axis because the smoothing process distorts the data at the edges excellence in science PHY WE T Nanoscale characteristics by TESS HYWE Scanning Tunneling Spectroscopy expert di dU I nA mV nA 800 700 600 500 400 300 200 100 0 100 20
9. Onm K 136nm Fig 13 Topography of MoS2 Fig 14 Reduced band structure of MoS2 rPH f WE excellence in science 6 PHYWE Systeme GmbH amp Co KG All rights reserved P2533500 Nanoscale characteristics by Scanning Tunneling Spectroscopy TESS PHYWE expert Prepare a measurement by clicking on Point and putting the cursor on the surface of your scanned image where you want to take the spectroscopy data Press the Start button After a few seconds the end of the measurement is indicated when the Stop button changes back to Start and when your data curve doesn t change anymore The Probe Status should be green during the whole measurement If the Probe Status changes to orange switch to the Imaging Window approach the sample again and take a new surface scan afterwards switch back to the Spectroscopy window After the measurement is finished you can see the I U curve in the line graph If your curve does not look as desired repeat the measurement by pressing start again If you still don t get the desired curves switch back to the imaging mode and repeat the process To analyze the current voltage curves right click the line graph and click Copy data to clipboard Now open the analyzing software PHYWE meas ure and paste the data by pressing ctrl v or using Measurement Import Data You should receive a graph as shown in Fig 15 I nA 40 800 600
10. Scan forward Topography Scan forward Raw data 2 9nm Line fit 156pm Topography range Topography range X 178nm Fig 12 Surface of TaS2 Fig 13 CDW at 31nm P2535000 PHYWE Systeme GmbH amp Co KG All rights reserved 7 TEP Quantum Mechanics by STM Tunneling TESS oa Effect and Charge Density Waves expert PH WE In theory the lattice parameter of TaS is a Topography Scan forward 3 346 A From the scan shown in fig 13 one can determine the measured lattice parame ter and period of CDW e g by using the 6 05nm Measure Length tool E As shown in fig 15 we receive a mean hori zontal period of an 3 433 and a vertical period of ay 4 633A The mean period of the CDW maxima is 1 216 nm horizaontally and 2 078 nm vertically leading to lattice periods an 1 216 nm V13 3 377 and ay 2 078 nm V13 5 772 Line ft 803pm Furthermore we can measure the angles of the structure using the Measure Angle tool lt as in fig 16 We receive an angle of 109 5 between the maxima of the CDW 333pm From fig 11 we know the angle between the 5 43nm rele hexagonal arranged CDW maxima is 120 Fig 14 CDW on TaS2 Topography Scan forward m Topography Scan forward z FF Cursor position X Pos 66 34n 5 e 5 Yom 120m e h Z Pos 61 04pm Z Pos 9 156pm 3 ae i Length 4 463nm F it DettaZ 57 99pm Angle a E Width 4 264nm o Height 1
11. TEP 5 3 70 00 sheet on the nano scale One sees the shiny side consists of longish structures whereas the dark side more likely has island like structures Figure 7 shows the surface of a Copper sheet which has also been rolled but its two surfaces do not differ One immediately sees the similarities in Fig 5 and Fig 7 Topography Scan forward 568nm Line ft 19mm Topography range x Fig 6 Ta dark Fig 8 shows a coined copper surface of a European 1 cent coin The structures are round and irregular Figure 9 shows HOPG which is grown and consist of single layers which are strongly bonded within themselves but only bonded among each other by the weak van der Waals force Topography Scan forward 567nm Line fit 22 9nm Topography range Onm PHYWE Systeme GmbH amp Co KG All rights reserved 5 Investigation in roughness and nano TESS morphology of different metal samples by STM expert gt HYWE Topography Scan forward Topography Scan forward 560nm 568nm Line ft 17 5nm Line fit 1 93nm Topography range Topography range Onm 149nm 4 38nm Ae 416nm Fig 8 1 Euro Cent Coin Iron based alloy with copper coating Fig 10 Gold Table 1 shows the outputs of S when applying the area Furthermore the Gold in Fig 10 is also grown This image roughness tool on shows many mono atomic steps and some of higher or the scans shown der above One sees the LA E
12. image should be almost free of strong gradients one plane of the sample surface should have the same color in every point Theory and evaluation Tunneling Effect Tunneling is a functioning concept that arises from quan tum mechanics Classically an object hitting an impenetr able barrier will not pass through In contrast objects with a very small mass such as the electron have wave like characteristics which permit such an event referred to as tunneling In other words the probability to find an electron behind a barrier is unequal zero Inside the bar rier the wave function of the electron decays exponential ly Fig 3 Sketch of the tunneling of an electron through a barrier illustrated by a ball and a wave function STM Scanning Tunneling Microscopy The size of an atom in relation to the tip is that of a golf ball to a mountain In your PHYWE measurement system a platinum iridium tip is moved in three dimensions using piezo crystal translators that are driven with sub nanometer precision Fig 4 The sample to be examined approaches the tip within a distance of about 1 nanome ter 1 nm 1 1 000 000 000 m Classical physics would prohibit the appearance of electrons in the small gap be tween the tip and the sample but if a sharp tip and a conducting surface are put under a low voltage U 0 1 V a very small tunneling current I 1 nA with 1 nm tip sam ple distance though may flow between tip and sampl
13. mapped image The strength of the tunneling current depends exponentially on the distance between the tip and the sample d usually referred to as z distance the applied bias U and constant factors c and c2 Fig 4 Sketch of the piezo electric device driving the tip movement I c Uexp c2d 1 In I U spectroscopy the distance between tip and sample is constant Only the applied voltage is changed Regarding 1 one would expect a linear curve from the spectroscopic measurement However the results differ from the expectation because of the band structure of the investigated materials The electrons of a single atom can occupie discrete ener gy levels forming atomic orbitals Several atoms together into a molecule will form molecular orbitals The number of Conduction Band molecular orbitals is proportional to the number of atoms is the molecule When large numbers of atoms gt 10 are brought togeth Band Gap er to form a solid there are large numbers of energy levels occupied by electrons and the difference between them becomes very small forming energy bands However de pending on the atoms forming the solid there are energy intervals which cannot be occupied by electrons no matter how many atoms are aggregated Such intervals are called band gaps Fig 5 Band gaps only occur at semi condutors small band gap lt 3eV and insulator large band gap Solids which have no band gap meaning Conduction band and Valan
14. quick start guide QSG or the operating manual OM Chapter 5 You ll find an electronic version of the QSG and OM under the help menue of the measure nano software In most cases it is enough to cleave the graphite sample once in a few weeks If you have problems to find a clean area or you don t get good images with several freshly prepared tips clean the sample surface using scotch tape as described in the QSG or OM Sample Cleaning others than HOPG The other samples normally need not to be cleaned However if you experience bad scans throughout or acci dently touched the sample you can lay a lint free cloth on top of the sample surface and drip some alcohol on it Remove the wet cloth by carefully wiping and wait until the alcohol has dried completely To avoid any scratches or other irreversible surface modifications it is important that the surface does not experience pressure at any time Assembling your own samples Additionally to the supplied samples you can assemble samples of different materials on your own Basi cally every conducting and not too rough surface is suited To be able to scan the material you need to cut off a small piece of about the size of a free sample support supplied P2537000 TESS oavwe expert with the STM and clean the sample using the above given procedure Now glue the material on the sample holder using the silver paint It s important the surface of the material which will b
15. scan on HOPG to check if the tip is good by resolving single atoms For more de tails please refer to the experimental guide of P2532000 Atomic resolution of the graphite HOPG surface by STM available at www phywe com 2 Change to the rough sample and start scanning with a low image size e g 10nm x 10nm 3 If you receive artificial structures in the scanned im ages try increasing the time line or if this won t help in crease the tip voltage and or lower the set point with draw the sample and re approach it 4 If still receiving artifacts repeat 3 until you receive good scans If not receiving good scans even with tip vol tage gt 5V and set point lt 0 5nA withdraw the sample and try scanning the surface at a different location 5 Increase the scanning area 6 If receiving good scans repeat 5 Otherwise start over at 3 7 When reaching the desired image size adjust the gain values to smooth the image You can also try to increase the points line for a better scan quality When you are satisfied grab a good image by clicking the Photo put ton 8 Now you can apply some filters to reduce noise adjust scan line levels and remove a background distortion The recommended procedure would be Apply glitch filter Apply noise filter Do a background substraction and or correct scan line PHYWE Systeme GmbH amp Co KG All rights reserved 3 TEP 5 3 70 00 levels Your corrected
16. tance instead of a measurement from point to point Measure Length Now determine the spacing between atoms You can either draw lines for the measurement between neighboring atoms Fig 2 and 6 or between atoms of the next following row Fig 6 and 8 It is also possible to gain the desired information from a cross sectioning Draw a line through a row of atoms and do the distance measurement in the graph image Here you can decide again if you want to measure from one atom to the other hill to valley distance approx d 140 pm or from atom row to atom row hill to hill see Fig 9 or valley to valley distance approx d 245 pm Topography Section Topography dccaslati 25 9pmi Parabola ft 107pm m t i o Topography range 80 9pm Section Fig 9 Cross section through a row of atoms see Fig 17 Distance between lines d 248 5 pm hill to hill AZ 15 3 pm hill Fig 10 Angle measurement Angle be top to valley tween lines a 118 8 i Cc j E i me IE a 1 E i 7 E H Hy a 7 To further increase the accuracy of your measurement and reduce systematic errors measure the dis tance of 5 to 10 rows and divide it by the number of rows and you also end up with the distance from row to row For example a single measurement in Fig 9 gives a distance from row to row of d 248 5 pm doing the same measurement over 5 rows d 1231 5 pm reduces this value to d 246 3 pm Do the measur
17. the 16 measurements Notice e The device may cut off the graphs due to high tunnel currents In this case adjust your Parame ters to decrease the current e g lower set point e High tunnel currents may also influence the structure of the tip and or the surface Therefore try choosing a lower set point or lower start and end values if the curve looks not as expected e As long as you are in the spectroscopy mode the piezo controllers are inactive Because of ther mal drift you might receive different results when measuring the same point multiple times espe cially when using high resolutions Also modifications of the local tip geometry can occur during the measuring procedure indicated by noisy and jumpy spectra Therefore check the topogra phy after every spectroscopic measurements to evaluate the results excellence in science PHY WE 5 P2533500 PHYWE Systeme GmbH amp Co KG All rights reserved Nanoscale characteristics by TESS 9HYWE Scanning Tunneling Spectroscopy expert Topography Scan forward E Cc o 2 F w 5 Z S 5 i E i E H ooh E ii E Ha g E Hy r a aH Ae oo a al 6 260m x 66 6nm Fig 9 Topography of HOPG Fig 10 Reduced band structure of HOPG E 5 M E 5 E a z m E 2 q 5 3 E i E E Onm Onm x 150m Fig 11 Topography of gold Topography Scan forward 138rnm Line fit 1 7 7nm Energy eV 3 JE Ji E E E i f5
18. the sample surface Make sure you adjust the parameters of the feedback loop to achieve good image quality You may switch to the standard or advanced level of the software user interface Op wdata 146mm Ra Topography range retract the sample from the tip and Fig 12 Original images of different atomic terraces a b c and their processed images d e f Yellow bars indicating the place of the cross sectioning tions gt Config User Interface to see below Time Line 0 2 s Set point 1 2 nA P gain 1000 I gain obtain the possibility to adjust the P 1200 PHY WE excellence in science 6 PHYWE Systeme GmbH amp Co KG All rights reserved P2532000 TESS SHYWE Atomic resolution of the graphite surface by STM expert and the l gain individually Too high values in the P and l gain Topography will be noticeable in the line graph as very high peaks and a very rough line structure Adjust the gain values to smooth the line graph and your image respectively When you are sa ra Photo button F 5 8nm tisfied grab a good image by clicking the Now you can apply some filters to reduce noise adjust scan line levels and remove a background distortion The recommended procedure would be Apply glitch filter Apply noise filter Section Do a background substraction and or correct scan line Fig 13 Cross section of image d see above levels Distance Az 332 2 pm If you ar
19. you have the tip current as the z signal What you can do you can correlate values of the tip current to a step height of a terrace you measured before in the constant current mode by a cross sec tioning For this you can use the Measure Length tool to look at Tip current Scan forward the Az values displayed in the Tool Results panel see Fig 30 E aR Measuring distances follows the same procedure as mentioned above Choose your measurement points wisely avoid tilted planes and scanning artifacts Remember the exponential dependence of the tunneling current and the gap distance c U exp c d_ Measuring the dif ference in the current in Fig 30 gives A 578 6 pA and corres ponds to the step height of one atomic terrace with Az 334 8 pm The cross sectioning through an atomic row Fig 32 reveals that the difference in the current from the hilltop to valley is A 81 68 pA and corresponds to the Az 15 3 pm we found earlier Fig 26 Height differences tip sample distance of 0 1 nm will increase the Fig 29 Constant height image Image measured current by about one order of magnitude size 40 nm Time Line 0 2 s Set In Fig 31 an example for a measurement from one row to the next es a Tip Pos 0 05nm one atom to atom gives a distance of d 150 1 pm Here we find an error of about 6 and is out of the bounds to be in good agree ment with expected values from the theory 140 pm
20. you want to take the spectroscopy data Press the Start button After a few seconds the end of the measurement is indicated when the Stop button changes back to Start and when your data curve doesn t change anymore The Probe Status should be green during the whole measurement If the Probe Status changes to orange switch to the Imaging Window approach the sample again and take a new surface scan afterwards switch back to the Spectroscopy window After the measurement is finished you should be able to see two curves in the dual line graph like in Fig 6 One for the forward measurement and one for the backward measurement You should see an expo nential dependence of current to distance for both of them If not repeat the same measurement pressing Start again If you still don t get the desired curves switch back to the imaging mode and repeat the process Tip current Spec forward HS Increase Chart Data Range Chart Properties 47 Spe Copy to Clipboard Raw data 53 4p Fig 6 Sketch of the dual line graph To analyze the current distance curves right click the dual line graph and click Copy data to clipboard Fig 6 Now open the analyzing software PHYWE measure and paste the data by pressing ctrl v or using 5 P2533000 PHYWE Systeme GmbH amp Co KG All rights reserved Nanoscale work function measurement TESS SHYWE Scanning Tunneling Spectroscopy exp
21. 0 300 400 500 600 700 800 900 Fig 17 Specific resistivity of HOPG di dU 2 nA mV nA 600 500 400 300 200 100 0 100 200 300 400 500 600 Fig 18 Specific resistivity of gold di dU I nA mV nA 300 200 100 0 100 200 300 400 500 600 700 600 900 1000 1100 1200 Fig 19 Specific resistivity of MoS2 with marked band gap rPH f WE excellence in science 8 PHYWE Systeme GmbH amp Co KG All rights reserved P2533500 TESS ye Nanoscale characteristics by expert Scanning Tunneling Spectroscopy Task 4 Interpret the results regarding to the bandstructure The first thing we see is that the LDOS of gold is described by a linear curve while HOPG and MoS are not This is the result of gold being a conductor This means it has no band gap due to the valence and con duction band overlap meaning electrons can move freely along the metal The I U spectroscopy curve therefore images Ohm s law U RI Regarding the band structure of HOPG and MoS2 one sees there are regions where the LDOS is near zero These regions are where the band gaps are Theoretically we would expect the LDOS to be exactly zero but impurities and the non ideal measurement environment air high temperature influences the recorded data here Furthermore one sees the characteristics of the two curves e g extrema differ Especially remarkable is that the curve of Graphite is approximately symmetric to OV whereas the curve of MoS is
22. 000 TESS Quantum Mechanics by STM Tunneling TEP expert Effect and Charge Density Waves Theory and Evaluation Tunneling Tunneling is a functioning concept that arises from quantum mechanics Classically an object hitting an impenetrable bar rier will not pass through In contrast objects with a very small mass such as the electron have wavelike characteristics which permit such an event referred to as tunneling In other words the probability to find an electron behind a barrier is unequal zero Inside the barrier the wave function of the elec Fig 3 Sketch of the tunneling of an electron tron decays exponentially Fig 3 through a barrier STM Scanning Tunneling Microscopy The size of an atom in relation to the tip is that of a golf ball to a mountain In your PHYWE measure ment system a platinum iridium tip is moved in three dimensions using piezo crystal translators that are driven with sub nanometer precision Fig 4 The sample to be examined approaches the tip within a distance of about 1 nanometer 1 nm 1 1 000 000 000 m Classical physics would prohibit the appearance of electrons in the small gap between the tip and the sample but if a sharp tip and a con ducting surface are put under a low voltage U 0 1 V a very pe lt gt small tunneling current I 1 nA with 1 nm tip sample distance f al though may flow between tip and sample The resulting tunne N ling current is a function of
23. 2000 PHYWE Systeme GmbH amp Co KG All rights reserved 5 Atomic resolution of TESS SHYWE the graphite surface by STM expert STM Scanning Tunneling Microscopy The size of an atom in relation to the tip is that of a golf ball to a mountain In your PHYWE measurement system a platinum iridium tip is moved in three dimensions using piezo crystal translators that are driven with sub nanometer precision Fig 11 The sample to be ex amined approaches the tip within a distance of about 1 nanometer 1 nm 1 1 000 000 000 m Classical physics would prohibit the appearance of electrons in the small gap between the tip and the sample but if a sharp tip and a conducting surface are put under a low voltage U 0 1 V a very small tunneling current I 1 nA with 1 nm tip sample distance though may flow between tip and sam ple The resulting tunneling current is a function of tip po Fig 11 Sketch of the piezo electric device driving the sition applied voltage and the local density of states tp movement LDOS of the sample this information is displayed on the computer monitor as 2D color mapped image The strength of the tunneling current depends exponen tially on the distance between the tip and the sample d usually referred to as z distance the applied bias U and constant factors c and c2 Sample holder Backward scan _ _ _ J Forward scan I c U exp c d 1 This extreme dependence on t
24. 3 Power cord and adapter 4 MeasureNano Software 5 PHYWE STM Tool Box 1 Wire cutter Flat nose pliers Pointed tweezers Rounded tweezers Pt Ir wire d 0 25mm 30 cm 10 oe et dh Sample Holder Graphite sample HOPG Gold sample 4 Spare sample support eo et oe ed ee 13 Fig 2 STM set User Manual Scanning Tunneling Microscopy STM Operating Instruction and Experiments Tasks 1 Prepare a Pt Ir tip and the graphite HOPG sample and approach the tip to the sample 2 Investigate the topography of clean terraces and the step height between neighboring terraces in constant current mode 3 Image the arrangement of graphite atoms on a clean terrace by optimize tunneling and scanning pa rameters Interpret the structure by analyzing angles and distances between atoms and atomic rows and by using the 2D and 3D graphite model 4 Measure and compare images in the constant height and constant current mode Set up and Procedure To prepare a fresh tip for STM measurements follow the steps mentioned here or have a look in the Quick Start Guide or chapter 5 of the PHYWE STM user manual respectively Make sure you have al ready set up your microscope on a very steady table in a quiet environment with a low level of building vibrations e g basement Your microscope should be set up on a separate table than your PC or lap top Keep some free space on the table to prepare the samp
25. E AE surfaces can be cate Rough Plain gorized by their Cu HOPG i roughness as done in Ta shiny Gold 2 table 2 Ta dark TaS2 Regarding the manu Coin 2 facturing procedures Sn of the surfaces in ta ble 2 the rolled and coined materials are rough and the grown materials are plain Tab 2 Materials catego rized by roughness Conclusion Topography range The longish nanoscopic structure of rolled materials can be explained by the stretching process the materials ex perience when they are flattened 118nm 220nm Fig 9 HOPG Tantalum disulfide TaS Fig 11 which is also grown con The chaotic structures of the coin surface are reasoned in sists of layers like HOPG but is much more brittle This the fast coining process during which the surface is ex means the single layers are not as strong bonded within posed to high forces and immediately cools down after themselves as those of HOPG Fig 12 wards Material Cu Ta shiny Ta dark HOPG Gold Coin Sn TaS2 Sy nm 33 3 38 2 56 1 0 4 4 4 31 3 133 0 14 0 Tab 1 Area Roughness PKs WE excellence in science 6 PHYWE Systeme GmbH amp Co KG All rights reserved P2537000 TEP 5 3 70 00 Investigation in roughness and nano morpholo TESS PHYWE j expert of different metal samples by STM Topography Scan forward Furthermore we have seen that macroscopically shiny materials are not necessary plain on the nanoscopic scale and vice ve
26. Evaluation Tunneling Tunneling is a functioning concept that arises from quantum mechanics Classically an object hitting an impenetrable bar rier will not pass through In contrast objects with a very small mass such as the electron have wavelike characteristics which permit such an event referred to as tunneling In other words the probability to find an electron behind a barrier is unequal zero Inside the barrier the wave function of the elec Fig Sketch vf the tunn ling of an electron tron decays exponentially Fig 3 through a barrier STM Scanning Tunneling Microscopy The size of an atom in relation to the tip is that of a golf ball to a mountain In your PHYWE measurement system a platinum iridium tip is moved in three dimensions using piezo crystal translators that are driven with sub nanometer precision Fig 4 The sample to be examined approaches the tip within a distance of about 1 nanometer 1 nm 1 1 000 000 000 m Classical physics would prohibit the appearance of electrons in the small gap between the tip and the sample but if a sharp tip and a conducting surface are put under a low voltage U 0 1 V a very small tunneling current I 1 nA with 1 nm tip sample distance though may flow between tip and sample The resulting tunneling current is a function of tip position the applied voltage and the local density of states LDOS of the sample this information is displayed on the computer monitor as 2D color
27. Graphite Model ee excellence in science Using the 2D Graphite Model to understand high resolution STM im ages of Graphite HOPG Extract of the Experiment P2532000 Atomic resolution of the graphite surface by STM Following the detailed instruction given in the experiments description of P2532000 you should be able Topography Scan forward Topography Scan forward 3 07nm Parabola fit 143pm Parabola fit 107pm Topography range Topography range Onm yt 3 07nm Onm yt 10 2nm Fig 1 Image size 10 nm Time Line Fig 2a Image size 3 nm Time Line 0 03 s Set point 1 2 nA P gain 0 03 s Set point 1 2 nA P gain 1200 I gain 1500 1300 I gain 850 Distance be tween lines d 138 pm to observe atomic arrangements like those shown in the pictures The imaged data corresponds to a topographic representation of the LDOS near the Fermi edge of sur face atoms in the HOPG sample Before we start to interpret the images let s have a look at the atomic arrangement of HOPG and the question what are the bright protrusions in the images we observe Hexagonal Structures The lattice structure of graphite is the so called hexagonal closest packing h c p with a ABA pattern side view top view Fig 4 Sketch of sp hybridized carbon atoms The 3D model of the graphite lattice shows the arrangements of Fig 3 PHYWE 3D model of the gra neighboring atoms and graphene sheets one layer Fi
28. TESS SHYWE Atomic resolution of expert the graphite surface by STM Related Topics Tunneling effect Hexagonal Structures Scanning Tunneling Microscopy STM Imaging on the subna nometer scale Piezo electric devices Local Density of States _DOS Constant Height and Constant Current Mode Principle Approaching a very sharp metal tip to an electrically conduc tive sample by applying an electrical field leads to a current Caution between tip and sample without any mechanical contact This _ Set up your system on a very so called tunneling current is used to investigate the electron steady table ic topography on the sub nanometer scale of a fresh prepared graphite HOPG surface By scanning the tip line by line across the surface graphite atoms and the hexagonal struc ture are imaged Do your experiments in a calm vibrational free environment Equipment Compact Scanning Tunneling Microscope complete set incl 1 09600 99 tools sample kit and consumables in aluminum case 1 Graphite model 2D 09620 00 1 Crystal lattice kit graphite 39840 00 Additionally needed 1 PC Windows XP or higher Adhesive tape Fig 1 Experimental setup P2532000 PHYWE Systeme GmbH amp Co KG All rights reserved 1 Atomic resolution of TESS EE PHY WE the graphite surface by STM expert PHYWE Compact STM set Control unit with mounted scan head 1 Magnifying cover glass 10X Magnification 2 USB cable
29. ample kit and consumables in aluminum case 1 MoS2 sample 09609 00 Additionally needed 1 PC Windows XP or higher Adhesive tape PHYWE measure toner g MOIUA rv i li we l i Fig 1 Experimental setup excellence in science PHY WE 1 P2533500 PHYWE Systeme GmbH amp Co KG All rights reserved Nanoscale characteristics by TESS PH WE Scanning Tunneling Spectroscopy expert PHYWE Compact STM set 1 Control unit with mounted scan head 1 1 Magnifying cover glass 10X Magnification 2 1 USB cable 3 1 Power cord and adapter 4 1 MeasureNano Software 5 1 PHYWE STM Tool Box Wire cutter Flat nose pliers Pointed tweezers Rounded tweezers Pt Ir wire d 0 25 mm 30 cm Sample Holder Graphite sample HOPG Gold sample Spare sample support KA A Am Am Am mm om om 14 Fig 2 Content of 09600 99 Scanning Tunneling Microscopy STM 15 Operating Instructions and Experiments Tasks 1 Prepare a Pt Ir tip and the sample surfaces Approaching the tip towards the sample 2 Investigate the topography of the gold HOPG and MoS sample in constant current mode 3 Switch to spectroscopy mode Measure and compare images recorded on the different materials in Tip voltage mode l U spectroscopy 4 Interpret the results regarding to the bandstructure Set up and Procedure Task 1 Prepare a Pt ir tip and th
30. art PELL EL eee pra at Click Zoom in the upper tool bar move the mouse cursor to a flat region similar color in the color map and click on it The software will now draw a square that indicates the new scan range The size of the new scan range is displayed in the Tool Results panel Change the size of the new scan range to about 30 50 nm Fig 16 by clicking and dragging a corner of the square with the mouse cursor Double click the color map when the new scan area is set as you want it or press Zoom in the Tools Result panel Fig 16 Image size 30 nm Time Line The imaging settings are now set in such a manner that the 0 13 s Set point 1 2 nA P gain new measurement will correspond to the area that was indi fee oa ae cated by the square you have set Let the topography re produce stably again Topography Scan forward Parabola ft 143pm i Lm i H a H ito H e H D H E H O To achieve atomic resolution the image size should be decreased even further considering that one nanometer is the diameter of between four and eight atoms Atomic arrangements can normally be rec ognized at an image size of about 10 3 nm Fig 17 Therefore Set the image size in the imaging panel to 3 nm or use the Zoom option on your last image Fig 18 Some parts of the scan head react to the slightest temperature changes Since these thermal fluctuations in fluence the measurement on th
31. at you translated correctly The atoms from the surface layer with a neighbor in the layer below appear darker as those without a direct neighbor they appear brighter This view corresponds to the imaging data of your HOPG sample you collect with your STM Every second surface atom is imaged The distance between two rows of atoms of the same type is 245 pm From one atom to its next neigh bor this distance is 140 pm Fig 6 Measurements on atomic arrangements Keep in mind that you can t see every atom of a Cs ring as a bright spot but every second Use the 2D model of the graphite surface to compare it with the images you took also see Fig 7 Every second atom has a neighbor in the layer beneath it Each atom from the upper layer loses electron density to the direct neighbor in the layer below it making them to appear darker in STM images Atoms without a di rect neighbor in the layer beneath have the full electron density and appear as bright spots rH ff WE excellence in science 2 PHYWE Systeme GmbH amp Co KG All rights reserved 09620 00 PH WE 2D Graphite Model ee excellence in science E t o E a a o ii m 0 Topography range 3 07nm j Fig 8 Distance measurement Distance Fig 2b Hexagonal pattern reveals elec between lines d 252 7 pm tronic and topographic structure of surface atoms Due to the accuracy of measurement values it is useful to do measurements with lines Measure Dis
32. ce band overlap are called conductors Valance Band Density of states Fig 5 Sketch of the band structure of an insulator semi conductor excellence in science PHY WE 3 P2533500 PHYWE Systeme GmbH amp Co KG All rights reserved Nanoscale characteristics by TESS PH WE Scanning Tunneling Spectroscopy expert Regarded in detail the electronic band structure of a solid is very complex It depends on the properties of the underlying crystal lattice and is described by graphs as shown in Fig 6 Here the energy is plotted as a function of the wave vector k which describes the motion of the electrons in the lattice The wavevector k takes on values within the Brillouin Zone which is a model to describe unit cells corresponding to the crystal lattice Particular direc tions points in the Brillouin Zone are assigned conven tional names like A A 2 etc The areas electrons are allowed to be are highlighted grey Regarding the possible configurations in Fig 7 one can see the difference between the different types of con ductors The materials used in our measurements are a conductor metal gold a semi condutor MoS and a Fig 6 Reduced band structure of Si with marke semi metal HOPG pantgep es The electron distribution among the allowed energies is determined by the local density of states _ DOS The LDOS is a space Ere resolved description of the number of states at each energy level
33. ckground correction and or correct scan line levels Topography Scan forward Topography Scan backward Fi Raw data 1 68nm Line ft 1 5mm Topography range Topography range Fig 5 Examples of terraces on HOPG left and gold right recorded at tunnel current 0 5 nA tip voltage 0 1 V Task 3 Switch to spectroscopy mode Measure and compare images recorded on terraces and defects in Z Axis mode l z spectroscopy l z spectroscopy After grabbing a good image switch to the Spectroscopy mode by clicking the Spectros copy button in the navigator or by clicking PE in the Imaging bar In a spectroscopic measurement the tunneling current is measured as a function of either the z distance or WE i rH excellence in science 4 PHYWE Systeme GmbH amp Co KG All rights reserved P2533000 Spectroscopy TESS we Nanoscale work function measurements by expert Scanning Tunneling Spectroscopy the tip voltage To determine the work function select Z Axis as modulated output Pin down values of your measurement parameters accurately If you experience problems in measuring the desired behavior try to adjust your parameter values For example you can start experimenting with the following set of parameters for HOPG Set point 0 5 nA P Gain 500 Gain 500 Tip voltage 0 1 V Start value 0 nm End Value 5 nm Modulation time 0 2 s Data points 128 Averages 16 By using this
34. d above it is useful to do measurements with lines Measure Distance in stead of a measurement from point to point Measure Length Now determine the spacing between atoms You can either draw lines for the measurement between neighboring atoms Fig 23 or Linbh 1a pm T pnyaphieberah Fig 24 Hexagonal pattern reveals elec tronic and topographic structure of surface atoms rH INE excellence in science 10 PHYWE Systeme GmbH amp Co KG All rights reserved P2532000 TESS SHYWE Atomic resolution of the graphite surface by STM expert between atoms of the next following row Fig 25 It is also possi ble to gain the desired information from a cross sectioning Draw a line through a row of atoms and do the distance measurement in the graph image Here you can decide again if you want to meas ure from one atom to the other hill to valley distance approx d 140 pm or from atom row to atom row hill to hill see Fig 26 or val ley to valley distance approx d 245 pm To increase the accuracy of your measurement and reduce syste matic errors measure the distance of 5 to 10 rows and divide it by the number of rows and you also end up with the distance from row to row For example a single measurement in Fig 26 gives a dis tance from row to row of d 248 5 pm doing the same measure ment over 5 rows d 1231 5 pm reduces this value to d 246 3 pm one i Fig 25 Distance measurement Dis
35. dia 25mm 87004 10 1 Water distilled 5l 31246 81 PHYWE Compact STM set 09600 99 ref to Fig 2 1 Control unit with mounted scan head 1 1 Magnifying cover glass 10X Magnification 2 1 USB cable 3 1 Power cord and adapter 4 1 measureNano Software 5 1 PHYWE STM Tool Box consisting of 1 Wire cutter 6 1 Flat nose pliers 7 1 Pointed tweezers 8 1 Rounded tweezers 9 1 Pt Ir wire d 0 25mm 30 cm 10 1 Sample Holder 11 1 Graphite sample HOPG 12 1 Gold sample 13 4 Spare sample support 14 Scanning Tunneling Microscopy STM Operating 15 Instruction and Experiments Tasks 1 Preparation of Pt Ir tunneling tips and different sample surfaces Approaching the tip towards the sample 2 Investigate the topography of different rolled coined tempered grown etched or polished samples in constant current mode 3 Compare the scanned images and apply the Rough ness analysis tools 4 Interpret the results Pre l wE excellence in science 2 PHYWE Systeme GmbH amp Co KG All rights reserved Investigation in roughness and nano TESS ouywe expert Content of 09600 99 Set up and Procedure Task 1 Preparation of Pt Ir tunneling tips and different sample surfaces Approaching the tip towards the sam ple For preparation of tunneling tips and the approaching procedure please refer to experiment P2532000 Atomic Resolution of the graphite surface by STM the
36. e The resulting tunneling current is a function of tip posi tion applied voltage and the local density of states j a y WE excellence in science 4 PHYWE Systeme GmbH amp Co KG All rights reserved Investigation in roughness and nano morphology of different metal samples by STM TESS mvo expert LDOS of the sample this information is displayed on the computer monitor as 2D color mapped image The strength of the tunneling current depends exponentially on the distance between the tip and the sample d usually referred to as z distance the applied bias U and constant factors c and c3 I amp cr U exp cz dz 1 This extreme dependence on the distance makes it possi ble to measure the tip sample movement very precisely One of the three piezo crystals the z piezo can now be used in a feedback loop that keeps the tunneling current constant by appropriately changing the z distance The elongation of a piezo crystal is proportional to a device specific constant which is a temperature dependent pa rameter and the applied voltage Elongation piezo con stant Voltage e g A 500 pm V 10 V 5 nm Always keep in mind to do measurements preferably at the same room temperature Temperature dependencies create uncertainty and errors in the expected values of your measurements Sample holder Backward scan ee oot Forward scan Fig 4 Sketch of the piezo electric device driving the
37. e nanometer scale the sample has to be scanned as fast as possible Set the Time Line in the imag ing panel to 0 03s with 128 Points Line for atomic resolution With a good tip and properly set parameters you should be able to observe atomic arrangements like those shown in the pictures The imaged data corresponds to a topographic representation of the LDOS near the Fermi edge of surface atoms in the HOPG sam ple Before we start to interpret the images let s have a look at the atomic arrangement of HOPG and the question what are the bright protrusions in the images we observe Parabola fit 143pm i a m C p E i G i HD Hp H O H Fig 17 Image size 10 nm Time Line 0 03 s Set point 1 2 nA P gain 1200 l gain 1500 D Sands Introduction to Crystallography Benjamin Cummings Reading Massachusetts 1969 PHY WE excellence in science 8 PHYWE Systeme GmbH amp Co KG All rights reserved P2532000 TESS sHYWE Atomic resolution of the graphite surface by STM expert Hexagonal Structures The lattice structure of graphite is the so called hexagonal closest packing h c p with a ABA pattern Have a look at your 3D model of the graphite lattice to get used to arrangements of neighboring atoms and graphene sheets one layer Assemble your 3D model according to the instructions Fig 19 A graphene sheet consists of carbon atoms black balls in your 3D model in the
38. e not sure that the corrected image is the better one go a step backwards and start over again Your corrected image should be almost free of strong gradients one plane of the sample surface should have the same color in every point Raw data Topography range f 6 1mm Estimating the step height of terraces Then you can start to analyze your data At first use the Create Cross section tool Place a line on the image you want to analyze If you want to estimate the step height of ter Topography races you should make sure the line is perpendicular to the edge of the terrace Click Cut out line This will create a new graph image with the z information corresponding to the cross sectioning line you have drawn Fig 13 Avoid creating cross sections from spots with scan or correcting artifacts strange gradients or other jumps in the coloring of the surface Now 62 6om Raw data Topography range you can use the fo Measure Distance tool It allows you to draw two straight lines between which the distance is measured The advantage of using the line measurement instead of a point measurement is that it is possible to reduce Fig 14 Cross section of image e see above the influence of the surface roughness or noisy data by Distance AZ 333 9 pm placing the lines between the lowest and the highes values of each terrace When you do measurements keep in mind that you always have errors influencing your results E
39. e sample surfaces Approaching the tip towards the sample For preparation of tunneling tips and the approaching procedure please refer to experiment P2532000 Atomic Resolution of the graphite surface by STM the quick start guide QSG or the operating manual OM Chapter 5 You ll find an electronic version of the QSG and OM under the help menue of the measure nano software In most cases it is enough to cleave the graphite sample once in a few weeks If you have problems to find a clean area or you don t get good images with several freshly prepared tips clean the sample surface using scotch tape as described in the QSG or OM The gold sample normally need not to be cleaned However if you experience bad scans throughout or accidently touched the sample you can lay a lint free cloth on top of the sample surface and drip some alcohol on it Remove the wet cloth by carefully wiping and wait until the alcohol has dried completely To avoid any scratches or other irreversible surface modifications it is important that the surface does not experience pressure at any time Normally the MoS2 sample have not to be prepared by a special procedure If you still have problems to image the surfaces carefully cleave the samples with the scotch tape method PHY WE excellence in science 2 PHYWE Systeme GmbH amp Co KG All rights reserved P2533500 TESS siio Nanoscale characteristics by expert Scanning Tunneling Spectroscopy Theory and
40. e scanned and the sample holder are as pa rallel as possible and you do not touch the sam ple surface with your fingers or scratch it with the tweez ers Afterwards wait at least 15 minutes to let the silver paint harden Now you can scan your own sample as usual The set nanomorphology contains 8 different metal sam ples and the necessary tools and accessories to prepare and store the samples Please follow the procedure men tioned above to prepare your own sam ples with the ex ception of the cent coin It can be used without mounting on a sample support Additionally the HOPG and Gold Sample both part of the Compact STM set can also be analysed Sample etching Beside the nano morphology of grown rolled coined and tempered samples also the effect of etching on a nanos copic scale can be investigated For this please use the equipment given in the list and refer to the procedure given in the Experimental Guide P5510200 Metallo graphic Sample Preparation Chemical Etching available at www phywe com Sample polishing Another standard procedure to process samples is the po lishing of the surface As an example the na noscopic ef fect of polishing on copper foil before mounting on the sample support can be investigated For this please use the equipment given in the list and refer to the procedure given in the Experimental Guide P5510100 Metallo graphic Sample Preparation Grinding and Polishing of Metals availab
41. e units of b into account Now that you are familiar with the procedure apply the spectroscopy mode on some points on clean terraces and defects like step edges or holes on HOPG and Gold and compare the measured work func tions The measured data will vary for each tip In order to compare results you should use the same tip for the different measurements For example the measurement on a clean terrace on the HOPG sample Fig 5 point 3 gives the curve in Fig 7 From the fit we know that b 8 78 nm SO b 8 787 5eV 0 73 eV A 10 25 F X a Function fitting r oS es Function Analyse exponential function left axis r a ebx c Tip current Parameters Fit Tip current e Results Parameters a 0 259 a b c b 8 78 c 0 0112 max iterations 225 Scattering 0 00158537591606012 Iterations 105 max relative error NEGRAN 1 1E 6 Calculate Close Parameters gt Fig 7 Function fitting window in PHYWE measure PHY WE excellence in science 6 PHYWE Systeme GmbH amp Co KG All rights reserved P2533000 TESS w Nanoscale work function measurements by expert Scanning Tunneling Spectroscopy For the other points in Fig 12 one can calculate the work function in the same way The results are HOPG ee a 0 27 eV 0 11 eV E 0 26 eV 0 12 eV o 3 l 1 2 3 You can see that the measured effective work functions for gold are l
42. ect they can occur in higher dimensions However the theoretical background is much more complex CDW on TaS2 TaS is a transition metal chalcogenide in the 1T phase This means each layer of Ta is packed between layers of S as shown in Fig 10 The weak van der Waals bonding between the single layers are the rea son for CDW in the two dimensional layers Fig 10 Layer packing of samples Fig 11 Nearly commensurable phase CDW on TaS2 O S O S Ta Ta The CDW can form a commensurable or incomesurable superlattice Commensurable means the ratio between CDW period and atomic lattice period is rational whereas it is incommensurable when it s irra WE iret g excellence in science 6 PHYWE Systeme GmbH amp Co KG All rights reserved P2535000 TESS Quantum Mechanics by STM Tunneling TEP Effect and Charge Density Waves expert tional Furthermore semicommensurable phases are possible in which there are areas with commensur able period as well as incommensurable Below 183K TaS forms a commensurable phase Between 183K and 353K TaS forms a commensura ble phase and an incommensurable V13 x V13 superlattice This nearly commensurable phase is what we will observe and is shown in Fig 11 Above 353K TaS forms an incommensurable phase The new setting of the electrons can be directly investigated by STM In the scans the CDW superim poses the atomic structure To see both structures it is needed to bring
43. ements as exact as possible minimal differences in the line drawing only a few pixel can make a big difference in your resulting values To further support your results do a quick estimation of mean values see above from at least three to five measurements more will increase the accuracy even further The bonding angle between atoms you are able to determine by using the Measure Angle tool Fig 10 09620 00 PHYWE Systeme GmbH amp Co KG All rights reserved 3 09620 00 2D Graphite Model PHY WE excellence in science After finishing the measurements you also have the possibility to present the data in 3D Fig 11 see chapter 4 4 3 and 4 5 of the user manual Select 3D View in the Select Chart Type drop down menu Then adjust the appearance until you are satisfied with the look Always click and hold the left mouse button on the 3D view chart while changing the 3D view The sur face is reduced in feature complexity as long as the left mouse button is pressed Press the following additional keys buttons to determine what chart property is changed Surface rotation mouse left right Surface tilt mouse up down Size displayed surface Cirl key mouse up down Surface position Shift key mouse up down left right Z scale magnification left mouse button right mouse button mouse up down Light source direction 860 Shift Ctrl key mouse left right Light source height 090
44. ert Measurement Import Data A dialog will pop up Select Sort data Ask again if any x values occur twice You should receive a graph as shown in Fig 7 Now go back to PHYWE measure nano and switch the spectroscopy signal of the dual line graph by right click Signal Export this curve to PHYWE measure in the same way Now compare the curves for forward and backward measurement You will see the both curves to be different because of surface adsorbates which impact the tunnel cur rent When retracting the tip from the surface these adsorbates will provide a direct connection decreas ing the actual work function Therefore we will only analyze the backward spec data which are less af fected by this effect and leave the forward spec data aside Task 4 Determine the local work function for the different areas and interpret the results To determine the effective work function we need to fit an exponential function Therefore go to Analysis Function fitting in PHYWE measure and select exponential function in the uppermost drop down dialog After clicking calculate you will obtain the fit parameters By clicking Add new curve the fit curve will be drawn into the diagram so you can visually evaluate the quality of the fit Fig 7 From 1 we know for our fit parameter b A and therefore b2 P Tr To obtain correct values for the work function it is important to take th
45. g 3 phite lattice Crystal lattice kit A graphene sheet consists of carbon atoms black balls in the 3D graphite 39840 00 model in their sp hybridized state with an angle of 120 for each bonding in the x y plane Fig 4 These are covalent bonds indi cated with white connection pieces from atom to atom within one www phywe com 09620 00 PHYWE Systeme GmbH amp Co KG All rights reserved 1 TESS we Nanoscale work function measurements by Scanning Tunneling Spectroscopy expert Related Topics Tunneling effect Defects Scanning Tunneling Microscopy STM Scanning Tunneling Spectroscopy STS Local Density of States LDOS Work function Surface activation Catalysis Principle Scanning tunneling microscopy is used to image the electronic topography of a freshly prepared graphite Caution HOPG and gold surface By spectroscopic measurements Set up your system on a very steady l z the local effective work function can be determined in table dependence of the material and the nanomorphology of the Do your experiments in a calm sample The results are discussed with respect to surface vibrational free environment activation and catalysis Equipment Compact Scanning Tunneling Microscope complete set incl l i 09600 99 tools sample kit and consumables in aluminum case Additionally needed 1 PC Windows XP or higher Adhesive tape PHYWE measure totana m r M i
46. g etching polishing etc and therefore their nano morphology widely varies while on a macroscopic scale the sample surfaces appear similar Caution Set up your system on a very steady table Do your experiments in a calm vibrational free environment Equipment Compact Scanning Tunneling Microscope complete Adhesive tape 1 setincl tools sample kit and consumables in alu 09600 99 Acetone Alcohol and cleaning cloths minum case 1 Set samples nanomorphology 09613 00 For further investigation optionally needed Polishing of samples Cu 1 Microscopic slides 50 pcs 64691 00 1 Diamond suspension 1um 250g 70042 25 Additionally needed PC Windows XP or higher Fig 1 Set up of experiment P2537000 P2537000 PHYWE Systeme GmbH amp Co KG All rights reserved 1 TEP pa morphology of different metal samples by STM 1 Diamond suspension 0 25um 250g 70043 25 1 Lubricant RED 1l 70061 70 1 Cleaning and polishing tissues pkg of 50 46417 00 1 Gloves disposable pkg of 100 39175 03 1 Wash bottle plastic 1000ml 33932 00 1 Magnifier 10x dia 25mm 87004 10 Etching of samples Cu 1 Hydrochloric acid 30 500ml 48451 50 1 Iron Ill chloride 250g 30069 25 1 Safety goggles 46333 01 1 Pasteur pipettes 3ml PE 500pcs 36616 00 1 Beaker low 250ml 46054 00 1 Cleaning and polishing tissues pkg of 50 46417 00 1 Protective gloves 46347 00 Fig 2 1 Wash bottle plastic 1000ml 33932 00 1 Magnifier 10x
47. he distance makes it possible to measure the tip sample movement very precisely One of the three piezo crystals the z piezo can now be used in a feedback loop that keeps the tunneling current constant by appropriately changing the z distance The elongation of a piezo crystal iS proportional to a device specific constant which is a temperature dependent parameter and the applied voltage Elongation piezo constant Voltage e g AZ 500 pm V 10 V 5 nm Always keep in mind to do measurements preferably at the same room temperature Temperature dependencies create un certainty and errors in the expected values of your measurements Finding atomic terraces on the sample surface To activate the full measurement range of your device click the amp Full button in the imaging window For good results you can use image sizes of about 0 2 um When you are lucky you find a terrace in your first measurement If that is not the case yo u ca n e ith e r u se th e Topography Scan forward Topography Scan forward Topography Scan forward rae alc ey D MOVE tool in the imaging window f P Onm c2 d to start the measurement at a different spot on the surface or you Jf A I z a fit 1 12m Mean ft 583pm turn carefully the sample holder with the black plastic handle Afterwards you approach the sample again Topography range In Fig 12 you can find examples of terrace like structures at different spots on
48. he measurement signal is the tip current flowing between the tip and the sample Be careful When you change scanning parameters too drastically it is likely that you crash the tip into the sample At first enable the advanced user interface of the software Op tions gt Config User Interface Now you will find several options and parameters to adjust To do a measurement in the constant height mode check the Const Height mode in the Imaging Mod es panel in the imaging window Fig 28 Now you can adjust the Rel Tip Pos this is the distance by which the tip is moved away or towards the sample from the position that corresponds to the set point A negative setting will move the tip away from the sample The scanner now scans along a straight line that should be parallel to the sample surface The slope of the line is defined by the x and y slope parameters in the imaging options section of the imaging panel The height of the line is determined at the start of each scan line First the z controller is turned on Once the tip position is sta ble the z controller is turned off and the tip is moved away from the sample by the distance set by the parameter Rel Tip Pos Fig 27 Angle measurement Angle be tween lines a 118 8 P2532000 PHYWE Systeme GmbH amp Co KG All rights reserved 11 Parabola ft 107pm HE np 2 i ce E H oa E 1 g oo i o yo il 3 07nm Atomic resol
49. htclick the graph and select Copy data to clipboard Fig 5 and paste the data in PHYWE measure analysis software by pressing ctrl v Here you can fit an exponential function by clicking Analysis Function fitting and selecting expo nential function in the uppermost drop down dialog Fig 6 Now click calculate to receive fitting para meters and Add new curve to draw the calculated curve into the diagram amp Chart Properties Copy Copy Data to Clipboard Fig 5 Sketch of the dual line graph You should be able to see the exponential function fits the measured data pretty good One can assume the exponential correlation is correct PHY WE excellence in science 4 PHYWE Systeme GmbH amp Co KG All rights reserved P2535000 TESS _ Quantum Mechanics by STM Tunneling Effect and Charge Density Waves expert Furthermore you can determine the effective workfunction for we know for our fit parameter b AJo and therefore b 2 p Az F n ps y R Function fitting sr ALCES Function Analyse exponential function left axis e la e bx c v Tip current Parameters Fit Tip current e Results Parameters a b c max iterations 225 Scattering 0 00158537591606012 Iterations 105 max relative error i 1 1E 6 Calculate Close Parameters gt Fig 6 Function fitting window in PHYWE
50. illed and empty _ O1 Set up and Procedure Task 1 Prepare a Pt ir tip and the sample surfaces Approaching the tip towards the sample For preparation of tunneling tips and the approaching procedure please refer to experiment P2532000 Atomic Resolution of the graphite surface by STM the quick start guide QSG or the operating manual OM Chapter 5 You ll find an electronic version of the QSG and OM under the help menue of the measure nano software In most cases it is enough to cleave the graphite sample once in a few weeks If you have problems to find a clean area or you don t get good images with several freshly prepared tips clean the sample surface using scotch tape as described in the QSG or OM The gold sample normally need not to be cleaned However if you experience bad scans throughout or accidently touched the sample you can lay a lint free cloth on top of the sample surface and drip some alcohol on it Remove the wet cloth by carefully wiping and wait until the alcohol has dried completely To avoid any scratches or other irreversible surface modifications it is important that the surface does not experience pressure at any time Normally the other samples have not to be prepared by a special procedure If you still have problems to image the surfaces carefully cleave the samples with the scotch tape method PHY WE excellence in science 2 PHYWE Systeme GmbH amp Co KG All rights reserved P2535
51. ing a pair of tweezers Stick a piece of adhesive tape to the graphite surface and apply very little pressure with your thumb or the end of the tweezers Use the tweezers to go under the adhesive tape and press the sample down to the table P2532000 PHYWE Systeme GmbH amp Co KG All rights reserved 3 Atomic resolution of TESS HYWE the graphite surface by STM expert bo tis Fig 6 Step by step pictures of the HOPG sample surface preparation Pull off the adhesive tape gently The topmost layer of the sample should stick to the tape If you are not satisfied with the cleaving e g the surface looks uneven or there are too many flakes re maining start from the beginning The middle of the sample surface should be very flat and mirror like Any loose flakes in the outer regions of the sample can be removed with the tweezers The graphite sample is now ready to use and should not be touched anymore Now that you have prepared the sam ple you need to mount it onto the sample holder Mounting the sample and the sample holder Unpack the Sample Holder touching only its black plastic handle Use the tweezers to push the sample to the edge of the supporting magnet in the sample package Grab the sample with the tweezers as shown in Fig 7 and place it on the magnet of the sample holder Put the sample holder down on to the sample holder guide bars first Fig 8 a and release it gently on to the approach
52. ir sp hybridized state with an angle of 120 for each bonding in the x y plane Fig 20 These are covalent bonds indicated with white connection pieces from atom to atom within one layer Cez rings From one layer to the other we observe electrostatic interactions Van der Waals forces In the 3D model this loose connection from p orbital repulsion is indicated by the violet connection pieces The distance between two adjacent graphene sheets in the graphite lattice is 9 18 aoe ee id a sete 0 3348 nm 334 8 pm see Fig 21 1300 paste 850 Deane be tween lines d 138 pm Parabola ft 107pm Topography range Onm 3 07nm side view top view Fig 20 Sketch of sp hybridized carbon atoms Fig 19 PHYWE 3D model of the graphite lattice Crystal lattice kit graphite 39840 00 Fig 21 3D sketch of the graphite lattice structure side view P2532000 PHYWE Systeme GmbH amp Co KG All rights reserved 9 Atomic resolution of the graphite surface by STM i ieee z te bes Es sxt efeneses seet n oe Pe Pe PE OF Se oe be be eee ee el ele Se be Oe de be be be Sober be be be ele lalel ele see ee oe ae Om i353 ne setetes PH WE expert La gt Sa we P 2 i yV A 4 LS mt ee i S gt 03 fee on TER gt ED opory 0292080 6061 Sa r a P 7 IEP It 2 43 e 6 9 ae oe rere wu ones ererney eee 2 8 9 0 6 8 6 8 0
53. le and the measurement tips Have a look in the STM user manual make sure the software starts cleanly and that you have set appropriate parameters to do measure Pt Ir wire ments on the graphite HOPG sample Chapter 5 3 3 in the Me rst user manual e g Set point 1 nA Tip voltage 50 mV direction Start the tip and sample preparation by taking the necessary tools out of the toolbox You will need parts 6 to 12 from the list mentioned above and some adhesive tape scotch tape not in cluded Flat nose Ay Tip preparation Use the pointed tweezers to carefully remove the old tip from the tip holder Hold the end of the wire tightly with the pliers Holding the wire with the pliers move the cutters at a m wire cutter length of approximately 4 mm as obliquely Fig 3 Sketch of the tip preparation PHY WE excellence in science 2 PHYWE Systeme GmbH amp Co KG All rights reserved P2532000 TESS SHYWE Atomic resolution of the graphite surface by STM expert as possible in a very sharp angle Close the cutters until you can feel the wire but do not cut the wire In order to obtain the required sharpness the tip needs to be torn off by pulling the wire cutter quickly away from you rather than cutting cleanly through the wire Use the pointed tweezers to hold the tip wire right behind the tip Release the flat pliers Transfer the tip to the microscope Tip mounting Put the tip wire undernea
54. le at www phywe com Instead a grinding and polishing machine use two microscopic slides Put the sample together with the polishing material on one of the sildes Use the second slide for polishing the surface of the sample by a circular movement of the slide Task 2 Investigate the topography of different rolled coined tempered grown etched or polished samples in constant current mode The difficulty in scanning extremely rough surfaces as we will do in this experiment is that the piezo crystals have a limited speed and range of operation Therefore extreme P2537000 Investigation in roughness and nano morphology of different metal samples by STM TEP 5 3 70 00 height differences in a small area can lead to scanning ar tifacts or even crashing the tip into the surface One way to prevent this from happening and achieving good images is to decrease the scanning speed meaning to increase the time per line of scanning Furthermore it s recommended to start scanning just a small area of few 10nm slowly increasing the image size as desired When receiving artifacts one can try to furthermore de crease the image size or scanning again with a higher tip surface distance i e increasing the tip voltage or decreas ing the set point followed by with drawing and re approaching the sample To receive good scans of large areas of a rough sample the sequence of procedure should be as followed 1 Prepare a tip and do a surface
55. measure Task 3 Imaging and characterization of charge density waves at different substrates Interpret the results regarding to the bandstructure Charge Density Waves CDW Low Dimensional metals quasi 1D or 2D can undergo a phase transition involving electron phonon coupling Hereby the atoms of the lattice change their equilibrium position This is only possible if the cost of elastic energy needed for this deformation is compensated by the gain of the electron energy The phase transition results in a new electronic band structure and periodicity of the lattice CDW a are interesting for a couple of reasons including separ ag ccs the propose that such a state could lead to su p _ 7 m perconductivity and a special AC DC response L Fig 7 Lattice with Period Fig 7 shows a 1D lattice with lattice parameter a a A possible second state is shown in Fig 8 Here the atoms are successively displaced left or right a 2h a 2b by b lt lt a The new Lattice Parameter is 2a Other cya b displacements with different lattice parameter are i S O E S possible too Eventual the lattice will configure in the energetically most favorable setting That is ra also the reason why only certain materials are _ observed to form CDW Fig 8 Lattice after displacement of atoms Rudolf Peierls was the first one to explain this effect In one dimension the periodicity of the crystal creates energy band gaps in the E k diagram at mul
56. motors support Be careful the magnet that holds the sample holder in its place can drag the sample off the sample holder make sure you bring the sample behind it Fig 7 Place the sample in the middle of the sample holder Guidebars rotor device Guidebars Motor device Fig 8 Sketch of the manual approach of the sample to the tip ri f WE excellence in science 4 PHYWE Systeme GmbH amp Co KG All rights reserved P2532000 TESS SHYWE Atomic resolution of the graphite surface by STM expert Push the sample holder carefully in the direction of the tip b but don t let it touch the tip 1 cm distance See also the step by step pictures in Fig 9 lt Approach the sample lt 2 Use the advance button io UGAT ELNA i 1 LS NET CR to move ppi w O hE towards the ti TUES _ _ y ne Recween the tip and the sampple ae almost touching you Wke mirror Phe in the s te Start the automati finaly S Surface should be visible approach j pie a We E e i oe gs Always check the distance 4 When tip ch image ah Fig 9 Step by step pictures of the software supported approaching of the sample to the tip Now that you have approached the sample manually use the approach panel in the measurement soft ware to drive the sample towards the tip Advance button To determine the distance between the tip and the sample adjust the lightning conditions in a
57. not symmetric at all This comes from the underlying band structure MoS has two band gaps at 1 29V and 1 95V We cannot image the 1 95V gap because our voltage range is to limited but the 1 29V gap can be seen in Fig 19 We have determined typical properties of semi conductors from which we can determine the kind of a material by just seeing it s I U spectroscopy curve This experiment demonstrate the potential of the scanning tunneling microscope to investigate not only the electrical characteristics in general but also at the nanoscopic scale Especially for artifical nanos structures STS can be used to investigate the electrical characteristics excellence in science PHY WE 9 P2533500 PHYWE Systeme GmbH amp Co KG All rights reserved TEP 5 3 70 00 Investigation in roughness and nano morpholo of different metal samples by STM expert Related Topics Tunneling effect Scanning Tunneling Microscopy STM Nano morphology Roughness Coining Rolling Tempering Growing Polishing Etching Principle One crucial requirement for a successfull functionalization of surfaces at the nanometer scale is their morphology at this scale The tunneling current between a metallic tip and different metal surfaces is used to investigate the mor phology on a nanoscopic scale of the samples by scanning across the surface and image the electronic topography The samples has undergone different processes like coining temperin
58. ower than for graphite although the actual work function should be higher for gold This comes from the different outside influences men tioned above and different tips which have been used for the different samples One can see that for similar regions on one sample the work functions will not scatter a lot Yet more important is the observation that the work function is generally smaller at defects than on clean terraces This can be explained by Smoluchowski s model In this model the valence electrons in a metallic metallike solid are almost free and flow along the atomic cores Defects in the solid s surface result in the expose of atomic cores while the electrons flow smooth across the defect Fig 8 This results in an abundance of electrons in certain areas negative charged regions with higher potential energies of electrons for example the lower edge of step edges which then can be extracted easier Fig 8 Sketch of the charge distribution at a surface defect Interpretation The result of a smaller work function is a higher reactivity of a solid surface and therefore defects in the surface make solids more chemically active One application of this effect is to increase the effectiveness in catalysts by using materials with rough surfaces where rough means the surface has a high density of defects It was shown that STM is a powerfool tool to determine work functions of nano structures This ability can be used for many pu
59. quality of the tip with the graphite sample before using the gold sample To activate the full measurement range of your device click the H Fullbutton in the imaging window For good results you can use image sizes of about 0 2 um When you are lucky you find a terrace in your first measurement If that is not the case you can either use the MOV tool in the imaging window to start the measurement at a different spot on the surface or you retract the sample from the tip and care fully turn the sample holder with the black plastic handle Afterwards you approach the sample again In Fig 5 you can find examples of terrace like structures at different spots on the HOPG and gold sample surface Make sure you adjust the parameters of the feedback loop to achieve good image quality You may switch to the standard or advanced level of the software user interface Options gt Config User Interface to obtain the possibility to adjust the P and the I gain individually Too high values in the P and l gain will be noticeable in the line graph as very high peaks and a very rough line structure Adjust the gain values to smooth the line graph and your image respectively When you are satisfied grab a good image by clicking the Photo button Now you can apply some filters to reduce noise adjust scan line levels and remove a background distortion The recommended procedure would be Apply glitch filter Apply noise filter Doa ba
60. rposes e g measuring the work function of optically stimulated surfaces or na nostructures P2533000 PHYWE Systeme GmbH amp Co KG All rights reserved 7 09620 00 2D Graphite Model PH WE excellence in science layer C rings From one layer to the other only electrostatic interactions take effect Van der Waals forces In the 3D model this loose connection from p orbital repulsion is indicated by the violet connec tion pieces The distance between two adjacent graphene sheets in the graphite lattice is 0 3348 nm 334 8 pm see Fig 21 A 0 335 nm B A xX Fig 5 3D sketch of the graphite lattice structure side Fig 6 2D sketch of the graphite lattice structure top view view The second aid you can use to get along on the atomic scale is the 2D model of graphite 09620 00 Fig 7 sy fe es alle liek aot E T raat eta tate a Tah A ki d SEE oe 0 0 8 9 6 0 6 0 0 0 5 00 Si Topai i h qi oe i aia Hi opa IPEPE LEPE Eaa DEPED WPT Y 6620 84 4 enters a ata es o Ti s TS on ace oir a a q we _ at p 0 0 04 ere ae EEE E E E E E PEERED atele E EE Fig 7 2D model of the graphite surface atoms and an underlying graphene sheet top view 09620 00 Start by bringing the foil matching on top of the paper the marker should show two circles Then trans late the foil diagonally Afterwards the markers should show three circles indicating th
61. rrors arise from the scanning itself temperature dependence of the piezo electric device but even more due to a bad z leveling or background correction Your results should not exceed a relative error of about 5 to literature values the lower the better When you have completed some measurements it is recom mended to calculate the mean value of your data for each proposed step size Add up every value v and divide the sum by the number of Topography Section values you added up 362pm Onm Section 43 40m Tt m gt v 2 y 2 m t i fa of The standard deviation is another calculation which tells you the accuracy of your measurement Topography range 30 4nm 1 a lt a Onm Sertion s 6 m I3 7 Fig 15 Cross section of image f see above Distance Az 686 1 pm P2532000 PHYWE Systeme GmbH amp Co KG All rights reserved 7 Atomic resolution of TESS CE PHYWE the graphite surface by STM expert This allows you to overview quickly your results in this form m s Table 1 Results Measurement No Step height Literature No atomic layers Error Az single step Fig 12 Az pm own data ee single graphene 333 05 1 20 pe 333 9 layer step 0 99 gt 1 pm Az 334 8 pm Atomic resolution on graphite To decrease the imaging area Eea AL EET iaaii Ta Click the color map chart to activate it A blue square is now sk FA aaa eS 5 oar 8 drawn around the color map ch
62. rsa Line ft 16 4nm Fig 11 TaS2 The plain surfaces of the grown materials are rooted in the growing process in which the single atoms lay down onto the surface slowly after each other Because the smallest possible surface is energetically favorable the material will form plain structures Topography Scan forward Raw data 742pm Fig 12 HOPG An important result of our measurements is that the sur face roughness of coined materials does not differ from rolled but when looking at the topography one can identi fy the coined material by the round chaotic structures whereas rolled materials have longish structures Also the nanoscopic roughness of grown materials uses to be low er than that of rolled and coined materials P2537000 PHYWE Systeme GmbH amp Co KG All rights reserved 7 TEP 5 3 70 00 Investigation in roughness and nano TESS HYWE morphology of different metal samples by STM expert a lf WE excellence in science 8 PHYWE Systeme GmbH amp Co KG All rights reserved P2537000
63. rt natural 09612 00 1 TaSe on sample support 09611 00 1 WSe on sample support 09610 00 Additionally needed 1 PC Windows XP or higher Adhesive tape PHYWE measure Fig 1 Experimental setup P2535000 PHYWE Systeme GmbH amp Co KG All rights reserved 1 TEP Quantum Mechanics by STM Tunneling TESS Effect and Charge Density Waves expert PH WE PHYWE Compact STM set Control unit with mounted scan head Magnifying cover glass 10X Magnification USB cable Power cord and adapter MeasureNano Software 1 PHYWE STM Tool Box Wire cutter Flat nose pliers Pointed tweezers Rounded tweezers Pt Ir wire d 0 25 mm 30 cm Sample Holder Graphite sample HOPG Gold sample Spare sample support NN oORON we Ne ee NA KA A Am Am mr mr mr or N a a AN AN AN AN a a a 3 3 32 32 OO ON O Nee WS Fig 2 Content of 09600 99 Scanning Tunneling Microscopy STM Operating Instructions and Experiments Tasks 1 Prepare a Pt Ir tip and the sample surfaces Approaching the tip towards the sample 2 Investigate the tunneling effect at HOPG and Gold by Scanning Tunneling Spectroscopy Current Distance Spectroscopy 3 Imaging and characterization of charge density waves at different substrates Interpret the results regarding to the bandstructure 4 Investigate in charge density waves at different voltages and interpret the imaged states f
64. set of parameters the device will divide the distance from Onm to 5nm from the current tip position into 128 points and measure the tunnel current at each of them The time used for one series of measurements is 0 2 s This process will be repeated 16 times The resulting curve is the average of the 16 measurements Notice For gold you will need higher gain values than for HOPG to be able to see the step structure as shown in Fig 5 Positive Start and End values will bring the tip closer to the surface 1 nm will change the tunnel cur rent about 1 magnitude High tunnel currents may influence the structure of the tip and or the surface Therefore try choosing a lower set point or lower start and end values if you receive high currents gt 20nA and the curve is not exponential As long as you are in the spectroscopy mode the piezo controllers are inactive Because of thermal drift you might receive different results when measuring the same point multiple times especially when using high resolutions Also modifications of the local tip geometry can occur during the mea suring procedure indicated by noisy and jumpy spectra Therefore check the topography after every spectroscopic measurements to evaluate the results Before you start measuring switch to a dual line graph first right click chart type Then prepare a measurement by clicking on Point and putting the cursor on the surface of your scanned image where
65. surement range Now Zoom in to atomic resolution to check the tip Adjust the parameters so that you can see a detailed picture of the surface You can save the picture by clicking the ca Photo button If you do you can apply some filters to reduce noise adjust scan line levels and remove a background distortion The recommended procedure would be Apply glitch filter Apply noise filter Doa background correction and or correct scan line levels The samples surfaces are shown in Fig 9 11 and 13 Furthermore the corresponding reduced band structers are shown in Fig 10 12 and 14 Change back to full measurement range an switch to the spectroscopy mode Choose Tip voltage as modulated output to record an I U curve in a plane region Pin down values of your measurement pa rameters accurately If you experience problems in measuring the desired behavior try to adjust your parameter values For example you can start experimenting with the following set of parameters Set point 1 0 nA P Gain 1000 Gain 2000 Tip voltage 0 05 V Start value 0 5 V End Value 0 5 V Modulation time 0 2 s Data points 128 Averages 16 By using this set of parameters the device will divide the interval from 0 5 V to 0 5 V into 128 points and measure the tunnel current at each of them The time used for one series of measurements is 0 2 s This process will be repeated 16 times The resulting curve is the average of
66. t Wy I yt Fig 1 Experimental setup excellence in science PHY WE P2533000 PHYWE Systeme GmbH amp Co KG All rights reserved 1 Nanoscale work function measurement TESS PH WE Scanning Tunneling Spectroscopy expert PHYWE Compact STM set 1 Control unit with mounted scan head 1 1 Magnifying cover glass 10X Magnification 2 1 USB cable 3 1 Power cord and adapter 4 1 MeasureNano Software 5 1 PHYWE STM Tool Box Wire cutter Flat nose pliers Pointed tweezers Rounded tweezers Pt Ir wire d 0 25 mm 30 cm Sample Holder Graphite sample HOPG Gold sample Spare sample support KA A om A mA mm om o 14 Fig 2 Content of 09600 99 Scanning Tunneling Microscopy STM 15 Operating Instructions and Experiments Tasks 1 Prepare a Pt Ir tip and the sample surfaces Approaching the tip towards the sample 2 Investigate the topography of the gold and HOPG sample on clean terraces and defects in constant current mode 3 Switch to spectroscopy mode Measure and compare images recorded on terraces and defects in Z Axis mode l z spectroscopy 4 Determine the local work function for the different areas and interpret the results Set up and Procedure Task 1 Prepare a Pt Ir tip and the sample surfaces Approaching the tip towards the sample For preparation of tunneling tips and the approaching procedure please refer to experiment P2532000 Atomic Resolu
67. t nelgiat Image mage from hilltop to valley corresponds to the size 5 nm Time Line 0 03 s Set d AT 81 68 DA point 1 2 nA Rel Tip Pos MEASTE OR ADA 0 14nm Distance between lines d 150 1 pm When you have finished your measurements you also have the possibility to present your data in 3D Fig 33 see chapter 4 4 3 and 4 5 of the user manual Select 3D View in the Select Chart Type drop down menu Then adjust the appearance until you are satisfied with the look Always click and hold the left mouse button on the 3D view chart while changing the 3D view The sur face is reduced in feature complexity as long as the left mouse button is pressed Press the following additional keys buttons to determine what chart property is changed Surface rotation mouse left right Surface tilt mouse up down Size displayed surface Ctrl key mouse up down Surface position Shift key mouse up down left right Z scale magnification left mouse button right mouse button mouse up down Light source direction 360 Shift Ctrl key mouse left right Light source height 0 90 Shift Ctrl key mouse up down Topography Scan forward Tip current Scan forward Line fit 436pm Line fit 1 17nA Fig 33 3D representation of constant current data left and constant height data right P2532000 PHYWE Systeme GmbH amp Co KG All rights reserved 13 PH WE 2D
68. tance For comparison of the constant current and the constant height between lines d 252 7 pm mode measure the distance from the hilltop to the valley in Fig 26 the distance is Az 15 3 pm Do the measurements as exact as possible minimal differences in the line drawing only a few pixel can make a big difference in your resulting values To further support your results do a quick estimation of mean values see above from at least three to five mea surements more will increase the accuracy even further The bonding angle between atoms you are able to determine by using the Measure Angle tool Fig 27 Parabola ft 107pm Topography range Topography Section 25 99pm Imaging with the constant height mode All images so far have been acquired in the constant current mode where the current between the tip and the sample is kept constant The distance between them is proportional to the flowing tunneling current so the distance is also kept con stant The measurement signal here is the elongation of the z piezo which drives the tip movement and follows the sur Raw data Topography range 60 Spm Fig 26 Cross section through a row of atoms face topography Let us now switch to the constant height see Fig 17 Distance between lines mode It is an advanced measurement mode where you fix d 248 5 pm hill to hill AZ 15 3 pm hill the tip height above the sample surface to a specific value top to valley T
69. th the clamp on the tip A B holder A parallel to the groove and push it all the way to the end a 2 Move the tip wire sideways until it is in the groove and held securely under the clamp B It should stick out about 1 2 mm beyond the tip holder See lt M also Fig 5 for step by step pictures i ai m l a Fig 4 Sketch of the tip mounting an n E 1 Place the tip on _ 2 Move the tip sideways i 3 Always hold the tip the right side under the clamp towards the groove tightly with the tweezers i t avoid snapping eT a ion ee ay gt a 4 Adjust the position of the 5 Make sure the tip is straight 6 If necessary push the tip tipsinside the middle groove and points towards the sample backwards the tip should only stick out 1 2 mm Fig 5 Step by step pictures of the tip mounting Now that you have prepared a new a tip and mounted it in the tip holder s groove proceed with preparing the sample surface Sample preparation In most cases it is enough to cleave the graphite sample once in a while If you have problems to find a clean area or you don t get good images with several freshly prepared tips clean the sample surface as described below Highly oriented pyrolytic graphite HOPG consists of weakly bonded van der Waals bonding layers Due to this layered structure of graphite it can easily be cleaved using a piece of adhesive tape Put the sample on the table us
70. the probability to find an electron behind a barrier is unequal zero Inside the barrier the wave function of the elec maS Sketch of the tunneling oF an electron tron decays exponentially Fig 3 through a barrier STM Scanning Tunneling Microscopy The size of an atom in relation to the tip is that of a golf ball to a mountain In your PHYWE measurement system a platinum iridium tip is moved in three dimensions using piezo crystal translators that are driven with sub nanometer precision Fig 4 The sample to be examined approaches the tip within a distance of about 1 nanometer 1 nm 1 1 000 000 000 m Classical physics would prohibit the appearance of electrons in the small gap between the tip and the sample but if a sharp tip and a conducting surface are put under a low voltage U 0 1 V a very small tunneling current I 1 nA with 1 nm tip sample distance though may flow between tip and sample The resulting tunneling current is a function of tip position applied voltage and the local density of states LDOS of the sample this information is displayed on the computer monitor as 2D color mapped image The strength of the tunneling current depends exponentially on the distance between the tip and the sample d usually referred to as z distance and the applied bias U From Schrddinger s equation one can find that Fig 4 Sketch of the piezo electric device driving the tip movement I f U exp A d 1 where
71. the tip very close to the sample surface The recommended procedure to achive good scans is 1 178nm Prepare a tip and do a surface scan on HOPG to check if the tip is good by resolving single atoms set point 1nA tip voltage 50mV Change to a sample which forms CDW Set image size to approximately 100nm tip voltage to 20mV set point to 4nA and approach the sample You should see terrace like structures comp fig 12 if not try using the cleaning pulse button or withdraw the sample and reapproach a different spot on the sample Zooming into a clean terrace you should be able to see the CDW at a resolution of about 80nm fig 13 Continue zooming into an area with high periodicity until you receive the desired resolution To obtain atomic resolution it is needed to increase the set point up to about 30nA withdraw and reapproach the sample When increasing the set point remember decreasing the l and P gain respectively When obtaining the desired image adjust the gain values to smooth the image You can also try to increase the points line for even better scan quality When you are satisfied grab a good im age by clicking the Photo button Now you can apply some filters to reduce noise adjust scan line levels and remove a background distortion The recommended procedure would be Apply glitch filter Apply noise filter Doa background substraction and or correct scan line levels Topography
72. tion of the graphite surface by STM the quick start guide QSG or the operating manual OM Chapter 5 You ll find an electronic version of the QSG and OM under the help menue of the measure nano software In most cases it is enough to cleave the graphite sample once in a few weeks If you have problems to find a clean area or you don t get good images with several freshly prepared tips clean the sample surface using scotch tape as described in the QSG or OM The gold sample normally need not to be cleaned However if you experience bad scans throughout or accidently touched the sample you can lay a lint free cloth on top of the sample surface and drip some alcohol on it Remove the wet cloth by carefully wiping and wait until the alcohol has dried completely To avoid any scratches or other irreversible surface modifications it is important that the surface does not experience pressure at any time PHY WE excellence in science 2 PHYWE Systeme GmbH amp Co KG All rights reserved P2533000 TESS Nanoscale work function measurements by PHYWE expert Scanning Tunneling Spectroscopy Theory and Evaluation Tunneling Tunneling is a functioning concept that arises from quantum mechanics Classically an object hitting an impenetrable bar rier will not pass through In contrast objects with a very small mass such as the electron have wavelike characteristics which permit such an event referred to as tunneling In other words
73. tip movement P2537000 TESS PHYWE expert Task 3 Compare the scanned images and apply the Roughness analysis tools After scanning a sample use the integrated function Cal culate Area Roughness to Rs mine the nanoscopic roughness of the sample This will result in an output as shown in Fig 12 You can look up the meaning of the sin Todak o o 6 gle values in the Operat ing Instructions and Expe Pene j vas riments Scanning Tunnel Aea 121 9 9m Sa 106 5om Microscopy 4 3 6 nual ma Sq 1376 5om Sy 12 46 nm Sp 7 21430m Sv 4 6525nm Sm 3 736 1fm Store We are mostly interested in S which is the maxi mum height difference of any two points on the sur face because it reflects the roughness of the sam ple Fig 12 Area Roughness measurement Comparison of Surfaces Due to the different procedures of manufacturing we ex pect the surfaces to have characteristic structures In the following you can see pictures taken from grown rolled and coined surfaces Some of the images are displayed as shaded map in PHYWE measure nano right click gt Chart Type gt Shaded map Topography Scan forward 513nm Line fit 34 1mm Topography range 44 2nm Fig 5 Ta shiny Tantalum sheets are rolled and have a shiny and a dark side Images 5 and 6 show the two surfaces of such a P2537000 Investigation in roughness and nano morphology of different metal samples by STM
74. tip position the applied voltage mee and the local density of states LDOS of the sample this iN Fig 4 Sketch of the piezo electric device formation is displayed on the computer monitor as 2D color driving the tip movement mapped image Task 2 Investigate the tunneling effect at HOPG and Gold by Scanning Tunneling Spectroscopy Current Distance Spectroscopy The strength of the tunneling current depends exponentially on the distance between the tip and the sample dz usually referred to as z distance and the applied bias U From Schrddinger s equation one can find that I f U exp A d 1 where 2Me a 10 25nm eV A 2 Here is the effective work function This is an actual workfunction mainly influenced by surface adsor bates and the high electrical field density at the tip The extreme dependence on the distance makes it possible to measure the tip sample movement very precisely By keeping the tip voltage constant and measuring the tunnel current in dependency of d the effective work function can be determined by fitting an exponential function to the measured tunnel cur rent 1 Always keep in mind to do measurements preferably at the same room temperature Tempera ture dependencies create uncertainty and errors in the expected values of your measurements To evaluate 1 the following procedure can be used After grabbing a good image switch to the Spectroscopy mode by clicking the
75. tiples of the value k Tr a In the model the ions each contribute one electron then the band will be filled up to the Fermi energy Ep i e up to values of kf 11 2a in the ground state as shown in Fig 9 a 1 For details see Nanoscale workfunction measurements by scanning tunnel spectroscopy by PHYWE P2535000 PHYWE Systeme GmbH amp Co KG All rights reserved 5 TEP Quantum Mechanics by STM Tunneling TESS PHYWE Effect and Charge Density Waves expert If the lattice period changes to 2a by lattice distortions as in Fig 9 b this has the effect of introducing new band gaps at k 11 2a This causes the electrons to be at lower energy than in the original lattice Hence this lattice distortion becomes energetically favorable when the energy savings because of the new band gaps is larger than the elastic energy cost of the lattice deformation This effect will only be noticeable when the electrons are arranged in states close to the ground state meaning the lattice needs to be under a characteristic temperature the Peierls temperature Pe jg ot b p Pit O oO OU O O O oO Q a atoms e K I l f K i w a Kp QO Ke a Kf 0 Ke w 20 metal insulator Fig 9 Band structure of a 1D lattice with Period a As we can see also from Fig 9 the CDW state leads to a transition from a conductor to a semiconductor insulator because of the new bandgap Although CDW typically are an one dimensional eff
76. unnel currents may influence the structure of the tip and or the surface Therefore try choosing a lower set point or lower start and end values if you receive high currents gt 20nA and the curve is not exponential As long as you are in the spectroscopy mode the piezo controllers are inactive Because of thermal drift you might receive different results wnen measuring the same point multiple times especially when using high resolutions Also modifications of the local tip geometry can occur during the mea suring procedure indicated by noisy and jumpy spectra Therefore check the topography after every spectroscopic measurements to evaluate the results To evaluate the exponential correlation between tunneling current and d click Point in spectroscopy mode and select a point in the topography Now press the Start button After a few seconds the end of the measurement is indicated when the Stop button changes back to Start and when your data curve doesn t change anymore The Probe Status should be green during the whole measurement If the Probe Status changes to orange switch to the Imaging Window approach the sample again and take a new surface scan afterwards switch back to the Spectroscopy window H Increase Chart Data Range Tip current Spec forward After the measurement is finished you should already be able to identify a exponential cuve in the line graph Now you can rig
77. ution of TESS CE PHY WE the graphite surface by STM expert Start imaging with relatively large image size 40 60 nm and get used to Wiens Sirs this mode Fig 29 Switch to the appropriate measurement signal Tip Current in the Select Signal drop down menu located in the main tool Scan mode bar Most of the times you need some attempts to get a good value for Continuous F the relative tip position Start with safe values of about 25 nm if you end area up with a blank black imaging window you can decrease the value in Measurement mode small increments to 15 5 1 0 5 0 25 0 125 0 075 nm etc Scan Forward Watch the imaging window carefully and wait until you get a reproducing tip current signal Adjustments to the scanning can only be done by L Const Heigtt mode changing the tip current set point the tip voltage and the relative tip po Rel Tip Pas 100nm s sition above the sample surface The images you take seem to be af EE fected by noise much more than images taken in the constant current mig 28 mano modes re ip mode because the feedback loop is turned off and slightest topographic the imaging window ad differences will give a change in the tip current signal Some other fea vanced user interface tures of your sample like special electronic states of surface atoms might only be visible in this mode Step heights of terraces can t be measured directly within this mode because
78. way you can see the mirror image of the tip in the sample surface During this procedure you should also check the probe status light in the software The status light hast to be orange If the probe status switches to red you crashed the sample into the tip You can try to use the Withdraw button to drive the sample a small step backwards and start with the final approach but it is likely that you have to start again with the preparation of a new tip When the tip and its mirror image are about to touch you can proceed with the automatic final approach press the Approach button The automatic approach can take several minutes please be patient as the duration depends on the gap you left over from the manual approach After the successful approach the probe status light will switch to green Then the software will do a slope correction and starts measuring right away Theory and Evaluation Tunneling Tunneling is a functioning concept that arises from quantum mechanics Classically an object hitting an impenetrable bar rier will not pass through In contrast objects with a very small mass such as the electron have wavelike characteristics which permit such an event referred to as tunneling In other words the probability to find an electron behind a barrier is unequal zero Inside the barrier the wave function of the elec tron decays exponentially Fig 10 Sketch of the tunneling of an electron through a barrier P253
79. which are available to be occupied in a electronic system The LDOS is proportional to the slope of the characteristic curve re i ceived from I U Spectroscopy when regarding the tip s density of states to be with no structure We will use this to image the band structure of semi conductors The measured curves are a overlay of the bands in the corresponding band schematic The expected results are shown in Fig 8 Occupied states Electronic Structure Fig 7 did A Direct band gap semi conductor B Indirect bandgap semi conductor C Sami metal Fig 8 Properties of different kinds of conductors Always keep in mind to do measurements preferably at the same room temperature Temperature de pendencies create uncertainty and errors in the expected values of your measurements PHY WE excellence in science 4 PHYWE Systeme GmbH amp Co KG All rights reserved P2533500 TESSI siio Nanoscale characteristics by expert Scanning Tunneling Spectroscopy Task 2 and 3 Investigate the topography of the gold HOPG and MoS2 sample in constant current mode Switch to spectroscopy mode Measure and compare images recorded on the different materials in Tip voltage mode l U spectroscopy Investigation of band structure The procedure imaging the I U spectroscopy curve is the same for all three samples After you ap proached the sample click the m Fullbutton in the imagaing window to activate the full mea

5.3.20- 00 Atomic resolution of the graphite surface by STM

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