Instrumentation and Operation

Transcription

1 Instrumentation and Operation 1 STM Instrumentation COMPONENTS sharp metal tip scanning system and control electronics feedback electronics (keeps tunneling current constant) image processing system data points image (off-line data analysis) 2 1

2 etched W STM Tips α profile for wide scans good for imaging of steep edges 50 µm not perfect for atomic resolution cut Pt/Ir 80:20 α profile good for atomic resolution (possible) cutting with e.g. scissors no universal recipe! 3 Tip Preparation cutting cleaning heating, oscillation (may produce blutness or lead to melting) voltage pulses continuous scanning e.g. increasing voltage while scanning with Au tip 25 Å elongation higher resolution sputtering however: good results also without 4 cleaning! 2

3 Chemical Composition of STM Tip Surface (1) _ + Na resolution depends on polarity due to adsorbed impurity atom 5 STM Tip Composition theoretically better resolution for d elements more localized electrons (e.g. Na W) however Au (s band) also high resolution identity of tip atom not known noble metals less susceptible to contaminations e.g. W always layer of oxide + other contaminations but high activation energy for surface diffusion (1,8 ev) 6 3

4 Quality Control of STM Tips course optical microscope: bad performance can be expected! STM experiment image quality (check different tips) [field ion microscope] more information AFM 7 Single Tube Scanner material: lead zirconium titanate ceramics (PZT) metal electrodes by vapor deposition -X Y X ν res khz (bar: 1 8 khz) non linear Z cross talk translation of tip or sample 8 4

5 Scan Parameters X frequency: 0,1 122 Hz (NanoScope, DI) Y frequency according to number of lines per image 128 x 128 data points per image 256 x 256 data points per image 512 x 512 data points per image 1 second to 1,5 hours for 1 image typically 30 seconds to 1 minute scan angle, size, frequency,... can be varied 9 Feedback System bias voltages: some 10 mv (up to several volts) tunneling current: some na modes of operation constant height mode small areas high scan rates possible elimination of thermal drifts, high resolution imaging constant current mode low scan rates wide area scans lower risk for tip crashes 10 5

6 Image Processing System interface: analogue digital (data to PC) lateral resolution 128 x 128 pixels 256 x 256 pixels 512 x 512 pixels z scale: 64 k resolution off line analysis image analysis filtering zooming etc. 11 AFM Instrumentation COMPONENTS sharp tip + soft spring scanning system and control electronics detection for spring defl. feedback electronics (keeps force constant) image processing system data points image (off-line data analysis) 12 6

7 Tips and Springs (Cantilevers) historical: gold foil (Binnig) first developments: fine wires (e.g. W) with diamond tips (glued onto wire) W wire diamond tip today: microfabricated cantilevers (not to scale!) 4 mm 200 µm 1,5 mm Si 3 N 4 glass substrate 100 µm integrated pyramidal tip fabrication: photolithographic mulitstep process starting from a silicon wafer k = 0,1 1 N/m ν 0 = khz 13 Vibration Problem typical vibrations of buildings: ν < 20 Hz damping factor = (ν/ν 0 ) 2 for ν << ν 0 amplitude of tip < 0,01 Å forces from N can be detected high frequency vibrations and noise must be eliminated! 14 7

8 Vibration Isolation heavy stone support on bungy cords low resonance frequencies optional noise isolation 15 Si 3 N 4 Cantilever Wafer approximately 500 cantilever substrates 16 8

9 Cantilever Break-Off 17 Cantilever Mount (2) Digital Instruments Veeco, Santa Barbara, CA, USA 18 9

10 SEM Image of AFM Tip and Cantilever pyramidal silicon nitride tip radius of curvature 20 nm 19 Si Cantilevers different apex angles important parameters: spring constant (0, N/m) apex angle resonance frequency (6 600 khz) length ( µm) thickness (1 7 µm) k = 0,1 N/m + d = 0,1 Å F min = N 20 10

11 Tip Quality Control at the atomic level AFM experiment HOPG (graphite) with atomic resolution (two types of atoms visible) optical microscope simulations 21 AFM Image Simulation (2) 142 pm 20 pm images for different two atomic tips 22 11

12 Detection of Spring Deflection (5) optical lever scheme I I PD1 PD1 + I I PD2 PD2 in commercial instruments detector far away from measuring cell measurements under liquids stable against influences from outside atomic resolution z resolution 0,1 Å signal for deflection (= force) further systems: e.g. piezolever 23 Force Detection by Optical Lever X,Y adjusting knobs for laser position photodiode adjusting knob filter laser prism mirror split photo diode detector cantilever Institute of Analytical Chemistry Vienna University of Technology 24 12

13 Commercial Liquid Cell laser in/out cantilever glass cover piezo scanner sample Institute of Analytical Chemistry Vienna University of Technology silicone o-ring 25 AFM Modes of Operation constant height mode height position of sample unchanged variation of cantilever deflection is detected small areas high scan rates possible elimination of thermal drifts, high resolution imaging (atomic resolution!) constant force mode cantilever deflection kept constant by feedback loop low scan rates wide area scans lower risk for tip crashes 26 13

14 Other AFM Components scan system: piezo elements see STM feedback loop deflection from sub Ångstrøm range to several micrometers modes of operaton analogeous to STM constant height mode constant force mode image processing system see STM 27 Commercial Instrument (3) NanoScope Multimode SPM Digital Instruments Veeco Santa Barbara, CA USA 28 14

15 AFM Head (2) Digital Instruments Veeco, Santa Barbara, CA, USA 29 Optical Head mirror laser diode photo detector cantilever sample Digital Instruments Veeco, Santa Barbara, CA, USA 30 15

16 Cantilever Mount (2) Digital Instruments Veeco, Santa Barbara, CA, USA 31 Laser Alignment with Paper Method (2) 32 16

17 Laser Alignment with Paper Method (1) 33 Laser Alignment with Paper Method (3) 34 17

18 Force Optimization also additional information from force distance curves 35 NanoScope III System Digital Instruments Veeco, Santa Barbara, CA, USA 36 18

19 NanoScope III TM-AFM (1) Digital Instruments Veeco, Santa Barbara, CA, USA 37 NanoScope III TM-AFM (2) Digital Instruments Veeco, Santa Barbara, CA, USA 38 19

20 NanoScope III Optical Head Digital Instruments Veeco, Santa Barbara, CA, USA 39 NanoScope III AFM Scanner Digital Instruments Veeco, Santa Barbara, CA, USA 40 20

21 NanoScope III Cantilever Holder (2) Digital Instruments Veeco, Santa Barbara, CA, USA 41 NanoScope III Cantilever Holder (4) Digital Instruments Veeco, Santa Barbara, CA, USA 42 21

22 Liquid Cell Digital Instruments Veeco, Santa Barbara, CA, USA 43 Analytical Properties Analytical Aspects 44 22

23 STM Properties for Analytical Chemistry (1) atomic resolution, but also microscopic range real space imaging local probe topography with direct depth information (above atomic level) on atomic scale electronic structure LDOS EF in-situ measurements in gases or liquids possible chemical reactions in-situ electrochemistry (potential control!) at electrode surfaces (in-situ) 45 STM Properties for Analytical Chemistry (2) in many cases simple sample preparation ( as it is or cleavage) also local spectroscopy possible Scanning Tunneling Spectroscopy (STS) local barrier height imaging l-v curves DRAWBACKS: no direct element specific information possible artefacts by asymmetric tips 46 23

24 AFM Properties for Analytical Chemistry (1) atomic resolution, but also microscopic range real space imaging local probe topography with direct depth information both conductors and insulators (also organic and biological samples!) in-situ measurements in gases or liquids possible chemical reactions in-situ electrochemistry (potential control!) at electrode surfaces (in-situ) 47 AFM Properties for Analytical Chemistry (2) in many cases simple sample preparation ( as it is or cleavage, no coating necessary!) additional information from force distance curves further material properties by special techniques (e.g. elasticity by force modulation, friction by LFM) DRAWBACKS: no direct element specific information possible artefacts by asymmetric tips 48 24

25 Artefacts and Solutions (1) from tip: objects sharper than tip image of tip e.g. pyramids representing the image of the tip edges parallel to scan direction 49 Artefacts and Solutions (1a) 50 25

26 Artefacts and Solutions (2) from tip: asymmetric tips bad resolution along one direction e.g. check e.g. rotation of scan direction 51 Artefacts and Solutions (3) from tip: multiple tips multiple images e.g

27 Artefacts and Solutions (4) from tip: convolution of tip and sample geometry 53 Artefacts and Solutions (5) vibrations: e.g. line structures, not from sample e.g. check image changes with: scan rate constant image scan angle image rotation scan size correct distances 54 27

28 Artefacts and Solutions (6) thermal drifts: e.g. Y scan disabled α α wait for stabilization 55 Artefacts and Solutions (7) impurities: e.g. particle particles can disturb motion of tip distortion of single scan lines perturbation clean samples imaging under liquids 56 28