STM and AFM Tutorial. Katie Mitchell January 20, 2010

Transcription

1 STM and AFM Tutorial Katie Mitchell January 20, 2010

2 Overview Scanning Probe Microscopes Scanning Tunneling Microscopy (STM) Atomic Force Microscopy (AFM) Contact AFM Non-contact AFM RHK UHV350 AFM/STM

3 Scanning Probe Microscopes Scanning tunneling microscope Sharp probe tip is scanned across surface Piezoelectric materials provide atomic scale control of x,y,z motion Detect signal that reflects topography or other spatially varying property of interest Good vibration isolation required

4 Scanning Tunneling Microscopy V ~ V I ~ pa na Image from Will's Nanotechnology Blog can operate in air, liquid or vacuum sample and tip must be conducting

5 E F E F Scanning Tunneling Microscopy Tip 1D Potential Barrier a) Sample 0 2 exp w n 2z E F Tip ev b) e - Sample E F E F Tip e - 2m c) z 2 Sample ev E F

6 Natural Log of Tunneling Current Current-Distance Dependence 1 0 W and Pt-Ir tips on Si(111)-7x = 1.0±0.1 Å Tip Displacement (Å) M. Rempel, MSc Thesis, Univ. of Saskatchewan, 2003

7 Scanning Tunneling Microscopy 3D Picture E E M w 2 2 d M E ev E e I F F ev d E ev E e I F F ev 0 4 Transition probability Tunneling current Assuming s- wave tip state

8 STM on Au(111) x 100 nm, 25 mv, 1.15 na

9 Au(111): High Resolution 50 x 50 nm, 25 mv, 1.15 na 5 x 5 nm, 90 mv, 1.09 na

10 Highly Ordered Pyrolytic Graphite (HOPG) 5 nm x 5 nm STM image 0.1V, 1.0 na Pt-Ir tip Images, Mark Rempel HOPG lattice

11 Si(111) 7x7-1.5 V, 0.42 na - filled states Model of Si(111)-7x7 surface structure; the side length of the surface unit cell is 26.6 Å +1.5 V, 0.50 na - empty states STM images, Dale Heggie

12 Si(111) 7x V, 0.12 na +1.5 V, 0.78 na +1.5 V, 0.80 na +2.0 V, 0.34 na STM images, Dale Heggie

13 InP(100) V, 0.35 na -2.0 V, 0.23 na STM images, Dale Heggie

14 InP(100) ghost tip images noise -1.8 V, 0.58 na

15 Scanning Tunneling Spectroscopy Measure I(V) Calculate sample density of states di dv I V E ev F

16 Si(111)-7x7 (di/dv)/(i/v) as a function of sample bias, for Si(111)-7x7 (a) (d) W tip (e) (f) Pt-Ir tip features can be compared to expected density of states for Si(111)-7x7 surface Figure, Mark Rempel

17 STM Tips Most common tips Electrochemically etched W wire Mechanically cut Pt-Ir wire Desirable properties Conducting Hard Inert (for imaging in air) Best shape varies with application

18 SEM images Tungsten STM tip imaged at 150x and 20,000x magnification a) b) 100 m 1 m

19 DC Drop-off Method for Tungsten Anode (W wire) and cathode submerged in aqueous solution of NaOH or KOH W wire etches and breaks at the air/solution interface Remaining wire is used as the tip C.J. Chen, Introduction to Scanning Tunneling Microscopy. OUP (1993) 19

20 Etching Mechanism Anode W(s) + 6OH - WO 3 (s) + 6e - + 3H 2 O WO 3 (s) + 2OH - WO 2-4 (aq) + H 2 O Cathode 6H 2 O + 6e - 3H 2 (g) + 6OH - Overall W(s) + 2OH - + 2H 2 O 3H 2 (g) +WO 4 2- (aq) G. Stephen Kelsey, J. Electrochem. Soc. 124(6), 814 (1977). 20

21 Etching Apparatus 21

22 Atomic Force Microscopy measure deflection of a cantilever of known spring constant, due to tip-surface interaction forces F ~ nn can operate in air, liquid or vacuum Image from Will's Nanotechnology Blog sample and tip may be conducting or insulating

23 Atomic Force Microscopy Wikipedia Contact Mode Operate in region of repulsive tip-sample forces Scanning at constant cantilever deflection results in contour map of constant force, which represents topography Twisting of cantilever is a measure of lateral (friction) forces Contact mode can be very useful but can also be damaging to delicate surfaces

24 Atomic Force Microscopy Wikipedia Non-Contact Mode (NC AFM) Operates in region of attractive (van der Waals) forces Cantilever is forced to oscillate at its resonant frequency ( khz) Change in force experienced by tip changes the resonant frequency Change in resonant frequency is used as the feedback signal Very high Q cantilevers allow very small changes in resonant frequency to be detected, and thus small changes in force Ideal for delicate samples, and can give atomic resolution

25 Force-Distance Curves Shahin, V. et al. J Cell Sci 2005;118:

26 AFM Cantilevers and Tips SEM image of cantilever Microfabricated cantilevers with integrated tips Silicon nitride, silicon oxide, silicon Spring constants: N/m (contact), N/m (non-contact) Resonance frequencies: 1-50 khz (contact), khz (non-contact) Coatings depend on application: eg. conducting, magnetic, functionalized (specific molecules)

27 Cantilever Example Nanosensors PPP-NCL-10 Intended for non-contact mode Fabricated from n-type silicon no coating Specifications t = 7.00 microns w = 38 microns l = 229 microns h = microns c = 47 N/m f = 184 khz Tip radius ~ 7 nm

28 RHK UHV 350 AFM/STM full range of AFM acquisition modes AFM, STM or simultaneous AFM/STM Position sensitive detector RHK Technology Inc. Cantilever Sample holder

29 Design Features Sample stationary, tip/cantilever scanned Beetle or Walker design Piezo legs walk down ramp due to slip/stick Piezo legs used for approach and scanning Central piezo stack used for z motion Laser deflection and 4-quadrant PSD AFM, Wikibooks

30 Design Features Head accepts STM tip or AFM cantilever holders Tip and cantilever holders can be transferred on and off in vacuum Sample may be heated or cooled while scanning Vibration isolation External airlegs Viton stack supporting sample stage Scan head designed to have high resonant frequencies Scan head decoupled from chamber

31 Sample Holders Sapphire washer to hold sample in place Thermocouple connections Ramps for piezo legs

32 SPM 1000 controller Primary interface Control Hardware Front-panel controls PLL Pro Digital controller for AFM PMC 100 Piezo Motor Control Controls laser lens and position sensitive detector motors for AFM Small piezo control box (for X,Y,Z)

33 XPMPro Control Software STM/AFM data acquisition, processing and analysis May be used free for data analysis on any computer running Windows (XP, Vista)* PLLPro Controls the AFM mode, and all setup and parameters for AFM operation *

34 Specifications From RHK product brochure