STM and AFM Tutorial

STM and AFM TutorialKatie MitchellJanuary 20, 2010

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

Scanning Probe MicroscopesScanning tunneling microscope• Sharp probe tip is scannedacross surface• Piezoelectric materialsprovide atomic scalecontrol of x,y,z motion• Detect signal that reflectstopography or otherspatially varying propertyof interest• Good vibration isolationrequired

Scanning Tunneling Microscopy• V ~ 0.1 - 2 V• I ~ pA – nAImage from Will's Nanotechnology Blog• can operate inair, liquid orvacuum• sample and tipmust beconducting

E FE FScanning Tunneling MicroscopyTip1D Potential Barriera)Sample0 2 exp w n2zeVe -TipSample Tipb)c)E Fe - 2m E FE FE Fz 2SampleeV

Natural Log of Tunneling CurrentCurrent-Distance Dependence10W and Pt-Ir tips onSi(111)-7x7-1-2-3 = 1.0±0.1 Å -1-4-5-6-70.0 0.5 1.0 1.5 2.0 2.5 3.0Tip Displacement (Å)M. Rempel, MSc Thesis, Univ. of Saskatchewan, 2003

Scanning Tunneling Microscopy3D Picturew2M2 EETransition probabilityI4eeV02E eV EMdFFTunneling currentI4eeV0E eV E dFFAssuming s-wave tip state

STM on Au(111)120°60°100 x 100 nm, 25 mV, 1.15 nA

Au(111): High Resolution50 x 50 nm, 25 mV, 1.15 nA5 x 5 nm, 90 mV, 1.09 nA

Highly Ordered Pyrolytic Graphite(HOPG)5 nm x 5 nmSTM image0.1V, 1.0 nAPt-Ir tipImages,Mark RempelHOPG lattice

Si(111) – 7x7-1.5 V, 0.42 nA - filled statesModel of Si(111)-7x7surface structure; the sidelength of the surface unitcell is 26.6 Å+1.5 V, 0.50 nA - empty statesSTM images, Dale Heggie

Si(111) – 7x7+1.5 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

InP(100)-1.81 V, 0.35 nA -2.0 V, 0.23 nASTM images, Dale Heggie

InP(100)“ghost” tip imagesnoise-1.8 V, 0.58 nA

Scanning Tunneling Spectroscopy• Measure I(V)• Calculate sample density of statesdIdVIV E eV F

Si(111)-7x7• (dI/dV)/(I/V) as a function ofsample bias, for Si(111)-7x7• (a) – (d) W tip• (e) – (f) Pt-Ir tip• features can be comparedto expected density of statesfor Si(111)-7x7 surfaceFigure, Mark Rempel

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

SEM imagesTungsten STM tip imaged at 150x and 20,000x magnificationa) b)100 m 1 m

DC Drop-off Method for Tungsten• Anode (W wire) and cathodesubmerged in aqueoussolution of NaOH or KOH• W wire etches and breaks atthe air/solution interface• Remaining wire is used as thetipC.J. Chen, Introduction to Scanning Tunneling Microscopy. OUP (1993)19

Etching MechanismAnodeW(s) + 6OH - WO 3 (s) + 6e - + 3H 2 OWO 3 (s) + 2OH - WO2-4 (aq) + H 2 OCathode6H 2 O + 6e - 3H 2 (g) + 6OH -OverallW(s) + 2OH - + 2H 2 O 3H 2 (g) +WO 42-(aq)G. Stephen Kelsey, J. Electrochem. Soc. 124(6), 814(1977).20

Etching Apparatus21

Atomic Force Microscopy• measure deflection ofa cantilever of knownspring constant, due totip-surface interactionforces• F ~ nN• can operate in air,liquid or vacuumImage from Will's Nanotechnology Blog• sample and tip may beconducting or insulating

Atomic Force MicroscopyWikipedia• Contact Mode– Operate in region of repulsive tip-sample forces– Scanning at constant cantilever deflection results in contour mapof 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 todelicate surfaces

Atomic Force MicroscopyWikipedia• Non-Contact Mode (NC AFM)– Operates in region of attractive (van der Waals) forces– Cantilever is forced to oscillate at its resonant frequency (100-300 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 frequencyto be detected, and thus small changes in force– Ideal for delicate samples, and can give atomic resolution

Force-Distance CurvesShahin, V. et al. J Cell Sci 2005;118:2881-2889

AFM Cantilevers and TipsSEM image of cantilever• Microfabricated cantilevers with integrated tips– Silicon nitride, silicon oxide, silicon– Spring constants: 0.1-1 N/m (contact), 10-100 N/m (non-contact)– Resonance frequencies: 1-50 kHz (contact), 100-300 kHz (non-contact)– Coatings depend on application: eg. conducting, magnetic,functionalized (specific molecules)

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 = 10 – 15 microns• c = 47 N/m• f = 184 kHz• Tip radius ~ 7 nm

RHK UHV 350 AFM/STM• full range of AFM acquisition modes• AFM, STM or simultaneousAFM/STMPosition sensitivedetectorRHK Technology Inc.CantileverSample holder

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 PSDAFM, Wikibooks

Design Features• Head accepts STM tip or AFM cantilever holders• Tip and cantilever holders can be transferred on and off invacuum• 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

Sample HoldersSapphire washer to hold sample in placeThermocouple connectionsRamps for piezo legs

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

• XPMProControl Software– STM/AFM data acquisition, processing and analysis– May be used free for data analysis on any computerrunning Windows (XP, Vista)*• PLLPro– Controls the AFM mode, and all setup andparameters for AFM operation*http://www.rhk-tech.com/downloads_files_V2.php

SpecificationsFrom RHKproduct brochure