Atomic Force and Scanning Tunneling Microscopy for Atomic Scale Investigations of 2D Materials

Eric W. Hudson The Pennsylvania State University Department of Physics

NSF-MIP 2D Crystal Consortium Center for Nanoscale Science (MRSEC) The 2DCC Platform - Three Facilities

Thin Films and In-Situ Characterization

Bulk Crystal Growth Theory and Simulation MBE #2: Multi-Module Growth/Analysis Cluster

Cryogenic LT Nanoprobe 4 STM tips + SEM

EVO 50 2” wafer MBE system

Load Lock is docking station for vacuum DA 30L suitcase Cryogenic ARPES

System Currently in Installation

Wafer-scale MBE with in vacuo transfer to cryogenic 4-probe STM + ARPES. Atomic Force Microscopy: MCL Two Bruker Icons • Modes: – Contact Mode – Lateral Force Microscopy – Conductive AFM – Piezo Response Microscopy – Scanning Capacitance Microscopy – Tapping Mode w/ Phase Imaging – Electrostatic Force Microscopy – Magnetic Force Microscopy – Kelvin Probe Force Microscopy Contact: Tim Tighe – PeakForce Tapping w/ ScanAsyst – PeakForce Quantitative Nanomechanical Mapping – PeakForce TUNA – PeakForce Kelvin Probe Force Microscopy • AFM 2: Dimension heater/cooler stage enables: – -35ºC up to 100ºC – room temperature up to 250ºC Atomic Force Microscopy: MCL

Two Bruker Icons Contact: Tim Tighe

Wide variety of AFM techniques; heater/cooler stage available Scanning Nanoscale Interface Probe Ensemble

Variable temperature, UHV AFM/STM with sample prep and optical access Introduction to AFM/STM

What can you do with these 3 instruments? • Introduction to SPM: Technology & Techniques • Scanning Tunneling Microscopy (STM) • Atomic Force Microscopy (AFM)

Along the way: Applications to variety of systems Introduction to Scanning Probe Microscopy Scanning Probe Microscopy

Techniques for imaging (spatially resolving) material properties at microscopic (down to sub-nm) length scales using a probe scanned over the surface SPM Tech: Piezo Scanner

Commonly used “tube” scanner

Motion ~ 10-100 nm/V xy, 1-10 Å/V in z Range ~ 1-10 µm x 10-100 nm æö200 V Precision ~ nm (10V )ç÷20 : 2pm; 20 fm in z! èø2 Scanning Probe Microscopy

Techniques for imaging (spatially resolving) material properties at microscopic (down to sub-nm) length scales using a probe scanned over the surface SPM Tech: Feedback

What is feedback source (held constant)? Need something sensitive to atomic scale changes in displacement SPM Tech: Feedback

• Tunneling Current

Using WKB approximation: h Idµ-exp( kk) -1 º » 0.1 nm 2meF SPM Tech: Feedback

• Cantilever deflection (“force”)

Quad sensor

Other ways: Fabry-Perot Interferometer, piezoelectric Scanning Probe Microscopy

Techniques for imaging (spatially resolving) material properties at microscopic (down to sub-nm) length scales using a probe scanned over the surface SPM Techniques: A Small Sample

Force-based Probes Non-Force-Based AFM (atomic force) STM (tunneling) LFM (lateral) SNOM (near-field optical) CFM (chemical) SCM (capacitance) KPFM (Kelvin probe) SSM (SQUID) MFM (magnetic) SThM (thermal) MRFM (mag. resonance) SGM (gate) Introduction to STM

Scanning Tunneling Microscopy Topography

Atomic Scale Imaging STM is a High Resolution Microscope

3.45 Å

Zooming in on NbSe2 250 mK 50 pA, 50 mV STM Images Electrons – Not Atoms

NbSe2: Charge Density Wave 250 mK 50 pA, 50 mV Sometimes Electron Wave-Behavior Dominates

Electron Standing Waves at Terraces on Cu(111) 500 Å, 4.3 K 1 nA, 50 mV Spectroscopy

Thinking about the Density of States Spectroscopy Thinking about Tunneling

•Apply V

EF shifts by eV •Measure I(V) z eV - éùz I(V)» êúò LDOS(E)dE e 0 ëû0

dIµ LDOS(E)dV

positive sample bias Typical STM Measurements

Topography

DOS Map

Line Cut

Conductance Local Density of States(X,Y,E) of Density Local Spectrum Dopants in Graphene BN doped Graphene

2 nm

2 nm Pristine Graphene 1 nm Collaboration: Terrones Group Topological Insulator Thin Films

83 nm thick Cr doped (BiSb)Te film on insulating InP substrate Cleaved in-situ

Collaboration: Samarth Group k-Space via STM

Not just ARPES… Study Standing Waves

Eigler, “Amber Waves” Cu(111) Study Standing Waves

k q=kout-kin in Get dispersion k(E)? kout =2p/l

Eigler, “Amber Waves” Cu(111) Study Standing Waves

Get dispersion k(E)? y Yes! Measure dI/dV(y,E)

FT is dI/dV(qy=2ky,E)

Ag(110): JI Pascual et al, PRB 63 (2001) Study Standing Waves

y FT is dI/dV(qy=2ky,E) Why 2ky? Contours Constant Energy

Ag(110): JI Pascual et al, PRB 63 (2001) Study Standing Waves

y FT is dI/dV(qy=2ky,E) Why 2ky? Contour Constant ky Energy

Ag(110): JI Pascual et al, PRB 63 (2001) Introduction to AFM

Force Microscopy Techniques Idea of Force Microscopy

• Force-based measurement – material interaction leads to force • That force should ideally not destroy material F <1nN

• But we need to be able to measure it Detecting Small Forces

• Probe sensitivity to force • Displacement sensor sensitivity Displacement Sensors

Initial Approach: STM of AFM cantilever

Binnig, Quate, Gerber, "Atomic Force Microscope" Displacement Sensors Laser beam Fabry-Perot deflection Interferometry

Noise floor for all now 1 fm : Hz Piezoelectric Cantilever Probe Sensitivity

112 Sensitivity set by thermal noise: 22kxD= kTB 1 2 1 æö2kk T éùN ÞS 2 = B F ç÷êú(Think 4kTRB mech B) èøpQf0 ëûHz For a rectangular cantilever (l x w x t): 1 2 2 11æö 11 1 22wt 42 2 FSBmin ==FBç÷( Er ) ( kTB) [N] èølQ 123 14243 14 2 43 Material Thermal Cantilever properties energy dimensions

Stowe…Rugar, APL 71, 288 (97) Probe Sensitivity If you want to minimize: 1 2 2 11æö 11 1 22wt 42 2 FSBmin ==FBç÷( Er ) ( kTB) [N] èølQ 123 14243 14 2 43 Hard to control Low T, thin, long independent of Q laser power µN low loss k = 260 m

Thermally driven B=0.03Hz, @ 300 mK Mamin & Rugar, APL 79, 3358 (01) Fmin ~ 240 zN Modes of Operation Contact AFM

What is going on here? Constant deflection à constant force Can damage surface (not good for soft surfaces) What does contact mean?

Contact mode Force Distance Curves

Measure force while sweeping z Force Distance Curves Non-contact mode

Contact mode

Non-Contact mode Oscillations drive tip

drive tip drive tip Afæöx Ax==drivef tan-1 ç÷ where º drive 2 2 2 ç÷Qx1- f 1-x2 +x èø( ) tip ( ) ( Q) How do interactions appear?

Contact mode

Non-Contact mode

How does F impact oscillations? Impact of Attractive Force

FkzFkzkzkkz=-DÞ=-D+intD=--D( int ) Frequency Shift: How do interactions appear?

Contact mode

Non-Contact mode

Keep frequency shift constant (PLL) keeps force constant Tapping mode

Contact mode

Tapping mode

Non-Contact mode Improvement over STM: Topography on Insulators LaVO3 thin films on LSAT T > 139 K T < 139 K

30 nm 100 nm RMS Roughness: 0.1 nm RMS Roughness: 1 nm Why? Structural Phase Transition @ T = 139 K

Collaboration: Engel-Herbert Group More than topography… More than topography… Force-based Probes AFM (atomic force) LFM (lateral) CFM (chemical) KPFM (Kelvin probe) MFM (magnetic) MRFM (mag. resonance)

Can even combine simultaneously… Topography + Dissipation “Topography” “Dissipation”

Si 7x7

Z Feedback: Drive Fback: constant constant Freq shift amplitude

H. Hug Dual-PLL Imaging

Co-Pd Multilayer

Schwenk, Hug APL 104 (2014) Dual-PLL Imaging

Bit patterned media (200 nm spacing)

Schwenk, Hug APL 104 (2014) Dual-PLL Imaging

B in fundamental, large motion Topo in first harmonic, small motion

Schwenk, Hug APL 104 (2014) Kelvin Probe Microscopy Kelvin Probe: The Idea

Difference in work functions à contact potential Electrostatic Force

Tip/Surface is capacitor Butt, Surf. Sci. Rep 59, 1 (‘05)

dU dC UCVF=1122Þ=== V ? 22dD dD

R2 R FV» pe 2 FV» pe 2 far 0 D2 close 0 D Bias Dependence Kelvin Probe In Practice

Visualizing variations in work function, driven by variations in topography

Peres, J. Phys D (50) 2017 Introduction to SPM

Wide variety of SPM techniques provide powerful access to material properties at atomic-scale

MIP has several SPMs currently running, another tied to MBE coming online shortly. Consider collaborations!