Scanning Tunneling Microscopy - Tsinghua University

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Scanning Tunneling Microscopy

陈曦清华大学物理系

• Quantum Tunneling

• A Brief History of Microscopy

• Invention of STM

• STM Instrumentation

• Scanning Tunneling Spectroscopy

1Quantum Tunneling

Transmission rate:

Quantum Mechanical Tunneling

T = exp

p2m(V (x) � E)

Three earliest examples (1928):

• α decay (Gamow)

• Field ionization of hydrogen (Oppenheimer)

• Field emission from metal surface (Fowler & Nordheim)

α decay

ln ⌧ = � lnA +12⇡B

Field Ionization

Field Emission

EF + W

2⇡hWexp

�8⇡

p2mW 3/2

Fowler-Nordheim formula

W+EF-eFx

1973 Nobel Prize in Physics

L Esaki

I Giaever

B D Josephson

To be able to measure a tunneling current the two metals must be spaced no more than about 100Å apart, and we decided early in the game not to attempt to use air or vacuum between the two metals because of problems with vibration. ...... After a few months we hit on the correct idea: to use evaporated metal films and to separate them by a naturally grown oxide layer.

Nobel lecture, I Giaever, 1973

Quantum Mechanical Tunneling between Metals

Insulator

Empty Band

Filled Band

Partially

Filled Band

Energy GapFermi Level EF

Density of States (DOS):

The number of states per

interval of energy at an

energy level

DOS of 3D free electrons

E =~2k2

energy level

DOS of 2D free electrons

E =~2k2

energy level

DOS of 2D Dirac electrons

E = ~vF k

Tunneling between two metals

No tunneling current

Tunneling between two metals

I ⇠ e�2d

2m(W � E)~

�d ⇠ 1A ! I reduced by 10

Tunneling spectroscopy

Tunneling probabilityV

Ew(E) / DOS(E)

Tunneling spectroscopy

V I I+∆IV

⇓�I / DOS(V ) · �V

dI(V )dV

/ DOS(V )

w(E) / DOS(E)

Tunneling: an approach to measure DOS

Example: Coulomb interaction + disorder ⇒ DOS ~ E1/2 at EFAl’tshuler & Aronov (1979)

Ge1-xAux

McMillan & Mochel (1981)

Giaever, Hart & Megerle (1962)

Inelastic Electron Tunneling Spectroscopy (IETS)

Inelastic excitations in barrier

1 ML Molecules

Jaklevic & Lambe (1966)

dI(V )dV

/ DOS(V )

Number of available tunneling channels at V

dI/dV: change of current after V is increased by dV

Elastic tunneling

Inelastic tunneling: additional tunneling channels

d2 I/d

2A Brief History of Microscopy

Development of Microscopy

ItalySpectacles

JanssenFirst Microscope

Roman Glass, Lenses

1st Century 14th Century 1590 1665 1676

Leeuwenhoek Bacteria

Hooke: English father of microscopy

Leeuwenhoek: father of microscopy

Lister Spherical

Aberration

1830 1878 1932

ZernikePhase contrastMicroscopeNobel 1953

RuskaNobel 1986

Atomic Era of Microscopy

1951 Oct. 11, 1955 1970

Erwin W. Müller

STEM image of Th atoms

Albert Crewe

1981 STM

3Invention of STM

Gerd Binnig Heinrich RohrerIBM Research Laboratory, Zurich, Switzerland

Nobel Prize in Physics, 1986

Gerd Binnig: superconductivityHeinrich Rohrer: superconductivity, Kondo, phase transitionChristoph Gerber: joined IBM in 1966, worked with HR, craftsmanly, inventor of AFM

“...... gave us the courage and lightheartedness to start something which should not have worked in principle”

None in microscopy or surface science

Motivation: Local study of growth and electrical properties of thin insulating layers as tunneling junctions

Goal: not to build a microscope, but to perform spectroscopy locally on an area less than 100 Å in diameter

Contact over insulating film

Instead of scanning tip in contact over a surface, a small gap of a few angstroms was maintained and controlled by the

tunneling current

Not only a local spectroscopic probeBut spectroscopic and topographic imaging

Profilometer

March 16, 1981 First demonstration of vacuum tunneling

Si(111) 7x7 (1982) 1998

Main instrumental problems:

How to avoid mechanical vibrations that move tip and sample

against each other?

How strong are the forces between tip and sample?

How to move a tip on such a fine scale?

How to move the sample on a fine scale over long distances?

How to avoid strong thermally excited length fluctuations of

sample and tip?

Vibration isolation

1st generationsuperconducting levitation

2st generationspring

Tip movementThe continuous deformation of piezomaterial in the angstrom and sub-angstrom range was established only later by the tunneling experiments themselves.

Piezoelectric Materials

Scanning and rough positioning

AFM, atomic force microscopy

BEEM, ballistic electron emission microscopy

EFM, electrostatic force microscope

FMM, force modulation microscopy

KPFM, kelvin probe force microscopyMFM, magnetic force microscopy

MRFM, magnetic resonance force microscopy

NSOM, near-field scanning optical microscopy

PFM, piezo force microscopy

PTMS, photothermal microspectroscopy/microscopySAP, scanning atom probe

SCM, scanning capacitance microscopy

SECM, scanning electrochemical microscopy

SGM, scanning gate microscopy

SICM, scanning ion-conductance microscopySThM, scanning thermal microscopy

SVM, scanning voltage microscopy

SHPM, scanning Hall probe microscopy

SSM, scanning SQUID microscope

Scanning

Family

Who is who: Don Eigler

Who is who: Wilson Ho

Who is who: Roland Wiesendanger

Who is who: Seamus Davis

4STM Instrumentation

IV: 10 μV ~ 1 V I : pA ~ nA

Pre-amplifier

109 Ω

GainNoise levelBand width

dI/dV: Lock-in amplifier

excitation

experimental system

response

+noise

singnal

+noisetransducer

dc+acexcitation

experimental system

response

+noise singnal+noise

transducer

reference

output

low-pass filterV+v

I = I(V + v cos !t) = I(V ) +

dI(V )

dVv cos !t

⇥ cos (!t + �)

I(V ) cos (!t + �) +

dVv(cos (2!t + �) + cos �)/2

= I(V ) cos (!t + �) +

dVv cos (2!t + �) +

dVv cos �

low pass filter

X-Y scanning

Z motion

Feedback

Piezoelectricity

x＋δx

z＋δzz

�z = d33V

�x = (x/z)d31V

d ~ 1 Å/V

Lead Zirconate Titanate：PbTiO3+PbZrO3

EBL#2:

d33=3.80 Å/V

d31=-1.73 Å/V

Tube scanner

δL=d31VzL / t ~ 10 Å/V

Tube scanner

δx=2√2 d31VxL2 / πDt ~10 Å/V

Vx -Vx

ZLow sensitivity

Higher spatial resolutionMore invulnerable to noise

Smaller scanning range

Scanning range ~ micron

Sensitivity: displacement per volt

Resonance frequenciesScanning speed

X-Yx-y control

z control

Electronics

feedbackPre-amp

Tip approach

Earlier time:mechanical mechanism vibration, thermal leak

shear piezo

δx=d15V1 mm: 103~104 steps

HexagonalPrism

ExteriorShell

PositionDetector

Shear PiezoStack

Pan Type

Besocke Type

Vibration noise

General rules:

Low noise environment

Vibration isolation

Rigid design

Vibration noise

Low noise environment:

Basement with solid foundation

Turn off mechanical pumps, turbo pumps, etc

Acoustic-isolation room

Without solid foundation

With solid foundation

Vibration noise

Vibration isolation and rigid design

Vibration noise

UHV STM-Clean Surcace

High vacuum: 10-7 to 10-9 torrUltra high vacuum: < 10-9 torr

Mean free path• 10-9 torr: 105 m• 10-10 torr: 106 m• 10-11 torr: 107 m

Monolayer formation time• 10-9 torr: 103 s• 10-10 torr: 104 s• 10-11 torr: 105 s ~ days

Cryogenic STM

• Investigate phenomena only at low temperature

• Higher energy resolution

• Low thermal drift

• Slow down dynamics

Cryogenic STM

Higher energy resolution

kBTA level on

Sample

peak width @10K

dI/dV 3.5kBT 3 meV

d2I/dV2 5.4kBT 5 meV

Cryogenic STM

Low thermal drift

ΔL=α(T)·L·ΔT 0.01Å stability

10-5K fluctuation @ 300K

10-2K fluctuation @ 4K

50 100 150 200 250

Temperature (K)

Cryogenic STM

Cryostats:

LN2: 77 K

LHe: 4.2 K

He-3: ~0.3 K

Dilution: ~10 mK

He-3 Dilution

Cryogenic STM

He-3 Cryostats

He3 pot

1K pot

Sorptionpump

He3 vessel

Cryogenic STM-Continuous Flow

• Variable T• Rapid cooling down• Compact• High LHe consumption• No magnet

Cryogenic STM-Bath Cryostat

Top loading• Conventional• Magnet• Ultra low temperature• Bulky

Cryogenic STM-Bath Cryostat

Bottom loading• Low LHe

consumption• Compact

Ultra low temperature STM

Y. J. Song, et al, RSI 81, 121102 (2010)

1.0 m 10 mK 15 T @NIST

Tsinghua

1.0 m 10 mK 15 T @NIST 1.52 m 1.52 m

1.41 m

1.17 m dia.

4.30 m 5.60 m

3.12 m

1.91 m3.05 m

0.56 m

Mixing Chamber

SSMC connectors

Heat exchanger

1K pot condenser

Volume for 250 L

Magnetic bore

Baffles

JT condenser

Shields (Still & ICP)

Ag clamping ring

plastic guide

SPM receptacle

Mixing Chamber(MC)

Ag extension rods

1K pot

SSMCconnectors

JT loop

precoolingheat exchanger

thermal anchor

strain relief grooves

STM Tip

Chemistry of tungsten• WO2 and WO3

• WO3 soluble in strong base• Oxide reduced to metal

Etching tungsten tip• AC etching: blunt but less oxide• DC etching: sharp but more oxide• Optimal procedure: AC+DC+Strong acid

Tip etcher

DC etching

STM Tip

1.5mA e-

Ar+ Ar+

Repeated cycles of heating and self-sputtering in vacuum to remove oxide and sharpen tip

STM Tip

400Vkeep I < 20uA 5 min

HV field emission in vacuum to remove oxide

STM Tip

LV field emission in vacuum to fix blunt, multiple tip

5 ~ 10V 1 sec

STM Tip

Controlled crash to sharpen tip

Sample

Single crystal metal: sputter+anneal

0-2kV 0-2kV

Sample

Single crystal metal: sputter+anneal

Sample

Sample holder

Pyrometer

Sample

Single crystal metal

Au(110) Cu(100) Pt(111) 250 nm x 250 nm

Sample

Silicon

Flash to 1200ºC

Sample

Cleaving BiO

Sample

Thin film growthHeater

Knudsen Cells

RHEED Gun

RHEED Screen

LN2Shroud

Sample

Thin film growth10 nm

Bi2Te3

KFe2Se2

5Scanning Tunneling Spectroscopy

dI(V )dV

/ DOS(V )

Looking for structures (peak, dip, step) in dI/dVSpectroscopic imaging

-13.6 eV

-3.4 eV

-1.5 eVd

Electron orbitals of H atom

50 meV

100 meV

150 meV

300 meV

200 meV

EnergySpectroscopic imaging

Electronic States-Landau Quantization

Conventional 2DES

En = ~!c

✓n +

2m(p + eA)2 H = vF [(p + eA) ⇥ �] · z

En = ED + sgn(n)vF

p2eB~|n|

n=0 1 2 3

Massless Dirac fermion

n=0 1 2 E E

Graphene

Miller, et al, Science 324, 924 (2009)

TI: Bi2Se3

-300 -250 -200 -150 -100 -50 0 50Sample Bias (mV)

Dirac Point

Fermi level

1 T2 T

En ⇠p

LL0 independent of B

í��

��

��7��7

� � � � � �� √nB

(QHUJ\��P

Electronic States-Quantum Confinement

Sub-band of Pb /Si(111)

1.5Bind

Wave vector

Van Hove Singularity

-1.0 -0.5 0.0 0.5 1.0 1.5

Sample Bias(V)

19ML18ML17ML16ML15ML14ML13ML12ML11ML

STSQuantum confinement in thin film

Building confined systems by manipulation

Quantum corral

Eigler, et al, Science 262, 218 (1993)

9Å offcenter

Circle’scenter

Openterrace

-0.6 -0.4 -0.2 0.0 0.2 0.4Voltage (V)

l=0 (hard wall model)l=1l=2

Peaks at circle’s centerExtra peaks 9Å off center

nl / Jl(knl⇢)eil�

Enl = ~2k2nl/2m⇤

Atomic chainHo et al, Science 297, 1853 (2002)

Au13 Au15 Au20

Atomic chain

-8 -4 0 4 8Wave Vector (10 m )9 -1

m = 0.5 meff e0.0

Electronic States-Molecules

Sample Bias (V) -2.0 -1.0 0.0 1.0 2.0

Bare Ag(100)

C60/Ag(001)-1.7 V 0.02 & 0.41 V 1.62 V

HOMO split LUMO LUMO+1

Theory

-1.7 V 0 V 0.4 V 1.6 V

Crommie et al, PRL 2003

Electronic States-Kondo

density of states

Electronic States-Kondo

Quantum mirage

Eigler, et al, Nature 2000

Electronic States-Standing Wave

Eigler, et al, Nature 363, 524 (1993)

Cu(111)

0 20 40 60 80 100 120

Distance (Å)

400 mV300 mV200 mV100 mV50 mV

Fourier transform gives the momentum transfer q

q only in Γ-M directions

Backscattering is forbidden in topological insulator

d2 I/d

Ho, et al, Science 280, 1732 (1998)

Cu(100)

C2H2 on tip

1 on C2H2 2 on bare surface

358 meV: stretching mode of C2H2

358 mV

266 mV 311 mV

Cu(111)

trimer

-40 -20 0 20 40

12C16O13C16O

Sample voltage (mV)

30 35 40

12C16O13C16O

VAC= 2 mVRMS

Sample voltage (mV)

STM Topography of array of CO dI/dV image at 35.5 mV

Single bond formation

Ho, et al, Science 286, 1719 (1999)

d2 I / d

280260240220200180

Sample Bias (mV)

Fe(12C16O)

Fe(12C16O)2

Identification of bond formation

Δ=gµBS·B

Spin flip spectroscopy

0 Δ-ΔV

Δ~1 mV at 10 T, need He3 fridge

H=JS1·S2

0 J-JV

4 5 6 70.4

6�(m

0.0 0.2 0.4 0.6 0.8 1.0 1.2

Voltage (mV)

B=0B=2.8TB=4.2TB=5.6TB=7T

Voltage (mV)

Mn on metal

bare metal

B=0B=7T

1.0Mn on oxide

bare oxide

B=0B=7T

Voltage (mV)

NiAl(110)

Al2O3Mn

4th layer

3rd layer

2nd layer

1st layer

2nd layer

3rd layer

4th layer

5th layer

-40 -20 0 20 40

Sample Bias (mV)-40 -20 0 20 40-40 -20 0 20 40

Spin flip IETS of spin chains

2 spins 3 spins 4 spins

1.37JS = 1/2

S = 1/2

S = 3/2

JS = 1/2J = 18 meV

H = J S1 . S2+J S2 . S3H = J S1 . S2 H = J S1 . S2+J S2 . S3+J S3 . S4

Sample Bias (mV)17 18 19 20 21 22 23

1234567

Sz=-1Sz=0Sz=1

B=0 B=0

Singlet to triplet transition

Anisotropy CuN

BBBBBBBB

???? ????????

????????

-10 -8 -6 -4 -2 0 2 4 6 8 10

Voltage (mV)

BBBBBBBB

????????

-10 -8 -6 -4 -2 0 2 4 6 8 10

Voltage (mV)

1086420-2-4-6-8-10

Voltage (mV)

B ∥ N directionBBBB

BBBBBBBB

???? ????????

????????

-10 -8 -6 -4 -2 0 2 4 6 8 10

Voltage (mV)

BBBBBBBB

????????

-10 -8 -6 -4 -2 0 2 4 6 8 10

Voltage (mV)

B ⊥ N direction

1086420-2-4-6-8-10

Voltage (mV)

H = gµB

B · S + DS2z

+ E(S2x

� S2y

Heinrich, et al, Science 317, 1199 (2007)

D =-1.55 meVE = 0.31 meV

Spin Polarized STM

Tip SampleE E

DOS DOS

emptystates

filledstates

Spin Polarized STM

dI/dV = (dI/dV )0 + (dI/dV )SP MT · MA

Tip SampleE E

DOS DOS

emptystates

filledstates

Spin Polarized STM

1 ML of antiferromagnetic Mn on W

[011]-

Wiesendanger, et al, Science 288, 1805 (2000)

Spin Polarized STM

W tip coated with Fe

[011]-

Spin Polarized STM

All-spin-based logic operations atom by atom

Readout Output

Bbias Bpulse

Spin Lead Spin Lead

Input _Input `

JlJisl

Readout Output

CoCoFe

Spin Polarized STM

All-spin-based logic operations atom by atom

0.4 T0

weak coupling between output atom and input α

Spin Polarized STM

Strong coupling between output atom and both inputs: OR

1 10 1 01

0 110 00

-0.39 T

-0.385 T

0.75 T

Superconductivity

Perfect diamagnetism

Oct. 26, 1911

No DC resistivity Energy gap

T(K)0 0.5 1.0 1.5 2.0TC

illijo

Phillips, PR 114, 676 (1959)

Superconductivity

CeCu2Si2 UBe13 UPt3UPd2Al3 CeCoIn5

CNT diamondYbC6

CNTCNTPuRhGa5

PuCoGa5

Li@33GPa

YbPd2B2C

RbCsC60

Cs3C60@1.4GPa MgB2

FeAsYBaCuO

BiSrCaCuO

TlBaCaCuOHgBaCaCuO

HgTlBaCaCuO

HgBaCaCuO @30GPa

≈ ≈

≈≈

Liquidhelium

Liquidhydrogen

Liquidneon

Liquidnitrogen

2010200520001995199019851980194019000

Superconductivity

-6 -4 -2 0 2 4 6

50 4.2 K

600 mK

240 mKDiff

tial C

Sample Bias (mV)

NbSe2 TC = 7.2 K Vortex, 0.25 T, 300 mK

dI/dV map at -0.21 mV

Superconducting gap

Hudson, PhD thesis

Superconductivity

Quasi-particles S anti-parallel to m

U = JS · � J > 0

Superconductivity

Impurity induced sub-gap states

0 2 4 6-2-4-6Bias (mV)

Si(111)

Nb tip

0 2 4 6-2-4-6Bias (mV)

Superconductivity

-200 -100 0 100 2000.0

Sample bias (mV)

Zn (nomagnetic) induced bound state in BSCCO

Davis, et al, Nature 403, 746 (2000)

-1.5 mV

Signature of d-wave

Superconductivity

Quasiparticle scattering in BSCCO

Davis, et al, Nature 422, 592 (2003)

kx (2π/a)

k y (2π/

X=(1/2,1/2)M=(0,1/2)

q4 q5q6

-1 -1/2 0 1/2 10

q4q5q6q7

qx (2π/a)

q y (2π/

3-3 -1 1

d-wave superconductor

nodal point

Superconductivity

0 10 20 30 40 50 60 70 80 900

θk∆(θ)

FT-STS (-)FT-STS(+)ARPES

dI/dV image

Superconductivity

Phase separation in KxFe2-ySe2

0 2 4 6 8 10-2-4-6-8-10

Bias (mV)

0.43 V

0 0.2 0.4 0.6-0.2-0.4-0.6Bias (V)

KFe2Se2

K2Fe4Se5

Reference books:

• C. J. Chen, Introduction to scanning tunneling microscopy

• J. A. Stroscio & W. J. Kaiser, Scanning tunneling microscopy

• R. Wiesendanger, Scanning probe microscopy and spectroscopy

Scanning Tunneling Microscopy - Tsinghua University

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