Microscopy and Spectroscopy with

Tunneling Electrons

STMSTM

Sfb Kolloquium – 23rd October 2007

The Tunnel effect

)exp()( ESET −Φ×−∝

BarrierBarrier

widthwidth BarrierBarrier

heigthheigth

s

Development: The Inventors

1981

Development: Almost Got it!

Gustev Schmalz, Zeitschrift des Vereinesdeutscher Ingenieure, Oct 12, 1929, pp. 1461-1467

R. Young, J. Ward, F. Scire, The Topografiner: An Instrument for Measuring Surface Microtopography, Rev. Sci. Inst., Vol43, No 7, p 999, 1972.

1929 1971Stylus Profiler

Topografiner

Tricks and Tips

The Tips

The Movement

The Feedback

PID digital or analog.

The Surface

Conducting/semiconducting.

¡CLEAN!.

Single Crystals.

Piezoelectrics.

W, Pt/Ir.

Silicon (111) 7x7 and Silicon Carbide.

Representation of STM data

Advantages of Low Temperature

Heat shield.Heat shield..

5 K cold 5 K cold

room.room.

100 K cold 100 K cold

room.room.

L. HeL. N

2

Wobble Stick Wobble Stick

port.port.

Window. Window.

MBE port.MBE port.

Manipulator port.Manipulator port.

Vacuum Vacuum pumspums

port.port.

High stability

High energy resolution (<2 meV)

Low mobility of adsorbates

Extremely high vacuum

UHV-STM working at 4-5 K

Control of sample temperature

Local Spectroscopy

Microscopy on single adsorbates: Atoms and

Molecules

Benzene on Cu(100)D. D. EiglerEigler..

IBM, USA.

Single Atoms Single Molecules

PVBA / Ag(110)

R. Berndt.R. Berndt.

Kiel, Alemania.

1-nitronaftaleno Au(111)

Microscopy on molecules: Self-Assembling

Nucleation centres.

Structure and conformation.

Chirality.

Basic mechanism for growth of molecular films

We see topography?

►Atomicsteps

►Atomic structureof surfaces

►AdsorbatesD. D. EiglerEigler..

IBM, USA.

STM is NOT a true Microscopy!.

Benzene on Cu(100)

Benzene on

Ag(110)

The adsorbate’s shape depends

on the chemical interaction with

the surface!.

STM is NOT a true Microscopy!.

EF+eV

E

F

Tip

Sample

dE )z,T(eV,eV)(E(E)I p

eVE

E

F

F

φρρ ×−×∝ ∫+

TipSample

Empty States.

Filled States.

Si(111) 7x7

Modeling the tunneling current

►1961: Bardeen solution to the problem of tunneling in one dimension

Modeling the tunneling current

►1985: Tersoff and Hamann approximation to the problem of imaging plane waves on surfaces

(Bias →0 V)

+Tip: S-wave (spherical)

Surface: Bloch wave

Current: Density of States at the tip position

From molecular orbitals to resonances

Ea

Ip

LUMO

HOMO

EF

Ev

Free molecule Molecule at the Surface

-

∆∆+

2U

+2U

00

►SCREENING

►BROADENING

►SPLITTING

►ALIGNMENT & Hybridization

Seeing molecular orbitals?

Intramolecular Structure:

We do not see the atoms, but the states.

C60 on Si(111):

Predominantly covalent bond.C60 sobre Ag(110):

Predominantly ionic bond.Constant density of States surfaces for the:

HOMOHOMO LUMOLUMO

•Ab Initio calculations with SIESTA by: D. Sánchez-Portal, E. Artacho, P. Ordejón and J.M. Soler.

STMSTMDOSDOS--LUMOLUMO **

DO

SD

OS

-- HO

MO

H

OM

O **

ST

MS

TMS2

S5

S2

S3S3

C60

Simulation of STM images

TersoffTersoff--HamannHamann

simulations maysimulations may

reproduce STM reproduce STM

images, butimages, but……..

•Ab Initio calculations with VASP by: Manuel Cobian and Nicolas Lorente

Principle of scanning tunneling spectroscopy

►1961: Bardeen solution to the problem of tunneling in one dimension

dE )z,T(eV,eV)(E(E)I p

eVE

E

F

F

φρρ ×−×∝ ∫+

TipSample

Local Density of states atenergy E and position x,y

Methods of scanning tunneling spectroscopy

Three magnitudes are controlled by the user

I - The tunnelling current

V - The energy of tunnelling electrons

Z - The width of the tunnelling barrier

dE )z,T(V,eV)(E(E)I p

eVE

E

s

F

F

φρρ ×−×∝ ∫+

p

I I vsvs Z Z -- V fixed.V fixed.

-2 -1 0 1 2-60

-40

-20

0

20

40

I - T

un

nel

ing

cu

rren

t (n

A)

V - Sample voltage (V)-20 -15 -10 -5 0 5 10 15 200

5

10

15

20

25

30

Z -

Tip

-sam

ple

dis

tan

ce (

Å)

V - Sample voltage (V)

FFFF

0

d Z

/ dV

(u.a.)

0 1 2

5

10

15

20

25

30

FFFFFFFF

FFFF =

5.4

eV

= 4.

5 eV

= 3.

7 eV

I- T

un

nel

ing

cu

rren

t (n

A)

Z Tip-sample distance (Å)

I vs. V I vs. V –– Z fixed.Z fixed. Z vs. V Z vs. V –– I fixed.I fixed.

Differential conductance is proportional to the Surface Density of States (DOS).

... )z,T(eV,,...)(EeV,...)(EdV

dIeV)z,y,G(x, pFF +××+∝≡ φρρ TipSample

STS Fundaments: Energy Resolution.

dE )z,T(eV,eV)(E(E)I p

eVE

E

F

F

φρρ ×−×∝ ∫+

TipSample

Tunnelling current (I) does not provide true Energy Resolution.

Spectroscopy of molecular states

-2 -1 0 1 2

Tun

nelli

ng

Cu

rre

nt

(nA

)

Sample Bias (V)

Diffe

ren

tial C

ond

ucta

nce

LU

MO

GAP

HO

MO

STS STS spectrumspectrumI (V)

dI/dV (V)

EF

Filled Empty

Single-molecule vibrational spectroscopy.

Less than 10% of electrons Less than 10% of electrons interact inelastically with an adsorbate

Molecular Vibrations: Chemical

fingerprint of a single adsorbate.

%5.4G

∆G=

%5.1G

∆G=

-400 -200 0 200 400

0

30

n(C-H)

n(C-H)

356 mV

Sample Bias (mV)

d2 I/dV

2 (nA

/V2 )

C2H

2

Cu(100)

Intramolecular

vibrational modes:Adsorbate-substrate bond

vibrational modes:

C2H2 on Cu(100) CO on Cu(100)

Vibrational Spectroscopy of one molecule.

-100 -50 0 50 100

-100

0

100

d2 I/dV

2 (nA

/V2 )

Sample Bias (mV)

C60 on Ag(110)

Cavity-breathing

vibrational mode:

Resonant scattering.Resonant scattering.•Short range•Resonances close to EF

%12G

∆G<

Excitation Mechanisms

Dipolar scattering.Dipolar scattering.•Long range

%1G

∆G<

Vibrational Spectroscopy of one molecule.

W. Ho, UCI, USA. D. Eigler, IBM, USA.

Magnetic imaging with atomic resolution.

Manipulation of Atoms and Molecules

Fabrication of atomic scale devices

Electron induced processes

Manipulation of Atoms and Molecules

D. D. EiglerEigler ..

IBM Almaden, USA

1990Silver atoms on Ag(111).

K.K.--H. H. RiederRieder..

FU Berlin.

Electron induced reactions

Multipleelectrons

10-010-15 10-310-12 10-9 10-6

fs ps µs sns ms

Electronic excitations

1 nA

Vibrational Tunnelling rate

Electron induced reactions

10-010-15 10-310-12 10-9 10-6

fs ps µs sns ms

Electronic excitations

1 nA

Vibrational

inelPtunn

vibinel

tP

τ⋅×

tunn

vibinel

tP

τ⋅×

tunn

vibinel

tP

τ⋅×Yr α

1n

r

tunn

I αY t

eI

−⇒=

nI α I) Y( Rate r ×=

)n(⋅⋅⋅×

Tunnelling rate

Electron induced reactions

n

d I α I) Y( Rate ×=

Stipe et al. PRL 78, 4410 (97)

300 meV < E dissociation < 400 meV

Excitation of O-O stretch

Dissociation of O2 on Pt(111)Fix electron energy (voltage bias)

Probe statistical dissociation rate vs. Tunnel current

Summary:

microscopy that can measure properties.

STMSTM

and a tool to manipulate atoms and molecules.

Spectroscopy with high spatial resolution,

more than a

Is a