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P. Grutter STM as a Tool to Understand the Electronic Properties of Molecules Peter Grutter Physics Department McGill University Part of SPM lecture series in 534A ‘Nanoscience and Nanotechnology’

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Page 1: P. Grutter STM as a Tool to Understand the Electronic Properties of Molecules Peter Grutter Physics Department McGill University Part of SPM lecture series

P. Grutter

STM as a Tool to Understand the Electronic Properties of Molecules

Peter Grutter

Physics Department

McGill University

Part of SPM lecture series in 534A ‘Nanoscience and Nanotechnology’

Page 2: P. Grutter STM as a Tool to Understand the Electronic Properties of Molecules Peter Grutter Physics Department McGill University Part of SPM lecture series

P. Grutter

Outline

• Motivation and Intro

• History of tunneling

• STM and STS theory

• Wires

• Molecules

• Chemically and atomically defined contacts

Page 3: P. Grutter STM as a Tool to Understand the Electronic Properties of Molecules Peter Grutter Physics Department McGill University Part of SPM lecture series

P. Grutter

Page 4: P. Grutter STM as a Tool to Understand the Electronic Properties of Molecules Peter Grutter Physics Department McGill University Part of SPM lecture series

P. Grutter

History

Page 5: P. Grutter STM as a Tool to Understand the Electronic Properties of Molecules Peter Grutter Physics Department McGill University Part of SPM lecture series

Binnig and Rohrer obtained the Nobel prize in 1986 for the discovery of the STM

First STM

Page 6: P. Grutter STM as a Tool to Understand the Electronic Properties of Molecules Peter Grutter Physics Department McGill University Part of SPM lecture series

But: 1972!

Topografiner

System very similar to today’s STM, but atomic resolution was not achieved

30 A vertical, 4000 A lateral resolution

Page 7: P. Grutter STM as a Tool to Understand the Electronic Properties of Molecules Peter Grutter Physics Department McGill University Part of SPM lecture series

How does it work?

• Tunneling current between tip and sample

I ~ (V/s) exp (- A√φ*s)

Page 8: P. Grutter STM as a Tool to Understand the Electronic Properties of Molecules Peter Grutter Physics Department McGill University Part of SPM lecture series

Tunneling current• Exponential dependence on distance

I ~ (V/s) exp (- Aφ1/2s)

“Proof of concept”

March 18th, 1981

Binnig et al, APL 1982

Very sensitive to gap size!

Page 9: P. Grutter STM as a Tool to Understand the Electronic Properties of Molecules Peter Grutter Physics Department McGill University Part of SPM lecture series

First STM image

• Binnig et al. 1982, PRL• First atomic resolution image of the Si (111) 7x7

reconstruction

Page 10: P. Grutter STM as a Tool to Understand the Electronic Properties of Molecules Peter Grutter Physics Department McGill University Part of SPM lecture series

Tip preparation• Tip must be as sharp and narrow as possible

Chemically etched or mechanically cut.

Page 11: P. Grutter STM as a Tool to Understand the Electronic Properties of Molecules Peter Grutter Physics Department McGill University Part of SPM lecture series

Tip effects

The shape of the tip may affect the image

-More than one tip

-“flat” or irregular shape

-Structure change during scan

Page 12: P. Grutter STM as a Tool to Understand the Electronic Properties of Molecules Peter Grutter Physics Department McGill University Part of SPM lecture series

Scan resolution

STM Large Range

Comparable (or better) to most techniques

Page 13: P. Grutter STM as a Tool to Understand the Electronic Properties of Molecules Peter Grutter Physics Department McGill University Part of SPM lecture series

P. Grutter

Operation of an STM1,2

[1] C. Julian Chen, Introduction to Scanning Tunnelling Microscopy, Oxford (1993)[2] G.A.D. Briggs and A. J. Fisher, Surf. Sci. Rep. 33, 1 (1999)

Page 14: P. Grutter STM as a Tool to Understand the Electronic Properties of Molecules Peter Grutter Physics Department McGill University Part of SPM lecture series

P. Grutter

Current theoretical modelsTheoretical methods:

Landauer formula or Keldysh non-equilibrium Green’s functions 1-4

Transfer Hamiltonian methods5

Methods based on the properties of the sample surface alone6

[1] R. Landauer, Philos. Mag. 21, 863 (1970)

M. Buettiker et. al. Phys. Rev. B 31, 6207 (1985)

[2] L. V. Keldysh, Zh. Eksp. Theor. Fiz. 47, 1515 (1964)

[3] C. Caroli et al. J. Phys. C 4, 916 (1971)

[4] T. E. Feuchtwang, Phys. Rev. B 10, 4121 (1974)

[5] J. Bardeen, Phys. Rev. Lett. 6, 57 (1961)

[6] J. Tersoff and D. R. Hamann, Phys. Rev. B 31, 805 (1985)

Page 15: P. Grutter STM as a Tool to Understand the Electronic Properties of Molecules Peter Grutter Physics Department McGill University Part of SPM lecture series

P. Grutter

Landauer formula for the STM1,2

[1] Y. Meir and N. S. Wingreen, Phys. Rev. Lett. 68, 2512 (1992)[2] A.A. Abrikosov, L.P. Gorkov and I.E. Dzyaloshinski, Methods of Quantum Field Theory in Statistical Physics, Dover, NY (1975)[3] M. Buettiker et al. Phys. Rev. B 31, 6207 (1985)

The tunnel current for non-interacting electrons3:

)],(),([),(2

)( 00 eVEfEfVETdEh

eVI RRLL

Page 16: P. Grutter STM as a Tool to Understand the Electronic Properties of Molecules Peter Grutter Physics Department McGill University Part of SPM lecture series

P. Grutter

Transfer Hamiltonian method1,2

[1] J. Pendry et al. J. Phys. Condens Matter 3, 4313 (1991)[2] J. Julian Chen, Introduction to Scanning Tunneling Microscopy Oxford (1993) pp. 65 - 69

**

2

4

2

0

2

S

eV

FTFs

Sdm

M

MEEEeVEdEe

I

M…overlap of wavefunctions (--> resolution!)

…. DOS ( --> spectroscopy !)

Page 17: P. Grutter STM as a Tool to Understand the Electronic Properties of Molecules Peter Grutter Physics Department McGill University Part of SPM lecture series

P. Grutter

Bardeen approach1,2

EE

dSm

eI

s

2

,

**2

24

tip

sample

[1] C.J. Chen, Introduction to Scanning Tunneling Microscopy, Oxford Univ. Press (1993)[2] W.A. Hofer and J. Redinger, Surf. Sci. 447, 51 (2000)

Page 18: P. Grutter STM as a Tool to Understand the Electronic Properties of Molecules Peter Grutter Physics Department McGill University Part of SPM lecture series

P. Grutter

Tunneling Current

)exp( zAVI

…. Workfunction, typically 3-5 eV

z….. Tip-sample separation, typically 4-10 A

z = 1 A --> I one one order of magnitude !

Page 19: P. Grutter STM as a Tool to Understand the Electronic Properties of Molecules Peter Grutter Physics Department McGill University Part of SPM lecture series

P. Grutter

Small V approximation!

Simmon’s equation (Simmon, 1963)

Fowler-Nordheim regime (V>>

)exp( zAVI

)/exp(2 VconstVIMeasure log I vs log V -> resonances!

Resolution due to exp dependence! (not so on metals -> later)

Page 20: P. Grutter STM as a Tool to Understand the Electronic Properties of Molecules Peter Grutter Physics Department McGill University Part of SPM lecture series

P. Grutter

Unknown/Challenges:

1. Chemical nature of STM tip (problem for spectroscopy, corrugation)

2. Relaxation of tip/surface atoms (tip sample separation not equal to piezo scale)

3. Effect of tip potential on electronic

surface structure (quenching of surface states)

4. Influence of magnetic properties

on tunnelling current/surface corrugation (is spin-STM possible?)

5. Relative importance of the effects

Page 21: P. Grutter STM as a Tool to Understand the Electronic Properties of Molecules Peter Grutter Physics Department McGill University Part of SPM lecture series

P. Grutter

1. Chemical nature of the tip1

[1] P. Varga and M. Schmid, Appl. Surf. Sci. 141, 287 (1999)

Page 22: P. Grutter STM as a Tool to Understand the Electronic Properties of Molecules Peter Grutter Physics Department McGill University Part of SPM lecture series

P. Grutter

Model of the STM tip1,2,3

Number of layers: 7Free standing film

Numerical method: DFT Relaxations: VASP [1]Electronic structure: FLEUR [2]

Lattice constant: 6.016 au (GGA)Exchange/correlation: PW91[3]

Brillouin-zone sampling: 10 k-pointsConvergence parameter: < 0.01 e/au3

[1] G. Kresse and J. Hafner, Phys. Rev. B 47, R558 (1993)[2] Ph. Kurz et al. J. Appl. Phys. 87, 6101 (2000)[3] J. P. Perdew et al. Phys. Rev. B 46, 6671 (1992)

Page 23: P. Grutter STM as a Tool to Understand the Electronic Properties of Molecules Peter Grutter Physics Department McGill University Part of SPM lecture series

P. Grutter

Electronic properties of the tip:non-magnetic tip models

Page 24: P. Grutter STM as a Tool to Understand the Electronic Properties of Molecules Peter Grutter Physics Department McGill University Part of SPM lecture series

P. Grutter

Chemical contrast on PtRh(100)1,2

[1] P.T. Wouda et al. Surf. Sci. 359, 17 (1996)[2] P. Varga and M. Schmid Appl. Surf. Sci. 141, 287 (1999)

Experiments: 22 pm contrastSimulations: interval EF +/- 80 meV

Page 25: P. Grutter STM as a Tool to Understand the Electronic Properties of Molecules Peter Grutter Physics Department McGill University Part of SPM lecture series

P. Grutter

2. The influence of forces in STM scans1

[1] W.A. Hofer, A.J. Fisher, R.A. Wolkow, and P. Grutter, Phys. Rev. Lett 87, 236104 (2001)[2] G. Cross, A. Schirmeisen, P. Grutter, U. Durig, Phys. Rev. Lett. 80, 4685 (1998)

Force measurement on Au(111)2 Simulation of forces:

Simulation: VASPGGA: PW914x4x1 k-points

Page 26: P. Grutter STM as a Tool to Understand the Electronic Properties of Molecules Peter Grutter Physics Department McGill University Part of SPM lecture series

P. Grutter

Tip relaxation effects

W tip on Au(111) surfaceThe force on the apex atom isone order of magnitude higherthan forces in the second layer

Substantial Relaxations occur only in a distance range below 5ASubstantial Relaxations occur only in a distance range below 5A

Page 27: P. Grutter STM as a Tool to Understand the Electronic Properties of Molecules Peter Grutter Physics Department McGill University Part of SPM lecture series

P. Grutter

Tip relaxation effects

Hofer, Fisher, Wolkow and Grutter Phys. Rev. Lett. 87, 236104 (2001)W tip on Au(111) surface

The real distance is at variance with the piezoscale by as much as 2AThe real distance is at variance with the piezoscale by as much as 2AThe surplus current due to relaxations is about 100% per AThe surplus current due to relaxations is about 100% per A

Page 28: P. Grutter STM as a Tool to Understand the Electronic Properties of Molecules Peter Grutter Physics Department McGill University Part of SPM lecture series

P. Grutter

Corrugation enhancement

STM simulation: bSCAN Bias voltage: - 100mVEnergy interval: +/- 100meVCurrent contour: 5.1 nA

Due to relaxation effects in the low distance regime the corrugation of the Au(111) surface is enhanced by about 10-15 pm1

[1] V. M. Hallmark et al., Phys. Rev. Lett. 59, 2879 (1987)

Page 29: P. Grutter STM as a Tool to Understand the Electronic Properties of Molecules Peter Grutter Physics Department McGill University Part of SPM lecture series

P. Grutter

3. Change of electronic surface properties1

[1] W.A. Hofer, J. Redinger, A. Biedermann, and P. Varga, Surf. Sci. Lett. 466, L795 (2000)[2] V. L. Moruzzi et al. Phys. Rev. B 15, 6671 (1977)

System: Fe(100) bcc latticeDFT calculation: FLEURLattice constant: 2.78 A

LDA: Moruzzi et al [2]No of k-points: 36

Page 30: P. Grutter STM as a Tool to Understand the Electronic Properties of Molecules Peter Grutter Physics Department McGill University Part of SPM lecture series

P. Grutter

Quenching of surface states

Simulation of quenching: distance dependent reduction of the occupationnumber of single Kohn-Sham states of the surface, 2nd order polynomial

Page 31: P. Grutter STM as a Tool to Understand the Electronic Properties of Molecules Peter Grutter Physics Department McGill University Part of SPM lecture series

P. Grutter

5. Importance of different effects

Page 32: P. Grutter STM as a Tool to Understand the Electronic Properties of Molecules Peter Grutter Physics Department McGill University Part of SPM lecture series

P. Grutter

Tunneling Spectroscopy (cartoon version)

Elastic: linear I-V Inelastic: non-linear I-V

Page 33: P. Grutter STM as a Tool to Understand the Electronic Properties of Molecules Peter Grutter Physics Department McGill University Part of SPM lecture series

P. Grutter

Tunneling Spectroscopies

• I(V) at constant z or variable z

• dI/dV at constant z or constant average I

• d (log I)/dz (barrier height measurement)

Page 34: P. Grutter STM as a Tool to Understand the Electronic Properties of Molecules Peter Grutter Physics Department McGill University Part of SPM lecture series

P. Grutter

Tunneling Spectroscopy: an example

Hyrogen on SiC surface: goes from insulator -> conductor

Derycke et al., Nature Mater. 2, 253 (2003)

UPS

Page 35: P. Grutter STM as a Tool to Understand the Electronic Properties of Molecules Peter Grutter Physics Department McGill University Part of SPM lecture series

P. Grutter

Geometric and Electronic Properties of Molecules I

P. Weiss et al., Science 271, 1705 (1996)

Y. Sun, H. Mortensen, F.

Mathieu, P. Grutter (McGill)

Porphrin on Au(111)

Alkane thiols

Page 36: P. Grutter STM as a Tool to Understand the Electronic Properties of Molecules Peter Grutter Physics Department McGill University Part of SPM lecture series

P. Grutter

Geometric and Electronic Properties of Molecules II

J. Mativietsky, S. Burke, Y.Sun, S. Fostner, R. Hoffmann, P. Grutter

C60 on Au(111)

Page 37: P. Grutter STM as a Tool to Understand the Electronic Properties of Molecules Peter Grutter Physics Department McGill University Part of SPM lecture series

P. Grutter

Single-Molecule Vibrational Spectroscopy and Microscopy

B.C. Stipe, M.A. Rezaei, W. Ho Science 280, 1733 (1998)

25 averages, 2 minutes per spectrum

= 4.2% (1-2)

= 3.3% (3, different molecule)

Page 38: P. Grutter STM as a Tool to Understand the Electronic Properties of Molecules Peter Grutter Physics Department McGill University Part of SPM lecture series

P. Grutter

Single-Molecule Vibrational Spectroscopy and Microscopy

B.C. Stipe, M.A. Rezaei, W. Ho Science 280, 1733 (1998)

C2H2 and C2D2 comparison

Page 39: P. Grutter STM as a Tool to Understand the Electronic Properties of Molecules Peter Grutter Physics Department McGill University Part of SPM lecture series

P. Grutter

Geometric and Electronic Properties of Nanowires I

Whitman et al, PRL 66, 1338 (1991)0.3 ML Cs on GaAs

and InSb (fig. C)

Page 40: P. Grutter STM as a Tool to Understand the Electronic Properties of Molecules Peter Grutter Physics Department McGill University Part of SPM lecture series

P. Grutter

Geometric and Electronic Properties of Nanowires II

Ohbuchi and Nogami, PRB 66, 165323 (2003)

0.36 ML Ho on Si, 400 nm image

Anisotropic lattice mismatch --> wires. Are they conductive?

Page 41: P. Grutter STM as a Tool to Understand the Electronic Properties of Molecules Peter Grutter Physics Department McGill University Part of SPM lecture series

P. Grutter

Geometric and Electronic Properties of Nanowires III

Evans and Nogami, PRB 59, 7644 (1999)

0.04 ML In on Si(001), 14 nm image

However: In wires are NOT conductive !

Nogami, Surf. Rev. & Letters, 6, 1067 (1999)

Page 42: P. Grutter STM as a Tool to Understand the Electronic Properties of Molecules Peter Grutter Physics Department McGill University Part of SPM lecture series

P. Grutter

Defined, reproducible, understandable I-V of molecules

Chemically reliable contact

Cui et al. Nanotechnology 13, 5 (2002), Science 294, 571 (2001)

Page 43: P. Grutter STM as a Tool to Understand the Electronic Properties of Molecules Peter Grutter Physics Department McGill University Part of SPM lecture series

P. Grutter

Other spectroscopies of molecules: may the force be with you

Ch. Joachim and J. Gimzewski, Chem. Phys. Lett 265, 353 (1997)

Experimental variation o f the conductance of C60 modulated by Vin (t). The time variation o f the voltage Vz piezo applied to the piezoelectric actuator is shown as a dashed line and the experimental C60(t) conductance response as a solid line.

Page 44: P. Grutter STM as a Tool to Understand the Electronic Properties of Molecules Peter Grutter Physics Department McGill University Part of SPM lecture series

P. Grutter

Interpretation of C60 amplifier

Ch. Joachim and J. Gimzewski, Proc. IEEE 86, 184 (1998)

Calculated variations of surface resistance of C60 on Au(110) as a function of applied force

Page 45: P. Grutter STM as a Tool to Understand the Electronic Properties of Molecules Peter Grutter Physics Department McGill University Part of SPM lecture series

P. Grutter

STM/STS and conductivity

So So

if STM/STS is so powerful if STM/STS is so powerful

- can we use it to determine the conductivity - can we use it to determine the conductivity of molecules???of molecules???

Page 46: P. Grutter STM as a Tool to Understand the Electronic Properties of Molecules Peter Grutter Physics Department McGill University Part of SPM lecture series

P. Grutter

‘Traditional’: infinite, structureless leads -> periodic boundary conditions.

but:

- result depends on lead size!

- bias not possible due to periodic boundary condition!

Calculating Conductance

Jellium lead Jellium leadmolecule

Page 47: P. Grutter STM as a Tool to Understand the Electronic Properties of Molecules Peter Grutter Physics Department McGill University Part of SPM lecture series

P. Grutter

Calculation of electrical transport

)],(),([),(2

)( 00 eVEfEfVETdEh

eVI RRLL

O f t e n o n e a s s u m e s t h a t T i s n o t a f u n c t i o n o f V , i . e . :

)(),( ETVET

a n d s t i c k s a l l t h e V d e p e n d e n c e i n t o t h e F e r m if u n c t i o n f

Page 48: P. Grutter STM as a Tool to Understand the Electronic Properties of Molecules Peter Grutter Physics Department McGill University Part of SPM lecture series

P. Grutter

ab-initio modelling of electronic transport

lead

Hong Guo’s research group, McGill Physics

Page 49: P. Grutter STM as a Tool to Understand the Electronic Properties of Molecules Peter Grutter Physics Department McGill University Part of SPM lecture series

P. Grutter

DFT plus non-equilibrium Green’s Functions

J. Taylor, H. Guo , J. Wang, PRB 63, R121104 (2001)

1. Calculate long, perfect lead.

Apply external potential V by shifting energy levels

-> create electrode data base and get potential right

lead

Page 50: P. Grutter STM as a Tool to Understand the Electronic Properties of Molecules Peter Grutter Physics Department McGill University Part of SPM lecture series

P. Grutter

2. Solve Poisson equation for middle part

(device plus a bit of leads); match wavefunctions

and potential as a function of V to leads

(use data base) in real space.

3. calculated with non-equilibrium Green’s functions (necessary as this is an open system). This automatically takes care of bound states

Page 51: P. Grutter STM as a Tool to Understand the Electronic Properties of Molecules Peter Grutter Physics Department McGill University Part of SPM lecture series

P. Grutter

STM/STS and conductivity• STM measures DOS(EFermi)

• DOS related to conductivity

• BUT: how does the tunneling current couple to molecular conductivity?– Very indirect:

• function of – DOS, E (where does potential drop off?)– symmetry/coupling (electrode vs. complex molecule)– k vector (lateral vs. perpendicular conductivity)– internal transport mechanism (tunneling, hopping, ballistic)

Page 52: P. Grutter STM as a Tool to Understand the Electronic Properties of Molecules Peter Grutter Physics Department McGill University Part of SPM lecture series

P. Grutter

So is SPM useful in molecular electronics?

Page 53: P. Grutter STM as a Tool to Understand the Electronic Properties of Molecules Peter Grutter Physics Department McGill University Part of SPM lecture series

P. Grutter

Molecular electronics: the issues

• Contacts• Structure-function

relationship between transport process and molecular structure

• Dissipation

• Crosstalk (interconnects)

• Architecture • I-O with a trillion

processors• Fault tolerance• Manufacturing costs

Page 54: P. Grutter STM as a Tool to Understand the Electronic Properties of Molecules Peter Grutter Physics Department McGill University Part of SPM lecture series

P. Grutter

Does atomic structure of the contact matter?

YES !

Page 55: P. Grutter STM as a Tool to Understand the Electronic Properties of Molecules Peter Grutter Physics Department McGill University Part of SPM lecture series

P. Grutter

Does atomic structure of the contact matter?

Mehrez, Wlasenko, et al, Phys. Rev. B 65, 195419 (2002)

Page 56: P. Grutter STM as a Tool to Understand the Electronic Properties of Molecules Peter Grutter Physics Department McGill University Part of SPM lecture series

P. Grutter

Electronic Properties of Molecules: Requirements

R. Reifenberger

Page 57: P. Grutter STM as a Tool to Understand the Electronic Properties of Molecules Peter Grutter Physics Department McGill University Part of SPM lecture series

P. Grutter

Low-T UHV STM/AFM/FIM

140K,

10-11mbar

quick change between

FIM - AFM/STM mode

Stalder, Ph.D. Thesis 1995

Cross et al. PRL 80, 4685 (1998)

Schirmeisen et al. NJP 2, 29.1 (2000)

Sun, Lucier, Mortensen, Schaer

Page 58: P. Grutter STM as a Tool to Understand the Electronic Properties of Molecules Peter Grutter Physics Department McGill University Part of SPM lecture series

P. Grutter

Field Ion Microscopy

(FIM)

E. Muller, 1950’s

Page 59: P. Grutter STM as a Tool to Understand the Electronic Properties of Molecules Peter Grutter Physics Department McGill University Part of SPM lecture series

P. Grutter

Page 60: P. Grutter STM as a Tool to Understand the Electronic Properties of Molecules Peter Grutter Physics Department McGill University Part of SPM lecture series

P. Grutter

FIM of W(111) tip

Imaging at 5.0 kV

A. Schirmeisen,

G. Cross,

A. Stalder,

U. Durig

Page 61: P. Grutter STM as a Tool to Understand the Electronic Properties of Molecules Peter Grutter Physics Department McGill University Part of SPM lecture series

P. Grutter

FIM of W(111) tip

Imaging at 5.0 kV

Manipulating at 6.0 kV

Page 62: P. Grutter STM as a Tool to Understand the Electronic Properties of Molecules Peter Grutter Physics Department McGill University Part of SPM lecture series

P. Grutter

FIM of W(111) tip

Imaging at 5.0 kV

Manipulating at 6.0 kV

Page 63: P. Grutter STM as a Tool to Understand the Electronic Properties of Molecules Peter Grutter Physics Department McGill University Part of SPM lecture series

P. Grutter

FIM of W(111) tip

Imaging at 5.0 kV

Manipulating at 6.0 kV

Page 64: P. Grutter STM as a Tool to Understand the Electronic Properties of Molecules Peter Grutter Physics Department McGill University Part of SPM lecture series

P. Grutter

Single Au atom on W(111) tip

Imaged at 2.1 KV

Page 65: P. Grutter STM as a Tool to Understand the Electronic Properties of Molecules Peter Grutter Physics Department McGill University Part of SPM lecture series

P. Grutter

Anne-Sophie Lucier

Page 66: P. Grutter STM as a Tool to Understand the Electronic Properties of Molecules Peter Grutter Physics Department McGill University Part of SPM lecture series

P. Grutter

W(111) tip on Au(111)

Cross et al.

PRL 80, 4685 (1998)

Schirmeisen et al,

NJP 2, 29.1 (2000)

Page 67: P. Grutter STM as a Tool to Understand the Electronic Properties of Molecules Peter Grutter Physics Department McGill University Part of SPM lecture series

P. Grutter

W(111) trimer tip on Au(111)

Ead = 21 eV

= 0.2 nm

Page 68: P. Grutter STM as a Tool to Understand the Electronic Properties of Molecules Peter Grutter Physics Department McGill University Part of SPM lecture series

P. Grutter

Molecular Dynamics Simulations

U. Landman et al, Science 248, 454 (1990)

Page 69: P. Grutter STM as a Tool to Understand the Electronic Properties of Molecules Peter Grutter Physics Department McGill University Part of SPM lecture series

P. Grutter

Force and Current vs. Distance

Sun et al, subm. PRL

Page 70: P. Grutter STM as a Tool to Understand the Electronic Properties of Molecules Peter Grutter Physics Department McGill University Part of SPM lecture series

P. Grutter

Making contact

2.0 1.5 1.0 0.5 0.0 -0.5 -1.0 -1.5-7

-6

-5

-4

-3

-2

-1

0

Fo

rce

[nN

]

Tip-Sample Separation [Å]

10

100

1000

Cu

rren

t [n

A]

elastic

C2

~±0.2Å

~±0.2Å

C1

~0.1G0,50mVbias

Page 71: P. Grutter STM as a Tool to Understand the Electronic Properties of Molecules Peter Grutter Physics Department McGill University Part of SPM lecture series

P. Grutter

8.0 7.0 6.0 5.0 4.0 3.0 2.0 1.0 0.0 -1.0 -2.010-4

10-3

10-2

10-1

100

101

102

103

104

Cu

rren

t [n

A]

Tip-Sample Separation [Å]

Work Function vs. Apparent Barrier Height

Hofer, Fisher, Wolkow, Grutter, Phys.Rev. Lett., 87, 2001, 236104

Ze~ 0.4eV

~ 4.5eV

~9.4eV

ÅW tip-Au surface

dlnI/dz=-(2m)1/2/ħ 1/2

=0.95(dlnI/dz)2

I[nA] and Z[Å]

Vbias=0.05V

Vbias=0.1V

Page 72: P. Grutter STM as a Tool to Understand the Electronic Properties of Molecules Peter Grutter Physics Department McGill University Part of SPM lecture series

P. Grutter

Atomic Structure Matters

-10 -9 -8 -7 -6 -5 -4 -30

2

4

6

8

10

12

14

Without relaxation

Atop site

Hollow site

W(111) tip, Au(111) surface

Ap

pa

ren

t Ba

rrie

r H

eig

ht [

eV

]

Tip-sample Separation [Å]

W.A. Hofer, U. of Liverpool, unpublished

Page 73: P. Grutter STM as a Tool to Understand the Electronic Properties of Molecules Peter Grutter Physics Department McGill University Part of SPM lecture series

P. Grutter

Major Conclusions:• Forces cannot be neglected!

– Different decay lengths -> non-local, non-uniform!

– Substantial (nN)

– Major relaxation effects

• Point of contact determined both electronically and mechanically: they are identical to within measurement error.

• W an atomically very robust electrode material.

• In tunneling regime: modeling in quantitative agreement with experiment.

Page 74: P. Grutter STM as a Tool to Understand the Electronic Properties of Molecules Peter Grutter Physics Department McGill University Part of SPM lecture series

P. Grutter

Beware of PowerPoint Engineering

or Cartoon Physics!!!

Page 75: P. Grutter STM as a Tool to Understand the Electronic Properties of Molecules Peter Grutter Physics Department McGill University Part of SPM lecture series

P. Grutter

Storing information atom by atom

• Ultra high density (library of congress on a pin head)

• Ultra slow (needs life time of universe to write)

• Huge footprint (UHV 4K STM)

D. Eigler, IBM Almaden

Page 76: P. Grutter STM as a Tool to Understand the Electronic Properties of Molecules Peter Grutter Physics Department McGill University Part of SPM lecture series

P. Grutter

Conductance via dissipation imaging?

Stowe et al., APL 75, 2785 (1999)

Denk and Pohl, JAP 59, 2171 (1991)

zyx

stx VCvP

222

Page 77: P. Grutter STM as a Tool to Understand the Electronic Properties of Molecules Peter Grutter Physics Department McGill University Part of SPM lecture series

P. Grutter

Summary

• Tools, both experimental and theoretical, drive our capabilities to understand the nanoworld!

• STM spectroscopy very powerful, but big challenge to extract conductivity.

• STM and AFM have only started to make an impact in the field of nanoelectronics.