Imaging and manipulation of single atoms and molecules: the science of the nanoscale

worldMiquel Salmeron

Materials Science DivisionLawrence Berkeley National Laboratory

University of California, Berkeley, CA. USA

How does STM work ?Principles of atomic manipulationSTM as a writing toolMaps of atoms or maps of electronic states?Rotating moleculesMaking and breaking molecules Movies of molecular motion

Content:

eVbias

sample

tip

Principle of operation of theScanning Tunneling Microscope

I V N eV e A z . ( ).e .

Z-resolution 0.1 pm

XY-resolution 100 pm

Z = 5-10 Å

Attraction(pull)

Repulsion(push)

Vibrational excitation

Electric field

Transfer by voltage pulses

((( ) ( )))

We need to unravel the mechanisms of interaction between our probe tip and single atoms /molecules

+

+1 V -1 V

+/-

Using the STM tip as a tool for atomic scale manipulation

Repulsive interaction manipulation:

Plowing:

Hammering:

Herman Hesse poem "Stages“ written in PMMA from The Glass Bead Game; image size: 1.6 µm x 1.6 µm, height scale: 26 nm). The storage capacity is much higher than for the CD shown in the background at the same magnification.

AFM as a writing tool

Writen by Markus Heyde

Xe atoms on Ni(110)

Building structures atom-by-atom

STM images courtesy of Don Eigler, IBM, San Jose

Building of a quantum “corral” with Fe atoms on Cu

C

C

H

H

Calculated image (Philippe Sautet)

orbital

pz

TIP

H

O+

Imaging: acetylene on Pd(111) at 28 K

Molecular Image Tip cruising altitude ~700 pmΔz = 20 pm

Surface atomic profile

Tip cruising altitude ~500 pm

Δz = 2 pm

1 cm(± 1 μm)

The STM image is a map of the pi-orbital of distorted acetylene

Why don’t we see the Pd atoms?Because the tip needs to be very close to image the Pd atoms and would knock the molecule away

If the tip was made as big as an airplane, it would be flying at 1 cm from the surface and waving up an down by 1 micrometer

Excitation of frustrated rotational modes in acetylene molecules on Pd(111) at T = 30 K

Tip

e-

((( ) ( )))

0.1

1

10

100

-300-250-200-150-100-500Tip Bias (mV)

Log

(Hop

s/s) 253 pA

-37mV

0

8

16

24

32

0 50 100 150 200 250 300 350 400 450current (pA)

rota

tion

s p

er

secon

d

1.72 seconds

V = 20 mV

0

50

100

150

200 1

2,3

Pd

Pd

Pd1

23

Pd

Pd

Pd

2

Measuring the excitation rate

Tip fixed at position 1:

Curr

ent

(pA

)

((( ) ( )))

x

Center of molecule

Excitation of translations of C2H2 molecules:

R = 150 M R = 94 M R = 0.55 G

Rotation by electron excitation:

R = 10.5 M

Translation by direct contact (orbital overlap):

z ~ +0.8 Åz ~ -0.2 Å

z ~ - 1 Å

Tip

z

((( ) ( )))

Trajectories of molecule pushed by the tip

Molecular oxygen at 30K

2 nm

No dissociation using low tunnel current and low energy electrons

molecule

tip

2 nm

Atomic oxygen

High dissociation rate at high current and

energy

atom

tip

TIP

O2

2O

2.58 eV

Tip-induced dissociation of O2

Tip electrons

Lifetime = 10-12 s1 nA 10-10 s

((( ) ( )))

I = 11 nA

T = 43 K

Images at 1 nA, 100 mV

Equilibration of hot O atoms

molecules pairs of atoms

2

18

12

3

O-pair separation hystogram

Lifetime of O atoms in the excited state:EOads ~ 4 eV; distance traveled ~ Pd lattices 1 fs de-excitation mechanism is by creation of e-h pairs in the Pd substrate

Distribution of O-atoms after dissociation of several molecules

Positions color-coded for distance

Diffusion of water molecules on Pd(111)b)a)

Atom-trackingMovies

Hopping rate, r = v·exp(-Hopping rate, r = v·exp(-E/kT)E/kT)

Energy barrier, E = 126 ± 7 Energy barrier, E = 126 ± 7 meVmeV

(2.9 kcal/mol)

Attempt frequency, v = 10Attempt frequency, v = 1012.0 ± 0.6 12.0 ± 0.6 s s -1-1

water molecules

Trajectory of the tip following a water molecule

a b c

def

2 M Dimer

Trimer5-H2O

Clustering and diffusion at 40 K

diffusion coefficients at 40 K:diffusion coefficients at 40 K:

• monomer ~ 0.0023 Åmonomer ~ 0.0023 Å22/s /s

• dimer > 50 Ådimer > 50 Å22/s /s

• trimer, tetramer ~ 1.02 Åtrimer, tetramer ~ 1.02 Å22/s /s

The most stable cluster: hexagonal

6-H2O

Why dimers move faster than monomers:

Jim DunphyClaude ChapelierStefan BehlerAnne Borg Mark RoseToshi MitsuiEvgueni FominFrank OgletreeMarkus Heyde

Collaborators:

Funding by the US Department of Energy