Spectroscopy ---STM

• Probe HOMO and LUMO at the sub-molecular and atomic scale.

• Probe metal oxidation (Co (II) Co(III))

• Allows determination of energy gap (homo –lumoseparation, instead of using the optical energy gap).

• Gives the LDOS (local density of states ≠ UPS-averaged value)

• Electron path during electron transfer (metal ion vs. ring).

Tunneling in an STM

)exp( EUdcVI −−=

STM imagesconvolution of topography (physical size) and density of states

The PE due to the attraction of the electron by a positive nuclear charge

+e-

Hydrogen Atom

rerV

oπε4)(

2−=

+ + + +

PE(x) = -e /(x-x )2

Spatial Picture

Potential EnergyPicture

PE = 0 ev

PE = -20 eV

atomic core valence electron

iΣi

+ + + + Spatial Picture

Box Potential

Energy Picture

PE = 0 ev

PE = -20 eV

atomic core valence electron

"Electrons in a Box" Energy Picture for a Metal

PE = 0 ev

PE = -10 eV

N electrons in the box!HOWEVER: Electrons are quantum mechanical beasts, so

1) Only certain energies (states) are possible2) At most, 2 electrons can be in each state

Fermi Energy(E )F

N nuclei with 1 valence electron each, require ...

E F

Potential barrier to electron flowbetween metals

Metal MetalVacuum Gap

d

φ

path of tipConstant current contour

Negative Contrast

Positive Contrast

I(V) = ∫ ρs(E) ρt(E+V) T(r,E,V)dE0

V

Non-resonant tunneling

Ni(II) tetraphenylporphyrin

T

S

TS

φφ

φ φ

Metal Tip

Vacuum Space

Conducting Support

AdsorbateState Density

E

E

vacuum

Fermi

Negative Bias

Positive Bias

Vbias

Vbias

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

dI/d

V

STM-OMTS at 298K

unoccupied orbital

5.26.27.2 4.2 3.2Energy Below Vacuum Level (eV)

occupied orbital(transient oxidation)

(transient reduction)

EF

ro

= Δ

= Δ

r

o

cobalt(II) tetraphenylporphyrin

Energy Below Vacuum Level (eV)7.2 6.2 5.2 4.2 3.2

STM

UPS

TunnelDiode

E = 4.7 eVF

E = 5.2 eVF

E = 4.3 eVFa

a

a

b

b

c

c

Assignments:

a) Pc-2 ⇒ Pc-1

(π ionization)

b) Co+2 ⇒ Co+3

(dz2 ionization)

c) Pc-2 ⇒ Pc-3

(π* affinity level)

7.2 6.2 5.2 4.2 3.2

CoTPP

Energy Below Vacuum Level (eV)

dI/dV (Arbirary Units)Inte

nsity

(Arb

itrar

y Un

its)

STMUPS

E = 5.2 eVF

E = 4.7 eVF

E = 4.3 eVF

A B

C

Electrochemical Model for OMTS

Δr is the bias voltage relative to EF at which the first transient reduction (LUMO tunneling) is observed.

Δo is the bias voltage relative to EF at which the first transient oxidation (HOMO tunneling) is observed.

By using the measured value of EF(from UPS), the OMTS bands can be located both relative to the vacuum level and also to electrochemical potentials.

Richardson, Inorg. Chem. 29 (1990) 3213.Schmickler, “Interfacial Electrochemistry” (1996)Loutfy et al. Can, J, Chem. 62 (1984) 1877 A molecule adsorbed on Au(111).

vacuum level

-1.0

-2.0

-3.0

-4.0

+1.0

0.0

1/2

EF

E1/2

E (

sce)

red

+2.0gI

0.0

1.0

2.0

3.0

4.0

5.0

6.0

gA

E1/2ox

cA

Ic

A f

IfEn

ergy

(eV)

r

o

7.0

(Au)

0.0 2.0 4.0 6.0 8.0

0.0

2.0

4.0

6.0

8.0

OMTS Energy (Volts)

Elec

troch

emica

l Pot

entia

l(v

acuu

m)

STM [Au(111)] data

Al-Al O -Pb diode data2 3

E(NHE)=0.0

Ionization and Oxidation Potentials Relative to the Vacuum Level.

Molecule OMTSEnergy(STM)

UPSthin film

UPSvapor phase

Eoxelectrochemical

CoPc (metal oxidation) 5.40 5.471

CoTPP (metal oxidation) 5.30 5.201

ZnPc2 (ring oxidation) 5.35 6.39 5.34CoPc (ring oxidation) 5.80 5.80 6.383 5.851

CuPc (ring oxidation) 5.80 5.701

NiOEP (ring oxidation) 6.34 6.38 6.354 5.51CoTPP (ring oxidation) 6.40 6.50 5.76NiTPP (ring oxidation) 6.50 6.60 6.625 5.80 Pc = phthalocyanine, TPP = tetraphenylporphyrine, and OEP = octaethylporphyrin 1] Wolberg, A.; Manassen, J.; J. Amer. Chem. Soc. 1970, 92, 2982-2991.2] Schlettwein, D.; Hesse, K.; Gruhn, N.E.; Lee, P.A.; Nebesny, K.; Armstrong, N. J.Phys. Chem. B 2001, 105, 4791-4800.3] Berkowitz, J.; J. Chem. Phys. 1979, 70, 2819-2828.

4] Westcott, B.L.; Gruhn, N.; Michelsen, L.; Lichtenberger, D.; J. Am. Chem Soc. 2000,122, 8083-8084.5] Khandelwal, S.C.; Roebber, J.L.; Chem. Phys. Lett. 1975, 34, 355-359.

M

M+

E0

Ep

KE(electron) + E = h

M + h = M + e+ -

UPS

5.26.27.2 4.2 3.2Energy (eV)

OM

TS (d

I/dV)

UPS

NiOEP on gold

1 nm

A Quantum Dot (QD) is a nanometer sized structure that is capable of trapping electrons in three dimensions. Quantum dots are made by creating an island of conductive material surrounded by insulator. Electrons that enter the QD will be confined because of the high potential required to escape through the insulator. The energy required to add one electron to a QD is e2/2C, where C is the capacitance of the QD. Each new electron entering the quantum dot must overcome this energy. So, to add 3 electrons to a QD we must supply 3e2/2C of electronic energy. This charge blocking effect is called the Coulomb Blockade. In order for the QD to be thermally stable, e2/2C >> kT(~30meV at room T)

Bakkers, E.; Hens, Z.; Kouwenhoven, L.; Gurevich, L.; Vanmaekelbergh, D.; Nanotechnology 13 (2002) 258.

Double Barrier Tunnel JunctionΓ: electron tunneling rate from the tip to an empty orbital

a) Tunneling spectra of InAsQuantum Dots (QD) ~2nm in radius. The solid curve was for QD/HOPG while the dashed curve is for a QD/thiol-linker/Au sample. (DBTJ)

b) Calculated spectra:Γout/Γin =1 dashed lineΓout/Γin = 10 solid line

Katz, D.; Millo, O.; Kan, S.; Banin, U. Appl. Phys. Lett. 79 (2001) 117-119.

Controlled Manipulation of atoms and molecules

• Parallel Processes1. Field assisted diffusion (intense E field between tip-surface (0.2 to 2

V/Å compares with 5 V/Å; ionization and desorption of an atom).2. Sliding (tip exerts a force on the adsorbate which is bound to the

surface (bond is not broken). Force needed to overcome the lateral force between adsorbate and surface.

• Perpendicular Processes1. Transfer On –or Near contact (adsorbate surface bond is broken. No

need of E, Potential difference or flow of current between tip-sample)

2. Field Evaporation (intense E needed to lower the potential E (barrier) a sort of FIM, field ion microscopy).

3. Electromigration (flow of current induce the migration of impurities or other defects through the bulk of the sample).

Tip-Surface Interaction

Surface

Tip

Fine

Pos

ition

Coa

rse

Appr

oach

InteractionSensing

FeedbackControl

Erro

r

Sign

al

x,y

Computer: drives x,y scan; saves data; generates images.

Feedback System:holds interactionbetween tip and surface constant. The coarse approach brings

the tip and surface 'closeenough' for the fine mechanism to maintain a constant desiredinteraction.

Fine Position & Scanningare usually performed with one or more piezo-electric elements.

Parallel process

E

Perpendicular process

B

Full monolayer coverage obtained at impedance gap of about 2.8GΩ.

2D Lattice of HOPG ~2.4Ǻ

Vertical Manipulation of NiOEP with a STM tip under ambient conditions

Clearing of NiOEP with the STM tip operated at about 120 MΩ. This STM image reveals a molecule free center region.

20 nm

Perfect monolayer coverage of HOPG Same STM image after one 80 x 10 nm2

scan at high setpoint current (gap of 120MΩ).

The surface of HOPG is revealed after removal of the NiOEP

20 nm

30 nm

L. Scudiero and K.W. Hipps, J. Phys. Chem. C (2007), 111, 17516-17520.