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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.