Sub-Monolayer Sn/Cu(100)

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Electronic Energy Levels Alignment of Dye
Sensitized Oxide Surfaces
Sylvie Rangan Rutgers University Department of
Physics and Astronomy 136 Frelinghuysen Road,
Piscataway, NJ 08854
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Introduction
A Dye Sensitized Solar Cells (DSSCs) a potential
low cost alternative to Si solar cells.

• -Sunlight enters the structure,
• excites electron-hole pairs
• in dye.
• -Electron is transferred into
• TiO2 conduction band, and
• hole is filled by I- ions
• (3I- ? 2e- I3-).
• -Electron travels through circuit,
• reduces I3- (2e- I3- ? 3I-)

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Energetics N3 molecule on TiO2
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New experimental approach of the subject
A few examples to show you it works!
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Experimental setup
Occupied and unoccupied states in the same UHV
system
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Experimental setup
Scanning Tunnel Microscope
I
VT-SPM OMICRON
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Outline
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Outline
Band alignment tuning strategies
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TiO2 (110) monocrystal
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TiO2 (110)
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N3 on TiO2 (110)
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Energy diagram
LUMO 1
LUMO
0.5 eV
3.2 eV
Ti3d
HOMO
0.9 eV
LUMO
HOMO
TiO2(110)
N3 TiO2(110)
N3
TiO2
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Influence of intermediate steps
HOMO and LUMO due to N3
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Anatase TiO2 nanoparticles
Ti3d
TiO2 nano
N3 TiO2 nano
Dunbar P. Birnie III group, Rutgers University

• Same HOMO-LUMO gap
• Anatase gap 0.2 eV larger
• than rutile gap

100 Å
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Comparison with DSSC perfomances
Incident Photon to Current conversion Efficiency
N3nanocrystals TiO2
From Photoelectrochemical cells, Michael Grätzel,
Nature, nov 2001, 338
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ZnO substrates
Monoatomic step edge 2 Å
100 nm x 100 nm
Length (nm)
ZnO (11-20) epitaxial film

• ZnO nanorods
• Grown by Metalorganic
• chemical vapor deposition
• Length 1.8 µm
• Diameter 100 nm

Yicheng Lu group, Rutgers University
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N3 on ZnO(11-20)
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N3 on ZnO - nanorod
LUMO 1
LUMO
2.7 eV
2.0 eV
(2.1 eV)
0.4 eV
(0.4 eV)
4.5 eV
Zn3d
O2p
(4.6 eV)
3.4 eV
(3.2 eV)
HOMO
1.3 eV
(1.3 eV)

• Smaller dye coverage
• Similar to single crystal ZnO

N3
ZnO
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Isonicotinic acid N3 linker analogue
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N3-INA comparison
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INA on TiO2 model calculations
(1)
(2)
(1) DFT Study of Bare and Dye-Sensitized TiO2
Clusters and Nanocrystals Lundqvist, Nilsing,
Persson, Lunell Intern. Journal of Quantum
Chemistry, Vol 106, 32143234 (2006)
(2) Anchor group influence on moleculemetal
oxide Interfaces Periodic hybrid DFT study of
pyridine bound to TiO2 via carboxylic and
phosphonic acid M. Nilsing , P. Persson, L.
Ojama Chemical Physics Letters 415 (2005) 375380
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First Conclusion

• UPS and IPS in the same UHV system
• The most direct method to characterize the ground
  state electronic structure
• UV-visible absorption(exciton)/NEXAFS(core hole)
  typically used in the field
• First measurement of the electronic
  occupied/unoccupied structure of dye molecules on
  surfaces
• Can help improve theoretical treatment of
  dye/oxides systems.

• Energy level alignment of N3 on TiO2(110) and
  ZnO(11-20)
• N3 on TiO2 nanoparticles and ZnO nanorods
• INA vs N3 linker group good model for LUMO

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Zn-TetraPhenylPorphyrin

• Appropriate HOMO/LUMO levels position in energy.
• Functional groups added to the phenyl groups have
  not much influence on the TPP absorption
  properties.

ZnTPP Zinc TetraPhenylPorphyrin
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ZnTPP vs ZnP
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Energy levels alignment
E. Galoppini, Rutgers
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Approaches for band alignment tuning
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Approaches for band alignment tuning
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ZnTPP1 on TiO2
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Energy level alignment
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Comparison with other methods 1/2
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Comparison with other methods 2/2
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ZnTPP1 electronic structure
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Tunnel Microscopy Approach
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ZnTPP3 adsorption
Effect on the electronic structure Adsorption
modes? Aggregation?
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Another conclusion
ZnTPP derivatives good candidate to study
fundamental properties related to energy level
alignment.
UPS/IPS useful and direct way for measuring
energy level alignment.
Still a lot of work to do!
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THANKS !!!
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Absorption properties
Typical UV-visible absorption spectrum

• S1 and S2 first and second excited states of the
  molecule.
• Fast internal conversion S2 ? S1
• B and Q bands both arise from p to p transitions
  and can be explained by considering the four
  frontier orbitals of the porphyrin.

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Goutermann Four-Orbital Model
Orbitals
LUMOs
HOMOs
Energy states
Soret (B) band
S2
Q bands
S1

• Transitions between these orbitals gives rise to
  two 1Eu excited states.
• Orbital mixing splits these two states into a
  high energy state with a high oscillator strength
  and a low energy state with a low oscillator
  strength.

S0
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Pivalic acid covered surface
6 Å
3 nm x 2 nm
15 nm x 15 nm
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Zn-based dyes
ZnTPP2
ZnTPP1
These dye are believed to adsorb perpendicular to
the substrate and might form clusters of parallel
molecules.
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Zn-based dyes
ZnTPP3
This dye is believed to adsorb flat on the
substrate.
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ZnTPP1 on TiO2 and ZnO
Small effect on ZnTPP elecronic structure by
changing the substrate.
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Approaches for band alignment tuning 3/3