II. Results from Prior NSF Support
Bartynski: DMR #### NEW GRANT FOR XPS SYSTEM
ECS-0224166: “Development of ZnO Based Room Temperature Spintronics” [$255,000 (09/02 – 08/06)
w/ Y. Lu] We have investigated both epitaxial ZnO thin films (c-axis in plane on r-sapphire) and
nanopillars (c-axis  substrate) grown by MOCVD Fig. 1). We examined a wide range of transition metal
dopants, finding Mn and Fe most promising. We have incorporated
dopants by diffusion, ion implantation, and in-situ during film growth with
appropriate precursors. Both the implantation doped and the in-situ
doped samples exhibit ferromagnetic behavior at room temperature.
More recent studies are focusing on the effects of co-doping with various
metals to controlling carrier type (i.e. n- or p-) and concentration.
x 10
-5
2
Laser
Pulse
0
Fourier Amplitude
DMR-0421177, “Development of a high resolution and low current
density inverse photoemission spectrograph for research and
education” [$331,560 (9/04-8/07), with R. Opila]:
We have
constructed a novel inverse photoemission spectrograph in a nearnormal incidence geometry that takes advantage of parallel photon
energy detection, both in and out of the dispersion plane, so as to
obtain good count rates while maintaining either high electron energy
resolution or low electron flux. We have developed a new electron
source (Fig. 3), modified an existing system to accept the new source,
grating and detector, we have tested the high-resolution electron source
in the early Spring.
Fig. 1: As-grown (a, b)
and Fe-implanted (c, d)
MOCVD-grown ZnO epifilms (a, c) and nanopillars
(b, d).
Relative Reflectivity Change R/R
DMR-0304432: “NER: Development of an Ultrafast Scanning Probe
Microscope,” [$100,000 (07/03-6/06), w/ F. Zimmermann] seeking to
combine apertureless near-field optical microscopy with femtosecond
pump-probe methods to obtain femtosecond time-resolved “movies” of
photoexcited dynamical processes at surfaces. We have successfully
built a femtosecond pump-probe apparatus to probe ultrafast dynamics.
We have used this apparatus to probe coherent phonon dynamics in
manganite crystals (Fig. 2). In addition, we have built a combined
apertureless near-field optical microscope/atomic force microscope
that has yielded excellent topographic images of a test sample. We
are currently combining the femtosecond pump-probe apparatus with
the microscope to obtain time-resolved microscopic images.
-2
-4
0
200
400
600
800
Frequency (cm-1)
-6
-1
0
1
2
3
Pump-Probe Delay (ps)
Fig. 2: Coherent phonon
excitation of LuMnO3 with 20fs
laser pulses observed in pumpFig.reflectivity
3: Left: High
res e-gun.
probe
studies.
Right: 1mm x 5mm linear ebeam with 50 meV resolution.
and will commission the system
DMR-0421028 “Development of an integrated thin film growth system with comprehensive in-situ
characterization for research and education” [$525,000 (9/04-8/07), with Y. Chabal] The objective of this
project is the development of a modular system for Atomic Layer Deposition (ALD) with in-situ IR
characterization and UHV sample transfer to a wide array of analytical techniques, in particular our novel
Medium Energy Ion Scattering (MEIS). The system, which is now fully designed and currently under
construction, will be used primarily to grow epitaxial high-K dielectric films on a variety of substrates.
DMR-9801681: "Auger-Photoelectron coincidence spectroscopy (APECS) of surface alloys, ultrathin films
and layered compounds" [$264,000 (7/98 – 6/04)] We have used the novel technique of APECS, in
conjunction with a range of other approaches, to investigate surface alloys, oxide surfaces, and metallic
quantum wells: systems where engineering the electronic structure gives rise to interesting physical
properties. We have also used APECS to probe magnetic oxides in a spin-specific manner and
performed the first angle resolved APECS measurements.
Galoppini PI. NSF-NIRT-CHE0303829; $1,030,760; 09/1/0307/31/08 Electronic Interactions in Hybrid OrganicNanoparticle Materials The objective of this project was to tune
the interfacial electronic interactions through systematic molecular1
Fig. 4: FIGURE 4: Caption needed.
Will fix numbering and size later.
level variation between dyes and dyes@host and nanoparticles. Two classes of molecular scaffolds were
studied: rigid linkers with multiple anchoring groups for surface binding; and “hemicarcerands” cage hosts
for encapsulation of organic guests (azulene). TiO 2 and ZnO nanocrystals, assembled in colloidal
solutions and in mesoporous thin films were functionalized with the organic dyes. Electronic interactions
of the hybrid organic-nanoparticle interface were probed electrochemically, spectroscopically on ultrafast
time scales. The study showed that the electronic processes are influenced by varying the conjugation
and length of the linkers, the size of the nanoparticle or hemicarcerand, and by the multipodal footprint.
The work resulted in twenty eight publications, five to six papers in preparation, numerous presentations
at meetings, and in the collaborative work of students, postdoctoral associates and high school students,
with frequent exchanges between labs. Six Ph.D. theses and 2 M.S. were based on this work. 2 project
SEED students participated to the research and 6 undergraduate. Travel supplements by REU allowed
summer research in Sweden for one undergraduate student and the team to take turns in work visits of
laboratory of collaborators abroad (Berlin, Stockholm).
NSF-MRI-0443538 Acquisition of x-Ray Instrumentation for Materials Chemistry at Rutgers-Newark
$100,700(02/2/05-01/31/08) [EG co-PI.] A new single-crystal X-ray CCD diffractometer with fast and
reliable data collection at low T replaced a 32-year old instrument, allowing the new faculty to routinely
have access to this technique, resulting in numerous publications. The instrument has been used for
experiments for an advanced undergraduate laboratory course and in minority outreach programs (ACS
SEED and local high schools).
NSF-Chem.Instr.-0342432 Acquisition of a Dual Non-Collinear Optical Parametric Amplifier (NOPA)
System for Ultrafast Electron Transfer Studies 07/2004/-06/2007; $220,000 [EG co-PI.] Two sub-15
fs NOPAs were integrated with an existing kHz Ti:sapphire amplifier to expand tenability and are now
being used to study a wide variety of ultrafast photo-chemical processes. This grant has greatly enhanced
fs spectroscopy resources at Rutgers University and has had a crucial impact on the progress of NSFsponsored projects.
Tailoring the Electronic Energy Level Alignment of Metalloporphyrins Adsorbed on Oxide
Semiconductor Surfaces
Robert A. Bartynski Rutgers University New Brunswick (PI) and Elena Galoppini, Rutgers University
Newark (co-PI)
III. Preliminary Remarks
III. A: Introduction. In this proposal we describe a collaborative research program aimed at establishing
the atomistic and molecular level properties of novel porphyrin-bridge-anchor dyes bound to wide band
gap metal oxide semiconductor surfaces (ZnO, TiO 2). The metal oxide surfaces will include crystalline
planar surfaces, and for comparison nanoparticle films. This class of molecule/semiconductor systems
has shown great promise for photovoltaic applications. Although the majory of the work is done on
nanoparticle films, recent work on porphyrin arrays suggests that under appropriate binding conditions
efficient light harvesting may be obtained even when the porphyrins are bound to planar crystal surfaces,
such as the ones investigated in this proposal. However, while there are numerous studies about the
properties of devices prepared from porphyrins/semiconductor materials, a detailed understanding of, for
example, the surface binding geometry, electronic structure, energy level alignment, and intermolecular
interactions, and how these properties influence optical absorption, charge transfer and electrochemistry
is very much needed. In the proposed work we will employ a series of systematic synthetic modifications
to prepare model porphyrin-bridge-anchor group compounds to (a) engineer the molecular energy level
spacing and alignment with the substrate, (b) modify orbital densities to influence charge transfer
efficiency, (c) control the surface bonding geometry, and (d) regulate molecular-molecule interactions.
The dye molecule/metal oxide semiconductor interface will be explored using a series of advanced
surface characterization techniques including direct and inverse photoemission, low temperature
scanning tunneling microscopy and spectroscopy, near edge x-ray absorption spectroscopy, and other
synchrotron radiation-based techniques. This work will be complemented by UV-vis absorption
spectroscopy, FT-IR-ATR spectroscopy, fluorescence emission, and cyclic voltammetry studies of the
2
proposed compounds. The dye-semiconductor system will be modeled using density functional theory to
facilitate the interpretation of the experimental results. The objective is to perform a comprehensive study
of this important class of hybrid systems and obtain a fundamental understanding of the key properties
that underlie their practical use.
The goal of the program described in this proposal is to develop synthetic and adsorption
strategies that control the binding and tailor the properties of novel porphyrin-based complexes bound to
oxide-semiconductor substrates in order to enhance their desirable properties for practical applications
including light harvesting arrays for solar energy applications.
The proposed research comprises a novel combination of synthetic and analytical approaches.
Two entirely new classes of porphyrin-bridge-anchor group compounds with tunable intramolecular
dipoles or protective ligand structures
will be synthesized. These molecules
will open new possibilities for
engineering the properties of the
molecule-surface interface.
In
addition,
we
will
perform
a
comprehensive
photophysical
study of the model compounds.
Investigations of adsorption in highly
controlled
ultrahigh
vacuum
conditions on well characterized
oxide surfaces and
theoretical
models using density functional
formalism, will be correlated with
solution-based studies so that the
fundamental properties can be directly
related to performance in more
Figure 1:. Figure 5: this will be modified and needs caption.
practical environments. The objective
is to develop principles that are
transferrable and will have a broad
impact on other areas, ranging from
photovoltaics and photocatalysis to
organic electronics and interface engineering, where the properties of the organic
molecule/semiconductor interface are critical.
To achieve this goal, we propose a collaborative program that brings together two PI’s with
highly complementary expertise one (EG, co-PI) an organic chemist who will synthesize a series of
porphyrin model compounds with novel functionalizations, designed to have differing orbital energies,
tunable intermolecular interactions and directed surface bonding, and will study their optical absorption
and electrochemical properties in solution and bound to semiconductor surfaces; the other (RAB, the PI)
a surface scientist who will determine the bonding geometries and measure the electronic and geometric
properties of the same porphyrin complex/oxide surface systems under ultrahigh vacuum conditions.
This research team has been collaborating informally during the past year. This exploratory work already
yielded a joint publication and provides a springboard for the proposed research. The two Rutgers
campuses are a 40 min drive away and exchanges of students from each group is routinely taking place.
The PIs also have on-going collaborations with theoreticians [involving P. Persson (Lund University,
Sweden). [71, 67, 72, 73] ] who can provide additional insight into the properties of these systems as well
as groups experienced in the development of solar device (Anders Hagfeldt, Uppsala University,
Sweden) [references] and ultrafast charge transfer studies at the interface (Villy Sundstrom, Lund
University, Sweden) [references]
3
Rational design and comprehensive characterization of metalloporphyrins adsorbed on oxide
semiconductor surfaces
Robert A. Bartynski and Elena Galoppini, Rutgers University
II. Results from Prior NSF Support
Bartynski:
DMR-0923246: “MRI: Acquisition of a state-of the-art X-ray photoelectron spectrometer for
research, training and education” [$547,279 (8/1/09 – 7/31/10) w/ 4 coPIs] Funding has just been
received for the acquisition of a state-of-the-art XPS system to replace an aging system in the Rutgers
Laboratory for Surface Modification. This instrument will be part of a central facility that provides surface
analytical services to Rutgers and the neighboring scientific
community. We are currently in negotiation with vendors to obtain
the best instrument with the funds allocated.
ECS-0224166: “Development of ZnO Based Room Temperature
Spintronics” [$255,000 (09/02 – 08/06) w/ Y. Lu] We have
investigated both epitaxial ZnO thin films (c-axis in plane on rsapphire) and nanopillars (c-axis  substrate) grown by MOCVD Fig.
1). We examined a wide range of transition metal dopants, finding
Mn and Fe most promising. We have incorporated dopants by
diffusion, ion implantation, and in-situ during film growth with
Fig. 5: As-grown (a, b) and
appropriate precursors. Both the implantation doped and the in-situ
Fe-implanted (c, d) MOCVDdoped samples exhibit ferromagnetic behavior at room temperature.
grown ZnO epi-films (a, c) and
More recent studies are focusing on the effects of co-doping with
nanopillars (b, d).
various metals to controlling carrier type (i.e. n- or p-) and
concentration.
DMR-0304432: “NER: Development of an Ultrafast Scanning
Probe Microscope” [$100,000 (07/03-6/06), w/ F. Zimmermann]
seeking to combine apertureless near-field optical microscopy with
femtosecond pump-probe methods to obtain femtosecond timeresolved “movies” of photoexcited dynamical processes at surfaces.
We have successfully built a femtosecond pump-probe apparatus to
probe ultrafast dynamics. We have used this apparatus to probe
coherent phonon dynamics in manganite crystals (Fig. 2). In addition,
we have built a combined apertureless near-field optical
Fig. 6: Coherent phonon
microscope/atomic force microscope that has yielded excellent
excitation of LuMnO3 with 20fs
topographic images of a test sample. We are currently combining the
laser pulses observed in
femtosecond pump-probe apparatus with the microscope to obtain
pump-probe
reflectivity
time-resolved microscopic images.
studies.
DMR-0421177, “Development of a high resolution and low current
density inverse photoemission spectrograph for research and education” [$331,560 (9/04-8/07),
with R. Opila] We have constructed a novel inverse photoemission
spectrograph in a near-normal incidence geometry that takes
advantage of parallel photon energy detection, both in and out of the
dispersion plane, so as to obtain good count rates while maintaining
either high electron energy resolution or low electron flux. We have
Fig. 7: Left: High res e-gun.
developed a new electron source (Fig. 3), modified an existing
Right: 1mm x 5mm linear e-beam
system to accept the new source, grating and detector, we have
with 50 meV resolution.
tested the high-resolution electron source and will commission the
system in the early spring.
DMR-0421028 “Development of an integrated thin film growth system with comprehensive in-situ
characterization for research and education” [$525,000 (9/04-8/07), with Y. Chabal] The objective of
this project is the development of a modular system for Atomic Layer Deposition (ALD) with in-situ IR
x 10
-5
Laser
Pulse
0
Fourier Amplitude
Relative Reflectivity Change R/R
2
-2
-4
0
200
-1
0
1
Pump-Probe Delay (ps)
4
400
600
800
Frequency (cm-1)
-6
2
3
characterization and UHV sample transfer to a wide array of analytical techniques, in particular our novel
Medium Energy Ion Scattering (MEIS). The system, which is now fully designed and currently under
construction, will be used primarily to grow epitaxial high- dielectric films on a variety of substrates.
DMR-9801681: "Auger-Photoelec¬tron coincidence spectroscopy (APECS) of surface alloys,
ultrathin films and layered compounds" [$264,000 (7/98 – 6/04)] We have used the novel technique of
APECS, in conjunction with a range of other approaches, to investigate surface alloys, oxide surfaces,
and metallic quantum wells: systems where engineering the electronic structure gives rise to interesting
physical properties. We have also used APECS to probe magnetic oxides in a spin-specific manner and
performed the first angle resolved APECS measurements.
Galoppini:
NSF-NIRT-CHE0303829;
$1,030,760;
09/1/03-07/31/08
“Electronic Interactions in Hybrid Organic-Nanoparticle
Materials” The objective of this project was to tune the interfacial
electronic interactions through systematic molecular-level variation
between dyes and dyes@host and nanoparticles. Two classes of
molecular scaffolds were studied: rigid linkers with multiple
anchoring groups for surface binding; and “hemicarcerands” cage
hosts for encapsulation of organic guests (azulene). TiO 2 and ZnO
Fig. 8: Text here.
nanocrystals, assembled in colloidal solutions and in mesoporous
thin films were functionalized with the organic dyes. Electronic
interactions of the hybrid organic-nanoparticle interface were
probed electrochemically, spectroscopically on ultrafast time
scales. The study showed that the electronic processes are influenced by varying the conjugation and
length of the linkers, the size of the nanoparticle or hemicarcerand, and by the multipodal footprint. The
work resulted in twenty eight publications, five to six papers in preparation, numerous presentations at
meetings, and in the collaborative work of students, postdoctoral associates and high school students,
with frequent exchanges between labs. Six Ph.D. theses and 2 M.S. were based on this work. 2 project
SEED students participated to the research and 6 undergraduate. Travel supplements by REU allowed
summer research in Sweden for one undergraduate student and the team to take turns in work visits of
laboratory of collaborators abroad (Berlin, Stockholm).
NSF-MRI-0443538 Acquisition of x-Ray Instrumentation for Materials Chemistry at Rutgers-Newark
$100,700(02/2/05-01/31/08) [EG co-PI.] A new single-crystal X-ray CCD diffractometer with fast and
reliable data collection at low T replaced a 32-year old instrument, allowing the new faculty to routinely
have access to this technique, resulting in numerous publications. The instrument has been used for
experiments for an advanced undergraduate laboratory course and in minority outreach programs (ACS
SEED and local high schools).
NSF-Chem.Instr.-0342432 Acquisition of a Dual Non-Collinear Optical Parametric Amplifier (NOPA)
System for Ultrafast Electron Transfer Studies 07/2004/-06/2007; $220,000 [EG co-PI.] Two sub-15
fs NOPAs were integrated with an existing kHz Ti:sapphire amplifier to expand tenability and are now
being used to study a wide variety of ultrafast photo-chemical processes. This grant has greatly enhanced
fs spectroscopy resources at Rutgers University and has had a crucial impact on the progress of NSFsponsored projects.
5
III. Preliminary Remarks
III. A: Introduction
In this proposal we describe a collaborative research program aimed at establishing the atomistic and
molecular level properties of novel porphyrin-bridge-anchor dyes bound to oxide semiconductor surfaces.
This class of systems has shown great promise as, for example, light harvesters for photovoltaic
applications. [REFS] Recent work on porphyrin-based arrays suggests that under appropriate binding
conditions efficient energy gathering may be obtained even in planar structures. [REFS] However, a
detailed understanding of, for example, the surface binding geometry, electronic structure, energy level
alignment, and intermolecular interactions, and how these properties influence optical absorption, charge
transfer and electrochemistry is lacking. In the proposed work we will employ a series of systematic
synthetic modifications to model porphyrin-bridge-anchor group compounds to (a) engineer the molecular
energy level spacing and alignment with the substrate, (b) modify orbital densities to influence charge
transfer efficiency, (c) control the surface binding geometry, and (d) regulate molecular-molecule
interactions. The dye molecule/oxide semiconductor interface will be explored using a series of advanced
surface characterization techniques including direct and inverse photoemission, low temperature
scanning tunneling microscopy and spectroscopy, near edge x-ray absorption spectroscopy, and other
synchrotron radiation-based techniques. This work will be correlated UV-vis absorption spectroscopy, FTIR-ATR spectroscopy, fluorescence emission and cyclic voltammetry studies of dye molecules bound to
oxide surfaces in solution. The dye-semiconductor system will be modeled using density functional
theory to facilitate the interpretation of the experimental results. The objective is to perform a
comprehensive study of this important class of dye/semiconductor systems and obtain a fundamental
understanding of the key properties that underlie the practical use of these systems.
The goal of the program described in this proposal is to develop synthetic and adsorption
strategies that tailor the properties of novel porphyrin-based complexes bound to oxide-semiconductor
substrates in order to enhance their desirable properties for practical applications including light
harvesting arrays for solar energy applications.
The proposed research comprises a
novel combination of synthetic and
analytical approaches as indicated in Fig.
5. An entirely new class of porphyrinbridge-anchor group compounds with
tunable
intramolecular
dipoles
or
protective ligand structures will be
synthesized. These molecules will open
new possibilities for engineering the
properties of the molecule-surface
interface. In addition, we will perform a
comprehensive study of these new
molecules. Investigations of adsorption in
highly controlled ultrahigh vacuum
conditions on well characterized oxide
surfaces and theoretical models using
density functional formalism will be
correlated with solution-based studies so
that the fundamental properties can be
directly related to performance in more
practical environments. The objective is
to
develop
principles
that
are
transferrable and will have a broad impact
on
other
areas,
ranging
from
photovoltaics and photocatalysis to
Fig. 9: Text
6
organic electronics and interface engineering, where the properties of the organic
molecule/semiconductor interface are critical.
To achieve this goal, we propose a collaborative program that brings together two PI’s with highly
complementary skills: one (EG) an organic chemist who will synthesize a series of porphyrin model
compounds with novel functionalizations, designed to have differing orbital energies, tunable
intermolecular interactions and directed surface binding, and will study their optical absorption and
electrochemical properties in solution; the other (RAB) a surface scientist who will determine the bonding
geometries and measure the electronic and geometric properties of the same porphyrin complex/oxide
surface systems under ultrahigh vacuum conditions. This research team, separated by only a 40 min.
drive, has been collaborating informally during the past year. Students and postdocs from Rutgers-New
Brunswick and Rutgers-Newark have spent brief periods working in each other’s labs. This exploratory
effort has yielded promising results leading to a joint publication {REF} and providing the springboard for
the proposed research. The PIs also have on-going collaborations with theoreticians [involving P.
Persson (Lund University, Sweden). [71, 67, 72, 73] ] who can provide additional insight into the
properties of these systems. [Should we also mention device people here???]
B: Motivations: WHY PORPHYRIN-BASED SYSTEMS: The study of the interface between an organic
molecule (e.g., dye, redox active compound) and wide bandgap semiconductor (e.g., TiO 2, ZnO) is both
fundamentally important and of great technical and practical interest for the development of sensors, solar
cells, electrochromic windows, and other devices. Metalloporphyrins and metalloporphyrin oligomeric
arrays of varying complexity bound to semiconductors have attracted considerable attention in various
fields of chemistry and biochemistry owing to: (a) their role as active sites for oxygen transfer
(hemoglobin), electron transfer (cytochrome) or energy conversion (chlorophyll) in biological systems;
[21, 22] (b) their ability to bind and release gases and to act as active catalytic centers in nanoporous
catalytic materials [23-25]; and (c) their photo- and chemical stability and the close match of their
photoabsorption properties to the solar spectrum, which makes them excellent sensitzers for efficient
photovoltaic applications [26-30].
In addition, porphyrin model systems are particularly well-suited for molecular level studies because
of their synthetic versatility. The porphyrin macrocyle easily accepts different transition metal ions (Zn2+,
Mg2+, etc..) to form metalloporphyrins, and can be synthetically modified and functionalized to form
model compounds of varying complexity, from simple monomers to sophisticated oligomeric arrays [ref
here]. Recent research has shown that the porphyrin macrocycle is particularly suitable for the
preparation of model anchor-bridge-porphyrin model dyes designed to control the orientation of the
porphyrin unit with respect to the surface (for instance planar or perpendicular to the surface, See Fig. 13)
as recently shown by the co-PI (refs). Recent exciting results have found that well-designed porphyrin
arrays (often called antennae) , can be prepared by using a building-block approach or by self assembly.
[ref of Drain] It has been suggested that the controlled binding of such arrays to the surface of planar
semiconductor surfaces, such as the ones investigated in this proposal could greatly increase the light
harvesting and, most interestingly, it could lead to configurations (branched, linear, with an energy
gradient etc…) that possess great control of energy and electron transfer processes. In summary,
porphyrins are promising building-block molecules, not only for nanoparticle semiconductor films, but also
for the sensitization of planar semiconductor surfaces that could ultimately lead to “thin” solar devices.
Furthermore, the type of tetraphenyl porphyrin systems (ZnTPP) investigated by this work are particularly
attractive because the properties of the phenyl rings (typically used for binding to the surface) are nearly
decoupled from those of the porphyrin macrocycle (the portion of the molecule typically responsible for its
desirable optical properties). This suggests that one can separately explore these two critical aspects of
the molecule/surface interaction: binding and electronic properties. By appropriate functionalization, one
can attempt to facilitate photoexcited charge transfer by tuning the macrocycle to absorb efficiently at a
desired wavelength, as well as directing the phenyl group through which surface binding is thought to
occur. However, to determine the underlying properties that govern molecular orientation at the surface,
energy level alignment, intermolecular distance, surface binding site, and many other critical aspects of
these molecules requires a comprehensive experimental and theoretical study.
7
THE NEED FOR COMPREHENSIVE CHARACTERIZATION AND ANALYSIS: The interaction of dye
molecules at oxide surfaces is highly complex, depending on parameters including adsorption site,
molecular geometry (orientation, distance) and charge redistribution at the surface. In the following
paragraphs, we give a brief description of the properties that will be characterized in the proposed
research.
ELECTRONIC STRUCTURE: The electronic states of ZnTPP that are within a few eV of the HOMOs and
LUMOs are shown in Fig. 6. For the isolated molecule, the
states on the macrocycle are well separated spatially and
energetically from those of the phenyl rings. While this picture
may be useful as a guideline, many additional factors must be
taken into account when considering the molecule bound to a
surface. For example, functionalizing the phenyl rings in an
effort to influence the molecule-molecule interactions at a
surface can also shift the phenyl levels so that they are nearly
degenerate with the frontier orbitals of the macrocycle. This
may have the deleterious effect of disrupting a favorable band
alignment, or it may have a beneficial effect of actually
enhancing molecule-to-substrate charge transfer, once again
illustrating the need for a broad-based study of these systems.
Fig. 10: Text
ENERGY LEVEL ALIGNMENT: When bound to semiconductor
substrates, the alignment of the frontier orbitals of the porphyrin
molecule with the conduction band and valance band edges of
the semiconductor is a key factor in determining molecule-surface
charge transfer. Fig. 7 schematically illustrates this situation for
dye-sensitized solar cells (DSSCs). In such n application, improved
energy alignment of the LUMO with the oxide conduction band edge,
and the HOMO with the redox potential of the electrolyte, could
improve efficiency and provide new options in the design of
photovoltaic cells. While the HOMO – LUMO gap may depend
largely on the nature of the porphyrin macrocycle, factors that
depend on binding geometry, such as intermolecular interactions,
can significantly alter the alignment. In addition, the introduction of
dipoles between the macrocycle and the anchoring group can shift
the levels of the molecule with respect to those of the substrate.
Here it will be critical to correlate spectroscopic studies of the
Fig. 11: Text
molecule bound to an oxide surface in ultrahigh vacuum to, for
example, electrochemical measurement of oxidation and reduction
potentials.
OPTICAL ABSORPTION: Zn(II)-porphyrin models will be central to the studies described in this proposal.
The structure of the molecule, along with electron distribution diagrams of two of the frontier HOMO and
LUMO orbitals primarily involved in optical absorption, are shown in Fig. 8. The typical UV-Vis absorption
spectrum of porphyrins is composed of two main bands: The Soret (or B band) roughly at 400 nm and a
weaker Q band around 550 nm. Both Q and B bands arise from totransitions that can be understood
within the Goutermann model[31] as transitions between
the frontier orbitals shown in Fig. 8. The ground state
electronic structure of the metalloporphyrin is related to, but
differs from, that of the free base porphyrin. The extent of
eg
modification depends to first order on the relative energy
position of the d-orbitals of the metal and the free base
porphyrin orbitals. The calculated HOMO-LUMO gap of a
series of metals (Fe, Co, Ni, Cu and Zn) can vary from 1.7
Zn-Porphyrin
eV for Ni to 2.5 eV for Co and Zn. [32] The fact that the
N
N
Zn
N
N
a)
a1u
8
a2u
b)
Fig. 12: (a) Molecular structure of ZnP; (b)
eg LUMOs and a2u and a1u HOMOs of the
molecule, and important optical transitions.
optical transitions involving these orbitals are in the visible, and can be modified with relative ease by
metal insertion, is part of what makes these molecules so attractive as mediators of photocatalytic and
photovoltaic processes. Recent results showing the promise of porphyrin-based arrays that may allow
effective light harvesting on planar surfaces makes such studies all the more compelling.
BINDING GROUPS AND BINDING GEOMETRY: While the electronic structure and ultimately the
performance of ZnTPP-derived molecules can be anticipated from the properties and behavior of the
unbound molecule, upon adsorption on a semiconductor surface, a variety of changes can occur. For
example, binding to the surface through a phenol ring can rearrange the energy ordering of molecular
orbitals. Moreover, the site on the surface at which the molecule binds, or the anchoring group, can
influence binding stability and residence lifetime. As surface coverage increases, molecule-molecule
interactions can result in interaction between the -complexes of neighboring molecules (J or H
aggregates) and change the tilt angle between the molecular axis and the surface normal. Such effects
usually dramatically alter the optical absorption and charge transfer properties. As such it will be
imperative to obtain a clear picture of how both the model and the novel TPP-derived molecules
examined in this project bind to the surface and how that binding changes as a function of adsorbate
coverage. Previous studies optical absorption studies indicate that by functionalization of the phenyl
groups with bulky alkyl substituents (Me, tBu), aggregation of the dye at the surface can be avoided,
leading to more control over the response in optical absorption.
SUMMARY: This brief review underscores the relevance of the porphyrin/semiconductor hybrid systems
to a variety of fields of chemistry, including renewable energy-related applications, and the current need
for a comprehensive, atomistic study of the adsorption properties of ZnTPP-derived dye molecules at
metal oxide surfaces.
IV. Research Plan
IV. A: Overview
The overall approach of the proposed research is to apply a wide range of analytical techniques to
determine the fundamental properties that govern the adsorption behavior of dye molecules at oxide
surfaces.[36-45] This study will focus on novel transition metalTPP dyes specifically synthesized for this
project bound to single crystal planar surfaces (list which ones specifically TiO2(110 …).
By judicious functionalization, we will attach anchoring groups to one of the peripheral phenyl rings,
through which the molecule is expected to attach to the surface. By functionalizing the other peripheral
phenyl groups, we will attempt to modify intermolecular interactions at the surface. Moreover, we will
explore ways to tailor the relative energy of the porphyrin HOMO and LUMO levels with respect to the
band edges of the substrate. This will be done by selectively functionalizing a linker chain between the
anchor group and the porphyrin ring so as to introduce a dipole. We will also investigate the effect of
attaching electron withdrawing and donating groups as -substituents to the pyrrolic portion of the
porphyrin macrocycle [add to the refs: For a recent reference about β-pyrrolic substitutions see” An
alternative synthesis of β-pyrrolic acetylene-substituted porphyrins” Adam W.I. Stephenson, Pawel
Wagner, Ashton C. Partridge , Kenneth W. Jolley, Vyacheslav V. Filichev and David L. Officer: Tet. Lett.,
2008, 49, 5632-5635 and refs herein] and insertion of different transition metal ions other than Zn2+in the
porphyrin ring (which ones?must be specific. Or say: see section ###). To maintain a high degree of
binding control in this part of the research, we will initially perform these measurements using atomically
well-ordered single crystal TiO2(110) and ZnO (###?) surfaces.
In addition to characterizing these molecules on the single crystal surfaces, we will expand our
studies to include adsorption on anatase TiO2 , ZrO2 and ZnO nanoparticles and ZnO nanorods films as
the ones illustrated in Fig.1 cast on glass, ITO or sapphire, depending on the experiments. [refs]. We
have preliminary results confirming that the experimental approach described below will be successful
with these systems as well. [46] The colloidal nanoparticle films will be prepared by the co-PI, using wellknown solution methods (see refs from EG synthetic paragraph: Langmuir 2008, 5366; JACS 2007
4655). The co-PI’s group has worked on the sensitization of such films for the past decade, and her
laboratory is equipped for their preparation and characterization.
In addition to being more
technologically relevant (i.e. DSSCs applications), the mesoporous films ensure a much higher (about
9
x1000) dye uptake, and will allow experiments that may not be conducted on single crystal surfaces.
Examples include UV-vis absorption spectra, FT-IR-ATR studies for characterization of the COOH binding
mode, and electrochemical measurements on the models/MOn/ITO films, to complement electrochemical
measurements in solution. Insulating ZrO2 (Eg = 5 eV) nanoparticle films will be used to probe porphyrin
aggregation effects on a substrate similar to mesoporous ZnO and TiO 2 films, but where emission is not
quenched (see for instance the same refs above: Langmuir 2008, 5366; JACS 2007 4655))
IV. B: Experimental Approach
IV. B1: Design and Synthesis
The co-PI’s synthetic group at Rutgers/Newark has
developed a variety of dye-linker-anchor group model
compounds, including, more recently, rigid-rod and planar
(“spider”) porphyrins-derivatives designed to study the
influence of binding geometry (distance and orientation) on
charge transfer and solar cells efficiency (Fig. 9). [FIG 9]
For this project, three types of models will be designed and
Fig. 13: Text
synthesized, two of which are totally new. The synthesis of
some of the model porphyrins is completed or is in progress.
Furthermore, the feasibility of the proposed approaches for the new molecules has been demonstrated,
as they employ well known published synthetic methods or strategies already successfully used by the
PI’s group.
a) Novel Porphyrin–Ipa rods with built-in dipole The synthetic strategy for the preparation of the
porphyrins in Fig. 11 [FIG 10] will be similar to what we have developed for the preparation of rigid-rod
porphyrins [48, 35, 49] except that the central phenyl ring in the oligophenylenethynylene (OPE) bridge
will be substituted with a donor and an acceptor.
Fig. 15: Text SHOULD BE CALLED FIG 10
The relative position of the donor (D)
and acceptor (A) substituents will be
switched by varying the order of the
cross-coupling procedures involving
10
Fig. 14: Minimized geometry (Spartan ’08, MM2) of the tripodal
anchor group (see ref 11) with meta (left) or para (right) position of
the COOH groups. THIS IS FIG 11
the TMS monoprotected OPE moiety carrying the D and A groups and the bromo or iodo-terminated Ipa
or tripodal anchor so that the dipole orientation will respect to the semiconductor surface can be reversed.
The methyl groups on the meso phenyl rings (mesityl-derivative) will avoid aggregation of the models (for
the synthesis see for instance ref 1). The Ipa rods are expected to bind to the oxide surface through both
the COOH groups but at an angle of about ~45 deg. [reference] Variations of the anchoring groups is
possible and it will be conducted at a later stage. The tripodal units will ensure an anchoring geometry to
the surface that is close to the normal. We will use the more recently developed expanded tripodal units
(~180 to 250 Å2) that will be useful to determine how the position of the anchoring groups can result in a
better “match” with the crystal surface (Fig. 10). [there was a ref here]
b) Models to study binding and orientation effects: Spiders and Rods. We propose to study new
aspects of a selected number of “spider” and “rod” model compounds (Fig. 13), which are already
available in our group. The objective is to obtain atomic-level information about their electronic structure
and binding to oxide surfaces. The structural difference between the porphyrins has been shown by us
and others (see for instance recent work by Yeh, Lin, Diau and Lindsey) to be of great influence for the
efficiency of TiO2 nanotubes, ZnO nanorods and TiO2 nanoparticle DSSC devices and on
injection/recombination processes on such semiconductors.(refs1-6) However, the interaction with the
semiconductor surface and the true orientation of the models is not established. The proposed study is
important to complement and rationalize the studies that have already been conducted or that are in
progress (DSSCs studies and
ultrafast
injection/recombination
dynamics) on such materials. For
instance: we observed in two
independent studies of DSSCs (refs
2 and 3a) that the “spider” with a
biphenyl unit is by far the most
efficient among the dyes in Fig. 13
and
exhibited
improved
photovoltages.
A
better
understanding of the interaction with
the semiconductor surfaces will be of
crucial importance to rationalize such
results and design improved dyes.
c) Novel Capped porphyrins
Charge
transfer
studies
by
Fig. 17: Text THIS IS FIG 12 something weird in my file about the
Sundström
[REFS]
and
calculations
pics with purple squares showing orientation
by Persson [REFS] suggests that
rotational
freedom
about
the
anchoring legs in the tetrachelate porphyrins (the spiders in Fig. 13) greatly complicate the ability to
correlate interfacial charge-transfer kinetics with the expected binding geometry. These results strongly
suggest that more sophisticated considerations and model design have to be applied for controlling the
dye binding and understanding how it influences the electron dynamics. To address this question, in the
past year we are developing two types of novel capped
porphyrins (Fig. 12). The purpose of the cap is to restrict
the conformational freedom of the linker group, to prevent
H
N
(or limit) direct contacts of the porphyrin ring with the
Propionic acid
semiconductor
and
decrease
porphyrin-porphyrin
stacking. The synthetic pathways for these molecules
have not been included in Fig. 12 because they are rather
lengthy, but we have recently successfully synthesized
Fig. 16: Text THIS IS FIG 13 CAN MAKE
and started to study one of them in solution and also on
SMALLER w/LARGER LETTERING
TiO2(110) surfaces (Bob, which techniques ? ) with the
PI’s group; [MAKE THIS SOUND MORE LIKE IT’S YET
O
O
O
O
O
O
O
O
O
O
N
O
O
Zn
N
O
O
O
N
O
O
O
N
O
O
O
O
O
O
11
O
O
O
O
TO COME] the synthesis of the second one is almost completed.
IV. B2: Analysis and characterization methods. To ascertain the key aspects of the interaction of the
proposed metalloporphyrin-derived dye molecules with oxide surfaces, we will undertake a series of
controlled studies on well characterized single crystal surfaces. For these measurements, atomically flat
and well-ordered single crystals surfaces of, primarily, TiO 2 and ZnO will be prepared under ultrahigh
vacuum (UHV) conditions. To ensure that adsorption of the dye molecules will be using a method as
close as possible to what is used in practical applications, the surfaces will be sensitized by immersion
into a solution of the dye. The sensitized sample will then be reintroduced into the UHV chamber for
analysis. Where appropriate, the oxide surface will first be passivated by exposing the surface to pivalic
acid (t-BuCOOH) vapor in the UHV chamber prior to removal for sensitization. We have used this
procedure with great success in exploratory studies., and an example of this method is given in the next
section.
{BRING STUFF FROM END TO HERE?}To complement these UHV studies, and to further relate our
UHV results to practical applications, we will also perform solution-based optical absorption and
electrochemical measurements of dyes prepared in the same synthesis batch. These may be performed
using single crystal surface when appropriate and when signal levels (i.e. optical density) are satisfactory.
Otherwise, we will use nanoparticle films to increase the surface area on which the dye is adsorbed so as
to boost the signal to be measured. The UHV experimental work, particularly the determination of
electronic structure and binding geometry, will be supplemented by theoretical studies using density
functional theory.
A brief description of our experimental and theoretical approach is outlined below. Note that all of these
capabilities are already in place in the PI’s laboratory and are available for immediate use for the
proposed research.
XPS-UPS-IPS: Central to our studies is the use of direct and inverse photoemission to determine the
molecule’s electronic structure, the orbital energies, and their alignment with the substrate bands when
adsorbed on an oxide surface. The basic principles of UPS and IPA are illustrated in Fig. 14. In UPS, a
photon is absorbed by the sample and an electron is ejected.
The kinetic energy distribution of ejected electrons contains
features about the occupied electronic states of the system,
particularly the HOMO levels of interest in these studies. In
IPS, a monoenergetic (~ 20 eV) beam of electrons is directed
towards the sample and these electrons couple to high lying
unoccupied states of the system. A small fraction undergoes
optical transitions to lower-lying unoccupied states, in
Fig. 18: (a) UPS and (b) IPS schematic
particular the LUMOs, emitting a photon in the process. By
processes WILL WORK AS SMALLER
measuring the energy distribution of emitted photons, one
FIG if have LARGER LETTERING.
can determine the energy of the unoccupied states in a way
that is analogous to how the occupied levels are determined
in UPS. An important novel aspect of our instrumentation is that UPS, IPS, and x-ray photoemission
[XPS] capabilities are housed in a single UHV experimental chamber.[52, 51] In that way, not only can
we explore the HOMO and LUMO energy alignment on an equal footing, but measurements of the
occupied and unoccupied electronic states of the surface resulting from a particular preparation can be
performed sequentially on the sample, thus removing any artifacts that may arise from variations in
sample preparation conditions.
ELECTRONIC STRUCTURE CALCULATIONS: The interpretation of the experimental data will be
facilitated by the use of ab-initio techniques to calculate the electronic structure of the gas phase and
adsorbed dyes, using the GAMESS(US) and GAUSSIAN03 packages. Important parameters such as
ionization potentials, electron affinities or ground state density of states will be computed.
12
SYNCHROTRON-BASED TECHNIQUES: Synchrotron radiation techniques allowing a high control over
the photon source polarization and energy, offer a large array of
spectroscopic studies. Using NEXAFS spectroscopy, where a core
level electron is promoted to an unoccupied orbital upon photon
absorption (as represented Fig. 15(a)), the unoccupied states can
be probed selectively by choosing the appropriate core level.
NEXAFS spectroscopy is thus a powerful chemical probe.
Moreover, due to the dipolar selection rules applying to the NEXAFS
transitions, using a polarized photon source allows the
determination of the unoccupied states orientation with respect to
the sample surface, and thus the adsorption geometry of adsorbates
Fig. 19: Schematic NEXAFS
(Fig. 15(b)). Also using the polarization properties of synchrotron
processes: (a) core level excitation
light, angle resolved UPS might be a useful tool for the
of an electron to an unoccupied
determination of adsorption modes, and could be directly compared
level upon photon absorption; (b)
to Rutgers-housed UPS-IPS data. [WHERE BEAM TIME??]
Geometry selection rule applying
ROOM TEMPERATURE AND LOW TEMPERATURE STM: STM
for a * orbital: the NEXAFS
experiments will be carried out either in a room temperature or in a
intensity is maximized when the
electric field of the incident source
low temperature (4K to 90K) STM, under UHV conditions. These
is parallel to the *, and zero when
studies will focus on determining the local geometry and ordering of
perpendicular.
individual molecules on single crystal substrates (TiO 2(110) and
ZnO(11-20)). Where possible we will also probe the local density of states using tunneling spectroscopy.
ELECTROCHEMICAL METHODS and UV-VIS ABSORPTION: Electrochemical techniques will be used
in parallel with electron spectroscopies to determine the affinity and ionization levels of dyes bound to
oxide surfaces in solution. In addition, UV-vis absorption spectroscopy will provide a direct evaluation of
the light absorption properties of the sensitizers considered in this study, and will allow a characterization
of excitonic effects, when compared to UPS-IPS results.
The compounds’ photophysical and
photoelectrochemical properties will be studied at Rutgers Newark though a variety of methods that we
have routinely used both in solution and bound: FT-IR-ATR to determine the anchoring mode of the
COOH groups to the surface, UV-Vis absorption spectra on the surface of the materials of interest will
help determine surface coverage and aggregation effects (J or H aggregates). Steady state emission will
be studied on insulating materials such as ZrO2 (Eg = 5 eV) or sapphire, as characteristic shifts in the
emission are useful to determine aggregation effects (ref 1,2). Cyclic voltammetry in solution and of the
compounds anchored in films cast on ITO will help determining the redox potential and the excited state
position relative to the Ebg[???] of the studied materials. Measurements requiring high surface coverage
(i.e. UV-Vis) will be conducted on nanoparticle films as it is not possible to obtain them from planar crystal
surfaces.
These results will be correlated with the UHV studies alluded to above, particularly for
nanoparticle films.
IV. C: Studies of Novel Porphyrin-based dyes on oxide surfaces
13
In the following sections, we describe a series of
experiments designed to determine the key properties and
interactions that determine the behavior of several
specifically designed TMP-based molecules (described in
section…###) bound to semiconducting oxide substrates..
In some cases, preliminary results illustrating proof-ofconcept and the ability to perform the proposed
measurements are presented. To fully explore all of the
options and variations described in the following sections
would require time and resources beyond the scope of this
program; however we include them in the discussion to set
the appropriate context. Our priority is to synthesize and
measure the properties of two or three molecules, for each
of the sections below, to establish the range with which we
can control the key binding properties in these systems.
Fig. 20: (a) Schematic diagram showing
dipole layer formed by functionalizing the
Then, for the most promising approaches, we will explore a
linker chain; (b) expected effect of such a
wider range of molecular complexes.
dipole layer on the energy of molecular
IV C1: Electronic Structure and Band Alignment
levels; (c) one possible scheme for
Our initial studies will focus on the electronic structure of a
establishing a permanent dipole in the
set of porphyrin-Ipa-rod complexes described in Section
linker chain. The molecule should have
the methyl groups if we say that will be
IV.B.1(a). As illustrated in Fig. 16(a) and Fig. 10,
TMP also the atoms font is supertiny
functionalizing the central phenyl ring of the linker with
appropriate electron withdrawing or electron donating
substituents (as in Fig 10) will introduce a dipole moment between the anchor group and the porphyrin
ring. For an overlayer of molecules at the surface, an effective dipole layer is formed, generating a
change in electrostatic potential that would shift the molecular orbitals on the porphyrin ring with respect
to the substrate levels. In the schematic diagram of Fig. 16(a), the electric dipole is oriented towards the
surface, thus increasing the electrostatic potential energy of the levels in the porphyrin ring, resulting in an
upward shift of the levels while maintaining the HOMO-LUMO gap unchanged, as illustrated in Fig. 16(b).
The first molecule that we will with this approach is shown in Fig. 16(c). We will begin by sensitizing
single crystal TiO2(110) surfaces using the solution immersion method. In this case a rigid linker unit,
consisting of ONE?? phenyl rings, will be appended to a phenyl group of the TPP molecule (ZnTMP-Ipa
in Fig 17 ??). The terminal phenyl ring is functionalized with a carboxylic group that will serve as the
anchor to the oxide surface. To establish the baseline condition, will first determine the behavior of a
porphyrin where the central phenyl ring of the linker group is not functionalized. This will give the natural
alignment of the porphyrin levels with respect to the oxide band edges, and enable us to determine the
adsorption geometry in the absence of any built-in dipoles.
We will then proceed to study porphyrin models where the phenyl ring of the linker will be
functionalized with an electron withdrawing NO 2 group on one side and an electron donating N(CH3)2
group on the other as shown in Fig. 16(c). From preliminary ab-initio calculation, we estimate that this
modification produces a dipole moment of about 6.5 D, oriented along a line connecting the ligands,.
Even though the orientation of this dipole is not parallel to the linker chain, a simple capacitor model using
realistic molecular dimensions suggests that an ordered array of such molecules would result in a ~ 1 eV
shift of the porphyrin homo-lumo? Energy? (Need a word here) levels. We anticipate that the presence of
the dipole on the linker chain will shift the HOMO and LUMO energy levels, as observed in UPS and IPS,
with respect to the band edges of the TiO 2, but not with respect to each other. To verify the anticipated
effect, additional studies will be made on molecules where the position of the NO2 and N(CH3)2 groups on
the central phenyl ring in the linker are reversed. This would reverse the sign of the dipole on the linker
unit, and should reverse the shift of the porphyrin levels. Such UHV studies will be correlated with UV-vis
and optical emission experiments (the latter requires a wide gap and will be performed on ZrO 2) to verify
that the effect maintains in when the molecule is bound to an oxide in solution. As part of a long term
plan we will explore other changes including varying the electronegativity of the ligands on the linker
14
which is expect to impact the magnitude of the shift of the porphyrin levels. Another important point is that
the dipole of the linker phenyl group is not oriented along the linker’s axis. Therefore, modifying the
anchor groups could change the binding geometry and affect the dipole layer of an ordered array of such
molecules, even if the internal structure of the linker and the porphyrin are not changed. Clearly there is a
large parameter space ??? rewrite??? to explore with this class of molecules.
Hints as to the effect of functionalization, and changes in binding structure, on the electronic
properties
of
adsorbed
porphyrin
complexes is given by an exploratory
study we have conducted comparing
“spider and rod” ZnTPP derivatives of the
type described in Section IV.B1(a). This
example, summarized in Fig. 17,
illustrates the power of combining UPS
and IPS data with theoretical models to
obtain a detailed understanding of the
origin
of
these
effects[WHAT
EFFECTS??].
The
three
ZnTPP
derivatives used in this study are shown
in Fig. 17(a), (b), and (c). The occupied
and unoccupied states obtained using
respectively using UPS and IPS,
measured for the a clean ZnO(11-20)
surface before and after sensitization with
each of the three ZnTPP derivatives, is
shown Fig. 17(d). A comparison of the
calculated (Gamess, B3LYP, 6-31*)
density of states for each molecule is
presented in the Fig. 17(e). One can see
Fig. 21: Text
that the HOMOs and LUMO levels
(indicated by ZnP), are relatively
insensitive to the functionalization of the meso-phenyl groups of the porphyrin macrocycle. In contrast, the
phenyl levels show a pronounced shift. In fact, for M-ZnTCPP they are nearly degenerate with the
LUMOs. The extent to which this is the result of functionalization, as opposed to enhanced macrocyclesubstrate interaction resulting from the flat adsorption geometry is exactly the sort of question we plan to
explore in the proposed research.
From results such as these, the energy level alignment of the ZnTPP derivative can be directly
extracted from the UPS and IPS spectra (as shown Fig. 17(f)). For the three dyes shown in Fig. 17, the
HOMOs centroid is found 1.6 eV above the substrate VB while the LUMOs centroid is found 1.9 eV
deeply above the CB. In this particular case, functionalization of the phenyl group had little effect on the
HOMOs and LUMOs position. Other types of functionalization, as described below, are expected to have
a significant effect on these orbital energies. Moreover, these data were obtained for a monolayer of
bound dye molecules. How do the level energies change as a function of dye coverage? Can moleculemolecule interactions be controlled and even designed, a priori, into the molecule? These are questions
the proposed research will address.
In addition to using built-in dipoles to modify electronic structure of the dye/surface comples, we will
also explore -pyrrolic functionalization of the porphyrin ring, as well as variations of the transition metal
as ways of shifting key orbitals and changing the HOMO-LUMO gap. Functionalizing the macrocycle on
the -pyrrolic positions with electron withdrawing groups is expected to raise the electrostatic potential of
the ring and thus lower the energy of the HOMOs and LUMOs in a relatively rigid manner. In contrast,
incorporating different transition metal ions within the macrocycle is expected to have profound effects on
the HOMO – LUMO gap, as well as change the nature of the levels themselves (for example, from being
the delocalized -manifold to an orbital with substantial metal d-character) which can have a large effect
15
on absorption efficiency and eventual charge transfer to the substrate. I think this last paragraph lacks
specificity and looks like an “afterthought” (and why not doing also this kind of experiment). This can bite
us back. We should indicate a SPECIFIC example (one metal in one case, one EWG in the other, for
instance) Or , we should indicate that this part is planned as a potential development at a later time…
IV. C1: Adsorption geometry and control of intermolecular coupling coupling has a specific
meaning… not sure here is used properly. Should it be interactions? Or electronic coupling?
While UPS and IPS provide detailed information about the electronic properties of the dye/oxide surface,
it is essential to characterize the binding geometry and binding mode of the dye molecules to the oxide
surface. Do the molecules favor a particular binding site and geometric configuration, or is there a range
of competing sites that the molecules occupy? Does the geometry crystal lattice type ??? of the metal
oxide substrate (e.g. TiO2 (###0 vs TiO2 (###) or …??? impose order on the bound dye? Could then
different substrate types be used to impose a desirable molecular alignment? Does the orientation of the
porphyrin ring change with increasing dye coverage, as the porphyrin rigid-rods stack more closely
forming a closely packed monolayer? Answering such questions can have a significant impact on the
photophysical and photochemical properties of the dye/oxide interface, as it controls dye-dye interactions
on the surface of the semiconductor that regulate phenomena such as energy transfer, emission
quenching, band shifts etc… .
To address these issues we will study the binding geometry of the ZnTPP derivatives, described in
Sect. IV.B, to oxide surfaces. Much of what is known about the binding of functionalized porphyrins at
oxide surfaces is deduce from measurements of molecular ensembles. Few studies have directly
investigated the binding of individual molecules at the surface. Using STM we will explore how the
location of the COOH anchoring group on different meso-phenyl rings of the molecule (for instance meta
vs. para) affects molecular orientation and binding site. We will also study how steric effects controlled by
functionalizations of the meso-phenyl rings with bulky alkyl groups, and through capping the porphyrin
macrocycle, can control intermolecular distances and orientation, and thus influence molecule-molecule
interactions. At high coverages, where STM imaging may be ineffective, we will use NEXAFS to address
these questions. Moreover, as NEXAFS can be performed at different absorption edges (e.g., N or C)
one can probe the local geometry and interactions at specific locations within the molecule.
The ZnTPP derivatives of Fig. 17 are expected to bind to the surface through an anchoring group
(typically a COOH group) attached on one or more of the meso-phenyl rings. Moreover, the particular
position of the anchoring group on the meso-phenyls is expected to lead to different adsorption
geometries when the molecules are bound to the surface of the oxide. For example, the ZnTPP-Ipa and
ZnTMP-Ipa molecules of Fig. 17(a) and (b) possesses one isophthaic acid- (Ipa-) like termination in the
meso position of the macrocycle, and thus should only bind to the surface through this functionalized
phenyl. As a consequence, these molecules are
expected to bind nearly upright, with a large
angle between the plane of the porphyrin ring
and the plane of the substrate surface (Persson
has calculated a ~45 deg angle fore the Ipa unit,
reference).
In contrast, the m-ZnTCPP
tetrachelate porphyrin of Fig. 17(c) has four
identical meso-phenyls functionalized with a
COOH anchoring group in the meta position
which is expected to result in a flat adsorption
geometry on the oxide surface.
Preliminary STM measurements obtained for
solution-deposited m-ZnTCPP on pivalic acidpassivated TiO2(110) are shown in Fig. 18
(pivalic acid is t-BuCOOH). In this case, a highly
dilute (~ ### M) dye solution and short binding
times (~ # Seconds?) were used to avoid
16
Fig. 22: Text
aggregation of m-ZnTCPP molecules on the surface. In the large-scale image of Fig. 18(a), a series of
bright four-fold features are evidenced along with a regular periodic array of dimmer features. From STM
images of pivalic acid passivated surfaces that have not been sensitized, we associate the latter features
with pivalate species bound to the TiO2(110) surface (in rectangles). During solution sensitization of the
pivalic acid passivated TiO2(110) surface, a few m-ZnTCPP dye molecules displaced the pivalic acid,
resulting in the brighter 4-fold features (in circles). A close-up image, as shown in Fig. 18(c), strongly
suggests that the four bright features are associated with the phenyl groups of adsorbed m-ZnTCCP,
indicating that it does indeed assume the anticipated flat adsorption geometry. Fig. 18(d) shows a
tentative model for the adsorption of m-ZnTCPP with respect to the underlying surface. Interestingly, the
in-plane orientation of the m-ZnTCPP dye is aligned along the high symmetry directions of the surface.
Along with imaging, local spectroscopy would be further studied and compared with the non-local
techniques that are UPS and IPS.
We will perform similar STM studies of the novel dyes proposed in this project. While the ZnTPP-Ipa
and ZnTMP-Ipa rods are expected to bind essentially upright on the surface, the nature of the binding will
likely differ at low and high coverages. In particular, owing to the methyl functionalization of the phenyl
groups of ZnTMP-Ipa, the binding geometry of this molecule will likely be more (is more the missing
word?) influenced by neighboring molecules at lower surface concentrations than ZnTPP-Ipa. STM
investigations of this point will be complemented by NEXAFS studies. The dipole selection rules of
NEXAFS predict a well-defined behavior of the absorption as a function of electric field orientation. From
this (…??), changes in the orientation of the molecular axis with respect to the surface can be
determined. In the novel capped porphyrins that we will investigate (Fig. 13), the motion of the phenyl
rings is significantly constrained. As a result, it may not be able to conform to the substrate geometry as
easily as m-ZnTCPP and thus may lose the strong registry with the surface displayed by m-ZnTCPP in
Fig. 18. Such geometrical changes may have a considerable impact on molecule-to-substrate charge
transfer efficiencies and influence, for instance DSSCs efficiencies.
We will study whether the tripodal anchor groups will allow to achieve near perpendicular binding to
the surface, and whether the large footprint size can prevent dye-dye aggregation. It will be also
interesting to determine whether variations of the anchoring COOH groups position in the phenyl rings (
meta or para position) results in difference in surface adsorption, due to difference in crystal lattice
matching. Computational work by P. Perrson (private communication) seem to suggest that this is the
case. Finally, in the the case of the models with built-in dipole, a near-to perpendicular surface orientation
would be advantageous to……
RELEVANT APPLICATIONS, FUTURE WORK. It has been demonstrated that all aspects of the dye/MOn
interface (i.e. distance, orientation with respect to the surface, nature of the chemical bond of the
anchoring group with the metal oxide) influence important parameters such as electron transfer rates and
electronic coupling, as well as solar cells performance (such as efficiency or photovoltage), as
demonstrated by extensive work on porphyrin-bridge-anchor models (refs on phorphy by others),
including recent work by the co-PI [refs on porphys by us]. In addition, although a linker can weaken the
electronic coupling between the dye and the semiconductor, injection through linkers usually remains fast
and efficient, due to the fact that the injection processes are ultrafast (fs range). Therefore, the proposed
work, which is investigating key aspects of this interface, is of relevance to more practical applications
such as DSSCs development.
As part of the longer-term plans, the PIs will investigate ultrafast charge transfer dynamics and solar cells
properties of the porphyrins proposed in this study, through collaborators. Well established collaborations
with research groups that are highly specialized in such measurements and studies are already in place.
For instance, ultrafast injection and recombination dynamics studies with the “rods” and “spider”
porphyrins shown in Figure # are already close to completion by the Sundström group (Lund university),
[ref] and the photoelectrochemical properties of the “spiders” have been reported in collaboration with the
Hagfeldt group (KTH, Stockholm) [ref]. The co-Pi has visited the laboratory in Lund and spent her
sabbatical in the Hagefeldt’s laboratory. While the main focus of the proposed work remains an area that
is relatively less developed, i.e. the characterization of the porphyrin/MOn crystal surfaces interfaces by
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the methods outlined in Fig 1, the proposed work will ultimately lead to experiments that are more directly
connected to the applicative aspects of this work.
V. Educational and Outreach Activities we need to say something about Newark (after all is an
urban environment..) and interactions between campuses (see my outreach for the other NSF) can
some sentences be added here and there?
Our education and outreach activities will focus on photovoltaic applications of organic dyes similar to
those studied in the research portion of this program, but readily found in everyday items such as berry
juices. Due to dwindling fossil fuel reserves it is generally recognized that a transition to alternative
energy sources will become absolutely necessary during this century. In addition to advancing the
technology toward this transition, the research activities proposed here provide a unique opportunity to
educate young people – the nation’s
future decision makers – about the
importance, opportunities, and challenges
of alternative and renewable energy
technologies. Toward this important goal
we will implement a new program to
integrate advanced solar-cell science into
high school and undergraduate science
curricula. DSSC research-based activities
are ideally suited for outreach and
Fig. 23: Undergraduate students involved in making DSSC’s.
education at the high-school and
Students apply tape to define the active layer area (left) and
undergraduate levels, since a working
spray on a titania-nanoparticle suspension (right)
photovoltaic device can be constructed
relatively simply by the students from inexpensive and nontoxic materials [75, 76]. Based on this idea, we
will develop three hands-on experimental laboratory modules and a technical demonstration of
nanotechnology in photovoltaic devices. These lab modules will be integrated into undergraduate
laboratory courses of several physical sciences departments of Rutgers New Brunswick and Newark,
aspects will be used as demonstrations in undergraduate lecture courses, while others will be tailored
toward high-school science courses and K-12 outreach activities.
Undergraduate Education: For the lab modules, FTO coated glass slides will be provided as electrodes.
The students will coat one of the electrode surfaces with TiO 2 sol-gel colloidal solution, and sensitize the
TiO2 nanoparticles with natural flavonoid dyes (anthocyamine) by dipping in blackberry juice. Upon adding
a droplet of electrolyte and clamping the two electrode slides together with paper clips, a working solar
cell is obtained.
We are already in the process of bringing this kind of solar cell into the classroom this semester in the
Materials Science and Engineering department at RU-New Brunswick. The students are using a simple
spray coating process with commercially available powders to build working solar cells. Fig. 19 shows
students in the process of making DSSC’s. Deliberate variations in the coating thickness, sample
geometry, and electrolyte used are being tested by various student teams in the class. Future examples
of valid class participation projects include varying the dye concentration to look at the importance of dye
coverage on the titania surface, comparing different dyes, and changing the heat treatment/sintering
conditions of the titania to understand the importance of microstructure.
K-12 Education: The high school lab module will be developed and integrated into high-school curricula
in collaboration with two high school science teachers who are also our research collaborators, Dr. A.
Rajagopal at South Brunswick High School, and Dr. A. Danese of Morristown High School.
In addition to hands-on lab experiences for the
students, the lab modules will be used as a
basis for classroom demonstrations that will
be used to complement existing outreach
activities within Rutgers, illustrated in Fig.
20.The outreach activities have been
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Fig. 24: The solar cell demonstration will be integrated
into a wide range of outreach and student support
programs at Rutgers (see text for details).
developed to address specific demographic deficiencies in science & engineering [77-82]. The PV
demonstration will be used in the following outreach activities aimed at increasing the participation of
women and minorities in technical disciplines:
 TEEM (The Engineering Experience for Minorities), a residential program for African-American, Latino
and Native American high school juniors. During the residential program the students have the
opportunity to attend lectures and tour departments. It is proposed that the PV demonstration be used
during the department tours to illustrate the importance of nanotechnology and renewable energy.
 TARGET (The Academy at Rutgers for Girls in Engineering and Technology). This is a series of
lectures and experimental demonstrations intended to introduce girls to the exciting World of
Engineering and Technology. The PV demonstration will be used in support of lectures on technology
and the environment.
 The New Jersey Governor’s School of Engineering and Technology for high achieving students
across New Jersey. This is an opportunity for outstanding high school students to work directly with
professors and their research students over a 10 week summer period. It is proposed that two or
more of these high school students work with our team on the fabrication and characterization of
titania based PV devices.
 Nanodays. Each year Rutgers runs open-house events where high school students and teachers can
participate in a series of lectures, demonstrations and departmental tours highlighting
nanotechnology. Specifically, Nanodays emphasize the current and future importance of nanoscale
devices and phenomena in everyday situations. The titania PV demonstration will be an ideal
example of how nanotechnology can help reduce the problems of fossil fuel dependency.
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