Assembly
of Composite Nanomaterials and Characterization of Photoinduced Interfacial
Electron Transfer Reactions
In our approach to materials assembly, nanometer-scale
components are organized into composite materials through coordinate covalent
bonding interactions. Current research projects focus on the
following aspects of materials assembly and interfacial electron-transfer
reactivity: |
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Adsorption of inorganic
nanoparticles to metal oxide films. Metallic and semiconducting nanoparticles are
tethered to metal oxide surfaces through bifunctional molecular linkers. The
specificity of surface-attachment interactions enables precise control over
the materials assembly process. The color and optical density of materials
can be controlled by varying the size and fractional surface coverage of
surface-adsorbed nanoparticles. Similarly, the distance and electronic
coupling between nanoparticles are tunable by varying the properties of the
molecular linker. We typically use electron microscopy, UV/visible
(electronic) spectroscopy, and infrared (vibrational) spectroscopy to
characterize the composite materials. Our ongoing research
is focused on optimizing the materials assembly process and developing novel
composite materials with unique physical properties and chemical
reactivity. Recent materials
assembly publication: Mann, J.R.;
Watson, D.F. "Adsorption of CdSe Nanoparticles to Thiolated TiO2 Surfaces: Influence of Intralayer Disulfide
Formation on CdSe Surface Coverage." Langmuir 2007, 23, 10924-10928. |
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Photocatalytic
patterning of monolayers for the site-selective deposition of molecules and nanoparticles
onto surfaces. We have developed a new method for the patterned deposition of
metallic and semiconducting nanoparticles, as well as organic dyes, onto
substrate surfaces. Our approach involves monolayer photolithography,
followed by surfactant-mediated self-assembly. The TiO2-catalyzed photocatalytic oxidation of
surfactant monolayers turns off the attachment of nanoparticles to surfaces.
Illumination through a photomask enables optical pattern formation. To date,
we have demonstrated micron-scale pattern formation using this solution-phase
patterning technique. Our ongoing research
is focused on further elucidating the photochemical mechanism and expanding
the scope of this photopatterning process. Recent photopatterning
publications: Soja, G.R.; Watson,
D.F. “TiO2-Catalyzed Photodegradation of Porphyrins: Mechanistic Studies
and Application in Monolayer Photolithography. Langmuir 2009, 25, 5398-5403. Click here to access
the article from the ACS Publications website. Dibbell, R.S.; Soja,
G.R.; Hoth, R.M.; Watson, D.F. “Photocatalytic Patterning of Monolayers
for the Site-Selective Deposition of Quantum Dots onto TiO2 Surfaces.” Langmuir 2007, 23,
3432-3439. |
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Spectroscopic
characterization of electron transfer reactions at interfaces. We use time-resolved
spectroscopic techniques to probe interfacial electron transfer reactions
within tethered assemblies of semiconductor nanoparticles. We have
recently characterized the interfacial electron transfer reactivity of
tethered assemblies of CdS quantum dots, mercaptoalkanoic acid linkers, and
TiO2 nanoparticles.
We discovered that the efficiency of excited-state electron transfer, or
electron injection, from CdS to TiO2 varies dramatically with the alkyl chain length of the
mercaptoalkanoic acid linkers. Our findings may have implications for the
design of quantum dot solar cells and photocatalysts. More generally, they
lend fundamental insight into the factors governing the efficiency of
excited-state electron transfer between nanoparticles. Our ongoing research
is focused on fundamental studies of excited-state electron transfer
reactions between nanoparticles. Recent electron
transfer publication: Dibbell, R.S.; Watson,
D.F. “Distance-Dependent Electron Transfer in Tethered Assemblies of
CdS Quantum Dots and TiO2 Nanoparticles.”
J. Phys. Chem. C, 2009, 113, 3139-3149. |
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Mixed monolayers on
surfaces. Mixed-monolayer-functionalized surfaces may have a range of
applications in molecular recognition, molecular electronics, and as
substrates for materials assembly. The preparation of mixed monolayers with
well-controlled compositions, however, remains a challenge. We recently
discovered a unique class of mixed monolayers on TiO2 surfaces, which undergo time-dependent
compositional changes after the initial establishment of saturation surface
coverages. Dimerization reactions between the terminal functional groups of adsorbed
molecules cause increased affinity of certain components of mixed monolayers,
leading to the time-dependent compositional changes. Our ongoing research
is focused on elucidating the structural and compositional factors that
influence this mechanism, characterizing the kinetics of compositional
changes, and exploring the relevance of the mechanism in a range of mixed
monolayer systems. Recent publication on
mixed monolayers: Soja, G. R.; Mann,
J.R.; Watson, D.F. “Temporal Evolution of the Composition of Mixed
Monolayers on TiO2 Surfaces: Evidence
for a Dimerization-Induced Chelate Effect.” Langmuir 2008, 24,
5249-5252. |
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Collaborative
research projects
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New organic
sensitizers. In collaboration with the research groups of Prof. Michael
Detty and Prof. Jochen Autschbach in our department, we are developing novel
classes of organic sensitizers. Our goals are to optimize the
light-harvesting efficiency of dyes by controlling the dye-surface
orientation and the aggregation state of surface-adsorbed dyes. We recently
demonstrated that controlled H-aggregation of chalcogenorhodamine dyes leads
to enhanced light-harvesting and photocurrent
efficiencies in dye-sensitized solar cells. Our ongoing research
is focused on understanding and controlling the electron-transfer reactivity
of these and related organic sensitizers. Recent publication on
the organic dyes: Mann, J.R.;
Gannon, M.K.; Fitzgibbons, T.C.; Detty, M.R.; Watson, D.F. "Optimizing
the Photocurrent Efficiency of Dye-Sensitized Solar Cells through the
Controlled Aggregation of Chalcogenoxanthylium Dyes on Nanocrystalline
Titania Films." J. Phys. Chem. C 2008, 112,
13057-13061. Click
here to access the article from the ACS Publications website. |
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Chemical fate and
transport of inorganic nanomaterials. In collaboration with the research groups of
Profs. Diana Aga, Sarbajit Banerjee, and Luis Colón, we are
investigating the chemical fate and transport of engineered inorganic
nanomaterials. Our research is
focused on understanding the influence of structural and compositional
properties on chemical fate and transport. Recent publication on environmental
research: Navarro, D.A.G.; Watson, D.F.; Aga, D.S.; Banerjee, S. "Natural
Organic Matter-Mediated Phase Transfer of Quantum Dots in the Aquatic
Environment." Environ. Sci. Technol. 2009, 43, 677-682. Click here
to access the article from the ACS Publications website. |
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