-
Echem Gradients <download slide pdf>
Current injection of the order 1-100 mA into ultrathin (5 nm ≤ d ≤ 80 nm) Au films yields
significant in-plane voltage drops and makes possible a new kind of experiment,
in which the working electrode (WE), rather than assuming a single value of
potential, displays an in-plane potential gradient, V(x). The size and position of the potential gradient can be tuned
by adjusting the magnitude of the injected current and the voltage offset, respectively.
The potential-distance relationship, V(x),
is determined by the magnitude of the injected current, and the voltage offset, V0, according to,
where i is the injected
current, A is the Au film WE
cross-sectional area, and r(l) is the
film resistivity at position, l. We
have used this in-plane electric potential gradient in two different ways. Exploiting the thermodynamic driving force
for heterogeneous electron transfer, electrochemical reactions occur at spatial
positions where the local potential is favorable, i.e. where V(x) exceeds
the Nernstian value, E0. Alternatively, the spatial potential
distribution, V(x), can be coupled to
the chemistry through a spatially varying electron transfer rate constant, k(x).
- Stimulus-Responsive Materials <download slide pdf>
Spatially graded physical structures are of interest, because the strength of interaction, Gint, can be continuously varied in space, providing a useful alternative to the more common practice of temporally varying the interaction strength. A key goal is to develop laterally graded surfaces and thin films, i.e. structures in which the composition, physical properties, or both, vary spatially. The principal goals of the work are:
- to combine lateral composition variations with controlled gradients in physical properties to produce structures in which both chemistry and topography are controlled spatially;
- to characterize the structural and dynamic properties of such structures, especially in response to externally imposed perturbations; and
- to understand their capabilities and limits in molecular recognition applications, especially in comparison to molecular array approaches.
-
Porous Optoelectronic Materials <download slide pdf>
Our interest in porous
optoelectronic materials is motivated by the prospect for significant advances
in optoelectronics, chemical and biochemical sensing, and matrices for
characterization of macromolecules. Recently electroless methods for the
etching of GaN have been developed in our laboratory in which ultrathin (3 nm
< d < 10 nm) Pt islands are
deposited on the semiconductor, followed by etching in a CH 3OH:HF:H 2O 2 solution under UV illumination. Catalytic reduction of H 2O 2 at the Pt islands
combines with UV radiation to inject holes deep into the valence band, which
subsequently produces an etched porous structure. PGaN exhibits a number of
interesting and advantageous properties - including blue-shifted luminescence,
spatially differentiated bandgap emission, extraordinarily high
surface-to-volume ratio, chemically derivatizable surfaces, enhanced
photoconductivity and an electrically sensitive surface dipole layer – which
make it extremely interesting for chemical and biochemical sensing
applications.
For more information about these efforts, please see the graphics gallery that follows and these original papers:
Terrill, R.H; Balss, K.M.; Zhang, Y.; Bohn, P.W. “Dynamic Monolayer Gradients: Active Spatiotemporal Control of Alkanethiol Coatings on Thin Gold Films,” J. Amer. Chem. Soc. 2000, 122, 988-989.
Li, X.; Bohn, P.W “Metal-Assisted Chemical Etching in HF/H2O2 Produces Porous Silicon,” Appl. Phys. Lett. 2000, 77, 2572-2574.
Harada, Y.; Li, X.; Bohn, P.W.; Nuzzo, R.G “Catalytic Amplification of the Soft Lithographic Patterning of Si. Non-electrochemical Orthogonal Fabrication of Photoluminescent Porous Si Pixel Arrays,” J. Amer. Chem. Soc., 2001, 123, 8709-8717.
Balss, K.M; Coleman, B.D.; Lansford, C.H.; Haasch, R.; Bohn, P.W. “Active Spatiotemporal Control of Electrochemical Reactions by Coupling to In-Plane Potential Gradients,” J. Phys. Chem. B 2001, 105, 8970-8978.
Chattopadhyay, S.; Li, X.; Bohn, P.W. “In-Plane Control of Morphology and Tunable Photoluminescence in Porous Silicon Produced by Metal-Assisted Electroless Chemical Etching,” J. Appl. Phys. 2002, 91, 6134-6140.
Balss, K.M.; Fried, G.A.; Bohn, P.W. “Chemically Selective Force Mapping of Electrochemically Generated
Two-Component w-Substituted Alkanethiol Monolayer Gradients by Pulsed-Force-Mode
Atomic Force Microscopy,” J. Electrochem. Soc., 2002, 149, C450-C455.
Li, X.; Kim, Y.W.; Bohn, P.W.; Adesida, I. “In-plane Bandgap Control in Porous GaN through Electroless Wet Chemical Etching,” Appl. Phys. Lett. 2002, 80, 980-982.
Balss, K.M.; Kuo, T.C.; Bohn, P.W. “Direct Chemical Mapping of Electrochemically Generated Spatial Composition Gradients on Thin Gold Films with Surface-Enhanced Raman Spectroscopy,” J. Phys. Chem. B 2003, 107, 994-1000.
Díaz, D.J.; Williamson, T.L.; Adesida, I.; Bohn, P.W.; Molnar R.J. “Morphology and Luminescence of Porous GaN Generated via Pt-assisted Electroless Etching,” J. Vac. Sci. Technol. B 2002, 20, 2375-2383.
Plummer, S.T.; Wang, Q.; Bohn, P.W.; Stockton, R.; Schwartz, M.A. “Electrochemically-Derived Gradients of the Extracellular Matrix Protein Fibronectin on Gold,” Langmuir 2003, 19, 7528-7536.
Rittenhouse, T.L.; Hossain, T.K.; Lindesay, J.; Marcus, A.; Bohn, P.W.; Adesida, I. “Surface-state Origin for the Blueshifted Emission of Anodically Etched Porous Silicon Carbide,” J. Appl. Phys. 2004, 95, 490-496.
Coleman, B.D.; Finnegan, N.; Bohn, P.W. “Sharply-Defined Lateral Composition Gradients of Copper on Gold by Spatiotemporal Control of the In-Plane Electrochemical Potential Distribution,” Thin Solid Films 2004, 467, 121-126.
Williamson, T.L.; Díaz, D.J.; Bohn, P.W.; Molnar, R.J. “Structure-Property Relationships in Porous GaN Generated by Pt-Assisted Electroless Etching Studied by Raman Spectroscopy,” J. Vac. Sci. Technol. B, 2004, 22, 925-931.
Wang, X.; Bohn, P.W. “Anisotropic in-plane gradients of poly(acrylic acid) formed by electropolymerization with spatiotemporal control of the electrochemical potential,” J. Amer. Chem. Soc. 2004, 126, 6825-6832.
Chattopadhyay, S.; Bohn, P.W. “Direct-Write Patterning of Microstructured Porous Silicon Arrays by Focused Ion Beam Pt Deposition and Metal-Assisted Electroless Etching,” J. Appl. Phys. 2004, 96, 6888-6894.
Wang, Q.; Bohn, P.W. “Surface Composition Gradients of Immobilized Cell Signaling Molecules. Epidermal Growth Factor on Gold,” Thin Solid Films 2006, 513, 338-346.
Wang, X.; Tu, H.; Braun, P.V.; Bohn, P.W. “Length scale heterogeneity in lateral gradients of poly(N-isopropylacrylamide) polymer brushes prepared by surface-initiated atom transfer radical polymerization coupled with in-plane electrochemical potential gradients,” Langmuir 2006, 22, 817-823.
Guo, X.; Williamson, T.L.; Bohn, P.W. “Enhanced ultraviolet photoconductivity in porous GaN prepared by metal-assisted electroless etching,” Solid State Comm. 2006, 140, 159-162.
Wang, X.; Bohn, P.W. “Spatiotemporally controlled formation of two-component counterpropagating lateral graft density gradients of mixed polymer brushes on planar Au surfaces,” Adv. Mater. 2007, 19, 515-520.
Lokuge, I.; Wang, X.; Bohn, P.W. “Temperature Controlled Flow Switching in Nanocapillary Array Membranes Mediated by Poly(N-isopropylacrylamide) Polymer Brushes Grafted by Atom Transfer Radical Polymerization,” Langmuir 2007, 23, 305-311.
|
