Ruxandra Vidu

Nanocable structures for high-efficiency solar cells can be created by template synthesis and electrodeposition. An important challenge in synthesizing tubular nanostructures is the mass transfer and chemical reaction for a multiscale nanomaterial: one dimension on the nanoscale with another dimension on the micron-scale. Our work includes the formation of CdTe inside Au nanotube membranes. This "nanocable" structure (CdTe core in Au nanotube) advanced our understanding of the conditions to create this particular metal/semiconductor interface in a context that had broader applicability beyond solar cells. The same conditions could be used to create Au/CdTe, i.e Au core in CdTe nanotube. We used polycarbonate track-etched membranes (PCTEs) although other types of porous materials with monodisperse diameters and lengths such as alumina membranes can be used.

References:
NSF SBIR Phase I Award #0539336 (2005).

Au nanotubes can be created by electroless deposition on polycarbonate track-etched membranes (PCTEs). Electroless metal deposition involves the use of a chemical reducing agent to plate a metal from solution onto a surface. The advantage of the electroless method over electrochemical plating is that surfaces to be coated need not be conductive. The key requirement of electroless deposition is that the kinetics of homogeneous electron transfer from reducing agent to the metal ion are slow, otherwise the metal ion would simply be reduced in the bulk solution. Prior art in electroless deposition was very useful in developing this chemistry. However, we made significant modifications to the prior art to make this process capable of plating gold uniformly in nanoscopic pores.

References:
NSF SBIR Phase I Award #0539336 (2005).

Our method uses a novel fabrication route for Au-Te nanocables, where the working electrode is the metal nanotubes membrane. Electroless plating is used first to plate nonconductive surfaces of membrane pores and make them conductive for further electrochemical applications. Then, electrodeposition is used to fill in the nanotubes.

Here we show an example of using the Au nanotube membrane as a second template to deposit Te on the inner surface of the Au nanotubes by slow electrochemical deposition. The deposition rate is slow compared to that of the axial mass transfer to grow nanocables coaxially within the Au nanotubes. The Au-Te nanocables are Au(shell)-Te(core)nanocables and have radial dimensions consistent with those of the starting templates.

References:
J.-R. Ku, R. Vidu et al., Journal of the American Chemical Society, 2004, 126(46), 15022-15023. R. Vidu, J-R. Ku, P. Stroeve, "Fabrication of Multiscale Nanostructures from Polymeric Membrane Templates" in Polymeric Nanostructures and Their Applications edited by H. S. Nalwa, 2007, Vol. 1: "Polymeric Nanostructures", Chapter 3, p.124-149.

We developed a completely new fabrication method for low-cost, highly efficient radial p-n junction nanorod arrayed solar cells using CVD techniques. We started with patterning silicon-on-insulator SOI wafers using nanoimprint lithography (NIL). Nanoimprinting has enabled tailoring the patterns and seed placement to our specific device requirements. Besides solar energy materials, the resulting structures would also have broad usefulness in the areas of thermo-electrics and precision sensors, to name only a few. A device consisting of arrays of radial p-n junction nanorods may provide a solution to device design and optimization and use of low-cost materials is an important factor to commercial viability.

References:
NSF SBIR Phase I Award #0712688 (2007).

Surfaces and interfaces become very important in devices like sensors and solar cells as the use of advanced nanomaterials involves structures that are characterized by surface and not by volume properties.

This project successfully demonstrated feasibility of two new techniques for surface modification never before applied to nanostructures: surface smoothing and surface roughening. Surface smoothing treatment is based on a surface diffusion process while surface roughening treatment uses electrochemically controlled alloying/de-alloying (ECAD) process. The second method is based on our findings that ECAD process applied under potential control in the under-potential (UPD) region results in surface porosity.

The benefit of controlling the surface structure and properties of nanostructures goes far beyond the immediate objectives of this project. Combining atomic-level surface modification processes with manufacturing processes at nano-scale is of great interest for advancing our understanding of optic and electronic properties of nanostructured platforms.

References:
NSF SBIR Phase I Award #0741095 (2007). Vidu, R. et al., Journal of Electroanalytical Chemistry, 1999. 475(2): p. 171-180. Vidu, R. et al., JVST B, 1999. 17(6): p. 2423-2430. Vidu, R. et al., Electrochemistry, 1999,67(12): p. 1240 Vidu, R. et al., Surface Science, 2000.452(1-3): p. 229 Vidu, R. et al., JPCCP, 2001. 3(16): p. 3320-3324.

Thermoelectrics are renewable energy materials that benefit from low dimensionality. Cobalt antimonides are considered to be the most suitable thermoelectric materials for applications in the range around 600 K. CoSb3 exhibits excellent electrical transport properties, one of the highest values for hole mobility in a semiconductor due to a high degree of covalent bonding.

This work presents a comparison between thin film and nanowire deposition, both grown on a gold-coated polycarbonate track-etched (PCTE) membrane. First, a thin layer of Au was deposited on one side of the PCTE membrane. The Au-coated template was then placed on a copper tape and mounted in between two plastic tapes, exposing either the Au layer for thin film deposition or the un-coated side of the PCTE membrane for nanowire growth. Thin films of cobalt antimonide were grown on the nanostructured Au surface. Nanowires were grown inside the pores of the PCTE membrane.

References:
NSF SBIR Phase I Award #0712688 (2007). D.V. Quach, R. Vidu, J.R. Groza, P. Stroeve, Electrochemical Deposition of Cobalt Antimonide Thin Films and Nanowires, to be published 2010.

Underpotential deposition (UPD) is the electrodeposition of a metal cation onto another metal at a potential more positive than the equilibrium (Nernst) potential for the reduction of this metal.

The occurrence of underpotential deposition is often interpreted as a result of a strong interaction between the electrodepositing metal M with the substrate S (of which the electrode is built). The M-S interaction needs to be energetically favored to the M-M interaction in the crystal lattice of the pure metal M. This mechanism is deduced from the observation that UPD typically occurs only up to a monolayer of M (sometimes up to two monolayers). The formation of metal monolayers through underpotential deposition depends on the interactions between the electrode surface and the depositing species. Other adsorbable species, if present in the solution, will influence the formation of UPD metal films through their adsorption on the electrode surface, taking place simultaneously.

Our work includes Cd and Te UPD on Au single crystals. Additionally, we observed and studied the surface alloying in the Cd-Au system.

References:
Ikemiya N., et al., Surface Science, 369 (1996), p. 199. Vidu, R. et al., Journal of Electroanalytical Chemistry, 1999. 475(2): p. 171-180. Vidu, R. et al., JVST B, 1999. 17(6): p. 2423-2430. Vidu, R. et al., Electrochemistry, 1999,67(12): p. 1240 Vidu, R. et al., Surface Science, 2000.452(1-3): p. 229 Vidu, R. et al., JPCCP, 2001. 3(16): p. 3320-3324.

Epitaxial growth is one of the most important technologies used in the fabrication of nanometer-scale devices when uniform and defect-free thin films are sought. Understanding the atomic structures formed at ordered metal surfaces is an important topic in the preparation of epitaxial thin films by EC deposition.

Electrochemical deposition represents an alternate methodology to prepare layered structure compounds at room temperature, which could avoid interdiffusion problems associated with the high temperature required in current deposition techniques. However, the strong adatom-substrate interaction at the metal/ionic solution interface may lead to intermixing at interface in many underpotential deposition systems. Therefore, many systems cannot be characterized any longer by the conventional growth modes commonly used to describe epitaxial growth since interdiffusion has been observed to take place at the interface even at room temperature.

When the favorable conditions are met and the surface alloying occurs, it interferes with the underpotential deposition (UPD). However, the two processes can be distinguished one from the other because the UPD atoms are limited to 1-2 monolayers while a limit in atom coverage should not theoretically exist for the surface alloy. Moreover, the kinetics of the UPD adsorption process is much faster than the alloy formation. The alloy formation is likely to take place under thermodynamically favorable conditions whenever two different metals are in contact with each other.

References:
Vidu, R. et al., Scripta Materialia, 1999. 41(6): p. 617. Vidu, R. et al., Surface Science, 2000.452(1-3): p. 229