Photovoltaic cells fabricated by electrophoretic deposition of CdSe nanocrystals

Nathanael J. Smith, Kevin J. Emmett, and Sandra J. Rosenthal

Applied Physics Letters 93, 043504 2008

Purpose: To use Electrophoretic depsosition (EPD) to deposite CdSe nanocrystals onto TiO₂ for use in photovoltaic cells.

Methods: The CdSe nanocrystals were synthesized using a solution of CdO, trioctylphosphineoxide (TOPO), hexadecylamine, and dodecylphosphonic acid which was heated to 320 °C. The Se was injected via a Se in tributylphosphine solution. The nanocrystals were grown, isolated, and finally stored in hexanes. A voltage of 500V was established between the two electrodes and the electrodes were then placed in the solution of nanocrystals in hexanes. The nanocrystals were deposited onto a variety of materials including TiO₂, Indium/Tin Oxide, glass, and Si.

Key Findings:
1. The EPD was extremely sensitive to the preparation conditions of the nanocrystals. Although the same basic procedure was used, the minor differences that are inherent in the synthesis and isolation caused some nanocrystals to deposit while others did not at all.
2. Deposition stops once the population of either the negatively or positively charged nanocrystals is depleted. (In this case is was the negative species)
3. Unreacted precursors (esp. TOPO) cause the nanocrystals to favor staying in solution rather than forming films on the selected material.
4. Formation of films on the materials is complete after 1 minute.
5. More Cd is deposited on the negative electrode.
6. Nanocrystals do indeed play a role in the functioning of a photovoltaic cell, even though the efficiency of the CdSe nanocrystal cells is ~10^-6%

Microwave plasma chemical vapor deposition of nano-composite C/Pt thin-films

Marek Marcinek, Xiangyun Song, Robert Kostecki, Electrochemistry Communications 9 (2007) 1739–1743

Purpose: To present a new, one step method of synthesizing nano-composite C/Pt thin films using MPCVD.  The new method is boasted to be inexpensive, quick and easy.   The films are also said to be able to uniformly distribute the Pt nano-particles in the carbon substrate.

Methods: Pt precursor was placed on one end of a slide and HOPG was placed 5mm away from the Pt precursor on the same glass slide.  This slide was located in a chamber which was then purged with Ar, which was in turn positioned close to a magnetron.  The magnetron generated Ar plasma such that the hot edge of the plasma was near the Pt precursor and the cold edge near the HOPG (substrate).  The microwave radiation caused the Ar plasma to suddenly discharge which evaporated the Pt precursor.  This lead to the deposition of the Pt in the HOPG substrate, creating a nano-composite film.

Key Findings:

Found to be reproducable using the same precursor and deposition times.

The broad peaks in the Raman spectrum result from the sp2 hybridized carbons.

The thickness of the films is uniform for similar conditions: ~2 micrometers.

Definitions:

MPCVD: microwave plasma chemical vapor deposition

HOPG: highly-oriented, pyrolitic graphite

Aberration-Corrected Z-Contrast Scanning Transmission Electron Microscopy of CdSe Nanocrystals

James R. McBride, Tadd C. Kippeny, Stephen J. Pennycook, and Sandra J. Rosenthal

Nano Letters 2004, Vol. 4, No. 7 1279-1283

Purpose: To use aberration-corrected atomic number constrast scanning transmission electron microscopy (Z-STEM) to study the differences in CdSe nanocrystals that were prepared using different solvents, and thus, ligands attached to the nanocrystal surface.  With the best probe size being 0.8 Angstroms, the lattice structure and surface structure can be imaged.

Method: CdSe nanocrystals were synthesized using exclusively trioctylphosphine oxide (TOPO) and then using TOPO and hexadecylamine (HDA).  After the synthesis and cleaning up the nanocrystals, they were dissolved in TOluene and placed on an ultrathin carbon-coated TEM grid.  The Z-STEM image was taken of the nanocrystals and the darker spots on the image corresponded to the Cd atoms and the ligher to the Se atoms.

Key Findings:

1. The addition of HDA seems to make the nanocrystals more uniform in shape and size

2.  The addition of HDA seems to make the nanocrystals more spherical.

3. The addition of HDA to the reaction mixture seems to slow down the growth rate of the nanocrystals, allowing for the surface to rearrange itself into lower energy (spherical) conformations.

Review: Homogeneously alloyed CdSxSe1-x nanocrystals: Synthesis, characterization, and composition/size-dependent band gap

Swafford, Laura A. ; Weigand, Lauren A.; Bowers II, Michael J.; McBride, James R.; Rapaport, Jason L.; Watt, Tony L.; Dixit, Sriram K.; Feldman, Leonard C.; Rosenthal, Sandra J.

Journal of the American Chemical Society, v 128, n 37, Sep 20, 2006, p 12299-12306

Purpose: The puprose of this experiment was to synthesize homogenous CdS_xSe_1-x nanocrystals and characterize nanocrystals utilizing a variety of techniques that would precisely determine the size, structure and exact composition of the nanocrystals. The composition of the nanocrystals were also varied by changing the reaction conditions.

Method: After the nanocrystals were synthesized using previous methods with minor modifications, the nanocrystals were characterized using UV-visable absorption spectroscopy, TEM, X-Ray diffractrometry, and Rutherford backscattering spectometry.

Vary reaction method and conditions in order to tailor the composition of the nanocrystals.

Key Findings: The nanocrystals synthesized using this method were very homogeneous.  The crystals were very homogeneous thoughout, with a possible small CdS core, which scarcely contributed to the total volume of the nanocrystal.  By varying the amounts of Se and S added into the solution, the compostion of the nanocrystals could be controlled.  The rates in which the Se and S solutions were added to the Cd solution also effected the homogeneity.  The amount of tributylphosphine that was used in the reaction also has an effect on the compostion of the nanocrystals, from allowing the nanocrystals to grow into spherical shapes or into rods.

Definitions: TEM: Transmission Electron Microscopy

Key Note: This was the first time that CdS_xSe_1-x nanocrystals were homogeneously synthesized in for all values of x.