Photovoltaic cells fabricated by electrophoretic deposition

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

Purpose

The purpose of this paper is to observe the efficiency of photovoltaic cells made with CdSe nanocrystals that have been electrophoretically deposited onto titanium oxide. The efficiency of these cells are lower to other solar cells (~10^-6%).

Methods/Procedures Used

Electrophoresis was used to deposit the CdSe nanocrystals in a fast and non-destructive manner. Rutherford backscattering was used to compare the composition of the films on both the positive and negative electrodes during the deposition.

Key Findings

1. Too much TOPO solvent made electrophoresis difficult. This is due to an effect seen in previous literature where CdSe prefer to stay in solution.
2. An interesting effect was observed where cadmium precursor attaches to the negative electrode during deposition in a greater concentration than onto the positive electrode. This concentration difference was determined by Rutherford backscattering
3. Nanometer thick photovoltaic cells were successfully created within one minute with this technique.

Biomimetic subwavelength antireflective gratings on GaAs

Chih-Hung Sun,

 

1 Brian J. Ho,1 Bin Jiang,2 and Peng Jiang1,*

OPTICS LETTERS / Vol. 33, No. 19 / October 1, 2008 p 2224-2226

Purpose of Study-

The authors developed a simple effective method in order to make a patterned surface that decreases the reflectivity of a surface.  In this manuscript they apply this method to GaAs wafers.

Methods-

Silica colloidal spheres are dispersed on a wafer using a simple spin-coating technique.  The size of the colloids can be controlled by varing the speed of the spin-coater.  In order to apply this method to GaAs surfaces, a single-layer reduction technique was employed.  Once the silica has  been deposited, a PDMS stamp is created.   Using the stamp as a mold, they are able create a template to form a patterned polymer film.

Key Findings-

• Subwavelength antireflective gratings can be easily formed on a variety of substrates.
• Silica colloids can be uniformily dispersed across a substrate, and be used to create a pattern template.

Integrated molecular targeting of IGR1R and HER2 surface receptors and destruction of breast cancer cells using single wall carbon nanotubes

Ning Shao, Shaoxin Lu, Eric Wickstrom, Balaji Panchapakesan, Nanotechnology 18 (2007) 315101 (9pp).

Purpose of the Study:   

Demonstrate the in vitro ability of single walled carbon nanotubes (SWCNT) functionalized with anti-IGF1R and anti-HER2 monoclonal antibodies to destroy breast cancer cells upon excitation with near infrared light (808nm). 

Methods:

 

•  SWCNTs having an average diameter of 1.4nm and lengths ranging from 500nm to 1mm were fabricated with a self-assembled monolayer of polyethylene glycol (PEG) to prevent undesirable binding with other biomolecules.
• Transmission electron micrography was used to characterize the SWCNTs.
• SWCNTs were divided into three groups and functionalized with a non-specific antibody (control), anti-IGF1R antibody, or anti HER2 antibody.
• Cytotoxicity was minimized, consistent with published research, by chemically modifying SWCNT surface with –OH and –COOH groups in addition to the PEG conjugates.
• SWCNTs were mixed with fluorescent dyes (phalloidin 488 & 555) to form fluorescent SWCNT-antibody hybrids.
• SWCNTs were incubated with two different human breast cancer cell lines:  (a) BT474, which exhibit high expression of HER2 receptors and lower expression of IGF1R receptors, and (b) MCF7, which exhibit high expression of IGF1R receptors and lower expression of HER2 receptors.  Each cell line was incubated with both anti-HER2 SWCNTs and anti-IGF1R SWCNTs.
• Confocal fluorescent microscopy was used to characterize the fluorescent SWCNT-antibody hybrids and the cellular uptake thereof.
• An 808nm laser at 800mW cm^-2 and 3 min exposure time was used for the photo-thermal therapy regimen. 
• Trypan blue assay was used to verify and quantify cell death (i.e. cell viability).  

Key Findings:

 

1. SWCNT-receptor specific antibody conjugates were readily internalized into the cells over large areas while the SWCNTs with the non-specific antibody were not.  The authors hypothesize the following mechanism (consistent w/ previous research):  upon receptor-specific attachment, stresses are generated due to free energy release which generates a pressure differential across membrane pores, thereby facilitating endocytosis.
2. Cells receiving only the NIR light without internalized SWCNT survived the photo-thermal therapy whereas those incubated with the SWCNT anti-HER2 and anti-IGF1R conjugates did not survive (100% kill rate).  Those cells to which SWCNT-non specific antibody conjugates were attached, 50% survived (most likely due to fewer attached).
3. Cells were killed due to the localized increase in temperature resulting from high absorption by the SWCNT that occurs with NIR light while normal cells remain transparent to NIR.   
4. The energy used to destroy the cancer cells was estimated to be ~200nW per cell, too low to create any damage to the normal cells.  Due to the high specificity, antibody-directed targeting, the authors were able to use half the laser power of past nanotube based cell killing techniques. 

Definitions:

Her2:  human epidermal growth factor receptor (often over expressed by breast cancer cells)

IGF1R:  insulin-like growth factor receptor (often over expressed by breast cancer cells)

Direct Fabrication of Nanoscale Light Emitting Diode on Silicon Probe Tip for Scanning Microscopy

Kazunori Hoshino, Member, IEEE, Lynn J. Rozanski,
David A. Vanden Bout, and Xiaojing Zhang, Member, IEEE

JOURNAL OF MICROELECTROMECHANICAL SYSTEMS, VOL. 17, NO. 1, FEBRUARY 2008

Purpose

Near-field scanning optical microscopy (NSOM), combines scanning probe technology with optical microscopy. Conventionally, a fiber optic tip is assembled on the probe tip. This slows the fabrication process and requires an external light source. This purpose of this work is to integrate a nano-sized LED on the tip of the scanning probe.

Method

The fabrication can be divided into two parts: tip fabrication and LED fabrication.

Tip fabrication: started with a SoI wafer using the oxide as resist. An electrode was patterned on one side. The tip was subsequently etched to a tip.

LED fabrication: First the electrode part was doped, then trimmed with a focused ion beam (FIB) creating a 100nm gap. The probe was then charged in a solution of CdSe/ZnS core-shell nanoparticles of about 5nm in diameter. Electrostatic forces caused the nanoparticles to assemble and bridge the gap.

Results

The probe was successfully tested in a standard NSOM setup with the following observations.

- The electrode tip exhibited diode electrical behavior.

- A broad emission frequency was observed due to doping of the electrode in addition to the use of the ion beam for fabrication. (which can be mitigated by using an e-beam)

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%

Enhanced field emission characteristics of zinc oxide mixed carbon nano-tubes

J.Y. Pan, C.C. Zhu, Y. L. Gao. Enhanced field emission characteristics of zinc oxide mixed carbon nano-tube films. Applied Surface Science 254(2008) 3787-3792.

Purpose of The Study

To improve the field emission characteristics by mixing Zinc oxide and carbon nano tubes(CNT) in a composite material. CNT are potential cathode materials in flat emission display. CNT FED are expected to be candidate of a flat plane display. However CNT have a high turn-on electric field, low emission stability and emission uniformity. These factors are slowing the progress of CNT. This study is a proposed method to reduce the turn-on electric field and increase emission stability and uniformity.

Methods

Using Chemical Vapor Deposition (CVD) to synthesize multi-wall CNTs, these CNTs were used as emitters of screen printed cathode films. CNT were mixed with ZnO, Terpineol(dispersant) and ethyl cellulose(binder) heated to 120 celcius and stirred. The CNT film is prepared by screen printing the solution on indium tin oxide (ITO) coated glass. Dried and baked to remove organic binder. The printing area is 2.5 cm x 2.5cm. Field-emission scanning electron microscopy was used to analyze the morphologies of the prepared films.

Key Findings

1. ZnO grains fill into the interspaces of CNTs (shown by SEM images).

2. Potential emitters in ZnO-CNT film would also increase due to the distribution of ZnO flakes.

3. Compared to usual CNTs, ZnO-CNT film has a lower turn-on electric field.(from 1.70 to 1.17 V/micrometer)

4. The emission current of ZnO-CNTs is 505 microamps, which is higher compared to the usual CNT (60 microamps).

5. ZnO is helpful as a field emission material to improve field emission characteristics of CNT films.

Entrapment of Photosystem I within Self-Assembled Films

“Entrapment of Photosystem I within Self-Assembled Films”

Helen A. Kincaid, Tom Niedringhaus, Madalina Ciobanu, David E. Cliffel, and G. Kane Jennings

Purpose

This paper discusses the ability to extract photosystem proteins from plants and incorporate them into devices that take advantage of the light harvesting ability of the proteins to generate electricity.  It focuses on the method of forming a SAM on the surface of a gold substrate and using this SAM to attach photosystem I in a patterned structure.  Alkanethiol SAMs with chain lengths longer than 12 carbons were shown to be too long and prevented charge transfer to the Au substrate.

Methods

Gold substrates were prepared using a silicon wafer that was cleaned and coated with evaporated chromium and gold layers.  SAMs were formed on the Au surface by immersion of the substrate in a 1 mM alkanethiol and ethanol solution.  The same process was done in a solution of PSI to get a thin layer of PSI on the SAM.  After finishing this process, the SAMs were backfilled with a long-chain alkanethiol in different solvents depending on the experiment.  Reflectance-Absorbance Infrared Spectroscopy (RAIRS), Spectroscopic Ellipsometry (SE), and Electrochemical Impedance Spectroscopy (EIS) were used to do electrochemistry on the resulting SAM/PSI covered substrate.

Key Findings

Backfilling the SAM is found to be essential in controlling the coverage of the PSI on the substrate.  Also time of exposure and concentration of the PSI solution are found to affect the type of coverage that the PSI will have on the SAM.  The choice of solvent (polarity) is involved in the quality of the backfilled SAM that is formed.  Polar solvents are found to be more effective than nonpolar solvents in getting a densely packed SAM.

Microwave-assisted rapid synthesis of anisotropic Ag nanoparticles by solid state transformation

S. Navaladian, B. Viswanathan, T. K. Varadarajan and R. P. Viswanath

 

 

Microwave irradiation is used to synthesize anisotropic silver nanoparticles via decomposition of silver oxalate in a glycol medium using polyvinyl pyrolidone as the capping agent. The concentration of Ag nanoparticles formed by thermal decomposition shows to be higher than that of polyol-reduced Ag nanoparticles. These nanoparticles have been characterized and show an average of size around 30 nm following irradiation for 75s and 5-6 nm after 60s. Ethylene glycol (EG) was found to be a better medium than diethylene glycol (DEG). Microwave-assisted synthesis is manifold faster than conventional heating.

 

Methods

 

Characterization of silver oxalate was carried out using a powder x-ray diffraction (XRD), Perkin Elmer thermo-gravimetric analysis (TGA)7 and an FEI (Model: Quanta 200) scanning electron microscope (SEM). Irradiation was carried out in a Panosonic Inverter (1100 W, 2.45 GHz) microwave oven. Collodial Ag nanoparticles were characterized by UV-visible spectroscopy, transmission electron microscopy (TEM), and high resolution transmission electron microscopy (HRTEM).

 

Key Findings

 

1.) Nanoparticle size can be manipulated by microwave exposure.

2.) Suggest that SPR band of nanoparticles vary depending on dielectric constant,         refractive index and size.

3.) Suggest mechanism for nanoparticle growth: Ag₂C₂O₄ (s) à 2 Ag(s) + 2 CO₂

4.) Suggest that EG is a more suitable dielectric medium than DEG which is due to the smaller size of EG to effectively enter pores of silver oxalate and reduce Ag⁺ on surface.

Graphene-stabilized copper nanoparticles as an air-stable substitute for silver and gold in low-cost ink-jet printable electronics

Norman A Luechinger, Evagelos K Athanassiou and Wendelin J Stark

Institute for Chemical and Bioengineering, Department of Chemistry and Applied

Biosciences, ETH Zurich, CH-8093 Zurich, Switzerland

NANOTECHNOLOGY   NOV 5 2008

 

The purpose of this study as can be surmised from the title was to find a cost effective alternative to the noble metal nanoparticles used in ink-jet printers that would be just as stable in air.  There are a few interesting concepts behind this.  The first is that most metals now used in bulk, have either an oxide coating or other coating keeping them from reacting.  Similar to this is how nanoparticles are capped to alter their properties, and specifically there has recently been copper and cobalt nanoparticles that have been capped with graphene.

 

Reducing flame synthesis was used to created the graphene passivated copper nanoparticles.  These particles were then characterized by microanalysis, thermogravimetric analysis, and X-ray diffraction.  These particles were then dissolved in water with a dispersion agent that also decreased the surface tension, and then sonicated to ensure that there were no aggregates.  This ink was then used, and the results tested for conductivity.

 

There were a number of findings in this study.  The copper and graphene nanoparticle ink was successfully used in a standard ink-jet printer.  The result produced an ink with a metallic luster, and enough conductivity to operate an LED.  However, there was not nearly as much conductivity as bulk copper.  The TGA analysis confirmed that the nanoparticles were inert to 165C.  This means that elements that normally would be too reactive to be used for nanoparticles, can be used with a graphene coating

Micro- and nanoscale characterization of hydrophobic and hydrophilic leaf surfaces

Bhushan B., Jung Y.C

Nanotechnology 17 (2006) 2758–2772

Purpose

 

Characterization of micro- and nanoscale topographic features of hydrophobic leaves, and analyze their roll in the superhydrophobic behavior. The study also makes a comparison of hydrophilic leaves with the purpose to understand the importance of low energy surface materials in the improvement of hydrophobic behavior.

Methods

 

Four type of leaves were studied: lotus and colocasia with hydrophobic characteristics and, fagus and magnolia with hydrophilic characteristics. Topographical characterization of the leaves was performed using SEM. Height of the micro and nano features was measured using optical profilometry and AFM in both modes, contact and tapping. Finally wetting properties were determined using a contact angle goniometer.

Key Findings

·         Comparison between hydrophobic and hydrophilic leaves revealed that the later have a thin wax film, which suggests that low energy constituents over this surfaces influence the hydrophobic behavior.

·         Combination of low surface energy materials and roughness are the responsible factors for superhydrophobic behavior.

·         Superhydrophobic behavior combines micro- and nanoscale roughness. However nanoscale roughness is the main responsible for the increase in the contact angle due to the increase of air pockets under the drops.