Vasyl P. Kunets

Light-harvesting using high density p-type single wall carbon nanotube/n-type silicon heterojunctions.

Photovoltaic conversion was achieved from high-density p-n heterojunctions between single-wall carbon nanotubes (SWNTs) and n-type crystalline silicon produced with a simple airbrushing technique. The semitransparent SWNT network coating on n-type silicon substrate forms p-n heterojunctions and exhibits rectifying behavior. Under illumination the numerous heterojunctions formed between substrate generate electron-hole pairs, which are then split and transported through SWNTs (holes) and n-Si (electrons), respectively. The nanotubes serve as both photogeneration sites and a charge carriers collecting and transport layer. Chemical modification by thionyl chloride of the SWNT coating films was found to significantly increase the conversion efficiency by more than 50% through adjusting the Fermi level and increasing the carrier concentration and mobility. Initial tests have shown a power conversion efficiency of above 4%, proving that SOCl(2) treated-SWNT/n-Si configuration is suitable for light-harvesting at relatively low cost.

SOCl2 enhanced photovoltaic conversion of single wall carbon nanotube/n-silicon heterojunctions

We report solar cells based on high-density p-n heterojunctions between single wall carbon nanotubes (SWCNTs) and a n-type silicon wafer. Chemical modification by thionyl chloride of the SWCNT coating films was found to significantly increase the conversion efficiency by more than 45% through adjusting the Fermi level and increasing the carrier concentration and mobility. Electron-hole pairs are optically excited in the numerous heterojunctions formed between SOCl2-treated SWCNTs thin coating and n-type silicon substrate, and then split and transported through SWCNTs (holes) and n-Si (electrons), respectively.

Multicolor photodetector based on GaAs quantum rings grown by droplet epitaxy

Photoresponse from multicolor photodetector was measured in the spectral range of 0.4–6.0μm as a function of temperature and bias voltage. Devices were fabricated from wafers with an active region of five periods of GaAs quantum rings grown by droplet epitaxy technique on lattice matched Al0.3Ga0.7As barriers. The photoresponse spectra exhibit two broad bands in the visible-near-infrared and midinfrared spectral regions. The visible-near-infrared band, which is due to interband transitions, was observed at temperatures as high as room temperature. On the other hand, the midinfrared band, which is due to intersubband transitions, was observed at temperature lower than 80K.

Intermediate-band material based on GaAs quantum rings for solar cells

The intermediate-band concept is invoked to explain the photoresponse spectra obtained for unbiased devices fabricated from GaAs quantum rings grown by a droplet epitaxy technique on lattice-matched Al0.3Ga0.7As barriers. The photoresponse spectra where measured at room temperature in the visible-near-infrared spectral range. The presence of the intermediate band in the device active region is confirmed by measuring the mid-infrared photoresponse, which is attributed to the intersubband transitions in the conduction band. The photocurrent was measured at room temperature and found to be about four orders of magnitude larger than the dark current in the voltage range of ± 4.0 V.

Carbon-covered magnetic nanomaterials and their application for the thermolysis of cancer cells

Three types of graphitic shelled-magnetic core (Fe, Fe/Co, and Co) nanoparticles (named as C-Fe, C-Fe/Co, and C-Co NPs) were synthesized by radio frequency-catalytic chemical vapor deposition (RF-cCVD). X-ray diffraction and X-ray photoelectron spectroscopy analysis revealed that the cores inside the carbon shells of these NPs were preserved in their metallic states. Fluorescence microscopy images indicated effective penetrations of the NPs through the cellular membranes of cultured cancer HeLa cells, both inside the cytoplasm and the nucleus. Low RF radiation of 350 kHz induced localized heating of the magnetic NPs, which triggered cell death. Apoptosis inducement was found to be dependent on the RF irradiation time and NP concentration. It was showed that the Fe-C NPs had a much higher ability of killing the cancer cells (over 99%) compared with the other types of NPs (C-Co or C-Fe/Co), even at a very low concentration of 0.83 μg/mL. The localized heating of NPs inside the cancer cells comes from the hysteresis heating and resistive heating through eddy currents generated under the RF radiation. The RF thermal ablation properties of the magnetic NPs were correlated with the analysis provided by a superconducting quantum interference device (SQUID).

Polymer functionalized n-type single wall carbon nanotube photovoltaic devices

Photovoltaic conversion was achieved from high-density p-n heterojunctions formed between polymer functionalized n-type single wall carbon nanotubes (SWNTs) and underlying p-type Si substrate. Functionalization of SWNTs by amine-rich polymers results in the evolution of tubes from p-type to n-type, and the polyethylene imine (PEI) functionalized SWNT film can serve as both photogeneration sites and a charge carrier collecting/transport layer. Photoremoval of oxygen adsorbed on the nanotubes prior to PEI functionalization can increase the conversion efficiency of the polymer functionalized n-type SWNT photovoltaic devices.

Polarization doping: Reservoir effects of the substrate in AlGaN graded layers

High electron sheet concentrations of ∼1015 cm−2 result from polarization doping with compositionally graded AlGaN films grown on unintentionally doped GaN templates which exhibit background electron concentrations of ∼1016 cm−3. Similar graded films grown on semi-insulating (SI), free standing GaN substrates exhibited carrier concentrations two orders less. Transport studies of the as-grown materials using temperature dependent Hall effect revealed a very weak temperature dependence of the carrier concentration and mobility as compared to traditionally doped films using Si as a dopant. And qualitative modeling of the electron mobility indicates that alloy scattering and charged dislocation scattering are the most significant contributors to limiting the mobility over the entire temperature range.

Room Temperature Near-Infrared Photoresponse Based on Interband Transitions in

Near-infrared photoresponse is observed in the temperature range of 77-300 K for a photodetector fabricated from undoped In0.35Ga0.65As/GaAs multiple quantum dots grown in a molecular beam epitaxy system. The detectivity is estimated to be on the order of 3.70 times 109 and 2.70 X 107 cm .radicHz/W at 77 and 300 K, respectively. The reduction of the detectivity is attributed to the increase of the dark current as the temperature is increased. The photoresponse is explained in terms of several interband transitions. These transitions are found to be in good agreement with the self-consistent theoretical calculations.

\hbox{In}_{0.35}\hbox{Ga}_{0.65}\hbox{As}

A simple and easily processible photovoltaic device has been developed based on boron-doped single-walled carbon nanotubes (B-SWNTs) and n-type silicon (n-Si) heterojunctions. The SWNTs were substitutionally doped with boron atoms by thermal annealing, in the presence of B2O3. The samples used for these studies were characterized by Raman spectroscopy, thermal gravimetric analysis, transmission electron microscopy, and x-ray photoelectron spectroscopy. The fully functional solar cell devices were fabricated by airbrush deposition that generated uniform B-SWNT films on top of the n-Si substrates. The carbon nanotube films acted as exciton-generation sites, charge collection, and transportation while the heterojunctions formed between B-SWNTs and n-Si acted as charge dissociation centers. The current-voltage characteristics in the absence of light and under illumination, as well as optical transmittance spectrum are reported here. It should be noted that the device fabrication process can be made amenable t...

Multiple Quantum Dot Photodetector

Intermediate band solar cells were realized using a GaAs (311)A p-i-n junction with Si as both the p- and n-type dopant, where the intermediate band was realized with a stack of InGaAs quantum wires. This quantum wire photovoltaic device demonstrates a non-trivial increase in solar cell efficiency over a reference p-i-n GaAs (311)A junction resulting from a significant increase in short circuit current and an only slight decrease in open circuit voltage. Presented are optical and electrical characterizations of these devices.