Rene G. Rodriguez

Post-Consumer Plastic Identification Using Raman Spectroscopy

Raman spectroscopy is evaluated as a spectroscopic method for identification of common household plastics for recycling purposes. The methods of K-nearest neighbor (KNN), cyclic subspace regression (CSR), and library searching are compared for computerized plastic classification. Plastics studied consist of polyethylene terephthalate, high-density polyethylene, polyvinyl chloride, low-density polyethylene, polypropylene, and polystyrene. With principal component analysis (PCA), visual distinction between the different plastics becomes possible. Correct class membership to all six plastic types is provided by KNN. To date, all development and uses of CSR have been based on building models for each prediction property analogous to the form of partial least-squares known as PLS1. Cyclic subspace regression is modified in this paper to also allow modeling of multiple properties, as does PLS2. The new form of CSR was able to correctly classify all six plastic types when seven-factor models were used. This paper reports that key observations made in comparing PCR to PLS1 are verified for the interrelationships of PCR and PLS2 models. Most notable is that even though PLS2 uses spectral responses and plastic identifications to form factors, PLS2 eigenvector weights are not much different from PCR eigenvector weights where PCR only uses spectral responses to form eigenvector weights. Library searching showed less significant results than KNN and CSR. Regardless of the identification approach, polyethylene samples could be identified as either being high density or low density with the use of Raman spectroscopy.

Increasing PCB Radiolysis Rates in Transformer Oil

Abstract The kinetics of Aroclor 1242 radiolysis in transformer oil, using high-energy electrons, was found to be analogous to that previously measured for individual polychlorinated biphenyl (PCB) congeners irradiated with γ -rays. The plot of the pseudo-first-order rate constant for PCB decomposition versus initial PCB concentration is a power function, with high rate constants for low concentrations. The addition of alkaline isopropanol to transformer oil was found to increase the pseudo-first-order rate constant for PCB decomposition. The rate constant under these conditions is independent of concentration. This may be explained by the establishment of chain reaction dechlorination in the oil.

A high-yield synthesis of chalcopyrite CuInS 2 nanoparticles with exceptional size control

We report high-yield and efficient size-controlled syntheses of Chalcopyrite CuInS2nanoparticles by decomposing molecular single source precursors (SSPs) via microwave irradiation in the presence of 1, 2-ethanedithiol at reaction temperatures as low as 100°C and times as short as 30 minutes. The nanoparticles sizes were 1.8nm to 10.8 nm as reaction temperatures were varied from 100°C to 200°C with the bandgaps from 2.71 eV to 1.28 eV with good size control and high yields (64%-95%). The resulting nanoparticles were analyzed by XRD, UV-Vis, ICP-OES, XPS, SEM, EDS, and HRTEM. Titration studies by 1H NMR using SSP 1 with 1, 2-ethanedithiol and benzyl mercaptan were conducted to elucidate the formation of Chalcopyrite CuInS2nanoparticles.

A Large-Scale Synthesis and Characterization of Quaternary CuIn x Ga 1− x S 2 Chalcopyrite Nanoparticles via Microwave Batch Reactions

Various quaternary CuInxGa1−xS2 (0≤x≤1) chalcopyrite nanoparticles have been prepared from molecular single-source precursors via microwave decomposition. We were able to control the nanoparticle size, phase, stoichiometry, and solubility. Depending on the choice of surface modifiers used, we were able to tune the solubility of the resulting nanoparticles. This method has been used to generate up to 5 g of nanoparticles and up to 150 g from multiple batch reactions with excellent reproducibility. Data from UV-Vis, photoluminescence, X-ray diffraction, TEM, DSC/TGA-MS, and ICP-OES analyses have shown high reproducibility in nanoparticle size, composition, and bandgap.

Coherent Raman Spectroscopic Monitoring of Pulsed Radio Frequency PECVD of Silicon Nitride Thin Films

During pulsed plasma enhanced chemical vapor deposition, (PECVD), of silicon nitride thin films, depletion of silane reactant was measured by coherent anti-Stokes Raman scattering, (CARS), spectroscopy as a function of radio frequency pulse width, peak power, and delay time after the rf pulse. The results were correlated with “goodness of deposition parameters” including film thickness, deposition rate, and N-H and Si-H film content. The pulse width and peak power affected the plasma similarly, as silane depletion, film thickness, and rate of film growth all increased with both pulse width and peak power for a 10 Hz repetition rate. The CARS measured silane depletion also increased proportionally with pulse width for short rf pulses but not for long ones. Although the film properties changed with both power and pulse width, there were differences in the effects. A decreasing SiH/NH ratio resulted from increasing peak power, but increases in pulse width lead to an increasing ratio in some cases. The delay studies showed the CARS-measured silane depletion was higher 2 ms after the rf pulse had ended than 0.5 ms within the pulse. This result was accounted for by the flow rate, shower head design, and placement of the focus of the laser beams. Based on the results of the CARS-measured silane depletion as a function of pulse width and delay time, it is evident that the reactants move in a segregated-flow with a significant proportion of the velocity component directed toward the lower electrode, at least for the conditions of this experiment. Thus differences noted between pulsed and continuous PECVD processing likely depend largely on the transit time of the molecules in the plasma region.

Fabrication and characterization of thin film solar cell made from CuIn 0.75 Ga 0.25 S 2 wurtzite nanoparticles

CuIn0.75Ga0.25S2 (CIGS) thin film solar cells have been successfully fabricated using CIGS Wurtzite phase nanoparticles for the first time. The structure of the cell is Glass/Mo/CIGS/CdS/ZnO/ZnO:Al/Ag. The light absorption layer is made from CIGS Wurtzite phase nanoparticles that are formed from single-source precursors through a microwave irradiation. The Wurtzite phase nanoparticles were converted to Chalcopyrite phase film through a single-step annealing process in the presence of argon and sulfur at 450°C. The solar cell made from Wurtzite phase nanoparticles showed 1.6% efficiency and 0.42 fill factor.

Point defect characterization in CdZnTe

Measurements of the defect levels and performance testing of CdZnTe detectors were performed by means of Current Deep Level Transient Spectroscopy (I-DLTS), Transient Charge Technique (TCT), Current versus Voltage measurements (I–V), and gamma-ray spectroscopy. CdZnTe crystals were acquired from different commercial vendors and characterized for their point defects. I-DLTS studies included measurements of defect parameters such as energy levels in the band gap, carrier capture cross sections, and defect densities. The induced current due to laser-generated carriers was measured using TCT. The data were used to determine the transport properties of the detectors under study. A good correlation was found between the point defects in the detectors and their performance.

GeS2 and GeSe2 PECVD from GeCl4 and Various Chalcogenide Precursors

The plasma enhanced chemical vapor depositions of germanium chalcogenide thin films from germanium tetrachloride, hydrogen sulfide and alkyl chalcogenides were studied to determine the viability of these reagents for thin film deposition. Hydrogen sulfide is a commonly used reagent for this technique and was used to determine optimal reaction conditions for thin film deposition. Germanium tetrachloride, alkylsulfides and alkylselenides were also employed because of their lower potential toxicities and higher availabilities compared to their more typical congeners: germane, hydrogen sulfide and hydrogen selenide in the formation of germanium chalcogenides. Alkylsulfides were found to be unsuitable for the deposition of germanium sulfides, however alkylselenide precursors were used successfully for the deposition of germanium selenides. The relative mass flow rates, reactor pressure, substrate temperature and plasma power density were studied for their effects on germanium chalcogenide deposition. These parameters affected the composition, deposition rate, film quality, and spectroscopic properties of the deposited films.

Defect measurements of CdZnTe detectors using I-DLTS, TCT, I-V, C-V and γ-ray spectroscopy

In this work we measured the crystal defect levels and tested the performance of CdZnTe detectors by diverse methodologies, viz., Current Deep Level Transient Spectroscopy (I-DLTS), Transient Current Technique (TCT), Current and Capacitance versus Voltage measurements (I-V and C-V), and gamma-ray spectroscopy. Two important characteristics of I-DLTS technique for advancing this research are (1) it is applicable for high-resistivity materials (>106 Ω-cm), and, (2) the minimum temperature for measurements can be as low as 10 K. Such low-temperature capability is excellent for obtaining measurements at shallow levels. We acquired CdZnTe crystals grown by different techniques from two different vendors and characterized them for point defects and their response to photons. I-DLTS studies encompassed measuring the parameters of the defects, such as the energy levels in the band gap, the carrier capture cross-sections and their densities. The current induced by the laser-generated carriers and the charge collected (or number of electrons collected) were obtained using TCT that also provides the transport properties, such as the carrier life time and mobility of the detectors under study. The detectors electrical characteristics were explored, and its performance tested using I-V, C-V and gamma-ray spectroscopy.

Incorporation of Novel Nanostructured Materials into Solar Cells and Nanoelectronic Devices

Each of the investigators on this project has had significant accomplishments toward the production of semiconductor nanoparticles, particles, and thin films and attempts to incorporate these materials into photovoltaics or sensors; to use them for improving fluorescence diagnostics; or to employ them as cancer fighting agents. The synthesis and characterization of the nanomaterials, and more recently the device construction and testing of these materials, have been the subject of several publications and presentations by team members. During the course of the investigations, several students were fully involved as part of their graduate and undergraduate training. The nature of these projects in material development dictates that the students have gained significant experience in a diverse array of material-related topics.