Matching Items (4)

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Growth and characterization of pyrite thin films for photovoltaic applications

Description

A series of pyrite thin films were synthesized using a novel sequential evaporation

technique to study the effects of substrate temperature on deposition rate and micro-structure of

the deposited material. Pyrite was

A series of pyrite thin films were synthesized using a novel sequential evaporation

technique to study the effects of substrate temperature on deposition rate and micro-structure of

the deposited material. Pyrite was deposited in a monolayer-by-monolayer fashion using

sequential evaporation of Fe under high vacuum, followed by sulfidation at high S pressures

(typically > 1 mTorr to 1 Torr). Thin films were synthesized using two different growth processes; a

one-step process in which a constant growth temperature is maintained throughout growth, and a

three-step process in which an initial low temperature seed layer is deposited, followed by a high

temperature layer, and then finished with a low temperature capping layer. Analysis methods to

analyze the properties of the films included Glancing Angle X-Ray Diffraction (GAXRD),

Rutherford Back-scattering Spectroscopy (RBS), Transmission Electron Microscopy (TEM),

Secondary Ion Mass Spectroscopy (SIMS), 2-point IV measurements, and Hall effect

measurements. Our results show that crystallinity of the pyrite thin film improves and grain size

increases with increasing substrate temperature. The sticking coefficient of Fe was found to

increase with increasing growth temperature, indicating that the Fe incorporation into the growing

film is a thermally activated process.

Contributors

Agent

Created

Date Created
  • 2014

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Determining carrier mobilities in GaAs and natural pyrite using geometrical magnetoresistance measurement

Description

Measurements of the geometrical magnetoresistance of a conventional semiconductor, gallium arsenide (GaAs), and a more recently developed semiconductor, iron pyrite (FeS2) were measured in the Corbino disc geometry as a

Measurements of the geometrical magnetoresistance of a conventional semiconductor, gallium arsenide (GaAs), and a more recently developed semiconductor, iron pyrite (FeS2) were measured in the Corbino disc geometry as a function of magnetic field to determine the carrier mobility (μm). These results were compared with measurements of the Hall mobility (μH) made in the Van der Pauw configuration. The scattering coefficient (ξ), defined as the ratio between magnetoresistance and Hall mobility (μm/μH), was determined experimentally for GaAs and natural pyrite from 300 K to 4.2 K. The effect of contact resistance and heating on the measurement accuracy is discussed.

Contributors

Agent

Created

Date Created
  • 2016

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Development of a co-deposition method for deposition of low-contamination pyrite thin films

Description

Pyrite is a 0.95 eV bandgap semiconductor which is purported to have great potential in widespread, low–cost photovoltaic cells. A thorough material selection process was used in the design of

Pyrite is a 0.95 eV bandgap semiconductor which is purported to have great potential in widespread, low–cost photovoltaic cells. A thorough material selection process was used in the design of a pyrite sequential vapor deposition chamber aimed at reducing and possibly eliminating contamination during thin film growth. The design process focused on identifying materials that do not produce volatile components when exposed to high temperatures and high sulfur pressures. Once the materials were identified and design was completed, the ultra–high vacuum growth system was constructed and tested.

Pyrite thin films were deposited using the upgraded sequential vapor deposition chamber by varying the substrate temperature from 250°C to 420°C during deposition, keeping sulfur pressure constant at 1 Torr. Secondary Ion Mass Spectrometry (SIMS) results showed that all contaminants in the films were reduced in concentration by orders of magnitude from those grown with the previous system. Characterization techniques of Rutherford Back–scattering Spectrometry (RBS), X–Ray Diffraction (XRD), Raman Spectroscopy, Optical Profilometry and UV/Vis/Near–IR Spectroscopy were performed on the deposited thin films. The results indicate that stoichiometric ratio of S:Fe, structural–quality (epitaxy), optical roughness and percentage of pyrite in the deposited thin films improve with increase in deposition temperature. A Tauc plot of the optical measurements indicates that the pyrite thin films have a bandgap of 0.94 eV.

Contributors

Agent

Created

Date Created
  • 2016

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Unearthing the antibacterial activity of a natural clay deposit

Description

The discovery and development of novel antibacterial agents is essential to address the rising health concern over antibiotic resistant bacteria. This research investigated the antibacterial activity of a natural clay

The discovery and development of novel antibacterial agents is essential to address the rising health concern over antibiotic resistant bacteria. This research investigated the antibacterial activity of a natural clay deposit near Crater Lake, Oregon, that is effective at killing antibiotic resistant human pathogens. The primary rock types in the deposit are andesitic pyroclastic materials, which have been hydrothermally altered into argillic clay zones. High-sulfidation (acidic) alteration produced clay zones with elevated pyrite (18%), illite-smectite (I-S) (70% illite), elemental sulfur, kaolinite and carbonates. Low-sulfidation alteration at neutral pH generated clay zones with lower pyrite concentrations pyrite (4-6%), the mixed-layered I-S clay rectorite (R1, I-S) and quartz.

Antibacterial susceptibility testing reveals that hydrated clays containing pyrite and I-S are effective at killing (100%) of the model pathogens tested (E. coli and S. epidermidis) when pH (< 4.2) and Eh (> 450 mV) promote pyrite oxidation and mineral dissolution, releasing > 1 mM concentrations of Fe2+, Fe3+ and Al3+. However, certain oxidized clay zones containing no pyrite still inhibited bacterial growth. These clays buffered solutions to low pH (< 4.7) and oxidizing Eh (> 400 mV) conditions, releasing lower amounts (< 1 mM) of Fe and Al. The presence of carbonate in the clays eliminated antibacterial activity due to increases in pH, which lower pyrite oxidation and mineral dissolution rates.

The antibacterial mechanism of these natural clays was explored using metal toxicity and genetic assays, along with advanced bioimaging techniques. Antibacterial clays provide a continuous reservoir of Fe2+, Fe3+ and Al3+ that synergistically attack pathogens while generating hydrogen peroxide (H2O¬2). Results show that dissolved Fe2+ and Al3+ are adsorbed to bacterial envelopes, causing protein misfolding and oxidation in the outer membrane. Only Fe2+ is taken up by the cells, generating oxidative stress that damages DNA and proteins. Excess Fe2+ oxidizes inside the cell and precipitates Fe3+-oxides, marking the sites of hydroxyl radical (•OH) generation. Recognition of this novel geochemical antibacterial process should inform designs of new mineral based antibacterial agents and could provide a new economic industry for such clays.

Contributors

Agent

Created

Date Created
  • 2015