Light-induced metastability of amorphous/microcrystalline (micromorph) silicon tandem solar cell, in which the microcrystalline bottom cell was deposited in a single-chamber system, has been studied under a white light for more than 1000 hours. Two different light-induced metastable behaviors were observed. The first type was the conventional light-induced degradation, where the open-circuit voltage (Voc ), fill factor (FF), and short-circuit current density (Jsc ) were degraded, hence the efficiency was degraded as well. This phenomenon was observed mainly in the tandem cells with a bottom cell limited current mismatch. The second type was with a light-induced increase in Voc , which sometimes resulted in an increase in efficiency. The second type of light-induced metastability was observed in the tandem cells with a top cell limited current mismatch. The possible mechanisms for these phenomena are discussed.

Polyaniline is one of the most studied conducting polymers and its properties have been used in many sensor applications including gas sensors and pH sensors. In this work, we present a method that is able to embed a conducting polyaniline film in a desired pattern onto a nonconducting polymer matrix. We have developed an embedded polyaniline film on a polydimethylsiloxane (PDMS) matrix using a cast molding technique. The polyaniline film was grown by electrochemically polymerizing polyaniline from an electrolyte containing sulfuric acid and aniline monomer. A three-electrode cell system was used with a standard Ag/AgCl reference electrode, a gold auxiliary electrode, and a patterned gold working electrode. After the polyaniline film was formed on the patterned working electrode, PDMS was poured atop and cured. Cured PDMS was then debonded along with the polyaniline film embedded on the PDMS layer or block. The gold layer of the working electrode can be patterned using a photolithography steps so that the designed pattern can be transferred by the polyaniline growth on the working electrode. The greenish color of the polyaniline film indicated that the embedded polyaniline was in the conducting state (emeraldine salt form) and the conductivity of the polyaniline has also been verified by applying a DC voltage across the film and measuring the current through the polyaniline layer. The measured conductivity of the PANi layer at room temperature was in the range of 4.5 ~ 4.6 S/m. The advantage of this technique is that the PDMS matrix holds the polyaniline in place while allowing the polyaniline film to undergo reversible doping/dedoping chemistry when electrolyte solution comes into contact with the surface of the film. The developed technology can be used for various chemical sensor applications, for example, a pH sensor or gas sensors where a flexible and all-polymer apparatus is needed.

Graphene grown by Chemical Vapor Deposition (CVD) on nickel subsrate is oxidized by means of oxygen plasma and UV/Ozone treatments to introduce bandgap opening in graphene. The degree of band gap opening is proportional to the degree of oxidation on the graphene. This result is analyzed and confirmed by Scanning Tunnelling Microscopy/Spectroscopy and Raman spectroscopy measurements. Compared to conventional wet-oxidation methods, oxygen plasma and UV/Ozone treatments do not require harsh chemicals to perform, allow faster oxidation rates, and enable site-specific oxidation. These features make oxygen plasma and UV/Ozone treatments ideal candidates to be implemented in high-throughput fabrication of graphene-based microelectronics.

The Density Functional Theory has been used to analyze an inter-granular segregation of Cu and Mg. The stability of Cu and Mg atoms in the aluminum matrix, intermetallic phases and symmetric twist grain boundaries has been compared. The quantitative description of solubility of Cu and Mg atoms in the nano-crystalline aluminum has been proposed. The calculations have been carried out to investigate the properties of symmetric twist boundaries in aluminum with and without Cu/Mg atoms. The phenomena of are discussed and its effect on the stability of precipitates containing these elements.

In this work, we report the characterization of Spin-On Glass (SOG) as low temperature gate insulator. Our SOG film was deposited at temperature of 200°C, which is compatible to use on flexible substrates. The optical and electrical characterization showed that the refractive index and dielectric constant are very close to those of thermally grown SiO2. Also, analysis of surface roughness by AFM is presented. We demonstrated the use of SOG as gate insulator, fabricating and characterizing inverted staggered a-SiGe:H TFTs. The observed results are promising and suggest that SOG films deposited at 200°C in the Laboratory of Microelectronics of INAOE could be an alternative to improve electrical characteristics of TFTs on low temperature flexible substrates.

The surrounding ambient introduces a gaseous boundary to many nanotechnology applications such as nanosensors, nanoelectromechanical systems and nanocoatings. Despite the large surface area to volume ratio of nanostructures, a formal study of the surface scattering effects induced by a gaseous boundary has received little attention. In this work, we consider the perturbing effects to the electron cloud or jellium of conducting nanostructures when submitted to a gaseous interface of varying interaction energies. Specifically, we incorporate the novel experimental method of Dynamic Electron Scattering (DES) to measure electronic thermal conductivity of 30 nm thick Au and Cu metal films in He and Ar atmospheres. The gas particle impact energy is varied by changing the flow speed from stationary (non-moving gas field) to high speed flow over the metal films. The scattering effects of each gas are clearly observable through electronic thermal conductivity reductions as the gas impact energy increases. We find the high collision density of He to induce greater reductions in thermal conductivity than the much heavier Ar with lower collision density. The perturbed transport properties of the Au and Cu thin films are explained by kinetic surface scattering mechanisms that dominate the scattering landscape of high surface area to volume ratio materials as suggested by comparative measurements on bulk Cu.

Significant improvement of mechanical properties was observed recently in graphene platelet-epoxy nanocomposites relative to unfilled epoxy, such as an increase of the fracture toughness by 50% and dramatic decrease of fatigue crack growth rate. In this work, thin films of 0.1 wt.% of graphene platelet (GPL) – epoxy nanocomposites were fabricated and the nanoscale mechanical properties of the nanocomposite were investigated by nanoindentation. This provides information about the presence of characteristic length scales induced by the microstructure and the strength of the filler-matrix interface.

LiFePO4 has shown considerable promise as a cathode material in Li-ion batteries due to its stability, low toxicity and high cyclability. However, the data on thermodynamic stability of olivine phase FePO4 (o-FePO4), the delithiated form of o-LiFePO4, remains scarce and contradictory. In this work, o-FePO4 was synthesized by chemical delithiation of o-LiFePO4 and characterized structurally and thermally. X-ray diffraction and absorption data indicate pure olivine phase, but with residual amount of Fe2+, most likely due to incomplete delithiation. Differential scanning calorimetry and thermal gravimetric analysis reveal that o-LixFePO4 decomposes exothermally above 550 °C with about 9% weight loss, the products being trigonal phase FePO4, Fe7(PO4)6, and LiPO3.

We report results from an experimental and theoretical study of the room temperature (RT) compression of the ternary alloy Ti-6Al-4V. In this work, we have extended knowledge of the equation of state (EOS) from 40 GPa to 221 GPa, and observed a different sequence of phase transitions to that reported previously for pure Ti.

This paper reports the development of micromachining processes, as well as electrical and mechanical evaluation of a stimuli-responsive, mechanically-dynamic polymer nanocomposite for biomedical microsystems. The nanocomposite, which consists of a cellulose nanofiber network embedded in a poly(vinyl acetate) matrix, was shown to display a switchable stiffness comparable to bulk samples, with a Young’s modulus of 3570 MPa in the dry state, which reduced to ~25 MPa in the wet state, with a stiff-to-flexible transition-time dependent on exposed surface area. Upon immersion in phosphate buffered saline, the ac resistance through the PVAc-TW thickness was found to reduce from 8.04 MΩ to ~17 kΩ. Electrochemical impedance of an Au electrode on PVAc-TW was found to be ~178 kΩ at 1 kHz, and this was found to be stable as the probe shank was flexed to compress the metal, but increased with increasing flex angle when the metal was flexed into a tensile state.

Multiferroics, the study of materials which possess ferromagnetic and ferroelectric ordering in a single phase, has become an area of prominent research. Moreover, this behavior has been extensively studied in materials which possess a perovskite crystal structure such as BiFeO3 and YMnO3. Due to their weak saturation magnetic moment, many rare-earth orthoferrites are currently of extreme interest. Utilizing a solid-state reaction between Y2O3 and Fe2O3 we have developed the rare-earth orthoferrite YFeO3 and conducted a bulk material study to determine this material’s availability for thin film multiferroic research. The absence of Y2O3 and Fe2O3 impurities was confirmed using Copper-Kα XRD. Examination of the dependence of the magnetization M on the temperature T was conducted to determine the reliability of multiferroic behavior across varying temperatures in conjunction with the investigation of the dependence of M on the electric field strength H. Results clearly display ferromagnetic behavior in our bulk material, providing ample evidence that our bulk material is an excellent candidate for thin film studies. Future studies on multiferroic YFeO3 thin films grown via pulsed laser deposition on Lanthanum Aluminate substrates will be conducted. Detailed data will be provided via XRD and SQUID to confirm magnetic properties while impurities are non-existent in our thin films.

In order to realize the magnetic refrigeration system, it is necessary to develop a 100 W class refrigerator with COP > 7.5. This requires us to find new magnetic refrigerant materials, of which cooling capacity is 2.5 times higher than that of Gd. In this paper, first we discuss the cooling capacity of magnetic refrigerant materials to achieve COP = 7.5. Then, we compare the experimental results of MnAsSb, MnFe(PGe) and La(FeCoSi)13 compounds with the calculated cooling capacity. It is suggested that a composite layer material of MnFe(PGe) would show excellent cooling capacity in the temperature span of 20 K.

Recently, a wide range of new applications of diamond materials such as spintronics, field emission, and bio-sensing have been proposed. These applications often require the precise patterning of diamonds, which is not trivial because diamonds are the hardest materials known in nature. Among various patterning techniques, the focused ion beam milling method has been proven to provide flexibility as well as high resolution in the pattern design. In this study, a focused beam of 30 kV Ga+ ions was utilized to create sub-micrometer size patterns out of crystalline diamonds. The sputtering rate, re-deposition, and surface roughening of diamond structure have been closely monitored with various milling parameters during the milling process. Our study revealed a low milling yield of 0.02 μm3/nC, high Ga content re-deposition, and the formation of sub-micron scale terracing on the sidewall of patterned diamonds.

A large-area graphene layer identification technique was developed for research and industrial applications. It is based on the analysis of optical microscopy images using computational image processing algorithms. The initial calibration is performed with the micro-Raman spectroscopy. The method can be applied to the wafer-scale graphene samples. The technique has the potential to be the gateway in the development of fully automated statistical process control methods for the next generation thin-film materials used by the semiconductor industry. The proposed technique can be applied to graphene on arbitrary substrates and used for other atomically thin materials.

Electron spin resonance study of low-κ insulating layers reveals that from a defect perspective these materials resemble oxygen-rich silicon dioxide matrices. The films fabricated using chemical vapor deposition in combination with porogen technology also contain a considerable amount of residual carbon in the form of clusters. Furthermore, ion sputtering damage generates additional defects provisionally identified as dangling bonds in the silicon oxycarbide clusters. The density of these defects is found to increase with increasing porosity of the low-κ insulator. Nevertheless, a lower defect density may be attained if using a porogen-free self-assembly technology.

CZTS (Copper zinc tin sulfide) thin films have been synthesized on transparent conducting oxide (TCO) back contacts on a glass substrate, allowing sun light to pass through the entire solar cell. Aqueous solution based co-electrodeposition followed by elevated temperature sulfurization, was used to grow CZTS on transparent fluorinated tin oxide. Loss in conductivity of FTO is observed after sulfurization, causing reduced device efficiency. Increased resistivity of the FTO is likely due to outdiffusion process. A systematic study of resistivity of back contact at various sulfurization temperatures and times is discussed. Various remedial measures for improved conductivity of back contact were proposed and conducted.

Cahn-Hilliard type of phase-field (PF) model coupled with elasticity equations is used to study the instabilities in multilayer thin films. The governing equations of the solid state phase transformation include a 4th order partial differential equation representing the evolution of the conserved PF variable (concentration) coupled to 2nd order partial differential equations representing the mechanical equilibrium. A mixed order Galerkin finite element (FE) model is used including C0 interpolation functions for the displacement, and C1 interpolation functions for the concentration. It is shown that quadratic convergence, expected for conforming elements, is achieved from this coupled mixed-order FE model.

Using the PF – FE model, first, we studied the effect of compositional strain on the PF interface thickness and the results of simulations are compared with the analytical solutions of an infinite thin film diffusion couple with a flat interface.

Morphological instabilities in binary multilayer thin films are investigated. The alloys with and without intermediate phase are considered, as well as the cases with stable and metastable intermediate phase. Maps of transformations in multilayer systems are carried out considering the effects of initial thickness of layers, compositional strain, and growth of a stable/unstable intermediate phase on the instability of the multilayer thin films. It is shown that at some cases phase transformation, intermediate phase nucleation and growth, or deformation of layers due to high compositional strain can lead to the coarsening of the layers which can result in different mechanical and materials behaviors of the original designed multilayer.