The contact resistivity, ρ c, between phase change material (PCM) and an electrode plays an important role in the operation of highly scaled phase change random access memory (PCRAM). We investigated the effect of surface cleaning on the ρ c between a W electrode and amorphous GeCu2Te3 (GCT) which shows high thermal stability. The surface cleaning of the amorphous GCT was conducted by Ar reverse sputtering. The ρ c of the amorphous GCT whose surface was cleaned with Ar reverse sputtering was 6.7×10-3Ω cm2. Meanwhile, the ρ c of the amorphous GCT with no surface cleaning was 8.0×10-5Ω cm2. The low ρ c in the amorphous GCT with no surface cleaning was apparently due to the existence of a low resistance Cu-rich underlayer which was formed as a consequence of surface oxidation of the amorphous GCT. These results indicate that the surface of a PCM must be treated carefully to accurately measure the contact resistivity between the PCM and electrodes.

Surface smoothing of Ru used as underlayer of magnetic tunneling junctions (MTJ)in magneto-resistive random access memory (MRAM) was carried out with gascluster ion beam (GCIB) in order to improve device characteristics. For Rufilms, surface smoothing with 5 kV N2-GCIB irradiation was effective,and CoFe films deposited on smoothed Ru surface also showed smooth surface. Fromthe hysteresis loop measurements of MTJ formed on smoothed Ru withN2-GCIB, it showed improvement of inter-layer coupling magnetic field(Hin) with decreasing the surface roughness of underlayer Ru. Itis expected that surface roughness of MgO in MTJ was also improved by smoothingof underlayer Ru with N2-GCIB.

The inclusion of a spinel structure in the layered-layered composite cathode material is currently explored to enhance the cycling stability and electrochemical properties of lithium ion batteries. Li2MnO3 based composite cathodes are one of the most widely investigated positive electrodes due to their high discharge capacity and rate capability. In our studies, we have synthesized the cobalt-free layered-layered-spinel composite cathode material, 0.5Li2MnO3-0.25LiMn2O4-0.25LiNi0.5Mn0.5O2 (LLNMO), via the sol-gel method. The structure of the composition was characterized using XRD and Raman Spectroscopy in which peaks corresponding to the layered and spinel structures were identified. The morphology along with the elemental analysis were studied with SEM/EDX. The SEM images exhibited agglomerates with particle size in the nano range and the EDX analysis confirmed the presence of manganese, nickel and oxygen in the structure. The electrochemical performance was analyzed by charge/discharge studies (CD) and cyclic voltammetry (CV). The composite cathode material showed high capacity retention and good cycle stability with a coulombic efficiency of 98%. The discussed results demonstrated that LLNMO is a promising cathode material for the next generation of Li-ion batteries.

Surfactants are commonly used as additives in cleaning formulations during acoustic cleaning of semiconductor surfaces. Since surfactants are surface active, they can affect cavitation characteristics and, therefore, influence cleaning efficiency and damage to the surface. In this work, stable and transient cavitation pressures were characterized in Triton® X-100 containing aqueous solutions subjected to single frequency and dual-frequency sound fields using acoustic emission measurements. The hydrophone based technique allowed quantitative measurements of the absolute pressure values from stable and transient cavitation fields. The ratio of stable cavitation pressure to transient cavitation pressure under different conditions indicated that surfactants can play an important role in reducing feature damage while maintaining particle removal efficiency. Damage studies conducted on aluminum coated glass samples further confirmed these results.

In this study, the initial AlN layer and the vertical-direction breakdown voltage (VDBV) of AlGaN/GaN high-electron-mobility transistors (HEMTs) were characterized. Prior to the formation of the interface between the AlN layer and the Si substrate, only trimethylaluminum (TMA) was introduced without ammonia to control the crystal quality of initial AlN layer (TMA preflow). HEMT structures were simultaneously grown on identical AlN layers on Si substrates (AlN/Si templates) grown using different TMA preflow temperatures. The density of screw- or mixed-type dislocations in the initial AlN layer decreased as the TMA preflow temperature increased. Further, the VDBV of the HEMT structure increased as the TMA preflow temperature increased. It is supposed that the screw- or mixed-type dislocations are the possible source of the vertical leakage current in the HEMT structures. The improvement in the crystal quality of the initial AlN layer affects the increase in the VDBV of the AlGaN/GaN HEMTs on Si substrates.

Magnetic core shell nanoparticles (NPs) have potential for applications in magnetic random access memory, spintronic devices, and drug delivery systems. Our investigations are focused on the synthesis of inverted core shell nanoparticles and characterization of their structural and magnetic properties. By using our hydrothermal nanophase epitaxy technique, we are able to synthesize well-ordered α-Cr2O3@α-MxCr2-xO3 (M = Co, Ni, Mn, Fe) inverted core-shell nanoparticles. This typically results in the formation of novel phases of MxCr2-xO3 shells having ferromagnetic/ferrimagnetic (FM/FiM) spin ordering and an antiferromagnetic (AFM) Cr2O3 core structure. The combined results from XRD and high-resolution TEM (HRTEM) provide evidence of the presence of corundum phase both in the shell and in the core regions. HRTEM results also show a sharp interface exhibiting epitaxial atomic registry of shell atoms over highly ordered core atoms whereas TEM-EDX analyses show that the M atoms reside predominantly in the shell regions. The XPS analyses of the NPs indicate the M transition metals incorporated in the shell are in the +2 oxidation state. Magnetic measurements show well developed hysteresis loops: The field cooled hysteresis loops reveal horizontal shifts in the applied field axis and vertical shifts in the magnetization axis, relative to the zero-field cooled hysteresis loops. This provides direct evidence for the exchange bias effect between the AFM α-Cr2O3 core and the FM/FiM α-MxCr2-xO3 shell. The XPS data are consistent with oxygen vacancy formation in order to maintain charge neutrality upon substitution of the M2+ ion for the Cr3+ ion in the α-MxCr2-xO3 shell. The FM/FiM ordering in the shell may at least partially result from the F-center exchange coupling between the oxygen-vacancy induced bound magnetic polaron and nearby cations.

Niobium-doped TiO2 films as highly transparent conducting oxidic electrodes were prepared by reactive magnetron sputtering from a titanium target in an argon-oxygen gas flow. As-deposited films were amorphous and exhibited high resistivities ranging from 10 to 1×105 Ω cm in dependence on the deposition parameters. We stabilized the reactive magnetron sputtering deposition by adjusting the magnetron discharge voltage at a constant oxygen gas flow. The precise process control during the preparation of the as-deposited films was essential to gain low resistivities (10-3 Ω cm) and low optical absorption coefficients (α550nm < 2×103 cm-1) after annealing. These polycrystalline TiO2:Nb films on borosilicate glass show a quite high electron concentration > 1×1020 cm-3 and a high carrier mobility (≈ 8 cm2 V-1 s-1).

Periodic mesoporous materials possess high surface to volume ratio and nano-scale sized pores, making them potential candidates for heterogeneous catalysis, ion exchange, gas sensing and other applications. In this study, we use in situ small angle x-ray scattering (SAXS) and molecular dynamics (MD) simulations to investigate the mechanical and hydrothermal stability properties of periodic mesoporous SBA-15 silica and SBA-15 type aluminosilica (Al-SBA-15) to extreme conditions. The mesoporous SBA-15 silica and Al-SBA-15 aluminosilica possess amorphous frameworks and have similar pore size distribution (pore size ∼9-10 nm). The in situ SAXS measurements were made at the B1 beamline, at the Cornell High Energy Synchrotron Source (CHESS). The mesoporous SBA-15 silica and Al-SBA-15 aluminosilica specimens were loaded in a diamond anvil cell (DAC) for pressure measurements, and, separately, with water in the DAC for hydrothermal measurements to high P-T conditions (to 255 °C and ∼ 114 MPa). Analyses of the pressure-dependent SAXS data show that the mesoporous Al-SBA-15 aluminosilica is substantially more mechanically stable than the SBA-15 silica. Hydrothermal measurements show a small net swelling of the framework at elevated P-T conditions, due to dissolution of water into the pore walls. Under elevated P-T conditions, the Al-SBA-15 aluminosilica shows significantly greater hydrothermal stability than the SBA-15 silica. Our MD simulations show that the bulk modulus value of periodic mesoporous SBA-15 silica varies exponentially with percentage porosity. Molecular dynamics simulations are being made in order to better understand how the pore architecture and the chemical composition of the host structure govern the stability properties of the mesoporous materials.

We have used reverse nonequlibrium molecular dynamics modeling to study the impact of uniaxial stretching on the thermal conductivity of monolayer molybdenum disulfide (MoS2) and hexagonal boron nitride (h-BN). Our results predict an anomalous response of the thermal conductivity of these materials to normal strain. Thermal conductivity of h-BN increases under a tensile strain whereas thermal conductivity of MoS2 remains fairly constant. These are in striking contrast to the impact of tensile strain on the thermal conductivity of three dimensional materials whose thermal conductivity decreases under tensile strain. We investigate the mechanism responsible for this unexpected behavior by studying the impact of tensile strain on the phonon dispersion curves and group velocities of these materials.

We demonstrate Ag-free transparent OLED (TOLED) fabricated by organic vapor phasedeposition (OVPD) using thin Au contacts. Three types of TOLED devices have beenstudied. The first one has been deposited on ITO substrates to compare thin Agand Au films as top cathodes. A 6-fold increase in operational lifetime(LT50, 4 mA/cm2) from 27 h to 172 h can be observedwhen replacing Ag by Au while maintaining similar electro-opticalcharacteristics. Furthermore, a second type of TOLED on thin Au films, replacingITO and suppressing laterally guided modes [1], has been studied. TOLED on ITOsubstrates and on thin Au films exhibit very low onset voltages of 2.2 V. Bothtypes show about 30% transparency in the VIS light region and emit orange lightwith a peak wavelength of 608 nm from either side with a total EQE of about 9%(measured at 1000 cd/m2 in sum). The third type of TOLED wasfabricated with an inverted structure, with the aim to further increaseoperational lifetime by burying the reactive LiF/Al electron injection layer(EIL). This will make the EIL less accessible for oxygen and moisture. Ourresults show difficulties in electron injection when depositing the organicstack on Al/LiF, which may be attributed to an insufficient thermal activationof the EIL.

A simple model to describe the fundamental absorption of amorphous hydrogenated silicon carbide thin films based on band fluctuations is presented. It provides a general equation describing both the Urbach and Tauc regions in the absorption spectrum. In principle, our model is applicable to any amorphous material and it allows the determination of the bandgap. Here we focus on the bandgap engineering of amorphous hydrogenated silicon carbide layers. Emphasis is given on the role of hydrogen dilution during the deposition process and post deposition annealing treatments. Using the conventional Urbach and Tauc equations, it was found that an increase/decrease of the Urbach energy produces a shrink/enhancement of the Tauc-gap. On the contrary, the here proposed model provides a bandgap energy which behaves independently of the Urbach energy.

The nonlinear property of SiC multilayer devices under Ultra Violet (UV) irradiation is used to design an optical processor for indoor positioning. The transducers combine the simultaneous demultiplexing operation with the photodetection and self-amplification. Moreover, we present a way to achieve indoor positioning using the parity bits and the navigation syndrome. A 4 bit representation with the original string colour message and the transmitted 7 bit string, the encoding and decoding accurate positional information processes and the design of SiC navigation syndrome generators are discussed. The visible multilateration method estimates the device’s position by using the MUX signal received from several, non-collinear transmitters. The location and motion information is found by mapping position and estimates the location areas.

Recent studies on graphene hydrogenation processes showed that hydrogenation occurs via island growing domains, however how the substrate can affect the hydrogenation dynamics and/or pattern formation has not been yet properly investigated. In this work we have addressed these issues through fully atomistic reactive molecular dynamics simulations. We investigated the structural and dynamical aspects of the hydrogenation of graphene membranes (one-side hydrogenation, the so called graphone structure) on different substrates (graphene, few-layers graphene, graphite and platinum). Our results also show that the observed hydrogenation rates are very sensitive to the substrate type. For all investigated cases, the largest fraction of hydrogenated carbon atoms was for platinum substrates. Our results also show that a significant number of randomly distributed H clusters are formed during the early stages of the hydrogenation process, regardless of the type of substrate. These results suggest that, similarly to graphane formation, large perfect graphone-like domains are unlikely to be formed. These findings are especially important since experiments have showed that cluster formation influences the electronic transport properties in hydrogenated graphene.

In this study, density functional theory (DFT) is employed to investigate the electronic properties of armchair silicene nanoribbons perforated with periodic nanoholes (ASiNRPNHs). The dangling bonds of armchair silicene nanoribbons (ASiNR) are passivated by mono- (:H) or di-hydrogen (:2H) atoms. Our results show that the ASiNRs can be categorized into three groups based on their width: W = 3P − 1, 3P, and 3P + 1, P is an integer. The band gap value order changes from “EG (3P − 1) < EG (3P) < EG (3P + 1)” to “EG (3P + 1) < EG (3P − 1) < EG (3P)” when edge hydrogenation varies from mono- to di-hydrogenated. The energy band gap values for ASiNRPNHs depend on the nanoribbons width and the repeat periodicity of the nanoholes. The band gap value of ASiNRPNHs is larger than that of pristine ASiNRs when repeat periodicity is even, while it is smaller than that of pristine ASiNRs when repeat periodicity is odd. In general, the value of energy band gap for ASiNRPNHs:2H is larger than that of ASiNRPNHs:H. So a band gap as large as 0.92 eV is achievable with ASiNRPNHs of width 12 and repeat periodicity of 2. Furthermore, creating periodic nanoholes near the edge of the nanoribbons cause a larger band gap due to a strong quantum confinement effect.

An innovative model based on spinodal-like decomposition of supercooled multi-component liquids was developed for prediction of the glass forming ability (GFA) of ternary alloys. The up-hill diffusion in the initially homogeneous freezing ternary liquid was considered as a necessary condition for solidification of crystalline phases. New generalized criteria of spinodal decomposition of ternary alloys for time/space correlated fluctuations were formulated. These criteria take into account both thermodynamic and kinetic properties of the system. The introduced criteria were found to provide adequate GFA evaluation of different compositions in the Mg-based ternary alloys.

The cubic polytype of silicon carbide is a stimulating candidate for Micro-Electro-Mechanical-Systems (MEMS) applications due to its interesting physical and chemical properties. Recently, we demonstrated the possibility to elaborate 3C-SiC membranes on 3C-SiC pseudo-substrates, using a silicon epilayer grown by Low Pressure Chemical Vapor Deposition as a sacrificial layer. Such structures could be the starting point for the elaboration of new MEMS devices. However, the roughness still represents a major concern. Therefore, in this contribution, we investigate the influence of an excimer laser irradiation on the Si epilayer surface prior to the 3C-SiC epilayer growth. We compare these results with the 3C-SiC epilayer grown directly on the as-grown Si epilayer.

Nanocrystalline cellulose (NCC) solid films obtained from evaporated aqueous heterogeneous mixtures retain the self-assembled chiral nematic order formed in the suspension. These semi-translucent films are iridescent and reflect or transmit circularly polarized visible light (400-700nm) due to the chiral properties of the self-assembled nanostructure. This effect occurs at different wavelengths depending on the pitch of the helical structure. In this paper, NCC films have been fabricated from different recipes to produce various helix pitches. The corresponding red-shift in the optical wavelength has been obtained by means of Spectrophotometry measurements. Preliminary experiments have been performed to investigate optical polarization effects as function of angles of incidence using ellipsometry. Finally, laser micromachining results on NCC films may suggest feasibility for integration as tunable light polarizer components.

Collagen is the most abundant protein present in the human body and found in connective tissues, bone, and tendon. It is also known as a natural resource for healing damaged skin tissues [1]. In this study, under specific microenvironment conditions, mimetic collagen gels were successfully formed synthetically from reconstituted Bovine type I collagen monomers. This was achieved by controlling ionic strength, temperature and pH, allowing fibrils with native mimetic D periodic banding structure to assemble spontaneously within the gels. In addition, by providing appropriate aging temperatures and times, mature collagen fibril growth is also realized in the gels in vitro. Mimetic gels were subsequently formed into fibers through a wet-spinning process. These spun fibers were found to preserve the native mimetic D periodic banding and fibrillar structure formed in the initial gels. As a result, the synthetic fibers resemble native tendon. Here structural development within the gel samples and fibers as a function of processing was analyzed by scanning electron microscopy (SEM). Results in this study also show a potentially new route for the fabrication of synthetic collagen fibers mimicking tendon, which may find applications as engineered tissues or scaffolding materials.

WSe2 films have been fabricated using a low-temperature, two-step method involving the reaction of W(CO)6 and elemental selenium in refluxing (110 °C) toluene to form a nanocrystalline precursor consisting of amorphous tungsten and trigonal crystalline selenium. Drop cast or airbrush-deposited films of this precursor were annealed in an argon atmosphere using a two-step temperature ramp (250 °C for 15 min followed by 550 °C for 30 min). Raman and x-ray diffraction (XRD) characterization as well as the measured bandgaps of the resulting films are consistent with (002)-oriented WSe2 and are compared to the characterization of films produced via selenization of sputtered tungsten films in closed quartz tubes at 875 °C.

Spent nuclear fuel from Finnish power plants is planned to be deposited deep in the crystalline bedrock in Olkiluoto, Finland. The bedrock needs to be well characterized to assess the risks inherent to the long term safety of the site. In the bedrock the possibly released radionuclides are mainly transported by water conducting fractures and their transport is retarded by diffusion and sorption. In porous materials these properties are typically linked to microscopic pore structure (pore size distribution, tortuosity and constrictivity) and chemical nature of the minerals and groundwater.

In this work transport properties of veined gneiss (VGN) and pegmatitic granite (PGR) samples from Olkiluoto were studied using various through diffusion experiments and the C-14-PMMA autoradiography. Through diffusion experiments were performed on rock cores using HTO and 36Cl in water phase and He in gas phase as tracers. The effective diffusion coefficients (De) determined for the VGN were found to be dependent on the tracer molecule (De(HTO) < De(He) < De(Cl)) whereas for the PGR such a dependence was not found. The porosity distributions determined by the C-14-PMMA autoradiography revealed the difference in the pore structure between the samples. The porosity of VGN consists mostly of grain boundary pores and pores between biotite lamellae. Due to a high content of nanometer scale pores anion exclusion affected the results of 36Cl and Knudsen diffusion the ones of He for VGN. Furthermore, in the PGR micrometer scale intra- and intergranular fissures form a connected network for diffusive transport and thus all tracers diffuse at the same rate.