Advances in Science and Technology Vol. 77

Abstract: We present a performance assessment of graphene/hexagonal Boron Nitride heterojunctions based transistors able to provide large current modulation. The study is performed by means of a multi-scale approach leveraging ab-initio simulations to capture the physics at the atomic scale, and tight-binding simulations to compute transport. In particular, we focus on two technological solutions, a vertical and a planar structure both able to provide large Ion/Ioff ratios. As we will show, due to reduced capacitative coupling, the planar structure outperforms the vertical device as far as digital applications are concerned.

Abstract: The ‘DLC-GFET’, a graphene-channel field effect transistor with a diamondlike carbon (DLC) top-gate dielectric film, is presented. The DLC film was formed ‘directly’ onto the graphene channel without forming passivation interlayers using our photoemission-assisted plasma-enhanced chemical vapor deposition (PA-CVD), where the plasma was precisely controlled by significant photoemission from the sample with quite low electric power, minimizing plasma damage to the channel. The DLC-GFET exhibits clear ambipolar characteristics with a slightly positive shift of the neutral points (Dirac voltages). Relatively high transconductances were obtained as 14.6 (8.8) mS/mm in the n (p) channel modes, respectively, with a thick DLC gate dielectric of 48 nm and a long gate length of 5 μm, promising vertical scaling-down to improve the high-frequency performance. The positive shift of the Dirac voltage is due to unintentional hole doping from an oxygen species like H₂O in the DLC film into the graphene channel, suggesting that a modulation-doped DLC structure with a δ-doped oxygen (nitrogen) species for the p (n) mode will overcome high access resistance.

Abstract: Our focus is on the graphene under a magnetic field with Landau levels i.e. quantum Hall regime where a ‘confining potential’ is imposed by a finite electric field. In our theory, the graphene is modelled by a conventional fermion on a honeycomb lattice and the finite electric field is by a static potential. We reveal the fate/breakdown of the quantum Hall regime. A possible candidate of this kind of system is a grapheneQuantumDot, which we also discussed in the light of our theory.

Abstract: ZnO nanorods are currently studied for variety optoelectronic applications. Typically, thin film and bulk ZnO show a strong light absorption in the ultra violet (UV) range. For devices that operate in the visible and infrared range such as optoelectronic sensors and photovoltaic cells, it is necessary to modify the absorption profile from UV to higher wavelengths in the visible region. In this study we investigate optical absorption of ZnO nanorods doped with cobalt using a modified chemical bath deposition technique. The light absorption properties of Cobalt doped ZnO nanorods were studied using Photoluminescence (PL) and Raman Spectroscopy. For doping of Cobalt ranging from 3 to 10 percentage of the total weight, the PL intensity shows a suppression of the prominent UV peak at 383 nm with increase in doping concentration. This reduction in PL intensity at 383 nm is accompanied by an increase in the PL intensity at 429 nm and 469 nm. We will discuss details of ZnO-Cobalt structures using Raman spectroscopy on cobalt doped ZnO nanorod samples of 1 -20% doping concentration.

Abstract: Thermoelectric generator is expected as an independent source, an energy converter for co-generation with Refuse Derived Fuel (RDF) and so on. Thermoelectric materials were required high Seebeck coefficient, low electrical resistivity and low thermal conductivity. Thermoelectric oxides are suitable at the high temperature range because of chemical stability. Authors focus attention on Aurivillius compounds. The Aurivillius compounds consist of Perovskite layers and Bi-O layers. It is expected that nano-layered structure shows high Seebeck coefficient due to the quantum confinement of electron in Perovskite layers. It was reported that the Seebeck coefficient of Aurivillius phase Bi₂VO_5.5 was high value of -28.3 mVK^-1 at 1010 K, and the electrical resistivity of the one was also high value of 0.033 Ωm at 1010 K. We investigated about element substitution effects at V site on thermoelectric properties. Bi₂V_1-xM_xO_5.5 (M=Cu, Cr, x=0, 0.05, 0.1, 0.2) were prepared by solid-state reaction and hot pressing. From the results of the electrical resistivities and the Seebeck coefficients, Cu and Cr behaved as acceptor to Bi₂VO_5.5. Cr was effective for reducing the thermal conductivity of Bi₂VO_5.5. The maximum value of dimensionless figure of merit for Bi₂VO_5.5 was 0.06 at 910 K.

Abstract: The α and β phases of In₂Se₃ are attractive thermophotovolβtaic (TPV) materials. For example, they have a direct transition and an optimal bandgap for TPV systems. We investigated about the influence of the growth conditions on the crystal forms of In-Se prepared by vapor transport. Firstly, only bulk In₂Se₃ was encapsulated in a quartz ampoule and crystals were grown by vapor transport. Hollow hexagonal cylinders that were approximately 10 μm in diagonal dimension were obtained. They were identified as a mixed phase of In₂Se₃ by XRD. Secondly, the deposition surface configuration was varied by using an ampoule with a smaller internal diameter and a silicon oxide substrate. Hollow hexagonal cylinders that were approximately 20 μm in diagonal dimension were obtained on the inner wall of the ampoule with a smaller internal diameter. Solid hexagonal crystals of approximately 10 μm in diagonal dimension grew perpendicular to the silicon oxide substrate, whereas hollow hexagonal cylinders grew on the inner wall of the ampoule. Finally, the surface energy of the deposition surface was varied for silicon substrate. Nanoparticles with diameters of approximately 50 nm were sparsely deposited on the textured silicon surface.

Abstract: Photovoltaic energy can be expensive if the solar radiation in a particular region is not abundant. When the solar radiation is scarce in a region, there is presence of winds and rainfall. If flexible solar cells are coupled with flexible piezoelectric films then the hybrid structure can generate energy from solar radiation, wind and rainfall. Hybrid piezoelectric-photovoltaic devices have been developed which are capable of generating electricity from solar as well as wind and rain energy. This work focuses on non-transparent hybrid structure which contains copper and aluminium electrodes and eliminates the used of costly indium tin oxide (ITO).These hybrid films are made by depositing organic photovoltaic cell based on P3HT and PCBM on a commercial PVDF film. The hybrid piezoelectric-photovoltaic film was first tested under a solar simulator with 1.5 AM filter at one sun solar intensity. The film produced an open circuit voltage, Voc of 0.43V and a short circuit current density, Isc of 4.48mA/cm2. It was then subjected to a turbulent wind speed of 10m/sec (36km/hour) in a custom built wind tunnel. A peak voltage of 52V was generated by the PVDF substrate due to the oscillations created by the wind. Peak power was also measured using a variable resistor and was recorded to be 85 µW. In order to check if the film was not damaged when it was subjected to the turbulent wind speed, the film was again tested under the solar simulator and did not show any changes in its open circuit voltage or short circuit current.

Abstract: Great focus has been directed towards double-layer capacitance and Faradic, redox reactions because of their long device lifetimes and their high power densities, respectively. Our novel approach to combining these mechanisms in a tri-layered composite electrode promises to increase the energy densities of the device, without sacrificing the supercapacitance and the high power densities attributed with it. Initial analysis of the interfacial interactions of graphene oxide (GO) and manganese oxide (MnO2) were promising. This paper aims to further demonstrate the tri-layered composite by forming a layer of reduced graphene oxide (rGO) on MnO2 nanowires and cobalt oxide nanorods. We have successfully created the first of a kind supercapacitor electrode material as a scalable device. In this paper, in addition to analysis of the composite electrode, we present modifications to the traditional electrophoretic deposition process and optimizations to the thermal reduction of GO in order to create rGO surfaces on substrates that are normally difficult to adhere it to.

Abstract: Comparative investigation of the thermo-induced unconstrained shape-memory (SM) recovery of a multi-phase semi-crystalline covalent network was performed using cross-linked polyolefin based blends of linear high density polyethylene (HDPE) and/or short-chain branched ethylene octen copolymers (EOC) as well as polycyclooctene (polyoctenamer, TOR). Different phase morphologies of the blends were generated by variation of blend composition and different pathways for sample preparation: Melt mixing of blends, compression molding of films, slowly cooling or quenching of films and subsequent cross-linking by electron irradiation at room temperature. Partly well developed triple- and quadruple SM behavior after one-step programming process was demonstrated for binary and ternary HDPE/EOC/TOR blends, which exhibit a morphology with segregated phases, where the matrix has the lower melting and correspondingly switching temperature (Tm and Tsw) in comparison to the disperse phases. These blends show pronounced steps of SM strain recovery and storage modulus as well as distinct DSC melting peaks. The peaks of SM recovery rate are located in the Tm range of the blend phases. The quenching procedure resulted in a better phase separation at nano-level and correspondingly in a more pronounced triple-shape behavior of the blends. All HDPE/EOC/TOR blends showed high values of strain fixing and strain recovery ratios of 95 to 99 %.

Abstract: A series of crosslinked poly(ε-caprolactone) (PCL) materials were obtained starting from linear, three- and four-arm star PCL functionalized with methacrylate end-groups, allowing to tune the melting temperature (Tm) on a range between 36 and 55°C. After deforming the specimens at 50% above Tm, the materials are seen to fully restore their original shape by heating them on a narrow region close to Tm; further, when the shape memory effect is triggered under fixed strain conditions, the materials are able to exert stress on a range between 0.2 and 7 MPa. The materials also display two-way shape memory features, reversibly moving between two shapes when cooled and heated under a fixed load. Finally, to investigate the application of the PCL materials as self-expandable stents, one-way shape memory experiments are currently carried out on tubular specimens.