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Large area epitaxy of two-dimensional (2D) layered materials with high material quality is a crucial step in realizing novel device applications based on 2D materials. In this work, we report highquality, crystalline, large-area gallium selenide (GaSe) films grown on bulk substrates such as c-plane sapphire and gallium nitride (GaN) using a valved cracker source for Se. (002)-Oriented GaSe with random in-plane orientation of domains was grown on sapphire and GaN substrates at a substrate temperature of 350-450 °C with complete surface coverage. Higher growth temperature (575 °C) resulted in the formation of single-crystalline ε-GaSe triangular domains with six-fold symmetry confirmed by in-situ reflection high electron energy diffraction and off-axis x-ray diffraction. A two-step growth method involving high temperature nucleation of single crystalline domains and low temperature growth to enhance coalescence was adopted to obtain continuous (002)-oriented GaSe with an epitaxial relationship with the substrate. While six-fold symmetry was maintained in the two step growth, β-GaSe phase was observed in addition to the dominant ε-GaSe in cross-sectional scanning transmission electron microscopy images. This work demonstrates the potential of growing high quality 2D-layered materials using molecular beam epitaxy and can be extended to the growth of other transition metal chalcogenides. [ABSTRACT FROM AUTHOR]

This paper presents an approach of 'film separation' and 'transfer' to fabricating functional films or devices on polymeric substrates. For either a ferroelectric La-doped Pb(Zr,Ti)O3, PLZT thin film or a ZnO thin film, using laser lift-off (LLO), film separation from the transparent rigid substrate (sapphire or glass) was achieved. The film transfer to a polymeric receptor substrate was obtained by laminating the polymeric substrate on the film stack prior to LLO. The introduction of a sacrificial layer between the film and the substrate mostly resolved concerns to induce damages during LLO. Electrical testing of the films before and after LLO demonstrated the film properties on the original substrate were satisfactorily retained even for the transferred films on the polymeric substrate. [ABSTRACT FROM AUTHOR]

A rotating ball display was tested with a thin monolayer sheet. An oil cavity was formed in the shape of a spheroid in a thin elastomer sheet. The rotation of a bichromal ball was investigated from the point of rotation time from one color to the other color. Bichromal balls were under friction during rotation. The friction caused an abnormal change in the rotation time. The rotation time of the elastomer sheet was compared with that of a microcup rotating ball display. The friction explained the difference of rotation time variation between the elastomer sheet display and the microcup display. [ABSTRACT FROM AUTHOR]

We report on the synthesis and properties of wafer-scale two-dimensional/three-dimensional (2D/3D) n-SnSe2/n-GaN(0001) heterojunctions. The hexagonal crystal structure of crystalline SnSe2 grown by molecular beam epitaxy was confirmed via in-situ reflection high-energy electron diffraction and off-axis X-ray diffraction. Current-voltage (I-V) measurements of SnSe2/GaN diodes exhibited 9 orders of magnitude rectification, and the SnSe2/GaN heterojunction barrier height was estimated to be 1 eV using capacitance-voltage measurements and internal photoemission measurements. Vertical electronic transport analyzed using temperature-dependent I-V measurements indicates thermionic field emission transport across the junction. This work demonstrates the potential of epitaxial growth of large area high quality 2D crystals on 3D bulk semiconductors for device applications involving carrier injection across 2D/3D heterojunctions. [ABSTRACT FROM AUTHOR]

The integration of two-dimensional materials such as transition metal dichalcogenides with bulk semiconductors offer interesting opportunities for 2D/3D heterojunction-based device structures without any constraints of lattice matching. By exploiting the favorable band alignment at the GaN/MoS2 heterojunction, an Esaki interband tunnel diode is demonstrated by transferring large area Nb-doped, p-type MoS2 onto heavily n-doped GaN. A peak current density of 446 A/cm2 with repeatable room temperature negative differential resistance, peak to valley current ratio of 1.2, and minimal hysteresis was measured in the MoS2/GaN non-epitaxial tunnel diode. A high current density of 1 kA/cm2 was measured in the Zener mode (reverse bias) at -1V bias. The GaN/MoS2 tunnel junction was also modeled by treating MoS2 as a bulk semiconductor, and the electrostatics at the 2D/3D interface was found to be crucial in explaining the experimentally observed device characteristics. [ABSTRACT FROM AUTHOR]

Transfer of epitaxial, two-dimensional (2D) MoS2 on sapphire grown via synthetic approaches is a prerequisite for practical device applications. We report centimeter-scale, single crystal, synthesized MoS2 field effect transistors (FETs) transferred onto SiO2/Si substrates, with a field-effect mobility of 4.5 cm² V-1 s-1, which is among the highest mobility values reported for the transferred large-area MoS2 transistors. We demonstrate simple and clean transfer of large-area MoS2 films using deionized water, which can effectively avoid chemical contamination. The transfer method reported here allows standard i-line stepper lithography process to realize multiple devices over the entire film area. [ABSTRACT FROM AUTHOR]

In this paper, research describing a high-efficiency vertical axis wind turbine (VAWT) system is presented. The proposed cycloidal vertical axis wind turbine (CWT) is straight-bladed; adopting the cycloidal blade system that is used as the propulsion system in a cyclocopter. The CWT actively controls the pitch angle of its rotor blades to improve turbine efficiency according to wind conditions. It can self-start by changing its pitch angle pattern and is able to achieve maximum operating efficiency. An optimization scheme based on the response surface method was used to find the most efficient design variables while performing computational fluid dynamics (CFD) analysis with variables including the number of blades, chord length, tip speed ratio, and maximum pitch angle. For the experiments, a 600W (at 10 m/s of wind speed) class cycloidal VAWT was designed and developed. Two types of experiments were carried out and compared with the CFD results. The first test was a constant wind speed test. The constant wind speed was achieved by driving a truck at constant speed along a straight road with the CWT mounted on the truck. The second test was a field test that was conducted in a certification test site for small wind turbine systems at the Korea Institute of Energy Research in Jeju Island. The result of the constant wind speed test verified the CFD analysis and the field test demonstrated its reliability. Furthermore, another CWT with a flap control device is added in the tip of blades is also studied and designed for the highest efficiency. The flap control device, operated with simple mechanical linkages, can transform the blade airfoil's camber between positive and negative. [ABSTRACT FROM AUTHOR]

Electrical and optical characterization of two-dimensional/three-dimensional (2D/3D) p-molybdenum disulfide/n-gallium nitride (p-MoS2/n-GaN) heterojunction diodes are reported. Devices were fabricated on high-quality, large-area p-MoS2 grown by chemical vapor deposition on sapphire substrates. The processed devices were transferred onto GaN/sapphire substrates, and the transferred films were characterized by X-ray diffraction (XRD) and atomic force microscopy (AFM). On-axis XRD spectra and surface topology obtained from AFM scans were consistent with previously grown high-quality, continuous MoS2 films. Current-voltage measurements of these diodes exhibited excellent rectification, and capacitance-voltage measurements were used to extract a conduction band offset of 0.23 eV for the transferred MoS2/GaN heterojunction. This conduction band offset was confirmed by internal photoemission measurements. The energy band lineup of the MoS2/GaN heterojunction is proposed here. This work demonstrates the potential of 2D/3D heterojunctions for novel device applications. [ABSTRACT FROM AUTHOR]

The growth and electrical characterization of the heterojunction formed between two-dimensional (2D) layered p-molybdenum disulfide (MoS2) and nitrogen-doped 4H silicon carbide (SiC) are reported. The integration of 2D semiconductors with the conventional three-dimensional (3D) substrates could enable semiconductor heterostructures with unprecedented properties. In this work, direct growth of p-type MoS2 films on SiC was demonstrated using chemical vapor deposition, and the MoS2 films were found to be high quality based on x-ray diffraction and Raman spectra. The resulting heterojunction was found to display rectification and current-voltage characteristics consistent with a diode for which forward conduction in the low-bias region is dominated by multi-step recombination tunneling. Capacitance-voltage measurements were used to determine the built-in voltage for the p-MoS2/n-SiC heterojunction diode, and we propose an energy band line up for the heterostructure based on these observations. The demonstration of heterogeneous material integration between MoS2 and SiC enables a promising new class of 2D/3D heterostructures. [ABSTRACT FROM AUTHOR]

We report on the first demonstration of p-type doping in large area few-layer films of (0001)-oriented chemical vapor deposited MoS2. Niobium was found to act as an efficient acceptor up to relatively high density in MoS2 films. For a hole density of 3.1s1020 cm-3, Hall mobility of 8.5 cm² V-1 s-1 was determined, which matches well with the theoretically expected values. X-ray diffraction scans and Raman characterization indicated that the film had good out-of-plane crystalline quality. Absorption measurements showed that the doped sample had similar characteristics to high-quality undoped samples, with a clear absorption edge at 1.8 eV. Scanning transmission electron microscope imaging showed ordered crystalline nature of the Nb-doped MoS2 layers stacked in the [0001] direction. This demonstration of substitutional p-doping in large area epitaxial MoS2 could help in realizing a wide variety of electrical and opto-electronic devices based on layered metal dichalcogenides. [ABSTRACT FROM AUTHOR]