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Although excellent milestones of III-nitrides in optoelectronic devices have been achieved, the focus on the optimization of their geometrical structure for multiple applications is very rare. To address this issue, we exclusively designed a prototype device to enhance the photoconversion efficiency and gas interaction capabilities of GaN nanorods (NRs) grown on a V-grooved Si(100) substrate with Si(111) facets for photodetector and gas sensor applications. Photoluminescence studies have demonstrated an increased surface-to-volume ratio and light trapping for GaN NRs grown on V-grooved Si(111). GaN NRs on V-grooved Si(100) with Si(111) facets exhibited high photodetection performance in terms of photoresponsivity (217 mA/cm2), detectivity (3 × 1013 Jones), and external quantum efficiency (2.73 × 105%) compared to GaN NRs grown on plain Si(111). Owing to the robust interconnection between NRs and a high surface-to-volume ratio, the GaN NRs grown on V-grooved Si(100) with Si(111) facets probed for NO2 detection with the assistance of photonic energy. The photo-assisted sensing makes it possible to detect NO2 gas at the ppb level at room temperature, resulting in significant power reduction. The device showed high selectivity to NO2 against other target gases, such as NO, H2S, H2, NH3, and CO. The device showed excellent long-term stability at room temperature; the humidity effect on the device performance was also examined. The excellent device performance was due to the following: (i) benefited from the V-grooved Si structure, GaN NRs significantly trapped the incident light, which promoted high photocurrent conversion efficiency and (ii) GaN NRs grown on V-grooved Si(100) with Si(111) facets increased the surface-to-volume ratio and thus improved the gas interaction with a better diffusion ratio and high light trapping, which resulted in increased response/recovery times. These results represent an important forward step in prototype devices for multiple applications in materials research.

Although excellent milestones of III-nitrides in optoelectronic devices have been achieved, the focus on the optimization of their geometrical structure for multiple applications is very rare. To address this issue, we exclusively designed a prototype device to enhance the photoconversion efficiency and gas interaction capabilities of GaN nanorods (NRs) grown on a V-grooved Si(100) substrate with Si(111) facets for photodetector and gas sensor applications. Photoluminescence studies have demonstrated an increased surface-to-volume ratio and light trapping for GaN NRs grown on V-grooved Si(111). GaN NRs on V-grooved Si(100) with Si(111) facets exhibited high photodetection performance in terms of photoresponsivity (217 mA/cm

We demonstrate improved surface pit and phase separation in thick InGaN grown on a GaN/Si (111) substrate, using plasma-assisted molecular beam epitaxy with an indium modulation technique. The formation of surface pit and compositional inhomogeneity in the InGaN epilayer are investigated using atomic force microscopy, scanning electron microscopy and temperature-dependent photoluminescence. Indium elemental mapping directly reveals that poor compositional homogeneity occurs near the pits. The indium-modulation epitaxy of InGaN minimizes the surface indium segregation, leading to the reduction in pit density and size. The phase separation in InGaN with a higher pit density is significantly suppressed, suggesting that the pit formation and the phase separation are correlated. We propose an indium migration model for the correlation between surface pit and phase separation in InGaN.

An effective method for high-quality molecular beam epitaxy growth of InGaN/GaN multiple quantum wells is demonstrated by inserting an ultrathin low temperature GaN (LT-GaN) interlayer between InGaN well and conventional high temperature GaN (HT-GaN) barrier layers. The LT-GaN interlayer is fabricated using metal migration enhanced epitaxy at the same growth temperature for InGaN. A smooth LT-GaN surface with a low defect density is obtained and indium decomposition is not observed. Large emission blueshift is significantly suppressed and narrow linewidth photoluminescence emission is achieved. The improved optical properties of the InGaN/GaN MQWs with LT-GaN interlayers are due to reduced compositional fluctuation and improved interface roughness.

Mesoporous Ni-Co oxide (NCO) nanosheet electrodes are fabricated on Ni foam via an electrodeposition technique. Their bifunctional activities for electrochemical energy storage and electro-catalysis for water splitting in strong alkaline media are optimized by varying the ratio of concentrations of the Ni and Co precursors. The ratio-based changes vary the pore size of the NCO nanosheets between 92.5 and 200 nm, and structural analyses reveal that the electrode films have a spinel NiCo2O4 structure. The obtained specific capacitance varies dramatically between 613 and 2704 Fg−1 at 2 mA cm−2, with good capacity retention (80–90%) after 2000 cycles. The NCO nanosheet electrodes also exhibit a good oxygen evolution reaction at the surface. The lowest overpotential (315 mV at 10 mA cm−2) is obtained with a Tafel slope of 59 mV dec−1. The observed bifunctional activities of the new NCO nanosheet electrode are superior to those of nanostructured NCO electrodes prepared via hydrothermal and SILAR methods. The analyses regarding the electrochemically active surface area and electrochemical impedance spectroscopy, together with the observed electrochemical performance, reveal that the most-optimized Ni and Co composition produces the synergetic effects of an electrochemically active surface area and great stability.

Ruthenium oxide (RuO 2 ) is fabricated on graphene (Gr)-coated Copper (Cu) foil by using a cathodic electroplating technique for flexible supercapacitor electrode applications. The electrochemical properties of the RuO 2 /Gr/Cu electrode are investigated with a conventional three electrode configuration in 0.5 M H 2 SO 4 electrolyte. The graphene insertion layer plays a key role in improving the structural and electrochemical properties of the RuO 2 electrode film under the bent condition. The electrode exhibits a specific capacitance of 1561 F g −1 (0.015 F cm −1 ) at a scan rate of 5 mV s −1 and a significantly improved retention of 98% under the bent condition. The flexible RuO 2 /Gr/Cu electrode exhibits a high energy density of ∼13 Wh kg −1 at a power density of ∼21 kW kg −1 . The excellent capacitance retention and electrochemical stability of the flexible RuO 2 /Gr/Cu electrode are due to the improved mechanical adhesion between the RuO 2 and the current collector. This flexible RuO 2 /Gr/Cu film could be used as a supercapacitor electrode with a high capacity and long-cycle life for the next-generation flexible electronic applications.

The dataset presented here is related to the research article entitled "Highly Efficient Electro-optically Tunable Smart-supercapacitors Using an Oxygen-excess Nanograin Tungsten Oxide Thin Film" (Akbar et al., 2017) [9] where we have presented a nanograin WO3 film as a bifunctional electrode for smart supercapacitor devices. In this article we provide additional information concerning nanograin tungsten oxide thin films such as atomic force microscopy, Raman spectroscopy, and X-ray diffraction spectroscopy. Moreover, their electrochemical properties such as cyclic voltammetry, electrochemical supercapacitor properties, and electrochromic properties including coloration efficiency, optical modulation and electrochemical impedance spectroscopy are presented.

Although gallium nitride (GaN) nanostructures are auspicious for photocatalytic activity, geometrical optimization has paid much attention for a significant light trapping in photoelectrochemical applications. To minimize the optical losses, we designed a prototype V-groove textured Si (100) with (111) facets, and GaN nanorods (NRs) were grown over a prototype substrate using plasma-assisted molecular beam epitaxy. The photocurrent density of V-groove textured GaN NRs in the NaOH electrolyte is found to be 801.62 μA/cm2 at 1.14 V vs reversible hydrogen electrode, which was 2.1-fold larger than that of GaN NRs on plain Si (111). Using this prototype V-groove textured Si(100) with (111) facets, a significant light can be trapped and modulated into GaN NRs. Furthermore, the heterostructure between GaN NRs and V-groove textured Si stimulates effective charge separation and transportation. These results represent an important forward step in solar photoelectrolysis.

A novel water-soluble inorganic Ca(NO 3 ) 2 salt electrode is investigated for its pseudocapacitance in an aqueous KOH electrolyte. Commercially available Ca(NO 3 ) 2 salt is directly used as the key electrode material. The supercapacitor electrode contains Ca(NO 3 ) 2 salt, carbon black, and polyvinylidene fluoride (PVDF) in a ratio of 80:10:10. The Ca(NO 3 ) 2 -based electrode demonstrates an exceptionally long life cycling stability, and a reasonably sound specific capacitance of 234 F/g is obtained at a current density of 3 A/g. Via chemical and electrochemical reactions, the in-situ activation of the Ca(NO 3 ) 2 forms an intermediate CaO which contributes to the pseudocapacitance of the electrode. The electrode undergoes a reversible redox reaction between Cu 2+ ↔ Cu + during the charge-discharge process. Superior rate capability and excellent specific capacitance retention of ∼120% over 2000 cycles are achieved compared with other inorganic salt electrodes.

A high quality, single phase InGaN film is fabricated on a GaN/Si (111) substrate by optimizing the pulse patterned In supply with a plasma-assisted MBE technique. Compositional phase separation in InGaN is considerably suppressed. The optical and structural properties of the single phase InGaN epitaxial film are consistently confirmed by atomic force microscopy, X-ray diffraction and photoluminescence measurements. We propose a growth mechanism for single phase InGaN in terms of optimal incorporation and surface migration of In atoms.

A smart supercapacitor shares the same electrochemical processes as a conventional energy storage device while also having electrochromic functionality. The smart supercapacitor device can sense the energy storage level, which it displays in a visually discernible manner, providing increased convenience in everyday applications. Here, we report an electro-optically tunable smart supercapacitor based on an oxygen-rich nanograin WO3 electrode. The nanostructured WO3 electrode is dark blue in color in the charged state and becomes transparent in its discharged state with a high optical modulation of 82%. The supercapacitor has a specific capacitance of 228 F g−1 at 0.25 Ag−1 with a large potential window (1.4 V). It is highly durable, exhibits good electrochemical stability over 2000 cycles, retains 75% of its initial capacitance, and exhibits high coloration efficiency (~170 cm2/C). The excellent electrochromic and electrochemical supercapacitor properties of the electrode is due to the synergetic effect between nanograin morphology and excess oxygen. A smart-supercapacitor fabricated with an oxygen-rich nanograin WO3 electrode exhibits a superb combination of energy storage and highly-efficient electrochromic features in one device that can monitor the energy storage level through visible changes in color.

This study investigates the transport and switching time of nonvolatile tungsten oxide based resistive-switching (RS) memory devices. These devices consist of a highly resistive tungsten oxide film sandwiched between metal electrodes, and their RS characteristics are bipolar in the counterclockwise direction. The switching voltage, retention, endurance, and switching time are strongly dependent on the type of electrodes used, and we also find quantitative and qualitative evidence that the electronegativity (χ) of the electrodes plays a key role in determining the RS properties and switching time. We also propose an RS model based on the role of the electronegativity at the interface.

Mn-doped ZnS thin films are synthesized on soda-lime glass substrates using magnetron co-sputtering technique. X-ray diffraction and atomic force microscopy measurements indicate that of the as-obtained films including the highest Mn (~11% relative to the Zn concentration) in the lattice of ZnS are amorphous with a granular morphology. X-ray photoelectron spectroscopy reveals the presence of Zn 2+ , Mn 2+ and S 2− chemical states in the films. The undoped ZnS film exhibits photoluminescence (PL) peaks at energies around 3.26 eV (wavelength ~379 nm) and 2.95 eV (~420 nm), which originate from the interplay between excited electron, defect (sulfur vacancy) states and the valence band. For the Mn-doped ZnS films, the band-to-band emission peak is quenched and shifts toward to higher energies at a rate of 11.7±2 meV/Mn%. We propose that Mn dopant-mediated structural phases and non-radiative deep traps in ZnS cause the modification in the optical transition.

We report on the effect of oxygen annealing for GaN surface on the Schottky barrier configuration and leakage current in Ni-AlGaN/GaN Schottky barrier diodes. After oxygen annealing, their turn-on voltage and reverse-bias leakage current characteristics are significantly improved due to a reduction in the Schottky barrier height (SBH) and suppression in the surface states respectively. Interface state density extracted from the Terman method was reduced by 2 orders of magnitude. X-ray photoelectron spectroscopy measurements show that the oxygen annealing induces Ga 2 O 3 on the GaN surface. The formation of Ga 2 O 3 reduces the interface state density as well as lowers the SBH through the modification of hybridized metal induced gap states.

Multi-wall carbon nanotube/polyaniline (CNT/PANI) nanocomposite thin films for electrochemical electrode applications are synthesized on flexible graphene-coated indium-tin-oxide (ITO) substrates by using a drop-casting technique. Graphene serves as an adhesion layer between the CNT/PANI nanocomposite film and the flexible ITO substrate. A nanoscale vermicular morphology of PANI films containing well-dispersed CNTs is formed on the surface of graphene. The electrochemical characteristics of the nanocomposite films are investigated in a 0.5-M LiClO4 + PC electrolyte. The electrical conduction of the CNT/PANI/graphene/ITO film is considerably superior to that of a PANI/ITO film. The cyclic voltammogram measurements indicate that the specific capacitance of the CNT/PANI film is ~134 F/g which is ~11% higher than that (~120 F/g) of the pure PANI film. Most importantly, the nominal capacitance loss of the PANI/CNT film (~1.2%) is significantly improved relative to that of the pure PANI film (~18.1%) after 100 charge-discharge cycles. We attribute the considerably improved capacity retention of the flexible CNT/PANI electrode to the graphene adhesion layer.

The PANI (polyaniline) thin films are synthesized on flexible ITO/PET (indium tin oxide/polyethylene terephthalate) substrates by using the drop-casting method. The amount of the PANI for the drop casting varies from 0.04 to 0.16 g. The morphology of the drop-casted PANI films shows a porous vermicular shape. The electrochemical supercapacitor properties of the PANI films are examined in a 0.5-M LiClO4 + PC electrolyte. The PANI films with 0.08 g of PANI show higher current density and considerably higher specific capacitance and capacity retention, compared with other PANI films. The highest specific capacitance of the films with 0.08 g of PANI is found to be ∼120 F/g, and the capacity retention is found to be as high as 70.51% after 100 charge-discharge cycles.

We report on the electrosynthesis of nanostructured zinc oxide (ZnO) thin films using organic molecule templates self-assembled on indium-doped tin oxide substrates in an aqueous medium. The organic molecule templates alter the morphology and microscopic defect configuration of the ZnO film having a hexagonal wurtzite structure. Low-temperature photoluminescence (PL) study reveals that the ZnO films exhibit ultraviolet (3.36 eV and 3.22 eV), violet (3.06 eV), blue (2.88 eV), and green (2.50 eV) emissions that originate from transitions related to deep-level (zinc interstitials and vacancies) and acceptor-like (ionized zinc vacancies) defect states. A defect energy level diagram based on the PL measurement is proposed.

We have investigated the role of the metal/oxide junction interface on the resistive switching (RS) characteristics in WO3+x films. The WOx films are fabricated on Pt substrates by magnetron sputtering at room temperature. Top metal contact (Au or Al) is fabricated by using thermal evaporator. The thicknesses of WOx films and top electrodes are 1 μm and 200 nm, respectively. It has been found that the bi-polar RS direction is dependent on the choice of top metal electrode, Au or Al. The sample with a Au top electrode shows clockwise (CW) RS mode whilst the sample with a Al top electrode shows counter-clockwise (CCW) RS mode. The on/off ratio is 10 times for Au/WOx/Pt and 100 times for Al/WOx/Pt. The bi-polar RS modes are modeled in terms of the difference in the electronegativity of the top and bottom electrodes.

Thermal properties of the AlGaN/GaN Schottky barrier diodes were investigated, using a pulsed-IV measurement technique. The thermally degraded mobility in the DC-bias configuration was restored, when the pulse-bias voltages were applied. It was observed that heat generation was minimized, using a pulse width of 500 ns and pulse period of 10 ms. For the SBDs consisting of 5 μm of anode–cathode distance, on-resistance measured by the pulse-IV and DC-IV were 1.6 and 6.2 Ω-mm, respectively. We also demonstrated the device-width dependence of the thermal properties of the SBDs. We found that the performance of the power devices can be greatly influenced by the heat generation.

The multi-valued resistive switching (RS) characteristics and mechanism in a WO x /AlO y heterojunction-based nonvolatile memory have been investigated. Although Al/WO x /Pt and Al/AlO y /Pt show different RS characteristics, bipolar and unipolar modes, respectively, the Al/WO x /AlO y /Pt structure shows both bipolar and unipolar RS characteristics, depending on the magnitude of the applied voltage. Three resistance states, which are defined as the low-resistance state (LRS), the mid-high-resistance state (mid-HRS) and the high-resistance state (HRS), are observed. A model combining two different RS mechanisms, oxygen ions involving the redox process at the interface and filamentary conduction, is proposed, and provides a simple physical concept to understand the multi-valued RS behavior. Our findings will be useful to design and optimize oxide heterojunction-based non-volatile RS memory devices.

We investigate the resistive switching power from unipolar resistive switching current-voltage characteristics in various binary metal oxide films sandwiched by different metal electrodes, and find a universal feature (the so-called universality) in the switching power among these devices. To experimentally derive the switching power universality, systematic measurements of the switching voltage and current are performed, and neither of these correlate with one another. As the switching resistance (R) increases, the switching power (P) decreases following a power law P ∝ R−β, regardless of the device configurations. The observed switching power universality is indicative of the existence of a commonly applicable switching mechanism. The origin of the power universality is discussed based on a metallic filament model and thermo-chemical reaction.

This work reports on a mechanism for irreversible resistive switching (RS) transformation from bipolar to unipolar RS behavior in SrRuO3 (SRO)/Cr-doped SrZrO3 (SZO:Cr)/Pt capacitor structures prepared on a Ti/SiO2/Si substrate. Counter-clockwise bipolar RS memory current-voltage (I–V) characteristics are observed within the RS voltage window of −2.5 to +1.9 V, with good endurance and retention properties. As the bias voltage increases further beyond 4 V under a forward bias, a forming process occurs resulting in irreversible RS mode transformation from bipolar to unipolar mode. This switching mode transformation is a direct consequence of thermally activated Ti out-diffusion from a Ti adhesion layer. Transition metal Ti effectively out-diffuses through the loose Pt electrode layer at high substrate temperatures, leading to the unintended formation of a thin titanium oxide (TiOx where x < 2) layer between the Pt electrode and the SZO:Cr layer as well as additional Ti atoms in the SZO:Cr layer. Cross-sectional scanning electron microscopy, transmission electron microscopy and Auger electron spectroscopy depth-profile measurements provided apparent evidence of the Ti out-diffusion phenomenon. We propose that the out-diffusion-induced additional Ti atoms in the SZO:Cr layer contributes to the creation of the metallic filamentary channels.

We have investigated the temperature dependence of ballistic mobility in a 100 nm-long InGaAs/InAlAs metamorphic high-electron-mobility transistor designed for millimeter-wavelength RF applications. To extract the temperature dependence of quasi-ballistic mobility, our experiment involves measurements of the effective mobility in the low-bias linear region of the transistor and of the collision-dominated Hall mobility using a gated Hall bar of the same epitaxial structure. The data measured from the experiment are consistent with that of modeled ballistic mobility based on ballistic transport theory. These results advance the understanding of ballistic transport in various transistors with a nano-scale channel length that is comparable to the carrier's mean free path in the channel.