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Scaling-down the size of semiconductor cavity lasers and engineering their electromagnetic environment in the Purcell regime can bring about spectacular advance in nanodevices fabrication. We report here an unprecedented observation of a coherent Cathodoluminescence from GaN nanocavities (20–100 nm). Incident lower energy (8/cm2) cavity lasers that are addressable by simple deflection and tuning of incident electron beams.

Inelastic mean free path (IMFP) of the electron is a very important parameter for quantitative analysis of several electron spectroscopies and transport properties. In spite of being a fundamental material property, its experimental determination is not trivial due to complexity of the various electron scattering processes in matter. In this report, we demonstrate a procedure to determine the IMFP of 300 keV electrons in GaN, using the log-ratio technique where the local specimen thickness needs to be accurately known. The GaN nanorod morphology of the sample used here allows the accurate measurement of thickness by 'thickness map' under EFTEM measurements which enable the site specific determination of IMFP. IMFP for different collection semi angles have also been measured to validate the angular dependence. Our experimental results estimates the IMFP of GaN for 300 keV electrons to be 143 ± 11 nm at no-aperture condition and exhibit a strong inverse angular dependence at smaller collection semi angles (β20 mrad) and a near angular independence at larger collection semi angles (β30 mrad). We discuss these results in the light of three different theoretical models prevalent in the literature.The inelastic mean free path (IMFP) of electrons is defined as the average distance traveled between two successive inelastic collisions by an electron moving with a particular energy in a given material. It is a very fundamental material parameter for surface science and description of electron transport processes in solids. IMFP is particularly useful for quantifying several electron spectroscopies such as Auger electron spectroscopy, X-ray photoelectron spectroscopy, electron energy loss (EEL) spectroscopy and elastic peak electron spectroscopy etc. However, it is difficult to determine IMFP experimentally or theoretically as the interaction process has strong angular dependence that varies with electron energy, density and dielectric properties of the material. Particularly for EEL which is carried out in TEM at using electron with few hundred kilovolts, knowledge of IMFP at those high energies is required. In this report, we demonstrate a general procedure to determine IMFP of 300 keV electrons in Gallium Nitride inside TEM. Single crystalline Gallium Nitride nanorod has just been taken as an example to demonstrate the process. The procedure can be extended to any other materials of both crystalline and amorphous in nature.

(In1−xDyx)2O3 (x = 0, 0.05, 0.10) nanoparticles with a particle size of 15–22 nm were obtained by an auto-combustion method. The powder x-ray diffraction (XRD), Raman and x-ray absorption near edge spectral (XANES) analysis revealed a cubic bixbyite structure (space group Ia-3). The presence of Dy3+ ions in the In2O3 host lattice was confirmed from XPS data. The high-resolution transition electron microscopic (HRTEM) and XANES revealed the single phase (solid solution) nature of the nanoparticles. The optical band gap was found to increase from 3.57 eV to 3.64 eV by doping Dy into the In2O3 lattice. Photoluminescence spectra shows emission in the yellow region probably due to oxygen related vacancies. A weak ferromagnetic property was seen for Dy-doped In2O3 at room temperature, while pure In2O3 is diamagnetic. It seems that Dy:In2O3 is not a suitable␣system to consider for dilute magnetic semiconductors in spintronic devices.

Structural, optical and surface properties of epitaxially grown 2D GaN nanowall network using Molecular Beam Epitaxy have been compared to those of non-epitaxially grown single-crystalline 1D GaN nanowires using Chemical Vapour Deposition. The kinetics of growth mechanisms and formed morphology are shown to significantly influence the respective band structures and consequently their luminescence properties. X-ray diffraction and Raman spectroscopy reveal that the epitaxial 2D nanowall network experiences a hydrostatic strain in addition to a compressive strain, whereas non-epitaxial 1D nanowires possess a morphology-dependent tensile/compressive strain and a negligible hydrostatic strain. Slightly blue-shifted photoluminescence emission from both these nanostructures is markedly enhanced compared to that from an epilayer. The epitaxial nanowall network exhibits the highest enhancement in the band edge emission among them. X-ray photoelectron spectroscopy spectra show shifts in the valence band features and in the hybridization of shallow core levels. Using the XRD, Raman, PL and XPS data, a variation in the band structure of these differently kinetically formed GaN nanostructures is also sketched.

Integrating silicon and III-nitride technologies for high-speed and large bandwidth communication demands optically interconnected active components that detect, process, and emit photons and electrons. It is imperative that multifunctional materials can enhance the performance and simplify fabrication of such devices. Spontaneously grown GaN in the nanowall network (NwN) architecture simultaneously displays unprecedented optical and electrical properties. A two-order increase in band-edge emission makes it suitable for high-brightness light-emitting diodes and laser applications. Decorating this NwN with silver nanoparticles further enhances emission through plasmonic interactions and renders it an excellent surface-enhanced Raman spectroscopy substrate for biomolecular detection. The observation of very high electron mobility (approximately 104 cm2/Vs) and large phase-coherence length (60 μm) is a consequence of two-dimensional (2D) electron gas formation applicable for high electron mobility transistors. Detecting ballistic transport in the nanowalls confirms proximity-induced superconductivity (

For spintronic devices, the room temperature ferromagnetism in dilute semiconductors is essential. We report here the successful preparation of (In1−xGdx)2O3 [x = 0, 0.05, 0.10] by auto-combustion method. The powder XRD and SAED data confirmed the formation of single-phase cubic bixbyite In2O3. The particles are in the range of 18–22 nm size. Raman spectra showed doping of Gd3+ ions in the In2O3 lattice. The 3+ valence state of In and Gd was confirmed from XPS and XAS techniques. The magnetic hysteresis loop and EPR spectra showed the weak ferromagnetism in (In0.90Gdn0.10)2O3, while the pure In2O3 is diamagnetic and (In0.95Gdn0.05)2O3 is paramagnetic. To our knowledge, it is the first study to report the RTFM in Gd-doped In2O3 nanoparticles.

The ever-increasing demand for renewable and clean energy sources has prompted the development of novel materials for photoelectrochemical (PEC) water splitting, but efficient solar to hydrogen conversion remains a big challenge. In this work, we report a bio-nanohybrid strategy in a photo-system to simultaneously enhance the charge separation and water splitting efficiency of photoanode (PA) by introducing Bacteriorhodopsin (bR), a natural proton pumping photosystem and GaN nanowall network (NWN), a direct band gap and corrosion-resistant semiconductor. The experimental study reveals that this combination of bR and GaN NWN has huge potential as a light-activated sensitizer as well as proton pumping source to achieve enhance photocurrent density in hydrogen evolution reaction (HER). Consequently, this synergistic effect in bR/GaN NWN PA gives rise to largely enhanced applied bias photon-to-current efficiency (ABPE) ~7.8% and photocurrent density (28.74 mA/cm2 at 1.0 V vs RHE). It is worth mentioning that the photocurrent density of bR/GaN NWN, to the best of our knowledge, is superior to previously reported bR-based PAs and bio-photoelectric devices reported till today for solar-to-hydrogen fuel generation.

Co-sensitized photoanode with nitrogen doped graphene particles (NGPs) and CdS capable of harvesting light efficiently in the visible region is combined with a poly(3,4-ethylenedioxypyrrole) (PEDOP)@carbon(C)-cloth based counter electrode (CE) for the first time. The conduction band of NGPs is wedged between that of CdS and TiO2, and charge separation is improved in TiO2/NGPs/CdS compared to TiO2/CdS, thus resulting in a power conversion efficiency (PCE), greater by 37% and 305% in comparison to TiO2/CdS and TiO2/NGPs cells. The electrolyte is a polysulfide/silica gel modified with a polyanionic additive of sodium poly(4-styrenesulfonate) (NaPSS). NaPSS polymer chains serve as a passivating layer at the photoanode/electrolyte interface and reduce back electron transfer, while simultaneously allowing efficient hole transfer to the electrolyte. An optimal NaPSS content is ascertained from the highest PCE, conductivity and longest photovoltage decay time achieved for the electrolyte with 5 wt% NaPSS, compared to electrolytes with lower and higher NaPSS contents. The efficacy of PEDOP@C-cloth as a CE for the solar cell is confirmed from a lower charge transfer resistance and a higher diffusion current density for polysulfide reduction at the CE/electrolyte interface compared to sole C-cloth. This amplified solar cell performance is attained owing to the synergy of all the three components: NGPs at the photoanode, NaPSS in the electrolyte and PEDOP at the CE thus manifesting in a PCE of 7.7% for the TiO2/NGPs/CdS/polysulfide/silica/NaPSS gel/PEDOP@C-cloth cell relative to 3.5% attained for the TiO2/CdS/polysulfide/silica gel/C-cloth cell.

Recently, the electron mobility in wedge-shaped c-GaN nanowall networks has been estimated to cross the theoretical mobility limit for bulk GaN. Significant blue-shift of the bandgap has also been observed. Both the findings are explained in terms of two-dimensional electron gas (2DEG) formed at the central vertical plane of the walls due to the polarization charges at the two inclined faces. Carrier concentration and mobility have earlier been determined from thermoelectric power and conductivity measurements with the help of a statistical model. Due to the network nature of the system, direct measurements of these quantities from Hall experiments are not possible. Search for a better way to estimate mobility in this system thus becomes important. Since, strain can also lead to the blue-shift of the bandgap, it is also imperative to evaluate carefully the role of strain. Here, using Raman spectroscopy, we have estimated carrier concentration and mobility in these nanowall networks with varied average tip-widths. Depth distribution of strain and luminescence characteristics are also studied. The study reveals that strain has no role in the bandgap enhancement. Moreover, the electron mobility, which is determined from the lineshape analysis of the A1(LO)-plasmon coupled mode in Raman spectra, has been found to be significantly higher than the theoretical limit of mobility for bulk GaN for the same electron concentration. These results thus corroborate the picture of polarization induced vertical 2DEG formation in these walls as predicted theoretically.

Epitaxial metal/semiconductor superlattices with atomically sharp interfaces and tunable Schottky barrier heights have attracted significant interest in recent years for thermionic emission-based high-temperature thermoelectric devices, optical hyperbolic metamaterials, hot-electron photocatalysis, and optoelectronic heterostructures for visible-to-terahertz frequency range applications. ZrN/ScN is a demonstration of such epitaxial metal/semiconductor superlattices and exhibits atomically sharp lattice-matched interfaces, albeit with the presence of threading dislocations on MgO substrates. Along with its influence on structural integrity and atomic diffusion, the presence of such defects significantly impacts electron and phonon transport in these metamaterials with carrier trapping, scattering, shunt path, etc. Therefore, an in-depth analysis of the atomistic structure and the composition of such defects is extremely necessary to design devices with improved efficiencies. In this paper, high-resolution scanning transmission electron microscopy and atom-probe tomography are employed to determine the structure and three-dimensional (3D) spatial distribution of oxide defect clusters along the voids in ZrN/ScN superlattices. $\mathrm{Sc}{\mathrm{O}}^{+}$ and $\mathrm{Sc}{\mathrm{O}}^{++}$ ions are found to cluster predominantly along such 3D interface defects with zirconium and scandium atoms surrounding them. Defect regions are also found to be depleted of nitrogen atoms and rich with a high concentration of oxygen. The oxygen content was found to be higher inside the ScN layers compared to ZrN. First-principles modeling analysis verified the clustering of oxygen at high oxygen partial pressure and demonstrated a higher affinity of scandium towards oxygen than for zirconium towards oxygen. These results mark significant progress in understanding the atomic structure and composition of defects in nitride superlattices.

A Lithium (Li)‐Selenium (Se)‐alkali activated carbon hybrid cell with a tungsten oxide interlayer is implemented for the first time. The Se hybrid at a Se loading of 70% in the full Li‐Se cell, delivers a large reversible capacity of 625 mAh gSe‐1 as opposed to 505.8 mAh gSe‐1 achieved for the pristine Se cell. This clearly shows the advantage of the carbon in improving the capacity of the Li‐Se cell. To further circumvent the issues of polyselenide dissolution and shuttle which cause severe capacity fade, a tungsten oxide interlayer is drop‐cast over the battery separator. While the oxide layer conducts Li‐ions which is evidenced from a Li‐ion diffusion coefficient of 4.2 × 10‐9 cm2 s‐1, it simultaneously blocks the polyselenide cross‐over for it is impermeable to polyselenides, thereby reducing the capacity fade with cycling. The outcome of this unique approach is reflected in the reversible capacities of 808 and 510 mAh gSe‐1, achieved for the Li‐oxide@separator/Se‐ alkali activated carbon cell before and after 100 cycles, thus demonstrating that carbon and oxide can efficiently restrict the capacity fade and improve the performances of Li‐Se cells.

A cost effective, low temperature, scalable route is presented for the synthesis of Selenium nanoparticles-cetyltrimethylammonium bromide-decorated multiwalled carbon nanotubes (Se NPs@CTAB-MWCNTs) composite. The Se content in the carbon black (CB) free Li-Se NPs@CTAB-MWCNTs cell is ∼72 wt%, which is much higher than that reported in previous studies on Li-Se composite cells, where the Se loadings typically vary between 40 and 50 wt% in full cells. The high loading of Se imparts a high capacity to the composite cell due to a high electrical conductivity of Se NPs (∼0.1 S cm−1). The initial Li-ion storage capacity delivered by the same CB free cell is ∼709 mAh gSe−1 (at 0.5 current (C)-rate), and a reversible capacity of 157 mAh gSe−1 is retained after 500 cycles. In comparison, the reversible capacities offered by the Li-Se NPs-CB cell (at 80 wt% Se loading), and the Li-Se NPs@CTAB-MWCNTs-CB cell (at 64 wt% Se loading) are 13.8 and 172.5 mAh gSe−1 at the same C-rate, after 500 cycles. For the CB free cell, the capacity decay is 0.14% per cycle from 8th to 500th cycle. Though present in small quantities of 18 and 16 wt% in the two composite based cells without and with CB, CTAB-MWCNTs confer the following functional attributes to the cells. They (a) restrict polyselenide shuttle, (b) buffer the volume expansion during discharge and (c) allow Li-ion storage, without the use of any conducting CB additive (due to the composite's high inherent electrical conductivity). The electrochemical performance of the CB free Li-Se composite cell opens up opportunities to develop eco-friendly and robust Li-Se batteries.

Cu 2 ZnSiS 4 thin film has been synthesized using co-sputtering technique followed by sulphurization at 800 °C for 20 min. The relatively lower time of sulphurization employed are advantageous over earlier reported work in which the same material has been prepared in bulk form with a sulphurization time of ~ 24 h. The structural, optical and electrical properties of Cu 2 ZnSiS 4 have been studied using macroscopic and nanoscopic characterization methods. Raman results confirm the formation of Cu 2 ZnSiS 4 phase and X-ray diffraction spectra shows that its diffraction pattern is similar to Sn based (Cu 2 ZnSnS 4 ) kesterite phase. The direct band gap of this new material is found to be ~ 2.71 eV. The conductivity-temperature analysis in the high temperature range (380 K–150 K) shows hopping conduction via defect states to be the conduction mechanism. The average work function of this material is 4.92 eV and Kelvin probe force microscopy results show an upward band bending at grain boundaries in comparison to intra-grain. X-ray photoelectron spectroscopy confirms the + 4 oxidation state of Si in the Cu 2 ZnSiS 4 thin film sample. Further, results of Kelvin probe force microscopy, UV–visible and Ultra-voilet photoelectron spectroscopy measurements have been used to obtain energy band diagram of this new material which also reveal p-type semiconducting nature of the Cu 2 ZnSiS 4 and indicate its usefulness as a photoelectrochemical material.

We report here the influence of Mg and Si-doping during growth, on the morphology, structure, and optical properties of single-crystalline GaN nanorods (NRs) grown on Si substrates using plasma-assisted molecular beam epitaxy. Mg-doping is shown to enhance the lateral growth of the NRs, leading to a higher degree of coalescence. Si-doping during the nucleation stage of the growth enhances the mutual misorientation of the NRs. Strain profile measurements along the length of individual NR by transmission electron microscopy shows that the top regions are relaxed. Evaluation of carrier concentration by Raman spectroscopy reveals that Si-doping leads to an increase of carrier concentration from 1016 to 1017 cm−3, and the optimal Mg incorporation for the realisation of p-doping is confirmed by photoluminescence spectroscopy. These results will significantly help in understanding and tuning the structural and optical properties of GaN NRs through doping in the fabrication of NR based optoelectronic devices.

Heterojunction nanorods having dissimilar semiconductors possess charge transfer (CT) properties and are proposed as active elements in optoelectronic systems. Herein, we describe the synthetic methodologies for controlling the charge carrier recombination dynamics in CdSe–CdTe heterojunction nanorods through the precise growth of CdTe segment from one of the tips of CdSe nanorods. The location of heterojunction was established through a point-by-point collection of the energy-dispersive X-ray spectra using scanning transmission electron microscopy. The possibilities of the growth of CdTe from both the tips of CdSe nanorods and the overcoating of CdTe over CdSe segment were also ruled out. The CT emission in the heterojunction nanorods originates through an interfacial excitonic recombination and was further tuned to the near-infrared region by varying the two parameters: the aspect ratio of CdSe and the length of CdTe segment. These aspects are evidenced from the emission lifetime and the femtosecond transient absorption studies.

A novel yet simple approach of carbonate (CBN) treatment of TiO2 films is performed, and quantum dot solar cells (QDSCs) with high power conversion efficiencies (PCEs), reasonably good stabilities, and good fill factors (FFs) are fabricated with TiO2–CBN films. The ability of carbonate groups to passivate defects or oxygen vacancies of TiO2 is confirmed from a nominally enhanced band gap, a lowered defect induced fluorescence intensity, an additional Ti–OH signal obtained after carbonate decomposition, and a more capacitive low frequency electrochemical impedance behavior achieved for TiO2–CBN compared to untreated TiO2. A large area QDSC of 1 cm2 with a TiO2–CBN/CdS/Au@PAA (poly(acrylic acid)) photoanode delivers an enhanced PCE of 4.32% as opposed to 3.03% achieved for its analogous cell with untreated TiO2. Impedance analysis illustrates the role of carbonate treatment in increasing the recombination resistance at the photoanode/electrolyte interfaces and in suppressing back-electron transfer to the el...

The intrinsic physical properties of nanostructures of metals and their oxides are altered when they are prone to surface oxidation in ambient atmosphere. To overcome this limitation, novel synthesis methodologies are required. In this study, solid octahedral shapes of MnO limit the inward oxygen diffusion compared to that of the MnO-nanoparticle-assembled octahedra. In addition to morphology control, which restricts the thickness of the Mn3O4 surface layer, the binding chemistry of the surfactants plays an essential role. For example, the Mn3O4 surface layer is 0.4 nm thinner with trioctylphosphine oxide than with trioctylamine as the surfactant. The nanostructures were prepared by varying the surfactants, surfactant-to-precursor molar ratio, accelerating agent, and reaction heating rate. The surface oxidation of MnO nano-octahedra was probed by Rietveld analysis of X-ray diffraction patterns and X-ray photoelectron spectroscopy and characterized by magnetic measurements, as the presence of ferrimagnetic Mn3O4 shell on the antiferromagnetic MnO core provides an exchange coupling at the core–shell interface. Thicker the Mn3O4 shell, higher is the exchange-biased hysteresis loop shift.

A hybrid system consisting of Ag nanoparticles dispersed onto a GaN nanowall network (GaN NWN) exhibited characteristic optical properties and electronic band structure. Surface-sensitive XPS studies of this high-surface-area system revealed the presence of a high surface charge carrier concentration due to dangling bonds, which resulted in a high metal-like surface conductivity. The low coverage of absorbed Ag led to the nanocluster formation, facilitating charge transfer from GaN to Ag, and thereby further increasing the surface charge carriers. Photoluminescence studies revealed the presence of a high density of band tail states at the conduction band, which is significantly (14-fold) larger than in the GaN epilayer. Raman studies show an increase (2.46-fold) in the interfacial strain at the Ag/GaN interface after the deposition of the Ag nanoparticles. We show that these surface modifications increase the density of hot spots, resulting in an intense Raman signal with an enhancement factor of 107. The role of the charge transfer between Ag nanoparticles and GaN NWN in the enhancement of Raman signal has been demonstrated.

Intentional doping and unintentional impurities in intrinsic semiconductors generate carriers that enable device operations. Fundamental to the electronic activity of dopants and impurities is the introduction of defect states inside the forbidden energy gap of semiconductors having shallow and/or deep characteristics, which fundamentally define the ability to engineer its physical properties and associated device performance. Here we demonstrate that unintentional electron doping by oxygen (${\mathrm{O}}_{\mathrm{N}}$) impurities and intentional magnesium hole doping ($\mathrm{M}{\mathrm{g}}_{\mathrm{Sc}}$) in scandium nitride (ScN) do not introduce any defect states inside its fundamental bandgap and that the rigid-band electronic structure remains unchanged. Employing a combination of spectroscopic techniques as well as first-principles density functional theory analysis, we show that the ${\mathrm{O}}_{\mathrm{N}}$ and $\mathrm{M}{\mathrm{g}}_{\mathrm{Sc}}$ defect states in ScN are located inside the bands, which leaves behind the virgin ScN bandgap as well as the valence and conduction band edges that are important for electronic transport. The rigid-band electronic structure of ScN with respect to the electron and hole doping results in high electron and hole concentrations due to the free movement of Fermi level and results in tunable electronic and thermoelectric properties necessary for device applications.

Lithium-sulfur (Li-S) batteries are technologically significant for sulfur is cheap, and offers high gravimetric capacity and a large energy density. But achieving long term cyclability with moderate capacity loss, and scalability pose formidable challenges. A solution phase approach for the preparation of a composite of sulfur with hydroxyl groups functionalized multiwalled carbon nanotubes (MWCNTols) and coated with poly(3,4-ethylenedioxypyrrole) (PEDOP) is presented for the first time. Comparison of the Li-S performances at 0.1 C current-rate show that the S based cell with a S-loading of 80% retains a low capacity of 122 mAh gsulfur−1 after 100 cycles, whereas cells with S/MWCNTols and S/MWCNTols/PEDOP composites with sulfur loadings of 73 and 70% respectively, retain capacities of 384 and 624 mAh gsulfur−1 after 200 charge-discharge cycles, with Coulombic efficiencies of 96 and 98.7% respectively. This performance differential illustrates the role of PEDOP in inhibiting sulfur loss and in maximizing cell response. The polymer provides electrical interconnects between the insulating sulfur clusters and facilitates Li+ transfer at the interface. The ease of the synthesis, coupled with the remarkable cycling performance delivered by this composite at a high sulfur-loading, demonstrate the promise that this S/CNT/conducting polymer composite has for practical Li-S batteries.

We report the growth of a Mg incorporated GaN nanowall network (NwN) by using a plasma assisted molecular beam epitaxy (PA-MBE) system that was characterized by photoluminescence (PL) spectroscopy, Raman spectroscopy, high-resolution X-ray diffraction (HR-XRD), X-ray photoelectron spectroscopy (XPS) and secondary ion mass spectroscopy (SIMS). We record a PL enhancement (≈3.2 times) in nominally and moderately Mg incorporated GaN compared to that of an undoped NwN. Two distinct (and broad) blue luminescence peaks appear at 2.95 and 2.70 eV for the heavily Mg incorporated (1020 atoms per cm3) GaN, of which the 2.95 eV peak is sensitive to annealing. The incorporated Mg concentration is estimated to be 1020 atoms per cm3 while retaining its band edge emission at ≈3.4 eV. XPS valence band spectra shows a shift in the Fermi level towards Valence Band Maximum (VBM) and the increase in Mg flux confirms the formation of p-type GaN. Mg accumulation at the nanowall surface and the GaN/Al2O3 interface was observed from SIMS measurements.

Point defects create exotic properties in materials such as defect-induced luminescence in wide-bandgap semiconductors, magnetism in nonmagnetic materials, single-photon emission from semiconductors, etc. In this article, oxygen defect formation in metallic TiN and semiconducting rock salt-(Al,Sc)N is investigated with a combination of first-principles density functional theory, synchrotron-based x-ray absorption spectroscopy (XAS) analysis, and scanning transmission electron microscopy–energy-dispersive x-ray spectroscopy mapping. Modeling results show that oxygen in TiN and rock salt-(Al,Sc)N prefers to be in the defect complex of substitutional and interstitial oxygen (nON + Oi) types. While in TiN, the preferential interstitial sites of oxygen in ON + Oi are at the tetrahedral site, in rock salt-(Al,Sc)N, a split interstitial site along the [111] direction was found to be energetically preferable. Simulations performed as a function of the oxygen partial pressure show that under experimental growth conditions, four oxygen atoms at the substitutional sites of nitrogen (4ON), along with four Ti atoms, decorate around an interstitial oxygen atom at the tetrahedral site (Oi) in the energetically favored configuration. However, in rock salt-(Al,Sc)N, n in nON + Oi was found to vary from two to four depending on the oxygen partial pressure. Theoretical predictions agree well with the experimentally obtained XAS results. These results are not only important for a fundamental understanding of oxygen impurity defect behavior in rock salt nitride materials but will also help in the development of epitaxial metal/semiconductor superlattices with efficient thermionic properties.

We report on the growth of single crystalline InGaN nanostructures with minute compositional variation on sapphire at a low substrate temperature of 350 °C that shows good optical and electrical properties. We investigate the photoelectrochemical performance of InGaN with two different average In compositions (13 and 16%) and compare the properties by forming their heterostructures with TiO2 (InGaN/TiO2). A significant enhancement in photocurrent density has been observed by using the heterostructures with more In content, in comparison to the InGaN having the same In content and also with the heterostructure having lesser In content. Though the average In compositional difference is only 3%, the de-convoluted XRD (0002) curve shows significant change in one of the phases (In0.29Ga0.71N) with an increment in both indium composition (6%) and in the phase area, which results in the significant difference in photoelectrochemical properties. The transient photo response analysis shows that both samples are fast responsive, while the photocurrent density is significantly different with the fabrication of heterostructures. A suitable change in band diagram of the system is sketched, which shows the band bending enhances the separation and migration of the photogenerated carriers to the semiconductor/electrolyte interface, thereby the photocurrent density.

We determine atomic structure, electronic structure, formation energies, magnetic properties of native point defects, such as gallium (Ga) and nitrogen (N) vacancies, in bulk and at the nonpolar (10 1 ¯0) surface of wurtzite gallium nitride (w-GaN) using first-principles density functional theory (DFT) based calculations. In bulk and at the (10 1 ¯ 0) surface of GaN, N vacancies are significantly more stable than Ga vacancies under both Ga-rich and N-rich conditions. We show that within DFT-local density approximated N vacancies form spontaneously at the (10 1 ¯ 0) surface of GaN when doped to raise the Fermi level up to ≈ 1.0 eV above valence band maximum (VBM) while with valence band edge correction it is 1.79 eV above VBM. We provide experimental evidence for occurrence of N vacancies with electron energy loss spectroscopy measurements, which further hints the N vacancies at surface to the source of auto-doping which may explain high electrical conductivity of GaN nanowall network grown with molecular beam epitaxy.

1-dimensional GaN nanowires (NW) and nanorods (NR) grown using catalyst assisted Chemical Vapour Deposition are investigated for their structural and optical properties, and their electronic band structure. X-ray Diffraction (XRD) studies reveal that biaxial strain increases with aspect ratio, which is corroborated by Raman studies wherein phonon related confinement effects are observed for NR due to the crystal strain. Photoluminescence studies show 3 and 6 times enhancement in the intensity of band edge emission of NW and NR, respectively, signifying the presence of a large number of band tail states. A small red shift in NW (3.43 eV) and large red-shift in NR (3.28eV) is reported, which are attributed to the change in the band gap and band structure of these morphologies. Valence band studies by X-ray Photoelectron Spectroscopy reveal the presence of band tail states for NR and surface bond contraction for NW. The effect of crystal strain on the electronic band structure on optical properties of 1-D GaN nanostructures with different aspect ratios is elucidated.

Nitride alloys are considered potential candidates as photoelectrodes for photoelectrochemical water splitting. Here we show an In0.25Ga0.75N layer activated by indium nitride quantum dots as efficient photoanode for photoelectrochemical hydrogen generation by water splitting when directly grown on cheap silicon (111) substrates. Photocurrent measurements show more than five times enhancement by the indium nitride quantum dots compared to a bare In0.25Ga0.75N-on-silicon photoanode. The maximum incident photon-to-current conversion efficiency is 44% at 550 nm at 0.4 V, the applied-bias photon-to-current efficiency is 4.1% and the hydrogen and oxygen generation rates are 75 µmol h−1 cm−2 and 33 µmol h−1 cm−2 at 0.2 V under 100 mW cm−2 white light illumination. Nitride based photoelectrodes are promising candidates for photoelectrochemical water splitting and hydrogen generation but suffer from quick degradation. Here the authors show indium gallium nitride activated by InN quantum dots on silicon which can be used as stable photoanode for efficient water splitting.

A hybrid system consisting of Ag nanoparticles dispersed on GaN nanowall network (GaN NWN) exhibits its characteristic optical properties and electronic band structure. XPS studies of this system reveal the presence of a high density of surface charge carriers due to charge transfer between the conduction bands of GaN and Ag. Surface Enhanced Raman Spectrocopy (SERS) studies show that these surface modifications increase the density of hot spots and result in an enhanced Raman signal with an enhancement factor of 107 and limit of detection of 10−10 M. Role of the optoelectronic properties of the substrate on the enhancement of SERS signal has been discussed.

This work proposes p-TiO

We study the native charge compensation effect in Mg doped GaN nanorods (NRs), grown by Plasma Assisted Molecular Beam Epitaxy (PAMBE), using Raman, photoluminescence (PL) and X-ray photoelectron spectroscopies (XPS). The XPS valence band analysis shows that upon Mg incorporation the E$_F$-E$_{VBM}$ reduces, suggesting the compensation of the native n-type character of GaN NRs. Raman spectroscopic studies on these samples reveal that the line shape of longitudinal phonon plasmon (LPP) coupled mode is sensitive to Mg concentration and hence to background n-type carrier density. We estimate a two order of native charge compensation in GaN NRs upon Mg-doping with a concentration of 10$^{19}$-10$^{20}$ atoms cm$^{-3}$. Room temperature (RT) PL measurements and our previous electronic structure calculations are used to identify the atomistic origin of this compensation effect., 5 pages, 5 Figure

The work presents a systematic study of energetic N 2 + ion interaction with the clean Al 2 O 3 surface at room temperature. Energetic N 2 + ions with energies ranging from 0.1 to 5 keV were bombarded onto the c -plane Al 2 O 3 surface in situ in a UHV system equipped with X-ray Photoelectron Spectroscopy. Survey scans and core level spectra of Al(2p), O(1s), N(1s) were recorded as a function of ion fluence. Survey scans of XPS are used for the compositional analysis, while deconvoluted core level spectra are used to identify the evolution of the chemical bonding. Energetic dependence of N 2 + ions occupying interstitial and substitutional sites in Al 2 O 3 lattice are probed to follow the surface evolution. Results show that maximum thickness of surface is nitride by 5 keV N 2 + ion with an optimal fluence of 1.5 × 10 15 ions/cm 2 . This modified surface can be used as a template for low defect III-nitrides growth, with enhanced lattice matching than on bare c -Al 2 O 3 .

Origin of unprecedentedly high electron mobility observed in the c-axis oriented GaN nanowall networks is investigated by studying the depth distribution of structural, electrical and optical properties of several such high mobility samples grown by molecular beam epitaxy.(MBE) technique for different time durations. It has been found that in two hour grown samples, walls are tapered continuously from the bottom to the top. While in four hour grown samples, walls are flat-topped with the top surface containing certain secondary tip structures. These additional features run along the length of the walls to form a well-connected network. Our study reveals that the carriers are quantum mechanically confined not only in the secondary tip structures but also in the wider part of the walls. The secondary tip structures, which are found to offer higher mobility than the rest of the network, are also identified as the regions of stronger confinement. The effect of mobility enhancement observed in these samples has been attributed to a 2D quantum confinement of electrons in the central vertical plane of the walls. (C) 2015 Elsevier Ltd. All rights reserved.

Effect of nanoparticle size and alloying has been well studied in Pd and Pd alloy nanoparticles and explained on the basis of modifications in the crystal structure and Pd 4d binding energy position. In the present study, the effect of carbon shell thickness on the hydrogenation properties of gas phase synthesized Pd–C core–shell nanoparticles having controllable nanoparticle core and shell thickness has been investigated. A large improvement in H/Pd ratio and increased hydrogen induced lattice expansion is observed in Pd–C nanoparticles. These results are explained on the basis of lowering of coordination number of Pd surface sites at Pd–C core–shell interface revealed by the increase in the Pd 4d binding energy on increase in shell thickness. By combining the modification in electronic properties due to the size and the alloying effects with that due to carbon shell thickness, as reported in the present study, Pd–H interaction can be significantly enhanced.

Surface nitridation of the c-sapphire substrate is used to improve the optical and structural quality of a GaN nanowall network film grown by plasma assisted molecular beam epitaxy. The nitridation results in the formation of a thin AlN layer on the sapphire surface. Several in-situ and ex-situ characterization are complementarily used to probe the changes in epitaxial growth, band edge emission and strain in the films. The GaN nanowall network layer formed on the pre-nitrided substrate shows a two order higher intensity of band edge luminescence than that of a GaN epilayer, demonstrating its potential for light-emission applications. Open image in new window

This work examines the changes in bonding of GaN nanowall network samples with varying morphology grown by rf-plasma assisted molecular beam epitaxy system. X-ray photoelectron spectroscopy is employed to study the valence band spectra of two samples and the difference in bonding with change in morphology is observed. Compared to a standard epilayer, the nanostructured samples display a higher binding energy for different hybridization levels appearing near the top of valence band. Higher than statistical branching ratio of Ga 2p spin split levels is also observed unlike in the case of epilayer. The higher bonding energy is understood as having more stable electronic structure which is possible because of reduction of defects in the nanostructured films.

A layer-by-layer (LbL) assembly of poly(butyl viologen) (PBV) and poly(peroxotungstate) was used for preparing a cathodically coloring dual electrochrome WO 3 –PBV film. A new formulation containing ferrocenecarboxaldehyde and potassium hexacyanoruthenate(II) was employed for the first time and anodically coloring ruthenium purple (RP) thin films were electrodeposited. X-ray photoelectron spectroscopy and Fourier transform infrared analyses confirmed the RP film structure to be an inorganic coordination polymer: K x’ Fe x III [Ru II (CN) 6 ] y , wherein Fe 3+ and Ru 2+ color centers are flanked by CN − ligands. Nanoscale electrical conductivities determined by conducting atomic force microscopy were deduced to be 1.03 and 147.4 mS cm −1 , for the RP and WO 3 –PBV LbL films respectively, which confirmed their ability to function as mixed conductors, attributes pertinent during optical switching. Reversible bias induced transition between purple and colorless hues was attained in the RP film and between deep blue and colorless states was achieved for the WO 3 –PBV LbL film with coloration efficiencies of 188 and 380 cm 2 C −1 respectively. A complementarily coloring electrochromic device of WO 3 –PBV/RP was fabricated with an environmentally benign, inexpensive aqueous electrolyte, which switched between blue, purple, and colorless states. The device exhibited an electrochromic efficiency of 476 cm 2 C −1 at 580 nm, an outstanding transmission modulation of 49%, and short switching times (~1 s); all under remarkably low operating voltages of ±1 V. The WO 3 –PBV/RP device can deliver high optical contrast under low operating bias, and is highly scalable and durable, which demonstrates it to be ideal for practical optical switching devices like fast changing displays or smart windows.

In the present experiment, two GaN nanowall network (NWN) samples with different porosity were grown on c-sapphire substrates using plasma assisted molecular beam epitaxy (PA-MBE). Ag nanoparticles were deposited on both the samples using a physical vapour deposition (PVD) system. Annealing the samples at different temperatures resulted in a change in Ag nanoparticle size due to diffusion and Ostwald ripening which had significant effect on the photoluminescence and SERS activity of GaN NWN. It was observed that the photoluminescence yield increased by more than five times in both cases at 200 °C. The SERS activity for thiophenol is higher in the as-deposited case for the sample with higher porosity, but after annealing to 200 °C the activity increased for the sample with lower porosity. It is also interesting to observe that the sample with higher porosity shows SERS signals even after being annealed to higher temperatures. Studies are also done for other analytes such as R6G and BSA. The results are discussed in terms of plasmonic effects of Ag nanoparticles on the excitonic emission from the GaN surface, which is also simulated using 2D-FDTD simulations.

Low energy argon ion bombardment of different fluence onto a GaN film grown on Al 2 O 3 (0 0 0 1) surface induces interesting compositional and morphological modifications. While atomic force microscopy (AFM) studies reveal the formation of a circular flat islands, Energy-dispersive X-ray spectroscopy (EDX) and X-ray photoelectron spectroscopy (XPS) core-level and valence-band spectra confirm that these nanodroplets consist of pure Ga. Detailed XPS studies suggest breaking of Ga N bond, that releases Ga to diffuse on the surface and nucleate at the threading dislocation sites, defined by the interfacial surface free energy. The formation of droplet pattern formation is discussed in the light of the existing theories on ion induced surface modification.

A combined set of thermo-mechanical steps recommended for high strength beta Ti alloy are homogenization, deformation, recrystallization, annealing and ageing steps in sequence. Recrystallization carried out above or below beta transus temperature generates either beta annealed (lath type morphology of alpha) or bimodal (lath+globular morphology of alpha) microstructure. Through variations in heat treatment parameters at these processing steps, wide ranges of length scales of features have been generated in both types of microstructures in a near beta Ti alloy, Ti-5Al-5Mo-5V-3Cr (Ti-5553). 0.2% Yield strength (YS) has been correlated to various microstructural features and associated heat treatment parameters. Relative importance of microstructural features in influencing YS has been identified. Process parameters at different steps have been identified and recommended for attaining different levels of YS for this near beta Ti alloy. (C) 2014 Elsevier B.V. All rights reserved.

We present the surface modification of Si(111) into silicon nitride by exposure to energetic N2+ ions. In-situ UHV experiments have been performed to optimize the energy and fluence of the N2+ ions to form silicon nitride at room temperature (RT) and characterized in-situ by X-ray photoelectron spectroscopy. We have used N2+ ion beams in the energy range of 0.2–5.0 keV of different fluence to induce surface reactions, which lead to the formation of SixNy on the Si(111) surface. The XPS core level spectra of Si(2p) and N(1s) have been deconvoluted into different oxidation states to extract qualitative information, while survey scans have been used for quantifying of the silicon nitride formation, valence band spectra show that as the N2+ ion fluence increases, there is an increase in the band gap. The secondary electron emission spectra region of photoemission is used to evaluate the change in the work function during the nitridation process. The results show that surface nitridation initially increases rapidly with ion fluence and then saturates.

Small changes in electrical conductance of quasi-2 dimensional samples often need to be measured at low temperatures, T ∼ 1 K and lower. Care needs to be taken to minimize self-heating due to the excitation current itself and the possibility of any unexpected source of variation of the electrochemical potential around the contacts need to be understood. It is not only the low resistance of the ohmic contacts but also their “noninvasive” character that must be ensured. While the importance of the “noninvasiveness” of the voltage probes is well appreciated for mesoscopic devices—it turns out that these considerations continue to remain important even in macroscopic samples that are a few millimeters in size. We have done low temperature measurements on different types of gallium nitride samples with contacts made of pure indium (popularly called indium dots), which have a superconducting transition and contacts made of Ti/Au, which remain nonsuperconducting. We show that there are qualitative differences in the measured four-terminal resistance in the two cases. Obviously, correct conclusions about phase relaxation times, localization lengths, etc., will require ensuring that there are no effects arising from the “invasiveness” of the probes.

A simple strategy to improve the efficiency of a ZnO-nanorod-based dye-sensitized solar cell (DSSC) by use of Au-encapsulated carbon dots (Au@C-dots) in the photoanode is presented. The localized surface plasmonic resonance of Au in the 500-550 nm range coupled with the ability of C-dots to undergo charge separation increase the energy-harvesting efficiency of the DSSC with ZnO/N719/Au@C-dots photoanodes. Charge transfer from N719 dye to Au@C-dots is confirmed by fluorescence and lifetime enhancements of Au@C-dots. Forster resonance energy transfer (FRET) from the gap states of ZnO nanorods to N719 dye is also ratified and the energy transfer rate is 4.4×108 s-1 and the Forster radius is 1.89 nm. The overall power conversion efficiency of the plasmonic and FRET-enabled DSSC with ZnO/N719/Au@C-dots as the photoanode, I2/I- as the electrolyte and multiwalled carbon nanotubes as the counter electrode is 4.1 %, greater by 29 % compared to a traditional ZnO/N719 cell. Covering all possibilities: Au@C-dots function as plasmonic electron conduits and also undergo photo-induced charge separation in FRET-enabled high-performance ZnO based dye-sensitized solar cells

We probe the parametric dependence of the nitridation mechanism of Al 2 O 3 surface by radio frequency nitrogen plasma in a molecular beam epitaxy system. Our quantitative analysis by XPS and RHEED shows that the chemical composition varies with exposure time and is independent of nitrogen plasma fluence and substrate temperature. Here we show that nitrogen incorporation into the host material is diffusion limited process and involves the conversion of Al 2 O 3 into AlN, which has a higher rate initially and then saturates at 6 h of nitrogen plasma exposure. We deposit a thin AlN layer on this saturated nitrided sapphire substrate, which was found to consist of nanorods with a bimodal diameter distribution and apex morphology. By RHEED, XRD and TEM studies we attribute that the faceted nanocolumns are formed at misfit-induced dislocations with the dislocations propagating axially in the nanorods, while the oval apexed features are diffusion mediated. We find that nanorods of both the morphologies are c-oriented, single crystalline and strain relaxed, but possess different in-plane orientation.

Highly dispersed Pd(0) nanoparticles were successfully immobilized into a stable, crystalline and porous covalent organic framework (COF), TpPa-1, by a solution infiltration method using NABH4 as a reducing agent. High resolution and dark field TEM images confirmed the uniform loading of the Pd(0) nanoparticles into the TpPa-1 matrix without aggregation. This hybrid material exhibited excellent catalytic activity towards the Cu free Sonogashira, Heck and sequential one pot Heck–Sonogashira cross-coupling reactions under basic conditions, and with superior performance compared to commercially available Pd supported on activated charcoal (i.e., 1, 5 and 10 wt%). Additionally, the precursor Pd(II)-doped COF also displayed competitive catalytic activity for the intramolecular oxidative biaryl synthesis under acidic conditions. Both catalysts were found to be highly stable under the reaction conditions showing negligible metal leaching, non-sintering behavior, and good recyclability. To the best of our knowledge, the organic support used in this work, TpPa-1, constitutes the first COF matrix that can hold both Pd(0) nanoparticles and Pd(II) complex without aggregation for catalytic purposes under both highly acidic and basic conditions.

We report on very high brightness of high quality, dislocation free GaN nanowall network. The GaN nanowall network sample was grown by plasma assisted molecular beam epitaxy and the optical properties were investigated by photoluminescence (PL) and absorption spectroscopy. Other nanostructures and flat structures were grown for comparison and a standard epilayer was used as a reference sample. The PL intensity of the nanowall network structure was observed to be a hundred times more than nanostructures consisting of tubes, as well as flat films. To explain the broadness and the peak position values, strain was calculated by X-ray diffraction studies and a band diagram was proposed to elucidate the structure of these films using PL, absorption and XPS valence band values.

We are able to grow high quality InN films on indium metal induced 1 × 1 and 7 × 7 superstructural phases of Si(111) at lower growth temperatures than those previously employed. With multi-technique characterization probes, we show the promising route to obtain better crystallinity and emission properties for films that are grown on the 1 × 1 surface reconstruction at 200 °C, as compared to those grown on 7 × 7 reconstruction. These effects are observed in a narrow growth parametric region, and attributed to the better integral match of the 2D unit cells with the substrate reconstruction. Further improvement in the quality of InN films is accomplished by growing InN on the above formed epitaxial layers as intermediate layers which display PL emission even at room temperature. The effect of structural quality, carrier concentration, and stoichiometry on band edge emission and electron mobility of these films, show us directions towards obtaining improved quality InN films.

The crystal-structure-dependent luminescence properties of cadmium selenide quantum dots (QDs) in their cubic zincblende (Zb-CdSe) and hexagonal wurtzite (Wz-CdSe) phases have been investigated by maintaining their optical band gaps as well as the relative ratios of capping ligands the same. The Zb-CdSe QDs exhibited excellent photostability and high photoluminescence quantum yield (ϕL = 0.375) compared with Wz-CdSe QDs (ϕL = 0.046). Detailed X-ray photoelectron spectroscopic (XPS) investigation revealed an important finding that the surface of Zb-CdSe is rich with Cd2+ ions, which leads to the formation of CdO layer. This was further confirmed by analyzing the (i) Cd-to-Se stoichiometric ratio of zincblende QDs (2:1 on surface and 1:1 after sputtering) and (ii) core-level XPS spectra of oxygen. In contrast, the Cd-to-Se stoichiometric ratio was found to be same (1:1) for Wz-CdSe QDs throughout the crystal. Thus, the high luminescence of Zb-CdSe is attributed to the formation of a thin layer of CdO, leadi...