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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.

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 (

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.

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 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.

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.

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.

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.

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

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 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.

Orofacial granulomatosis is a clinical entity presenting with swelling of the facial and/or oral tissues in association with histologic evidence of noncaseating granulomatous inflammation. Exclusion of other entities like sarcoidosis, mycobacterial infections, foreign body reactions and biopsy helps to establish a correct diagnosis. Here with reporting a case of lip swelling in 35-year old female along with palatal lesion, which was treated lucratively with steroids.

Indium induced surface reconstructions of Si(1 1 1)-7 × 7 are used as templates to grow high quality InN. We grow InN on Si(1 1 1)-7 × 7, Si(1 1 1)-4 × 1-In and Si(1 1 1)-1 × 1-In reconstructed surfaces and study the quality of the films formed using complementary characterization tools. InN grown on Si(1 1 1)-1 × 1-In reconstruction shows superior film quality with lowest band-edge emission having a narrow full width at half maximum, intense and narrow 0 0 0 2 X-ray diffraction, low surface roughness and carrier concentration an order lower than other samples. We attribute the high quality of the film formed at 300 °C to the integral matching of InN and super lattice dimensions, we also study the reasons for the band gap variation of InN in the literature. Present study demonstrates the proposed Superlattice Matched Epitaxy can be a general approach to grow good quality InN at much lower growth temperature on compatible In induced reconstructions of the Si surface.

We demonstrate spontaneous growth of a high density of self-organized, oriented, and epitaxial GaN low-dimensional nanostructures on Al2O3(0001) by sheer kinetic control without involving lithography, catalysts, buffer layers, or any surface pretreatment and consequently reducing process steps. GaN thin films grown by plasma assisted-molecular beam epitaxy (PA-MBE) form wurtzite GaN as a nanowall hexagonal network of flat 2-D films. In a narrow parametric window, 1-D nanocolumns of high density (1 × 108 cm–2) with excellent structural and optical properties are observed. The reduced adatom diffusion in the high nitrogen rich conditions is proposed to cause supersaturation and nucleation at edge and screw dislocations, forming nanowalls and nanocolumns.

We have identified crystal growth conditions in plasma-assisted molecular beam epitaxy (PAMBE) that leads to spontaneous formation of GaN nanostructure like nanowalls, nanotubes, and nanorods on c-plane sapphire. We have grown GaN on c-plane Al2O3 substrate at temperatures 630, 680 and 780 °C. The rf plasma forward power Wf =375 W and nitrogen content ∼4.5 sccm was kept constant for all growths. The micro-structural and structural information is extracted by high-resolution X-ray diffractometry (HRXRD) and reflection high-energy electron diffraction (RHEED). For studying optical properties of nanostructures photoluminescence (PL) spectroscopy is employed. X-ray diffraction study shows that the formed GaN nanostructures are of high crystalline quality and also well aligned along the symmetric (0 0 0 2), (0 0 0 4) peaks of wurtzite GaN. The morphology of grown nanostructures is characterized by field emission scanning electron microscopy (FE-SEM). The morphology of GaN nanostructures in a plan view of FE-SEM images shows that the nanostructures seem to be formed out of a well separated matrix layer containing deep faceted holes. The shape of fully strain relaxed nanostructure diameter varies from 50 to 200 nm at different temperatures. In this study we have optimized the deposition conditions needed to form self-assembled nanostructures of various sizes and assembly. The growth controlled by kinetics thus enables us to form spontaneous GaN nanostructures without use of catalysts or buffer layers.

In the advent of GaN photonic devices, there is renewed interest in looking at the growth of nanostructures of III-Nitrides that can yield novel properties. To grow a high density of self-assembled GaN nanostructures on a large scale, there is a need to use templates with well-defined nanometer scale features. The Si (5 5 12)-2 × 1 surface is a naturally faceted stable surface that has several nanotrenches of (225) and (337) with a large unit cell of 5.35 × 0.77 nm2. In this work we present here, the first report on the X-ray photoelectron spectroscopy study of the adsorption kinetics of Ga on Si (5 5 12)-2 × 1 reconstructed surface. The uptake curve which is the plot of the ratio of Ga (2 p ) to Si (2 p ) intensity with deposition time, shows that Ga grows in a Stranski–Krastnov growth mode on Si (5 5 12) surface. The core level spectra of Si (2 p ) and Ga (2 p ) have been deconvoluted to see the evolution of the interface during the adsorption process of Ga . The valence-band spectra show the modification of the electronic structure and the secondary electron emission spectra yield the change in work function during Ga adsorption. Correlating the electronic and structural aspects of the interface formation results the study demonstrates the possibility of integrating Si and group III-nitride technologies while exploiting the novel properties due to reduced dimensions.

This work proposes p-TiO2/n-GaN as a new semiconductor heterostructure, which holds great promise as a photoelectrode material. To fabricate p-TiO2/n-GaN heterostructures, wurtzite GaN is grown in two different morphologies by molecular beam epitaxy, and a TiO2 overlayer is formed by atomic layer deposition. The XRD and Raman experiments confirm the anatase crystal structure of TiO2 and SEM shows the conformal coating of TiO2 on GaN. The system with GaN nanowall network morphology (TiO2/GaN NWN, calls as TGN) showed better photoelectrochemical response with a photocurrent density of ∼0.65 mA cm-2 and an IPCE of 17% compared to 0.24 mA cm-2 and 6% of the planar heterostructure (TiO2/GaN epilayer, called TGE), at an applied bias of 1.24 V and at an incident power of only 13 mW cm-2. Chronoamperometric analysis show that electrodes are highly stable. The p-n diagram derived by using the data from cathodoluminescence (CL) and valence band (VB) spectra showed higher barrier height for the TGN structure due to interfacial band bending, and thus, favoring photogenerated charge separation. The evolution of hydrogen in the form of bubbles is visually evident at an applied bias of -0.56V for TGN. The enhanced photocurrent density, cathodic shift in the onset potential and the stability show the superiority of TGN to TGE in charge separation as well as in the photoelectrochemical performance.

Coherent transmission of Cooper pairs through a non-superconducting medium is a key challenge for hybrid electronics with superconductors, normal metals and semiconductors. While superconductor–normal metal–superconductor (SNS) junctions have been known for quite sometime, including a low carrier density region or a two-dimensional electron gas (2DEG) in the path of superconducting electrons is relatively less explored. Indeed, this is due to the limited choice of materials that would make ohmic contacts to such systems, while simultaneously supporting a superconducting phase. In this paper we show a coherent transmission of supercurrent through a degenerate semiconductor over a length ≈2 μm with a critical magnetic field B c ≈ 8 T at 1.6 K and T c ≈ 5 K at zero magnetic field. This length scale is much larger than the typical thickness of a Josephson junction. Our system is a fragment of a GaN nanowall network that has been shown to support a high mobility 2DEG (μ n > 104 cm2 V−1 s−1 ). The current and voltage probes were superconducting tungsten–gallium composite electrodes and the measurements could be done in four-probe geometry. We demonstrate ballistic type carrier transport with a near ideal transparency of 1 and a critical current (I c ) large enough such that the Josephson coupling parameter . Some features in magneto-transport data suggest that there is possibly a small magnetic moment forming in the semiconductor fragment. In addition the combination of a T c typical of elemental metallic superconductors, but a critical field that appears to be higher than the Clogston–Chandrasekhar limit, may be indicative of the emergence of a triplet pairing mechanism in these structures.

This work reports the interaction of aliphatic (triethyl amine, butyl amine) and aromatic amines (PPD, aniline) with CdSe quantum dots of varied sizes. The emission properties and lifetime values of CdSe quantum dots were found to be dependent on the oxidation potential of amines and crystallite sizes. Smaller CdSe quantum dots (size ∼5 nm) ensure better surface coverage of amines and hence higher quenching efficiency of amines could be realized as compared to larger CdSe quantum dots (size ∼14 nm). Heterogeneous quenching of amines due to the presence of accessible and inaccessible set of CdSe fluorophores is indicated. PPD owing to its lowest oxidation potential (∼0.26 V) has been found to have higher quenching efficiency as compared to other amines TEA and aniline having oxidation potentials ∼0.66 and >1.0 V, respectively. Butyl amine on the other hand, plays a dual role: its post-addition acts as a quencher for smaller and enhances emission for larger CdSe quantum dots, respectively. The beneficial effect of butyl amine in enhancing emission intensity could be attributed to enhance capping effect and better passivation of surface-traps.

The initial stages of room-temperature Mg film growth of Si (111) were studied using low-energy electron diffraction, electron energy-loss spectroscopy, and differential reflectance spectroscopy. In the studied range of magnesium film thickness (h = 0–0.2 nm), the formation of semiconductor magnesium silicide (Mg2Si), which is a promising material for silicon-silicide thermocouples, was detected. At small thicknesses of the deposited film, identical Mg2Si clusters are formed. An increase in the number of Mg atoms results in the formation of two-dimensional and then three-dimensional Mg2Si islands. Spectra of the optical response function δΛ″θ were measured for all structures; these spectra represent a characteristic of opticalproperties of these structures.

A novel route to achieve two dimensional (2D) carrier confinement in a wedge shaped wall structure made of a polar semiconductor has been demonstrated theoretically. Tapering of the wall along the direction of the spontaneous polarization leads to the development of charges of equal polarity on the two inclined facades of the wall. Polarization induced negative (positive) charges on the facades can push the electrons (holes) inward for a n-type (p-type) material which results in the formation of a 2D electron (hole) gas at the central plane and ionized donors (acceptors) at the outer edges of the wall. The theory shows that this unique mode of 2D carrier confinement can indeed lead to a significant enhancement of carrier mobility. It has been found that the reduced dimensionality is not the only cause for the enhancement of mobility in this case. Ionized impurity scattering, which is one of the major contributer to carrier scattering, is significantly suppressed as the carriers are naturally separated from the ionized centers. A recent experimental finding of very high electron mobility in wedge shaped GaN nanowall networks has been analyzed in the light of this theoretical reckoning.

The performance of perovskite solar cells is strongly influenced by the composition and microstructure of the perovskite. A recent approach to improve the power conversion efficiencies utilized mixed-halide perovskites, but the halide ions and their roles were not directly studied. Unraveling their precise location in the perovskite layer is of paramount importance. Here, we investigated four different perovskites by using X-ray photoelectron spectroscopy, and found that among the three studied mixed-halide perovskites, CH3 NH3 Pb(I0.74 Br0.26 )3 and CH3 NH3 PbBr3-x Clx show peaks that unambiguously demonstrate the presence of iodide and bromide in the former, and bromide and chloride in the latter. The CH3 NH3 PbI3-x Clx perovskite shows anomalous behavior, the iodide content far outweighs that of the chloride; a small proportion of chloride, in all likelihood, resides deep within the TiO2 /absorber layer. Our study reveals that there are many distinguishable structural differences between these perovskites, and that these directly impact the photovoltaic performances.

This paper addresses the issue related to morphology of CdSe nanoparticles capped with organic molecules. Semiconducting CdSe nanoparticles of 5–16 nm are synthesized using CdO precursor, capped with trioctyl phosphine (TOP)/trioctyl phosphine oxide (TOPO) using different starting precursor ratios of Cd:Se. At an optimum ratio of Cd/Se-2:1, highly luminescent and small sized (∼5 nm) nanoparticles are obtained. At other Cd/Se precursor ratios (0.5:1, 1:1, 3:1) larger particles are formed with varying photoluminescence (PL) intensity and optical absorption (UV–VIS). X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) are used to determine the crystallinity and stoichiometry of the system, respectively. It is shown that the blue shifts of the optical absorption edge concurrent with the CdSe nanocrystal size reduction, for sizes measured by XRD with respect to the bulk semiconductor, agree perfectly with the strong quantum confinement model. The optical edge shifts are significantly higher for CdSe nanocrystallite as measured by transmission electron microscopy (TEM) than the theoretical prediction based on the strong quantum confinement model. This is understood on the basis of agglomeration effects as observed by TEM for CdSe nanocrystallites. The nano-sized CdSe growth island thus formed comprises of several TOP/TOPO passivated nanocrystals.

We report the molecular beam epitaxy growth of device quality InN films on GaN epilayer and nano-wall network (NWN) templates deposited on c-sapphire by varying the film thickness up to 1 μm. The careful experiments are directed towards obtaining high mobility InN layers having a low band gap with improved crystal quality. The dislocation density is quantified by using high resolution X-ray diffraction rocking curve broadening values of symmetric and asymmetric reflections, respectively. We observe that the dislocation density of the InN films grown on GaN NWN is less than that of the films grown on the GaN epilayer. This is attributed to the nanoepitaxial lateral overlayer growth (ELOG) process, where the presence of voids at the interface of InN/GaN NWN prevents the propagation of dislocation lines into the InN epilayers, thereby causing less defects in the overgrown InN films. Thus, this new adaptation of the nano-ELOG growth process enables us to prepare InN layers with high electron mobility. The obt...

Acute pyelonephritis (APN), although a common clinical entity, still not much is known about the clinical profile in the Indian scenario. We prospectively collected clinical, biochemical, and radiological data of patients hospitalized with a diagnosis of APN from March 2014 to June 2016. A total of 296 cases were included in the study. Mean age was 53.85 ± 9.78 years. Male to females ratio was 1.93:1. Among the risk factors recognized for complicated pyelonephritis (PN), diabetes mellitus (DM) (54.4%) was the most common factor followed by renal calculi (14.4%), benign prostatic hyperplasia (6.7%), immunocompromised state (3.3%), stricture urethra and meatal stenosis (3.3%), and neurogenic bladder (2%). Urinary culture was negative in 153 (51.7%) and positive in 143 patient (48.3%). Most common organism isolated was Escherichia coli (29.7%), followed by Klebsiella pneumoniae (5.4%), pseudomonas (5.4%), Enterococcus (4.4%), and Proteus in 10 (3.4%). Serum creatinine of more than 1.5 mg/dl at admission was seen in 96.3% patients; 40% of them had underlying chronic kidney disease with DM being the most common. Multiorgan dysfunction either at admission or during the course in hospital stay was seen in 31.8% patients. Twelve (2%) had emphysematous PN. Six patients had Class II, 4 had Class III, 1 with Class I, and another with Class IV. A total of 18 deaths were noted (6.1%). Hemoglobin 1.5 mg/dl, HbA1c% >10%, and immunosuppression had statistically significant association with the development of multiorgan dysfunction on univariate analysis, but on multivariate analysis, only hemoglobin, HbA1c%, and immunosuppression reached statistical significance. Even with attributable risk of mortality, only hemoglobin, HbA1c%, and immunosuppression reached statistical significance on multivariate analysis. HbA1c% adds to the predictive parameters to recognize at-risk patients to intensify the treatment and avoid complications.

The work significantly optimizes growth parameters for nanostructured and flat GaN film in the 480–830 °C temperature range. The growth of ordered, high quality GaN nanowall hexagonal honeycomb like network on c-plane sapphire under nitrogen rich (N/Ga ratio of 100) conditions at temperatures below 700 °C is demonstrated. The walls are c-oriented wurtzite structures 200 nm wide at base and taper to 10 nm at apex, manifesting electron confinement effects to tune optoelectronic properties. For substrate temperatures above 700 °C the nanowalls thicken to a flat morphology with a dislocation density of 1010/cm2. The role of misfit dislocations in the GaN overlayer evolution is discussed in terms of growth kinetics being influenced by adatom diffusion, interactions and bonding at different temperatures. The GaN films are characterized by reflection high energy electron diffraction (RHEED), field emission scanning electron (FESEM), high resolution X-ray diffraction (HRXRD) and cathodoluminescence (CL).

In this article, we propose the formation of GaN nanowall morphology on Si (111) surface by varying nitrogen flux from 2, 4.5 and 6sccm in PA-MBE growth. The optical, structural and electrical properties of these films are investigated. Cathodo-luminescence data suggests that the nanowall network grown at 6sccm show high optical emission and high crystalline quality. The current-voltage (I-V) characteristics of the GaN films exhibit varying rectifying behavior at 1V with forward to reverse current ratios of 39, 155, 388 for the GaN layers grown at 2, 4.5 and 6 sccm while the reverse leakage current values are 1.2×10−3, 2.6×10−4 and 9×10−6 A respectively for these junctions. The good optical and electrical properties observed suggest that GaN/Si heterojunction grown at 6sccm nitrogen flow comprise of low defect density.

In the present study, GaN nanowall network (NWN) structures are grown using rf-plasma assisted molecular beam epitaxy (PA-MBE) system under highly nitrogen rich conditions over nitrided and bare c-plane sapphire substrates. Nitridation is carried out prior to growth for two of the samples and its effect on the crystal quality of the resultant nanowall structure is studied and compared with the same structure grown on bare sapphire. It is found that nitriding the c-plane sapphire prior to growth results in an improvement of crystalline quality of the resulting nanowall as observed by the reduction in FWHM of (0002) XRC.

A two step kinetically controlled single-growth process for the formation of InN 2D flat films and an array of nano rods (NRs) on GaN nano wall network (NWN) template by Plasma Assisted Molecular Beam Epitaxy (PAMBE) is demonstrated here. Surface and bulk diffraction studies reveal that the rods and film exhibit high quality structural and optical properties, in comparison to those formed on bare sapphire or GaN epilayer.

The transport and optical properties of wedge-shaped nanowall network of GaN grown spontaneously on cplane sapphire substrate by Plasma-Assisted Molecular Beam Epitaxy (PAMBE) show interesting behavior. The electron mobility at room temperature in these samples is found to be orders of magnitude higher than that of a continuous film. Our study reveals a strong correlation between the mobility and the band gap in these nanowall network samples. However, it is seen that when the thickness of the tips of the walls increases to an extent such that more than 70% of the film area is covered, it behaves close to a flat sample. In the sample with lower surface coverage (≈40% and ≈60%), it was observed that the conductivity, mobility as well as the band gap increase with the decrease in the average tip width of the walls. Photoluminescence (PL) experiments show a strong and broad band edge emission with a large (as high as ≈ 90 meV) blue shift, compared to that of a continuous film, suggesting a confinement of carriers on the top edges of the nanowalls. The PL peak width remains wide at all temperatures suggesting the existence of a high density of tail states at the band edge, which is further supported by the photoconductivity result. The high conductivity and mobility observed in these samples is believed to be due to a “dissipation less” transport of carriers, which are localized at the top edges (edge states) of the nanowalls.

We demonstrate here that nanostructuring of GaN thin film significantly enhances the band-edge emission, due to structural and geometrical effects. Films of increasing roughness are formed by kinetic control in a PA-MBE system and their morphological, structural and optical properties are compared by complementary characterization probes. The nanowall configuration with largest pore size (≈215 nm) shows a two orders of magnitude enhancement of integrated PL intensity in comparison to a GaN epilayer. Finite difference time domain (FDTD) simulation is performed to explain the role of total internal reflection and scattering on light extraction. The extended defects terminate proximal to interface, leading to most regions of the nanowalls are defect free, enhancing light generation. The observation of broad HRXRD rocking curves is attributed to a mosaicity that originates due to mutual misorientation of the nanowalls. Thus, the low dislocation density in the nanowalls and their suitable geometry promotes high light emission and extraction, respectively, offering this nanostructure as a potential material for high brightness LED fabrication.

An In-flux dependent study of the nature of epitaxy, compositional phase separation and band-edge emission of spontaneously formed c-oriented InGaN nanorods on c-sapphire is performed. At higher In flux-rates, m-faceted thick nanorods (≈700 nm) form with two in-plane epitaxial orientations, and display compositional phases with In composition varying from 14 to 63%. In these rods, photo-luminescent (PL) emission is seen to originate only from the localized high-In phase (63%) that is embedded in the low-In (14%) InGaN matrix. As the In flux-rate is reduced, nanorods of smaller diameter (≈60 nm) and a coalesced nanorod network are formed, with In incorporation of 15% and 9%, respectively. These faceted, c-aligned thinner nanorods are of a single compositional phase and epitaxy and display room-temperature PL emission. Optical absorption and emission properties of these nanostructures follow Vegard's law of band-gaps, and the observed bowing parameter and Stokes shifts correlate to the observed compositional inhomogeneity and carrier localization.

A kinetically controlled two-step growth process for the formation of an array of dislocation free high mobility InN nanorods (NRs) on GaN nanowall network (NWN) by Molecular Beam Epitaxy is demonstrated here. The epitaxial GaN NWN is formed on c-sapphire under nitrogen rich conditions, and then changing the source from Ga to In at appropriate substrate temperature yields the nucleation of a self assembled spontaneous m-plane side faceted-InN NR. By HRTEM, the NRs are shown to be dislocation-free and have a low band gap value of 0.65 eV. Hall measurements are carried out on a single InN NR along with J-V measurements that yield mobility values as high as ≈4453 cm2/V s and the carrier concentration of ≈1.1 × 1017 cm−3, which are unprecedented in the literature for comparable InN NR diameters.

Transport and optical properties of random networks of c-axis oriented wedge-shaped GaN nanowalls grown spontaneously on c-plane sapphire substrates through molecular beam epitaxy are investigated. Our study suggests a one dimensional confinement of carriers at the top edges of these connected nanowalls, which results in a blue shift of the band edge luminescence, a reduction of the exciton-phonon coupling, and an enhancement of the exciton binding energy. Not only that, the yellow luminescence in these samples is found to be completely suppressed even at room temperature. All these changes are highly desirable for the enhancement of the luminescence efficiency of the material. More interestingly, the electron mobility through the network is found to be significantly higher than that is typically observed for GaN epitaxial films. This dramatic improvement is attributed to the transport of electrons through the edge states formed at the top edges of the nanowalls. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4755775]

We present here a report on a role of initial nitridation of Si(111) surface on GaN nanorod growth. High quality wurtzite GaN nanorods are grown by Molecular Beam Epitaxy on bare Si(111)-7x7, crystalline and amorphous silicon nitride at 750oC, under nitrogen rich conditions. Using in-situ reflection high energy electron diffraction and ex-situ X-ray photoelectron spectroscopy, field emission scanning electron microscopy and photoluminescence, the structural and chemical properties are monitored. In the first part of the study, we have optimized the conditions of the N2* RF plasma, for formation of crystalline and amorphous silicon nitride on Si(111)-7x7 surface. While in the second part, GaN nanorods are grown on clean and these modified Si(111) substrates. Anisotropic spots are observed by RHEED for GaN grown on clean Si and on the amorphous silicon nitride, while circular, sharp and intense RHEED spots have been observed for GaN grown on crystalline Si3N4. FESEM results show nanorod growth in all the three different conditions. However, GaN nanorods grown on crystalline Si3N4 surface are observed to be self aligned and oriented along direction, while those grown on amorphous silicon nitride and bare Si(111) surfaces show great disorder increasing, respectively. Overall, the results clearly demonstrate that high quality of dense and self aligned c-oriented GaN nanorods can be formed on Si(111) surface by modifying it by appropriate nitridation.

Effect of growth temperature on the morphology, structural and optical properties of layers grown under nitrogen-rich conditions, by plasma assisted molecular beam epitaxy, on c-plane Sapphire have been studied. By varying the substrate temperature we are able to obtain tapered as well as flat-top nanorod and interconnected random network morphology. An increase of indium incorporation, determined by HRXRD, from 1% to 23% with decrease in growth temperature has been obtained. Room temperature photoluminescence (PL) and absorption measurements on the grown samples follow Vegardʼs law. A band bowing parameter of 1.54 eV has been determined for the observed composition range. A good quality film having no phase separation with significant PL emission intensity is obtained at a growth temperature much lower than that conventionally used in MBE for growth on sapphire.

The difference in the evolution of dislocation mediated low-dimensional GaN nanostructures on differently pre-nitrided c-plane sapphire surfaces is demonstrated. We show that GaN growth on c-plane sapphire, after forming an AlN layer by exposure to N*2 plasma, leads to the formation of nano-petal-like morphologies of the GaN nanostructures. When an AlN intermediate layer is grown on the nitrided surface, under nitrogen rich conditions, AlN grows with a bimodal distribution of nanocolumns. We observe that GaN grown on this AlN intermediate layer prefers to grow only on single crystalline AlN nanocolumns with sizes in the range 80–120 nm. We observe that these nano-features are epitaxial with high crystallinity and demonstrate a thirty times increase in band edge emission compared to GaN film grown on bare sapphire. The threading screw dislocation is seen to propagate through both the AlN layer and GaN nanorods with their morphologies depending on the proximity of the dislocations. Our results show that the shape, structure and mosaicity of GaN nano-features are dependent on the morphology of the AlN intermediate layer.

We study the surface charge accumulation on InN thin films that strongly effects mobility of charge carriers. The films are formed by MBE in the temperature range (400–470 °C) yielding films with different morphology, crystallinity, and optical properties. The band-gap values determined by optical absorption and PL studies are found to depend on the Hall carrier concentration as per the Moss-Burstein relation. The magnitude of the near surface band bending is measured on InN thin films by knowing the surface and bulk Fermi level position with respect to the valence band maximum using X-ray photoelectron spectroscopy and optical absorption measurements, respectively. The studies show that the surface charge occurs due to the In adlayer on the film causing band-bending is up to 1 eV for low band gap single crystalline films and nearly a flat band for the highly degenerate polycrystalline films. The absence of the band bending is seen to occur when the bulk carrier concentration related to the crystalline quality of the InN films, tends to equate with the surface charge density.

The issue of ambiguous values of the band gap (0.6 to 2 eV) of InN thin film in literature has been addressed by a careful experiment. We have grown wurtzite InN films by PA-MBE simultaneously on differently modified c-plane sapphire substrates and characterized by complementary structural and chemical probes. Our studies discount Mie resonances caused by metallic In segregation at grain boundaries as the reason for low band gap values (≈ 0.6 eV) and also the formation of Indium oxides and oxynitrides as the cause for high band gap value (≈ 2.0 eV). It is observed that polycrystallinity arising from azimuthal miss-orientation of c-oriented wurtzite InN crystals increases the carrier concentration and the band gap values. We have reviewed the band gap, carrier concentration, and effective mass of InN in literature and our own measurements, which show that the Moss-Burstein relation with a non-parabolic conduction band accounts for the observed variation of band gap with carrier concentration.

We address the carrier concentration, strain, and bandgap issue of InN films grown on c-sapphire at different N-flux by molecular beam epitaxy using x-ray diffraction and x-ray photoelectron spectroscopy. We demonstrate that the strain in InN films arises due to point defects like nitrogen interstitials and nitrogen antisites. We report minimal biaxial strain due to relaxed growth morphology and a minimal hydrostatic strain arising due to interstitial nitrogen atoms being partially compensated by nitrogen antisites. We find that the variation in absorption edge can be attributed to defect induced carrier concentration and that nitrogen interstitials and nitrogen antisites act as donors that yield the respective absorption edge and Moss-Burstein shift. Our studies are a step towards the ability to form low carrier concentration strain-relaxed films and to determine the intrinsic band gap value for this technologically important material.

We report the change in electronic properties of the Ga/Si interface by monitoring the Ga(3d) core-level photoelectron spectra and electron diffraction induced by submonolayer Ga adsorption on Si(111)-7×7 surface. The spectra shows a flat band for submonolayer coverages, attributed to the metallic nature of the Si(111)-7×7 reconstruction and a premetallic band structure of two-dimensional Ga islands. At 1 ML, electron diffraction pattern shows metallic (7×7) to semiconducting (1×1) phase-transition and the spin-orbit split branching ratio of Ga(2p) core level attain the metallic bulk value, and the barrier assumes the Schottky–Mott value while full width half maxima and branching ratio attain bulk values.

Etude experimentale. Couches preparees par evaporation. Mesure de la resistance de la couche en courant alternatif pour des frequences comprises entre 100 Hz et 1 MHz

In situ electrical resistivity measurements in palladium films over the thickness range 25 A to 370 A are reported. The Fuchs curve agrees with experimental p versus t curve for specularity parameter p = 0.6. The thickness dependence of the resistivity gives the bulk mean free path and resistivity as 255 A and 11 μ Ω cm, respectively.