A. S. Bhatti

Structural characterization of GaAs and InAs nanowires by means of Raman spectroscopy

We report Raman studies of GaAs and InAs nanowires (NWs) grown on SiO2 and GaAs surfaces by means of catalyst-assisted molecular beam epitaxy. We have investigated several tens of NWs grown using either Mn or Au as a catalyst. The LO and TO phonon lines of the NWs showed an energy downshift and a broadening as compared to the lines usually observed in the corresponding bulk materials. A doublet is sometimes observed in the LO region due to the observation of a signal attributed to the surface optical (SO) phonon. The energy position of the SO phonon agrees with the values expected considering the section diameter of the NWs. LO and TO downshifts are due to the presence of structural defects within the NWs. The larger the energy downshift, the smaller the dimension of the defect-free regions. The results demonstrate that different catalysts provide wires with comparable crystal quality. The measurements also point out that differences in defect density can be found in wires coming from the same batch indic...

Effects of Mg doping on optical and CO gas sensing properties of sensitive ZnO nanobelts

We report the synthesis, optical characterization and enhanced carbon monoxide (CO) gas sensing properties of magnesium (Mg) doped 1D zinc oxide (ZnO) nanobelts obtained via a vapor transport method. The structural, morphological and compositional properties of the samples were investigated by powder X-ray diffraction (p-XRD), field emission scanning electron microscopy (FESEM) and energy dispersive X-ray (EDX) analysis. Optical characterization was carried out using Raman spectroscopy, Photoluminescence (PL), diffuse reflectance spectroscopy (DRS), UV sensing and CO gas sensing. Crystalline nanobelts were obtained with average thickness of about 34 nm, width of 290 nm, and length of 3.25 μm. Significant changes in the bandgap energy due to Mg doping were observed. The gas sensing properties of undoped and Mg doped ZnO nanostructures were tested based on the resistance change upon exposure to air and CO gas. The ability of Mg doped ZnO nanobelts to sense 20 ppm of CO gas at 350 °C was enhanced fivefold, with good stability, indicating that Mg doping is very effective in improving the CO sensing of ZnO nanobelts. In addition, a model that describes the CO gas sensing mechanism of both undoped and Mg doped ZnO nanostructures is presented.

Electrochemical immunosensor for prostate-specific antigens using a label-free second antibody based on silica nanoparticles and polymer brush

In this paper, we propose a sensitive electrochemical immunosensor synthesized using a surface-initiated atom transfer radical polymerization process for the detection of prostate-specific antigen (PSA). Electrochemical immunosensors based on polymer brush [oligo(ethylene glycol)methacrylate-co-glycidyl methacrylate] (OEGMA-co-GMA) were grown on plane Au and nanostructured (NS) Au electrodes, characterized and compared for their sensitivity to detect PSA. Due to a large capacity for antibody loading and high resistance to nonspecific antibody adsorption of POEGMA-co-GMA brush, the Au-NS immunosensor exhibited detection in a wide dynamic range of five orders of magnitude with an improved lower limit of detection of 2pgml(-1), which was better than the synthesized immunosensor with the polymer brush grown on plane Au electrode. The Au-NS electrode showed improved detection sensitivity of 4.9μAng(-1)ml for PSA detection, which was almost 2 times better than the plane Au electrode. Finally, the use of silica nanoparticles (Si-NPs) conjugated with polyclonal antibody enhanced the response of the immunosensor. The proposed electrochemical immunosensor would be an exciting addition in medical diagnostics for the early detection of cancer biomarkers, e.g., PSA due to improved limit of detection (LOD); eventually helpful in circumventing cancer metastasis.

Synthesis of ZnO nanostructures for low temperature CO and UV sensing.

In this paper, synthesis and results of the low temperature sensing of carbon monoxide (CO) gas and room temperature UV sensors using one dimensional (1-D) ZnO nanostructures are presented. Comb-like structures, belts and rods, and needle-shaped nanobelts were synthesized by varying synthesis temperature using a vapor transport method. Needle-like ZnO nanobelts are unique as, according to our knowledge, there is no evidence of such morphology in previous literature. The structural, morphological and optical characterization was carried out using X-ray diffraction, scanning electron microscopy and diffused reflectance spectroscopy techniques. It was observed that the sensing response of comb-like structures for UV light was greater as compared to the other grown structures. Comb-like structure based gas sensors successfully detect CO at 75 °C while other structures did not show any response.

Lineshape analysis of Raman scattering from LO and SO phonons in III-V nanowires

Micro-Raman spectroscopy is employed to study the phonon confinement in Au- and Mn-catalyzed GaAs and InAs nanowires. The phonon confinement model is used to fit the LO phonon peaks, which also takes into account the contribution to the asymmetry of the line shape due to the presence of surface optical (SO) phonons and structural defects. This also allows us to determine the correlation lengths in these wires, that is the average distance between defects and the defect density in these nanowires. Influence of these defects on the SO phonon is also investigated. A good agreement between the experimental results and the calculations for the SO phonon mode by using the dielectric continuum model is also obtained.

Improvement of (004) texturing by slow growth of Nd doped TiO2 films

In this work, we present preferred (004) texturing of sputter deposited titanium dioxide (TiO2) films on glass substrates as a consequence of Nd doping at very slow growth rate. Nd concentration was varied from 0.0 to 2.0 atomic percent (at. %) in TiO2 thin films deposited under identical growth conditions, i.e., the growth rate of 0.6 A/s and at 500 °C. At 2 at. % Nd, complete texturing along (004) plane was observed, as indicated by the X-ray diffraction analyses. Morphology of the deposited TiO2 films from pure to 2 at. % Nd doped TiO2 films showed faceted to planar growth. Visualization of electrical and structural analysis (VESTA) software revealed that the change in the morphology was due to preferential growth in (004) orientation. Raman spectroscopy highlighted the phonon confinement in the Eg mode, and a red shift was observed due to an increase in the anti-symmetry in bonding with increased Nd concentration. XPS results confirmed the variation in oxygen vacancy concentration along with the reduc...

Anomalous photoluminescence behavior from amorphous Ge quantum dots produced by buffer-layer-assisted growth

The authors present photoluminescence results from amorphous Ge quantum dots formed using buffer-layer-assisted growth. Their sizes, shapes, and densities were controlled by varying the thickness of the Xe buffer layer, with sizes varying from 2to8nm. A relatively weak signal was observed at ∼3K at ∼0.91eV that was independent of size and was insensitive to laser intensity. Its temperature-dependent magnitude showed a Berthelot-type behavior that they associate with hopping of carriers between radiative tail states and shallow nonradiative states. These findings are similar to those from porous semiconductors.

Significance of postgrowth processing of ZnO nanostructures on antibacterial activity against gram-positive and gram-negative bacteria.

In this work, we highlighted the effect of surface modifications of one-dimensional (1D) ZnO nanostructures (NSs) grown by the vapor–solid mechanism on their antibacterial activity. Two sets of ZnO NSs were modified separately – one set was modified by annealing in an Ar environment, and the second set was modified in O2 plasma. Annealing in Ar below 800°C resulted in a compressed lattice, which was due to removal of Zn interstitials and increased O vacancies. Annealing above 1,000°C caused the formation of a new prominent phase, Zn2SiO4. Plasma oxidation of the ZnO NSs caused an expansion in the lattice due to the removal of O vacancies and incorporation of excess O. Photoluminescence (PL) spectroscopy was employed for the quantification of defects associated with Zn and O in the as-grown and processed ZnO NS. Two distinct bands were observed, one in the ultraviolet (UV) region, due to interband transitions, and other in the visible region, due to defects associated with Zn and O. PL confirmed the surface modification of ZnO NS, as substantial decrease in intensities of visible band was observed. Antibacterial activity of the modified ZnO NSs demonstrated that the surface modifications by Ar annealing limited the antibacterial characteristics of ZnO NS against Staphylococcus aureus. However, ZnO NSs annealed at 1,000°C or higher showed a remarkable antibacterial activity against Escherichia coli. O2 plasma–treated NS showed appreciable antibacterial activity against both E. coli and S. aureus. The minimum inhibition concentration was determined to be 0.5 mg/mL and 1 mg/mL for Ar-annealed and plasma-oxidized ZnO NS, respectively. It was thus proved that the O content at the surface of the ZnO NS was crucial to tune the antibacterial activity against both selected gram-negative (E. coli) and gram-positive (S. aureus) bacterial species.

Rhenium dichalcogenides (ReX2, X = S or Se): an emerging class of TMDs family

The two-dimensional transition metal dichalcogenides (TMDs) have been attracting increasing interest due to their unique structures and remarkable properties. As a new member of the TMDs family, rhenium dichalcogenides (ReX2, X = S or Se) possess many distinctive features because of an unusually distorted octahedral (1T) crystal structure with triclinic symmetry. In this unique crystal structure, each monolayer contains diamond-shaped chains (DS-chains) comprising interlinked Re4 clusters, which makes the structure anisotropic. This novel structure renders ReX2 with wide applications in (opto-)electronics, such as photodetectors and field effect transistors (FETs). ReX2 are new materials, so this review presents mainly basic and research work done in recent years. In the first part, the unique crystal and electronic structures are introduced. The second part summarizes the various growth methods for ReX2. Finally, the third part focuses on the applications in high-performance photodetectors and FETs based on 2D ReX2.

Properties of dominant electron trap center in n-type SiC epilayers by means of deep level transient spectroscopy

Characterization of dominant electron trap in as-grown SiC epilayers has been carried out using deep level transient spectroscopy. Two electron traps E1 and Z1 at Ec−0.21 and Ec−0.61 are observed, respectively; Z1 being the dominant level. Line shape fitting, capture cross section, and insensitivity with doping concentration have revealed interesting features of Z1 center. Spatial distribution discloses that the level is generated in the vicinity of epilayers∕substrate interface and the rest of the overgrown layers is defect-free. Owing to the Si-rich growth conditions, the depth profile of Z1 relates it to carbon vacancy. The alpha particle irradiation transforms Z1 level into Z1∕Z2 center involving silicon and carbon vacancies. Isochronal annealing study further strengthens the proposed origin of the debated level.