Krishna Hari Sharma

Optical characteristics of nonpolar a-plane ZnO thin film on (010) LiGaO2 substrate

We investigate the optical properties of non-polar a-plane ZnO film grown on (010) LiGaO2 substrate by chemical vapor deposition. X-ray diffraction indicates the a-plane orientation and Raman spectroscopy reveals the phase purity. Four distinct features in the near band-edge range are identified as neutral-donor-bound-exciton (D°X), free-exciton (FX), free-to-bound, and donor-acceptor pair transitions. The thermal activation energy of D°X is 12 meV while an acceptor energy level of 124 meV is estimated. Temperature evolution of photoluminescence (PL) shows that the room-temperature luminescence is a mixture of free-to-bound and FX transitions. The polarization dependence of the near-band-edge transitions were investigated. The largest degree of polarization of 95.5% occurs in FX transition. It can be attributed to anisotropic in-plane strain and the nonpolar a-plane orientation.

Full Solution‐Processed Synthesis and Mechanisms of a Recyclable and Bifunctional Au/ZnO Plasmonic Platform for Enhanced UV/Vis Photocatalysis and Optical Properties

The synthesis of noble metal/semiconductor hybrid nanostructures for enhanced catalytic or superior optical properties has attracted a lot of attention in recent years. In this study, a facile and all-solution-processed synthetic route was employed to demonstrate an Au/ZnO platform with plasmonic-enhanced UV/Vis catalytic properties while retaining strengthened luminescent properties. The visible-light response of photocatalysis is supported by localized surface plasmon resonance (LSPR) excitations while the enhanced performance under UV is aided by charge separation and strong absorption. The enhancement in optical properties is mainly due to local field enhancement effect and coupling between exciton and LSPR. Luminescent characteristics are investigated and discussed in detail. Recyclability tests showed that the Au/ZnO substrate is reusable by cleaning and has a long shelf life. Our result suggests that plasmonic enhancement of photocatalytic performance is not necessarily a trade-off for enhanced near-band-edge emission in Au/ZnO. This approach may give rise to a new class of versatile platforms for use in novel multifunctional and integrated devices.

Resonant Raman scattering and photoluminescent properties of nonpolar a-plane ZnO thin film on LiGaO2 substrate

Nonpolar a-plane ZnO thin films are grown on LiGaO2 substrates and their optical and vibronic characteristics are investigated. Raman spectroscopy under nonresonant and resonant excitations shows the phase purity and resonant multiphonon process. From multiphonon Raman peaks up to the 6th order in the resonant spectra, the LO phonon frequencies for wave vectors away from the Γ point are estimated using the cascade model. Polar LO phonons with E1 symmetry play an important role in the indirect transitions of hot excitons. The optical anisotropy has been studied on the basis of polarization-dependent photoluminescence. The origin of photoluminescence is studied and a donor binding energy of 74.6 meV is found.

Enhanced Photocatalytic Performance of ZnO Nanorods Coupled by Two-Dimensional α-MoO3 Nanoflakes under UV and Visible Light Irradiation

We exploit the utilization of two-dimensional (2D) molybdenum oxide nanoflakes as a co-catalyst for ZnO nanorods (NRs) to enhance their photocatalytic performance. The 2D nanoflakes of orthorhombic α-MoO3 were synthesized through a sonication-aided exfoliation technique. The 2D MoO3 nanoflakes can be further converted to substoichiometric quasi-metallic MoO3-x by using UV irradiation. Subsequently, 1D-2D MoO3 /ZnO NR and MoO3-x /ZnO NR composite photocatalysts have been successfully synthesized. The photocatalytic performances of the novel nanosystems in the decomposition of methylene blue are studied by using UV- and visible-illumination setup. The incorporated 2D nanoflakes show a positive influence on the photocatalytic activity of the ZnO. The obtained rate constant values follow the order of pristine ZnO NR<MoO3 /ZnO NR<MoO3-x /ZnO NR composites. The enhancement of the photocatalytic efficiency can be ascribed to a fast charge carrier separation and transport within the heterojunctions of the MoO3 /ZnO NRs. In particular, the best photocatalytic performance of the MoO3-x /ZnO NR composite can be additionally attributed to a quasi-metallic conductivity and substoichiometry-induced mid-gap states, which extend the light absorption range. A tentative photocatalytic degradation mechanism was proposed. The strategy presented in this work not only demonstrates that coupling with nanoscale molybdenum oxide nanoflakes is a promising approach to significantly enhance the photocatalytic activity of ZnO but also hints at new type of composite catalyst with extended applications in energy conversion and environmental purification.

Facile and Cost-Efficient Synthesis of Quasi-0D/2D ZnO/MoS2 Nanocomposites for Highly Enhanced Visible-Light-Driven Photocatalytic Degradation of Organic Pollutants and Antibiotics

Nanoscale transition-metal dichalcogenide materials showed promising potential for visible-light responsive photocatalysis. Here, we report our investigations on the synthesis of heterodimensional nanostructures of two-dimensional (2D) ultrathin MoS2 nanosheets interspersed with ZnO nanoparticles by using a facile two-step method consisting of sonication-aided exfoliation technique followed by a wet chemical process. The photocatalytic activity of the nanocomposites was examined by studying the degradation of different organic dye pollutants and tetracycline, a common antibiotic, under visible-light irradiation. It is found that within 30 min more than 90 % of the model organic dye was photodegraded by the optimized quasi-0D/2D hybrid nanomaterial. The reaction rate of pollutant degradation is about five and eight times higher than those of the pristine MoS2 naonosheets and P25 photocatalysts, respectively. The outstanding photocatalytic activity of the heterodimensional hybrids can be attributed to a few beneficial features from the synergetic effects. Most importantly, the intimate junction between ZnO and MoS2 facilitates the separation of photogenerated carriers, leading to the enhancement of photocatalytic efficiency. A tentative photocatalytic degradation mechanism was proposed and tested. Overall, the present work provides valuable insights for the exploration of cost-effective nanoscale heterodimensional hybrids constructed from atomically thin layered materials.

Comparative Photothermal Performance among Various Sub-Stoichiometric 2D Oxygen-Deficient Molybdenum Oxide Nanoflakes and In Vivo Toxicity

The present study deals with photothermal therapy of solid tumors using different forms of oxygen-deficient sub-stoichiometric two-dimensional (2D) molybdenum oxide nanoflakes (α-MoO3-x ). Upon exfoliation of molybdenum oxide power using fine gridding followed by ultrasonication, bluish green molybdenum oxide (BG α-MoO3 ) was obtained. Oxygen vacancies in BG were generated upon irradiation with an intense xenon lamp. Irradiating the BG for 3 and 5 h, deep blue (B) and olive green (G) oxygen-deficient nanoflakes were obtained respectively. All exhibited high NIR absorption, making these nanomaterials suitable for photothermal therapy. All three forms were functionalized with polypyrrole (PPy@BG, PPy@B, PPy@G) to boost the photothermal stability and transduction efficiency. After functionalization and irradiation with 808 nm laser, the enhancement of temperature for BG, B, G was 50, 65, 52 °C respectively and the corresponding photothermal transduction efficiencies (PTE) were 29.32, 44.42 and 42.00 %. Each of the nanoflakes were found to be highly biocompatible and photostable both in vitro and in vivo. There was substantial decrease in the size of tumors after seven days of treatment on tumor-bearing experimental mice models.

Morphology and photoluminescence of nanoscale few-layered MoS 2 prepared by liquid phase exfoliation

Owing to many exotic properties and potential applications, graphene and related two-dimensional (2D) nanomaterials have attract a lot of attention. Among the various types of transition metal dichalcogenides materials, molybdenum disulfide (MoS2) is of technological importance in optoelectronics, sensors, energy storage, and catalysis. Exfoliation of bulk MoS2 is an attractive route to large-scale synthesis of few-layered or monolayered crystals. Here, we report structural, morphological and optical properties of nanoscale MoS2 nanosheets prepared by sonication exfoliation. The influence of sonication parameters to material properties are investigated. The absorption spectra of MoS2 suspension are studied. Strong photoluminescence with the peak centered at 500 nm was observed and excitation-dependent PL is studied. Our results shows that quantum confinement effects is important in the photoluminescence and it can be a way to enhance the PL intensity.

Synthesis and characterization of two-dimensional carbon dots decorated with molybdenum oxide nanoflakes with various phases

The optical and morphological properties of the carbon dots (C-dots) decorated with reduced two-dimensional molybdenum oxide (MoO3−x) nanoflakes were investigated. Two dimensional flakes of MoO3−x were decorated with C-dots (C-dots@MoO3−x) using a probe sonication process at room temperature. In the initial stage of the synthetic process, MoO3 nanoflakes were studied before and after exfoliation. Furthermore, the optical and physical properties were studied and significant changes were found in the fabrication of the nanoflakes with varied sonication time intervals. The C-dots@MoO3−x nanoflakes exhibit fluorescent property with maximum absorbance at longer wavelength (850 nm) in comparison to MoO3−x nanoflakes.