Ujjal K. Gautam

Inorganic semiconductor nanostructures and their field-emission applications

Inorganic semiconductor nanostructures are ideal systems for exploring a large number of novel phenomena at the nanoscale and investigating the size and dimensionality dependence of their properties for potential applications. The use of such nanostructures with tailored geometries as building blocks is also expected to play crucial roles in future nanodevices. Since the discovery of carbon nanotubes much attention has been paid to exploring the usage of inorganic semiconductor nanostructures as field-emitters due to their low work functions, high aspect ratios and mechanical stabilities, and high electrical and thermal conductivities. This article provides a comprehensive review of the state-of-the-art research activities in the field. It mainly focuses on the most widely studied inorganic nanostructures, such as ZnO, ZnS, Si, WO3, AlN, SiC, and their field-emission properties. We begin with a survey of inorganic semiconductor nanostructures and the field-emission principle, and then discuss the recent progresses on several kinds of important nanostructures and their field-emission characteristics in detail and overview some additional inorganic semiconducting nanomaterials in short. Finally, we conclude this review with some perspectives and outlook on the future developments in this area.

ZnO and ZnS Nanostructures: Ultraviolet-Light Emitters, Lasers, and Sensors

ZnO and ZnS, well-known direct bandgap II–VI semiconductors, are promising materials for photonic, optical, and electronic devices. Nanostructured materials have lent a leading edge to the next generation technology due to their distinguished performance and efficiency for device fabrication. As two of the most suitable materials with size- and dimensionality-dependent functional properties, wide bandgap semiconducting ZnO and ZnS nanostructures have attracted particular attention in recent years. For example, both materials have been assembled into nanometer-scale visible-light-blind ultraviolet (UV) light sensors with high sensitivity and selectivity, in addition to other applications such as field emitters and lasers. Their high-performance characteristics are particularly due to the high surface-to-volume ratios (SVR) and rationally designed surfaces. This article provides a comprehensive review of the state-of-the-art research activities in ZnO and ZnS nanostructures, including their syntheses and potential applications, with an emphasis on one-dimensional (1D) ZnO and ZnS nanostructure-based UV light emissions, lasers, and sensors. We begin with a survey of nanostructures, fundamental properties of ZnO and ZnS, and UV radiation–based applications. This is followed by detailed discussions on the recent progress of their synthesis, UV light emissions, lasers, and sensors. Additionally, developments of ZnS/ZnO composite nanostructures, including core/shell and heterostructures, are discussed and their novel optical properties are reviewed. Finally, we conclude this review with the perspectives and outlook on the future developments in this area. This review explores the possible influences of research breakthroughs of ZnO and ZnS nanostructures on the current and future applications for UV light–based lasers and sensors.

High‐Performance Blue/Ultraviolet‐Light‐Sensitive ZnSe‐Nanobelt Photodetectors

Single-crystalline zinc selenide (ZnSe) nanobelts were fabricated via the ethylenediamine (en)-assisted ternary solution technique and subsequent thermal treatment. Individual ZnSe nanobelts were assembled into nanoscale devices, showing a high spectral selectivity and photocurrent/immediate-decay ratio and a fast time response, justifying effective utilization of the ZnSe nanobelts as blue/UV-light-sensitive photodetectors.

Rapid and Direct Conversion of Graphite Crystals into High‐Yielding, Good‐Quality Graphene by Supercritical Fluid Exfoliation

Graphene has attracted a great deal of attention in recent years due to its unusual electronic, mechanical, and thermal properties. Exploiting graphene properties in a variety of applications requires a chemical approach for the large-scale production of high-quality, processable graphene sheets (GS), which has remained an unanswered challenge. Herein, we report a rapid one-pot supercritical fluid (SCF) exfoliation process for the production of high-quality, large-scale, and processable graphene for technological applications. Direct high-yield conversion of graphite crystals to GS is possible under SCF conditions because of the high diffusivity and solvating power of SCFs, such as ethanol, N-methyl-pyrrolidone (NMP), and DMF. For the first time, we report a one-pot direct conversion of graphite crystals to a high yield of graphene sheets in which about 90-95% of the exfoliated sheets are < 8 layers with approximately 6-10% monolayers and the remaining 5-10% are > or = 10 layers.

Electrical Transport and High‐Performance Photoconductivity in Individual ZrS2 Nanobelts

Individual ZrS(2)-nanobelt field-effect transistors were fabricated using a photolithography process. Temperature-dependent electrical transport revealed different electrical conductivity mechanism at different working temperature regions. ZrS(2)-nanobelt photodetectors demonstrated a high-performance visible-light photoconductivity.

Heterostructures and superlattices in one-dimensional nanoscale semiconductors

One-dimensional (1D) semiconductor nanostructures are of prime interest due to their potentials in investigating the size and dimensionality dependence of the materials’ physical properties and constructing nanoscale electronic and optoelectronic devices. Recent advances in the design and control of heterostructures and superlattices in 1D nanoscale semiconductors have opened the door to new device concepts. 1D heterostructures consisting of two or more important functional materials are of prime importance for revealing unique properties and essential for developing potential nanoelectronic and optoelectronic devices. On the other hand, the controlled growth of twinned superlattices within a single nanostructure could facilitate bandgap engineering and reveal novel electronic behaviours. In addition, an attractive challenge is to achieve the entire growth control within an individual nanostructure, e.g. to make highly reproducible, periodically twinned superlattices with an adjustable twin spacing. This Highlight article reviews some recent key advances in the field and outlines potential future areas that require immediate research and development.

Asymmetric tungsten oxide nanobrushes via oriented attachment and Ostwald ripening

Tungsten oxide nanobrushes were synthesized using a solvothermal approach that lead to self-branching in the presence of citric acid and hexadecylamine as surfactants. Our synthetic approach yielded branched nanorods of tungsten oxide in a single synthetic step. Based on our results, we propose a phenomenological pathway for the formation, branching, and assembly of these tungsten oxide brushes. The formation of tungsten oxide brushes proceeds by thermal decomposition of ammonium tungstate in the presence of citric acid and hexadecylamine. The pale blue powder obtained after solvothermal reaction was analyzed by X-ray diffraction (XRD), transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM). The field emission (FE) properties of the tungsten oxide nanostructures which can be tailored by their aspect ratio and the hierarchical nanostructures follow a Fowler–Nordheim behavior.

Synthesis of metal–semiconductor heterojunctions inside carbon nanotubes

Heterojunctions between a metal and a semiconductor are at the core of all modern electronic devices. Recently, fabrication of such structures at the nanoscale has emerged as a hot topic due to their immense potential for the next generation of nanoscale devices and electronics. Here we report a high-temperature route for the synthesis of metal (In)–semiconductor (ZnS) nano-heterojunctions inside a carbon nanotube (CNT). As In is a superconductor at low temperatures, these ‘nanocables’ are also potential superconductor–semiconductor heterojunctions, synthesized for the first time inside a CNT. A noteworthy feature is that the majority of the heterostructure surface area is involved in forming interfaces such as In||ZnS, In||CNT and ZnS||CNT. Mastering these structural relations is critical to controlling its overall properties. Several interesting facts emerged from detailed structural characterization of the heterojunctions with high-resolution transmission electron microscopy. The growth direction of the wurtzite-type ZnS encapsulated segments is along [100], while [0001] is the commonly preferred growth direction in free-standing ZnS nanowires. Following the observation of smooth In||ZnS interfaces, the orientation relationship of these two segments was analysed. Another interesting finding is the presence of a few layers of cubic ZnS near its interface with the CNT. This peculiarity is suggested to be a key contributor to the unusual encapsulated nanowire growth axis. These complex In/ZnS/CNT materials should provide opportunities for fundamental studies of heterojunctions at the nanoscale, as well as providing the basis for the development of chemical and radiation-shielded electronic nanodevices.

Nucleation sequence on the cation exchange process between Y0.95Eu0.05PO4 and CePO4 nanorods

Nanorods of Y0.95Eu0.05PO4@CePO4 (Y0.95Eu0.05PO4 phase was nucleated first and then a CePO4 phase was nucleated) and CePO4@Y0.95Eu0.05PO4 (CePO4 phase was nucleated first and then Y0.95Eu0.05PO4 phase was nucleated) were prepared at a relatively low temperature of 140 °C in ethylene glycol medium. Based on XRD, TEM and Raman studies it has been inferred that Y0.95Eu0.05PO4@CePO4 sample consists of a mixture of bigger (length around 800-1000 nm and width around of 80-100 nm) and smaller (length around 70-100 nm and width around 10-20 nm) nanorods, having monoclinic CePO4 and tetragonal YPO4 structure, whereas CePO4@Y0.95Eu0.05PO4 sample consists of mainly small nanorods having a single phase CePO4 structure. From the detailed luminescence studies it has been established that there exists significant incorporation of Y3+/Eu3+ ions in the CePO4 phase in CePO4@Y0.95Eu0.5PO4 sample. This has been attributed to the cation exchange taking place between Ce3+ ions in CePO4 host and Eu3+ and Y3+ ions in solution during the synthesis stage. Unlike this, such an exchange is not possible for Y0.95Eu0.05PO4@CePO4 sample synthesized under identical conditions due to the higher solubility product (Ksp) value of YPO4 compared to CePO4. Incorporation of Eu3+ in the CePO4 lattice of CePO4@Y0.95Eu0.5PO4 sample is confirmed by the significant reduction in the lifetime of 5D0 level of Eu3+ and the luminescence intensity from Eu3+, arising due to the electron transfer between the Ce3+/Ce4+ and Eu3+/Eu2+ species. These results are further supported by the non-radiative decay rates and quantum yields calculated from the emission spectrum.

Clean superconducting In nanowires encapsulated within insulating ZnS nanotubes

We have synthesized indium (In) nanowires in pure form and large scale, encapsulated within insulating ZnS nanotubes, and examined the intrinsic superconductivity in one-dimensional limit. We demonstrate that the property of the superconducting nanowires encapsulated within insulating nanotubes can be controlled down to diameters much smaller than the characteristic lengths. The critical temperature and critical magnetic field of the one-dimensional In nanowires are not affected down to a diameter of 40 nm, almost 10% of the coherence length of bulk In. This study further suggests that superconducting interconnects, with controlled physical properties, in nanocircuits could be achieved by such encapsulation.