Novel approaches to the fabrication of nanoscale devices

This thesis describes the effects of a post-growth hydrogenation on as-grown samples and device structures based on III-N-V and III-V semiconductor compounds. The spectral response of quantum wells (QWs) or superlattices (SLs) are tuned by the control dissociation of N-H complexes using a focused laser beam (photon assisted dissociation) or by thermal annealing. These approaches could be implemented in other materials and heterostructure devices, and offer the advantage of enabling an accurate control of the spectral response of a device using a layer compound with a single N- concentration. A focused laser beam is also used to diffuse hydrogen from the p-type contact layer towards the III-N-V superlattice in the intrinsic region of a p-i-n diode, thus creating preferential injection paths for the carriers and creating nanoscale light emitting diodes. Opportunities for realizing a movable micron size-light emitting diode (-LED) are also demonstrated. Moreover, room temperature electroluminescence from semiconductor junctions formed from combinations of n-InSe, p-InSe, p-GaSe and n-In2O3 is demonstrated. These p-n junctions are fabricated using mechanical exfoliation of Bridgman-grown crystals and a simple mechanical contact method or thermal annealing. These results demonstrate the technological potential of mechanically formed heterojunctions and homojunctions of direct band gap layered GaSe and InSe compounds with an optical response over an extended wavelength range, from the near-infrared to the visible spectrum. These layered crystals could be combined in different sequences of layer stacking, thus offering exciting opportunities for new structures and devices

Sustainable supercapacitor with a natural rubber-based electrolyte and natural graphite-based electrodes

Supercapacitors are at the forefront of energy storage devices due to their ability to fulfill quick power requirements. However, safety and cost are important parameters for their real-world applications. Green materials-based electrodes and electrolytes can make them safer and cost-effective. Herein, a supercapacitor based on a methyl-grafted natural rubber/salt-based electrolyte and natural graphite (NG)-based electrodes are fabricated and characterized. Zinc trifluoromethanesulfonate [Zn(CF3SO3)2] is used as the salt for the electrolyte. A mixture of NG, activated charcoal, and polyvinylidenefluoride is used for electrodes. Our supercapacitor shows a single electrode specific capacitance, Csc of 4.2 Fg−1 from impedance measurement. Moreover, the capacitive and resistive features are dominant at low and high frequencies, respectively. The cyclic voltammetry test shows the dependence of Csc on the scan rate with a high value at slow scan rates. Performance of the supercapacitor during 5000 charge and discharge cycles at a constant current of 90 μA shows a rapid decrease of single electrode specific discharge capacitance at the beginning, but it starts to stabilize after about 2500 cycles. These findings are relevant to further developments of green materials-based supercapacitors, offering opportunities to expand the functionalities of supercapacitors in green technologies

Imaging shape and strain in nanoscale engineered semiconductors for photonics by coherent x-ray diffraction

Coherent x-ray diffractive imaging is a nondestructive technique that extracts three-dimensional electron density and strain maps from materials with nanometer resolution. It has been utilized for materials in a range of applications, and has significant potential for imaging buried nanostructures in functional devices. Here, we show that coherent x-ray diffractive imaging is able to bring new understanding to a lithography-based nanofabrication process for engineering the optical properties of semiconducting GaAs1-yNy on a GaAs substrate. This technique allows us to test the process reliability and the manufactured patterns quality. We demonstrate that regular and sharp geometrical structures can be produced on a few-micron scale, and that the strain distribution is uniform even for highly strained sub-microscopic objects. This nondestructive study would not be possible using conventional microscopy techniques. Our results pave the way for tailoring the optical properties of emitters with nanometric precision for nanophotonics and quantum technology applications

Room temperature electroluminescence from mechanically formed van der Waals III–VI homojunctions and heterojunctions

Room temperature electroluminescence from semiconductor junctions is demonstrated. The junctions are fabricated by the exfoliation and direct mechanical adhesion of InSe and GaSe van der Waals layered crystals. Homojunction diodes formed from layers of p- and n-type InSe exhibit electroluminescence at energies close to the bandgap energy of InSe (Eg= 1.26 eV). In contrast, heterojunction diodes formed by combining layers of p-type GaSe and n-type InSe emit photons at lower energies, which is attributed to the generation of spatially indirect excitons and a staggered valence band lineup for the holes at the GaSe/InSe interface. These results demonstrate the technological potential of mechanically formed heterojunctions and homojunctions of direct-bandgap layered GaSe and InSe compounds with an optical response over an extended wavelength range, from the near-infrared to the visible spectrum

Optical detection and spatial modulation of mid-infrared surface plasmon polaritons in a highly doped semiconductor

Highly doped semiconductors (HDSCs) are promising candidates for plasmonic applications in the mid-infrared (MIR) spectral range. This work examines a recent addition to the HDSC family, the dilute nitride alloy In(AsN). Post-growth hydrogenation of In(AsN) creates a highly conducting channel near the surface and a surface plasmon polariton detected by attenuated total reflection techniques. The suppression of plasmonic effects following a photo-annealing of the semiconductor is attributed to the dissociation of the N-H bond. This offers new routes for direct patterning of MIR plasmonic structures by laser writing

Epitaxial growth of γ-InSe and α, β, and γ-In2Se3 on ε-GaSe

We demonstrate that γ-InSe and the α, β and γ phases of In2Se3 can be grown epitaxially on ε-GaSe substrates using a physical vapour transport method. By exploiting the temperature gradient within the tube furnace, we can grow selectively different phases of InxSey depending on the position of the substrate within the furnace. The uniform cleaved surface of ε-GaSe enables the epitaxial growth of the InxSey layers, which are aligned over large areas. The InxSey epilayers are characterised using Raman, photoluminescence, X-ray photoelectron and electron dispersive X-ray spectroscopies. Each InxSey phase and stoichiometry exhibits distinct optical and vibrational properties, providing a tuneable photoluminescence emission range from 1.3 eV to ~ 2 eV suitable for exploitation in electronics and optoelectronics

Engineering p-n junctions and bandgap tuning of InSe nanolayers by controlled oxidation

Exploitation of two-dimensional (2D) van der Waals (vdW) crystals can be hindered by the deterioration of the crystal surface over time due to oxidation. On the other hand, the existence of a stable oxide at room temperature can offer prospects for several applications. Here we report on the chemical reactivity of ?-InSe, a recent addition to the family of 2D vdW crystals. We demonstratethat, unlike other 2D materials, InSe nanolayers can be chemically stable under ambient conditions. However, both thermal- and photo-annealing in air induces the oxidation of the InSe surface, which converts a few surface layers of InSe into In2O3, thus forming an InSe/In2O3 heterostructure with distinct and interesting electronic properties. The oxidation can be activated in selected areas of the flake by laser writing or prevented by capping the InSe surface with an exfoliated flake of hexagonal boron nitride. We exploit the controlled oxidation of p-InSe to fabricate p-InSe/n-In2O3 junction diodes with room temperature electroluminescence and spectral response from the near-infrared to the visible and near-ultraviolet ranges. These findings reveal the limits and potential of thermal- and photo-oxidation of InSe in future technologies

Emerging Applications of Novel Nanoparticles (Ed. by N Balakrishnan)

This book is a comprehensive and modern guide on emerging nanoparticles and their diverse applications in engineering, medicine, food safety, transportation, energy, agriculture, and environmental sustainability. Written by leading researchers from all over the world, it is designed to cover the full range of nanoparticles as well as provide in-depth detail regarding their development, characterization, processing, and synthesis. The book is divided into two sections: the first covers the development of advanced nanoparticles and the second is dedicated to their variety of cutting-edge applications. The authors also cover the unique properties and green synthesis of nanoparticles as well as their use as nanobiosensors, nanopesticide, nanofertilizer, and as energy storage and conversion devices, just to name a few. This book provides readers with insight onto the broad scope of computational, theoretical, and experimental research on novel nanoparticles and their applications. It is ideal for both young and experienced researchers and industry professionals in the field of nanotechnology

Memristive effects due to charge transfer in graphene gated through ferroelectric CuInP2S6

Ferroelectricity at the nanometre scale can drive the miniaturisation and wide application of ferroelectric devices for memory and sensing applications. The two-dimensional van der Waals (2D-vdWs) ferroelectrics CuInP2S6 (CIPS) has attracted much attention due to its robust ferroelectricity found in thin layers at room temperature. Also, unlike many 2D ferroelectrics, CIPS is a wide band gap semiconductor, well suited for use as a gate in field-effect transistors (FETs). Here, we report on a hybrid FET in which the graphene conducting channel is gated through a CIPS layer. We reveal hysteresis effects in the transfer characteristics of the FET, which are sensitive to the gate voltage, temperature and light illumination. We demonstrate charge transfer at the CIPS/graphene interface in the dark and under light illumination. In particular, light induces a photodoping effect in graphene that varies from n- to p-type with increasing temperature. These hybrid FETs open up opportunities for electrically and optically controlled memristive devices

Ferroelectric semiconductor junctions based on graphene/In2Se3/graphene van der Waals heterostructures

The miniaturization of ferroelectric devices offers prospects for non-volatile memories, low-power electrical switches and emerging technologies beyond existing Si-based integrated circuits. An emerging class of ferroelectrics is based on van der Waals (vdW) two-dimensional materials with potential for nano-ferroelectrics. Here, we report on ferroelectric semiconductor junctions (FSJs) in which the ferroelectric vdW semiconductor α-In2Se3 is embedded between two single-layer graphene electrodes. In these two-terminal devices, the ferroelectric polarization of the nanometre-thick In2Se3 layer modulates the transmission of electrons across the graphene/In2Se3 interface, leading to memristive effects that are controlled by applied voltages and/or by light. The underlying mechanisms of conduction are examined over a range of temperatures and under light excitation revealing thermionic injection, tunnelling and trap-assisted transport. These findings are relevant to future developments of FSJs whose geometry is well suited to miniaturization and low-power electronics, offering opportunities to expand functionalities of ferroelectrics by design of the vdW heterostructure