Intrinsic Optoelectronic Properties Research Articles

The Transient Photocurrent and Photovoltage Behavior of a Hematite Photoanode under Working Conditions and the Influence of Surface Treatments

Hematite (α-Fe2O3) is widely recognized as a promising candidate for the production of solar fuels via water splitting, but its intrinsic optoelectronic properties have limited its performance to date. In particular, the large electrochemical overpotential required to drive the water oxidation is known as a major drawback. This overpotential (0.4 – 0.6 V anodic of the flat band potential) has been attributed to poor oxygen evolution reaction (OER) catalysis and to charge trapping in surface states but is still not fully understood. In the present study, we quantitatively investigate the photocurrent and photovoltage transient behavior of α-Fe2O3 photoanodes prepared by atmospheric pressure chemical vapor deposition, under light bias, in a standard electrolyte, and one containing a sacrificial agent. The accumulation of positive charges occurring in water at low bias potential is found to be maximum when the photocurrent onsets. The transient photocurrent behavior of a standard photoanode is compared to photoanodes modified by either a catalytic or surface passivating overlayer. Surface modification shows a reduction and a cathodic shift of the charge accumulation, following the observed change in photocurrent onset. By applying an electrochemical model, the values of the space charge width (5–10 nm) and of the hole diffusion length (0.5–1.5 nm) are extracted from photocurrent transients’ amplitudes with the sacrificial agent. Characterization of the photovoltage transients also suggests the presence of surface states causing Fermi level pinning at small applied potential. The transient photovoltage and the use of both overlayers on the same electrode enable differentiation of the two overlayers’ effects and a simplified model is proposed to explain the roles of each overlayer and their synergetic effects. This investigation demonstrates a new method to characterize water splitting photoelectrodes—especially the charge accumulation occurring at the semiconductor/electrolyte interface during operation. It finally confirms the requirements of nanostructuring and surface control with catalytic and trap passivation layers to improve iron oxide’s performance for water photolysis.

Heterofunctional Nanomaterials: Fabrication, Properties and Applications in Nanobiotechnology

Nanotechnology and nanoengineering includes a novel class of materials that are gaining significant recognition to pursuit technological/biological advances in diverse fields including, biology, medicine, electronics, engineering etc. due to their unique size- and shape-dependent intrinsic physicochemical, optoelectronic and biological properties. Characteristics such as high surface to volume ratios and quantum confinement results in materials that are qualitatively different from their bulk counterparts. These properties not only make them suitable for numerous applications in existing and emerging technologies, but also have outstanding role in many fields that provide inspiration for their fabrication. In Today's trend nanotechnology is spreading vigorously where researchers all over the world are focusing towards their synthesis and applications. Therefore, this review is helpful for the researchers in the field of nanobiotechnology/nanomedicine, providing a brief overview of nanotechnology, covering nanomaterial synthesis methods (with emphasis on environmentally benign greener approaches), their properties, and applications; such as drug delivery, bio-labeling, nanotoxicity etc. The influence of synthesis methods and surface coatings/stabilizing agents and their subsequent applications is discussed, and a broad outline on the biomedical applications into which they have been implemented is also presented.

Room-Temperature Preparation, Characterization, and Photoluminescence Measurements of Solid Solutions of Various Compositionally-Defined Single-Crystalline Alkaline-Earth-Metal Tungstate Nanorods

The current report describes the systematic synthesis of single-crystalline alkaline-earth-metal tungstate AWO4 (A = Ca, Sr, Ba) nanorods as well as a series of their crystalline solid−solution analogues Sr1-xCaxWO4 and Ba1-xSrxWO4 (0 < x < 1) with controllable chemical composition and morphology using a modified template-directed methodology under ambient room-temperature conditions. Extensive characterization of the resulting nanorods has been performed using diffraction, X-ray photoelectron spectroscopy, electron microscopy, and optical spectroscopy. The composition-modulated luminescence properties of these alkaline-earth-metal tungstate solid−solution nanorods provide for a fundamental understanding of the intrinsic optical and optoelectronic properties of these systems, suggesting, therefore, the possibility of their rational incorporation into functional nanoscale devices.

Quantum chemical study of the initial surface reactions of atomic layer deposition GaAs for photonic crystal fabrication

III-V materials such as GaAs, GaP, and InP are deposited by atomic layer depostion (ALD) to form inverse opal due to their modifiable intrinsic optoelectronic properties, thus active photonic devices are possible to fabricate. ALD GaAs on the hydroxylated SiO2 surface was investigated under the framework of density functional theory. Our calculation results show that both half-reactions are thermodynamically and kinetically favored, exothermic by 33.73 and 25.90kcal∕mol compared to the reactants. However, longer AsH3 pulse time during the second half cycle of ALD procesis recommended since there is a high activation energy barrier existing in the second half cycle.

Photonic crystal waveguide modulators for silicon photonics: Device physics and some recent progress

We review the basic principles, structural designs, and electrical characteristics of photonic crystal waveguide modulators based on slow light Mach-Zehnder interferometers. A number of structural and electrical configurations of photonic crystal waveguide modulators are discussed. In addition to the advantages associated with the slow group velocity, photonic crystal waveguide modulators are found to exhibit other structural and optoelectronic merits for high speed modulation. We also briefly review past work on silicon modulators and present certain speed scaling of silicon modulators. The scaling is governed by certain intrinsic optoelectronic properties of silicon and fundamental nature of light, and plays a significant role in determining the minimum ac current density and power of silicon modulators as demonstrated in some recent experiments.

Nanowire Lasers

We review principles and trends in the use of semiconductor nanowires (NWs) as gain media for stimulated emission and lasing. Semiconductor nanowires have recently been widely studied for use in integrated optoelectronic devices, such as LEDs, solar cells, and transistors. Intensive research has also been conducted on the use of nanowires for sub-wavelength laser systems that take advantage of their quasi-one-dimensional nature, flexibility in material choice and combination, and intrinsic optoelectronic properties. First, we provide an overview on using quasi-one-dimensional nanowire systems to realize sub-wavelength lasers with efficient, directional, and low-threshold emission. We then describe the state-of-the-art for nanowire lasers in terms of materials, geometry, and wavelength tunability. Next, we present the basics of lasing in semiconductor nanowires, define the key parameters for stimulated emission, and introduce the properties of nanowires. We then review advanced nanowire laser designs from the literature. Finally, we present interesting perspectives for low-threshold nanoscale light sources and optical interconnects. We intend to illustrate the potential of nanolasers in many applications, such as nanophotonic devices that integrate electronics and photonics for next-generation optoelectronic devices. For instance, these building blocks for nanoscale photonics can be used for data storage and biomedical applications when coupled to on-chip characterization tools. These nanoscale monochromatic laser light sources promise breakthroughs in nanophotonics, as they can operate at room temperature, potentially be electrically driven, and yield a better understanding of intrinsic nanomaterial properties and surface state effects in low-dimensional semiconductor systems.