We report the rational synthesis of nitrogen-doped zinc oxide (ZnO:N) nanowire arrays, and their implementation as photoanodes in photoelectrochemical (PEC) cells for hydrogen generation from water splitting. Dense and vertically aligned ZnO nanowires were first prepared from a hydrothermal method, followed by annealing in ammonia to incorporate N as a dopant. Nanowires with a controlled N concentration (atomic ratio of N to Zn) up to ∼4% were prepared by varying the annealing time. X-ray photoelectron spectroscopy studies confirm N substitution at O sites in ZnO nanowires up to ∼4%. Incident-photon-to-current-efficiency measurements carried out on PEC cell with ZnO:N nanowire arrays as photoanodes demonstrate a significant increase of photoresponse in the visible region compared to undoped ZnO nanowires prepared at similar conditions. Mott−Schottky measurements on a representative 3.7% ZnO:N sample give a flat-band potential of −0.58 V, a carrier density of ∼4.6 × 1018 cm−3, and a space-charge layer of ∼...

Nitrogen-Doped ZnO Nanowire Arrays for Photoelectrochemical Water Splitting

Two-dimensional (2D) transition metal carbides and nitrides (MXenes) have shown outstanding performances in electrochemical energy storage and many other applications. Delamination of MXene flakes in water produces colloidal solutions that are used to manufacture all kinds of products (thin films, coatings, and electrodes, etc.). However, the stability of MXene colloidal solutions, which is of critical importance to their application, remains largely unexplored. Here we report on the degradation of delaminated-Ti3C2Tx colloidal solutions (T represents the surface functionalities) and outline protocols to improve their stability. Ti3C2Tx MXene solutions in open vials degraded by 42%, 85%, and 100% after 5, 10, and 15 days, respectively, leading to the formation of cloudy-white colloidal solutionss containing primarily anatase (TiO2). On the other hand, the solution could be well-preserved when Ti3C2Tx MXene colloidal solutionss were stored in hermetic Ar-filled bottles at 5 °C, because dissolved oxygen, th...

Oxidation Stability of Colloidal Two-Dimensional Titanium Carbides (MXenes)

Gas discharge plasmas and their applications

The wet etching of GaN, AlN, and SiC is reviewed including conventional etching in aqueous solutions, electrochemical etching in electrolytes and defect-selective chemical etching in molten salts. The mechanism of each etching process is discussed. Etching parameters leading to highly anisotropic etching, dopant-type/bandgap selective etching, defect-selective etching, as well as isotropic etching are discussed. The etch pit shapes and their origins are discussed. The applications of wet etching techniques to characterize crystal polarity and defect density/distribution are reviewed. Additional applications of wet etching for device fabrication, such as producing crystallographic etch profiles, are also reviewed.

Wet etching of GaN, AlN, and SiC : a review

Financial support by the Spanish Ministry of Economy and Competitiveness (MINECO, Severo Ochoa program, CTQ 2012-32315 and CTQ2010-18671) and Generalitat Valenciana (GV/2013/040) is gratefully acknowledged. A.D.M. thanks University Grants Commission, New Delhi for the award of Assistant Professorship under its Faculty Recharge Programme.

Carbocatalysis by Graphene-Based Materials

Understanding the chemical vapor deposition (CVD) kinetics of graphene growth is important for advancing graphene processing and achieving better control of graphene thickness and properties. In the perspective of improving large area graphene quality, we have investigated in real-time the CVD kinetics using CH4–H2 precursors on both polycrystalline copper and nickel. We highlighted the role of hydrogen in differentiating the growth kinetics and thickness of graphene on copper and nickel. Specifically, the growth kinetics and mechanism is framed in the competitive dissociative chemisorption of H2 and dehydrogenating chemisorption of CH4, and in the competition of the in-diffusion of carbon and hydrogen, being hydrogen in-diffusion faster in copper than nickel, while carbon diffusion is faster in nickel than copper. It is shown that hydrogen acts as an inhibitor for the CH4 dehydrogenation on copper, contributing to suppress deposition onto the copper substrate, and degrades quality of graphene. Additionally, the evidence of the role of hydrogen in forming C–H out of plane defects in CVD graphene on Cu is also provided. Conversely, resurfacing recombination of hydrogen aids CH4 decomposition in the case of Ni. Understanding better and providing other elements to the kinetics of graphene growth is helpful to define the optimal CH4/H2 ratio, which ultimately can contribute to improve graphene layer thickness uniformity even on polycrystalline substrates.

Graphene CVD growth on copper and nickel: role of hydrogen in kinetics and structure

G. Bruno, P. Capezzuto, and G. Cicala, Chemistry of Amorphous Silicon Deposition Processes: Fundamentals and Controversial Aspects: Some Fundamentals on Plasma Deposition. Chemical Systems for Amorphous Silicon andIts Alloys. Effect of Novel Parameters. Deposition Mechanisms and Controversial Aspects. G. Turban, B. Drevillon, D.S. Mataras, and D.E. Rapakoulias, Diagnostics of Amorphous Silicon (a-Si) Plasma Processes: Optical Diagnostics. Mass Spectrometry and Langmuir Probes. In Situ Studies of the Growth of a-Si:H by Spectroellipsometry. C.M. Fortmann, Deposition Conditions and the Optoelectronic Properties of a-Si:H Alloys: General Comments on Amorphous Alloy Growth. Relationship between Mobility and Device Performance. Concepts of Electronic Transport in Amorphous Semiconductors. Summary and Conclusions. J. Perrin, Reactor Design for a-Si:H Deposition: Power Dissipation Mechanisms in SiH4 Discharges. Material Balance and Gas-Phase and Surface Physicochemistry. Concepts of Reactors for a-Si:H Deposition. Summary and Conclusions. A. Madan, Optoelectronic Properties of Amorphous-Silicon Using the Plasma-Enhanced Chemical Vapor Deposition (PECVD) Technique: Effect of the Properties of a-SiH Due to Parametric Variations Using the PECVD Technique. Alternative Deposition Techniques. Surface States, Interface States, and Their Effect on Device Performance. Y. Hamakawa, W. Ma, and H. Okamoto, Amorphous Silicon-Based Devices: Significant Advantages of a-Si in Its Alloys as a New Optoelectronic Material. Progress in Amorphous Silicon Solar Cell Technology. Integrated Photosensor and Color Sensor. Aspects of a-Si Imaging Devices Application. a-Si Electrophotographic Applications. Visible-Light Thin-Film Light-Emitting Diode (TFLED). Subject Index. (Chapter Headings): Chemistry of Amorphous Silicon Deposition Processes: Fundamentals and Controversial Aspects. Diagnostics of Amorphous Silicon (a-Si) Plasma Processes. Deposition Conditions and the Optoelectronic Properties of a-Si:H Alloys. ReactorDesign for a-Si:H Deposition. Optoelectronic Properties of Amorphous Silicon Using the Plasma Enhanced Chemical Vapor Deposition (PECVD) Technique. Amorphous Silicon Based Devices.

Plasma deposition of amorphous silicon-based materials

Enhanced absorption in Au nanoparticles/a-Si:H/c-Si heterojunction solar cells exploiting Au surface plasmon resonance

The impact of the nitridation temperature on sapphire/GaN interface modifications and the structural, chemical, and optical properties of GaN epitaxial thin films with N plasma radicals is investigated. Based on ex situ spectroscopic ellipsometry and x-ray photoelectron spectroscopy analysis, it is found that the sapphire nitridation chemistry, specifically AlN versus oxynitride (NO) production, depends on the surface temperature. Nitridation at 200 °C produces a very thin AlN layer with 90% coverage, while high temperature nitridation leads to a 70% coverage of AlN layer containing NO. These initial stages of growth significantly impact the characteristics of the layers following the nitridation step, specifically the low temperature buffer, annealed buffer, and the GaN epitaxial layer. The annealed buffer on a 200 °C nitridation provides a homogeneous GaN thin layer covering most of the sapphire surface. This homogeneous GaN layer after annealing produces a superior template for subsequent growth, resulting in improved structural and optical properties of GaN epitaxial films. On the other hand, the annealed buffer grown on sapphire nitrided at temperatures lower or higher than 200 °C, has islands of GaN nuclei revealing the sapphire substrate, and ultimately, resulting in degraded GaN epitaxial film quality as demonstrated by photoluminescence and x-ray diffraction measurements. The results can be traced back to the chemistry of the nitridation process.

Role of sapphire nitridation temperature on GaN growth by plasma assisted molecular beam epitaxy: Part I. Impact of the nitridation chemistry on material characteristics

Indium–tin–oxide (ITO) films deposited by sputtering and e-gun evaporation on both transparent (Corning glass) and opaque (c-Si, c-Si/SiO2) substrates and in c-Si/a-Si:H/ITO heterostructures have been analyzed by spectroscopic ellipsometry (SE) in the range 1.5–5.0 eV. Taking the SE advantage of being applicable to absorbent substrate, ellipsometry is used to determine the spectra of the refractive index and extinction coefficient of the ITO films. The effect of the substrate surface on the ITO optical properties is focused and discussed. To this aim, a parametrized equation combining the Drude model, which considers the free-carrier response at the infrared end, and a double Lorentzian oscillator, which takes into account the interband transition contribution at the UV end, is used to model the ITO optical properties in the useful UV–visible range, whatever the substrate and deposition technique. Ellipsometric analysis is corroborated by sheet resistance measurements.

Pio Capezzuto

Papers

Graphene CVD growth on copper and nickel: role of hydrogen in kinetics and structure

Plasma deposition of amorphous silicon-based materials

Enhanced absorption in Au nanoparticles/a-Si:H/c-Si heterojunction solar cells exploiting Au surface plasmon resonance

Role of sapphire nitridation temperature on GaN growth by plasma assisted molecular beam epitaxy: Part I. Impact of the nitridation chemistry on material characteristics

Parametrization of optical properties of indium–tin–oxide thin films by spectroscopic ellipsometry: Substrate interfacial reactivity