Mehmet F. Cansizoglu

High optical absorption of indium sulfide nanorod arrays formed by glancing angle deposition.

Indium(III) sulfide has recently attracted much attention due to its potential in optical sensors as a photoconducting material and in photovoltaic applications as a wide band gap material. On the other hand, optical absorption properties are key parameters in developing photosensitive photodetectors and efficient solar cells. In this work, we show that indium sulfide nanorod arrays produced by the glancing angle deposition technique have superior absorption and low reflectance properties compared to conventional flat thin film counterparts. We observed an optical absorption value of approximately 96% for nanorods at wavelengths <500 nm in contrast to 79% for conventional thin films of indium sulfide. A superior photoconductivity (PC) response as high as about 40% (change in resistance upon illumination) was also observed in nanorod samples. This is mainly believed to be due to their high optical absorption, whereas only less than 1% PC change was detected in conventional thin films. We give a preliminary description of the enhanced light absorption properties of the nanorods by using the Shirley-George model, which predicts diffusion of light as a function of the roughness of the surface.

Enhanced Photocurrent and Dynamic Response in Vertically Aligned In2S3/Ag Core/Shell Nanorod Array Photoconductive Devices

Enhanced photocurrent values were achieved through a semiconductor-core/metal-shell nanorod array photoconductive device geometry. Vertically aligned indium sulfide (In2S3) nanorods were formed as the core by using glancing angle deposition technique (GLAD). A thin silver (Ag) layer is conformally coated around nanorods as the metallic shell through a high pressure sputter deposition method. This was followed by capping the nanorods with a metallic blanket layer of Ag film by utilizing a new small angle deposition technique combined with GLAD. Radial interface that was formed by the core/shell geometry provided an efficient charge carrier collection by shortening carrier transit times, which led to a superior photocurrent and gain. Thin metal shells around nanorods acted as a passivation layer to decrease surface states that cause prolonged carrier lifetimes and slow recovery of the photocurrent in nanorods. A combination of efficient carrier collection with surface passivation resulted in enhanced photocurrent and dynamic response at the same time in one device structure. In2S3 nanorod devices without the metal shell and with relatively thicker metal shell were also fabricated and characterized for comparison. In2S3 nanorods with thin metal shell showed the highest photosensitivity (photocurrent/dark current) response compared to two other designs. Microstructural, morphological, and electronic properties of the core/shell nanorods were used to explain the results observed.

PiN InGaN nanorod solar cells with high short-circuit current

We report on the photovoltaic characteristics of molecular beam epitaxy-grown PiN InGaN nanorod solar cells. The glancing angle deposition process was adapted to grow continuous transparent metal layers on discontinuous nanorods. A short-circuit current density of 4.6 mA/cm2 and an open-circuit voltage of 0.22 V with a power conversion efficiency of 0.5% under 1 sun, air-mass 1.5, illumination were observed. The excellent light-generated current in the InGaN nanorod solar cells is considered to stem from the improved crystal quality owing to the strain-free nature as well as the enhanced light concentration effects in the nanorod configuration.

Enhanced photoresponse of conformal TiO2/Ag nanorod array-based Schottky photodiodes fabricated via successive glancing angle and atomic layer deposition

In this study, the authors demonstrate a proof of concept nanostructured photodiode fabrication method via successive glancing angle deposition (GLAD) and atomic layer deposition (ALD). The fabricated metal-semiconductor nanorod (NR) arrays offer enhanced photoresponse compared to conventional planar thin-film counterparts. Silver (Ag) metallic NR arrays were deposited on Ag-film/Si templates by utilizing GLAD. Subsequently, titanium dioxide (TiO2) was deposited conformally on Ag NRs via ALD. Scanning electron microscopy studies confirmed the successful formation of vertically aligned Ag NRs deposited via GLAD and conformal deposition of TiO2 on Ag NRs via ALD. Following the growth of TiO2 on Ag NRs, aluminum metallic top contacts were formed to complete the fabrication of NR-based Schottky photodiodes. Nanostructured devices exhibited a photo response enhancement factor of 1.49 × 102 under a reverse bias of 3 V.

Enhanced light trapping and plasmonic properties of aluminum nanorods fabricated by glancing angle deposition

Vertically aligned arrays of aluminum (Al) nanorods were fabricated by glancing angle deposition (GLAD) method. Nanorods with maximum lengths of 200 and 350 nm were grown on 100 nm flat Al thin film. Total and diffuse reflectance profiles were measured using an ultraviolet–visible–near infrared (UV-Vis-NIR) spectrophotometer utilizing an integrating sphere to study detailed optical properties of Al nanorods in comparison to conventional planar Al thin film samples. Finite-difference-time-domain (FDTD) optical modeling method was utilized to simulate the optical response of Al nanorod array and thin film structures. FDTD simulations were carried out for periodic and random arrays of Al nanorods as well as for an isolated single nanorod in order to investigate effects of geometrical structure on plasmonic and light trapping effects. UV-Vis-NIR spectrum results reveal that total reflectance is inversely proportional with nanorod length, and decreases down to as low as ∼25%–30% in the visible spectrum at wave...

Enhanced light trapping and carrier collection in glancing angle deposited nanostructures

Nanostructured materials have become an attractive alternative to their thin film and bulk counterparts in photovoltaic (PV) research. They owe this attention mainly to their superior optical and electrical properties. Light trapping in vertically aligned nanostructures results in high optical absorption and core/shell type of nanostructured devices provide enhanced carrier collection by utilizing a radial junction. Combination of these two features can potentially lead to the development of high efficiency nanostructured solar cells. Here, results from optical absorption properties of indium sulfide (In2S3; n-type semiconductor) nanostructures as a model material system in different geometries and their photoconductive properties in an In2S3-nanorods-core/metal-shell device design are presented and discussed. Glancing angle deposition (GLAD) technique was used to grow In2S3 nanostructures in different shapes (i.e., zigzags, springs, screws, tilted rods, and vertical rods). Optical absorption was found to strongly depend on the shapes of semiconducting nanostructures through ultraviolet-visible (UV-Vis) spectroscopy measurements. Numerical solutions of finite difference time domain (FDTD) optical modelling show that diffracted light is distributed uniformly within the 3D nanostructure geometries, indicating an enhanced diffuse light scattering and light trapping. A high pressure sputter deposition method was used to get a conformal silver (Ag) layer around GLAD In2S3 nanorods and produce the nanostructured core/shell photoconductive devices. Core/shell geometry was observed to enhance radial interface and shorten charge carrier transit times. This provides efficient carrier collection and results in superior photocurrent and gain. Slow recovery of photocurrent arisen from prolonged carrier lifetimes due to high surface states in nanorods is also eliminated by the metal shell, which provides surface passivation and decreases surface states. Overall, we demonstrate that GLAD nanostructures provide both efficient charge carrier collection and enhanced light trapping, and therefore can lead to the utilization of low quality (i.e. low cost) materials for high efficiency solar cells.

SAD-GLAD core-shell nanorod arrays for fuel cell, photodetector, and solar cell electrode applications

The glancing angle deposition (GLAD) technique, unlike a conventional physical vapor deposition (PVD) process, incorporates a flux of atoms that are obliquely incident on a tilted and rotating substrate. Instead of a continuous thin film coating, these atoms can form arrays of three-dimensional nanostructures due to a shadowing effect. By simply controlling the deposition angle and substrate rotation speed, nanostructures of a large variety of materials in the shapes of rods, screws, or springs can be obtained easily that are otherwise difficult to produce by conventional lithographical techniques. In this study, a brief overview of the growth mechanisms of GLAD nanostructures is presented. In addition, a new small angle deposition (SAD) technique as a simple means of conformally coating nanorod or nanowire arrays is described. SAD utilizes a small tilt angle during PVD on nanostructured substrates, which allows the effective exposure of nanorod sidewalls to the incoming flux and leads to enhanced thin film conformality. In this work, some recent results on core-shell nanorod arrays obtained by coating GLAD nanorods with a SAD shell will be presented. It will be shown that core-shell nanostructured geometries obtained by the simple SAD-GLAD method can significantly enhance catalyst activity for fuel cell electrodes, and charge carrier collection efficiency in photoconductive/semiconductor nanostructured materials.

Enhanced Hydrogen Storage Properties of Magnesium Nanotrees with Nanoleaves

Hydrogen storage in advanced solid state materials has been an intense area of research due to many drawbacks in conventional high pressure or cryogenic liquid hydrogen storage methods. A practical hydrogen storing material is required to have high storage capacity and fast dehydrogenation kinetics. Among many solid state materials for hydrogen storage, magnesium hydride (MgH2) combines a hydrogen capacity of 7.6 wt % with the benefit of the low cost of production and abundance. The main difficulties for implementing MgH2 are slow absorption/desorption kinetics and high reactivity towards air and oxygen, which are also common issues in most lightweight metal hydrides. Previously, improvements in hydrogen storage and release properties have been reported by using nanostructured magnesium that can be obtained through various fabrication methods including ball-milling, mechanical alloying, and vapor transport. In this study, we investigate the hydrogen absorption and desorption properties of magnesium “nanotrees” fabricated by glancing angle deposition (GLAD) technique, and also conventional Mg thin films deposited at normal incidence. Mg nanotrees are about 15 μm long, 10 μm wide, and incorporate “nanoleaves” of about 20 nm in thickness and 1,2 μm in lateral width. A quartz crystal microbalance (QCM) gas absorption/desorption measurement system has been used for our hydrogen storage studies. Nanostructured and thin film Mg have been deposited directly on the surface of the gold coated unpolished quartz crystal samples. QCM hydrogen storage experiments have been performed at temperatures ranging between 100-300 C, and at H2 pressures of 10 and 30 bars. Our QCM measurements revealed that Mg nanotrees can absorb hydrogen at lower temperatures and also at a faster rate compared to Mg thin film. In addition, Mg nanotrees can reach hydrogen storage values of about 4.80 wt% at 100 C, and up to about 6.71 wt% (which is close to the theoretical maximum storage value of Mg) at temperatures lower than 150 C. The significant enhancement in hydrogen absorption properties of our Mg nanotrees is believed to originate from novel physical properties of their nanoleaves. These nanoleaves are very thin (~20 nm) and both surfaces are exposed to hydrogen enhancing the diffusion rate of hydrogen together with a decreased diffusion length. Based on X-ray diffraction measurements, individual nanoleaves have non-closepacked crystal planes that can further enhance the hydrogen absorption kinetics. In addition, our nanostructured Mg have been observed to quite resistant to surface oxidation, which is believed to due to the single crystal property of the Mg nanoleaves, which further improves the absorption kinetics of hydrogen. INTRODUCTION Increasing concerns about the dependence of world economy on current fossil based energy sources in the world have further stimulated research towards environmentally friendly and sustainable energy technologies. Among various candidates, hydrogen has been considered an ideal element for storage, transport, and Page 1 of 6

Engineering Morphology of Surfaces by Oblique Angle Etching

During a typical chemical etching process growth front morphology generally generates an isotropic rough surface. In this work, we show that it is possible to form a rippled surface morphology through a geometrical self-assembly process using a chemical oblique angle etching technique. We observe in our Monte Carlo simulations that obliquely incident reactive species preferentially etch the hills that are exposed to the beam direction due to the shadowing effect. In addition, species with non-unity sticking (etching) coefficients can be re-emitted from the side walls of the hills and etch the valleys, which at the end can lead to the formation of ripples along the direction of the beam. This mechanism is quite different than the previously reported ripple formation during ion-beam bombarded surfaces where the particles have much higher energies, lower incidence angle and ripple formation is mainly due to physical deformation of the surface. We investigate the ripple formation process in our simulated surfaces for a wide range of etching angle and sticking coefficient values.

Black holes enabled light bending and trapping in ultrafast silicon photodetectors

Micro and nanoscale holes on the surfaces of indirect band gap semiconductors such as silicon can enable perpendicular light bending and trapping of photons to enhance the light material interactions and absorption by orders of magnitude. The ‘bending’ of a vertically oriented light beam at nearly 90 degrees can be visualized as radial waves generated by a pebble dropped into a calm pool of water. Such bending and photon trapping result in an increased optical absorption path enabling very high light absorption coefficients. This observation led to the design of silicon photodetectors with high broadband efficiency above 50% and record ultrafast response contributing to more than 40 billion bits of data per second (Gb/s) communication speed.