Investigation of Wear Mechanism of Gallium Nitride

The optoelectronic properties of gallium nitride (GaN) has been extensively studied for decades, which has facilitated its application in many different areas, cementing it as one of the most important semiconductor materials in the world. However, in comparison to the study of its optoelectronic properties, there are few studies of its mechanical properties - especially the tribological performance. Knowing the tribological properties of GaN, such as friction and wear, is crucial for understanding its machinability, the implementation of GaN in MEMS, solar cells, and other devices, as well as the wear performance of these GaN-based devices when working under harsh environments. In our study, we reveal that GaN has an ultralow wear nature, and that its wear rate can approach that of diamond. We also discover that the wear rate of GaN is affected by its crystallographic orientation, humidity, and composition.For the crystallographic orientation dependence, we look into the physics by both experimental and computational methods. We demonstrate that both the friction coefficient and wear rate of GaN exhibits a 60° periodicity. We conclude that these periodic variations of wear rate and friction coefficient are the results of a periodic variation of the energy barrier.The moisture dependent wear mechanism of GaN has been investigated under dry, low humidity, and high humidity environments. The results show that the wear rate of GaN perfectly follows an increasing of the humidity which spans over two orders of magnitude when the testing environment switches from dry nitrogen to humid lab air. On the contrary, the friction coefficient gave a contrary response, i.e., the lowest friction coefficient was found under low humidity environment, dry nitrogen had the highest friction coefficient, and the humid environment had its friction in the middle. Various characterization techniques, including SEM/EDS, AFM and TEM were employed to interrogate the worn surfaces under each condition. Based on the results, we hypothesize that the wear under dry nitrogen environment is adhesive in nature whereas grooving abrasive wear dominates the wear behavior of GaN under a humid environment.The compositional study of GaN wear revealed that by alloying different elements into the GaN system, one can not only tune the bandgap, but also modify the wear rate. This finding can be useful for applications and design that require suitable electronic properties while keep the wear rate within an acceptable range.Furthermore, during the investigation of the GaN wear mechanism, we discovered that the tribological sliding can also alter the surface band bending of this material. Our results demonstrate that the environment, number of sliding cycles, and normal loads can effectively tune the surface band bending of GaN. This finding shows the capability of mechanical dynamic contact for surface electronic property modification, which can be used in various applications, such as gas sensing, photocatalysis, and photochemistry.Understanding of the wear mechanism of GaN as well as the shear-induced band bending on GaN can remarkably promote the applications of GaN in various fields other than the optoelectronic area. This also reinforces the important message that tribology is not only a discipline that focuses on investigation of protective coating and lubrication but also can be used in device design and fabrication

QCM Measurements of RH with Nanostructured Carbon-Based Materials: Part 2-Experimental Characterization

In this series of two papers, the humidity sensing of a carbon nanotube (CNT) network-based material is transduced and studied through quartz crystal microbalance (QCM) measurements. To this aim, quartzes functionalized with different amounts of sensing material were realized, exposed to different humidity levels, and characterized. In this second paper, the experimental results are presented and discussed. The sensing mechanisms are elucidated exploiting the theory presented in the first paper of this series. The presented results show that the investigated material functionalization induces a large response of QCM to humidity in terms of resonant frequency even at low RH levels, with a sensitivity of about 12 Hz/%RH (at RH < 30% and room temperature and 10 ug of deposited SWCNT solution) and an increase in sensitivity in the high RH range typical of nanostructured film. Regarding the response in terms of motional resistance, a large response is obtained only at intermediate and high humidity levels, confirming that condensation of water in the film plays an important role in the sensing mechanism of nanostructured materials

Template-based synthesis of aluminum nitride hollow Nanofibers via plasma-enhanced atomic layer deposition

Cataloged from PDF version of article.Aluminum nitride (AlN) hollow nanofibers were synthesized via plasma-enhanced atomic layer deposition using sacrificial electrospun polymeric nanofiber templates having different average fiber diameters (~70, ~330, and ~740 nm). Depositions were carried out at 200°C using trimethylaluminum and ammonia precursors. AlN-coated nanofibers were calcined subsequently at 500°C for 2 h to remove the sacrificial polymeric nanofiber template. SEM studies have shown that there is a critical wall thickness value depending on the template's average fiber diameter for AlN hollow nanofibers to preserve their shapes after the template has been removed by calcination. Best morphologies were observed for AlN hollow nanofibers prepared by depositing 800 cycles (corresponding to ~69 nm) on nanofiber templates having ~330 nm average fiber diameter. TEM images indicated uniform wall thicknesses of ~65 nm along the fiber axes for samples prepared using templates having ~70 and ~330 nm average fiber diameters. Synthesized AlN hollow nanofibers were polycrystalline with a hexagonal crystal structure as determined by high-resolution TEM and selected area electron diffraction. Chemical compositions of coated and calcined samples were studied using X-ray photoelectron spectroscopy (XPS). High-resolution XPS spectra confirmed the presence of AlN. © 2012 The American Ceramic Societ

Localized Charge Transfer Process and Surface Band Bending in Methane Sensing by GaN Nanowires

The physicochemical processes at the surfaces of semiconductor nanostructures involved in electrochemical and sensing devices are strongly influenced by the presence of intrinsic or extrinsic defects. To reveal the surface controlled sensing mechanism, intentional lattice oxygen defects are created on the surfaces of GaN nanowires for the elucidation of charge transfer process in methane (CH4) sensing. Experimental and simulation results of electron energy loss spectroscopy (EELS) studies on oxygen rich GaN nanowires confirmed the possible presence of 2(ON) and VGa-3ON defect complexes. A global resistive response for sensor devices of ensemble nanowires and a localized charge transfer process in single GaN nanowires are studied in situ scanning by Kelvin probe microscopy (SKPM). A localized charge transfer process, involving the VGa-3ON defect complex on nanowire surface is attributed in controlling the global gas sensing behavior of the oxygen rich ensemble GaN nanowires.Comment: 42 pages, 6 figures, Journa

Greenhouse Gas Sensors Fabricated with New Materials for Climatic Usage: A Review

With the increasing utilization of fossil fuels in today’s technological world, the atmosphere’s concentration of greenhouse gases is increasing and needs to be controlled. In order to achieve this goal, it is imperative to have sensors that can provide data on the greenhouse gases in the environment. The recent literature contains a few publications that detail the use of new methods and materials for sensing these gases. The first part of this review is focused on the possible effects of greenhouse gases in the atmosphere, and the second part surveys the developments of sensors for greenhouse gases with coverage on carbon nano-materials and composites directed towards sensing gases like CO2, CH4, and NOx. With carbon dioxide measurements, due consideration is given to the dissolved carbon dioxide gas in water (moisture). The density functional calculations project that Pd-doped single-walled carbon nanotubes are ideal for the development of NOx sensors. The current trend is to make sensors using 3D printing or inkjet printing in order to allow for the achievement of ppb levels of sensitivity that have not been realized before. This review is to elaborate on the need for the development of greenhouse gas sensors for climatic usage by using selected examples

Obtaining and Characterization of New Materials

At present, more and more procedures and technologies used to discover and characterize new materials are available, including advanced characterization techniques.This Special Issue covers a wide range of topics about obtaining and characterizing new materials, from the nano to macro scales, including for new alloys, ceramics, composites, biomaterials, and polymers and the procedures and technologies used to enhance their structure, properties, and functions. To select new materials for future use, we must first understand their structure and their characteristics using modern techniques such as microscopy (SEM, TEM, AFM, STM, etc.), spectroscopy (EDX, XRD, XRF, FTIR, XPS, etc.), and mechanical tests (tensile, hardness, elastic modulus, toughness, etc.) and their behaviors (in vitro and in vivo; corrosion; and thermal—DSC, STA, DMA, magnetic properties, and biocompatibility), among many others

Nano- and Microcomposites for Electrical Engineering Applications

In a dedicated Special Issue, the journal Polymers has compiled papers on the current trends and research directions within the preparation, characterization and application of polymer-based composite materials in electrical engineering applications. In recent times, this type of material has evolved to become one of the most thoroughly investigated materials, stimulated by the demand for the resource-efficient assembly of generators, transformers, communication devices, etc. Novel composites are to be used as insulating materials with high thermal conductivity and excellent temperature stability, through which premature ageing and degradation of devices shall be avoided or at least reduced. This Special Issue comprises twelve contributions by internationally renowned researchers; to mention Petru V. Nothinger (University Politehnica of Bucharest), Alun S. Vaughan (University of Southampton), Stanislaw M. Gubanski (Chalmers University of Technology), Michael Muhr (Graz University of Technology), Johan J. Smit (TU Delft), and Ulf W. Gedde (KTH Royal Institute of Technology) as prominent examples. The state-of-the-art research and technology of the area ‘micro- and nanocomposites for electrical engineering applications’ has been summarized in three review articles, while the current research trends and the development and characterization of novel materials have been described in eight original research articles. Stimulated by the vivid current interest in this topic, this Special Issue of Polymers has additionally been compiled in a book version

Advancements in microfabricated gas sensors and microanalytical tools for the sensitive and selective detection of odors

In recent years, advancements in micromachining techniques and nanomaterials have enabled the fabrication of highly sensitive devices for the detection of odorous species. Recent efforts done in the miniaturization of gas sensors have contributed to obtain increasingly compact and portable devices. Besides, the implementation of new nanomaterials in the active layer of these devices is helping to optimize their performance and increase their sensitivity close to humans’ olfactory system. Nonetheless, a common concern of general-purpose gas sensors is their lack of selectivity towards multiple analytes. In recent years, advancements in microfabrication techniques and microfluidics have contributed to create new microanalytical tools, which represent a very good alternative to conventional analytical devices and sensor-array systems for the selective detection of odors. Hence, this paper presents a general overview of the recent advancements in microfabricated gas sensors and microanalytical devices for the sensitive and selective detection of volatile organic compounds (VOCs). The working principle of these devices, design requirements, implementation techniques, and the key parameters to optimize their performance are evaluated in this paper. The authors of this work intend to show the potential of combining both solutions in the creation of highly compact, low-cost, and easy-to-deploy platforms for odor monitoringPostprint (published version

Multi-nanolayered VO2/Sapphire Thin Film via Spinodal Decomposition

Abstract Coating of VO2-based thin film has been extensively studied for fabricating energy-saving smart windows. One of the most efficient ways for fabricating high performance films is to create multi-nanolayered structure. However, it has been highly challenge to make such layers in the VO2-based films using conventional methods. In this work, a facile two-step approach is established to fabricate multilayered VO2-TiO2 thin films. We first deposited the amorphous thin films upon sputtering, and then anneal them to transform the amorphous phase into alternating Ti- and V-rich multilayered nanostructure via a spinodal decomposition mechanism. In particular, we take advantage of different sapphire substrate planes (A-plane (11–20), R-plane (1–102), C-plane (0001), and M-plane (10-10)) to achieve different decomposition modes. The new approach has made it possible to tailoring the microstructure of the thin films for optimized performances by controlling the disorder-order transition in terms of both kinetic and thermodynamic aspects. The derived thin films exhibit superior optical modulation upon phase transition, significantly reduced transition temperature and hysteresis loop width, and high degradation resistance, these improvements indicate a high potential to be used for fabricating the next generation of energy saving smart windows

Bacterial cellulose coated ST-cut quartz surface acoustic wave humidity sensor with high sensitivity, fast response and recovery

A Love mode surface acoustic wave (SAW) humidity sensor based on bacterial cellulose (BC) coated ST-cut quartz was developed in this study. The BC film is composed of ultrafine interwoven fibers to form a highly porous network, and its surface contains a large amount of hydroxyl groups, which significantly improve the adsorption capability of SAW sensing layer for water molecules. This results in significant mass loading effects and enhanced responsivity of the SAW sensor. The resonant frequency of the sensor changes linearly with RH at lower relative humidity (RH) values (e.g., RH30%), but when RH80%, an exponential increase in frequency shift as a function of RH is obtained due to the enhanced mass loading effect. A frequency shift of 89.8 kHz was measured using a sensor with a BC film with a thickness of 148 nm thick when the RH was increased from 30% to 93%. The frequency of the sensor can be fully shifted back to the original reading when the RH was returned back to 30%, with the response and recovery times of 12 s and 5 s, respectively. The SAW sensor also exhibits good short-term repeatability and long-term stability for humidity sensing