Hydrogenated amorphous carbon (a-C:H) films with a designed buffer layer of amorphous hydrogenated silicon carbide on the substrates were fabricated by plasma enhanced chemical vapor deposition (PECVD). The effect of radio frequency (RF) power on the structural and optical properties of a-C:H films was investigated. The ratios of sp 3 to sp 2 of carbon atoms and hydrogen contents in the RF power range of 75–175 W are determined and a similar trend as a function of power. The increase of sp 3 to sp 2 ratio leads to the increase of transmittance and optical gap of a-C:H films. a-C:H film under an RF power of 175 W possesses high transmissive ability (>80%) in the visible wave length, even the highest transmittance value of about 94.2% is achieved at the wave length 550 nm. These results show the optimal a-C:H films which are promising for the applications in the area of solar cells acting a window layer and antireflection layer.

This article provides a short review on computational modeling on the formation, thermodynamics, and elasticity of single-phase high-entropy alloys (HEAs). Hundreds of predicted single-phase HEAs were re-examined using various empirical thermo-physical parameters. Potential BCC HEAs (CrMoNbTaTiVW, CrMoNbReTaTiVW, and CrFeMoNbReRuTaVW) were suggested based on CALPHAD modeling. The calculated vibrational entropies of mixing are positive for FCC CoCrFeNi, negative for BCC MoNbTaW, and near-zero for HCP CoOsReRu. The total entropies of mixing were observed to trend in descending order: CoCrFeNi > CoOsReRu > MoNbTaW. Calculated lattice parameters agree extremely well with averaged values estimated from the rule of mixtures (ROM) if the same crystal structure is used for the elements and the alloy. The deviation in the calculated elastic properties from ROM for select alloys is small but is susceptible to the choice used for the structures of pure components.

Two-step crystallization process based on a low-Pb-content seed layer is proposed to form PZT films by the chemical solution deposition. The first crystallization step was performed after the deposition from precursor solutions with 0 and 5 wt% Pb excess, which provides a low nucleation rate and the strong perovskite (111) orientation. The bulk film was obtained from solutions with a 30 wt% Pb excess, which ensures a high growth rate and eliminates formation of pyrochlore residuals. Some films with a fixed Pb excess were prepared for comparison. It is shown that the low-Pb-content seed layer can sufficiently enhance the texture of perovskite (111) grains thus providing the highest polarization magnitudes as compared to films prepared with the fixed Pb content. The lead content and the crystallization of the seed layer are found to affect the grain-boundary conduction, which, in turn, influences the polarization dependence of transient currents.

Phosphorene has recently gained tremendous interest in the current decade, specifically, black phosphorus monolayer, a unique 2D material, investigation of which has led toward the creation of new scientific discoveries for future optoelectronic sensor devices. Beyond the success of graphene and other 2D layered materials research over the past decades, the increased interest toward this new emerging single-element structured material is because of its layer dependent 0.3–2.0 eV band gap modulation range which is also the band gap modulation range of single- and few-layered graphene and transition metal dichalcogenides (TMDs). Besides that, phosphorene allows strong light-matter interactions at resonance because of its unique physical structure and outstanding electrical and optical properties. Therefore, current advancements are being done to enhance the performance of phosphorene thin films because of its applicability in different fields. This paper is aimed to highlight key properties, applications, and future perspects and challenges incurred regarding the use of 2D layered phosphorene.

In the microsize regime, all crystalline metals studied to-date exhibit a “smaller-is-stronger” size effect. Here, we report an unusual weakest-size phenomenon in the precipitated alloy duralumin 2025, i.e., below a critical size of ∼7 μm, the strength increases as the size decreases, while above this size, the strength increases toward the bulk value with increasing size. At the critical size, strain-hardening is also slowest and the room-temperature creep is fastest. Interestingly, the reduction of strength at the weakest size is more significant for the peak-aged state of duralumin 2025 than its naturally aged state. Theoretical modeling shows that at the weakest size, both strengthening mechanisms of precipitation hardening and dislocation starvation are ineffective. The present results indicate that the conventional wisdom of precipitation hardening is not applicable in the micro-regime, and the common “smaller-is-stronger” understanding is incorrect when material microstructures impose internal length scales that can affect strength.

The unloading part of a load–displacement curve from instrumented indentation tests is usually approximated by a power law (Oliver and Pharr model), where the load is the dependent variable. This approach generally fits well the data. Nevertheless, the convergence is occasionally quite questionable. In this regard, we propose a different approach for the Oliver and Pharr model, called the inverted approach, since it assigns the displacement as the dependent variable. Both models were used to fit the unloading curves from nanoindentation tests on fused silica and aluminum, applying a general least squares procedure. Generally, the inverted methodology leads to similar results for the fitting parameters and the elastic modulus (E) when convergence is achieved. Nevertheless, this approach facilitates the convergence, because it is a better conditioned problem. Additionally, by Monte Carlo simulations we found that robustness is improved using the inverted approach, since the estimation of E is more accurate, especially for aluminum.

The epitaxial growth of two-dimensional oxide layers on metal surfaces is examined in view of the 1949 van der Merwe proposition that epitaxy requires a pseudomorphic monolayer. It is argued that this limitation is relaxed in the 2-D case and that ordered oxide phases can grow out of a variety of interface scenarios, ranging from pseudomorphic to incommensurate. Prototypical examples of binary and ternary oxides supported on noble metal surfaces are presented, and the structural peculiarities of 2-D oxide phases are emphasized. The usually strong coupling at the oxide–metal interface leads to the stabilization of novel structure concepts that are not encountered in the, respective, bulk phases. The structural flexibility of 2-D lattices is discussed, and their ability to accommodate strain in generating novel 2-D oxide phases is emphasized. In the case of weakly coupled systems, it is reported that more subtle interactions at the interface can create periodic nanoscale morphologies and particular growth patterns in subsequent layers.

Layered transition metal oxides are some of the most important materials for high energy and power density electrochemical energy storage, such as batteries and electrochemical capacitors. These oxides can efficiently store charge via intercalation of ions into the interlayer vacant sites of the bulk material. The interlayer can be tuned to modify the electrochemical environment of the intercalating species to allow improved interfacial charge transfer and/or solid-state diffusion. The ability to fine-tune the solid-state environment for energy storage is highly beneficial for the design of layered oxides for specific mechanisms, including multivalent ion intercalation. This review focuses on the benefits as well as the methods for interlayer modification of layered oxides, which include the presence of structural water, solvent cointercalation and exchange, cation exchange, polymers, and small molecules, exfoliation, and exfoliated heterostructures. These methods are an important design tool for further development of layered oxides for electrochemical energy storage applications.

This paper reviews our recent investigations of compound semiconductors and heterovalent interfaces using the technique of aberration-corrected scanning transmission electron microscopy. Bright-field imaging of compound semiconductors with a collection angle that is comparable in size to the incident-beam convergence angle is demonstrated to provide better atomic-column visibility for lighter elements in comparison with the more traditional high-angle annular-dark-field approach. Several pairs of Group II–VI/Group III–V compound semiconductors with zincblende structure have been studied in detail. These combinations are all valence-mismatched (i.e., heterovalent), and include CdTe/InSb (Δa/a ≤ 0.05%), ZnTe/InP (Δa/a = 3.8%), and ZnTe/GaAs (Δa/a = 7.4%). CdTe/InSb (001) interfaces are observed to be defect-free with a slight lattice contraction at the interface plane. For interfaces with larger lattice-parameter mismatch, the primary interfacial defects are identified as Lomer edge dislocations and perfect 60° dislocations. However, the atomic structure of the dislocation cores has not yet been unambiguously determined.

In this work, the high-performance silicon carbide particle SiCp[carbon nanotube (CNT)] hybrid reinforcement is currently explored to develop the advanced metal matrix composites. 17 wt% SiCp(CNT)/Al composites were fabricated by a powder metallurgy technique, in which SiCp(CNT) hybrid reinforcement with various CNT contents (e.g., 3, 6 and 9 wt%) were applied. Effects of CNT content on the morphology of SiCp(CNT) hybrid reinforcement, the microstructural characteristics, and the tensile mechanical behavior of SiCp(CNT)/Al composites were studied as well. Especially, the SiCp(CNT)/Al composites with 6 wt% CNT in SiCp(CNT) hybrid reinforcement exhibited the most significant enhancing effects in the elastic modulus and tensile strength. Meanwhile, the SiCp(CNT)/Al composites produced a synergistic strengthening effect of SiCp and CNT compared to SiCp/Al composites, while the SiCp(CNT)/Al composites with high CNT content in SiCp(CNT) hybrid reinforcement provided weak improvement in the tensile strength and ductility due to the forming agglomeration of CNT in the matrix.

We describe the main experimental challenges toward the metrological calibration of photodetectors based on single semiconductor nanowires, and we propose a method for the quantification of their photoresponse, focusing in particular on GaAs nanowires. Spatially resolved measurements of the device’s photocurrent were performed with a far-field scanning optical setup and a laser excitation at λ = 656 nm. The photoresponse was quantitatively described by fitting the two-dimensional mapping of the photocurrent at different positions along the main nanowire axis. Our results indicate that the device’s photoresponse strongly depends on the position along the nanowire, which is attributed to the inhomogeneous properties of the device’s contacts. Furthermore, we show that its spatial profile across the nanowire can be directly compared with the profile of the laser beam by taking into account the angle between the scanning direction and the main nanowire axis as a geometrical factor. Finally, we discuss the impacts of laser-induced heating effects on the calibration of such nanoscale devices.

In situ and operando measurement techniques combined with nanoscale resolution have proven invaluable in multiple fields of study. We argue that evaluating device performance as well as material behavior by correlative X-ray microscopy with <100 nm resolution can radically change the approach for optimizing absorbers, interfaces and full devices in solar cell research. In this article, we thoroughly discuss the measurement technique of X-ray beam induced current and point out fundamental differences between measurements of wafer-based silicon and thin-film solar cells. Based on reports of the last years, we showcase the potential that X-ray microscopy measurements have in combination with in situ and operando approaches throughout the solar cell lifecycle: from the growth of individual layers to the performance under operating conditions and degradation mechanisms. Enabled by new developments in synchrotron beamlines, the combination of high spatial resolution with high brilliance and a safe working distance allows for the insertion of measurement equipment that can pave the way for a new class of experiments. Applied to photovoltaics research, we highlight today’s opportunities and challenges in the field of nanoscale X-ray microscopy, and give an outlook on future developments.

Personal, multifunctional, and smart electronic devices/systems are indispensable components of the internet of things for modern information collection and exchange, which play a key role in facilitating the development of human civilization. Traditional technique for powering these sensor nodes mainly relies on batteries, which may not be favorable owing to the limited battery lifetime, large sensor population, wide distribution, as well as the potential of environmental detriment. Extricated from external power sources, triboelectric nanogenerators (TENGs) based active sensors have been extensively spread into a variety of fields for self-powered high-performance sensing, featured as being lightweight, extremely cost-effective, and environmentally friendly. In this article, current progress of TENGs as smart sensors for self-powered touch detection, vibration and acoustic sensing, biomedical applications, as well as human-machine interfacing, has been comprehensively reviewed, from aspects of materials usage, device fabrication to practical applications. The latest representative achievements regarding the TENG based self-powered sensing systems were also systematically presented. In the end, some perspectives and challenges for the TENG based self-powered smart sensors were also summarized.

Methylammonium-tin-iodide (MASnxI3, 0.9 ≤ x ≤ 1.4) systems were prepared by self assembly process in aqueous solutions. The “as-prepared” MASnxI3 systems exhibit a crystalline tetragonal structure (space group I4cm) with polyhedral-shaped crystallites. The as-prepared samples were annealed at T = 150 °C, t = 8 h under nitrogen and synthetic air. Under nitrogen, the CH3NH3SnxI3 systems adopted a cubic crystalline structure (space group P4mm) with crystallites of 2–4 μm length, whereas under air, the formation of noncrystalline phases was observed. The optical absorption spectra displayed absorption edges at 1107.0 nm (x = 0.9), 1098.6 nm (x = 1.0), and 1073.2 nm (x = 1.1), respectively, whereas at higher Sn-content (x ≥ 1.2), a broad tail of the absorbance profile was observed. The photoluminescence (PL) emission spectra (RT, λexc = 500 nm) showed major PL-events over 1 µm range and the appearance of additional bands at increasing the Sn-content. The fabrication of layers with a semiconducting behavior was demonstrated.

Pulsed laser deposition is one of the most flexible growth methods for high-quality epitaxial multifunctional thin films and short-period superlattices. The following examples of current research interest demonstrate the state-of-the art: First, it is shown that the magnetoelectric performance of multiferroic BiFeO3–BaTiO3 (001)-oriented superlattices depends on the crystalline coherence of the different layers at the interfaces. Second, it is exemplified that dielectric-plasmonic superlattices built from the electrically insulating oxide MgO and the metallically conducting nitride TiN are promising metamaterials with hyperbolic dispersion. As a third example, it is demonstrated that LaNiO3- and LaMnO3-based superlattices with (001)-, (011)-, and (111)-out-of-plane orientation and controlled single layer thickness from 2 to 15 atomic monolayers show metal-insulator transitions and tunable gaps, in partial agreement with density functional theory calculations. Underlined by these examples, it is shown that the precise control of an epitaxially coherent, or two-dimensional layer-by-layer growth, named after Jan van der Merwe, is a prerequisite to achieve the desired functionality of oxide–oxide and oxide–nitride superlattices.

GaN field-effect transistors with impressive power switching characteristics have been demonstrated. Preventing their widespread field deployment are reliability and instability concerns. Some emanate from the use of a dielectric in the gate stack. Under typical operation, the gate dielectric comes periodically under intense electric field. This causes trapping and detrapping of electrons and introduces transient shifts in the threshold voltage, a phenomenon known as Bias-Temperature Instability (BTI). A high electric field also results in the formation of defects inside the dielectric. Over time, the defects accumulate and eventually result in the abrupt creation of a conducting path that shorts the dielectric and renders the device inoperable. This process, known as Time-Dependent Dielectric Breakdown (TDDB), often imposes a maximum lifetime for the FET technology. This article presents a methodology for the study of BTI and TDDB in insulated-gate GaN FETs. Our findings paint a picture of BTI and TDDB that in many respects is similar to that of Si transistors but with some unique characteristics. Understanding the physics and developing appropriate lifetime models is essential to enabling the deployment of this important new power electronics technology.

The lithium ion battery is the most promising battery candidate to power battery electric vehicles. For these vehicles to be competitive with those powered by conventional internal combustion engines, significant improvements in battery performance are needed, especially in the energy density and power delivery capabilities. Promising substitutes for graphite as the anode material include silicon, tin, germanium, and various metal oxides that have much higher theoretical storage capacities and operated at slightly higher and safer potentials. In this critical review, metal oxides-based materials for lithium ion battery anodes are reviewed in detail together with the progress which is made in my lab on that topic. Their advantages, disadvantages, and performance in lithium ion batteries are discussed through extensive analysis of the literature, and new trends in materials development are also reviewed. Two important future research directions are proposed and performed in my lab, based on results published in the literature: the development of composite and nanostructured metal oxides to overcome the major challenge posed by the high capacity of metal oxide anodes.

Nb-based silicides are promising ultrahigh-temperature materials. However, the structural stability and mechanical properties of Nb-based silicides are markedly influenced by Nb3Si phase. Therefore, the improvement of the stability and mechanical properties of Nb3Si is a great challenge. To solve these key problems, in this work, we apply the first-principles calculations to investigate the influence of transition metals (TM = Mo, Re, Ta, W, Pt, and Ir) on the structural stability, mechanical, and thermodynamic properties of Nb3Si. Two possible doped sites: Nb site and Si site are considered. We find that these alloying elements not only can stabilize the Nb3Si phase but also effectively improve the mechanical properties of Nb3Si. The calculated electronic structure shows that high elastic modulus is attributed to the formation of the TM–Si bond. Importantly, these alloying elements improve the heat capacity of Nb3Si due to the vibration of TM atoms under high temperature. Therefore, our calculated results predict that alloying elements of Re and Ir are beneficial for improving the overall performances of Nb3Si.

A CrMnFeCoNi high-entropy alloy was investigated by nanoindentation from room temperature to 400 °C in the nanocrystalline state and cast plus homogenized coarse-grained state. In the latter case a 〈100〉-orientated grain was selected by electron back scatter diffraction for nanoindentation. It was found that hardness decreases more strongly with increasing temperature than Young’s modulus, especially for the coarse-grained state. The modulus of the nanocrystalline state was slightly higher than that of the coarse-grained one. For the coarse-grained sample a strong thermally activated deformation behavior was found up to 100–150 °C, followed by a diminishing thermally activated contribution at higher testing temperatures. For the nanocrystalline state, different temperature dependent deformation mechanisms are proposed. At low temperatures, the governing processes appear to be similar to those in the coarse-grained sample, but with increasing temperature, dislocation-grain boundary interactions likely become more dominant. Finally, at 400 °C, decomposition of the nanocrystalline alloy causes a further reduction in thermal activation. This is rationalized by a reduction of the deformation controlling internal length scale by precipitate formation in conjunction with a diffusional contribution.

The present study was focused to investigate mechanical properties of ZnO thin films deposited on fused quartz substrates at different sputtering deposition pressures (5, 10, 15, and 20 mTorr) using DC sputtering. The crystallinity and microstructure show a marked influence on the mechanical properties of ZnO thin films. The structural evolution of the thin films is in (002) plane and influenced by deposition pressure. The intensity of (002) peak of the films rises initially and decreases with further increasing deposition pressure. The mechanical properties such as hardness, Young’s modulus, and coefficient of friction of ZnO thin films were measured using three-sided pyramidal Berkovich nanoindentation. The adhesion strength of thin films was measured by using scratch test under ramp loading. Load–displacement profile of thin films at continuous indentation cycle without any discontinuity revealed no fracture, cracking event, and defects, which is a consequence of dense microstructure and good adherence of films to the substrate.