The electronic structure at the atomic scale of ultrathin gate oxides

Probing charge traps at the 2D semiconductor/dielectric interface

The family of 2-dimensional (2D) semiconductors is a subject of intensive scientific research due to their potential in next-generation electronics. While offering many unique properties like atomic thickness and chemically inert surfaces, the integration of 2D semiconductors with conventional dielectric materials is challenging. The charge traps at the semiconductor/dielectric interface are among many issues to be addressed before these materials can be of industrial relevance. Conventional electrical characterization methods remain inadequate to quantify the traps at the 2D semiconductor/dielectric interface since the estimations of the density of interface traps, Dit, by different techniques may yield more than an order-of-magnitude discrepancy, even when extracted from the same device. Therefore, the challenge to quantify Dit at the 2D semiconductor/dielectric interface is about finding an accurate and reliable measurement method. In this review, we discuss characterization techniques which have been used to study the 2D semiconductor/dielectric interface. Specifically, we discuss the methods based on small-signal AC measurements, subthreshold slope measurements and low-frequency noise measurements. While these approaches were developed for silicon-based technology, 2D semiconductor devices possess a set of unique challenges requiring a careful re-evaluation when using these characterization techniques. We examine the conventional methods based on their efficacy and accuracy in differentiating various types of trap states and provide guidance to find an appropriate method for charge trap analysis and estimation of Dit at 2D semiconductor/dielectric interfaces.

Transforming the Petroleum Industry through Catalytic Oxidation Reactions vis-à-vis Preceramic Polymer Catalyst Supports

Preceramic polymers, for instance, are used in a variety of chemical processing industries and applications. In this contribution, we report on the catalytic oxidation reactions generated using preceramic polymer catalyst supports. Also, we report the full knowledge of the use of the remarkable catalytic oxidation, and the excellent structures of these preceramic polymer catalyst supports are revealed. This finding, on the other hand, focuses on the functionality and efficacy of future applications of catalytic oxidation of preceramic polymer nanocrystals for energy and environmental treatment. The aim is to design future implementations that can address potential environmental impacts associated with fuel production, particularly in downstream petroleum industry processes. As a result, these materials are being considered as viable candidates for environmentally friendly applications such as refined fuel production and related environmental treatment.

Vertically grown ultrathin Bi2SiO5 as high-κ single-crystalline gate dielectric

Single-crystalline high-κ dielectric materials are desired for the development of future two-dimensional (2D) electronic devices. However, curent 2D gate insulators still face challenges, such as insufficient dielectric constant and difficult to obtain free-standing and transferrable ultrathin films. Here, we demonstrate that ultrathin Bi₂SiO₅ crystals grown by chemical vapor deposition (CVD) can serve as excellent gate dielectric layers for 2D semiconductors, showing a high dielectric constant (>30) and large band gap (~3.8 eV). Unlike other 2D insulators synthesized via in-plane CVD on substrates, vertically grown Bi₂SiO₅ can be easily transferred onto other substrates by polymer-free mechanical pressing, which greatly facilitates its ideal van der Waals integration with few-layer MoS₂ as high-κ dielectrics and screening layers. The Bi₂SiO₅ gated MoS₂ field-effect transistors exhibit an ignorable hysteresis (~3 mV) and low drain induced barrier lowering (~5 mV/V). Our work suggests vertically grown Bi₂SiO₅ nanoflakes as promising candidates to improve the performance of 2D electronic devices.

Dielectrics for Two-Dimensional Transition-Metal Dichalcogenide Applications

Despite over a decade of intense research efforts, the full potential of two-dimensional transition-metal dichalcogenides continues to be limited by major challenges. The lack of compatible and scalable dielectric materials and integration techniques restrict device performances and their commercial applications. Conventional dielectric integration techniques for bulk semiconductors are difficult to adapt for atomically thin two-dimensional materials. This review provides a brief introduction into various common and emerging dielectric synthesis and integration techniques and discusses their applicability for 2D transition metal dichalcogenides. Dielectric integration for various applications is reviewed in subsequent sections including nanoelectronics, optoelectronics, flexible electronics, valleytronics, biosensing, quantum information processing, and quantum sensing. For each application, we introduce basic device working principles, discuss the specific dielectric requirements, review current progress, present key challenges, and offer insights into future prospects and opportunities.

Wafer-scale high-κ dielectrics for two-dimensional circuits via van der Waals integration

The practical application of two-dimensional (2D) semiconductors for high-performance electronics requires the integration with large-scale and high-quality dielectrics-which however have been challenging to deposit to date, owing to their dangling-bonds-free surface. Here, we report a dry dielectric integration strategy that enables the transfer of wafer-scale and high-κ dielectrics on top of 2D semiconductors. By utilizing an ultra-thin buffer layer, sub-3 nm thin Al₂O₃ or HfO₂ dielectrics could be pre-deposited and then mechanically dry-transferred on top of MoS₂ monolayers. The transferred ultra-thin dielectric film could retain wafer-scale flatness and uniformity without any cracks, demonstrating a capacitance up to 2.8 μF/cm², equivalent oxide thickness down to 1.2 nm, and leakage currents of ~10^-7A/cm². The fabricated top-gate MoS₂ transistors showed intrinsic properties without doping effects, exhibiting on-off ratios of ~10⁷, subthreshold swing down to 68 mV/dec, and lowest interface states of 7.6×10⁹cm^-2 eV^-1. We also show that the scalable top-gate arrays can be used to construct functional logic gates. Our study provides a feasible route towards the vdW integration of high-κ dielectric films using an industry-compatible ALD process with well-controlled thickness, uniformity and scalability.

Composition and electronic structure of [Formula: see text]/[Formula: see text]/Al passivating carrier selective contacts on n-type silicon solar cells

Carrier-selective and passivating SiO[Formula: see text]/TiO[Formula: see text] heterocontacts are an attractive alternative to conventional contacts due to their high efficiency potentials combined with relatively simple processing schemes. It is widely accepted that post deposition annealing is necessary to obtain high photovoltaic efficiencies, especially for full area aluminum metallized contacts. Despite some previous high-level electron microscopy studies, the picture of atomic-scale processes underlying this improvement seems to be incomplete. In this work, we apply nanoscale electron microscopy techniques to macroscopically well-characterized solar cells with SiO[Formula: see text]/TiO[Formula: see text]/Al rear contacts on n-type silicon. Macroscopically, annealed solar cells show a tremendous decrease of series resistance and improved interface passivation. Analyzing the microscopic composition and electronic structure of the contacts, we find that partial intermixing of the SiO[Formula: see text] and TiO[Formula: see text] layers occurs due to annealing, leading to an apparent thickness reduction of the passivating SiO[Formula: see text]. However, the electronic structure of the layers remains clearly distinct. Hence, we conclude that the key to obtain highly efficient SiO[Formula: see text]/TiO[Formula: see text]/Al contacts is to tailor the processing such that the excellent chemical interface passivation of a SiO[Formula: see text] layer is achieved for a layer thin enough to allow efficient tunneling through the layer. Furthermore, we discuss the impact of aluminum metallization on the above mentioned processes.

Mechanisms of metal-insulator transitions in ultrathin SrMoO3films

As the thickness of a transition metal oxide thin film is reduced to several unit cells, dimensional and interfacial effects modulate its structure and properties, and initiate low-dimension quantum phase transitions different from its bulk counterparts. To check if a metal-insulator transition (MIT) occurs to a low-dimensional 4d2electron systems, we investigated SrMoO₃thin films by characterizing and analyzing their lattice structures, electric transport properties and electronic states. Among various dimensional effects and interfacial effects, quantum confinement effect (QCE) was discerned as the dominating mechanism of the thickness-driven MIT. Surface/interface scattering contributes to the residual resistivity while the competition of several interactions modulated by QCE governs the temperature dependence of the resistivity of SrMoO₃ultrathin films.

Roles of excess minority carrier recombination and chemisorbed O2 species at SiO2/Si interfaces in Si dry oxidation: Comparison between p-Si(001) and n-Si(001) surfaces

This study provides experimental evidence for the following: (1) Excess minority carrier recombination at SiO₂/Si interfaces is associated with O₂ dissociative adsorption; (2) the x-ray induced enhancement of SiO₂ growth is not caused by the band flattening resulting from the surface photovoltaic effect but by the electron-hole pair creation resulting from core level photoexcitation for the spillover of bulk Si electronic states toward the SiO₂ layer; and (3) a metastable chemisorbed O₂ species plays a decisive role in combining two types of the single- and double-step oxidation reaction loops. Based on experimental results, the unified Si oxidation reaction model mediated by point defect generation [S. Ogawa et al., Jpn. J. Appl. Phys., Part 1 59, SM0801 (2020)] is extended from the viewpoints of (a) the excess minority carrier recombination at the oxidation-induced vacancy site and (b) the trapping-mediated adsorption through the chemisorbed O₂ species at the SiO₂/Si interface.

Optimal Microstructure of Silicon Monoxide as the Anode for Lithium-Ion Batteries

Because of its metastable nature, silicon monoxide (SiO) consists of Si nanodomains in an amorphous matrix of SiO₂. The microstructure of SiO, including SiO₂, Si domains, and interphase (SiO_x) between domains, was modified via an annealing treatment in argon gas and thoroughly characterized by in-situ and ex-situ X-ray diffraction, pair distribution function, and electron energy loss spectroscopy. Two microstructure transformation routes were observed during the annealing process: (1) at a temperature of <800 °C, the annealing treatment was found to affect mainly the structural conformation of the amorphous SiO₂ matrix and the interphase, while (2) an annealing temperature of >800 °C led to significant Si nanodomain growth. We found that the microstructure has a great impact on the electrochemical performance of SiO. The optimized microstructure of SiO appears to be achieved through annealing treatment at 800 °C or less, which results in interphase (SiO_x) reduction without causing significant Si domain growth. This work provides a deep insight into the domain and interphase transformation of SiO upon heat treatment. The improved understanding of the relationship between SiO microstructure and its electrochemical behavior will enable proper design and development of high-energy SiO for lithium-ion batteries.

Structure of Amorphous Two-Dimensional Materials: Elemental Monolayer Amorphous Carbon versus Binary Monolayer Amorphous Boron Nitride

The structure of amorphous materials has been debated since the 1930s as a binary question: amorphous materials are either Zachariasen continuous random networks (Z-CRNs) or Z-CRNs containing crystallites. It was recently demonstrated, however, that amorphous diamond can be synthesized in either form. Here we address the question of the structure of single-atom-thick amorphous monolayers. We reanalyze the results of prior simulations for amorphous graphene and report kinetic Monte Carlo simulations based on alternative algorithms. We find that crystallite-containing Z-CRN is the favored structure of elemental amorphous graphene, as recently fabricated, whereas the most likely structure of binary monolayer amorphous BN is altogether different than either of the two long-debated options: it is a compositionally disordered "pseudo-CRN" comprising a mix of B-N and noncanonical B-B and N-N bonds and containing "pseudocrystallites", namely, honeycomb regions made of noncanonical hexagons. Implications for other nonelemental 2D and bulk amorphous materials are discussed.

Dislocation-induced stop-and-go kinetics of interfacial transformations

Most engineering materials are based on multiphase microstructures produced either through the control of phase equilibria or by the fabrication of different materials as in thin-film processing. In both processes, the microstructure relaxes towards equilibrium by mismatch dislocations (or geometric misfit dislocations) across the heterophase interfaces^1-5. Despite their ubiquitous presence, directly probing the dynamic action of mismatch dislocations has been unachievable owing to their buried nature. Here, using the interfacial transformation of copper oxide to copper as an example, we demonstrate the role of mismatch dislocations in modulating oxide-to-metal interfacial transformations in an intermittent manner, by which the lateral flow of interfacial ledges is pinned at the core of mismatch dislocations until the dislocation climbs to the new oxide/metal interface location. Together with atomistic calculations, we identify that the pinning effect is associated with the non-local transport of metal atoms to fill vacancies at the dislocation core. These results provide mechanistic insight into solid-solid interfacial transformations and have substantial implications for utilizing structural defects at buried interfaces to modulate mass transport and transformation kinetics.

Two-Dimensional Ultrathin Silica Films

Two-dimensional (2D) ultrathin silica films have the potential to reach technological importance in electronics and catalysis. Several well-defined 2D-silica structures have been synthesized so far. The silica bilayer represents a 2D material with SiO₂ stoichiometry. It consists of precisely two layers of tetrahedral [SiO₄] building blocks, corner connected via oxygen bridges, thus forming a self-saturated silicon dioxide sheet with a thickness of ∼0.5 nm. Inspired by recent successful preparations and characterizations of these 2D-silica model systems, scientists now can forge novel concepts for realistic systems, particularly by atomic-scale studies with the most powerful and advanced surface science techniques and density functional theory calculations. This Review provides a solid introduction to these recent developments, breakthroughs, and implications on ultrathin 2D-silica films, including their atomic/electronic structures, chemical modifications, atom/molecule adsorptions, and catalytic reactivity properties, which can help to stimulate further investigations and understandings of these fundamentally important 2D materials.

Measurement of Nanometre-Scale Gate Oxide Thicknesses by Energy-Dispersive X-ray Spectroscopy in a Scanning Electron Microscope Combined with Monte Carlo Simulations

A procedure based on energy-dispersive X-ray spectroscopy in a scanning electron microscope (SEM-EDXS) is proposed to measure ultra-thin oxide layer thicknesses to atomic scale precision in top-down instead of cross-sectional geometry. The approach is based on modelling the variation of the electron beam penetration depth and hence the depth of X-ray generation in the sample as a function of the acceleration voltage. This has been tested for the simple case of silica on silicon (SiO₂/Si) which can serve as a model system to study gate oxides in metal-on-semiconductor field-effect transistors (MOS-FETs). Two possible implementations exist both of which rely on pairs of measurements to be made: in method A, the wafer piece of interest and a reference sample (here: ultra-clean fused quartz glass for calibration of the effective k-factors of X-ray lines from elements O and Si) are analysed at the same acceleration voltage. In method B, two measurements of the apparent O/Si ratio of the same wafer sample need to be made at different acceleration voltages and from their comparison to simulations the SiO₂ layer thickness of the sample can be inferred. The precision attainable is ultimately shown to be limited by surface contamination during the experiments, as very thin carbonaceous surface layers can alter the results at very low acceleration voltages, while the sensitivity to ultra-thin surface oxides is much reduced at higher acceleration voltages. The optimal operation voltage is estimated to lie in the range of 3–15 kV. Method A has been experimentally verified to work well for test structures of thin oxides on Si-Ge/Si.

Polyhedron and Charge Ordering in Interfacial Reconstruction of a Hexagonal Ferrite/Sapphire Heterostructure

Interfacial reconstruction, emanating from the symmetry breaking at the interface, plays a key role in modulating the microstructures and properties of heterostructures. The appeal of revealing such a reconstruction resides in the underlying mechanism connected to the function of heterostructures and new insights into designing a new interface device. Here, we demonstrate an interfacial reconstruction in a large lattice-mismatch system, h-LuFeO₃/α-Al₂O₃ heterostructure. Combining the atomic-resolution imaging and spectroscopy of scanning transmission electron microscopy, the periodic variation of FeO immediate coordination and charge ordering of iron are revealed, indicating a strong lattice-charge coupling in the reconstruction. Such a reconstruction reported here suggests that polyhedral and electronic flexibility is important for the reconstruction formation and presents possibilities for further construction of more functional heterostructures.

Correlating the electronic structures of metallic/semiconducting MoTe2 interface to its atomic structures

Contact interface properties are important in determining the performances of devices that are based on atomically thin two-dimensional (2D) materials, especially for those with short channels. Understanding the contact interface is therefore important to design better devices. Herein, we use scanning transmission electron microscopy, electron energy loss spectroscopy, and first-principles calculations to reveal the electronic structures within the metallic (1T')-semiconducting (2H) MoTe2 coplanar phase boundary across a wide spectral range and correlate its properties to atomic structures. We find that the 2H-MoTe2 excitonic peaks cross the phase boundary into the 1T' phase within a range of approximately 150 nm. The 1T'-MoTe2 crystal field can penetrate the boundary and extend into the 2H phase by approximately two unit-cells. The plasmonic oscillations exhibit strong angle dependence, that is a red-shift of π+σ (approximately 0.3-1.2 eV) occurs within 4 nm at 1T'/2H-MoTe2 boundaries with large tilt angles, but there is no shift at zero-tilted boundaries. These atomic-scale measurements reveal the structure-property relationships of the 1T'/2H-MoTe2 boundary, providing useful information for phase boundary engineering and device development based on 2D materials.

Study of the Microstructure of Amorphous Silica Nanostructures Using High-Resolution Electron Microscopy, Electron Energy Loss Spectroscopy, X-ray Powder Diffraction, and Electron Pair Distribution Function

Silica has many industrial (i.e., glass formers) and scientific applications. The understanding and prediction of the interesting properties of such materials are dependent on the knowledge of detailed atomic structures. In this work, amorphous silica subjected to an accelerated alkali silica reaction (ASR) was recorded at different time intervals so as to follow the evolution of the structure by means of high-resolution transmission electron microscopy (HRTEM), electron energy loss spectroscopy (EELS), and electron pair distribution function (e-PDF), combined with X-ray powder diffraction (XRPD). An increase in the size of the amorphous silica nanostructures and nanopores was observed by HRTEM, which was accompanied by the possible formation of Si–OH surface species. All of the studied samples were found to be amorphous, as observed by HRTEM, a fact that was also confirmed by XRPD and e-PDF analysis. A broad diffuse peak observed in the XRPD pattern showed a shift toward higher angles following the higher reaction times of the ASR-treated material. A comparison of the EELS spectra revealed varying spectral features in the peak edges with different reaction times due to the interaction evolution between oxygen and the silicon and OH ions. Solid-state nuclear magnetic resonance (NMR) was also used to elucidate the silica nanostructures.

A Metal-Free Oxygenated Covalent Triazine 2-D Photocatalyst Works Effectively from the Ultraviolet to Near-Infrared Spectrum for Water Oxidation Apart from Water Reduction

Solar-driven water splitting is highly desirable for hydrogen fuel production, particularly if water oxidation is effectively sustained in a complete cycle and/or by means of stable and efficient photocatalysts of main group elements, for example, carbon and nitrogen. Despite extensive success on H₂ production on polymer photocatalysts, polymers have met with very limited success for the rate-determining step of the water splitting-water oxidation reaction due to the extremely slow "four-hole" chemistry. Here, the synthesized metal-free oxygenated covalent triazine (OCT) is remarkably active for oxygen production in a wide operation window from UV to visible and even to NIR (up to 800 nm), neatly matching the solar spectrum with an unprecedented external quantum efficiency (even 1% at 600 nm) apart from excellent activity for H₂ production under full arc irradiation, a big step moving toward full solar spectrum water splitting. Experimental results and DFT calculations show that the oxygen incorporation not only narrows the band gap but also causes appropriate band-edge shifts. In the end, a controlled small amount of oxygen in the ionothermal reaction is found to be a promising and facile way of achieving such oxygen incorporation. This discovery is a significant step toward both scientific understanding and practical development of metal-free photocatalysts for cost-effective water oxidation and hydrogen generation over a large spectral window.

Tissue Specific Fate of Nanomaterials by Advanced Analytical Imaging Techniques - A Review

A variety of imaging and analytical methods have been developed to study nanoparticles in cells. Each has its benefits, limitations, and varying degrees of expense and difficulties in implementation. High-resolution analytical scanning transmission electron microscopy (HRSTEM) has the unique ability to image local cellular environments adjacent to a nanoparticle at near atomic resolution and apply analytical tools to these environments such as energy dispersive spectroscopy and electron energy loss spectroscopy. These tools can be used to analyze particle location, translocation and potential reformation, ion dispersion, and in vivo synthesis of second-generation nanoparticles. Such analyses can provide in depth understanding of tissue-particle interactions and effects that are caused by the environmental "invader" nanoparticles. Analytical imaging can also distinguish phases that form due to the transformation of "invader" nanoparticles in contrast to those that are triggered by a response mechanism, including the commonly observed iron biomineralization in the form of ferritin nanoparticles. The analyses can distinguish ion species, crystal phases, and valence of parent nanoparticles and reformed or in vivo synthesized phases throughout the tissue. This article will briefly review the plethora of methods that have been developed over the last 20 years with an emphasis on the state-of-the-art techniques used to image and analyze nanoparticles in cells and highlight the sample preparation necessary for biological thin section observation in a HRSTEM. Specific applications that provide visual and chemical mapping of the local cellular environments surrounding parent nanoparticles and second-generation phases are demonstrated, which will help to identify novel nanoparticle-produced adverse effects and their associated mechanisms.

Oxygen K-edge X-ray Absorption Spectra

We review oxygen K-edge X-ray absorption spectra of both molecules and solids. We start with an overview of the main experimental aspects of oxygen K-edge X-ray absorption measurements including X-ray sources, monochromators, and detection schemes. Many recent oxygen K-edge studies combine X-ray absorption with time and spatially resolved measurements and/or operando conditions. The main theoretical and conceptual approximations for the simulation of oxygen K-edges are discussed in the Theory section. We subsequently discuss oxygen atoms and ions, binary molecules, water, and larger molecules containing oxygen, including biomolecular systems. The largest part of the review deals with the experimental results for solid oxides, starting from s- and p-electron oxides. Examples of theoretical simulations for these oxides are introduced in order to show how accurate a DFT description can be in the case of s and p electron overlap. We discuss the general analysis of the 3d transition metal oxides including discussions of the crystal field effect and the effects and trends in oxidation state and covalency. In addition to the general concepts, we give a systematic overview of the oxygen K-edges element by element, for the s-, p-, d-, and f-electron systems.

Interfacial effects on leakage currents in Cu/α-cristobalite/Cu junctions

As the miniaturization trend of integrated circuit continues, the leakage currents flow through the dielectric films insulating the interconnects become a critical issue. However, quantum transport through the mainstream on-chip interfaces between interconnects and dielectrics has not been addressed from first principles yet. Here, using first-principles calculations based on density functional theory and nonequilibrium Green's function formalism, we investigate the interfacial-dependent leakage currents in the Cu/α-cristobalite/Cu junctions. Our results show that the oxygen-rich interfaces form the lowest-leakage-current junction under small bias voltages, followed by the silicon-rich and oxygen-poor ones. This feature is attributed to their transmission spectra, related to their density of states and charge distributions. However, the oxygen-poor interfacial junction may conversely have a better dielectric strength than others, as its transmission gap, from -2.8 to 3.5 eV, is more symmetry respect to the Fermi level than others.

Local Modified Becke-Johnson Exchange-Correlation Potential for Interfaces, Surfaces, and Two-Dimensional Materials

The modified Becke-Johnson meta-GGA potential of density functional theory has been shown to be the best exchange-correlation potential to determine band gaps of crystalline solids. However, it cannot be consistently used for the electronic structure of nonperiodic or nanostructured systems. We propose an extension of this potential that enables its use to study heterogeneous, finite, and low-dimensional systems. This is achieved by using a coordinate-dependent expression for the parameter c that weights the Becke-Russel exchange, in contrast to the original global formulation, where c is just a fitted number. Our potential takes advantage of the excellent description of band gaps provided by the modified Becke-Johnson potential and preserves its modest computational effort. Furthermore, it yields with one single calculation band diagrams and band offsets of heterostructures and surfaces. We exemplify the usefulness and efficiency of our local meta-GGA potential by testing it for a series of interfaces (Si/SiO₂, AlAs/GaAs, AlP/GaP, and GaP/Si), a Si surface, and boron nitride monolayer.

Design Considerations of Efficient Photo-Electrosynthetic Cells and its Realization Using Buried Junction Si Thin Film Multi Absorber Cells

As is obvious from previous work on semiconductor photoelectrochemistry, single junction semiconductors do not provide either the required maximum photovoltage or a high photocurrent for solar water splitting, which is required for efficient stand-alone devices. From these experiences we conclude, that multi-junction devices must be developed for bias-free water splitting. In this article we present our design considerations needed for the development of efficient photo-electro-synthetic cells, which have guided us during the DFG priority program 1613. At first, we discuss the fundamental requirements, which must be fulfilled to lead to effective solar water splitting devices. Buried junction and photoelectrochemical arrangements are compared. It will become clear, that the photovoltaic (PV) and electrochemical (EC) components can be optimized separately, but that maximized conversion efficiencies need photovoltages produced in the photovoltaic part of the device, which are adapted to the electrochemical performance of the electrolyzer components without energetic losses in their coupling across the involved interfaces. Therefore, in part 2 we will present the needs to develop appropriate interface engineering layers for proper chemical and electronic surface passivation. In addition, highly efficient electrocatalysts, either for the hydrogen or oxygen evolution reaction (HER, OER), must be adjusted in their energetic coupling to the semiconductor band edges and to the redox potentials in the electrolyte with minimized losses in the chemical potentials. The third part of our paper describes at first the demands and achievements on developing multijunction thin-film silicon solar cells. With different arrangements of silicon stacks a wide range of photovoltages and photocurrents can be provided. These solar cells are applied as photocathodes in integrated directly coupled PV-EC devices. For this purpose thin Pt and Ni catalyst layers are used on top of the solar cells for the HER and a wire connected RuO2 counter electrode is used for the OER. Electrochemical stability has been successfully tested for up to 10,000 s in 0.1 M KOH. Furthermore, we will illustrate our experimental results on interface engineering strategies using TiO2 as buffer layer and Pt nanostructures as HER catalyst. Based on the obtained results the observed improvements, but also the still given limitations, can be related to clearly identified non-idealities in surface engineering either related to recombination losses at the semiconductor surface reducing photocurrents or due to not properly-aligned energy states leading to potential losses across the interfaces.

Integrated Devices for Photoelectrochemical Water Splitting Using Adapted Silicon Based Multi-Junction Solar Cells Protected by ALD TiO2 Coatings

In this study, we present different silicon based integrated devices for photoelectrochemical water splitting, which provide enough photovoltage to drive the reaction without an external bias. Thin films of titanium dioxide, prepared by atomic layer deposition (ALD), are applied as a surface passivation and corrosion protection. The interfaces between the multi-junction cells and the protective coating were optimized individually by etching techniques and finding optimal parameters for the ALD process. The energy band alignment of the systems was studied by X-ray photoelectron spectroscopy (XPS). Electrochemically deposited platinum particles were used to reduce the HER overpotential. The prepared systems were tested in a three-electrode arrangement under AM 1.5 illumination in 0.1 M KOH. In final tests the efficiency and stability of the prepared devices were tested in a two-electrode arrangement in dependence of the pH value with a ruthenium-iridium oxide counter electrode. For the tandem-junction device solar to hydrogen efficiencies (STH) up to 1.8% were reached, and the triple-junction device showed a maximum efficiency of 4.4%.

Characterization of Al Incorporation into HfO2 Dielectric by Atomic Layer Deposition

This study presents the characteristics of HfAlO films for a series of Al incorporation ratios into a HfO₂ dielectric by atomic layer deposition on a Si substrate. A small amount of Al doping into the HfO₂ film can stabilize the tetragonal phase of the HfO₂, which helps to achieve a higher dielectric constant (k) and lower leakage current density, as well as a higher breakdown voltage than HfO₂ film on its own. Moreover, assimilation of Al₂O₃ into HfO₂ can reduce the hysteresis width and frequency dispersion. These are indications of border trap reduction, which was also verified by the border trap extraction mechanism. X-ray photoelectron spectroscopy (XPS) analysis also verified the HfAlO microstructural properties for various Al compositions. In addition, higher amounts of Al₂O₃ in HfAlO resulted in better interface and dielectric behavior through trap minimization, although the equivalent-oxide-thickness (EOT) values show the opposite trend.

Close contacts at the interface: Experimental-computational synergies for solving complexity problems

The study of material science has been long devoted to the disentanglement of bulk structures which mainly entails finding the inner structure of materials. That structure is accountable for a major portion of materials’ properties. Yet, as our knowledge of these “backbones” enlarged so did the interest for the materials’ boundaries properties which means the properties at the frontier with the surrounding environment that is called interface. The interface is thus to be understood as the sum of the material’s surface plus the surrounding environment be it in solid, liquid or gas phase. The study of phenomena at this interface requires both the use of experimental and theoretical techniques and, above all, a wise combination of them in order to shed light over the most intimate details at atomic, molecular and mesostructure levels. Here, we report several cases to be used as proof of concept of the results achieved when studying interface phenomena by combining a myriad of experimental and theoretical tools to overcome the usual limitation regardind  atomic detail, size and time scales and systems of complex composition. Real world examples of the combined experimental-theoretical work and new tools, software, is offered to the readers.

Under what conditions does (SiO)N nucleation occur? A bottom-up kinetic modelling evaluation

Silicon monoxide (SiO) is a structurally complex compound exhibiting differentiated oxide-rich and silicon-rich nano-phases at length scales covering nanoclusters to the bulk. Although nano-sized and nano-segregated SiO has great technological potential (e.g. nano-silicon for optical applications) and is of enormous astronomical interest (e.g. formation of silicate cosmic dust) an accurate general description of SiO nucleation is lacking. Avoiding the deficiencies of a bulk-averaged approach typified by classical nucleation theory (CNT) we employ a bottom-up kinetic model which fully takes into account the atomistic details involved in segregation. Specifically, we derive a new low energy benchmark set of segregated (SiO)_N cluster ground state candidates for N ≤ 20 and use the accurately calculated properties of these isomers to calculate SiO nucleation rates. We thus provide a state-of-the art evaluation of the range of pressure and temperature conditions for which formation of SiO will or will not proceed. Our results, which match with available experiment, reveal significant deficiencies with CNT approaches. We employ our model to shed light on controversial issue of circumstellar silicate dust formation showing that, at variance with the predictions from CNT-based calculations, pure SiO nucleation under such conditions is not viable.

Low-temperature direct bonding of silicon nitride to glass

Direct bonding may provide a cheap and reliable alternative to the use of adhesives. While direct bonding of two silicon surfaces is well documented, not much is known about direct bonding between silicon nitride and glass. This is unfortunate since silicon nitride is extensively used as an anti-reflection coating in the PV industry, often in contact with a shielding layer made of glass. A series of bonding experiments between glass and SiN was performed. The highest bonding quality, manifested by the highest bonding energy and lowest void area, was obtained with pairs that had been activated by nitrogen plasma followed by post-contact thermal annealing at 400 °C. HRTEM imaging, HRTEM-EDS and EELS measurements performed on the thin films prepared from bonded samples by Focused Ion Beam (FIB) revealed a clear defect-free interface between the silicon nitride and the glass, 4 nm in thickness. ATR FT-IR measurements performed on activated surfaces prior to contact indicated the formation of silanol groups on the activated glass surface and a thin oxide layer on the silicon nitride. An increase in the bearing ratio of the glass following activation was noticed by AFM. A mechanism for bonding silicon nitride and glass is suggested, based on generation of silanol groups on the glass surface and on oxidation of the silicon nitride surface. The results point out the importance of exposure to air, following activation and prior to bringing the two surfaces into contact.

Suggested mechanism for direct bonding of SiN to glass: (A) raw materials, (B) surfaces after plasma activation and exposure to air, (C) surfaces in contact, (D) formation of water molecules, (E) covalent bond formation and water removal.

Effects of the c-Si/a-SiO2 interfacial atomic structure on its band alignment: an ab initio study

The crystalline-Si/amorphous-SiO₂ (c-Si/a-SiO₂) interface is an important system used in many applications, ranging from transistors to solar cells. The transition region of the c-Si/a-SiO₂ interface plays a critical role in determining the band alignment between the two regions. However, the question of how this interface band offset is affected by the transition region thickness and its local atomic arrangement is yet to be fully investigated. Here, by controlling the parameters of the classical Monte Carlo bond switching algorithm, we have generated the atomic structures of the interfaces with various thicknesses, as well as containing Si at different oxidation states. A hybrid functional method, as shown by our calculations to reproduce the GW and experimental results for bulk Si and SiO₂, was used to calculate the electronic structure of the heterojunction. This allowed us to study the correlation between the interface band characterization and its atomic structures. We found that although the systems with different thicknesses showed quite different atomic structures near the transition region, the calculated band offset tended to be the same, unaffected by the details of the interfacial structure. Our band offset calculation agrees well with the experimental measurements. This robustness of the interfacial electronic structure to its interfacial atomic details could be another reason for the success of the c-Si/a-SiO₂ interface in Si-based electronic applications. Nevertheless, when a reactive force field is used to generate the a-SiO₂ and c-Si/a-SiO₂ interfaces, the band offset significantly deviates from the experimental values by about 1 eV.

Intrinsic Resistance Switching in Amorphous Silicon Suboxides: The Role of Columnar Microstructure

We studied intrinsic resistance switching behaviour in sputter-deposited amorphous silicon suboxide (a-SiO_x) films with varying degrees of roughness at the oxide-electrode interface. By combining electrical probing measurements, atomic force microscopy (AFM), and scanning transmission electron microscopy (STEM), we observe that devices with rougher oxide-electrode interfaces exhibit lower electroforming voltages and more reliable switching behaviour. We show that rougher interfaces are consistent with enhanced columnar microstructure in the oxide layer. Our results suggest that columnar microstructure in the oxide will be a key factor to consider for the optimization of future SiOx-based resistance random access memory.

Computational Study of MoS2/HfO2 Defective Interfaces for Nanometer-Scale Electronics

Atomic structures and electronic properties of MoS₂/HfO₂ defective interfaces are investigated extensively for future field-effect transistor device applications. To mimic the atomic layer deposition growth under ambient conditions, the impact of interfacial oxygen concentration on the MoS₂/HfO₂ interface electronic structure is examined. Then, the effect on band offsets (BOs) and the thermodynamic stability of those interfaces is investigated and compared with available relevant experimental data. Our results show that the BOs can be modified up to 2 eV by tuning the oxygen content through, for example, the relative partial pressure. Interfaces with hydrogen impurities as well as various structural disorders were also considered, leading to different behaviors, such as n-type doping, or introducing defect states close to the Fermi level because of the formation of hydroxyl groups. Then, our results indicate that for a well-prepared interface the electronic device performance should be better than that of other interfaces, such as III-V/high-κ, because of the absence of interface defect states. However, any unpassivated defects, if present during oxide growth, strongly affect the subsequent electronic properties of the interface. The unique electronic properties of monolayer-to-few-layered transition-metal dichalcogenides and dielectric interfaces are described in detail for the first time, showing the promising interfacial characteristics for future transistor technology.

The Role of III-V Substrate Roughness and Deoxidation Induced by Digital Etch in Achieving Low Resistance Metal Contacts

To achieve low contact resistance between metal and III-V material, transmission-line-model (TLM) structures of molybdenum (Mo) were fabricated on indium phosphide (InP) substrate on the top of an indium gallium arsenide (InGaAs) layer grown by molecular beam epitaxy. The contact layer was prepared using a digital etch procedure before metal deposition. The contact resistivity was found to decrease significantly with the cleaning process. High Resolution Transmission & Scanning Electron Microscopy (HRTEM & HRSTEM) investigations revealed that the surface roughness of treated samples was increased. Further analysis of the metal-semiconductor interface using Energy Electron Loss Spectroscopy (EELS) showed that the amount of oxides (InxOy, GaxOy or AsxOy) was significantly decreased for the etched samples. These results suggest that the low contact resistance obtained after digital etching is attributed to the combined effects of the induced surface roughness and oxides removal during the digital etch process.

From Dose to Response: In Vivo Nanoparticle Processing and Potential Toxicity

Adverse human health impacts due to occupational and environmental exposures to manufactured nanoparticles are of concern and pose a potential threat to the continued industrial use and integration of nanomaterials into commercial products. This chapter addresses the inter-relationship between dose and response and will elucidate on how the dynamic chemical and physical transformation and breakdown of the nanoparticles at the cellular and subcellular levels can lead to the in vivo formation of new reaction products. The dose-response relationship is complicated by the continuous physicochemical transformations in the nanoparticles induced by the dynamics of the biological system, where dose, bio-processing, and response are related in a non-linear manner. Nanoscale alterations are monitored using high-resolution imaging combined with in situ elemental analysis and emphasis is placed on the importance of the precision of characterization. The result is an in-depth understanding of the starting particles, the particle transformation in a biological environment, and the physiological response.

Controlling the defects and transition layer in SiO2 films grown on 4H-SiC via direct plasma-assisted oxidation

The structural stability and electrical performance of SiO₂ grown on SiC via direct plasma-assisted oxidation were investigated. To investigate the changes in the electronic structure and electrical characteristics caused by the interfacial reaction between the SiO₂ film (thickness ~5 nm) and SiC, X-ray photoelectron spectroscopy (XPS), X-ray absorption spectroscopy (XAS), density functional theory (DFT) calculations, and electrical measurements were performed. The SiO₂ films grown via direct plasma-assisted oxidation at room temperature for 300s exhibited significantly decreased concentrations of silicon oxycarbides (SiO_xC_y) in the transition layer compared to that of conventionally grown (i.e., thermally grown) SiO₂ films. Moreover, the plasma-assisted SiO₂ films exhibited enhanced electrical characteristics, such as reduced frequency dispersion, hysteresis, and interface trap density (D_it ≈ 10¹¹ cm^-2 · eV^-1). In particular, stress induced leakage current (SILC) characteristics showed that the generation of defect states can be dramatically suppressed in metal oxide semiconductor (MOS) structures with plasma-assisted oxide layer due to the formation of stable Si-O bonds and the reduced concentrations of SiO_xC_y species defect states in the transition layer. That is, energetically stable interfacial states of high quality SiO₂ on SiC can be obtained by the controlling the formation of SiO_xC_y through the highly reactive direct plasma-assisted oxidation process.

Grassy Silica Nanoribbons and Strong Blue Luminescence

Silicon dioxide (SiO₂) is one of the key materials in many modern technological applications such as in metal oxide semiconductor transistors, photovoltaic solar cells, pollution removal, and biomedicine. We report the accidental discovery of free-standing grassy silica nanoribbons directly grown on SiO₂/Si platform which is commonly used for field-effect transistors fabrication without other precursor. We investigate the formation mechanism of this novel silica nanostructure that has not been previously documented. The silica nanoribbons are flexible and can be manipulated by electron-beam. The silica nanoribbons exhibit strong blue emission at about 467 nm, together with UV and red emissions as investigated by cathodoluminescence technique. The origins of the luminescence are attributed to various defects in the silica nanoribbons; and the intensity change of the blue emission and green emission at about 550 nm is discussed in the frame of the defect density. Our study may lead to rational design of the new silica-based materials for a wide range of applications.

Phonon-Enabled Carrier Transport of Localized States at Non-Polar Semiconductor Surfaces: A First-Principles-Based Prediction

Electron-phonon coupling can hamper carrier transport either by scattering or by the formation of mass-enhanced polarons. Here, we use time-dependent density functional theory-molecular dynamics simulations to show that phonons can also promote the transport of excited carriers. Using nonpolar InAs (110) surface as an example, we identify phonon-mediated coupling between electronic states close in energy as the origin for the enhanced transport. In particular, the coupling causes localized excitons in the resonant surface states to propagate into bulk with velocities as high as 10(6) cm/s. The theory also predicts temperature enhanced carrier transport, which may be observable in ultrathin nanostructures.

Vertically Integrated Nanowire-Based Unified Memory

A vertically integrated nanowire-based device for multifunctional unified memory that combine dynamic random access memory (DRAM) and flash memory in a single transistor is demonstrated for the first time. The device utilizes a gate-all-around (GAA) structure that completely surrounds the nanowire; the structure is built on a bulk silicon wafer. A vertically integrated unified memory (VIUM) device composed of five-story channels was fabricated via the one-route all-dry etching process (ORADEP) with reliable reproducibility, stiction-free stability, and high uniformity. In each DRAM and flash memory operation, the five-story VIUM showed a remarkably enhanced sensing current drivability compared with one-story unified memory (UM) characteristics. In addition to each independent memory mode, the switching endurance of the VIUM was evaluated in the unified mode, which alternatively activates two memory modes, resulting in an even higher sensing memory window than that of the UM. In addition to our previous work on a logic transistor joining high performance with good scalability, this work describes a novel memory hierarchy design with high functionality for system-on-chip (SoC) architectures, demonstrating the practicality and versatility of the vertically integrated nanowire configuration for use in various applications.

A Large-Area Transferable Wide Band Gap 2D Silicon Dioxide Layer

An atomically smooth silica bilayer is transferred from the growth substrate to a new support via mechanical exfoliation at millimeter scale. The atomic structure and morphology are maintained perfectly throughout the process. A simple heating treatment results in complete removal of the transfer medium. Low-energy electron diffraction, Auger electron spectroscopy, scanning tunneling microscopy, and environmental scanning electron microscopy show the success of the transfer steps. Excellent chemical and thermal stability result from the absence of dangling bonds in the film structure. By adding this wide band gap oxide to the toolbox of 2D materials, possibilities for van der Waals heterostructures will be broadened significantly.

Photoactivation of Luminescent Centers in Single SiO2 Nanoparticles

Photobleaching of fluorophores is one of the key problems in fluorescence microscopy. Overcoming the limitation of the maximum number of photons, which can be detected from a single emitter, would allow one to enhance the signal-to-noise ratio and thus the temporal and spatial resolution in fluorescence imaging. It would be a breakthrough for many applications of fluorescence spectroscopy, which are unachievable up to now. So far, the only approach for diminishing the effect of photobleaching has been to enhance the photostability of an emitter. Here, we present a fundamentally new solution for increasing the number of photons emitted by a fluorophore. We show that, by exposing a single SiO2 nanoparticle to UV illumination, one can create new luminescent centers within this particle. By analogy with nanodiamonds, SiO2 nanoparticles can possess luminescent defects in their regular SiO2 structure. However, due to the much weaker chemical bonds, it is possible to generate new defects in SiO2 nanostructures using UV light. This allows for the reactivation of the nanoparticle's fluorescence after its photobleaching.

An overview of chitin or chitosan/nano ceramic composite scaffolds for bone tissue engineering

Chitin and chitosan based nanocomposite scaffolds have been widely used for bone tissue engineering. These chitin and chitosan based scaffolds were reinforced with nanocomponents viz Hydroxyapatite (HAp), Bioglass ceramic (BGC), Silicon dioxide (SiO₂), Titanium dioxide (TiO₂) and Zirconium oxide (ZrO₂) to develop nanocomposite scaffolds. Plenty of works have been reported on the applications and characteristics of the nanoceramic composites however, compiling the work done in this field and presenting it in a single article is a thrust area. This review is written with an aim to fill this gap and focus on the preparations and applications of chitin or chitosan/nHAp, chitin or chitosan/nBGC, chitin or chitosan/nSiO₂, chitin or chitosan/nTiO₂ and chitin or chitosan/nZrO₂ in the field of bone tissue engineering in detail. Many reports so far exemplify the importance of ceramics in bone regeneration. The effect of nanoceramics over native ceramics in developing composites, its role in osteogenesis etc. are the gist of this review.

Derivation of the surface free energy of ZnO and GaN using in situ electron beam hole drilling

Surface free energy, as an intrinsic property, is essential in determining the morphology of materials, but it is extremely difficult to determine experimentally. We report on the derivation of the SE of different facets of ZnO and GaN experimentally from the holes developed using electron beam drilling with transmission electron microscopy. Inverse Wullf's construction is employed to obtain polar maps of the SE of different facets to study different nanomaterials (ZnO and GaN) in different morphologies (nanorod, nanobelt and thin film) to prove its versatility and capability. The results show that the SE of ZnO{10-13} is derived to be 0.99 J m(-2), and the SE of ZnO{10-10} is found to be less than {0002} and {11-20}. A GaN thin film also exhibits a similar trend in the SE of different facets as ZnO and the SE of GaN{10-13} is determined to be 1.36 J m(-2).

Band engineering for efficient catalyst-substrate coupling for photoelectrochemical water splitting

To achieve an overall efficient solar water splitting device, not only the efficiencies of photo-converter and catalyst are decisive, but also their appropriate coupling must be considered.

Ultra-thin silicate films on metals

Silica is one of the key materials in many modern technological applications. 'Surface science' approach for understanding surface chemistry on silica-based materials, on the one hand, and further miniaturization of new generation electronic devices, on the other, all these face the necessity of rational design of the ultrathin silica films on electrically conductive substrates. The review updates recent studies in this field. Despite the structural complexity and diversity of silica, substantial progress has recently been achieved in understanding of the atomic structure of truly 2D silicates.

Carrier Mobility Enhancement of Tensile Strained Si and SiGe Nanowires via Surface Defect Engineering

Changes in the carrier mobility of tensile strained Si and SiGe nanowires (NWs) were examined using an electrical push-to-pull device (E-PTP, Hysitron). The changes were found to be closely related to the chemical structure at the surface, likely defect states. As tensile strain is increased, the resistivity of SiGe NWs deceases in a linear manner. However, the corresponding values for Si NWs increased with increasing tensile strain, which is closely related to broken bonds induced by defects at the NW surface. Broken bonds at the surface, which communicate with the defect state of Si are critically altered when Ge is incorporated in Si NW. In addition, the number of defects could be significantly decreased in Si NWs by incorporating a surface passivated Al2O3 layer, which removes broken bonds, resulting in a proportional decrease in the resistivity of Si NWs with increasing strain. Moreover, the presence of a passivation layer dramatically increases the extent of fracture strain in NWs, and a significant enhancement in mobility of about 2.6 times was observed for a tensile strain of 5.7%.

Aligned Growth of Hexagonal Boron Nitride Monolayer on Germanium

A hexagonal boron nitride monolayer with aligned orientations is grown on reusable semiconducting germanium. The number of primary orientations of the h-BN domains depends on the symmetry of the underlying crystal face, and Ge (110) gives rise to only two opposite orientations. The structures and electrical properties of grain boundaries between h-BN domains with opposite orientations are also systematically analyzed.

Effect of interface atomic structure on the electronic properties of nano-sized metal-oxide interfaces

We report that the size dependence of electronic properties at nanosized metal-semiconducting oxide interfaces is significantly affected by the interface atomic structure. The properties of interfaces with two orientations are compared over size range of 20-200 nm. The difference in interface atomic structure leads to electronic structure differences that alter electron transfer paths. Specifically, interfaces with a higher concentration of undercoordinated Ti result in enhanced tunneling due to the presence of defect states or locally reduced tunnel barrier widths. This effect is superimposed on the mechanisms of size dependent properties at such small scales.

Photoluminescence of a single quantum emitter in a strongly inhomogeneous chemical environment

A comprehensive photoluminescence study of defect centers in single SiO₂ nanoparticles provides new insight into the complex photo-physics of single quantum emitters embedded into a random chemical environment.