III-Nitride Magnetron Sputter Epitaxy on Si: Controlling Morphology, Crystal Quality, and Polarity Using Al Seed Layers.

Group III-nitride semiconductors have been subject of intensive research, resulting in the maturing of the material system and adoption of III-nitrides in modern optoelectronics and power electronic devices. Defined film polarity is an important aspect of III-nitride epitaxy as the polarity affects the design of electronic devices. Magnetron sputtering is a novel approach for cost-effective epitaxy of III-nitrides nearing the technological maturity needed for device production; therefore, control of film polarity is an important technological milestone. In this study, we show the impact of Al seeding on the AlN/Si interface and resulting changes in crystal quality, film morphology, and polarity of GaN/AlN stacks grown by magnetron sputter epitaxy. X-ray diffraction measurements demonstrate the improvement of the crystal quality of the AlN and subsequently the GaN film by the Al seeding. Nanoscale structural and chemical investigations using scanning transmission electron microscopy reveal the inversion of the AlN film polarity. It is proposed that N-polar growth induced by Al seeding is related to the formation of a polycrystalline oxygen-rich AlN interlayer partially capped by an atomically thin Si-rich layer at the AlN/Si interface. Complementary aqueous KOH etch studies of GaN/AlN stacks demonstrate that purely metal-polar and N-polar layers can be grown on a macroscopic scale by controlling the amount of Al seeding.

Density Functional Theory-Fed Phase Field Model for Semiconductor Nanostructures: The Case of Self-Induced Core-Shell InAlN Nanorods.

The self-induced formation of core-shell InAlN nanorods (NRs) is addressed at the mesoscopic scale by density functional theory (DFT)-resulting parameters to develop phase field modeling (PFM). Accounting for the structural, bonding, and electronic features of immiscible semiconductor systems at the nanometer scale, we advance DFT-based procedures for computation of the parameters necessary for PFM simulation runs, namely, interfacial energies and diffusion coefficients. The developed DFT procedures conform to experimental self-induced InAlN NRs' concerning phase-separation, core/shell interface, morphology, and composition. Finally, we infer the prospects for the transferability of the coupled DFT-PFM simulation approach to a wider range of nanostructured semiconductor materials.

Molecularly-induced roughness and oxidation in cobalt/organodithiol/cobalt nanolayers synthesized by sputter-deposition and molecular sublimation.

Integrating interfacial molecular nanolayers (MNL) with inorganic nanolayers is of interest for understanding processing-structure/chemistry correlations in hybrid nanolaminates. Here, we report the synthesis of Co/biphenyldithiol (BPDT)/Co nanolayer sandwiches by metal sputter-deposition and molecular sublimation. The density and surface roughness of the Co layers deposited on the native oxide are invariant with the Ar pressure pAr during deposition. In contrast, the Co layer roughness rCo deposited on top of the BPDT MNL increases with pAr, and correlates with a higher degree of Co oxidation. Increased roughening is attributed to MNL-accentuated self-shadowing of low mobility Co atoms at high pAr, which consequently increases Co oxidation. These results indicating MNL-induced effects on the morphology and chemistry of the inorganic layers should be of importance for tailoring nanolayered hybrid interfaces and laminates.

Electronic and optical properties of core-shell InAlN nanorods: a comparative study via LDA, LDA-1/2, mBJ, HSE06, G0W0 and BSE methods.

Currently, self-induced InAlN core-shell nanorods enjoy an advanced stage of accumulation of experimental data from their growth and characterization as well as a comprehensive understanding of their formation mechanism by the ab initio modeling based on Synthetic Growth Concept. However, their electronic and optical properties, on which most of their foreseen applications are expected to depend, have not been investigated comprehensively. GW and the Bethe-Salpeter equation (BSE) are regarded as the state-of-the-art ab initio methodologies to study these properties. However, one of the major drawbacks of these methods is the computational cost, much higher than density-functional theory (DFT). Therefore, in many applications, it is highly desirable to answer the question of how well approaches based on DFT, such as e.g. the local density approximation (LDA), LDA-1/2, the modified Becke-Johnson (mBJ) and the Heyd-Scuseria-Ernzerhof (HSE06) functionals, can be employed to calculate electronic and optical properties with reasonable accuracy. In the present paper, we address this question, investigating how effective the DFT-based methodologies LDA, LDA-1/2, mBJ and HSE06 can be used as approximate tools in studies of the electronic and optical properties of scaled down models of core-shell InAlN nanorods, thus, avoiding GW and BSE calculations.

Toward an increased reliability of chemical bonding assignment in insulating samples by x-ray photoelectron spectroscopy.

X-ray photoelectron spectroscopy (XPS) spectra from solid samples are conventionally referenced to the spectrometer Fermi level (FL). While, in the case of metallic samples, alignment of the sample and the spectrometer FLs can be directly verified from the measured Fermi edge position, thus allowing to assess the surface electrical potential, this is not a workable option for insulators. When applied, it generates a large spread in reported binding energy values that often exceed involved chemical shifts. By depositing insulating amorphous alumina thin films on a variety of conducting substrates with different work functions, we show not only that FL referencing fails but also that the Al2O3 energy levels align instead to the vacuum level, as postulated in the early days of XPS. Based on these model experiments that can be repeated for all sorts of thin-film insulators, a solution to the binding energy reference problem is proposed for reliable assessment of chemical bonding.

Self-Induced Core-Shell InAlN Nanorods: Formation and Stability Unraveled by Ab Initio Simulations.

By addressing precursor prevalence and energetics using the DFT-based synthetic growth concept (SGC), the formation mechanism of self-induced InAlN core-shell nanorods (NRs) synthesized by reactive magnetron sputter epitaxy (MSE) is explored. The characteristics of In- and Al-containing precursor species are evaluated considering the thermal conditions at a typical NR growth temperature of around 700 °C. The cohesive and dissociation energies of In-containing precursors are consistently lower than those of their Al-containing counterparts, indicating that In-containing precursors are more weakly bonded and more prone to dissociation. Therefore, In-containing species are expected to exhibit lower abundance in the NR growth environment. At increased growth temperatures, the depletion of In-based precursors is even more pronounced. A distinctive imbalance in the incorporation of Al- and In-containing precursor species (namely, AlN/AlN+, AlN2/AlN2 +, Al2N2/Al2N2 +, and Al2/Al2 + vs InN/InN+, InN2/InN2 +, In2N2/In2N2 +, and In2/In2 +) is found at the growing edge of the NR side surfaces, which correlates well with the experimentally obtained core-shell structure as well as with the distinctive In-rich core and vice versa for the Al-rich shell. The performed modeling indicates that the formation of the core-shell structure is substantially driven by the precursors' abundance and their preferential bonding onto the growing edge of the nanoclusters/islands initiated by phase separation from the beginning of the NR growth. The cohesive energies and the band gaps of the NRs show decreasing trends with an increment in the In concentration of the NRs' core and with an increment in the overall thickness (diameter) of the NRs. These results reveal the energy and electronic reasons behind the limited growth (up to ∼25% of In atoms of all metal atoms, i.e., In x Al1-x N, x ∼ 0.25) in the NR core and may be qualitatively perceived as a limiting factor for the thickness of the grown NRs (typically <50 nm).

Chemical scissor-mediated structural editing of layered transition metal carbides.

Intercalated layered materials offer distinctive properties and serve as precursors for important two-dimensional (2D) materials. However, intercalation of non-van der Waals structures, which can expand the family of 2D materials, is difficult. We report a structural editing protocol for layered carbides (MAX phases) and their 2D derivatives (MXenes). Gap-opening and species-intercalating stages were respectively mediated by chemical scissors and intercalants, which created a large family of MAX phases with unconventional elements and structures, as well as MXenes with versatile terminals. The removal of terminals in MXenes with metal scissors and then the stitching of 2D carbide nanosheets with atom intercalation leads to the reconstruction of MAX phases and a family of metal-intercalated 2D carbides, both of which may drive advances in fields ranging from energy to printed electronics.

Nanolaminated Ternary Transition Metal Carbide (MAX Phase)-Derived Core-Shell Structure Electrocatalysts for Hydrogen Evolution and Oxygen Evolution Reactions in Alkaline Electrolytes.

The development of abundant, cheap, and highly active catalysts for the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) is important for hydrogen production. Nanolaminate ternary transition metal carbides (MAX phases) and their derived two-dimensional transition metal carbides (MXenes) have attracted considerable interest for electrocatalyst applications. Herein, four new MAX@MXene core-shell structures (Ta2CoC@Ta2CTx, Ta2NiC@Ta2CTx, Nb2CoC@Nb2CTx, and Nb2NiC@Nb2CTx), in which the core region is Co/Ni-MAX phases while the edge region is MXenes, have been prepared. Under alkaline electrolyte conditions, the Ta2CoC@Ta2CTx core-shell structure showed an overpotential of 239 mV and excellent stability during the HER with MXenes as the active sites. For the OER, the Ta2CoC@Ta2CTx core-shell structure showed an overpotential of 373 mV and a small Tafel plot (56 mV dec-1), which maintained a bulk crystalline structure and generated Co-based oxyhydroxides that formed by surface reconstruction as active sites. Considering rich chemical compositions and structures of MAX phases, this work provides a new strategy for designing multifunctional electrocatalysts and also paves the way for further development of MAX phase-based materials for clean energy applications.

Composition-dependent photoconductivities in indium aluminium nitride nanorods grown by magnetron sputter epitaxy.

Photoconduction (PC) properties were investigated for ternary indium aluminium nitride (In x Al1-x N) nanorods (NRs) with different indium compositions (x) from 0.35 to 0.68, as grown by direct-current reactive magnetron sputter epitaxy. Cross-sectional scanning transmission electron microscopy (STEM) reveals single-crystal quality of the vertically aligned In x Al1-x N NRs. Single-rod photodetector devices with good ohmic contacts were fabricated using the focused-ion-beam technique (FIB), where the In-rich In0.68Al0.32N NR exhibits an optimal photocurrent responsivity of 1400 A W-1 and photoconductive gain of 3300. A transition from a positive photoresponse to a negative photoresponse was observed, while increasing the In composition x from 0.35 to 0.57. The negative PC was further enhanced by increasing x to 0.68. A model based on the coexistence and competition of deep electron trap states and recombination centers was proposed to explain the interesting composition-dependent PC in these ternary III-nitride 1D nanostructures.

Out-Of-Plane Ordered Laminate Borides and Their 2D Ti-Based Derivative from Chemical Exfoliation.

Exploratory theoretical predictions in uncharted structural and compositional space are integral to materials discoveries. Inspired by M5 SiB2 (T2) phases, the finding of a family of laminated quaternary metal borides, M'4 M″SiB2 , with out-of-plane chemical order is reported here. 11 chemically ordered phases as well as 40 solid solutions, introducing four elements previously not observed in these borides are predicted. The predictions are experimentally verified for Ti4 MoSiB2 , establishing Ti as part of the T2 boride compositional space. Chemical exfoliation of Ti4 MoSiB2 and select removal of Si and MoB2 sub-layers is validated by derivation of a 2D material, TiOx Cly , of high yield and in the form of delaminated sheets. These sheets have an experimentally determined direct band gap of ≈4.1 eV, and display characteristics suitable for supercapacitor applications. The results take the concept of chemical exfoliation beyond currently available 2D materials, and expands the envelope of 3D and 2D candidates, and their applications.

Boridene: Two-dimensional Mo4/3B2-x with ordered metal vacancies obtained by chemical exfoliation.

Extensive research has been invested in two-dimensional (2D) materials, typically synthesized by exfoliation of van der Waals solids. One exception is MXenes, derived from the etching of constituent layers in transition metal carbides and nitrides. We report the experimental realization of boridene in the form of single-layer 2D molybdenum boride sheets with ordered metal vacancies, Mo4/3B2-xTz (where Tz is fluorine, oxygen, or hydroxide surface terminations), produced by selective etching of aluminum and yttrium or scandium atoms from 3D in-plane chemically ordered (Mo2/3Y1/3)2AlB2 and (Mo2/3Sc1/3)2AlB2 in aqueous hydrofluoric acid. The discovery of a 2D transition metal boride suggests a wealth of future 2D materials that can be obtained through the chemical exfoliation of laminated compounds.

Electrochemical Lithium Storage Performance of Molten Salt Derived V2SnC MAX Phase.

MAX phases are gaining attention as precursors of two-dimensional MXenes that are intensively pursued in applications for electrochemical energy storage. Here, we report the preparation of V2SnC MAX phase by the molten salt method. V2SnC is investigated as a lithium storage anode, showing a high gravimetric capacity of 490 mAh g-1 and volumetric capacity of 570 mAh cm-3 as well as superior rate performance of 95 mAh g-1 (110 mAh cm-3) at 50 C, surpassing the ever-reported performance of MAX phase anodes. Supported by operando X-ray diffraction and density functional theory, a charge storage mechanism with dual redox reaction is proposed with a Sn-Li (de)alloying reaction that occurs at the edge sites of V2SnC particles where Sn atoms are exposed to the electrolyte followed by a redox reaction that occurs at V2C layers with Li. This study offers promise of using MAX phases with M-site and A-site elements that are redox active as high-rate lithium storage materials.

Halogenated Ti3C2 MXenes with Electrochemically Active Terminals for High-Performance Zinc Ion Batteries.

The class of two-dimensional metal carbides and nitrides known as MXenes offer a distinct manner of property tailoring for a wide range of applications. The ability to tune the surface chemistry for expanding the property space of MXenes is thus an important topic, although experimental exploration of surface terminals remains a challenge. Here, we synthesized Ti3C2 MXene with unitary, binary, and ternary halogen terminals, e.g., -Cl, -Br, -I, -BrI, and -ClBrI, to investigate the effect of surface chemistry on the properties of MXenes. The electrochemical activity of Br and I elements results in the extraordinary electrochemical performance of the MXenes as cathodes for aqueous zinc ion batteries. The -Br- and -I-containing MXenes, e.g., Ti3C2Br2 and Ti3C2I2, exhibit distinct discharge platforms with considerable capacities of 97.6 and 135 mAh·g-1. Ti3C2(BrI) and Ti3C2(ClBrI) exhibit dual discharge platforms with capacities of 117.2 and 106.7 mAh·g-1. In contrast, the previously discovered MXenes Ti3C2Cl2 and Ti3C2(OF) exhibit no discharge platforms and only ∼50% of capacities and energy densities of Ti3C2Br2. These results emphasize the effectiveness of the Lewis-acidic-melt etching route for tuning the surface chemistry of MXenes and also show promise for expanding the MXene family toward various applications.

A general Lewis acidic etching route for preparing MXenes with enhanced electrochemical performance in non-aqueous electrolyte.

Two-dimensional carbides and nitrides of transition metals, known as MXenes, are a fast-growing family of materials that have attracted attention as energy storage materials. MXenes are mainly prepared from Al-containing MAX phases (where A = Al) by Al dissolution in F-containing solution; most other MAX phases have not been explored. Here a redox-controlled A-site etching of MAX phases in Lewis acidic melts is proposed and validated by the synthesis of various MXenes from unconventional MAX-phase precursors with A elements Si, Zn and Ga. A negative electrode of Ti3C2 MXene material obtained through this molten salt synthesis method delivers a Li+ storage capacity of up to 738 C g-1 (205 mAh g-1) with high charge-discharge rate and a pseudocapacitive-like electrochemical signature in 1 M LiPF6 carbonate-based electrolyte. MXenes prepared via this molten salt synthesis route may prove suitable for use as high-rate negative-electrode materials for electrochemical energy storage applications.

Atomic-Scale Tuning of Graphene/Cubic SiC Schottky Junction for Stable Low-Bias Photoelectrochemical Solar-to-Fuel Conversion.

Engineering tunable graphene-semiconductor interfaces while simultaneously preserving the superior properties of graphene is critical to graphene-based devices for electronic, optoelectronic, biomedical, and photoelectrochemical applications. Here, we demonstrate this challenge can be surmounted by constructing an interesting atomic Schottky junction via epitaxial growth of high-quality and uniform graphene on cubic SiC (3C-SiC). By tailoring the graphene layers, the junction structure described herein exhibits an atomic-scale tunable Schottky junction with an inherent built-in electric field, making it a perfect prototype to systematically comprehend interfacial electronic properties and transport mechanisms. As a proof-of-concept study, the atomic-scale-tuned Schottky junction is demonstrated to promote both the separation and transport of charge carriers in a typical photoelectrochemical system for solar-to-fuel conversion under low bias. Simultaneously, the as-grown monolayer graphene with an extremely high conductivity protects the surface of 3C-SiC from photocorrosion and energetically delivers charge carriers to the loaded cocatalyst, achieving a synergetic enhancement of the catalytic stability and efficiency.

Compromising Science by Ignorant Instrument Calibration-Need to Revisit Half a Century of Published XPS Data.

X-ray photoelectron spectroscopy (XPS) is an indispensable technique in modern materials science for the determination of chemical bonding as evidenced by more than 10 000 XPS papers published annually. A literature survey reveals that in the vast majority of cases an incorrect referencing of the binding energy scale is used, neglecting warnings that have been formulated from the early days of the technique. Consequences for the data reliability are disastrous and decades of XPS work require revisiting. The purpose of this Viewpoint is to highlight the existing problems, review the criticism and suggest ways forward.

Multielemental single-atom-thick A layers in nanolaminated V2(Sn, A) C (A = Fe, Co, Ni, Mn) for tailoring magnetic properties.

Tailoring of individual single-atom-thick layers in nanolaminated materials offers atomic-level control over material properties. Nonetheless, multielement alloying in individual atomic layers in nanolaminates is largely unexplored. Here, we report 15 inherently nanolaminated V2(A xSn1-x)C (A = Fe, Co, Ni, Mn, and combinations thereof, with x ∼ 1/3) MAX phases synthesized by an alloy-guided reaction. The simultaneous occupancy of the 4 magnetic elements and Sn in the individual single-atom-thick A layers constitutes high-entropy MAX phase in which multielemental alloying exclusively occurs in the 2-dimensional (2D) A layers. V2(A xSn1-x)C exhibit distinct ferromagnetic behavior that can be compositionally tailored from the multielement A-layer alloying. Density functional theory and phase diagram calculations are performed to understand the structure stability of these MAX phases. This 2D multielemental alloying approach provides a structural design route to discover nanolaminated materials and expand their chemical and physical properties. In fact, the magnetic behavior of these multielemental MAX phases shows strong dependency on the combination of various elements.