The composition, structure, and electrical properties of Ga 2 O 3 thin films, grown by atomic layer deposition (ALD) from GaI 3 and O 3 precursors, were characterized.

Germanium epitaxy on Si (100) was explored by using GeH4 in a hot-wall UHV-CVD system, aiming to develop an efficient and budget-friendly method for routine production of high-quality Ge on Si films. It was found that by adopting elevated growth pressures as compared to prior UHV-CVD studies, much higher growth rates over 20 nm/min could be achieved without sacrificing material quality. Low surface roughness of 0.90 nm has been observed on samples produced by conventional low-high temperature (LT-HT) method, which could be further suppressed to 0.38 nm by using a newly-developed 3-step (LT-HT1-HT2, with HT1>HT2) growth protocol. Threading dislocation densities of the Ge films could be reduced by thermal annealing treatments, from similar to 1 & times; 10(8) cm(-2) for the as-grown samples down to 5 similar to 8 & times; 10(6) cm(-2) for the annealed ones, significantly lower than commercial Ge on Si films produced by RPCVD. Strip waveguides have been fabricated by using optimized Ge on Si samples, and propagation loss of less than 2 dB/cm has been obtained at 2000 nm operating wavelength, surpassing prior results based on RPCVD-grown Ge. The above results demonstrated that the GeH4-based Ge deposition by using optimized UHV-CVD conditions could concurrently deliver high growth rates, sub-nanometer surface roughness and low TDDs in the 10(6) cm(-2) range, providing premium Ge/Si virtual substrates suitable for subsequent integration of Ge-based electronic and photonic devices on silicon.

The composition, structure, and electrical properties of Ga2O3 thin films, grown by atomic layer deposition (ALD) from GaI3 and O3 precursors, were characterized. The films were deposited on Si substrates and on Ru and TiN bottom electrodes at temperatures of 200-500 degrees C. Growth of kappa- and epsilon-Ga2O3 was observed on Ru and TiN at substrate temperatures >= 325 degrees C, while temperatures >= 425 degrees C were needed to deposit crystalline Ga2O3 on bare Si. The formation of both epsilon-Ga2O3 and predominant kappa-Ga2O3 phases in crystalline films was confirmed by high-resolution transmission electron microscopy studies. Films deposited at 375-450 degrees C exhibited low leakage current densities (down to 10-9 A cm-2 at an electric field strength of 0.1 MV cm-1), breakdown fields up to 6.5 MV cm-1, and permittivity values up to 22 at 10 kHz. These results demonstrate that Ga2O3 deposited in this ALD process is suitable for implementation as high permittivity dielectrics in advanced electronic devices. Furthermore, the findings highlight the importance of deposition temperature and substrate choice in optimizing the dielectric properties of Ga2O3 films deposited for these applications.

Achieving large two-dimensional (2D) sheets of any metal is challenging due to their tendency to coalescence or cluster into 3D shapes. Recently, single-atom-thick gold sheets, termed goldene, was reported. Here, we ask if goldene can be extended to include multiple layers. The answer is yes, and trilayer goldene is the magic number, for reasons of electronegativity. Experiments are made to synthesize the atomically laminated phase Ti 4 Au 3 C 3 through substitutional intercalation of Si layers in Ti 4 SiC 3 for Au. Density functional theory calculations suggest that it is energetically favorable to insert three layers of Au into Ti 4 SiC 3 , compared to inserting a monolayer, a bilayer, or more than three layers. Isolated trilayer goldene sheets, ~100 nanometers wide and 6.7 angstroms thick, were obtained by chemically etching the Ti 4 C 3 layers from Ti 4 Au 3 C 3 templates. Furthermore, trilayer goldene is found in both hcp and fcc forms, where the hcp is ~50 milli–electron volts per atom more stable at room temperature from ab initio molecular dynamic simulations.

The high-temperature oxidation resistance and mechanical properties of Ti1-xAlxBy (0.36 <= x <= 0.74, and 1.83 <= y <= 2.03) films grown by hybrid HiPIMS/DCMS co-sputtering from TiB2 and AlB2 targets at substrate temperatures lower than180 degrees C are studied. The air-annealing experiments conducted at temperatures ranging from 700 to 900 degrees C reveal a strong correlation between the starting Al concentration and the oxidation resistance. Low Al content films (x <= 0.49 +/- 0.03 in the as-deposited state) show higher oxidation rates and develop B-depleted porous oxide scales as the original film is consumed. In contrast, oxides growing on top of high-Al content films (x >= 0.58 +/- 0.03) are compact, composed of amorphous alumina (Al2O3) and borate (Al18B4O33), which effectively passivate the surface against oxidation . Oxide scales on films with x >= 0.58 +/- 0.03 are, on average, 60 % harder and have 18 % higher elastic moduli. The hardest scale grew on the Ti0.42Al0.58B1.87 film, with the nanoindentation hardness of 27.3 +/- 2.7 GPa, which is comparable to that of as-deposited TiAlN, used widely for high-temperature wear protection. Electron microscopy also shows that for x >= 0.58 +/- 0.03, the oxide scales adhere well to the unoxidized portions of Ti1-xAlxBy films, which is explained by a better match of the respective thermal expansion coefficients.

Altered and gangue quartz in hydrothermal veins from the Kubi Gold deposit in Dunkwa on Offin in the central region of Ghana are investigated for possible Au-associated indicator minerals and to provide the understanding and increase the knowledge of the mineral hosting and alteration processes in quartz. X-ray diffraction, air annealing furnace, differential scanning calorimetry, energy dispersive X-ray spectroscopy, and transmission electron microscopy have been applied on different quartz types outcropping from surface and bedrocks at the Kubi Gold Mining to reveal the material properties at different temperatures. From the diffraction results of the fresh and annealed quartz samples, we find that the samples contain indicator and the impurity minerals iron disulfide, biotite, titanium oxide, and magnetite. These minerals, under oxidation process between 574 and 1400 degrees C temperatures experienced hematite alterations and a transformation from alpha-quartz to beta-quartz and further to cristobalite as observed from the calorimetry scans for hydrothermally exposed materials. The energy dispersive spectroscopy revealed elemental components of Fe, S, Mg, K, Al, Ti, Na, Si, O, and Ca contained in the samples, and these are attributed to the impurity phase minerals observed in the diffraction. The findings also suggest that during the hydrothermal flow regime, impurity minerals and metals can be trapped by voids and faults. Under favorable temperature conditions, the trapped minerals can be altered to change color at different depositional stages by oxidation and reduction processes leading to hematite alteration which is a useful indicator minerals in mineral exploration.

M(n+1)AX(n) (MAX) phases have attracted significant attention due to their structural diversity and potential applications. Designing MAX phases with single -atom -thick A layers featuring 4d/5d-orbital electronic elements is interesting work. Here, we present a comprehensive report on noble metal -based M2(A(1-x)A'(x))C (M = V, Ti, Nb; A = Al, Sn, In, Ga, Ge; A' = Ru, Rh, Pd, Ir, Pt, Au and combinations thereof; 0 < x <= 0.4) phases featuring A sublayers of 4d/5d-orbital electronic elements through an A -site alloying strategy. The chemical composition of MAX phases can be adjusted by selecting different M- and A -site elements, with morphology tailored by distinct C sources. Furthermore, the V-2(Sn0.8Pt0.2)C (15.7 wt % Pt) catalyst showed better performance for hydrogen evolution reaction compared to the commercial Pt/C (20 wt % Pt) electrode. This study highlights the prospects of A -site alloying for the design of novel MAX phases with unique properties and promising applications in electrocatalysis and beyond.

Sample charging during X-ray photoelectron spectroscopy analyses of electrically insulating samples is a widely recognized challenge of this essential technique. If the electron loss caused by the photoelectric effect is not compensated due to specimens’ poor electrical conductivity, the positive charge building up in the surface region results in an uncontrolled shift of detected core level peaks to higher binding energy (BE). This seriously complicates chemical bonding assignment, which is based on measured peak positions, and accounts for a large spread in reported core level BE values. Here, we show that peaks from several industry-relevant oxides, serving as model insulators, typically displaced by several eV due to charging, shift back to positions characteristic of electrically-neutral samples following ex-situ capping with a few nm thick metallic layer with low affinity to oxygen. The effect is present only if the capping layers contain sufficiently large non-oxidized volume that provides long-range conduction paths to grounded Cu clamps, while being thin enough to allow for recording high quality spectra from the underlying insulators. The versatility of the charging elimination is demonstrated for different oxides/cap combinations, air exposure times, and sample types (including thin film and bulk specimens).

The synthesis of monolayer gold has so far been limited to free-standing several-atoms-thick layers, or monolayers confined on or inside templates. Here we report the exfoliation of single-atom-thick gold achieved through wet-chemically etching away Ti3C2 from nanolaminated Ti3AuC2, initially formed by substituting Si in Ti3SiC2 with Au. Ti3SiC2 is a renown MAX phase, where M is a transition metal, A is a group A element, and X is C or N. Our developed synthetic route is by a facile, scalable and hydrofluoric acid-free method. The two-dimensional layers are termed goldene. Goldene layers with roughly 9% lattice contraction compared to bulk gold are observed by electron microscopy. While ab initio molecular dynamics simulations show that two-dimensional goldene is inherently stable, experiments show some curling and agglomeration, which can be mitigated by surfactants. X-ray photoelectron spectroscopy reveals an Au 4f binding energy increase of 0.88 eV. Prospects for preparing goldene from other non-van der Waals Au-intercalated phases, including developing etching schemes, are presented. Atomically thin gold nanosheets are predicted to have interesting properties but their synthesis is challenging. Here the exfoliation of two-dimensional single-atom-thick gold, termed goldene, is achieved through wet-chemically etching Ti3C2 from Ti3AuC2. The synthesized goldene has promising properties as a heterocatalyst.

Intercalation of non-van der Waals (vdW) layered materials can produce new 2D and 3D materials with unique properties, but it is difficult to achieve. Here, we describe a structural editing protocol for 3D non-vdW layered ternary carbides and nitrides (MAX phases) and their 2D vdW derivatives (MXenes). Gap-opening and species-intercalating stages were mediated by chemical scissors and guest intercalants, creating a large family of layered materials with unconventional elements and structures in MAX phases, as well as MXenes with versatile termination species. Removal of surface terminations by metal scissors and stitching of carbide layers by metal atoms leads to a reverse transformation from MXenes to MAX phases, and metal-intercalated 2D carbides. This scissor-mediated structural editing may enable structural and chemical tailoring of other layered ceramics.

Abstract The quest to make free-standing monolayer gold has hitherto been limited to free-standing several-atoms-thick layers, or monolayers but confined on or inside another template. Examples are monolayers diffused into double hydroxides1, membranes framed in alloys under electron irradiation2, nano-ribbons suspended in graphene3, quantum dots on hexagonal BN4, monolayers in between SiC wafers and monolayer graphene5, and fragments produced via thermal dewetting on sapphire6. Here, we report the synthesis of free-standing single-atom-thick 2D gold (named goldene) by wet-chemically etching away Ti3C2 from Ti3AuC2, a nano-laminated MAX-phase ceramic initially formed by substituting Si in Ti3SiC2 with Au7. The free-standing goldene layers are revealed by scanning transmission electron microscopy. While ab initio molecular dynamics simulations show that 2D goldene is inherently stable, the experiments reveal a tendency for curling and agglomeration at edges. Prospects for preparing goldene from a series of non-van der Waals Au-intercalated MAX-phases, including developing etching schemes, are also presented.

Abstract The quest to make monolayer gold has hitherto been limited to a few atomic layers stabilized on or inside another material. Examples are monolayers diffused into double hydroxides1, sheets framed in Au-Ag alloys under electron irradiation2, nano-ribbons suspended in graphene3, single-atom-thick quantum dots on hexagonal BN4, monolayers in between SiC wafers and monolayer graphene5, and fragments produced via thermal dewetting on substrates6. Here, we report the synthesis of free-standing single-atom-thick 2D gold (named goldene) by wet-chemically etching away Ti3C2 from Ti3AuC2, a nano-laminated MAX-phase ceramic initially formed by substitution of Si in Ti3SiC2 with Au7. The free-standing goldene layers are revealed by scanning transmission electron microscopy. While ab initio molecular dynamics simulations show that 2D goldene is inherently stable, the experiments reveal a tendency for curling and agglomeration at edges. Prospects for preparing goldene from a series of non-van der Waals Au-intercalated MAX-phases, including developing etching schemes, are also presented.

Intercalation of noble metals into non-van der Waals solids provides a new avenue to synthesize novel nanolaminated compounds with distinct material properties. Herein, we use solid-state reaction at 400 °C to prepare Cr2AuC from two Cr-based Mn+1AXn phase precursors and demonstrate the formation of Cr2AuC upon full replacement of Ga layers with Au in Cr2GaC thin films via thermal substitution reaction. The resulting Cr2AuC exhibits 2.7% lattice expansion relative to the original Cr2GaC, whereas Ge in a Cr2GeC thin film was sparsely replaced by Au, as revealed by electron microscopy and x-ray diffraction analysis. To explain the observed differences, using ab initio calculations, we consider the bonding properties of Cr2GaC and Cr2GeC, and the energetic driving forces for substitution by evaluating the mixing free energy of Au on both A-sites of the MAX phases, and of both A-elements in the surrounding Au lattice. The results suggest that it is somewhat easier to mix Au on the A-site in Cr2GaC than in Cr2GeC, and substantially easier to mix Ga into the Au-lattice than Ge. Finally, we discuss how the gained insights can be consulted for exploring a wider class of Mn+1AXn phases with intercalated noble metals.

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.

Out-of-plane chemically ordered transitionmetal boride(o-MAB) phases, Ta4M & DPRIME;SiB2 (M & DPRIME; = V, Cr), and a structurally equivalent disordered solidsolution MAB phase, Ta4MoSiB2, are synthesized.DFT calculations are used to examine the dynamic stability, elasticproperties, and electronic density states of the MAB phases. We report on the synthesis of computationally predictedout-of-planechemically ordered transition metal borides labeled o-MAB phases, Ta4M & DPRIME;SiB2 (M & DPRIME; =V, Cr), and a structurally equivalent disordered solid solution MABphase Ta4MoSiB2. The boride phases were preparedusing solid-state reaction sintering of the constituting elements.High-resolution scanning transmission electron microscopy along withRietveld refinement of the powder-X-ray diffraction patterns revealedthat the synthesized o-MAB phases Ta4CrSiB2 (98 wt % purity) and Ta4VSiB2 (81 wt% purity) possess chemical ordering with Ta preferentially residingin the 16l position and Cr and V in the 4c position, whereas Ta4MoSiB2 (46wt % purity) was concluded to form a disordered solid solution. Densityfunctional theory (DFT) calculations were used to investigate thedynamic stability, elastic properties, and electronic density statesfor the MAB phases, confirming the stability and suggesting the boridesbased on Cr and Mo to be stiffer than those based on V and Nb.

Cr-N based materials, including stoichiometric CrN and Cr:N with a wide range of nitrogen contents, are commonly used as hard and corrosion-resistant coatings. Cr-rich films in this materials system can retain the bcc structure of metallic Cr with few percent of dissolved nitrogen, which can be used for tailoring the mechanical, thermal, and electrical properties. Here, we investigated low nitrogen containing Cr thin films deposited by high ion assisted magnetron sputtering with a substrate temperature of 200 degrees C. With the gas flow ratio maintained at f(N2/Ar) = 0.02, the substrate bias and the target power allows for control of the film composition (0.03 < N/Cr < 0.34). The films comprise a mixture of bcc-Cr and hexagonal Cr2N1-delta phases. The mechanical properties studied by nanoindentation and Brillouin inelastic light scattering revealed a hardening effect due to the multiphase nanostructure. The mechanical properties of the Cr:N films depend on the residual stress, on the amount of h-Cr2N1-delta phase and on the nanostructuring nature of the coatings. A maximum hardness of 37 GPa was achieved for a dense film with a Youngs modulus of 340 GPa, a shear modulus of 118 GPa, and a relatively low thermal conductivity of 7 W/mK.

Solid-state precipitation can be used to tailor material properties, ranging from ferromagnets and catalysts to mechanical strengthening and energy storage. Thermoelectric properties can be modified by precipitation to enhance phonon scattering while retaining charge-carrier transmission. Here, unconventional Janus-type nanoprecipitates are uncovered in Mg 3 Sb 1.5 Bi 0.5 formed by side-by-side Bi- and Ge-rich appendages, in contrast to separate nanoprecipitate formation. These Janus nanoprecipitates result from local comelting of Bi and Ge during sintering, enabling an amorphous-like lattice thermal conductivity. A precipitate size effect on phonon scattering is observed due to the balance between alloy-disorder and nanoprecipitate scattering. The thermoelectric figure-of-merit ZT reaches 0.6 near room temperature and 1.6 at 773 K. The Janus nanoprecipitation can be introduced into other materials and may act as a general property-tailoring mechanism.