Growth of black arsenic phosphorus thin films and its application for field-effect transistors.

Black arsenic phosphorus single crystals were grown using a short-way transport technique resulting in crystals up to 12 × 110µmand ranging from 200 nm to 2µmthick. The reaction conditions require tin, tin (IV) iodide, gray arsenic, and red phosphorus placed in an evacuated quartz ampule and ramped up to a maximum temperature of 630 °C. The crystal structure and elemental composition were characterized using Raman spectroscopy, x-ray diffraction, and x-ray photoelectron spectroscopy, cross-sectional transmission microscopy, and electron backscatter diffraction. The data provides valuable insight into the growth mechanism. A previously developed b-P thin film growth technique can be adapted to b-AsP film growth with slight modifications to the reaction duration and reactant mass ratios. Devices fabricated from exfoliated bulk-b-AsP grown in the same reaction condition as the thin film growth process are characterized, showing an on-off current ratio of 102, a threshold voltage of -60 V, and a peak field-effect hole mobility of 23 cm2V-1s-1atVd= -0.9 V andVg= -60 V.

Voltage-Controlled Antiferromagnetism in Magnetic Tunnel Junctions.

We demonstrate a voltage-controlled exchange bias effect in CoFeB/MgO/CoFeB magnetic tunnel junctions that is related to the interfacial Fe(Co)O_{x} formed between the CoFeB electrodes and the MgO barrier. The unique combination of interfacial antiferromagnetism, giant tunneling magnetoresistance, and sharp switching of the perpendicularly magnetized CoFeB allows sensitive detection of the exchange bias. We find that the exchange bias field can be isothermally controlled by magnetic fields at low temperatures. More importantly, the exchange bias can also be effectively manipulated by the electric field applied to the MgO barrier due to the voltage-controlled antiferromagnetic anisotropy in this system.

Rational control of WSe2 layer number via hydrogen-controlled chemical vapor deposition.

Tranistion metal dichalcogenides are a promising family of materials for electronics and optoelectronics, in part due to their range of bandgaps that can be modulated by layer number. Here, we show that WSe2 can be selectively grown with one, two, or three layers, as regulated by a one-step hydrogen-controlled chemical vapor deposition (H-CVD) process involving cyclical pulses of H2 flow. The physical and vibrational properties of the resulting mono-, bi-, and tri-layer WSe2 films are characterized by atomic force microscopy and Raman spectroscopy. Modifying the H-CVD process to include more than three H2 pulses results in thicker WSe2 films, however the thickness of these films is not well controlled and feature small, bulk-like pyramidal islands. Transmission electron microscopy analysis reveals that most of these islands exhibit a spiral structure and appear to be grown via screw-dislocation-driven growth, similar to other works. Therefore, the H-CVD process is demonstrated to be a powerful tool for controlling the layer thickness of WSe2, but its practicality is limited to the few-layer regime.

Thin-Film Deposition of Surface Passivated Black Phosphorus.

A single-step, direct silicon-substrate growth of black phosphorus (BP) crystals is achieved in a self-contained short-way transport technique under low-pressure conditions (<1.5 MPa). A 115 nm-thick BP hero single crystal is formed with lateral dimensions of 10 × 85 µm. The synthesis proceeds with Sn, SnI4, and red phosphorus and has a well-defined phosphorus phase dependency on the SnI4 concentration. Furthermore, in situ Sn passivation of BP occurs. This allows long-term stability with no sign of any degradation after 4 months of exposure to ambient conditions. Single-crystal BP flakes and multigrain flakes with high- and low-angle grain boundaries are achieved. Electron backscatter diffraction determined crystal growth to be independent of the substrate, which is further supported by successful growth on various substrates, including sapphire, silicon nitride, silicon, and silicon oxide. Cross-sectional transmission electron microscopy of a single crystal flake provides valuable insight into the growth mechanism. Elemental Sn encapsulates BP crystals, and crystalline SnI x inclusions are found to be scattered throughout the BP crystal. It is suggested that SnI x inclusions may provide the dominant mechanism for seeding vertical growth. IR absorption measurements for thin and bulk BP recipes show an equal response below Eg dominated by free carrier absorption. FET devices fabricated from thin-film and bulk BP recipes show improved device performance compared to unpassivated BP films of equal thickness with an on/off current ratio >102.

Thermal Stabilization of Metal-Organic Framework-Derived Single-Site Catalytic Clusters through Nanocasting.

Metal-organic frameworks (MOFs) provide convenient systems for organizing high concentrations of single catalytic sites derived from metallic or oxo-metallic nodes. However, high-temperature processes cause agglomeration of these nodes, so that the single-site character and catalytic activity are lost. In this work, we present a simple nanocasting approach to provide a thermally stable secondary scaffold for MOF-based catalytic single sites, preventing their aggregation even after exposure to air at 600 °C. We describe the nanocasting of NU-1000, a MOF with 3 nm channels and Lewis-acidic oxozirconium clusters, with silica. By condensing tetramethylorthosilicate within the NU-1000 pores via a vapor-phase HCl treatment, a silica layer is created on the inner walls of NU-1000. This silica layer provides anchoring sites for the oxozirconium clusters in NU-1000 after the organic linkers are removed at high temperatures. Differential pair distribution functions obtained from synchrotron X-ray scattering confirmed that isolated oxozirconium clusters are maintained in the heated nanocast materials. Pyridine adsorption experiments and a glucose isomerization reaction demonstrate that the clusters remain accessible to reagents and maintain their acidic character and catalytic activity even after the nanocast materials have been heated to 500-600 °C in air. Density functional theory calculations show a correlation between the Lewis acidity of the oxozirconium clusters and their catalytic activity. The ability to produce MOF-derived materials that retain their catalytic properties after exposure to high temperatures makes nanocasting a useful technique for obtaining single-site catalysts suitable for high-temperature reactions.

Accelerating Reactive Compatibilization of PE/PLA Blends by an Interfacially Localized Catalyst.

We show catalyst localized at the interface can compatibilize polyethylene (PE) and polylactide (PLA) blends. Telechelic hydroxyl functional PE was synthesized by ring opening metathesis polymerization, which reacted with PLA in melt mixing (shown by adhesion and droplet size reduction). Lewis acid tin catalysts were examined as interfacial reaction promoters, with the goal of interfacial localization. Stannous octoate was shown to localize at the interface by transmission electron microscopy with energy dispersive X-ray spectroscopy and improved dispersion of PLA in PE as compared to uncatalyzed materials and a nonlocalized tin chloride dihydrate.

Tracking surface evolution using ligand-assisted dissolution of cobalt oxyhydroxide.

The use of surface-specific reactions to probe reactive surface area is a promising direction in materials research. The work presented herein examines how the kinetics of dissolution can be used to quantify particle growth as well as the evolution of site-specific reactive surface area. The dissolution of heterogenite (ß-CoOOH) by IDA results in two geometric isomers of Co(IDA)(2)(-): the s-fac and u-fac isomers. The heterogenite particles studied here can generally be described as cylindrical plates, and the relative amount of s-fac isomer produced is found to increase as the height of the plates increases. The quantity of each isomer produced is shown to correlate with the relative number of two different types of surface sites, designated as edge and corner sites, while basal sites are seemingly unreactive. It is hypothesized that u-fac isomer results from the more accessible Co centers at the corner sites, while the s-fac isomer results from the less accessible edge sites. An empirical relationship is developed between the fraction of s-fac isomer produced and the height of the ß-CoOOH particles, and this relationship is used to quantify particle growth by analysis of kinetic data. Finally, this new information is used to modify a previously proposed pH-dependent growth model, resulting in a significant improvement in the fit and physical relevance of the model.