Limits to the strain engineering of layered square-planar nickelate thin films.

The layered square-planar nickelates, Ndn+1NinO2n+2, are an appealing system to tune the electronic properties of square-planar nickelates via dimensionality; indeed, superconductivity was recently observed in Nd6Ni5O12 thin films. Here, we investigate the role of epitaxial strain in the competing requirements for the synthesis of the n = 3 Ruddlesden-Popper compound, Nd4Ni3O10, and subsequent reduction to the square-planar phase, Nd4Ni3O8. We synthesize our highest quality Nd4Ni3O10 films under compressive strain on LaAlO3 (001), while Nd4Ni3O10 on NdGaO3 (110) exhibits tensile strain-induced rock salt faults but retains bulk-like transport properties. A high density of extended defects forms in Nd4Ni3O10 on SrTiO3 (001). Films reduced on LaAlO3 become insulating and form compressive strain-induced c-axis canting defects, while Nd4Ni3O8 films on NdGaO3 are metallic. This work provides a pathway to the synthesis of Ndn+1NinO2n+2 thin films and sets limits on the ability to strain engineer these compounds via epitaxy.

Scalable Characterization of 2D Gallium-Intercalated Epitaxial Graphene.

Scalable synthesis of two-dimensional gallium (2D-Ga) covered by graphene layers was recently realized through confinement heteroepitaxy using silicon carbide substrates. However, the thickness, uniformity, and area coverage of the 2D-Ga heterostructures have not previously been studied with high-spatial resolution techniques. In this work, we resolve and measure the 2D-Ga heterostructure thicknesses using scanning electron microscopy (SEM). Utilizing multiple correlative methods, we find that SEM image contrast is directly related to the presence of uniform bilayer Ga at the interface and a variation of the number of graphene layers. We also investigate the origin of SEM contrast using both experimental measurements and theoretical calculations of the surface potentials. We find that a carbon buffer layer is detached due to the gallium intercalation, which increases the surface potential as an indication of the 2D-Ga presence. We then scale up the heterostructure characterization over a few-square millimeter area by segmenting SEM images, each acquired with nanometer-scale in-plane resolution. This work leverages the spectroscopic imaging capabilities of SEM that allows high-spatial resolution imaging for tracking intercalants, identifying relative surface potentials, determining the number of 2D layers, and further characterizing scalability and uniformity of low-dimensional materials.

Tunable 2D Group-III Metal Alloys.

Chemically stable quantum-confined 2D metals are of interest in next-generation nanoscale quantum devices. Bottom-up design and synthesis of such metals could enable the creation of materials with tailored, on-demand, electronic and optical properties for applications that utilize tunable plasmonic coupling, optical nonlinearity, epsilon-near-zero behavior, or wavelength-specific light trapping. In this work, it is demonstrated that the electronic, superconducting, and optical properties of air-stable 2D metals can be controllably tuned by the formation of alloys. Environmentally robust large-area 2D-Inx Ga1- x alloys are synthesized byConfinement Heteroepitaxy (CHet). Near-complete solid solubility is achieved with no evidence of phase segregation, and the composition is tunable over the full range of x by changing the relative elemental composition of the precursor. The optical and electronic properties directly correlate with alloy composition, wherein the dielectric function, band structure, superconductivity, and charge transfer from the metal to graphene are all controlled by the indium/gallium ratio in the 2D metal layer.

Scalable Substitutional Re-Doping and its Impact on the Optical and Electronic Properties of Tungsten Diselenide.

Reliable, controlled doping of 2D transition metal dichalcogenides will enable the realization of next-generation electronic, logic-memory, and magnetic devices based on these materials. However, to date, accurate control over dopant concentration and scalability of the process remains a challenge. Here, a systematic study of scalable in situ doping of fully coalesced 2D WSe2 films with Re atoms via metal-organic chemical vapor deposition is reported. Dopant concentrations are uniformly distributed over the substrate surface, with precisely controlled concentrations down to <0.001% Re achieved by tuning the precursor partial pressure. Moreover, the impact of doping on morphological, chemical, optical, and electronic properties of WSe2 is elucidated with detailed experimental and theoretical examinations, confirming that the substitutional doping of Re at the W site leads to n-type behavior of WSe2 . Transport characteristics of fabricated back-gated field-effect-transistors are directly correlated to the dopant concentration, with degrading device performances for doping concentrations exceeding 1% of Re. The study demonstrates a viable approach to introducing true dopant-level impurities with high precision, which can be scaled up to batch production for applications beyond digital electronics.

Spontaneous Relaxation of Heteroepitaxial Thin Films by van der Waals-Like Bonding on Te-Terminated Sapphire Substrates.

Here the novel direct heteroepitaxial growth method of a 3D heteroepitaxial system is demonstrated on a 3D substrate, CdTe (111)/Al2 O3 (0001), which forms a spontaneous vdW-like bond at the interface, instead of the two 3D crystals being strongly bound. Despite a large lattice mismatch, the thin films are single crystals and maintain high quality due the compliance of the interface which accommodates strain. This weak bonding interface is accomplished by the self-assembly of a pseudomorphic chalcogenide layer on the sapphire surface during growth. Since the vdW-like interface forms spontaneously in situ during growth, it is easily scalable to large wafer sizes, without the need to layer transfer 2D materials onto the growth substrate for remote epitaxy. Further, the weak adhesion of the films on the substrates allow for epitaxial film transfer to a variety of other substrates, leaving the original growth substrate for future reuse. This type of 3D/3D vdW-like interface is exploitable as a compliant interlayer for additional epitaxy, and may even be observable directly in other material systems grown on complex oxides, allowing for the production of large area high quality freestanding and layer transferred epitaxial devices for material systems not currently possible by conventional processing techniques.

Unexpected Near-Infrared to Visible Nonlinear Optical Properties from 2-D Polar Metals.

Near-infrared-to-visible second harmonic generation from air-stable two-dimensional polar gallium and indium metals is described. The photonic properties of 2D metals, including the largest second-order susceptibilities reported for metals (approaching 10 nm/V), are determined by the atomic-level structure and bonding of two-to-three-atom-thick crystalline films. The bond character evolved from covalent to metallic over a few atomic layers, changing the out-of-plane metal-metal bond distances by approximately ten percent (0.2 Å), resulting in symmetry breaking and an axial electrostatic dipole that mediated the large nonlinear response. Two different orientations of the crystalline metal atoms, corresponding to lateral displacements <2 Å, persisted in separate micrometer-scale terraces to generate distinct harmonic polarizations. This strong atomic-level structure-property interplay suggests metal photonic properties can be controlled with atomic precision.

Bacteriophage ZCSE2 is a Potent Antimicrobial Against Salmonella enterica Serovars: Ultrastructure, Genomics and Efficacy.

Developing novel antimicrobials capable of controlling multidrug-resistant bacterial pathogens is essential to restrict the use of antibiotics. Bacteriophages (phages) constitute a major resource that can be harnessed as an alternative to traditional antimicrobial therapies. Phage ZCSE2 was isolated among several others from raw sewage but was distinguished by broad-spectrum activity against Salmonella serovars considered pathogenic to humans and animals. Lytic profiles of ZCSE2 against a panel of Salmonella were determined together with low temperature activity and pH stability. The morphological features of the phage and host infection processes were characterized using a combination of transmission electron and atomic force microscopies. Whole genome sequencing of ZCSE2 produced a complete DNA sequence of 53,965 bp. No known virulence genes were identified in the sequence data, making ZCSE2 a good candidate for phage-mediated biological control purposes. ZCSE2 was further tested against S. Enteritidis in liquid culture and was observed to reduce the target bacterium to below the limits of detection from initial concentrations of 107-108 Colony Forming Units (CFU)/mL. With a broad host-range against pathogenic Salmonella serovars, phage ZCSE2 constitutes a potential tool against a major cause of human and animal disease.