Wafer scale BN on sapphire substrates for improved graphene transport.

Wafer scale (2") BN grown by metal organic chemical vapor deposition (MOCVD) on sapphire was examined as a weakly interacting dielectric substrate for graphene, demonstrating improved transport properties over conventional sapphire and SiO2/Si substrates. Chemical vapor deposition grown graphene was transferred to BN/sapphire substrates for evaluation of more than 30 samples using Raman and Hall effects measurements. A more than 2x increase in Hall mobility and 10x reduction in sheet carrier density was measured for graphene on BN/sapphire compared to sapphire substrates. Through control of the MOCVD process, BN films with roughness ranging from <0.1 nm to >1 nm were grown and used to study the effects of substrate roughness on graphene transport. Arrays of graphene field effect transistors were fabricated on 2" BN/sapphire substrates demonstrating scalability and device performance enhancement.

Heterogeneous Pyrolysis: A Route for Epitaxial Growth of hBN Atomic Layers on Copper Using Separate Boron and Nitrogen Precursors.

Growth of hBN on metal substrates is often performed via chemical vapor deposition from a single precursor (e.g., borazine) and results in hBN monolayers limited by the substrates catalyzing effect. Departing from this paradigm, we demonstrate close control over the growth of mono-, bi-, and trilayers of hBN on copper using triethylborane and ammonia as independent sources of boron and nitrogen. Using density functional theory (DFT) calculations and reactive force field molecular dynamics, we show that the key factor enabling the growth beyond the first layer is the activation of ammonia through heterogeneous pyrolysis with boron-based radicals at the surface. The hBN layers grown are in registry with each other and assume a perfect or near perfect epitaxial relation with the substrate. From atomic force microscopy (AFM) characterization, we observe a moiré superstructure in the first hBN layer with an apparent height modulation and lateral periodicity of ∼10 nm. While this is unexpected given that the moiré pattern of hBN/Cu(111) does not have a significant morphological corrugation, our DFT calculations reveal a spatially modulated interface dipole layer which determines the unusual AFM response. These findings have improved our understanding of the mechanisms involved in growth of hBN and may help generate new growth methods for applications in which control over the number of layers and their alignment is crucial (such as tunneling barriers, ultrathin capacitors, and graphene-based devices).

A simple and selective colorimetric mercury (II) sensing system based on chitosan stabilized gold nanoparticles and 2,6-pyridinedicarboxylic acid.

The development of simple and cost-effective methods for the detection and treatment of Hg2+ in the environment is an important area of research due to the serious health risk that Hg2+ poses to humans. Colorimetric sensing based on the induced aggregation of nanoparticles is of great interest since it offers a low cost, simple, and relatively rapid procedure, making it perfect for on-site analysis. Herein we report the development of a simple colorimetric sensor for the selective detection and estimation of mercury ions in water, based on chitosan stabilized gold nanoparticles (AuNPs) and 2,6-pyridinedicarboxylic acid (PDA). In the presence of Hg2+, PDA induces the aggregation of AuNPs, causing the solution to change colors varying from red to blue, depending on the concentration of Hg2+. The formation of aggregated AuNPs in the presence of Hg2+ was confirmed using transmission electron microscopy (TEM) and UV-Vis spectroscopy. The method exhibits linearity in the range of 300nM to 5µM and shows excellent selectivity towards Hg2+ among seventeen different metal ions and was successfully applied for the detection of Hg2+ in spiked river water samples. The developed technique is simple and superior to the existing techniques in that it allows detection of Hg2+ using the naked eye and simple and rapid colorimetric analysis, which eliminates the need for sophisticated instruments and sample preparation methods.

Enzymatic glucose sensor based on Au nanoparticle and plant-like ZnO film modified electrode.

A novel electrochemical glucose sensor was developed by employing a composite film of plant-like Zinc oxide (ZnO) and chitosan stabilized spherical gold nanoparticles (AuNPs) on which Glucose oxidaze (GOx) was immobilized. The ZnO was deposited on an indium tin oxide (ITO) coated glass and the AuNPs of average diameter of 23 nm were loaded on ZnO as the second layer. The prepared ITO/ZnO/AuNPs/GOx bioelectrode exhibited a low value of Michaelis-Menten constant of 1.70 mM indicating a good bio-matrix for GOx. The studies of electrochemical properties of the electrode using cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) showed that, the presence of AuNPs provides significant enhancement of the electron transfer rate during redox reactions. The linear sweep voltammetry (LSV) shows that the ITO/ZnO/AuNPs/GOx based sensor has a high sensitivity of 3.12 µA·mM(-1)·cm(-2) in the range of 50 mg/dL to 400 mg/dL glucose concentration. The results show promising application of the gold nanoparticle modified plant-like ZnO composite bioelectrode for electrochemical sensing of glucose.

Robust longitudinal spin-Seebeck effect in Bi-YIG thin films.

In recent years, the coupling of magnetic insulators (bismuth-doped yttrium iron garnet, Bi-YIG) with platinum has garnered significant interest in spintronics research due to applicability as spin-current-driven thermoelectric coatings. These coatings bridge the gap between spintronics technologies and thermoelectric materials, providing a novel means of transforming waste heat into electricity. However, there remain questions regarding the origins of the spin-Seebeck effect (SSE) as well as claims that observed effects are a manifestation of magnetic proximity effects, which would induce magnetic behavior in platinum. Herewith we provide support that the voltages observed in the Bi-YIG/Pt films are purely SSE voltages. We reaffirm claims that magnon transport theory provides an ample basis for explaining SSE behavior. Finally, we illustrate the advantages of pulsed-laser deposition, as these Bi-YIG films possess large SSE voltages (even in absence of an external magnetic field), as much as twice those of films fabricated via solution-based methods.