No abstract

Two-dimensional transition metal dichalcogenides are promising candidates for ultrathin optoelectronic devices due to their high absorption coefficients and intrinsically passivated surfaces. To maintain these near-perfect surfaces, recent research has focused on fabricating contacts that limit Fermi-level pinning at the metal-semiconductor interface. Here, we develop a new, simple procedure for transferring metal contacts that does not require aligned lithography. Using this technique, we fabricate vertical Schottky-junction WS2 solar cells, with Ag and Au as asymmetric work function contacts. Under laser illumination, we observe rectifying behavior and open-circuit voltage above 500 mV in devices with transferred contacts, in contrast to resistive behavior and open-circuit voltage below 15 mV in devices with evaporated contacts. One-sun measurements and device simulation results indicate that this metal transfer process could enable high specific power vertical Schottky-junction transition metal dichalcogenide photovoltaics, and we anticipate that this technique will lead to advances for two-dimensional devices more broadly.

Interlayer excitons in heterobilayers of transition-metal dichalcogenides (TMDCs) have generated enormous interest due to their permanent vertical dipole moments and long lifetimes. However, the effects of mechanical strain on the optoelectronic properties of interlayer excitons in heterobilayers remain relatively uncharacterized. Here, we experimentally demonstrate strain tuning of Γ–K interlayer excitons in molybdenum disulfide and tungsten diselenide (MoS2/WSe2) wrinkled heterobilayers and obtain a deformation potential constant of ∼107 meV/% uniaxial strain, which is approximately twice that of the intralayer excitons in the constituent monolayers. We further observe a nonmonotonic dependence of the interlayer exciton photoluminescence intensity with strain, which we interpret as being due to the sensitivity of the Γ point to band hybridization arising from the competition between in-plane strain and out-of-plane interlayer coupling. Strain engineering with interlayer excitons in TMDC heterobilayers offers higher strain tunability and new degrees of freedom compared to their monolayer counterparts.

We describe a preconcentration device that may be suitable for quantitative analysis of trace volatile ketones and aldehydes in ambient air as well as in human breath. The approach is based on microreactor chips fabricated from silicon wafers. The microreactors have thousands of micropillars in microfluidic channels for uniformly distributing a gaseous sample flowing through the chips. The surfaces of the micropillars are functionalized with a quaternary ammonium aminooxy salt, [2-(aminooxy)ethyl]-N,N,N-trimethylammonium iodide (ATM), for trapping trace ketones and aldehydes by means of oximation reactions. ATM adducts and unreacted ATM are eluted from the microreactor with less than 40 μL of methanol and directly analyzed by nanospray Fourier transform ion cyclotron resonance (FTICR) mass spectrometry (MS). Ketones and aldehydes at levels of 1 ppbv have been detected using this microreactor and FTICR-MS system.

A straightforward "bottom-up" synthesis is described for efficient entrapment of inorganic hydrophobic nanoparticles (HNPs) consisting of iron oxide, gold, or quantum dots within the hydrophobic core of lipid nanoparticles (LNPs). These LNPs consist of hydrophobic "core" lipids such as triolein surrounded by a monolayer of amphipathic "surface" lipids, such as phosphatidylcholine and polyethylene-glycol-lipid. It is shown that rapid, controlled mixing of HNPs, core lipids and surface lipids in an organic solvent with an aqueous phase resulted in stable, monodisperse LNPs containing HNPs (LNP-HNP). This method allows 40-fold more hydrophobic iron oxide nanoparticles (IONPs) to be entrapped within an LNP than previous methods and can be readily extended to encapsulate other HNPs. The LNP-HNP diameter can be modulated over the range of 35-150 nm by varying the flow rate during particle synthesis or by varying the core-to-surface lipid ratio. LNP-IONPs can be generated using a variety of "core" lipids, including other triglycerides as well as cholesteryl-palmitate and tocopherol. Finally, it is shown that LNP-IONPs are accumulated in the liver, resulting in enhanced contrast for in vivo MRI. It is concluded that the bottom-up approach for encapsulating HNPs within LNPs has advantages of homogeneity, reproducibility and stability required for biomedical applications.