Stable and Fluid Multilayer Phospholipid–Silica Thin Films: Mimicking Active Multi-lamellar Biological Assemblies

Phospholipid-based nanomaterials are of interest in several applications including drug delivery, sensing, energy harvesting, and as model systems in basic research. However, a general challenge in creating functional hybrid biomaterials from phospholipid assemblies is their fragility, instability in air, insolubility in water, and the difficulty of integrating them into useful composites that retain or enhance the properties of interest, therefore limiting there use in integrated devices. We document the synthesis and characterization of highly ordered and stable phospholipid–silica thin films that resemble multilamellar architectures present in nature such as the myelin sheath. We have used a near room temperature chemical vapor deposition method to synthesize these robust functional materials. Highly ordered lipid films are exposed to vapors of silica precursor resulting in the formation of nanostructured hybrid assemblies. This process is simple, scalable, and offers advantages such as exclusion of ethanol and no (or minimal) need for exposure to mineral acids, which are generally required in conventional sol–gel synthesis strategies. The structure of the phospholipid–silica assemblies can be tuned to either lamellar or hexagonal organization depending on the synthesis conditions. The phospholipid–silica films exhibit long-term structural stability in air as well as when placed in aqueous solutions and maintain their fluidity under aqueous or humid conditions. This platform provides a model for robust implementation of phospholipid multilayers and a means toward future applications of functional phospholipid supramolecular assemblies in device integration

Optimizing Composition and Morphology for Large-Grain Perovskite Solar Cells via Chemical Control

We report solid iodine as a precursor additive for achieving purified organometallic perovskite crystals. By adding iodine, we found that the reaction can be pushed toward pure iodine phase rather than the kinetically favored chlorine phase. This approach can be applied in large crystalline perovskite solar cells and improved the average efficiency from 9.83% to 15.58%

Fluid and Resistive Tethered Lipid Membranes on Nanoporous Substrates

Cell membranes perform important biological roles including compartmentalization, signaling, and transport of nutrients. Supported lipid membranes mimic the behavior of cell membranes and are an important model tool for studying membrane properties in a controlled laboratory environment. Lipid membranes may be supported on solid substrates; however, protein and lipid interactions with the substrate typically result in their denaturation. In this report, we demonstrate the formation of intact lipid membranes tethered on nanoporous metal thin films obtained via a dealloying process. Uniform lipid membranes were formed when the surface defect density of the nanoporous metal film was significantly reduced through a two-step dealloying process reported here. We show that the tethered lipid membranes on nanoporous metal substrates maintain both fluidity and electrical resistivity, which are key attributes to naturally occurring lipid membranes. The lipid assemblies supported on nanoporous metals provide a new platform for investigating lipid membrane properties, and potentially membrane proteins, for numerous applications including next generation biosensor platforms, targeted drug-delivery, and energy harvesting devices

Experimental Determination of the Ionization Energies of MoSe₂, WS₂, and MoS₂ on SiO₂ Using Photoemission Electron Microscopy

The values of the ionization energies of transition metal dichalcogenides (TMDs) are needed to assess their potential usefulness in semiconductor heterojunctions for high-performance optoelectronics. Here, we report on the systematic determination of ionization energies for three prototypical TMD monolayers (MoSe₂, WS₂, and MoS₂) on SiO₂ using photoemission electron microscopy with deep ultraviolet illumination. The ionization energy displays a progressive decrease from MoS₂, to WS₂, to MoSe₂, in agreement with predictions of density functional theory calculations. Combined with the measured energy positions of the valence band edge at the Brillouin zone center, we deduce that, in the absence of interlayer coupling, a vertical heterojunction comprising any of the three TMD monolayers would form a staggered (type-II) band alignment. This band alignment could give rise to long-lived interlayer excitons that are potentially useful for valleytronics or efficient electron–hole separation in photovoltaics

Catalytic Activity in Lithium-Treated Core–Shell MoO_<i>x</i>/MoS₂ Nanowires

Significant interest has grown in the development of earth-abundant and efficient catalytic materials for hydrogen generation. Layered transition metal dichalcogenides present opportunities for efficient electrocatalytic systems. Here, we report the modification of 1D MoO_<i>x</i>/MoS₂ core–shell nanostructures by lithium intercalation and the corresponding changes in morphology, structure, and mechanism of H₂ evolution. The 1D nanowires exhibit significant improvement in H₂ evolution properties after lithiation, reducing the hydrogen evolution reaction (HER) onset potential by ∼50 mV and increasing the generated current density by ∼600%. The high electrochemical activity in the nanowires results from disruption of MoS₂ layers in the outer shell, leading to increased activity and concentration of defect sites. This is in contrast to the typical mechanism of improved catalysis following lithium exfoliation, i.e., crystal phase transformation. These structural changes are verified by a combination of Raman and X-ray photoelectron spectroscopy (XPS)

Spatially Resolved Photoexcited Charge-Carrier Dynamics in Phase-Engineered Monolayer MoS₂

A fundamental understanding of the intrinsic optoelectronic properties of atomically thin transition-metal dichalcogenides (TMDs) is crucial for its integration into high performance semiconductor devices. Here, we investigate the transport properties of chemical vapor deposition (CVD) grown monolayer molybdenum disulfide (MoS₂) under photoexcitation using correlated scanning photocurrent microscopy and photoluminescence imaging. We examined the effect of local phase transformation underneath the metal electrodes on the generation of photocurrent across the channel length with diffraction-limited spatial resolution. While maximum photocurrent generation occurs at the Schottky contacts of semiconducting (2H-phase) MoS₂, after the metallic phase transformation (1T-phase), the photocurrent peak is observed toward the center of the device channel, suggesting a strong reduction of native Schottky barriers. Analysis using the bias and position dependence of the photocurrent indicates that the Schottky barrier heights are a few millielectron volts for 1T- and ∼200 meV for 2H-contacted devices. We also demonstrate that a reduction of native Schottky barriers in a 1T device enhances the photoresponsivity by more than 1 order of magnitude, a crucial parameter in achieving high-performance optoelectronic devices. The obtained results pave a way for the fundamental understanding of intrinsic optoelectronic properties of atomically thin TMDs where ohmic contacts are necessary for achieving high-efficiency devices with low power consumption

Structural Design of Benzo[1,2‑<i>b</i>:4,5‑<i>b</i>′]dithiophene-Based 2D Conjugated Polymers with Bithienyl and Terthienyl Substituents toward Photovoltaic Applications

In this contribution, six conjugated polymers consisting of benzo­[1,2-<i>b</i>:4,5-<i>b</i>′]­dithiophene–bithiophene (BDT-BT) and benzo­[1,2-<i>b</i>:4,5-<i>b</i>′]­dithiophene–benzothiadiazle (BDT-BTD) as building blocks in the main chain were synthesized by coupling polymerization and utilized for photovoltaic applications. By directly attaching three kinds of alkylthienyl side chains to the conjugated main chain, the resulted two-dimensional configuration revealed a broader absorption range due to the ground state electron transition of their corresponding alkylthienyl units and polymer backbone. Temperature-dependent absorbance, emission spectra, and thermal annealing further verify that the shoulder band(s) were originated from the aggregated (crystalline) species of polymers. The photovoltaic properties of the donor–acceptor polymers revealed well-defined side chain geometries, physical, and electronic structures and showed the highest power conversion efficiency of 4.25% among polymer solar cells based on two-dimensional (2-D) bithienyl- or terthienyl-substituted benzodithiophene