Tuning the Fermi Level of Graphene by Two-Dimensional Metals for Raman Detection of Molecules.

Graphene-enhanced Raman scattering (GERS) offers great opportunities to achieve optical sensing with a high uniformity and superior molecular selectivity. The GERS mechanism relies on charge transfer between molecules and graphene, which is difficult to manipulate by varying the band alignment between graphene and the molecules. In this work, we synthesized a few atomic layers of metal termed two-dimensional (2D) metal to precisely and deterministically modify the graphene Fermi level. Using copper phthalocyanine (CuPc) as a representative molecule, we demonstrated that tuning the Fermi level can significantly improve the signal enhancement and molecular selectivity of GERS. Specifically, aligning the Fermi level of graphene closer to the highest occupied molecular orbital (HOMO) of CuPc results in a more pronounced Raman enhancement. Density functional theory (DFT) calculations of the charge density distribution reproduce the enhanced charge transfer between CuPc molecules and graphene with a modulated Fermi level. Extending our investigation to other molecules such as rhodamine 6G, rhodamine B, crystal violet, and F16CuPc, we showed that 2D metals enabled Fermi level tuning, thus improving GERS detection for molecules and contributing to an enhanced molecular selectivity. This underscores the potential of utilizing 2D metals for the precise control and optimization of GERS applications, which will benefit the development of highly sensitive, specific, and reliable sensors.

Light- and Redox-Dependent Force Spectroscopy Reveals that the Interaction between Plastocyanin and Plant Photosystem I Is Favored when One Partner Is Ready for Electron Transfer.

Photosynthesis is a fundamental process that converts photons into chemical energy, driven by large protein complexes at the thylakoid membranes of plants, cyanobacteria, and algae. In plants, water-soluble plastocyanin (Pc) is responsible for shuttling electrons between cytochrome b6f complex and the photosystem I (PSI) complex in the photosynthetic electron transport chain (PETC). For an efficient turnover, a transient complex must form between PSI and Pc in the PETC, which implies a balance between specificity and binding strength. Here, we studied the binding frequency and the unbinding force between suitably oriented plant PSI and Pc under redox control using single molecule force spectroscopy (SMFS). The binding frequency (observation of binding-unbinding events) between PSI and Pc depends on their respective redox states. The interaction between PSI and Pc is independent of the redox state of PSI when Pc is reduced, and it is disfavored in the dark (reduced P700) when Pc is oxidized. The frequency of interaction between PSI and Pc is higher when at least one of the partners is in a redox state ready for electron transfer (ET), and the post-ET situation (PSIRed-PcOx) leads to lower binding. In addition, we show that the binding of ET-ready PcRed to PSI can be regulated externally by Mg2+ ions in solution.

Ultrathin Metals on a Transparent Seed and Application to Infrared Reflectors.

Ultrathin metal films (UTMFs) are widely used in optoelectronic applications, from transparent conductors to photovoltaic cells, low emissivity windows, and plasmonic metasurfaces. During the initial deposition phase, many metals tend to form islands on the receiving substrate rather than a physically connected (percolated) network, which eventually evolves into continuous films as the thickness increases. For example, in the case of Ag and Au on dielectric surfaces, percolation begins when the thickness of the metal film is at least about 5 nm. It is known that the type of growth can be changed when a proper seed layer is used. Here, we show that a CuO layer directly deposited on a substrate can dramatically influence surface wetting and promote early percolation of polycrystalline Ag and Au UTMFs. We demonstrate that the proposed seed is effective even when its thickness is sub-nanometric, in this way maintaining the full transparency of the receiving substrate. The Ag and Au films seeded with CuO showed a percolation thickness close to 1 nm and were morphologically and optically characterized from an ultraviolet (λ = 300 nm) to a midinfrared (λ = 15 µm) wavelength. Infrared reflectors, a mirror and a resonant plasmonic structure, were also demonstrated and uniquely tuned by electrical gating, this being possible owing to the small thickness of the constituting Au UTMF.

NaCl substrates for high temperature processing and transfer of ultrathin materials.

Ultrathin materials often require high temperatures for growth and processing, which cannot be withstood by the substrate underneath. For example, polymers are widely used as a supporting layer but unfortunately have low strain-point temperatures. This is the case of polyethylene terephthalate (PET) which has glass transition and melting temperatures of 76 and 250 °C, respectively. In this paper we propose to use polished salt, a material that can withstand high temperatures during fabrication and, at the same time, can be sacrificed during the transfer onto the final substrates. More specifically, we demonstrate thermal dewetting of Au ultrathin metal films and growth of MoS2 on NaCl at 750 and 650 °C, respectively, and subsequent transfer onto PET films, after which the salt is easily dissolved by water. We believe that the proposed technique can be extended to fabrication of other ultrathin materials, e.g. graphene, as well as final substrates for a wide range of applications, including flexible electronic and optoelectronic devices.

Antireflective Transparent Oleophobic Surfaces by Noninteracting Cavities.

Oleophobic surfaces have been so far realized using complex microscale and nanoscale re-entrant geometries, where primary and secondary structures or overhang geometries are typically required. Here, we propose a new design to create them with noninteracting cavities. The suspension of liquid droplets relies on the mechanism of compression of air under the meniscus leading to stable composite oil-air-solid interfaces. To demonstrate the concept, we make oleophobic surfaces, with contact angle for oleic acid of about 130° (and hexadecane about 110°), using both microholes in silicon and nanoholes in glass. Thanks to the subwavelength dimensions and antireflection effect of the nanoholes, the glass substrate also shows a high degree of optical transparency with optical transmission exceeding that of the initial bare substrate. Crockmeter tests without any significant change in morphology, optical and wetting properties after more than 500 passes also confirm the high mechanical durability of the nanohole surface. The results indicate the possibility of using the proposed oleophobic surfaces for a wide range of applications, including self-cleaning transparent windows and windshields for automobiles and aircrafts.

An antireflection transparent conductor with ultralow optical loss (<2 %) and electrical resistance (<6 Ω sq-1).

Transparent conductors are essential in many optoelectronic devices, such as displays, smart windows, light-emitting diodes and solar cells. Here we demonstrate a transparent conductor with optical loss of ∼1.6%, that is, even lower than that of single-layer graphene (2.3%), and transmission higher than 98% over the visible wavelength range. This was possible by an optimized antireflection design consisting in applying Al-doped ZnO and TiO2 layers with precise thicknesses to a highly conductive Ag ultrathin film. The proposed multilayer structure also possesses a low electrical resistance (5.75 Ω sq-1), a figure of merit four times larger than that of indium tin oxide, the most widely used transparent conductor today, and, contrary to it, is mechanically flexible and room temperature deposited. To assess the application potentials, transparent shielding of radiofrequency and microwave interference signals with ∼30 dB attenuation up to 18 GHz was achieved.