Aggregation-Dependent Dielectric Permittivity in 2D Molecular Crystals.

The functionality of 2D molecular crystal-based devices crucially depends on their intrinsic properties, such as molecular energy levels, light absorption efficiency, and dielectric permittivity, which are highly sensitive to molecular aggregation. Here, it is demonstrated that the dielectric permittivity of the 2,7-dioctyl[1]benzothieno[3,2-b][1]benzothiophene (C8 -BTBT) molecular crystals on monolayer WS2 substrates can be tuned from 4.62 in the wetting layer to 2.25 in the second layer. Its origin lies in the different molecular orientations in the wetting layer (lying-down) and in the subsequently stacked layers (standing-up), which lead to a positive Coulomb coupling (JCoup ) value (H-aggregation) and a negative JCoup value (J-aggregation), respectively. Polarized optical contrast spectroscopy reveals that the permittivity of C8 -BTBT is anisotropic, and its direction is related to the underlying substrate. The study offers guidelines for future manipulation of the permittivity of 2D molecular crystals, which may promote their applications toward various electronic and optoelectronic devices.

Lithography-free and high-efficiency preparation of black phosphorous devices by direct evaporation through shadow mask.

Two-dimensional (2D) materials including black phosphorus (BP) have been extensively investigated because of their exotic physical properties and potential applications in nanoelectronics and optoelectronics. Fabricating BP based devices is challenging because BP is extremely sensitive to the external environment, especially to the chemical contamination during the lithography process. The direct evaporation through shadow mask technique is a clean method for lithography-free electrode patterning of 2D materials. Herein, we employ the lithography-free evaporation method for the construction of BP based field-effect transistors and photodetectors and systematically compare their performances with those of BP counterparts fabricated by conventional lithography and transfer electrode methods. The results show that BP devices fabricated by direct evaporation method possess higher mobility, faster response time, and smaller hysteresis than those prepared by the latter two methods. This can be attributed to the clean interface between BP and evaporated-electrodes as well as the lower Schottky barrier height of 20.2 meV, which is given by the temperature-dependent electrical results. Furthermore, the BP photodetectors exhibit a broad-spectrum response and polarization sensitivity. Our work elucidates a universal, low-cost and high-efficiency method to fabricate BP devices for optoelectronic applications.

UV Rewritable Hybrid Graphene/Phosphor p-n Junction Photodiode.

Graphene-based p-n junction photodiodes have a potential application prospect in photodetection due to their broadband spectral response, large operating bandwidth, and mechanical flexibility. Here, we report an ultraviolet (UV) rewritable p-n junction photodiode in a configuration of graphene coated with an amorphous phosphor of 4-bromo-1,8-naphthalic anhydride derivative polymer (poly-BrNpA). Under moderate UV irradiation, occurrence of photoisomerization reaction in the poly-BrNpA film leads to its drastically modified optical characteristics and a concurrent n-type doping in the underneath graphene. Meanwhile, the poly-BrNpA film, highly sensitive to water molecules, has a capability of restoring graphene to its initial p-type doping status by means of water adsorption. Based on these findings, a lateral graphene/poly-BrNpA p-n junction photodiode, responsive to visible light at the junction interface, can be written by UV irradiation and then erased via water adsorption. The p-n junction photodiode is rewritable upon such repetitive loops showing repeatable optoelectronic properties. This study provides a new scheme and perspective of making graphene-based rewritable p-n junction photodiodes in a flexible and controllable way, and it may contribute to expanding new families of optoelectronic devices based on two-dimensional materials.

Fast Photoelectric Conversion in the Near-Infrared Enabled by Plasmon-Induced Hot-Electron Transfer.

Interfacial charge transfer is a fundamental and crucial process in photoelectric conversion. If charge transfer is not fast enough, carrier harvesting can compromise with competitive relaxation pathways, e.g., cooling, trapping, and recombination. Some of these processes can strongly affect the speed and efficiency of photoelectric conversion. In this work, it is elaborated that plasmon-induced hot-electron transfer (HET) from tungsten suboxide to graphene is a sufficiently fast process to prevent carrier cooling and trapping processes. A fast near-infrared detector empowered by HET is demonstrated, and the response time is three orders of magnitude faster than that based on common band-edge electron transfer. Moreover, HET can overcome the spectral limit of the bandgap of tungsten suboxide (≈2.8 eV) to extent the photoresponse to the communication band of 1550 nm (≈0.8 eV). These results indicate that plasmon-induced HET is a new strategy for implementation of efficient and high-speed photoelectric devices.