Zero-Bias Photodetection in 2D Materials via Geometric Design of Contacts.

Structural or crystal asymmetry is a necessary condition for the emergence of zero-bias photocurrent in light detectors. Structural asymmetry has been typically achieved via p-n doping, which is a technologically complex process. Here, we propose an alternative approach to achieve zero-bias photocurrent in two-dimensional (2D) material flakes exploiting the geometrical nonequivalence of source and drain contacts. As a prototypical example, we equip a square-shaped flake of PdSe2 with mutually orthogonal metal leads. Upon uniform illumination with linearly polarized light, the device demonstrates nonzero photocurrent which flips its sign upon 90° polarization rotation. The origin of zero-bias photocurrent lies in a polarization-dependent lightning-rod effect. It enhances the electromagnetic field at one contact from the orthogonal pair and selectively activates the internal photoeffect at the respective metal-PdSe2 Schottky junction. The proposed technology of contact engineering is independent of a particular light-detection mechanism and can be extended to arbitrary 2D materials.

Terahertz Photoconductivity in Bilayer Graphene Transistors: Evidence for Tunneling at Gate-Induced Junctions.

Photoconductivity of novel materials is the key property of interest for design of photodetectors, optical modulators, and switches. Despite the photoconductivity of most novel 2d materials having been studied both theoretically and experimentally, the same is not true for 2d p-n junctions that are necessary blocks of most electronic devices. Here, we study the sub-terahertz photocoductivity of gapped bilayer graphene with electrically induced p-n junctions. We find a strong positive contribution from junctions to resistance, temperature resistance coefficient, and photoresistivity at cryogenic temperatures T ∼ 20 K. The contribution to these quantities from junctions exceeds strongly the bulk values at uniform channel doping even at small band gaps of ∼10 meV. We further show that positive junction photoresistance is a hallmark of interband tunneling, and not of intraband thermionic conduction. Our results point to the possibility of creating various interband tunneling devices based on bilayer graphene, including steep-switching transistors and selective sensors.

The influence of copper ions on the transport and relaxation properties of hydrated eumelanin.

Eumelanin, the human skin pigment, is a poly-indolequinone material possessing a unique combination of physical and chemical properties. For numerous applications, the conductivity of eumelanin is of paramount importance. However, its hydration dependent conductivity is not well studied using transport-relaxation methods. Furthermore, there is no such work taking into account the simultaneous control of humidity as well as metal ion concentration. Here we present the first such study of the transport and relaxation characteristics of synthetic eumelanin doped with various Cu ion concentrations while controlling the humidity with a frequency range of 10-3 Hz-1 MHz. We found that Cu ions do not cause the appearance of additional relaxation processes, but partially slow down those present in neat eumelanin. In addition, considering previously published work, the key relaxation process observed in doped and undoped materials is associated with the moisture-induced synthesis of uncharged semiquinones and a corresponding increase in the overall aromaticity of the material.

Multifunctional Elastic Nanocomposites with Extremely Low Concentrations of Single-Walled Carbon Nanotubes.

Stretchable and flexible electronics has attracted broad attention over the last years. Nanocomposites based on elastomers and carbon nanotubes are a promising material for soft electronic applications. Despite the fact that single-walled carbon nanotube (SWCNT) based nanocomposites often demonstrate superior properties, the vast majority of the studies were devoted to those based on multiwalled carbon nanotubes (MWCNTs) mainly because of their higher availability and easier processing procedures. Moreover, high weight concentrations of MWCNTs are often required for high performance of the nanocomposites in electronic applications. Inspired by the recent drop in the SWCNT price, we have focused on fabrication of elastic nanocomposites with very low concentrations of SWCNTs to reduce the cost of nanocomposites further. In this work, we use a fast method of coagulation (antisolvent) precipitation to fabricate elastic composites based on thermoplastic polyurethane (TPU) and SWCNTs with a homogeneous distribution of SWCNTs in bulk TPU. Applicability of the approach is confirmed by extra low percolation threshold of 0.006 wt % and, as a consequence, by the state-of-the-art performance of fabricated elastic nanocomposites at very low SWCNT concentrations for strain sensing (gauge factor of 82 at 0.05 wt %) and EMI shielding (efficiency of 30 dB mm-1 at 0.01 wt %).

Water-Activated Semiquinone Formation and Carboxylic Acid Dissociation in Melanin Revealed by Infrared Spectroscopy.

Eumelanin is a widespread biomacromolecule pigment in the biosphere and has been widely investigated for numerous bioelectronics and energetic applications. Many of these applications depend on eumelanin's ability to conduct proton current at various levels of hydration. The origin of this behavior is connected to a comproportionation reaction between oxidized and reduced monomer moieties and water. A hydration-dependent FTIR spectroscopic study on eumelanin is presented herein, which allows for the first time tracking the comproportionation reaction via the gradual increase of the overall aromaticity of melanin monomers in the course of hydration. We identified spectral features associated with the presence of specific "one and a half" C𝌁O bonds, typical for o-semiquinones. Signatures of semiquinone monomers with internal hydrogen bonds and that carboxylic groups, in contrast to semiquinones, begin to dissociate at the very beginning of melanin hydration were indicated. As such, we suggest a modification to the common hydration-dependent conductivity mechanism and propose that the conductivity at low hydration is dominated by carboxylic acid protons, whereas higher hydration levels manifest semiquinone protons.

Rotational coherence of encapsulated ortho and para water in fullerene-C60 revealed by time-domain terahertz spectroscopy.

We resolve the real-time coherent rotational motion of isolated water molecules encapsulated in fullerene-C60 cages by time-domain terahertz (THz) spectroscopy. We employ single-cycle THz pulses to excite the low-frequency rotational motion of water and measure the subsequent coherent emission of electromagnetic waves by water molecules. At temperatures below ~ 100 K, C60 lattice vibrational damping is mitigated and the quantum dynamics of confined water are resolved with a markedly long rotational coherence, extended beyond 10 ps. The observed rotational transitions agree well with low-frequency rotational dynamics of single water molecules in the gas phase. However, some additional spectral features with their major contribution at ~2.26 THz are also observed which may indicate interaction between water rotation and the C60 lattice phonons. We also resolve the real-time change of the emission pattern of water after a sudden cooling to 4 K, signifying the conversion of ortho-water to para-water over the course of 10s hours. The observed long coherent rotational dynamics of isolated water molecules confined in C60 makes this system an attractive candidate for future quantum technology.