The junctions formed at the contact between metallic electrodes and semiconductor materials are crucial components of electronic and optoelectronic devices
. Metal-semiconductor junctions are ...characterized by an energy barrier known as the Schottky barrier, whose height can, in the ideal case, be predicted by the Schottky-Mott rule
on the basis of the relative alignment of energy levels. Such ideal physics has rarely been experimentally realized, however, because of the inevitable chemical disorder and Fermi-level pinning at typical metal-semiconductor interfaces
. Here we report the creation of van der Waals metal-semiconductor junctions in which atomically flat metal thin films are laminated onto two-dimensional semiconductors without direct chemical bonding, creating an interface that is essentially free from chemical disorder and Fermi-level pinning. The Schottky barrier height, which approaches the Schottky-Mott limit, is dictated by the work function of the metal and is thus highly tunable. By transferring metal films (silver or platinum) with a work function that matches the conduction band or valence band edges of molybdenum sulfide, we achieve transistors with a two-terminal electron mobility at room temperature of 260 centimetres squared per volt per second and a hole mobility of 175 centimetres squared per volt per second. Furthermore, by using asymmetric contact pairs with different work functions, we demonstrate a silver/molybdenum sulfide/platinum photodiode with an open-circuit voltage of 1.02 volts. Our study not only experimentally validates the fundamental limit of ideal metal-semiconductor junctions but also defines a highly efficient and damage-free strategy for metal integration that could be used in high-performance electronics and optoelectronics.
Direct α-phosphonylation of an unprotected secondary amine in a single step is of practical importance to amino phophophates. However, this protocol is limited due to the high redox barrier of ...unprotected amine. In this paper, we report C–H phosphonylation of an unprotected secondary amine via an electrochemical approach in the presence of catalytic carboxylate salt. This metal-free and exogenous oxidant-free method furnishes diverse target molecules with satisfactory yield under mild reaction conditions. Successful application of the protocol in a gram-scale experiment demonstrates the potential utility for further functionalization.
Nanostructured materials have shown extraordinary promise for electrochemical energy storage but are usually limited to electrodes with rather low mass loading (~1 milligram per square centimeter) ...because of the increasing ion diffusion limitations in thicker electrodes. We report the design of a three-dimensional (3D) holey-graphene/niobia (Nb₂O₅) composite for ultrahigh-rate energy storage at practical levels of mass loading (>10 milligrams per square centimeter). The highly interconnected graphene network in the 3D architecture provides excellent electron transport properties, and its hierarchical porous structure facilitates rapid ion transport. By systematically tailoring the porosity in the holey graphene backbone, charge transport in the composite architecture is optimized to deliver high areal capacity and high-rate capability at high mass loading, which represents a critical step forward toward practical applications.
Two-dimensional layered semiconductors such as molybdenum disulfide (MoS2) have attracted tremendous interest as a new class of electronic materials. However, there are considerable challenges in ...making reliable contacts to these atomically thin materials. Here we present a new strategy by using graphene as the back electrodes to achieve ohmic contact to MoS2. With a finite density of states, the Fermi level of graphene can be readily tuned by a gate potential to enable a nearly perfect band alignment with MoS2. We demonstrate for the first time a transparent contact to MoS2 with zero contact barrier and linear output behavior at cryogenic temperatures (down to 1.9 K) for both monolayer and multilayer MoS2. Benefiting from the barrier-free transparent contacts, we show that a metal–insulator transition can be observed in a two-terminal MoS2 device, a phenomenon that could be easily masked by Schottky barriers found in conventional metal-contacted MoS2 devices. With further passivation by boron nitride (BN) encapsulation, we demonstrate a record-high extrinsic (two-terminal) field effect mobility up to 1300 cm2/(V s) in MoS2 at low temperature.
Single-atom catalysts represent a unique catalytic system with high atomic utilization and tunable reaction pathway. Despite current successes in their optimization and tailoring through structural ...and synthetic innovations, there is a lack of dynamic modulation approach for the single-atom catalysis. Inspired by the electrostatic interaction within specific natural enzymes, here we show the performance of model single-atom catalysts anchored on two-dimensional atomic crystals can be systematically and efficiently tuned by oriented external electric fields. Superior electrocatalytic performance have been achieved in single-atom catalysts under electrostatic modulations. Theoretical investigations suggest a universal "onsite electrostatic polarization" mechanism, in which electrostatic fields significantly polarize charge distributions at the single-atom sites and alter the kinetics of the rate determining steps, leading to boosted reaction performances. Such field-induced on-site polarization offers a unique strategy for simulating the catalytic processes in natural enzyme systems with quantitative, precise and dynamic external electric fields.
The electrochemical molecular intercalation of two-dimensional layered materials (2DLMs) produces stable and highly tunable superlattices between monolayer 2DLMs and self-assembled molecular layers. ...This process allows unprecedented flexibility in integrating highly distinct materials with atomic/molecular precision to produce a new generation of organic/inorganic superlattices with tunable chemical, electronic, and optical properties. To better understand the intercalation process, we developed an on-chip platform based on MoS2 model devices and used optical, electrochemical, and in situ electronic characterizations to resolve the intermediate stages during the intercalation process and monitor the evolution of the molecular superlattices. With sufficient charge injection, the organic cetyltrimethylammonium bromide (CTAB) intercalation induces the phase transition of MoS2 from semiconducting 2H phase to semimetallic 1T phase, resulting in a dramatic increase of electrical conductivity. Therefore, in situ monitoring the evolution of the device conductance reveals the electrochemical intercalation dynamics with an abrupt conductivity change, signifying the onset of the molecule intercalation. In contrast, the intercalation of tetraheptylammonium bromide (THAB), a branched molecule in a larger size, resulting in a much smaller number of charges injected to avoid the 2H to 1T phase transition. Our study demonstrates a powerful platform for in situ monitoring the molecular intercalation of many 2DLMs (MoS2, WSe2, ReS2, PdSe2, TiS2, and graphene) and systematically probing electronic, optical, and optoelectronic properties at the single-nanosheet level.
Metal–graphitic interfaces formed between metal nanoparticles (MNPs) and carbon nanotubes (CNTs) or graphene play an important role in the properties of such hybrid nanostructures. This Perspective ...summarizes different types of interfaces that exist within the metal–carbon nanoassemblies and discusses current efforts on understanding and modeling the interfacial conditions and interactions. Characterization of the metal–graphitic interfaces is described here, including microscopy, spectroscopy, electrochemical techniques, and electrical measurements. Recent studies on these nanohybrids have shown that the metal–graphitic interfaces play critical roles in both controlled assembly of nanoparticles and practical applications of nanohybrids in chemical sensors and fuel cells. Better understanding, design, and manipulation of metal–graphitic interfaces could therefore become the new frontier in the research of MNP/CNT or MNP/graphene hybrid systems.
The unique physical and chemical properties of single-walled carbon nanotubes (SWNTs) make them ideal building blocks for the construction of hybrid nanostructures. In addition to increasing the ...material complexity and functionality, SWNTs can probe the interfacial processes in the hybrid system. In this work, SWNT–TiO2 core/shell hybrid nanostructures were found to exhibit unique electrical behavior in response to UV illumination and acetone vapors. By experimental and theoretical studies of UV and acetone sensitivities of different SWNT–TiO2 hybrid systems, we established a fundamental understanding on the interfacial charge transfer between photoexcited TiO2 and SWNTs as well as the mechanism of acetone sensing. We further demonstrated a practical application of photoinduced acetone sensitivity by fabricating a microsized room temperature acetone sensor that showed fast, linear, and reversible detection of acetone vapors with concentrations in few parts per million range.
Abstract
Precise understanding of interfacial metal−hydrogen interactions, especially under in operando conditions, is crucial to advancing the application of metal catalysts in clean energy ...technologies. To this end, while Pd-based catalysts are widely utilized for electrochemical hydrogen production and hydrogenation, the interaction of Pd with hydrogen during active electrochemical processes is complex, distinct from most other metals, and yet to be clarified. In this report, the hydrogen surface adsorption and sub-surface absorption (phase transition) features of Pd and its alloy nanocatalysts are identified and quantified under operando electrocatalytic conditions via on-chip electrical transport measurements, and the competitive relationship between electrochemical carbon dioxide reduction (CO
2
RR) and hydrogen sorption kinetics is investigated. Systematic dynamic and steady-state evaluations reveal the key impacts of local electrolyte environment (such as proton donors with different p
K
a
) on the hydrogen sorption kinetics during CO
2
RR, which offer additional insights into the electrochemical interfaces and optimization of the catalytic systems.
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