Advanced beyond-silicon electronic technology requires discoveries of both new channel materials and ultralow-resistance contacts 1,2 . Atomically thin two-dimensional (2D) semiconductors have great potential for realizing high-performance electronic devices 1,3 . However, because of metal-induced gap states (MIGS) 4-7 , energy barriers at the metalsemiconductor interface, which fundamentally lead to high contact resistances and poor current-delivery capabilities, have restrained the advancement of 2D semiconductor transistors to date 2,8,9 . Here, we report a novel ohmic contact technology between semimetallic bismuth and semiconducting monolayer transition metal dichalcogenides (TMDs) where MIGS is sufficiently suppressed and degenerate states in the TMD are spontaneously formed in contact with bismuth. Through this approach, we achieve zero Schottky barrier height, a record-low contact resistance (R C ) of 123 Ω μm, and a recordhigh on-state current density (I ON ) of 1135 µA µm -1 on monolayer MoS 2 . We also demonstrate that excellent ohmic contacts can be formed on various monolayer semiconductors, including MoS 2 , WS 2 , and WSe 2 . Our reported R C values are a significant improvement for 2D semiconductors, and approaching the quantum limit. This technology unveils the full potential of high-performance monolayer transistors that are on par with the state-of-the-art 3D semiconductors, enabling further device down-scaling and extending Moore's Law.The electrical contact resistance at a metal-semiconductor (M-S) interface has been an increasingly critical, yet unsolved issue for the semiconductor industry, hindering the ultimate

Two-dimensional (2D) materials have generated great interest in the past few years as a new toolbox for electronics. This family of materials includes, among others, metallic graphene, semiconducting transition metal dichalcogenides (such as MoS2), and insulating boron nitride. These materials and their heterostructures offer excellent mechanical flexibility, optical transparency, and favorable transport properties for realizing electronic, sensing, and optical systems on arbitrary surfaces. In this paper, we demonstrate a novel technology for constructing large-scale electronic systems based on graphene/molybdenum disulfide (MoS2) heterostructures grown by chemical vapor deposition. We have fabricated high-performance devices and circuits based on this heterostructure, where MoS2 is used as the transistor channel and graphene as contact electrodes and circuit interconnects. We provide a systematic comparison of the graphene/MoS2 heterojunction contact to more traditional MoS2-metal junctions, as well as a theoretical investigation, using density functional theory, of the origin of the Schottky barrier height. The tunability of the graphene work function with electrostatic doping significantly improves the ohmic contact to MoS2. These high-performance large-scale devices and circuits based on this 2D heterostructure pave the way for practical flexible transparent electronics.

A Weyl semimetal (WSM) is a novel topological phase of matter [1][2][3][4][5][6][7][8][9][10][11][12][13], in which Weyl fermions (WFs) arise as pseudo-magnetic monopoles in its momentum space. The chirality of the WFs, given by the sign of the monopole charge, is central to the Weyl physics, since it directly serves as the sign of the topological number [5,12] and gives rise to exotic properties such as Fermi arcs [5,9,11] and the chiral anomaly [12][13][14][15][16]. Despite being the defining property of a WSM, the chirality of the WFs has never been experimentally measured. Here, we directly detect the chirality of the WFs by measuring the photocurrent in response to circularly polarized mid-infrared light. The resulting photocurrent is determined by both the chirality of WFs and that of the photons. Our results pave the way for realizing a wide range of theoretical proposals [12,13,[17][18][19][20][21][22][23][24][25][26] for studying and controlling the WFs and their associated quantum anomalies by optical and electrical means. More broadly, the two chiralities, analogous to the two valleys in 2D materials [27,28], lead to a new degree of freedom in a 3D crystal with potential novel pathways to store and carry information.infrared pump and a soft X-ray probe, which is technically very challenging. On the other hand, optical experiments on WSMs have remained very limited [36,37], although they are promising approaches to achieve these goals [12]. In this paper, we detect the chirality of the WFs in the WSM TaAs by measuring its mid-infrared photocurrent response. Circularly polarized light induced photocurrents, also called the circular photogalvanic effect (CPGE), have been previously measured in other systems [29-31] but have not been experimentally studied in WSMs.We first discuss the theoretical picture of the CPGE for optical transitions from the lower part of the Weyl cone to the upper part [17]. There are two independent factors important for the CPGE here. The first is the chirality selection rule (Figs. 1c,d). For a right circularly polarized (RCP) light propagating along +ẑ and a χ = +1 WF, the optical transition is allowed on the +k z side but forbidden on the −k z side [17]. The second is the Pauli blockade, which is only present when chemical potential is away from the Weyl node. FIG. 4: Detection and manipulation of chiral Weyl fermions by optical means. a, Our calculations (see SI.II.3) showĴ THY = χ W 1klight ×ĉ. b, By measuring the direction of the currentJ and knowing the polarization and propagation direction of the light, we obtainĴ EXP =k light ×ĉ from data. By comparing theory and data, we obtain χ W 1 = +1. The χ W 1 = +1 determined by the photocurrent agrees with that predicted by first-principles ( Fig. 1j), further confirming our experimental detection of WF chirality. c-e, Comparison between the chirality degree of freedom of the WFs and valley degree of freedom in gapped Dirac system. c, In a gapped Dirac system, an optical excitation with a particular handedness can only populate ...

Optical cavities with multiple tunable resonances have the potential to provide unique electromagnetic environments at two or more distinct wavelengths--critical for control of optical processes such as nonlinear generation, entangled photon generation, or photoluminescence (PL) enhancement. Here, we show a plasmonic nanocavity based on a nanopatch antenna design that has two tunable resonant modes in the visible spectrum separated by 350 nm and with line widths of ∼60 nm. The importance of utilizing two resonances simultaneously is demonstrated by integrating monolayer MoS2, a two-dimensional semiconductor, into the colloidally synthesized nanocavities. We observe a 2000-fold enhancement in the PL intensity of MoS2--which has intrinsically low absorption and small quantum yield--at room temperature, enabled by the combination of tailored absorption enhancement at the first harmonic and PL quantum-yield enhancement at the fundamental resonance.