One of the most fundamental devices for electronics and optoelectronics is the PN junction, which provides the functional element of diodes, bipolar transistors, photodetectors, LEDs, and solar cells, among many other devices. In conventional PN junctions, the adjacent p-and ntype regions of a semiconductor are formed by chemical doping. Materials with ambipolar conductance, however, allow for PN junctions to be configured and modified by electrostatic gating. This electrical control enables a single device to have multiple functionalities. Here we report ambipolar monolayer WSe 2 devices in which two local gates are used to define a PN junction exclusively within the sheet of WSe 2 . With these electrically tunable PN junctions, we demonstrate both PN and NP diodes with ideality factors better than 2. Under excitation with light, the diodes show photodetection responsivity of 210 mA/W and photovoltaic power generation with a peak external quantum efficiency of 0.2%, promising numbers for a nearly transparent monolayer sheet in a lateral device geometry. Finally, we demonstrate a lightemitting diode based on monolayer WSe 2 . These devices provide a fundamental building block for ubiquitous, ultra-thin, flexible, and nearly transparent optoelectronic and electronic applications based on ambipolar dichalcogenide materials.Next generation photodetectors and photovoltaic devices, as well as sensors, displays, and light emitting diodes, will all require new optoelectronic materials with superior characteristics to those currently in use. Candidate materials must be flexible for wearable devices, transparent for interactive displays, efficient for solar cells, and robust and low cost for broad distribution. Monolayer semiconducting transition metal dichalcogenides (TMDs) such as tungsten diselenide (WSe 2 ) are flexible 1 , nearly transparent 2,3 , high strength 1 , direct band gap 2 materials that have the potential to meet all of these criteria. Tungsten diselenide's crystal structure is comprised of stacks of trilayer sheets made of a single atomic layer of tungsten encapsulated by two layers of selenium. This structure leads to very strong intralayer bonding in the trilayer plane and weak interlayer bonding * These authors contributed equally to this work.arXiv:1310.0452v2 [cond-mat.mes-hall]

We report electronic transport measurements of devices based on monolayers and bilayers of the transition-metal dichalcogenide MoS 2 . Through a combination of in situ vacuum annealing and electrostatic gating we obtained ohmic contact to the MoS 2 down to 4 K at high carrier densities. At lower carrier densities, low temperature four probe transport measurements show a metal-insulator transition in both monolayer and bilayer samples. In the metallic regime, the high temperature behavior of the mobility showed strong temperature dependence consistent with phonon dominated transport. At low temperature, intrinsic field-effect mobilities greater than 1000 cm 2 /Vs were observed for both monolayer and bilayer devices. Mobilities extracted from Hall effect measurements were several times lower and showed a strong dependence on density, likely caused by screening of charged impurity scattering at higher densities.

We report on the etching of graphene devices with a helium ion beam, including in situ electrical measurement during lithography. The etching process can be used to nanostructure and electrically isolate different regions in a graphene device, as demonstrated by etching a channel in a suspended graphene device with etched gaps down to about 10 nm. Graphene devices on silicon dioxide (SiO 2 ) substrates etch with lower He ion doses and are found to have a residual conductivity after etching, which we attribute to contamination by hydrocarbons.