We report the production of a two-dimensional (2D) heterostructured gas sensor. The gas-sensing characteristics of exfoliated molybdenum disulfide (MoS2) connected to interdigitated metal electrodes were investigated. The MoS2 flake-based sensor detected a NO2 concentration as low as 1.2 ppm and exhibited excellent gas-sensing stability. Instead of metal electrodes, patterned graphene was used for charge collection in the MoS2-based sensing devices. An equation based on variable resistance terms was used to describe the sensing mechanism of the graphene/MoS2 device. Furthermore, the gas response characteristics of the heterostructured device on a flexible substrate were retained without serious performance degradation, even under mechanical deformation. This novel sensing structure based on a 2D heterostructure promises to provide a simple route to an essential sensing platform for wearable electronics.

Thermoelectric (TE) energy converters are solid-state devices that can generate electricity by harvesting waste thermal energy, thereby improving the efficiency of a system. The many advantages of TE devices include solid-state operation, zero-emissions, vast scalability, no maintenance, and a long operating lifetime. Nonetheless, because of their limited energy conversion efficiencies, thermoelectric devices currently have a rather limited set of applications. However, there is a reinvigorated interest in the field of thermoelectrics by identifying classical and quantum mechanical size effects, which provide additional ways to enhance energy conversion efficiencies in nanostructured materials 1,2 including superlattice thin films 3 and quantum dots. 4 For example, thermoelectric figure of the merit (ZT) up to 2.5 was achieved by synthesizing two-dimensional Sb 2 Te 3 /Bi 2 Te 3 superlattice thin films, exceeding pervious limits of ∼1 from bulk counterpart. 5 Even more exciting are the theoretical predictions for one-dimensional nanostructures including nanowires and nanotubes, which are thought to have ZT exceeding 5. 1,6 In the case of nanotubes, theoretical calculation predicts a further reduction in the thermal conductivity, because of a stronger phonon-surface scattering, compared to solid nanowire. 7 Limited works have been reported on the synthesis of thermoelectric nanotubes including hydrothermally grown Bi, Bi 2 Se 3 , and Bi 2 Te 3 nanotubes 8 and electrodeposited Bi nanotubes. 9 However, these processes have some limitations. For example, the nanotube production yield is very low (<30%). 8 In the template-directed method it is difficult to restrict the proceeding electrodeposition along the walls without filling up the whole pores. 9 The galvanic displacement reaction is an electrochemical process, which is induced by the difference in redox potentials between materials. Various metallic nanotubes have been synthesized via this reaction (e.g., gold nanotubes from silver nanowires); 10 however, no one to-date has demonstrated the synthesis of semiconducting thermoelectric nanotubes. In this paper, we demonstrate the synthesis of high-aspect ratio Bi 2 Te 3 nanotubes with controlled composition by galvanic displacement of nickel nanowires in acidic nitric electrolyte containing Bi 3+ and HTeO 2 + ions. Bi 2 Te 3 nanotubes were synthesized because Bi 2 Te 3 and its derivative compounds are considered to be the best materials used in thermoelectric refrigeration at room temperature. In addition to synthesis, we also demonstrate the fabrication method to create individual Bi 2 Te 3 nanotube-based devices by combining the magnetic assembly of single nickel nanowire across microfabricated electrodes, followed by a galvanic displacement reaction. Figure 1A shows the schematic illustration of the galvanic displacement reaction of Ni nanowires to Bi 2 Te 3 nanotubes. The detailed experimental conditions are provided in the Supporting Information. When Ni nanowires are immersed into an acidic nitric solution con...

Heterostructures of compositionally and electronically variant two-dimensional (2D) atomic layers are viable building blocks for ultrathin optoelectronic devices. We show that the composition of interfacial transition region between semiconducting WSe2 atomic layer channels and metallic NbSe2 contact layers can be engineered through interfacial doping with Nb atoms. WxNb1-xSe2 interfacial regions considerably lower the potential barrier height of the junction, significantly improving the performance of the corresponding WSe2-based field-effect transistor devices. The creation of such alloyed 2D junctions between dissimilar atomic layer domains could be the most important factor in controlling the electronic properties of 2D junctions and the design and fabrication of 2D atomic layer devices.

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