Esaki's discovery of NDR in heavily doped semiconducting germanium p-n junctions in 1958 was the first experimental evidence of quantum mechanical tunneling transport of electrons in all-condensedmatter systems 3,4 . This discovery motivated Giaever's tunneling experiments that proved the existence of the superconductive energy gap predicted by the then-newly formulated Bardeen-Cooper Schrieffer (BCS) theory of superconductivity 5 . After these initial breakthroughs, tunneling in various classes of crystalline matter has been observed, and forms the basis for several practical applications. For example,Josephson junctions exploit tunneling in superconductors for exquisitely sensitive magnetic flux detectors in superconducting quantum interference devices (SQUIDs) 6 , and are now being investigated as the building blocks of quantum computers 7 . Electron tunneling forms the basis for low-resistance ohmic contacts to heavily doped semiconductors for energy-efficient transistors, as low-loss cascade elements in multi-junction solar cells, and for coherent emission of long-wavelength photons in quantum-cascade lasers 8,9 . In addition to such practical applications, the extreme sensitivity of tunneling currents to various electronic, vibrational, and photonic excitations of solids makes tunneling spectroscopy one of the most sensitive probes for such phenomena 10 .Recently, interband tunneling in semiconductors has been proposed as the enabler for a new class of semiconductor transistors called tunnel field-effect transistors (TFETs) that promise very low-power operation. The heart of such devices is an Esaki tunnel diode, with preferably a near broken-gap band alignment at the source-channel heterojunction 11,12 . As these heterojunction TFETs are scaled down to the nanometer regime, the increase in bandgap barrier due to quantum confinement may significantly prohibit the desired tunneling currents, because tunneling current decreases exponentially with the barrier height. Layered semiconductors with a sizable bandgap and a wide range of band alignments can potentially avoid such degradation, and have been proposed as ideally suited for such applications 13,14 .This class of devices distinguishes themselves from the graphene-based SymFET by offering a desired low off-current 15,16 . Compared to traditional 3D heterojunctions, such structures are expected to form high-quality heterointerfaces due to the absence of dangling bonds [1][2][3]20 . The weak vdW bonding in principle does not suffer from lattice mismatch requirements and makes strain-free integration possible.Among the previous reports on vdW solids, heterojunctions of type-I (straddling) and type-II (staggered) band alignments have been demonstrated [17][18][19] . The Esaki diode device structure is schematically shown in Fig. 1a. The devices are fabricated using a dry transfer process with flake thicknesses of ~50-100 nm for both BP and SnSe 2 24 . BP is the p-type semiconductor, and SnSe 2 the n-type semiconductor of the vdW Esaki diode. A detailed de...

Transition metal dichalcogenides are relevant for electronic devices owing to their sizable band gaps and absence of dangling bonds on their surfaces. For device development, a controllable method for doping these materials is essential. In this paper, we demonstrate an electrostatic gating method using a solid polymer electrolyte, poly(ethylene oxide) and CsClO4, on exfoliated, multilayer 2H-MoTe2. The electrolyte enables the device to be efficiently reconfigured between n- and p-channel operation with ON/OFF ratios of approximately 5 decades. Sheet carrier densities as high as 1.6 × 10(13) cm(-2) can be achieved because of a large electric double layer capacitance (measured as 4 μF/cm(2)). Further, we show that an in-plane electric field can be used to establish a cation/anion transition region between source and drain, forming a p-n junction in the 2H-MoTe2 channel. This junction is locked in place by decreasing the temperature of the device below the glass transition temperature of the electrolyte. The ideality factor of the p-n junction is 2.3, suggesting that the junction is recombination dominated.

The properties of multilayer exfoliated MoTe 2 field-effect transistors (FETs) on SiO 2 were investigated for channel thicknesses from 6 to 44 monolayers (MLs). All transistors showed p-type conductivity at zero back-gate bias. For channel thicknesses of 8 ML or less, the transistors exhibited ambipolar characteristics. ON/OFF current ratio was greatest, 1 Â 10 5 , for the transistor with the thinnest channel, 6 ML. Devices showed a clear photoresponse to wavelengths between 510 and 1080 nm at room temperature. Temperature-dependent current-voltage measurements were performed on a FET with 30 layers of MoTe 2. When the channel is turned-on and p-type, the temperature dependence is barrier-limited by the Au/Ti/MoTe 2 contact with a hole activation energy of 0.13 eV. A long channel transistor model with Schottky barrier contacts is shown to be consistent with the common-source characteristics. V

To deposit an ultrathin dielectric onto WSe2, monolayer titanyl phthalocyanine (TiOPc) is deposited by molecular beam epitaxy as a seed layer for atomic layer deposition (ALD) of Al2O3 on WSe2. TiOPc molecules are arranged in a flat monolayer with 4-fold symmetry as measured by scanning tunneling microscopy. ALD pulses of trimethyl aluminum and H2O nucleate on the TiOPc, resulting in a uniform deposition of Al2O3, as confirmed by atomic force microscopy and cross-sectional transmission electron microscopy. The field-effect transistors (FETs) formed using this process have a leakage current of 0.046 pA/μm(2) at 1 V gate bias with 3.0 nm equivalent oxide thickness, which is a lower leakage current than prior reports. The n-branch of the FET yielded a subthreshold swing of 80 mV/decade.

We report on the fabrication and characterization of synthesized multiwall MoS 2 nanotube (NT) and nanoribbon (NR) field-effect transistors (FETs). The MoS 2 NTs and NRs were grown by chemical transport, using iodine as a transport agent. Raman spectroscopy confirms the material as unambiguously MoS 2 in NT, NR, and flake forms. Transmission electron microscopy was used to observe cross sections of the devices after electrical measurements and these were used in the interpretation of the electrical measurements allowing estimation of the current density. The NT and NR FETs demonstrate n-type behavior, with ON/OFF current ratios exceeding 10 3 , and with current densities of 1.02 µA/µm, and 0.79 µA/µm at V DS = 0.3 V and V BG = 1 V, respectively. Photocurrent measurements conducted on a MoS 2 NT FET, revealed short-circuit photocurrent of tens of nanoamps under an excitation optical power of 78 W and 488 nm wavelength, which corresponds to a responsivity of 460 A/W. A long channel transistor model was used to model the common-source characteristics of MoS 2 NT and NR FETs and was shown to be consistent with the measured data. , sensors 7,8 , field-effect transistors 9,10,11,12 , and logic circuits 13,14 . The absence of surface dangling bonds, the excellent gate electrostatics of the few-layer transistor, and the potential for large area planar processing are all motivating this research. While planar processing is desirable for manufacturing, at the limits of scaling, the properties of these materials may be compromised by unpassivated dangling bonds at the sheet edges. Edges introduce traps, which can degrade subthreshold swing and increase tunneling leakage, 1/f noise, and variability in the device characteristics. Edges can be substantially eliminated by using nanotubes (NTs), and nanoribbons (NRs) formed from collapsed NTs. . The silica ampoule containing MoS 2 powder and iodine in amount of 1.5 mg/cm 3 was evacuated and sealed at a pressure of 7 x 10 -4 Pa. The transport reaction using iodine as a transport agent ran from 1133 K to 1010 K with a temperature gradient of 6.2 K/cm in a two-zone furnace. After three weeks of growth, the silica ampoules were cooled to room temperature with a controlled cooling rate of 60º C/hour. Approximately a few percent of the starting material was transported by the reaction to form nanotubes, while the rest of the transported material grows as strongly undulated thin plate-like crystals. The nearly equilibrium growth conditions enable the synthesis of nanotubes of different diameters, length, and wall thickness, but with extremely low density of structural defects. They grow up to several millimeters in length. The Raman measurements, shown in Fig. 3(a), were performed on the NR, the NT, and a flake that was exfoliated from the same material source. Measurements were done in the backscattering configuration using a WITec Alpha 300 system at room . The conduction of heat out of the NT may be expected to be less than the NR because of smaller contact area to the substrate...