Femtosecond transient absorption spectroscopy and microscopy were employed to study exciton dynamics in suspended and Si 3 N 4 substrate-supported monolayer and few-layer MoS 2 2D crystals. Exciton dynamics for the monolayer and few-layer structures were found to be remarkably different from those of thick crystals when probed at energies near that of the lowest energy direct exciton (A exciton). The intraband relaxation rate was enhanced by more than 40 fold in the monolayer in comparison to that observed in the thick crystals, which we attributed to defect assisted scattering. Faster electronÀhole recombination was found in monolayer and few-layer structures due to quantum confinement effects that lead to an indirectÀdirect band gap crossover. Nonradiative rather than radiative relaxation pathways dominate the dynamics in the monolayer and few-layer MoS 2 . Fast trapping of excitons by surface trap states was observed in monolayer and few-layer structures, pointing to the importance of controlling surface properties in atomically thin crystals such as MoS 2 along with controlling their dimensions.

Atomically thin molybdenum disulfide (MoS2) offers potential for advanced devices and an alternative to graphene due to its unique electronic and optical properties. The temperature-dependent Raman spectra of exfoliated, monolayer MoS2 in the range of 100–320 K are reported and analyzed. The linear temperature coefficients of the in-plane E 2g 1 and the out-of-plane A 1g modes for both suspended and substrate-supported monolayer MoS2 are measured. These data, when combined with the first-order coefficients from laser power-dependent studies, enable the thermal conductivity to be extracted. The resulting thermal conductivity κ = (34.5 ± 4) W/mK at room temperature agrees well with the first-principles lattice dynamics simulations. However, this value is significantly lower than that of graphene. The results from this work provide important input for the design of MoS2-based devices where thermal management is critical.

Terahertz technology promises myriad applications including imaging, spectroscopy and communications. However, one major bottleneck at present for advancing this field is the lack of efficient devices to manipulate the terahertz electromagnetic waves. Here we demonstrate that exceptionally efficient broadband modulation of terahertz waves at room temperature can be realized using graphene with extremely low intrinsic signal attenuation. We experimentally achieved more than 2.5 times superior modulation than prior broadband intensity modulators, which is also the first demonstrated graphene-based device enabled solely by intraband transitions. The unique advantages of graphene in comparison to conventional semiconductors are the ease of integration and the extraordinary transport properties of holes, which are as good as those of electrons owing to the symmetric conical band structure of graphene. Given recent progress in graphene-based terahertz emitters and detectors, graphene may offer some interesting solutions for terahertz technologies.

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...

We report the realization of field-effect transistors (FETs) made with chemically-

We demonstrate a graphene-based electro-absorption modulator achieving extraordinary control of terahertz reflectance. By concentrating the electric field intensity in an active layer of graphene, an extraordinary modulation depth of 64% is achieved while simultaneously exhibiting low insertion loss (∼2 dB), which is remarkable since the active region of the device is atomically thin. This modulator performance, among the best reported to date, indicates the enormous potential of graphene for terahertz reconfigurable optoelectronic devices.

We determined the band alignment of a graphene-insulator-semiconductor structure using internal photoemission spectroscopy. From the flatband voltage and Dirac voltage, we infer a 4:6 Â 10 11 cm À2 negative extrinsic charge present on the graphene surface. Also, we extract the graphene work function to be 4.56 eV, in excellent agreement with theoretical and experimental values in literature. Electron and hole injection from heavily doped p-type silicon (Si) are both observed. The barrier height from the top of the valence band of Si to the bottom of the conduction band of silicon dioxide (SiO 2) is found to be 4.3 eV. The small optical absorption in graphene makes it a good transparent contact to enable the direct observation of hole injection from Si to graphene. The barrier height for holes escaping from the bottom of Si conduction band to the top of SiO 2 valence band is found to be 4.6 eV. V

The modulation depth of 2-D electron gas (2DEG) based THz modulators using AlGaAs/GaAs heterostructures with metal gates is inherently limited to < 30%. The metal gate not only attenuates the THz signal (> 90%) but also severely degrades the modulation depth. The metal losses can be significantly reduced with an alternative material with tunable conductivity. Graphene presents a unique solution to this problem due to its symmetric band structure and extraordinarily high mobility of holes that is comparable to electron mobility in conventional semiconductors. The hole conductivity in graphene can be electrostatically tuned in the graphene-2DEG parallel capacitor configuration, thus more efficiently tuning the THz transmission. In this work, we show that it is possible to achieve a modulation depth of > 90% while simultaneously minimizing signal attenuation to < 5% by tuning the Fermi level at the Dirac point in graphene.