The conversion of light into electrical signals is at the heart of a plenty of technologies related with everyday life, such as optical communication, remote sensing, security, and many other fields. [1] The continuous extension of application areas requires progressive development of portable photodetectors with high performance in terms of speed, efficiency, or active wavelength range. [2] Recent years have witnessed the burgeoning interests in exploiting the optical and electronic properties of layered materials due to their weak interlayer van der Waals (vdW) interaction and dangling-bond-free surfaces for high-quality integration. [3] Compared with traditional materials, such as GaAs, Si, and Ge, the quantum behavior of electrons in 2D materials can be distinctive different from their bulk parental materials, which shows strong layer-dependence and composed properties in vdW heterostructure, and revolutionizes the way of manipulation, conversion of nonequilibrium carriers under strong light-matter interaction. Over the past few years, 2D materials such like graphene, [4] black phosphorus, [5] h-BN, [6] and MoS 2 [7] and their heterostructures have been widely implemented in a variety of photonic and electronic systems, showing appealing novel properties for single photon source, exciton/plasmon/phonon polaritons, artificial diode for transparent electronic and photovoltaic devices, and open up novel feasibility for realizing optoelectronic devices across a very wide range of electromagnetic spectrum. Up to now, significant efforts have been devoted to develop photodetectors capable of working toward longer wavelength in the 2D quantum materials family, based on a number of distinct characteristics of graphene and related materials. [8] However, the detecting wavelength of these detectors is mainly limited within visible to short-wavelength infrared due to their unmatched dark current, carrier mobility, and bandgap properties. For example, the detectable wavelength of WS 2 is shorter than 647 nm [9] and the black phosphorus can work at wavelength shorter than 7.7 µm. [10,11] For longer wavelength photodetection, liquid helium cooling or even dilute cooling is required in order to inhibit noise from intrinsically large dark current or the thermally induced transition among closely lying energylevels. [12] In compared with other electromagnetic radiation Recent years have witnessed rapid progresses made in the photoelectric performance of two-dimensional materials represented by graphene, black phosphorus, and transition metal dichalcogenides. Despite significant efforts, a photodetection technique capable for longer wavelength, higher working temperature as well as fast responsivity, is still facing huge challenges due to a lack of best among bandgap, dark current, and absorption ability. Exploring topological materials with nontrivial band transport leads to peculiar properties of quantized phenomena such as chiral anomaly, and magnetic-optical effect, which enables a novel feasibility for an advanced optoele...

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We report the results of a 27 Al and 51 V nuclear magnetic resonance study of Fe 2 VAl at temperatures between 4 and 550 K. This material has been a subject of current interest due to indications of possible heavy fermion behavior. The low-temperature NMR relaxation rate follows a Korringa law, indicating a small density of carriers at the Fermi level. At elevated temperatures, the shifts and relaxation rates go over to a thermally activated response, a semiconductorlike behavior, consistent with separate low-lying bands removed from the Fermi-level. These results are consistent with recent electronic structure calculations, and can explain both the reported activated resistivity as well as the Fermi cutoff exhibited in photoemission studies. While we observe nonstoichiometric samples of (Fe 1Ϫx V x ) 3 Al to be magnetic, the xϭ0.33 composition is nonmagnetic, with narrow NMR linewidths. ͓S0163-1829͑98͒07739-X͔

Emergent topological Dirac semimetals afford fresh pathways for optoelectronics, although device implementation has been elusive to date. Specifically, palladium ditelluride (PdTe2) combines the capabilities provided by its peculiar band structure, with topologically protected electronic states, with advantages related to the occurrence of high-mobility charge carriers and ambient stability. Here, we demonstrate large photogalvanic effects with high anisotropy at terahertz frequency in PdTe2-based devices. A responsivity of 10 A/W and a noise-equivalent power lower than 2 pW/Hz0.5 are achieved at room temperature, validating the suitability of PdTe2-based devices for applications in photosensing, polarization-sensitive detection, and large-area fast imaging. Our findings open opportunities for exploring uncooled and sensitive photoelectronic devices based on topological semimetals, especially in the highly pursuit terahertz band.

Transition-metal dichalcogenides showing type-II Dirac fermions are emerging as innovative materials for nanoelectronics. However, their excitation spectrum is mostly unexplored yet. By means of high-resolution electron energy loss spectroscopy and density functional theory, here, we identify the collective excitations of type-II Dirac fermions (3D Dirac plasmons) in PtTe_{2} single crystals. The observed plasmon energy in the long-wavelength limit is ∼0.5  eV, which makes PtTe_{2} suitable for near-infrared optoelectronic applications. We also demonstrate that interband transitions between the two Dirac bands in PtTe_{2} give rise to additional excitations at ∼1 and ∼1.4  eV. Our results are crucial to bringing to fruition type-II Dirac semimetals in optoelectronics.

PtTe 2 is a novel transition-metal dichalcogenide hosting type-II Dirac fermions that displays application capabilities in optoelectronics and hydrogen evolution reaction. Here it is shown, by combining surface science experiments and density functional theory, that the pristine surface of PtTe 2 is chemically inert toward the most common ambient gases (oxygen and water) and even in air. It is demonstrated that the creation of Te vacancies leads to the appearance of tellurium-oxide phases upon exposing defected PtTe 2 surfaces to oxygen or ambient atmosphere, which is detrimental for the ambient stability of uncapped PtTe 2 -based devices. On the contrary, in PtTe 2 surfaces modified by the joint presence of Te vacancies and substitutional carbon atoms, the stable adsorption of hydroxyl groups is observed, an essential step for water splitting and the water-gas shift reaction. These results thus pave the way toward the exploitation of this class of Dirac materials in catalysis.

The successful growth of high-quality SnSe 2 crystals by chemical vapour transport (CVT) and epitaxial films by molecular beam epitaxy (MBE) was reported by several groups 1-2 . However, the productivity by CVT and MBE is far below than that by the Bridgman technique. Compared with the high cost of equipment of MBE, both CVT and Bridgman methods are relatively faster and more economic. However, the CVT growth would introduce a trace of transport agent, with an obvious influence on the carrier density for semiconductor, and would also produce chalcogen vacancies at high temperature. For the CVT-grown SnSe 2 crystal, the suppression of Se vacancies can be achieved by adding excess Se and lowering the temperature gradient. The Se vacancies can be controlled by turning the source beam flux ratio in the MBE growth, while the lattice-matched substrate must be carefully chosen. Therefore, we can conclude the Bridgman growth is the most suitable method for up scaling for the case of SnSe 2 .

We present a systematic study of the DO 22 -structure trialuminide intermetallic alloys using 27 Al NMR spectroscopy. The quadrupole splittings, Knight shifts, and spin-lattice relaxation times on Al 3 Ti, Al 3 V, Al 3 Nb, and Al 3 Ta have been identified. Knight-shift tensors were isolated by observation of quadrupole satellite lines and fitting to the central-transition powder patterns. The results are associated with the local electronic density of states for each crystallographic site. Universally small isotropic Knight shifts and long T 1 's are consistent with low Fermi-surface densities of states indicating the importance of Fermi-surface features for the phase stability of these alloys. Larger anisotropic Knight shifts occurring at aluminum site I indicate strong hybridization at this site, and the electric-field-gradient tensors confirm the strong ab plane bonding configuration. Local-moment magnetism is found in Al 3 V, yet electrically this material appears very similar to the other DO 22 aluminides. ͓S0163-1829͑98͒07112-4͔