Atomically thin PtSe 2 films have attracted extensive research interests for potential applications in high-speed electronics, spintronics and photodetectors. Obtaining high quality, single crystalline thin films with large size is critical. Here we report the first successful layer-by-layer growth of high quality PtSe 2 films by molecular beam † The authors declare no competing financial interest. 1 arXiv:1703.04279v2 [cond-mat.mtrl-sci] 15 Mar 2017 epitaxy. Atomically thin films from 1 ML to 22 ML have been grown and characterized by low-energy electron diffraction, Raman spectroscopy and X-ray photoemission spectroscopy. Moreover, a systematic thickness dependent study of the electronic structure is revealed by angle-resolved photoemission spectroscopy (ARPES), and helical spin texture is revealed by spin-ARPES. Our work provides new opportunities for growing large size single crystalline films for investigating the physical properties and potential applications of PtSe 2 .
KeywordsPtSe 2 , Molecular beam epitaxy (MBE), Raman, ARPES, Transition metal dichalcogenide (TMDC) Layered transition metal dichalcogenides (TMDCs) have attracted extensive interests for applications in electronics, optoelectronics and valleytronics due to the strong spin-orbit coupling, sizable band gap and tunability of the electronic structure by quantum confinement effect. [1][2][3][4] In the past decade, this has been witnessed by the significant efforts conducted on the atomically thin MoS 2 film. 5-7 However, its low mobility has limited applications, for inbstance, in high speed electronics. 8,9 Finding thin films of other TMDC with better properties is highly desirable. PtSe 2 has emerged as an interesting compound that belongs to TMDC.Although the bulk crystal is a semimetal, 10,11 monolayer (ML) platinum diselenide (PtSe 2 ) has been revealed to be a semiconductor with a band gap of ≈ 1.2 eV. 12 Importantly, the charge-carrier mobility of PtSe 2 has been predicted among the highest in TMDCs 9 and has been experimentally shown to be comparable to black phosphorene 13 yet with the advantage of much improved stability. 14 This makes PtSe 2 a promising candidate for high-speed electronics. Moreover, the hidden helical spin texture with spin-layer locking in monolayer PtSe 2 has been recently revealed, 15 and such spin physics induced by a local Rashba effect has great potential for electric field tunable spintronic devices. 16 In addition, remarkable performance
The nutrient ecological stoichiometry of plants and soil is important for the growth and dynamics of species, but the stoichiometric relationships among leaf, litter, and soil remain poorly understood. We analyzed the carbon (C), nitrogen (N), and phosphorus (P) stoichiometry of the leaves, litter, and soil for 31 species at 140 sites in warm temperate forests in northwestern China to document the patterns of nutrient traits and their relationships with climatic factors. The average concentrations of C, N, and P in the combined forests were 462.97, 18.04, and 1.32 g kg−1 for leaves, 365.12, 12.34, and 0.87 g kg−1 for litter, and 15.72, 1.29, and 0.54 g kg−1 for soil, respectively. The concentrations differed significantly among the leaves, litter, and soil. Leaf and soil nutrients were not significantly correlated, whereas leaf and litter nutrients and litter and soil nutrients were significantly correlated, indicating that litter provided a link between leaves and soil and demonstrating the nutrient associations among leaves, litter, and soil. Soil nutrients were strongly correlated with climatic factors, and precipitation had a larger impact than temperature on the plants and soil. This study will help to predict the growth and dynamics of species under environmental changes.
Two-dimensional (2D) semiconductors are promising in channel length scaling of field-effect transistors (FETs) due to their excellent gate electrostatics. However, scaling of their contact length still remains a significant challenge because of the sharply raised contact resistance and the deteriorated metal conductivity at nanoscale. Here, we construct a 1D semimetal-2D semiconductor contact by employing single-walled carbon nanotube electrodes, which can push the contact length into the sub-2 nm region. Such 1D–2D heterostructures exhibit smaller van der Waals gaps than the 2D–2D ones, while the Schottky barrier height can be effectively tuned via gate potential to achieve Ohmic contact. We propose a longitudinal transmission line model for analyzing the potential and current distribution of devices in short contact limit, and use it to extract the 1D–2D contact resistivity which is as low as 10−6 Ω·cm2 for the ultra-short contacts. We further demonstrate that the semimetal nanotubes with gate-tunable work function could form good contacts to various 2D semiconductors including MoS2, WS2 and WSe2. The study on 1D semimetal contact provides a basis for further miniaturization of nanoelectronics in the future.
Low-dimensional
semiconductors have shown great potential in switches
for their atomically thin geometries and unique properties. It is
significant to achieve new tunneling transistors by the efficient
stacking methodology with low-dimensional building blocks. Here, we
report a one-dimensional (1D)–two-dimensional (2D) mixed-dimensional
van der Waals (vdW) heterostructure, which was efficiently fabricated
by stacking an individual semiconducting carbon nanotube (CNT) and
2D MoS2. The CNT–MoS2 heterostructure
shows specific reconfigurable electrical transport behaviors and can
be set as a nn junction, pn diode, and band-to-band tunneling (BTBT)
transistor by gate voltage. The transport properties, especially BTBT,
could be attributed to the electron transfer from MoS2 to
CNT through the ideal vdW interface and the 1D nature of the CNT.
The progress suggests a new solution for tunneling transistors by
making 1D–2D heterostructures from the rich library of low-dimensional
nanomaterials. Furthermore, the reconfigurable functions and nanoscaled
junction show that it is prospective to apply CNT–MoS2 heterostructures in future nanoelectronics and nano-optoelectronics.
This work describes the design and fabrication of free-standing carbon nanotube-palladium (CNT-Pd) composite sheets for hydrogen gas sensing. The CNT-Pd composites were made by electroplating palladium onto a solvent-densified and oxygen plasma-treated CNT sheet. The latter was prepared using high purity CNTs drawn from a dense, vertically aligned array grown by chemical vapor deposition on silicon substrates. The CNT-Pd sheets were characterized by energy-dispersive spectroscopy, scanning electron microscopy, and X-ray diffraction. The amount of palladium in the composite was 16.5 wt % as measured via thermogravimetric analysis. Thin strips of the CNT-Pd sheets were assembled as chemiresistor sensors and tested for hydrogen gas detection. The sensors demonstrated a limit of detection of 0.1 mol % and displayed signal reversibility without the need for oxygen removal or heat treatment. A decrease in signal reversibility was observed after multiple exposure cycles; however, redensification with ethanol significantly restored the original reversibility. The sensor showed the Freundlich adsorption isotherm behavior when exposed to hydrogen. The material's potential application toward a wearable, flexible sensor was demonstrated by integrating the chemiresistor onto a fabric material using hot-press processing and testing the composite for hydrogen sensitivity.
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