No abstract

Black phosphorus (BP) has been recently rediscovered as an elemental two-dimensional (2D) material that shows promising results for next generation electronics and optoelectronics because of its intrinsically superior carrier mobility and small direct band gap. In various 2D field-effect transistors (FETs), the choice of metal contacts is vital to the device performance, and it is a major challenge to reach ultralow contact resistances for highly scaled 2D FETs. Here, we experimentally show the effect of a work function tunable metal contact on the device performance of BP FETs. Using palladium (Pd) as the contact material, we employed the reaction between Pd and H to form a Pd-H alloy that effectively increased the work function of Pd and reduced the Schottky barrier height (Φ) in a BP FET. When the Pd-contacted BP FET was exposed to 5% hydrogen concentrated Ar, the contact resistance (R) improved between the Pd electrodes and BP from ∼7.10 to ∼1.05 Ω·mm, surpassing all previously reported contact resistances in the literature for BP FETs. Additionally, with exposure to 5% hydrogen, the transconductance of the Pd-contacted BP FET was doubled. The results shown in this study illustrate the significance of choosing the right contact material for high-performance BP FETs in order to realize the real prospect of BP in electronic applications.

Single-ion detection has, for many years, been the domain of large devices such as the Geiger counter, and studies on interactions of ionized gasses with materials have been limited to large systems. To date, there have been no reports on single gaseous ion interaction with microelectronic devices, and single neutral atom detection techniques have shown only small, barely detectable responses. Here we report the observation of single gaseous ion adsorption on individual carbon nanotubes (CNTs), which, because of the severely restricted one-dimensional current path, experience discrete, quantized resistance increases of over two orders of magnitude. Only positive ions cause changes, by the mechanism of ion potential-induced carrier depletion, which is supported by density functional and Landauer transport theory. Our observations reveal a new single-ion/CNT heterostructure with novel electronic properties, and demonstrate that as electronics are ultimately scaled towards the one-dimensional limit, atomic-scale effects become increasingly important.

We report plasmonic enhancement of photocatalysis by depositing 5 nm Au nanoislands onto tungsten diselenide (WSe2) monolayer films. Under 532 nm wavelength illumination, the bare WSe2 film produces a relatively small photocurrent (20 nA). With the addition of Au nanoparticles, we observe enhancements of up to 7× (0.14 μA) in the measured photocurrent. Despite these relatively small photocurrents, it is remarkable that adequate charge separating fields are generated over just 7.3 Å of material. Here, the improvement in the photocatalytic performance is caused by the local electric field enhancement produced in the monolayer WSe2 monolayer by the plasmonic Au nanoislands, as verified by electromagnetic simulations using the finite different time domain (FDTD) method. The near-field optical enhancement increases the electron–hole pair generation rate at the surface of WSe2, thus, increasing the amount of photogenerated charge contributing to photoelectrochemical reactions. Despite reducing the effective surface area of WSe2 in contact with the electrolytic solution by 70%, the plasmonic nanoislands couple the incident light very effectively from the far field to the near field in the plane of the monolayer WSe2, thereby improving the overall photoconversion efficiency from 3.5% to 24.7%.

We report photoluminescence (PL) spectroscopy of air-suspended and substrate-supported molybdenum disulfide (MoS 2) taken before and after exposure to proton radiation. For 2-, 3-, and 4-layer MoS 2 , the radiation causes a substantial (>10Â) suppression of the indirect bandgap emission, likely due to a radiation-induced decoupling of the layers. For all samples measured (including the monolayer), we see the emergence of a defect-induced shoulder peak at around 1.7 eV, which is redshifted from the main direct bandgap emission at 1.85 eV. Here, defects induced by the radiation trap the excitons and cause them to be redshifted from the main direct band emission. After annealing, the defect-induced sideband disappears, but the indirect band emission remains suppressed, indicating a permanent transition into a direct bandgap material. While suspended 2-, 3-, and 4-layer MoS 2 show no change in the intensity of the direct band emission after radiation exposure, substrate-supported MoS 2 exhibits an approximately 2-fold increase in the direct bandgap emission after irradiation. Suspended monolayer MoS 2 shows a 2-3Â drop in PL intensity; however, substrate-supported monolayer MoS 2 shows a 2-fold increase in the direct band emission.