van der Waals heterostructures (vdWHs) of metallic (m-) and semiconducting (s-) transition-metal dichalcogenides (TMDs) exhibit an ideal metal/semiconductor (M/S) contact in a field-effect transistor. However, in the current two-step chemical vapor deposition process, the synthesis of m-TMD on pregrown s-TMD contaminates the van der Waals (vdW) interface and hinders the doping of s-TMD. Here, NbSe 2 /Nb-doped-WSe 2 metal-doped-semiconductor (M/d-S) vdWHs are created via a one-step synthesis approach using a niobium molar ratio-controlled solution-phase precursor. The one-step growth approach synthesizes Nb-doped WSe 2 with a controllable doping concentration and metal/doped-semiconductor vdWHs. The hole carrier concentration can be precisely controlled by controlling the Nb/(W + Nb) molar ratio in the precursor solution from ∼3 × 10 11 /cm 2 at Nb-0% to ∼1.38 × 10 12 /cm 2 at Nb-60%; correspondingly, the contact resistance R C value decreases from 10 888.78 at Nb-0% to 70.60 kΩ.μm at Nb-60%. The Schottky barrier height measurement in the Arrhenius plots of ln(I sat /T 2 ) versus q/K B T demonstrated an ohmic contact in the NbSe 2 /W x Nb 1−x Se 2 vdWHs. Combining p-doping in WSe 2 and M/d-S vdWHs, the mobility (27.24 cm 2 V −1 s −1 ) and on/off ratio (2.2 × 10 7 ) are increased 1238 and 4400 times, respectively, compared to that using the Cr/pure-WSe 2 contact (0.022 cm 2 V −1 s −1 and 5 × 10 3 , respectively). Together, the R C value using the NbSe 2 contact shows 2.46 kΩ.μm, which is ∼29 times lower than that of using a metal contact. This method is expected to guide the synthesis of various M/d-S vdWHs and applications in future high-performance integrated circuits.

Covid-19 lockdowns have improved the ambient air quality across the world via reduced air pollutant levels. This article aims to investigate the effect of the partial lockdown on the main ambient air pollutants and their elemental concentrations bound to PM 2.5 in Hanoi. In addition to the PM 2.5 samples collected at three urban sites in Hanoi, the daily PM 2.5 , NO 2 , O 3 , and SO 2 levels were collected from the automatic ambient air quality monitoring station at Nguyen Van Cu street to analyze the pollution level before (March 10th–March 31st) and during the partial lockdown (April 1st–April 22nd) with “current” data obtained in 2020 and “historical” data obtained in 2014, 2016, and 2017. The results showed that NO 2 , PM 2.5 , O 3 , and SO 2 concentrations obtained from the automatic ambient air quality monitoring station were reduced by 75.8, 55.9, 21.4, and 60.7%, respectively, compared with historical data. Besides, the concentration of PM 2.5 at sampling sites declined by 41.8% during the partial lockdown. Furthermore, there was a drastic negative relationship between the boundary layer height (BLH) and the daily mean PM 2.5 in Hanoi. The concentrations of Cd, Se, As, Sr, Ba, Cu, Mn, Pb, K, Zn, Ca, Al, and Mg during the partial lockdown were lower than those before the partial lockdown. The results of enrichment factor (EF) values and principal component analysis (PCA) concluded that trace elements in PM 2.5 before the partial lockdown were more affected by industrial activities than those during the partial lockdown.

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In this study, selective Nb doping (P-type) at the WS 2 layer in a WS 2 −MoS 2 lateral heterostructure via a chemical vapor deposition (CVD) method using a solution-phase precursor containing W, Mo, and Nb atoms is proposed. The different chemical activity reactivity (MoO 3 > WO 3 > Nb 2 O 5 ) enable the separation of the growth temperature of intrinsic MoS 2 to 700 °C (first grown inner layer) and Nb-doped WS 2 to 800 °C (second grown outer layer). By controlling the Nb/(W+Nb) molar ratio in the solution precursor, the hole carrier density in the p-type WS 2 layer is selectively controlled from approximately 1.87 × 10 7 /cm 2 at 1.5 at.% Nb to approximately 1.16 × 10 13 / cm 2 at 8.1 at.% Nb, while the electron carrier density in n-type MoS 2 shows negligible change with variation of the Nb molar ratio. As a result, the electrical behavior of the WS 2 −MoS 2 heterostructure transforms from the N−N junction (0 at.% Nb) to the P−N junction (4.5 at.% Nb) and the P−N tunnel junction (8.1 at.% Nb). The band-toband tunneling at the P−N tunnel junction (8.1 at.% Nb) is eliminated by applying negative gate bias, resulting in a maximum rectification ratio (10 5 ) and a minimum channel resistance (10 8 Ω). With this optimized photodiode (8.1 at.% Nb at V g = −30 V), an I photo /I dark ratio of 6000 and a detectivity of 1.1 × 10 14 Jones are achieved, which are approximately 20 and 3 times higher, respectively, than the previously reported highest values for CVD-grown transition-metal dichalcogenide P−N junctions.