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Monolayer MoS 2 has long been considered as the most promising candidate for wearable photovoltaic devices. However, its photovoltaic efficiency is restricted by its large band gap (2.0 eV). Though the band gap can be reduced by increasing the number of layers, the indirect band gap nature of the resulting multilayer MoS 2 is unfavorable. Herein, we report a theoretical discovery of the hitherto unknown symmetry-broken phase (denoted as 1T d ) of monolayer MoS 2 through a swarm structure search. The 1T d phase has a distorted octahedral coordinated pattern of Mo, and its direct band gap of 1.27 eV approaches the optimal value of 1.34 eV that gives the Shockley–Queisser limit for photovoltaic efficiency. Importantly, the direct band gap nature persists in thin films with multilayers owing to extremely weak vdW forces between adjacent 1T d layers. The theoretical photovoltaic efficiency at 30 nm thickness reaches ∼33.3%, which is the highest conversion efficiency among all the thin-film solar cell absorbers known thus far. Furthermore, several feasible strategies including appropriate electron injection and annealing methods were proposed to synthesize the 1T d phase. Once synthesized, the superior photovoltaic properties of the 1T d phase may lead to the development of an entirely new line of research for transition metal dichalcogenide solar cells. 

Abstract. Mixing ratios of volatile organic compounds (VOCs) were recordedin two field campaigns in central Beijing as part of the Air Pollution andHuman Health in a Chinese Megacity (APHH) project. These data were used tocalculate, for the first time in Beijing, the surface–atmosphere fluxes ofVOCs using eddy covariance, giving a top-down estimation of VOC emissionsfrom a central area of the city. The results were then used to evaluate theaccuracy of the Multi-resolution Emission Inventory for China (MEIC). TheAPHH winter and summer campaigns took place in November and December 2016and May and June 2017, respectively. The largest VOC fluxes observed were ofsmall oxygenated compounds such as methanol, ethanol + formic acid andacetaldehyde, with average emission rates of 8.31 ± 8.5, 3.97 ± 3.9 and 1.83 ± 2.0 nmol m−2 s−1, respectively, in the summer.A large flux of isoprene was observed in the summer, with an average emissionrate of 5.31 ± 7.7 nmol m−2 s−1. While oxygenated VOCs madeup 60 % of the molar VOC flux measured, when fluxes were scaled by ozoneformation potential and peroxyacyl nitrate (PAN) formation potential thehigh reactivity of isoprene and monoterpenes meant that these speciesrepresented 30 % and 28 % of the flux contribution to ozone and PANformation potential, respectively. Comparison of measured fluxes with theemission inventory showed that the inventory failed to capture the magnitudeof VOC emissions at the local scale.