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van der Waals two-dimensional layered heterostructures have recently emerged as a platform, where the interlayer couplings give rise to interesting physics and multifunctionalities in optoelectronics. Such couplings can be rationally controlled by dielectric, separation, and stacking angles, which affect the overall charge or energy-transfer processes, and emergent potential landscape for twistronics. Herein, we report the efficient Förster resonance energy transfer (FRET) in WS2/hBN/MoSe2 heterostructure, probed by both steady-state and time-resolved optical spectroscopy. We clarified the evolution behavior of the electron-hole pairs and free electrons from the trions, that is, ∼59.9% of the electron-hole pairs could transfer into MoSe2 by FRET channels (∼38 ps) while the free electrons accumulate at the WS2/hBN interface to photogate MoSe2. This study presents a clear picture of the FRET process in two-dimensional transition-metal dichalcogenides' heterojunctions, which establishes the scientific foundation for developing the related heterojunction optoelectronic devices. Ministry of Education (MOE) National Research Foundation (NRF) Accepted version Q.X. gratefully acknowledges the Singapore Ministry of Education Tier3 Programme “Geometrical Quantum Materials” (MOE2018-T3-1-002), AcRF Tier2 grant (MOE2017-T2-1-040), and Tier1 grant (RG 194/17). Q.X. also acknowledges strong support from Singapore National Research Foundation Competitive Research Programme “Integrated On-chip Planar Coherent Light Sources” (NRF-CRP-21-2018-0007), and National Research Foundation-Agence Nationale de la Recherche (NRF-ANR) Grant (NRF2017-NRF-ANR005 2DCHIRAL). K.W. and T.T. acknowledge support from the Elemental Strategy Initiative conducted by the MEXT, Japan, Grant Number JPMXP0112101001, JSPS KAKENHI Grant Numbers JP20H00354 and the CREST(JPMJCR15F3), JST. Author Contributions: Q.X. supervised the research. Z. H. conceived the idea. Z.H. and X.L. prepared the heterostructures. P.H.M. and H.V.D. performed the numerical simulation. Z.H., X.L., and M.R.A. performed the micro-spectroscopy experiments. K.W. and T.T. provided the h-BN bulk crystals. Z.H., X.L., and Q.X. analyzed the data. Z.H. wrote the manuscript with input from all authors.

Lead-halide perovskites have been attracting attention for potential use in solid-state lighting. Following the footsteps of solar cells, the field of perovskite light-emitting diodes (PeLEDs) has been growing rapidly. Their application prospects in lighting, however, remain still uncertain due to a variety of shortcomings in device performance including their limited levels of luminous efficiency achievable thus far. Here we show high-efficiency PeLEDs based on colloidal perovskite nanocrystals (PeNCs) synthesized at room temperature possessing dominant first-order excitonic radiation (enabling a photoluminescence quantum yield of 71% in solid film), unlike in the case of bulk perovskites with slow electron-hole bimolecular radiative recombination (a second-order process). In these PeLEDs, by reaching charge balance in the recombination zone, we find that the Auger nonradiative recombination, with its significant role in emission quenching, is effectively suppressed in low driving current density range. In consequence, these devices reach a maximum external quantum efficiency of 12.9% and a power efficiency of 30.3 lm W-1 at luminance levels above 1000 cd m-2 as required for various applications. These findings suggest that, with feasible levels of device performance, the PeNCs hold great promise for their use in LED lighting and displays. This research is supported by the National Research Foundation, Prime Minister’s Office, Singapore under its competitive Research Programme (CRP Award No. NRF-CRP14-2014-03). H.V.D. gratefully acknowledges the financial support from NRF Investigatorship Grant No. NRF-NRFI2016-08 and additional support from TUBA. Q.H.X. acknowledges financial support from Singapore National Foundation via an Investigatorship Award (NRF-NRFI2015-03) and Singapore Ministry of Education through grants (MOE2015-T2-1-047 and MOE2015-T1-001-175). G.C.X. acknowledges financial support from the Science and Technology Development Fund from Macau SAR (FDCT-116/2016/A3, FDCT-091/2017/A2), Start-up Research Grant (SRG2016-00087-FST) from Research & Development Office at University of Macau, and the Natural Science Foundation of China (91733302, 61605073, 2015CB932200).

Public attention to the food scandals raises an urgent need to develop effective and reliable methods to detect food contaminants. The current prevailing detections are primarily based upon liquid chromatogra- phy, mass spectroscopy, or colorimetric methods, which usually require sophisticated and time-consuming steps or sample preparation. Herein, we develop a facile strategy to assemble the vertically aligned monolayer of Au nanorods with a nominal 0.8 nm gap distance and demonstrate their applications in the rapid detection of plasticizers and melamine contamina- tion at femtomolar level by surface-enhanced Raman scattering spectros- copy (SERS). The SERS signals of plasticizers are sensitive down to 0.9 fM concentrations in orange juices. It is the lowest detection limit reported to date, which is 7 orders of magnitude lower than the standard of United States (6 ppb). The highly organized vertical arrays generate the reproducible "SERS-active sites" and can be achieved on arbitrary substrates, ranging from silicon, gallium nitride, glass to flexible poly(ethylene naphthalate) substrates.