The niobium oxide dihalides have recently been identified as a new class of van der Waals materials exhibiting exceptionally large second-order nonlinear optical responses and robust in-plane ferroelectricity. In contrast to second-order nonlinear processes, third-order optical nonlinearities can arise irrespective of whether a crystal lattice is centrosymmetric. Here, we report third harmonic generation (THG) in two-dimensional (2D) transition metal oxide iodides, namely NbOI2and TaOI2. We observe a comparable THG intensity from both materials. By benchmarking against THG from monolayer WS2, we deduce that the third-order susceptibility is approximately on the same order. THG resonances are revealed at different excitation wavelengths, likely due to enhancement by excitonic states and band edge resonances. The THG intensity increases for material thicknesses up to 30 nm, owing to weak interlayer coupling. After this threshold, it shows saturation or a decrease, due to optical interference effects. Our results establish niobium and tantalum oxide iodides as promising 2D materials for third-order nonlinear optics, with intrinsic in-plane ferroelectricity and thickness-tunable nonlinear efficiency.
Xin Xu,Peigeng Han,Daoyuan Zheng,Kaimin Du,Chunxia Li,Feng Liu,Ruiling Zhang,Keli Han
Recently, lanthanide-doped halide perovskites have attracted increasing attention due to their unique optical properties, such as upconversion (UC) and near-infrared (NIR) emission. However, these emitters suffer from low efficiency, limiting further practical applications. Herein, a Bi 3+ -alloyed double perovskite, Cs 2 NaEr 0.5 Bi 0.5 Cl 6 , with strong NIR-triggered UC and NIR-II (1000–1700 nm) emission is reported. Cs 2 NaEr 0.5 Bi 0.5 Cl 6 exhibits efficient green UC emission with a high quantum yield up to 2.1% and NIR-II emission peaking at 1541 nm with a quantum yield of 37.9% under 980 nm excitation. The strong UC emission is related with the efficient energy transfer between Er 3+ and self-trapped exciton states induced by corporation of Bi 3+ ions. In addition, Bi 3+ ions can act as diluters, suppressing the energy loss caused by undesirable successive energy transfer among Er 3+ ions, enhancing the UC and NIR-II emission. Furthermore, a water-resistant Cs 2 NaEr 0.5 Bi 0.5 Cl 6 /polydimethylsiloxane film is prepared. Excitingly, Cs 2 NaEr 0.5 Bi 0.5 Cl 6 not only shows the potential as a UC thermometry with high sensitivity but also exhibits a high signal to noise ratio in NIR imaging.
Quantum dot light-emitting diodes (QLEDs) exhibit pronounced thermal- and size-dependent charge injection, characterized by significantly enhanced current under elevated temperatures and reduce device areas. In this study, the underlying mechanisms are elucidated by examining the influence of thermal energy on carrier transport dynamics. This analysis reveals that hole transport is governed by a hopping mechanism, leading to markedly increased hole mobility at higher temperatures, which is a primary contributor to the thermal-dependent current enhancement. This improved hole injection promotes better charge balance and increase luminance. Furthermore, both thermal effect and lateral resistance is identified as key contributors to the size-dependent current behavior. Smaller-area QLEDs exhibit reduced lateral resistance and more pronounced thermal effect, resulting in higher current injection. By leveraging these thermally and geometrically enhanced hole injection characteristics, the application of QLEDs is demonstrated as multi-dimensional visual temperature sensors and realize significantly stronger emission from high-energy excitonic state (1P) transitions in small-area devices.
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