All-inorganic colloidal perovskite quantum dots (QDs) based on cesium, lead, and halide have recently emerged as promising light emitting materials. CsPbBr QDs have also been demonstrated as stable two-photon-pumped lasing medium. However, the reported two photon absorption (TPA) cross sections for these QDs differ by an order of magnitude. Here we present an in-depth study of the TPA properties of CsPbBr QDs with mean size ranging from 4.6 to 11.4 nm. By using femtosecond transient absorption (TA) spectroscopy we found that TPA cross section is proportional to the linear one photon absorption. The TPA cross section follows a power law dependence on QDs size with exponent 3.3 ± 0.2. The empirically obtained power-law dependence suggests that the TPA process through a virtual state populates exciton band states. The revealed power-law dependence and the understanding of TPA process are important for developing high performance nonlinear optical devices based on CsPbBr nanocrystals.

Low-dimensional metal halides have recently attracted extensive attention owing to their unique structure and photoelectric properties.H erein, we report the colloidal synthesis of all-inorganic low-dimensional cesium copper halide nanocrystals (NCs) by adopting ah ot-injection approach.U sing the same reactants and ligands,b ut different reaction temperatures,b oth 1D CsCu 2 I 3 nanorods and 0D Cs 3 Cu 2 I 5 NCs can be prepared. Density functional theory indicates that the reduced dimensionality in 1D CsCu 2 I 3 compared to 0D Cs 3 Cu 2 I 5 makes the excitons more localized, which accounts for the strong emission of 0D Cs 3 Cu 2 I 5 NCs. Subsequent optical characterization reveals that the highly luminescent, strongly Stokes-shifted broadband emission of 0D Cs 3 Cu 2 I 5 NCs arises from the self-trapped excitons.O ur findings not only present am ethod to control the synthesis of low-dimensional cesium copper halide nanocrystals but also highlight the potential of 0D Cs 3 Cu 2 I 5 NCs in optoelectronics. Scheme 1. Colloidal synthesis of cesium copper halide nanocrystals.Supportinginformation and the ORCID identification number for one of the authors of this article can be found under: https://doi.

Hybrid two-dimensional (2D) organic-inorganic perovskites continue to draw increased attention in view of their outstanding performance in optoelectronic devices such as solar cells and light-emitting devices. Herein, for the first time, we report the synthesis and characterization of lead-free, 2D mixed Ge-Sn halide perovskites, (PEA)GeSn I (where PEA = CHCHCHNH), and demonstrate that the bandgaps decrease linearly with increasing Sn content. Most importantly, among them, (PEA)GeSnI possesses the smallest bandgap of 1.95 eV. Density functional theory calculations confirm that Sn substitution induces a smaller bandgap and more dispersed band structure, which are desirable characteristics of light-absorbing materials. In addition, conductivity and stability of (PEA)GeSnI have also been assessed.

All-inorganic zero-dimensional (0D) metal halides have recently received increasing attention due to their excellent photoluminescence (PL) performance and high stability. Herein, we present the successful doping of copper(I) into 0D Cs 2 ZnBr 4. The incorporating of Cu + cations enables the originally weakly luminescent Cs 2 ZnBr 4 to exhibit an efficient blue emission centered at around 465 nm, with a high photoluminescence quantum yield (PLQY) of 65.3 %. Detailed spectral characterizations, including ultrafast transient absorption (TA) techniques, were carried out to investigate the effect of Cu + dopants and the origin of blue emission in Cs 2 ZnBr 4 :Cu. To further study the role of the A-site cation and halogen, A 2 ZnCl 4 :Cu (A = Cs, Rb) were also synthesized and found to generate intense sky-blue emission (PLQY % 73.1 %). This work represents an effective strategy for the development of environmentally friendly, low-cost and high-efficiency blueemitting 0D all-inorganic metal halides. Low-dimensional metal halides have been widely studied as optoelectronic materials in the past two decades. [1] Zerodimensional (0D) metal halides have recently attracted intense interest due to their high photoluminescence quantum yields (PLQYs) and color tunability. [2] In terms of the 0D structure, various metal halide species, including tetrahedral BX 4 , pyramidal BX 5 , and octahedral BX 6 , have been reported to form highly crystalline materials. [3] Inspiringly, green, yellow and red luminescent 0D metal halides with high PLQYs have been realized in Cu-, Sn-, Sb-and In-based halide single crystals. [4] Nevertheless, blue-emitting 0D metal halides with high efficiency and stability remain challenging. Recently, several lead-free all-inorganic 0D metal halides with high efficient deep-blue emission in short wavelength blue light region (< 460 nm) have been reported. [5] However, all-inorganic 0D metal halides with intense emission in pureblue spectral region (460-480 nm), [6] which are more desirable for display and solid-state lighting, [7] remain largely unexplored. Therefore, it is of great significance to develop environmentally friendly, stable and high-efficiency 0D metal halides with PL peaks located at pure-blue region. Recent studies have demonstrated that doping is an effective strategy for the preparation of highly luminescent and stable metal halides. [8] Among the various alternatives, first row transition metals are of great interest as the majority are inexpensive, earth abundant, relatively nontoxic, and tend to form low-dimensional metal halides. [9] In this work, we chose Cs 2 ZnBr 4 as host and Cu + as the dopant for the 0D all-inorganic, stable, and efficient blueemitting metal halides. By introduction Cu + into the weakly luminescent Cs 2 ZnBr 4 (PLQY % 3.6 %, PL peak % 465 nm), an unprecedented improvement of PLQY (% 65.3 %) was realized without affecting the emission peak position. Detailed spectral characterizations including ultrafast transient absorption (TA) techniques reveal that the bright...

A lead-free double perovskite, (CH3NH3)2AgBiI6, which is rather stable, was investigated using a combination of experiment and density functional theory.

Low‐dimensional metal halides have recently attracted extensive attention owing to their unique structure and photoelectric properties. Herein, we report the colloidal synthesis of all‐inorganic low‐dimensional cesium copper halide nanocrystals (NCs) by adopting a hot‐injection approach. Using the same reactants and ligands, but different reaction temperatures, both 1D CsCu2I3 nanorods and 0D Cs3Cu2I5 NCs can be prepared. Density functional theory indicates that the reduced dimensionality in 1D CsCu2I3 compared to 0D Cs3Cu2I5 makes the excitons more localized, which accounts for the strong emission of 0D Cs3Cu2I5 NCs. Subsequent optical characterization reveals that the highly luminescent, strongly Stokes‐shifted broadband emission of 0D Cs3Cu2I5 NCs arises from the self‐trapped excitons. Our findings not only present a method to control the synthesis of low‐dimensional cesium copper halide nanocrystals but also highlight the potential of 0D Cs3Cu2I5 NCs in optoelectronics.

Recently, two-dimensional organic-inorganic perovskites have attracted increasing attention due to their unique photophysical properties and high stability. Here we report a lead-free, two-dimensional perovskite, (PEA)GeI (PEA = CH(CH)NH). Structural characterization demonstrated that this 2D perovskite structure is formed with inorganic germanium iodide planes separated by organic PEAI layers. (PEA)GeI has a direct band gap of 2.12 eV, in agreement with 2.17 eV obtained by density functional theory (DFT) calculations, implying that it is suitable for a tandem solar cell. (PEA)GeI luminesces at room-temperature with a moderate lifetime, exhibiting good potential for photovoltaic applications. In addition, 2D (PEA)GeI is more stable than 3D CHNHGeI in air, owing to the presence of a hydrophobic organic long chain. This work provides a direction for the development of 2D Ge-based perovskites with potential for photovoltaic applications.

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