Quantum confinement of 2D perovskite nanoplatelets for light emission

术语“钙钛矿”是指具有共同晶体结构和通式ABX3的一族化合物。典型的金属卤化物钙钛矿(MHP)在A位有一价阳离子，在B位有二价金属阳离子，在X位有卤化物阴离子。金属卤化物钙钛矿的晶体结构为BX64-八面体，其中B元素位于中心，卤化物阴离子占据八面体位置。A位的阳离子被笼罩在八面体之间，这对于形成钙钛矿晶体结构是重要的。MHPS具有优异的光电性能，在光电子领域的应用引起了人们极大的研究兴趣。MHP的主要材料之一是卤化铅钙钛矿，即铅(铅)作为B离子。由于它们具有可调的带隙、易于生产和高缺陷容忍度，因此在光伏和发光应用中具有巨大的潜力。基于MHP的光电子器件具有很好的性能，例如在发光二极管(LED)中的外量子效率(EQE)为20.3%，在单结太阳能电池中的记录功率转换效率(PCE)为25.2%。虽然块状钙钛矿薄膜显示出巨大的潜力，但纳米钙钛矿材料往往还表现出其他有趣的特性。为了探索钙钛矿材料更多的潜在应用，人们对纳米钙钛矿的形貌进行了广泛的研究。图4显示了已被合成为纳米立方体(NCS)、纳米小片(NPL)、纳米棒(NRS)以及根据其大小和形状分类的量子点(QD)的钙钛矿型纳米材料。在纳米半导体材料中，量子限域通过精细地控制带隙，对其光电性能起着决定性的作用。在经典量子井模型中，半导体形成了“井”，而外部环境则充当了“墙”。因此，能级被上移到更高的水平。因此，在相同的成分下，钙钛矿光发射体可以通过纳米级的尺寸调节提供不同的发射波长，为目标发射波长提供替代选择。钙钛矿型纳米碳管具有典型的立方形边缘长度为8-15 nm，是目前研究最多的钙钛矿型纳米碳管。CsPbX3 NCS提供尺寸和成分调整，覆盖400-710 nm的发射波长。计算了CsPbBr3材料的激子玻尔半径为3.5 nm。钙钛矿纳米片由几层钙钛矿晶格组成，表现出很强的量子限制效应，发射波长相对于体相钙钛矿有显著的蓝移，最高可达0.6 eV(~100 nm)。二维结构可以由它们自己的分子组成来表示，例如单层Cs2PbBr4和双层CsPb2Br5。二维受限结构带来了激子结合能的增加，并允许通过厚度控制来调节发射。由于较大的接触面积，增强了堆叠NPL之间的电荷转移。由于较大的配体覆盖率，NPL被认为比NCS对环境湿度更稳定[47]。然而，非发光二极管在溶液中的稳定性较差，因为它们容易团聚，其独特的性质受到损害。与纯绿色和红色LED相比，蓝光发光存在寿命短、发光量子效率低、效率低等问题，对LED照明和显示行业来说是一个具有挑战性的课题。蓝色LED是照明和显示应用的基本组成部分，因为三原色以最通用的方式组成白光。

The term "perovskite" refers to a family of compounds with a common crystal structure and the general formula ABX3. Typical metal halide perovskites (MHP) have a monovalent cation at the A site, divalent metal cation at the B site, and halide anion at the X site. The crystal structure of metal halide perovskite is characterised by the BX64- octahedra, where the B element sits at the centre and halide anions occupy the octahedral sites. The A-site cation is caged in between the octahedra, which is important for enabling the perovskite crystal structure to form. MHPs have attracted significant research interest for optoelectronic applications due to their outstanding optoelectronic properties. One of the main MHP materials is lead halide perovskite, i.e., lead (Pb) as B cation. They have great potential in photovoltaic and light emission applications due to a tuneable band gap, facile production, and high defect tolerance. MHP-based optoelectronic devices have promising performances, for example, external quantum efficiency (EQE) of 20.3% in light-emitting diodes (LEDs) and record power conversion efficiency (PCE) of 25.2% in a single junction solar cell.While bulk perovskite films have been showing great potential, nanoscale perovskite materials often exhibit other interesting properties. In order to explore more potential applications of perovskite material, nanoscale perovskite morphologies have been investigated extensively. Figure 4 shows perovskite nanomaterials that have been synthesized as nanocubes (NCs), nanoplatelets (NPLs), nanorods (NRs), as well as quantum dots (QDs) classified according to their size and shape. In nanoscale semiconductor materials, quantum confinement plays a decisive role in the optoelectronic properties by finely manipulating the band gap. The semiconductor forms the "well" while the outer environment serves as the "wall" in the classic quantum well model. Therefore, the energy levels are upshifted to a higher level. As a result, within the same composition, perovskite light emitters can provide different emission wavelengths by nanoscale size tuning, providing alternative choices for targeted emission wavelengths. Perovskite NCs have typical edge length of 8-15 nm in cubic shape, and is the mostly studied form of perovskite material due to well-established protocols. CsPbX3 NCs offers both size and compositional tuning, covering emission wavelengths from 400-710 nm. It is noted that the exciton Bohr radius for CsPbBr3 materials is calculated as 3.5 nm. Perovskite nanoplatelets (NPLs) consist of few-layer perovskite lattices and these NPLs been shown to exhibit strong quantum confinement effect, with a significant blue-shift in emission wavelength up to 0.6 eV (~100 nm) relative to bulk perovskite. 2D structures can be represented by their own molecular compositions, e.g., single layer Cs2PbBr4 and double layer CsPb2Br5. The 2D confined structure brings increased exciton binding energy, and it allows emission tuning by thickness control. Charge transfer between stacked NPLs is enhanced due to large contact area. NPLs are proposed to be more stable than NCs against environmental moisture due to large ligand coverage[47]. However, NPLs tends to be less stable in solution as they agglomerate easily, having its unique properties compromised.Blue emission is a challenging topic for LED lighting and display industry as it suffers from many problems, such as shorter lifetime, lower luminescence quantum yield, and lower efficiency compared with pure green and red LEDs. Blue LED is an essential composition of lighting and display applications as the three primary colour compose white light in the most versatile way.