Optical Tuning of Low-Dimensional Materials under High Pressure
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摘要: 压力作为独立于物质温度和组分的热力学参量，为物质科学的研究和创新提供了新的维度，已成为发展新概念、创造新理论及探索新材料的重要源泉。本文主要概述了作者近年来在高压下低维材料的光学特性调控方面所取得的一些进展。通过压力改变激子结合能和卤素八面体的扭曲行为，实现了低维卤化物钙钛矿纳米材料发光从“0”到“1”的突破，提出了压力诱导发光的概念；通过引入压力效应，利用压力对纳米材料表面配体的调控，改变了表面配体与CdSe量子点的相互作用和能级耦合，促进了Hirshfeld电荷转移，从而实现了CdSe量子点的荧光大幅度增强近一个数量级；借助高压手段调控能带结构，成功实现了CdSe/CdS半导体纳米晶由准Ⅱ型核壳结构向Ⅰ型核壳结构的构型转变。上述工作加深了对发光材料在极端压缩条件下构效关系的深入理解和认识，研究成果为设计和制备具有特定功能的低维材料提供了新方法。Abstract: As a thermodynamic parameter, independent of temperature and composition, pressure provides a new dimension for material science research and innovation. Pressure has become an important source for developing new concepts, creating new theories and exploring new materials. Here, some advances in optical properties regulation of low-dimensional materials under high pressure are summarized. By changing the exciton binding energy and the distortion behavior of halide octahedra under pressure, the luminescence of low-dimensional halide perovskites experienced a stark change from "0" to "1". Meanwhile, we innovatively put forward the new concept of "pressure-induced emission (PIE)". Through introducing the pressure effect, it is able to regulate the surface ligands of nanomaterials, change the interaction and energy level coupling between the surface ligands and CdSe quantum dots. This will promote the Hirshfeld charge transfer, thus realizing the significant emission enhancement of CdSe quantum dots by nearly one order of magnitude. With the help of high-pressure regulation on energy band structure, we successfully achieved the core/shell configuration transition of CdSe/CdS semiconductor nanocrystals from quasi-type Ⅱ to type Ⅰ core-shell structure. The above work will deepen the understanding of the structure-property relationship of luminescent materials under extreme compression conditions. The research results provide new methods for the design and preparation of low-dimensional materials with specific functionality.
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图 1 不同压力下Cs4PbBr6纳米晶的荧光光谱演化（a）～（c）及发光的CIE色标（d）
Figure 1. Changes in the photoluminescence spectra of Cs4PbBr6 nanocrystals under pressure （a）–（c） and pressure-dependent chromaticity coordinates of the emissions （d）[8]
图 2 Cs4PbBr6纳米晶与激子自陷相关的压力诱导发光机制[8]：常压(a)和4 GPa (b)条件下Cs4PbBr6纳米晶发光的位形坐标模型（A到B为光激发后的吸收跃迁，C到D表示自陷态激子复合发射，B和C态之间的红色和绿色路径分别表示激子自陷和去自陷。Edetrap为去自陷能，Eex、Eex1和Eex2为激发态，ST为激子自陷态，G为基态，S1/2为黄昆因子反应电子-声子耦合强度。）
Figure 2. Pressure-induced emission mechanism associated exciton self-trapping in Cs4PbBr6 nanocrystals[8]：configuration coordinate model of emission for the Cs4PbBr6 nanocrystals at 0 GPa (a) and 4 GPa (b)(Herein, the absorption transition upon excitation from A to B is described. The STE recombination emission is depicted from C to D. The path between B and C refers to exciton self-trapping (red) and detrapping (green). Edetrap is activation energy for detrapping. Eex, Eex1 and Eex2 are splitting of the bound exciton state. ST，G and S1/2 are self-trapped state, ground state and Huang-Rhys parameter，respectively.)
图 4 高压下4.8 nm（a）、2.5 nm（b）和2.0 nm（c） CdSe纳米晶的发射光谱演化（覆盖了红光到蓝光范围，每个图片中右侧插图表示金刚石对顶砧中的光学照片），不同尺寸CdSe纳米晶随压力变化的色度变化（d）[13]
Figure 4. Photoluminescence spectra of different sized CdSe NCs 4.8 nm(a) , 2.5 nm (b) and 2.0 nm (c) with increasing pressure, covering the color range from red to blue (right hand insets in each figure show micrographs of theDAC interior)；color gamut changes of the different sized CdSe NCs with increasing pressure (d)[13]
图 5 第一性原理计算[13]：(a) 常压和高压下CdSe纳米晶和油酸分子的分波态密度（费米能级EF位于0 eV处）；(b)缺陷态D、油酸分子的LUMO能级L和新杂化能级N的电荷密度分布（电荷密度分布的等值面为0.0005 e/Å3。球棍模型中，粉球和绿球分别代表Cd原子和Se原子，棕色链表示油酸分子）；(c) 高压下油酸分子转移到CdSe纳米晶的Hirshfeld电荷变化，hv表示光辐射
Figure 5. First-principles calculations[13]：(a) partial density of states of CdSe and OA at ambience and high pressure (An energy of 0 eV indicates the position of the Fermi level (EF = 0 eV))；(b) electron density distributions of the trap states D, LUMO level L of OA, and new hybridized states N, which are labeled in partial density of states (The isosurface of electron distribution is 0.0005 e/Å3. Therein, the Cd and Se atoms are denoted as pink and green, respectively, in the ball-and-stick models. The brown chain represents the OA molecule.)；(c) Hirshfeld charge transferred from OA to CdSe NCs vs. the distance between them when the applied pressure was increased to 7.5 GPa in computations, hv represents the light irradiation
图 6 不同压力下具有3个原子壳层厚度CdSe/CdS核壳纳米晶的荧光寿命曲线（a）和平均寿命演化（b）；高压下CdSe/CdS纳米晶荧光衰减曲线拟合得到的单激子相关慢成分寿命 τ slow 和多激子相关快成分寿命τ fast的变化(c)；高压下CdSe/CdS核壳纳米晶中慢成分（单激子）寿命和快成分（多激子）寿命随压力变化的对比(d)[15]
Figure 6. Pressure-dependent photoluminescence decay curves (a) and average lifetimes (b) of CdSe/CdS NCs with a 3 ML CdS shell thickness；pressure dependence of the lifetime of single-exciton τslow and multiexciton τfast, two of which from fits to the decay curves of CdSe/CdS nanocrystals (c)；pressure dependence of comparison between the lifetimes of single-exciton τslow and multiexciton τfast (d)[15]
图 7 CdSe/CdS核壳纳米晶在常压（a）、随着壳生长（b）以及外加压力作用下（c）的电子能级结构示意图(红线表示在应力或压力下的能级结构，虚线为本征能带排列)[15]
Figure 7. Schematic illustration of electronic energy levels for CdSe/CdS core-shell nanocrystals (a) under ambient conditions, (b) with shell growth, and (c) under external pressure (The red lines depict the results under strain or pressure, and dashed lines are the intrinsic band alignments)[15]
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