甲基铵三碘化铅的光热诱导可逆相变
Photothermally induced, reversible phase transition in methylammonium lead triiodide
摘要：
已知金属卤化物钙钛矿（MHP）在加热时会经历几种结构相变，从较低的对称性到较高的对称性。虽然结构相变已被广泛的光学，热学和电学的方法进行了研究，大多数测量是准静态的，因此不提供直接的信息有关的基本时间尺度的相变在这一类新兴的半导体。在这里，我们调查的正交到四相相变的原型金属卤化物钙钛矿，甲基铵三碘化铅（CH3NH3PbI3或MAPbI3）使用低温纳秒瞬态吸收光谱的时间尺度。通过使用中红外泵浦脉冲加热的材料在略低于相变温度和探测作为延迟时间的函数的瞬态光学响应，我们观察到一个干净的签名的瞬态，可逆的正交到四相相变。发现正向相变在几十纳秒的时间尺度上进行，之后反向相变在与从膜到下面的衬底的热耗散相称的时间尺度上进行。观察到高度的瞬态相变占稳态相变的三分之一。与完全无机的相变材料如VO2相比，MAPbI 3中缓慢相变的数量级可以归因于与有机阳离子和无机框架之间的强氢键相关的大的能垒。我们的方法为解开MHP和其他混合半导体材料的相变动力学铺平了道路。
英文摘要：
Metal halide perovskites (MHPs) are known to undergo several structural phase transitions, from lower to higher symmetry, upon heating. While structural phase transitions have been investigated by a wide range of optical, thermal and electrical methods, most measurements are quasi-static and hence do not provide direct information regarding the fundamental timescale of phase transitions in this emerging class of semiconductors. Here we investigate the timescale of the orthorhombic-to-tetragonal phase transition in the prototypical metal halide perovskite, methylammonium lead triiodide (CH3NH3PbI3 or MAPbI3) using cryogenic nanosecond transient absorption spectroscopy. By using mid-infrared pump pulses to impulsively heat up the material at slightly below the phase-transition temperature and probing the transient optical response as a function of delay time, we observed a clean signature of a transient, reversible orthorhombic-to-tetragonal phase transition. The forward phase transition is found to proceed at tens of nanoseconds timescale, after which a backward phase transition progresses at a timescale commensurate with heat dissipation from the film to the underlying substrate. A high degree of transient phase transition is observed accounting for one third of the steady-state phase transition. In comparison to fully inorganic phase-change materials such as VO2, the orders of magnitude slower phase transition in MAPbI3 can be attributed to the large energy barrier associated with the strong hydrogen bonding between the organic cation and the inorganic framework. Our approach paves the way for unraveling phase transition dynamics in MHPs and other hybrid semiconducting materials.
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