甲基銨溴化鉛單晶鈣鈦礦材料的光物理探討

Abstract

本論文合成出單晶CH3NH3PbBr3鈣鈦礦材料，並且對其進行變溫XRD、變溫螢光光譜、吸收光譜與時間解析光譜的量測，以及藉由拉曼光譜觀察材料之晶格震動，與過去的文獻不同的是本研究中，我們進行了大範圍的溫度量測，以XRD結果為基礎，討論晶相對於光學性質的影響。於XRD的實驗中，可以觀察到材料於145 K與220 K皆有相變的發生，分別是立方相-四方相與四方相-正交相的相變溫度；在光激螢光的實驗中，可以清楚觀察到因相變而導致的光譜變化，有趣的是溫度低於四方相-正交相的相變溫度時，仍然可以觀察到單晶CH3NH3PbBr3鈣鈦礦於四方相的訊號，我們推論其結果是因為相共存的發生。變溫吸收的實驗中我們可以觀察到隨著溫度的下降，螢光訊號會有藍移的現象發生，以及可以看到因為相變而造成的光譜變化；至於螢光放光機制，我們推論是來自載子於能隙間躍遷所放出的螢光訊號，以及載子被晶體表面的溴原子空缺捕捉，並且複合的結果；由1.5 K下量測的磁光光譜，可以觀察到螢光訊號並不會受到外加磁場大小而有所影響，證實我們對螢光訊號來源的推論的正確性。時間解析光譜的量測中，可以觀察到真空的環境中，螢光生命期變得非常的短，我們推論造成生命期的變化原因是在大氣中，晶體將會吸附氧氣分子，而在真空下氧氣分子將會脫附於晶體表面。在拉曼光譜的量測結果可以發現光譜中的拉曼位移會因為鹵素離子的不同而有所差異，此外並沒有觀察到有任何前驅物的拉曼訊號，證實晶體並沒有任何前驅物的殘留。

In this work, we have evaluated the structural, photo-physical characteristics of MAPbBr3 single crystals using temperature dependent XRD, absorption, time-resolved/steady-state photoluminescence, and Raman spectroscopy techniques The perovskite single crystals are synthesized by using inverse temperature crystallization method. In contrast to previous research reports on perovskite materials, we study changes in optical properties accompanied with the phase changes in crystal lattice structures expanded throughout a wide temperature range from 295 K to 1.5 K. Temperature dependent X-ray diffraction studies reveal phase transitions from cubic to tetragonal and from tetragonal to orthorhombic phase at 220 K and 145 K, respectively. Furthermore, measurements of temperature dependent photoluminescence clearly indicate that the observed luminescence peaks from 4.5 K to 295 K can be attributed to the band gap of MAPbX3 single crystals. Interestingly, at a temperature lower than 80 K, the multiple emission peaks are observed and they are attributed to co-exist of two different crystallographic phases, which is also evident in the temperature dependent XRD measurements. The luminescence peaks are asymmetric in line shape with a weak and broad shoulder near the absorption edge. It is resulted from the presence of the Br atoms vacancy on the surface of crystals. Further to demonstrate, we also investigate the time-resolved photoluminescence on the MAPbX3 single crystals. Wherein, the obtained different luminescence lifetimes in vacuum and in air could be due to the desorption/adsorption of oxygen molecules at the crystal surface. Thereafter, Raman spectroscopic measurements are carried out and compared between MAPbBr3 and MAPbCl3 single crystals. Energy shifts in the Raman spectra are observed when the Br atoms were replaced with the Cl atoms. This is due to the strong interplay between the halide ions with the specifically aligned MA groups, particularly in the vibration modes related to the N atoms. It is also well confirmed that there is no spectral signals related to PbBr2 vibronic stretching. Therefore, there should be no PbBr2 residual remained in the crystals. Lastly, we also investigate the ferromagnetic property of the MAPbBr3 crystals by place the crystal at low temperature and in high magnetic field. The photoluminescence spectra are unaffected by the applied magnetic field up to 5 T.