ICE Issue 9

8 Scientific Article The Israel Chemist and Chemical Engineer Issue 9 · January 2023 · Tevet 5783 present LFRS as a handy tool to identify and characterize phase transformations within such highly sensitive materials. Our preliminary work showed that the LFR modes are dependent on the crystallinity of the analyte material. It was observed that even in-situ changes in the material matrix upon laser excitation can successfully be captured with the right optical set-up and laser power. This was observed in examination of MaPbI3 single crystals. It is well-known from the literature that MAPbI3 exhibits different stable crystalline phases at different temperatures. At temperatures below 164 K it is orthorhombic, between 165-330 K tetragonal and at higher temperatures cubic. Additionally, there is a general agreement within the literature on thermally induced phase transitions in MAPbI3 [8]. In one of our earlier works, we presented a photoinduced phase transition [9]. A mere 2-minute excitation of MAPbI3 single crystals in the tetragonal phase with a 532 nm laser above 15 mW power resulted in the formation of cubic phase at ambient conditions and this phase transformation was confirmed using the photoluminescence (PL) shift as shown in Figure 1. Such dynamic phase transitions are hard to capture in conventional crystallographic techniques such as XRD. However, LFRS makes a good candidate for such thermal and photo-induced phase transition observations. When lower laser power can be used for characterizing the former, sufficiently high laser power facilitates both phase transition and detection. In addition to these unique characteristics of phase identification and phase transition detection, the study also revealed some interesting characteristics of the material such as Raman stimulation of iodine vapor signals in addition to stimulated spontaneous Raman signals. The overlap of the excitation laser with the electronic absorption band of the material results in resonance Raman scattering (RRS). This overlap results in a higher scattering intensity compared to the fundamental spontaneous Raman bands and in many cases leads to appearance of overtone bands [10]. On the other hand, stimulated Raman scattering (SRS) is a third-order nonlinear process that exhibits narrow-line emission from existing Raman shifts. In a regular experimental framework, this process is induced using two synchronized pulsed lasers as single-frequency excitation sources, or a narrowband source synchronized with a broadband source for multiplex excitation [11]. Certain materials under certain specific conditions self-induce SRS when subjected to a high enough laser power, even when the exciting laser is continuous wave (CW). This self-stimulated phenomenon is generally referred to as impulsive stimulated Raman scattering, or cascaded Raman process. Self-induced SRS is extensively investigated for ionic crystals, and it is understood that non-linear Raman gain is governed mainly by large crystal size and ionic radius of the cation; both of which are true for the MAPbI3 crystals used for this study [12]. Figure 2 represents the SRS from the resonant vibrational modes when the excitation laser power exceeds 15 mW. Yaakov R. Tischler leads the Device Spectroscopy Laboratory (DSL) at Bar-Ilan’s Institute for Nanotechnology and Advanced Materials. The lab is focused on studying and tailoring light-matter interactions in nanoscale devices and nanostructured materials. This involves research on microcavity polaritonic devices, organicbased lasers, near-field scanningmicroscopy, and applied vibrational spectroscopy. One of themain thrusts of DSL is Raman spectroscopy, which includes development of new spectroscopic techniques and applications thereof. Yaakov opened DSL 12.5 years ago and personally holds 13 US Patents. His former students and post-docs have gone on to make an impact in government, academic and high-tech sectors, particularly in the semiconductor and photonics industries as well as in start-ups. Figure 2. SRS from the resonant vibrational mode when laser power crosses 15 mW (green plot), and RRS obtained from high laser power excitation of PbI2(s) (pink plot).§ § Figure 1 and Figure 2 are reprinted with permission under © license 5338940298014.