You are here

FIP Seminar Student Speaker Awardee: Reversible crystal-glass switching and underlying transformation kinetics in hybrid perovskites

Mar 1

Wednesday, March 1, 2023 - 12:00pm to 1:00pm

Wilkinson 132

Add to calendar »


Akash Singh, PhD Candidate, MEMS, Duke University

Metal halide perovskites (MHPs) are the widely celebrated family of crystalline semiconductors that has led to momentous advancement in the fields of photovoltaics, emitters, and sensors. Though most of the research is condensed towards studying the crystalline MHPs, the study of MHPs that can access a glassy/amorphous state could provide unique opportunities to extend the associated structure-property relationships and broaden their application space. Despite significant efforts to amorphize MHPs under high pressure, an immediate reversal to a crystalline state upon pressure removal has so far impeded the study of the glassy MHP state and the associated practical application, thus necessitating alternative routes. Further, the ability to melt-quench hybrid MHPs has largely been limited due to (i) significant decomposition of the organic component upon heating above 200ºC and (ii) super-fast ordering kinetics of these melts. Herein, we demonstrate the first example of glass formation at ambient pressure and reversible switching between glassy and crystalline states in hybrid MHP. Drawing inspiration from structure-property studies of chiral vs. racemic organic systems, we exploit bulky aromatic chiral organic cations to modify the bonding and packing characteristics and thereby enable an exceptionally low melting temperature (Tm = 175ºC) below the degradation point (Td ≈ 205ºC), as well as slow ordering kinetics in the exemplary 2D S-(-)-1-(1-naphthyl)ethylammonium lead bromide chiral MHP.1 These factors provide facile access to a stable glassy state of the said chiral MHP by melt-quenching both in thin-film and monolith configurations. Furthermore, the components of the melt can recrystallize (at Tx ≈ 100°C) upon heating above the glass-transition temperature (Tg ≈ 67°C), allowing reversible switching between the glassy and crystalline states with characteristic temperatures obeying the typical sequence for phase change materials, i.e., Tg < Tx < Tm < Td (Fig. 1).1 In addition, the underlying kinetic effect of glass crystallization is studied using iterative calorimetric experiments and numerical modelling techniques, such as the Ligero and Kissinger kinetic models, to extract the activation energy (~350 kJ/mol) and Avrami parameter (~2), which suggests a heterogeneous surface-mediated nucleation mechanism with 2D crystal growth, as also corroborated by in-situ and ex-situ microscopy (Fig. 2).


Burns, August