Persistent Ionic Photo-responses and Frank-Condon Mechanism in Proton-transfer Ferroelectrics

TL;DR

This paper reports on persistent ionic photo-responses in ferroelectric croconic acid caused by proton metastable states, revealing a nuclear quantum mechanism that influences material properties through photoexcitation.

Contribution

It uncovers a novel ionic metastable state mechanism driven by proton transfer and quantum effects, expanding understanding of photo-tunability in ferroelectric materials.

Findings

Structural and ferroelectric responses relax in about 1000 s

Photoconductivity decays within 1 s

Internal bias field persists after polarization switching

Abstract

Photoexcitation is well-known to trigger electronic metastable states and lead to phenomena like long-lived photoluminescence and photoconductivity. In contrast, persistent photo-response due to ionic metastable states is rare. In this work, we report persistent structural and ferroelectric photo-responses due to proton metastable states via a nuclear quantum mechanism in ferroelectric croconic acid, in which the proton-transfer origin of ferroelectricity is important for the ionic metastable states. We show that, after photoexcitation, the changes of structural and ferroelectric properties relax in about 1000 s, while the photoconductivity decays within 1 s, indicating the dominant ionic origin of the responses. The photogenerated internal bias field that survives polarization switching process suggests another proton transfer route and metastable state, in addition to the metastable states resulting from proton transfer along the hydrogen bonds proposed previously. Analysis based on Frank Condon principle reveals the quantum mechanical nature of the proton-transfer process both within the hydrogen bonds and out of the hydrogen bonds, where small mass of proton and significant change of potential landscape due to the excited electronic states are the key. The demonstration of persistent photo-responses due to the proton metastable states unveils a nuclear quantum mechanism for photo-tunability of materials, which is expected to impact many material properties sensitive to ionic positions.