Understanding ultrafast charge transfer processes in SnS and SnS2: using the core hole clock method to measure attosecond orbital-dependent electron delocalisation in semiconducting layered materials

Abstract

SnS and SnS₂ are earth abundant layered semiconductors that owing to their optoelectronic properties have been proposed as materials for different photovoltaic, photosensing and photocatalytic applications. The intrinsic efficiency of these materials for such applications is driven by their charge transfer dynamics, which in turn depend on their electronic structure and the interaction of the molecular orbitals involved in the charge transfer process. In this publication, we provide a step-by-step description of the use of the core hole clock method to obtain orbital dependent charge transfer times for SnS₂ and SnS down to the attosecond time scale. We use both S 1s and Sn 3d core holes, with natural core hole lifetimes of 1.3 fs and 300 as, as time references to obtain a complete picture of electron delocalisation times across the conduction band of both materials. Ultrafast electron delocalisation times, lower than 5 femtoseconds and as low as tens of attoseconds, are measured for electrons excited to the unoccupied molecular orbitals of both materials. SnS delocalisation times are found to be shorter than those for SnS₂ for all probed molecular orbitals, with delocalisation times being more than an order of magnitude shorter for higher lying molecular orbitals. DFT calculations show that charge transfer dynamics are strongly influenced by the interlayer interaction within these materials. The slower delocalisation of electrons in SnS₂ can be linked to the restricted out of plane spatial dispersion of the molecular orbitals in the conduction band and consequent limitation of the electron delocalisation pathways. The results presented in this paper highlight the high potential of combining the core hole clock method with high-level DFT calculations to study orbital dependent charge transfer in semiconductor materials for optoelectronic applications down to the attosecond scale.