Exciton‐Dominated Ultrafast Optical Response in Atomically Thin PtSe₂

Abstract

semiconducting systems. [1,2] A representative example is layered 2D semiconductors, where reduced dielectric screening leads to huge exciton binding energies up to several hundreds of meV, enabling strong excitonic light-matter interactions even at room temperature. Thus, excitons in 2D materials govern fundamental optical properties such as light absorption and emission. [3,4] Furthermore, excitons in 2D systems often couple with their unique physical properties, such as a thickness-dependent change in the bandgap nature and polarization-governed optical selection rules. These relationships provide rich physics and proper functionality for novel optoelectronic applications. [3][4][5] To fully exploit the excellent properties of excitons, it is essential to explore their behavior on ultrashort timescales. Many fundamental excitonic phenomena such as recombination, many-body interactions, and scattering mechanisms occur on femtosecond or picosecond timescales. [6][7][8][9][10][11][12][13] Also, ultrafast light-matter interactions can modulate excitons, enabling novel quantum memory and switching effects. [14][15][16][17][18][19] Therefore, understanding exciton behaviors on Strongly bound excitons are a characteristic hallmark of 2D semiconductors, enabling unique light-matter interactions and novel optical applications. Platinum diselenide (PtSe 2 ) is an emerging 2D material with outstanding optical and electrical properties and excellent air stability. Bulk PtSe 2 is a semimetal, but its atomically thin form shows a semiconducting phase with the appearance of a band-gap, making one expect strongly bound 2D excitons. However, the excitons in PtSe 2 have been barely studied, either experimentally or theoretically. Here, the authors directly observe and theoretically confirm excitons and their ultrafast dynamics in mono-, bi-, and tri-layer PtSe 2 single crystals. Steady-state optical microscopy reveals exciton absorption resonances and their thickness dependence, confirmed by first-principles calculations. Ultrafast transient absorption microscopy finds that the exciton dominates the transient broadband response, resulting from strong exciton bleaching and renormalized band-gap-induced exciton shifting. The overall transient spectrum redshifts with increasing thickness as the shrinking bandgap redshifts the exciton resonance. This study provides novel insights into exciton photophysics in platinum dichalcogenides.The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/smll.202103400.

Anomalous Oscillating Behavior of Ultrafast Spatiotemporal Hot Carrier Diffusion in Two-Dimensional PtSe2

“…The overall sign of the differential reflection signal is negative, agreeing with a previous study . Considering that most of the differential transmission measurements on few-layer PtSe 2 have observed photoinduced bleaching (PB) over broadband ranges near the 800 nm probe wavelength, − the observed negative sign of the differential reflection can be interpreted as PB. Note that PB has exhibited negative signs in differential reflection measurements of various 2D materials on transparent substrates, − such as sapphire used in this work.…”

“…Note that, while the previous study observed an additional standing wave CP mode in chemical vapor-deposited PtSe 2 , 44 it was not identified in our experiment, probably due to the relatively weak PtSe 2 − substrate coupling of the mechanically exfoliated flake sample 54,55 (see the extended discussion in the Supporting Information, Figure S3). The decaying background in the −ΔR/R 0 trace corresponds to the dynamics of the pumpgenerated carriers at the 800 nm probe wavelength, as demonstrated in previous studies 45,47,48,56 (see the extended discussion and associated broadband transient responses in the Supporting Information, Figure S4). The full −ΔR/R 0 trace can be modeled by the following function,…”

“…The oscillating part is due to the layer-breathing CP mode that arises from the strong interlayer lattice coupling. 44,45,47,53 This assignment is confirmed by the fact that the thickness dependence of the CP frequency agrees well with the 1D chain model of the layer-breathing mode (Supporting Information, Figure S2). Note that, while the previous study observed an additional standing wave CP mode in chemical vapor-deposited PtSe 2 , 44 it was not identified in our experiment, probably due to the relatively weak PtSe 2 − substrate coupling of the mechanically exfoliated flake sample 54,55 (see the extended discussion in the Supporting Information, Figure S3).…”

“…57 The second term denotes the biexponential decay dynamics of the carriers (blue line in Figure 1c), where A 1 and A 2 are the amplitudes, and τ 1 = 1. in previous ultrafast studies on 2D PtSe 2 . These studies found that the fast component is due to the carrier trapping by defects, 47,48,58 and the slow component is caused by the defectassisted carrier−carrier scattering or carrier−phonon scattering. 49,56,58,59 The full curve from eq 1 well reproduces the −ΔR/R 0 trace (green line in Figure 1c).…”

Momentum-Dependent Oscillator Strength Crossover of Excitons and Plasmons in Two-Dimensional PtSe2

“…While the transport and catalytic properties of PtX 2 have been thoroughly investigated, its electronic excitation spectrum, including the collective excitations (excitons and plasmons), remains largely unexplored. Understanding these excitations − is the key to controlling the optical properties of the material and for future photonic or optoelectronic applications. ,− The optical excitations in the MoX 2 and WX 2 semiconducting TMDs have been intensely studied during the past years because of the unique exciton physics found in their mono- and few-layer structures. , In comparison, the optical properties of few-layer PtX 2 have only recently gained attention, and only the most basic properties of their excitons have so far been addressed. − The plasmon excitations of bulk and thin layers of PtTe 2 were reported by Ghosh et al However, in these measurements the samples were not atomically thin and substrate–sample interactions could not be ruled out. To date, the electronic excitation spectrum of freestanding few-layer PtX 2 remains unexplored, and it is unclear how it depends on the number of layers and the metal–semiconductor transition taking place as the thickness decreases to the monolayer limit.…”