Probing the upper band gap of atomic rhenium disulfide layers. Issue 1 (December 2018)

• Record Type: Journal Article
• Authors: Dhakal, Krishna; Kim, Hyunmin; Lee, Seonwoo; Kim, Youngjae; Lee, JaeDong; Ahn, Jong-Hyun
• Is Part Of: Light, science & applications. Volume 7:Issue 1(2018:Jan.)
• Page Start: 1
• Page End: 11
• Publication Date: 2018-12
• Subjects: Optics -- Research -- Periodicals; Photonics -- Periodicals; 535.05
• DOI: 10.1038/s41377-018-0100-3
• Languages: English
• ISSNs: 2047-7538

Abstract

Abstract Here, we investigate the ultrafast carrier dynamics and electronic states of exfoliated ReS2 films using time-resolved second harmonic generation (TSHG) microscopy and density functional theory (DFT) calculations. The second harmonic generation (SHG) of layers with various thicknesses is probed using a 1.19-eV beam. Up to ~13 nm, a gradual increment is observed, followed by a decrease caused by bulk interferometric light absorption. The addition of a pump pulse tuned to the exciton band gap (1.57 eV) creates a decay-to-rise TSHG profile as a function of the probe delay. The power and thickness dependencies indicate that the electron–hole recombination is mediated by defects and surfaces. The two photon absorptions of 2.38 eV in the excited state that are induced by pumping from 1.57 to 1.72 eV are restricted because these transitions highly correlate with the forbiddend –d intrasubshell orbital transitions. However, the combined usage of a frequency-doubled pump (2.38 eV) with wavelength-variant SHG probes (2.60–2.82 eV) allows us to vividly monitor the variations in TSHG profiles from decay-to-rise to rise-to-decay, which imply the existence of an additional electron absorption state (s -orbital) at an approximate distance of 5.05 eV from the highest occupied molecular orbital states. This observation was critically examined by considering the allowance of each electronic transition and a small upper band gap (~0.5 eV) using modified DFT calculations. Materials: The 3D material that behaves like a 2D material Scientists have investigated the electronic and optical properties of ultrathin films of rhenium disulfide (ReS2 ) and could lead to new devices for use in optoelectronic applications. Unlike other transition metal dichalcogenides like MoS2, ReS2 maintains its direct band gap even its bulk form allowing it to emit and absorb light easily and making it attractive for use in photovoltaic devices. This led Jong-Hyun Ahn from Yonsei University and the collaborators in DGIST (Hyunmin Kim and Jaedong Lee) in South Korea to explore the properties of ReS2 crystals using pump-probe time-resolved second harmonic generation microscopy. Observing the ultrafast intra-conduction band transitions, the researchers confirmed the existence of the high-energy band gap in bilayers of ReS2, leading to their potential for use in a range of optoelectronic applications, including solar cells and light emitting diodes.