Cooperite or platinum sulfide (PtS) is a rare mineral with nonlayered structure. Its general existence in small size and irregular shape with inevasible impurities either in natural or artificial synthetic type has hindered the study of its intrinsic optical and photonic properties.

Recently, a collaborative study, led by Prof. WANG Jun from Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, investigated the PtS films fundamental optical properties, ultrafast carrier dynamics, nonlinear absorption and refraction properties, and analyzed the trend and origin of the thickness-dependent optical bandgap and carrier lifetime. Their study was published in ACS Nano.

Their study prepared large scale, uniform and thickness-controllable PtS films without other element impurities by employing the thermally assisted conversion method.

In their experiment, PtS films, which are much closer in stoichiometry to intrinsic PtS than cooperite mineral, approaching the 2D limit (2D-PtS) from bulk, were obtained by controlling the thickness of precursor Pt metal used. An abnormal trend of the optical bandgap narrowing with decreasing film thickness was attributed to the presence of surface energy bands within the forbidden band.

The transient transmission measurements were performed using a 380-fs-pulsed laser with a wavelength of 520 nm, whose frequency was doubled from 1040 nm. The ultrafast carrier dynamics and the thickness-dependent carrier lifetime could be attributed to surface states assisting the carrier recombination in the inner bulk, hence resulting in the fast lifetime at the surface and the slow one in the bulk.

A model based on the quantum mechanical wave function was used to estimate the occupation probability of electrons and holes. Good agreement between the experiment results and the calculations indicated that the surface behaviour played a leading role in the Auger process.

Nonlinear optical measurements were performed with 340 fs pulses at 515 nm. Ultrafast saturable absorption and self-defocusing properties in the visible region were observed, which were dominated by the resonant electronic nonlinearities.

This work provides a comprehensive understanding of the intrinsic nonlinear optical property and carrier recombination in 2D nonlayered PtS, which are of profound significance for the design or modulation of optoelectronic devices based on PtS.

This work was supported by the Chinese National Natural Science Foundation, the Strategic Priority Research Program of CAS, the Key Research Program of Frontier Science of CAS, and the Program of Shanghai Academic Research Leader.

Fig. 1 Ultrafast carrier dynamics of PtS film. A schematic diagram of surface and inner bulk recombination of 2D PtS. (Image by SIOM)

Related website: https://pubs.acs.org/doi/10.1021/acsnano.9b06782

Contact:
Mr. Cao Yong
General Administrative Office
Shanghai Institute of Optics and Fine Mechanics, CAS
Email: caoyong@siom.ac.cn