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Benefiting from the attractive high carrier mobility, large absorption coefficient, and ambient stability, group IV–VI compounds show excellent potential applications for optoelectronic devices. Herein, a germanium telluride (GeTe) film with a nanosphere structure has been controllably grown by facile physical vapor deposition and utilized into a photoelectrochemical (PEC)-type photodetector. The GeTe-based PEC-type photodetector possesses an excellent photoresponse and high responsivity in the visible region. Meanwhile, density functional theory calculations were employed to investigate the electronic characteristics of GeTe, supplying evidence of the broadband absorption. The improvement of photoelectric response can be attributed to the thicker films due to the larger light harvesting and higher carrier concentration, which is also verified by the visible–near-infrared absorption spectra and electrochemical impedance spectroscopy. The photocurrent density (Iph) and photoresponsivity (Rph) values of the GeTe-based photodetector can reach 22.8 μA/cm2 and 228.9 μA/W. Generally, our work provides an effective way to extend semiconductor materials in applications of PEC-type photodetectors. [ABSTRACT FROM AUTHOR]

Ultrafast charge transfer in van der Waals heterostructures can effectively engineer the optical and electrical properties of two-dimensional semiconductors for designing photonic and optoelectronic devices. However, the nonlinear absorption conversion dynamics with the pump intensity and the underlying physical mechanisms in a type-II heterostructure remain largely unexplored, yet hold considerable potential for all-optical logic gates. Herein, two-dimensional ReSe 2 /ReS 2 heterostructure is designed to realize an unusual transition from reverse saturable absorption to saturable absorption (SA) with a conversion pump intensity threshold of approximately 170 GW/cm 2 . Such an intriguing phenomenon is attributed to the decrease of two-photon absorption (TPA) of ReS 2 and the increase of SA of ReSe 2 with the pump intensity. Based on the characterization results of X-ray photoelectron spectroscopy, ultraviolet photoelectron spectroscopy, femtosecond transient absorption spectrum, Kelvin probe force microscopy, and density functional theory calculation, a type-II charge-transfer-energy level model is proposed combined with the TPA of ReS 2 and SA of ReSe 2 processes. The results reveal the critical role of ultrafast interfacial charge transfer in tuning the unusual nonlinear absorption and improving the SA of ReSe 2 /ReS 2 under different excitation wavelengths. Our finding deepens the understanding of nonlinear absorption physical mechanisms in two-dimensional heterostructure materials, which may further diversify the nonlinear optical materials and photonic devices.