Two-Dimensional Cs ₂AgBiBr ₆/WS ₂ Heterostructure-Based Photodetector with Boosted Detectivity via Interfacial Engineering

The past decade has witnessed an extraordinary increase in research progress on ultrathin two-dimensional (2D) nanomaterials in the fields of condensed matter physics, materials science, and chemistry after the exfoliation of graphene from graphite in 2004. This unique class of nanomaterials has shown many unprecedented properties and thus is being explored for numerous promising applications. In this Perspective, I briefly review the state of the art in the development of ultrathin 2D nanomaterials and highlight their unique advantages. Then, I discuss the typical synthetic methods and some promising applications of ultrathin 2D nanomaterials together with some personal insights on the challenges in this research area. Finally, on the basis of the current achievement on ultrathin 2D nanomaterials, I give some personal perspectives on potential future research directions.

Layered materials of graphene and MoS₂, for example, have recently emerged as an exciting material system for future electronics and optoelectronics. Vertical integration of layered materials can enable the design of novel electronic and photonic devices. Here, we report highly efficient photocurrent generation from vertical heterostructures of layered materials. We show that vertically stacked graphene-MoS₂-graphene and graphene-MoS₂-metal junctions can be created with a broad junction area for efficient photon harvesting. The weak electrostatic screening effect of graphene allows the integration of single or dual gates under and/or above the vertical heterostructure to tune the band slope and photocurrent generation. We demonstrate that the amplitude and polarity of the photocurrent in the gated vertical heterostructures can be readily modulated by the electric field of an external gate to achieve a maximum external quantum efficiency of 55% and internal quantum efficiency up to 85%. Our study establishes a method to control photocarrier generation, separation and transport processes using an external electric field.

The Raman scattering of single- and few-layered WS2 is studied as a function of the number of S-W-S layers and the excitation wavelength in the visible range (488, 514 and 647 nm). For the three excitation wavelengths used in this study, the frequency of the A1g(Γ) phonon mode monotonically decreases with the number of layers. For single-layer WS2, the 514.5 nm laser excitation generates a second-order Raman resonance involving the longitudinal acoustic mode (LA(M)). This resonance results from a coupling between the electronic band structure and lattice vibrations. First-principles calculations were used to determine the electronic and phonon band structures of single-layer and bulk WS2. The reduced intensity of the 2LA mode was then computed, as a function of the laser wavelength, from the fourth-order Fermi golden rule. Our observations establish an unambiguous and nondestructive Raman fingerprint for identifying single- and few-layered WS2 films.