Ruddlesden−Popper perovskites have been demonstrated to possess great potential for optical and optoelectronic devices. Because they exhibit better ambient stability than three-dimensional (3D) perovskites, they have been considered as potential substitutes for 3D perovskites as light absorbing layers to improve the photoresponsivity of monolayer transition metal dichalcogenide (TMDC)-based photodetectors. Investigation of the optoelectronic properties of TMDC monolayer/2D perovskite vertical heterostructures is however at an early stage. Here, we address the photovoltaic effect and the photodetection performance in tungsten disulfide (WS 2 ) monolayer/2D perovskite (C 6 H 5 C 2 H 4 NH 3 ) 2 PbI 4 (PEPI) vertical heterostructures. A vertical device geometry with separate graphene contacts to both heterointerface constituents acted as a photovoltaic device and self-driven photodetector. The photovoltaic device exhibited an open circuit voltage of −0.57 V and a short circuit current of 41.6 nA. A photoresponsivity of 0.13 mA/W at the WS 2 /PEPI heterointerface was achieved, which was signified by a factor of 5 compared to that from the individual WS 2 region. The current on/off ratio of the self-driven photodetector was approximately 1500. The photoresponsivity and external quantum efficiency of the self-driven photodetector were estimated to be 24.2 μA/W and 5.7 × 10 −5 , respectively. This work corroborates that 2D perovskites are promising light absorbing layers in optoelectronic devices with a TMDC-based heterointerface.

Photodetectors usually operate in the wavelength range with photon energy above the bandgap of channel semiconductors so that incident photons can excite electrons from valence band to conduction band to generate photocurrent. Here, however, we show that monolayer WS 2 photodetectors can detect photons with energy even lying 219 meV below the bandgap of WS 2 at room temperature. With the increase of excitation wavelength from 620 to 680 nm, photoresponsivity varies from 551 to 59 mA/W. This anomalous phenomenon is ascribed to energy upconversion, which is a combination effect of one-photon excitation and multiphonon absorption through an intermediate state created most likely by sulfur divacancy with oxygen adsorption. These findings will arouse research interests on other upconversion optoelectronic devices, photovoltaic devices, for example, of monolayer transition metal dichalcogenides (TMDCs).

Photoluminescence (PL) upconversion is a phenomenon involving light–matter interactions, where the energy of emitted photons is higher than that of the incident photons. PL upconversion is an intriguing process in two-dimensional materials and specifically designed 2D heterostructures, which have potential upconversion applications in optoelectronic devices, bioimaging, and semiconductor cooling. In this review, we focus on the recent advances in photoluminescence upconversion in two-dimensional materials and their heterostructures. We discuss the upconversion mechanisms, applications, and future outlook of upconversion in two-dimensional materials.

Electrochemical intercalation and storage of alkali metal ions into the graphite interlayer space is a key ingredient for commercial rechargeable ion batteries. While this has been exceptionally fruitful for lithium, the use of other more abundant chemical species such as potassium has fallen behind due to their less favorable ionic radius and host-guest interactions. Here, we deploy a device level on-chip cell architecture to study the reversible intercalation of potassium into the single van der Waals gallery of a graphene bilayer at room temperature and monitor in a time-dependent manner the electronic properties of the graphene bilayer during the insertion and extraction processes. The potassium diffusion is highly reversible and a charge transfer up to about 2.5×1013 cm-2 is achieved. The diffusion coefficient well exceeds 10-7 cm2 s-1 at room temperature, an order of magnitude larger than previously reported values in graphite and other compounds.