Van der Waals (vdW) integration affords semiconductor heterostructures without constrains of lattice matching and opens up a new realm of functional devices by design. A particularly interesting approach is the electrochemical intercalation of two-dimensional (2D) atomic crystal and formation of superlattices, which can provide scalable production of novel vdW heterostructures. However, this approach has been limited to the use of organic cations with non-functional aliphatic chains, therefore failed to take the advantage of the vast potentials in molecular functionalities (electronic, photonic, magnetic, etc.). Here we report the integration of 2D crystal (MoS₂, WS₂, highly oriented pyrolytic graphite (HOPG), WSe₂ as model systems) with electrochemically inert organic molecules that possess semiconducting characteristics (including perylene-3,4,9,10-tetracarboxylic dianhydride (PTCDA), pentacene and fullerene), through on-chip electrochemical intercalation. An unprecedented long-range spatial feature of intercalation has been achieved, which allowed facile assembly of a vertical MoS₂-PTCDA-Si junction. The intercalated heterostructure shows significant modulation of the lateral transport, and leads to a molecular tunneling characteristic at the vertical direction. The general intercalation of charge neutral and functional molecules defines a versatile platform of inorganic/organic hybrid vdW heterostructures with significantly extended molecular functional building blocks, holding great promise in future design of nano/quantum devices.

Due to its controlled reaction with water and biofluids, Mg as a dissolvable conductor has enabled the development of many transient electronic devices. In addition, Mg is a novel plasmonic material with high extinction efficiency, but its transientoptical properties have not been explored thoroughly. In this study, for the first time, we exploit the transient and tunable plasmonic properties of Mg in environmental and biomedical sensor applications. We used soft nanoimprint lithography to fabricate flexible and large-area Mg plasmonic structures that can be applied on the human skin. Their resonance (or color) can be tuned in the visible range by gradual Mg dissolution in a water fluid or vapor-rich environment; these structures can be easily implemented as passive optical sensors without the need for complex electronic circuits or a power supply. We demonstrate the applications of our optical sensors in the accurate monitoring of environmental humidity and physiological detection of sweat loss on the human skin during exercise. Our devices could be used as decomposable/resorbable optical sensors and can help minimize long-term health effects and environmental risks associated with consumer device waste, which will lead to many new possibilities in transient photonic device applications.

Van der Waals (vdW) heterojunctions based on two-dimensional (2D) atomic crystals have been extensively studied in recent years. Herein, we show that both vertical and lateral vdW heterojunctions can be realized with layered molecular crystals using a two-step physical vapor transport (PVT) process. Both types of heterojunctions show clean and sharp interfaces without phase mixing under atomic force microscopy (AFM). They also exhibit a strong interfacial built-in electric field similar to that of their inorganic counterparts. These heterojunctions have greater potential for device applications than individual materials. The lateral heterojunction (LHJ) devices show rectifying characteristics due to the asymmetric energy barrier for holes at the interface, while the vertical heterojunction (VHJ) devices behave like metal–insulator–semiconductor tunnel junctions, with pronounced negative differential conductance (NDC). Our work extends the concept of vdW heterojunctions to molecular materials, which can be generalized to other layered organic semiconductors (OSCs) to obtain new device functionalities.

Chemical reduction of graphene oxide represents an important route towards large-scale production of graphene sheets for many applications. Thus far, gas-phase reactions have been demonstrated to efficiently reduce graphene oxide, but a molecular understanding of the reaction processes is largely lacking. Here, using molecular dynamics simulations, we compare the reduction of graphene oxide in different environments. We find that NH₃ affords more efficient reduction of hydroxyl and epoxide groups than H₂ and vacuum annealing partly due to lower energy barriers. Various reduction paths of oxygen groups in NH₃ and H₂ are quantitatively identified. Furthermore, we show that with the combination of vacancies and oxygen groups, pyridinic- or pyrrolic-like nitrogen can readily be incorporated into graphene. All of these nitrogen configurations lead to n-doping of the graphene. Our results are consistent with many previous experiments and provide insights towards doping engineering of graphene.