Due to the large gap in timescale between volatile memory and nonvolatile memory technologies, quasi‐nonvolatile memory based on 2D materials has become a viable technology for filling the gap. By exploiting the elaborate energy band structure of 2D materials, a quasi‐nonvolatile memory with symmetric ultrafast write‐1 and erase‐0 speeds and long refresh time is reported. Featuring the 2D semifloating gate architecture, an extrinsic p–n junction is used to charge or discharge the floating gate. Owing to the direct injection or recombination of charges from the floating gate electrode, the erasing speed is greatly enhanced to nanosecond timescale. Combined with the ultrafast write‐1 speed, symmetric ultrafast operations on the nanosecond timescale are achieved, which are ≈106 times faster than other memories based on 2D materials. In addition, the refresh time after a write‐1 operation is 219 times longer than that of dynamic random access memory. This performance suggests that quasi‐nonvolatile memory has great potential to decrease power consumption originating from frequent refresh operations, and usher in the next generation of high‐speed and low‐power memory technology.

The search for ultrafast photonic memory devices is inspired by the ever‐increasing number of cloud‐computing, supercomputing, and artificial‐intelligence applications, together with the unique advantages of signal processing in the optical domain such as high speed, large bandwidth, and low energy consumption. By embracing silicon photonics with chalcogenide phase‐change materials (PCMs), non‐volatile integrated photonic memory is developed with promising potential in photonic integrated circuits and nanophotonic applications. While conventional PCMs suffer from slow crystallization speed, scandium‐doped antimony telluride (SST) has been recently developed for ultrafast phase‐change random‐access memory applications. An ultrafast non‐volatile photonic memory based on an SST thin film with a 2 ns write/erase speed is demonstrated, which is the fastest write/erase speed ever reported in integrated phase‐change photonic devices. SST‐based photonic memories exhibit multilevel capabilities and good stability at room temperature. By mapping the memory level to the biological synapse weight, an artificial neural network based on photonic memory devices is successfully established for image classification. Additionally, a reflective nanodisplay application using SST with optoelectronic modulation capabilities is demonstrated. Both the optical and electrical changes in SST during the phase transition and the fast‐switching speed demonstrate their potential for use in photonic computing, neuromorphic computing, nanophotonics, and optoelectronic applications.

Van der Waals heterostructure is formed by two-dimensional materials, which applications have become hot topics and received intensive exploration for fabricating without lattice mismatch. With the sustained decrease in dimensions of field effect transistors, van der Waals heterostructure plays an important role in improving the performance of devices because of its prominent electronic and optoelectronic behavior. In this review, we discuss the process of assembling van der Waals heterostructures and thoroughly illustrate the applications based on van der Waals heterostructures. We also present recent innovation in field effect transistors and van der Waals stacks, and offer an outlook of the development in improving the performance of devices based on van der Waals heterostructures. Crystals 2018, 8, 8 2 of 23bond and trap state on the surfaces [6], monolayer are free of these disadvantages (Figure 1a,b) and exhibit extraordinary electronic and optoelectronic properties. Additionally, without free dangling bonds, the interface between neighbor 2DLM layers are assembled by van der Waals forces, which is much weaker than chemical bond force. Therefore, the van der Waals heterostructure can be easily isolated by exfoliation with the help of taps [7]. Although some highly disparate materials have a great lattice mismatch in creating heterostructure, those can be assembled together by van der Waals force. This allows diverse 2DLMs to construct various van der Waals heterostructures (vdWHs) with completely novel properties and functions. Crystals 2018, 8, 8 2 of 22 In general, heterostructures formed by 2DLM monolayers are assembled by covalent bond force within layers and stacked together by van der Waals force between layers. Thus, there is no free dangling bond between 2DLM layers. In contrast to typical nanostructures persecuted by dangling bond and trap state on the surfaces [6], monolayer are free of these disadvantages (Figure 1a,b) and exhibit extraordinary electronic and optoelectronic properties. Additionally, without free dangling bonds, the interface between neighbor 2DLM layers are assembled by van der Waals forces, which is much weaker than chemical bond force. Therefore, the van der Waals heterostructure can be easily isolated by exfoliation with the help of taps [7]. Although some highly disparate materials have a great lattice mismatch in creating heterostructure, those can be assembled together by van der Waals force. This allows diverse 2DLMs to construct various van der Waals heterostructures (vdWHs) with completely novel properties and functions.