**1. Introduction**

Since the successful exfoliation of graphene [1], a group of materials with two-dimensional structures have revived and are attracting explosive interests from a variety of fields, including transistors [2], photodetectors [3], chemical sensors, memories, and artificial synapses [4, 5]. This is benefited from the versatile properties, of 2D materials defined not only by their crystal structure (1 T, 2H, etc.) but also by their layer number, i.e., the electrical conductivity and optical bandgaps [6]. The transition metal chalcogenides (TMDs) of 1 T or 1 T' phase usually manifest metallic behavior, while in 2H phase, they are semiconductor and can be transformed into insulator by field-effect modulation [7]. Meanwhile, monolayer MoS2, WSe2, and MoTe2 are transformed into direct band semiconductor with greatly improved photoluminescence yield compared to their indirect bulk form, rendering the further fabrication of light emitting diodes [8, 9]. The recent appearance of 2D ferroelectric materials from direct chemical synthesis or atom doping has further enriched the physical properties of 2D semiconductors [10, 11]. These rapid evolution of 2D materials with diverse physical and chemical properties motivates enduring efforts to explore various property tuning and integration strategies in functional devices, e.g., via chemical doping, alloying, or constructing heterostructures [12].

An indispensable feature of the 2D materials is their van der Waals interlayer coupling, which is weak enough compared to covalent or ionic bonding to enable mechanical or electrochemical exfoliation [13]. The exfoliated 2D materials in monolayer or few layer thicknesses can then be artificially stacked, either laterally or vertically, making heterostructures in various forms that are not possible in conventional semiconductors with 3D crystal lattice (Si, III-V, and oxides) due to the lattice mismatch. The great flexibility in assembling 2D materials thus renders unprecedented opportunity in discovering novel nanoscale transport phenomenon [14] and carrier dynamics and stimulates the exploration of 2D functional devices via deliberately designing the heterostructures. In optoelectronics, this enabled the tailoring of charge separation characteristics of photogenerated electron–hole pairs in semiconductors [15], thereby allowing innovated designs of heterostructured transistors [16, 17], tunneling diode for photodetection [18, 19], and further optoelectronic memories with float gate structures [20].

In this chapter, we first introduce the basic design of heterostructures for optoelectronics and the pick-transfer methods for their artificial assembly and then discuss the recent progress in fabricating novel 2D vdW heterostructures for functional devices. In view of the rapid progress in this field, the chapter is not intended to cover all aspects of the field but focus on optoelectronic-related application, typically photodiode and phototransistors for photodetection and optoelectronic memories that integrate both light sensing and memory function.
