**3. Dry transfer methods for artificial 2D stacking**

The deterministic transfer of two-dimensional materials constitutes a crucial step toward the fabrication of heterostructures based on the artificial stacking of two-dimensional materials. To stack multiple 2D materials into heterostructure, one needs to transfer 2D materials into a specified position on substrate. This is usually done under an optical microscope, in which one could identify the ultrathin 2D materials through their slight color contrast with substrates. A 3D moving stage is usually equipped to fine adjust the stacking position of each layer, as indicated in **Figure 2a** [30]. So far, a lot of methods and processes have been developed to achieve high-quality assembly of 2D materials in devices and multilayer heterostructures. For 2D materials initially grown on substrates, e.g., graphene on copper, MoS2 on sapphire, they are usually first etched free from the substrates via polymer (typically poly(methyl methacrylate), known as PMMA)-assisted handling and wet-chemical etching processes [31]. However, the residual of PMMA and wet etching chemicals often deteriorate material performance and also degrades the cleanness of stacked interface, which can be serious in multilayered heterostructures. All-dry transfer of 2D materials is thus desired for making high-performance devices.

To make a heterostructure based on vertical stacking, 2D materials can be typically exfoliated from single crystals by Scotch tape and then transferred to viscous

*Smart Nanosystems for Biomedicine, Optoelectronics and Catalysis*

optoelectronic memories with float gate structures [20].

memories that integrate both light sensing and memory function.

**2. Type I, II, and III heterostructures for optoelectronics**

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

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

The interfacial energy band alignment in heterostructures governs the carrier dynamics in devices and therefore determines directly their functional performances. Depending on the relative positions of conduction band and valance band of constituting materials, there are generally three types of band alignments, including type I (straddling gap), type II (staggered gap), type III (broken gap), as illustrated in **Figure 1a** [21]. The different band offsets make them perform differently in optoelectronic devices [22]. In type I alignment, the bandgap of a semiconductor is located within the bandgap of another one; thus both electrons and holes tend to relax in the first narrow bandgap semiconductor. It is therefore widely used in light emitting diodes for higher light illumination efficiency by confining electron and hole pairs within the narrow bandgap semiconductor [23]. In contrary, in type II alignment, both the conduction band minimum (CBM) and valance band maximum (VBM) are higher or lower than the other, which forces electrons and

*Band positions and alignments for 2D materials and heterostructures. (a) Heterostructures of type I, II and III interfacial band alignments, reproduced with permission from Ref. [21], Copyright 2016 American Physical Society. (b) Conduction and valance band positions of selected 2D materials collected from literatures.*

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**Figure 1.**

**Figure 2.**

*Setup and typical dry transfer processes for 2D vdW stacking. (a) Schematic of the experimental setup and (b) the processes employed for the all-dry transfer process, reproduced with permission from Ref. [30], Copyright 2014 IOP publishing Ltd. (c) Schematic of the van der Waals (vdW) technique for polymer-free assembly of layered materials, reproduced with permission from Ref. [33], Copyright 2013 Science.*

elastomer stamp (poly dimethyl siloxane, PDMS), as illustrated in **Figure 2b** [30]. The transparent PDMS stamp is then used to handle the exfoliated 2D flakes. Under optical microscope, it is then aligned to a target position, e.g., the position of already transferred 2D layer. The position of the stamp is then fine-tuned in all three dimensions to approach the target substrate, until the full contact. It is then slowly peeled off from substrate leaving 2D material behind. Sometimes slight heating of the substrate is necessary to reduce the viscosity of PDMS stamp and promote the successful transfer of 2D material onto target substrate. Instead of the common used PDMS stamps, thermal release tapes can be also used as the handle [32]. Though no wet-chemical etching processes is adopted in the above procedures, the surface of PDMS is full of silane groups and may contaminate the 2D material during transfer and make the contacting substrate hydrophobic. It may therefore deteriorate the material and interface quality in device.

An improved polymer-free method was reported by Wang et al., which adopted the clean h-BN as the buffer layer to attach graphene (**Figure 2c**) [33]. This is based on the stronger interaction between graphene and h-BN compared to SiO2, so that elastomer stamps with h-BN layer could pick up graphene layer from substrate. Note that the graphene layer is initially transferred onto SiO2 substrate by tape exfoliation; both the top and bottom surface are free from polymer residuals due to the fresh exfoliation when peeling off the tape. Through this method, the graphene layer during all transfer processes is protected by h-BN and thus could form clean interfaces with both the top and bottom h-BN layers. The as-prepared h-BN-encapsulated graphene manifests unprecedented room temperature mobility up to 140,000 cm2 V<sup>−</sup><sup>1</sup> s<sup>−</sup><sup>1</sup> , with long ballistic transport distance over 15 μm at 40 K, demonstrating the ultrahigh interface quality formed in such polymer-free transfer methods.

Recently, the pick-transfer methods have been also modified to transfer metal electrodes onto 2D materials, avoiding the interdiffusion of elements within the contact interface with 2D materials from traditional physical deposition of metal electrodes (via magnetic sputtering, thermal evaporation, etc.) [34]. Importantly, the formed electrical contact with MoS2 using different metal electrodes displayed

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*Emerging Artificial Two-Dimensional van der Waals Heterostructures for Optoelectronics*

ideal Schottky barriers defined by the work function difference between metal and MoS2, which have not been achieved in conventional Si devices. It is therefore undoubted that the versatile usages of pick-transfer methods in assembling 2D devices hold vast potential in reforming existing technologies from many aspects.

There are several kinds of photodetectors that convert incident light signal to electrical signals, including detectors that rely on photoelectric effect, pyroelectric effect, and photothermal effects. Among the various detectors, the photoelectric detectors exhibit fast response dynamics based on simple separation of electron– hole pairs and are mostly used in commercial products. The photoelectric detectors can be further categorized into photodiode and phototransistors. In photodiodes, the photogenerated electron–hole pairs are separated by the built-in electric field in space charge region, while in phototransistors, an external electric field is applied to

In 2D heterostructures, both photodiodes and phototransistors can be built up by vdW stacking of different materials. Because of the presence of band offset at the interface, heterostructured junctions tend to enable efficient charge separation compared to homojunctions, which requires deliberate control of their p and n doping states. In this section, we discuss several typical heterostructures in type I–III

To fabricate heterostructured diode, one kind of 2D material is exfoliated and transferred onto the other one. For the charge separation in vertical direction, type II band alignment is desired. However, this is not naturally obtained, especially when one adopts a narrow bandgap semiconductor for infrared applications, e.g., BP. However, since the work function of ultrathin 2D materials can be dramatically modulated by electrostatic methods, the behavior of 2D diodes was demonstrated tunable by applying gate voltages [35–37]. As illustrated in **Figure 3a**, BP and WSe2 form type I band alignments [38], with slight conduction band offset (~0.1 eV). When increasing the back-gate voltage from negative to positive, the WSe2 layer is tuned sequentially from p to i and n states by the injection of gate-coupled electrons and then forming, respectively, p–p, pi, and p-n junctions with the p-typed BP. Further increasing gate bias also tunes BP to n type and results in n-n junction. Accordingly, these junctions manifest different rectification ratios under gate bias. **Figure 3b** displays the forward and reverse channel current (at *V*ds = 1 V and −1 V); the different onset threshold gate voltage under forward and reverse bias results in a window of −30 V < Vg < 10 V, in which high rectification ratio is obtained by the formation of p-n junction. The widely tuned doping characteristic of 2D bipolar semiconductor thus renders feasible modification of the diode characteristics in the device via various kinds of field effects, including using ionic liquids and ion gels [39]. By choosing appropriate 2D semiconductors, p-n junctions can be formed directly without gate bias. Wang et al. reported such diode based on p-typed gate and n-type MoS2 [37]. It displays apparent photovoltaic effect under light illumination, as indicated in **Figure 3c**. The extracted ideal factor of the junction is as low as 2 at room temperature, corresponding to Shockley-Read-Hall (SRH) recombination-dominated carrier loss during transport. So far, various kinds of p-n junctions have been made based on such type II band alignments, including BP/MoS2 [38], MoS2/MoTe2 [40], MoS2/WSe2 [41], etc. The open circuit voltage by photovoltaic effect in such type II

generate photodetection gain >100% for highly sensitive detection.

band alignments and their behavior in photodetection.

**4.1 Heterostructured diodes**

*DOI: http://dx.doi.org/10.5772/intechopen.88433*

**4. Heterostructures for photodetection**

ideal Schottky barriers defined by the work function difference between metal and MoS2, which have not been achieved in conventional Si devices. It is therefore undoubted that the versatile usages of pick-transfer methods in assembling 2D devices hold vast potential in reforming existing technologies from many aspects.
