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

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

#### **Figure 1.**

*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.*

**151**

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

holes residing in different semiconductors. The separation of electron–hole pairs in type II aligned heterostructures allows the fabrication of rectifying diodes with photovoltaic effects and is usually adopted for photoelectric detectors that transform incident light into electrical signals [3]. In the case of type III band alignment, the bandgap of a semiconductor lies outside of the other one, with its CBM lower than the VBM of the other. There is no more forbidden gap at the interface compared to the bulk semiconductor. Such type III band alignment is useful in tunnel-

Since the conduction band is usually related to the cations while valance band is related to the anions, designing the band offsets is traditionally mostly achieved in superlattices of semiconductor alloys with widely tuned bandgap and suppressed lattice mismatches, e.g., AlxGa1−xAs/GaAs [25]. However, by using 2D materials, the lattice mismatch between adjacent heterostructured materials is in principle eliminated due to the weak interlayer coupling via van der Waals force. Various 2D materials of different energy band structures and gaps, e.g., graphene, MXenes, black phosphorous (BP), TMDs, and hexagonal boron nitride (h-BN), can thus be artificially stacked to make multiple kinds of heterostructures [13, 26]. **Figure 1b** illustrates the energy bandgap position of several 2D semiconductors [21, 27, 28]. Due to the zero-bandgap characteristic of graphene, it could not be directly used for high on–off switching devices, e.g., transistors, diodes, but is often used as elec-

other 2D semiconductors have been found as alternatives, with widely distributed bandgaps from 0.2 eV to 2–3 eV [4, 6]. The electron affinity also varies largely from 3 to <5 eV, thus rendering the possibility to make all kinds of heterostructures (types I, II, III) with different band offsets, i.e., by choosing suitable 2D semiconductors. For example, WSe2/SnS2 constitutes a type III heterostructure, while MoS2/ WSe2 forms a typical type II structure. Notably, the number of stacked layers is also not limited to two but can be facilely increased for multilayer heterostructures for tunneling diodes or device encapsulation [13]. The continuously increasing 2D material family thus incubates infinite possibilities in 2D heterostructures and

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

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

 V<sup>−</sup><sup>1</sup> s<sup>−</sup><sup>1</sup>

[29]. Recently,

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

ing field-effect transistors with large current density [24].

trode contacts for its ultrahigh carrier mobility >10,000 cm2

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

2D materials is thus desired for making high-performance devices.

extremely rich functions.

#### *Emerging Artificial Two-Dimensional van der Waals Heterostructures for Optoelectronics DOI: http://dx.doi.org/10.5772/intechopen.88433*

holes residing in different semiconductors. The separation of electron–hole pairs in type II aligned heterostructures allows the fabrication of rectifying diodes with photovoltaic effects and is usually adopted for photoelectric detectors that transform incident light into electrical signals [3]. In the case of type III band alignment, the bandgap of a semiconductor lies outside of the other one, with its CBM lower than the VBM of the other. There is no more forbidden gap at the interface compared to the bulk semiconductor. Such type III band alignment is useful in tunneling field-effect transistors with large current density [24].

Since the conduction band is usually related to the cations while valance band is related to the anions, designing the band offsets is traditionally mostly achieved in superlattices of semiconductor alloys with widely tuned bandgap and suppressed lattice mismatches, e.g., AlxGa1−xAs/GaAs [25]. However, by using 2D materials, the lattice mismatch between adjacent heterostructured materials is in principle eliminated due to the weak interlayer coupling via van der Waals force. Various 2D materials of different energy band structures and gaps, e.g., graphene, MXenes, black phosphorous (BP), TMDs, and hexagonal boron nitride (h-BN), can thus be artificially stacked to make multiple kinds of heterostructures [13, 26]. **Figure 1b** illustrates the energy bandgap position of several 2D semiconductors [21, 27, 28]. Due to the zero-bandgap characteristic of graphene, it could not be directly used for high on–off switching devices, e.g., transistors, diodes, but is often used as electrode contacts for its ultrahigh carrier mobility >10,000 cm2 V<sup>−</sup><sup>1</sup> s<sup>−</sup><sup>1</sup> [29]. Recently, other 2D semiconductors have been found as alternatives, with widely distributed bandgaps from 0.2 eV to 2–3 eV [4, 6]. The electron affinity also varies largely from 3 to <5 eV, thus rendering the possibility to make all kinds of heterostructures (types I, II, III) with different band offsets, i.e., by choosing suitable 2D semiconductors. For example, WSe2/SnS2 constitutes a type III heterostructure, while MoS2/ WSe2 forms a typical type II structure. Notably, the number of stacked layers is also not limited to two but can be facilely increased for multilayer heterostructures for tunneling diodes or device encapsulation [13]. The continuously increasing 2D material family thus incubates infinite possibilities in 2D heterostructures and extremely rich functions.
