**Author details**

**Device group Device structure Voc (V) Jsc (mA/cm2**

56 Two-dimensional Materials - Synthesis, Characterization and Potential Applications

*Note*: Reproduced with permission from Ref. [63].

**6. Future directions and conclusion**

discussed in this chapter.

this study.

A Axial junction 0.18 13.14 38.43 0.91 1.93 B Radial junction 0.20 16.20 40.24 1.32 1.91 C Radial junction with passivation 0.26 17.16 55.32 2.51 1.54

**Table 4.** The main performance and figure of merits obtained from the NW array on graphene solar cell presented in

In summary, this chapter has provided a detailed overview of the recent progress of the QvdW heteroepitaxial growth of III–V semiconductors on layered materials. The growth demonstra‐ tions of several III–V semiconductors, such as GaAs, GaN, InAs, are mainly included in this chapter. Most of these demonstrations used graphene as a buffer 2D material. Many studies have reported nonplanar nanostructures, for example nanowire arrays, whereas few of them are focused on planar structures, for example thin films. The suitability of such grown structures in various applications in electronics, photonics, and optoelectronics is also

The more versatility of the vdWE over conventional heteroepitaxy is the possibility to achieve epitaxial growth of III–V semiconductors without the strict requirements of lattice matching. On top of that, the idea of vdWE can be applied to rather a wide variety of materials. The materials extend from 1D to 3D and even to organic. In such a way, it is possible to integrate 3D/2D to create new types of junctions. Such inherent features of vdWE could have widespread consequences and real-life applications. These are the reasons why the research area of vdWE

In spite of great technological advances and efforts, nanostructure-based devices are still suffering from several carrier loss mechanisms, surface-states induced band bending, Fermi level pinning, poor ohmic contacts, and controlled incorporation of *n*- and *p*-type dopants. Due to the poor performance caused by these aforementioned issues, thin films or planar structures are still gaining more attention. In this context, vdWE, historically developed for planar structures, can be a great solution on the way toward successful heteroepitaxial growth on a target substrate. However, the main challenge is the low surface energy of the 2D materials. Therefore, finding a way to increase the wettability and the surface free energy of the 2D

Although most of the literature studies so far are focusing on using graphene, we consider other 2Ds such as TMDs have the potential to be a good fit for this integration, as long as they are stable at high temperature. The reason is that they have higher surface free energy, can be monolithically integrated at UHV chamber making the interfaces purer from any organics, and

is gaining extensive interest recently in the research community.

materials will make integration much easier in the future.

**) FF (%) PCE (%)** *n*

Yazeed Alaskar1,2\*, Shamsul Arafin3 , Isaac Martinez-Velis1 and Kang L. Wang1

\*Address all correspondence to: alaskar@ucla.edu

1 Department of Electrical Engineering, University of California at Los Angeles, CA, USA

2 National Nanotechnology Research Center, King Abdulaziz City for Science and Technol‐ ogy, Riyadh, Saudi Arabia

3 Department of Electrical and Computer Engineering, University of California at Santa Bar‐ bara, CA, USA
