**3. III-V nanowires QvdWE on layered materials**

Munshi et al. proposed a generic atomic model that relates the epitaxial growth configurations of various III-V nanowires on graphene to the lattice mismatch between graphene and the grown semiconductor, as in **Figure 2**(**a**–**d**) [47]. The experimental verification of the model was undertaken through the use of molecular beam epitaxy (MBE). In this study, the self-catalyzed vapor liquid technique was used to create regular hexagonal cross-sectional shapes with

**Figure 2.** (a–d) Different possible arrangements of semiconductor atoms in the (111) plane of cubic crystal on graphene. (b) III/V semiconductor bandgaps versus lattice. The vertical lines represent the lattice matching condition for different III/V semiconductor arrangements in (a–d) on graphene. Reproduced with permission from Ref. [47].

uniformity in diameter and length. To increase the nanowire density on graphene, twotemperature growth strategy was used to facilitate the nucleation prior to the nanowire growth. A TEM image of GaAs nanowire grown on graphene shows that the bottom part of the nanowire has a mixture of ZB and WZ segments with twins and stacking faults, whereas the rest of the nanowire is nearly defect-free ZB configuration. In this demonstration, authors showed that despite the quite large mismatch in two different configurations, that is, cubic GaAs on graphene, 6.3% lattice mismatch for blue configuration and 8.2% for the green configuration (**Figure 2e**), they were still able to achieve high quality epitaxial growth, hinting that QvdWE can overcome the lattice mismatch constraints.

**3. III-V nanowires QvdWE on layered materials**

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

Munshi et al. proposed a generic atomic model that relates the epitaxial growth configurations of various III-V nanowires on graphene to the lattice mismatch between graphene and the grown semiconductor, as in **Figure 2**(**a**–**d**) [47]. The experimental verification of the model was undertaken through the use of molecular beam epitaxy (MBE). In this study, the self-catalyzed vapor liquid technique was used to create regular hexagonal cross-sectional shapes with

**Figure 2.** (a–d) Different possible arrangements of semiconductor atoms in the (111) plane of cubic crystal on graphene. (b) III/V semiconductor bandgaps versus lattice. The vertical lines represent the lattice matching condition for different

III/V semiconductor arrangements in (a–d) on graphene. Reproduced with permission from Ref. [47].

**Figure 3.** (a) Tilted scanning electron microscopy (SEM) image of In0.39Ga0.61As NWs, with a higher magnification im‐ age shown in the inset. (b) Normalized XRD rocking curves of different In compositions plotted on a logarithmic scale. The B peak represents the position of the InGaAs peak. Reproduced with permission from Ref. [48].

Mohseni et al. have reported a self-organized method for the formation of coaxially hetero‐ structured InAs/In*x*Ga1–*x*As NWs, over a wide tunable ternary compositional range grown directly on graphene substrates [48]. In this growth, metalorganic chemical vapor deposition (MOCVD) was used to integrate the III-V semiconductor on the graphene. The growth of InGaAs nanowires led to spontaneous separation of phase from the start of the growth, which eventually led to a well-defined InAs/In*x*Ga1–*x*As (0.2 < *x* < 1) core-shell structure. The axial growth of InAs-In*x*Ga1–*x*As (0.2 < *x* < 1) started without change in diameter of nanowires. After about 2 μm in height, those nanowires were grown in the form of core-shell structures. The shell composition of InAs-In*x*Ga1–*x*As changes as a function of indium flow; however, the thickness of the shell and core and the start of nonsegregated InAs-In*x*Ga1–*x*As are independent of the composition of indium (**Figure 3**). Moreover, the authors found out that no InGaAs phase segregation was observed when growing on MoS2 or through the Au-assisted vapor-liquidsolid (VLS) technique on graphene. This work demonstrated that QvdWE of InAs on graphene could facilitate phase segregation phenomenon causing a self-organized InAs core/InGaAs shell nanowires. This phase segregation is driven mainly by lack of strain between InAs and graphene layers interface due to the weak vdW forces at this interface.

**Figure 4.** The percentage of vertically well-aligned InAs nanowires (*black solid circles*) and the density of nanowires (*blue empty squares*) relationship to the rms surface roughness of the graphene substrates. Reproduced with permission from Ref. [41].

Hong et al. found out that the density of InAs nanowires arrays integrated on graphene films increases with the number of graphene layers [41]. This is mainly because the nucleation of InAs is more likely on the surface potential wells formed by surface ledges in the graphene. However, the use of single-layer graphene as a substrate will encourage the growth of vertically well-aligned InAs arrays of nanowires with high levels of strong vdW attraction (**Figure 4**). The QvdW epitaxial relationship between InAs and graphene is confirmed by the InAs's hexagonal morphology with six ZB sidewall facets, aligned in a sixfold within a single domain of graphene.

QvdWE of III-V nanowires on 2D materials has brought a new momentum due to the intro‐ duction of new and attractive properties to the grown 1D nanostructure. However, the increased speed of investigation in developing nanostructures on 2D materials does not lack its issues and drawbacks. There is a challenge in accurate regulation of dopant circulation inside the semiconductor nanostructures, the direction of growth, dimensionality, and consistency in size. In addition, high surface recombination, poor thermal management, and highly resistive ohmic contacts are other few inherent aspects of nanostructures, which must be overcome to realize the potential of nanowires [49].
