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With the perspective of using two-dimensional materials as growth substrates for semiconductors, we explore the nucleation of GaN nanostructures on graphene. Using plasma-assisted molecular beam epitaxy, we investigate what happens during the long incubation time which precedes the epitaxy of the first GaN islands. After 30 min of nitrogen plasma exposure with no deposition, we find that graphene is modified, and we identify C–N bonds. We measure and model the variation of the incubation time with the growth parameters. These data support the idea that graphene must be modified before GaN nucleation becomes possible. We then test the adhesion at the interface between graphene and the GaN nanostructures. Our studies converge on the conclusion that GaN nanostructures nucleate on graphene from pyridinic N atoms incorporated in the lattice, which are responsible for strong binding between the two materials.

The growth of ZnTe nanowires and ZnTe–CdTe nanowire heterostructures is studied by in situ transmission electron microscopy. We describe the shape and the change of shape of the solid gold nanoparticle during vapor–solid–solid growth. We show the balance between one monolayer and two monolayer steps, which characterizes the vapor–liquid–solid and vapor–solid–solid growth modes of ZnTe. We discuss the likely role of the mismatch strain and lattice coincidence between gold and ZnTe on the predominance of two monolayer steps during vapor–solid–solid growth and on the subsequent self-regulation of the step dynamics. Finally, the formation of an interface between CdTe and ZnTe is described.

We investigate by in situ X-ray diffraction the early stage of GaN nanowire growth on a single layer of polycrystalline graphene transferred onto a Si(100)/SiO2carrier substrate. The experiment was carried out in a molecular beam epitaxy chamber installed on a synchrotron line equipped with a X-ray diffractometer and a nitrogen plasma source. The evolution of the in-plane lattice parameters of the crystals is monitored. The in-plane lattice parameter of graphene shows only a little variation between room temperature and GaN growth temperature (720 °C). As for GaN, an incubation time of about 1 h precedes nucleation. Just after nucleation, the GaN nanocrystals are under tensile strain, and they relax as growth proceeds. The critical nucleus size and the parameters governing nanowire growth dynamics are obtained from modeling. We discuss the possible incorporation of N atoms in the graphene lattice during the incubation time. These atoms could be involved in chemical bonds between GaN and graphene, which could explain the initial strain observed experimentally.

Crystal phase switching between the zincblende and wurtzite structures in III–V nanowires is crucial from the fundamental viewpoint as well as for electronic and photonic applications of crystal phase heterostructures. Here, the results of in situ monitoring of self-catalyzed vapor–liquid–solid growth of GaAs nanowires by molecular beam epitaxy inside a transmission electron microscope are presented. It is demonstrated that the occurrence of the zincblende or wurtzite phase in self-catalyzed nanowires is determined by the sole parameter, the droplet contact angle, which can be finely tuned by changing the group III and V fluxes. The zincblende phase forms at small (<100°) and large (>125°) contact angles, whereas pure wurtzite phase is observed for intermediate contact angles. Wurtzite nanowires are restricted by vertical sidewalls, whereas zincblende nanowires taper or develop the truncated edge at their top. These findings are explained within a dedicated model for the surface energetics. These results give a clear route for the crystal phase control in Au-free III–V nanowires. On a more general note, in situ growth monitoring with atomic resolution and at the technological-relevant growth rates is shown to be a powerful tool for the fine-tuning of material properties at the nanoscale.

We present a new approach to achieve selective area growth of GaN nanowires by plasma-assisted molecular beam epitaxy. The nanowires are grown on graphene nanodots, which are patterned by electron beam lithography from polycrystalline graphene patches transferred to SiO2substrates. The GaN nanowires grow on these graphene nanodomains with a perfect selectivity with respect to the SiO2surrounding surface. The results demonstrate that a single monolayer of graphene can withstand the lithography process without losing its ability to induce epitaxial growth. The nanowire length distribution and patterns’ fill factor are analyzed in the framework of a theoretical model, which takes into account an incubation time dependent on the graphene dot size. Overall, these results represent the first demonstration of selective area nanowire growth on a regular array of graphene nanodomains.