Control of the crystallization process is central to developing novel materials with atomic precision to meet the demands of electronic and quantum technology applications.Semiconductor nanowires grown by the vapor-liquid-solid process are a promising material system in which the ability to form components with structure and composition not achievable in bulk is well-established. Here we use in situ TEM imaging of GaAs nanowire growth to understand the processes by which the growth dynamics are connected to the experimental parameters. We find that two sequential steps in the crystallization processnucleation and layer growthcan occur on similar time scales and can be controlled independently using different growth parameters. Importantly, the layer growth process contributes significantly to the growth time for all conditions, and will play a major role in determining material properties. The results are understood through theoretical simulations correlating the growth dynamics, liquid droplet and experimental parameters.A central challenge in crystal growth is to understand the dynamic and transient processes underlying the nucleation and growth steps. The ability to independently control these two steps would greatly expand the potential to design the structure, morphology and properties of the resulting material. Understanding the steps in crystallization is particularly important in

Semiconductor nanowires offer the opportunity to incorporate novel structures and functionality into electronic and optoelectronic devices. A clear understanding of the nanowire growth mechanism is essential for well-controlled growth of structures with desired properties, but the understanding is currently limited by a lack of empirical measurements of important parameters during growth, such as catalyst particle composition. However, this is difficult to accurately determine by investigating post-growth. We report direct measurement of the catalyst composition of individual gold seeded GaAs nanowires inside an electron microscope as they grow. The Ga content in the catalyst during growth increased with both temperature and Ga precursor flux. A direct comparison of the calculated phase diagrams of the Au-Ga-As ternary system to the measured catalyst composition not only lets us estimate the As content in the catalyst but also indicates the relevance of phase diagrams to understanding nanowire growth.

Despite the numerous reports on the metal-catalyzed growth of GaN nanowires, the mechanism of growth is not well understood. Our study of the nickel-assisted growth of GaN nanowires using metalorganic chemical vapor deposition provides key insights into this process. From a comprehensive study of over 130 nanowires, we observe that as a function of thickness, the length of the nanowires initially increases and then decreases. We attribute this to an interplay between the Gibbs-Thomson effect dominant in very thin nanowires and a diffusion induced growth mode at larger thickness. We also investigate the alloy composition of the Ni-Ga catalyst particle for over 60 nanowires using energy dispersive X-ray spectroscopy, which along with data from electron energy loss spectroscopy and high resolution transmission electron microscopy suggests the composition to be NiGa. At the nanowire growth temperature, this alloy cannot be a liquid, even taking into account melting point depression in nanoparticles. We hence conclude that Ni-assisted GaN nanowire growth proceeds via a vapor-solid-solid mechanism instead of the conventional vapor-liquid-solid mechanism.

We report the optimized synthesis and electrochemical characterization of a composite of few-layered nanostructured MoS2 along with an electroactive metal oxide BiVO4. In comparison to pristine BiVO4, and a composite of graphene/BiVO4, the MoS2/BiVO4 nanocomposite provides impressive values of charge storage with longer discharge times and improved cycling stability. Specific capacitance values of 610 Fg−1 (170 mAhg−1) at 1 Ag−1 and 166 Fg−1 (46 mAhg−1) at 10 Ag−1 were obtained for just 2.5 wt% MoS2 loaded BiVO4. The results suggest that the explicitly synthesized small lateral-dimensioned MoS2 particles provide a notable capacitive component that helps augment the specific capacitance. We discuss the optimized synthesis of monoclinic BiVO4, and few-layered nanostructured MoS2. We report the discharge capacities and cycling performance of the MoS2/BiVO4 nanocomposite using an aqueous electrolyte. The data obtained shows the MoS2/BiVO4 nanocomposite to be a promising candidate for supercapacitor energy storage applications.

Crystal growth of III-V semiconductor nanowires assisted by a liquid particle/droplet occurs at the solid-liquid interface. This makes the stability of a droplet on the top of a nanowire crucial for successful nanowire growth. Using in-situ transmission electron microscopy together with theoretical analysis of the capillary forces involved, we conclude that truncation of the solid-liquid interface extend the stability range for a droplet in contact with the nanowire top interface. This provides insights to the limits of nanowire growth and is used to experimentally estimate the surface energy of the wurtzite {1 1 2 0} facet of GaAs.Epitaxial crystal growth of semiconductor nanowires assisted by a liquid-particle relies, in a simple perspective, on two fundamental principles: nucleation of material, and a liquid covering the growth front. As in many cases of crystal growth from a melt, the wetting angle, or the contact angle between the growing crystal and the melt, is of importance for crystal formation [1,2]. For nanowires grown by the vapor-liquid-solid (VLS) mechanism, a fundamental stability criterion for having a droplet at the nanowire top has been proposed by Nebol'sin and Shchetinin[3] based on ex-situ observations and earlier theoretical work [4][5][6].Since their report on this stability limit, several experimental[7-11] and theoretical [12][13][14][15] investigations of nanowire growth focusing on the wetting properties of the liquid metal catalyst have been reported, often with focus on its influence on nucleation [8,9] rather than the droplet wetting dynamics. Still, this Nebol'sin-Shchetinin stability criterion remains generally accepted, perhaps due to the simplicity of the model. The Nebol'sin-Shchetinin model predicts an upper bound for having a droplet on the top nanowire facet by relating the ratio of the surface energies of the solid and liquid phases in contact with the vapor (γ sv and γ lv ) to the wetting angle and tapering of the nanowire[3].Although the model is widely accepted, it has important limitations: for instance growth of self-assisted GaAs [16] and InAs [17] have been extensively reported, although the relevant surface energy ratios are in these cases greater than the predicted upper bound (γ sv /γ lv ∼ 2 compared to √ 2 for un-tapered nanowires[3]). A limitation of the existing model is the assumption that the interface between droplet and nanowire is flat, which, according to experimental results [8,9,18], is not always the case during growth. These experimental reports have shown the formation of a truncation of the top nanowire facet during growth,