Non-classical nucleation in vapor–liquid–solid growth of monolayer WS2 revealed by in-situ monitoring chemical vapor deposition

The very early nucleation stage of a transition metal dichalcogenide (TMD) was directly observed with in-situ monitoring of chemical vapor deposition and automated image analysis. Unique nucleation dynamics, such as very large critical nuclei and slow to rapid growth transitions, were observed during the vapor–liquid–solid (VLS) growth of monolayer tungsten disulfide (WS2). This can be explained by two-step nucleation, also known as non-classical nucleation, in which metastable clusters are formed through the aggregation of droplets. Subsequently, nucleation of solid WS2 takes place inside the metastable cluster. Furthermore, the detailed nucleation dynamics was systematically investigated from a thermodynamic point of view, revealing that the incubation time of metastable cluster formation follows the traditional time–temperature transformation diagram. Quantitative phase field simulation, combined with Bayesian inference, was conducted to extract quantitative information on the growth dynamics and crystal anisotropy from in-situ images. A clear transition in growth dynamics and crystal anisotropy between the slow and rapid growth phases was quantitatively verified. This observation supports the existence of two-step nucleation in the VLS growth of WS2. Such detailed understanding of TMD nucleation dynamics can be useful for achieving perfect structure control of TMDs.

Orientation-Dependent Conversion of VLS-Grown Lead Iodide Nanowires into Organic-Inorganic Hybrid Perovskites

In this study, we demonstrate Sn-assisted vapor-liquid-solid (VLS) growth of lead iodide (PbI2) nanowires with van der Waals layered crystal structure and subsequent vapor-phase conversion into methylammonium lead iodide (CH3NH3PbI3) perovskites. Our systematic microscopic investigations confirmed that the VLS-grown PbI2 nanowires display two major growth orientations of [0001] and [1¯21¯0], corresponding to the stacking configurations of PbI2 layers to the nanowire axis (transverse for [0001] vs. parallel for [1¯21¯0]). The resulting difference in the sidewall morphologies was correlated with the perovskite conversion, where [0001] nanowires showed strong localized conversion at top and bottom, as opposed to [1¯21¯0] nanowires with an evenly distributed degree of conversion. An ab initio energy calculation suggests that CH3NH3I preferentially diffuses and intercalates into (112¯0) sidewall facets parallel to the [1¯21¯0] nanowire axis. Our results underscore the ability to control the crystal structures of van der Waals type PbI2 in nanowire via the VLS technique, which is critical for the subsequent conversion process into perovskite nanostructures and corresponding properties.

AgNPs decorated 3D bionic silicon nanograss arrays pattern with high-density hot-spots for SERS sensing via green galvanic displacement without additives

The Si nanograss arrays are directly grown on Si substrate via catalyst-assisted VLS growth and subsequent plasma interaction. AgNPs were rapidly immobilized on Si nanograss arrays for SERS sensing, without any organic reagents and additives.

One Dimensional ZnO Nanostructures: Growth and Chemical Sensing Performances

Recently, one-dimensional (1D) nanostructures have attracted the scientific community attention as sensitive materials for conductometric chemical sensors. However, finding facile and low-cost techniques for their production, controlling the morphology and the aspect ratio of these nanostructures is still challenging. In this study, we report the vapor-liquid-solid (VLS) synthesis of one dimensional (1D) zinc oxide (ZnO) nanorods (NRs) and nanowires (NWs) by using different metal catalysts and their impact on the performances of conductometric chemical sensors. In VLS mechanism, catalysts are of great interest due to their role in the nucleation and the crystallization of 1D nanostructures. Here, Au, Pt, Ag and Cu nanoparticles (NPs) were used to grow 1D ZnO. Depending on catalyst nature, different morphology, geometry, size and nanowires/nanorods abundance were established. The mechanism leading to the VLS growth of 1D ZnO nanostructures and the transition from nanorods to nanowires have been interpreted. The formation of ZnO crystals exhibiting a hexagonal crystal structure was confirmed by X-ray diffraction (XRD) and ZnO composition was identified using transmission electron microscopy (TEM) mapping. The chemical sensing characteristics showed that 1D ZnO has good and fast response, good stability and selectivity. ZnO (Au) showed the best performances towards hydrogen (H2). At the optimal working temperature of 350 °C, the measured response towards 500 ppm of H2 was 300 for ZnO NWs and 50 for ZnO NRs. Moreover, a good selectivity to hydrogen was demonstrated over CO, acetone and ethanol.

Photocatalytic Bi2O3/TiO2:N Thin Films with Enhanced Surface Area and Visible Light Activity

Bi2O3 nanocone films functionalized with an overlayer of TiO2 were deposited by d.c. reactive magnetron sputtering. The aforementioned nanocone structures were formed via a vapour-liquid-solid (VLS) growth, starting from a catalytic bismuth seed layer. The resultant nanocones exhibit an improved surface area, measured by atomic force microscopy, when compared to non-VLS deposition of the same metal oxide. X-ray diffraction texture analysis enabled the determination of the crystallographic β-phase of Bi2O3. A very thin TiO2 overlayer (6 nm thick), undoped and doped with nitrogen, was deposited onto the nanocones template, in order to functionalize these structures with a photocatalytic, self-cleaning, cap material. N-doped TiO2 overlayers increased the selective absorption of visible light due to nitrogen doping in the anatase cell, thus, resulting in a concomitant increase in the overall photocatalytic efficiency.

Synthesis of Multiwall Boron Nitride (BN) Nanotubes by a PVD Method Based on Vapor–Liquid–Solid Growth

Multiwall boron nitride (BN) nanotubes were synthesized by a novel physical vapor deposition (PVD) method, in which the BN nanotubes grow on a compact substrate composed of AlN, γ-Al2O3, Y2O3, and carbon powders. The obtained BN nanotubes assemble in an orderly manner with a typical length of over one millimeter and a diameter of one-hundred nanometers. The hollow multiwall tubes have a spherical tip, which is presumed to be a liquid drop at the synthesis temperature, indicating the vapor–liquid–solid (VLS) growth mechanism.