Metal Nanodroplets Research Articles

Spontaneous growth of III-nitride 1D and 0D nanostructures on to vertical nanorod arrays

In this work, we report on the growth of AlGaN nanostructures by the droplet epitaxy process. Initially, well-oriented vertical AlN nanorod clusters were grown on to c-plane sapphire substrates by Plasma Assisted Molecular Beam Epitaxy (PA-MBE). On top of these AlN nanorods an Al0.76Ga0.24N layer was deposited, followed by 40 pairs of Al0.76Ga0.24N/AlN Multiple Quantum Wells (MQWs). Spontaneously formed nanodots and nanowires were observed by Field Emission Scanning Electron Microscopy (FESEM) on the top planes of these vertical nanorod arrays. For nearly stoichiometric conditions, 20 nm diameter AlGaN nanodots were formed selectively at the step edges generated during step-flow growth. For growth under excess group III conditions, lateral nanowires were formed perpendicular to the edges of the vertical nanorods, with widths varying from 20 nm near the center to 70 nm at the edge for a length of ∼150 nm. An enhancement of at least 15 times was obtained in the Cathodoluminescence (CL) emission peak intensity (290 nm) from the top of the nanorod structures. We believe these spontaneously formed well-ordered nanodot and horizontal nanowire structures are generated by the sequential formation and Nitridation of metal nanodroplets on the growth surface. Their properties can be controlled by optimization of the deposition parameters by varying the surface diffusivity of Ga and Al adatoms.

Growth Dynamics of Gallium Nanodroplets Driven by Thermally Activated Surface Diffusion.

The growth of catalytic liquid-metal nanodroplets on flat substrates is essential for many technological applications. However, the detailed nucleation and growth dynamics of these nanodroplets remain unclear. Here, using in situ transmission electron microscopy (TEM) imaging, we track in real time the growth of individual Ga nanodroplets from a beam of Ga vapor. We show that the nucleation and growth are driven by thermally activated surface diffusion of Ga adatoms, with the diffusion activation energy of ED = 95 ± 10 meV on a SiNx surface. More importantly, our analysis shows that Ga dimers serve as the critical nucleation clusters and that the nanodroplet growth follows a power-law of the form R(t) ∝ e-ED/kBT(t - t0)1/2. These insights into the growth dynamics of metallic nanodroplets are essential for tailoring their size and density for their application in self-catalyzed growth of nanomaterials.

Early-stage dynamics of metallic droplets embedded in the nanotextured Mott insulating phase of V2O3

Unveiling the physics that governs the intertwining between the nanoscale self-organization and the dynamics of insulator-to-metal transitions (\textit{IMT}) is key for controlling on demand the ultrafast switching in strongly correlated materials and nano-devices. A paradigmatic case is the \textit{IMT} in V$_2$O$_3$, for which the mechanism that leads to the nucleation and growth of metallic nano-droplets out of the supposedly homogeneous Mott insulating phase is still a mystery. Here, we combine X-ray photoemission electron microscopy and ultrafast non-equilibrium optical spectroscopy to investigate the early stage dynamics of isolated metallic nano-droplets across the \textit{IMT} in V$_2$O$_3$ thin films. Our experiments show that the low-temperature monoclinic antiferromagnetic insulating phase is characterized by the spontaneous formation of striped polydomains, with different lattice distortions. The insulating domain boundaries accommodate the birth of metallic nano-droplets, whose non-equilibrium expansion can be triggered by the photo-induced change of the 3$d$-orbital occupation. We address the relation between the spontaneous nanotexture of the Mott insulating phase in V$_2$O$_3$ and the timescale of the metallic seeds growth. We speculate that the photoinduced metallic growth can proceed along a non-thermal pathway in which the monoclinic lattice symmetry of the insulating phase is partially retained.

Size Dependence of the Surface Tension of Nanoparticles

The linear relationship between surface tension γs and radius $$~{{R}_{{\text{s}}}}$$ of small particles (nanoparticles) was derived in the 1960s by Rusanov as $${{{\gamma }}_{{\text{s}}}} = K{{R}_{{\text{s}}}},$$ where K is the coefficient of proportionality. In this work, a more generalized dependence is obtained in the context of Gibbs’ theory of thermodynamics for curved interfaces on the same size scales for a randomly selected interface, including an equimolecular surface with radius $${{R}_{{\text{e}}}}{\text{.}}$$ It is shown that when $${{R}_{0}} \geqslant R \geqslant {\delta }$$ (where $${\delta } = {{R}_{{\text{e}}}} - {{R}_{{\text{s}}}},$$ $${{R}_{0}}$$ is a characteristic radius limiting the range of the Rusanov equation’s applicability), linear dependence $${\gamma } = KR$$ should be valid for all $$R \in \left[ {{{R}_{{\text{s}}}},{{R}_{{\text{e}}}}} \right].$$ Parameter $$K~$$ is estimated for metal nanodroplets and solid metal nanoparticles as well.

Shape-controlled synthesis of liquid metal nanodroplets for photothermal therapy

The capping agents for liquid metal (LM) nanodroplets in aqueous solutions are restricted to thiol-containing and positively-charged molecules or macromolecules. However, both thiolate-metal complex and electrostatic interaction are liable to detachment upon strong mechanical forces such as sonication, leading to limited stability and applications. To address this, we utilized ultrasmall water soluble melanin nanoparticles (MNPs) as the capping agent, which exhibited strong metal binding capability with the oxide layer of gallium based LMs and resulted in enhanced stability. Interestingly, shape-controlled synthesis of LM nanodroplets can be achieved by the incorporation of MNPs. Various EGaIn nanostructures including nanorice, nanosphere and nanorod were obtained by simply tuning the feed ratio, sonication time, and suspension temperature. Among these shapes, EGaIn nanorice has the best photothermal conversion efficiency, which could be leveraged for photothermal therapy.

Solution processable liquid metal nanodroplets by surface-initiated atom transfer radical polymerization.

Eutectic gallium indium (EGaIn) is a liquid metal alloy at room temperature. EGaIn microdroplets can be incorporated into elastomers to fabricate highly stretchable, mechanically robust, soft multifunctional composites with high thermal stability1 and electrical conductivity2-4 that are suitable for applications in soft robotics and self-healing electronics5-7. However, the current methods of preparation rely on mechanical mixing, which may lead to irregularly shaped micrometre-sized droplets and an anisotropic distribution of properties8. Therefore, procedures for the stabilization of sub-micrometre-sized droplets of EGaIn and compatibilization in polymer matrices and solvents have attracted significant attention9-12. Here we report the synthesis of EGaIn nanodroplets stabilized by polymeric ligand encapsulation. We use a surface-initiated atom transfer radical polymerization initiator to covalently functionalize the oxide layer on the surface of the EGaIn nanodroplets13 with poly(methyl methacrylate) (PMMA), poly(n-butyl acrylate) (PBMA), poly(2-dimethylamino)ethyl methacrylate) (PDMAEMA) and poly(n-butyl acrylate-block-methyl methacrylate) (PBA-b-PMMA). These nanodroplets are stable in organic solvents, in water or in polymer matrices up to 50 wt% concentration, enabling direct solution-casting into flexible hybrid materials. The liquid metal can be recovered from dispersion by acid treatment. The nanodroplets show good mechanical, thermal and optical properties, with a remarkable suppression of crystallization and melting temperatures (down to -80 °C from 15 °C).

Engineering hot electrons of localized surface plasmon on InGaN photoanode for solar-powered water splitting

We demonstrate an effective approach to enhancing the performance of III-nitride-based photoelectrode by utilization of hot electrons generated from localized surface plasmons (LSPs). The device designed as a TiO2 / Au / InGaN sandwich structure is fabricated by cladding Au nanodroplets with a thin TiO2 layer to provide a large interfacial layer, which is beneficial for the hot electron collection efficiency by electron reflection via an additional energy barrier. The applied bias photo-to-current efficiency is enhanced by up to four times due to the contribution of hot electrons generated from the absorbed light with wavelengths >470 nm, corresponding to photon energies below the band gap of InGaN (2.67 eV) recognized from incident photon-to-current efficiency spectrum. The LSPs of our photoelectrodes also exhibit a suppressed size-dependence on the metal nanodroplets, feasible for mass production.

Liquid Metal Nanodroplets: Light‐Induced Shape Morphing of Liquid Metal Nanodroplets Enabled by Polydopamine Coating (Small 9/2019)

Liquid Metal Nanodroplets: Light‐Induced Shape Morphing of Liquid Metal Nanodroplets Enabled by Polydopamine Coating (Small 9/2019)

Light-Induced Shape Morphing of Liquid Metal Nanodroplets Enabled by Polydopamine Coating.

Shape morphing nanosystems have recently attracted much attention and a number of applications are developed, spanning from autonomous robotics to drug delivery. However, the fabrication of such nanosystems remains at an early stage owing to limited choices of strategies and materials. This work reports a facile method to fabricate liquid metal (LM) nanodroplets by sonication of bulk LM in an aqueous dopamine hydrochloride solution and their application in light-induced shape morphing at the nanoscale. In this method, dopamine acts as a surfactant, which stabilizes the LM nanodroplets dispersion during the sonication, and results in downsizing of the nanodroplets. Furthermore, by adding 2-amino-2-(hydroxymethyl)-1,3-propanediol to the suspension, self-polymerization of dopamine molecules occurs, resulting in the formation of polydopamine (PDA)-coated LM nanodroplets. Owing to the high photothermal conversion of the PDA, PDA-coated LM nanodroplets are transformed from spherical shapes to ellipsoids by NIR laser irradiation. This study paves a simple and reliable pathway for the preparation of functional LM nanodroplets and their application as shape-morphing nanosystems.

Metal nanodroplets catalyzed growth of ZnS nanowires with a high aspect ratio via long-pulse-width laser ablation in the liquid phase

ZnS nanowires with a high aspect ratio were successfully prepared by long-pulse-width millisecond laser ablation of Cu/Zn binary metal target immersed in dodecyl mercaptan (DDM) liquid under room conditions. The formation of ZnS nanowires was determined by the Cu nanodroplets formed upon laser ablation and laser parameters. The Cu nanodroplets formed upon laser ablation were proposed to act as catalyst to promote the growth of ZnS nanowires via vapor-liquid-solid mechanism in relatively stable micro-environment that close to the surface of the target.

An experimental and computational study of size-dependent contact-angle of dewetted metal nanodroplets below its melting temperature

Decorating 1D nanostructures (e.g., wires and tubes) with metal nanoparticles serves as a hierarchical approach to integrate the functionalities of metal oxides, semiconductors, and metals. This paper examines a simple and low-temperature approach to self-assembling gold nanoparticles (Au-np)—a common catalytic material—onto silicon nanowires (SiNWs). A conformal ultra-thin film (i.e., <15 nm thick) is deposited onto SiNWs and thermally dewetted, forming nanoparticles in the 6–70 nm range. Two parameters of its morphology are dependent upon dewetting conditions: particle size and particle contact angle. Using transmission electron microscopy imaging, it is found that annealing temperature profile has a strong effect on the particle size. Additionally, the contact angle is found to be dependent on particle size and temperature even below the eutectic temperature of the Au-Si alloy. Molecular dynamics simulations were performed to investigate potential explanations for such experimental observation. In this temperature regime, the simulations reveal the formation of an amorphous phase at the interface between the catalyst and SiNW that is sensitive to temperature. This amorphous layer increases the adhesion energy at the interface and explains the contact angle dependence on temperature.

Nanodroplets for Stretchable Superconducting Circuits

The prospective utilization of nanoscale superconductors as micro/nanocoils or circuits with superior current density and no electrical resistance loss in next‐generation electronics or electromagnetic equipment represents a fascinating opportunity for new microsystem technologies. Here, a family of superconducting liquid metals (Ga–In–Sn alloys) and their nanodroplets toward printable and stretchable superconducting micro/nanoelectronics is developed. By tuning the composition of liquid metals the highest superconducting critical temperature (Tc) in this family can be modulated and achieved as high as 6.6 K. The liquid metal nanodroplets retain their bulk superconducting properties and can be easily dispersed in different solvents as inks. The printable and stretchable superconducting micro/nano coils, circuits and electrodes have been fabricated by inkjet printer or laser etching by using superconducting nanodroplets inks. This novel superconducting system greatly promotes the commercial utilization of superconductors into advanced flexible micro/nanoelectronic devices and offers a new platform for developing more application with superconductors.

Metallic Nanodroplet Induced Coulomb Catalysis for Off-Resonant Plasmonic Enhancement of Photoemission in Semiconductors.

The enhancement of light from semiconductors due to surface plasmons coupled resonantly to its emission is limited because of dissipation in the metal and is also restricted by the dielectric characteristics and homogeneity of the metal–semiconductor interface. We report a new mechanism based on electrostatic interactions of carriers and their image charges in metals to generate more photons from optical sources at frequencies that are off-resonant to the localized plasmon frequency. Coulomb catalysis of carrier accumulation resulting from the inhomogeneity of metal nanodroplets on a semiconductor’s surface can result in an enhancement of light that is nondissipative and does not require resonant coupling of plasmons to the emission wavelength. The enhancement occurs because of an increase in the ratio of radiative to nonradiative recombination in the vicinity of metal nanoparticles. It is equally effective with any type of metal and enhances radiation at any frequency, a property that is of principal importance for the realization of widely tunable semiconductor emitters. This fundamental mechanism provides a new perspective for improving the efficiency of light emitters and controlling carrier concentration on the nanoscale. The structural characteristics of the hybrid metal–semiconductor emitters are studied using electron microscopy and atomic force microscopy. We demonstrate the electrostatic mechanism by studying steady-state and transient photoluminescence from two-dimensional semiconductors, such as GaAs/AlGAs quantum wells, and bulk semiconductors, such as ZnO thin films, emitting in the near-IR and UV wavelength regimes, respectively.

Extrapolating Dynamic Leidenfrost Principles to Metallic Nanodroplets on Asymmetrically Textured Surfaces.

In an effort to enhance our knowledge on how to control the movement of metallic nanodroplets, here we have used classical molecular dynamics simulations to investigate whether Cu nanostructures deposited on nanopillared substrates can be made to jump at desired angles. We find that such control is possible, especially for Cu nanostructures that are symmetric; for asymmetric nanostructures, however, control is more uncertain. The work presented here borrows ideas from two seemingly different fields, metallic droplets and water droplets in the dynamic Leidenfrost regime. Despite the differences in the respective systems, we find common ground in their behavior on nanostructured surfaces. Due to this, we suggest that the ongoing research in Leidenfrost droplets is a fertile area for scientists working on metallic nanodroplets.

Dewetting Properties of Metallic Liquid Film on Nanopillared Graphene

In this work, we report simulation evidence that the graphene surface decorated by carbon nanotube pillars shows strong dewettability, which can give it great advantages in dewetting and detaching metallic nanodroplets on the surfaces. Molecular dynamics (MD) simulations show that the ultrathin liquid film first contracts then detaches from the graphene on a time scale of several nanoseconds, as a result of the inertial effect. The detaching velocity is in the order of 10 m/s for the droplet with radii smaller than 50 nm. Moreover, the contracting and detaching behaviors of the liquid film can be effectively controlled by tuning the geometric parameters of the liquid film or pillar. In addition, the temperature effects on the dewetting and detaching of the metallic liquid film are also discussed. Our results show that one can exploit and effectively control the dewetting properties of metallic nanodroplets by decorating the surfaces with nanotube pillars.

Dry‐Released Nanotubes and Nanoengines by Particle‐Assisted Rolling

Surface tension of self-assembled metal nanodroplets can be applied to overcome the deformation barriers of strain-engineered nanomembranes and produce extremely nanoscale tubes. Aggregated nanoparticles stress nanomembranes and subsequently integrate on the walls of rolled-up nanotubes, which can speed up the tubular engines owing to the enhanced electrocatalytic activity.

Crystallization kinetics of Ga metallic nano-droplets under As flux

We present an experimental investigation of the crystallization dynamics of Ga nano-droplets under As flux. The transformation of the metallic Ga contained in the droplets into a GaAs nano-island proceeds by increasing the size of a tiny ring of GaAs which is formed just after the Ga deposition at the rim of a droplet. The GaAs crystallization rate depends linearly on the liquid–solid interface area. The maximum growth rate is set by the As flux impinging on the droplet, thus showing an efficient As incorporation and transport despite the predicted low solubility of the As in metallic Ga at the crystallization temperatures.

From nanoparticle chains to nanorods: control of ZnO nanostructures by laser ablation

Pb-doped ZnO nanostructures were prepared using millisecond pulse-width laser ablation. Nanoparticle chains formed when the laser ablation was performed in air or on a pure Zn target, and when Ar flow was introduced it could be used to mediate the nucleation and growth of crystalline nanorods from a composite Zn/Pb target. Based on the results of control experiments, a mechanism was proposed to explain the formation of the observed nanostructures. Namely, the millisecond laser pulse can generate metal nanodroplets in contrast to a plasma or vapor plume, which then react with the controlled environmental gas to give rise to different nanostructures. The final morphology is dependent on the target composition and oxygen content of the ambient atmosphere during ablation.

Uptake of water droplets by non-wetting capillaries

We present direct experimental evidence that water droplets can spontaneously penetrate non-wetting capillaries, driven by the action of Laplace pressure due to high droplet curvature. Using high-speed optical imaging, microcapillaries of radius 50 μm to 150 μm, and water microdroplets of average radius between 100 μm and 1900 μm, we demonstrate that there is a critical droplet radius below which water droplets can be taken up by hydrophobised glass and non-wetting polytetrafluoroethylene (PTFE) capillaries. The rate of capillary uptake is shown to depend strongly on droplet size, with smaller droplets penetrating the tube more quickly. Droplet size is also shown to influence meniscus motion in a pre-filled non-wetting capillary, and quantitative measurements of this effect result in a derived water-PTFE static contact angle between 96° and 114°. Our measurements confirm recent theoretical predictions and simulations for metal nanodroplets penetrating non-wetting carbon nanotubes (CNTs). The results are relevant to a wide range of technological applications, such as microfluidic devices, ink-jet printing, and the penetration of fluids in porous materials.

Laser fabrication of 2D and 3D metal nanoparticle structures and arrays

A novel method for fabrication of 2D and 3D metal nanoparticle structures and arrays is proposed. This technique is based on laser-induced transfer of molten metal nanodroplets from thin metal films. Metal nanoparticles are produced by solidification of these nanodroplets. The size of the transferred nanoparticles can be controllably changed in the range from 180 nm to 1500 nm. Several examples of complex 2D and 3D microstructures generated form gold nanoparticles are demonstrated.