Ga Nanodroplets Research Articles

Synthesis of Planet‐Like Liquid Metal Nanodroplets with Promising Properties for Catalysis

AbstractLiquid metal nanodroplets are an emerging class of underexplored materials with significant potential in many applications, including catalysis, bio‐therapeutics, and phase‐change materials. These nanostructures are generally synthesized by mechanical agitation via ultrasonication of low‐melting metals like Ga. Once these materials are successfully synthesized, they can be suspended in a vast array of different solvents. However, one major issue arises specifically with liquid metal alloys which are found to de‐alloy in the sonication process. Here, it is demonstrated that this challenge can be overcome by undertaking sonication at high temperatures, suspending nanodroplets within molten sodium acetate (NaOAc). After cooling, the nanostructures become planet‐like nanodroplets which are covered by an interfacial oxide crust, feature a liquid metal mantle, and a solid core. The molten salt solvent can effectively be removed rendering this approach to be ideal, especially for catalysts. The proof‐of‐concept application is demonstrated by carrying out electrocatalytic ethanol oxidation, using the Cu–Ga system. The superior performance of the Cu–Ga nanodroplets highlights potential in catalyzing a vast array of reactions. Aside from the Cu–Ga system, this facile process can be applied to multiple other systems, including Ag–Ga, Zn–Ga, Bi–Ga, In–Cu, and Sn–Cu.

Ga-Pt supported catalytically active liquid metal solutions (SCALMS) prepared by ultrasonication - influence of synthesis conditions on n-heptane dehydrogenation performance.

Supported catalytically active liquid metal solution (SCALMS) materials represent a recently developed class of heterogeneous catalysts, where the catalytic reaction takes place at the highly dynamic interface of supported liquid alloys. Ga nuggets were dispersed into nano-droplets in propan-2-ol using ultrasonication followed by the addition of Pt in a galvanic displacement reaction - either directly into the Ga/propan-2-ol dispersion (in situ) or consecutively onto the supported Ga droplets (ex situ). The in situ galvanic displacement reaction between Ga and Pt was studied in three different reaction media, namely propan-2-ol, water, and 20 vol% water containing propan-2-ol. TEM investigations reveal that the Ga-Pt reaction in propan-2-ol resulted in the formation of Pt aggregates on top of Ga nano-droplets. In the water/propan-2-ol mixture, the desired incorporation of Pt into the Ga matrix was achieved. The ex situ prepared Ga-Pt SCALMS were tested in n-heptane dehydrogenation. Ga-Pt SCALMS synthesized in pure alcoholic solution showed equal dehydrogenation and cracking activity. Ga-Pt SCALMS prepared in pure water, in contrast, showed mainly cracking activity due to oxidation of Ga droplets. The Ga-Pt SCALMS material prepared in water/propan-2-ol resulted in high activity, n-heptene selectivity of 63%, and only low cracking tendency. This can be attributed to the supported liquid Ga-Pt alloy where Pt atoms are present in the liquid Ga matrix at the highly dynamic catalytic interface.

Smart Adhesive Patches of Antibacterial Performance Based on Polydopamine-Modified Ga Liquid Metal Nanodroplets

Patches have been widely used in medical and healthcare, motion detection, and other fields. Patches are required to possess strong adhesive forces to keep them effectively adhered to the skin during use and to be removed from the skin with relative ease after usage. Moreover, a large number of bacteria will proliferate at the interface between the patch and the skin, when the patch is used for lengthy durations, which is harmful to human health. Hence, there is an increasing demand for patches with combined switchable adhesions and excellent antibacterial performances. Inspired by the natural adhesive performance of snail slime in wet (liquid) and dry (solid) states, smart Ga liquid metal nanodroplet-based polymer patch (GxPP, x = 10, 30, 50) with switchable adhesion properties were prepared by incorporating polydimethylsiloxane (PDMS) into polydopamine (PDA)-modified Ga nanodroplets (PDA-Ga). Since the melting point of Ga is 29.8 °C, solid Ga melts to the liquid state at temperatures above 29.8 °C, and this significantly decreases the modulus of GxPP, resulting in significantly decreased adhesive strength of GxPP. The adhesion of G30PP decreased from 40.4 to 22.1 kPa. In addition to this obvious change in the adhesion properties of GxPP, the unique antibacterial performances of Ga imparted GxPP with excellent antibacterial effects. Compared with the PDMS, the antibacterial rates of G30PP against Escherichia coli and Staphylococcus aureus were 98 and 99%, respectively. The excellent antibacterial performance and smart switchable adhesion properties of GxPP revealed its high application prospects in the field of wearable flexible skin electronics.

XPS modeling of GaN/GaAs nanostructure grown by the droplet epitaxy technique

The contribution of XPS spectroscopy and its associated modeling to the study of metallic and semiconductor nanostructure design is highlighted in this work. We first give a brief description of the growth kinetics of pure and binary materials on hydrophilic substrates showing the real shape of the nanodroplets to be a spherical cap. This morphology is implemented in the XPS signal modeling of pure and binary nanodroplets. Then, the theoretical calculations will be compared to the XPS intensities in situ carried out on the Ga/GaAs(111)A and GaN/GaAs(111)A nanostructures. This comparison allows the shape of Ga and GaN nanodroplets to be determined, giving their radius, height and coverage. Finally, XPS results are compared to AFM and SEM studies to confirm the XPS modeling. Moreover, the influence of using inelastic mean free paths or effective attenuation lengths in the XPS signal modeling is discussed.

Gallium Nanodroplets are Anti-Inflammatory without Interfering with Iron Homeostasis.

Gallium (Ga) compounds, as the source of Ga ions (Ga3+), have been historically used as anti-inflammatories. Currently, the widely accepted mechanisms of the anti-inflammatory effects for Ga3+ are rationalized on the basis of their similarities to ferric ions (Fe3+), which permits Ga3+ to bind with Fe-binding proteins and subsequently disturbs the Fe homeostasis in the immune cells. Here in contrast to the classic views, our study presents the mechanisms of Ga as anti-inflammatory by delivering Ga nanodroplets (GNDs) into lipopolysaccharide-induced macrophages and exploring the processes. The GNDs show a selective inhibition of nitric oxide (NO) production without affecting the accumulation of pro-inflammatory mediators. This is explained by GNDs disrupting the synthesis of inducible NO synthase in the activated macrophages by upregulating the levels of eIF2α phosphorylation, without interfering with the Fe homeostasis. The Fe3+ transferrin receptor-independent endocytosis of GNDs by the cells prompts a fundamentally different mechanism as anti-inflammatories in comparison to that imparted by Ga3+. This study reveals the fundamental molecular basis of GND-macrophage interactions, which may provide additional avenues for the use of Ga for anti-inflammatory and future biomedical and pharmaceutical applications.

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.

X-ray photoelectron spectroscopy study of Ga nanodroplet on silica-terminated silicon surface for nanowire growth

In this paper the early stages of the self-catalyzed Vapor-Liquid-Solid (VLS) growth of GaAs nanowires on Si substrates by Molecular Beam Epitaxy (MBE) are studied. The interaction of Ga nanodroplets (NDs) with the silica overlayer is investigated by X-ray Photoemission Spectroscopy (XPS) and Atomic Force Microscopy (AFM). We show how Ga NDs drill the silica overlayer and make contact with bulk Si to allow GaAs nanowires (NWs) epitaxial growth by studying each of the three steps of the NW growth process sequentially: Ga NDs pre-deposition, post-deposition annealing to reach the NW growth temperature, and finally NW growth itself. The pre-deposition temperature allows to control the density and morphology of the NDs. A high enough annealing temperature enhances the reduction of SiOx by Ga oxidation and leads to the formation of holes which is seen by a new component in XPS spectra, assumed to be the consequence of the GaSi interaction. Finally, these nano-holes act as nucleation sites for the epitaxial GaAs VLS growth. During the GaAs growth, three different chemical environments of Ga are identified in Ga2p core level: metallic Ga due to the droplets at the top of the NWs, Ga oxide in contact with the SiOx overlayer and Ga arsenide from NWs.

Liquid gallium encapsulated in carbon nanofibers for high performance lithium storage

Freestanding membrane built from interwoven carbon fibers with encapsulated liquid gallium (Ga) nanodroplets has been demonstrated via the ultrasonic cell disrupting and electrospinning approaches along with a subsequent annealing process. Employing this membrane as a freestanding anode for lithium storage, the severe volume changes of Ga can be effectively accommodated due to the unique encapsulated 3D network structure. As a result, the membrane exhibits superior electrochemical performance, including high reversible capacity (450 mAh g−1), enhanced cycling stability (247 mAh g−1 at 0.65 A g−1 over 250 cycles), as well as good rate performance (200 mAh g−1 at 2 A g−1).

Epitaxial GaN nanotripods: morphology and crystal structure

The goal of this work is to study the morphology and crytal structure of novel GaN epitaxial nanostructures (Y-shape nanotripods) grown on silicon (111) by plasma assisted molecular beam epitaxy (PA-MBE). Prior to the nanowire synthesis epitaxial GaN nanoparticles were formed on the Si(111) via nitridation of Ga nanodroplets on Si substrate surface (droplet epitaxy technique). Effect of the seeding layer on futher nanostructure growth, GaN/Si heterointerface formation, crystal structure and morphology is studied by use of different microscopic techniques.

Ion-Beam-Directed Self-Ordering of Ga Nanodroplets on GaAs Surfaces.

Ordered nanodroplet arrays and aligned nanodroplet chains are fabricated using ion-beam-directed self-organization. The morphological evolution of nanodroplets formed on GaAs (100) substrates under ion beam bombardment is characterized by scanning electron microscopy and atomic force microscopy. Ordered Ga nanodroplets are self-assembled under ion beam bombardment at off-normal incidence angles. The uniformity, size, and density of Ga nanodroplets can be tuned by the incident angles of ion beam. The ion beam current also plays a critical role in the self-ordering of Ga nanodroplets, and it is found that the droplets exhibit a similar droplet size but higher density and better uniformity with increasing the ion beam current. In addition, more complex arrangements of nanodroplets are achieved via in situ patterning and ion-beam-directed migration of Ga atoms. Particularly, compared to the destructive formation of nanodroplets through direct ion beam bombardment, the controllable assembly of nanodroplets on intact surfaces can be used as templates for fabrication of ordered semiconductor nanostructures by droplet epitaxy.

Single-fluxon controlled resistance switching in centimeter-long superconducting gallium-indium eutectic nanowires.

The ability to manipulate a single quantum object, such as a single electron or a single spin, to induce a change in a macroscopic observable lies at the heart of nanodevices of the future. We report an experiment wherein a single superconducting flux quantum, or a fluxon, can be exploited to switch the resistance of a nanowire between two discrete values. The experimental geometry consists of centimeter-long nanowires of superconducting Ga-In eutectic, with spontaneously formed Ga nanodroplets along the length of the nanowire. The nonzero resistance occurs when a Ga nanodroplet traps one or more superconducting fluxons, thereby driving a Josephson weak-link created by a second nearby Ga nanodroplet normal. The fluxons can be inserted or flipped by careful manipulation of the magnetic field or temperature to produce one of many metastable states of the system.

Site-controlled fabrication of Ga nanodroplets by focused ion beam

Ga droplets are created by focused ion beam irradiation of GaAs surface. We report that ordered Ga droplets can be formed on the GaAs surface without any implantation damage. The droplets are characterized with bigger sizes than those droplets formed on damaged area. These aligned Ga droplets are formed via the migration of Ga atoms from ion irradiation area to the edge of undamaged GaAs surface and further nucleation into droplets. The morphological evolution and size distribution of these nanodroplets are investigated systematically with different beam irradiation time and incident angles. Based on this method, well positioned Ga nanodroplets, such as chains, are achieved by using focus ion beam patterning. The controllable assembly of droplets on undamaged semiconductor surface can be used to fabricate templates, to fabricate quantum structures and quantum devices by droplet epitaxy technique.

Formation and nitridation of InGa composite droplets on Si(1 1 1): In-situ study by high resolution X-ray photoelectron spectroscopy

We demonstrate applying synchrotron radiation based high resolution X-ray photoelectron spectroscopy as a versatile in-situ tool to study the step-wise formation and nitridation of indium gallium (InGa) composite droplets on Si(111)7 × 7. This includes analysis of initial interactions of Ga and In with Si(111)7 × 7, and characterization of formation and nitridation of InGa droplets. The results show that after stabilization of Si(111)7 × 7 structure with Ga and In at 750°C and formation of Ga nanodroplets as the bases, the InGa composite droplets (In:Ga ≈ 3:1) were formed at RT and 200°C in a coverage range 3–14 monolayers. The nitridation efficiency and structure of the droplets/Si(111) with NH3 were temperature dependent. At or below 350°C, the droplets were hardly nitridated. At 670°C, the nitridation of InGa was more completed, which however caused an unavoidable nitridation of Si surface. The optimum nitridation occurred around 480°C and proceed in a 2D mode. Further simultaneous growth and nitridation of the InGa droplets on the 2D nitridated surface at 400°C resulted in a complex surface composition and structure. An ex-situ atomic force microscopy reveals both the aligned metallic droplet-island assemblies and the fractured nitridated island bases with small droplets on top for the final surface.

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.

Dynamic nanoscale in situ TEM tribology

ABSTRACTNanotribology, the study of friction, wear and lubrication at the nanoscale is an important area of research; however in practice due to the size scale, requires specifically designed tools to characterize nanoscale contacts. We have developed a TEM triboprobe incorporating an advanced nano-positioner with 3D programmable motion inside a transmission electron microscope (TEM) which allows us to selectively apply multiple reciprocating wear cycles to a nanoscale surface, and observe in real-time dynamical changes and the evolution of wear around a sliding nano-contact.Nanoscale cyclic rubbing of an automotive aluminum-silicon alloy processed by focused ion beam (FIB) reveals dynamical surface fragmentation and the generation of nanoscale debris particles. The nanoparticles undergo complex motion as they interact with the sliding nano-contact. Over hundreds of reciprocating cycles, frictional heating leads to a phase separation of the Ga+ ions implanted by FIB forming liquid Ga nano-droplets and liquid bridges. The addition of nanoscale debris particles and liquid droplets dramatically changes the wear dynamics and transforms a 2-body sliding contact into a complex 4-body solid–liquid system exhibiting time-dependent, non-equilibrium kinetic behavior.TEM nanotribology opens up new possibilities for the real-time quantification of cyclic friction, wear and dynamic solid–liquid nano-mechanics, which will have widespread applications in many areas of nano-science and nanotechnology.

Friction-formed liquid droplets

The formation of nanoscale liquid droplets by friction of a solid is observed in real-time.This is achieved using a newly developed in situ transmission electron microscope (TEM)triboprobe capable of applying multiple reciprocating wear cycles to a nanoscale surface.Dynamical imaging of the nanoscale cyclic rubbing of a focused-ion-beam (FIB) processedAl alloy by diamond shows that the generation of nanoscale wear particles is followed by aphase separation to form liquid Ga nanodroplets and liquid bridges. The transformation ofa two-body system to a four-body solid–liquid system within the reciprocating weartrack significantly alters the local dynamical friction and wear processes. Movingliquid bridges are observed in situ to play a key role at the sliding nanocontact,interacting strongly with the highly mobile nanoparticle debris. In situ imagingdemonstrates that both static and moving liquid droplets exhibit asymmetricmenisci due to nanoscale surface roughness. Nanodroplet kinetics are furthermoredependent on local frictional temperature, with solid-like surface nanofilamentsforming on cooling. TEM nanotribology opens up new avenues for the real-timequantification of cyclic friction, wear and dynamic solid–liquid nanomechanics, which willhave widespread applications in many areas of nanoscience and nanotechnology.

Optical characteristics of highly ordered gallium oxide nano-islands on GaAs

A periodic array of Ga oxide islands was obtained by annealing the highly ordered Ga nano-droplets on GaAs surface at 400°C under an oxygen atmosphere for 7 hours. These Ga oxides are a mixture of α-Ga2O3 and β-Ga2O3 confirmed by Raman spectroscopy study. Enhanced optical transmission of GaAs with such ordered Ga oxide nano-islands was obtained. Both dielectric and dimensional confinement effects were considered in analysis of the electromagnetic characteristics of the nanostructured materials. Finite-difference time-domain method was used to numerically study the light transmission through the patterned Ga oxide on GaAs surface. Based on the calculated results, the light transmission enhancement is attributed to the formation of the ordered nano Ga oxides.

Highly Ordered Ga Nanodroplets on a GaAs Surface Formed by a Focused Ion Beam

The morphological evolution of a GaAs surface induced by a focused ion beam (FIB) has been investigated by in situ electron microscopy. Under off-normal bombardment without sample rotation, Ga droplets with sizes from 70 to 25 nm in diameter on the GaAs surface can self-assemble into a highly ordered hexagonal pattern instead of Ostwald ripening or coalescence. The mechanism relies on a balance between anisotropic loss of atoms on the surface of droplets due to sputtering and an anisotropic supply of atoms on the substrate surface due to preferential sputtering of As. The ratio of wavelength to the droplet diameter predicted by this model is in excellent agreement with experimental observations.

Real-time x-ray studies of gallium adsorption and desorption

Real-time grazing-incidence small-angle x-ray scattering has been employed to study the adsorption and desorption of Ga on c-plane sapphire and Ga-polar GaN surfaces. Formation of self-organized liquid Ga nanodroplets has been observed on sapphire during Ga exposure from an effusion cell at high flux. Subsequent to the Ga deposition, the nanodroplets were nitridated in situ by a nitrogen plasma source, which converted the droplets into GaN nanodots. In addition to the droplet studies, at lower Ga flux, the adsorption and desorption of Ga have been studied in the predroplet regime. For identical processing conditions, significantly different Ga adsorption∕desorption rates were observed on sapphire and GaN surfaces.