Atomic Mixing Research Articles

Impacts of various interfacial nanostructures on spectral phonon thermal boundary conductance

Nanoengineering of interfaces has become an effective way to tune the thermal boundary conductance (TBC) of heterostructures. However, the same nanostructure design can have opposite impacts on TBCs for different systems. To provide a clue toward a unified explanation, in this work, we directly and explicitly reveal the impacts of nanostructures on mode-dependent phonon TBC contributions. We study four representative types of nanostructures, i.e., (1) an intermediate layer, (2) interfacial interlaced teeth, (3) interfacial atomic mixing, and (4) interfacial atomic defects on two example heterostructures: 28Si/Ge and 6Si/Ge, which have moderate and large phonon frequency mismatches, respectively. We find that most of these nanostructures reduce the TBC of 28Si/Ge while increasing the TBC of 6Si/Ge. Each nanostructure is found to have two competing impacts on an interface—one tends to increase TBC while the other tends to decrease TBC. For example, adding an intermediate layer provides a phonon bridging effect, which tends to increase both elastic and inelastic phonon transmission, but it adds one more interface and, thus, more phonon reflection. As a result, an interlayer decreases the TBC of the 28Si/Ge interface by decreasing the inelastic transmission while increasing both elastic and inelastic transmissions of the 6Si/Ge interface. Other nanostructures with atomic disorder can increase transmission by increasing the contact area but can also decrease transmission by phonon-disorder backscattering. This work unveils the fundamental thermal transport physics across interfaces with nanostructures and sheds light on future interface nanoengineering for electronic devices such as high-power transistors, photodiodes, and supercomputing architectures.

Rapid mixing of metal atoms ejected in water by pulsed laser ablation of the boundary between jointed Au and Ag target metal plates

We prepared metal alloy nanoparticles by pulsed laser ablation of the boundary between jointed target metal plates of Au and Ag in purified water. When the spot size radius of the focused laser on the target was 2.5 mm, separate Au nanoparticles and Ag nanoparticles were mainly observed with slight alloying. In contrast, when the spot size radius was 0.25 mm, the ejected Au and Ag atoms were found to be more mixed, efficiently forming Au and Ag alloy nanoparticles. The mixing distance of Au and Ag atoms was estimated to be ∼0.5 mm. The dynamics of the plasma plume and the cavitation and their interaction with Au and Ag atoms were quite complex. Hence, we assumed that during plasma plume formation and cavitation formation, Au and Ag atoms diffuse in the plasma plume and cavitation as a uniform medium with a maximum temperature and minimum viscosity. It was suggested that the fast mixing of Au and Ag atoms occurred inside the plasma plume, taking the high temperature, several thousand K of the plasma plume into account. Moreover, the mixing was promoted by the overlapping of plasma plumes, which were generated on Au and Ag plates separately, owing to explosive plasma motions. Then, the alloy nanoparticles were formed in the cavitation bubble where Au and Ag atoms were spatially distributed by plasma plumes. The mixing of the atoms in the cavitation bubble was not likely based on the mixing distance.

Effect of confinement on weakly nonlinear temporal instability analysis of a viscous planar liquid sheet sandwiched between two co-flowing gas streams at equal velocities

Extensive number of applications like rocket jet engine and gas turbines carry out atomization within a confined environment as it provides faster atomization and good mixing efficiency. Literature suggests that the effect of confinement on breakup of droplets, liquid threads, and annular jets is not elementary, but it varies with liquid viscosity, surrounding medium viscosity, and the confinement strength. Through the present weakly nonlinear temporal analysis, we unveil if a similar dynamic effect of confinement persists on the breakup of planar two-dimensional liquid sheets of varying viscosities. Moreover, to mimic air assisted atomization, the surrounding gases are considered to be flowing with non-zero velocities. Our study reveals that presence of confinement at higher gas-to-liquid velocity ratios of 2.5 and 3 produces no apparent improvement in atomization at all liquid Reynolds numbers. However, at relatively low gas-to-liquid velocity ratios of 2 and 2.25, confinement produces faster breakup as compared to an unconfined one, more so when Reynolds number is relatively low (Re < 100). The overall influence of confinement on non-dimensional breakup time is, however, weak as compared to how it affects the linear growth rate. A counteracting effect of confinement on the linear maximum growth rate and normalized second order disturbance amplitude is identified as the primary reason. Our study also identifies the minimum confinement height CH-1 which is just enough for the liquid sheet to achieve breakup without touching the solid walls. Interestingly, the maximum linear growth rate and minimum breakup time for a particular flow condition are always registered when confinement height equals to CH-1. The values of CH-1 display a strong dependence upon Reynolds number and gas-to-liquid velocity ratios.

Localization of the Optical Phonon Modes in Boron Nitride Nanotubes: Mixing Effect of 10B Isotopes and Vacancies.

We explored the mixing effect of 10B isotopes and boron (B) or nitrogen (N) vacancies on the atomic vibrational properties of (10,0) single-wall boron nitride nanotubes (BNNTs). The forced oscillation technique was employed to evaluate the phonon modes for the entire range (0–100%) of 10B isotopes and atomic vacancy densities ranging from 0 to 30%. With increasing isotope densities, we noticed a blue shift of the Raman-active A1 phonon peak, whereas an increased density of mixed or independent B and N vacancies resulted in the emergence of a new low-frequency peak and the annihilation of the A1 peak in the phonon density of states. High-energy optical phonons were localized as a result of both 10B isotopes and the presence of mixing defects. We found an asymmetrical nature of the localization length with increasing 10B isotope content, which corresponds well to the isotope-inherited localization length of carbon nanotubes and monolayer graphene. The localization length falls abruptly with the increase in concentration of both atomic vacancies (B or N) and mixing defects (10B isotope and vacancies). These findings are critical for understanding heat conduction and nanoscopic vibrational investigations such as tip-enhanced Raman spectra in BNNTs, which can map local phonon energies.

High-entropy transition metal nitride thin films alloyed with Al: Microstructure, phase composition and mechanical properties

Deviation from equimolar composition in high-entropy multielement ceramics offers a possibility of fine-tuning the materials’ properties for targeted application. Here, we present a systematic experimental and theoretical study on the effects of alloying equimolar pentanary (TiHfNbVZr)N and hexanary (TiHfNbVZrTa)N high-entropy nitrides with Al. Although being predicted to be metastable by ab initio density-functional theory calculations, single-phase fcc NaCl-structured solid solution thin films with Al solubility limits as high as x ∼ 0.51–0.61 in (TiHfNbVZr)1-xAlxN and x ∼ 0.45–0.64 in (TiHfNbVZrTa)1-xAlxN are synthesised utilizing a hybrid deposition technique that offers dynamic mixing of film atoms from Al+ subplantation and non-equilibrium growth conditions leading to quenching of the desired film structure. In experimental studies supplemented with density-functional theory calculations, it is demonstrated that Al concentration in alloys with the multielement compositions of high-entropy nitride thin films determine hardness, yield strength, toughness, and ability to deform plastically up to fracture due to different deformation mechanisms arising from the electronic structure and phase compositions.

Significant reduction in semiconductor interface resistance via interfacial atomic mixing

The contact resistance between two dissimilar semiconductors is determined by the carrier transmission through their interface. Despite the ubiquitous presence of interfaces, quantitative simulation of charge transport across such interfaces is difficult, limiting the understanding of interfacial charge transport. This work employs Green's functions to study the charge transport across representative Si/Ge interfaces. For perfect interfaces, it is found that the transmittance is small and the contact resistance is high, not only because the mismatch of carrier pockets makes it hard to meet the momentum conservation requirement, but also because of the incompatible symmetries of the Bloch wave functions of the two sides. In contrast, atomic mixing at the interface increases the carrier transmittance as the interface roughness opens many nonspecular transmission channels, which greatly reduces the contact resistance compared with the perfect interface. Specifically, we show that disordered interfaces with certain symmetries create more nonspecular transmission. The insights from our study will benefit the future design of high-performance heterostructures with low contact resistance.

Evidencing Pt-Au alloyed domains on supported bimetallic nanoparticles using CO desorption kinetics

We explored here the possibility to use CO desorption kinetics monitored by IR spectroscopy as a tool to characterize the mixing of Au and Pt atoms at the surface of supported nanoparticles. CO desorption of CO monitored at 50 °C showed that a bimetallic sample exhibited an intermediate behavior between those of monometallic Au and Pt samples, reflecting the alloyed nature of the nanoparticles. Our data also show that alumina-supported Au nanoparticles led to the formation of surface carbonates at 50 °C, possibly formed through Boudouard reaction (2 CO → C + CO2), in contrast to the case of the Pt-based sample.

An experimental setup to generate narrowband bi-photons via four-wave mixing in cold atoms

We present our recently-built experimental setup designed to generate near-infrared and narrow-band correlated photon pairs by inducingfour-wave mixing in a cold gas of 87Rb atoms confined in a magneto-optical trap. The experimental setup and its automation and control approach are described in detail. A characterization of the optical density of the atomic ensemble as well as the basic statistical measurements of the generated light are reported. The non-classical nature of the photons pairs is confirmed by observing a violation of Cauchy-Schwarz inequality by a factor of 5.6 × 10 5 in a Hanbury Brown – Twiss interferometer. A 1/e coherence time for the heralded, idler photons of 4.4 ± 0.1 ns is estimated from our observations. We are able to achieve a value of 104 s−1pair-detection-rate, which results in a spectral brightness of 280 (MHz s)−1. The combination of high brightness and narrow-band spectrum makes this photon-pair source a viable tool in fundamental studies of quantum states and opens the door to use them in quantum technologies.

Electrochemical synthesis of high entropy hydroxides and oxides boosted by hydrogen evolution reaction

Conventional synthesis techniques for high entropy materials often require prolonged processes or the use of complex fabrication steps. Here, we report on a scalable method to synthesize high entropy hydroxide and oxide nanoparticles using electrochemical synthesis in an aqueous environment. The current density parameter was set to create a non-equilibrium condition where the hydrogen evolution reaction results in a turbulent environment facilitating the formation of nanoparticles within the electrolyte. Using this approach, we demonstrate the synthesis of both binary (Fe, Mn) and quinary (Fe, Mn, Ni, Ca, Mg) metal hydroxide and oxide nanoparticles. The chemical and microscopy analysis shows that the high entropy nanoparticles can be synthesized as a hydroxide without any post processing and as an oxide through a calcination process with atomic scale mixing of all elements. The synthesis of high entropy nanoparticles is important for applications targeting catalysis, energy storage and conversion, filtration, and environmental engineering. • Aqueous electrochemical synthesis of high entropy hydroxides under ambient conditions • Green and substrate-free nanoparticle synthesis using inexpensive metal salts • Fast transformation from metal hydroxides to metal oxides with calcination process • Ability to scale to industrially promising applications Ritter et al. report an electrochemical process for quick and scalable synthesis of high entropy hydroxide and oxide nanoparticles that avoids long synthesis times and high temperatures. The large variety of metal salt precursors allows for tuning of the composition for applications in catalysis, energy storage, and conversion.

Chemical reaction of Ni/Al interface associated with perturbation growth under shock compression

The exothermic reaction of Ni/Al laminates always starts from the interface, and the role of interfacial instability in the shock-induced chemical reaction has not been clarified. This work reports the Richtmyer–Meshkov (RM) instability growth, atomic diffusion, and chemical reaction of Ni/Al interface under shock compression based on atomistic simulations. For shocking from Al to Ni, the interface experiences finite collapse and exhibits weak localized reaction. The diffusion of solid Ni to molten Al will be inhibited due to the formation of NiAl phase, and continuous inter-diffusion occurs with the melting of Ni. For shocking from Ni to Al, a small amount of NiAl structure is formed due to the atomic residue during defect collapse. RM instability growth is observed at higher shock intensity, which significantly promotes the atomic mixing and results in a power-law increase in the number of diffusing atoms. Meanwhile, the chemical reaction propagates rapidly from the vortex to the head of the spike accompanied by the decomposition of many clusters, with the nonlinear development of RM instability. The number and the size of Ni clusters no more satisfy the simple power-law relationship for which we propose an improved power-law distribution. Interestingly, the growth of nanoscale perturbation approximately satisfies the logarithmic law with time, but the linear growth stage is inhibited due to significant inter-diffusion, especially for the small wavelength. Thus, the mixing width and the reaction degree are positively correlated with the initial wavelength in our simulation scale, which is contrary to the RM growth law of the free surface.

Experimental realization of efficient nondegenerate four-wave mixing in cesium atoms.

Nondegenerate four-wave mixing (FWM) in diamond-type atomic systems has important applications in a wide range of fields, including quantum entanglement generation, frequency conversion, and optical information processing. Although the efficient self-seeded nondegenerate FWM with amplified spontaneous emission (ASE) has been realized extensively, the seeded nondegenerate FWM without ASE is inefficient in reported experiments so far. Here we present the experimental realization of the seeded nondegenerate FWM in cesium atoms with a significantly improved efficiency. Specifically, with two pump lasers at 852 and 921 nm and a seed laser at 895 nm, a continuous-wave laser at 876 nm is efficiently generated via FWM in a cesium vapor cell with a power up to 1.2 mW, three orders of magnitude larger than what has been achieved in previous experiments. The improvement of the efficiency benefits from the exact satisfaction of the phase-matching condition realized by an elaborately designed setup. Our results may find applications in the generation of squeezing and entanglement of light via nondegenerate FWM.

Spatial Manipulation via Four-Wave Mixing in Five-Level Cold Atoms

In a recent publication [S. Gan, Laser Phys. 31, 055401 (2021)], a scheme for controlling the vortex four-wave mixing (FWM) in a five-level atomic system has been put forward. Based on this work, we propose a new scheme for the spatial manipulation via four-wave mixing in a five-level atomic system when the radial index is considered. It is found that the phase and intensity of the FWM field can be spatially manipulated. More importantly, we show the superposition modes created by the interference between the FWM field and a same-frequency Gaussian beam, which can also be controlled via the corresponding parameters. Our research is helpful to understand and manipulate optical vortices and can be widely used in quantum computation and communication.

Nonvolatile High-Speed Switching Zn-O-N Thin-Film Transistors with a Bilayer Structure.

Zinc oxynitride (ZnON) has the potential to overcome the performance and stability limitations of current amorphous oxide semiconductors because ZnON-based thin-film transistors (TFTs) have a high field-effect mobility of 50 cm2/Vs and exceptional stability under bias and light illumination. However, due to the weak zinc-nitrogen interaction, ZnON is chemically unstable─N is rapidly volatilized in air. As a result, recent research on ZnON TFTs has focused on improving air stability. We demonstrate through experimental and first-principles studies that the ZnF2/ZnON bilayer structure provides a facile way to achieve air stability with carrier controllability. This increase in air stability (e.g., nitrogen non-volatilization) occurs because the ZnF2 layer effectively protects the atomic mixing between ZnON and air, and the decrease in the ZnON carrier concentration is caused by a shallow-to-deep electronic transition of nitrogen deficiency diffused from ZnON into the interface. Further, the TFT based on the ZnF2/ZnON bilayer structure enables long-term air stability while retaining an optimal switching property of high field-effect mobility (∼100 cm2/Vs) even at a relatively low post-annealing temperature. The ZnF2/ZnON-bilayer TFT device exhibits fast switching behavior between 1 kHz and 0.1 MHz while maintaining a stable and clear switching response, paving the way for next-generation high-speed electronic applications.

Controlling transition metal atomic ordering in two-dimensional Mo1−x W x S2 alloys

The unique optical and electronic properties of two-dimensional transition metal dichalcogenides (2D TMDs) make them promising materials for applications in (opto-)electronics, catalysis and more. Specifically, alloys of 2D TMDs have broad potential applications owing to their composition-controlled properties. Several important challenges remain regarding controllable and scalable fabrication of these alloys, such as achieving control over their atomic ordering (i.e. clustering or random mixing of the transition metal atoms within the 2D layers). In this work, atomic layer deposition is used to synthesize the TMD alloy Mo1−x W x S2 with excellent composition control along the complete composition range 0 ⩽ x ⩽ 1. Importantly, this composition control allows us to control the atomic ordering of the alloy from well-mixed to clustered while keeping the alloy composition fixed, as is confirmed directly through atomic-resolution high-angle annular dark-field scanning transmission electron micrography imaging. The control over atomic ordering leads to tuning of the bandgap, as is demonstrated using optical transmission spectroscopy. The relation between this tuning of the electronic structure and the atomic ordering of the alloy was further confirmed through ab-initio calculations. Furthermore, as the atomic ordering modulates from clustered to well-mixed, the typical MoS2 and WS2 A1g vibrational modes converge. Our results demonstrate that atomic ordering is an important parameter that can be tuned experimentally to finely tune the fundamental properties of 2D TMD alloys for specific applications.

A lightweight refractory complex concentrated alloy with high strength and uniform ductility

Advanced alloys with lightweight, gigapascal strength, and sufficient ductility are vitally crucial for engineering applications. However, the intrinsic conflicts between strength and ductility or strength and density cause a big challenge to improve the comprehensive performance of alloys. In the current study, we adopt a two-step method to overcome this trade-off by tailoring the atomic mixing of lightweight elements and refractory elements to form a lightweight refractory complex concentrated alloy (LRCCA). Then, a lightweight, high-strength, intrinsically ductile complex concentrated matrix (LRCCM) alloy is developed, modified with nano/submicron multicomponent intermetallic compound particles (MICPs). The developed MICPs-LRCCM exhibits a high yield strength, sufficient ductility and a relative low density (∼5.9 g/cm3), as well as high specific strength of ∼230 MPa·cm3/g. High solution strengthening together with precipitation are simultaneously operated in the MICPs-LRCCM. Our results provide a guidance for designing high-performance lightweight alloys for structural applications.

Atomic diffusion at the Ni–Ti liquid interface using molecular dynamics simulations

ABSTRACT Nickel Titanium (NiTi) alloys are produced by heating raw Ni and Ti in high temperature. In this stage, Ni and Ti atoms diffuse and mix to form an alloy. Despite the importance of technology, Ni and Ti atoms’ diffusion mechanism, however, still remains unelucidated. We performed molecular dynamics simulations for 1 ns to investigate the diffusion process of Ni–Ti liquid at temperatures of 2000, 2200, 2400, and 2600 K. Our results show that the diffusion coefficient increases with increasing temperature. In the 2000–2400 K temperature range, Ni atoms diffuse faster by 1.4% to 16.3% than Ti atoms. On the other hand, the liquid Ni structure becomes less dense at higher temperatures (T = 2600 K), such that Ti atoms with a smaller mass can diffuse more rapidly. From the calculations, the diffusion activation energy of Ni is 0.2 eV higher than that of Ti.

Effect of interfacial atomic mixing on the thermal conductivity of multi-layered stacking structure

Multi-layered stacking structures and atomic mixing interfaces were constructed. The effects of various factors on the thermal conductivity of different lattice structures were studied by non-equilibrium molecular dynamics simulations, including the number of atomic mixing layers, temperature, total length of the system, and period length. The results showed that the mixing of two and four layers of atoms can improve the thermal conductivities of the multi-layer structure with a small total length due to a phonon “bridge” mechanism. When the total length of the system is large, the thermal conductivity of the multi-layer structure with atomic mixing interfaces decreases significantly compared with that of the perfect interfaces. The interfacial atom mixing destroys the phonon coherent transport in the multi-layer structure and decreases the thermal conductivity to some extent. The thermal conductivity of the multi-layer structure with perfect interfaces is significantly affected by temperature, whereas the thermal conductivity of the multi-layer structures with atomic mixing is less sensitive to temperature.

Design and synthesis of single phase Hf0.25Zr0.25Ce0.25Y0.125Si0.125O2-δ high-entropy ceramics

We design and synthesise a new high-entropy ceramic Hf0.25Zr0.25Ce0.25Y0.125Si0.125O2-δ through a hybrid mixing of non-metal and metal atoms. First-principles calculations predict the thermal stability of the single-crystal phase at high temperatures. XRD and EDS mapping confirm a single cubic fluorite phase with a uniform elemental distribution. In-situ high-temperature XRD shows that the phase is stable with no phase separation or secondary phases. The measured mechanical and thermal properties are comparable with reported high-entropy fluorite oxides. Our results add to the compositional space of high-entropy ceramics for a wide range of applications from thermal insulators to electronics.

Mechanical alloying in the Co-Fe-Ni powder mixture: Experimental study and molecular dynamics simulation

The formation dynamics of medium- entropy alloy CoFeNi has been studied both experimentally and theoretically in terms of molecular dynamic simulation. The results of experiments and modelling showed good qualitative agreement and supplemented each other. The synthesis was found to occur in two stages. (a) Refinement of crystallites and accumulation of structural defects take place due to severe plastic deformation and friction of metal particles that induce abnormally fast diffusive mixing of different metal atoms. This process results in formation of nanocrystalline intermediate alloy with a crystallite size of about 10 nm. (b) Recrystallization that leads to the nucleation and growth of multicomponent phase with a simple FCC crystal structure. A critical time for the onset of recrystallization depends on planetary milling regime and gaseous atmosphere in the milling jar.

Strain Relaxation in Metal Alloy Catalysts Steers the Product Selectivity of Electrocatalytic CO2 Reduction.

Strain engineering in bimetallic alloy structures is of great interest in electrochemical CO2 reduction reactions (CO2RR), in which it simultaneously improves electrocatalytic activity and product selectivity by optimizing the binding properties of intermediates. However, a reliable synthetic strategy and systematic understanding of the strain effects in the CO2RR are still lacking. Herein, we report a strain relaxation strategy used to determine lattice strains in bimetal MNi alloys (M = Pd, Ag, and Au) and realize an outstanding CO2-to-CO Faradaic efficiency of 96.6% and show the outstanding activity and durability toward a Zn-CO2 battery. Molecular dynamics (MD) simulations predict that the relaxation of strained PdNi alloys (s-PdNi) is correlated with increases in synthesis temperature, and the high temperature activation energy drives complete atomic mixing of multiple metal atoms to allow for regulation of lattice strains. Density functional theory (DFT) calculations reveal that strain relaxation effectively improves CO2RR activity and selectivity by optimizing the formation energies of *COOH and *CO intermediates on s-PdNi alloy surfaces, as also verified by in situ spectroscopic investigations. This approach provides a promising approach for catalyst design, enabling independent optimization of formation energies of reaction intermediates to improve catalytic activity and selectivity simultaneously.