Metal Single Crystals Research Articles

In situ conversion of a MOG to a crystalline MOF: a case study on solvent-dependent gelation and crystallization.

Herein, we report in situ transformation of a metal-organic gel (MOG) to a crystalline metal-organic framework (MOF) and solvent-dependent gelation/crystallization via solvothermal reactions of a tetracarboxylic acid, namely 4,4'-dinitro-2,2',6,6'-tetracarboxybiphenyl, and ZnSO4. The results provide structural insights into MOGs at the molecular level and also help in the synthesis of crystalline MOFs that are otherwise difficult to obtain.

Theoretical Evidence for the Single-Ended-Source Controlled Yield Strengths of Micropillar FCC and BCC Metal Single Crystals

Theoretical Evidence for the Single-Ended-Source Controlled Yield Strengths of Micropillar FCC and BCC Metal Single Crystals

Free-standing 2D ironene with magnetic vortex structure at room temperature

•Te promoter and space-confined setup are two key factors for synthesizing 2D ironene •2D ironene nanoflakes exhibit intrinsic ferromagnetism at room temperature •Thickness and geometrical shape-dependent magnetic vortex structures are demonstrated •Magnetic domain evolution and domain wall motion are observed in situ Free-standing two-dimensional (2D) elemental transition metal single crystals are an important kind of nanomaterial, with abundant fascinating theoretical properties, such as magnetism. To date, however, they rarely have been grown and studied because of the difficulty in controllable synthesis, since the existence of surface dangling bonds makes the 2D growth of these crystals challenging. Herein, we discuss developing a Te-assisted chemical vapor deposition (CVD) method and growing 2D Fe single-crystal nanoflakes (termed ironene) with different thicknesses, illustrating that Te powder and space-confined setup are the two key factors. Notably, thickness and geometrical shape-dependent magnetic domain structure of the obtained 2D ironene at room temperature was explored. Furthermore, this interesting domain configuration was confirmed by Lorentz transmission electron microscopy (LTEM), and magnetic domain manipulation was performed in situ. This work provides a valuable case study for synthesizing 2D elemental transition metal nanoflakes with room-temperature ferromagnetism and paves the way for developing new diverse spintronics. Free-standing two-dimensional (2D) elemental transition metal single crystals are an important kind of nanomaterial, with abundant fascinating theoretical properties, such as magnetism. To date, however, they rarely have been grown and studied because of the difficulty in controllable synthesis, since the existence of surface dangling bonds makes the 2D growth of these crystals challenging. Herein, we discuss developing a Te-assisted chemical vapor deposition (CVD) method and growing 2D Fe single-crystal nanoflakes (termed ironene) with different thicknesses, illustrating that Te powder and space-confined setup are the two key factors. Notably, thickness and geometrical shape-dependent magnetic domain structure of the obtained 2D ironene at room temperature was explored. Furthermore, this interesting domain configuration was confirmed by Lorentz transmission electron microscopy (LTEM), and magnetic domain manipulation was performed in situ. This work provides a valuable case study for synthesizing 2D elemental transition metal nanoflakes with room-temperature ferromagnetism and paves the way for developing new diverse spintronics.

Wafer-Scale Uniform Synthesis of 2D Transition Metal Dichalcogenides Single Crystals via Chemical Vapor Deposition

ConspectusTwo-dimensional (2D) semiconducting transition metal dichalcogenides (TMDCs), most with a formula of MX2 (M = Mo, W; X = S, Se, etc.), have emerged as ultrathin channel materials in next-generation electronics, due to their atomic thickness, tunable bandgap, and relatively high carrier mobility, etc. To propel their practical applications in integrated circuits, large-scale preparation of large-domain MX2 single crystals and single-crystal films is significantly important. Among all the synthetic methods, chemical vapor deposition (CVD) has shown great promise for the controllable syntheses of large-area uniform 2D materials with electronic-grade quality and reasonable cost. So far, intensive efforts have been devoted to the controllable growth of MX2 single crystals with large sizes, controlled thicknesses, fast growth rates, and unidirectional orientations. However, due to the complexity of the precursor species (mostly like solid powders) and their gradient feeding distributions along the gas flow direction, it is still a huge challenge to synthesize large-area MX2 samples with centimeter-scale uniformity, which inevitably hinders their practical applications in industry. More significantly, due to the noncentrosymmetric structures of MX2, antiparallel domains and twin grain boundaries usually evolved on most substrates (e.g., mica, C-sapphire). These defective grain boundaries usually exhibit metallic characteristics and serve as conducting channels, seriously impairing the electrical and optical properties of related devices.This review thereby focuses on the recent progresses on the CVD syntheses of large-scale uniform, large-domain MX2 flakes and single-crystal films, primarily on the single-crystal insulating substrates, amorphous insulating substrates and single-crystal metal substrates. Some promising growth strategies toward improving the thickness uniformity, domain size and crystal quality of MX2 are highlighted, through modulating the key parameters in the CVD processes, such as precursor type, substrate category/crystallinity or growth promoter. The key mechanisms that directing the uniform orientations of individual domains and their merging toward single-crystal films are also highlighted. Besides, the applications of the 2D MX2 in high-performance electronic devices are also discussed. Finally, the current challenges for the wafer-scale syntheses of MX2 single crystals and their applications are prospected. The potential opportunities in this field are also proposed in terms of the wafer-scale production and thickness control of 2D MX2, epitaxial growth of wafer-scale MX2 single-crystals, damage-free, and easy-processing transfer of wafer-scale MX2, as well as the high-performance electronics/optoelectronics based on wafer-scale MX2. We believe this review can provide a basic guidance for the controllable growth of wafer-scale MX2 single crystals, and inspire more researchers to design innovative synthesis routes to promote their practical applications in new-generation electronic devices.

A novel mechanism to generate metallic single crystals

Generally, the evolution of metallic single crystals is based on crystal growth. The single crystal is either produced by growing a seed single crystal or by sophisticated grain selection processes followed by crystal growth. Here, we describe for the first time a fully new mechanism to generate single crystals based on thermo-mechanically induced texture formation during additive manufacturing. The single crystal develops due to two different mechanisms. The first step is a standard grain selection process due to directional solidification, leading to a pronounced fiber texture. The second and new mechanism bases on successive thermo-mechanically induced plastic deformations and texture formation in FCC crystals under compression. During this second step, the columnar grain structure transforms into a single crystal by rotation of individual grains. Thus, the single crystal forms step by step by merging the originally columnar grain structure. This novel, stress induced mechanism opens up completely new perspectives to fabricate single crystalline components and to accurately adjust the orientation according to the load.

Dual-coupling-guided epitaxial growth of wafer-scale single-crystal WS2 monolayer on vicinal a-plane sapphire.

The growth of wafer-scale single-crystal two-dimensional transition metal dichalcogenides (TMDs) on insulating substrates is critically important for a variety of high-end applications1-4. Although the epitaxial growth of wafer-scale graphene and hexagonal boron nitride on metal surfaces has been reported5-8, these techniques are not applicable for growing TMDs on insulating substrates because of substantial differences in growth kinetics. Thus, despite great efforts9-20, the direct growth of wafer-scale single-crystal TMDs on insulating substrates is yet to be realized. Here we report the successful epitaxial growth of two-inch single-crystal WS2 monolayer films on vicinal a-plane sapphire surfaces. In-depth characterizations and theoretical calculations reveal that the epitaxy is driven by a dual-coupling-guided mechanism, where the sapphire plane-WS2 interaction leads to two preferred antiparallel orientations of the WS2 crystal, and sapphire step edge-WS2 interaction breaks the symmetry of the antiparallel orientations. These two interactions result in the unidirectional alignment of nearly all the WS2 islands. The unidirectional alignment and seamless stitching of WS2 islands are illustrated via multiscale characterization techniques; the high quality of WS2 monolayers is further evidenced by a photoluminescent circular helicity of ~55%, comparable to that of exfoliated WS2 flakes. Our findings offer the opportunity to boost the production of wafer-scale single crystals of a broad range of two-dimensional materials on insulators, paving the way to applications in integrated devices.

Growing 3D-nanostructured carbon allotropes from CO2 at room temperature under the dynamic CO2 electrochemical reduction environment

Synthesis of nanostructured carbon allotropes using CO2 as a cheap carbon source is challenging and usually limited by high temperature activation. Herein, formation of 3D nanostructured carbon allotropes including nano-graphene and single crystalline nanodiamond films is feasible at room temperature on various single crystal metals (e.g., Bi, Ag, Zn, and Co) that were formed under the dynamic CO2 electrochemical reduction reaction environment at relatively low applied potential (−1.1 to −1.6 V vs. Ag/AgCl). The nanocrystalline carbon was obtained as the major products from CO2 (≥96% selectivity, ∼1 μm thickness) without any liquid products. Upon applying the negative potential, nanoclustering of the self-limiting ultrathin metal oxide layers of metal particles on the highly conductive substrate could lead to formation of negatively charged metal clusters, which were well stabilized by the ternary electrolyte system containing [BMIm]+[BF4]-/propylene carbonate/water. This system allows the reduction of CO2 into single atoms C∗ and the subsequently electrocrystallization of C∗ into carbon allotropes on the crystallographic planes of the single crystal metals formed as the building blocks. The CO2-derived Ag–C/epoxy composites show promising thermal conductivity. The results present a breakthrough advancement in the growth of nanostructured carbon allotropes from CO2 by the viable negative CO2 emission approach.

RKKY coupled local-moment magnetism in NaFe1−xCuxAs

A central question in a large class of strongly correlated electron systems, including heavy fermion compounds and iron pnictides, is the identification of different phases and their origins. It has been shown that the antiferromagnetic (AFM) phase in some heavy fermion compounds is induced by Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction between localized moments, and that the competition between this interaction and Kondo effect is responsible for quantum criticality. However, conclusive experimental evidence of the RKKY interaction in pnictides is lacking. Here, using high resolution $^{23}$Na NMR measurements on lightly Cu-doped metallic single crystals of NaFe$_{1-x}$Cu$_{x}$As ~($x \approx 0.01$) and numerical simulation, we show direct evidence of the RKKY interaction in this pnictide system. Aided by computer simulation, we identify the $^{23}$Na NMR satellite resonances with the RKKY oscillations of spin polarization at Fe sites. Our $^{23}$Na spin-lattice and spin-spin relaxation data exhibits signature of an itinerant and inhomogeneous AFM phase in this system, accompanied by a simulation of Cu-induced perturbation to the ordered moments on the Fe sites. Our NMR results indicate coexistence of local and itinerant magnetism in lightly Cu-doped NaFe$_{1-x}$Cu$_{x}$As.

Comparative study of Fe(Se,Te) thin films on flexible coated conductor templates and single-crystal substrates

In this paper, we compare epitaxial Fe(Se,Te) thin films grown by pulsed laser deposition on CaF2, SrTiO3, MgO single crystals as well as on different metallic templates having a CeO2 based top surface. In particular, we performed a detailed structural and superconducting analysis. X-ray diffraction studies showed highly textured films on all templates. The superconducting transition temperatures are between 21 K and 14 K for Fe(Se,Te) films on CaF2 and on MgO single-crystal substrates, respectively, whereas films on the metal templates show T c values up to 18 K. The critical current density (J c) was determined from magnetization loops in fields up to 7 T. Calculations in the framework of the extended critical state model showed J c values over 2 MA cm−2 and 0.9 MA cm−2 at 5 K in self-field on single-crystal substrates and metal templates substrates, respectively. Detailed transmission electron microscopy studies reveal smooth layers on all templates. Whereas small defects were found in films on single crystals, pronounced grain boundaries with higher misorientation angle were visible in the layers on metallic templates.

General Microwave Route to Single-Crystal Porous Transition Metal Nitrides for Highly Sensitive and Stable Raman Scattering Substrates.

The synthesis of metallic transition metal nitrides (TMNs) has traditionally been performed under harsh conditions, which makes it difficult to prepare TMNs with high surface area and porosity due to the grain sintering. Herein, we report a general and rapid (30 s) microwave synthesis method for preparing TMNs with high specific surface area (122.6-141.7 m2 g-1) and porosity (0.29-0.34 cm3 g-1). Novel single-crystal porous WN, Mo2N, and V2N are first prepared by this method, which exhibits strong surface plasmon resonance, photothermal conversion, and surface-enhanced Raman scattering effects. Different from the conventional low-temperature microwave absorbing media such as water and polymers, as new concept absorbing media, hydrated metal oxides and metallic metal oxides are found to have a remarkable high-temperature microwave heating effect and play key roles in the formation of TMNs. The current research results provide a new-concept microwave method for preparing high lattice energy compounds with high specific surface.

Bending stress relaxation of microscale single-crystal copper at room temperature: An in situ SEM study

The bending stress relaxation of microscale single-crystal copper was studied by in situ SEM bending experiments to elucidate the time-dependent plasticity involving the effect of the strain gradient and the sample size. Three specimens with varied heights of square cross sections were fabricated using a focused ion beam. Interestingly, an obvious bending plateau was observed in the load-displacement curves. The calculated flow stress on the cross section indicated an inverse relationship between the specimen size and strength. In addition, the proportion of decay stress is approximately 2.7% for the beam with a height of h = 1.82 μm and 13.9% for that of h = 0.94 μm in the bending plateau. Moreover, during the dwell period, a sudden stress drop occurred, which is ascribed to the strain burst or stress jerky during the deformation of single-crystal metals. Microbending is usually related to a significant increase in geometrically necessary dislocations (GNDs), which is supposed to improve the resistance for further deformation. However, the bending plateau implied perfect plastic flow and an evolving balance in the microstructures. On the other hand, thermal activation theory was employed to interpret the time-dependent plasticity. The apparent activation volume of the dwell periods in the bending plateau suggested that cross slip dominates the inelastic strain rate evolution. Finally, the inelastic strain rate for stress relaxation was derived from the curve of decay stress, and it was inversely proportional to the height of the microbeam, which is in complete contrast with the trend observed in the previous compression tests of micropillars. In current microscale bending specimens, the thinner beam containing fewer initial dislocation sources exhibits a larger creep ductility with the assistance of GNDs.

Peak states of molybdenum single crystals shock compressed to high stresses

To determine crystal anisotropy effects at high stresses, peak states behind the plastic shock waves were examined in BCC single crystals. Using plate impact experiments, molybdenum (Mo) single crystals were shock compressed up to 190 GPa elastic impact stress along [100], [110], and [111] orientations. Laser interferometry was used to measure wave velocities and particle velocity profiles at the Mo–LiF window interface. These data were analyzed to obtain in-material quantities in the peak states. The Hugoniots for [100] and [110] orientations were comparable, but the Hugoniot for the [111] orientation was different from the other two orientations. Also, these Mo single crystal Hugoniots display differences from the polycrystalline Mo Hugoniots. Although none of the differences can be considered large, the present results demonstrate that, unlike FCC metal single crystals (Cu, Al), some anisotropy is preserved in Mo single crystal Hugoniots even at high stresses.

Stabilizing Metastable Polymorphs of van der Waals Solid MoS2 on Single Crystal Oxide Substrates: Exploring the Possible Role of Surface Chemistry and Structure

In this work we address the question of growth of thin films of a soft van der Waals solid (2D MoS2) on a hard, crystalline metal oxide substrates. Thus, MoS2 thin films are grown on different single-crystal metal oxide substrates (SrLaAlO4, c-Al2O3, SrTiO3, LaAlO3) by pulsed laser deposition (PLD) and characterized by different techniques to confirm and quantify their phase constitution (2H, 1T′). We observe that, on the SrLaAlO4 (001) substrate, a mixed 1T′+2H phase of MoS2 grows with a dominant (∼75%) 1T′ phase, while on c-Al2O3 (0001) pure 2H phase grows. On the SrTiO3 and LaAlO3 substrates also the mixed phase grows, but with a much lesser component of the 1T′. Higher 1T′ contribution in SLAO and STO points to the chemical role of strontium in the early growth. It is also noted that, with increased film thickness on SrLaAlO4, the contribution of the pure 2H phase is enhanced, indicating the pivotal role of lattice strain in stabilizing the initial layer(s). The mixed 1T′+2H phase in the ultrathin film shows significantly lower room temperature resistivity (∼2 mΩ cm) with respect to that of the pure 2H phase (∼14 mΩ cm). The substrate selective polymorphism with distinct electronic features could invite multiple application interests.

Surface Structure-Sensitive Enantioselectivity: Aspartic Acid Reaction Kinetics on All Surfaces Vicinal to Cu(111)

Enantioselective reactions on chiral, single-crystal metal surfaces can provide novel insight into the origins of enantioselectivity in chiral molecular adsorption and catalysis. Using a high-throughput approach to study the surface of a spherical Cu single crystal, we have measured the surface reaction kinetics of aspartic acid decomposition on 169 different Cu(hkl) crystal planes vicinal to the Cu(111) orientation. These span both R- and S-surface chiralities comprehensively. The decomposition kinetics of d- and l-aspartic acids were measured and their decomposition rate constants were estimated on all 169 different surface orientations. The half-times, t1/2(hkl), for the decomposition of aspartic acid reveal that the reactivity for both enantiomers is lower along achiral directions exposing surfaces with (100) step edges than the reactivity along directions exposing surfaces with (110) step edges. The orientation-resolved rate constants, k(hkl), clearly reveal structure sensitivity on surfaces vicinal to Cu(111) but enantiospecificity is only observed for the initiation rate constant, ki(hkl).

Synthesis of Wafer‐Scale Graphene with Chemical Vapor Deposition for Electronic Device Applications

AbstractThe first isolation of graphene opens the avenue for new platforms for physics, electronic engineering, and materials sciences. Among several kinds of synthesis approaches, chemical vapor deposition is most promising for the growth at wafer‐scale, which is compatible with the Si‐based electronic device integration protocols. In this review, the types, properties, and synthesis methods of graphene are first introduced. Many details of wafer‐scale graphene synthesis by chemical vapor deposition strategies are given, including the wafer‐scale single crystal metal and alloy preparation, roll to roll synthesis over Cu, roll to roll electrochemical transfer technique. Besides, the batch‐to‐batch synthesis are highlighted for direct graphene over dielectric substrates such as sapphire and Si/SiO2. The electronic transport and transparent conductance of the wafer‐scale graphene are compared with high‐quality single crystal. The progress and proof‐of‐the‐concept are briefly recalled in graphene‐based electronics such as transistors, sensors, integrated circuits, and spin transport valves. Eventually, the readers are provoked with the current challenges as well as the future opportunities.​

Theoretical Justification of Single-Ended Dislocation-Source-Controlled Deformation of Micropillar fcc Crystals.

It was established at the beginning of the 21st century that the critical resolved shear stress of small-sized (diameter from 50nm to 10 μm) metallic crystals fabricated from bulk crystals increases drastically with decreasing specimen diameter. Dou and Derby [Scr. Mater. 61, 524 (2009)SCMAF71359-646210.1016/j.scriptamat.2009.05.012] showed that, the critical shear stresses of small-sized single crystals of various fcc metals obeyed a universal power law of specimen size with an exponent of -0.66. In this study, we succeeded in reproducing almost perfectly the above universal relation without any adjustable parameters, based on a deformation process controlled by the operation of single-ended dislocation sources.

Tensile mechanical performance of Al/Ni dissimilar metals bonded by self-propagating exothermic reaction based on molecular dynamics simulation

To provide new possibilities for electrode materials, a new welding method of Al/Ni dissimilar metals was studied using molecular dynamics in this research. The effective bonding between aluminum and nickel was achieved by Al-Ni reactive nano-multilayers (RNMLs). The tensile strength of the bonded component was 57 % higher than that of diffusion-bonded Al/Ni nanowires. First, compared to the two single-crystal metals before bonding, the Young’s modulus of the bonded component was 48.88 GPa higher than that of single-crystal aluminum and 68.09 GPa lower than that of single-crystal nickel. Furthermore, a large number of stair-rod dislocations and nano-coherent twins were generated in the process of tensile loading, improving the mechanical performance of the bonded component. Second, as the atomic ratio of Al-Ni RNMLs increased, the tensile strength decreased first to 6.22 GPa and then continuously increased to 7.48 GPa, and the Young’s modulus increased from 113.73 to 141.73 GPa and remained stable. When the atomic ratio was 1.98, Shockley dislocations and stair-rod dislocations were more easily generated. Similarly, an ignition temperature of 830 K resulted in the highest tensile strength of the bonded components among all of the RNMLs ignition temperatures.

Effect of stable stacking fault energy and crystal orientation on fracture behaviour of thin metallic single crystals

ABSTRACT Understanding the evolution of dislocations and twinning at the crack front is critical for designing micro-mechanical systems with improved performance. In this work, the dislocation evolution at the crack front in thin pre-cracked FCC single crystals is correlated with the associated fracture toughness, which is shown to be dependent on material specific properties such as stable stacking fault energy and crystal orientation using atomistic simulations. For materials with high value, sessile dislocations form at the crack front causing increased localised plastic flow stress that leads to low fracture toughness. Whereas the fracture process in materials with low value is governed by the motion of glissile dislocations and stress-induced twinning leads to high fracture toughness. For this case, twinning occurs at high stress levels followed by un-twinning due to stress relaxation at crack front by twinning. The crystal orientation influences the type of dislocations emitted (screw/edge) from the crack front which governs the mode of crack propagation. The Mode-III crack propagation by the emission of screw type dislocations causes significant decrease in the fracture toughness compared to Mode I crack propagation which is caused by simultaneous emission of edge type dislocations on the two symmetrically inclined slip planes at the crack front. For certain pre-cracked crystal orientations, twinning is seen during the early stages of plastic deformation in materials with high value. However, un-twinning is not observed in crystal orientation-based twinning at the crack front.

Deformation of Nickel Titanium Single Crystal under Compressive Pulse

Nickel titanium single crystal is used to show the importance of the volume compressibility anisotropy in analyzing the elastoplastic deformation processes in metal single crystals with the cubic symmetry. The process of uniform bulk deformation corresponds to the process of nonuniform stress-strain state of metal single crystals of cubic symmetry for several orientations of the theoretical coordinate system relative to crystallographic directions of the main axes. The indicative surface of the volume compressibility (or its reciprocal variable of bulk modulus) has an aspherical shape and is the function of the Euler angles. This is shown for the first time based on the solution of the model problem, viz. determination of the stress-strain state of a spherical body made of nickel titanium single crystal during the transmission of a compressive pulse through the tested material. In general case of orientation of the theoretical coordinate system relative to main crystallographic axes, a spherical body made of nickel titanium single crystal undergoes deformation due to the compressive load and acquires the shape of a two-axial ellipsoid.

Preparation of single-crystal metal substrates for the growth of high-quality two-dimensional materials

Recent advances on preparing single-crystal metals and their crucial roles in controlled growth of high-quality 2D materials are reviewed.