Single-crystalline Morphology Research Articles

Influence of Single-Crystalline Morphology on High-Nickel Layered Oxide Cathodes for Lithium-Ion Batteries

Single-crystal, high-nickel layered oxides have seen increasing interest due to their potential to address the surface reactivity and mechanical resilience issues inherent to these cathode compositions. Great strides have been made in methods for synthesizing single-crystal layered oxides, but much is still unknown about the impact of single-crystalline morphology on the electrochemical behavior of these materials.Here, we report a comparative study on the electrochemical operation of single- and polycrystalline LiNiO2, respectively termed SCLNO and PCLNO. SCLNO is synthesized with a molten-salt-assisted method, allowing the crystals to form at the same temperature as used for solid-state synthesis of PCLNO. As a result, the cathodes possess similar lattice structures with pure hexagonal layered phase and comparable levels of cation mixing (~1.5%), enabling a robust comparison of single-crystalline and polycrystalline morphology with minimal confounding variables. Charge/discharge performance, rate capability, and cycling stability reveal insights on the behavior single-crystal layered oxides and the performance tradeoffs they elicit. Particular attention is directed towards Li+ diffusion and phase evolution dynamics within the two morphologies, as characterized by in-situ and ex-situ X-ray diffraction, differential capacity analysis, and galvanostatic intermittent titration technique (GITT).As has been reported in other cathode systems,1 single-crystal LNO (SCLNO) delivers a lower initial capacity of about 200 mA h g-1, compared to nearly 230 mA h g-1 for polycrystalline LNO (PCLNO), most of which is lost in the low-voltage discharge region known to be particularly susceptible to kinetic hindrance.2 Indeed, GITT indicates a lower effective Li+ diffusivity in SCLNO. Interestingly, the single-crystal cathode exceeds the polycrystalline cathode in rate capability; SCLNO delivers 78% capacity at 10C discharge rate (158 mA h g-1) while PCLNO achieves only 25% capacity (57 mA h g-1). Analysis of the charge/discharge profiles of the materials reveals the H2-H3 transition is still present in SCLNO at high discharge rates, while it disappears completely in PCLNO. Despite the conflicting nature of the observed lower diffusivity, yet improved rate capability, further analysis suggests that the elimination of grain boundaries in single-crystalline cathodes may be responsible for both phenomena. Details of the experimental techniques, and results from the electrochemical analysis will be presented during the meeting. The findings from this study will contribute to understanding single-crystalline layered oxides and aid the design of higher-performance cathode materials.References Sun, X. Cao, H. Zhou, Advanced single-crystal layered Ni-rich cathode materials for next-generation high-energy-density and long-life Li-ion batteries, Phys. Rev. Mater. 6 (2022) 1–11Grenier, P.J. Reeves, H. Liu, I.D. Seymour, K. Märker, K.M. Wiaderek, P.J. Chupas, C.P. Grey, K.W. Chapman, Intrinsic Kinetic Limitations in Substituted Lithium-Layered Transition-Metal Oxide Electrodes, J. Am. Chem. Soc. 142 (2020) 7001–7011.

Influence of Single-Crystalline Morphology on the Electrochemical Behavior of High-Nickel Layered Oxide Cathodes

High-Nickel layered oxides are being pursued for their high specific capacities. With increasing nickel content, the layered oxides also suffer from structural and morphological instabilities, which degrade the electrochemical performance. Single-crystalline morphology presents an attractive solution to these problems, simultaneously reducing surface area available for parasitic reactions and preventing electrolyte penetration into the bulk with the removal of grain boundaries. Single-crystal cathodes have already shown promising results with NMC composition cathodes, but there are still gaps in the fundamental understanding of how single-crystalline morphology alters electrochemical behavior. We attempt to fill some of these gaps by studying in detail the electrochemical operation of polycrystalline and single-crystalline LiNiO2 (LNO). Single-crystal LNO is prepared through a molten salt method yielding high-quality, distinct crystals with comparable lattice chemistry to polycrystalline LNO. The single-crystal LNO achieves far greater stability during long-term cycling, reaching 500 cycles with 82.5% capacity retention. Interestingly, the single-crystalline LNO also displays superior rate performance, delivering 157 mA h g−1 at 10C discharge rate. Investigation of the phase evolution behavior during cycling strongly suggests that the absence of grain boundaries in single-crystalline LNO is responsible for the superior performance.

Heuristics for Molten-Salt Synthesis of Single-Crystalline Ultrahigh-Nickel Layered Oxide Cathodes.

In pursuit of Li-ion batteries with higher energy density, ultrahigh-nickel layered oxides are a leading candidate for next-generation cathode materials. Single-crystalline morphology offers a neat solution to the poor stability of ultrahigh-Ni cathodes; a lower active surface area mitigates electrolyte decomposition at high voltages, and the elimination of grain boundaries improves mechanical resilience and increases volumetric energy density. However, single-crystal cathodes possess their own challenges, several of which originate from synthesis at elevated temperatures meant to induce grain growth. Molten-salt synthesis is an alternative method for obtaining single crystals, accelerating grain growth through the presence of a molten flux without the need for increased temperature. Herein, we offer heuristic guidelines for molten-salt synthesis, discussing key factors for designing reaction mixtures and the necessary exploratory research for novel molten salt/cathode systems. The influence of different salts and synthesis conditions on the morphology and properties of single-crystal LiNiO2 is presented. It is found that oxidative salts, such as Li2O2 and LiNO3, are crucial to supplementing dissolution of gaseous oxygen into the molten phase. Through these discussions, this work aims to provide a set of overarching principles for obtaining higher-quality single-crystal layered oxide cathodes and engender more rigorous and impactful investigation into their fundamental nature and applications.

Mechanical densification synthesis of single-crystalline Ni-rich cathode for high-energy lithium-ion batteries

The intergranular microcracking in polycrystalline Ni-rich cathode particle is led by anisotropic volume change and stress corrosion along grain boundary, accelerating battery performance decay. Herein, we have suggested a simple but advanced solid-state method that ensures both uniform transition metal distribution and single-crystalline morphology for Ni-rich cathode synthesis without sophisticated co-precipitation. Pelletization-assisted mechanical densification (PAMD) process on solid-state precursor mixture enables the dynamic mass transfer through the increased solid-solid contact area which facilitates the grain growth during sintering process, readily forming micro-sized single-crystalline particle. Furthermore, the improved chemical reactivity by a combination of capillary effect and vacancy-assisted diffusion provides homogeneous element distribution within each primary particle. As a result, single-crystalline Ni-rich cathode with PAMD process has eliminated a potential evolution of intergranular cracking, thus achieving superior energy retention capability of 85% over 150 cycles compared to polycrystalline Ni-rich particle even after high-pressure calendering process (corresponding to electrode density of ∼3.6 g cm−3) and high cut-off voltage cycling. This work provides a concrete perspective on developing facile synthetic route of micron-sized single-crystalline Ni-rich cathode materials for high energy density lithium-ion batteries (LIBs).

Underlying limitations behind impedance rise and capacity fade of single crystalline Ni-rich cathodes synthesized via a molten-salt route

Layered oxide LiNixMnyCozO2 (NMC) cathodes are often synthesized as polycrystalline secondary particles. Due to intergranular fracture stemming from volume changes of randomly oriented primary particles during charge/discharge, the synthesis of larger single-crystalline cathodes is of high interest. In this work, molten salt assisted growth of micron-sized Ni-rich crystals is achieved with excellent crystallinity, low cation mixing, and negligible impurities. However, electrochemical performance is compromised by high surface reactivity resulting in decomposition of electrolyte and subsequent formation of a thick CEI layer. While intergranular fracture is eliminated, planar gliding and severe intragranular fracture along the (003) plane occurs in the high voltage region within the first few cycles and is associated primarily with H2 to H3 structural transitions. In addition, H2 to H3 transitions are highly irreversible with cyclic voltammograms revealing polarization growth within <5 cycles. Subsequently, the single-crystalline material exhibited markedly reduced available capacity and enhanced capacity fade from sharp impedance growth compared to its polycrystalline counterpart. This work furthers a fundamental understanding into the limitations of single-crystalline Ni-rich cathodes, and the obstacles limiting the advantages offered by the single-crystalline morphology.

A well-controlled cracks and gliding-free single-crystal Ni-rich cathode for long-cycle-life lithium-ion batteries

Single-crystalline (SC) high energy nickel (Ni)-rich cathodes play a key role as a potential cathode material in lithium-ion batteries (LIBs) to address the challenges in a hierarchical structure of their secondary particles by decreasing phase boundaries and materials surfaces. The SC LiNi0.78Mn0.12Co0.1O2 (SC-NMC78) cathode with primary particles of several micron-sized particles are developed and thoroughly investigated in this study, demonstrating superior cycling performance, along with significantly enhanced structural reliability after long-term cycling. The improved SC-NMC78 has an octahedral SC morphology with a modest grain size, which reduces the lithium-ion diffusion route and enhances structural stability. The SC-NMC78 offers a high discharge capacity of 175 and 155 mAh g−1 at 0.2 and 1 C, respectively, and better capacity retention of 132 mAh g−1 after 200 cycles at 1 C as a cathode in LIBs. The cycled SC-NMC78 particles exhibited no lattice gliding and micro-cracks, demonstrating that the SC shape may substantially reduce anisotropic micro-strain. This efficient, repeatable, and customizable method for producing SC Ni-rich cathodes without any additives should accelerate their commercialization. The density functional theory also proved that the low global hardness of Ni2+ in SC-NMC78 and optimized content of Ni/Li exchange were well-consistent with the experimental findings.

Crystalline Ti-nanostructures prepared by oblique angle deposition at room temperature

Nanostructured Ti thin films are fabricated by oblique angle deposition (OAD) in combination with electron beam evaporation in an ultrahigh vacuum chamber at room temperature on thermally and natively oxidized Si(100) substrates. The incidence angle θOAD of the incoming particle flux with respect to the substrate normal is varied between 70° ≤ θOAD ≤ 84°. This highly oblique deposition geometry leads to the formation of highly porous, thin, crystalline films consisting of separated Ti columns that are oriented toward the incoming particle flux. The influence of the incidence angle on the texture, morphology, and columnar tilt angles of these Ti thin films is investigated. It is found that the texture depends on the angle θOAD of the incoming particle flux. High-resolution transmission electron microscopy reveals that highly oblique deposited Ti columns tend to grow with single crystalline morphology. The orientation of the lattice planes in the Ti columns with respect to the substrate normal changes remarkably as the incidence angle is varied. Moreover, the orientation of the lattice planes differs from the growth direction of the columns. The samples are analyzed using x-ray diffraction measurements such as in-plane pole figure measurements and θ-2θ x-ray diffraction patterns, as well as by using transmission electron microscopy and scanning electron microscopy.

Enhanced Long-Term Cycling Performance of Single Crystalline LiCo0.95Ni0.05O2 cathode Material at High Cut-Off Voltage in Li-Ion Cell

A significant interest in developing high volumetric energy density and high power energy storage systems (ESSs) has facilitated development of ideal cathode material in Lithium-ion batteries. Among various cathode materials, LiCoO2 (LCO) is one of the promising candidates because of single crystalline morphology with relatively high electrode density (>3.9g cm-3) and superior electrochemical performance. However, Increased cut-off voltage, higher than the conventional operating voltage of LCO (~4.3 V vs. Li/Li+), to maximize energy storage capacity gives rise to severe electrochemical degradation at the electrode-electrolyte interface and irreversible structural deformation at highly delithiated states of cathode material, which result in a rapid decrease of capacity and nominal voltage in a continuous cycle. Herein, for an enhanced cycle life of LCO particularly in high cut-off voltage, we proposed LiCo0.95Ni0.05O2 (LCNO) which not only enabled to demonstrate remarkable cycling performance and but also to be synthesized as a single crystalline with layered structure(R-3m) by the solid-state method in air condition. For comparison, the electrochemical behaviors of active materials, LCO and LCNO, charged up to 4.45V, were investigated. Even though LCO showed an initial reversible capacity of ∼182 mAh/g, higher than that of LCNO (∼176 mAh/g), LCNO exhibited much better capacity retention of 93.6% than that of LCO (74.3%) under continuous 100 cycles. We identified the less formation of irreversible byproducts on the surface of LCNO, compared to that of LCO, indicating the reactivity of interface between LCNO-electrolyte was highly stabilized with the assistance of Ni-ions. Furthermore, the evolution of interfacial change between LCNO-electrolyte after electrochemical cycling has been also investigated by high-resolution scanning transmission electron microscopy (HR-TEM). Based on electron energy-loss spectroscopy measurements (EELS), we confirmed that Ni-ions in LCNO may have a potential ability to initiate the oxidation state of Co ions around the surface to be partially reduced to Co2+/Co3+ mixed-valence state during cycles with a change of surface structure from layered structure to spinel-like phase, thus reducing surface side reaction simultaneously.

Glancing angle deposition of sculptured thin metal films at room temperature

Metallic thin films consisting of separated nanostructures are fabricated by evaporative glancing angle deposition at room temperature. The columnar microstructure of the Ti and Cr columns is investigated by high resolution transmission electron microscopy and selective area electron diffraction. The morphology of the sculptured metallic films is studied by scanning electron microscopy. It is found that tilted Ti and Cr columns grow with a single crystalline morphology, while upright Cr columns are polycrystalline. Further, the influence of continuous substrate rotation on the shaping of Al, Ti, Cr and Mo nanostructures is studied with view to surface diffusion and the shadowing effect. It is observed that sculptured metallic thin films deposited without substrate rotation grow faster compared to those grown with continuous substrate rotation. A theoretical model is provided to describe this effect.

A Wide Band Gap Naphthalene Semiconductor for Thin‐Film Transistors

This study reports a new simple organic semiconductor 2,6‐bis(4‐methoxyphenyl)naphthalene (BOPNA) with unprecedentedly large band gap of 3.35 eV and an apparent hole mobility of ≈1 cm2 V–1 s–1 measured in thin‐film organic field‐effect transistors in a saturation regime. This large band gap leads to complete optical transparency in the visible range (&gt;370 nm), very high stability and independence of the device current with illumination conditions; this is in contrasts to the behavior of common organic semiconductors that have the band gap in the visible or near‐IR range of the spectrum. Crystal structure of BOPNA reveals a highly isotropic electronic coupling in two directions of a herringbone‐packing plane. A uniform, nearly single crystalline morphology can be achieved in highly crystalline BOPNA films through a rapid thermal annealing. Temperature‐dependent studies reveal increase of the hole mobility as the temperature is reduced from +25 to –25 °C, suggesting a band‐like transport, followed by a nearly temperature‐independent mobility down to at least –150 °C.

Synthesis of 1D Sn-doped ZnO hierarchical nanorods with enhanced gas sensing characteristics

Pure and Sn-doped hierarchical ZnO based 1D single-crystalline nanorods were produced by chemical vapour transport and condensation process in Ar/O2 atmosphere. Conventional analytical techniques, such as field emission scanning electron microscopy (FESEM), X-ray diffraction, transmission electron microscopy, photoluminescence spectra and high angle annular dark field image (HAADF) of the as-grown nanostructures confirm tin doping in ZnO single crystals. A possible formation mechanism of the nanostructures was proposed. It was established that the incorporation of Sn in the ZnO branched nanorods significantly improves their sensitivity towards acetone and ethanol vapours. The response and recovery times were measured to be ~7s and ~38s when exposed to 50ppm acetone at the test temperature of 400°C. The acetone and ethanol sensing performance of the doped ZnO nanorod sensors were better than that undoped ZnO nanostructured sensors under comparable operating conditions. The sensitivity/response of acetone and ethanol vapours for Sn-doped hierarchical ZnO nanorod was ascribed to the single-crystalline morphology, increased O-vacancy and other defect density and higher surface areas, which in turn accelerates a fast and effective diffusion process for acetone or ethanol vapours as compared to undoped ZnO nanostructures.

Crystallization-Driven Self-Assembly of Block Copolymers with a Short Crystallizable Core-Forming Segment: Controlling Micelle Morphology through the Influence of Molar Mass and Solvent Selectivity

Three well-defined asymmetric crystalline-coil poly(ferrocenyldimethylsilane-block-2-vinylpyridine) (PFS-b-P2VP) diblock copolymers (PFS44-b-P2VP264, PFS75-b-P2VP454, and PFS102-b-P2VP625) with similar block ratios (r = NP2VP/NPFS = ca. 6.0 ± 0.1) but different overall molar masses (Mn = 38 700, 65 800, and 90 400 g mol–1) were synthesized by sequential anionic polymerization, and their solution self-assembly behavior was explored as a function of (i) molar mass and (ii) the ratio of common to selective solvent. When self-assembly was performed in isopropanol (i-PrOH), a selective solvent for P2VP, a decrease in the rate of the crystallization-driven transition from the initially formed spheres (with amorphous PFS cores) into cylinders (with crystalline cores) was detected with an increase in molecular weight. This trend can be explained by a decrease in the rate of crystallization for the PFS core-forming block as the chain length increased. In contrast, when a mixture of i-PrOH with increasing amounts of THF, a common solvent for both blocks, was used, spheres, cylinders, and also narrow lenticular platelets consisting of crystallized PFS lamellae sandwiched by two glassy coronal P2VP layers were formed from the same PFSx-b-P2VP6x sample. The most likely explanation involves the plasticization of the PFS core-forming block which facilitates crystallization, possibly complemented by contraction of the coils of the P2VP coronal block which otherwise limit of the lateral growth of the crystalline PFS core as THF is a poorer solvent for P2VP than i-PrOH. Selected area electron diffraction studies indicated that the PFS cores of the spherical micelles were amorphous but were consistent with those of the cylindrical micelles existing in a state approximating to that of a monoclinic PFS single crystal. In contrast, in the platelets formed in THF/i-PrOH, the PFS cores were found to be polycrystalline. The formation of narrow lenticular polycrystalline platelets rather than a regular, rectangular single crystalline morphology was attributed to a poisoning effect whereby the interference of the long P2VP coronal blocks in the growth of a rectangular PFS single crystalline core introduces defects at the crystal growth fronts.

Temperature-dependent growth mechanism and microstructure of ZnO nanostructures grown from the thermal oxidation of zinc

We report a detailed study on the growth morphologies and microstructure of ZnO nanostructures formed from the oxidation of Zn at different temperatures. ZnO shows bicrystalline nanowire morphology for oxidation below the melting point of Zn, and single-crystalline morphology between the melting and boiling points of Zn, and tetrapod morphology above the boiling point of Zn. The morphological and microstructural variations are attributed to the temperature-dependent oxide growth mechanisms, i.e., the oxidation below the melting point of Zn is dominated by a solid–solid transformation process, a liquid–solid process between the melting and boiling points of Zn, and a vapor–solid process above the boiling point of Zn. The understanding of the oxide growth mechanisms from these results may have practical implications for rational control of the morphology, crystallinity, preferential growth directions, shape and aspect ratio of ZnO nanostructures

Single-crystalline Te microtubes: Synthesis and NO 2 gas sensor application

Tellurium tubular crystals were grown by direct thermal evaporation of tellurium metal in an inert atmosphere on quartz substrates at ambient pressure without employing any catalyst. Tellurium powder was evaporated by heating at 600 °C and was condensed at a substrate temperature of 300–350 °C in the downstream of argon gas at a flow rate of 100 mL/min. The structure and chemical composition of the as-synthesized samples were examined by X-ray diffraction analysis, scanning electron microscopy, energy-dispersive X-rays microanalysis and micro-Raman spectroscopy. Scanning electron microscopy images and X-ray diffraction patterns showed that the as-synthesized Te had a tubular single-crystalline morphology with a hexagonal cross-section. The Te microtubes were typically 0.5–6 mm long, 30–70 μm in external diameter, and 5–20 μm thick. NO 2 gas-sensing properties of the Te microtubes at room temperature were also investigated. They showed a promising sensitivity and response towards tested gas.

Structural modification of a multiply twinned nanoparticle by ion irradiation: A molecular dynamics study

We study the possibility of modifying the structure of a multiply twinned nanoparticle by ion irradiation. Molecular dynamics simulations are carried out for the prototypic case of a metastable icosahedral Pt particle bombarded with He and Xe ions in the energy range of 0.1–10keV. A single xenon impact can be used to melt the particle. It can also induce partial melting, which causes a collapse of the twin boundary structure in the solid part and transformation to single crystalline morphology. Under He irradiation, we observe a saturation of the vacancy concentration, but no untwinning.

Diastereospecific Photocyclization of a Isopropylbenzophenone Derivative in Crystals and the Morphological Changes

Reaction of crystals of 2,4,6-triisopropylbenzophenone derivative with the (S)-phenylethylamide group caused diastereospecific Norrish type II photocyclization by UV irradiation to give (R,S)-cyclobutenol as a sole product. In contrast, the solution photolysis gave an almost 1:1 mixture of (R,S)- and (S,S)-cyclobutenol. The specific diastereodifferentiation in the crystalline state is attributed to the smooth transformation with minimum molecular motion due to the very similar molecular shapes as well as the 2-fold helical arrangements between the reactant crystal and the product (R,S)-cyclobutenol crystal. UV irradiation of the bulk crystals led to cracking and breaking into small fragments. In contrast, the microcrystals maintained the single-crystalline morphology in the course of photocyclization, suggesting the single-crystal-to-single-crystal transformation.

Formation of Single-Crystalline Aragonite Tablets/Films via an Amorphous Precursor

Thin tablets and films of calcium carbonate have been grown at the air-water interface via an amorphous precursor route using soluble process-directing agents and a Langmuir monolayer based on resorcarene. By using appropriate concentrations of poly(acrylic acid-sodium salt) in combination with Mg2+ ion, an initially amorphous film is deposited on the monolayer template, which subsequently crystallizes into a mosaic film composed of a mixture of single-crystalline and spherulitic patches of calcite and aragonite. Of particular importance is the synthesis of single-crystalline "tablets" of aragonite (approximately 600 nm thick), because this phase generally forms needle-like polycrystalline aggregates when grown in vitro. To our knowledge, a tabular single-crystalline morphology of aragonite has only been observed in the nacreous layer of mollusk shells. Therefore, this in vitro system may serve as a useful model for examining mechanistic issues pertinent to biomineralization, such as the influence of organic templates on nucleation from an amorphous phase.

Morphology, structures and properties of ZnO nanobelts fabricated by Zn-powder evaporation without catalyst at lower temperature

Uniform ZnO normal nanobelts and toothed-nanobelts have been successfully synthesized respectively through pure zinc powder evaporation without catalyst at 600°C. Experimental results indicate that the key to the fabricating method is to control the gas flow rates and the partial pressures of argon, oxygen and zinc vapor. Scanning electron microscopy and high-resolution transmission electron microscopy observations show that the ZnO nanobelts have several types of single crystalline morphology. HRTEM images reveal that there are numerous screw dislocations and the growth is around the dislocations in the toothed-nanobelts. The growth of ZnO nanobelts is controlled by vapor-solid and screw dislocation mechanisms. Room temperature photoluminescence spectra of the toothed-nanobelts showed a UV emission at ∼390 nm and a broad green emission with 4 subordinate peaks at 455–495 nm.

Thermal and morphological studies of chemically prepared emeraldine‐base‐form polyaniline powder

AbstractIn this study, emeraldine base (EB)‐form polyaniline (PANI) powder was chemically prepared in 1M HNO3 aqueous solution. The thermal characteristics and chemical structures of this powder were studied by differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), Fourier transform infrared spectroscopy (FTIR), and X‐ray diffraction (XRD). A polarizing optical microscope was also used to examine the crystalline morphology of this sample. The results indicated that the EB‐form PANI powder had a discernible moisture content. Moreover, in the first run of DSC thermal analysis, the exothermic peak at 170–340°C was due to the crosslinking reaction occurring among the EB‐form PANI molecular chains. FTIR and XRD examinations further confirmed the chemical crosslinking reaction during thermal treatment. TGA results illustrated that there were two major stages for weight loss of the EB‐form PANI powder sample. The first weight loss, at the lower temperature, resulted from the evaporation of moisture. The second weight loss, at the higher temperature, was due to the chemical structure degradation of the sample. The degradation temperature of the EB‐form PANI powder was around 420–450°C. The degradation temperature of emeraldine salt (ES)‐form PANI powder was lower (around 360–410°C) than that of the EB form (around 420–450°C). From the TGA results, I roughly estimated that 2.74 aniline repeat units, on average, were doped with 1 HNO3 molecule in the ES‐form PANI. I found a single crystalline morphology of EB‐form PANI, mostly like a conifer leaf. More complex, multilayered dendritic structures were also found. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 89: 2142–2148, 2003

Single-Crystalline Copper Nanowires Produced by Electrochemical Deposition in Polymeric Ion Track Membranes

The fabrication of copper nanowires by electrodeposition in ion track membranes is reported. The large-grain copper needle deposited at 40 °C shown in the Figure is a transition between the polycrystalline needles formed at room temperature and the single-crystalline morphology at 60 °C. A systematic study of the DC deposition parameters required for single-crystalline growth is presented.