Coherent Epitaxial Growth Research Articles

Nest-Scheme RuIrLa Nanocrystals by NP-to-NP Oriented Assembly: Coherent Strain Fields-Driven Band Structure Splitting for Efficient Acidic Water Oxidation.

Atomic substructure engineering provides new opportunities for the designing newly and efficient catalysts with diverse atom ensembles, trimmed electron bands, and way-out coordination environments, creating unique contributing to concertedly catalyze water oxidation, which is of great significance for proton exchange membrane water electrolysis (PEMWE). Herein, nest-scheme RuIrLa nanocrystals with dense coherent interfaces as built-in substructures are firstly fabricated by using commercial ZnO particles as acid-removable templates, through a La-stabilized coherent epitaxial growth of nanoparticles (NPs). The obtained nests exhibit a low overpotential of 198mV at 10mA cm-2, and the RuIrLa||Pt/C module equipped in PEMWE operates stably at a cell voltage potential of 1.69V at 100mA cm-2 in 0.5M H2SO4 for 55h, which is far beyond the current IrO2||Pt/C. Within the nests, the position at the interface shows high tensile/compressive strain, significantly reducing the OER activation energy. More importantly, the La termination-stabilized coherent interfaces within the nests creates a unique self-healing process for the outstanding long-term stability. This work provides a promising substructure engineering to develop efficient catalysts with abundant substructures, such as coherent interfaces, dislocations, or grain boundaries, thereby realizing concerted improvement of activity and durability toward water oxidation.

Tuning mechanical properties of TiAlSiN/TiAlN multilayers by interfacial structure

Multilayer structure as an effective way to tailor the structure and performance of hard coatings has attracted wide attentions. Here, we aim to elucidate the change of the interface, mechanical properties and oxidation resistance for TiAlSiN/TiAlN multilayers by varying modulation ratios from ML (tTiAlSiN/tTiAlN = 1:1) to HML (tTiAlSiN/tTiAlN = 1:2). Increasing TiAlN sublayer thickness stabilizes the cubic growth of TiAlSiN layers, where the HML exhibits coherent epitaxial growth in contrast to the cubic-wurtzite mixed structure of ML. This coherent strengthening effect is responsible for the noteworthy enhancement of the hardness values from ∼30.9 GPa of ML to ∼33.2 GPa of HML. Significantly, the presence of a coherent interface effectively withstands external pressure, thereby endowing HML with exceptional adhesive strength and fracture toughness. Nevertheless, the oxidation resistance of the coating is reliant on the chemical composition, with the multilayers displaying lower oxidation resistance compared to the TiAlSiN monolayer.

Self-Luminous Probe with One-Step Energy Conversion from Bioluminescence to NIR-IIb.

Self-luminous probes with near-infrared (NIR) emission are powerful tools for deep-penetration and autofluorescence-free imaging, owing to the joint optimization of both excitation and emission. However, the limited emission wavelength and requirement for multistep energy transfer limit its potency. In this study, the concept of direct wavelength conversion is established from visible light (vis) to NIR-IIb using an exquisitely designed sensitizer-activator ion pair. The manipulation of the doping hosts enables a pair of energy levels between the sensitizer and activator. Based on this a class of broadband vis-responsive nanocrystals with intense NIR-II emission is prepared. The stability and quantum yield (up to 7.4%) of the nanocrystals are further enhanced by ZnS passivation via coherent epitaxial growth. By coupling luciferase, the self-luminous probe can convert bioluminescence to NIR-IIb luminescence (>1500nm) through a one-step energy transfer. A maximum penetrable thickness of 6mm is achieved in the porcine tissue model. Collectively, the distinctive photon-conversion performance of this probe offers the prospect of high-resolution labeling of deep-seated lesions.

Microstructures, mechanical properties and corrosion resistance of TiN/AlN multilayer films

TiN/AlN multilayer films with different modulation periods (Λ) were prepared by RF-reactive magnetron sputtering. The microstructures, mechanical property and corrosion resistance of TiN/AlN multilayer films were investigated by X-ray diffraction (XRD), transmission electron microscopy (TEM), nanoindentation and electrochemical comprehensive tester. The results show that the TiN/AlN multilayer films are composed of face centered cubic (fcc) TiN and tightly packed hexagonal (hcp) AlN. The multilayer films are coherent epitaxial growth as Λ increases from 2 to 12 nm. The hardness and the corrosion resistance increase with Λ increases from 2 to 12 nm. The optimum hardness is 29.27 GPa and the strongest corrosion resistance displays with Λ = 12 nm. The coherent interface could be destroyed, and the alternating stress field as well as the hardness decreases sharply during the modulation period of 26 nm–36nm. The self-corrosion potential decreases and the self-corrosion current density increases. The corrosion resistance of the film decreases with Λ increase from 26 nm to 36 nm.

Effect of SiC thickness on microstructure and mechanical properties of (AlCrTiZrV)N/SiC nano-multilayers film synthesized by reactive magnetron sputtering

High-entropy alloy nano-multilayers film have been increasingly used in protective coatings due to the excellent performance. Among the many factors that affect the performance of the nano-multilayers film, the thickness of the modulation layer has a significant effect on the performance of the high-entropy alloy nano-multilayers film. In this work, the microstructure and mechanical properties of (AlCrTiZrV)N/SiC nano-multilayers films with different thicknesses of the SiC layers prepared by using reactive magnetron sputtering are studied systematically. The results show that under the influence of the template layer, the amorphous SiC layer undergoes crystallization transformation and grows co-epitaxially with the template layer, resulting in the enhancement in the hardness. The maximum hardness and modulus of elasticity of (AlCrTiZrV)N/SiC nano-multilayers film reach 31.5 GPa and 290.6 GPa when the thickness of SiC is 0.5 nm, which are significantly improved compared to the single layer (AlCrTiZrV)N. As the thickness of the SiC layer further increases, the cubic steady-state structure of the SiC layer returns to the amorphous state and the coherent epitaxial growth of the multilayers film is destroyed, resulting in a decrease in the hardness of the nano-multilayers film.

In-plane ferroelectricity and enhanced Curie temperature in perovskite BaTiO3 epitaxial thin films

In-plane ferroelectric polarization in BaTiO3 (BTO) epitaxial thin films on MgAl2O4(001) (MAO) is reported. The directional dependence of both in-plane polarization curves and Raman spectroscopy shows that the films have an orthorhombic structure at room temperature, in contrast to the tetragonal structure of the corresponding bulk. The largest in-plane polarization value among BTO-based tensile-strained films is obtained. The temperature dependence of the lattice constants shows that the Curie temperature of the thin films is as high as 220 °C, which is higher than that of the bulk by 100 °C. The significant enhancement of the Curie temperature is attributed to high-quality coherent epitaxial growth due to perfect matching between the lattice parameter of the c-axis of BTO and that of MAO.

Coherent epitaxial growth of superconducting NbN ultrathin films on AlN by sputtering

We investigated the structural and electrical properties of superconducting NbN films epitaxially grown on AlN single-crystalline films using a sputtering technique. The NbN(111) films grown on AlN under optimized temperatures exhibited clear peaks with Pendellösung fringes attributed to the growth of the atomically flat surfaces in 2θ/ω X-ray diffraction patterns. Scanning transmission electron microscopy also confirmed the formation of sharp NbN/AlN interfaces. Reciprocal space mapping revealed that the NbN films were coherently grown on the AlN templates, which indicates that the NbN films have the same in-plane lattice constants as AlN. It was also determined that the shape of the unit cell of NbN depends strongly on the epitaxial growth temperature. The NbN films coherently grown on AlN exhibited superconducting transition temperatures (Tc) ranging from 12 to 16 K, which also depends on the epitaxial growth temperature. These results indicate that the epitaxial strain (or change in crystal structure) in NbN modifies Tc.

Straightforward Approach for Epitaxial Growth of CuO over ZnO Nanorods: Heterostructure Study and Photoelecterochemical Water Splitting Performance

Optimization of the band gap in semiconductors through tailored heterojunctions has received immense research interests for advancement in photovoltaic and photoelectochemical hydrogen production. Heteroepitaxial superstructures are considered as crucial for charge carrier flow at the semiconductor interface. The present research attempt disclosed the method of facile fabrication of perfectly aligned ZnO nanorods, followed by their uniform decoration with CuO nanoparticles. In consequence, heterojuction was developed, which was studied through XPS and TEM. High resolution TEM images provides the evidence of coherent epitaxial growth due to overlapping lattices of both materials across the interface by highlighting the lattice mismatch of 7%. The 7 degree plane loss is expected due to less matching of lattice constants and misorientation of the planes at the interface. This assembly provided better collection of photo-generated charge carriers in extended light absorption range, resulting in better performance of photelectrochemical (PEC) water oxidation device. Figure 1

Residual stresses and HRTEM phase interface structures of AlN structured PVD coatings on plate-grained WC–Co substrate

A fine plate-like grained WC–12Co based alloy with (0001)WC texture coefficient of 2.73 for the sinter skin was used as the substrate. Single layer Al0.66Cr0.34N and two layers Al0.65Cr0.30Si0.04W0.01N/Al0.66Cr0.34N (AlCrSiWN/ AlCrN) approximate 1.6 μm in total thickness for each were separately deposited on the substrate in an industrial-scale arc evaporation system. This study aims to investigate the strengthening mechanism for the adhesion and the resistance against cohesive failure of the films and to prove a hypothesis of stress buffering function of the binder phase in the substrate to the film. Accordingly, coating related phase structure and residual stress change in the substrate and HRTEM phase interface structures of the film/substrate system were investigated. The results show that stress buffering by the Co phase (binder phase) is realized by energy absorption in a form of residual stress build-up and fcc to hcp phase transformation, accompanied by an atomic disorder Co zone in several atomic layer thickness adjacent to the AlCrN/Co interface. Coherent and semi-coherent epitaxial growth of AlCrN on WC surfaces is observed and the superiority of plate-grained WC–Co substrate is confirmed. Methods of exploiting the potential of the stress buffering function are proposed.

Structure, mechanical and friction behavior of laminar Ti-incorporated Al2O3 thermal control films deposited on polyethylene terephthalate flexible substrates

Nano-multilayered Ti-Al2O3 films with different Ti layer thicknesses were fabricated on polyethylene terephthalate flexible substrates by roll to roll magnetron sputtering. The morphologies and structure of the multilayer films were investigated by scanning electron microscopy and X-ray diffraction. Multilayer films show dense and coherent epitaxial growth due to the mutual growth-promoting effect. Bruker tester, nano-indenter and tribometer were used to examine the adhesion strength, mechanical and tribological properties of Ti-Al2O3 multilayer films. Results indicate that multilayer films can be deformed together with PET substrate and withstand the larger strain without cracking. The mean value of adhesion strength of the multilayer films obtained from the scratch tests are in the range of 14.5–16.3 mN. The multilayer films with Ti layer thickness of 8.3 nm have the highest hardness of 21.3 GPa, lowest friction coefficient of 0.32 and lowest wear rate of 2.46×10−6 mm3/Nm.

Effects of Ar/O2 ratio on preparation and properties of multilayer Cr2O3/α-Al2O3 tritium permeation barrier

A multilayered Cr2O3/α-Al2O3 tritium permeation barrier coating containing a total of 10 nanoscale layers was prepared by radiofrequency magnetron sputtering without intentional heating. X-ray diffraction, Auger electron spectroscopy, and spherical aberration corrected transmission electron microscopy studies showed a multilayer structure and a Cr2O3 (104)/α-Al2O3 (104) coherent epitaxial growth. The effect of the Ar/O2 gas flow ratio (Λ) on the structure and properties of the Cr2O3/Al2O3 multilayer coating were investigated. The results show that the crystallinity of the α-Al2O3 phase increased with decreasing Λ. The mechanical properties, He+ ion irradiation tolerance, and corrosion resistance of the coating began to improve greatly when Λ was decreased to 1:1. The coating with Λ = 1:2 gave α-Al2O3 with the best properties, i.e., high nano-hardness (19.6 GPa), good corrosion resistance (icorr = 10 nA/cm2), and excellent irradiation resistance. The deuterium permeability of the Cr2O3/α-Al2O3 multilayer coating was also investigated.

Phase interfaces between AlTiN and WC/Mo2C in WC–Mo2C substrate and the related characteristics

To explore the synergistic effect of the hard phase and the ductile binder phase in hardmetal substrate to the coatings, a binderless WC–6Mo2C–0.68Cr3C2–0.37VC (bWC) model alloy and a WC–10Co–0.65Cr3C2–0.35VC reference alloy with similar grain size (~0.4 μm) were used as substrates. Ti0.94Si0.06N/TiAlSiN/Al0.52Ti0.48N coating was deposited by DC magnetron sputtering. Scratch testing (loading range 0–30 N) results show that complete removal of the coating on bWC substrate occurs under a substantially higher critical load LC2 (28.3 N vs. 21.4 N), but accompanied by a quite earlier cohesion failure expressed by LC1 (LC2 − LC1 = 16.4 N vs. 1.0 N). To reveal the reason behind the high adhesion strength, phase interfaces between AlTiN and WC/Mo2C based phase in bWC substrate were investigated by HRTEM. Coherent and semi-coherent epitaxial growth relationships at the AlTiN/WC interfaces and semi-coherent epitaxial growth relationships at the AlTiN/Mo2C interfaces are easily identified. The minimum lattice misfit (1.9%) is observed when AlTiN growths on (0001)WC along its 4¯22¯ direction. The reasons behind the related phenomena are discussed.

Laser fluence and spot size effect on compositional and structural properties of BiFeO3 thin films grown by Pulsed Laser Deposition

We investigated the effect of laser fluence and spot size on the structure and composition of BiFeO3 (BFO) epitaxial thin films grown on SrTiO3 substrates by Pulsed Laser Deposition. X-ray diffraction shows that BFO's out of plane lattice parameter increases with the laser fluence. A coherent epitaxial film growth is observed for all tested laser fluences and spot sizes for thicknesses up to 16nm. The critical thickness at which relaxation occurs depends either on the laser fluence or spot size. The fluence dependence of the out of plane lattice parameter is accompanied with a cationic composition variation. Bi vacancies are evidenced at lower fluences while as Bi/Fe tends towards 1 a higher relaxation critical thickness is observed. An optimum Bi/Fe ratio is obtained for a fluence of 1.72J/cm2. This result was confirmed by wavelength-dispersive x-ray spectroscopy (WDS) scans over a 1cm2 film. An excellent thickness and composition uniformity is attained over the entire sample area.

Prediction of first-order martensitic transitions in strained epitaxial films

Coherent epitaxial growth allows us to produce strained crystalline films with structures that are unstable in the bulk. Thereby, the overlayer lattice parameters in the interface plane, (a, b), determine the minimum-energy out-of-plane lattice parameter, . We show by means of density-functional total energy calculations that this dependence can be discontinuous and predict related first-order phase transitions in strained tetragonal films of the elements V, Nb, Ru, La, Os, and Ir. The abrupt change of can be exploited to switch properties specific to the overlayer material. This is demonstrated for the example of the superconducting critical temperature of a vanadium film which we predict to jump by 20% at a discontinuity of .

Suppressed polar distortion with enhanced Curie temperature in in-plane 90°-domain structure of a-axis oriented PbTiO3 Film

PbTiO3 thin films were epitaxially grown on (001) KTaO3 single crystal substrates by metalorganic chemical vapor deposition. The coherent epitaxial growth introduced a large in-plane tensile strain to the PbTiO3 film. This tensile strain increased TC and directed the polarization to one of the in-plane ⟨100⟩ axes below TC, resulting in the formation of perfect a1/a2/a1/a2 domain structure. We found that the polar distortion is appreciably suppressed in such a1/a2/a1/a2 domain structure while TC is enhanced due to the strain.

Preparation and characterization of CrAlN/TiAlSiN nano-multilayers by cathodic vacuum arc

Superhard CrAlN/TiAlSiN nano-multilayers deposited at various substrate bias voltages were produced on the surface of Si (100) wafers and high-speed steels by cathodic vacuum arc. The microstructures of these coatings were investigated systematically by X-ray diffraction (XRD), X-ray photoelectron spectroscope (XPS), scanning electron microscope (SEM) and high-resolution transmission electron microscope (HRTEM), in association with mechanical property characterization. The HRTEM observation shows that these coatings have a nano-multilayer structure containing alternatively fcc-(Cr,Al)N sublayer and amorphous sublayer with some nanocrystalline fcc-(Ti,Al)N phases embedded in this amorphous matrix. The coherent epitaxial growth structure is observed along the {100} crystal plane of the fcc-(Cr,Al)N phase and the {100} crystal plane of the fcc-(Ti,Al)N phase. The orientation relationship between them is as follows: {100}(Cr,Al)N//{100}(Ti,Al)N and [001](Cr,Al)N//[001](Ti,Al)N. Mechanical property characterization shows that the coating hardness is dependent on the substrate bias voltages, and the friction coefficients of these coatings with a higher substrate bias voltage are found to be much lower than those of the coatings deposited at a lower substrate bias voltage. The improvement of the friction coefficient is attributed mainly to the formation of tribochemical reaction films. In addition, the much lower friction coefficient can also be related to the higher hardness and fracture toughness.

The Bain library: A Cu-Au buffer template for a continuous variation of lattice parameters in epitaxial films

Smallest variations of the lattice parameter result in significant changes in material properties. Whereas in bulk, lattice parameters can only be changed by composition or temperature, coherent epitaxial growth of thin films on single crystals allows adjusting the lattice parameters independently. Up to now only discrete values were accessible by using different buffer or substrate materials. We realize a lateral variation of in-plane lattice parameters using combinatorial film deposition of epitaxial Cu-Au on a 4-in. Si wafer. This template gives the possibility to adjust the in-plane lattice parameter over a wide range from 0.365 nm up to 0.382 nm.

Influence of VN 2-D Inserted Layers Thickness on Microstructure and Mechanical Property of TiSiN Nanocomposite Film

Using V and TiSi targets, a series of TiSiN nanocomposite films being inserted by VN nano-layers with different thickness were prepared by RF magnetron sputtering method. Using X-ray diffraction (XRD), high resolution transmission electron microscope (HRTEM) and nano-indentation techniques, the influences of VN inserted layer thickness on microstructure and mechanical property of TiSiN nanocomposite film were investigated. The results show that, when the VN inserted layer thickness is small, the TiSiN film transforms from nanocomposite structure into nanomultilayered structure, leading to the decrease of hardness. As the VN inserted layers thicken, the hardness of film renewedly increases. When the VN inserted thickness is 0.5 nm, the film presents columnar crystals which continuously crosses through the nanomultilayers. TiSiN and VN layers exhibit coherent epitaxial growth structure, and the hardness accordingly reaches 37.2 GPa. With further increase of the VN inserted layer thickness, the coherent epitaxial growth structure is broken and the hardness of film decreases.

Effects of modulation periodicity on microstructure, mechanical and tribological properties of NbN/AlN nanostructured multilayer films

NbN/AlN nano-multilayer films with modulation periodicity (Λ) ranging from 3.9 to 31.6nm have been deposited on Si (100) substrate by reactive magnetron sputtering in Ar/N2 mixtures. The Λ dependent structural, mechanical and tribological properties for resulting NbN/AlN multilayers have been explored. As Λ varies from 3.9 to 31.6nm, the crystal structures of NbN and AlN are face-centered cubic and hexagonal in multilayer films, respectively, favoring formation of a coherent epitaxial growth with crystallographic relationship of 1 1 1NbN1 1 0NbN//0 0 0 2AlN1 1 2¯ 0AlN due to total energy minimization, and large Λ can improve mutual coherence and promote coherent growth of fcc-NbN(111)/w-AlN(0002). The remarkable hardness enhancement implements in a wide range of Λ from 3.9 to 31.6nm for NbN/AlN multilayer system, which can be mainly attributed to hindering the dislocation motion caused by heterostructure coherent interface of fcc-NbN(111)/w-AlN(0002). Although all multilayers show lower friction coefficient than constituent monolayers, great improvements in the wear behaviors only appear in multilayers with Λ=3.9 and 7.4nm due to their large content of interfaces.

Nanometer scale chemistry and microstructure of CrN/AlN multilayer films

Nanometerscale multilayer CrN/AlN thin films were deposited by pulsed DC magnetron sputtering. Nanoscale resolution chemistry and microstructure of CrN/AlN films with modulation periods (Λ) of 4nm and 18nm were studied using advanced microscopy and diffraction. At the nanometer level, the interface between the CrN and AlN nanolayers contains a concentration gradient of Al and Cr. Inter-diffusion of Cr and Al atoms into the AlN and CrN layers was identified. Microstructural characterization revealed the CrN nanolayers epitaxially stabilize the AlN layers (2nm) to a cubic structure for the Λ=4nm film. The rapid repetition of the interfacial energy across the AlN nanolayers is critical for the coherent epitaxial growth. The film at Λ=4nm exhibited a superhardness of 42GPa. In contrast, the AlN layers (12.5nm) exhibited a hexagonal structure in the Λ=18nm film as the bulk energy becomes dominating, in which misorientated and incoherent interfaces were found. The hardness of the film dropped down to 23GPa.