Intermixing Of Layers Research Articles

INTERDIFFUSION IN CHALCOGENIDE SEMICONDUCTOR SUPERLATTICE NANOSTRUCTURES

The diffusion intermixing of layers during annealing of epitaxial superlattice nanostructures based on chalcogenide semiconductors (PbS, PbSe, PbTe, EuS, EuSe, SrS) was studed by X-ray diffraction technique. The interdiffusion coefficients were determined basing on changing of the intensity of near-Bragg reflection satellites in X-ray diffraction pattern. Layer materials in superlattices EuS-PbTe and PbS-PbTe are not intermixed.

Effect of annealing on layer intermixing in nanoscale Cr/Ti multilayers depending on the period value

The effect of annealing on the intermixing of thin Ti and Cr layers and their crystallinity in multilayer structures was studied using XPS, TEM, XRR and XRD methods. Multilayer structures with varios period values (sum thickness of the Ti and Cr layers) in the range from 1.4 nm to 4.3 nm were considered. The analysis revealed the formation of TiCrx at the interfaces of the layers. The contribution of the formed chromide relative to the base layers increases with decreasing period of the structure. The layers in the structure with thinnest period were found to be X-ray amorphous, while the structure with a thicker period exhibited the appearance of large crystallites of chromium and titanium. Annealing of the multilayer structures induced additional intermixing, leading to an increase in the extension of the chromide regions. Additionally, it resulted in a reduction of internal stresses within the structures. When the multilayer structures were annealed at a temperature of 700 °C, complete degradation occurred, with all layers intermixing with each other and with the Si substrate.

Microstructure and thermal stability of crystalline/amorphous Fe/FeW nanomultilayers

The thermal stability of crystalline-amorphous interfaces was investigated in Fe/FeW nanomultilayers (NMs), where the alloy layers were amorphous in the as-sputtered state with concentrations of Fe-38 at.% W or Fe-67 at.% W. Compositionally driven devitrification, layer breakdown, and recrystallization were compared using both single-layer and multilayer configurations at temperatures ranging from 250 °C to 750 °C. Annealing of the NMs to 500 °C revealed destabilization in the Fe-67 W layers with the formation of crystalline-crystalline interfaces (CCIs) whereas the Fe-38 W layers remained intact with stable crystalline-amorphous interfaces (CAIs). Further annealing to 750 °C resulted in multilayer evolution and recrystallization, where breakdown of the CAIs was attributed to layer intermixing while the CCIs experienced intermetallic grooving and pinch-off. The influence of amorphous stability, composition, and intermetallic formation are discussed with respect to the NM breakdown mechanisms. This work highlights a promising strategy for exploring compositionally driven stability at the nanoscale in crystalline-amorphous alloys.

Domain-Selective Enzymatic Cross-linking and Etching for Shape-Morphing DNA-Linked Nanoparticle Films.

The highly programmable and responsive molecular recognition properties of DNA provide unparalleled opportunities for fabricating dynamic nanostructures capable of structural transformation in response to various external stimuli. However, they typically operate in tightly controlled environments because certain conditions (ionic strength, pH, temperature, etc.) must be met for DNA duplex formation. In this study, we adopted site-specific enzymatic ligation and DNA-based layer-by-layer thin film fabrication to build shape-morphing DNA-linked nanoparticle films operational in a broad range of environments. The ligated films remained intact in unusual conditions such as pure water and high temperature causing dissociation of DNA duplexes and showed predictable and reversible shape morphing in response to various environmental changes and DNA exchange reactions. Furthermore, domain-selective ligation combined with photoinduced interlayer mixing allowed for the fabrication of unusual edge-sealed double-layered films through midlayer etching, which is difficult to realize by other methods.

An experimental and numerical investigation of thermofluidic properties on the novel curved microchannel heat sink with slotted section

The current work proposes the development of a novel curved microchannel heat sink by modifying the existing flat microchannel heat sink for enhancing the heat dissipation in the electronic device without significant pressure drop penalty. The inspiration for the curved channel is taken from a NACA airfoil. The microchannel comprises two sections: a curved section and a slotted straight section. Experimental and numerical investigations are performed on the proposed geometry to study the thermofluidic properties for Reynolds number (Re) ranging from 200 to 500 using DI water as working fluid. Fluid flow and heat transfer are analysed by carrying out a full domain numerical simulation in ANSYS Fluent 19.0. The thermofluidic properties of the curved microchannel heat sink (CMCHS) and straight microchannel heat sink (SMCHS) are compared numerically by keeping the geometrical parameters (hydraulic diameter, foot area and fin area) and inlet parameters (inlet velocity and heat flux) identical. The average heat transfer coefficient (havg) of CMCHS is enhanced over SMCHS due to the formation of vorticities in the slotted region owing to the fluid intermixing and boundary layer disruption. The presence of curved section leads to reducing pressure drop penalty pertaining to gravitational effects. Together, the curved section and slotted section enhance the performance factor of CMCHS over SMCHS. The vortices formed increase as the Reynolds number increases, enabling an increase in the heat transfer coefficient. Therefore, the performance factor also increases as the Reynolds number is increased. An enhancement of ≈ 67.6 % in havg is achieved for Re = 500 in CMCHS compared to SMCHS with DI water as the working fluid.

DeepFreeze 3D-biofabrication for Bioengineering and Storage of Stem Cells in Thick and Large-Scale Human Tissue Analogs.

3D bioprinting holds great promise for meeting the increasing need for transplantable tissues and organs. However, slow printing, interlayer mixing, and the extended exposure of cells to non-physiological conditions in thick structures still hinder clinical applications. Here the DeepFreeze-3D (DF-3D) procedure and bioink for creating multilayered human-scale tissue mimetics is presented for the first time. The bioink is tailored to support stem cell viability, throughout the rapid freeform DF-3D biofabrication process. While the printer nozzle is warmed to room temperature, each layer solidifies at contact with the stage (-80 °C), or the subsequent layers, ensuring precise separation. After thawing, the encapsulated stem cells remain viable without interlayer mixing or delamination. The composed cell-laden constructs can be cryogenically stored and thawed when needed. Moreover, it is shown that under inductive conditions the stem cells differentiate into bone-like cells and grow for months after thawing, to form large tissue-mimetics in the scale of centimeters. This is important, as this approach allows the generation and storage of tissue mimetics in the size and thickness of human tissues. Therefore, DF-3D biofabrication opens new avenues for generating off-the-shelf human tissue analogs. It further holds the potential for regenerative treatments and for studying tissue pathologies caused by disease, tumor, or trauma.

Increasing the thermal stability of a Cr/Sc multilayer by nitriding

The effect of nitridation of Cr, Sc or both layers in the ultrathin multilayer structure Sc/Cr on the intermixing of thin Cr and Sc layers, as-deposited and after annealing at different temperatures were considered in details. The processes of passivation by nitrogen and synthesis in nitrogen medium of chromium and scandium layers are also compared. It was established that during passivation process the nitrogen interacts with surface of scandium film forming a ScNx compound and does not interact at all with the chromium layer. A net scavenging of nitrogen is observed in the case of CrNx/Sc system, while in the Sc/CrNx system the interaction of scandium with nitrogen gas before chromium layer deposition with formation of some amount of ScNx is added.Based on theoretical analysis of the effect of nitriding the system and detailed analysis carried out by photoelectron spectroscopy a multilayer system [Sc/CrNx]300 was synthesized and analyzed at different temperatures of annealing. Analysis of X-ray diffraction data showed that the sample consists of X-ray amorphous layers, the crystallinity of which practically does not change with increasing temperature. The results show that the [Sc/CrNx]300 system with small nitrogen content is very resistant to elevated temperatures up to 450 °C.

Microstructures and nanomechanical properties of nanolaminated Ta/Co composites and their strengthening mechanisms

In this study, nanolaminated tantalum (Ta)/cobalt (Co) composites (NTCCs) with an individual layer thickness (h) ranging from 5 nm to 100 nm were fabricated via magnetron sputtering. The microstructures and nanomechanical properties of the NTCCs were affected by variation in h. The NTCCs showed nanograin structures in Ta and Co layers, with Ta and Co textures that were randomly oriented. Nanohardness (H) and corresponding yield strength, σys=H/2.7, of the NTCCs gradually increased from 5.75 ± 0.15 GPa to 7.20 ± 0.13 GPa and from 2.12 ± 0.06 GPa to 2.67 ± 0.05 GPa, respectively, with reducing h from 100 nm to 5 nm. NTCC showed an extraordinarily high yield strength (∼ 2.67 GPa) at h = 5 nm due to its reduced individual layer thickness and non-defective microstructures, which is well above the maximum yield strength of studied nanolaminated materials (comprised of at least one hcp constituent), to date. The creep depth of 5 nm NTCC was lower than that of the 100 nm NTCC, and the creep deformation of 5 nm NTCC is related to the bending, breaking, and intermixing of Ta and Co layers, whereas the 100 nm NTCC exhibited bending and thinning of Ta and Co layers with more deformation. Strain rate sensitivity (m) of NTCC increased from 0.0666 to 0.2076 with increasing h from 5 nm to 100 nm. The Hall–Petch and Confined Layer Slip strengthening mechanisms governed the strength of the NTCCs for h = 25–100 nm and h = 10 nm, respectively. It is worth noting that the 5 nm NTCC did not follow any of the strengthening mechanisms and became independent of h; rather, the strength at this length scale was greatly influenced by grains, layer thickness, and microstructural variations at the interfaces. The increased σys and E at h = 5 nm may facilitate tailoring the mechanical properties of NTCCs with high strength and ductility.

Green Solvent Selection for All Solution-Processed Inverted Quantum Dot Light Emitting Diode.

Quantum-dot light-emitting diodes (QD-LEDs) have gained attention as potential display technologies. However, the solvents used to dissolve a polymeric hole transport layer (HTL) are hazardous to both humans and the environment. Additionally, intermixing the HTL and QD layers presents a significant challenge when fabricating inverted QD-LEDs. Here, a green solvent selection procedure to achieve good device performance and environmental safety in QD-LEDs is established. This procedure utilizes Hansen solubility parameters and surface roughness to identify a set of solvents that do not lower the device performance by avoiding interlayer mixing or a rough interface. The CHEM21 solvent selection guide is used to screen for environmentally hazardous solvents. Finally, cyclopentanone (CPO) is selected as the optimal HTL solvent from among 16 candidates. Using CPO improves the maximum luminescence by ≈1.6 times and the maximum current efficiency by ≈12.6 times, compared to that of conventional devices using hazardous chlorobenzene. Solvent selection is critical for the fabrication of green and high-performance inverted QD-LEDs, particularly for large display panels that require n-type oxide thin-film transistors.

Control of the Crystallization and Phase Separation Kinetics in Sequential Blade‐Coated Organic Solar Cells by Optimizing the Upper Layer Processing Solvent

AbstractSequential deposition of the active layer in organic solar cells (OSCs) is favorable to circumvent the existing drawbacks associated with controlling the microstructure in bulk‐heterojunction (BHJ) device fabrication. However, how the processing solvents impact on the morphology during sequential deposition processes is still poorly understood. Herein, high‐efficiency OSCs are fabricated by a sequential blade coating (SBC) through optimization of the morphology evolution process induced by processing solvents. It is demonstrated that the device performance is highly dependent on the processing solvent of the upper layer. In situ morphology characterizations reveal that an obvious liquid–solid phase separation can be identified during the chlorobenzene processing of the D18 layer, corresponding to larger phase separation. During chloroform (CF) processing of the D18 layer, a proper aggregation rate of Y6 and favorable intermixing of lower and upper layers results in the enhanced crystallinity of the acceptor. This facilitates efficient exciton dissociation and charge transport with an inhibited charge recombination in the D18/CF‐based devices, contributing to a superior performance of 17.23%. These results highlight the importance of the processing solvent for the upper layer in the SBC strategy and suggest the great potential of achieving optimized morphology and high‐efficiency OSCs using the SBC strategy.

Role of C and B4C barrier layers in controlling diffusion propagation across the interface of Cr/Sc multilayers.

The optical performance of low-bilayer-thickness metallic multilayers (ML) can be improved significantly by limiting the intermixing of consecutive layers at the interfaces. Barrier layers are supposed to exhibit a decisive role in controlling diffusion across the interfaces. The element-specific grazing incidence extended X-ray absorption fine structure technique using synchrotron radiation has been used in conjunction with grazing incidence X-ray reflectivity and diffuse X-ray scattering measurements to study the impact of the two most common barrier layers, viz., C and B4C, at the interfaces of Cr/Sc MLs. The diffusion propagation is reduced by both the barrier layers; however, it is found that the improvement is more significant with the B4C barrier layer. It is seen that C forms an intermixed layer with Sc and leads to carbide formation at the interface, which then acts as shielding and prevents further interdiffusion, while B4C hardly penetrates into Sc and stops the overlap between Sc and Cr directly by wetting the corresponding interface. Thus, the above measurements reveal crucial and precise information regarding the elemental diffusion kinetics at the interfaces of Cr/Sc MLs in a non-destructive way, which is very important for technological applications of these MLs as X-ray optical devices.

Mechanically robust stretchable semiconductor metallization for skin-inspired organic transistors.

Despite recent remarkable advances in stretchable organic thin-film field-effect transistors (OTFTs), the development of stretchable metallization remains a challenge. Here, we report a highly stretchable and robust metallization on an elastomeric semiconductor film based on metal-elastic semiconductor intermixing. We found that vaporized silver (Ag) atom with higher diffusivity than other noble metals (Au and Cu) forms a continuous intermixing layer during thermal evaporation, enabling highly stretchable metallization. The Ag metallization maintains a high conductivity (>104 S/cm) even under 100% strain and successfully preserves its conductivity without delamination even after 10,000 stretching cycles at 100% strain and several adhesive tape tests. Moreover, a native silver oxide layer formed on the intermixed Ag clusters facilitates efficient hole injection into the elastomeric semiconductor, which transcends previously reported stretchable source and drain electrodes for OTFTs.

Unraveling the degradation mechanism of LiNi0.8Co0.1Mn0.1O2 at the high cut-off voltage for lithium ion batteries

LiNi0.8Co0.1Mn0.1O2 (NCM811) layered oxides have been regarded as promising alternative cathodes for the next generation of high-energy lithium ion batteries (LIBs) due to high discharge capacities and energy densities at high operation voltage. However, the capacity fading under high operation voltage still restricts the practical application. Herein, thecapacity degradationmechanismof NCM811 at atomic-scale isstudied in detail under various cut-off voltages using aberration-corrected scanning transmission electron microscopy (STEM).It is observed that the crystal structure of NCM811 evolution from a layered structure to a rock-salt phase is directly accompanied by serious intergranular cracks under 4.9 V, which is distinguished from the generally accepted structure evolution of layered, disordered layered, defect rock salt and rock salt phases, also observed under 4.3 and 4.7 V. The electron energy loss spectroscopy analysis also confirms the reduction of Ni and Co from the surface to the bulk, not the previously reported only Li/Ni interlayer mixing. The degradation mechanism of NCM811 at a high cut-off voltage of 4.9 V is attributed to the formation of intergranular cracks induced by defects, the direct formation of the rock salt phase, and the accompanied reduction of Ni2+ and Co2+ phases from the surface to the bulk.

Refined thermal stability of Cr/Sc multilayers with Si(Be) barrier layers

Effect of Si and Be barrier layers on the intermixing of thin Cr and Sc layers, as-deposited and after annealing in a wide range of temperatures was considered using X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), X-ray reflectometry (XRR) and grazing-incidence X-ray diffraction (GIXRD). It was established that annealing of a Si/[Cr/Sc]200 system increases mixing. In the sample heated at a temperature of 450°C during 1h as a result of a full intermixing of the Cr and Sc layers a segregation of scandium on the sample surface is traced. The structure becomes textured with Sc [001] preferred orientation perpendicular to the substrate. The insertion of the Be barrier layer in the Si/[Cr/Sc]200 system limits the intermixing between chromium and scandium layers during annealing at temperatures up to 350°C, but full degradation of the structure occurs at 450°C. Be barrier layer in the Cr/Sc system prevents a texturing and grain growth in the system, but does not oppose the crystallization process. A silicon layer inserted between the scandium and chromium layers restricts their intermixing. The multilayer retains an amorphous structure.

Low-temperature copper–copper quasi-direct bonding with cobalt passivation layer

In this work, an ultra-thin cobalt film is used as a passivation layer to achieve a Cu–Cu quasi-direct bonding process, and successful bonding was achieved at the bonding temperature of 230 °C with the bonding pressure of 0.5 MPa in non-vacuum conditions. A tensile bonding strength of over 12.8 MPa can be achieved. With the cobalt layer, the total film roughness is greatly reduced to around 1 nm, and the cross-sectional transmission electron microscope images reveal the presence of a void-free bonding interface. Time-of-flight secondary ion mass spectrometry depth analysis is carried out using a PVD deposited multilayer sample, which simulates the bonding process without pressure. It is found that a Cu–Co intermixing layer formed between the Cu and Co interfaces during annealing, and Cu has diffused into the Co layer. Due to the nanocrystalline structure of the Co film and the low roughness of the system, a low bonding temperature can be achieved. These results demonstrate that Co, which can be selectively grown on the Cu surface, can be used as a passivation layer to achieve low-cost and high-quality interconnect for the three-dimensional integrated circuits.

Enhancing all-optical switching of magnetization by He ion irradiation

All-optical switching (AOS) of magnetization by a single femtosecond laser pulse in Co/Gd based synthetic ferrimagnets is the fastest magnetization switching process. On the other hand, He ion irradiation has become a promising tool for interface engineering of spintronic material platforms, giving rise to significant modification of magnetic properties. In this paper, we explore the use of He ion irradiation to enhance single pulse AOS of Co/Gd bilayer-based synthetic ferrimagnets. The intermixing of the constituent magnetic layers by He ion irradiation was both numerically simulated and experimentally verified. We theoretically modeled the effects of intermixing on AOS using the layered microscopic 3-temperature model and found that AOS is enhanced significantly by breaking the pristine Co/Gd interface through intermixing. Following this notion, we studied the threshold fluence of AOS as a function of He ion irradiation fluence. We found that the AOS threshold fluence can be reduced by almost 30%. Our study reveals the control of AOS by He ion irradiation, which opens up an industrially compatible approach for local AOS engineering.

Amorphous to Polycrystalline Phase Transition in La2O3 Films Grown on a Silicon Substrate Forming Si‐Doped La2O3 Films

Herein, La2O3 films are fabricated on a Si substrate without a La–Sr intermixing layer at the La2O3/Si interface using a pulsed laser deposition method. X‐ray diffraction data shows only two discernible peaks of the La2O3 films: hexagonal La2O3 (10‐1) and cubic La2O3 (222), indicating polycrystalline character. During film growth, the reflection high‐energy electron diffraction pattern from the La2O3 surface changes from an initial column shape to a complicated distributed dot pattern with narrow lines, suggesting possible structural property changes in the La2O3 film. The occurrence of a structural transition is confirmed by high‐resolution transmission electron microscopy (HRTEM), which exhibits a clear crystalline phase change from an initial ≈10 nm thick amorphous La2O3 film to polycrystalline La2O3 film on Si. Rutherford backscattering shows a reduced La–Si intermixing between La2O3 and Si. Furthermore, the results of X‐ray photoelectron spectroscopy atomic depth profile analysis show that observation of La‐silicate over the whole La2O3 film indicates that Si diffuses through whole thick La2O3 films forming Si‐doped La2O3 films. This study of the well‐defined structural characteristics and sharp interface of La2O3/Si will enable further understanding of high dielectric constant materials grown on Si by the introduction of advanced film growth technique.

Effect of B4C barrier layer on layer intermixing in nanoscale W/Be multilayers before and after annealing

The effect of an ultrathin B4C barrier layer on the content of beryllides formed as a result of W and Be mixing during deposition in a short-period W/Be multilayers, and, as a consequence, on the reflectivity of the multilayers and their thermal stability is discussed in the paper. All the multilayers were prepared by means of DC magnetron sputtering. Chemical composition analysis and evaluation of thickness extensions were carried out utilizing X-ray photoelectron spectroscopy and transmission electron microscopy, respectively. The results obtained showed that the insertion of the ultrathin B4C barrier layer led to a decrease in the content of beryllides in multilayers due to the formation of tungsten compounds with boron/carbon. In the case of the B4C deposited onto the beryllium layer it was also associated with the additional formation of Be2C. It was suggested that the formation of the borides and carbides is more optically favorable compared to the beryllides in the multilayer without the barrier layer, which causes an increase in reflectivity. Ex situ annealing of the multilayer structures at temperatures up to 300 °C did not provoke additional mixing of the layers, but only increased a surface BeO, which led to decreasing in reflectivity of the multilayers.

Nanophotonic control of thermal emission under extreme temperatures in air.

Nanophotonic materials offer spectral and directional control over thermal emission, but in high-temperature oxidizing environments, their stability remains low. This limits their applications in technologies such as solid-state energy conversion and thermal barrier coatings. Here we show an epitaxial heterostructure of perovskite BaZr0.5Hf0.5O3 (BZHO) and rocksalt MgO that is stable up to 1,100 °C in air. The heterostructure exhibits coherent atomic registry and clearly separated refractive-index-mismatched layers after prolonged exposure to this extreme environment. The immiscibility of the two materials is corroborated by the high formation energy of substitutional defects from density functional theory calculations. The epitaxy of immiscible refractory oxides is, therefore, an effective method to avoid prevalent thermal instabilities in nanophotonic materials, such as grain-growth degradation, interlayer mixing and oxidation. As a functional example, a BZHO/MgO photonic crystal is implemented as a filter to suppress long-wavelength thermal emission from the leading bulk selective emitter and effectively raise its cutoff energy by 20%, which can produce a corresponding gain in the efficiency of mobile thermophotovoltaic systems. Beyond BZHO/MgO, computational screening shows that hundreds of potential cubic oxide pairs fit the design principles of immiscible refractory photonics. Extending the concept to other material systems could enable further breakthroughs in a wide range of photonic and energy conversion applications.

A comprehensive study on melting enhancement by changing tube arrangement in a multi-tube latent heat thermal energy storage system

In this study, the paraffin wax's melting dynamics in a multi-tube latent heat thermal energy storage system are investigated. The performance of ten arrangements of five heat transfer fluid tubes is compared. The radial eccentricity of four planetary tubes is varied around a fixed central tube. The study aims to find an appropriate arrangement of multiple HTF tubes keeping the same PCM content and heating surface. A simplified two-dimensional computational domain was considered, an enthalpy-porosity based melting model was designed, and the governing equations were solved using ANSYS-Fluent. The melting sequence in various cases is represented as the liquid fraction contours, flow streamlines, and temperature contours. The performance of proposed designs is assessed on the basis of transient plots of liquid fraction, average domain velocity, and heat flux at the outer surface of the heat transfer fluid tube. It is observed that the melting dynamics are greatly influenced by the tube arrangement. The coalescence of small vortices and the intermixing of different temperature layers accelerate the melting. It is found that the tubes in the top half should be placed closer to the center to delay thermal stratification, and in the bottom half should be placed deeper into the poor melting zone. The staggered arrangement of planetary tubes outperforms the inline arrangement at given eccentricities. The fastest and slowest melting is realized by Case 9 and Case 1, which take 128 and 269 min, respectively, for melting. The proposed designs can store 1450 kJ/m of energy with 98 % theoretical efficiency.