Nanolaminate Structure Research Articles

Advances in Atomic Layer Deposition (ALD) Nanolaminate Synthesis of Thermoelectric Films in Porous Templates for Improved Seebeck Coefficient.

Thermoelectrics is a green renewable energy technology which can significantly contribute to power generation due to its potential in generating electricity out of waste heat. The main challenge for the development of thermoelectrics is its low conversion efficiency. One key strategy to improve conversion efficiency is reducing the thermal conductivity of thermoelectric materials. In this paper, the state-of-the-art progresses made in improving thermoelectric materials are reviewed and discussed, focusing on phononic engineering via applying porous templates and ALD deposited nanolaminates structure. The effect of nanolaminates structure and porous templates on Seebeck coefficient, electrical conductivity and thermal conductivity, and hence in figure of merit zT of different types of materials system, including PnCs, lead chalcogenide-based nanostructured films on planar and porous templates, ZnO-based superlattice, and hybrid organic-inorganic superlattices, will be reviewed and discussed.

Super high-dielectric-constant oxide films for next-generation nanoelectronics and supercapacitors for energy storage

Abstract

MOS Capacitance Measurements for PEALD TiO2 Dielectric Films Grown under Different Conditions and the Impact of Al2O3 Partial-Monolayer Insertion.

In this paper, we report the plasma-enhanced atomic layer deposition (PEALD) of TiO2 and TiO2/Al2O3 nanolaminate films on p-Si(100) to fabricate metal-oxide-semiconductor (MOS) capacitors. In the PEALD process, we used titanium tetraisopropoxide (TTIP) as a titanium precursor, trimethyl aluminum (TMA) as an aluminum precursor and O2 plasma as an oxidant, keeping the process temperature at 250 °C. The effects of PEALD process parameters, such as RF power, substrate exposure mode (direct or remote plasma exposure) and Al2O3 partial-monolayer insertion (generating a nanolaminate structure) on the physical and chemical properties of the TiO2 films were investigated by Rutherford backscattering spectroscopy (RBS), Raman spectroscopy, grazing incidence X-ray diffraction (GIXRD), and field emission scanning electron microscopy (FESEM) techniques. The MOS capacitor structures were fabricated by evaporation of Al gates through mechanical mask on PEALD TiO2 thin film, followed by evaporation of an Al layer on the back side of the Si substrate. The capacitors were characterized by current density-voltage (J-V), capacitance-voltage (C-V) and conductance-voltage (G-V) measurements. Our results indicate that RF power and exposure mode promoted significant modifications on the characteristics of the PEALD TiO2 films, while the insertion of Al2O3 partial monolayers allows the synthesis of TiO2/Al2O3 nanolaminate with well-spaced crystalline TiO2 grains in an amorphous structure. The electrical characterization of the MOS structures evidenced a significant leakage current in the accumulation region in the PEALD TiO2 films, which could be reduced by the addition of partial-monolayers of Al2O3 in the bulk of TiO2 films or by reducing RF power.

Atomic layer deposition of nitrogen incorporated molybdenum oxide: Unveiling carrier transport mechanism and its application in Li-ion battery

Nitrogen incorporated molybdenum oxide (MoOx:N) thin films are obtained by stacking of atomic layer deposited grown MoOx and MoNx in a nanolaminate architecture without any postdeposition processing. Evolution of stoichiometrically dependent electron conductivity is obtained that is efficiently controlled by varying the ratio of the number of layers (MoOx to MoNx) in the nanolaminate structure. The carrier type, the free carrier concentration, and the mobility of the as-grown materials are studied by temperature-dependent Hall-effect measurement. The authors hypothesize that the addition of N-atom creates oxygen vacancies in the films that in turn act as a shallow donor level that results in improved conductivity in the ternary material. As-grown materials, both in the form of planar architecture and coaxially grown on multiwalled carbon nanotubes are investigated as a prospective anode material in the Li-ion battery. The comparison reveals a substantially improved performance of N-incorporated samples than that of the pristine materials. The highest discharge capacity of 1287 mA h g−1 was achieved in the first cycle at a current rate of 0.1 A g−1, and a stable capacity of 974 and 610 mA h g−1 is achieved when discharged at 0.1 and 2 A g−1, respectively, from the core-shell structures.

Optical and mechanical properties of nanolaminates of zirconium and hafnium oxides grown by atomic layer deposition

Nanolaminates of ZrO2 and HfO2 were grown by atomic layer deposition, using metal halides and water as precursors, on silicon and fused quartz substrates at 300 °C. The crystalline phase composition, optical refraction, and mechanical performance of the multilayers were influenced by the relative contents of the constituent metal oxides. The crystal growth in as-deposited HfO2 dominantly led to the monoclinic phase, whereas ZrO2 was partially crystallized as its metastable and hard tetragonal polymorph. The hardness and elasticity of the nanolaminate structures could be modified by varying the amounts of either oxide contributing to the crystallographic order formed in the solid films. The refractive indexes depended on the nanolaminate structure.

Solution-Processed Li2O-Al2O3/TiO2 Nanolaminate Stacks Containing Mobile Lithium Ions and with Increased Breakdown Voltages.

An aqueous solution approach has been utilized to prepare nanolaminates of TiO2 and ionically conductive Li2O-Al2O3 (LiAlO). This new approach utilizes low curing temperatures, resulting in fully oxidized films as demonstrated by Fourier-transform infrared spectroscopy and X-ray photoelectron spectroscopy. The layered structures have been characterized by scanning electron microscopy, X-ray diffraction, and X-ray reflectivity. Incorporation of sufficiently thick (13 and 27 nm) ion blocking TiO2 layers into nanolaminate structures with LiAlO layers resulted in an increase in breakdown voltage by more than a factor of two, relative to LiAlO. Nanolaminate structures also preserve the large double layer capacitance of the ionically conductive layer. Increased breakdown strength coupled with large capacitances results in a doubling of ultimate charge storage capacity, illustrating how nanolaminates can be used to improve properties relevant for energy/charge storage applications.

P.316 Systematic evaluation of bipolarity in Parkinson's disease and the rates of comorbidity and clinical implications

The proven superior ductility of nanoglasses (NGs) makes them a promising second phase for metallic glass (MG) matrix composites. Here we evaluate the mechanical properties of MG-NG nanolaminate composites by performing molecular dynamics simulations of tensile loading. We focus on the effects of NG layer thickness and separation as well as the loading direction on the predicted strength and inelastic deformation profile. Our results reveal that nanolaminates with NG layers separated by more than 50 nm fail by shear banding. Meanwhile, the predicted nanolaminate strength versus MG volume fraction follows an inverse Hall-Petch relationship rather than the linear rule-of-mixtures. In contrast, by closely packing NG layers to 4.8 or 6.5 nm, the nanolaminates exhibit enhanced tensile ductility for tensile loading perpendicular or parallel to the MG-NG interfaces, respectively. Our results further reveal that the change in the loading direction causes the differences not only in the location of SB initiation but also the critical distance between NG layers for failure mode transition. Finally, the MG-NG nanolaminate structure with NG layers closely packed and interfaces oriented parallel to the loading direction is identified as the most effective heterostructure, which preserves superplasticity while producing a maximum strength of 2.35 GPa, a value 15% greater than that of monolithic NG with a grain size of 5 nm. Our work demonstrates that a nanolaminate combining MG and NG layers of suitable thicknesses is able to withstand large plastic deformations while maintaining the structural stability, and we expect that these results will inspire the development of novel strong and superplastic MG matrix composites that will broaden the possible applications of MGs.

Comparison of the interface and border traps of nanolaminate and bilayer structures of Al2O3 and HfO2 on In0.53Ga0.47As

This study is a comparison of interface and border traps of nanolaminate and bilayer structures of Al2O3 and HfO2 with those of single layers on an In0.53Ga0.47As substrate. The nanolaminate structure shows better trap minimalization in both trap cases along with lower hysteresis and leakage current density and higher breakdown field than the bilayer structures. X-ray photoelectron spectroscopy (XPS) was used to quantify the microstructural properties, and a combination of XPS and reflected electron energy loss spectrum analysis was used to calculate the bandgap of deposited films. Al incorporation into HfO2 increases the bandgap and band offsets.

Wandering and nanolaminated structures of grain boundary triple junctions in tungsten

The peculiarities of nanomorphology triple junctions have been studied by the method of field ion microscopy in fine-grained high-textured tungsten using the phenomenon of atom-by-atom field evaporation. It was found evidence of the formation of double-kinks at the triple junction lines accompanied by a local intrusion of several atomic layers of one grain into another. Observed a double-kink mode of the wandering is geometrically related to revealed nanolaminated structures of triple junctions bounded by pairs of twist boundaries. Molecular statics calculations have shown that the grain boundary expansion and energetics for the laminated grains with incommensurate twist grain boundaries are essentially independent of their thickness in nano-scale region.

Deformation induced grain boundary segregation in nanolaminated Al–Cu alloy

A gradient nanostructured surface layer was formed in an Al–4 wt%Cu alloy processed by means of surface mechanical grinding treatment at liquid nitrogen temperature. Within the deformed surface layer, laminated structures with a wide range of thickness were formed. With a decreasing depth from the treated surface, lamellae thickness decreases accompanied by an increased fraction of high angle grain boundaries (HAGBs) from 10% to 70%. In the topmost surface layer, nanolaminated (NL) structures were found with an average thickness as small as 28 nm and a HAGB fraction of 70%. Composition analysis indicated that Cu atoms segregate at NL boundaries in the as-prepared sample, Cu concentration is about 3–4 times higher than that in the lattice. The obvious grain boundary (GB) segregation of Cu induced by cryogenic plastic deformation is attributed dynamic interaction between solute atoms with gliding dislocations. GB segregation of Cu is responsible for the stabilization of the NL structures with a much finer structural size than that in pure Al, resulting in higher hardness. The deformation-induced GB segregation provides an alternative strategy to achieving stable high strength nanostructures in Al alloys.

Diffusion-controlled intercalation approach to synthesize the Ti2AlC MAX phase coatings at low temperature of 550 °C

Mn+1AXn (MAX) phase materials have unique nano-laminated structures and exhibit interesting hybrid properties with ceramics and metals. However, high synthesis temperatures, e.g. typically ≥700 °C for Ti2AlC coatings, are the key barriers to their wide application on various substrates. In this study, Ti2AlC MAX phase coatings were firstly fabricated by annealing the pre-designed α-TiC0.65/Al multilayer coatings at temperatures as low as 550 °C till now, which is the lowest synthesis temperature compared to already-reported literatures. Different with the common used short-time annealing (1–3 h), noted that the long-time annealing from 15 h to 400 h was particularly conducted based on the sluggish diffusion kinetics at lower temperature. Moreover, the dependence of structural evolution upon annealing proposed that Al diffusion-controlled intercalation process played the key factor to reduce the the formation temperature of Ti2AlC MAX phase. The results provide a strategy to fabricate high-quality Ti2AlC MAX phase coatings at 550 °C for wide application of temperature-sensitive substrates.

Preparation of bulk nanolaminated aluminum alloy with high strength

Bulk nanolaminated (NL) structures were obtained in a low alloying commercial aluminum alloy 5052, via a two-step process: grain refinement firstly by equal-channel angular pressing (ECAP) and secondly by liquid nitrogen rolling (LNR). The microstructure and the properties including hardness, tensile properties and thermal stability were characterized. The results show that the average lamellar spacing of the NL structure is ~49 nm with high dislocation density (~6.4 × 1014 m−2) and high strength (σ0.2 ≈ 515 MPa). Decrease of lattice parameter of the ECAP 8P + LNR sample, calculated from XRD data, unveils strong segregation of Mg to the lamellar boundaries. Close analysis indicates, although solid solution strengthening, grain boundary strengthening and dislocation strengthening accounts most of the high strength, strengthening related to solute segregation to lamellar boundaries during deformation process may also play an important role for this high strength.

Hardness and modulus of elasticity of atomic layer deposited Al2O3-ZrO2 nanolaminates and mixtures

Atomic layer deposition was used to produce 90–105 nm thick alumina-zirconia mixtures and nanolaminate structures on soda-lime glass substrate. The resultant chemical and structural compositions of the thin films were characterized. Hardness and modulus of elasticity were determined by instrumented nanoindentation. The hardness of mixtures and nanolaminates were in the range of 11–15 GPa and moduli in the range of 140–180 GPa ZrO2 with 3.7 mol.-% Al2O3 crystallized in pure tetragonal phase and measured hardness reached about 15 GPa on glass substrate at indentation displacement of about 13 nm. Similar mechanical properties were measured in most thin films, except pure ZrO2, demonstrating insensitivity of mechanical properties to deposition receipt.

Deposition and tribomechanical study of nanolaminate Ti/TiN/AlTiSiN/(AlTiSiN/TiAlSiN)n/AlTiSiN hard coating

The application of Physical Vapor Deposition (PVD) hard coatings is one time-tested practice in the modern industry. Two of the main tendencies that promise a progress in this area are the combination of more chemical elements in one coating and the creation of nanolaminate structures. Since many combinations are possible in the design of the coatings, it could be expected they would have many different properties which are difficult to predict. In an attempt to work simultaneously in both of these directions, one nanolaminate Ti/TiN/AlTiSiN/(AlTiSiN/TiAlSiN)n/AlTiSiN coating was developed and its mechanical properties were tested. The results obtained give a reason to believe that it could be widely practically applied.

P‐9.1: QD based color converter with DBR Structure and its application on Micro‐LED

An enhancement of light extraction efficiency of quantum dots (QD) in light‐emitting diodes (LEDs) with Bi‐functional TiO2/Al2O3 distributed Bragg reflector (DBR) nanolaminate structure grown by atomic layer deposition (ALD) has been demonstrated. The DBRs were simulated and optimized with TFCalc, and they exhibited excellent and tunable optical properties, as well as reliable moisture barrier performance. These DBRs were integrated in the QD‐LED, enabling an obvious increase in red emission and a strong decrease in blue light transmittance, which can achieve color conversion greatly. Furthermore, these DBRs can prolong the lifetime of QDs evidently by isolating the QDs from the moisture vapor. These results highlight the potential application of DBRs in the QLEDs and QD‐LEDs.

Solid solution hardening in nanolaminate ZrN-TiN coatings with enhanced wear resistance

Coating architectures with a microstructure controlled on nanoscale enables the attainment of enhanced mechanical, tribological and other properties. In the present study, we fabricated nanolaminate ZrN/TiN systems with various modulation periods, L, ranging from 1 to 100 nm using pulsed DC magnetron sputtering. It was observed that a modulation period of L = 10 nm is a threshold value for the transition from a periodic to a solid solution structure. We demonstrate that the coatings with L<10 nm displayed a single-phase solid solution with a Zr-rich composition of Ti0.35Zr0.65N. This indicates the effect of mixing and/or diffusion between the two phases that occurred during the film growth. The crystallite size was found to vary with the modulation periodicity: we obtained 11–12.5 nm for the Ti0.35Zr0.65N solid solution, and 14.5–25 nm for the laminate structure. The solid solution and interface strengthening were found to be the main cause of the hardening of the nanolaminate structure presenting a hardness of 32–35 GPa, significantly higher than for the individual TiN (21 GPa) and ZrN (16 GPa) single-layer coatings. The hard, solid solution system was found to exhibit the highest wear resistance corresponding to the highest values of the H/E and H3/E2 ratios related to a high resistance to plastic deformation and a high toughness. Finally, a model for the Ti0.35Zr0.65N solid solution phase formation related to internal residual stress is proposed.

Evolution of nano-laminated structure formed by the thermally-assisted plastic deformation in dry sliding wear

Abstract The evolution of nano-laminated structure generated in dry sliding wear of a high strength martensite steel was investigated by using Scanning Electronic Microscopy (SEM) and Transmission Electron Microscopy (TEM). Gradient microstructure along the depth was observed with the top layer consisting of nano-laminated structure. During the sliding wear-induced plastic deformation, grains were refined and lamellar structures were formed as a result of generation of dislocation cell blocks bounded by geometrically necessary boundaries (GNBs) and incidental dislocation boundaries (IDBs). With increasing plastic shear strain and strain gradient, there is a continuous reduction in the spacing and length in dislocation cell blocks, leading to nano-laminated structures. The ultrafine nano-laminated structure with laminate spacing of 20 nm was observed at the sliding time of 120 min. However, the grain refinement is not a merely mechanical deformation process. The formation of Fe3O4 and FeO gave direct evidence that the highest temperature due to frictional heat could reach up to 600 °C. The result suggested that the formation of nano-laminated structure was governed by thermally-assisted plastic deformation.

Low-angle grain boundary structures and size effects of nickel nanolaminated structures

Recent experiments have demonstrated the fabrication of nickel nanolaminated structures with low-angle grain boundaries and that these nanolaminated structures exhibit an ultrahigh hardness of 6.4 GPa and excellent thermal stability. However, the detailed microstructure of the low-angle grain boundary in these nanolaminated structures and plastic deformation mechanisms underlying their ultrahigh hardness have remained elusive to date. Thus, we performed a series of large-scale atomistic simulations to explore the microstructures, thermal stability and energies of the low-angle grain boundary in nanolaminated nickel and to further mimic the plastic deformation of nanolaminated nanopillars. First, we examined the thermal stability of low-angle grain boundaries in nanolaminated structures with three typical textures observed in previous experiments, and we found that the {111}<110>-textured nanolaminated structure is the most stable and has a critical temperature of approximately 750 K for stability. The low-angle grain boundary in such nanolaminated structures consists of a periodic arrangement of three types of dislocations with Burgers vectors of 1/2[101], 1/3[11¯1] and 1/6[14¯1]. Our simulations also showed that the grain boundary energy of the low-angle grain boundary with misorientation of 5° is approximately 0.70 J/m2 and that the interaction energy between two low-angle grain boundaries follows a power law as the lamellar thickness, with an exponent of −1.14. We further simulated the uniaxial compression and tension of nanolaminated nanopillars with a texture of {111}<110> and different lamellar thicknesses. The simulation results showed that as the lamellar thickness decreases, both compressive and tensile yield stresses significantly decrease, while their flow stresses slightly increase. The size effect on yield stress is attributed to dislocation escape from the low-angle grain boundary due to some typical reactions of constituent dislocations, while the size effect on flow stress is related to a complex reaction and interaction of high-density dislocations. Our study provides a fundamental understanding of the grain boundary structure and plastic deformation in textured nanolaminated metals, which offers insights for the design of stronger nanolaminated structures.

Scratching Cu|Au Nanolaminates

We used molecular dynamics simulations to study the scratching of Cu|Au nanolaminates of 5 nm layer thickness with a nanoscale indenter of 15 nm radius at normal forces between 0.5 μ N and 2 μ N. Our simulations show that Au layers wear quickly while Cu layers are more resistant to wear. Plowing was accompanied by the roughening of the Cu|Au heterointerface that lead to the folding of the nanolaminate structure at the edge of the wear track. Our explorative simulations hint at the complex deformation processes occurring in nanolaminates under tribological load.

Shape-preserving machining produces gradient nanolaminate medium entropy alloys with high strain hardening capability

A high density of grain boundaries can potentially increase structural materials' strength, but at the expense of losing the materials' strain hardening ability at high flow stress levels. However, endowing materials with grain size gradients and a high density of internal interfaces can simultaneously increase the strength and strain hardening ability. This applies particularly for through-thickness gradients of nanoscale interface structures. Here we apply a machining method that produces metals with nanoscale interface gradients. Conventional bulk plastic deformation such as rolling, a process applied annually to about 2 billion tons of material, aims to reduce the metal thickness. We have modified this process by introducing severe strain path changes, realized by leading the sheet through a U-turn while preserving its shape, an approach known as ‘hard turning’. We applied this process at both room temperature and 77 K to a NiCrCo medium entropy alloy. Micropillar compression was conducted to evaluate the mechanical response. After hard turning at room temperature, the surface microstructure obtained a ∼50% increase in yield stress (0.9 GPa) over the original state with homogeneous grain size (0.4 GPa), but the initial strain hardening rate did not show significant improvement. However, after hard turning at 77 k, the gradient nanolaminate structure tripled in yield stress and more than doubled its initial strain hardening rate. The improvements were achieved by introducing a specific microstructure that consists of gradient nanolaminates in the form of nanospaced twins and martensite in the face center cubic (fcc) phase. This microstructure was formed only at cryogenic temperature. It was found after turning at room temperature that only nanospaced twins were present in the fcc phase inside nanolaminates that had formed at the surface. The origin of the enhanced strain hardening mechanism was studied. Joint density functional theory (DFT) and axial next nearest neighbor Ising (ANNNI) models were used to explain the temperature-dependent phase formation of the NiCrCo nanolaminate at the surface of the hard-turned material.