Nanolaminated composite materials: structure, interface role and applications

Abstract

This article is a review on the nanolaminate composite materials from a materials science perspective. In fact, nanolaminate composite materials are a category of two-dimensional nanomaterials in the fields of chemistry, materials science and condensed matter physics. Miniaturization of materials to the nanometer scale is improving remarkably in science and technology. Nanolaminates constitute a unique class of nanomaterials that illustrate fascinating mechanical, physical, chemical and electrical properties and these attributes are increasingly being prospected for promising applications. Nanolaminates can typically be developed using bottom-up techniques that are designed with various stacking series along with layer thicknesses. The particular properties of fabricated nanolaminates can be determined by their arrangements, compositions and thicknesses. The properties can be established during the fabrication process by size control of each coating layer and interfacial chemical reactions between layers. Hence, fundamental understanding of nanoscale layered-engineering during the creation of a nanolaminate structure enables tailoring of the overall performance of these composite materials. This work demonstrates the bottom-up physical and chemical approaches for the fabrication of nanolaminates. The influence of the interface layer in the nanolaminate composite materials is considered from the viewpoint of conferring high performance characteristics. The desirable physical attributes will be pursued by incorporation of dopants and site-engineering techniques on various materials that would improve the nanolaminate properties. The final section concludes with an outlook on future directions in this technological field.

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Jalal Azadmanjiri

Jalal Azadmanjiri received his PhD (full scholarships) in Materials Science and Engineering from Monash University, Australia (2012) followed by postdoctoral research fellow at Swinburne University of Technology. His current research interests include nanomaterials and nanotechnology, especially focusing on design, preparation and characterization of nanomaterials in zero-, one- and two-dimensional structures. He is also interested in the development and improvement of hybrid materials with thin film and layered structures for energy storage devices and the study of their interfaces.

Christopher C. Berndt

Professor Berndt has been involved in teaching and research within the materials engineering and mechanical engineering disciplines for the past 37 years. He has taken on leadership roles within professional societies for the past 20 years, which includes the Presidency of ASM International and the Australian Ceramic Society. He has impacted many thousands of undergraduates through his teaching, as well as some 60 graduate students and post docs. Berndt is especially proud of his students and post docs who have achieved professional prominence and earned good lives.

James Wang

James Wang obtained his PhD from the School of Electrical and Computer Engineering, RMIT University in 2003, and since then he has been working as a research fellow and senior lecturer in the Faculty of Science, Engineering and Technology, Swinburne University of Technology, Australia. His research fields include the preparation, characterisation and application of various thin solid films and coatings, magnetic materials, and biomaterials.

Ajay Kapoor

Ajay Kapoor obtained his BTech and MTech at IIT, BHU, Varanasi (1980 and 1983 respectively), and his PhD at Cambridge University, UK (1987). Subsequently, he lectured at IIT BHU before moving back to Cambridge in 1990. He spent time at Leicester University (1994–1995), Sheffield University (1995–2004), Newcastle University (2004–2007) and moved to Swinburne University of Technology, Australia in 2007. Currently he is Pro Vice Chancellor (International Research Engagement) in Swinburne. His research interests include mechanics of materials, wear and rolling contact fatigue.

1. Introduction

1.1. Overview

Nanolaminates that represent a class of composite materials are fabricated from alternating nanometer-scale layers of at least two materials. Sublayer thickness in nanolaminates may vary from a few atomic sheets to several tens of nanometers. There has been a great demand regarding design and fabrication of next-generation structural nanomaterials that can supply and provide outstanding chemical, mechanical and physical behaviors. Among materials with a nanostructure, the nanolaminates with two-dimensional (2D) systems have attracted excellent interest in both fundamental and applied scientific research. Fig. 1 shows publications in the past several years. It is expecting that the publications on nanolaminated composites growing fast for the next years because 2D nanosheets are exceptionally rich in both physical and chemical properties as well as structural diversity, resulting their potential applications in electrical,^1,2 optical,^3–5 thermal⁶ and mechanical properties⁷ that do not exist in their bulk counterparts.

Fig. 1 Histogram detailing the number of nanolaminates publications that have appeared in peer-reviewed English-language journals between 2000 and 2015; data were obtained using Scopus, searched on “Nanolaminates”.

2D nanosheets and nanolaminates that are absolutely dense and ultra-fine grained could be produced for engineering applications to bring benefit of enhanced mechanical properties for devices such as energy storage. Nanolaminates can be fabricated using bottom-up^8–12 deposition and in some cases by top-down¹³ techniques that are designed with various stacking series and layer thicknesses. The properties of created nanolaminates be determined by their compositions, arrangements and thicknesses. These can be demonstrated within the synthesis process through thickness control of each layer and interfacial chemical reaction between layers.

The rapid advances in nanoscience and technology provide new opportunities in achieving highly efficient nanolaminates. Particularly, physical properties such as a significant surface area and high efficient interface structure markedly improve the efficiency of such structures. Therefore, nanolaminated structures are increasingly important for the growing of innovative devices and have attracted excellent interest. Towards this aim, the present review article is focused on the developments in the structure, interface role and applications of nanolaminates; with an emphasis on recently published works.

1.2. The scope of this review

This review initially classifies the nanostructured laminates and the interfacial role on the behaviors of these structures. The crucial role of the interface with regard to the chemical, mechanical and physical behavior of nanolaminates is discussed. Subsequently, the field of nanolaminate materials on the basis of recent investigations and applications on ceramic-based will be determined. The current condition and development of artificial materials for high performance applications, such as energy storage (e.g., dielectrics of capacitors and batteries), gas-diffusion barriers and energy conversion (e.g., thermoelectric) will be highlighted. The final section concludes with an outlook on future directions for nanolaminate composites.

2. Overview of bottom-up approaches in fabrication of nanosheets

A common approach for the synthesis and fabrication of nanostructured functional materials is the bottom-up method. The bottom-up technique constructs the desired nanostructure by placing atoms or molecules one at a time. This approach has been studied extensively to develop new and promising organic and inorganic zero-dimensional (0D), one-dimensional (1D), two-dimensional (2D) and three-dimensional (3D) nanostructured materials (NSMs), with the desired mechanical,^14,15 electrical,^9,16 magnetic,^17–20 optical,²¹ etc. properties. The bottom-up approaches have been widely documented,^21–34 Table 1. The most common bottom-up method; that is the physical and chemical methods, along with their interests and drawbacks with an emphasis on 2D nanostructure laminates, will be explained in the following section.

Table 1 Some bottom-up methods, building block and their application

Bottom-up	Building block	Applications	Reference
Self-assembly	p-type oligo (p-phenylenevinylene)s and n-type perylenebisimides thin films	Photoelectronic applications	22
Self-assembly	Gold–polypyrrole amphiphiles	Electrocatalysis, chemical sensors and microelectronic devices	22
Langmuir–Blodgett	Biomimetic lamellar lipid	Biological membranes	26
Langmuir–Blodgett	Poly(3-hexyl thiophene) film	Glucose biosensor	27
Layer-by-layer	Graphene/polyaniline (PANI) hybrid electrode	Supercapacitors	28
Layer-by-layer	Graphene/palladium (Pd) nanoparticles films	Sensors	28
Atomic layer deposition	Metal oxides (TiO2, Al2O3, etc.) thin films	Dielectric layers, insulating layer, anti-reflective layers, etc.	29
Atomic layer deposition	Poly(methyl methacrylate) (PMMA), polyethylene, etc.	Polymer surface functionalization, creation of composites, diffusion barriers, etc.	30
Electrochemical deposition	Platinum on alloys (such as Co-super alloy, Inconel, pure Al, Al–Ti alloy, graphite)	Batteries, fuel cells and capacitors	31
Chemical vapor deposition	Gallium arsenide (GaAs)	Electronic (such as, ICs) and photovoltaic devices	32
Chemical vapor deposition	Tungsten carbide	Wear resistance	33
Magnetron sputtering	Nd-doped SnO2 thin films	Solar sell devices	34
Magnetron sputtering	Al/CuO	MEMS energy sources, micro-initiators	35

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2.1. Synthesis of 2D nanostructure laminates by bottom-up physical methods

Nanosheets and nanolaminates are currently of intensive interest in modern materials research due to their special properties that are conferred, for instance, by the small size and large surface-to-volume ratio in nanolaminates compared to the 1D NSMs. Hence, physical methods have been intensively used to fabricate 2D NSMs.^35–41

2.1.1. Pulsed laser deposition. Pulsed laser deposition (PLD) is a physical gas-phase technique for depositing thin films. PLD has become a popular technique to fabricate high-purity NSMs of any composition. In this technique, the target is vaporized from the target using a pulsed laser in an ultra-high vacuum chamber or in the presence of a reagent gas. A schematic view of pulsed laser deposition for the synthesis of NSMs is demonstrated in Fig. 2. Target atoms may be either evaporated under the vacuum condition and deposited directly onto the substrate;^42,43 or the target atoms may interact in the presence of a reagent gas to form the desired compound; e.g., oxide in the case of oxygen, nitride for nitrogen or ammonia, carbide for methane, etc. The elemental composition and size distribution of nanoparticles may be altered by changing experimental parameters, including composition of the inert gas and the reagent gas in the chamber and varying the temperature gradient and laser pulse power.

Fig. 2 A schematic of the pulsed laser deposition (PLD) technique.

PLD is a fast technique (e.g., 1.3 and 2.0 monolayers for iron and silicon deposition per minute, respectively, using a Nd:YAG laser⁴⁴) and the stoichiometry of the deposited film is very close to that of the target material; hence, it is possible to fabricate stoichiometric thin films from a single alloy bulk target.⁴⁵ The properties of the fabricated thin film can also be controlled in terms of crystalline structure, thickness and composition.⁴⁵ PLD does not need expensive or corrosive precursors, and large volume targets are not necessary with this technique compared to the other thin film methods such as magnetron sputtering. However, clusters and droplets may be observed at the surface of the deposited films that lead to higher roughness and this is a main drawback for optical, and electrical, etc.⁴⁶ applications of the deposited film.

2.1.2. Thermal evaporation deposition. Thermal evaporation is a common physical approach for thin film deposition. Fig. 3 indicates a typical thermal evaporation arrangement. The source material is evaporated under low pressure conditions of about 10⁻⁵ to 10⁻⁶ Torr. The vacuum environment prevents reaction between the vapor and ambient atmosphere and, furthermore, allows evaporated atoms or molecules to transfer onto the substrate surface. The material vapor condenses as a thin film layer onto the substrate surface. This technique is an inexpensive and simple method to deposit material onto a substrate. As well, the substrate surface damage is less and the deposited thin film exhibits high purity. However, poor adhesion, low density, and the inability to evaporate dielectric materials are some disadvantages of the evaporation method.

Fig. 3 A schematic drawing of thermal evaporation technique for deposition thin film.

2.1.3. Magnetron sputtering technique. There are many sputter deposition techniques, such as ion-beam sputtering, reactive sputtering, ion-assisted deposition, high-target-utilization sputtering, magnetron sputtering, and gas flow sputtering. Magnetron sputtering is the most common sputtering technique for fabricating thin film coatings (Fig. 4). Magnetron sputtering is a physical vapor deposition process that can be employed to manufacture multilayered materials. The atoms are formed in the vapor phase by bombardment of a solid source material with energetic particles. In this fashion, a single layer of material is deposited onto the substrate; which is normally a silicon wafer. Current experimental work is focused on repeated magnetron sputtering to manufacture disparate layers.

Fig. 4 A schematic that depicts depositing multilayer thin films by magnetron sputtering.

The microstructure and properties of the nanolaminates depend on their composition and the total nanolaminated thickness depends on the thickness of the individual layers. The structure and composition can be manipulated by adjusting the process parameters. Sputtered thin films manufactured by magnetron sputtering have excellent chemical and microstructural uniformity. As well, almost any metal target material can be sputtered without decomposition. The water-cooled target minimizes the effects of radiated heat so that thermal microstructural effects are diminished. However, slow deposition speed, low adhesion, and low density are several drawbacks for magnetron sputtering compared to other technologies.

2.2. Synthesis of 2D nanostructure laminates by bottom-up chemical methods

Chemical approaches play a significant role in developing nanostructured materials, by mixing materials and solutions at the molecular level; hence providing good chemical homogeneity in comparison to physical methods.²⁵ Additionally, chemical processing design and synthesis of new materials can be refined into the final end products.²⁵ However, toxic reagents and solvents are frequently used for the synthesis of NSMs⁴⁷ and the chemical methods are relatively slow to actualize. There are many bottom-up chemical methods used for fabricating 2D NSMs; of which several important ones will be explained below.

2.2.1. Chemical vapor deposition. Chemical vapor deposition (CVD) is a chemical procedure for producing 0D, 1D, 2D and 3D NSMs on the substrate surface. In this method, gaseous molecules transform into solid materials. The CVD-produced NSMs demonstrate high quality, high-performance and dense films. The CVD process coats the substrate at low deposition rates in the order of micrometers per hour. The CVD process is similar to the physical vapor deposition (PVD) method with a prime difference being that the precursors are solid compounds rather than gases. CVD is a conventional process for depositing films in the semiconductor and optoelectronics industries because of the low costs required to produce high purity films.

CVD may be implemented in different equipment formats; however, most CVD processes may be classified as either low pressure CVD (LPCVD) or ultra-high vacuum CVD (UHCVD) processes. LPCVD is carried out under sub-atmospheric pressures to prevent unwanted reactions and confer a more uniform coating thickness. UHCVD is performed under low atmospheric pressures (∼10⁻⁶ Pa). The CVD precursor gases that react with the substrate must be highly volatile, otherwise it is difficult to deliver them to the reaction chamber. The gaseous by-products of the CVD process are usually toxic, which is a disadvantage that can be mitigated by appropriate design of the CVD reaction chamber and post-process disposal that employs gas cleaning technology.

Fig. 5 demonstrates direct deposition of graphene onto a quartz dielectric substrate, which offers superior electronic properties, by a single-step CVD process.¹⁰ The graphene layer is formed using surface catalytic decomposition of hydrocarbon precursors on thin copper films pre-coated onto a quartz substrate.¹⁰ The copper films evaporated during or instantly after graphene growth, thereby resulting in a graphene layer that is coated directly onto the quartz substrate.

Fig. 5 A schematic for demonstration of CVD process for deposition of graphene on quartz substrate. (a) A thin film of copper is deposited on the dielectric surface, firstly, (b) during the CVD process, (c) the copper thin film evaporates and (d) leaving the graphene layer on the quartz substrate. Reproduced with permission from data published in ref. 10 Copyright 2010, American Chemical Society.

2.2.2. Electrochemical deposition. Since electrochemistry is a reliable and inexpensive method, it has been employed to deposit a composite layer onto a substrate. Electrochemical deposition (Fig. 6) has been used to (i) deposit a nanostructured material onto a substrate surface, (ii) create a nanostructure on the surface of a substrate, and (iii) functionalize a nanostructured template. Electrodeposition involves the use of relatively simple equipment that can be configured in either a two- or three-electrode arrangement. The method can be performed at or near room temperature and atmospheric pressure, as well as being readily scaled for large areas.

Fig. 6 A schematic of electrochemical deposition technique for deposition of Cu₂ZnSnS₄ (CZTS) and a multi-bath electrodeposition–annealing method.

2.2.3. Atomic layer deposition. Atomic layer deposition (ALD) or atomic layer epitaxy (ALE) is a chemical deposition method for producing high-quality, large surface area and thin film layers with atomic scale precision.¹¹ Fig. 7 illustrates an ALD method where the thin film layers cyclically grow for a binary compound from gaseous precursors. ALD is a surface controlled layer-by-layer thin film deposition process. It has the greatest potential for producing thin films where thickness and film composition can be controlled at the atomic level. ALD can produce a wide range of film materials with high density, low impurity and accurate control of thickness. As well, a low deposition temperature can mitigate adverse effects on sensitive substrates. ALD has been applied for the deposition of oxide materials such as high dielectric constant insulators.¹²

Fig. 7 A schematic illustration of an ALD method where the thin film layers cyclically grow for a binary compound from gaseous precursors.

The ALD method has been widely considered by researchers in nanotechnology areas due to its control over the deposited layer thickness of materials and the potential for modification of chemical and physical properties in the nanoscale range.¹¹ There is a comprehensive review on metal and nitride thin films fabricated by ALD for semiconductor device applications by Kim et al.⁴⁸ The surface chemistry of the trimethylaluminum [Al(CH₃)₃] (TMA) and H₂O process by ALD is elaborated by Puurunen et al.⁴⁹ Moreover, HfO₂ is considered for electronic applications⁵⁰ and the electrical and mechanical properties of some metal oxides such as ZnO,⁵¹ TiO₂ ⁵² and Al₂O₃ ⁵³ have been investigated systematically. Other groups have developed deposition processes for metals such as platinum and ruthenium for electrode material applications.⁴⁹ Slowness of deposition is the major limitation of the ALD technique; usually a fraction of coverage is deposited in each cycle. However, a very thin layer is required for next-generation electronic devices and thus the slowness of ALD method may not a critical issue.

2.2.4. Langmuir–Blodgett and self-assembled monolayer. The Langmuir–Blodgett (LB) method is a technique for producing mono- and multi-molecular films that is implemented by transferring Langmuir films to surface of a solid substrate (Fig. 8). Langmuir films consist of a monolayer of amphiphilic compounds formed on the surface of a liquid. The self-assembled monolayer (SAM) technique ensues when molecular assemblies are formed spontaneously on surfaces by adsorption and are then organized into relatively large ordered domains (Fig. 9).⁵⁴ These two methods enable the fabrication of closely packed, well-ordered and organized multilayer films from oriented monolayers and allow for the immobilization of several functional molecules onto surfaces.⁵⁵

Fig. 8 A schematic demonstrating LB technique for producing mono- and multimolecular films and 3 types of LB deposited layers on a hydrophobic surface.

Fig. 9 A schematic for SAM technique.

These methods have some disadvantages. LB is an expensive and time consuming method that requires specialized instrumentation. It also requires specific and limited molecules (e.g., amphiphilic molecules) to prepare the films.⁵⁴ Additionally, a weak molecular interaction between the film and the solid support is a disadvantage for LB.⁵⁴ Deposited films using the SAM process also have limited stability and strength under ambient and physiological conditions.

2.2.5. Layer-by-layer assembly approach. The layer-by-layer (LbL) assembly technique includes the consecutive adsorption of supplementary multivalent molecules on a surface, giving arise to either electrostatic or non-electrostatic interaction.⁵⁵ LbL assembly is an effective, easy, reproducible, robust, flexible method to fabricate highly ordered nanoscale structures or patterns with extended functionalities and activities. This technique may be employed to fabricate ordered nanostructured polymeric multilayer thin films and nanocomposites with tailored thicknesses, compositions, structures, properties and functionalities on any substrate.⁵⁵ In order to synthesis a high quality thin film and avoid any contaminations from the previous adsorption step, as well as prevent weakly adsorbed layers, washing steps are required between the deposition of each molecule layer.⁵⁶

The LbL method generally occurs through a variety of deposition techniques such as dip-coating, spin-coating, spraying, and perfusion that have been extensively reviewed in previous work.^57–60 The architecture and properties of the deposited films by the LbL method can be well-controlled at the nanometer-scale level.⁵⁵ There are several molecular interactions between thin film layers for the LbL assembly technique that have been overviewed by J. Borges and J.F. Mano.⁵⁵ Fig. 10 shows a schematic for a LbL self-assembly of a multilayer coating by sequential adsorption of oppositely charged polyelectrolytes.

Fig. 10 A schematic of LbL self-assembly of a multilayer coating by consecutive adsorption of oppositely charged polyelectrolytes.

3. Overview on binary and ternary nanostructure laminates and interfacial roles

Ultrathin 2D nanomaterials that contain uniform layered structural features with outstanding properties are attracting interest in the fields of condensed matter physics, materials science and chemistry. Therefore, ultrathin 2D nanomaterials have become a key class of materials for fabricating many types of 2D crystals such as graphene,^61,62 metal organic frameworks (MOFs),^62,63 covalent organic frameworks (COFs),⁶⁴ and MXenes,¹³ etc. Ultrathin 2D nanomaterials demonstrate unique characteristics compared to their counterparts with different dimensionality.⁶⁵ Some of special characteristics of ultrathin 2D nanomaterials are: (i) the atomic thickness of ultrathin 2D nanomaterials provides them with maximum mechanical flexibility and optical transparency,⁶⁵ and (ii) the large lateral size of these nanomaterials offer a high specific surface area that make them favorable for surface active applications.⁶⁶

Fig. 11 exhibits a schematic of some typical ultrathin 2D nanomaterials, such as transition metal dichalcogenides (TMDs), hexagonal boron nitride (h-BN), covalent organic frameworks (COFs), metal–organic frameworks (MOFs), layered double hydroxides (LDHs), black phosphorus (BP), MXenes, metals and oxides.

Fig. 11 A schematic demonstration of various kinds of the typical ultrathin 2D nanomaterials, such as LDHs, h-BN, metals, oxides, BP, TMDs, Mxenes, COFs and MOFs. Reproduced from data published in ref. 65 Copyright 2015, American Chemical Society.

Nanolaminates are a subclass of ultrathin 2D nanomaterials that incorporate binary and ternary elements of various layer thicknesses, Fig. 12. They are atomic size sandwiches made with simply 2 or even up to 200 000 layers. The nanolaminate structures are fully dense, ultra-fine grained solids that provide an increased concentration of interface defects. The properties of the nanolaminates depend on their particular arrangements, compositions, thicknesses and are always different from those of the bulk form components. The different properties arise due to the nanometer-scale environment of the atoms in each layer. Nanolayers with binary and ternary compounds, as well as atoms within nanolayers that are powerfully affected by the interfaces between the component sublayers, will be described in the following section.

Fig. 12 A schematic illustration of nanolaminate structure with 2 different sublayer.

3.1. Binary nanolaminated compounds

Nanolaminated structures made from sublayers and/or bilayers of binary compounds, such as binary metal oxides⁶⁷ and MXenes,¹³ can display different functional properties than the initial ingredients.

Binary metal oxides can be divided in two categories of dielectric binary oxides (e.g.; MgO, Al₂O₃, SiO₂, TiO₂, ZrO₂, HfO₂, V₂O₅, Nb₂O₅, Ta₂O₅, Sc₂O₃, Y₂O₃, La₂O₃, CeO₂, Nd₂O₃, Er₂O₃),^68–71 and conductors/semiconductors of binary oxides (e.g.; WO₃, MnO_x, Co₃O₄, NiO, ZnO, Ga₂O₃, In₂O₃, SnO₂, Sb₂O₃).^68,72,73

It has been shown that the nanolaminate properties of several binary oxides can be merged advantageously.⁶⁷ Hard coatings, optical filters and X-ray mirrors are examples that exploit binary metal oxide multilayers.^74–76 In addition to good chemical stability, excellent mechanical and optical properties; advanced dielectric properties can be achieved with multilayered structures. As-deposited binary metal oxides usually show relatively large leakage currents and low breakdown resistance, which are essential for energy storage applications.

For example, Al₂O₃, which is an excellent insulator with high band gap (5.95 eV and κ = 9), may be combined with oxides of high dielectric constant (high-κ), like HfO₂ (κ = 25), TiO₂ (κ = 80) or ZrO₂ (κ = 25).^77–81 If the single binary layer thickness attains several nanometers, then the dielectric with a relatively high κ compared to that of the reference SiO₂ (κ = 3.9) can be constructed without strong negative dependence on the voltage or temperature.⁶⁷ Nevertheless, the quantities of dielectric constant that can be attained are maximized, resulting in values of up to 8, 20 and 26 in amorphous Al₂O₃/TiO₂ nanolaminates,⁸⁰ nanocrystalline Al₂O₃/TiO₂ nanolaminates⁷⁹ and Al₂O₃/TiO₂/Al₂O₃ trilayers,⁸² respectively.

Li et al.⁸³ indicated that modifying the interfacial composition of nanolaminate structures, including layers of two different materials (AlO_x and TiO_y) with low dielectric constant, can contribute to dielectric polarization under an externally applied field. Hence, the dielectric constant of the layered structures is enhanced, Fig. 13; which may be attributed to the internal barrier layer capacitance because of the attendance of electrical inhomogeneity in thin film materials. These results demonstrate that the dielectric properties of materials can be altered considerably by employing the design concept of nanolaminate structures.

Fig. 13 (a) A schematic of the Al₂O₃ and TiO₂ nanolaminates, (b) titanium and aluminum elemental maps obtained from energy filtered TEM images, and (c) bright field TEM image of AlO_x–TiO_y nanolaminates. Adapted with permission from data published in ref. 83 Copyright 2011, AIP Publishing LLC.

ZnO/Al₂O₃ nanolaminates fabricated by atomic layer deposition (ALD) is another nanolaminate with sublayers of ZnO (semiconductor binary oxide, refractive index at λ = 400 nm is ∼2.4 for ZnO with 50 nm) and Al₂O₃ (dielectric binary oxide, refractive index at λ = 400 nm is ∼1.74 for Al₂O₃ with 50 nm).⁸⁴ The third-order optical nonlinearity of ZnO/Al₂O₃ nanolaminates can be enhanced by nanoscale engineering of the thin film structure (refractive index for ZnO and Al₂O₃ in Al₂O₃/ZnO with 2 (50/50 nm) at λ = 400 nm is: 2.5 and 1.79, respectively).⁸⁴ The grain size of the crystalline ZnO film was controlled by varying the thickness of the ZnO layers within the nanolaminate in which thin (2 nm) amorphous Al₂O₃ layers impede ZnO crystal growth.⁸⁴ Nanoscale engineering on the nanolaminates was altered to attain a strong optical “third-harmonic generation” from the optimized nanolaminate structure compared to a ZnO reference film of comparable thickness.⁸⁴

MXenes are a new family of 2D transition metal carbides and/or nitrides that are produced by etching and extracting an “A” layer from MAX phases¹³ (Note: MAX phases in the form of ternary nanolaminated compounds are mentioned in the next section). Fig. 14 shows a schematic of the MXene phase. The etching process is performed by immersing the MAX phase in strong etching solutions, such as hydrofluoric acid (HF) or ammonium bifluoride (NH₄HF₂) at room temperature. MXenes with binary compounds could be produced with compositions of M₂X, M₃X₂, and M₄X₃.^13,85

Fig. 14 A schematic illustration of exfoliation of “A” out of MAX phase to form MXene. Reproduced with permission from data published in ref. 93 Copyright 2012, American Chemical Society.

MXene monolayers are predicted to be metallic since they show a high electron density near the Fermi level.^86–90 Hence, multilayer MXenes are electronically conductive with a conductivity similar to multilayer graphene. In contrast to graphene, MXenes show hydrophilic behavior that offers dispersion in aqueous solutions.¹³ Since MXenes are materials with layered solids and the bonding strength between the layers is weak, then intercalation of the guest molecules in MXenes is possible. Therefore, it has been shown that MXenes can easily be intercalated with a range of organic molecules and inorganic salts that not only enable synthesis of various intercalation compounds, but also lead to new applications for these materials.¹³ MXene phases are being investigated further; for example their synthesis by exploring the structure and surface termination of MXenes to define their chemical formulas and control chemical composition.¹³ MXenes materials could be used for a broad range of applications including electronic devices, gas sensors, photocatalysis, reinforcement for composites, and energy storage materials.^13,91,92

3.2. Ternary nanolaminated compounds

The ternary M_n+1AX_n phases [where “M” is an early transition metal, “A” is mainly a group IIIA or IVA (i.e., groups 13 or 14) element, “X” is carbon (C) and/or nitrogen (N), and n = 1, 2 or 3] are a large family with more than 70 different pure MAX phases^94–96 available. There are almost 50 M₂AX (or 211), 5 M₃AX₂ (or 312) and a growing number of M₄AX₃ (or 413) phases since the structure was first established for Ti₃AlN₂.⁹⁷ The significant difference in the structures of the 211, 312 and 413 phases is the number of M layers between every two “A” layers. All identified MAX phases are layered hexagonal with P6₃/mmc symmetry, where the “M” layers are nearly closed packed, and “X” atoms fill the octahedral sites.¹³ “A” atoms are interleaved between the layers. In the other words, the structure of a MAX phase can be described as 2D layers of early transition metal carbides and/or nitrides attached together with an “A” element (Fig. 15). The M–X bond has a strong mixed covalent/metallic/ionic character, whilst the M–A bond is metallic, so that the M–A bonds are weaker than the M–X bonds.^13,98 These strong bonds in MAX phases are in contrast with the other layered materials, such as graphite and transition metal dichalcogenides, where weak van der Waals interactions bind the atomic structure. Hence, the bonds between the layers in the MAX phases are too strong to be broken using shear or other mechanical forces.

Fig. 15 A schematic illustration of structure of the MAX phases and the corresponding MXenes. Adapted with permission from data published in ref. 13 Copyright 2013, John Wiley and Sons.

Metals are generally recognized using getting thermally and electrically conductive, plastically deformable at room temperature, machinable, thermal shock resistant, etcetera. On the other hand, ceramics are usually defined by high elastic moduli, high oxidation and corrosion resistance, good mechanical properties at high temperature, and so on. Recently, investigators have fabricated ternary M_n+1AX_n (or MAX) phases with a naturally nanolaminated structure and unique combination of properties.^{94,95,97,99,100} In fact, the MAX phases are attractive because of their significant amalgamation of chemical, physical, electrical and mechanical properties, which combines the characteristics of metals and ceramics.⁹⁵ For instance, MAX phases are usually resistant to oxidation and corrosion as well as being elastically stiff. However, simultaneously they display high electrical and thermal conductivities and are machinable. These properties origin from an intrinsically nanolaminated crystal structure, with M_n+1X_n slabs intercalated with pure A-element layers.⁹⁵

The research on MAX phases has advanced by using innovative fabrication techniques of thin films. Magnetron sputtering and arc deposition have been employed to synthesis single-crystal materials by epitaxial growth that have been further studied in terms of fundamental material properties.⁹⁵ The required synthesis temperature for deposition of V₂GeC and Cr₂AlC using sputtering techniques is ∼450 °C ^101,102 whereas the deposition of MAX phases from the Ti–Si–C and Ti–Al–N systems usually requires ∼800 °C.⁹⁵

First-principle calculations for predicting hypothetical MAX phases and their properties are in progress. New MAX phases with desired properties, such as Mo₂Ga₂C and Nb₂GeC  ^97,100 have been discovered. Theoretical and experimental approaches are aimed at introducing dopants and forming solid solutions in a controlled manner. In this fashion, MAX phase solid solutions with attractive and novel properties may be identified in the new thin-film materials.

3.3. Interface crucial role in properties of nanolaminates

Nanolaminates can exhibit different degrees of crystallinity and are often comprised of different elements, for example A and B in an A/B binary laminate system. The two main structural features to describe the nanocrystalline nanolaminate are: (i) the characteristic layer pair spacing, which is known as the composition wavelength for a repeating sequence of A and B layers, and (ii) the grain size of sublayers.

The role of interfaces between bilayers in materials with a nanolaminated structure is important and has been investigated in the fields of catalysis, corrosion science and inorganic semiconductor devices.¹⁰³ However, the formation of interfacial layers in nanolaminates are not understood well because of an insufficient in situ characterization; thereby leading to poor control of the significant properties.¹⁰⁴ Hence, these interfaces perform a critical role concerning the properties of these materials; which is in addition to the large lateral surface area that is a desirable characteristic for nanolaminates. For example, a chemical and/or physical interaction can occur at the interface¹⁰³ when metal oxides and metals build an atomic-scale sandwich structure. The nanometer scale chemical and/or physical interaction will influence the chemical, physical or mechanical properties of the nanolaminates,^14,103–110 Table 2.

Table 2 Some nanolaminates and interfacial effect on their chemical, physical and mechanical properties

Composition of the bilayers	Interfacial effect	Reference
Metal/molybdenum trioxide	Effective on chemical, electronic properties and work function	103
Aluminum/copper(II) oxide	Controls the onset reaction temperature, reaction kinetic, and stability at low temperature	104 and 106
Aluminum/copper/copper(II) oxide	Increases flame velocity and decreases reaction temperature	105
Copper/zirconium	Enhances strength, tensile ductility and toughness	14
Silicon nitride/titanium nitride	Enhances hardness and strength	107
Copper/niobium	Enhances strength, tensile ductility and toughness	108
Aluminum oxide/titanium dioxide/aluminum oxide	Promote the dielectric constant value	109
Organic/clay	Ultralow thermal conductivity	110

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3.3.1. Role of interface structure on chemical behavior of nanolaminates. Various methods have been demonstrated to fabricate reactive multilayer nanolaminates.^111–114 Understanding the formation of interfacial layers at an atomic level between reactive and energetic materials plays a significant role in the chemical and nanoenergetic system performances.^103–106

In an early effort, Kwon et al.¹⁰⁴ employed a magnetron sputtering technique to fabricate high-quality aluminium/copper(II) oxide (Al/CuO) bilayers that were grown onto oxidized N type (100) double-side polished silicon, and studied chemical behavior of interfacial layer. Two sets of Al/CuO and Al/Al₂O₃/CuO (Al₂O₃, between Al and CuO) nanolaminates were fabricated to examine the interfacial chemistry and the role of exothermic reaction between these nanostructured materials. The synthesis of Al/CuO nanolaminates was implemented via magnetron sputtering on a piece of cleaned silicon wafer that was rinsed in deionized water and dried in a nitrogen gas steam. The substrate temperature was 10 °C during sputter deposition and the base pressure of the chamber was less than 10⁻⁵ Pa. CuO and Al layers were deposited using Cu and Al targets with purities >99.999%. Sputtering was carried out under an argon (Ar) and oxygen plasma at 400 W for Cu and only argon at 800 W for Al. The magnetron sputtering chamber was fully pumped out after each Al or CuO deposition cycle to avoid cross contamination and Al oxidation from residue oxygen during CuO deposition. The consecutive deposition of CuO and Al was performed without venting the chamber. The thicknesses of each CuO and Al layer were 100 nm and these were under less than 30 MPa stress.

High resolution transmission electron microscopy (HRTEM) on Al/CuO nanolaminates (Fig. 16) shows that an intermixed interfacial layer (a mixture of Cu, O and Al), which was irregular in thickness, is evident between the Al and CuO layers. The thickness of the interface layer is negligible in some regions and as thick as ∼5 nm in other regions. This varying structure arose due to the highly textured and 15 nm rough CuO surface.

Fig. 16 HRTEM images of the interface between sputter-deposited Al and CuO. Adapted with permission from data published in ref. 104 Copyright 2013, American Chemical Society.

Further characterization of the intermixed interface layer region was executed on a 2 monolayer (ML) thin film of Al, which is equal to 0.5 nm, deposited onto a CuO surface. The thin layer was characterized via in situ XPS. The in situ XPS results (Fig. 17) show that the Cu 2p reduced from a higher binding energy of 933.5 eV (black pattern) to 932 eV (red pattern) after Al deposition.

Fig. 17 The (black) graph shows XPS Cu 2p spectra of CuO, the (blue) pattern is after 2 ALD cycles with trimethylaluminum (TMA, Al₂(CH₃)₆) and water, and e-beam evaporated 2 monolayer of Al (red). The two dotted lines at 943.7 and 962 eV display the satellite peaks of Cu(II) of the underlying CuO. Adapted with permission from data published in ref. 104 Copyright 2013, American Chemical Society.

These results illustrate that CuO [Cu(II)] is reduced to Cu₂O [Cu(I)] in the interfacial region. These reduction reactions and the intermixed interface layer between Al and CuO are both effective with regard to the reduced onset reaction temperature, the reaction kinetics and stability at low temperature; all of which are important for reactive and energetic materials.

The mechanism of interface formation for Al/CuO nanolaminates, obtained from theoretical and experimental data by Kwon et al.,¹⁰⁴ may be summarized as the deposition of isolated Al atoms on CuO surfaces leads to amorphization of the surface/interface through the insertion of Al atoms into substantial Cu sites (Fig. 18b). This mechanism results in a mixed interface composed of Al, Cu and O. The significant rearrangement of oxygen atoms around Al atoms confirms the experimental observation of CuO reduction. Fig. 18 depicts the Al/CuO reaction process, from the initial physical vapor deposition (Fig. 18a) to the formation of reduced products in the interface layer. The differential scanning calorimetric (DSC) thermal analysis data (Fig. 18c) supports this proposed mechanistic model.

Fig. 18 A schematic of (a) Al deposition on CuO, (b) Al/CuO reaction and amorphization process upon Al subsurface penetration and zoom around the Al site revealing the tetrahedral AlO₄ structure and (c) DSC thermal analysis pattern of the Al/CuO bilayers without Al₂O₃ diffusion barrier. Reproduced with permission from data published in ref. 104 Copyright 2013, American Chemical Society.

Kwon et al.¹⁰⁴ in addition, fabricated Al/Al₂O₃/CuO nanolaminates to control the composition, thickness and conformity of the interface between the reactive materials of Al and CuO. Hence, an ultrathin Al₂O₃ interface was deposited between Al and CuO. Al₂O₃ was deposited using the ALD technique on CuO at 120 °C with alternating pulses of trimethylaluminum [TMA, Al₂(CH₃)₆] for 2 seconds and heavy water (D₂O) for 0.5 seconds. A separate N₂ purge was applied for 10 minutes.¹¹⁵ The incoming Al species are CH₃ decorated by implementation of the ALD method; hence the interface chemistry would be expected to differ from the previous deposition method that described the dual layer Al/CuO system.

Thermal analysis on the Al/Al₂O₃/CuO system displayed the presence of Al₂O₃ layers as thin as 0.5 nm that offered a barrier layer and which impact the long range diffusion of Al, Cu and O atoms; thereby enabling material combustion.¹⁰⁴ The reactivity properties do not count on the Al₂O₃ thickness. Therefore, the composition and conformal coverage of the initial few Al₂O₃ monolayers on CuO are essential to confer thermal stability on this system. The TMA surface reactions with CuO in the Al/Al₂O₃/CuO system are the same as the previous dual Al/CuO system. TMA leads to the extraction of oxygen atoms from CuO and thus to reduction of CuO. Nevertheless, the presence of the surface methyl ligands (R–CH₃) prevents Al diffusion into CuO.¹⁰⁴ Fig. 19 diagrams the Al/Al₂O₃/CuO reaction process; from the beginning of ALD deposition to the final reduced products in the interface layer. The DSC data complement these results.

Fig. 19 A schematic of (a) Al₂O₃ deposition on CuO, (b) CuO reduction mechanism induced by the decomposition of TMA and (c) DSC thermal analysis pattern of the Al/CuO bilayers with Al₂O₃ diffusion barrier. Reproduced with permission from data published in ref. 104 Copyright 2013, American Chemical Society.

In summary, there is a major exothermic reaction (at 530 °C) in Al/CuO nanolaminate with another minor exothermic reaction at lower temperatures (315 and 400 °C), while there are only two minor exothermic reactions (at 300 and 550 °C) for Al₂O₃ with 0.5 nm and (at 250 and 550 °C) for Al₂O₃ with 2.4 nm. The reactions arise because an interface layer is formed during physical deposition of Al. A mixture of Cu, O and Al is created through Al diffusion into CuO and this mixture of elements and a compound presence a low diffusion barrier. In contrast, deposition an Al₂O₃ layers, using TMA by the ALD method, produces a conformal coating that efficiently hampers Al diffusion; even for ultrathin layer thickness of ∼0.5 nm. This barrier results in better stability and durability at low temperature and reduced reactivity between Al and CuO.

In another report by Marín et al.¹⁰⁵ the reactivity of thermite Al/CuO multilayered systems studied. Thermite multilayered systems are interesting structures as energetic layers on a chip that have potential applications such as micropropulsion in space, nanoairbags to drive fluids,¹⁰⁵ MEMS energy sources,¹¹⁶ pressure-mediated molecular delivery^117,118 and microinitiators.^119,120 Multilayered structures are simple to control with regard to the thickness of each layer and the number of layers.¹⁰⁵ These structural changes in a thermite multilayered system can be utilized to tune their energetic performance. Al/CuO was studied¹⁰⁵ because the Al/CuO system has an energy density of 21 kJ cm⁻³, that is almost 3 times higher than trinitrotoluene (“TNT” at 7.6 kJ cm⁻³).¹⁰⁶ The measured flame propagation velocity versus bilayer thickness in Al/CuO nanolaminates exhibit that the reaction-propagation velocity decreases from 90 to 2 m s⁻¹ as the nanolaminate thickness increases from 50 to 2000 nm.¹⁰⁵ If the bilayer thickness is decreased to below 25 nm, then the flame propagation velocity falls to zero. The flame velocity changes in the nanolaminates of less than 25 nm is due to spontaneous and uncontrolled intermixing at the interface during deposition (Fig. 20).

Fig. 20 A schematic of (a) Al/CuO nanolaminate with self-propagation rate and reaction heat of 44 m s⁻¹ and 1 kJ g⁻¹, respectively, (b) Al/Cu/CuO nanolaminate with improved self-propagation rate and reaction heat of 72 m s⁻¹ and 1/3 kJ g⁻¹, respectively. The heat flow curves also show that the reaction temperature is reduced for the Al/Cu/CuO nanolaminate and it accrued at two steps. The flame velocity of the both nanolaminates also is decreasing with increasing thickness of the bilayers (c). Reproduced with permission from data published in ref. 105 Copyright 2015, American Chemical Society.

Marín's group has demonstrated that a thin layer of Cu of around 5 nm, deposited between each Al and CuO layer of an Al/CuO nanolaminates (i.e., the dual Al/CuO system changes to Al/Cu/CuO), alters the system reactivity. The flame velocity increases to 72 ± 1 m s⁻¹ with the Al/Cu/CuO nanolaminate system, compared to 44 ± 1 m s⁻¹ for the Al/CuO nanolaminates. The increase in flame temperature arises due to spontaneous diffusion of Cu into Al layers, which incubates the formation of an interfacial Al : Cu alloy with a melting temperature lower than pure Al.¹⁰⁵ The newly-created alloy is responsible for the enhanced reactivity of the system.

Fig. 20 displays the enhanced reactivity of the Al/CuO nanolaminates. These results for Cu deposition between Al and CuO layers open new insights with regard to the interface engineering of nanoenergetic materials that incorporate a nanolaminate structure. The investigations such as Al/CuO and Al/Al₂O₃/CuO systems highlight the importance of the chemical characteristic of the diffusion barrier with regard to the interface structure of nanolaminates, also the crucial role of these interfacial layers concerning the behavior of nanostructured laminates. Thus, selective appropriate elements, synthesis process and precise characterization can be used to increase efficiency of reactive materials that are improving remarkably in science and technology.

3.3.2. Role of interface structure on mechanical behavior of nanolaminates.
3.3.2.1. Electron structure model for the interfacial role in superhard nanolaminates. Design and synthesis of superhard (H ≥ 40 GPa) nanostructured materials, such as nanocomposites and nanolaminates, are of intense interest in modern materials research.^107,121,122 Superhard nanocomposites and nanolaminates can be used as wear-resistant coatings on tools, mechanical components and scratch-resistant thin films on optics, and so forth.^107,122 Si₃N₄/TiN nanolaminates have been considered^107,122 as a model system to fabricate superhard materials. It is essential to recognize and understand the nature of internal interfaces of the bilayer because the interface layer frequently determines the bulk properties.

Patscheider et al.¹⁰⁷ sputtered several nanolayers of Si₃N₄, Si and Ti onto epitaxial TiN layers on MgO(001) to fabricate Si₃N₄/TiN(001), Si/TiN(001) and Ti/TiN(001). The hardness of the nanolaminates and the role of bilayer interfaces were investigated. The fabricated nanolaminates were characterized using in situ angle-resolved X-ray photoelectron (AR-XPS) spectroscopy. In situ AR-XPS allowed satellite intensity to be monitored; i.e., the positioning and local atomic configuration, which is expected to confer the physical properties of the nanolaminate ensemble. The satellite peak is collected from photoemission interaction of the X-ray with valence electrons of a sample. The satellite intensity is a direct measure of the interface valence electron density which is, in turn, linked to the interfacial bond strength and hardness.¹⁰⁷ The thickness of the overlay materials (Si₃N₄, Ti and Si) on TiN was selected to be very thin to insure sufficient electron transparency so that the interfaces could be probed. Hence, only 4 monolayers (ML) were grown on the substrate [MgO(001)] within an ultrahigh vacuum system by using reactive magnetron sputter deposition.

The XPS spectra results indicate that both the width and intensity of Ti 2p satellites increased once TiN(001) was covered with 4 ML of Si₃N₄ at a substrate bias voltage (V_b) of −7 V (Fig. 21a). The development in intensity of Ti 2p satellites in Si₃N₄/TiN(001) is due to a high nitrogen (N) concentration around the surface and near-surface Ti atoms, thereby resulting in negative polarization because of the higher electronegativity of N than Ti.¹⁰⁷ Electrons flow from the valence band of TiN to the overlayer material at the interface of Si₃N₄/TiN(001). Thus, electrostatic polarization along with covalent bonding is introduced that strengthens the Si₃N₄/TiN(001) interface.

Fig. 21 (a) XPS spectra of TiN(001) covered with 4 ML of Si₃N₄ at substrate bias voltage (V_b) of −7 V, (b) XPS spectra of Si₃N₄/TiN(001) at different V_b (−7, −150 and −250 V) as well as comparison of average bond coordination around Ti interface atoms at V_b of −7 V and −150 V. (c) XPS spectra of deposited Ti/TiN(001) and Si/TiN(001), and (d) demonstrate a model system for valance electron density at interface as well as band structure of Si₃N₄/TiN(001), Si/TiN(001) and Ti/TiN(001). Reproduced with permission from data published in ref. 107 Copyright 2011, American Physical Society.

Si₃N₄/TiN(001) was also examined at different V_b (−7, −150 and −250 V). A distinct growth in the satellite intensity was observed when V_b increased (Fig. 21b). The increase of intensity of the Ti 2p satellites suggests that large interfacial electron accumulation and electrostatic polarization that is directly related to interfacial bonding and, hence, higher hardness could be obtained (Fig. 21b).¹⁰⁷ On the other hand, XPS spectra from deposited Si/TiN(001) show that the intensities of the Ti 2p satellites are markedly reduced (Fig. 21c). Indeed, deposition of 4 ML of Ti leads to an almost complete loss of the satellite structure in Ti/TiN(001). Hence, based on the XPS intensities, it can be considered that the electron density in the interfaces of the fabricated nanolaminates are in this order: Si₃N₄/TiN(001) > Si/TiN(001) > Ti/TiN(001).

There is a direct relationship between satellite intensity and interfacial charge accumulation with band gap.¹⁰⁷ Band gap values for Si₃N₄, Si and Ti can be found in Fig. 21. Materials with a high band gap offer larger interfacial charge accumulation.¹⁰⁷ Fig. 21d demonstrates the interfacial band structure of Si₃N₄/TiN(001), Si/TiN(001) and Ti/TiN(001). A wide band gap in the thin film signifies that a larger number of free electrons are available to populate the interface valence band; hence, there is enhanced polarization, electron valence bonding and hardness. In contrast, when the contacting overlayer has a smaller band gap (Si), or none at all (Ti), electrons are donated by the overlayer coated to TiN and thereby reducing the interfacial polarization and interface strength.

It was found that satellite intensity, characterized by in situ AR-XPS, is a direct measure of the interface valence electron density. The latter has a direct relationship with interfacial bond strength and, thus, hardness. A Si₃N₄/TiN(001) nanolaminate with high N concentration at the interface, which accepts free electrons, can develop polarization of the interface. This effect is further enhanced with increasing the substrate bias voltage. Conversely, Si and Ti overlayers donate electrons to the TiN valence band and lead to charge depletion in the interfacial layers. Therefore, polarization at the interface of the Si₃N₄/TiN(001) nanolaminate increases the interfacial strength and provides enhanced hardness. On the other hand, low nitridation and polarization in Si/TiN(001) and Ti/TiN(001) results in an increasing contribution of Si and Ti atoms in contact with TiN and, thus, a weakened interface.

3.3.2.2. Interfacial role in crystalline–amorphous nanolaminates. Wang et al.¹⁴ investigated the interfacial role on the mechanical properties of nanolaminates by fabricating alternating layers of high purity Cu (99.999%) and Zr (99.7%) using DC magnetron sputtering. The reaction between Cu and Zr layers results a solid state amorphization phase in the growth of an interfacial nanolayers of Cu/Zr metallic glass, between the Cu layers.

The mechanical properties of crystalline Cu/Cu–Zr glass nanolaminates were investigated.¹⁴ Single-phase bulk nanocrystalline Cu with an average grain size of 30 nm reveals a tensile strength of less than 800 MPa (Fig. 22a).¹⁰⁶ In contrast, the tensile strength for the Cu/Zr nanolaminate with 5 nm Cu_≈3Zr metallic glass thickness and a 35 nm nanocrystalline Cu layer is 1090 ± 20 MPa (Fig. 22a).¹⁴ The variance in tensile strengths illustrates that the amorphous interface layers in Cu/Zr nanolaminate have enhanced the strength compared to only nanocrystalline copper layers. In addition to this, the nanolaminate also exhibits a significant tensile ductility, with an elongation to failure of ε% = 13.8 ± 1.7% ¹⁴ where ε is the strain.

Fig. 22 Microstructures and tensile properties of Cu/Zr nanolaminates. (a) Room-temperature tensile true stress–strain curves of the pure nanocrystalline (∼30 nm) of Cu, nanocrystalline–amorphous Cu/Zr nanolaminate and Cu/304 stainless steel crystalline multilayer with a sublayer thickness of 25 nm, all tested at the strain rate of 1 × 10⁻⁴ s⁻¹. (b) TEM images from cross-sectional and top surface view of the as-deposited nanocrystalline Cu and amorphous Cu/Zr intermixing multilayer nanostructures. (c and d) MD simulations of Cu/Zr nanolaminates under periodic boundary conditions. Reproduced with permission from data published in ref. 14 Copyright 2007, National Academy of Sciences, USA.

Other series of results swapped the amorphous Cu_≈3Zr metallic glass layers with crystalline phase layers; e.g., 304 stainless steel (SS) or niobium (Nb), to make Cu/304 SS or Cu/Nb nanolaminates. The tensile strength increased to ∼1500 MPa for the Cu/304 SS nanolaminate (Fig. 22a). However, the elongation is <2%.^123–125 A reduced ductility is the main limitation for all high strength crystalline/crystalline multilayer materials with nanometer scale size bilayers (bilayer ≤ 40 nm), and also in many single phase nanocrystalline metals such as nanocrystalline Cu (Fig. 22a).^{106,123–126} High-strength and high-ductility nanostructured Cu has been reported previously.^127,128 Nevertheless, high tensile ductility in Cu/Zr nanolaminates with amorphous Cu_≈3Zr is unexpected due to the near zero tensile ductility of bulk metallic glasses, and a typical low ductility in nanoscale microstructured materials.¹⁴

A further notable feature of a nanocrystalline–amorphous nanolaminate is toughness. The key to toughness is an appropriate blending of strength and ductility. Hence, high ductility in Cu/Zr nanolaminates would allow improved toughness due to significant plastic deformation (Fig. 22a), which is an uncommon behavior for metallic glasses. The room temperature behavior in Cu/Zr nanolaminate with a high flow stress of ∼1.09 GPa at a higher strain rate of 1 × 10⁻⁴ s⁻¹ conflicts with the observations in pure nanocrystalline materials that indicated an alternative deformation mechanism.¹⁴ The plastic behavior observed in Cu/Zr nanolaminate is also in contrast with the Considère criterion that predicts the mathematical onset of necking when dσ/dε ≤ σ is reached at constant strain rate.¹⁴

In order to recognize the large tensile ductility and plastic behavior of the nanolaminates, the Cu/Zr nanolaminate was tested under HRTEM at different tensile strains.¹⁴ HRTEM shows that numerous dislocation storage or pileups was not revealed in the deformed nanocrystalline layers, except for a few deformation twins (Fig. 22b). Fig. 22b shows that one of the deformed twins tends to be terminated at the amorphous–crystalline interfaces (ACIs) or the Cu/Cu grain boundaries (GBs).¹⁴ Thus, in addition to GBs, the interface layer may have become a source for dislocation nucleation.¹⁴

Interaction between nanocrystalline layers and the nanoscale interface metallic glass in Cu/Zr nanolaminate may account for the large ductility enhancement in both phases.¹⁴ Wang et al. employed molecular dynamic (MD) simulations to validate this reasoning.¹⁴ The MD simulations display that dislocations are nucleated from ACIs or from GBs or intersections between ACIs and GBs; slipping across the nanocrystalline layer and being absorbed at the opposite ACI (Fig. 22c). The simulation results in addition illustrated that ACIs behave as inherent sinks of dislocations; that is, absorbing the dislocation value in the nanocrystalline Cu after plastic work is accomplished.¹²⁹

The MD simulations indicate that the amorphous layers can impact dislocation structures created in the nanocrystalline layers (Fig. 22d). Many dislocations have been nucleated at the beginning of tensile deformation in the system, resulting in dense sessile dislocation forests (Fig. 22d). The dislocations form from intersections of two nonparallel stacking faults or twining systems.¹⁴ As the simulation time progresses, however, the dislocation density reduces significantly due to the attraction and annihilation of dislocations in the metallic glass layers (Fig. 22d).

Another method that often accompanies dislocation absorption is ACI sliding that spreads the dislocation core along the ACI.^14,130 Therefore, according to experimental and simulation results, the nanoscale metallic glass layer not only retains large tensile plasticity itself, but plays the dominant mechanistic role; exhibiting the ability to behave as both a dislocation source and sink to intercede inelastic shear/slip transfer while avoiding extreme stress concentrations that would have led to fracture initiation.¹⁴

The extensive tensile ductility and nearly ideal plastic flow behavior in Cu/Zr nanolaminate propose that the nanocrystalline–metallic glass composite approach is an effective method towards developing materials with mechanical performance beyond those achievable from single phase elemental materials. The solid-state amorphization process that creates a crystalline–amorphous nanolaminate is evident in several material systems such as Cu/Ti, Ni/Ti, Cu/Hf, Ag/Zr, Ni/Zr, Ag/Hf, Ni/Nb, Ti/Si, Pd/Si and Cu/Nb.^14,108

3.3.3. Role of interface structure on physical behavior of nanolaminates. Transition metal oxides that are composed of oxygen atoms bound to transition metals, are often used in organic electronic devices to enhance charge transfer between electrodes and organic semiconductors.¹⁰³ They often must be connected with the metal during use⁶² (Fig. 23a). On the other hand, oxide layers are no more than a couple of nanometers thick in these devices since minimal thickness is necessary to reduce the series resistance that oxides contribute to a device. Therefore, metal contact with the metal oxides can influence the oxide's chemical reactivity, conductivity and energy-level alignment properties; where these effects may change the metal oxides ability to perform their intended function (Fig. 23b). For instance, in metal-oxide-semiconductor field-effect transistors (MOS-FETs), chemical reactions and defects at a metal/metal-oxide interface can produce an oxide gate-dielectric that exhibits high leakage current.¹³¹ Hence, selecting a contact metal and its interface role with the metal oxide can tailor the physical behavior and properties of metal oxide thin films.

Fig. 23 Schematics for (a) a multilayered organic electronic device and a metal oxide shield layer, (b) layered structure of MoO₃ oxidation states near an interface and plot of work function and conductivity versus MoO₃ thickness for reactive and non-reactive interfaces between MoO₃ and electrode materials. Reproduced with permission from data published in ref. 103 Copyright 2012, John Wiley and Sons.

Greiner et al.¹⁰³ studied the effect of the metal/metal oxide interface on the work function and electronic band structure. They examined MoO₃ (purity 99.99%) films grown by vacuum sublimation on gold, nickel, molybdenum, vanadium and copper. MoO₃ was selected since it is generally used as a hole-injection layer in organic devices to move an electron out of the device.^132–135 Metals are used because they have different work functions and oxidation potentials.¹⁰³

Mo cations have a nominal oxidation state of 6+ (Mo⁶⁺) in stoichiometric MoO₃. However, the Mo cations of MoO₃ are reduced to a lower oxidation state of Mo⁵⁺ and/or Mo⁴⁺ in the first few nanometers of each metal/MoO₃ interface.¹⁰³ The average oxidation state at each metal/MoO₃ interface, work function and free energy of oxidation of the metals are summarized in Table 3.

Table 3 Summary of work function and free energy of oxidation in MoO₃ films near the various metal interfaces¹⁰³

Substrate material	Mo^6+	Mo^5+	Mo^4+	Work function (eV)	Free energy of oxidation (kJ mol^−1)
Au	60%	40%	0%	5.3	∼−9
Ni	50%	50%	0%	5.0	−419
Mo	45%	55%	0%	4.5	−528
V	7%	36%	57%	4.2	−800
Cu	66%	34%	0%	4.7	−293

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Molybdenum exhibits 6+ and 5+ oxidation states when MoO₃ is in contact with the metals of Au, Ni, Mo and Cu; whereas Mo has 6+, 5+ and 4+ oxidation states when MoO₃ is in contact with V. Copper is a unique electrode metal because it reacts with MoO₃ to form a Cu–Mo–O alloy that incorporates both Mo⁶⁺ and Mo⁵⁺ valences.^136,137

Oxidation-state and work-function profiles of each metal/MoO₃ interface are shown in Fig. 24. These profiles display the relative Mo 3d peak areas for Mo⁶⁺, Mo⁵⁺ and Mo⁴⁺ peaks for each MoO₃ film thickness, which were obtained from XPS experiments. Oxidation-state profiles show that the low-oxidation-state Mo cations persist further from the interface on the more reactive substrates than on the less-reactive substrates. The profiles also indicate that the d_90% value is 6.4 nm for V, 6.0 nm for Mo, 33 nm for Ni and 2.8 nm for Au. Copper alloy (Cu–Mo–O) never achieves 90% (Note: d_90% is assigned as the MoO₃ thickness for which the Mo⁶⁺ peak is 90% of the total peak area).

Fig. 24 Profiles of (a) oxidation state and (b) work function for MoO₃ films grown on various metal (Au, Ni, Mo, V and Cu) substrates. Adapted with permission from data published in ref. 103 Copyright 2012, John Wiley and Sons.

The work-function depends on the MoO₃ thickness (Fig. 24b).¹⁰³ Fig. 24b indicates that work function tends to increase with distance far away from the metal/MoO₃ interface and finally plateaus at a value of ∼6.85 to 6.9 eV (Cu–Mo–O alloy work-function plateaus at ∼6.25 eV). The work-function profiles follow a similar trend to the oxidation-state profiles and suggest that these profiles are correlated. The d_6 eV values of metals are 1.1 nm for V, 0.88 nm for Cu, 0.85 nm for Mo, 0.68 nm for Ni and 0.28 nm for Au (Note: d_6 eV is assigned as the thickness at which the work-function reaches 6.0 eV). The fact that work function depends on MoO₃ thickness exemplifies the importance of oxide thickness and substrate choice for the energy-level alignment of molecular adsorbates.¹⁰³

The results of Greiner et al.¹⁰³ indicate that the interfacial MoO₃ reduction occurs close to the metal/MoO₃ interface function under two possible mechanisms: (i) charge transfer from the metal Fermi level to the MoO₃ conduction band, and (ii) reduction of MoO₃ driven by oxidation of the metal substrate.¹⁰³ These two mechanisms assist comprehension concerning (i) how a noble metal, such as Au, can reduce Mo⁶⁺ within the first few nanometers of the Au/MoO₃ interface, and (ii) how a reactive metal, such as V, can cause severe reduction of Mo⁶⁺, several nanometers from the interface of V/MoO₃.

The results of Greiner et al.,¹⁰³ demonstrate that the metal/metal-oxide interfaces critically influence the work function and band structure. It is indicated that the reduction of Mo⁶⁺, in interfacial metal/MoO₃, results in changes to the MoO₃ valence band structure so that the oxide semi-metal is close to the metal interface. The valence band changes can be illustrated in terms of electrons filling the previously empty Mo 4d states of MoO₃; thereby resulting in donor states close to the Fermi level.¹⁰³ The reduced Mo⁶⁺, to Mo⁵⁺ and Mo⁴⁺, also changes the work function of MoO₃ to one close to the metal/MoO₃ interface.¹⁰³ The design and selection the appropriate oxide buffer thickness can be optimized from the above investigated data that is relevant for organic electronic devices.

Another study by Losego et al.¹¹⁰ synthesized an ultralow thermal conductivity nanolaminate system via a simple self-assembly method. They used organoclays produced by alkylammonium cation (R–NH₃⁺) exchange with colloidal dispersed montmorillonite [(Na, Ca)_0.33(Al, Mg)₂(Si₄O₁₀)(OH)₂·nH₂O] clay sheets, pursued by solvent casting. The interface was investigated concerning the influence on the thermal conductivity of organoclay nanolaminates.

Fig. 25a shows a schematic of the sample geometry used to measure thermal conductivity of organoclay nanolaminates via time-domain thermoreflectance (TDTR). The results demonstrate that thermal conductivity is comparatively independent of nanolaminate spacing.¹¹⁰ A series resistance model was developed to better understand the behavior and influence of the interfacial thermal conductance for the organoclay nanolaminates:¹¹⁰ i.e.,

where Λ_effective is the measured thermal conductivity, d is the unit cell spacing in the heat flow direction, t_i is the layer thickness (organic or aluminosilicate), Λ_i is the layer thermal conductivity and G_i is the interfacial thermal conductance.

Fig. 25 (a) Schematic of the sample geometry used for TDTR measurements, (b) measured values for the cross-planar thermal conductivity of unmodified clay and organoclay nanolaminates prepared from varying alkylammonium chain lengths. The blue dashed line is an “effective medium” model that excepts interfacial conductance, the red dotted line supposes only interfacial conductance with G = 110 MW m⁻² K⁻¹, and the red solid line is a series resistance model that includes organic layer conductance (aluminosilicate conductance) and interfacial conductance with G = 150 ± 10 MW m⁻² K⁻¹.¹¹⁰ The thermal conductivity of WSe₂ is plotted at its equivalent interface density (for reference).¹¹⁰ Inset images indicate the presumptions of 1 interface per unit cell for the unmodified clay and 2 interfaces per unit cell for the organoclays. The unmodified clay is plotted using its measured d spacing but has an assumed interface density of 0.85 interfaces per nm. Adapted with permission from data published in ref. 110 Copyright 2013, American Chemical Society.

An interfacial thermal conductance of ∼150 MW m⁻² K⁻¹ was obtained for the organic/clay interface by Losego's model, which exhibits similar organic–inorganic interfaces. This interfacial conductance value presents opportunity for designing nanolaminate materials with lower thermal conductivities.¹³⁸ The average thermal conductivity has been documented as 0.09 W m⁻¹ K⁻¹ for organoclay nanolaminates (Fig. 25b).¹¹⁰ This value is close to the lowest thermal conductivity ever reported for a dense solid; i.e., 0.06 W m⁻¹ K⁻¹ at room temperature for WSe₂ films that were fabricated using more complex molecular beam evaporation techniques.¹³⁹

The series resistance modeling, suggested by Losego et al., illustrate that the interface densities achieved in the organoclays (1–1.5 nm) are adequate to gain the ultralow thermal conductance. A material is thought to reach its lowest thermal conductivity when amorphous, since phonons are forced to transit by means of a random walk through atomic vibrations.¹⁴⁰ Nevertheless, the amorphous limit can be surpassed once nanoscale features have been introduced.^141–144 Interfaces in nanolaminates scatter phonons and provide additional thermal resistance to a system. Interface densities in nanolaminates can be sufficiently high to decrease the thermal conductivity to below the amorphous limit.^145–147

Interfaces hinder phonon transport of thermal energy and its nanostructure can transform fully dense solids into ultralow thermal conductivity materials.¹¹⁰ Consequently, the above results suggest that organoclays can be developed as an experimental platform to explore nanolaminate materials and the role of interfaces in thermal physics.

4. Fabrication and properties of nanolaminates derived from ceramic-based multilayer phases

Section 3.2 has explained that MAX phases are naturally nanolaminated structures that exhibit unique combined metal and ceramic characteristics. Therefore, the focus of this section concerns multilayer thin films and nanosheets of MAX phases and ceramic materials that are used for energy storage, gas barriers and thermoelectrics. MAX phases are attractive owing to their combination of chemical, physical, electrical and mechanical properties. To date, research towards energy storage materials based on MAX phases are narrow. However, more interest is being devoted in this direction; for example, in creating 2D supercapacitor electrode materials. Conversely, fundamental researches has been increasingly concentrated upon inorganic nanosheets due to their potential as conductors, semiconductors and insulators. Most inorganic nanosheets are electrically insulated or exhibit a wide semiconducting band gap,^148,149 as well as having high chemical and thermal stabilities.^150,151 Thus, the functionality of powerful ceramic-based materials by investigating hybrid nanolaminates containing different layers is being pursued; as will be demonstrated in the following section.

Most 2D materials are usually produced by exfoliation or delamination of van der Waals solids,^152,153 or by direct reaction between different elements or compounds.^154–157 Recently, a selective extraction method has been designed to produce 2D materials derived from MAX phases.¹³ This method holds great interest because it operates at room temperature and has the capability of obtaining well-controlled morphologies and structures.^{13,148,158,159} As well, new materials can be produced by the selective extraction method that cannot be fabricated by other methods.^{13,148,158,159} For instance, MXenes and carbide-derived carbons (CDCs) are the most studied materials that are achieved by the selective extraction technique.^160–162 Structures of type “A”, “M and A” have been selectively extracted from the MAX phases, MXene and CDCs materials, respectively.

These new extracted materials present a combined metal conductivity and hydrophilicity that could be used in energy storage (Li-ion batteries, supercapacitors) or in polymer nanocomposites.^159–162 Recently, the selective extraction method, by electrochemical techniques at room temperature has been employed to extract the “M” element from Ti₂SC MAX phase and produce an “AX” nanolaminated structures.¹⁶³ The extraction method is applied at low potentials (0.3, 0.6, 1.0, 1.4 and 2.0 V) and ammonium fluoride (NH₄F) aqueous solution was selected as the electrolyte. The bonding force between Ti and S in Ti₂SC is weak, being van der Waals in nature, and S displays relatively little electrochemical interaction. Therefore, it is reasonable to predict that Ti would be easier to extract than S. The Ti extraction from Ti₂SC results in carbon/sulfur (C/S) nanolaminates (Fig. 26a).

Fig. 26 (a) A schematic indicating the selective extraction of Ti (M) from Ti₂SC (MAX phase) using electrochemical etching. (b) Raman spectra of Ti₂SC before (bottom pattern) and after etching at various potentials. (c) Dependence of etched C/S layer thickness (after 1 hour) on etching voltage. (d) High-resolution TEM images of amorphous (left image) and graphene-like (right image) C/S flakes. Reproduced with permission from data published in ref. 163 Copyright 2015, John Wiley and Sons.

The Raman spectra of Ti₂SC after etching at 0.3, 0.6 and 1.0 V are almost the same and all of the Ti₂SC peaks disappeared with just the C/S composite peaks revealed (Fig. 26b). The Raman spectra peaks are identified as C and small traces of TiO₂ at the higher potential of 1.4 V. However, the intensity of TiO₂ is enhanced at potential of 2.0 V, in addition to the C intensity. The TiO₂ fabricated at higher voltages is thought to arise from the hydrolysis of the extracted Ti ions that are trapped within the C/S framework.¹⁶³ It could also be suggested that both Ti and S are extracted from Ti₂SC at the higher voltages. The potential rate also influenced the thickness of the C/S nanolaminate layers.

The results exhibited that after 1 h etching at 0.3, 0.6 and 0.8 V the thickness of C/S layers increased from 7 to 25 μm (Fig. 26c).¹⁶³ HRTEM characterization shows that the fabricated C/S nanolaminates are significantly amorphous; however, 5% of the C/S displays a graphene-like structure (Fig. 26d).¹⁶³ Li/S cells were fabricated and discharge capacities were measured for both materials to compare the fabricated C/S nanolaminates against sandwich-type graphene/S nanocomposites. The results illustrated that the fabricated C/S nanolaminates presented better cycling stability when tested under the same conditions. In additional to Ti₂SC, some other MAX phases, such as Ti₃AlC₂, Ti₃SnC₂ and Ti₂GeC were examined by the electrochemical etching process to produce “AX” nanolaminated structures.¹⁶³ The products all revealed the “AX” layered structures, after extraction of the “M” constituent by using electrochemical etching. The established structure and characterization of nanolaminates derived from MAX phases reveal a research area with the potential of substantial important outcomes.

5. Current conditions, challenges and applications in ceramic-based nanolaminates

Nanolaminates may exhibit beneficial electronic, physical, chemical and optical properties owing to their unique structures, which suggest potential for a wide range of applications. The many variations of nanolaminates with versatile properties allow the selection of materials that are specific to a desired application. Recent investigations show progress of nanolaminates and ultrathin 2D nanosheets in the fields of chemistry, materials science and condensed matter physics. Some current applications and challenges of ceramic-based nanolaminates for energy storages (i.e., dielectrics of capacitors, batteries), gas-diffusion barriers and energy conversion (thermoelectric) are highlighting in below.

5.1. Dielectrics

Thin film and flexible capacitors manufactured from dielectrics with large dielectric constant are under intensive investigation because of their potential integration into a broad range of electronic devices.¹⁶⁴ Hence, a key challenge in microelectronic research is synthesizing of high permittivity dielectrics for microelectronic devices such as memory devices,^165–167 capacitors,^168,169 gate insulators,^131,170,171 energy storage^172,173 and high frequency applications in communication devices.¹⁷⁴ It is critical to achieve a combination of high dielectric constant (ε), lowest possible loss tangent (tan δ), and reduced leakage current density (J) for in the dielectric material.¹⁶⁴

There are different binary oxides (HfO₂, ZrO₂, and TiO₂, etc.) and ternary oxides (Hf_xAl_1−xO_y, and Ti_xAl_1−xO_y, etc.) that are used in dielectric materials.^164,175 However, these materials have a moderate dielectric constant (ε = 10 to 50), so that the desirable combination of high ε and low tan δ values is difficult for these single-phase materials.¹⁶⁴ The reported ε values for perovskite oxides (BaTiO₃, SrTiO₃, etc.) are in ∼10⁴ higher;¹⁷⁵ however, perovskites have a complex structure that is difficult to grow onto Si substrates.¹⁷⁵ Therefore, the nanolaminated forms of the binary oxides are an effective way to increase ε and reduce dielectric losses.¹⁶⁴ Hence, the research studies have focused on mixed-phase or nanolaminate materials such as TiO₂/Ta₂O₅, TiO₂/Nb₂O₅, Nb₂O₅/Ta₂O₅, HfSO_x/ZrSO_x.^{164,175–177} The high ε value in nanolaminates (i.e., large permittivity of 330 at the frequency about 10⁵ Hz in HfO₂/ZnO nanolaminate when ZnO sublayer thickness is 13.6 nm ¹⁷⁸) (while relative permittivity of thin film HfO₂ with 50 nm thickness at 100 Hz is ∼26 and for bulk ZnO is ∼8.66), was pointed out to be owing to the Maxwell–Wagner (MW) polarization via charge accumulation at the interfaces of insulating and semiconducting sublayers where space charge polarization increases ε.¹⁷⁵ It is also reported that MW polarization can increase the measured value of ε in chemically homogenous systems.¹⁷⁹

A dielectric with high ε and low leakage current density has been produced by the synthesis of amorphous nanolaminates of TiO₂ (as the semiconductor) and Al₂O₃ (as an insulator) with individual layer thickness as low as 0.2 nm.¹⁷⁵ An enhancement of ε value in the order of 10⁴ up to a frequency of 10 kHz, dielectric losses smaller than 1 and extremely low leakage currents were reported for this nanolaminate.^180,181 The high band gap of Al₂O₃ (8.8 eV) may hinder the leakage of the mobile charges and the tunneling can be controlled by the ultra-thin insulating layers for nanolaminate materials that use MW polarization. The amorphous character of nanolaminates also facilitates integration into existing technologies. Thus, this material may also be a promising candidate for integrated passive devices.¹⁷⁵

5.2. Batteries

Lithium (Li) is the lightest of all metals and has an excellent electrochemical potential that provides the largest energy density. Therefore, Li-ion batteries are important for portable electronics and electric vehicles due to their large energy density.^182–185 Another benefit of Li-ion batteries relates to their low maintenance in comparison to other energy storage devices.^186,187 Additionally, the self-discharge is less than half compared to nickel–cadmium (Ni–Cd).

Despite the overall advantages, the large-scale application of Li-ion batteries are limited by technological barriers, such as high cost, poor cycling stability and low electrical conductivity.¹⁸⁸ A solution to these challenges employs 2D nanosheets and their composites with high conductive carbon materials for developing rate ability and cycling performance of Li-ion batteries.

Xie et al.^189,190 have demonstrated that 2D nanosheets with an active and large surface area exhibit rapid Li storage and good cycling life. For instance, cobalt oxide (Co₃O₄) 2D nanosheets with a 1.5 nm thickness present a specific capacity equal to 812.8 mA h g⁻¹ with little capacity loss per cycle and is an excellent anode material.¹⁸⁹ Moreover, Li et al.¹⁹¹ reported that graphene/NiO 2D nanosheets could significantly increase the capacity, rate capability and cycling stability of Li-ion batteries. They have demonstrated that the enhancements in graphene/NiO are due to synergistic effects resulting from their good interfacial interaction that induces better performance rather than each individual component.¹⁹¹ Furthermore, Xie et al.¹⁹² illustrated that ultrathin graphene/β-Ni(OH)₂ nanosheets could confer fast electron transfer and improve the high rate electrochemical performance. Against these attractive benefits of graphene/2D nanosheet composites, there are some drawbacks of graphene-based composites in Li-ion batteries.¹⁹³

Two main limitations of graphene-based composites are: (i) preparation of graphene usually requires a time consuming and complex process through graphite exfoliation, or very high carbonization temperature is required from carbonaceous materials,^194,195 and (ii) decoration of the graphene surface by active materials avoids their aggregation because of the lack of oxygen bridges between graphene and the active materials.^191,195 Therefore, 2D nanosheets made from carbon-based composites could improve the Li-ion battery performance, compared to the complex synthesis process for graphene.

Xie et al.¹⁹³ have synthesized sandwich-like nanosheets of carbon-anchored ultrathin TiO₂ as a superior platform to achieve ultrafast Li storage kinetics and superior cycling stability. They employed a facile method and fabricated the ultrathin TiO₂ nanosheets using a precursor of lamellar TiO₂ and octylamine (C₈H₁₉N) (Fig. 27a). Firstly, lamellar hybrid nanosheets of TiO₂–octylamine were fabricated by mixing precursors and hybridization at 180 °C.¹⁹³ The lamellar hybrid nanosheets were then annealed at 450 °C for 2 h. The annealing process leads to growing anatase TiO₂ and in situ carbonization of the intercalated organic components in the lamellar hybrid nanosheets (Fig. 27a).

Fig. 27 (a) A schematic from formation procedure of nanolaminate carbon-anchored ultrathin TiO₂. (b) TEM image for the as-synthesized precursor lamellar TiO₂–octylamine hybrid nanosheets (left image), TEM, inset HRTEM images (middle and right) corresponding elemental mapping for the nanolaminate carbon-anchored ultrathin TiO₂. (c) A schematic illustration of advantage of using nanolaminate carbon-anchored ultrathin TiO₂ nanosheets as the lithium ion battery electrode that facilitates fast lithium insertion/extraction, while anchored carbon provides conducting paths through the Ti–O–C bridge. Reproduced with permission from data published in ref. 193 Copyright 2014, The Royal Society of Chemistry (RSC).

Fig. 27b exhibits TEM, HRTEM and elemental mapping of the as-synthesized precursor lamellar hybrid of TiO₂–octylamine and sandwich-like nanosheets of carbon-anchored ultrathin TiO₂. These images show that the as-synthesized precursor and sandwich-like morphologies have sizes of about 100 nm and 3–5 nm, respectively. The sandwich-like morphology also confirms that their structure did not change compared with the lamellar hybrid nanosheets of TiO₂–octylamine. The corresponding elemental mapping analysis (Fig. 27b) also confirms the dispersion of the carbon component on the surface of the ultrathin TiO₂ nanosheets. The cyclic voltammograms, cycling performance, efficacy of rate capacity and Nyquist values of carbon-anchored ultrathin TiO₂ and bare TiO₂ nanosheets have been studied and analyzed. The carbon-anchored ultrathin TiO₂ nanosheets delivered a capacity of 101.9 mA h g⁻¹ at discharge rates as high as 40 Coulombs at 6.8 A g⁻¹; while the capacity after 1200 cycles at a large current density of 5 C is 150.4 mA h g⁻¹.¹⁹³ These are superior results compared to those reported for TiO₂ nanosheets, with a reversible capacity of 82.2 mA h g⁻¹ at the 630th cycle at a current density of 2000 mA g⁻¹.¹⁹⁶

Xie et al. reported that the ultrafast Li storage kinetics and higher capacity in carbon-anchored ultrathin TiO₂ nanosheets may be attributed to a synergistic effect that improves Li storage.¹⁹³ The TiO₂ nanosheets with ultrathin thickness and large surface area assist to shorten the Li-ion diffusion length and increase the surface area contact with the electrolyte; hence, leading to faster kinetics and higher capacity¹⁹³ (Fig. 27c). In addition, carbon derived from the carbonization of octylamine also increases conductivity and better provides a strain match. Thus, the carbon phase provides superior cycling stability and rate capability.¹⁹³

5.3. Gas-diffusion barriers

Gas-diffusion barriers play a significant role in applications such as flat-panel displays, organic solar cells, OLEDs and organic thin film transistors (OTFTs).^197–200 It is broadly identified that organic or electrode devices are highly prone to the permeation of water and oxygen that is demonstrated to cause device degradation and limited lifetime.²⁰¹ For long-term stable device operation (i.e., a lifetime of >10 000 h), a low water vapor transmission rate (WVTR) on the order of <10⁻⁵ g per m² per day is required in commercial interests.²⁰² This value of WVTR is six orders of magnitude under what is used in conventional food packaging.²⁰² Organic devices encapsulation with a glass or metal lid is a common technique to protect the device from moisture and oxygen.²⁰¹ Metal oxides (Al₂O₃, SiO₂, TiO₂) and nitrides (SiN) can also be used as moisture diffusion barriers.²⁰⁰ However, glass or metal lid encapsulation are not appropriate for large-area, flexible or transparent applications. In addition, inorganic protective materials are known to crack and exhibit pinhole defects in the layer surface.²⁰⁰ The WVTR values for such materials is reported to be about 10⁻¹ g per m² per day.²⁰⁰ Hence, a highly impermeable protection technology with high productivity and a low cost fabrication process is required.

Thin films are promising candidates for moisture and oxygen barriers. Meyer et al.^202,203 employed the ALD method and fabricated Al₂O₃/ZrO₂ nanolaminates to examine them as gas-diffusion barriers. The Al₂O₃/ZrO₂ nanolaminates were grown at 80 °C. The measured permeation rate of the fabricated nanolaminate with 130 nm thickness, under a controlled environment of 70% humidity and 70 °C, was 4.7 × 10⁻⁵ g per m² per day for water and 1.6 × 10⁻² cm³ per m² per day for oxygen.²⁰³ The Al₂O₃/ZrO₂ nanolaminate with a ∼40 nm thickness also exhibited an ultralow WVTR of 3.2 × 10⁻⁴ g per m² per day at 80% humidity and 80 °C.²⁰² Choi et al.²⁰⁰ fabricated a SiO₂/Al₂O₃ nanolaminate on a plastic substrate by RF-magnetron sputtering and then characterized its gas-diffusion barrier properties.²⁰⁰ The SiO₂/Al₂O₃ gas barrier film of 480 nm thickness displayed a WVTR of 3.79 × 10⁻⁵ g per m² per day in an environment of 20 °C and relative humidity of 50%. Lee et al. also demonstrated that the WVTR of the Al₂O₃ and ZrO₂ layers were 9.5 × 10⁻³ g per m² per day and 1.6 × 10⁻² g per m² per day, respectively, however when deposited alternatively with 1 cycle of each layer, the WVTR decreased to 9.9 × 10⁻⁴ g per m² per day.²⁰⁴ The mechanism for the enhanced barrier efficiency can be attributed to the microstructure. The surface and cross-sectional characterization indicates suppressed formation of voids and extended crystals as a result of the alternating multilayer structure.²⁰³ In addition, the second phases of ZrO₂ and SiO₂ in the fabricated nanolaminate efficiently hamper the corrosion of Al₂O₃ by water as defined in the below chemical reactions.²⁰⁵

Al₂O₃(s) + 6H⁺(aq) + 3H₂O(l) → 2[Al(H₂O)₃]³⁺(aq)

Al₂O₃(s) + 2OH⁻(aq) + 3H₂O(l) → 2[Al(OH)₄]⁻(aq)

Thus, the nanolaminated structures provide a mechanism for thin-film encapsulation of organic electronic devices and other applications because of the low permeation rates. In addition, the films exhibit close conformity to the intended device, which improves their tolerances and overall performance since encapsulation can be assured.

5.4. Thermoelectric

Improved thermoelectric systems depend on enhancing the efficiency of existing p- and n-type semiconducting thermoelectric materials and increase their thermoelectric figure of merit (ZT).²⁰⁶ ZT is defined as S²σT/κ, where S, σ, T and κ represent the Seebeck coefficient, electrical conductivity, absolute temperature and thermal conductivity, respectively. S²σ is the power factor.²⁰⁷ The thermal conductivity is a principal parameter for measuring the performance of thermoelectric materials and can be expressed as κ = κ₁ + κ_e, where κ₁ represents the phonon contribution and κ_e the electronic contribution.²⁰⁶

Therefore, both phonons and electrons contribute to the thermal conductivity. Hence, decoupling of these contributors for the thermal conductivity provides a design opportunity to suppress the phonon contribution (κ₁) without a noticeable change in S and σ. This effect could be achieved by engaging phonon-blocking/electron-transmitting material structures.²⁰⁶ Hence, nanolaminated structures could boost the performance of thermoelectric materials because thermal conductivity may be suppressed without hindering the electrical transport properties.^207,208 Ultrathin multilayer materials including organic/inorganic superlattices are attracting attention as potential thermoelectric materials for flexible power generation.^206–208 These multilayered structures could block phonon transport by interfacial coupling and scattering effects without adversely influencing electric properties.^207,208

Progress has been made on n-type organic/inorganic superlattice materials with flexible structures by using organic intercalation of layered transition metal dichalcogenides, TiS₂.²⁰⁷ Koumoto et al. employed an electrochemical intercalation process and fabricated a TiS₂/[(hexylammonium)_x(H₂O)_y(DMSO)_z] hybrid superlattice (Fig. 28a).²⁰⁷ Fig. 28b, indicates that the layers in the prepared nanolaminate hybrid superlattice are distorted and formed in a wavy structure. This wavy structure forms because the inorganic nanolayers tend to roll up and form nanotubes under a certain processing treatments.²⁰⁷ The equilibrium molecular dynamics simulations have indicated that the thermal conductivity is less than the experimental observation.²⁰⁷ The lower simulated thermal conductivity may be attributed to the wavy structure layers which provide an additional mechanism for reducing thermal conductivity of the organic/inorganic multilayer structure.

Fig. 28 (a) A schematic indicating synthesis of TiS₂-based inorganic/organic superlattices. Firstly, TiS₂ single crystal electrochemically intercalated into a TiS₂[(HA)_x(DMSO)_y] superlattice where a bilayer structure of the hexylammonium ions was formed due to DMSO stabilization. After that, superlattice of TiS₂[(HA)_x(H₂O)_y(DMSO)_z] formed by the solvent exchange process after immersion in water where the hexylammonium ions change to a monolayer configuration. (b) HAADF-STEM images of the TiS₂[(HA)_x(H₂O)_y(DMSO)_z] hybrid superlattice displaying a wavy structure. (c–f) Electrical conductivity, thermal conductivity, Seebeck coefficient and ZT values of TiS₂ single crystal and TiS₂[(HA)_0.08(H₂O)_0.22(DMSO)_0.03], respectively. Reproduced with permission from data published in ref. 207 Copyright 2015, Macmillan Publishers Ltd: Nature Materials.

The electrical conductivity, thermal conductivity, Seebeck coefficient and ZT values of TiS₂ single crystal and TiS₂[(HA)_0.08(H₂O)_0.22(DMSO)_0.03] are shown in Fig. 28c–f. An electrical conductivity of 790 S cm⁻¹ and a power factor of 0.45 mW m⁻¹ K⁻² with an in-plane lattice thermal conductivity of 0.12 ± 0.03 W m⁻¹ K⁻¹ are obtained in the multilayer hybrid superlattice. The ZT value at 373 K for this material is 0.28, which is the highest quantity for n-type flexible composites. It is a challenge to obtain high values of ZT for n-type doping organic semiconductors due to their low electron affinity.²⁰⁷ The value of ZT in the multilayer n-type structure is close to the most promising PEDOT-PSS p-type organic material that exhibits the highest ZT of 0.42.²⁰⁸

Many other nanolaminate structures such as Bi₂Te₃, NbS₂, TaS₂, VS₂, CrS₂, MoS₂ and Na_xCoO₂ could also be intercalated to make both p- and n-type thermoelectric materials.²⁰⁷ The ultimate selections of organic molecules and inorganic materials with different sizes and functional groups could provide strategies to optimize the thermoelectric efficiency.²⁰⁷

6. Nanolaminates in the future of manufacturing

The exceptional mechanical properties of nanolaminates that arise from their unique structural features offer potential for many applications. Many applications for a nanolaminate incorporate their high strength to weight ratio. Broad areas include offshore facilities, pipelines, flow lines, process facilities and good constructions. These large scale nanolaminates will not be manufactured by vacuum techniques. However, Lomasney²⁰⁹ has used electrochemistry methods to manufacture laminated metals that have been named as “Modumetal”. This class of nanolaminated materials is based on the interfacial interaction of different materials to enhance the structural, corrosion and high temperature performance of coatings, bulk materials and components. Additionally, modumetal is a nanolayered alloy that is stronger and lighter than current metals (i.e., steel) and can be used in defense and maritime, automotive, oil and gas, aerospace, construction and infrastructure.²⁰⁹

Ti₃SiC₂ can be added onto ceramic armor²¹⁰ to act (i) as a laminar material combined with a metal layer or (ii) as a functionally gradient material (FGM) with other ceramics. Such materials technology may replace currently used ceramic working elements by conferring a benefit of improved crack resistance and fracture toughness. For instance, Ti₃SiC₂–Al₂O₃ nanolaminates demonstrate high damage energy and increased fracture toughness.²¹⁰ The design of a FGM requires all of the constituents to be well defined in terms of elastic properties (i.e., Young's and shear module and Poisson's ratio) and thermal properties (i.e., thermal expansion coefficient and thermal conductivity).

Nanolaminates may be coated onto, or manufactured into, thin-shell curved mirrors for high resolution imaging in visible and infrared light for terrestrial or extra-terrestrial applications to create lightweight adaptive imaging mirrors.²¹¹ These mirrors may have diameters of the order of a meter and include metal film reflectors on nanolaminate substrates supported by multiple distributed piezoceramic “piston”-type actuators for micron-level control.²¹¹ Whilst the conventional glass mirrors with equal size and precision have mass densities between 50 and 150 kg m⁻², the nanolaminate mirrors, including not only the reflector/shell portions but also the actuators and the backing structures needed to react the actuation forces, would have mass densities that may approach ∼5 kg m⁻².²¹¹ Moreover, whereas fabrication of a traditional glass mirror of equivalent precision requires several years, the reflector/shell portion of a nanolaminate mirror could be fabricated in less than a week, and its actuator system could be fabricated in 1 to 2 months.²¹¹

7. Conclusions and future prospects

This article reviews current research activities in the area of nanolaminate composite materials. State-of-the-art studies are highlighted, in which the concepts of structural design, the critical role of interfacial interaction, the ceramic-based nanolaminates and their applications is emphasized. The identification of new ultrathin multilayered composites is an emerging research topic, especially for materials such as MXenes, BP, and MOFs. Hence, research directions are oriented to synthesize new ultrathin materials with nanolaminate structures. One of key interest materials are ceramic-based nanolaminates.

MXene materials are important 2D structures that can be included in the ceramic- and magnetic-based category of nanolaminates. They are 2D transition metal carbides and/or nitrides that are fabricated by extracting “A” layer from MAX phases and which integrate metal and ceramic properties.¹³ MXenes by exhibiting good electronic conductivity and a 2D morphology are suitable for storage energy devices, such as Li-ion batteries, hybrid cells and supercapacitors. The experimental development of MXene materials can address current issues¹³ such as (i) a full understanding of surface chemistry, (ii) characterization of chemical and thermal stabilities under different environmental conditions, (iii) electronic, magnetic, optical, thermal and mechanical characterization of single layers of the MXenes, (iv) exploring potential applications (i.e., electronic, magnetic, sensors, etc.), and (v) investigation of new MXenes.

In addition to MXene phases, applied and fundamental research on other ceramic-based nanolaminates for energy storage (dielectrics of capacitors,¹⁶⁴ batteries^193,212), gas-diffusion barriers²⁰⁰ and energy conversion (thermoelectric)^206,207 is underway. For instance, sandwich-like carbon-anchored ultrathin TiO₂ nanosheets, proposed as a superior platform to achieve ultrafast Li storage kinetics and superior cycling stability, has been explored.¹⁹³ Progress has also been reported on n-type organic/inorganic superlattice materials with flexible structures that have been formed by organic intercalation of layered transition metal dichalcogenides TiS₂, which will be of interest for thermoelectric applicable.²⁰⁷

There are numerous applications in many fields of technology and engineering for nanolaminate composite materials. It is anticipated that current and future nanolaminates will require continued identification and exploration for specific applications. The expansion of nanolaminate structures face several challenges, such as (i) better understanding of structure–property relationships that predict the performance of a given nanolaminate structure, and (ii) identification of large-scale, low-cost and simple methods for industrial production. Overcoming these barriers will provide nanolaminates with excellent assure for applications in nanoelectronics, sensing, catalysis, gas separation and energy related areas.

Acknowledgements

The authors acknowledge the financial support through the Australia-India Strategic Research Fund (AISRF) ST060048.

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