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The development of heterostructure engineering of two-dimensional (2D) layered materials is driven by substantial interest and their potential applications. Atomically molecule arrangement of thin 2D layered materials can provide fascinating physical, chemical and mechanical properties that can be prospected for promising applications. The properties of fabricated 2D layered can be established within the synthesis process by composition and thickness control of layers. Hence, control of material growth at the atomic and molecular scales is an intense ambition for researchers. This review demonstrates the versatile and famous bottom-up chemical approaches in the fabrication of heterostructures based on 2D layered organic-inorganic hybrid materials. The tunable properties of nanolayered organic-inorganic materials are assessed from the view of admitting high performance characteristics. The suitable chemical attributes will be followed using combination of advanced techniques on various materials that would enhance the nanolayered organic-inorganic properties. The article is then focused on current conditions and challenges of the nanolayered organic-inorganichybrid materials. The final section gives some detailed discussion on applications of nanolayered organic-inorganic hybrid materials in clean energy (i.e., batteries, supercapacitors, water splitting, perovskite solar cells and thermoelectric).
The quick progress in nanoscience and technology provide new opportunities in accomplishing highly efficient nanostructures. A nanostructure with a large surface area and high efficient interface structure particularly, enhance the efficiency of such structures. Hence, new hybrid nanolayered architectures are increasingly significant for the expansion of new devices and have attracted excellent interest. Hybrid layered nanomaterials based on two-dimensional (2D) structure are developing demand for design and fabrication next-generation structural nanomaterials that can provide outstanding physical, chemical and mechanical behaviors. Among materials with nanostructure, the 2D layered organic-inorganic hybrid architectures are one of attract great interest in both substantial and applied scientific research for their potential applications in fields such as, energy storage [1-3], energy conversion [4, 5], catalysis [6, 7], sensors  and electrochemistry . Research in layered organic-inorganic hybrid nanostructure is relatively new and the publication records to date are still confined . However, it certainly developing rapidly because of emerging high efficiency clean energy devices fabricated by incorporating such novel nanostructures. Generally, the inorganic section in layered organic-inorganic materials can supply favorable physical, chemical and mechanical properties . The organic component also provides flexibility, reduced density and lowers the cost . Therefore, heterostructures based on 2D layered organic-inorganic offer opportunities to develop new materials with tunable properties intermediate between its components.
There are several bottom-up deposition techniques for synthesizing layered organic-inorganic hybrid nanomaterials including sol-gel , solution-processing [13, 14], Langmuir-Blodgett (LB) , layer-by-layer (LbL) assembly [16, 17], molecular layer deposition (MLD) [18, 19] and atomic layer deposition (ALD) [20, 21] techniques. The gas-phase techniques are borrowed to control the film thickness and composition at an atomic level; thereby having a capability to coat conformally high aspect ratio structures . Among each of the mentioned techniques, solution-processing, ALD and MLD are three such approaches that have been developed for many applications [10, 11, 22]. The fabricated hybrid organic-inorganic using ALD demonstrate a conformal structure at the atomic level control (~1 Å) on high aspect ratio structures [23-27]. The fabricated thin films by ALD are continuous and pinhole-free [26, 27]. The ALD technique is normally used in metal oxides and metal nitride binary materials . If a layered organic-inorganic hybrid nanomaterial grows in a molecular fashion layer-by-layer, rather than by an atom mechanism, then the deposition method is known as MLD. The MLD technique was observed during deposition of organic polymers in the 1990s [28, 29]. The investigation on MLD is still fairly limited and according to Meng’s report , there are just 100 research papers entirely recorded in the literature because of the difficulty in discovering appropriate coupling precursors. However, due to its unlimited feasibilities for new polymers and hybrid composites with accurately controllable growth, as well as its recent effective applications in numerous regions including new clean energies and surface engineering, MLD is extremely an encouraging technique . In fact, MLD is an approach for developing advanced organic and hybrid organic-inorganic materials to handle a lot of technical challenges with unusual properties in large diversity applications. Figure 1 displays schematics of ALD (Figure 1a) and MLD (Figure 1b) growth using sequential and self-limiting surface reactions. Metal alkoxide films could be grown from metal precursors and various organic alcohols by using the MLD technique . These metal alkoxide films are illustrated as “metalcones” . The first metalcones were the “alucones’ that were fabricated by Dameron et al.  and are based on trimethylaluminum (C6H18Al2, TMA as the metal precursor) and ethylene glycol (C2H6O2, EG as an organic precursor). A broad range of other hybrid organic-inorganic films have been fabricated by other organic precursors such as carboxylic acids (R-COOH) [21, 32] or alkylsilanes [R-Si(OR)3] [33-36].
Figure 1. A schematic illustration of the procedure to fabricate (a) aluminium oxide (Al2O3) and (b) alucone thin films using ALD and MLD techniques, respectively.
In the late 2000s, ALD and MLD approaches were combined to generate organic-inorganic hybrid composites . The feasibility to mix and match organic and inorganic precursors in the multilayer thin films leads to a broad spectrum of film properties . The layered organic-inorganic nanostructures may not only possess properties combined from the constituent films, but also have completely new properties. The layered organic-inorganic metalcone can display tunable mechanical, chemical and other physical properties. The properties may vary from soft, low dense metal alkoxides representative of the organic constituent to hard, dense metal oxides that are based on the inorganic constituent. The tunable properties of engineered hybrid organic-inorganic multilayers may be of value in fabricating advanced multilayer thin films with desirable properties, despite their similarities in process. Hence, the scope of this review paper is to discuss the role and importance of layered organic-inorganic hybrid nanostructures in clean energy. This review initially defines the versatile and well-known bottom-up chemical approaches for the fabrication of heterostructures based on 2D layered organic-inorganic materials. Subsequently, tunable properties of layered organic-inorganic nanomaterials using two advanced MLD and ALD techniques will be discussed. The field of layered organic-inorganic hybrid nanostructures is then segmented on the basis of the current status, challenges and high-performance applications, such as batteries, supercapacitors, water splitting, perovskite solar cells (PSCs) and thermoelectric devices. The final section concludes with an outlook on future directions for layered organic-inorganic hybrid nanomaterials.
2. Overview of bottom-up chemical approaches in fabrication of heterostructures based on 2D layered organic-inorganic materials
Bottom-up chemical approaches play a substantial role in developing nanostructured materials, by mixing components and solutions in the atom or molecular level. Therefore, those methods deliver superior chemical homogeneity in comparison to bottom-up physical techniques . Furthermore, chemical processing design and synthesis of new materials may be refined into the final end products . Nevertheless, toxic reagents and solvents are regularly employed for the synthesis of nanostructured materials . There are several bottom-up chemical methods used for fabricating 2D and layered hybrid organic-inorganic nanostructures; of which some significant ones will be described briefly in the following section.
Solution-processing for depositing transparent thin films over the surface of a substrate from chemical solution is a versatile technique. It permits the researchers to tune composition and therefore the properties of the final devices. Figure 2a illustrates a flow chart of the typical solution-processing method. A solution deposition processing is started with the precursor. Precursor could be a chemical reagent that includes the cation of interest and reacts to form the desirable phase and material. The choice of a precursor is dependent upon the selection of formulation, solvent and preferred processing route. Solution-processing route on transparent conducting oxide thin films usually engage acetates, chlorides or nitrates as precursors fabricated by these compounds are cheap and easily available to be provided from commercial suppliers . There are another various precursor candidates that have been reviewed and outlined extensively by Pasquarelli et al. . Coating solution formulation is the second step in solution-processing route. Typically, this phase is formulated by excessing of a solvent and other additive after selection of precursors. There are two main forms of formulations in coating solution step: (i) metal-organic/ionic solutions and (ii) colloidal/nanoparticle suspensions. Every coating solution formulation has its own advantages and disadvantages . Thin film deposition that can be done at atmospheric pressure upon minimal equipment price is the next step in a solution-processing technique. There are several scalable deposition approaches that have been named in Figure 2a and make it possible for uniform and large-area coverage. Those deposition techniques have been detailed by Pasquarelli et al. . Thermal processing of thin films that is performed for (i) decomposing the precursors and removal any organics, (ii) crystallization or phase transformation and (iii) controlling electronic carrier concentration, is the last step of solution-processing approach. Thermal processing can be carried out by different methods such as on a hotplate or under an adjusted environment with a tube furnace. In the thermal processing of nanoparticle thin films, typically there is a difference between curing and sintering . The curing temperature is described as a temperature where particles drop their organic capping shell and begin displaying conductivity using direct physical contact. However, sintering is additional heating and it happens at a higher temperature and leads necks are formed between particles. Therefore, sintering results in a dense film rather than nanoparticulate networks.
Chen at al.  reported a facile solution-processing method for producing a 2D and layered organic-inorganic perovskite (Figure 2b and 2c). In order to synthesis those nanostructures, briefly, they dissolved PbBr2 and C4H9NH3Br in dimethylformamide (DMF) via a two-step solution process to yield the perovskite precursor. The precursor directly crystallizes into a 2D cross star and layered cubic (C4H9NH3)2PbBr4 perovskites upon evaporation (Figure 2b and 2c). The fabricated 2D cross star nanostructure has large sizes of ultrathin 2D perovskites with high stability and significant photoelectric conversion efficiency. That pioneering research work provides a controllable synthesis of new 2D and layered perovskite structures and advantages the improvement of optoelectronic devices primarily based on ultrathin 2D perovskites.
Figure 2. (a) A flow chart of the typical solution-processing methods. (b) two-step solution process in the fabrication of 2D (C4H9NH3)2PbBr4 perovskite. (c) SEM images represent the different morphologies of 2D and layered (C4H9NH3)2PbBr4 perovskites that grown at different DMF ratios. Panel (a) reproduced with permission from data published in ref. . Copyright 2011, The Royal Society of Chemistry (RSC). Panel (b and c) reproduced with permission from data published in ref. . Copyright 2017, John Wiley & Sons, Inc.
ALD or atomic layer epitaxy (ALE) is usually a chemical deposition technique for fabricating a high-quality, large surface area and thin film layers with atomic size precision . Figure 3a demonstrates an ALD method where the thin film layers cyclically grow to get a binary compound from gaseous precursors. ALD is often a surface controlled LbL thin film deposition approach. It has the greatest prospective for developing a thin film where thickness and film composition is normally controlled at the atomic level. ALD can create a wide variety of film components with high density, low impurity and precise control of thickness. As the same time, a low deposition temperature can relieve adverse influences on sensitive substrates. ALD has been employed for the deposition of selective metals or metal oxide components on 2D nanomaterials such as deposition of platinum (Pt) at graphene line defects  or Al2O3 on 2D transition metal chalcogenides (TMDs) . The ALD technique has been extensively considered by researchers in nanotechnology areas because of its govern over the deposited thin film thickness of materials and the possible for adjustment of chemical and physical properties in the nanoscale range . For instance, Wirtz et al., deposited ~ 2 nm Al2O3 on pristine molybdenum disulfide (MoS2) and tungsten disulfide (WS2) via ALD method (Figure 3b) that is crucial for electronic applications. Their study revealed that it is feasible to reliably seed the deposition using non-covalent functionalization with perylene derivatives such as perylene bisimide as anchor unit (Figure 3b and 3c). Their finding also indicated that the deposition reactivity is dependent on the number of TMD layers.
The slowness of deposition is the main restriction of the ALD method; usually, a portion of coverage is deposited in each cycle. Nevertheless, a very thin film layer is required for next-generation electronic devices and hence the slowness of ALD technique may not a crucial issue.
Figure 3.(a)a schematic illustration of an ALD approach where the thin film layers cyclically develop for a binary compound from gaseous precursors. (b.i) AFM evaluation of TMDs just after ALD. (b.i) the topography of a WS2 triangle immediately after 27 cycles of TMA and H2O. The monolayer component of the triangle lies lower than the surrounding SiO2. (b.ii) line profile along the marked line in (b.i). The step height in between monolayer WS2 and SiO2 is 2.3 nm and between monolayer WS2 and bilayer WS2 is 3.7 nm. (b.iii) a schematic illustration of the surface structure in (b.i) as displayed by the AFM scan. (b.iv) the topography of MoS2 triangles after 27 cycles of ALD, after seeding with a perylene bisimide derivative. The flakes are higher than the surrounding substrate. (b.v) line profile along the line marked in (b.iv). The step height is 1.9 nm. (b.vi) a schematic illustration of the structure of (b.iv) as demonstrated by the scan. (c) chemical structure of the perylene bisimide that has been used for non-covalent functionalization and ALD seeding. Panel (b and c) adapted with permission from data published in ref.  Copyright 2015, The Royal Society of Chemistry (RSC).
MLD is another chemical deposition technique. Similar to ALD, MLD is also a gaseous-phase process and was emerged for deposition organic constituents in nanoscale films. In fact, MLD has opened a new venue for synthesizing novel nanostructured materials. This method also can grow high-quality thin film layers with controllable thickness and composition on the molecular-scale size. Figure 4a illustrates a MLD technique using two homobifunctional precursors. The molecules of one precursor react with surface by corresponding interlocking and chaining chemistry to add molecular layer of the precursor on the surface with new reactive sites . This process is continuing with the molecules of another precursor until the thin film layers cyclically grow to get a binary compound from gaseous precursors. MLD method has been combined with ALD technique to fabricate new types of organic-inorganic hybrid nanostructures . Jaggernauth et al.  functionalized surface of nanographene oxide with organic-inorganic polymers using MLD method with TMA (as metal precursor) and ethylene glycol (EG, as counter reactant) reagents (Figure 4b). The obtained analysis showed that an approximate thickness of 5, 8.5 and 12.5 nm were gained after 20, 50 and 80 cycles, respectively, of MLD with TMA and EG precursors. Figure 4c indicates the scanning electron microscopy (SEM) image of the deposited hybrid polymers after 80 MLD cycles. The functionalized nanographene oxide is highly favorable for the development of novel advanced applications such as energy-related materials, sensors or biomedical applications, because of its superior electrical, mechanical and thermal properties .
Figure 4.(a)A schematic illustration of an MLD approach where the thin film layers cyclically develop from gaseous precursors. (b) A schematic illustration of growing hybrid organic-inorganic polymers on nanographene oxide using TMA and EG precursors, by MLD technique. (c) SEM image of the deposited hybrid polymers on nanographene oxide after 80 MLD cycles. Panel (a) adapted with permission from data published in ref. . Copyright 2009, The American Chemical Society (ACS). Panel (b and c) reproduced with permission from data published in ref. . Copyright 2016, The American Chemical Society (ACS).