Coating strategies for atomic layer deposition

2.1 Zero-dimensional nanomaterials

The fabrication of low-dimensional materials is one of the key requirements for nanofabrication technology. Because of the high conformality of ALD films on complex structures with precise thickness control, the nanocoating has emerged as an important application of ALD. It is particularly interesting as the nanocoating can change the optical, chemical, and mechanical properties of the nanoparticles. For example, titanium dioxide (TiO₂) ALD layer is used for protective coating of Si-based anodes for high-energy Li-ion batteries [41]. As the TiO₂ buffer layer effectively affords Si volume changes inside the spheres during the repeated charge-discharge process, the aggregation of Si particles was greatly suppressed. Therefore, high conductivity without sacrificing structural properties was achieved. Further fine tuning the film structure by post-annealing is supposed to stabilize the nanoparticles with accessibility to the surface sites and contributes to the formation of bimetallic materials. For instance, O’Neill et al. [42] confirmed that high-temperature treatment opens porosity in the overcoat by forming crystallites of γ-Al₂O₃ on Cu nanoparticles. This ALD overcoat on copper shows remarkable stability by selective armoring the under-coordinated copper atoms on the nanoparticle surface. Lu and co-workers [26] further verified it by annealing the Pd catalyst at a high temperature after the Al₂O₃ coating. The diffuse reflectance infrared spectra of CO clearly indicated that the Pd catalyst with ~8-nm ALD Al₂O₃ overcoats became porous after the high-temperature treatment, which contained ~2-nm pores after activation as depicted by the model in Figure 4C. Tests showed that the porous Al₂O₃ film selectively blocked the edge and corner atoms of Pd particles, and separated the catalyst and the reaction product, while keeping the catalyst surface accessible. As pictured in Figure 4D, the porous Al₂O₃ overcoat trapped and stabilized the Pd NPs, inhibiting Pd NP leaching and coke formation, which greatly enhanced ethylene formation. While for the uncoated Pd/Al₂O₃ catalyst during oxidative dehydrogenation of ethane (ODHE) reaction, the filamentous carbon (thick black lines) plugged the reactor, and there was substantial sintering and leaching of Pd NPs from the support (barred white lines), as pictured in Figure 4B. Thus, the catalyst clearly inhibited coke formation and improved the thermal stability of Pd NPs at high temperature. Ramachandran et al. [43] further utilized the energy provided by thermal treatment to rationally design and tailor bimetallic catalysts. They first deposited a Pt/In₂O₃ bilayer by ALD and then thermally treated the bilayer in hydrogen. The ALD-deposited bilayer transformed into alloyed Pt-In particles because of atomic migration and coalescence at the support surface. This established a new way for fully tailored synthesis of bimetallic nanoparticles, with accurate control over particle size and alloy composition by changing the thickness of the ALD-grown Pt/In₂O₃ bilayer.

Figure 4:

Schematic model of Pd/Al₂O₃ catalysts with and without an ALD Al₂O₃ overcoat during the oxidative dehydrogenation of ethane reaction at 675°C. (A) The uncoated Pd/ Al₂O₃ catalyst. (B) The uncoated Pd/ Al₂O₃ catalyst during ODHE reaction. (C) The Pd catalyst with an ~8-nm ALD Al₂O overcoat. (D) The ALD Al₂O₃-overcoated Pd/Al₂O₃ catalyst during ODHE reaction. Reprinted with permission from Ref. [26]. Copyright 2012 American Association for the Advancement of Science.

Because of the high surface area of 0D nanoparticles, the duplication of the particle structures is particularly interesting. With a sacrificial carbon nanosphere template (Figure 5), Hong and Sang [40] fabricated nanocrystalline anode materials made of mesoporous anatase TiO₂. They accomplished conformal deposition of a nanoscale-thick TiO₂ layer on a carbon black template surface and, subsequently, removed the template by thermal calcination. The calcination and subsequent sintering process generated mesoporous TiO₂ and further transformed the TiO₂ particles into spherical nanocrystallites. This mesoporous nanostructure with a large surface area exhibited a greatly enhanced reversible capacity, approaching the theoretical value. Furthermore, owing to the thermally activated catalytic effect of the TiO₂ shell, the inner carbon black template can be removed without leaving any residue at relatively low temperature. However, the pure carbon black template without an ALD coating could only be completely burned at a high temperature over 700°C.

Figure 5:

(A) (Top) schematic illustration of ALD assisted sacrificial template method for TiO₂ nanocrystalline anode synthesis. (Down) SEM and TEM images of the materials obtained at each processing step. HRTEM images of TiO₂ produced from the carbon black encapsulated by ALD coating layers with (B) 3- and (C) 10-nm thicknesses. Reprinted with permission from Ref. [40]. Copyright 2015 Elsevier B.V.

2.2 One-dimensional nanomaterials

The 1D materials with nanoscale structures have shown a variety of applications due to the large surface-to-volume ratio and quantum confinement [44], [45] effects. One of the widely used ways to form the 1D structure is through template. Based on the template, coating the surface of a 1D template is able to confine lateral dimensions of the materials, and subsequently, hybrid and hollow structures with novel physical and chemical properties can be synthesized by etching of the core and reacting with the shell.

Direct coating on the template is the common way for the preparation of 1D nanomaterial preparation. For example, Woo et al. [46] successfully fabricated sub-5-nm nanotrenches and nanowires by transferring a CNT pattern. Borbón-Nuñez et al. [47] fabricated TiO₂ tubes by coating the multiwalled carbon nanotube (MWCNTs) surface with TiO₂ using ALD. They demonstrated that thick-walled (20 nm) TiO₂ tubes can be obtained by direct ALD TiO₂ coating, while for thinner-wall tubes (10 nm), a very thin Al₂O₃ buffer layer was inserted to enhance the structural stability of the TiO₂ tubes.

Among the 1D templates, polymer-based templates have the advantage for their being easy to remove and their flexibility when constructing various structures. Its template is often formed by electro-spinning or sputtering to maintain the nanoscale-sized templates. The surface of these densely packed polymers is often covered with highly reactive groups, and the inorganic film rapidly nucleates on the polymer surface, thus, forming a conformal inorganic coating. Because of the 1D morphology of spun polymers, they are more applicable to 1D core-shell structures, as well as the hollow structure by etching the core of their structures, for example, the organic-inorganic core-shell nanofibers of ALD GaN on electrospun polymeric nanofibers [48] and the hollow nanotubes with tunable diameters and wall [49]. This generation of hollow nanofibers often involves three processes: electrospun polymer templates, ALD shell layer formation, and selective removal of the polymer templates, as depicted in Figure 6. Kayaci used ZnO as a model semiconductor and systematically characterized the effects of calcination on the morphology and crystallinity of ALD ZnO on nylon 6,6 nanofibers [50]. They suggested that an excessive increase in surface area cannot result in an equivalent enhancement of the photocatalytic activity, and a careful balance of the surface area and defect density is required. However, if the polymer is amorphous, and the functional groups are nonreactive, the precursors can diffuse into the polymer and react within the film, resulting in an embedded inorganic phase in the organic bulk.

Figure 6:

Schematic diagram depicting the steps involved in the production of core-shell and hollow nanofibers. Reprinted with permission from Ref. [50]. Copyright 2015 Elsevier B.V.

As for organic materials, the template is often removed by solution dissolve or post calcination. Guan et al. [51] coated ZnO and TiO₂ on the SnO₂ nanowires by ALD, then dissolved the ZnO sacrificial layer to form a hollow and conformal TiO₂ shell. This surface-engineered method used an SnO₂ nanowire as the template and obtained a new type of hollowed SnO₂-in-TiO₂ wire-in-tube nanostructure. This structure radically improved the rate capability and cycling stability, compared to both bare SnO₂ nanowire electrodes. It can be attributed to the uniform TiO₂ shell protection, as this protection layer can simultaneously stabilize the solid electrolyte interphase layers and accommodate the volume expansion of the SnO₂ core wire. Deng et al. [45] systematically investigated the formation of porous metal oxides (Al₂O₃, TiO₂, V₂O₅, and ZnO) with different morphologies using atomic layer deposition on multi-walled carbon nanotubes followed by post deposition calcination. The morphological transformations during calcination are summarized in Figure 7. In addition, they pointed that the crystallization temperature and the surface coverage of the metal oxides and the oxidation temperature of the carbon nanotubes have significant influence on the final morphology. Lotfabad et al. [52] coated TiO₂, TiN, and Al₂O₃ ALD films on the outer or inner surfaces of hollow Si nanotubes and demonstrated that the coated composites have a positive influence on the cycling performance of Si nanotube electrodes, and they show relatively little structural damage and good cycling stability.

Figure 7:

Schematic demonstration of the formation of porous metal oxide nanostructures by ALD on MWCNTs followed by calcination. Reprinted with permission from Ref. [45]. Copyright 2014 Royal Society of Chemistry.

Except for the eliminating process combined with ALD coating for 1D nanostructure fabrication, other processes on inorganic templates are also important for 1D nanostructure fabrication. For instance, Guan et al. [53] constructed a novel hollowed wire-in-tube nanostructure of CoO-in-CoSnO₃ through ALD. They first obtained Co₂(OH)₂CO₃ nanowires by a hydrothermal method and then deposited ALD SnO₂ on it. This CoO wire-void-CoSnO₃ tube was formed through a proper thermal treatment, in which the Co₂(OH)₂CO₃ decomposed and reacted with the outer SnO₂ layer simultaneously. Because of the hollow feature of CoO-in-CoSnO₃, it showed much better electrochemical performance compared with the bare CoO nanowire. Jin fan et al. [54] demonstrated the fabrication of high-aspect-ratio nanotubes based on the Kirkendall effect. They transferred the ZnO-Al₂O₃ core-shell nanowires with a uniform ALD coating of the amorphous Al₂O₃ into a single-crystal ZnAl₂O₄ spinel nanotube through a solid-state reaction, which is based on the Kirkendall effect. The principle is that when the out-diffusion of the core material through the spinel is faster than the in-diffusion of the shell material, the Kirkendall voids generate. As the reaction is completed, it leads to hollow spinel nanotubes. The use of the Kirkendall effect allows a rational design of nanoscale hollow objects, providing another approach to the fabrication of nanotubes of a variety of materials.

2.3 Complex nanostructures

Because of the self-limiting reactions, ALD is able to grow uniform and conformal films on all coated surface in a precisely controlled way. This capability provides a straightforward way for tuning the properties of nearly any surface. In this sense, ALD is a powerful tool for the synthesis of nanostructured materials or functionalized nanostructured materials.

Replication of porous template structure is a valid way for nanostructure fabrication and has been used for surface functionalization. Anodic aluminum oxide (AAO) template has high potentials in various nanofabrication processes for its high aspect ratio and porous features. For example, Haider’s group [55] grew electrochemical polycrystalline bismuth (Bi) nanowires in highly ordered porous AAO templates. They replicated the AAO template structure by ALD SiO₂ and electrochemically depositing Bi nanowires on it. Results showed that the ALD SiO₂ not only replicated the AAO structure with accurate size control (30 nm) but also acted as a protective layer on the surface of the wires. Another classical application of AAO is the nanoarray fabrication through structure replication. For example, Yao’s group [44] fabricated TiO₂ nanowire arrays by replicating the AAO template (Figure 8). After the fabrication of nanoporous AAO templates, TiO₂ thin films were grown by ALD and sputtering, followed by selective removal of the aluminum substrate and AAO template via a wet etching process. Through this method, large-scale and free-standing TiO₂ nanowire arrays were formed. They also confirmed that the sputtered TiO₂ formed mushroom-like structures on the AAO template surface, while the ALD-coated TiO₂ filled the AAO holes finely, exhibiting clear crystal grain boundaries. By using diverse AAO templates, it is simple to synthesize highly ordered TiO₂ nanowire arrays with desired parameters like diameter and density.

Figure 8:

Schematic diagram of the processes to produce nanowire arrays. Reprinted with permission from Ref. [44]. Copyright 2015 Springer.

In addition to nanowire arrays, other complex structures based on the AAO template have also been presented, e.g. the sharp-edged crescent-shaped half-nanotubes (HNTs) [56], the tube-in-tube nanostructures and multisegmented nanotubes. The process is depicted in Figure 9. By controlling the removal of the sacrificial ALD layer onto the AAO template pore walls, this method allows convenient control over the morphological properties, such as edge sharpness, gap size, and wall thickness of HNTs. Gu et al. [57] accomplished successive fabrication of tube-in-tube nanostructures by growing ALD nanotubes on the interior template walls of the AAO nanochannels. By alternate removal of Al₂O₃ sacrificial spacers to release nanotubes replicated from the AAO template, multiple-walled coaxial nanotubes of five nested layers were synthesized within AAO templates. The resultant coaxial nanotube structures are highly ordered and absolutely conformal with the template pore structure and can achieve very high aspect ratios. Bae et al. [58] further described a general strategy for fabricating multisegmented nanotubes and nanopores via sequential, surface-selective modification with organic and inorganic layers combined with in situ formation of nanopores by electrochemical anodization. First, the octadecyltrichlorosilane (OTS) self-assembled monolayers (SAMs) were formed on the alumina templates as the first organic segment. Subsequently, the SAM-coated pores were separated from the underlying Al when the Al foil was anodized. As the inert surfaces of methyl-terminated OTS-SAMs hinder the desired ALD reactions, the freshly grown pore section was selectively coated with an inorganic layer (TiO₂, ZnO, or ZrO₂) using ALD. In this way, the inner surfaces of nanopores can be chemically and physically modified in a segmented manner with organic/inorganic materials.

Figure 9:

(A) Cross-sectional SEM image of an AAO template after electrodeposition of Au into the crescent-shaped nanochannels created with ZnO sacrificial layer. (B, C) Top-view SEM images of the HNT arrays released by dissolving the template. (D) A close-up of the HNTs. Reprinted with permission from Ref. [56]. Copyright 2011 American Chemical Society.

Based on the templates and ALD technology, combining with another solution growth, it is capable of constructing complex architecture. For example, Karuturi and co-workers [59] designed a new type of nanowire-inverse opal hybrid architecture involving the assembled colloidal sphere templates, the ALD TiO₂ film, the ALD ZnO seed layer, and the ZnO nanorods by standard solution growth. The process is depicted in Figure 10. The bilayer nanostructures were in intimate physical contact with high crystallinity, and the photocurrent performance of this bilayer was evidently improved. Kayaci et al. [60] explored the electrospun poly(acrylonitrile) nanofibrous as the templates, and grew the single crystalline ZnO nanoneedles with high optical quality on the ALD seeds via hydrothermal process. This novel structure showed enhanced photocatalytic activity because of the catalytic activity at surface defects (on ALD seed), valence band, and conduction band (of ZnO nanoneedles). Cheng et al. [61] fabricated the 3D hollow urchin nanowire structures by a combination of colloidal sphere self-assembly, atomic layer deposition, and subsequent hydrothermal growth. ALD TiO₂ and ZnO were deposited on self-assembled monolayer polystyrene microspheres at low temperature. The ALD TiO₂ acts as a scaffold, and the ALD ZnO layer is used as a “seed layer” for promoting ZnO nanorods growth. Because of the advantages of the mixed structure, it offers a higher surface area for quantum dot loading. Accordingly, experiments demonstrated that these 3D urchin-like structures generate a photocurrent higher than that of 2D TiO₂/ZnO inverse opals.

Figure 10:

Schematic of the fabrication procedure of bilayer photoanodes in which TiO₂ inverse opal is coupled with ZnO nanowire arrays. Reprinted with permission from Ref. [59]. Copyright 2012 Elsevier.

Except for the AAO-directed coaxial nanostructures and inverse opal, many other 3D networks have been reported in the past years, including metal foam, carbon fiber nanotube cloth, and ultrathin graphite foam (GF) [62]. For example, Guan et al. [62] directly confined Fe₂O₃ nanoparticles on 3D GF and utilized them as anode for Li-ion batteries. Because of the 3D nanostructure, this GF@Fe₂O₃ electrode demonstrated outstanding rate capability and cycling stability as the porous GF provides a stable, highly conductive support and a short ion diffusion length for fast reversible reaction. This function can further be extended to flexible carbon substrate, for instance, the NiO nanoparticles on flexible graphite foam-CNT forest substrate [63] and Co₃O₄ nanolayers on the flexible carbon nanotube/carbon cloth substrate [64]. The mechanically robust carbon-based substrate, the uniform ALD coating layers, together with the unique hierarchical structure contribute to the high areal capacitance and excellent cycling ability for supercapacitors. In addition, other porous materials with ALD coating layers are presented. Jang et al. [65] created the 3D hollow rigid lattices involved with the ALD TiN layer. The constituent solids attain tensile strengths of 1.75 GPa without failure even after multiple deformation cycles. Therefore, they suggested that the hierarchical design principles can be applied to create damage-tolerant lightweight engineering materials. Werner et al. [66] demonstrated the applicability of atomic layer deposition for the fabrication of multifunctional ordered mesoporous core-shell nanocomposites. Their results suggest that ALD can homogenously coat 3D co-continuous gyroidal mesoporous templates with well-defined pore sizes below 50 nm and thicknesses above 10 mm. However, challenges for the deposition of nanoscale 3D surface still exist. Incomplete saturation reaction may occur when the substrate has small openings and spaces, which limits the quality of the coating layers. Well understanding the precursor-surface reactions are needed to achieve the well controlled deposition within 3D structure, including the size and volatility of precursor and the thickness and composition of the substrate. As for the experimental aspect, varying exposure time or working pressure may achieve good saturation to reach the ideal conformality [67].

3.1 Surface activation

According to the growth mechanism, the ALD nucleation reaction only occurs at surface-active sites and defects. However, native sites are often not reactive enough to modify the desired reaction. For example, coatings on sp² carbon materials suffer non-coating or incomplete coating [68] due to the lack of dangling bonds for adsorption. It is shown that ALD reactions only occur at the step edges [69], [70] and surface defects [71] of pristine carbon materials, which form nanoparticles or nanowires. Therefore, surface activation is proposed as a method for creating or increasing the density of active sites and defects to promote nucleation.

Surface activation can carry out directly and indirectly. The direct surface activation often involves chemical covalent surface treatments, which manipulate the nucleation sites to increase and homogenize the growth of a film. This surface engineering aims at increasing the nucleation sites and defects, and it is a straightforward way to promote film deposition and help to transform the growth mode. For example, it has been demonstrated that the growth mode of the film can be controlled from selective decoration up to full coating, simply by controlling the degree of surface-defect engineering in the CNTs [72]. Zhu et al. [73] found that the rate of nucleation of Al₂O₃ is correlated with the amount of oxygen. They oxidized the black phosphorus samples by ambient air to modify the phosphorus oxide concentrations and found that this surface pretreatment promoted the Al₂O₃ nucleation and the uniform film growth. After the formation of phosphorus oxide, the surface reactivity changed, and the defects increased, which is helpful for Al₂O₃ nucleation. In addition, the authors also suggested that adventitious contamination of the surface could enhance the nucleation of Al₂O₃. This oxidized method works by decreasing the chemical inertness of a substrate [74] and creating new oxygen-incorporated nucleation sites [75], [76]. The oxidants include the mineral acids and the strong oxidants such as O₃ [77] and O₂ plasma [78]. Meng et al. [79] chemically functionalized CNTs prior to ALD-Fe₂O₃ by HNO₃, and the surface reactivity of CNTs is significantly enhanced. However, the as-deposited Fe₂O₃ was random and non-uniform, while the incorporation of N atoms in CNTs induces the deposition of uniform ALD-Fe₂O₃.

Although the chemical covalent functionalization method provides a powerful way to enhance the surface activity, it has some limitations, for example, the deterioration of electrical and structural properties of the pristine nanotubes and graphene. Thus, non-destructive functionalization methods are preferable. This indirect functionalization method is mainly based on supramolecular complexation [72], and it belongs to the class of secondhand surface pretreatments. By using various adsorptive and anchoring forces, precursor adsorption on the substrate is enhanced. One example is the use of hydrogen bonding, through which the precursor is physically adsorbed on the sp² carbon substrate. This creates nucleation sites for the latter reaction, even when the deposition temperature is relatively low. Zheng and co-workers [80] grew Al₂O₃-doped HfO₂ films on graphene by H₂O-based atomic layer deposition. Their results showed that predosing graphene with water increased the concentration of nucleation sites and introduced no defects. The dielectric film could act as a seed-like layer for the following film growth at 200°C. Moreover, Al₂O₃-doping could act as a network modifier to maintain the amorphous structure of Al₂O₃-based HfO₂ stacks on graphene, even at 800°C. Besides the hydrogen bonding, other forces such as van der Waals and π-π interactions, are also helpful for reinforcing the bonding between the surface and precursor. For instance, through nondestructive π-π stacking of a reactive molecule (4-mercaptophenol) on a graphene surface, Han et al. [81] obtained uniform and conformal ZnO thin films on graphene, which have good quality with a low density of pinholes.

It is reported that ALD deposition occurs preferentially in areas where the material has already been deposited in preceding cycles [72]. Therefore, the ALD seeding layer is of considerable importance to thin-film formation. The seed layer can even provide a non-destructive way to determine the overall properties of the resulting film [82]. Farmer and Gordon [83] found that trimethylaluminum (TMA) reacts with NO₂, forming a stable complex on the single-walled carbon nanotube (SWNT) surface at room temperature. After the completion of the 50 cycles of NO₂-TMA functionalization and 5 cycles of Al₂O₃ ALD stabilization steps, the structure can be used as a scaffold for growing many other materials by ALD. Young and co-workers [84] confirmed that Al₂O₃ islands produced by NO₂/TMA pretreatment help to increase the density of nucleation sites and facilitate the onset of Al₂O₃ growth. Furthermore, the Al₂O₃ films grown after the NO₂/TMA pretreatment also showed a reduced thickness and better resistance. Bielinski and co-workers [85] showed that ALD is a reliable surface modification method to guide nanowire orientation. They grew hierarchical ZnO nanowires by ALD seeding. The seed layer was used to promote heterogeneous nucleation of the nanowires. By simply varying the cycles of ALD ZnO, the morphology of hydrothermally grown ZnO nanowires could be tuned reliably. They concluded that the texture of the seed layer influenced the orientation of the resulting nanowires.

3.2 Surface blocking

In contrast to surface activation with the aim of promoting surface reactions, surface blocking is designed to block the surface site areas selectively. The selective process is unique in that no etching steps are needed for selective deposition. From the perspective of basic principle, the key for achieving this blocking is to weaken the dissociative chemisorption of co-reactant molecules on substrate surface. On this basis, several strategies are adopted. One is to adjust the ALD process parameters, such as the temperature, pressure, and substrate materials [30]. Weber et al. [86] reported a novel strategy for synthesizing Pd/Pt core/shell nanoparticles supported on Al₂O₃ substrates. As the dissociative chemisorption of O₂ only takes place on the platinum group metals but not on oxide support material, they achieved selective deposition of Pt only on the surface of Pd. Furthermore, they found that the shell growth takes place only when the size of the Pd core is larger than 1 nm. Their results clearly demonstrated that ALD enables precise and independent control of the core and shell sizes of bimetallic core/shell NPs. Lu et al. [87] explored a variety of deposition temperatures, co-reactants, and pulse-sequencing strategies to achieve the desired selectivity. They synthesized supported bimetallic nanoparticles by properly selecting the deposition temperature and the appropriate co-reactant. The second metal only deposited on the surface of the first metal, while the monometallic nanoparticle formation is avoided. Meanwhile, the size, composition, and structure of the bimetallic nanoparticles were precisely controlled by tailoring the precursor pulse sequence.

Unlike controlling the ALD process parameters, Yanguas-Gil et al. [88] proposed a different surface-functionalization step to control the dopant distribution and achieve higher doping efficiencies. By adding ethanol (EtOH) to the conventional precursor/coreactant ALD sequence, they reduced the density of surface reactive sites and, therefore, effectively achieved controlled, submonolayer saturation coverage. As EtOH displaces surface hydroxyls and blocks potential adsorption sites for the TMA, the growth of Al₂O₃ was inhibited, but they are removed during the H₂O exposures, allowing the Al₂O₃ ALD to proceed. When used to deposit Al-doped ZnO, this functionalization/precursor/coreactant sequence ALD films exhibited carrier concentrations two times higher, on average, than conventional ALD films of similar Al concentration.

However, the main stream to achieve area selectivity is to use the spatially controlled blocking resist [89]. According to the molecular weight, the selective strategy can be divided into processes involved with molecular blocking agents and self-assembled monolayers (SAMs). The molecular blocking agent originates from the large molecular groups or generated by-products. These bound groups are chemically inert to precursors that occupy the active sites of the substrate and prevent the subsequent adsorption process. In the study of the formation of copper nanoparticles on zinc oxide powder [90], results showed that when the surface runs out of available hydrogen, the remained hexafluoroacetylacetonate ligands hinder continuous copper film formation. In addition, novel nanostructures can be fabricated with the blocking agent. For instance, Ray and co-workers [91] developed a strategy to synthesize nanobowls by utilizing this blocking effect. This method involves the growth of uniform and dispersed ALD Pd nanoparticles, blocking agent for protecting the particles, the use of ALD in the formation of metal oxide structure around the nanoparticle, and the removal of blocking agent. This nanobowl is very advantageous in catalysis [92] because the unique, feathered structure imparts high catalytic activity and facilitates simultaneous separation of reactants and catalysts. Cheng et al. [93] designed a novel structure to stabilize Pt catalysts by area-selective ALD. They formed a nanocage structure (Figure 11) by selectively growing ALD zirconia around the Pt NPs. The Pt NPs were initially deposited on nitrogen-doped carbon nanotube (NCNT) by ALD, followed by the application of a blocking agent (oleylamine) to the Pt NP surface. The ALD zirconia was not deposited on the Pt surface due to the blocking agent. As the zirconia nanocages prevented the migration and agglomeration of Pt NPs on the support, the Pt NPs encapsulated in a zirconia nanocage show nine and 10 times more stability than ALD Pt/NCNT and Pt/C catalysts, respectively. Additionally, this catalyst exhibited an oxygen reduction reaction (ORR) activity 1.4 and 6.4 times greater than that of ALD Pt/NCNT and Pt/C, respectively, due to the small-size Pt NPs and the synergetic effects of Pt and ZrO₂.

Figure 11:

Schematic diagram of zirconia-encapsulated platinum nanocage structure fabricated by area-selective ALD. Reprinted with permission from Ref. [93]. Copyright 2014 John Wiley and Sons.

In addition to the molecular blocking agents, SAMs are the common blocking agents used for area-selective deposition. With SAMs, area-selective ALD greatly expands the applicability of ALD for fabricating novel structures and can be readily applied to the growth of other compositions. Generally speaking, SAMs consist of the following three parts: a long organic molecule backbone, head, and tail groups. SAMs offer a way to achieve area-selective ALD by alternating the appropriate tail group to block ALD precursors from attaching to the surface. For instance, through the deposition of octadecyltrichlorosilane (ODTS), the SiO₂ substrate was changed from hydrophilic to hydrophobic with the replacement of -CH₃ to -OH to cover the substrate [94]. Generally, SAMs are hydrophobic and serve as a layer to inhibit nucleation. For example, Cao and co-workers [94] utilized the commonly used ODTS SAMs to modify the surface. As the active sites were occupied by SAMs, the pinholes on the SAMs were used as active sites for the initial core nucleation. Subsequently, the second metal was selectively deposited as the shell layer. In the second stage of deposition, as the nucleation sites can be effectively blocked by surface ODTS SAMs, Pd/Pt and Pt/Pd NPs with uniform core-shell structures and a narrow size distribution were successfully formed. As both processes were conducted by ALD, sample transfer was avoided, and independent engineering of the dimensions of the catalyst can be easily achieved.

Except for the nucleation-inhibition layer for 0D coatings, SAMs can be utilized to grow planar or even more complex structures. For instance, Minaye Hashemi et al. [95] employed a strategy to selectively deposit films on Cu/SiO₂ substrates, as shown in Figure 12A. By using the intrinsically selective adsorption of octadecylphosphonic acid SAMs on Cu over SiO₂ surfaces, they selectively patterned a resist layer only on the Cu surface. Then, they applied ALD Al₂O₃ on the patterned surface and removed any residual dielectric film deposited on the Cu surface, leaving the dielectric Al₂O₃ film on SiO₂ unaffected. Moreover, it greatly expanded the thickness limits of selective deposition of dielectric materials on a metal/dielectric pattern, with an increase from 6 nm to 60 nm. However, things changed when ALD was applied to the non-planar surfaces. Chopra et al. [96] found that SAMs formed defects in the high curvature region, allowing nucleation of ALD TiN films. This is in contrast to the SAM covering planar surface, which exhibits a complete blocking TiN film. Similar to Minaye Hashemi et al. [95], Dong et al. [97] achieved the selective coating of the vertical surfaces of standing nanopillars. By utilizing the hydrophobic SAMs, they selectively inhibited the ALD ZnO coating on the modified horizontal regions, ensuring that only the vertical surfaces of the vertical standing nanorods were coated, as depicted in Figure 12B. These results further confirmed that ALD is a feasible process by artfully exploiting hydrophobic SAMs to tune the area-selective formation of ALD films on the horizontal and vertical surfaces of substrates.

Figure 12:

(A) Schematic demonstration of self-aligned patterning through a combination of selective deposition and selective removal of dielectric material. Reprinted with permission from Ref. [95]. Copyright 2015 American Chemical Society. (B) Schematic diagrams of the 3D (area-selective) ALD of a metal oxide on the vertical surfaces of standing nanopillar structures. Reprinted with permission from Ref. [97]. Copyright 2014 Nature Publishing Group.

Direct patterning the SAMs presents another effective way for area-selective ALD as the morphology of the patterned films depends on the SAM structures. This direct manufacturing on SAMs can be accomplished through various means such as microcontact, energetic beams, and so on. For instance, Färm et al. [98] prepared patterned SAMs using an elastomeric stamp, which transferred the patterned SAMs to the surface within just seconds. This fast microcontact printing process eliminates the need for a separate patterning process. They verified the passivation properties of the printed SAMs of iridium and TiO₂ grown by ALD processes and found the SAMs to be a well-patterned mask layer, with film growth occurring only in the non-SAM areas. Hua et al. [99] achieved pattern definition by thermal-probe-based lithography strategies. They utilized heated cantilever probes to locally decompose the polymer film, which served as a template for subsequent ALD growth, resulting in line patterns to obtain nanowire structures. In addition to rational utilization of their properties, SAMs can aid the construction of nanostructures. Huang and co-workers [100] applied the electron-beam to the previous self-assembled ODTS, then, selectively grew ALD films on places that are not covered with OTDS. With SAMs, area-selective ALD greatly extends the applicability of ALD for fabricating novel structures and can be readily applied to the growth of complex compositions.

However, challenges still exist as the SAMs are sensitive to high temperature and degrade over time. For example, if the exposure time is long enough, the ODTS will degrade. Therefore, decreasing the ALD film deposition temperature while maintaining the film qualities is of great importance [89]. For example, the good-quality Al₂O₃ thin films were prepared on polyethylene naphthalate substrates at 35°C without damaging the substrate [101], and the compact ALD TiO₂ layers were fabricated on perovskite solar cells with a power conversion efficiencies as high as 11.2%. Therefore, the growth of high-quality thin films at low temperature is very attractive for heat-sensitive films [102].

4.1 Reactor extension and redesign

4.1.2 Fluid bed reactor

The novel chemical and mechanical properties of nanoparticles are of interest, but their high surface energy limits their applications. Coating particle surface by ALD has proven to be an effective way to maintain their unique properties. However, due to the difficulty of precursor infiltration, the general ALD process does not result in uniform coating of the unsupported particle surface. Changing the reactor design by adding the fluid bed reactors (FBRs) is expected to address this problem. With FBRs, ALD can directly modify the particle surface while retaining its properties. The fluidized bed reactor has many advantages, including good mixing, large gas solid contact area, high efficiency for mass and heat transfer, and large-batch processing capability. By rigorous mixing of the nanoparticles through suspension and vibration, FBRs transform nanoparticles into a fluid-like state. This intimate contact improves the vapor solid contact efficiency and the heat transfer coefficients, and reduces particle aggregation. Moreover, the operation increases the precursor residence time, thus, dramatically improving the precursor utilization from 78% to nearly 100% [111]. A comparison of conventional ALD and FBR-ALD is illustrated in Figure 13 [112].

Figure 13:

Removal of molecules in a “conventional” ALD reactor with a single substrate and a fluidized bed ALD reactor filled with particles. Reprinted with permission from Ref. [112]. Copyright 2016 Springer.

The assistance of FBR is essential for fabricating ALD thin films on core-shell nanoparticles, without the use of SAMs and templates [113]. Such a conformal and uniform film on fine particles is exemplified in Figure 14. The uniform, amorphous, and conformal iron oxide layers cover the entire surface of individual zirconia nanoparticles in the fluidized bed reactor. The excellent thickness performance, achieved through the violent mixing process, is especially suitable for functionalizing particle surfaces. For example, experiments have shown that dense and uniform nanoscale Al₂O₃ passivation layers can effectively protect Fe₃O₄ nanoparticles from oxidation, preserving their magnetic properties [111]. With this functional layer, this encapsulation strategy of FBRs is particularly appealing for improving the applications and performances of catalysts that require a resistance to thermal oxidation.

Figure 14:

HRTEM micrographs of iron oxide layers on zirconia nanoparticles. Reprinted with permission from Ref. [114]. Copyright 2008 Elsevier B.V.

4.2 Improved growth model

As changing the design of the reactor helps to promote the deposition process, modifications to conventional ALD growth model present new insights into film deposition and direct new ways for film coating. For binary films, the ALD process generally involves alternating purges of two precursors into the reaction chamber. The addition of another step to the growth model, namely, the ABC-type ALD becomes prevalent in several respects, such as control over particle size, further reducing and oxidizing of the species and the additional element to the film, etc. [115]. It must be noted that in the AB-type ALD, the particle size is controlled by the number of ALD cycles, while the ABC-type ALD has the capability to control the density of the nanoparticles within an extremely narrow size distribution. Masango and co-workers [116] verified that the ALD Ag from the ABC type maintained the particle size when increasing metal loading, while the ALD Ag from the AB type formed larger particles. In addition to the controlled particle size, the ABC-type ALD yields smoother and more continuous films at the same temperature. Griffiths et al. [31] reported the deposition of Au films using the AB-type ALD and ABC-type ALD, both at 120°C. With the additional H₂O purge step at this temperature, the ABC-type ALD process accelerates the transformation of gold oxide to gold and reduces film impurities by reacting with phosphate to form the volatile phosphoric acid. Kalutarage et al. [117] showed that the growth rate of a copper film is faster for the ABC-type ALD. As the three-step process can create nucleation sites on the surface, it does not even require Cu seed layers to promote the nucleation of Cu.

In addition, the ABC-type ALD can contribute to the low-temperature process as the growth of high-quality thin films at low temperatures generally involves the inactive reactive precursor, requiring plasma and other assisted means to lower the reaction energy. For example, at a low reaction temperature, O₂ will not react (or will react incompletely) in the AB-type ALD [118], but can do in the ABC-type ALD [115] as it changes the common ALD mechanism from ligand exchange to combustion and reduction. Lu and Elam [119] achieved ALD of Ru at a temperature as low as 150°, whereas the thermal ALD of noble metals is frequently performed at a relatively high temperature of 300°C. This ABC-type ALD sequence proceeds via dissociative chemisorption, combustion, and reduction for the Ru(EtCp)₂, O₂, and H₂ steps, respectively (Figure 15). This O₂+H₂ process is especially suitable for transforming the film from metal oxide to metal, for example, the formation of Ir [120] and Pt [121]. In general, the low-temperature ABC-type ALD involves a metal precursor, a strong acid, and an optional reducing reagent for metal ions. Metal ions in this category include silver, gold, palladium, platinum, rhodium, iridium, rhenium, tungsten, and others [122]. Except for the traditional reducing agents such as H₂, reagent combinations that release electrons during oxidative decomposition should also be included [123].

Figure 15:

Schematic model of ABC-type Ru ALD, using the sequence Ru(EtCp)₂-O₂-H₂ on a Ru metal surface. Reprinted with permission from Ref. [119]. Copyright 2015 American Chemical Society.