Nanostructure and Nanomaterial Characterization Review (PDF)

Nanotechnol Rev 2018; 7(2): 209–231 Review Chinedu I. Ossai* and Nagarajan Raghavan* Nanostructure and nanomaterial characterization, growth mechanisms, and applications https://doi.org/10.1515/ntrev-2017-0156 Received June 26, 2017; accepted August 19, 2017; previously...

Nanotechnol Rev 2018; 7(2): 209–231 Review Chinedu I. Ossai* and Nagarajan Raghavan* Nanostructure and nanomaterial characterization, growth mechanisms, and applications https://doi.org/10.1515/ntrev-2017-0156 Received June 26, 2017; accepted August 19, 2017; previously 1 Introduction ­published online November 27, 2017 The advances in nanotechnology over the years have made Abstract: Nanostructures are playing significant roles in nanostructures potential targets of the industries, due to the development of new functions and the enhancement of their unique characteristics that influence the physical, the existing functions of industrial devices such as sensors, chemical, electrical, and optoelectrical attributes of nano- transistors, diodes, lithium-ion batteries, and photovoltaic materials. The band gap, high exciton binding energy, cells, due to their piezoelectricity, biocompatibility, and piezoelectricity, biocompatibility, and pyroelectrical char- pyroelectrical characteristics. This research focused on the acteristics of nanomaterials such as ZnO, TiO2, SnO2, and review of the characteristics of different nanostructures Fe2O3 have made them suitable for innovative sensors, and nanomaterials such as ZnO, ZnS, MoS2, GO, TiO2, SnO2, anodes for lithium-ion batteries, photoanodes, and nano- and Fe2O3, their fabrication techniques, growth mecha- generators for electricity harvesting [1–5]. Nanostructures nisms, and applications. The factors affecting the growth such as nanowires, nanotubes, nanobelts, nanosprings, mechanisms, the crystallographic natures, growth models nanoribbons, nanorings, nanosheets, and nanoparticles of anisotropic nanostructures, and growth of nanocrystals, have found applications in non-linear optical devices, flat were also highlighted in this research. The existence of lat- panel displays, light-emitting diodes, lasers, logic gates, tice mismatch, differential thermal expansion, and high and transistors [6, 7]. These nanostructures have been deposition temperature, have affected uniform deposition able to acquire their distinctive characteristics via modifi- of nanoparticles on substrates and caused heteroepitaxy, cations in the morphology, sizes, and shapes, through the which has resulted in defective nanostructures. Although fabrication process that involves the application of tem- heteroepitaxy has negatively affected the characteristics perature, pressure, and chemicals such as catalysts and and potential applications of nanostructures, it has also substrates. Hence, the influence of temperature and opened new research frontiers for potential new applica- pH on a one-dimensional (1-D) polymer nanostructure can tions of nanostructures. The use of nanostructures for gas modify the material, to make it useable in drug delivery, sensing is attributed to the high specific area, change of electronic and optical devices, as well as a catalyst and resistance on exposure to gases, and high photoconduc- support for fabricating other nanostructures [7, 9–12]. tion abilities, while the photon-carrier collection abilities Fabrication of nanomaterials has been done by dif- and anti-reflectance qualities are vital for solar photovol- ferent physical, chemical, and mechanical methods that taic cells. Nanostructures have also been used as coating used temperature, pressure, and phase transformation to pigments to prevent corrosion of facilities, reduce urban create nanostructures that have greatly enhanced devices, heat islands and energy consumptions, due to the near- due to their revolutionized electronic, magnetic, optoelec- infrared (NIR) reflective characteristics. trical, and sensing capabilities. Different fabrication techniques such as wet chemical, physical vapor deposi- Keywords: anisotropic growth models; crystallographic tion, molecular beam epitaxy, pulsed laser deposition, nature; growth mechanisms; heteroepitaxy; nanofabrica- sputtering, metal organic deposition, electrode position, tion processes; nanostructures characteristics. and electrospinning have predominated the nanostruc- tures’ fabrication, to date [14–16]. However, there remain different problems such as the presence of defects in the *Corresponding authors: Chinedu I. Ossai and Nagarajan Raghavan, nanostructures, due to lattice mismatch that is caused Engineering Product Development Pillar, Singapore University of Technology and Design, Singapore 487372, Singapore, by heteroepitaxy in the fabrication process [17, 18]. This e-mail: [email protected] (C.I. Ossai); problem and others such as cost, poor throughput, non- [email protected] (N. Raghavan) uniformity of nanostructure, and risk, have impacted the 210 C.I. Ossai and N. Raghavan: Nanostructures and nanomaterials fabrication of nanostructures, despite the improvements from predominant nanomaterials such as ZnO, ZnS, SnO2, in their characteristics. Moreover, heteroepitaxial growth ZnTe, TiO2, and polymer hybrid materials. We also aim to of nanostructures and nanocrystals has been associated show the fundamental problems that have dogged experts with a strain relief mechanism that can result in surface working with nanostructures, to give insights to potential reconstruction, step bunching, faceting, and formation new areas of nanostructure research, while highlight- of lattice mismatch, which causes misfit dislocations ing the current applications and emerging applications. Despite the improvements achieved in solving the of nanostructures and nanomaterials. The models for problem of heteroepitaxy via fabrication of nanostruc- growth mechanisms, factors affecting the growth mecha- tures at low temperature, with buffer layers that can help nisms, and characteristics of the different nanofabrication in the distribution of the defects on the substrates, and the processes were also reviewed. It is expected that this work positive impacts of doping and reorientation of geometry will be a snapshot on the advances in nanostructure and [18–21], the nanostructures’ growth defect still stands as nanomaterial technology and a compendium for guiding the most difficult problem of nanoscience. As the defects future research. on the nanostructures affect their characteristics because of the non-uniformity of the distribution of the nanopar- ticles on the nanofabrication substrates , experts are consistently working out new techniques of fabricating 2 C  rystallographic nature nanostructures, to ensure that the defects are minimal. of nanostructures To this end, some researchers have utilized novel fabrica- tion techniques to affect the optical emission characteris- The crystallographic nature of nanostructures has been tic of ZnS nanowires , whereas the growth orientation fundamental to the qualities and characteristics that is of Silicon (Si) nanowires has been used to influence the exhibited by different nanostructures. The nucleation optical and electrical characteristics of semiconductor process, reaction of molecules, and crystallization of nan- devices developed with them. Similarly, the band gap, oparticles have been attributed to the structure of different electrical conductance, and mechanical properties have nanomaterials used in the industry today, hence, the need also been reported to vary in nanowires that were fabri- for understanding the crystallographic nature of nanoma- cated with the same techniques but have different growth terials. Anisotropic bonding in crystallographic structures orientations [13, 23–26]. Furthermore, varying the synthe- have been attributed to the 1-D nanostructures of many sis times in the fabrication process has also been used solid materials because it forms the building blocks to reduce the defects in the nanostructures , but the of the nanostructures in the nanofabrication process. problems associated with growth defects have not allowed Research has shown that surface bonding dominates the researchers to fully tap the potentials of nanostructures; anisotropic growth of ZnO nanoparticles [33, 34], which hence, the furtherance of research in this area. forms different pyramidal nanostructures and morpholo- The aspect ratio has been shown to favor nucleation gies at the critical bond length. Again, the existence reactions and the formation of nanocrystals that are of the unique symmetric atomic crystalline structures, fundamental to the enhanced characteristics of nano- in solid state atomic lattice, has been attributed to the structures. This characteristic has favored the develop- directional bonding interaction that have resulted in the ment of periodic nanostructures for photonic crystals, formation of different 3-D structures that exhibit differ- gratings, and light-trapping structures, which have ent chemical, physical, and mechanical characteristics enhanced anti-reflectance that have been used in pho-. Anisotropic nanoparticles have also interacted at tovoltaic cells. The aspect ratio, orientation, chemi- nanoscale to aid directional bonding, by the exchange of cal composition, and surface energy at the nanostructure valences and directional selective functionalization interfacial lattices and facets have made the materials to of particles, with the resultant geometric re-orientation of have low thermal conductivity [28–30], a quality that is nanostructures [36–38]. vital for thermoelectric devices and batteries with greater energy density. The objective of this study is to review the functional 2.1 ZnS structures and characteristic characteristics, growth mechanisms, and the industrial applications of some common anisotropic nanostructures ZnS, as an important semiconductor material with high such as nanocombs, nanorings, nanohelixes, nanosprings, index of refraction and transmittance in visible range, nanobelts, nanowires, and nanocages that are obtained has been a fundamental material in nanostructures,  C.I. Ossai and N. Raghavan: Nanostructures and nanomaterials 211 design and development of the new ZnS nanostructures and their applications. 2.2 ZnO structures and characteristic ZnO is one of the nanomaterials that have found wide- spread application due to the surface and quantum con- finement effects, which have made them show novel electrical, chemical, mechanical, and optical properties. These properties have usefulness in field effect transistors, resonators, sensors, and nanocantilevers. ZnO has a Figure 1: Geometrical structure of ZnS showing the zinc-blended wide band gap of 3.37 eV [30, 41–43], and the strong piezo- and the wurtzite-blended structure of ZnS nanostructure (Reprinted electric and pyroelectric properties caused by the lack of with permission from Ref. , copyright 2006 by the Royal Society of Chemistry). such as nanowire, nanohelics, nanobelts, and nano- combs [31, 39]. The physical nature of ZnS 1-D nanostruc- ture, synthesized with the vapor-solid process (VSP) was studied by some researchers to understand the polar surface of the growth phenomenon of the material [30, 31]. They showed that the crystallographic structure of ZnS has zinc-blended crystals at room temperature that transforms at a temperature of 1020°C from zinc blended to the hexagonal crystal structure known as wurtzite (Figure 1) [30, 31, 40]. Wurtzite has hexagonal unit cells of alternating tetrahedral coordinates of S2− and Zn2+ ions that are packed along one axis and causes the non-cen- Figure 2: ZnS wurtzite polar crystalline nanostructure projected trosymmetric and piezoelectrical characteristics of ZnS along (Reprinted with permission from Ref. , copyright nanostructures. 2006 by the Royal Society of Chemistry). The alternating positive and negative charges of Zn2+ and S2− in the ZnS crystalline structure resulted in the polarized surfaces-{2110}, {0110}, and ±{001} that contrib- ute to the dipolar and surface energy divergence that may bring about asymmetric growth of the ZnS nanostruc- tures, during the nanofabrication process. The surface polarity, surface energy of the crystalline planes, and chemical activities on the crystalline planes have been the major contributors to anisotropic growth of the ZnS nano- structures (Figure 2). It has also been shown that the ZnS nanostructures are dependent on the substrate temperature and the catalyst used for their fabrication. It was evident that differ- ent nanostructures of ZnS such as nanorods, nanowires, nanobelts, and nanosheets can be obtained with varying characteristics, when the temperature and catalysts were alternated, hence, providing a significant insight into the ZnS nanostructure shape, size, and morphological orien- Figure 3: ZnO wurtzite structure showing the polarities (Reprinted tation. This is a step that is vital for controlling the with permission from Ref. , copyright 2004 by IOP Publishing). 212 C.I. Ossai and N. Raghavan: Nanostructures and nanomaterials central symmetry of the wurtzite (Figure 3) [41, 44] makes crystallographic orientation of the nanobelts were influ- it suitable for short-wavelength optoelectronic application enced by the dissolution of the indium ion into the Au [41, 42]. The piezoelectric and pyroelectric properties make (used as a template) during the nucleation of the ZnO at the material suitable for sensors, transducers, energy gen- the solid-liquid interface. It was also affirmed by these erators, and photo-catalysts for hydrogen production, while researchers that the doped nanobelt have a higher pho- the biocompatibility and nontoxic characteristics make it ton-carrier concentration than the nanowire, after using suitable for medicine. The high exciton-binding energy photoluminescence and resonant Raman scattering tech- of 60 meV in the ZnO crystals [42, 44–46] ensures efficient nique to characterize the materials. excitonic emission at room temperature , whereas the ZnO is a polar-dominated nanostructure that is cen- transparency to visible light makes the material extremely trally constituted of nanowires, nanobelts, nanosprings, conductive after doping. The ZnO nanostructure has a polar nanoring, nanohelices, and nanobows that can be characteristic that is dominated with a basal plane and obtained by the variability of surface charges and chemi- other planes that include ZnO ± {0001} that is atomically cal compositions at the polar surfaces of the wurtzite flat, stable, and without reconstruction, ZnO {21̅ 10̅ }, and nanocrystal structure [41, 48]. The non-symmetric wurtzite ZnO {0110 ̅ } that are non-polar surfaces with lower energy shape of ZnO is made up of alternating tetrahedrally coor- than {0001} facets. dinated O2− and Zn2+ ions, arranged along the c-axis (011̅0) Per previous study in Ref. carried out at 860°C (Figure 3). The ZnO nanostructure is also character- using vapor transport deposition (VTD), under similar ized by spontaneous polarization along the c-axis, despite growth conditions, ZnO nanostructures can be switched the normal dipolar movement that is caused by the posi- between {0001} − axial nanowire and {112̅0} − axial tively charged ions of Zn2+ (0001) and negatively charged nanobelt, by adding indium to the source. The research O2− (0001̅) at the polar surfaces. ZnO crystals can be showed that the nanowires were observed as ordered formed via the hydrolysis (Eqs. (1)–(5)) of Zn salt in basic vertical arrays of pure ZnO, whereas the nanobelts had solution, to get a colorless compound, due to the 3d10 elec- no spatial ordering. Fan et al. concluded that the tron configuration that has zero crystal field stabilization A B C Figure 4: Examples of some ZnO nanostructures formed in nanofabrication process. (A) Nanobelt; (B) nanospring; (C) nanohelix (Reprinted with permission from Ref. , (A) copyright 2004 by Elsevier Ltd; (B) copyright 2004 by the American Association for the Advancement of Science; (C); copyright 2007 by Cambridge University Press).  C.I. Ossai and N. Raghavan: Nanostructures and nanomaterials 213 energy. Hence, the adsorption of the monolayers of controlling step for the ZnO film formation that results the precursor molecules on the polar surfaces transforms in the growth along the basal plane, due to the relatively the polar surfaces into inverted polarity. Generally, KOH high surface energy. is preferred to NaOH for the hydrolysis because K+ has a Minimization of the total energy contributed by sponta- larger ionic radius, which lowers the probability of fusing neous polarization and elasticity at the polar facets, due to to the ZnO lattice. the spontaneous polarization across the nanobelt thickness, may result in the formation of nanospring that forms, due to Zn 2+ + 2OH− ↔ Zn(OH)2 (1) the rolling of a single-crystal nanobelt. When the nano- spring is folded circularly as the opposite sides of the polar Zn(OH)2 + 2OH− ↔ [Zn(OH)4 ]2− (2) surface meet face to face, a nanoring will be formed, whereas the formation of nanohelix is caused by the chemical [Zn(OH)4 ]2− ↔ ZnO22− + 2H2O (3) bonding of crystal stripes of nanobelts, due to the alternating ZnO22− + H2O ↔ ZnO + 2OH− (4) and periodical distribution of the crystal strips, twisting and coiling along the growth direction (see Figure 4). ZnO + OH− ↔ ZnOOH− (5) Growing ZnO using pulsed laser ablation with Zn target in water, ammonia nitrate, and cetyltrimethylam- The growth of ZnO is generally influenced by the con- monium bromide (CTAB) at temperatures of 45°C, 65°C, centration of Zn(OH)2 and the pH of the solution [16, 50, and 85°C resulted in morphology variation from spheri- 51]. In the initial stage of the formation of ZnO, the Zn2+ cal to nanoflakes, nanosquares, and branched nanorods and OH− ions in Eq. (1) undergoes dehydration via a proton. It was also observed that the increased absorbance transfer that results in the formation of Zn2+ · · · O2− · · · Zn2+ of ZnO was possible, due to the increase in laser fluence bonding, which agglomerates to an octahedral shape when it was grown in water. There was also increased [Znx(OH)y](2x−y)− [16, 52]. Research has shown that the nanoparticle concentration with the increasing of the [Znx(OH)y](2x−y)− aggregates formed in Eq. (2) are initially fluence with the higher absorption of the spectra of the with < 50 ions, but the introduction of the O2− brings about ZnO resulting in the higher band gap and the smaller an increase in the ions to about 150 with a resultant for- the size of the nanoparticle (Figure 5). On the other mation of wurtzite , which increases in size with a hand, ZnO nanoparticles prepared in 0.01-m ammonium core of approximately 1 nm at an ion count of over 200 nitrate in the pulsed laser ablation produced a rod-like. Again, the formation of the ZnO nuclei according to wurtzite with hexagonal shape and cubic zincite ZnO the steps shown in Eqs. (1) to (5) is slow and can be the phase [16, 31]. 1.2 J/cm2 A B 2.4 J/cm2 2 3.6 J/cm2 6.3 J/cm2 3.6 J/cm 2.4 J/cm2 0.6 6.3 J/cm2 CB Eg = 3.45 eV Band gap (eV) Absorbance Eg = 3.48 eV 0.4 Eg = 3.51 eV 3.58 eV 3.51 eV 3.48 eV 3.45 eV 0.2 VB 0.0 200 300 400 500 600 700 0 5 10 15 20 25 30 Wavelength (nm) Size (nm) Figure 5: Variation of ZnO spectra absorbance with laser fluence wavelength and the effects of the band gaps on nanoparticle sizes. (A) Absorption spectra of ZnO obtained from laser ablation in deionized water at laser fluence 1.2, 2.4, 3.6, 6.3 J/cm2. (B) Band gap variation with size of ZnO nanoparticles (Reprinted with permission from Ref. , copyright 2017 by Elsevier). 214 C.I. Ossai and N. Raghavan: Nanostructures and nanomaterials 2.3 C  haracteristics of molybdenum disulfide such as Ag into the epitaxial geometrical orientation, due (MoS2) to the limited defects on the structure. The use of solution-based 2-D MoS2 nanosheets for directional epitaxial growth of some noble metals such as Pd, Pt, and Ag nanostructures at room temperature 3 Nanostructures fabrication has been achieved with a distinguishable (111) and (101) techniques structural orientation on MoS2 (001) surface. This nanosheet-templated epitaxial growth was achieved Nanostructures have been fabricated with a top-down via a wet chemical reaction of the nanostructures that approach that involved the use of lithographic etching had the MoS2 nanosheets providing the nucleation sites of materials, for shaping and patterning the orientation for the growth of the noble metals and prevented the of nanostructures, whereas the bottom-up technique aggregation of the nanoparticles. This epitaxial growth involves the growing of nanostructures with atomic-scale phenomenon had involved in situ synthesis of the noble systems that utilizes adatoms for deposition of vapor- metals on the surface of the MoS2 in a photochemical ized materials on substrates and templates [3, 16]. There reduction technique that involved the use of CTAB at are different bottom-up fabrication approaches such as some instances. the wet chemical method [14, 16], physical vapor deposi- Importantly, CTAB, which works as an orientation tion [64, 65], metal organic chemical deposition [66, 67], surfactant in nanofabrication, has various nanostructure molecular beam epitaxy , pulsed laser deposition , orientations at different concentrations of the substrates sputtering , flux method , and electrospinning and other experimental conditions with increased used for the fabrication of nanostructures. Some of concentrations resulting in higher epitaxial growth due to the major characteristics of these fabrication techniques the CTAB-micelle, which bond to the substrate ions [53, include controllability of fabrication conditions such as 54]. MoS2 has synergistic effects on the anode of lithium- temperature, pressure and deposition rate, uniformity of ion batteries, a characteristic that will makes it possible nanostructure, high-quality nanomaterial with limited to ascend battery capacity upon recharge, due to the oxi- impurities, reproducibility that will enable the scale-up of dation of MoS2 to MoS3. Again, with MoS2 nanosheets the process into commercial production, high throughput, showing a slight decrease in specific capacitance of 14.9% choice of substrate and template material for fabrication, in 500 cycles, it will make an excellent electrode material and excessive cost of fabrication [15, 16]. A summary of for electrochemical capacitors. some of the bottom-up and top-down fabrication tech- niques is shown in Table 1. 2.4 C  haracteristics of graphene oxide (GO) 3.1 F abrication concepts and attributes of nanostructures Graphene has a 0 ev band gap because of the empty valence band that touched the conductance band that also Nanotubular structures attain better anti-reflection than has no valency electrons, a characteristic that gave rise to nanoparticles or deposited materials due to the high the peculiarities of graphene oxide nanostructures [57–59], porosity that can be up to 70%. A nanoporous poly- hence, the application of graphene oxide nanosheets in imide template prepared by dry etching using anodized many electronics and catalytic energy storage devices alumina oxide membrane mask, followed by atomic [60, 61]. The presence of the lattice of equivalent sub-­ layer deposition of alumina, was used for fabricating lattices of carbon atoms that are sigma (σ) bonded with the high-density, free-standing alumina nanotube arrays each other in a π orbital altered the electronic configu-. The variation of the pore size, thickness of the nano- ration of the material. Graphene oxide-based tem- porous polyimide template, and deposition thickness of plates help in the formation of the hexagonal structure Au alumina, helped to obtain nanotubes with various diam- nanosheets by the absorption of amine molecules and eters, lengths, and wall thicknesses. Similarly, the optical for the assembly of Au nanodot chains using long-chain properties of these alumina nanotube arrays, modified by thiol molecules of the reduced graphene oxide. Simi- changing the wall thicknesses on glass, showed an anti- larly, pyrolytic graphite substrates used in nanofabrication reflectance with a 0.1% reflectance at the visible wave- guaranteed better fusion of thiol-capped nanoparticles length of 540 nm. The broadband and omnidirectional  C.I. Ossai and N. Raghavan: Nanostructures and nanomaterials 215 Table 1: Summary of the characteristic of some nanostructure fabrication techniques. Lithographic method – Uses electron beams to remove micro-scale structures from resists, which are precursor materials – Can fabricate nanostructures of ~45 nm – Transfers a mask image to a substrate by coating while exposing the resist to electron beams and applying appropriate chemicals to develop the image on the substrate – The technique uses physical masking that involves the resist surface being in contact or in proximity with the mask or by software mask, which uses computer programs for patterning the mask – The cost of using this technique is high, as equipment could cost from hundreds of thousands of dollars to millions of dollars Pulsed laser deposition – Involves physical vapor deposition of thin films using high-powered beam to strike a deposition material in a vacuum chamber – The technique can also work in the presence of oxygen when depositing complex oxides – It can work at a temperature that varies from room temperature to above 1000°C – Temperature, laser energy density, pulse repetition rate, pressure, type of gas inside the chamber, and substrate-to-target distance determine the quality of the nanostructures produced – There is no uniformity of produced nanostructures, due to the narrow forward angular distribution of the plasma plume [16, 73] Electrodeposition – Uses induced electrochemical reaction in aqueous solution to deposit nanomaterials on metal oxides and chalcogenides – Growth of electrodeposited materials is outward on the substrate and can be controlled by using an insulating mask – It is a cheap nanofabrication process that can be done at room temperature using water-based electrolytes – Controlling of charging rate and time can be used to monitor the rate of deposition of films on the substrate, which can range from a few nanometers to tens of microns – The technique has been exploited for the nanofabrication of complex 3-D materials such as the interconnections in computer chips – The nanofabrication can be scaled-up from an atom deposition to large-scale dimension [74, 75] Metal organic chemical deposition – Relies on the transfer of materials in gas phases for deposition on substrate surface through a chemical reaction – The temperature for the nanostructure fabrication is in the range of 500°C to 1500°C, and the sample can be rotated at 1500 rpm to maintain the uniformity and quality of the produced nanocrystals – The fabrication can be done at near atmospheric pressure – There is a limitation on the choice of substrate needed for the fabrication Wet chemical method – Temperature and other parameters that control the morphology of the nanostructure can be controlled during the fabrication process, to enhance the characteristics of the nanostructures – The fabrication can be done at low temperature using flexible organic substrates that may not require catalysts – The technique can be used with silicon technology, which has found many applications in semiconductors [14, 16] Physical vapor deposition – Requires elevated temperature deposition of sublimed source material – Can incorporate impurities to nanostructures from the reactor chamber or from the use of catalysts in the nanofabrication – May not be integrated with flexible organic substrate; however, as a self-assembly technique, it requires a material to be deposited on substrates – Temperature, pressure, and carrier gas control the catalyzed process of physical vapor deposition [16, 76] Flux method – This technique fabricates nanostructures at elevated temperature and can incorporate impurities to the nanostructure – As a bulk crystal growth phenomenon, reactants are dissolved at elevated temperatures in molten salt to form complex materials – Controlling and altering the concentration of the molten complex compound and the temperature of the reaction helps to obtain a 1-D nanotube and other nanostructures – Alkaline hydroxides are the widely used flux for synthesis of oxide nanostructures due to their low melting points [16, 77, 78] Molecular beam epitaxy – Provides a single crystal film of heated substrate under high ultralight vacuum condition – Can fabricate single atomic crystalline layers at a time with the possibility of controlling the morphology of the nanostructure in the growth process – The fabricated nanostructure may have a uniform morphology and high quality with reduced impurity – Performance of the devices fabricated by this technique depends on the dopant control and the hyper-abrupt interfaces of the crystalline film layer – Can be used to fabricate nanostructures of different geometries from 1-D to 3-D – The limited choice of substrates may have to contribute to the poor yield of nanostructures produced by this technique, which is not cost effective [16, 68] 216 C.I. Ossai and N. Raghavan: Nanostructures and nanomaterials Table 1 (continued) Electrospinning – Used for generating nanofibers from materials such as polymers, composites, and ceramics – Consists mainly of a high voltage supply, a spinneret, and an electrically conductive collector – Controlling of the humidity, temperature, and pressure is vital for nanofiber fabrication using ceramics – The electrospinning process helps to generate continuous charges on the surface of liquid droplets used for the nanostructure fabrication Sputtering – The technique is scalable for mass production and works at low pressure in a reactor chamber that contains inert gas such as argon – Electromagnetic signals in the reactor turns the inert gas into plasma and accelerate the positive ions toward the substrate with a kinetic energy of ~10 eV – The sputtered atoms pass through the plasma and deposit on the substrate causing growth of nanocrystals that may be influenced by the collision of the sputtered ions with heavy particles in the plasma, resulting in loss of energy, direction, and not depositing on substrate – Film morphology may be altered by the high kinetic energy of sputtered particles due to the atomic mobilization process anti-reflective characteristics of a passivated conical the 2.81% obtained when a dense HfO2 blocking layer was frustum array of 550-nm height, fabricated with polysty- inserted between the tin-doped indium oxide nanowire rene colloidal lithography on a crystalline silicon nitride surface and the porous TiO2 shell layer. This increase was of 80-nm thickness, caused the suppression of the Fresnel attributed to electron recombination, which was achieved reflection at a visible wavelength (400–1000 nm) at an at a nanowire length of 20 μm. Moreover, the decrease in incident angle of ~60°. Tseng et al. also showed the TiO2 annealing temperature, from 600°C to 400°C, that the power conversion efficiency of 13.39%, which is resulted in an increase in electron recombination, due to 9.13% enhancement from the conventional KOH-textured the higher dye loading on the TiO2 nanoparticles, resulting silicon cell, was achieved, while the measurement of the in an efficiency of 5.59% of the photoanodes. external quantum efficiency showed that photocurrent The block copolymer has stood out as a lithographic was driven primarily by the increased optical absorption template, separation membrane, and organic photovol- in near infrared. taic layer due to the self-assembly attributes, which makes α-Fe2O3 can make a potential anodic material for lith- it possible for nanofabrication. Different annealing ium-ion batteries because of the low cost, environmen- techniques such as thermal, solvent vapor, magnet, and tal friendliness, corrosion-resistive qualities, and high electrical field alignment have been used to self-assemble thermal capacity of ~1007 mAhg−1 [4, 28, 83]. The fabrica- block copolymers, whereas in situ X-ray and neuron scat- tion of the hollow-structured α-Fe2O3 nanofibers was done tering have been used for studying the thin film of the using the electrospinning technique with iron acetylace- block copolymer, annealed with different protocols. tonate (Fe(aCaC3) and polyvinyl pyrrolidine (PVP) used as Controlling the morphology, ordering, and orientation of precursors. The calcination of (Fe(aCaC3)-PVP compos- the block copolymers using X-ray and neuron scattering ite fiber at a high temperature helped to form the intercon- has the potentials of linking the thermodynamic pathway nected 1-D hollow structure of the α-Fe2O3 nanofiber that of the nanostructures’ growth mechanism over the macro- exhibited a reversible capacity of 1293 mAhg−1 at a current scopic areas, during nanofabrication. The use of X-ray density of 60 mAg−1 at the anode. This material also dis- and neuron scattering can also help to refine annealing played an excellent cycle stability and rate capacity at a techniques for block copolymer, foster robust fabrication large aspect ratio, which makes it a suitable material for procedures, while helping to develop the next generation lithium-ion batteries. of direct block copolymer assembly of nanostructures To overcome the charge transport limitation that is using hybrid systems that can combine top-down and peculiar with conventional nanoparticle-based photo- bottom-up fabrications. anodes [84, 85], tin-doped indium oxide nanowires were To fabricate hierarchical 3-D branched TiO2/Si nano- used to prepare the dye-sensitive solar cell photoanodes, wire arrays, some researchers combined nanoimprint by vertically growing tin-doped indium oxide nanowires, lithography and reactive ion etching process, while using a using the thermal evaporation process. The TiO2 shell layer high-resolution scanning electron microscope for examin- was also utilized to uniformly coat the nanowire surface ing the morphology of the nanostructures. They also used to ensure a high dye loading. These researchers X-ray diffraction for studying the crystalline structures of reported that the cell efficiency increased to 4.85% from the TiO2 nanowires, which were hydrodynamically grown  C.I. Ossai and N. Raghavan: Nanostructures and nanomaterials 217 on the Si substrate. The work of Noh et al. revealed that the wet chemical synthesis fabrication of the Pt-MoS2 the increased surface area of the electrochemical reaction hybrid nanostructure resulted in a higher electro-cata- [28, 30, 90] and enhanced charge transfer dynamics lytic activity of the material toward the hydrogen evolu- caused the improved photo-electrochemical performance tion reaction compared to the commercial catalysts made of the TiO2/Si nanowire arrays when compared with the with the same platinum (Pt) loading. Similarly, the TiO2 thin film-coated Si nanowire arrays. use of pulsed laser atomic probe tomography for analyz- The existence of neutrality levels of surface state ing dopants and other impurities in Si and Ge nanowires donor-acceptor continuum makes the characterization of fabricated with vapor liquid solid mechanism was used surface state, based on photoluminescence and cathodo- by other authors to determine the concentrations of luminescence possible, hence, the ease of forming elec- phosphorous in silicon (Si) nanowires and boron (B) in tron-collecting and hole-collecting nanocontacts for germanium (Ge) nanowires. However, impurities associ- nanostructures without p-n junctions. It is, therefore, ated with nanostructures used for developing semicon- important to passivate the Si interface control layers to ductors have the potentialities of showing homoepitaxy of enhance the power supply ability of the molecular beam atom-like structures and coherent attributes that may be epitaxy grown quantum wires, which have potential vital for future applications in optical quantum comput- applications in solar cells, as an intelligent chip, in pho- ing and communication networks. tovoltaic energy generation. Hence, after adding different atoms of metallic elements in a single-phase ternary metal chalcogenide nanowire of < 20-nm diameter, the scal- able ultrathin composition of tellurium selenium-doped 4 G  rowth mechanisms of ternary chalcogenide nanowire was fabricated by utilizing nanostructures the templates of the Te1−xSex alloys. This nanomaterial was found to have a high thermoelectric figure of merit of The growth of nanostructures during fabrication depends 0.75 and a power factor of 1023 μW/mK2, be temperature on different conditions that can affect the structural dependent, and with a setback coefficient that suggests growth along a given direction and results in different that the charge transfer mechanism was dominated by growth morphologies and characteristics. The growth an energy barrier between the nanowire-nanowire nano- direction can depend on the surface energy, which tends junctions. As the energy barriers, substrates, deposition to direct growth more, along the facets with higher energy time, and physical and chemical characteristics of the (because of their smaller surface areas) to the detriment chemicals and catalysts can potentially affect the char- of surfaces with lower energy [16, 30, 41]. This situation acteristic, morphology, and orientation of the fabricated is evidence in the anisotropic growth of ZnO nanostruc- nanostructures [2, 9, 16, 90], it is important to understand ture that is highest along the c-axis at the facets {011̅0} and their impacts on the nanostructures’ geometry and behav- {21̅ 1̅0} with growth directions of-(21̅1̅0) ( ±[21̅ 1̅0], ±[1̅21̅0], ior. To this end, some researchers aimed to clarify the ±[1̅ 1̅20]); (011̅0) ( ±[011̅0], ±[11̅00]), and ±. Other role of the Si organization on the surface orientation of the researchers also demonstrated the structural specificity of nanostructures by studying the first stage of growth of the nanostructures on colloidal synthesis using a method that Si on the Ag (001) surface at room temperature, using the spontaneously resulted in facet selective growth of ZnO Auger electron spectroscopy and low-energy electron dif- nanorods, on the {111}, rather than on the {100}, facet of Ag fraction. They confirmed the existence of a layer-by-layer, nanocrystals. The heterogeneity of the nanostructures non-long ordered structure at about one monolayer of the in this experiment that was performed by Fan et al. nanostructure and a complex overlaying pattern that has using seed-mediated method of growing ZnO nanorods no clear symmetry and growth pattern following. on Ag-truncated nanocube surface was confirmed with Other researchers that worked on nanostructure fab- select-area electron diffraction and convergent-beam elec- rication such as Wu et al. synthesized nitrogen-doped tron diffraction. The results indicate that the ZnO nanorod p-type ZnTe nanowires using chemical vapor deposition grew along {011̅0} and the Ag nanoparticles grew along and affirmed that metal oxide-semiconductor field effect {1̅12} with an epitaxial relationship as [011̅0]ZnO | |[1̅12]Ag transistors have enhanced subthreshold swings and and (0002)Zn | |(111)Ag. Again, Piñero et al. fab- threshold voltage compared with junction field effect tran- ricated nanowires by growing on the facet of the sistors. However, the junction field effect transistors have Si-oriented substrate, GaAs crystals using the molecular higher transconductance and Ion/Ioff ratio than metal oxide - beam epitaxy and vapor-liquid-solid fabrication process. semiconductor field effect transistors. On the other hand, They discovered that the nanowires were mainly growing 218 C.I. Ossai and N. Raghavan: Nanostructures and nanomaterials in alternative wurtzite and zinc-blended phases, a result 4.1 F actors affecting nanostructure growth that can be attributed to the enhanced attractive interac- and characteristics tion of the facet, which was caused by lattice mismatch of the GaAs on the Si substrate. It is noteworthy that lattice Nanostructure growth is affected by numerous variables, mismatch prevents nucleation and growth of over-layer which have been summarized in Table 2. on substrates, due to the existence of structural strains [99–101]. Furthermore, another research that involved the growth of noble metal-Pt on a single-layer MoS2 nanosheet 4.2 Growth mode of nanocrystals at room temperature, using wet chemical synthesis, pro- duced two epitaxial orientation, and , that coex- The growth of ultrathin films of nanostructures and isted for the metal nanoparticles. Huang et al. also nanocrystals is fundamental to achieving the expected affirmed that the anisotropic nanostructure of the Pt epi- morphology, size, orientation, uniformity of growth, taxially aligned on the MoS2 nanosheets with the and and other characteristics of the fabricated nanostruc- spots being aligned with the and diffrac- tures. Generally, the growth mode of nanocrystals and tion spots of MoS2, respectively. nanostructures involves an orientation attachment Table 2: Summary of some factors affecting the growth rate of nanostructures. Factor Impacts/remarks Ref Diffusion –G  rowth rate is determined by diffusion rate [102–106] – Nucleation of nanoparticles is influenced by electrical strain, surface energy, and surface diffusion kinetic – Liquid phase enhances the growth in vapor-liquid-solid mechanism – Nanowires with smaller diameters grow faster than those with bigger diameters for systems having limited-mass-transport growth mechanism else the reverse is the case due to Gibbs-Thomson effect – Diffusion of adatoms of nanoparticles on the sidewalls of the nanowire causes smaller diameter nanowires to grow faster than those of bigger diameters Buffer layers –B  uffer layers’ result in varying morphology due to the different growth rates and catalysts – Buffer layers can act as a seed layer to enhance the coverage and growth rate of the nanostructures – Catalysts facilitate the growth rate via increased collision and nucleation – Catalysts initiate and guide growth and epitaxial orientation between the nanostructures and the substrates – Introduction of polydopamine on carbon fiber increased adhesive strength between carbon fiber and ZnO nanowires and the interfacial shear strength – Impurities from catalysts can change the characteristic, size, and orientation of nanostructures – Quantity of catalyst and particle flux affect the growth of nanowires [41, 107–113] Temperature –N  anowire diameters increase with increased growth temperature – 1-D ZnS nanocrystals were obtained at low density and high temperature (1000–1100°C) [8, 114] and nanowires were obtained in low temperature (800–1000°C) for a thermal evaporation nanofabrication technique Surface energy –A  rea of high surface energy enhances nucleation and formation of colonies of nanoparticle and [13, 28] facilitates nanocrystal formation – High aspect ratio of nanostructures favors higher surface area for chemical reaction Orientation – Si nanowires prefer to grow along the orientation, followed by , , , [13, 115–120] , and – Growth orientation is determined at precipitation – Surface chemistry of substrates and reaction pressure determine orientation of nanostructures – Nanowire diameter and precursor molar ratio impact the orientation of nanostructures – Chemical composition of initially allowed droplets influences orientation of nanostructures Doping and –D  oping and alloying are used to influence the physical properties such as electrical conductivity, [41, 121, 122] alloying band gap, and ferromagnetism of nanostructures – Temperature and doping affect the nanocrystal size – N-type, p-type, and transition metal doping techniques have been used to influence the characteristics of nanostructures  C.I. Ossai and N. Raghavan: Nanostructures and nanomaterials 219 Figure 6: Nanocrystal orientation-based growth mechanism due to Oswald’s ripening of poly-dispersed nanocrystals. mechanism, crystallization process, and particle-based induced by particle rotation, which is dominant in a aggregation. weakly flocculated colloidal state, leads to the formation of complex structures after the orientation-based growth. As Oswald’s ripening process is continuous in a 4.2.1 Orientation-based attachment growth poly-dispersed medium, destabilization and sedimenta- tion could follow ; hence, it is important to stabilize Poly-dispersed nanocrystals can be influenced by dispersions to avoid coarsening. Oswald’s ripening, which is a procedure by which larger- sized nanocrystals grow at the expense of the smaller size ones because the chemical potential difference between 4.2.2 Nucleation process them triggers diffusion that causes the loss of the mass of the smaller particles to the larger particles. This ori- Nucleation, which is the build-up of molecule clusters entation-based attachment growth can involve the attach- before the formation of new phases in the transformation ment of smaller particles together in the growth process by of vapor to liquid to solid, is characterized by the decrease sharing crystallographic orientation and docking at differ- in enthalpy and entropy of the system after surmounting ent planar interfaces due to the energy difference in the free energy barrier per Eq. (6). The random colli- the particles, which make larger particles grow more than sion and rearrangement of atoms and molecules at a given the smaller ones. Oriented-based attachment also causes phase of transformation, and the growth of the nanopar- a reduction in the high-energy surfaces and the reduc- ticles in clusters, which can be a reversible and stepwise tion of surface free energy that plays a significant role in kinetic process, contribute to the nucleation process. the particle integration process. Orientation-based The critical size of the nucleus (r* = 2–3 nm nanoparticle) attachment growth is dependent on the effective collision is essential for the nucleation process and formation of of particles, and the number of collisions in each surface clusters per Figure 7. area determines the level of mutual orientation attach- ∆G = ∆H − T ∆S > 0 (6) ment growth, in a well-dispersed colloidal suspension (Figure 6). This collision of particles is expected to where ∆G is the change in Gibb’s free energy, ∆H is the result in a statistical growth process, whereas coalescence change in enthalpy, T is the temperature, and ∆S is the 220 C.I. Ossai and N. Raghavan: Nanostructures and nanomaterials Figure 7: Nucleation and cluster formation of nanocrystals showing the random collisions, re-arrangement of atoms, and clustering. change in entropy. ∆G* is the change in Gibbs free energy to take place, the nanocrystals must surmount the free at the critical radius. energy (∆G) barrier in the nucleation process and At the critical size of clustering of the nucleus, growth form clusters. The free energy at the catalyst-nanowire becomes spontaneous, despite the ease of dissociation of interface (∆Gcni) can only form the nanoparticle layer nec- the clusters due to the very slow short life span. However, essary for homo- and heteroepitaxy when the conditions a few of the clusters get to the critical size and continu- in Eq. (8) are met. ously grow spontaneously to form larger particles. This growth is governed by thermodynamic principles, ∆G cni = αN 0.5 − ∆µN (8) which is controlled by the chemical potential difference that is related to the pressure in the system at equilibrium where α is a geometrical weight coefficient for forming a per Eq. (7) [129, 130]. new surface from the cluster of adatoms (N). Generally, the nucleation in the nanofabrication P ∆µ = µv − µs = RT ln   (7) process is expected to take place when the thermal activa-  P0   tion energy (∆GB) is reached, and the critical nucleation (JC) can take place in an environment that depends on the where ∆μ is the thermodynamic driving force, μv is the temperature (T) and concentration of monomer (C) per Eq. chemical potential of the vapor, μs is the chemical poten- (9). tial of the substrate, P is the partial pressure at the gas  α2 phase, and P0 is the equilibrium partial pressure of the  ∆G B = components over the nanocrystal at a given growth  4∆µ  (9)  J = ZC ω exp  −∆GB    condition.     K bT   C Nanowire growth is also affected by the wetting angle,  surface polarity, surface energy, and chemical activities at the crystalline planes [31, 131]. At the crystalline planes, where Kb is the Boltzmann’s constant, and Z is the Zel- the surface energy and the catalyst-vapor triple-phase dovich factors. boundaries are important in growth facilitation because During the nucleation process, phase transition is the thermodynamic driving force (∆μ) of the nanocrystals vital for the estimation of the size and number of nuclei can vary with the wetting angles. Hence, for growth that is formed , and the nucleation rate (In) for a  C.I. Ossai and N. Raghavan: Nanostructures and nanomaterials 221 grow into iso-oriented crystals via orientated attachment Nucleation after fusion and can also form a mesoscale assemble with the formation of microcrystals. The schematic of the tion Mesoscale assembly Crystal growth tabiliza aggregate-based nanoparticle crystalline growth is shown ry s pora in Figure 8. Tem Nanoparticle (3–100 nm) Mesoscale assembly 4.3 H  eteroepitaxy of nanostructures Microcrystal Amplification In the growth of nanostructures, there is a tendency of order and disorder in the heteroepitaxy of structures due Single crystal to a small lattice mismatch of layers on the template or iso-oriented crystal substrate. This disorder usually stalls the improve- ment in the physical properties of the nanostructures Figure 8: Schematic of aggregation-based nanoparticle crystalline growth. that can be achieved via fabricating a uniform size, proper shape, and regularly positioned nanostructures. Hence, there is the need for control of the different para- multicomponent nanomaterial can be estimated with Eq. meters such as temperature, deposition time, pressure, (10). and chemical concentrations of precursors during the nanofabrication process. The nanostructure fabri-  m   In = nl   ∑ωk  exp(H 0 ) (10) cation process can result in the growth of 3-D interfaces   k =1   due to the strain mechanism on the substrates and the  templates, which results in the formation of Volmer- where ωk is the diffusion coefficient of the kth component Weber growth mode – the formation of 3-D islands of a nanomaterial of the m-components, H0 is a barrier to directly on bare substrates – and the Straski-krastanov the multicomponent, and nl is the initial phase density of growth mode – the formation of an initial flat wetting the molecules. layer on substrates and templates. As these growth modes can result in heteroepitaxy and quantum dots in 3-D nanostructures, experts are still working on tech- 4.2.3 C  lassical crystal growth mode niques to harness the advantages of heteroepitaxy in novel industrial applications. Although heteroepi- This process occurs by the atom to atom addition to a tem- taxy, which is also attributed, in part, by differentiation plate or by the dissolution of unstable phases and precipi- in thermal expansion and lattice mismatch of materials tation of more stable phases. This phase is critical to the used for nanofabrication [18, 132], could have potential nanostructure fabrication because the parametric control future applications in the industry, however, the strains of the process of adatom growth influences the character- induced on the nanostructures by heteroepitaxy have istics, orientation, and geometry of the nanostructures [16, resulted in defects and microcracks at the interfaces, with 30]. The precipitation and formation of the nanocrystals is performance efficiency reduction for charge-transferring preceded by nucleation that occurs by the dissolution of devices. A high deposition temperature, which make the nanoparticles at a high temperature and cooling to a deposited adatoms on substrate surfaces to diffuse easily lower temperature and by the addition of a necessary on the substrates have also hampered homoepitaxy in reactant to produce a supersaturation state. favor of heteroepitaxial growth of nanostructures. Mitigating against the influence of heteroepitaxy will be vital for the reduction in the level of uncertain- 4.2.4 A  ggregation of particles ties associated with nanostructure characteristic flaws, hence, the need for buffer layers in planar structures, to In this state of the nanocrystal growth, particles grow by aid the reduction of lattice mismatch and thermal expan- aggregation in a mesoscopic transformation process that sion strains during nanofabrication [133–136]. Again, the allows them to increase to a stable size and then start improvement of the interfacial qualities of the nanostruc- combining with unstable particles until the formation of tures via reduction of the footprints of crystals in growing a single nanocrystal structure. The nanoparticles can also thin-film nanostructures will also help to improve the 222 C.I. Ossai and N. Raghavan: Nanostructures and nanomaterials nanocrystal qualities, due to the reduction of the strains generalized equation for growth velocity Vi(θ,t) at a given on the sidewalls of nanotubes. shape of graphene island gi(θ) and growth-driving forces q(t) at a given time t shown in Eq. (12). Vi ( θ, t ) = gi (t )q(t ) (12) 5 G  rowth models of anisotropic The model in Eq. (12) is linked to measurable variables nanostructures in the graphene nanostructure over a given polar space, by considering the growth radius, area of growth, and nucle- Physical, statistical, physical-statistical, and cross- ation time. The driving force of the nanostructure growth domain models have been predominantly adopted for over a given time has been associated with the shaded area modeling the growth process of nanowires, due to the of the graphene nanoparticle growth s(t) and the number limited data availability and the complexities associated of graphene islands (Nt) at time t per Eq. (13). with the growth process of this nanostructure. Sta- ds(t ) tistically, different approaches have been adopted for pre- q( t ) = / 2 N t s(t ) (13) dt  dicting the process conditions affecting the morphology of nanowires based on the observed and unobserved vari- By assuming a constrained birth process that has the ables that can be predicted using prior conditions of the time of growth of the graphene nanoparticles as a Poisson system. Different statistical techniques such as fractional process, which functions as a non-homogenous intensity factorial design, multinomial regression models, Gauss- function, the growth-driving force at time t was modeled ian Markov random field, bivariate Gaussian distribution, per Eq. (14). and box-Behnken design have been adopted by numerous researchers [136–140].  1 17   [ α [1 − e ]]e 3 − αω − αt The growth of nanowires was described with a dif-  +  3 72  (14) fusion-induced phenomenon, with stationary equations q( t ) =  that depended on the decreasing radius of the nanowire [ Anuc β {1 − e − αt }]  (R) in the form of 1/Rp, where p has a value of 0.5–2 and depended on growth conditions. Size-dependent, where Anuc represents the area of graphene nanoparticle non-linear effects in nanowire growth of crystal structures, island, β represents the coefficient of unnormalized shape kinetics and nucleation statistics in individual nanostruc- of the graphene nanoparticle island, α > 0 represents the ture layers, length of distribution of nanowires, radius of parameter governing the growth on the tray, and ω > 0 rep- growth rate under varying conditions, number of growth resents the parameter governing the relationship between islands, nucleation time, diffusion constants, and diffu- α and the coverage on the tray. sion rates [141–145] are among the conditions that deter- As noted from Eq. (14), the increase in driving force of mine the diffusion-induced growth rate of nanowires. The the growth kinetic is caused by the reduction in the gra- approximate normalized diffusion flux of adatoms via a phene nanoparticle island, whereas the increase in the circular triple phase line at the nanowire top (Z = L) for a coefficient of the unnormalized shape of the graphene   dL   nanoparticle island leads to a decrease in the growth time-dependent nanowire growth     with radius   dt diff  driving kinetic over a given fixed area. R is shown in Eq. (11). In a vapor-liquid-solid fabrication mechanism, the physical growth rate  dL  of nanowires can be attrib-    dL  2Ωs dn  dt    =− Df f (11)  dt diff R dz uted to discrete deposition or diffusion as shown in Eq. Z −L  (15).  2ΩL( N ∞ − N 0 ) where L, Ωs, Df, nf, and Z represent nanowire length, ele- L ≤ Lf dL  τR mentary volume in the solid, diffusion coefficient, the = (15) dt  2ΩLf ( J − ( N 0 / τ)) z-dependent adatom concentration on the sidewall facets, L > Lf  R and vertical coordinate, respectively.  The growth kinetics of a graphene nanostructure fabricated with chemical vapor deposition was deter- where τ, Ω, J, R, N0, N∞, Lf, D represent the mean lifetime mined with a stochastic mathematical framework with a of the adatom on a nanowire sidewall, atomic volume of  C.I. Ossai and N. Raghavan: Nanostructures and nanomaterials 223 Si, impingement flux, radius of nanowire, adatom con- error value, and Ks, Kw, and Kd are contribution factors from centration at liquid alloy, adatom concentration at base the substrate, sidewall, and droplet, respectively. substrate, diffusion length ( 2 Dτ ), and diffusion coeffi- cient, respectively. An absorption-induced and diffusion-induced model 6 Application of nanostructures of nanowire growth is shown in Eq. (16). Nanostructures have found numerous applications in the dL  R1  = V0  1 +  (∅ + 1) − industry, due to the unique characteristics of the nano- dt  Rcosh λ    () materials that have modified the characteristics of tradi-  R2   1 dR  tional materials used for different industrial purposes.  1 + tanh(λ)  ( ε + 1)   (1 − E )V Fabricated high-ratio silver nanowires have interesting  R   V* dt  (16) applications in plasmonic devices , and SnO2 nano- particle moderated surface area have large particles for where ε, R, ∅, V, V*, E, R1 and R2, and λ represent the super- light scattering and crystallization. This makes the SnO2 saturation nucleation, droplet radius, supersaturation of material efficient for charge transfer and, hence, have gaseous phase, deposition rate, kinetic parameters that found application in dye-sensitive solar cells. Simi- are related to the solid and liquid phase volume ratio, rela- larly, quantum dots, which are nanocrystals formed from tive difference between deposition rate and growth rate on lithographic techniques such as ion beam photolithogra- the substrate, constants used to consolidate various para- phy and chemical etching , are very small to result in meters in the diffusion-induced process, and the ratio of quantum confinement that can bring about discrete phys- whisker length L to the adatom diffusion length on the side ical and chemical states, which are vital for developing surface, respectively. new devices and applications [130–152]. The uncertainty in the growth morphology of the nanostructures and the local variabilities in the growth kinetics resulted in the use of statistical framework to 6.1 N  ear-infrared (NIR) reflective and anti- capture nanowire growth rate per Eq. (17). corrosive function of nanostructures X ( s, t ) = ηk −1 ( s, t ) + φ( s ) + ε (17) NIR reflective coatings have been utilized in many industrial where X(s, t) represents the nanowire length on substrate, sectors – military, construction, plastic, and petrochemical η(s, t) represents the morphology of the nanowire, φ(s) industries, due to their ability to minimize corrosion deg- represents the local process conditions affecting the mor- radation of facilities, minimize urban heat island effect, phology of the nanowire, and ε represents the error inher- enhance the life cycle duration of facilities, enhance aes- ent in the system. thetics, and minimize energy consumption. The NIR The mass continuity of the diffusion length of the reflective coatings with a nanomaterial blend of metal adatoms in the modeling of the vertical growth of nanow- oxides such as tin and zinc have shown remarkable NIR ires with the aim of optimizing the spatial arrangement of reflectance with better color choices and quality. catalyst arrays in the growth process was considered by Research has shown that paint coatings, developed Aghdam et al.. The authors proposed the probability from ZnO nanostructures via mechanical milling and of the influence zone for two nanowires per Eq. (18), consid- microwave irradiation of metallic zinc dust, have high cor- ering the diameters, the distance between the nanowires’ rosion resistance because of the improvement in the oxida- radius of interaction and surface collection area. tion kinetics, particle size distribution, morphology, phase P(dij < rint( i ) + rint( j ) = 1 − e 2 − δπ ( rint −r 2 ( i ) int ( j ) ) ) (18) purity, disperse and UV absorption characteristics. The research showed that 33% enhancement in the NIR shield- The vertical growth (h) of the nanowire can be deter- ing and 156% improvement in corrosion-resistant proper- mined with Eq. (19). ties (in comparison to uncoated surface) make the product have multifunctionality. Again, the ZnO nanopar- hi (t + ∆t ) − hi (t ) = ∆hi (t ) = J p ( K s + K w + K d )∆t + εh (19) ticle filler that were blended with cashew nut shell liquid where dij is the distance between nanowires i and j, rint is the resin as a matrix, using conventional thermal oxidation radius of interaction around a nanowire, h is the nanowire and microwave-assisted oxidation routes, also provided a height, Jp is the bulk equivalent planner growth rate, δ is the high-quality coating product, due to the minimal defects intensity parameter of Poison distribution, εh is the random in the nanostructures and morphology. However, the 224 C.I. Ossai and N. Raghavan: Nanostructures and nanomaterials crystalline purity of ZnO synthesized with the microwave- that plays an important role in light utilization efficiency, assisted oxidation route showed a higher purity than that via minimization of light loss during transmission. fabricated with the conventional thermal oxidation. The Hence, ensuring that optical surfaces efficiently conserve work of Echeverría et al. also showed that the water the energy available to them will improve the performance contact angle of the ZnO nanoparticle cum cashew nut shell of devices used in solar cells and other light energy- liquid resin-mixed coated glass was 91°, a value that is about dependent components. The use of micro-reactor-assisted three times higher than the water contact angle of uncoated nanomaterial deposition process to grow ZnO nanostruc- glass that is 34°. It is important to note that a small water tured anti-reflective coatings on Si substrate in an aqueous contact angle (90°) reduces the residual time, temperature, and concentration of precur- chances of water retention on surfaces, due to the ease of sor solution control to obtain a reflectance suppression of flow out. As water molecules play significant roles in published silicon, between wavelengths of 400–900 nm corrosion, the retention on corrosion vulnerable surfaces from 10.6% to 3.4% by adding ZnO nanoparticles. enhances the corrosion and deterioration of the materials. The water-repellent ability of this ZnO nanoparticle cum cashew nut shell liquid resin mixed paint could be attrib- 6.2 Nanostructure application in photo­ uted to the reduction of cardanol that has high hydrophilic voltaic cells characteristics due to the reduced number of active surface hydroxyl group in the mix. Nanomaterials can improve the potency of solar cells Application of nanomaterials for anti-reflective by improving light trapping and photo-carrier collec- coating is an important source of energy conservation tion abilities of devices used in photovoltaic cells. Table 3: Summary of nanomaterials and nanostructure photovoltaic (PV) devices. Type of nanomaterial Remarks References Single nanowire PV – Consist mainly of quasi-1-D materials such as nanowires, nanopillars, and [3, 158, 159] devices nanotubes – This includes integrated, nanoelectronics, and photoelectronic materials – The materials have controllable carrier transport along the axis – Low efficiency of PV devices is due to the large surface-to-volume- ratio and high surface recombination of Si – Au catalyst used for the fabrication of Si nanowires in vapor-liquid-solid growth introduces impurities that affect the band gaps, deteriorate photon-carrier life cycle, and reduce PV device performance Nanowire and – Si, InP, CdS, GaAs, and ZnO have been used as active materials in the devices [160, 161] nanopillar array PV – Fabrication has been mainly by vapor-liquid-solid technique, dry etching, top-down devices etching, and photolithography – p-n junction-masked nanowire, microwire, and nanopillar devices have improved photon-carrier collection – Si-fabricated nanowires have increased efficiency in PV devices CdS, CdTe, and – CdS thin film and CdTe thin film have untreated surface recombination velocity of [3, 162] nanopillar thin-film 108 cmS−1 and 104 cmS−1, respectively PV devices –A  lumina membranes have been used as template for fabricating these thin-film PV devices – Nanoimprint-assisted anodization process has been used to grow ordered hexagonal pore array of alumina membrane – Nanopillar solar fabrication can be implemented on bendable plastics for PV application Nanoparticles and – These nanomaterials are noted for unique properties such as size-dependent [3, 163, 164] quantum dot solar band gap and multi-excitation generation, which enables new PV mechanism to cells potentially break current thermodynamic limits –C  ompatible with solution-based process and can be fabricated using low temperature at low cost and high throughput  C.I. Ossai and N. Raghavan: Nanostructures and nanomaterials 225 Nanostructures enhance the performance of photovoltaic to the decrease in the density of the oxygen resulting from performance by facilitating the photon-carrier collection electron captured by the invading environmental oxygen. via ensuring that the direction of light propagation is Furthermore, SnO2 hierarchical nanostructures, prepared orthogonal on the carrier collection. Table 3 summarizes by the hydrothermal process, using stannous sulfate some of the nanostructures and nanomaterials that have (SnSO4) and trisodium citrate dihydrate (Na3C6H5O7·2H2O) been predominantly used for photovoltaic cells. in an ethanol-water system were used for testing butanol gas sensing ability by Zhang et al.. The researchers affirmed that the nanostructure could sense butanol gas 6.3 G  as-sensing application of concentration from 1 to 200 ppm (parts per million) with nanostructures a stable electrical signal over a period of 60 days. The sensing ability of the nanostructure was also attributed Nanostructures have found excellent applications in gas to the ability to absorb atmospheric oxygen, ionizing and sensing, due to the high specific area , change of capturing of free electrons from the SnO2 conductance resistance on exposure to gases , and high photo- band per Eq. (20). conduction abilities. Different nanomaterials such  Ο 2 (gas) + e −1 → Ο 2− (ads) as nanowires, nanoribbons, nanorods, nanotubes, and  − (20) Ο 2 (ads) + e → 2Ο (ads) −1 − nanobelts made from ZnO, TiO2, WO3, In2O3, Fe2O3, CuO,  NiO, Ga2O3, SnO2, and V2O5 have typical characteristics that make them very good materials for gas-sensing applica- The presence of O2−(ads) and O−(ads) ions on the tion. The work of numerous researchers has attested nanocrystal of SnO2 enhances the receptor functions of to the usability of the above-mentioned nanomaterials in the nanostructure and its ability to sense gases as the gas sensing, as exemplified in literature [167, 169–171]. exposition of sensors made with the material, which has Exposing ZnO nanostructures to ambient gases that a reducing ability, reacts with the molecules of the gas, have oxidizing tendencies causes the surface of the mate- absorbing oxygen, and releasing trapped electrons on the rial to adsorb the gas molecules, which captures an elec- conductance band [171, 175, 176]. α-Fe2O3 nanotubes with tron from the ZnO conduction band and causes an increase uniform growth morphology have been shown to have a in the resistance of the material [167–169, 172]. The work of high specific area and exhibits good sensitivity to gases Pan et al. on ZnO hierarchical nanostructures using as well as excellent electrochemical characteristic that the complementary metal oxide-semiconductor technique culminates in a discharge capability of 1415 mAhS−1 at to form a patterned triple layer nanostructure was about 100 mAS−1 and 20°C. 2800% more efficient for gas sensing than regularly used nanowire gas sensors. The researchers ensured a local growth of the ZnO hierarchical nanostructure on a single Si chip at ambient temperature using photolithography, 7 Conclusions physical vapor deposition, and lift-off. A Au catalyst con- tained in a triple-layer metal electrode in the fabrication This research focused on the advances in nanotechnology of the nanostructure facilitated the reaction. Similarly, as it applies to nanostructure characterization, nanofab- individual single-crystals of the SnO2 nanoribbons sense rication, growth mechanism, and industrial application. NO2 gas at room temperature due to the strong photocon- The unique characteristics of nanostructures and nano- duction abilities of the nanostructure, which resulted in materials, which includes high band gap, piezoelectrical, electron trapping and adsorption of NO2 and increasing biocompatible, and pyroelectrical properties made them of the conductivity of the nanoribbons. Wang et al. suitable for sensors, photoanodes, transistors, light-emit- used three types of TiO2 nanobelts – Na2Ti3O2, H2Ti3O7, ting diodes, lasers, and logic gate devices. and TiO2 prepared by hydrothermal process to obtain The use of different physical, chemical, and mechani- nanobelts that were used for gas sensing. They connected cal techniques such as the wet chemical method, elec- the nanobelts with hydrogen bonds and bridged oxygen trodeposition, metal organic chemical deposition, flux atoms to enhance their properties for gas sensing at sub method, molecular beam epitaxy, electrospinning, and part per billion (ppb) levels. As TiO2 is an n-type semi-con- sputtering have helped to develop unique nanostructures. ductor , the concentration of oxygen in the nanobelt These nanostructures that include nanowires, nano- was affected by the oxygen partial pressure in the environ- tubes, nanobelts, nanohelices, nanorings, nanocage, ment. This caused a higher resistance in the nanobelts due nanosheets, and nanorods have different orientations, 226 C.I. Ossai and N. Raghavan: Nanostructures and nanomaterials morphologies, and characteristics. 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