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 [8]. 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 [13]. 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 [17]. 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 [22], experts are consistently working out new techniques of fabricating 2 Crystallographic 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 [8], 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 [31]. The nucleation optical and electrical characteristics of semiconductor process, reaction of molecules, and crystallization of nan- devices developed with them [13]. 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 [32] because it forms the building blocks to reduce the defects in the nanostructures [27], 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 [35]. Again, the existence reactions and the formation of nanocrystals [28] 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- [36]. Anisotropic nanoparticles have also interacted at tovoltaic cells [29]. The aspect ratio, orientation, chemi- nanoscale to aid directional bonding, by the exchange of cal composition, and surface energy at the nanostructure valences [36] 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,