Synthesis of Nanowire Using Glancing Angle Deposition and Their Applications

Nanowires are highly attractive for advanced nanoelectronics and nanoscience applications, due to its novel properties such as increased surface area, large aspect ratio, and increased surface scattering of electrons and phonons. The design and fabrication of nanowires array provide a great platform to overcome the challenges/limitation of its counter partner. This chapter focuses on the synthesis of metal oxide nanowire and axial heterostructure nanowire array using the Glancing angle deposition (GLAD) technique. The structural, optical and electrical properties are studied. This GLAD technique offers control over one-dimensional (1D) nanostructure growth with self-alignment capability. It is also reviewed in an effort to cover the various application in this area of optoelectronic devices and wettability applications that had been synthesized using GLAD.


Introduction
Low-dimensional nanostructures such as zero-dimensional (0D), one-dimensional (1D) and two-dimensional (2D) have attracted enormous attention from three-dimensional or bulk structure due to the novel physical and chemical properties caused by size and quantum effects. Figure 1 illustrates the schematic representation of electron system in bulk structure and low-dimensional nanostructures. Quantum dot are 0D nanostructures with three quantum-confined directions. Quantum plane are 2D nanostructures with one quantum-confined direction, while two unconfined directions is available for movement of particle. Bulk structures are 0D structure with no quantum-confined directions. Nanowires (NWs) are 1D nanostructures with a large aspect ratio (length/diameter), with diameters in the 1-200 nm scale and lengths ranging from some hundreds of nanometers up to several tens of micrometer. Owing to their nanoscale dimensions, they have size confinement effects, which give them novel properties as compared to the bulk materials. Nanowires have two quantum-confined directions but one unconfined direction which is available for electrical conduction. The 1D geometry on the nanoscale of the nanowires provides an increased surface area, very high density of states, diameter dependent bandgap, and increased surface scattering of electrons and phonons [1]. These anisotropic properties are advantageous in nanoelectronics, photonics, optoelectronics, and bioengineering and have also generated great research interest [1,2]. The concept of many advanced nanowire-based optoelectronics devices, including photodetectors, photovoltaic cells, has also been demonstrated, making nanowires promising material for advanced optoelectronics. Thus, nanowires draw considerable attention from those trying to apply nanotechnology as well as investigating in nanoscience.
Nanowires are the result of anisotropic, 1D growth on a nanometer scale. Therefore, the critical issue related to the growth of the nanowires is how to synthesize in a controlled manner. Regarding many approached have been employed, including the use of the nanolithography-based method [3], solution-based method [4], vapor-based methods [5], template-based methods [6], and glancing angle deposition (GLAD) technique [7]. Among these, the GLAD is a physical vapor deposition technique, which is a combination of oblique angle deposition and substrate positional control, which is most attractive owing to its simplicity and synthesis of different nanostructures with controlled porosity and shapes [7].
This chapter reviews the synthesis of nanowires by the GLAD technique and their applications. Furthermore, this chapter focuses on the GLAD technique. This chapter seeks to explain the understanding of the GLAD technique in the synthesis of nanowires. Accordingly, the chapter first reviews the fundamentals of the GLAD technique and the synthesis of nanowires. They are followed by some examples of optoelectronic devices and wettability applications of NWs that have been synthesized based on the GLAD technique.

Oblique angle deposition (OAD)
The glancing angle deposition (GLAD) technique is developed by Robbie and Brett [8], which is an extension to oblique angle deposition (OAD), where the substrate position is manipulated during the deposition. This section reviews the oblique angle deposition.
In OAD, the collimated evaporated beam is incident on the substrate surface normal with an incident angle α, as shown in Figure 2(a). The incident vapor flux is treated as vector denoted by F, as shown in Figure 2(b) with its two components, a vertical component, and a lateral component. The arrival of incident vapor flux is a random process. During the deposition process, the impinging atoms will form islands on the substrate surface at random. The initial deposited seed will act as shadowing centers, and the tallest islands will receive more atoms as compared to the shorter ones. This phenomenon is known as the shadowing effect. With this procedure, only the tallest deposited material will grow into columns and thus result in a nanocolumn thin film formation. The lateral component is responsible for the shadowing effect. This leads to the inclination of the tilting of the nanostructure towards the direction of the incoming vapor flux, and the tilt angle is given by the tangent rule, 1 tan tan 2 b = q . Figure 3 shows how this effect takes place.
In general, the columnar tilt angle β is less than the vapor flux incident angle [7]. The thin films deposited by OAD shows the following properties: Porous thin films acquiring nano-columnar structures. The nanocolumns tilt away from the substrate surface normal and towards the incident vapor flux direction.

Glancing angle deposition
The GLAD technique is developed based on the OAD with the only addition, which is the manipulation of the substrate position by using two stepper motors, one controls the incident angle, α, and the other motor controls the azimuthal rotation of the substrate with respect to the substrate surface normal. Figure 4 shows the GLAD setup. The tilting of the nanocolumn structure found in OAD is mitigated primarily to the rotation of the substrate, which cancels out the lateral component of the incident vapor flux during the deposition process. By changing the speed and phase of the azimuthal rotation along with the deposition rate, different sculptures of nanocolumns such as C-shape, S-shape, helical or vertical nanocolumns can be achieved.
The nanocolumns synthesized by GLAD show the following properties: The porosity of the film is controlled by changing the incident angle. Self-alignment is  due to the shadowing effect. The shape and in-plane alignment of nanocolumns can be modified. GLAD is compatible with a large number of materials [9]. Table 1 summarizes the synthesis of nanowires using the GLAD technique. The following sections give examples of the various applications of nanowires synthesized using the GLAD technique. The general experimental conditions are the following: the experiments were performed in an electron-beam evaporator (BC 300 HHV India) incorporated with GLAD under high vacuum pressure (6 × 10 −6 mbar) inside the chamber. The deposition rate varied from 0.5-1 Å/s, and rotation speed was maintained at 20-30 rpm. The substrate holder was oriented at 85° with respect to the source. During the deposition process, the growth rate was monitored by using a quartz crystal-based digital thickness monitor inside the chamber.

Properties of nanowires synthesized using GLAD
This section discusses the structural and optical properties of nanowires. For various applications, characterizing the structural, optical, and electrical properties of nanowires is important so that the interrelationship can be investigated and established. Nanowires synthesized using the same material under the same experimental conditions may possess different properties due to the differences in structural properties.

Structural characterization
Structural properties of a nanowire help in determining the various attribute like optical and electrical properties. X-ray diffraction (XRD), scanning electron microscope (SEM), transmission electron microscope (TEM) are used to investigate the structural properties of nanowires. XRD characterization provides crystal structure information. The peak in XRD provides the crystal phase structure long with growth direction. SEM produces images of the nanowires down to the length scale of ~100 nm, which gives information regarding the structural arrangement, geometrical features of the nanowire. For example, SEM images of TiO 2 nanowire arrays grown on Si substrate provide evidence for shadowing effect as well as TEM is used for studying the nanowire at the atomic scale. Furthermore, selected area electron diffraction (SAED) patterns provide information on the crystal structure of nanowires. For example, TEM image of TiO 2 provide information on the geometry of the nanowires at nanoscale along with growth direction as shown in inset Figure 5(c).

Optical characterization
The optical characterization of nanowires provides information on properties different from those of the bulk forms. Optical measurements like absorption and photoluminescence provide information on the absorption and emission of light in frequency range varying from UV-Vis-NIR spectra. Absorption measurement aids in determining the bandgap of the grown nanowire shown in Figure 6. The difference in the optical properties is due to the geometric differences such as the diameter and length of nanowires.
Photoluminescence (PL) measurement studies the optical bandgap, oxygen vacancies, and defect states in nanowires. For example, Figure 7 investigates the PL spectrum of TiO 2 NW under 250 nm wavelength excitation.

Applications
In the preceding sections, we have discussed the central characteristics of nanowires, which attracts attention to find the application by using its novel properties compared to their bulk materials. Based on the GLAD technique, many conceptual devices have already been reported. In this section, selected applications of GLAD synthesized nanowires such as photodetectors and wettability applications are discussed. These conceptual devices were investigated based on an array of nanowires. First, the silicon (Si) substrate is subjected to a 3-step cleaning process using electronic grade acetone, methanol, and deionized (DI) water, and then an array of nanowires is synthesized on the Si substrate. The top metal contact electrode is synthesized on the nanowire arrays through an Al mask with a hole diameter ~1 mm and ITO, which is used as the back-contact electrode.  Room temperature PL spectrum of as deposited TiO 2 nanowire. Adapted from Ref. [15].

Photodetector
Semiconductor photodetectors are devices used for the detection of light. More specifically, photodetectors have applications in optical communication, flame detection, chemical, and biological detection [26]. The PN junction is one of the most commonly used configurations for semiconductor photodetectors. Many researchers have synthesized nanowire and axial heterostructure nanowire array-based photodetectors using GLAD technique. Table 2 gives a summary of the figure of merit for the photodetectors. These photodetectors are used for the detection of UV and visible light. The performance of photodetector is evaluated by investigation of various parameters such as photosensitivity, responsivity, detectivity, and noise equivalent ratio. The photosensitivity for photodetectors is calculated from the ratio of light current to dark current given by equation (1) The responsivity at a particular wavelength is computed from the ratio of photocurrent to incident optical power defined below: where I Photo is the photocurrent and P opto is the optical power. The detectivity and NEP give the noise performance of photodetectors. Detectivity is given by where J Dark is the dark current density, e is the charge of electron and R λ is the responsivity at a particular wavelength. The NEP is expressed as where A is the area of device and B is the bandwidth. The bandwidth is assumed to be 1 kHz as the flicker noise. With these relations, detectivity and NEP are plotted as a function of voltage to evaluate the performance of photodetector. Furthermore, the photocurrent-time response gives the temporal response under light on-off to study the switching behavior at a fixed voltage. The cumulative analysis of the figure of merit for photodetectors supports that the synthesis of nanowire and axial heterostructure nanowire array-based photodetectors using the GLAD technique as a potential prospect to fulfill the requirements of commercial UV-Visible photodetectors.

Wettability application
In today's world, various technologies have been obtained from nature. Among them, self-cleaning technology is one of it. Many surfaces found in nature show self-cleaning properties. The leaves of plants such as lotus [27] and wings of butterflies [28] are a few examples. Self-cleaning technology obtained a lot of attention in the late 20th century for applications ranging from solar panel cleaning, windowpane cleaning, and textiles to cement. In recent years, many research works are carried out to develop durable and efficient self-cleaning coating surfaces with improved optical qualities. The self-cleaning coating can be classified into two categories: hydrophobic and hydrophilic coatings. In the hydrophobic coating technique, water droplets roll and slide over the surfaces and clean them, while in the hydrophilic technique, water forms a sheet of water over the surfaces and carries the dirt and other impurities away. The phenomenon of self-cleaning is associated with surface contact angle, which is the angle formed between the surfaces of the liquid droplet to the solid surface. Generally, when the contact angle is less than 90 0 , the solid surface is defined as a hydrophilic surface (Figure 8(a)). If the contact angle is greater than 90 0 , the surface is termed as a hydrophobic surface (Figure 8(b)). Likewise, the surface with a water contact angle close to zero is defined as super hydrophilic, and surface with a contact angle greater than 150 0 is categorized as super hydrophobic (Figure 8(c)).
Young in 1805 proposed a model to define the state of liquid droplet on an ideal rigid surface [29]. The equation defined by Young's model is given as: θ Y is the water contact angle and ϒ SG , ϒ SL and ϒ LG are the interfacial surface tensions of solid and gas, solid and liquid, liquid and gas, respectively. This equation shows the water contact angle of a liquid droplet on solid surface from the three surface tensions. As seen in Figure 9(a), an equilibrium state is reached between these three surface tensions, and the contact angle is given by the angle between ϒ SL and ϒ LG . In 1936, Wenzel further purposed a model to describe homogeneous wetting regimes [30]. This wetting regime for water contact angle on rough surfaces is defined by the equation given below: where θ W and θ Y represent apparent contact angle and Young's contact angle for an ideal rigid surface respectively while r represents the surface roughness factor, which is the ratio of true solid surface area to the apparent surface area. This model states on the affiliation between the structure and surface tension of a homogeneous surface. In Figure 9(b), the true surface area is larger than the apparent surface area, thus the value of r is greater than 1. Wenzel's model is applicable to surface with single chemical component and thermodynamically stable state, which limits its applications in heterogeneous surfaces. In 1944, Cassie and Baxter defined an equation to describe contact angle for composite surfaces [31].
( ) Where θ C is apparent contact angle in Cassie-Baxter model, θ 1 and θ 2 are intrinsic contact angle of first and second components, respectively. f 1  cos f cos f f cos 1 1 q = q -= q + -(8) Figure 9(c) displays a water droplet on surface showing hydrophobic property due to surface composed of air and hydrophobic component.
Metal oxides such as TiO 2 , SnO 2, and ZnO have been used as a self-cleaning and antifogging surface [32,33]. Metal oxides are known to have good stability and transparency, which enable tuning wettability on application of proper radiation. Amidst them, TiO 2 is vastly studied due to its effective photocatalytic activity as it washes off dirt or decomposes organic contaminants from surfaces [34]. Moreover, TiO 2 is known for its nontoxicity, chemical, and thermal stability [16,35,36]. Moreover, growth of perpendicularly aligned coaxial TiO 2 /In 2 O 3 NW on Si substrate is problematic due to limited growth techniques. Here, coaxial TiO 2 /In 2 O 3 NW assembly deposited employing GLAD technique inside electron beam evaporator [37] showed comparatively faster photo-induced wettability tuning within 10 min illumination than the reported SnO 2 doped with Fe thin film deposited using spin coating [38], TiO 2 films deposited using metal-organic chemical vapor deposition process [39], grapheme films prepared using chemical vapor deposition (CVD) method [40], TiO 2 films were fabricated on stainless steel substrates via electrophoretic deposition (EPD) [41]. Based on Cassie-Baxter relation, the water contact angle of TiO 2 /In 2 O 3 NW was found to be 129°. Figure 10(a) shows coaxial TiO 2 / In 2 O 3 NW FESEM side view and inset of Figure 3(b) schematic of TiO 2 /In 2 O 3 NW. In 1D NW heterostructure, there is more surface area to volume ratio and allows charge carriers to flow with less scattering enabling more carriers interaction with water as compared with uniform thin films. Vertically aligned NWs also acquires  characteristics such as low reflectivity and multiple scattering of light, which increase the carriers generation and thus more interaction of separated photogenerated charge carriers with water molecules to adsorb on the surface comparing with thin film. Moreover, TiO 2 /In 2 O 3 NW (1.2 × 10 −3 degree −1 min −1 ) showed better wettability transition rate than TiO 2 NW (3.2 × 10 −4 degree −1 min −1 ) and In 2 O 3 NW (9.2 × 10 −5 degree −1 min −1 ) due to the interfacial surface modification between TiO 2 and In 2 O 3 and effective interaction between photogenerated charge carriers with water molecules (Figure 10(b)) [37]. All these assure prospective applications of coaxial TiO 2 /In 2 O 3 NW grown using the GLAD technique for smart surfaces, with controlled switchable wettability by external stimuli for self-cleaning applications.
Further, TiO 2 /In 2 O 3 NW surface wettability had been tuned by annealing treatment, without changing the surface with extra chemical coating or by external light stimuli [42]. Figure 10(c) shows 600°C Annealed TiO 2 /In 2 O 3 NW FESEM side view. TiO 2 /In 2 O 3 NW samples annealed at 600°C shows nearly superhydrophilic with static water contact angle of 12° (Figure 10(d)). The surface of TiO 2 /In 2 O 3 NW had been controlled to acquire desired water contact angles, which is paramount for designing practical application in self-cleaning, electronic, and biomedical fields.

Conclusion
In this capture, we have reviewed the synthesis of nanowires using GLAD and their applications. We have showed that GLAD is a simple, cost effective, and catalytic free technique where well-defined vertically aligned nanostructures can be synthesized which cannot be achieved easily by nanolithography-based method, solution-based method, vapor-based methods, template-based methods. Nanowires synthesized by GLAD technique has the following advantages: Growth of vertically aligned nanostructure, the shadowing effect introduces self-alignment effect, the porosity of the nanostructure film can be controlled by changing the incident angle. Various applications of nanowire and axial heterostructure nanowire array-based photodetectors as well as wettability applications synthesized using the GLAD technique have been discussed. The performance of these applications can be improved further with different structural and growth parameters. Therefore, it can be concluded that these nanoscale-based applications have potential for future industrial and commercial applications.