Synthesis of Nanowire Using Glancing Angle Deposition and Their Applications

2.1 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.

2.2 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.

2.2.1 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 TiO₂ nanowire arrays grown on Si substrate provide evidence for shadowing effect as well as information on the geometry of the nanowires, as shown in Figure 5(a) and (b). 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 TiO₂ provide information on the geometry of the nanowires at nanoscale along with growth direction as shown in inset Figure 5(c).

2.2.2 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 TiO₂ NW under 250 nm wavelength excitation.

3.1 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 below:

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.

3.2 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 90⁰, the solid surface is defined as a hydrophilic surface (Figure 8(a)). If the contact angle is greater than 90⁰, 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 150⁰ 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, θ₁ and θ₂ are intrinsic contact angle of first and second components, respectively. f₁ and f₂ are apparent area fraction of first and second component, respectively. In general, if either one of surface component is air, then θ₂ = 180⁰.

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 TiO₂, 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, TiO₂ is vastly studied due to its effective photocatalytic activity as it washes off dirt or decomposes organic contaminants from surfaces [34]. Moreover, TiO₂ is known for its nontoxicity, chemical, and thermal stability [16, 35, 36]. Moreover, growth of perpendicularly aligned coaxial TiO₂/In₂O₃ NW on Si substrate is problematic due to limited growth techniques. Here, coaxial TiO₂/In₂O₃ 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 SnO₂ doped with Fe thin film deposited using spin coating [38], TiO₂ films deposited using metal-organic chemical vapor deposition process [39], grapheme films prepared using chemical vapor deposition (CVD) method [40], TiO₂ films were fabricated on stainless steel substrates via electrophoretic deposition (EPD) [41]. Based on Cassie-Baxter relation, the water contact angle of TiO₂/In₂O₃ NW was found to be 129°. Figure 10(a) shows coaxial TiO₂/In₂O₃ NW FESEM side view and inset of Figure 3(b) schematic of TiO₂/In₂O₃ 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, TiO₂/In₂O₃ NW (1.2 × 10⁻³ degree⁻¹ min⁻¹) showed better wettability transition rate than TiO₂ NW (3.2 × 10⁻⁴ degree⁻¹ min⁻¹) and In₂O₃ NW (9.2 × 10⁻⁵ degree⁻¹ min⁻¹) due to the interfacial surface modification between TiO₂ and In₂O₃ and effective interaction between photogenerated charge carriers with water molecules (Figure 10(b)) [37]. All these assure prospective applications of coaxial TiO₂/In₂O₃ NW grown using the GLAD technique for smart surfaces, with controlled switchable wettability by external stimuli for self-cleaning applications.

Further, TiO₂/In₂O₃ 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 TiO₂/In₂O₃ NW FESEM side view. TiO₂/In₂O₃ NW samples annealed at 600°C shows nearly superhydrophilic with static water contact angle of 12° (Figure 10(d)). The surface of TiO₂/In₂O₃ NW had been controlled to acquire desired water contact angles, which is paramount for designing practical application in self-cleaning, electronic, and biomedical fields.