Perspective Chapter: Effect of Gold Seed Layer Annealing on the Surface Roughness and Nanostructure Growth

Abstract

ZnO has gain a great attention in many applications due to its wide band gap. Orientation and alignment of ZnO nanorods are the key objectives of fundamental applied research. They may be produced by both physical and chemical methods, however the chemical method has the advantages of low temperature and pressure conditions. The electronic properties of ZnO nanorods are more superior then the thin films. Most of the applications of ZnO nanorods depends on the morphology, orientation and interspacing among them. Seed layer on the substrate has a key role in the morphology of ZnO nanorods. In this chapter the, orientation, alignment and a clear mechanism of ZnO nanorods production in hydrothermal method is presented. The experimental results deduced that the ZnO nanorods are produced in the precursor solution and move down to the substrate through 001 face stab between the successive grains generated through annealing of gold seed layer, and as a result an oriented and aligned array of the nanorods are formed on the substrate.

1. Introduction

ZnO is a distinctive material and has been extensively studied by researchers for its potential in variety of applications such as piezoelectric, pyroelectric, dielectric, plate panel display sensor devices, field effect transistor and ultraviolet light emitting devices [1]. In nanostructure form, ZnO is extensively explored over the past decades exhibit unique properties as compared to its bulk form. Nanostructured ZnO consists of structures with in a range of 100 nm in diameter [2]. Zinc oxide is a compound semiconductor which consists of Zn+2 and O-2. Zinc belongs to group II of transition metals and oxygen to group VI element, therefore it is called a metal oxide semiconductor. The nanostructured materials have been broadly studied due to their potential uses in fabricated micro and nanoscale devices. The research on ZnO peaked around the end of 1970s and the start of 1980s. The field is incited by theoretical predictions and feasibly experimental conformations of ferromagnetism at room temperature for potential spintronics applications. Then the interest was ended, partly because it was not possible to dope ZnO both n- and p-type, which is a crucial requirement for applications of ZnO in optoelectronics, and the interested moved to structures of reduced dimensionality. Scientists have created numerous nanostructures, but it is unknown which type nanostructure is ideal for electronic and optoelectronic device applications. The ZnO thin film has a vital role in sensors, catalyst and transducer since 1960. Figure 1 indicates that the literature survey about ZnO nanomaterials increasing significantly for the chemical and gas sensors [3, 4].

Figure 1.

Web of knowledge database lists on the number of publications of ZnO nanostructures for chemical and gas sensors.

The special properties of 1D ZnO nanostructures are their chemical stability, and electrochemical selectivity that provide an adjustable platform essential for the sensors. Therefore, the ZnO nanostructures NRs/SWs are extensively investigated for gas sensor, humidity sensor, biosensor, biomarker, pressure/force/load sensors, pH sensors and UV sensors. During the last decades nanotechnology has focused its attention to one dimensional (1-D) materials. By reducing the size to nanoscale, novel electrical, optical, mechanical, and chemical properties are introduced, which are mostly believed to be the increase surface area and quantum confinement effect. For instance, it has been experimentally proved that single crystalline ZnO nanorods can have an electron field mobility 10 times greater than ZnO thin film transistor.

This chapter presents the importance of oriented zinc oxide (ZnO) nanorods in various applications and devices. ZnO is a semiconducting material at room temperature, has a wide direct band gap (3.37 eV) and a large exciton binding energy (60 meV) [5]. The high exciton binding energy (60 meV) of ZnO at room temperature is responsible for stable electron–hole pair recombination and excellent luminescence properties. The wide band gap materials are more effective in high temperature and high power applications for their ability to withstand high power due to their substantially higher breakdown voltage.

The performance of nanoscale optoelectronic devices and biosensing are significantly influenced by orientation and alignment of ZnO nanorods, however despite of prodigious research there is no clear mechanism for the formation and orientation of ZnO nanorods has been identified. In this study the oriented ZnO nanorods were fabricated through hydrothermal technique from the lowest precursor concentration. The concentrations of precursors were varied from 1 to 9 mM at same deposition temperature (90°C) and time (6 hours) on annealed and un annealed gold coated substrate. The root mean square roughness of the seed layer increased from 6.11 to 8.839 nm through annealing at 500°C.

Recently, the well-aligned ZnO nanorods have obtained a lot of curiosity due to their potential demand in nanodevices for electronics, optoelectronics, photonics, and electrochemical nanodevices [6]. A lot of effort has gone into getting the aligned and controlled morphology of the ZnO nanostructures. The use of well aligned ZnO nanorods has motivated the researchers to control the alignment of ZnO nanorods for the performance of nanoscaled-based optoelectronics devices [7], nanolaser [8], chemical and gas sensor [9], solar cell [10] and Schottky diodes [11]. The need and advantage of ZnO nanorods to be aligned in biosensing applications is to make way to attachment of antibodies on a maximum number of sites on the nanorods in a way to improve the sensitivity of the device. The nanorod overlapping could prevent antibodies from adhering to the spot if the nanorods are not well aligned. The orientation and uniform spacing of ZnO nanorods can also improve the light ensnaring and charge separation in polymer solar cell, and a result the efficiency and field emission characteristics are significantly enhanced [12]. The vertically aligned ZnO nanorods creates more surface defects and oxygen vacancies, have a better photocatalytic efficiency due to their increased specific surface area, which makes the nanorod an effective photocatalyst in photocatalytic applications [13]. By growing vertically oriented ZnO nanorods there is a probability to increase the electron transport rate [14]. Lattice matching of the substrate with ZnO is a key factor for the synthesizing of vertical aligned nanorods. Wenjie Mai et al. [15] used several substrates (GaN, SiC, Si, and sapphire), and concluded that the vertical aligned wurtzite ZnO (a = 0.3249 nm, c = 0.5207 nm) nanorods can be produced on wurtzite SiC (a = 0.3076 nm, c = 0.3046 nm) and GaN (0.3189 nm, 0.5185 nm) Fan et al. [16] used GaN and sapphire, to produce the vertical aligned ZnO nanowires and concluded that GaN is more suitable for the growth of aligned nanorods due the similar crystal structure and lattice matching. There are some recent researches reported on the growth of oriented ZnO nanorods on Transparent conducting oxide (TCO) glass substrates 1.e FTO (F-doped SnO), ITO (Sn doped In₂O₃) and IZO:Ga without using. The results indicated that the orientation and alignment of ZnO nanorods on IZO:Ga is more better than the other two with a sequence i.e. ITO < FTO < IZO:Ga.

2. Preference of 1-D ZnO on other nanostructures

The research on the nanostructures is increased after the discovery of carbon nanotubes in 1991. ZnO was investigated in 1912 for the first time, however its practical applications were focused after discovering its piezoelectric properties in 1960 [17], this resulted in the first electronic use of zinc oxide as a thin layer for surface acoustic wave devices. After significant research phases in the 1950s and 1970s, zinc oxide as a semiconducting material is currently experiencing a revival. The electrical and optical properties ZnO with pencil like crystal with hexagonal shape were reported in 1960. After that a variety of nanostructures (Plates, columns, pyramids, spheres, stellar-shaped crystals, dendrites and whiskers) were obtained by Yamada and Tobisawa under extreme nonequilibrium conditions of a converging shock-wave using an explosive charge [18]. The size of nanostructure is in between the range of 1–100 nm (100,000 times smaller than the human hair diameter). ZnO is one of the mostly research metal oxide. It is an inorganic semiconducting material that is rarely seen in nature and appears as a white crystalline powder. ZnO is usually referred to as zinc white because it appears as a “white powder.” Due to the presence of manganese and/or other impurities, it can seem yellow to reddish. Zinc, like magnesium and iron, is a naturally occurring metallic element and is found in the body’s and skin and is essential for maintaining health and balance. Since the discovery of quantum confinement effect that enhanced the properties of bulk material, this material the attention due to its large number of nanostructures (Nanoparticle, nanostrips, nanorods, nanowires, nanorings, nanocombs, nanocages nanosprings) [19].

With the discovery of semiconducting metal oxide nanobelts in 2001, the one dimensional nanostructures has attracted the attention due to their unique applications in optoelectronics, optics, catalysis and piezoelectricity [19]. Nanostructures have a large surface area, and surface processes have a big influence on electronic processes. The uniform and ordered growth of ZnO nanostructures is highly desirable and is potentially applicable in many devices, however the complex, morphologies of ZnO nanostructures complicate their functionality, i.e. limit their practical applications [20]. The 1-D ZnO nanostructures can provide a favorable combination of chemical selectivity, electrochemically and chemically tunable platform for the sensor response. This leads the 1-D ZnO nanostructures an active candidate for potential applications in gas sensor, biosensor, biomarkers, humidity sensor, pH sensor, chemical sensors and UV sensors due to their high surface area and low cost of fabrication. In the last few decades nanotechnology has focused its attention to one dimensional (1-D) materials. By reducing the size to nanoscale, novel optical, mechanical, chemical and electrical properties are introduced, which are mostly believed to be the increase surface area and quantum confinement effect. For instance, it has proved that the electron field mobility of single crystalline ZnO nanorod can be 10 times higher than that of ZnO thin film transistors [21]. 1-D ZnO is one of the most intensively studied nanorods (NRs) material. Peulon, was the first who developed ZnO nanorods in 1996 using an electrodeposition method. Direct carrier conduction path and large surface to volume ratio of 1-D ZnO nanostructures are the key factors in getting an edge over other types of nanostructures. ZnO nanorods are flexible metal oxide nanomaterials which have been successfully used in different fields such as chemical and biological sensing, optoelectronics, energy harvesting and ultraviolet (UV) photodetection [22]. ZnO nanorods are potential material for optoelectronic applications due to have less defects then thin-film structures.

3. Effect of annealing on gold seed layer and orientation of ZnO

3.1 Chromium (Cr) and Gold Seed layer annealing

The ZnO nanorods were produced on two substrates with different roughness through same synthesis method and using same parameters. A Cr/Au (20/50 nm) coated glass substrates were selected for the growth of ZnO nanorods. Some of the Cr/Au (20/50 nm) coated glass substrates were subjected to a preheated oven, annealed at 500°С for 30 minutes. The seed layer is annealed for three reasons: increased adhesion strength, improved crystallinity, density of peaks and valleys [23, 24] and roughness. The 3D, AFM images of annealed and un annealed Cr/Au coated glass substrate is shown in Figure 2, where different colors on the seed layer image corresponds to different height structures. The Au seed layer for nonannealed sample is non continues with large void spaces and less heights entity (maximum height 20 nm). The Au seed layer is substantially rebuilt, its surface morphology changes, and islands of various sizes are created after 30 minutes of annealing at 500°С. The annealed seed layer, which is crystalline and has several grains, offers sites where ZnO nanorods can be oriented. The increase in the diffusion of gold particles at high temperatures and their agglomeration into massive structures may be the cause of this alteration after annealing.

Figure 2.

AFM images of Au seed layer roughness (a) Un annealed and (b) Annealed.

In Table 1 various roughness parameters, Rpv, Rq, Ra, and RZ represent peak to valley value (difference between minimum and maximum), root mean square roughness, roughness average, and 10 points average, which is the arithmetic of 5 highest peaks and 5 lowest valleys in line, respectively, the Min and Max values represent the deepest valley and highest peaks in the scanned area. Table 1 represents that the root mean square roughness (Rq) and roughness average are increase from 6.771 to 8.839 nm and 4.074 to6.506 nm respectively.

Surface parameters	Rpv(nm)	Ra(nm)	RZ(nm)	Rq(nm)	Max(nm)	Min(nm)	Mid(nm)
Annealed	78.008	6.506	76.503	8.839	49.184	−28.824	10.180
Un Annealed	100.734	4.074	98.954	6.711	81.694	−19.040	31.327

Table 1.

The various roughness parameters of annealed and un-annealed seed layer.

3.2 ZnO nanorods growth

ZnO nanorods produced on both annealed and un annealed Cr/Au seed layer present a completely different morphology as discussed by Mustafa et al. [25] and Younas et al. [26]. The nanorods produced on annealed Cr/Au seed layer has better orientation then the nanorods produced on un annealed seed layer with same precursors concentration as shown in Figures 3 and 4. Figure 3 shows FESEM images of nanorods produced on un annealed seed layer from 8 mM precursors concentration zinc nitrate hexahydrate (Zn(NO₃)₂6H₂O) and hexamethylenetetramine (C₆H₁₂N₄), at same deposition time and temperature at various magnifications. The (Zn(NO₃)₂6H₂O) provide the Zn²⁺ ion essential for the production of ZnO nanorods, whereas the hexamethylenetetramine is used for capping the non-polar facets and providing the hydroxide source.

Figure 3.

Surface morphology of ZnO nanorods for 8 mM produced on un annealed seed layer at (a) 1000× (b) 5000× (c) 10,000 (d) 25,000 (e) 50,000 (f) 100,000 magnifications.

Figure 4.

Surface morphology of ZnO nanorods for 8 mM produced on annealed seed layer at (a) 1000× (b) 5000× (c) 10,000 (d) 25,000 (e) 50,000 (f) 100,000 magnification.

As can be seen in Figure 4, with the same concentrations, deposition times, and temperatures as the un annealed gold seed layer, the nanorods formed at the annealed seed layer have better alignment. The lower magnification image of Figures 3(a) and 4(a) indicates that whole substrates are filled with dense nanorods.

The improvement in orientation and alignment of nanorods confirm that the ZnO nanorods synthesized in hydrothermal method does not grow on the substrate, but these nanorods are formed in the solution, and when these nanorods become denser they come down in solution through (002) face collected on the substrate. It means that along with heterogeneous growth of ZnO nanorods on the substrate there is a homogeneous nucleation of ZnO nanorods inside the solution along the polar axis. The homogeneous nucleation starts inside the solution and terminate the growth on a substrate by heterogeneous nucleation. In Figure 4 some nanorods penetrates into other which confirm that the ZnO nanorods formed in growth solution are not in completely solid form, but are in soft crustal which become harder and rigid with growth time and cooling.

The main reason for the growth in (001) polar faces is that the wurtzite ZnO is electrostatically unstable and as a result redistributes its surface charge and lowers its surface energy. Figure 5 shows the presence of a few additional nanorods fall upon the initially formed nanorods film from the growth solution, reveals that the formation nanorods are started in the growth solution.

Figure 5.

Cross section representation ZnO nanorods at 8 mM precursor solution.

4. Internal mechanism of ZnO nanorods growth and orientation

The main reasons for the researchers trying to produce the vertically aligned ZnO nanorods are their best performance in nanoscaled-based laser [27] Optoelectronic devices [7] solar cells [28] and Schottky diodes [29]. In literature, various techniques with their benefits and drawbacks have been employed for the deposition of vertically aligned nanorods, however they required a very sophisticated equipment and manufacturing process. In order to produce well aligned and oriented ZnO nanorods of a substrate, a seed layer is sprayed before the nanorods growth. The seed layer surfaces are either positive or negative charged, therefore will attract ions of opposite charges (Zn⁺² or OH-1). These ions will successively attract ion with opposite charge to cover the surface and will react to form ZnO. Thus the nanorods grow layer by layer promoting good alignment. ZnO has polar (001, 00-1) and nonpolar surfaces {01-1, 2-1-1}. At the beginning there are both lateral and axial growth through the nonpolar and polar surfaces respectively, however when the nanorods growth reached certain limit then the axial growth is more significant than lateral growth. The preferred direction is along the (001) direction because the dipole moment is along this direction.

The high atomic organization order, which provides sites for heterogeneous ZnO nanorods nucleation on the substrates, is responsible for this behavior. The initial growth of the nanorods is started on the grains produced on the annealed substrate, and further growth is proceeded by the heterogeneous nucleation in the solution. It’s also possible that some of the nanorods growth took place in the fluid through homogenous nucleation, and then fall down on the seed layered substrate [26]. The increase of the nucleation sites on the annealed seed layer improves the alignment and orientation of the nanorods. As a result of the energetically advantageous nucleation sites for heterogeneous nucleation, oriented nanorods are generated on annealed gold seed multilayer substrates, as shown in Figure 6. The exposed basal planes of ZnO nanorods are polar, attracts more ion species due to high surface energy and as a result faster growth occurs in this direction through homogeneous nucleation in the growth solution and there is probability of vertically aligned ZnO nanorods to be produced. The developed nanorods become denser with the passage of time and as a result come down in the solution towards the substrate and penetrate between the grains produced due to the annealing, and as a result an oriented and aligned nanorods array is produced on the substrate.

Figure 6.

Proposed growth mechanism of ZnO nanorods in hydrothermal method.

5. Conclusion

The 1D nanostructure of ZnO (nanorods) has been used for a variety of applications due to their high surface to volume ratio, novel electrical properties and electronic conduction. The shape and configuration of ZnO nanorods plays a vital role in its performance and device application. The ZnO nanorods produced on annealed and un annealed gold seed layer reveals that nanorods produced on annealed seed layer has a better alignment and orientation as compared to the nanorods produced on un annealed seed layer. In this chapter the role and importance of the seed layer roughness on the orientation and alignment is presented. A proposed mechanism for the production of ZnO nanorods through hydrothermal method is presented.