Optimization of the CVD parameters for ZnO nanorods growth: Its photoluminescence and field emission properties
Graphical abstract
Introduction
As emerging electronic, optoelectronic and spintronic systems require the development of functional materials with multi-functionality, various research groups are working on the synthesis of novel nanostructured materials and their integrations into various device platforms [1,2]. Nanostructures of metal oxides are extremely important in developing modern devices as they show extraordinary multifunctionality [2,3]. Recently, one-dimensional (1D) nanostructures such as nanorods, nanowires and nanobelts have been attracting enormous attention in the field of nanotechnology [[4], [5], [6]]. Zinc oxide (ZnO), a direct wide bandgap (3.37 eV) semiconducting, photonic, magnetic and piezoelectric material, has a great potential for the wide range of applications in spintronic, solar cells, sensors, photonic devices, plasmonic devices and photocatalysis [7,8]. Well aligned and single crystalline 1D ZnO nanorods of high aspect ratio (length/diameter) are finding potential applications in nanogenerators, UV sensors, light emitting diodes, lasers and field-emission devices [[8], [9], [10], [11]].
Therefore, it is desirable to investigate the growth mechanism and various factors affecting the morphology of grown nanostructures, in order to get single crystalline 1D nanostructures. In recent years, the aligned growth of single crystalline ZnO nanorods has been successfully achieved on solid substrates via chemical vapor deposition (CVD) method with the use of metal catalysts, in which the metal particles in molten state provide sites for diffusion of reactant species and after super-saturation state of catalyst liquid droplet with reactant species it nucleates and form ZnO nanorods. The growth of nanorods is very much guided by the metal catalysts [[11], [12], [13], [14], [15], [16], [17]]. There are several reports related to the growth of ZnO nanorods in which various techniques such as metal organic vapor-phase epitaxial growth, template assisted growth, sputtering, electrodeposition and hydrothermal have been employed [[18], [19], [20], [21], [22], [23], [24], [25]]. However, in spite of this significant advancement in synthesis techniques, a large number of nucleation and growth related problems remain unresolved. This includes difficulty in the growth of single crystalline nanorods with a uniform diameter over the length, problems related to intentionally doping ZnO especially by p-type impurity(ies) and the control over nucleation and growth in order to form the high quality of nanostructures for applications in devices [26,27]. In CVD, the use of Au catalyst facilitates the growth of single crystalline nanorods with remarkably uniform diameter over the length of nanorods; metal catalyst particles also provide a simple parameter to control the diameter of nanorods. However, optimization of parameters for the growth of single crystalline ZnO nanorods using CVD reactor is challenging, because there are many parameters including metal catalyst layer thickness/morphology, substrate temperature during growth, growth time period, flow rates of carrier gas through which reactant species (vapor) get transported to substrate and supply rate of reactant species and their stoichiometry playing their roles, simultaneously. But, the nanostructures grown by CVD method are of high crystallinity and are found to have excellent adhesion with the substrate, as compared to low-temperature growth techniques, which make it a novel method for the growth of such nanostructures [[28], [29], [30], [31]].
Having single crystalline oriented structure, free from bulk and surface defects, high aspect ratio, good electrical and thermal properties and chemical inertness of CVD grown ZnO nanorods make them a potential candidate for electron source applications like field emission display, field emission electron microscope, microwave amplifiers in satellite communications and miniature X-ray tubes [11,32]. Important features of a field emission electron source for application in device fabrications are low turn-on field, high field emission current density and high temporal stability of field emission current density [[33], [34], [35]]. Due to single crystalline nature of ZnO nanorods, the defect concentration along the length is low and grain boundaries are also not present, therefore, it reduces the scattering of electrons during transport along its length and field emission current gets enhanced along with low threshold electric field. The Au-metal nanoparticles on the tip of ZnO nanorods change their electronic structure by increasing density of states near the Fermi level and enhances field emission properties [11].
In the present work, high quality single crystalline ZnO nanorods of various lengths have been grown by CVD method. To the best of our knowledge, we are presenting the first article on optimization of largest number of key parameters of the CVD method altogether for the growth of ZnO nanorods, i.e., substrate temperature, metal catalyst thickness and growth time duration. The room temperature photoluminescence (PL) spectra of these samples show strong and narrow bandwidth UV emission at 382 nm and the weak visible broad emission centered at the green region. These PL results also reveal the qualitative idea of defect concentrations and crystal quality of the samples. The field emission properties were also investigated for nanorods arrays. It reveals that the ZnO nanorods with 20 min growth time (keeping Au film thickness 5.0 nm and substrate temperature 800 °C) show good field emission property. Hence, the grown nanorods arrays may find many other promising applications including nano-sensors for chemicals and biomolecules, excitonic lasers and spintronic devices.
Section snippets
Preparation of films
The vapor-liquid-solid (VLS) synthesis process involves three main steps: metal alloying, crystal nucleation and axial growth. In general, Au catalyst is used to guide ZnO growth and define the diameter and position of the nanorods. The Au thin films of various thicknesses (5.0, 20.0 and 100.0 nm) were sputter deposited on the Si substrates using dc-sputtering. The Si substrates with Au thin film (Au/Si) were loaded in a horizontal tube furnace (CVD reactor) for ZnO nanostructure growth. The
Optimization of CVD parameters
In this work, the observations of nanostructured film growth were conducted by varying some key parameters of CVD technique. Although, the morphology of grown nanostructures depends on the various parameters of CVD experiment, such as (i) the concentration of precursor (zinc and oxygen in carrier gas) and their stoichiometry, (ii) flow rate of carrier gas, (iii) substrate temperature during growth (iv) size of the Au droplets as catalyst, (v) growth time duration, etc. [[37], [38], [39], [40],
Conclusions
In summary, single-crystalline ZnO nanorods were synthesized by VLS process using CVD technique. The nanorods, with diameters of 70.0 nm and lengths of several micrometers has been grown and the growth direction was observed along [0001] by HRTEM analysis. The CVD growth parameters such as substrate temperature, Au catalyst film thickness, and growth duration is optimized. The critically observed parameters are the temperature 800 °C for optimum growth and the Au catalyst film thickness 5.0 nm
Acknowledgements
We gratefully acknowledge the partial financial support by Inter University Accelerator Centre (IUAC), Delhi – 110067, under project ID UFR # 54306 and University of Delhi, Delhi – 110007 under the R&D scheme (Project # RC/2015/9677). We are also thankful to Prof. Santanu Ghosh and M. Sreekanth, IIT-Delhi, India for providing field emission facility. One of us (Sudhisht Kumar) is thankful to IUAC for a research fellowship.
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