A catalyst-free growth of aluminum-doped ZnO nanorods by thermal evaporation
© Suhaimi et al.; licensee Springer. 2014
Received: 16 February 2014
Accepted: 1 May 2014
Published: 23 May 2014
The growth of Al:ZnO nanorods on a silicon substrate using a low-temperature thermal evaporation method is reported. The samples were fabricated within a horizontal quartz tube under controlled supply of O2 gas where Zn and Al powders were previously mixed and heated at 700°C. This allows the reactant vapors to deposit onto the substrate placed vertically above the source materials. Both the undoped and doped samples were characterized using scanning electron microscopy (SEM), field emission scanning electron microscopy (FESEM), energy-dispersive X-ray spectroscopy (EDX), high-resolution transmission electron microscopy (HRTEM) and photoluminescence (PL) measurements. It was observed that randomly oriented nanowires were formed with varying nanostructures as the dopant concentrations were increased from 0.6 at.% to 11.3 at.% with the appearance of ‘pencil-like’ shape at 2.4 at.%, measuring between 260 to 350 nm and 720 nm in diameter and length, respectively. The HRTEM images revealed nanorods fringes of 0.46 nm wide, an equivalent to the lattice constant of ZnO and correspond to the (0001) fringes with regard to the growth direction. The as-prepared Al:ZnO samples exhibited a strong UV emission band located at approximately 389 nm (E g = 3.19 eV) with multiple other low intensity peaks appeared at wavelengths greater than 400 nm contributed by oxygen vacancies. The results showed the importance of Al doping that played an important role on the morphology and optical properties of ZnO nanostructures. This may led to potential nanodevices in sensor and biological applications.
Zinc oxide (ZnO) is an interesting and a well-known wide band gap II-VI semiconductor with a direct band gap of approximately 3.3 eV with large exciton binding energy (60 eV). The immense excitement in this area of research arises from understanding the fact that ZnO gives rise to new phenomena and multifunctionality which ultimately leads to unprecedented integration density with nanometer-scale structures . Since the structural, optical, magnetic and electrical properties of ZnO are dependent on growth parameters, hence their applications. So, the prime interest here is to synthesize catalyst-free doped ZnO and learn the influence of dopant concentrations on the structural and optical properties. Over the time, researchers have used various dopants to dope ZnO NSs.
Doping semiconductor NWs with foreign elements to manipulate their electrical and magnetic properties is an important aspect for the realization of various types of advanced nanodevices . Aluminum (Al) is one dopant that can be used to enhance phonon scattering promoted by Al induced grain reinforcement. The conductivity of the doped NWs is also increased.
Materials, method, and instruments
The choice of deposition temperature was arrived at by keeping in mind the melting point of Al being 660.32°C. This could ensure abundant Al vapors during the deposition process. So, the need was to maintain the temperature of the furnace just above melting point of both Zn and Al. As the furnace temperature reached the set value, high purity O2 and Ar in the ratio of 20:80 was introduced into the quartz tube. Flow rate of O2 was maintained at 200 sccm (standard cubic centimeters per second). The purity of O2 and Ar were 99.8% and 99.999%, respectively. The duration of heating was maintained at 120 min for all samples based on the preliminary results. The flow rate of reactant gas (O2) and carrier gas (Ar), duration of growth time, growth temperature, and pressure were carefully set and monitored since slightest change in these parameters may affect the result of the samples obtained. Exactly at the end of 120 min of heating, the flow of reactant and carrier gases were stopped and the furnace was set to cool down to room temperature before removing the sample. Once the furnace got cooled to near room temperature, the sample was removed from it. Grayish white deposits were observed on the silicon substrate. The same procedure was repeated for all samples of different dopant concentrations.
Doping mechanism of ZnO:Al
where ao = Bohr radius of H atom (0.53 Å), ϵr = relative permittivity of ZnO (81), m* = effective mass of an electron in ZnO (0.318), mo = mass of an electron, and aH = Bohr radius of ZnO.
Theory in reference  suggests that of ZnO in Equation (2.3) is approximately 14 Å. Since donated electrons orbit around charged donor with the radius, the repulsion force between electrons belonging to adjacent donors could suppress the donation of additional electrons. The Coulomb repulsion force between adjacent charged donors may also cause decrease of carrier concentration in the same manner. Thus, these repulsion forces could cause the effective field for doping around each donor. These effective fields probably limit the doping efficiency of Al atoms within a single Al2O3 layer.
Alloying evaporation method
The atomic ratio of Zn:O on the tip and root of a NR was not the same. Concentration of oxygen on the tip of the ZnO NRs exceeded the root . The fact is attributed to the alloying of Al/Zn mixed sources during the growth of NRs. The Al vapor pressure is much lower than that of Zn at the same temperature range. However, Zn and Al sources in the process would form a certain quantity of Zn-Al alloy by interdiffusion through the Zn/Al interface. Since the bond energy of Zn-Al, 0.101 eV, is higher than that of Zn-Zn, 0.054 eV, which may cause the decreasing of Zn vapor pressure in the quartz tube with the alloying of Zn and Al during the deposition process. On the other hand, the flow rate of oxygen in the furnace is constant. As a result, the tip of ZnO NRs exhibits lower zinc concentration than the root. This particular process has contributed to unique optical properties of the NRs as described below. With higher zinc and lower oxygen concentration at the root of NRs, it exhibits green emission that is attributed to the existence of oxygen vacancy.
Results and discussion
Synthesis ZnO:Al nanowires
Structural properties of undoped ZnO nanowires
Effect of dopant concentration on ZnO:Al nanostructure
Varying dopant concentrations at constant temperature, growth time, and flow rate of O 2
Growth time (min)
Growth temperature (°C)
Flow rate of O2gas (sccm)
Dopant concentration (at.%)
Effect of dopant on ZnO:Al optical properties
PL spectra were recorded at room temperature using Luminescence LS55 model-Perkin Elmer (Waltham, MA, USA) at the Physic Department, Universiti Teknologi Malaysia.
The peaks appear nearly identical in shape for all samples except that they differ in the intensity only. The intensity of the peaks increases and become sharper as the dopant concentration increase. For undoped, UV emission peaks are slightly broader whereas the peaks are narrower and sharper and of higher intensity for all doped samples and become sharper as the dopant concentrations increase. From here, we know that the optical properties of nanostructures also differ with the aspect ratio of the nanostructure in which we observe only UV emission for low aspect ratio and vice versa. The increase in peak intensity with the corresponding increase in dopant concentration can be attributed to near band-edge emission from crystalline ZnO and recombination of free excitons. This is in good agreement with the findings reported in .
It is obvious that well-doped ZnO nanostructures have been obtained especially sample ZnO:Al 4 which was doped with 2.4 at.% Al. From the EDAX result, it is very well confirmed that Al was incorporated into the NSs. In fact, the NRs contained 0.05 at.% Al as can be known from the Figure 9b inset table. During the doping process, rather than of Zn atoms being substituted by Al atoms, we believe that oxygen vacancies (Vo) and zinc interstitials (Zn i ) were formed as Al atoms combined with oxygen in ZnO. Indeed, it was a deviation from the conventional doping mechanisms in which Al is thought to substitute Zn atoms. Our idea is well supported by the PL spectra in Figure 12 in which emissions peaks in visible range can be attributed to formation of oxygen vacancies and zinc interstitials which also agrees with reference .
Dopant plays an important role on controlling the morphology of ZnO NWs. As evident from the result, it indicates that the optimum dopant concentration to be about 2.4 at.% where a ‘pencil-like’ hexagonal NSs were formed. We also obtained very interesting NSs at 1.2 at.% which appear pencil-like but having a tail. We assume 2.4 at.% to be an optimum dopant concentration necessary which resulted in the formation of defined hexagonal shaped pencil-like NSs. Once again, we would like to stress on the proposed method to obtain Al-doped ZnO (ZnO:Al) NSs. The intensity of UV emission increases with increase in doping which is observed on the PL spectra presented before. Especially, the UV emission is enhanced which is an indication of its practicality in optical sensing application. From SEM, FESEM, and PL images, we felt that the doping mechanism occurs via formation of oxygen vacancies (Vo) and zinc interstitials (Zn i ) rather than substitution as is the case for conventional methods.
The authors thank the Department of Physics, Faculty of Science and Ibnu Sina Institute, Universiti Teknologi Malaysia, Johor, for all facilities provided as well as to Malaysian Government (GUP) under vote 08 J25 for funding the project.
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