A study of photoluminescence properties and performance improvement of Cd-doped ZnO quantum dots prepared by the sol–gel method
© Zhang et al.; licensee Springer. 2012
Received: 27 April 2012
Accepted: 5 July 2012
Published: 18 July 2012
In the present work, ZnO quantum dots (QDs) have been prepared by the sol–gel method, and the performance of the QDs has been improved. The effect of Cd concentration on the structural and luminescent properties of the QDs, as well as the effect of the mass ratio of trioctylphosphine oxide (TOPO)/octadecylamine (ODA), has been investigated. The ZnO and Cd-doped ZnO QDs have hexagonal wurtzite structures and are 3 to 6 nm in diameter. When the Cd content was increased, the QD particle size was reduced; this effect was confirmed in the corresponding ultraviolet–visible spectra. The fluorescence intensity was simultaneously enhanced significantly. Both the UV and fluorescence spectra were blue-shifted. The luminous intensity was further enhanced when the QDs were modified with TOPO/ODA. Fourier transform infrared and X-ray diffraction techniques proved that the polymer successfully coated the surfaces of the QDs. A TOPO/ODA mass ratio of 1:2 was determined to result in the best optical performance among the different ratios examined. The results showed that the described synthetic method is appropriate for the preparation of doped QDs with high-fluorescence quantum efficiency.
KeywordsCd-doped ZnO Quantum dots Sol–gel method Photoluminescence properties
Sol–gel-derived materials have received particular interest as chemical receptor matrices because of their optical transparency, mechanical stability, chemical resistance and flexibility in sensor morphological configurations. The materials have a wide spectrum of advantages: they may be set up at room temperature, they are not degraded electrochemically or by light, they are open to a wide variety of chemical modifications, they can be obtained in a variety of forms (monoliths, thin films, fibres or powders), and they are able to respond rapidly. The combination of these factors makes these materials desirable for use in the pharmaceutical, food and chemical industries .
Semiconductor quantum dots, also known as semiconductor nanocrystals, are semiconductor materials with a particle size similar to that of the exciton Bohr radius or the de Broglie wavelength . These quantum dots (QDs) have attracted considerable attention from researchers because of their unique optical and electronic properties and have become a bright spot in nanotechnology research [3–9].
ZnO is a direct and wide-band-gap semiconductor material of group II-VI with a hexagonal wurtzite structure (a = 0.325 nm, c = 0.521 nm). As a result, ZnO has unique electrical and optical properties, such as a wide band gap (Eg = 3.37 eV) at room temperature and a large exciton binding energy of 60 meV, which can produce a significant quantum confinement effect . Particles aggregate easily during the preparation of ZnO QDs because of their large specific surface area and high surface activity. This aggregation can create an irregular surface, which causes many disadvantages in the final products. These issues have an intense effect on the luminescence properties of ZnO QDs. A useful approach to solving this problem is to dope ZnO QDs with another metal. CdO belongs to the cubic system (a = 0.467 nm) of II-VI direct-band-gap semiconductor materials and exhibits a band gap of 2.3 eV . The formation of Zn1-xCd x O alloys from ZnO and CdO can cause the band gap of ZnO to red-shift into the blue and green spectral range. The suitable incorporation of Cd enables the band gap to be tuned for various potential applications . In contrast, the lattice mismatch between ZnO and a Zn1-xCd x O alloy with a lower Cd content is small, and this feature allows the preparation of Zn1-xCd x O/ZnO heterojunctions .
We previously synthesised ZnO QDs capped with Cd using the sol–gel method in a dilute, water-free solution with PVP K30 (Damao Chemical Reagent Co., Tianjin, China) as a stabiliser and thiourea as a surface modification agent. The optimisation of the Cd matrix and the mass ratio of trioctylphosphine oxide (TOPO)/octadecylamine (ODA) on the optical properties of ZnO QDs have not yet been investigated. In this paper, the Cd content in the alloy was intentionally enhanced in an effort to improve the fluorescence efficiency and stability of the resulting QDs. A comparison between modified QDs and unmodified Cd-doped ZnO QDs is discussed below.
Zinc acetate dihydrate (99.0%), cadmium acetate dihydrate (99.5%) and lithium hydroxide (90.0%) were procured from Kermel Chemical Reagent Co. (Tianjin, China). Absolute ethanol (99.7%) and n-hexane (98.0%), produced by Zhiyuan Chemical Reagent Co. (Tianjin, China), were used to synthesise Cd-doped ZnO QDs. Octadecylamine, 1-octadecene and TOPO, produced by Aladdin Chemistry Co. (Shanghai, China), were used to modify Cd-doped ZnO QDs. All chemicals were directly used without further treatment.
In a typical sol–gel method, appropriate amounts of Zn(CH3COO)2·2H2O and Cd(CH3COO)2·2H2O were placed in 100 mL of anhydrous ethanol and heated to reflux at 80°C for 3 h. The appropriate amount of Cd(CH3COO)2·2H2O was then added in accordance with the desired molar ratio (x = Cd / (Cd + Zn) = 0.0%, 2.0%, 5.0%, 10.0%, or 20.0%) to bring the total amounts of Zn(CH3COO)2·2H2O and Cd(CH3COO)2·2H2O to 15 mmol. Simulta-neously, LiOH (0.3024 g) was dissolved in 20 mL of anhydrous ethanol and kept in an ultrasonic bath for 30 min. This solution was slowly added to the Zn2+ and Cd2+ solution under a reflux condenser at 50°C for 1 h. The solution was repeatedly washed with n-hexane (volume ratio = 1:2) to remove unwanted ions, and the obtained precipitate was dried in a vacuum oven to afford Cd-doped ZnO QDs as white powders. Next, 1-octadecene, octadecylamine, TOPO and Cd-doped ZnO QDs were slowly mixed in anhydrous ethanol with constant stirring at 50°C for 1 h. The transparent solution turned into a white solid when cooled to room temperature and was washed repeatedly with chloroform to provide the final product.
The synthesised powder samples were dissolved in ethanol, and a drop of this dilute ethanolic solution was placed on the surface of a copper grid. Microstructural investigations of dried samples were performed using a transmission electron microscope (TEM, JEM-2010, JEOL Ltd., Akishima-shi, Japan) with an acceleration voltage of 200 kV. Optical absorption measurements in the UV-visible range were performed at room temperature using a TU-1901 spectrophotometer equipped with a deuterium lamp as a UV light source and a tungsten halogen lamp as a visible source (Beijing Purkinje General Instrument Co., Ltd., Beijing, China). The wavelength range used in the experiment was 200 to 600 nm. X-ray diffraction (XRD) profiles of QD samples were obtained on an Ultima-III apparatus equipped with Cu Kα radiation source (Rigaku Corporation, Beijing, China). For fluorescence spectroscopy measurements, the excitation wavelength for QDs was identified as 341 nm using a Hitachi F-7000 fluorimeter (Chiyoda-ku, Japan). Samples were prepared using the KBr press disc method, and optical transmittance was determined by Fourier transform infrared (FT-IR) analysis. In addition, a Canon EOS-500D (Ohta-ku, Japan) was used for digital photos under a fluorescent lamp and under UV light at room temperature.
Results and discussion
where D is the particle size, λ is the wavelength of ra-diation used, θB is the Bragg diffraction angle and B is the peak width at half maximum . The XRD data, along with the TEM data presented previously, establish that doping with Cd suppresses the growth of ZnO QD particles. Furthermore, the XRD peaks the diffraction profiles of TOPO/ODA-modified Cd-doped ZnO QDs were not as sharp as in the case of the 5% Cd-doped ZnO sample, which indicated that the polymer coated the surfaces of the Cd-doped ZnO QDs. These results were also consistent with those from the FT-IR analysis.
In conclusion, we have successfully synthesised QDs of ZnO, Cd-doped ZnO and TOPO/ODA-modified Cd-doped ZnO. Blue shifts of the UV absorption peaks of the Cd-doped ZnO QDs were observed with increasing Cd concentration, which indicated a reduction of the QD size and a strengthening of the quantum confinement effect after doping. Through various measurements, including UV, PL, XRD and FT-IR, the surfaces of the QDs were found to play an important role in their optical properties. Both the TOPO and Cd coatings on the ZnO QD surfaces and an excessive amount of TOPO led to a reduction in luminous intensity. Thus, the TOPO/ODA mass ratio strongly influences the luminous intensity. A mass ratio of 1:2 was found to be the optimal ratio for maximal luminous intensity. These results provide strong support for the further development of extensive optical device applications, including bio-imaging and bio-sensors, based on the conjugation of these nanocrystals to biological entities.
Dr. SQZ is working as a professor in Guangdong University of Technology and is the director of the Department of Pharmacy Engineering. Professor Zhao has been studying about building immune determination method for toxic chemical with luminescent markers.
This work was financially supported by the National Natural Science Foundation of China (41071176) and the Guangdong Province 211 project.
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