Quantum confinement effect of   ZnO nano-particles

Cao Li, Su Xiyu, Wu Zhenyu^#, Zou Bingsuo^#, Dai Jiahua^#, Xie Sishen^#
(Department of Physics, Qufu Normal University, Qufu, Shandong, 273165; ^#Institute of  Physics, CAS, Beijing 100080 )

Received Mar.28, 2002.

Abstract Zinc oxide nanocrystals were prepared with different size and shape by coating of dodecyl benzene sulfonate (DBS) . Good evidences can prove that quantum confinement effect is the special quality for this nanosystem. As an indication of quantum confinement effect, excellent emissions from bandedge have been observed, which is necessary for the laser nanodevices.
Keywords Quantum confinement effect, Zinc oxide, Nanocrystals.

1. INTRODUCTION
It is well known, nano-materials will find more and more applications in the technological development in the future. As a typical quantum solid, nano-semiconductor materials have more novel quality than their bulk materials. II-VI semiconductor materials, which have many new properties that attract both fundamental and technological interests to many researchers today, such as solar cells ^[1-4], chemical sensors ^[5-6], luminescent and electrical devices ^[7], and some kinds of display units^[8]. Among them, zinc oxide is one of useful materials with optically active property. ZnO dots and wires in nano-scale span a wide range of electrical and optical properties that depend sensitively on both size and shape. Quantum confinement effect can modify the energy bands with the change of the size and shape of the nano-particles. Many physical and chemical methods can be used to change the particles size and shape, such as the control of the growth kinetics which is a good method to vary the CdSe particles shapes from a nearly spherical morphology to a rod-like ones ^[9], Dodecyl benzene sulfonate (DBS) and poly (vinylpyrrolidone) (PVP) are better stabilizers, which can effectively change the morphology of zinc oxide nano-particles. For example, many novel properties can be found through varying the molar ratio of the ion of Zn and PVP, with this method, the rod shape particles are made, many optical properties are observed ^[10].
    Here we find another way to obtain the anisotropic nanocrystal of ZnO by control the pH of microemulsion pool for the ZnO nanocrystal formation, quantum confinement effects can be observed for the absorption and emission properties of this system. Particles with different size and shape and their concrete changes about energy band and optical phenomena are clearly presented.

2. EXPERIMENT
0.03mol zinc chloride (AR grade) were dissolved in 300mL deionized water. Then DBS solution was added to form mixed solution. The HCl and NaOH solution were used to control the pH value of this solution. Toluene was used as the extraction solvent to form the organosol of Zn(OH)₂ nanoparticles. To control the particle size and shape, pH value of the mixed solution were adjusted from 3.5 to 5.8. In this pH range the ZnO nanoparticles can be extracted into toluene easily. While pH>=6.2, it is hard to form homogeneous micelle. When the coated ZnO was extracted, then it was refluxed at about 110ºC for 2-3hr and to remove excess water. The size of ZnO organosol was detected by TEM of JEOL 2010, its electric voltage is 200Kv and amplification multiple is 0.2 million; The TU-1901 UV-Visible absorption spectrometer and PTI-C-70 fluorescence spectrometer were used to study the absorption and emission of the samples.

3. RESULTS AND DISCUSSION
We have prepared zinc oxide nano-particles with different shapes. Figure 1 is the TEM figure of the two different shape samples, a rod-like one at high pH with above 10nm (a), and a sphere-like one at low pH (<4) (4-5nm) (b). The TEM of ZnO nanoparticle with 1-2 nm has been reported elsewhere ^[11]. The rod-like one have the quality of one-dimension semiconductor material ^[2], this can be verified by the following description.
    Figure 2 is the change of the energy bandgap with the size changing. From the figure we can find that the samples with the different size have evident shift as shown. A significant blue shift of the absorption edge compared to that of bulk ZnO in the absorption spectra is observed when the particles' size decreases. This shift firmly attests the theory of quantum confinement effect. Quantum confinement effect can be described with the Brus formula ^[12]:
(a)     (b)
Fig.1 The TEM photographs of the ZnO nano-particles with different size and shape. From the figure we know that we can prepare the shape we needed.

Where is the absorption band gap of nano-semiconductor particles, is the bulk ZnO band gap energy, r is the radius of the parcicle,is the exciton reduced mass , where and are the effective masses of the electron and hole ,e^*=e⁰/e¹  is the relative dielectric constant, where e⁰ and e¹ is the dielectric constants of the bulk ZnO and the capping DBS), , which is the Rydberg energy. Generally, it is accepted that the relative dielectric constant  e^*=2.47 and the effective masses of an electron and a hole expressed in free electron mass are 0.24 and 0.45 and the bulk band energy we used is 3.2eV in ZnO solid, the reference ^[13] gave the units of 156 meV and the exciton Bohr radius a_B=1.25 nm.

Fig.2 The change of the band gap energy Eg with the particle size. When the particle size is in the range of nanometer, the band energy increases with the size decreasing as the figure shown.

    Evidently, from the formula and Figure 2, we can see that the energy band change upwards, therefore optical absorption and blue shift correspondingly with the size change.


   Fig.3 A clearly quantum confinement effect is provided, we can see from the absorption spectra that the ZnO quantum dots (~2nm,5nm,>10nm) we have provided can give a good description about the absorption spectra band blue-shifting following the particles size decreasing, at the same time the exciton absorption peak becomes more apparent and blue shift with the size decrease. Furthermore it can be seen the exciton effect is evident when the size is decreased.


Fig.4 The fluorescence spectra of the nano-particles. (A) is excitation spectra and (B) is the photoluminescence spectra. Excitation peak is at 360nm, and the emission peak at 404nm (which is attributed to the electron hole recombination in the band-gap).

    With the same coating, the inner and the cover have different dielectric constants, so the change of nano-particles shape can bring about the following change of the ground states of the bounded exciton in the nano-particles, the change of the surface polarization effect acting on the bounded action of the exciton comes from the discontinuousness of the dielectric constants. From the formula and Figure 2, we can find that the deviation of the theoretical and the experimental values mainly come from the second and the last term in the right part of the formula, because, theoretically, the values we used are the bulk materials, including the effective mass of the electron, the hole and the dielectric constant, furthermore we do not think about the coating can affect the reduced mass of electron and hole. All of these are the reasons that can affect the deviation.
    As we all known, stronger exciton effect is an important character of quantum confinement effect in nano-semiconductors, the reason mainly comes from the carriers being confined in a very small district, this makes the electron and hole move only in a potential well. At the same time, it can enhance the coupling interaction with each other. Then the exciton bounded stronger and the probability of binding increased, so we can observe more apparent exciton absorption peak when the particles' size decreased. Therefore we can see the exciton absorption and their blueshifts in Fig.3.
    Figure 4 is the fluorescence spectra of the ZnO nano-particles. Curve A is the excitation spectrum and curve B is the emission spectrum. The excitation peak is about 360nm and the value of the photoluminescence peak is 404nm. Obviously, the band gap luminescence of ZnO nanocrystal is predominant in the emission spectrum. The blue shift takes place when the values are relative to the bulk samples and the enhancement of the bandedge resonant fluorescence shows the contribution of the quantum confinement effect. Surface states are of importance for radiative transitions for ZnO particles in the ref. ^[14-15], which is also the main radiationless channel. The luminescence of ZnO is very sensitive to the surface states, but the surface states can be controlled by the preparation methods, that is to say, different preparation methods can lead to different optical phenomena ^[14-19]. In our system, the organic coating occupies the surface states and forms the surface potential barrier, which can prevent the formation of the surface states and hence the resonance radiation transition of the exciton dominates. This property will be eventually applied in ZnO laser devices.

4. CONCLUSIONS
In conclusion, with the control of the pH of the mixed microemulsion ZnO solution, nanocrystal with different size and shape are prepared. Our results can be described in terms of the quantum confinement effect. The better zinc oxide nano-particles with the good method of surface decoration have been prepared, so our samples have no obvious surface states that lead to the quality undesired. Therefore, this surface decoration can improve the emission property of ZnO, which is very important for the developing high quality nano-devices.

Acknowledgments The authors would like to thank National Natural Science Foundation of China (Term No 20173073) and Hundred Talent Project of Chinese Academy of Science for the financial support.

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