Analysis of the formation of rod-like CdSe microstructure via low-temperature annealing

HUANG Zhao-Ling, BAI Zhong-Chen, SHANG Ye, SUN Ji-Chao, QIN Shui-Jie,

(1. College of Mechanical Engineering, Guizhou University, Guiyang 550025, China;2.Guizhou Key Laboratory of Optoelectronic Technology and Application, Guizhou University, Guiyang 550025, China)

Abstract: To investigate the effect of new nanostructures on luminescence, the arrays of rod-like CdSe microstructure was prepared successfully by the method of self-assembly in the case of low-temperature (170 ℃) annealing and the fluorescence spectra was measured with a centre wavelength at 365 nm. The experimental results suggested that the length and diameter as well as the array spacing of rod-like CdSe microstructure are 505.91 nm, 205 nm and 309 nm, respectively and a 36.16 nm blue shift of fluorescence spectra is found. Most importantly, the mechanism of its formation has been analyzed through 1-D Fokker-Planck equation in non-equilibrium statistical physics system. Thus, a novel route for preparation of new materials is put forward.

Key words: Rod-like CdSe microstructure； Self-assembly; Low-temperature annealing; Fluorescence

1 Introduction

Quantum dots, nanowires, nanorodsand other nano-microstructures have shown many excellent and unique optoelectronic properties, such as superior mechanical strength, higher luminous efficiency, and better thermoelectric properties[1-4]. Moreover, they have been studied extensively for application in the field of photosensitive cell[5], detector[6], semiconductor laser[7], etc. Owing to the topography and crystal structure of nano materials playing an important role in its physical properties and application of micro devices[8], the synthesis of nanomaterials with new special morphologies and structures will have an important significance in the research of nano electronic[9]and optical devices[10].

Nowadays, the preparation of microstructure is divided mainly into two methods: chemical methods and physical methods. The chemical methods mainly include solvothermal route, electrochemical deposition method, colloid-chemical and so on[11-13]. The physical methods mostly include surfactant modified, template control, self-assembly method and so on[14-16]. However, the chemical methods have many disadvantages, such as complex process, a large number of by-products, and many toxic preparation processes. Considering the advantages of simple process, strong anti oxidation ability, and easy transplant, etc. the physical method of self-assembly would have a great prospect in the field of microstructure preparation. Due to the limitation of measurement accuracy and experimental process, the self-assembly behavior of molecules and atoms has remained in theoretical analysis, and few experimental reports have been reported. Especially on the self-assembly behavior of cluster quantum dots.

In this paper,the rod-like CdSe microstructures were prepared by the method of self-assembly in the case of low-temperature annealing on surface of the thin film. The fluorescence spectra and surface morphology of rod-like microstructure were characterized by scanning near-field optical microscopy (SNOM) and atomic force microscopy (AFM). Meanwhile, the formation mechanism of rod-like CdSe microstructure is analyzed via 1-D Fokker-Planck equation in non-equilibrium statistical physics system.

2 Experimental

2.1 Preparation of rod-like CdSe microstructure sample

The experimental section was divided into two steps. Step 1, the synthesized zinc-blende CdSe QDs[17]were injected to n-hexane with the proportion of 1:3. Then colloidal CdSe QDs were dispersed evenly by the mean of ultrasonic dispersion (solution A). And isopropyl alcohol was added to solution A withVn-hexane:VQDs:Visopropanol=3:1:1. At the same time, the mixed solution was dispersed evenly (solution B). Step 2, 32 ul B solutions were drop on the surface of porous alumina substrate (aperture for 65 nm and deep 5 um) with the length of 100 * 100 mm. Staying 2 hours at room temperature, then the sample was put into the vacuum drying oven at 80 ℃. After two hours, the temperature was increased to 170 ℃. After annealing for 10 hours, the temperature was cooled to room temperature and the sample was kept in vacuum condition.

2.2 Measurement principle

We adopt the line scan mode of scanning near-field optical microscope (Nanonics, MV4000) for observing the topographic map of nano microstructure. Through using the 360 nm continuous laser of linear polarization (Changchun new industry), the polarized laser is concentrated by the half wave plate and lens. After the focused light passes the ultraviolet visible single-mode optical fiber with mutual coupling compensation, coupling light enters into the endpoint of the SNOM tip[18]. The morphology and fluorescence spectra of rod-like microstructure on surface of thin film are characterized by SNOM and Ocean Optics QE6500 with a centre wavelength at 365 nm. In order to get better analysis, we also use AFM (AFM, Benyuan Inc., CSPM 5500) to measure the surface morphology of sample film.

3 Results and discussion

3.1 Raw material analysis

CdSe QDs raw materials[18]were prepared via oleic acid-paraffin system. As shown in Fig.1, the existent form of colloidal CdSe QDs is not the single nanocrystal, but the clusters dispersed in the system. Quantum dot clusters appear an approximate sphere in the colloidal solution and its morphology and size (about 129.14 nm) are consistent with the collective effect of quantum dot clusters.

Fig. 1 The AFM map of CdSe QDs on the mica sheet (a) and the size of QDs micelle (b) ，lateral dimensions size is 129.14 nm and longitudinal size is 5.28 nm

3.2 Formation and analysis

We first use AFM to measure one point randomly on the surface of the sample. It can be clearly seen from the Fig.2 that the surface morphology of the thin film is different from Fig.1. Basically, Fig.1 show the quantum dot clusters on mica are homogeneous distribution. However, after quantum dot clusters go through low-temperature annealing on porous alumina, the surface morphology of thin film presents a clear array arrangement of rod-like. It is obvious that the fluorescence spectra of rod-like microstructure have a blue-shift of 36.16 nm, compared with the fluorescence spectra of colloidal QDs in Fig.3. At the same time, the fluorescence intensity of the thin film is increased, because the QDs are aggregated together in the process of annealing, In addition, the fluorescence peak of 658.9 nm and 458 nm also indicates that there are other larger or smaller nanostructures formed during the annealing process, which can make the fluorescence peak shift.

Fig. 2 The three-dimensional image of AFM (a) is the surface topography of sample film after annealing at 170 ℃. The length (c) and diameter as well as array spacing (b) of rod-like microstructure are 505.91 nm, 205 nm and 309 nm in the circle of the sample (a), respectively.

In order to characterize rod-like CdSe microstructure more accurately, we use the scanning near-field optical microscope to measure randomly one site at the edge of the thin film. We use the sample scan mode and AFM-fluorescence mode (sampling points was still 256*256 of pixels, each step of the integration time is 8 ms) to measure simultaneously the surface morphology and fluorescence imaging of thin film. As shown in Fig.4, we could see clearly that the location of the rod-like microstructure formed will also be the position of glow sticks in the fluorescence imaging. The above results show that the rod-like microstructure have formed by the self-assembly of QDs on surface of the thin film.

Fig. 3 Fluorescence spectra of colloidal CdSe QDs and sample film

Fig. 4 Surface morphology (a) and near-field fluorescence imaging (b) at the edge of the thin film

In the process of self-assembly of nanoparticles on the surface of sample film[19], there are three main types of force to colloidal self-assembly: inter-atomic interaction, Brown diffusion motion of particles and crystal lattice distortion. In non-equilibrium statistical physics system, the evolution of self-assembly could be described by 1-D Fokker-Planck equation[20-23]

(1)

where probability density distributionP(x)can be considered as equilibrium state,Kbis the Boltzmann constant, andTis the thermodynamic temperature.

The diffusion coefficient is described via equation

(2)

where the characteristic lengthl(x) represents a periodic variable in the equation, the work functionΔWacts on the particles as determined by the position of the particles and is expressed as

(3)

whereF(t) andFi(t)represent microscopic total stress and component force, respectively.Δxis the variety ofxposition.

Fig. 5 Scheme picture of the formation of rod-like CdSe microstructure

With the change of temperature, surface energy and gradient force, the system would present different properties, and then show different probability distribution to form a new microstructure. Scheme picture of the formation of rod-like CdSe microstructure is showed in Fig.5. In case of the incomplete evaporation, the mutual attraction and Brown diffusion of the particles accounts for the main part of total forceF(t)and the nanoparticles in the solvent will increase the chance of polymerization due to the gradual evaporation of the solvent. Lattice deformation will promote the formation of new microstructure as a result of the anisotropic lattice in zinc-blende CdSe QDs during the annealing process[24]. In this paper, there are more advantages to form a new structure in process of self-assembly. Because CdSe nanoparticles coated with oleic acid are single and dispersed agglomerates. At the same time, because CdSe nanoparticles polymer could be regarded as an oily particle, particles will aggregate randomly due to diffusion and inter-atomic interaction of particles and become a bigger oily polymer, which are advantageous to microstructural evolution of the CdSe nanoparticles. We also introduce porous alumina substrate which could also be regarded as a myriad of small heat sources during the process of particles aggregation.

Fig. 6 AFM image (a) is the surface topography of sample film after drying at 80 ℃. Figure (b) shows that the horizontal dimension (142.45 nm) and vertical dimension (4.38 nm) of the QDs polymer in which are located in the oval of sample (a).

According to the thermodynamics theory and 1-D Fokker-Planck equation[25-27], particles will be distributed along a specific direction. After drying at 80℃, the AFM images of the sample are shown in Fig.6. It could be seen from the figure (a) that QDs polymers aggregate slowly and regularly along one direction on the surface of the sample and figure (b) shows that the size of QDs polymer increases due to polymerizing regularly, which also confirms our above analysis. QDs in oily polymer will be affected by the thermal gradient and Brownian motion, which moved slowly along the specific direction in the process of low-temperature annealing, and so on. When the solvent have evaporated completely, the polymer would shrink and deform under the influence of thermal gradient force and other forces, and then the corresponding microstructure will be formed. The inevitable result is that the aggregation of QDs would make the crystal lattice distance more and more close, and then formed a new nano microstructure[28]. Fig.7 is AFM image of the sample film (heated for 3 hours at 170 ℃). It is shown that QDs polymers on the surface of the sample film shrinks slowly and deforms regularly towards the same direction[29]. This also confirms our above analysis. Through the analysis of the mechanism of self-assembly annealing on surface of thin film, we can provide a more effective theoretical method to guide the preparation of more regular and complex nano microstructures for the next step.

Fig. 7 AFM image (a) is the surface topography of sample film after drying at 170 ℃ for 3 hours , (b) and (c) show that the radial length(431.28 nm) and the horizontal width(729.07 nm)of the microstructure in which are located in the oval of sample (a), respectively.

4 Conclusions

In this article,the Formation and analysis of the rod-like CdSe microstructure is investigated on the surface of porous alumina template by the experimental method of colloidal self-assembly. The research shows the length and diameter as well as the array spacing of rod-like CdSe microstructure are 505.91 nm, 205 nm and 309 nm, respectively. This novel rod-like microstructure not only brings about a blue shift (36.16 nm) of peak wavelength, but also enhances the fluorescence radiation via low-temperature annealing (This article used 170 ℃). The formation mechanism of rod-like microstructure is also explored with the inter-atomic interaction and the Brown diffusion motion of particles, as well as crystal lattice distortion via 1-D Fokker-Planck equation in non-equilibrium statistical physics system.