Characterization of CdS and ZnS Quantum Dots Prepared via a Chemical Method on SBR Latex

The synthesis of CdS and ZnS quantum dots via a chemical route is reported. CdS and ZnS specimens were produced by chemical reactions where Styrene Butadane Rubber (SBR) latex matrix plays the key role in controlling particle growth during synthesis. The samples have been analyzed by UV/VIS absorption spectroscopy and High Resolution Transmission Electron Microscopy. These characterization techniques indicate the formation of quantum dots with a particle size less than 10 nm. Impedance analysis of the samples is also carried out to reveal the variation of admittance (impedance) with frequency. These results indicate their potential application in electronics as nano tuned devices and nano high pass filters.

Introduction

Synthesis of semiconductor quantum dots, characterization and their various applications are the frontier research areas at present [1-12]. Different techniques like MBE (Molecular beam epitaxy), RF (Radio Frequency) sputtering, LPE (Liquid Phase Epitaxy) etc. can be used to synthesize semiconductor quantum dots but the chemical route has been found to be the most attractive method due to its numerous advantages [2-3].

In the present investigation, the chemical method has been adopted for fabrication of quantum dots at room temperature. Styrene Butadane Rubber (SBR) latex which acts as matrix, plays the key role in controlling the size and shape of quantum dots [3] during sample fabrication. After synthesis, the samples have been characterized by different techniques to reveal their nano nature.

The samples have been analyzed by impedance (admittance) analysis to test for their possible application in Electronics as a “Nano Device”. It is observed that changes in admittance as a function of frequency of CdS quantum dots occurs in a manner exactly like that of a tuned circuit. On the other hand, the admittance behavior as a function of frequency of ZnS quantum dots mimics that of a high pass filter. This is in contrast to bulk specimens (both CdS and ZnS) where the admittance variation is almost constant with changes in frequency.

These studies infer that CdS quantum dots can act as a “nano tuned device” and ZnS quantum dot can function as “nano high pass filter”, which is our original and novel finding that has never been focused in any previous report.

Materials and Methods

To prepare CdS quantum dots, one coat of Styrene butadane rubber (SBR) latex is drawn over a glass substrate and then dried slowly to avoid spilling. The coated glass substrate is dipped into a CdCl₂ solution mixed with a few drops of HNO₃ and held for one hour and then taken out This is followed by ammonia passivation for half an hour. Finally the glass substrate is dipped into freshly prepared 2wt% Na₂S D/D water solution, until it appears fully yellow. The yellow thin film contains the semiconductor CdS quantum dots, embedded in SBR latex [1]. Similarly to synthesize ZnS quantum dots, ZnCl₂ is substituted for CdCl₂.

The samples have been characterized by UV/VIS absorption spectroscopy (using a Perkin Elmer Lamda 351.24), High Resolution Transmission Electron microscopy (HRTEM) (using a JEM 1000 C XII) and impedance analysis (using a Solartron SI 1260) as shown in Fig 2, 3, 4 and 5 respectively. Earlier synthesis of semiconductor quantum dots by different techniques and their characterizations have been reported by many workers [5-9] but preparation of CdS and ZnS quantum dots by the chemical route on SBR latex matrix and their characterizations by using HRTEM (for size determination) and impedance analysis (to reveal their changes in admittance with frequency) has never been focused in any earlier report earlier. The physical properties of the SBR latex matrix are provided in table 1 and the structure of the SBR latex material is provided in figure 1.

Table 1. Physical properties of the SBR latex matrix material.

Results and Discussion

Absorption spectroscopy (Fig 2) of the samples shows a strong blue shift in the absorption edge in comparison to that of bulk specimen [7-9]. From the blue shifted absorption edge, particle size has been assessed by using the hyperbolic band model [4].

(1)

Where:

R is quantum dot radius (2R is the diameter and hence the particle size)
E_gb is the bulk band gap
E_gn is quantum dot band gap (calculated from the strong absorption edge which is 375 nm for CdS and 200 nm for ZnS as shown in fig 2)
h is Planck’s constant
m* is effective mass of specimen (1.82 x 10^-31 kg for CdS and 3.64 x 10^-31 Kg for ZnS).

Finally, the quantum dot size has been estimated directly from their images in HRTEM (Fig 3).

All these studies infer that prepared quantum dot sizes (diametera) are approximately 10 nm. Sizes obtained from these characterizations are presented in table 2. It is observed that there are few discrepancies among the sizes estimated from the three different techniques. For accurate size assessment by UV/VIS absorption spectroscopy [11] (Shown in Fig 2), particles should be exactly circular in shape. But it is evident from HRTEM images that most of the quantum dot particles are not exactly circular. We believe that due to non circular shape, exact size determination by absorption spectroscopy has not been possible and only the approximate (and average) particle size calculation has been possible form absorption spectroscopy. But more accurate size assessment is easily made directly from the HRTEM images of the samples.

Table 2. CdS and ZnS particle sizes estimated sing high resolution TEM (HR TEM) and UV/VIS spectroscopy.

We carried out impedance analysis of CdS and ZnS quantum dots and bulk specimens. It was observed that with quantum dots, admittance rises slowly with increase in frequency up to a certain frequency range and after that at a particular critical frequency, admittance increases more rapidly. This critical frequency for CdS is 19 MHz and for ZnS is 20 MHz (Shown in Fig 4).

Interestingly, the CdS admittance data shows a steep fall at another, slightly higher critical frequency of 21 MHz. This behavior is not observed for ZnS quantum dots. Rather, the admittance remains almost constant at higher frequencies (32 MHz in the present investigation). Literature [7] reports that quantum dots are associated with capacitance and quantum dot admittance (or impedance) is basically due to capacitance present in the specimen. Thus capacitance (hence capacitive admittance) is a function of quantum dot size, shape and material [12]. Due to differences in materials and variations in size as well as in shape, capacitive admittance and critical frequencies for CdS and ZnS samples are different. The reason [12] for the steep fall in admittance observed with CdS quantum dot samples is probably due to the loading effect of charge carriers which is not the case with ZnS quantum dot samples.

Impedance analysis indicates that CdS quantum dots display a steep rise and steep fall of admittance at two close frequencies (around 19 MHz and 21 MHz ) with maximum admittance (that is impedance is minimum) at another frequency (20 MHz). This is the property of an electronic tuned circuit and the frequency at which maximum admittance is attained may be compared to the resonant frequency of a conventional tuned circuit and this frequency may be called ‘equivalent resonant frequency’.

In a conventional tuned circuit, resonance frequency is adjusted by tuning passive components (R & C), whereas in a quantum dot tuned device the ‘equivalent resonant frequency’ can be adjusted by controlling the size and shape of CdS quantum dots.

With ZnS quantum dots, its is observed that after a steep rise at a critical frequency of 20 MHz, admittance remains almost constant up to 32 MHz, (the frequency limit of our impedance analyzer). This is obviously the property of a high pass electronic filter that produces high admittance (minimum impedance) to higher frequencies and minimum admittance (maximum impedance) to the lower frequencies.

We have also studied the changes of bulk impedance [7] (shown in Fig 5) with frequency for both CdS and ZnS specimens. Unlike quantum dot impedance, bulk impedance does not change with changes in frequency and impedance (or admittance) remains constant. This is because bulk admittance (impedance) is basically resistive in nature which is not a function of frequency [7] and hence no variation in admittance with frequency occurs.

Conclusion

Quantum dots were successfully prepared on SBR latex are approximately 10 nm in diameter and of almost spherical shape. Impedance analysis shows that admittance of CdS quantum dots and ZnS quantum dots changes with changes in frequency while bulk admittance (of both CdS and ZnS) remains almost constant with frequency variation. Moreover, impedance analysis demonstrates that CdS quantum dots can act as electronic tuned circuits with an equivalent resonant frequency of 23 MHz. meanwhile ZnS quantum dots can act as high pass filters with a cut off frequency of 20 MHz.

Acknowledgement

The authors thank Prof. A Choudhury, Vice Chancellor, Gauhati University, Guwahati, Assam, India and Dr. H Chander, Dr S Chawla, LMD Group, Division of Electronic Materials, NPL, New Delhi India for their suggestions and assistances during the work.

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Contact Details

S.S. Nath*, D. Chakdar, G. Gope

Department of Physics
National Institute of Technology Silchar
Silchar – 10, Assam
India.

E-mail: [email protected]

D. K. Avasthi

Material Science
IUAC
New Delhi-10
India