Introduction

In the present article, we report the synthesis of ZnS quantum particle by using a chemical method. These samples have been irradiated by 100 MeV Nickel ion (Ni ⁺⁷ Swift heavy ion) at four different fluencies 1×10 ¹¹ ,3×10 ¹¹ , 1×10 ¹² and 3×10 ¹² ion/cm ² . The irradiated samples were analyzed by High resolution transmission electron microscopy (HRTEM), UV/VIS optical absorption spectroscopy, photoluminescence spectroscopy [₁,₂,₃,₄,₅,₆,₇,₈,₉,₁₀,₁₁,₁₂] and Impedance analysis [₅, ₂₃] . Optical absorption of 100 MeV Cl ⁺⁹ ion irradiated ZnO quantum dots (prepared by quenching method) has been reported else where ¹⁸ but optical absorption, photoluminescence study and impedance analysis of ion irradiated ZnS quantum dots (prepared by chemical route) have not been focused earlier. Hence we believe this report is highly important from the view point of quantum dot characterizations ²² .

Synthesis

5wt% D/D water solution of PVA is prepared by moderate heating and stirring. 1.36wt% aqueous solution of ZnCl₂ and 0.75wt% solution of Na₂S are prepared so that the molecular weight ratio of ZnCl₂ and Na₂S becomes 1:1. With this, few drops of HNO₃ is added. The solutions of PVA and ZnCl₂ are mixed in the ratio of 2:1 and then stirred at 250 rpm at 60 ⁰ C while with dropping funnel, Na₂S solution is put into it, until the whole solution appears completely milky. The prepared solution is kept in dark chamber at room temperature for 12 hours for stabilization. Finally, the solution is cast over laboratory glass substrate and then dried in oven at 40 ⁰ C. The film over substrate contains the quantum dots of ZnS specimens embedded in PVA matrix. Thus ZnS quantum dot samples were developed. These were irradiated in the Material Science chamber under high vacuum (4.6× 10 ^-6 Torr) by using the 100-MeV Ni ⁺⁷ ion beam with four ion doses with doses 1×10 ¹¹ ,3×10 ¹¹ , 1×10 ¹² and 3×10 ¹² ions/cm ² ,available from the 15UD tandem pelletron accelerator at IUAC(formerly NSC),New Delhi ,India.

Characterization

High resolution transmission electron microscopy (using JEM 1000 C XII) it is evident that in polyvinyl alcohol (PVA) matrix, the virgin quantum dot samples are orderly distributed [₁,₂,₃,₄,₅] with very small inter particle distances separated by only a thin wall of polymer matrix as displayed in fig 1.

Figure 1

Figure 1: HRTEM images of ZnS specimens: a, b, c,d and e stand for the virgin sample and the samples irradiated by 1 , 2 , 3 and 4 dose respectively.

When swift heavy ion (SHI) irradiates the sample, it gets heated up resulting in sudden rise in sample temperature that reaches the melting point of the specimen [₁,₂] . Due to this phenomenon, quantum dots melt and as they start to agglomerate to form bigger particles (fig1). Also, the red shifts (fig 2) in strong absorption edges of irradiated samples in UV/VIS absorption spectra (using Perkin Elmer Lamda 351.24) indicate the formations of bigger particles [₆,₇,₈,₉,₁₀,₁₁] . Different sizes also have been estimated form the spectra by using hyperbolic band model [₉,₂₂] .These sizes agrees (with little discrepancies) the quantum dot sizes obtained from Transmission Electron Microscopy.

Figure 2

Table 1: Sizes of samples calculated from absorption spectra.

Figure 3

Figure 2: UV/VIS absorption spectra of ZnS specimens: a, stands for virgin sample and b, c, d e for samples irradiated by 1stm 2 , 3 and 4 dose.

Figure 4

Figure 3: spectra of ZnS samples: a stands for virgin sample and b, c, d e for samples irradiated by 1 , 2 , 3 , and 4 ion dose respectively.

We carried out the photoluminescence study (using HITACHI-F-2005) of the samples. Photoluminescence spectra of ZnS samples excited with 200 nm source (fig.3) reveal that virgin ZnS quantum dots possess the Zn ²⁺ vacancy related luminescence ²³ with peak position at 500 nm. It is observed from the spectra of irradiated samples that emission intensity increases with no shift in emission peak positions with higher ion doses. This infers that the unlike luminescence in virgin quantum dot [₁₄, ₂₃] , luminescence in irradiated quantum dot is also due to the Zn ²⁺ vacancy but we believe that Ni ⁺⁷ ion produces more numbers defects in the form of Zn ²⁺ vacancies in the ZnS specimen resulting in higher intensities of emission out put .

Figure 5

Figure 4: Impedance spectra of ZnS sample: a stands for virgin sample and b, c, d e for samples irradiated by 1 , 2 , 3 , and 4 ion dose respectively.

Impedance analysis (Using Solartron SI 1260) of the sample reveals that after ion irradiation admittance Vs frequency curves of the ZnS quantum dot modifies. It is already reported ²² that quantum dot impedance is basically due to capacitance which varies directly with particle size. After ion irradiation, particle agglomerates resulting in formation of bigger particle of larger capacitance ²¹ (fig 4). Due to this phenomena, quantum dot impedance changes resulting in modification in admittance Vs frequency curves of irradiated samples.

Conclusion

After Ion irradiation bigger particles form. Photoluminescence study infers that luminescence intensity of irradiated specimen becomes higher and Impedance of the sample changes after irradiation, which are our new findings.

Acknowledgement

Correspondence to

Siddhartha. S. Nath Affiliations: Lecturer Department of Physics National Institute of Technology Silchar, Silchar, Assam, India. Ph. No:+919864280392. email- nathss1@rediffmail.com