Influence of Mg on Structural and Optical Properties of ZnSe Nanocrystals Synthesized by Microwave Assisted Technique

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ScienceDirect Materials Today: Proceedings 21 (2020) 1943–1948

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ISFM-2018

Influence of Mg on Structural and Optical Properties of ZnSe Nanocrystals Synthesized by Microwave Assisted Technique Shiv Kumar Choubey1, Anurag Kaushik1, K. P. Tiwary2* 1

Department of Electronics and Communication Engineering, Birla Institute of Technology, Mesra, Patna Campus, Patna, India 800014 2 Department of Physics, Birla Institute of Technology, Mesra, Patna Campus, Patna, India 800014

Abstract ZnSe is an important semiconductor material having applications in electronic and optoelectronic fields like nonlinear optics, light emitting diodes (LEDs), flat panel displays, lasers, logic gates, transistors, etc. Pure ZnSe and Mg doped ZnSe have been synthesized by microwave assisted method. In the synthesis of pure and Mg doped ZnSe nanoparticles the chemicals used were Zinc Chloride (ZnCl2), Selenium Powder, ethylenediamine and Magnesium Chloride. The synthesized nanoparticles have been characterized to investigate the morphological, structural and optical properties. The crystal structure of the nanocrystallites have been calculated by X- ray diffraction pattern and the size of the crystalliltes have been observed using Debye-Scherrer formula. The crystallite size was obtained around 6 nm for pure ZnSe and around 11 nm for Mg doped ZnSe nanocrystals. UV- Visible spectrophotometer was used for the measurement of optical bandgap. The value of bandgap was found to be 3.42 eV for ZnSe and it was 3.48 eV for Mg-doped ZnSe. The XRD spectrum also confirms the crystallinity of the samples. © 2019 Elsevier Ltd. All rights reserved. Peer-review under responsibility of the scientific committee of the International Symposium on Functional Materials (ISFM-2018): Energy and Biomedical Applications. Keywords: ZnSe; microwave; XRD; SEM; Band gap

* Corresponding author. Tel.: 91-612-2223538. E-mail address: [email protected] 2214-7853 © 2019 Elsevier Ltd. All rights reserved. Peer-review under responsibility of the scientific committee of the International Symposium on Functional Materials (ISFM-2018): Energy and Biomedical Applications.

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1. Introduction Since last few decades, there has been huge effort devoted to the discovery of semiconductor materials and their unique properties. ZnSe, one of them, is a II-VI semiconductor material which is one of the most important optoelectronic materials discovered in present era. It has many prominent applications in nonlinear optical devices, light emitting diodes, transistors, flat panel displays, logic gates, lasers, etc[1-3]. The various methods used for the synthesis of nanoparticles of ZnSe are Co precipitation method, reverse micelle synthesis, Vacuum bath technique, etc. [4-6]. ZnSe can be found in two allotropic forms, hexagonal wurtzite (W) structure and cubic zinc-blende (ZB) structure. The band gap of ZnSe is 2.7 eV. Due to its excellent optical luminescent property, it has gained so much importance [7-12]. The applications include LED, sensors, infrared detectors and Lasers. Recently, a lot of research has been carried out for development of photovoltaic materials. Further other alkaline elements and transition metals have been doped successfully by many researchers for various applications [13-21]. Magnesium, an alkaline earth metal, is a grey colored shiny, resemblance with silver has light weight and is strong. It has density of 1.738 g/ml and is capable of being shaped or bent. It is relatively soft and exhibits a shine or glow. It has atomic radius of 0.57Ǻ which is very close to the radius of Zn which is 0.60 Ǻ. Hence ZnSe can be easily doped with Mg. Hence it can be easily tuned by varying the Mg concentration. By doping Mg in ZnSe, the compound gains much importance for enhanced functional characteristics and potential applications. 2. Experimental 2.1 Preparation of pure ZnSe Microwave assisted solvothermal method was used for preparation of ZnSe nanoparticles. The chemicals used for preparations of nanoparticles were of analytical grade (AR) and used without further purification. For the synthesis, 4 grams of Zinc Chloride (ZnCl2) was taken as an aqueous solution. 0.2 g of selenium powder was put into a 50 mL beaker, which was filled with ethylenediamine (en) up to 85% of its total volume. The prepared solution of selenium and ethylenediamine was put on magnetic stirrer at 70 0C with 600-650 RPM for 2 hrs. This resulted in brown colored homogeneous solution of selenium and ethylenediamine. Now the aqueous solution of ZnCl2 and the prepared solution was mixed and put on magnetic stirrer for 30 minutes at 160 0C with 650 RPM. Then the solution was microwaved for 12 cycles with duty cycle of 25%. The prepared sample was filtered out and dried in oven for 10 hrs at 700C. 2.2 Preparation of Mg doped ZnSe Mg doped ZnSe was prepared by Microwave-Assisted Solvothermal method. All the chemicals used in the synthesis were of analytical grade. For this 4 grams of Zinc Chloride (ZnCl2) was taken as an aqueous solution. Now the aqueous solution of Magnesium Chloride was prepared and mixed with the solution of Zinc Chloride dropwise. Then the solution was kept on a magnetic stirrer for the next 2-3 hours. The magnetic stirrer was set on 950 RPM for two and half hours at 1600C. Then, 0.2 g of selenium powder was dissolved into ethylenediamine. The solution of Selenium was added drop by drop during stirring process. After that the entire solution was kept in microwave for 20 second and rest period for 60 second. The process was repeated for around 13 cycles to get the precipitate. The solution was then filtered out and dried for further characterization. 3. Result and discussion The Pure ZnSe and Mg-doped ZnSe nanoparticles were characterized by different characterization techniques to analyze their structural, morphological and optical characteristics. The analysis used for this purpose were X-ray diffraction (XRD), Scanning Electron Microscopy (SEM) and UV-visible (UV-Vis) absorption spectroscopy.

S.K. Choubey et al./ Materials Today: Proceedings 21 (2020) 1943–1948

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3.1 X-Ray Diffraction analysis The structural properties of the synthesized ZnSe NPs and Mg-doped ZnSe NPs were analysed by recording the XRD pattern by model Brucker AXS D8 Advance with Cu Kα radiation having wavelength 1.5406 Å. The XRD pattern peaks were indexed for the cubic phase of ZnSe. The peaks in the XRD pattern were indexed to the cubic phase of ZnSe as shown in Figure 1a and Fig 1b. There were very few peaks of magnesium in the XRD pattern of Mg-doped ZnSe. This means that the magnesium may have got filled in the interstitial spaces among ZnSe compound and were not able to be incorporated properly at the lattice points. The particle size was found to be around 6 nm for pure ZnSe and 11 nm for Mg doped ZnSe which was measured by Debye-Scherrer formula (Table 1-2). The Debye-Scherrer formula is D=

.

where λ is the wavelength of X-ray, β is called FWHM (Full Width at Half Maxima), θ is the angle of diffraction.

Fig.1a. XRD of pure ZnSe

Fig.1b. XRD of Mg dopedZnSe

Table 1. XRD analysis of pure ZnSe 2θ (deg.)

θ (deg.)

Sin θ

Cos θ

FWHM

d (Å.)

D (crystalline Size) (nm)

22.357

11.178

0.194

0.981

0.251

3.973

5.622

29.731

14.865

0.256

0.966

0.316

3.000

4.540

31.767

15.883

0.273

0.962

0.237

2.814

6.080

32.933

16.466

0.283

0.958

0.209

2.717

6.922

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S.K. Choubey et al. / Materials Today: Proceedings 21 (2020) 1943–1948

Table 2. XRD analysis of Mg-doped ZnSe 2θ(deg.)

θ (deg.)

Sin θ

Cos θ

FWHM

d (Å)

D(crystalline Size)(nm)

22.541

11.2705

0.1954

0.9807

0.223

3.941

6.337

29.930

14.965

0.2582

0.9660

0.143

2.983

3.474

31.961

15.980

0.2753

0.9613

0.130

2.797

11.094

33.123

16.556

0.2850

0.9585

0.120

2.702

12.056

3.2 SEM analysis

Fig.2a. SEM of pure ZnSe

Fig.2b. SEM of Mg doped ZnSe

The scanning electron microscopy image is used for observing the morphological property of prepared nanoparticles. The micrograph of scanning electron microscopy (SEM) of prepared sample is taken which is shown in Figure 2a and 2b. From this figure, well-structured crystalline grains are observed. The SEM images of ZnSe and Mg doped ZnSe nanoparticles sample showed that the prepared samples were crystalline in nature. 3.3 UV –Visible analysis The UV-Vis absorption spectrum of the nanoparticles of pure ZnSe and Mg doped ZnSe was recorded for studying the optical behavior. The Rayleigh UV-2601 spectrometer was used for taking UV-visible absorption spectrum of sample which gave the optical bandgap of materials. The wavelength range of 200-800 nm was taken into account to study optical properties.

S.K. Choubey et al./ Materials Today: Proceedings 21 (2020) 1943–1948

Fig.3a. UV-Vis spectroscopy of pure ZnSe

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Fig.3b.UV-Vis spectroscopy of Mg doped ZnSe

The UV-visible spectra are shown in Figure 3a and 3b. As per the graph plotted in Figure 3a, the intercepts on Xaxis is at 3.42 eV and in Figure 3b it is at 3.48 eV. Hence the energy bandgap of Mg-doped ZnSe is equal to 3.48 eV. The increase in band gap shows that band to band absorption is intense in UV region. The blue shift in absorption spectra indicates the formation of smaller sized ZnSe nanoparticles. The graph between (αhv)2 vs hv gives the energy band gap of pure and Mg doped ZnSe nanoparticles. This graph can be extrapolated in linear region of curve to energy axis.

α hυ ∝ ( hυ − Eg )

1

2

Here, α denotes absorption coefficient, hv denotes photon energy, Eg denotes direct band gap energy, and h is constant. In Figure 3 the graph between ( α h υ )2 versus hν have also been plotted, where the intercept of the graph on X-axis gives the value of band gap which is equal to 3.42 eV for pure ZnSe and 3.48 eV for Mg-doped ZnSe. 4. Conclusion The synthesized nanoparticles show the formation of cubic ZnSe crystal structure. The pure ZnSe nanocrystallites were of size around 6 nm and Mg-doped ZnSe nanocrystallites were of size around 11 nm which was calculated by Debye Scherrer formula. The doping of Mg in ZnSe was considered to be very efficient method for tuning of energy band gap. Thus by controlling the band gap its light emission color can be controlled effectively. In this study, the quantum confinement effect was found to have strong impact as the energy band gap of pure ZnSe was 3.42 eV and Mg doped ZnSe was 3.48 eV. The UV–visible absorption spectrum also showed slight apparent blue shifts. The doping of ZnSe with alkaline earth metal Mg enhances the functional and structural property, thus making it important for future applications. Acknowledgement The authors express their sincere thanks to Central Instrumentation Facility, BIT Mesra, for providing XRD and SEM facilities and also to the Department of Chemistry, BIT Patna for providing the UV-Visible spectroscopy facility.

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S.K. Choubey et al. / Materials Today: Proceedings 21 (2020) 1943–1948

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