Zns Mn Dhqg A0 Hoan Chinh Nhat

THE OPTICAL PROPERTIES OF THE POLYMER-CAPPED ZnS:Mn NANO THIN FILMS Tran Minh Thi(a), Pham Van Ben(b), Nguyen Minh Vuong(c) (a)

Faculty of Physics, Hanoi National University of Education, Vietnam (b) Faculty of Physics, National University of Hanoi, Vietnam (c) Laboratory of center, Quy Nhon University, Vietnam

Abstract: The ZnS:Mn nanopowder and the polyvinyl alcohol (PVA)-capped ZnS:Mn thin films were prepered by wet chemical process and dip-coating method. The microstructure of samples was investigated by X-ray diffraction (XRD) and transmission electron microscopy (TEM). The results show that the prepared samples were the Sphalerite structure with average particle size about 3 nm. The X ray diffraction spectra shown that PVA does not affect the microstructure of ZnS powder. The optical properties of samples were studied by measuring the absorption and photoluminescence spectra in the wavelength range from 300 nm to 900 nm at room temperature. The direct band gap of PVAcapped Mn doped ZnS nanocrystalline thin films were calculated for different PVA concentration in the samples. The values of direct band gap is about 3.75 eV. The time-resolved-luminescence spectra and fluorescence lifetime of polymer-capped ZnS:Cu nanocrystalline thin films were also investigated. The results allow us to explain their optical properties by quantum confinement effect of ZnS nanoparticles in the polymer PVA matrix [4][6].

• Introduction The direct band gap of nano-materials may be controled by doping, polymer coating and change of preparing conditions [1][2]. Polymer also has a role as the protected environment for ZnS nanoparticles. The optical propertiers of ZnS nanoparticles change considerably when dispersing them into polymer matrix with optimal concentration. In this paper we report results on the optical properties of polymer-capped 9% Mn doped ZnS thin film with different PVA concentrations. The study on influence of PVA concentration to luminescence spectra, grain size and direct band gap of this film is also presented

II- Experiments The high pure chemical solutions were used such as: first solution was Zn(CH3COO)2.2H2O 0.1M, second solution was Mn(CH3COO)2.4H2O. 0.1M and third solution was Na2S.9H2O. 0.1M. Catalysis was CH3OH:H2O for first and second solutions (ratio 1:1). Produce polymer-capped Mn doped ZnS nanocrystalline thin films with different PVA concentration: First solution and second were mixed with appropriate ratio. Then PVA amount of 1g, 2g, 4g, 5g was disolved in 100 ml of first and second mixed solution. The mixed solution with PVA heated at 80oC, then decrease to 300K. The third solution was added drop by drop in a reaction vessel. The PVA-capped Mn doped ZnS thin films were produced by spincoating method in glass substrate with centrifugation speed 3000 rpm.

III- Result and discussion

 The 9% Mn doped ZnS nano-powder was analysed by X-ray diffraction (XRD) in Fig 1. The analysed results show that : - Samples have Sphalerite structure. Lattice constant a = 5.4 Å. - The diffraction 3 peaks: (111), (220), (311). - The crystallites of size is about 3 nm by Scherrer formula.

2

2

2

h 1,8e ∆ E g = E g ( film) − E g (bulk ) = − (2) 2 εr 8µ r

me*mh* µ= * * me + mh

m = 0,34mo , m = 0,24mo * e

* h

m0 is the mass of free electron

 From (1) equation and Fig. 3, we can calculete the direct band gap of the M-PVA2, M-PVA4 and M-PVA5 samples. From (2) equation, We calculated the crystallites of size for this thin films.  The calculated results were given in table 1 . Table 1: The direct band gap and the crystallites of size of M-PVA2, M- PVA4, M- PVA5 samples

Thin film Eg(eV) r(nm)

M-PVA2 3.86 3.9

M-PVA4 3.77 5.5

M-PVA5 3.73 7.4

⇒ The results show that the values Eg decreased and the crystal size increased when PVA concentration increases. ⇒ The results show M-PVA2 thin film is best with Eg = 3.86 eV and r = 3.9 nm.

Fig. 5: The PL spectra of M-PVA2 sample at 300K Fig. 6: The dependent of Ln(IPL ) on the Ln(Iex) with the exciting wavelengh of 325nm and different of M-PVA2 sample exciting power density

Fig. 5 presents the PL spectra of the M-PVA2 thin film with the exciting wavelengh of 325nm and different exciting power density. Fig. 1: The XRD of ZnS sample (a)

Fig. 2: The SEM image of M-PVA4 thin film

The maximum positions of the D1, D2, D3 bands were nearly not changed when the power density increased from 0,13 W/cm2 to 0,64 W/cm2. But their intensity changed as following law: n I PL = A.( I EX ) with n ≈ 0,7 (Fig. 6).

and 9%Mn doped ZnS sample (b)

Fig. 2 is the SEM of M-PVA4 thin film. - Grain size about 100 nm. - That is the polymer in sphere form which coated outside the ZnS:Mn nano- particles .

3.2. The time-resolved-luminescence spectra and fluorescence lifetime of M-PVA2 thin film

3.1- Absorption and PL spectra 6000

D1

a b c

5000

Intensity(a.u)

I PL = A.( I EX ) n

α

• K is a constant α= (1) • Eg is the direct band gap hν • n = 1 for direct band gap semiconductor  The crystallites of size could be determined using the relation given below [4][6]: Eg (bulk) = 3.6 (eV)

K ( hν − E g )

n

M-PVA2 M-PVA4 M-PVA5

4000

3000

a 2000

b

D3

c

1000

0 300

D2

400

500

600

Wavelength(nm)

700

800

Fig. 7: the time-resolved-luminescence spectra of the green band at 300 K with 337 nm exciting wavelength for M-PVA2

Fig. 8: The decreasing of photoluminescence of M-PVA2 thin film at wavelength 450 at 300 K with exciting wavelength of 337nm.

⇒ Fig.7. The peaks of band shifted towards the lower energy and their intensity decreased (about 5 times) when the delay time  Fig. 3 presents the absorption spectra of the M-PVA2, M-PVA4 and increase from 60 ns to 114 ns. It is the typical characteristic of the M-PVA5 thin films. The relation between absorption coefficient and donor-acceptor emission recombination. energy of photon can be presented by following equation [3][4][6]: ⇒ Fig.8. The luminescence lifetime about 22 ns at wavelength 450 nm. References Fig. 3: The absorption spectra of M- PVA2MPVA4 and M-PVA5 thin films

• • • •

Fig. 4: The spectra of thin films with exciting wavelength 325nm and 0.32W/cm2

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