Effect of fabrication parameters on luminescent properties of ZnS:Mn nanocrystals

S. Shaari*a, Muhammad S. A. Rahmanb, Noor A. A. M. Arifb

aInstitute of Micro Engineering & Nanoelectronics, Universiti Kebangsaan Malaysia, 43600 UKM Bangi, Selangor, Malaysia;

bDept. of Electrical, Electronic & Systems Engineering, Universiti Kebangsaan ,Malaysia, 43600 UKM Bangi, Selangor, Malaysia

ABSTRACT

In this work, we mainly focused on the luminescence properties of ZnS:Mn nanocrystals. Various samples of ZnS:Mn have been characterized at different doping concentration, annealing temperature, spin speed and time. The present study shows the application of spin speed, spin time, doping concentration and temperature affect the luminescent intensity performance. Luminescent intensity becomes higher with the increasing film thickness. Spin speed and spin time are two major concerns for coating film to a demanded thickness on the glass slide. Film thickness is the main reasons of the increasing intensity with spin speed and time. Temperature dependent PL measurements provide thermally activated energy transfer from other defects to Mn2+ ions. As the temperature increase, the carriers can be trapped at Mn sites, enhancing the luminescence spectra. Meanwhile, the quenching process influenced the PL intensity with doping concentration. This process occurs at high Mn concentration which the energy transfer from Mn ions to the other nearest Mn atom is weak. Therefore, the luminescence of transition from 4T1 to 6A1 of ions becomes stronger. From this reason, it is shown that luminescent intensity increased with higher doping concentration but decreased with higher annealing temperature, spin speed and spin time during spin coating process.

Keywords: Luminescence properties, doping concentration, spin speed, spin time, annealing temperature, ZnS:Mn, nanocrystals, film thickness, spin coating process.

1. INTRODUCTION In recent years, research work has been focused on semiconductor nanocrystals since they can exhibit novel optical and electrical properties, which are significantly different from those of bulk materials, arising from quantum confinement effects1. In particular, doped nanocrystalline II-IV semiconductors have been extensively studied on their luminescence properties due to their potential applications in future optoelectronic devices2. Doped semiconductors have wide range of applications in sensors, displays, electronic devices, laser devices, and nonlinear optical devices3. ZnS is a typical II-IV semiconductor material for commercial especially if doped with divalent manganese ions. Usually, Mn2+ ion used as a dopant in many luminescent materials because it has d5 configuration and can exhibit a broad emission peak2. Doping with manganese ions at room temperature will exhibit a characteristic of orange luminescence attributed to the 4T1 – 6A1 transition of Mn2+ ions in the ZnS cubic lattices4. Although, this extensive studies on ZnS:Mn nanocrystals have been carried out, a lot of controversial questions are still waiting for further confirmation. The most one controversy is the effect of manganese ion concentration on the luminescence properties as reported by Peng et. al1. A few research found that the PL intensity of ZnS:Mn nanocrystals showed a maximum at a higher Mn2+ concentration which related with concentration quenching. The other top issues have attracted many researchers are the PL spectra dependence annealing temperature, spin speed and spin time. All the detail discussion and explanations with this topic has been reported in previous publications. However, certain researcher claimed that annealing temperature did not affect the luminescence properties. In the present work, we are showing that the PL spectrum is increasing with higher Mn concentration in the host ZnS but is decreasing with annealing temperature, spin speed and spin time during spin coating process.

*sahbudin@vlsi.eng.ukm.my; phone 60389216308; fax 60389216146: ukm.my

Optoelectronic Materials and Devices IV, edited by Jian-Jun He, Guang-Hua Duan, Fumio Koyama, Ming C. Wu, Proc. of SPIE-OSA-IEEE Asia Communications and Photonics, SPIE Vol. 7631, 76310U · © 2009 SPIE-OSA-IEEE

CCC code: 0277-786X/09/$18 · doi: 10.1117/12.852944

SPIE-OSA-IEEE Vol. 7631 76310U-1

Figure. 1 Flow chart of the procedure of the ZnS:Mn film preparation.

2. METHODOLOGY The preparation procedure started by adding zinc nitrate, thiourea and manganese acetate to a 2-propanol and distilled water at room temperature5,6. Then, the mixed solution was stirred for 4 days. A clear and transparent solution was obtained after stirring process. The films were prepared by spin coating on glass slide substrates. The coated thin film samples then were heated up from room temperature to required temperature. The microstructures of the films were studied using Field Emission Scanning Electron Microscopy (FESEM). FESEM with including Energy Dispersive X-Ray (EDX) was used for compositional analysis of the prepared ZnS:Mn nanocrystallites. The photoluminescence (PL) spectrum of the Mn doped ZnS nanocrystals has been measured using luminescence spectrometer. In this work, PL properties were studied at different: (i) temperature: 300˚C, 400˚C and 500˚C. (ii) concentration of Mn2+: 0.05, 0.15, 0.25 and 0.35 (iii) spin speed and spin time during stirring process: 3000 rpm within 20 s, 4000 rpm within 20 s, 3000 rpm within 40 s and 4000 rpm within 40 s. Fig. 1 shows the flow chart of the preparation procedure.

3. RESULTS FESEM micrographs reveal that the nano-sized spherical particles with 20 nm in diameter were obtained in Fig. 2. From the Energy Dispersive X-ray (EDX) analysis it shows that high intensity peaks are associated to elemental zinc (Zn), sulphur (S) and manganese (Mn) with their percentage ratio 67.55 %, 28.50 % and 3.95 % respectively. Fig. 3 shows the room temperature photoluminescence spectra of ZnS:Mn at different temperature, Mn2+ concentration, speed and rotation time under 250 nm excitation. It was found that intensity of the emission spectra increases with temperature as well as doping concentration. The rotation speed and time also determine the strength of the spectra. The intensity of the PL peak is doubled when the temperature increased from 300°C to 500°C. Similarly, it has a significant increase when the doping concentration increases from 0.05% to 0.35%. For all the doped samples, the PL spectra are dominated by three different emission bands. The first emission band ranging from 350 to 450 nm can be assigned to the radiative recombination involving defect states in the ZnS nanocrystals5. Peng et. al1 explain this emission also exist in the PL spectrum of the undoped ZnS nanocrystals which this emission band should indeed originate from the host ZnS but not from Mn2+ ions. In the others related paper4, the stronger peak with blue shift of the nanocrystals is associated to the Mn centers. The mid peak with known as huge peak at 500 nm is due to the first harmonic of the excitation wavelength 500

Mn2+ concentration: 1. 0.05 2. 0.15 3. 0.25 4. 0.35

Temperature: 1. 300°C 2. 400°C 3. 500°C

Spin speed and spin time: 1. 3000 rpm (20 s) 2. 3000 rpm (40 s) 3. 4000 rpm (20 s) 4. 4000 rpm (40 s)

Zinc Nitrate Hexahydrate Thiourea Manganese Acetate

Mixed with 2-propanol and distilled water

ZnS:Mn sol

Spin Coating process

Annealed at required temperature

ZnS:Mn film

SPIE-OSA-IEEE Vol. 7631 76310U-2

Figure. 2 nano

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SPIE-OSA-IEEE Vol. 7631 76310U-3

ions. Mn2+ can substitute for Zn2+ ions in the ZnS crystal lattice because of their close ionic radii with 0.80 and 0.83Å for Mn2+ and Zn2+ respectively6. Thus, it can be concluded that the Mn2+ ions in our samples are indeed incorporated into the host ZnS nanocrystals.

4. DISCUSSION 4.1 The PL spectra spin speed and time dependence

Generally, spin coating is a technique that uses centrifugal force to apply a uniform liquid coating layer to glass slide7. This happened when ZnS:Mn sol as a liquid form was layered on the glass substrate. Basically, spin speed and spin time are one of the most important factors in spin coating process. This two major concerns for coating film to a demanded thickness on the substrate. Fig. 4, shows the relation between ZnS:Mn thickness and both the spin speed and spin time. From the graph, we can observe that under the same spin speed, the film becomes thinner as the duration time is taken longer which is similar with that recommended by Mohajerani et. al8. This shows, the film thickness is also affected by the duration time. However, for the identical spinning duration, the film thickness is inversely proportional to the spin speed. That is an insufficient spinning duration will cause ZnS:Mn transiently distributed on the substrate. The dependence of thickness, H on spin speed, w and spin time, t is in general as bH w−∝ . The value of exponent b would be equal to 1 if only centrifugal force is important. On the other hand, b would be 0.5 when evaporation of solvent is

dominant in the coating process7,8. Meanwhile 23H w

−∝ refers when evaporation rate is constant. In our case, the

centrifugal action is probably more important than evaporation action. As appears in the figure, the highest spin speed and spin time produce the thinnest film which this thinnest film produce the lowest luminescent intensity.

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Figure. 4 Variation of the ZnS:Mn film thickness with spin time and spin speed

4.2 The PL spectra temperature dependence

Environment temperature is another factor in performance emission intensity. As already state, thickness film influenced

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SPIE-OSA-IEEE Vol. 7631 76310U-4

the PL spectrenergy of thethe temperatuemission intefilm. The thideviated undagainst the apthin films annThe higher anthinner. Therintensity withthe conductioincreasing temat 590 nm divarious defecmatrix with a

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SPIE-OSA-IEEE Vol. 7631 76310U-5

Mn2+ ions are incorporated into the ZnS host structure. The higher doping concentration (0.35%), lower temperature (300˚C), lower spin speed and time (3000 rpm within 20 s) is found sufficient to produce a good quality of ZnS:Mn nanoparticles thin film with higher luminescent intensity via sol gel–self assembled technique.

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