Investigation of the photoluminescence properties of thermochemically synthesized CdS nanocrystals

Semiconductor nanocrystals have been attracting strong interest because of their size tunable electrical and optical properties.^1–6 Much attention has been drawn to II-IV nanocrystals such as CdS, CdTe, and CdSe because of their potential application in light emitting and photovoltaic devices.^7–12 Narrow band luminescence nanoparticles are useful for using in color saturated light emitting devices^6,7,13 and simple mixing of red- green- blue (RGB) of them can be used in white light emitting devices.² On the other hand broad band luminescence nanocrystals can be useful for using in white light emitting devices alone.¹⁴ Using a single broad band luminescence nanocrystal in white light emitting devices has more advantages than using a mixing of nanoparticles.¹⁵ There are a few articles that reported synthesis of white-color light emitting nanoprticles.^16–22 CdS nanocrystals were synthesized with a new thermochemical method using thioglycerol (TG) as capping agent in our group,²³ here also we have synthesized CdS nanocrystals with this method but thioglycolic acid (TGA) has been used instead of TG as capping agent molecule. X-ray diffraction (XRD) and TEM analyses demonstrated hexagonal phase CdS nanocrystals with an average size around 2 nm and a good size distribution. The synthesized nanocrystals showed a near white emission range of 400- 750 nm centered at 504 nm with a (0.27, 0.39) CIE coordinate. This luminescence can be attributed to the recombination of charges trapped in the surface state of CdS nanocrystals and our results showed that the luminescence peak can be attributed to the recombination of an electron in conduction band of CdS with a hole trapped in cadmium vacancies near to the valance band of CdS. By using this synthesis method we have synthesized nanocrystals with different band gap, emission wavelength and therefore different CIE coordinate. The best attained photoluminescence quantum yield of the nanocrystals was about 12% this amount is about 20 times higher than that for thioglycerol (TG) capped CdS nanocrystals.

All chemicals used were of analytical reagent grade without further purification. 3CdSO₄.5H₂O, Na₂S₂O₃.2H₂O, thioglycolic acid (TGA) (99.95%) and thioglycerol (TG) were purchased from Merck chemical Co.

CdS nanocrystals were synthesized using thermochemical method²³ through a thermally activated reaction between CdSO₄ and Na₂S₂O₃. Na₂S₂O₃ is a UV and heat sensitive material releasing S species and free electron needed for the CdS formation reaction.²³ For synthesis, 50 cc mixed solution of CdSO₄, Na₂S₂O₃ and TGA were prepared concentrations were 5 mM, 60 mM, 12 mM respectively. NaOH, 1 molar was used to adjust PH value of the solution to 8.5. Then solution heated at 80°C in air for 1.5h.

Optical transmission was measured using a Perkin Elmer, Lambada 25, UV-Visible (UV-vis) spectrometer. The Photoluminescence (PL) spectra of the samples were recorded with a varian spectrometer and X-ray diffraction (XRD) was performed on the centrifuged and extracted particles using a Philips MRD X`pert Pro system. (TEM) image were recorded by a JEOL JEM-1400 F transmission electron microscope with an accelerating voltage of 200 kV.

Figure 1 shows the XRD pattern of the synthesized CdS nanocrystals. There are two main peaks 28.3 and 48.1, correspond to (101) and (103) planes of hexagonal structure of CdS. Using the full width at half maximum (FWHM) of the first main XRD peak and Debye–Scherrer's formula, the particle size is estimated to be about 2 nm.

Figure 2(a) is a typical TEM image of the nanocrystals. It can be seen that the particles are round in shape with a good uniform size distribution, figure 2(b) gives a histogram of the size extracted from figure 2(a). As is shown in the figure, most of the particles have diameters about 2 nm.

Figure 3 shows the absorption and Photoluminescence spectra of CdS nanocrystals. It is clear from the big Stokes shift that this luminescence is not a band gap emission, luminescence spectra is a broad band emission from 400-750 nm centered in 504 nm and its CIE coordinate is (0.29,0.37). Earlier studies on CdS nanocrystals^24–27 also showed this broad emission in this range (400-750 nm), which was attributed to recombination of trapped charge carriers at Surface defects. The surface states in CdS could be due either to Sulfur vacancies or cadmium vacancies depending on the Availability of the cations or anions,²⁸ interstitial sulfur or cadmium vacancies yield green fluorescence with maxima at approx. 522 and 530 nm (d = 3 nm), However, red fluorescence (approx. 770 nm for d = 3.7 nm) is due to sulfur vacancy at the CdS surface,^19,29–31 here the luminescence peak of position is near to the Cd vacancies luminescence for 3 nm nanocrystals. Therefore it seems that the luminescence peak can be attributed to the Cd vacancies and the shift of peak position to smaller wavelength can be attributed to the smaller size of these nanocrstals. Changing the Cd vacancies by increasing Na₂S₂O₃ concentration also confirmed that. Figure 4(a) and 4(b) show the absorption and PL spectra of nanocrystals synthesized with different concentrations of Na₂S₂O₃, here solution heated for 90 min. absorption spectra shows that by increasing amount of Na₂S₂O₃ till 40 mM absorption shoulder will increase it shows that amount of CdS is increasing, but by increasing Na₂S₂O₃ concentration more than 40 mM, absorption shoulder doesn't change, it shows that free cadmium ions was consumed and increasing Na₂S₂O₃ concentration more than 40 mM will result in increasing cadmium vacancies, from PL spectra also it is clear that by increasing Cd vacancies the PL intensity will increase and shows a maximum in 80 mM and after that, it will decrease, increasing luminescence intensity by increasing Cd vacancies confirm that luminescence peak can be attributed to this vacancies and perhaps the decrease in PL intensity is related to poor crystal quality cause by deviation from stochiometry. In this case an electron in CdS conduction band will recombine with a hole trapped in Cd vacancies near valance band of CdS.

Figure 5(a) and 5(b) Show the PL and absorption spectra of synthesized nanocrystals at different heating time after starting reaction. By increasing heating time absorbance shoulder increase which shows formation of new nanocrystals. Nanocrystals band gap change from 3.41 eV to 3.08 eV that indicates growth of nanocrystals. The position of peak of PL changed from 479 nm To 535 nm, which is in agreement with growth of nanocrystals. Figure 6 indicates changing of the CIE coordinate for synthesized nanocrystals versus heating time. The color of emission changes from near blue to warm white.

Photoluminescence QY measured using this equation:

In this relation F_X and F_R are integrated fluorescence spectrum, A_X and A_R are absorbance in excitation wavelength, n_r and n_x are the refractive indices of the samples and references, respectively in low concentration they are equal.³² Sodium sulfide (Uranine) QY in 10^-6 molar is 92%, by preparing a 10^-6 molar solution of nanocrystals QY was measured. Figure 7 displays the measured PL QY of nanocrystals. The maximum amount of PL QY was obtained for the sample heated for 90 min after starting reaction.

Figures 8(a) and 8(b) show the absorption and PL spectra of synthesized nanocrystals. From absorption spectra it was found that at the same condition in presence of TGA much more amount of CdS will be formed, but the amount of band gap is 3.25 eV and 3.2 eV for TG capped nanocrystals and TGA capped nanocrystals respectively. Photoluminescence spectra indicate a broad band emission between 400-750 nm centered at 495 nm and 504 nm for TG capped nanocrystals and TGA capped nanocrystals respectively but at the same conditions photoluminescence QY of TGA capped nanocrystals is 20 times higher than that for TG capped nanocrystals.

These differences should be related to the difference of capping molecule functional groups.¹⁷ Two hydroxyl group of TG are negative at the basic media that prevent their undesirable growth. In the basic media terminal groups of TGA are partially charged and the carbonyl oxygen of TGA may coordinate with Cd cites of monomers or cluster to as secondary coordination, and also carbon chain of TGA is shorter than that of TG, therefore in the same concentration, TGA can capped non radiative paths of nanocrystals better than TG.³³ 

CdS nanocrystals were synthesized with TGA as capping agent via thermochemical method. Synthesized nanocrystals showed a near white emission centered in 504 nm with a (0.27, 0.39) CIE coordinate. Our results showed that this luminescence can be attributed to recombination of an electron in conduction band of CdS with a hole trapped in Cd vacancies. By heating initial solution at different times after starting reaction nanocrystals band gap changed from 3.41 eV to 3.08 and the peak of emission changed from 480 nm to 535 nm, in the best case PL QY of synthesized nanocrystals was obtained about 12 %, this amount is about 20 times higher than that for thioglycerol (TG) capped CdS nanocrystals.