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Highly luminescent Cd–free Zn doped CuInS2 nanocrystals (ZCIS NCs) were synthesized, and their properties were evaluated using X-ray diffraction, Raman, UV, and photoluminescence. The crystal structure of the ZCIS NCs was similar to the zinc blende, and the lattice constant decreased with increasing Zn concentration. By incorporation of Zn, the emission wavelength was tuned from 536 to 637nm with concomitant enhancement of the quantum yield up to 45%. A white light emitting diodes, integrating dual ZCIS NCs (λem = 567, and 617nm) and a 460nm InGaN LED, exhibited a high color rendering index of 84.1 with a warm color temperature of 4256.2K. The CIE-1931 chromaticity coordinates were slightly shifted from (0.3626, 0.3378) at 20mA to (0.3480, 0.3206) at 50mA.
©2012 Optical Society of America
1. Introduction
Semiconductor nanocrystals (NCs) exhibit the unique properties of tunable band gap, broad absorption, and photostability, making them highly attractive materials for a wide cross-section of applications. Semiconductor NCs became potential alternatives for use in light emitting diodes (LED), solar cell, and bio-imaging [1–4]. Research regarding NCs for color conversion in white LED has been particularly highlighted because of the tunability of the emission of these NCs over the entire visible region, their high quantum yield, and their facile fabrication.
Following the initial suggestion of CdSe NC-based color conversion LED by Klimov et al. [5], similar types of white LED have been widely explored for improving the quality of white light [6–10]. Chen et al. developed a white LED consisting of a CdSe/ZnSe NCs convertor pumped by a blue InGaN LED [6]. Demir et al. controlled the color rendering index (CRI) and color temperature of white light using a combination of various emissions of CdSe NCs [7, 8]; recently, Jang et al. fabricated a 41 lm/W white LED with a color temperature of 100,000 K for display backlighting using a multi-core–shell CdSe NCs structure [10].
However, the inherent toxicity of Cd makes its use undesirable in terms of human and environmental impacts, thus limiting the practical application of Cd-based LED. Cd-free CuInS2 semiconductor NCs are regarded as viable alternatives due to their direct band gap (1.5 eV), and large absorption coefficient. The band gap can also be tuned by particle size [11, 12], composite [13], or by alloying with ZnS [14–16]. Castro et al. synthesized CuInS2 NCs by decomposition of single source precursors [11], and Nakamura et al. enhanced the quantum yield of these NCs by doping Zn ions into the CuInS2 [14]. Reiss and associates further improved the quantum yield by using dodecanethiol as a sulfur source and stabilizing ligand [12], and Xie’s group demonstrated highly luminescent CuZnInS NCs without shell coatings [15]. However, despite the suitability of properties, the application of CuInS2 NCs was limited to employing red color convertor for white LED [17, 18].
In this study, yellow to red color tunable strongly luminescent ZnCuInS (ZCIS) NCs were prepared by a simple synthetic method, and were employed in a color convertor for white LED. The emission wavelength was tuned by introducing different molar ratio of Zn, and corresponding structural and optical properties were investigated by X-ray diffraction (XRD), Raman, and Photoluminescence (PL). A white LED was fabricated by integrating a blue InGaN LED with yellow and red emitting quaternary ZCIS NCs, and the characteristics were examined.
2. Experimental
Copper(I) iodide (CuI, 99.999%), indium(III) iodide (InI3, 99.99%), sulfur powder (S, 99.99%), tri-n-octylphosphine (TOP, 90%, technical grade), oleylamine (OLA, 70%, technical grade), oleic acid (OA, > 99%) and 1-octadecene (ODE, 90%, technical grade) were purchased from Sigma-Aldrich. zinc diethyldithiocarbamate (DDCZ, 97%), containing four S atoms for one molecular, was purchased from Tokyo Chemical Industry. The ZCIS NCs were synthesized based on previous literature with modification of precursors [14]. CuI (0.2mmol), InI3 (0.2mmol), and OA (0.5mL) were dissolved in OLA (6mL); DDCZ (0.4mmol) was separately dissolved in TOP (5mL) and ODE (15mL). The Cu, In, and Zn-S solutions were mixed in a flask, and heated to 120°C. A sulfur solution, S powder (0.2mmol) dissolved in OLA (2mL), was rapidly injected into reaction solution at 120°C. The DDCZ was decomposed to ZnS, and excess sulfur could lead to burst nucleation of quaternary nanocrystals at this temperature. Then the mixture was heated at 220°C for 10 min under Ar inert condition for improving the crystallinity and quantum yield. The obtained NCs were washed with methanol and ethanol several times. For the various Cu( = In):Zn ratio, the composition of Cu and In precursors was changed, while other reaction conditions were maintained. A white LED was fabricated by combining a blue LED chip with ZCIS NCs color convertor. The ZCIS NCs and silicone gel mixtures were coated onto LED chip, and cured at 80°C for 5hr.
3. Results and discussion
Figure 1 shows the powder XRD patterns (Philips XPERT MPD) of the ZCIS NCs with various Cu( = In)/Zn ratios. The diffraction patterns were broadened which was indicative of nano-sized particles, and three apparent peaks were observed corresponding to the (1 2 2)/(1 0 3), (2 2 0), and (1 1 6)/(3 1 2) planes of the tetragonal phase or (1 1 1), (2 2 0), and (3 1 1) planes of the zinc blende phase. As the Zn ratio increased, the location of dominant peak was slightly moved to higher angle indicating that the crystalline phase became closed to bulk zinc blende. The ionic radius of Zn2+ (0.88nm) is smaller than that of Cu+ (0.91nm) or In3+ (0.94nm), thus, the substitution of Zn2+ for Cu+ or In3+ led to lattice contraction, and the lattice constant of the a-axis decreased from 0.5534, 0.5530, 0.5476, and 0.5457 to 0.5372nm with increasing Zn content. In addition, the particle sizes calculated based on the Scherrer formula were less than 3nm regardless of the Cu:Zn ratio.
Fig. 1 The Powder X-ray diffraction patterns of ZnCuInS nanocrystals with different Cu( = In):Zn ratios.
Figure 2 shows the HR-TEM (Tecnai G2 F30) image of ZCIS NCs. The prepared NCs were nearly spherical in shape, and the diameter of Cu/Zn = 0.2, 0.6, and 1.0 of ZCIS NC were 3.69, 3.56, and 3.82nm, respectively, measured from dynamic light scattering (DLS). The Cu( = In):Zn ratio had no significant effect on particles size, and their size was closed to XRD results.
Fig. 2 HR-TEM image of ZCIS NCs with different Cu( = In):Zn ratios. (a) 0.2, (b) 0.6, and (c) 1.0. (insets: size distribution of ZCIS NCs from DLS)
Raman scattering is a nondestructive method to identify crystalline phases. In general, CuInS2 can appear in two different crystalline phases, chalcopyrite (CH) and Cu-Au like (CA) ordering. According to Riedle [19] and Alvarez-Garcia [20], A1 mode peaks of CH and CA ordering are observed at 293 and 305cm−1, respectively. As shown in Fig. 3 , broad peaks were located at 290-310cm−1 suggesting that the obtained ZCIS NCs comprised a mixture of CH and CA ordering. The other peaks that were observed at around 264 (E3LO, B22LO), 326 (E1TO, B21TO), and 342cm−1(E1LO) were assigned to the CH ordering [21], and the peak at 478cm−1, observed in the Cu/Zn = 0.8 and 1.0, originated from the Cu2-xS impurity phase [19]. Increasing the concentration of Zn led to weakening of the broad A1 peak at around 300cm−1, and the strong peak was observed at about 353cm−1 characteristic of the ZnS structure [22]. These raman spectroscopy results were consistent with the XRD results.
Figure 4(a) shows the absorption spectra (PerkinElmer Lambda 35) of the Zn doped CuInS2 NCs with different Cu/Zn ratios. The absorption spectra were broad, and no distinct first excitonic peaks were observed due to their irregular elemental distribution and/or intra band gap states. Besides, as expected, the absorption onsets were blue shifted with increasing Zn ratio. Uehara et al. [13] reported that the emission of CuInS2 originates from the donor-acceptor (D-A) transition related to crystal defects. Therefore, enhancement of the emission intensity and adjustment of the emission wavelength are possible through control of crystal defects. In the CuInS2 structure, the strength of the Cu-S bond is weaker than that of In-S, and the ionic radius of Zn2+ is similar to that of Cu+. Thus, Zn2+ is readily incorporated into the CuInS2 lattice. Figure 4(b) shows the photoluminescence (PL, Perkin Elmer LS55) properties of ZCIS NCs with different Cu(In)/Zn compositions. Given that the band gap of ZnS (3.6eV) is larger than that of CuInS2 (1.5eV), the incorporation of the Zn ion into CuInS2 resulted in a blue shift. The center of the emission maximum was tuned from 637, 617, 592, 567, to 546nm as the Cu(In)/Zn ratio was varied from 1.0, 0.8, 0.6, 0.4, to 0.2, respectively, which covered the green to red region of the visible spectrum. The full width at half maximum (FWHM) of the ZCIS emission was approximately 90nm which was relatively broad compared to that of excitonic emission semiconductor NCs such as CdSe (~40nm). The wide FWHM might be the results of inhomogeneous distribution of incorporated Zn or broad nanocrystals size distribution as well as intrinsic defects such as Cu vacancies, and anti-site In substitution, etc. Moreover, increasing incorporation of Zn enhanced quantum yield by preventing the anti-site defects. The quantum yields (Φ) of ZCIS NCs relative to rhodamine 6G (Φ = 95%, in ethanol) standard reached 45% without shell formation, whereas the previously reported quantum yield of CuInS2 less than 5% [11, 12]. The calculated quantum yield of ZCIS NCs at Cu:Zn ratios of 0.2, 0.4, 0.6, 0.8, and 1.0 were 6, 40, 42, 41, and 45%, respectively.
Fig. 4 (a) Absorption and (b) emission spectra (λex = 460nm) of ZnCuInS nanocrystals with different Cu( = In):Zn ratios.
A white LED was fabricated by combining a 460nm InGaN LED chip with ZCIS NCs. Either single 567nm yellow emitting or dual integration of 567nm and 617nm yellow/red emitting ZCIS NCs was employed in wavelength convertor. Figures 5(a) , and 5(b) showed the emission spectra (Lab sphere) of the ZCIS NCs based white LED. The single yellow ZCIS NCs based white LED had CIE-1931 chromaticity coordinates of (0.3195, 0.3642), and correlated color temperature of Tc = 6011.7K at 20mA. As a result of the broad FWHM of the ZCIS NCs, the CRI was higher than that of CdSe NCs based white LED. The CRI of the single yellow ZCIS NCs phosphor white LED was 66.5, compared to values of 14.6 and 15.3 reported by Demir et al. [7, 8] and Cho et al. [9], respectively, using a single yellow CdSe NCs color convertor white LED. The warm and high CRI of white LED was developed using combination of a 460nm blue LED and yellow/red dual emitting ZCIS NCs. The addition of 617nm emitting ZCIS NCs enhanced the red spectral deficiency, thus, the strong red component of R9 was largely increased, and special CRI reached to 84.1. In the case of CdSe NCs based white LED, more than 4 different NCs emission wavelength were required for generating a CRI of white light above 70, which impacted negatively on the overall efficiency due to reabsorption within the NCs. The color temperature was 4256.2K in warm white region, and acceptable color stability was observed with various applied currents. The CIE-1931 chromaticity coordinates were changed from (0.3626, 0.3378) at 20mA to (0.3480, 0.3206) at 50mA. Although the luminous efficacy of 7.8 lm/W was still low, the color properties such as the CRI and color temperature were comparable to those of commercial white LED. Furthermore, the nontoxic, Cd-free nature of the ZCIS NCs makes them promising alternatives to Cd-containing NCs for application in opto-electronic devices.
Fig. 5 Emission spectra of fabricated white LED using an integrating sphere. (a) 460nm InGaN LED pumped with single yellow ZnCuInS nanocrystals, and (b) dual yellow/red ZnCuInS nanocrystals.
4. Conclusion
Color tunable strongly luminescent Cd-free ZCIS NCs were successfully synthesized via a facile route. Variation of the Cu/Zn molar ratio in the range 0.2–1.0 induced incremental shifts in the emission wavelength in the range 546–637 nm, with a large stoke shift and broad FWHM of approximately 90 nm. The incorporation of Zn improved the quantum yield significantly to 45.1%, without shell formation. The crystal structure of ZCIS NCs was close to zinc blende, and the lattice constant was decreased with increasing Zn ratio. Furthermore, excellent quality of white LED with CRI of 84.1, and warm color temperature of 4256.2K was achieved by employing dual 560/620nm emitting ZCIS NCs. These results suggest that the Cd-free ZCIS NCs developed in this study hold promise for application to solid-state lighting as viable alternatives to toxic Cd-based NCs.
Acknowledgment
This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (2012R1A1A2007908). This work was supported by the Human Resources Development the Korea Institute of Energy Technology Evaluation and Planning (20114010203050) grant funded by the Korean government Ministry of Knowledge Economy.
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