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Synthesis of CuInS2 quantum dots using polyetheramine as solvent.

Shei SC, Chiang WJ, Chang SJ - Nanoscale Res Lett (2015)

Bottom Line: An excess of group VI elements facilitated precipitation, whereas an excess of group I elements resulted in CuInS2 QDs with high photoluminescence quantum yield.Our results demonstrate that the band gap of the CuInS2 QDs is tunable with size as well as the composition of the reactant.We also determined some important physical parameters such as the band gaps and energy levels of this system, which are crucial for the application of CuInS2 nanocrystals.

View Article: PubMed Central - PubMed

Affiliation: Department of Electrical Engineering, National University of Tainan, Tainan, 70005 Taiwan.

ABSTRACT
This paper presents a facile solvothermal method of synthesizing copper indium sulfide (CuInS2) quantum dots (QDs) via a non-coordinated system using polyetheramine as a solvent. The structural and optical properties of the resulting CuInS2 QDs were investigated using composition analysis, absorption spectroscopy, and emission spectroscopy. We employed molar ratios of I, III, and VI group elements to control the structure of CuInS2 QDs. An excess of group VI elements facilitated precipitation, whereas an excess of group I elements resulted in CuInS2 QDs with high photoluminescence quantum yield. The emission wavelength and photoluminescence quantum yield could also be modulated by controlling the composition ratio of Cu and In in the injection stock solution. An increase in the portion of S shifted the emission wavelength of the QDs to a shorter wavelength and increased the photoluminescence quantum yield. Our results demonstrate that the band gap of the CuInS2 QDs is tunable with size as well as the composition of the reactant. The photoluminescence quantum yield of the CuInS2 QDs ranged between 0.7% and 8.8% at 250°C. We also determined some important physical parameters such as the band gaps and energy levels of this system, which are crucial for the application of CuInS2 nanocrystals.

No MeSH data available.


XRD patterns of CIS QDs synthesized with Cu/In ratios of 1/1, 3/4, and 1/2.
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Fig7: XRD patterns of CIS QDs synthesized with Cu/In ratios of 1/1, 3/4, and 1/2.

Mentions: We also examined the CIS QDs synthesized starting solution of various concentrations with different Cu/In molar ratios and at a fixed temperature of 150°C and for durations of 8 h. Figure 7 presents the XRD patterns of QDs with a CIS core produced at various Cu/In molar ratios of 1/1, 3/4, and 1/2. No discernible difference in reflection peak angle was observed, despite a relatively large variation in Cu content. Three distinct reflection peaks with 2θ values of 28.0°, 46.5°, and 54.9° were well indexed along the (112), (220), and (312) planes, respectively, of a known tetragonal chalcopyrite structure of the CuInS2 phase [30,31]. These XRD results are consistent with those in previous studies, in which the XRD patterns of highly off-stoichiometric (Cu/In ratio of 1/2) CIS QDs were the same as those of the stoichiometric QDs (Cu/In ratio of 1/1) [5]. The generation of a metastable In-rich CIS phase, such as CuIn3S5, could be expected under the Cu-deficient synthesis; however, the identification of such a metastable phase is challenging in practice, due to its structural similarity to the CuInS2 phase and XRD peak broadening. Nevertheless, according to Raman spectroscopic results obtained from CIS QDs with various degrees of Cu deficiency provided by Uehara et al. [5], the off-stoichiometric CIS QDs in this study, with Cu/In ratios of 3/4 and 1/2, are believed to possess the same chalcopyrite framework as stoichiometric QDs, despite the fact that such Cu-deficient QDs are likely to include Cu-related defects concentrated at high density (e.g., Cu vacancy and In interstitial at the Cu site). Figure 7 presents a TEM image of representative CIS QDs with a Cu/In ratio of 1/1, with widely dispersed size concentration in the range of 10 to 20 nm. Figure 8a presents the absorption spectra of CIS QDs with various Cu/In ratios, showing a blue shift in absorption far more pronounced than that observed in Cu-deficient CIS QDs. As mentioned previously in the context of the TEM results, this blue shift is not related to size variation. This variation in Cu/In composition-dependent band gap in the CIS QDs is consistent with previous reports, which generally attributed this effect to a lowering of the valence band due to the weakened repulsion between Cu d and S p orbitals in Cu-deficient material, ultimately leading to a widening of the band gap [26-28]. As shown in the normalized emission spectra and fluorescence images of CIS QDs in Figure 8b, all core QDs produced emissions in the orange-red region (with peak wavelengths of 600 nm for Cu/In = 1/1 and 580 nm for Cu/In = 1/2) with broad emission bandwidths of 110 to 125 nm. The systematic blue shift in emissions observed in the QDs with greater Cu deficiency is likely associated with a widening in the band gap, as described above. A large Stokes shift in emissions versus absorption wavelength up to approximately 650 meV implies that the radiative decay is unlikely to stem from carrier recombination between quantized electron-hole levels, but rather is associated with internal and/or surface defect sites that serve as intragap states. Nonetheless, the accurate assignment of electron-hole recombination channels in CIS QDs remains ambiguous. A commonly accepted transition mechanism is the so-called DAP recombination [22,27,29,30] in which InCu (In substituted at the Cu site) and/or VS (S vacancy) probably act as donor states with VCu (Cu vacancy) as an acceptor state [29]. The explanation provided by Li et al. [26] regarding the alternative carrier recombination between the quantized conduction band minimum and defect (acceptor) trap level in CuInSe QDs is also persuasive. The broad bandwidth of emissions is characteristic of defect-related radiative transitions, as in the case of CIS QDs. Inhomogeneity in the size of QDs could lead to the broadening of emissions. However, previous findings suggest that even in a comparison of size-selective precipitated CIS QDs, an improvement in size inhomogeneity rarely alters the emission bandwidth [29]. The emission QY of CIS QDs with Cu/In ratios of 1/2, 3/4, and 1/1 were measured at 6.3%, 7.5%, and 8.8%, respectively. This increase in QY with more Cu-deficient QDs is in agreement with the results reported by Uehara and Nam et al. [5]. A higher density of Cu-related defect states through the formation of Cu-deficient QDs would increase the probability of carrier recombination, resulting in enhanced efficiency. From the above discussions, we conclude that the best growth condition is at the temperature of 250°C, reaction time of 8 h, and Cu/In ratios of 1/1 to obtain the highest PLQY of CIS QDs.Figure 7


Synthesis of CuInS2 quantum dots using polyetheramine as solvent.

Shei SC, Chiang WJ, Chang SJ - Nanoscale Res Lett (2015)

XRD patterns of CIS QDs synthesized with Cu/In ratios of 1/1, 3/4, and 1/2.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4385220&req=5

Fig7: XRD patterns of CIS QDs synthesized with Cu/In ratios of 1/1, 3/4, and 1/2.
Mentions: We also examined the CIS QDs synthesized starting solution of various concentrations with different Cu/In molar ratios and at a fixed temperature of 150°C and for durations of 8 h. Figure 7 presents the XRD patterns of QDs with a CIS core produced at various Cu/In molar ratios of 1/1, 3/4, and 1/2. No discernible difference in reflection peak angle was observed, despite a relatively large variation in Cu content. Three distinct reflection peaks with 2θ values of 28.0°, 46.5°, and 54.9° were well indexed along the (112), (220), and (312) planes, respectively, of a known tetragonal chalcopyrite structure of the CuInS2 phase [30,31]. These XRD results are consistent with those in previous studies, in which the XRD patterns of highly off-stoichiometric (Cu/In ratio of 1/2) CIS QDs were the same as those of the stoichiometric QDs (Cu/In ratio of 1/1) [5]. The generation of a metastable In-rich CIS phase, such as CuIn3S5, could be expected under the Cu-deficient synthesis; however, the identification of such a metastable phase is challenging in practice, due to its structural similarity to the CuInS2 phase and XRD peak broadening. Nevertheless, according to Raman spectroscopic results obtained from CIS QDs with various degrees of Cu deficiency provided by Uehara et al. [5], the off-stoichiometric CIS QDs in this study, with Cu/In ratios of 3/4 and 1/2, are believed to possess the same chalcopyrite framework as stoichiometric QDs, despite the fact that such Cu-deficient QDs are likely to include Cu-related defects concentrated at high density (e.g., Cu vacancy and In interstitial at the Cu site). Figure 7 presents a TEM image of representative CIS QDs with a Cu/In ratio of 1/1, with widely dispersed size concentration in the range of 10 to 20 nm. Figure 8a presents the absorption spectra of CIS QDs with various Cu/In ratios, showing a blue shift in absorption far more pronounced than that observed in Cu-deficient CIS QDs. As mentioned previously in the context of the TEM results, this blue shift is not related to size variation. This variation in Cu/In composition-dependent band gap in the CIS QDs is consistent with previous reports, which generally attributed this effect to a lowering of the valence band due to the weakened repulsion between Cu d and S p orbitals in Cu-deficient material, ultimately leading to a widening of the band gap [26-28]. As shown in the normalized emission spectra and fluorescence images of CIS QDs in Figure 8b, all core QDs produced emissions in the orange-red region (with peak wavelengths of 600 nm for Cu/In = 1/1 and 580 nm for Cu/In = 1/2) with broad emission bandwidths of 110 to 125 nm. The systematic blue shift in emissions observed in the QDs with greater Cu deficiency is likely associated with a widening in the band gap, as described above. A large Stokes shift in emissions versus absorption wavelength up to approximately 650 meV implies that the radiative decay is unlikely to stem from carrier recombination between quantized electron-hole levels, but rather is associated with internal and/or surface defect sites that serve as intragap states. Nonetheless, the accurate assignment of electron-hole recombination channels in CIS QDs remains ambiguous. A commonly accepted transition mechanism is the so-called DAP recombination [22,27,29,30] in which InCu (In substituted at the Cu site) and/or VS (S vacancy) probably act as donor states with VCu (Cu vacancy) as an acceptor state [29]. The explanation provided by Li et al. [26] regarding the alternative carrier recombination between the quantized conduction band minimum and defect (acceptor) trap level in CuInSe QDs is also persuasive. The broad bandwidth of emissions is characteristic of defect-related radiative transitions, as in the case of CIS QDs. Inhomogeneity in the size of QDs could lead to the broadening of emissions. However, previous findings suggest that even in a comparison of size-selective precipitated CIS QDs, an improvement in size inhomogeneity rarely alters the emission bandwidth [29]. The emission QY of CIS QDs with Cu/In ratios of 1/2, 3/4, and 1/1 were measured at 6.3%, 7.5%, and 8.8%, respectively. This increase in QY with more Cu-deficient QDs is in agreement with the results reported by Uehara and Nam et al. [5]. A higher density of Cu-related defect states through the formation of Cu-deficient QDs would increase the probability of carrier recombination, resulting in enhanced efficiency. From the above discussions, we conclude that the best growth condition is at the temperature of 250°C, reaction time of 8 h, and Cu/In ratios of 1/1 to obtain the highest PLQY of CIS QDs.Figure 7

Bottom Line: An excess of group VI elements facilitated precipitation, whereas an excess of group I elements resulted in CuInS2 QDs with high photoluminescence quantum yield.Our results demonstrate that the band gap of the CuInS2 QDs is tunable with size as well as the composition of the reactant.We also determined some important physical parameters such as the band gaps and energy levels of this system, which are crucial for the application of CuInS2 nanocrystals.

View Article: PubMed Central - PubMed

Affiliation: Department of Electrical Engineering, National University of Tainan, Tainan, 70005 Taiwan.

ABSTRACT
This paper presents a facile solvothermal method of synthesizing copper indium sulfide (CuInS2) quantum dots (QDs) via a non-coordinated system using polyetheramine as a solvent. The structural and optical properties of the resulting CuInS2 QDs were investigated using composition analysis, absorption spectroscopy, and emission spectroscopy. We employed molar ratios of I, III, and VI group elements to control the structure of CuInS2 QDs. An excess of group VI elements facilitated precipitation, whereas an excess of group I elements resulted in CuInS2 QDs with high photoluminescence quantum yield. The emission wavelength and photoluminescence quantum yield could also be modulated by controlling the composition ratio of Cu and In in the injection stock solution. An increase in the portion of S shifted the emission wavelength of the QDs to a shorter wavelength and increased the photoluminescence quantum yield. Our results demonstrate that the band gap of the CuInS2 QDs is tunable with size as well as the composition of the reactant. The photoluminescence quantum yield of the CuInS2 QDs ranged between 0.7% and 8.8% at 250°C. We also determined some important physical parameters such as the band gaps and energy levels of this system, which are crucial for the application of CuInS2 nanocrystals.

No MeSH data available.