- Nano Express
- Open Access
Synthesis of CuInS2 quantum dots using polyetheramine as solvent
© Shei et al.; licensee Springer. 2015
- Received: 30 June 2014
- Accepted: 27 January 2015
- Published: 12 March 2015
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.
- CuInS2 quantum dots
Nanoparticles have attracted considerable attention due to their unique properties and applicability in biomedicine [1-4], renewable energy [5-9], and optical devices [10-13]. The ability to control the optical properties of semiconducting nanoparticles, otherwise referred to as quantum dots (QDs), has prompted a great deal of research in bioimaging [14,15] and opto-electronic devices [16-19]. Cadmium-based QDs, including CdSe and CdTe, have been widely examined for their excellent physical properties; however, the toxicity of cadmium limits their applicability. QDs from the periodic groups III-V [20-24] and I-III-VI [19,25-27], referred to as cadmium-free QDs, have been proposed as an alternative by many research groups. In fact, InP/ZnS core/shell QDs, which are representative of III-V QDs, have been extensively studied and are close to commercialization. Other I-III-VI QDs, including copper indium sulfide (CuInS2) (CIS), CuInSe2 (CISe), AgInS2 (AIS), and AgInSe2 (AISe), have also been widely studied [25-30]. According to Peng et al. , the photoluminescence of the CIS core cannot be maintained for more than 4 days and the quantum yield is <4%. Thus, type-I core/shell structures, such as the CIS/ZnS were developed to enhance the stability and quantum yield of CIS QDs. Zinc ethylxanthate and zinc diethyldithiocarbamate have been used for ZnS shell growth on a CIS core, and interestingly, this has led to a blue shift in photoluminescence [28,31]. This phenomenon differs considerably from that of the general type-I core/shell structures, which show a red shift in emissions with an increase in the thickness of the particle shell. However, the explanation that has been posited is not convincing, despite the strong suggestion of surface reconstruction by Reiss et al. and the inter-diffusion of Zn atoms by Pons et al. [27,29]. Their findings suggest that cation exchange may explain the blue shift . Since cation exchange was first reported, a number of studies have reported interesting results from a wide range of nanoparticles. According to these reports, the original shape and anion sublattice remain intact during the exchange of cations throughout the reaction. This paper presents a facile solvothermal method of synthesizing CuInS2 quantum dots (QDs) via a non-coordinated system using polyetheramine as a solvent. We demonstrated that this is a more effective way to produce monodisperse nanocrystals. Thus, a copper-thiol solution was injected into the solution, which included an In precursor. Our results give a clear indication that the blue shift in photoluminescence is caused by cation exchange.
Indium (III) chloride (InCl3 · 3H2O, 99.99%), copper (II) chloride (CuCl2 · 2H2O, 99.99%), sulfur (S2, 99.99%), and D400 were purchased from Sigma-Aldrich (St. Louis, MO, USA). For the preparation of CuInS2 QDs, InCl3 · 3H2O (0.1 mol), CuCl2 · 2H2O (0.1 mol), sulfur (0.1 mol), and ODE (200 ml) were loaded into a 500-ml three-necked flask. The solution was degassed at 100°C for 1 h. In the first case, the temperature was gradually increased to 150°C, 200°C, or 250°C over a period of 8 h under N2 atmosphere. In the second case, the temperature was maintained at 250°C and the reaction time was varied between 1 and 8 h under an N2 atmosphere without stirring. It should be noted that extending the growth to beyond 8 h resulted in severe agglomeration of the QDs. In the third case, CIS QDs were synthesized using various ratios of Cu/In (/1, 3/4, and 1/2), which were prepared by fixing the amount of In precursor and varying the amount of Cu precursor. For the synthesis of CIS QDs with a Cu/In ration of 1/1 (i.e., 0.5 mmol of CuCl2 or 0.5 mmol of InCl3), we loaded 200 mL of D400 into 500 mL of a three-necked flask. The reaction mixture was then degassed during heating to 100°C and backfilled with N2 followed by further heating to 250°C over a period of 8 h. CIS core QDs were then allowed to grow for 5 min at that temperature. CIS QDs with Cu/In ratios of 3/4 and 1/2 were synthesized in precisely the same manner except that we employed 0.75 and 0. 5 mmol of CuI, respectively. After cooling the resulting CIS QD crude solution to room temperature, as-reacted CIS core QDs were precipitated with an excess of ethanol, purified repeatedly with a chloroform/ethanol solvent by centrifugation (8,000 rpm/30 min), and dispersed in chloroform. For characterization, the CIS core particles were purified by precipitation in an excess of acetone and dispersed in alcohol.
Ultraviolet-visible (UV-vis) and photoluminescence (PL) spectra were obtained using a Shimadzu UV-2450 UV-vis spectrophotometer (Shimadzu, Kyoto, Japan) and a Cary Eclipse (Varian Medical Systems, Palo Alto, CA, USA) fluorescence spectrophotometer, respectively. The room-temperature PL quantum yields (QYs) of nanocrystals (NCs) were determined by comparing the integrated emissions of the NC samples in solution with those of a fluorescent dye (rhodamine 6G in ethanol or rhodamine 101 in 0.01% HCl ethanol solution) with identical optical density. The known QY of the NCs in solution was used to measure the PL efficiency of other NCs by comparing their integrated emissions. Fluorescence lifetime was determined using an Edinburgh FL 900 single-photon counting system (Edinburgh Instruments Ltd., Livingston, West Lothian, UK) equipped with a Hamamatsu C8898 ps light pulser (Hamamatsu, Hamamatsu City, Shizuoka Pref., Japan). Excitation light was obtained using a 441-nm laser light with the luminescence time range set to 200 ns. Data were analyzed using a non-linear least squares fitting program, with deconvolution of the exciting pulse at approximately 200 ps. Transmission electron microscopy (TEM) was performed by depositing the NCs from dilute toluene solutions onto copper grids with a carbon support by slowly evaporating the solvent in air at room temperature. TEM images were acquired using a JEOL JEM-1400 transmission electron microscope (JEOL Ltd., Akishima-shi, Japan) operating at an acceleration voltage of 120 kV. Powder X-ray diffraction (XRD) measurements were obtained under wide-angle X-ray scattering using a Siemens D5005 X-ray powder diffractometer (Siemens, Munich, Germany) equipped with graphite-monochromatized Cu Kr radiation (λ = 1.54178 Å). XRD samples were prepared by depositing NC powder on a Si (100) wafer.
This study synthesized highly luminescent CuInS2 QDs. The emission quantum yield (QY) of CIS QDs synthesized at 150°C, 200°C, and 250°C reached 3.7%, 5.0%, and 8.8%, respectively, when calculated at an excitation wavelength of 450 nm. As expected, an increase in the photoluminescence peaks was observed as the temperature was increased from 150°C to 250°C. In addition, QDs with CIS core were solvothermally prepared by varying the growth time. All of the CIS QDs exhibited a size-dependent red shift in emissions with longer growth time; however, the emission wavelengths (550 to 600 nm) fell deep within the red region with PL QYs of 0.7% to 8.8%. The radiative transition was described according to the DAP recombination of photo excited carriers resulting in a large Stokes shift and broad PL band. All CIS core QDs exhibited relatively red shift in emission. This red shift in emissions can be ascribed to a shift of the donor and acceptor levels associated with band gap widening as well as an increased probability of DAP recombination between closely spaced pairs. Finally, we characterized QDs with a CIS core at Cu/In molar ratios of 1/2, 3/4, and 1/1. A widened band gap in Cu-deficient CIS QDs led to a systematic increase in the blue shift in emissions according to the degree of Cu deficiency. CIS QDs exhibited broadband defect emissions with peak wavelengths of 560 to 600 nm and QY of 6.3% to 8.8%. The factor which will be the most important one to influence the PLQY is the reaction temperature. It was also found that XRD peak intensities and PLQY increased with the synthesis temperature. This seems to suggest that CIS cores synthesized at higher temperatures could provide a better crystal quality.
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