CuFeS2 Quantum Dots and Highly Luminescent CuFeS2 based Core ...
CuFeS2 Quantum Dots and Highly Luminescent CuFeS2 based Core/Shell Structures: Synthesis, Tunability and Photophysics Supplementary Information Biswajit Bhattacharyya and Anshu Pandey* *[email protected] Solid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore 560012, India
MATERIALS AND METHODS Synthetic Methods: Iron Chloride anhydrous (FeCl2, 97%), Copper acetate anhydrous [Cu(CH3COO)2, 98%], Cadmium acetate [Cd(CH3COO)2], Sulphur (S). 1-Octadecene (ODE, technical grade 90%), Oleic acid (OA, 99%), Oleylamine (technical grade, 70%), Dodecanethiol (DDT) were purchased from Sigma-Aldrich. All the chemicals were used without any purification. Preparation of CuFeS2 Quantum Dots: This material was prepared in two steps. First step consisted with preparation of anion precursor (preparation of sulphur in oleylamine). In Flask 1, sulphur powder (3.2 mg, 0.1 mM) was added to 50 mL three-necked flask containing 2.5 mL oleylamine and 1 mL of 1octadecene. This flask was held at 160 °C under argon for 30 min. In the second step, Iron chloride (13.85 mg, 0.1 mM), copper acetate (18.1 mg, 0.1 mM) were added to a 50 mL three necked flask (Flask 2) containing 2 mL ODE as solvent and 2 mL oleic acid as ligand. The reaction mixture was heated to 100 °C under vacuum for 5 min to get rid of all water if present in the reaction environment. Metal precursors were heated under argon atmosphere at 120 °C for 10 min to get dissolved in the solution as metal oleate form. First copper will form a blue colour solution and after some time the iron will start dissolving as the color changes from blue to green and finally it will turn to brown colour solution. 1.5 1
mL of dodecanethiol (DDT) was added to the reaction mixture and the reaction mixture was heated at 180 °C to initiate the nucleation of CuFeS2 QDs. After adding the thiol, the solution colour changes to bright yellow in few seconds. Then sulfur in oleyl amine was injected dropwise (0.1 ml per minute) to this solution (because in thiol CuFeS2 QDs can grow but the reaction kinetics of iron and copper towards thiol are different , so there is chance of side nucleation of pure Cu2S and FeS2). The growth solution darkens rapidly upon the addition of the sulfur precursor. These color changes are associated with the appearance of CuFeS2 QDs. Preparation of CuFeS2/CdS Quantum Dots: In a 20 mL three necked flask, 4 mL of freshly prepared CuFeS2 QDs was taken and it was heated at 150 °C under argon atmosphere. Cadmium oleate (1 mL of a 0.1 mM CuFeS2 solution) was added slowly into it over 4 min. This approach is similar to the one adopted for the synthesis of lead chalcogenide core/shell architectures. As we observed that addition of cadmium oleate etches the dots, so the temperature for this preparation should be low. To obtain different emission colours, we started from different sizes of CuFeS2 QDs. This process causes a shift of approximately 0.5 eV in the band edge position. The QDs were cleaned after synthesis (before analysis and characterization) to remove solvent, excess ligand and unreacted precursors. The cleaning was done in three steps. Initially 5 mL of methanol was added to 5 mL QDs solution. It was heated with a heat gun and the mixture was centrifuged for 1 min at 1000 RCF. Same procedure was repeated for two times. In the second step 5 mL of ethanol and 0.5 mL of methanol were added to the mixture and centrifuged for 1 min at 1000 RCF. Finally the supernatant was removed and a QDs precipitate on the walls of the reaction tube. Spectroscopic data were collected by dissolving cleaned QDs in tetrachloroethylene (spectroscopic grade). XRD Analysis: Films for XRD analysis were prepared by dissolving cleaned QDs in Hexane solutions, and then drop casting on a glass substrate. For further cleaning of the QDs film, a few drops of a mixture of n-butyl amine and acetone were added. A 0.154 nm X-ray source was used to collect all data. Quantum Yield Measurements: Absolute quantum yields were determined using an integrating sphere on an Edinburgh Instruments, FLS 920 spectrofluorimeter. A spectralon coated integrating sphere has been used to measure QYs in the VIS/NIR region. The sphere was previously calibrated with a standard UV/VIS/NIR light source to correct for any reflectivity artifacts. Samples were cleaned of all excess ligands and dissolved into an infrared compatible solvent (tetrachloro ethylene, TCE). An UV/VIS/IR transparent quartz cuvette containing TCE was introduced into the integrating sphere to determine lamp intensity and scattering characteristics of the sphere at excitation and emission wavelengths. 2
An identical volume of the sample in TCE was subsequently put into the same cuvette and introduced into the sphere. The light absorbed and emitted is measured directly and the quantum yield is estimated as the light emitted to light absorbed. Time resolved PL data were collected in the self-same instrument using a laser diode excitation. The integrating sphere was removed and sample emission was collected in a right angle geometry for the latter measurements. Cross-section Measurements: The absorption cross-section of the material was measured by first dissolving samples in hexane to take absorption spectra. The solvent was then evaporated, and the NC residue was treated with concentrated nitric acid. The resultant solution was diluted to 25 mL with water. Composition was then determined using Inductively Coupled Plasma Spectroscopy (ICPOES). The particle size distributions were estimated from TEM images. Bulk material density was taken to be 4.19 g/cc. The extinction coefficient was assumed to be identical to the optical absorption as is standard practice for semiconductor particles much smaller than the wavelength of light. Transient Absorption: Transient absorption measurements were performed using 100 fs pulses derived from a Coherent Libra amplified femtosecond laser. Samples were stirred with a magnetic stirrer to avoid photoinduced charging or degradation effects. Cleaned, purified QDs in a hexane solution were irradiated with a 400 nm pump obtained by frequency doubling of the fundamental. The transient spectra were collected using a broad band white light probe. Spectra were collected by using an Andor Shamrock 303i spectrograph. XPS Measurements: Substrates were cleaned with 1-propanol at 50 °C thoroughly 3 times. Next, the substrate cleaned above was again cleaned with acetone. Then purified QDs were drop casted on to the substrate. XPS spectra were measured by Kratos Axis Ultra Photo Electron Spectroscopy system. Effective beam size at sample was 20 microns. Selective Etching Experiment: Cleaned CuFeS2/CdS QDs were taken into volumetric flask in hexane. The solvent was then evaporated. 10 ml of 2N nitric acid (aqueous) was added to the QDs. After 1 minute, the acid was quantitatively transferred into a different volumetric flask. The acid treatment of the residual QDs was then repeated. This process was repeated a total of four times. The total quantity of metal ions in each aliquot as well as the cation ratios were determined by ICPOES. Using TEM imaging, the average size of the QD batch was determined. The combined knowledge of QD size, the total ion content of each aliquot and the bulk material density makes it possible to estimate the ion ratio in various regions of the QD.
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Quantum Yields: Table 1. Quantum yields of QD emission PL Maximum
Absolute QY (%)
(eV)
(Error ±2%)
2.11 1.87 1.73 1.65 1.43 1.09
54 87 63 42 17 21
Comparison of XRD and TEM sizing Table 2. Comparison of particle size as determined from XRD and TEM analysis Calculated size of
Size of the QDs from
the QDs using XRD
TEM imaging
(nm)
(nm)
5.0
4.3
5.8
5.0
11.2
9.2
12.5
12.3
In the above table, the QD sizes were calculated from the XRD FWHM using a DebyeScherrer analysis (Column 1). The corresponding TEM sizes are shown in Column 2. We observe a minor, 14% disagreement between the sizes estimated from XRD and TEM. Since the sizes predicted by XRD exceed the TEM predicted sizes, it is apparent that the crystallinity of the particles is not the reason for this discrepancy. Indeed, we observe a high degree of crystallinity in the TEM images, consistent with this interpretation. There are however two other possible sources of error. Firstly, particle shape in the Debye-Scherrer equation is assumed to be spherical. The assumption of a spherical shape factor is thus one 4
source of error. Second, this type of analysis assumes a homogenous size dispersion. The existence of a 11.9% size dispersion in the actual material gives rise to an additional source of error. It is thus apparent that the sources of error arise due to the assumptions made in the Scherrer equation and are not due to the properties of the sample.
Figure S1. Tauc Plots used to estimate band gap in figure 2b of the main manuscript.
Figure S2. STEM Elemental mapping showing co-location of Cu, Fe, Cd and S on an ensemble of CuFeS2/CdS QDs. The elemental maps correspond to the region of the red square.
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Figure S3. STEM Elemental mapping showing co-location of Cu, Fe, Cd and S on a single CuFeS2/CdS QD. Non-Radiative decay in cores:
Figure S4. A summary of radiative and nonradiative decay channels in CuFeS2 QDs. We observe negligible emission from pure CuFeS2 cores, while CuFeS2/CdS core/shell structures have high quantum yields. This can be interpreted in terms of a competition between a radiative decay at an internal defect center, and a non-radiative decay at a surface recombination site. For CuFeS2/CdS core/shell structures, typical lifetimes are ~500 ns, 2
implying
. The nonemissive nature of the pure CuFeS2 core can thus be
accounted by a surface defect with
20
, that is sufficient to cause the quantum
yield to drop below 0.01%. The growth of a CdS layer passivates the outer defect, suppressing
.
6
Stability:
Figure S5. Air stability of a film of CuFeS2 QDs. The XRD pattern of a film of CuFeS2 QDs is initially recorded. The film is observed to exhibit a pure chalcopyrite phase of CuFeS2. No impurity phases are observable. The film is subsequently stored under ambient conditions (25-30 oC, 40-60% relative humidity, non-condensing) for 60 days. No degradation is observed, and the XRD pattern is identical to the original, implying the stability of CuFeS2 QDs under terrestrial conditions.
Figure S6. Emission stability of CuFeS2/CdS QDs in hexane under continuous laser irradiation. In this experiment, QDs were illuminated with a continuous wave 405 nm laser diode with an irradiance of 0.35 W/cm2. No drop in emission intensity is observed even after one hour of continuous irradiation.
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MATERIALS AND METHODS Synthetic Methods: Iron Chloride anhydrous (FeCl2, 97%), Copper acetate anhydrous [Cu(CH3COO)2, 98%], Cadmium acetate [Cd(CH3COO)2], Sulphur (S). 1-Octadecene (ODE, technical grade 90%), Oleic acid (OA, 99%), Oleylamine (technical grade, 70%), Dodecanethiol (DDT) were purchased from Sigma-Aldrich. All the chemicals were used without any purification. Preparation of CuFeS2 Quantum Dots: This material was prepared in two steps. First step consisted with preparation of anion precursor (preparation of sulphur in oleylamine). In Flask 1, sulphur powder (3.2 mg, 0.1 mM) was added to 50 mL three-necked flask containing 2.5 mL oleylamine and 1 mL of 1octadecene. This flask was held at 160 °C under argon for 30 min. In the second step, Iron chloride (13.85 mg, 0.1 mM), copper acetate (18.1 mg, 0.1 mM) were added to a 50 mL three necked flask (Flask 2) containing 2 mL ODE as solvent and 2 mL oleic acid as ligand. The reaction mixture was heated to 100 °C under vacuum for 5 min to get rid of all water if present in the reaction environment. Metal precursors were heated under argon atmosphere at 120 °C for 10 min to get dissolved in the solution as metal oleate form. First copper will form a blue colour solution and after some time the iron will start dissolving as the color changes from blue to green and finally it will turn to brown colour solution. 1.5 1
mL of dodecanethiol (DDT) was added to the reaction mixture and the reaction mixture was heated at 180 °C to initiate the nucleation of CuFeS2 QDs. After adding the thiol, the solution colour changes to bright yellow in few seconds. Then sulfur in oleyl amine was injected dropwise (0.1 ml per minute) to this solution (because in thiol CuFeS2 QDs can grow but the reaction kinetics of iron and copper towards thiol are different , so there is chance of side nucleation of pure Cu2S and FeS2). The growth solution darkens rapidly upon the addition of the sulfur precursor. These color changes are associated with the appearance of CuFeS2 QDs. Preparation of CuFeS2/CdS Quantum Dots: In a 20 mL three necked flask, 4 mL of freshly prepared CuFeS2 QDs was taken and it was heated at 150 °C under argon atmosphere. Cadmium oleate (1 mL of a 0.1 mM CuFeS2 solution) was added slowly into it over 4 min. This approach is similar to the one adopted for the synthesis of lead chalcogenide core/shell architectures. As we observed that addition of cadmium oleate etches the dots, so the temperature for this preparation should be low. To obtain different emission colours, we started from different sizes of CuFeS2 QDs. This process causes a shift of approximately 0.5 eV in the band edge position. The QDs were cleaned after synthesis (before analysis and characterization) to remove solvent, excess ligand and unreacted precursors. The cleaning was done in three steps. Initially 5 mL of methanol was added to 5 mL QDs solution. It was heated with a heat gun and the mixture was centrifuged for 1 min at 1000 RCF. Same procedure was repeated for two times. In the second step 5 mL of ethanol and 0.5 mL of methanol were added to the mixture and centrifuged for 1 min at 1000 RCF. Finally the supernatant was removed and a QDs precipitate on the walls of the reaction tube. Spectroscopic data were collected by dissolving cleaned QDs in tetrachloroethylene (spectroscopic grade). XRD Analysis: Films for XRD analysis were prepared by dissolving cleaned QDs in Hexane solutions, and then drop casting on a glass substrate. For further cleaning of the QDs film, a few drops of a mixture of n-butyl amine and acetone were added. A 0.154 nm X-ray source was used to collect all data. Quantum Yield Measurements: Absolute quantum yields were determined using an integrating sphere on an Edinburgh Instruments, FLS 920 spectrofluorimeter. A spectralon coated integrating sphere has been used to measure QYs in the VIS/NIR region. The sphere was previously calibrated with a standard UV/VIS/NIR light source to correct for any reflectivity artifacts. Samples were cleaned of all excess ligands and dissolved into an infrared compatible solvent (tetrachloro ethylene, TCE). An UV/VIS/IR transparent quartz cuvette containing TCE was introduced into the integrating sphere to determine lamp intensity and scattering characteristics of the sphere at excitation and emission wavelengths. 2
An identical volume of the sample in TCE was subsequently put into the same cuvette and introduced into the sphere. The light absorbed and emitted is measured directly and the quantum yield is estimated as the light emitted to light absorbed. Time resolved PL data were collected in the self-same instrument using a laser diode excitation. The integrating sphere was removed and sample emission was collected in a right angle geometry for the latter measurements. Cross-section Measurements: The absorption cross-section of the material was measured by first dissolving samples in hexane to take absorption spectra. The solvent was then evaporated, and the NC residue was treated with concentrated nitric acid. The resultant solution was diluted to 25 mL with water. Composition was then determined using Inductively Coupled Plasma Spectroscopy (ICPOES). The particle size distributions were estimated from TEM images. Bulk material density was taken to be 4.19 g/cc. The extinction coefficient was assumed to be identical to the optical absorption as is standard practice for semiconductor particles much smaller than the wavelength of light. Transient Absorption: Transient absorption measurements were performed using 100 fs pulses derived from a Coherent Libra amplified femtosecond laser. Samples were stirred with a magnetic stirrer to avoid photoinduced charging or degradation effects. Cleaned, purified QDs in a hexane solution were irradiated with a 400 nm pump obtained by frequency doubling of the fundamental. The transient spectra were collected using a broad band white light probe. Spectra were collected by using an Andor Shamrock 303i spectrograph. XPS Measurements: Substrates were cleaned with 1-propanol at 50 °C thoroughly 3 times. Next, the substrate cleaned above was again cleaned with acetone. Then purified QDs were drop casted on to the substrate. XPS spectra were measured by Kratos Axis Ultra Photo Electron Spectroscopy system. Effective beam size at sample was 20 microns. Selective Etching Experiment: Cleaned CuFeS2/CdS QDs were taken into volumetric flask in hexane. The solvent was then evaporated. 10 ml of 2N nitric acid (aqueous) was added to the QDs. After 1 minute, the acid was quantitatively transferred into a different volumetric flask. The acid treatment of the residual QDs was then repeated. This process was repeated a total of four times. The total quantity of metal ions in each aliquot as well as the cation ratios were determined by ICPOES. Using TEM imaging, the average size of the QD batch was determined. The combined knowledge of QD size, the total ion content of each aliquot and the bulk material density makes it possible to estimate the ion ratio in various regions of the QD.
3
Quantum Yields: Table 1. Quantum yields of QD emission PL Maximum
Absolute QY (%)
(eV)
(Error ±2%)
2.11 1.87 1.73 1.65 1.43 1.09
54 87 63 42 17 21
Comparison of XRD and TEM sizing Table 2. Comparison of particle size as determined from XRD and TEM analysis Calculated size of
Size of the QDs from
the QDs using XRD
TEM imaging
(nm)
(nm)
5.0
4.3
5.8
5.0
11.2
9.2
12.5
12.3
In the above table, the QD sizes were calculated from the XRD FWHM using a DebyeScherrer analysis (Column 1). The corresponding TEM sizes are shown in Column 2. We observe a minor, 14% disagreement between the sizes estimated from XRD and TEM. Since the sizes predicted by XRD exceed the TEM predicted sizes, it is apparent that the crystallinity of the particles is not the reason for this discrepancy. Indeed, we observe a high degree of crystallinity in the TEM images, consistent with this interpretation. There are however two other possible sources of error. Firstly, particle shape in the Debye-Scherrer equation is assumed to be spherical. The assumption of a spherical shape factor is thus one 4
source of error. Second, this type of analysis assumes a homogenous size dispersion. The existence of a 11.9% size dispersion in the actual material gives rise to an additional source of error. It is thus apparent that the sources of error arise due to the assumptions made in the Scherrer equation and are not due to the properties of the sample.
Figure S1. Tauc Plots used to estimate band gap in figure 2b of the main manuscript.
Figure S2. STEM Elemental mapping showing co-location of Cu, Fe, Cd and S on an ensemble of CuFeS2/CdS QDs. The elemental maps correspond to the region of the red square.
5
Figure S3. STEM Elemental mapping showing co-location of Cu, Fe, Cd and S on a single CuFeS2/CdS QD. Non-Radiative decay in cores:
Figure S4. A summary of radiative and nonradiative decay channels in CuFeS2 QDs. We observe negligible emission from pure CuFeS2 cores, while CuFeS2/CdS core/shell structures have high quantum yields. This can be interpreted in terms of a competition between a radiative decay at an internal defect center, and a non-radiative decay at a surface recombination site. For CuFeS2/CdS core/shell structures, typical lifetimes are ~500 ns, 2
implying
. The nonemissive nature of the pure CuFeS2 core can thus be
accounted by a surface defect with
20
, that is sufficient to cause the quantum
yield to drop below 0.01%. The growth of a CdS layer passivates the outer defect, suppressing
.
6
Stability:
Figure S5. Air stability of a film of CuFeS2 QDs. The XRD pattern of a film of CuFeS2 QDs is initially recorded. The film is observed to exhibit a pure chalcopyrite phase of CuFeS2. No impurity phases are observable. The film is subsequently stored under ambient conditions (25-30 oC, 40-60% relative humidity, non-condensing) for 60 days. No degradation is observed, and the XRD pattern is identical to the original, implying the stability of CuFeS2 QDs under terrestrial conditions.
Figure S6. Emission stability of CuFeS2/CdS QDs in hexane under continuous laser irradiation. In this experiment, QDs were illuminated with a continuous wave 405 nm laser diode with an irradiance of 0.35 W/cm2. No drop in emission intensity is observed even after one hour of continuous irradiation.
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