Thermal Stability of Colloidal InP Nanocrystals: Small Inorganic ...
Thermal Stability of Colloidal InP Nanocrystals: Small Inorganic Ligands Boost HighTemperature Photoluminescence Clare E. Rowland,1 Wenyong Liu,2 Daniel C. Hannah,1 Maria K.Y. Chan,3 Dmitri V. Talapin,2,3 and Richard D. Schaller1,3 1
2
Department of Chemistry, Northwestern University, Evanston, IL 60208
Department of Chemistry and James Frank Institute, University of Chicago, Chicago, IL 60637 3
Center for Nanoscale Materials, Argonne National Laboratory, Argonne, IL 60439
Supporting Information
S1. InP NCs synthesis InP NCs with first excitonic peak beyond 580 nm were capped with trioctylphosphine (TOP) and trioctylphosphine oxide (TOPO), as described in S1.1; those with first excitonic peak below 580 nm were capped with myristic acid (MA), as described in S1.2. (1.1)
The synthesis of TOP/TOPO capped InP NCs:1
1.030g of InCl3 was added into 9.8 g TOP and 0.90 TOPO in a 3-necked 50 mL round bottom flask. Clear solution was formed when the temperature was increased to 80°C, followed by the injection of 0.75g tris(trimethylsilyl)phosphine, (TMS)3P. Then the solution was annealed at
270°C for 24 hours before being cooled for the post-synthetic washing. Size distribution of the crude solution is around 15%. It can be further improved by size-selective precipitation with toluene and ethanol as the solvent and nonsolvent, respectively. (1.2)
The synthesis of MA capped InP NCs:2
In a typical reaction, 0.117 g of In(ac)3 and 0.331 g of myristic acid were dissolved into 4 g octadecene (ODE). The combination was dried at room temperature for 5 min before increasing temperature to 188°C. Then a mixture of 50 mg (TMS)3P, 0.309 g of 1-octylamine, and 1.5 mL ODE was injected into the hot solution within 1 s. The solution immediately turned red. The reaction was stopped after 20 s to make a sample with the first excitonic peak at around 505 nm. The NCs were very air sensitive and all the post synthesis procedures were performed inside the glovebox.
Intensity (a.u.)
S2. Ligand exchange process3-4 All ligand exchange procedures were carried out in a nitrogen-filled glovebox (1−2 ppm O2 and H2O levels) using anhydrous solvents. The molar ratio of the ligand to NC materials during the ligand exchange was around 1:1. In a typical example, for TOP/TOPO NCs, 2 mL of InP NC solution in hexane (4.2 mg/mL) was loaded on top of a 2 mL solution of Na2S (15.9 mg/mL) in N-methyl formamide (NMF). The solution was stirred for 30 to 60 min until the upper organic phase turned colorless and the lower hydrazine phase turned dark. The organic phase was carefully removed, and the NMF phase was purified by washing three times with anhydrous hexane. Na2S-capped InP NCs were precipitated from hydrazine solution by adding a minimal amount of anhydrous InP-606nm-TOPO-TOP acetonitrile/toluene mixture (1:4 by volume), InP-606nm-S2-before-HF-etching collected by centrifugation, redissolved into NMF, and filtered through a 0.2 µm PTFE filter. Similar procedures were also applied to the (N2H5)4Sn2S6 capped InP NCs.
300
400
500
600
Wavelength (nm)
700
Figure 3a. Absorption spectra of the InP sample before and after ligand exchange are unchanged.
S3. HF etching of InP NCs5 (3.1) HF etching of organically capped InP NCs A stock solution of 4.8% HF in butanol was prepared by diluting 0.1 mL of 48 wt.% HF in H2O into 0.9 mL butanol. In a typical experiment, 0.2 mL ~30 mg/mL MA capped InP NCs was diluted into 6 mL hexane with 0.4 g TOPO in it. The addition of 3 droplets of 4.8% HF in butanol led to the immediately enhancement of PL, whereas the pristine MA capped InP NCs showed the pronounced radiative recombination arising from the deep traps within the bandgap. Similarly, 0.2 mL 20 mg/mL InP NCs was diluted into 6 mL hexane with 0.3 g TOPO in it, and 0.2 mL 4.8% HF in butanol was added into the solution. The solution was stirred for 2 hours under the illumination of 1.5 mass solar simulating light through 610 nm filter glass. Three orders of magnitude enhancement of PL was observed. (3.2) HF treated inorganic ligands capped InP NCs S2- capped InP NCs were also treated with HF and showed improved PL after 1 hour illumination. The PL of Sn2S64- capped InP NCs was also improved with HF treatment with the similar approach.
S4. (a) The maximum emission wavelength of InP and InP/ZnS is plotted as a function of sample temperature. Red-shifting with increasing temperature is consistant with expected behavior based on the Varshini relation. (b) The maximum emission wavelength of a variety of samples with comparable core sizes is plotted as a function of sample temperature.
S5. PL decay times (1/e times) are shown here for InP, InP/ZnS, InP/S2-, and InP/Sn2S64subjected to cyclical heating. Data are plotted as a function of the measurement temperature, and symbols and colors correspond to the maximum temperature to which NCs were raised in the previous heating cycle.
S6. Comparing experimentally-determined PL intensity to PL intensity calculated based on 1/e times, we find that 1/e times consistently overestimate PL intensity as compared to the experimental values. This phenonmenon is expected when 1/e times are overestimated, as occurs when significant dynamics occur on a timescale too short to be captured by TCSPC.
S7. Kinetic traces from the bleach maxima of transient absorption spectra acquired at several temperatures are shown for InP, InP/ZnS, InP/S2-, and InP/Sn2S64- samples.
S8. (a) Plotting PL intensity at 300 K against the maximum sample temperature from cyclical heating experiments shows irreversible PL loss. Attrition is defined as the difference between PL intensity at a given temperature and initial PL intensity. (b-e) Total PL intensity for InP cores, InP/ZnS, InP/S2-, and InP/Sn2S64- is fitted with an −(T −300)
exponential function, y = Ae τ PL , where A=1 for normalized data, T is the temperature in K, and τPL is related to the quenching. Attrition is also plotted and fitted with the −(T −300)
function y = y 0 + Ae τ attr , where y0=-A=1 for normalized data, T is the temperature in K, and τattr is related to quenching by attrition.
S9. Thermogravimetric analysis of trioctylphosphine oxide (TOPO) shows one weight loss event beginning at 460 K and complete by 580 K. The sample exhibited 99.5% weight loss by 600 K. Data were collected using a temperature ramp of 5°C/ min from 25-100°C. Temperature was held at 100°C for 20 min to ensure total water weight loss, and temperature was subsequently ramped again from 100-590°C at 5°C/ min.
References 1. Mićić, O. I.; Curtis, C. J.; Jones, K. M.; Sprague, J. R.; Nozik, A. J., Synthesis and Characterization of InP Quantum Dots. J. Phys. Chem. 1994, 98, 4966-4969. 2. Xie, R.; Battaglia, D.; Peng, X., Colloidal InP Nanocrystals as Efficient Emitters Covering Blue to Near-Infrared. J. Am. Chem. Soc. 2007, 129, 15432-15433. 3. Nag, A.; Kovalenko, M. V.; Lee, J.-S.; Liu, W.; Spokoyny, B.; Talapin, D. V., Metal-free Inorganic Ligands for Colloidal Nanocrystals: S2–, HS–, Se2–, HSe–, Te2–, HTe–, TeS32–, OH–, and NH2– as Surface Ligands. J. Am. Chem. Soc. 2011, 133, 10612-10620. 4. Liu, W.; Lee, J.-S.; Talapin, D. V., III-V Nanocrystals Capped with Molecular Metal Chalcogenide Ligands: High Electron Mobility and Ambipolar Photoresponse. J. Am. Chem. Soc. 2013, 135, 1349-1357. 5. Talapin, D. V.; Gaponik, N.; Borchert, H.; Rogach, A. L.; Haase, M.; Weller, H., Etching of Colloidal InP Nanocrystals with Fluorides: Photochemical Nature of the Process Resulting in High Photoluminescence Efficiency. J. Phys. Chem. B 2002, 106, 12659-12663.
2
Department of Chemistry, Northwestern University, Evanston, IL 60208
Department of Chemistry and James Frank Institute, University of Chicago, Chicago, IL 60637 3
Center for Nanoscale Materials, Argonne National Laboratory, Argonne, IL 60439
Supporting Information
S1. InP NCs synthesis InP NCs with first excitonic peak beyond 580 nm were capped with trioctylphosphine (TOP) and trioctylphosphine oxide (TOPO), as described in S1.1; those with first excitonic peak below 580 nm were capped with myristic acid (MA), as described in S1.2. (1.1)
The synthesis of TOP/TOPO capped InP NCs:1
1.030g of InCl3 was added into 9.8 g TOP and 0.90 TOPO in a 3-necked 50 mL round bottom flask. Clear solution was formed when the temperature was increased to 80°C, followed by the injection of 0.75g tris(trimethylsilyl)phosphine, (TMS)3P. Then the solution was annealed at
270°C for 24 hours before being cooled for the post-synthetic washing. Size distribution of the crude solution is around 15%. It can be further improved by size-selective precipitation with toluene and ethanol as the solvent and nonsolvent, respectively. (1.2)
The synthesis of MA capped InP NCs:2
In a typical reaction, 0.117 g of In(ac)3 and 0.331 g of myristic acid were dissolved into 4 g octadecene (ODE). The combination was dried at room temperature for 5 min before increasing temperature to 188°C. Then a mixture of 50 mg (TMS)3P, 0.309 g of 1-octylamine, and 1.5 mL ODE was injected into the hot solution within 1 s. The solution immediately turned red. The reaction was stopped after 20 s to make a sample with the first excitonic peak at around 505 nm. The NCs were very air sensitive and all the post synthesis procedures were performed inside the glovebox.
Intensity (a.u.)
S2. Ligand exchange process3-4 All ligand exchange procedures were carried out in a nitrogen-filled glovebox (1−2 ppm O2 and H2O levels) using anhydrous solvents. The molar ratio of the ligand to NC materials during the ligand exchange was around 1:1. In a typical example, for TOP/TOPO NCs, 2 mL of InP NC solution in hexane (4.2 mg/mL) was loaded on top of a 2 mL solution of Na2S (15.9 mg/mL) in N-methyl formamide (NMF). The solution was stirred for 30 to 60 min until the upper organic phase turned colorless and the lower hydrazine phase turned dark. The organic phase was carefully removed, and the NMF phase was purified by washing three times with anhydrous hexane. Na2S-capped InP NCs were precipitated from hydrazine solution by adding a minimal amount of anhydrous InP-606nm-TOPO-TOP acetonitrile/toluene mixture (1:4 by volume), InP-606nm-S2-before-HF-etching collected by centrifugation, redissolved into NMF, and filtered through a 0.2 µm PTFE filter. Similar procedures were also applied to the (N2H5)4Sn2S6 capped InP NCs.
300
400
500
600
Wavelength (nm)
700
Figure 3a. Absorption spectra of the InP sample before and after ligand exchange are unchanged.
S3. HF etching of InP NCs5 (3.1) HF etching of organically capped InP NCs A stock solution of 4.8% HF in butanol was prepared by diluting 0.1 mL of 48 wt.% HF in H2O into 0.9 mL butanol. In a typical experiment, 0.2 mL ~30 mg/mL MA capped InP NCs was diluted into 6 mL hexane with 0.4 g TOPO in it. The addition of 3 droplets of 4.8% HF in butanol led to the immediately enhancement of PL, whereas the pristine MA capped InP NCs showed the pronounced radiative recombination arising from the deep traps within the bandgap. Similarly, 0.2 mL 20 mg/mL InP NCs was diluted into 6 mL hexane with 0.3 g TOPO in it, and 0.2 mL 4.8% HF in butanol was added into the solution. The solution was stirred for 2 hours under the illumination of 1.5 mass solar simulating light through 610 nm filter glass. Three orders of magnitude enhancement of PL was observed. (3.2) HF treated inorganic ligands capped InP NCs S2- capped InP NCs were also treated with HF and showed improved PL after 1 hour illumination. The PL of Sn2S64- capped InP NCs was also improved with HF treatment with the similar approach.
S4. (a) The maximum emission wavelength of InP and InP/ZnS is plotted as a function of sample temperature. Red-shifting with increasing temperature is consistant with expected behavior based on the Varshini relation. (b) The maximum emission wavelength of a variety of samples with comparable core sizes is plotted as a function of sample temperature.
S5. PL decay times (1/e times) are shown here for InP, InP/ZnS, InP/S2-, and InP/Sn2S64subjected to cyclical heating. Data are plotted as a function of the measurement temperature, and symbols and colors correspond to the maximum temperature to which NCs were raised in the previous heating cycle.
S6. Comparing experimentally-determined PL intensity to PL intensity calculated based on 1/e times, we find that 1/e times consistently overestimate PL intensity as compared to the experimental values. This phenonmenon is expected when 1/e times are overestimated, as occurs when significant dynamics occur on a timescale too short to be captured by TCSPC.
S7. Kinetic traces from the bleach maxima of transient absorption spectra acquired at several temperatures are shown for InP, InP/ZnS, InP/S2-, and InP/Sn2S64- samples.
S8. (a) Plotting PL intensity at 300 K against the maximum sample temperature from cyclical heating experiments shows irreversible PL loss. Attrition is defined as the difference between PL intensity at a given temperature and initial PL intensity. (b-e) Total PL intensity for InP cores, InP/ZnS, InP/S2-, and InP/Sn2S64- is fitted with an −(T −300)
exponential function, y = Ae τ PL , where A=1 for normalized data, T is the temperature in K, and τPL is related to the quenching. Attrition is also plotted and fitted with the −(T −300)
function y = y 0 + Ae τ attr , where y0=-A=1 for normalized data, T is the temperature in K, and τattr is related to quenching by attrition.
S9. Thermogravimetric analysis of trioctylphosphine oxide (TOPO) shows one weight loss event beginning at 460 K and complete by 580 K. The sample exhibited 99.5% weight loss by 600 K. Data were collected using a temperature ramp of 5°C/ min from 25-100°C. Temperature was held at 100°C for 20 min to ensure total water weight loss, and temperature was subsequently ramped again from 100-590°C at 5°C/ min.
References 1. Mićić, O. I.; Curtis, C. J.; Jones, K. M.; Sprague, J. R.; Nozik, A. J., Synthesis and Characterization of InP Quantum Dots. J. Phys. Chem. 1994, 98, 4966-4969. 2. Xie, R.; Battaglia, D.; Peng, X., Colloidal InP Nanocrystals as Efficient Emitters Covering Blue to Near-Infrared. J. Am. Chem. Soc. 2007, 129, 15432-15433. 3. Nag, A.; Kovalenko, M. V.; Lee, J.-S.; Liu, W.; Spokoyny, B.; Talapin, D. V., Metal-free Inorganic Ligands for Colloidal Nanocrystals: S2–, HS–, Se2–, HSe–, Te2–, HTe–, TeS32–, OH–, and NH2– as Surface Ligands. J. Am. Chem. Soc. 2011, 133, 10612-10620. 4. Liu, W.; Lee, J.-S.; Talapin, D. V., III-V Nanocrystals Capped with Molecular Metal Chalcogenide Ligands: High Electron Mobility and Ambipolar Photoresponse. J. Am. Chem. Soc. 2013, 135, 1349-1357. 5. Talapin, D. V.; Gaponik, N.; Borchert, H.; Rogach, A. L.; Haase, M.; Weller, H., Etching of Colloidal InP Nanocrystals with Fluorides: Photochemical Nature of the Process Resulting in High Photoluminescence Efficiency. J. Phys. Chem. B 2002, 106, 12659-12663.
