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Phase transformations in CdSe quantum dots induced by reaction time
Materials Letters 141 (2015) 67–69
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Phase transformations in CdSe quantum dots induced by reaction time Fehmida K. Kanodarwala a, Fan Wang b, Peter J. Reece b, John A. Stride a,c,n a b c
School of Chemistry, University of New South Wales, Sydney, NSW 2052, Australia School of Physics, University of New South Wales, Sydney, NSW 2052, Australia Bragg Institute, Australian Nuclear Science and Technology Organization, PMB 1, Menai, NSW 2234, Australia
art ic l e i nf o
a b s t r a c t
Article history: Received 1 September 2014 Accepted 12 November 2014 Available online 24 November 2014
This paper reports the synthesis of high-quality TOP/TOPO capped CdSe nanocrystals using a simple colloidal method. The effects of varying the reaction time on the size and crystallinity of the synthesized CdSe quantum dots were studied in detail. Powder X-ray diffraction (PXRD) analysis showed a dependence of the crystallite phases as a function of reaction time, from cubic zinc-blende to hexagonal wurtzite and back again to the cubic phase. PXRD data was found to be consistent with high resolution transmission electron microscopy (HRTEM) images of the particles, whilst optical spectroscopy demonstrated the quantum dot nature of the particles. The reaction time-dependent phase behavior is best accounted for by rapid growth along the {111} crystallite facets, after which the facial facets then ‘catch-up’, resulting once again in cubic symmetry. & 2014 Elsevier B.V. All rights reserved.
Keywords: Quantum dots Phase transition Reaction time
1. Introduction Quantum dots (QDs) are a special class of semiconductor materials in which the excitons are confined in all three spatial dimensions [1]. As a result, they have photo-electronic properties that lie between those of bulk semiconductors and discrete molecules [2] and have the potential to be employed in a wide range of applications from biological imaging and optoelectronics to solar energy and anti-counterfeiting. Although extensive research has been conducted on nanocrystals (NCs) of group II–VI, III–V, and IV–VI and metal oxide NCs, CdSe still remains the most thoroughly studied to date. Colloidal synthesis has the advantage of potentially being a low cost synthetic approach and it is thus a widely employed synthetic method. There have been a number of variations reported to the basic concept, these not only include variations in the cadmium precursors viz., CdO, CdCO3, Cd(Ac)2 [3], but also variations in the capping ligands: trioctylphosphine oxide, phosphonic acid, amines, fatty acids [4]. CdSe exists in two crystalline phases, namely the hexagonal wurtzite (WZ) and the cubic zinc-blende (ZB) structures. Most synthetic techniques currently employed, usually produce NCs having the WZ structure. More recently, ZB–CdSe NCs have been successfully obtained [5,6] but there is a lack of a systematic or
n Corresponding author at: School of Chemistry, University of New South Wales, Sydney, NSW 2052, Australia. Tel.: þ 61 405016555. E-mail address: [email protected] (F.K. Kanodarwala).
http://dx.doi.org/10.1016/j.matlet.2014.11.043 0167-577X/& 2014 Elsevier B.V. All rights reserved.
comparative study in relation to the more conventional WZ system. This article studies the transition of the crystallite phases obtained in ostensibly mixed-phase NCs from predominantly cubic ZB to hexagonal WZ and back again to the cubic ZB phase, simply by the changing the reaction time. PXRD and TEM were employed to study the crystal structure and morphology of the particles, respectively, whilst the optical properties were studied with UV–vis absorption and photoluminescence spectroscopies.
2. Materials and methods The CdSe nanoparticles were synthesized via colloidal synthesis using the Schlenk line method. The synthesis was based upon two previously reported methods of Yu et al. [7] and Murray et al. [8]; these were combined and modified to allow the study of various factors to determine whether phase or morphological control could be achieved. The selenium precursor (TOPSe) was prepared as follows: 0.25 mmol (0.197 g) powder selenium was dissolved in 2 mL of trioctylphosphine (TOP) and 2 mL of toluene in a sample tube. The mixture was then sonicated for 10 min until the selenium had dissolved in the TOP, giving rise to a colorless solution. In a typical synthesis of CdSe QDs, 0.125 mmol (0.160 g) of CdO was mixed with 2 g lauric acid in a 50 mL three-necked roundbottom flask and degassed for 30 min and then heated to 130 1C under nitrogen flow with constant stirring until all of the mixture became optically clear. The system was allowed to cool to room
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F.K. Kanodarwala et al. / Materials Letters 141 (2015) 67–69
temperature, before adding 2 g of TOPO. The flask was resealed and heated to 320 1C. The pre-prepared TOPSe was quickly injected into the reaction flask and the temperature of the reaction mixture was observed to decrease to 280 1C upon injection, at which it was maintained for the growth period of the QDs. Aliquots of the reaction solution were removed at various postinjection intervals and then cooled. The series of samples are referred to herein as QD-30S, QD-60S, QD-120S, QD-300S and QD600S for 30, 60, 120, 300 and 600 s samples respectively. Powder X-ray diffraction (PXRD) measurements were performed using a Philips X'pert Multipurpose X-ray Diffraction System (MPD) operated at 40 kV and 40 mA (λ ¼0.154056 nm) in the Bragg–Brentano geometry. A Philips CM200 field emission TEM operating at an accelerating voltage of 200 kV, with a Bruker SDD was used for TEM. A Cary 100 UV–vis spectrophotometer was used to carry out absorption spectra. Micro-photoluminescence (μPL) spectra were measured on a custom-built microscope coupled to an Acton 2300 spectrometer.
3. Results and discussions PXRD was employed to determine the crystal structure of the aliquots of CdSe QDs. The characteristic features of ZB–CdSe NCs with X-rays of λ ¼0.154056 nm, consist of a sharp (111) peak at 2θ¼25.41, a deep valley between (220) and (311) peaks, as well as a smaller (440) peak at 2θ¼60.91. As for the hexagonal WZ–CdSe diffraction pattern, there is a broad peak around 2θ ¼251 and the (103) peak at 2θ¼45.61 [9]. Fig. 1 shows the combined PXRD spectrum of the samples from aliquots taken at different time intervals. The data for QD-30S, QD-60S, QD-300S and QD-600S exhibit peaks typical of the cubic phase, whereas the one at QD120S exhibits the peaks characteristic of the hexagonal phase. The sharp diffraction peaks show the highly crystalline nature of all of the synthesized particles. Rietveld refinement of the PXRD spectra was performed to obtain details of the percentage phase compositions, whilst the size of the nanoparticles was derived using the Debye–Scherrer equation; these are listed in Table 1. It can be deduced from these values that a decrease in the percentage composition of the cubic phase and a subsequent increase in the hexagonal phase occurs with increasing growth time; except for the 120 s, when the opposite was observed. As such, the samples QD-30S, QD-60S are more cubic in character, the QD-120S more hexagonal and the QD-300S and QD-600S are once again more
Fig. 1. Combined PXRD spectrum of the aliquots taken at different time intervals.
cubic. It should be noted that all of the CdSe QDs in this series are a mixture of both phases, but that they vary in the percentage of the two phases. This observation is consistent with the growth of ZB– CdSe being reinforced along the WZ {001} direction along with some ZB lattice stacking faults. This WZ–ZB polytypism phenomenon in semiconductors is not rare, since the structural difference in WZ and ZB is subtle and moreover their internal energy difference is small [10]. Usually, the transition of WZ and ZB are determined by the details of synthetic parameters and since WZ is a metastable phase compared to ZB, it requires vigorous conditions for its growth [11,12]. The unique feature observed in this study is the discovery of the occurrence of a phase dependence of the QDs, from cubic ZB to hexagonal WZ and back again to ZB, simply as a function of reaction time. This is due to rapid growth along the {111} facets of the crystals in the early stages of particle formation, prior to the other facets ‘catching-up’ to re-introduce cubic symmetry. Such a change in crystalline phase as a function of particle growth has not been reported before in colloidal CdSe QDs. The only previously reported phase transition has been from the cubic to hexagonal phase, induced by annealing of the QDs at high temperatures. Fig. 2 shows the combined UV–vis absorption and photoluminescence emission spectrum of the CdSe nanoparticles. QD-30S, QD60S, QD-120S, QD-300S, QD-600S have absorption edges at wavelengths 561, 566, 591, 594 and 602 nm respectively and emission edges at the wavelengths 622, 619, 629, 640 and 644 nm respectively. The difference in the absorption edge and the emission peak relates to the energy loss due to non-emissive electronic relaxation upon absorption and re-emission.
Table 1 Percentage of the phases obtained from PXRD and size of the QDs deduced from PXRD and UV–vis. Sample
UV–vis size (nm)
PXRD size (nm)
PXRD cubic (%)
PXRD hexagonal (%)
QD-30S QD-60S QD-120S QD-300S QD-600S
3.28 3.41 4.21 4.33 4.67
3.87 70.13 3.74 70.11 3.91 70.39 5.57 72.21 2.6770.01
65.5 73.3 31.1 85.5 99.8
34.5 26.7 69.9 14.5 0.2
Fig. 2. UV–vis absorption and photoluminescence emission spectra of QD-30S, QD-60S, QD-120S, QD-300S and QD-600S.
F.K. Kanodarwala et al. / Materials Letters 141 (2015) 67–69
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Fig. 3. HRTEM images of (a) QD-30S, (b) QD-60S, (c) QD-120S, (d) QD-300S and (e) QD-600S, inset fast Fourier transform of the images.
The diameters of the nanoparticles were calculated from the wavelength of the absorption edges using the equation reported by Xia et al. [9]. It was found that there was a subsequent increase in the size of the QDs across the series, in the range of 3.28– 4.67 nm, which compares to those deduced from PXRD of 2.67– 3.87 nm; both size determinations are therefore in reasonable agreement with each other. Fig. 3 shows the HRTEM images of the particles obtained at different time intervals and clearly show the presence of spherical particles with lattice spacing of d ¼0.36 nm, matching that of the calculated lattice spacing of {100} planes in CdSe. The HRTEM images of QD-30S and QD-60S, Fig. 3(a) and (b) respectively, shows that the fast Fourier transform (FFT) patterns are hexagonal and the angles between different crystal planes are all equal to 601, consistent with those calculated for ZB–CdSe. As the reaction progressed, the HRTEM of the particles, QD-120S Fig. 3(c), shows that the FFT pattern is a rectangular with a 901 angle between the crystal planes. At longer reaction times the HRTEM images of QD300S and QD-600S, as shown in Fig. 3(d) and (e), the FFT pattern is a square with an angle of 901 between the lattice points for QD300S, whilst the FFT for QD-600S was hexagonal with an angle of 601, both of which are consistent with that of ZB–CdSe. These results for the phases are in agreement with those deduced earlier from the PXRD analysis that support the phase transition.
4. Conclusions In summary, we have observed phase shift of CdSe QDs obtained in a colloidal synthesis, from cubic ZB to hexagonal WZ and back to the cubic ZB structure, simply as an effect of reaction time at a given temperature. This provides a unique insight into the QD growth mechanisms and allows for greater control over the precise nature of the final CdSe QDs.
References [1] Dabbousi BO, Rodriguez VJ, Mikulec FV, Heine JR, Mattoussi H, Ober R, et al. J Phys Chem B 1997;101(46):9463–75. [2] Jun S, Jang EJ, Park J, Kim J. Langmuir 2006;22(6):2407–10. [3] Peng ZA, Peng XG. J Am Chem Soc 2001;123(1):183–4. [4] Qu LH, Peng ZA, Peng XG. Nano Lett 2001;1(6):333–7. [5] Lim SJ, Chon B, Joo T, Shin SK. J Phys Chem C 2008;112(6):1744–7. [6] Yang YA, Wu H, Williams KR, Cao YC. Angew Chem Int Ed 2005;44(41):6712–5. [7] Yu K, Singh S, Patrito N, Chu V. Langmuir 2004;20(25):11161–8. [8] Murray CB, Norris DJ, Bawendi MG. J Am Chem Soc 1993;115(19):8706–15. [9] Xia X, Liu ZL, Du GH, Li YB, Ma M. J Lumin 2010;130(7):1285–91. [10] Yeh CY, Lu ZW, Froyen S, Zunger A. Phys Rev B 1992;46(16):10086–97. [11] Talapin DV, Nelson JH, Shevchenko EV, Aloni S, Sadtler B, Alivisatos AP. Nano Lett 2007;7(10):2951–9. [12] Fiore A, Mastria R, Lupo MG, Lanzani G, Giannini C, et al. J Am Chem Soc 2009;131(6):2274–82.
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Phase transformations in CdSe quantum dots induced by reaction time Fehmida K. Kanodarwala a, Fan Wang b, Peter J. Reece b, John A. Stride a,c,n a b c
School of Chemistry, University of New South Wales, Sydney, NSW 2052, Australia School of Physics, University of New South Wales, Sydney, NSW 2052, Australia Bragg Institute, Australian Nuclear Science and Technology Organization, PMB 1, Menai, NSW 2234, Australia
art ic l e i nf o
a b s t r a c t
Article history: Received 1 September 2014 Accepted 12 November 2014 Available online 24 November 2014
This paper reports the synthesis of high-quality TOP/TOPO capped CdSe nanocrystals using a simple colloidal method. The effects of varying the reaction time on the size and crystallinity of the synthesized CdSe quantum dots were studied in detail. Powder X-ray diffraction (PXRD) analysis showed a dependence of the crystallite phases as a function of reaction time, from cubic zinc-blende to hexagonal wurtzite and back again to the cubic phase. PXRD data was found to be consistent with high resolution transmission electron microscopy (HRTEM) images of the particles, whilst optical spectroscopy demonstrated the quantum dot nature of the particles. The reaction time-dependent phase behavior is best accounted for by rapid growth along the {111} crystallite facets, after which the facial facets then ‘catch-up’, resulting once again in cubic symmetry. & 2014 Elsevier B.V. All rights reserved.
Keywords: Quantum dots Phase transition Reaction time
1. Introduction Quantum dots (QDs) are a special class of semiconductor materials in which the excitons are confined in all three spatial dimensions [1]. As a result, they have photo-electronic properties that lie between those of bulk semiconductors and discrete molecules [2] and have the potential to be employed in a wide range of applications from biological imaging and optoelectronics to solar energy and anti-counterfeiting. Although extensive research has been conducted on nanocrystals (NCs) of group II–VI, III–V, and IV–VI and metal oxide NCs, CdSe still remains the most thoroughly studied to date. Colloidal synthesis has the advantage of potentially being a low cost synthetic approach and it is thus a widely employed synthetic method. There have been a number of variations reported to the basic concept, these not only include variations in the cadmium precursors viz., CdO, CdCO3, Cd(Ac)2 [3], but also variations in the capping ligands: trioctylphosphine oxide, phosphonic acid, amines, fatty acids [4]. CdSe exists in two crystalline phases, namely the hexagonal wurtzite (WZ) and the cubic zinc-blende (ZB) structures. Most synthetic techniques currently employed, usually produce NCs having the WZ structure. More recently, ZB–CdSe NCs have been successfully obtained [5,6] but there is a lack of a systematic or
n Corresponding author at: School of Chemistry, University of New South Wales, Sydney, NSW 2052, Australia. Tel.: þ 61 405016555. E-mail address: [email protected] (F.K. Kanodarwala).
http://dx.doi.org/10.1016/j.matlet.2014.11.043 0167-577X/& 2014 Elsevier B.V. All rights reserved.
comparative study in relation to the more conventional WZ system. This article studies the transition of the crystallite phases obtained in ostensibly mixed-phase NCs from predominantly cubic ZB to hexagonal WZ and back again to the cubic ZB phase, simply by the changing the reaction time. PXRD and TEM were employed to study the crystal structure and morphology of the particles, respectively, whilst the optical properties were studied with UV–vis absorption and photoluminescence spectroscopies.
2. Materials and methods The CdSe nanoparticles were synthesized via colloidal synthesis using the Schlenk line method. The synthesis was based upon two previously reported methods of Yu et al. [7] and Murray et al. [8]; these were combined and modified to allow the study of various factors to determine whether phase or morphological control could be achieved. The selenium precursor (TOPSe) was prepared as follows: 0.25 mmol (0.197 g) powder selenium was dissolved in 2 mL of trioctylphosphine (TOP) and 2 mL of toluene in a sample tube. The mixture was then sonicated for 10 min until the selenium had dissolved in the TOP, giving rise to a colorless solution. In a typical synthesis of CdSe QDs, 0.125 mmol (0.160 g) of CdO was mixed with 2 g lauric acid in a 50 mL three-necked roundbottom flask and degassed for 30 min and then heated to 130 1C under nitrogen flow with constant stirring until all of the mixture became optically clear. The system was allowed to cool to room
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temperature, before adding 2 g of TOPO. The flask was resealed and heated to 320 1C. The pre-prepared TOPSe was quickly injected into the reaction flask and the temperature of the reaction mixture was observed to decrease to 280 1C upon injection, at which it was maintained for the growth period of the QDs. Aliquots of the reaction solution were removed at various postinjection intervals and then cooled. The series of samples are referred to herein as QD-30S, QD-60S, QD-120S, QD-300S and QD600S for 30, 60, 120, 300 and 600 s samples respectively. Powder X-ray diffraction (PXRD) measurements were performed using a Philips X'pert Multipurpose X-ray Diffraction System (MPD) operated at 40 kV and 40 mA (λ ¼0.154056 nm) in the Bragg–Brentano geometry. A Philips CM200 field emission TEM operating at an accelerating voltage of 200 kV, with a Bruker SDD was used for TEM. A Cary 100 UV–vis spectrophotometer was used to carry out absorption spectra. Micro-photoluminescence (μPL) spectra were measured on a custom-built microscope coupled to an Acton 2300 spectrometer.
3. Results and discussions PXRD was employed to determine the crystal structure of the aliquots of CdSe QDs. The characteristic features of ZB–CdSe NCs with X-rays of λ ¼0.154056 nm, consist of a sharp (111) peak at 2θ¼25.41, a deep valley between (220) and (311) peaks, as well as a smaller (440) peak at 2θ¼60.91. As for the hexagonal WZ–CdSe diffraction pattern, there is a broad peak around 2θ ¼251 and the (103) peak at 2θ¼45.61 [9]. Fig. 1 shows the combined PXRD spectrum of the samples from aliquots taken at different time intervals. The data for QD-30S, QD-60S, QD-300S and QD-600S exhibit peaks typical of the cubic phase, whereas the one at QD120S exhibits the peaks characteristic of the hexagonal phase. The sharp diffraction peaks show the highly crystalline nature of all of the synthesized particles. Rietveld refinement of the PXRD spectra was performed to obtain details of the percentage phase compositions, whilst the size of the nanoparticles was derived using the Debye–Scherrer equation; these are listed in Table 1. It can be deduced from these values that a decrease in the percentage composition of the cubic phase and a subsequent increase in the hexagonal phase occurs with increasing growth time; except for the 120 s, when the opposite was observed. As such, the samples QD-30S, QD-60S are more cubic in character, the QD-120S more hexagonal and the QD-300S and QD-600S are once again more
Fig. 1. Combined PXRD spectrum of the aliquots taken at different time intervals.
cubic. It should be noted that all of the CdSe QDs in this series are a mixture of both phases, but that they vary in the percentage of the two phases. This observation is consistent with the growth of ZB– CdSe being reinforced along the WZ {001} direction along with some ZB lattice stacking faults. This WZ–ZB polytypism phenomenon in semiconductors is not rare, since the structural difference in WZ and ZB is subtle and moreover their internal energy difference is small [10]. Usually, the transition of WZ and ZB are determined by the details of synthetic parameters and since WZ is a metastable phase compared to ZB, it requires vigorous conditions for its growth [11,12]. The unique feature observed in this study is the discovery of the occurrence of a phase dependence of the QDs, from cubic ZB to hexagonal WZ and back again to ZB, simply as a function of reaction time. This is due to rapid growth along the {111} facets of the crystals in the early stages of particle formation, prior to the other facets ‘catching-up’ to re-introduce cubic symmetry. Such a change in crystalline phase as a function of particle growth has not been reported before in colloidal CdSe QDs. The only previously reported phase transition has been from the cubic to hexagonal phase, induced by annealing of the QDs at high temperatures. Fig. 2 shows the combined UV–vis absorption and photoluminescence emission spectrum of the CdSe nanoparticles. QD-30S, QD60S, QD-120S, QD-300S, QD-600S have absorption edges at wavelengths 561, 566, 591, 594 and 602 nm respectively and emission edges at the wavelengths 622, 619, 629, 640 and 644 nm respectively. The difference in the absorption edge and the emission peak relates to the energy loss due to non-emissive electronic relaxation upon absorption and re-emission.
Table 1 Percentage of the phases obtained from PXRD and size of the QDs deduced from PXRD and UV–vis. Sample
UV–vis size (nm)
PXRD size (nm)
PXRD cubic (%)
PXRD hexagonal (%)
QD-30S QD-60S QD-120S QD-300S QD-600S
3.28 3.41 4.21 4.33 4.67
3.87 70.13 3.74 70.11 3.91 70.39 5.57 72.21 2.6770.01
65.5 73.3 31.1 85.5 99.8
34.5 26.7 69.9 14.5 0.2
Fig. 2. UV–vis absorption and photoluminescence emission spectra of QD-30S, QD-60S, QD-120S, QD-300S and QD-600S.
F.K. Kanodarwala et al. / Materials Letters 141 (2015) 67–69
69
Fig. 3. HRTEM images of (a) QD-30S, (b) QD-60S, (c) QD-120S, (d) QD-300S and (e) QD-600S, inset fast Fourier transform of the images.
The diameters of the nanoparticles were calculated from the wavelength of the absorption edges using the equation reported by Xia et al. [9]. It was found that there was a subsequent increase in the size of the QDs across the series, in the range of 3.28– 4.67 nm, which compares to those deduced from PXRD of 2.67– 3.87 nm; both size determinations are therefore in reasonable agreement with each other. Fig. 3 shows the HRTEM images of the particles obtained at different time intervals and clearly show the presence of spherical particles with lattice spacing of d ¼0.36 nm, matching that of the calculated lattice spacing of {100} planes in CdSe. The HRTEM images of QD-30S and QD-60S, Fig. 3(a) and (b) respectively, shows that the fast Fourier transform (FFT) patterns are hexagonal and the angles between different crystal planes are all equal to 601, consistent with those calculated for ZB–CdSe. As the reaction progressed, the HRTEM of the particles, QD-120S Fig. 3(c), shows that the FFT pattern is a rectangular with a 901 angle between the crystal planes. At longer reaction times the HRTEM images of QD300S and QD-600S, as shown in Fig. 3(d) and (e), the FFT pattern is a square with an angle of 901 between the lattice points for QD300S, whilst the FFT for QD-600S was hexagonal with an angle of 601, both of which are consistent with that of ZB–CdSe. These results for the phases are in agreement with those deduced earlier from the PXRD analysis that support the phase transition.
4. Conclusions In summary, we have observed phase shift of CdSe QDs obtained in a colloidal synthesis, from cubic ZB to hexagonal WZ and back to the cubic ZB structure, simply as an effect of reaction time at a given temperature. This provides a unique insight into the QD growth mechanisms and allows for greater control over the precise nature of the final CdSe QDs.
References [1] Dabbousi BO, Rodriguez VJ, Mikulec FV, Heine JR, Mattoussi H, Ober R, et al. J Phys Chem B 1997;101(46):9463–75. [2] Jun S, Jang EJ, Park J, Kim J. Langmuir 2006;22(6):2407–10. [3] Peng ZA, Peng XG. J Am Chem Soc 2001;123(1):183–4. [4] Qu LH, Peng ZA, Peng XG. Nano Lett 2001;1(6):333–7. [5] Lim SJ, Chon B, Joo T, Shin SK. J Phys Chem C 2008;112(6):1744–7. [6] Yang YA, Wu H, Williams KR, Cao YC. Angew Chem Int Ed 2005;44(41):6712–5. [7] Yu K, Singh S, Patrito N, Chu V. Langmuir 2004;20(25):11161–8. [8] Murray CB, Norris DJ, Bawendi MG. J Am Chem Soc 1993;115(19):8706–15. [9] Xia X, Liu ZL, Du GH, Li YB, Ma M. J Lumin 2010;130(7):1285–91. [10] Yeh CY, Lu ZW, Froyen S, Zunger A. Phys Rev B 1992;46(16):10086–97. [11] Talapin DV, Nelson JH, Shevchenko EV, Aloni S, Sadtler B, Alivisatos AP. Nano Lett 2007;7(10):2951–9. [12] Fiore A, Mastria R, Lupo MG, Lanzani G, Giannini C, et al. J Am Chem Soc 2009;131(6):2274–82.