Equilibrium size maps and novel aspects of the growth kinetics of CdSe nanocrystals
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Equilibrium size maps and novel aspects of the growth kinetics of CdSe nanocrystals E. Slejko*, A.Radivo**, F. Antoniolli** and V. Lughi* * University of Trieste, Department of Materials and Natural Resources, via Valerio, 6a - 34127 Trieste, Italy – [email protected] ** MaXun srl, Trieste, Italy Background Despite the vast literature on nucleation and growth dynamics of II-VI semiconductor nanocrystals synthesized by hot injection methods, uniform guidelines for a reliable nanocrystal (NC) synthesis have not been developed yet, and some apparently contrasting observations have yet to be accounted for. Within this context, it is of particular interest to study the “equilibrium” size that the NCs reach upon completion of the chemical reactions involved, as this would enable control of the NC size without sacrificing the efficiency of the synthesis. Experimental Conclusions • A rough map of the NC size at equilibrium – i.e. for the maximum synthesis efficiency – is provided as a function of the growth temperature and the ratio [cadmium]:[oleic acid]. This lays the basis for the development of a useful tool for the design and use of reliable synthesis protocols. • Larger NC size than that obtained with the synthesis protocols presented here could be reached by expanding the Cd:OA ratio in both directions, or by increasing the temperature while keeping high Cd:OA ratios; Smaller NC size cannot be obtained by expanding the range of Cd:OA ratios, while it could be obtained by lowering the temperature • The growth kinetics are thermally activated with an activation energy of 0.35 eV. Knowledge of the activation energies associated to the single mechanisms that concur to growth would enable identification of the limiting growth step References [C.D. Dushkin, S. Saita, K. Yoshie, Y. Yamaguchi; “The kinetics of growth of semiconductor nanocrystals in a hot amphiphile matrix”; Adv. Coll. Int. Sci.; Volume 88; 37; 2000. C. B. Murray, C. R. Kagan, M. G. Bawendi; “Synthesis and characterization fo monodisperse nanocrystals and close-paked nanocrystal assemblies”; Annu. Rev. Mater. Sci.; Volume 30; 545; 2000. Monodispersed Particles; Tadao Sugimoto; Elsevier, Amsterdam; 2001; Celso de Mello Donegà, P. Liljeroth, D. Vanmaekelbergh; “Physicochemical Evaluation of the Hot-Injection Method,a Synthesis Route for Monodisperse Nanocrystals”; Small; Volume 1; Number 12; 1152; 2005. C. R. Bullen and P. Mulvaney; “Nucleation and Growth Kinetics of CdSe Nanocrystals in Octadecene”; Nano Letters; Volume 4; Number 12; 2303; 2004. •The size of the nanocrystals is estimated from the absorption peak position via the calibration performed by Yu et al., 2003. •As previously observed, the size rapidly increases after injection, then reaching a plateau (figure below) •An empirical model based on classic AJMK kinetics fits the data (figure below) and is used to evaluate: • The initial nuclei size, d 0 , before growth starts • The final, pre-ripening size, d •The kinetic parameter k, associated to the growth rate We define the final size, d , as the size that the nanocrystals approach when the chemical reactions have reached equilibrium, before the onset of Ostwald ripening. •A size range between 3.6 and 4.6 nm is spanned when growth is performed between 210 and 270 °C and a Cd:OA ratio between 0.05 and 0.2 is used • d is normally larger for higher growth temperatures. This effect is more marked when the Cd:OA ratio is high. At 210°C the final size is markedly lower throughout the explored range of Cd:OA ratios • d reaches a minimum for Cd:OA ratios between 0.05 and 0.1. Two competing mechanisms concur to this peculiar behavior: On one hand an increase in oleic acid is known to limit nucleation thus making more precursor available for the growth phase, while on the other hand an increase of oleic acid enhances the solubility of Cd and Se, i.e. reduces the particle size NaNaX4 “Nanoscience with Nanocrystals”, Munich-Tutzing, April 11- 15, 2010 Data Analysis Synthesis: Hot injection in octadecene (precursors: Cd oleate and TOPSe). Systematic variation of cadmium/oleic acid (OA) ratios (from 1:19 to 1:5) and growth temperature (210 – 270 °C). Injection temperature: 225°C Characterization: Sampling of nanocrystal suspension during synthesis, followed by UV-VIS spectroscopy and photoluminescence Results and Discussion Objectives: Study the growth kinetics of CdSe nanocrystals, as a function of the synthesis parameters in order to: •Better understand the mechanisms governing the synthesis •Provide a support tool for synthesis design Size of nuclei, d 0 Particle size distribution Final nanocrystal size, d Growth rate Fig. 2 Initial size of nuclei, d 0 [nm] [Cd]:[OA] ratio •As expected, the initial particle size , d 0 , is not affected by the growth temperature • d 0 is nearly constant for low values of the Cd:OA ratio • d 0 is significantly larger when Cd:OA = 1:5 •The full width at half maximum of the NC’s photoluminescence peak is taken as a measure of the particle size distribution •The size distribution shows a marked minimum when NCs are grown at around 255°C •Narrowest size distribution is obtained for Cd:OA ratios of 1:10 or less Fig. 3 Cd:OA = 1:5 1:6.5 1:15 1:19 1:10 Temperature [K] FWHM of the PL peak [nm] Time [s] Size [nm] Injection of precursor d 0 d Growth temperature 270°C 255°C 235°C 210°C [Cd]:[OA] Final (equilibrium) size, d [nm] Growth temperature 270°C 255°C 235°C 210°C Growth rate parameter, k [1/s] [Cd]:[OA] ln(k) 1/T [1/K] •The parameter k can be directly associated to the growth rate. It is a measure of how fast the particle size reaches a plateau •k does not depend on the Cd:OA ratio (left) •k increases with temperature (right) •k follows Arrhenius’ law as shown by the linear dependence of ln(k) on 1/T (right), thus demonstrating a thermally activated behavior with a thermal activation energy, G a , of 0.35 Ev •The lack of dependence of the growth rate on the content of oleic acid suggests that the growth in this case is limited by surfaces mechanisms rather than by diffusion in the solution T k G k k B a exp 0 k B : Boltzmann constant D. V. Talapin, A. L. Rogach, A. Kornowski, M. Haase, H. Weller; “Highly Luminescent Monodisperse CdSe and CdSe/ZnS Nanocrystals Synthesized in a Hexadecylamine-Trioctylphosphine Oxide-Trioctylphospine Mixture”; Nano Letters; Volume 1; Number 4; 207; 2001. W. W. Yu, L. Qu, W. Guo, and X. Peng; “Experimental Determination of the Extinction Coefficient of CdTe, CdSe, and CdS Nanocrystals”; Chem. Mater.; Volume 15; Number 14; 2854; 2003. B. D. Dickerson, D. M. Irving, E. Herz, R. O. Claus, and W. B. Spillman, Jr., K. E. Meissner; “Synthesis kinetics of CdSe quantum dots in trioctylphosphine oxide and in stearic acid”; Appl. Phys. Lett.; Volume 86; 171915; 2005 N. Varghese, K. Biswas, C. N. R. Rao; “Investigations of the Growth Kinetics of Capped CdSe and CdS Nanocrystals by a Combined Use of Small Angle X-ray Scattering and Other Techniques”; Chem. Asian J.; Volume 3; 1435; 2008. C. B. Murray, D. J. Norris, and M. G. Bawendi; “Synthesis and characterization of nearly monodisperse CdE (E= sulfur, selenium, tellurium) semiconductor nanocrystallites”; J. Am. Chem. Soc.; Volume 115; Number 19, 8706; 1993.
description
NaNaX4 “ Nanoscience with Nanocrystals ” , Munich - Tutzing , April 11 - 15 , 20 10. Equilibrium size maps and novel aspects of the growth kinetics of CdSe nanocrystals. E. Slejko * , A.Radivo * * , F . Antoniolli ** and V. Lughi* - PowerPoint PPT Presentation
Transcript of Equilibrium size maps and novel aspects of the growth kinetics of CdSe nanocrystals
Equilibrium size maps and novel aspects of the growth kinetics of CdSe nanocrystals
E. Slejko*, A.Radivo**, F. Antoniolli** and V. Lughi** University of Trieste, Department of Materials and Natural Resources, via Valerio, 6a - 34127 Trieste, Italy – [email protected]
** MaXun srl, Trieste, Italy
BackgroundDespite the vast literature on nucleation and growth dynamics of II-VI semiconductor nanocrystals synthesized by hot injection methods, uniform guidelines for a reliable nanocrystal (NC) synthesis have not been developed yet, and some apparently contrasting observations have yet to be accounted for.
Within this context, it is of particular interest to study the “equilibrium” size that the NCs reach upon completion of the chemical reactions involved, as this would enable control of the NC size without sacrificing the efficiency of the synthesis.
Experimental
Conclusions• A rough map of the NC size at equilibrium – i.e. for the
maximum synthesis efficiency – is provided as a function of the growth temperature and the ratio [cadmium]:[oleic acid]. This lays the basis for the development of a useful tool for the design and use of reliable synthesis protocols.
• Larger NC size than that obtained with the synthesis protocols presented here could be reached by expanding the Cd:OA ratio in both directions, or by increasing the temperature while keeping high Cd:OA ratios; Smaller NC size cannot be obtained by expanding the range of Cd:OA ratios, while it could be obtained by lowering the temperature
• The growth kinetics are thermally activated with an activation energy of 0.35 eV. Knowledge of the activation energies associated to the single mechanisms that concur to growth would enable identification of the limiting growth step
References[C.D. Dushkin, S. Saita, K. Yoshie, Y. Yamaguchi; “The kinetics of growth of semiconductor nanocrystals in a hot amphiphile matrix”; Adv. Coll.
Int. Sci.; Volume 88; 37; 2000.C. B. Murray, C. R. Kagan, M. G. Bawendi; “Synthesis and characterization fo monodisperse nanocrystals and close-paked nanocrystal
assemblies”; Annu. Rev. Mater. Sci.; Volume 30; 545; 2000.Monodispersed Particles; Tadao Sugimoto; Elsevier, Amsterdam; 2001;Celso de Mello Donegà, P. Liljeroth, D. Vanmaekelbergh; “Physicochemical Evaluation of the Hot-Injection Method,a Synthesis Route for
Monodisperse Nanocrystals”; Small; Volume 1; Number 12; 1152; 2005.C. R. Bullen and P. Mulvaney; “Nucleation and Growth Kinetics of CdSe Nanocrystals in Octadecene”; Nano Letters; Volume 4; Number 12;
2303; 2004.
•The size of the nanocrystals is estimated from the absorption peak position via the calibration performed by Yu et al., 2003.
•As previously observed, the size rapidly increases after injection, then reaching a plateau (figure below)
•An empirical model based on classic AJMK kinetics fits the data (figure below) and is used to evaluate:•The initial nuclei size, d0, before growth starts
•The final, pre-ripening size, d
•The kinetic parameter k, associated to the growth rate
We define the final size, d, as the size that the nanocrystals approach when the chemical reactions have reached equilibrium, before the onset of Ostwald ripening.• A size range between 3.6 and 4.6 nm is spanned when growth is performed
between 210 and 270 °C and a Cd:OA ratio between 0.05 and 0.2 is used
• d is normally larger for higher growth temperatures. This effect is more marked when the Cd:OA ratio is high. At 210°C the final size is markedly lower throughout the explored range of Cd:OA ratios
• d reaches a minimum for Cd:OA ratios between 0.05 and 0.1. Two competing mechanisms concur to this peculiar behavior: On one hand an increase in oleic acid is known to limit nucleation thus making more precursor available for the growth phase, while on the other hand an increase of oleic acid enhances the solubility of Cd and Se, i.e. reduces the particle size
NaNaX4 “Nanoscience with Nanocrystals”, Munich-Tutzing, April 11- 15, 2010
Data Analysis
Synthesis:Hot injection in octadecene (precursors: Cd oleate and TOPSe).Systematic variation of cadmium/oleic acid (OA) ratios (from 1:19 to 1:5) and growth temperature (210 – 270 °C).Injection temperature: 225°C
Characterization:Sampling of nanocrystal suspension during synthesis,
followed by UV-VIS spectroscopy and photoluminescence
Results and Discussion
Objectives:
Study the growth kinetics of CdSe nanocrystals, as a function of the synthesis parameters in order to:
• Better understand the mechanisms governing the synthesis
• Provide a support tool for synthesis design
Size of nuclei, d0Particle size distribution
Final nanocrystal size, d
Growth rate
Fig. 2
Initi
al s
ize
of n
ucle
i, d 0
[nm
]
[Cd]:[OA] ratio
•As expected, the initial particle size , d0, is not affected by the growth temperature
•d0 is nearly constant for low values of the Cd:OA ratio
•d0 is significantly larger when Cd:OA = 1:5
•The full width at half maximum of the NC’s photoluminescence peak is taken as a measure of the particle size distribution
•The size distribution shows a marked minimum when NCs are grown at around 255°C
•Narrowest size distribution is obtained for Cd:OA ratios of 1:10 or less Fig. 3
Cd:OA = 1:5
1:6.5
1:15
1:19
1:10
Temperature [K]
FWH
M o
f the
PL
peak
[nm
]
Time [s]
Siz
e [n
m]
Injectionof precursor
d0
d
Growthtemperature
270°C
255°C
235°C
210°C
[Cd]:[OA]
Fina
l (eq
uilib
rium
) siz
e, d
[n
m]
Growthtemperature
270°C
255°C
235°C
210°C
Gro
wth
rate
par
amet
er, k
[1/s
]
[Cd]:[OA]
ln(k
)
1/T [1/K]
• The parameter k can be directly associated to the growth rate. It is a measure of how fast the particle size reaches a plateau
• k does not depend on the Cd:OA ratio (left)
• k increases with temperature (right)
• k follows Arrhenius’ law
as shown by the linear dependence of ln(k) on 1/T (right), thus demonstrating a thermally activated behavior with a thermal activation energy, Ga, of 0.35 Ev
• The lack of dependence of the growth rate on the content of oleic acid suggests that the growth in this case is limited by surfaces mechanisms rather than by diffusion in the solution
TkG
kkB
aexp0 kB: Boltzmann constant
D. V. Talapin, A. L. Rogach, A. Kornowski, M. Haase, H. Weller; “Highly Luminescent Monodisperse CdSe and CdSe/ZnS Nanocrystals Synthesized in a Hexadecylamine-Trioctylphosphine Oxide-Trioctylphospine Mixture”; Nano Letters; Volume 1; Number 4; 207; 2001.
W. W. Yu, L. Qu, W. Guo, and X. Peng; “Experimental Determination of the Extinction Coefficient of CdTe, CdSe, and CdS Nanocrystals”; Chem. Mater.; Volume 15; Number 14; 2854; 2003.
B. D. Dickerson, D. M. Irving, E. Herz, R. O. Claus, and W. B. Spillman, Jr., K. E. Meissner; “Synthesis kinetics of CdSe quantum dots in trioctylphosphine oxide and in stearic acid”; Appl. Phys. Lett.; Volume 86; 171915; 2005
N. Varghese, K. Biswas, C. N. R. Rao; “Investigations of the Growth Kinetics of Capped CdSe and CdS Nanocrystals by a Combined Use of Small Angle X-ray Scattering and Other Techniques”; Chem. Asian J.; Volume 3; 1435; 2008.
C. B. Murray, D. J. Norris, and M. G. Bawendi; “Synthesis and characterization of nearly monodisperse CdE (E= sulfur, selenium, tellurium) semiconductor nanocrystallites”; J. Am. Chem. Soc.; Volume 115; Number 19, 8706; 1993.
