Supporting Information
Colloidal Atomic Layer Deposition (c-ALD) using self-limiting
reactions at nanocrystal surface coupled to phase transfer between
polar and non-polar media
Sandrine Ithurria,1 Dmitri V. Talapin1,2*
1
Department of Chemistry and James Frank Institute, University of Chicago, IL 60637, USA
2
Center for Nanoscale Materials, Argonne National Lab, Argonne, IL 60439, USA
E-mail: [email protected]
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EXPERIMENTAL SECTION
Chemicals: Trioctylphosphine oxide (TOPO, 99%, Aldrich), octadecylphosphonic acid (ODPA,
99%, Polycarbon), trioctylphosphine (TOP, 97%, Strem), Selenium (powder, 99.99%, Aldrich),
Selenium dioxide (SeO2, Aldrich), cadmium oxide (99.995%, Aldrich), cadmium acetate hydrate
(Cd(Ac)2, Aldrich), oleic acid (OA, 90%, Aldrich), 1-octadecene (ODE, 90%, Aldrich),
didodecyldimethylammonium bromide (DDAB, 98%, Fluka), tributylphosphine (TBP, 97%,
Aldrich), propylphosphonic acid (PPA, 95%, Aldrich), octylamine (99%, Aldrich), sulfur
(powder, 99.998%, Aldrich), ammonium sulfide (Aldrich), potassium sulfide (K2S, Alpha
Aesar), formamide (FA, 99.5+%, Aldrich), N-Methylformamide (NMFA, Aldrich) were used as
received.
Nanocrystal Synthesis
Wurtzite CdSe quantum dots1 In a three neck flask, 120mg CdO, 560mg ODPA and 6g TOPO
were degassed for one hour at 150°C. The flask was purged with nitrogen and heated to 370°C.
Then 3g of TOP were injected and, when the temperature of the reaction mixture reached 370°C,
0.888g of 1.7M TOPSe solution in TOP were injected. The reaction was stopped after 5
minutes. The NCs were precipitate with ethanol three times and redispersed in toluene.
Zinc Blende CdSe quantum dots2 In a three neck flask, 3mmol Cd(myristate)2, 3mmol SeO2 and
15ml octadecene were degassed during 30 min under dynamic vacuum at room temperature.
Under nitrogen flow, the flask was heated to 240°C for 10 minutes. The NCs were washed three
times, once with ethanol and two times with acetone. They were finally dispersed in hexane.
Wurtzite CdS nanorods3,4 In a three neck flask, 210mg CdO, 1.07 g ODPA, 15mg of PPA and
3.5g TOPO were degassed at 120°C for one hour. Then under nitrogen flow the mixture is
heated to 300°C to obtain a clear solution of Cd(ODPA)2. The temperature was then decreased to
150°C and the mixture was degassed under vacuum for one hour. Then, under nitrogen flow, the
mixture was heated to 320°C. At 305°C, 2g TOP were injected followed by the injection of 620
mg TOPS at 320°C. The reaction mixture was kept at 320°C for 10 min. The nanorods were
washed one time with a mixture of toluene, hexane, octylamine and acetone, two times with a
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mixture of toluene, hexane, octylamine and ethanol and finally two times with a mixture of
toluene and ethanol. Purified NCs were redispersed in toluene.
Zinc Blende CdSe nanoplatelets5
- Nanoplatelets exhibiting the first excitonic transition at 510nm: In a three neck flask, 170mg
Cd(Myristate)2, 12mg Se and 15ml ODE were degassed for 30 minutes at room temperature.
Then, under nitrogen flow, the flask was set to heat to 240°C. When the temperature of the
reaction mixture approached 190°C, 40mg Cd(Ac)2 were quickly introduced. After 5 min at
240°C, the mixture was cooled and 2ml of oleic acid and 15ml of hexane were added. The
mixture was then centrifuged and the precipitate containing the nanoplatelets is washed three
times with ethanol. The nanoplatelets were re-suspended in a non polar solvent such as hexane or
toluene.
- Nanoplatelets exhibiting the first excitonic transition at 462nm: In a three neck flask, 240mg
Cd(Ac)2, 150µl oleic acid and 15ml ODE were degassed for one hour at 80°C. Then, under
nitrogen flow, the flask was heated to 180°C and 150µl of 1M TOPSe in TOP were quickly
injected. After 30 minutes, the reaction was stopped. The nanoplatelets were washed three times
with ethanol and re-dispered in hexane.
General remarks on c-ALD process:
The cadmium and sulfide precursors used in these syntheses can react together at room
temperature even if they are in two immiscible phases, such as Cd(OA)2 in toluene and
ammonium sulfide in formamide. Obtained CdS particles will be stabilized by excessive
precursor. For example, if there is an excess of sulfide, the as-formed CdS particles will be
stabilized by sulfide anions in formamide.
Cadmium rich nanocrystals can almost instantaneously react with sulfide precursors, to
form sulfide rich nanocrystals. The reaction is complete and limited by available sites the
nanocrystal surface. This is an atomic layer deposition. This step should be followed by the
elimination of free reagents to prevent secondary nucleation induced by the addition of cadmium.
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The sulfide precursors were always first dissolved in the polar phase (formamide or Nmethylformamide). The nanocrystals could either be transferred into the polar phase by exchange
or organic ligands with inorganic ligands or be stabilized in non-polar phase by adding
didodecyldimethylammonium bromide (DDAB). In the presence of DDAB, sulfide could be
solubilized in toluene in form of DDA-sulfide complex soluble in non-polar solvents. This
precursor binds covalently to the NC surface during c-ALD process.
If the NCs have been first transferred from toluene to formamide, they can be transferred
back to toluene with the addition of DDAB. As without transferring the NCs, the excess of
sulfide is extracted with formamide. In presence of oleylamine instead of DDAB the process of
purification is identical.
We could not use DDAB in NMF because it was soluble in toluene and not soluble in
hexane, while toluene was miscible with NMF.
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Supporting Figures
Figure S1. X-ray diffraction patterns for W- and ZB-CdSe nanocrystals used as the seeds in
c-ALD process.
Figure S2. Photoluminescence quantum yield of CdSe/CdS core-shell NCs as a function of the
number of c-ALD cycles. Black dots show the data obtained for nanostructures grown from
wurtzite cores while circles show the data obtained for nanostructures grown from zinc blende
cores. The syntheses have been carried out in the presence of oleylamine.
The photoluminescence quantum yield of CdSe/CdS nanostructures has been determined in
comparison with Rhodamine 6G reference dye using the following expression:
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QYQDs = QYDye
2
I QDs ODDye nQDs
2
I Dye ODQDs n Dye
,
where QYDye is the dye quantum yield for a given excitation wavelength, IQDs,Dye is the integrated
fluorescence intensity, ODQDs,Dye is the optical density of the QDs and dye solutions and nQDs,Dye
is the refractive index of the QDs and dye solutions respectively.6
Figure S3. TEM images of CdSe NCs with wurtzite (W-CdSe) and zinc blende (ZB-CdSe)
structure and CdSe/CdS core-shell NCs obtained after ten c-ALD cycles using W-CdSe and ZBCdSe cores. The shape of core-shell NCs was tuned by using different precursors and surface
ligands as discussed in the text and Figures 1, 2.
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Figure S4.
Absorption spectra of CdSe/CdS core-shells synthesized from (a) W-CdSe cores
and (b) ZB-CdSe cores using (NH4)2S and cadmium oleate as sulfur and cadmium precursors,
respectively, and didodecyldimethylammonium bromide (DDAB) as the phase-transfer agent.
The spectra were measured after each c-ALD cycle.
Figure S5. High Resolution TEM images of 4CdS/CdSe/4CdS nanoplatelets, with 2D Fourier
Transforms for the areas selected with colored frames. The FFT data on faceting of the
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nanoplatelets confirm their zinc blende structure and the orientation of <100> axis normal to the
plane of each nanoplatelet.
Figure S6. (a) Absorption and emission spectra 5 monolayers thick CdSe nanoplatelets and for
xCdS/CdSe/xCdS nanoplatelets. Solid lines correspond to the nanoplatelet samples grown in Nmethylformamide (spectra measured in NMFA). Dashed lines correspond to the samples grown
without phase transfer on nanoplatelets, in the presence of oleylamine, ammonium sulfide and
Cd(acetate)2 (spectra measured in toluene). The positions of the first excitonic peaks are very
similar for the two syntheses. TEM images of (b) rolled 5 monolayers thick CdSe nanoplatelets
in toluene and (c) 5CdS/CdSe/5CdS nanoplatelets grown from 5 monolayers thick CdSe
nanopatelets in N-methylformamide.
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Figure S7.
Additional TEM images of 7CdS/CdSe/7CdS nanoplatelets.
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Figure S8.
Photoluminescence
excitation
spectra
for
(a)
CdSe/CdS
dot-in-rod
nanostructures1,7 and (b) for 6 monolayers thick CdSe nanoplateletss. The PLE spectra were
recorded for different emission wavelengths shown by the arrows.
Figure S9. (a) Absorption spectra of CdS nanorods before and after 1,2..6 c-ALD cycles. The
absorption spectra show a red shift of the absorption onset and washing of the excitonic peaks
due to relaxation of quantum confinement as the nanorod diameter approached characteristic size
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of the Bohr exciton in CdS (~2.9 nm). TEM images of (b,c) original CdS nanorods and (d,e) CdS
nanorods after six c-ALD cycles.
Figure S10 Absorption spectra of CdS-x(ZnS) nanocrystals doped with Mn2+ ions into the third
ZnS layer and the emission spectrum of CdS-6(ZnS):Mn2+ nanocrystals showing characteristic
phosphorescence from d-d transition in Mn2+ ions.
Supporting References
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(2)
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(3)
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Alivisatos, A. P. Science 2007, 317, 355-358.
(4)
Kovalenko, M. V.; Scheele, M.; Talapin, D. V. Science 2009, 324, 1417-1420.
(5)
Ithurria, S.; Tessier, M. D.; Mahler, B.; Lobo, R. P. S. M.; Dubertret, B.; Efros, A. L.
Nature Materials 2011, 10, 936-941.
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(6)
Grabolle, M.; Spieles, M.; Lesnyak, V.; Gaponik, N.; Eychmüller, A.; Resch-Genger, U.
Analytical Chemistry 2009, 81, 6285-6294.
(7)
Talapin, D. V., Nelson, J.H., Shevchenko, E.V., Aloni, S., Sadtler, B., Alivisatos, A.P.
Nano Letters 2007, 7, 2951-2959.
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