Enhanced Tunability of the Multi-photon Absorption CrossSection in Seeded CdSe/CdS Nanorod Heterostructures
Guichuan Xing1, Sabyasachi Chakrabortty2, Kok Loong Chou1, Nimai Mishra2, Cheng Hon
Alfred Huan1, Yinthai Chan2# and Tze Chien Sum1*
1
Division of Physics and Applied Physics, School of Physical and Mathematical Sciences,
Nanyang Technological University, 21 Nanyang Link, Singapore 637371
2
Department of Chemistry, National University of Singapore, 3 Science Drive 3, Singapore
117543
1. Synthesis of the heterostructures
Chemicals: Cadmium acetylacetonate (Cd(acac)2, 99.9%), cadmium oxide (CdO, 99.5%),
1,2-hexadecanediol (HDDO, 90%), 1-hexadecylamine (HDA , 90%), 1-octadecene (ODE,
90%), sulfur (S, reagent grade), selenium (Se, 99.99%) and trioctylphosphine oxide (TOPO,
90%) were purchased from Sigma Aldrich. Trioctylphosphine (TOP, 90%) was purchased
from Alfa Aesar. Diisooctylphosphinic acid (DIPA, 90%) was purchased from Fluka. nOctadecylphosphonic acid (ODPA, 97%), trioctylphosphine oxide (TOPO, 99%) and nhexylphosphonic acid (HPA, 97%) were purchased from Strem. All the chemicals were used
as received without further purification. Unless stated otherwise, all the reactions were
conducted in oven-dried glassware under nitrogen atmosphere using standard Schlenk
techniques.
Synthesis of spherical CdSe seeds: Synthesis of monodispersed CdSe NCs proceeded based
on a previously reported procedure with slight modifications. [1] A bath of 9 g TOPO (90%),
6 g HDA and 0.25 ml of DIPA was degassed at 100 0C for 1.5 h. A precursor solution
1
comprising of 317 mg Cd(acac)2 and 567 mg HDDO in 6 mL of ODE was degassed at 120 0C
for 1.5 h, followed by addition of 4 mL of 1.5 M trioctylphosphine selenide at room
temperature. The precursor solution was then rapidly injected into the bath at 360 0C and
allowed to cool to 80 °C. As-synthesized CdSe QDs were subsequently processed by 3-4
cycles of precipitation in a butanol / methanol mixture and re-dispersion in hexane for further
use. Processed CdSe QDs were dispersed in a minimum amount of hexane. The hexane was
then removed under vacuum and TOP was added to make up a QD concentration of 80 μM.
This mixture will subsequently be referred to as the CdSe stock solution.
Synthesis of CdSe seeded CdS heterostructured nanorods: CdSe/CdS nanorods were
prepared according to the method of Manna. [2] 3g TOPO (99%), 65 mg CdO, 290 mg
ODPA and 80 mg HPA are mixed in a 50 mL three neck RBF and degassed at 150 °C for
about 1.5 h. The reaction mixture was then heated to 360 oC under N2 atmosphere. After the
formation of the Cd-ODPA complex at around 270 oC, the solution turned from reddish
brown to colorless. Separately, a mixture of S, TOP and CdSe seeds was derived by first
dissolving 80 mg S in 1.8 mL TOP at 50 °C before adding 200 μL of the prepared CdSe stock
solution. Upon reaching the desired temperature, 1.8 mL TOP was added, and the
temperature was allowed to recover to 360 °C before the mixture of S, TOP and CdSe was
swiftly injected. The temperature was again allowed to recover to 360 °C and the anisotropic
shell was grown at this temperature for about 6-8 minutes. The heating mantle was then
removed and the solution was allowed to cool to 80 °C. As-synthesized CdSe/CdS nanorods
(NRs) were then processed by repeated cycles of precipitation in methanol and re-dispersion
in toluene. Seeded CdSe/CdS nanorods of different lengths were synthesized by varying
either the amount of Cd and S precursors added and/or the reaction times required for shell
growth.
2
2. Size dispersions of the heterostructures
Fig S1 Diameter (blue) and length (red) size dispersions of the CdSe/CdS nanodot/nanorod
heterostructures with average lengths of (a) 8.5 nm, (b) 34 nm, (c) 39 nm and (d) 180 nm.
3. Determination of the CdSe core size
Fig S2 UV-Vis absorption (black line) and fluorescence (red line, excited at 400 nm) spectra
of the CdSe core in toluene solution.
3
The size of CdSe QDs can be calculated by an empirical relationship deduced by Peng et al
for the CdSe QDs in toluene: [3]
D  (1.6122 109 )4  (2.6575 106 )3  (1.6242 103） 2  0.4277  41.57
(S1)
where D is the diameter of the nanocrystals and λ is the wavelength of the first excitonic
absorption peak (1S(e)→1S3/2(h)), here is 509 nm. The diameter of the CdSe core is then
calculated to be 2.44 nm.
4. Multi-photon excited photoluminescence
The excitation source was a Coherent LegendTM regenerative amplifier that was seeded by a
Coherent MiraTM oscillator (150 fs, 1 KHz, 800 nm). The laser pulses were focused by a lens
(f = 30 cm) on the samples solution in a 2-mm-thick quartz cell (beam spot ~ 0.5 mm inside
the cell). The emission from the heterostructures was collected at a backscattering angle of
150° by a pair of lenses and optical fibers, and directed to a spectrometer (Acton, Spectra Pro
2500i coupled CCD Princeton Instruments, Pixis 400B). A short-pass filter with a cut-off
wavelength of 750 nm was placed before the spectrometer to minimize the scattered
excitation light. The time-resolved upconversion PL measurements were collected using an
Optronis OptoscopeTM streak camera system which has an ultimate temporal resolution of 6
ps.
In the Z-scan experiment, the same excitation source as that for the upconversion PL
experiment was used. The input laser pulses were focused onto the sample by a lens with 30
cm focus length. The beam waist at the focus point was 37 ± 3 um and was confirmed with a
standard two-photon absorption experiment on a 0.5 mm thick ZnSe bulk crystal. The
heterostructures toluene solution contained in 2 mm thick quartz cell was moved across the
focus point along the beam propagation axis. The transmittance at different position z was
recorded.
4
5. Tailoring the photoemission through controlling the core size in the heterostructures
Early reports on the optical spectra of CdSe/CdS heterostructures suggested that due to a
conduction band offset of ~0 eV between the CdSe core and the CdS shell, the electron is
delocalized throughout the nanorod while the relatively heavier hole is strongly localized in
the CdSe core due to its large valence band offset (0.884 eV). [4-9] Recent studies have
shown, however, that depending on the size of the core, the conduction band offset can be
larger than 0 eV and as high as 0.3 eV, [8,9] resulting in a transition from a quasi-Type II to a
Type I (or quasi-Type II) band offset as the size of the core increases. [8] One consequence of
a Type I core-shell energy profile and an electron that is not delocalized across the entire
length of the nanorod shell is that the PL emission should not redshift beyond a certain length
of the nanorod. In the following we further show that by enlarging the CdSe core size to 3.5
nm (which is evident from the linear absorption spectrum, see Fig S3), the photoemission
peak of 40 nm CdSe/CdS heterostructures can be easily tailored to 637 nm. As shown in Fig
S4.
Fig S3 UV-Vis absorption (black line) and fluorescence (red line, excited at 400 nm) spectra
of the 3.5 nm CdSe core in toluene solution.
5
Fig S4 UV-Vis absorption (black solid line), the enlarged long wavelength part (red line) and
fluorescence (black dotted line, excited at 400 nm) spectra of the CdSe/CdS nanodot/nanorod
heterostructures in toluene solution.
References:
1. P. T. Snee, Y. Chan, D. G. Nocera, M. G. Bawendi, Adv. Mater. 17, 1131 (2005).
2. L. Carbone, C. Nobile, M. De Giorgi, F. D. Sala, G. Morello, P. Pompa, M. Hytch, E.
Snoeck, A. Fiore, I. R. Franchini, M. Nadasan, A. F. Silvestre, L. Chiodo, S. Kudera,
R. Cingolani, R. Krahne, L. Manna, Nano Lett. 7, 2942 (2007).
3. W. W. Yu, L. Qu, W. Guo, X. Peng, Chem Mater. 15, 2854 (2003).
4. D. V. Talapin, R. Koeppe, S. Gotzinger, A. Kornowski, J. M. Lupton, A. L. Rogach,
O. Benson, J. Feldmann, and H. Weller, Nano Lett. 3, 1677 (2003).
5. W. W. Yu, L. Qu, W. Guo, X. Peng, Chem Mater. 15, 2854 (2003).
6. L. Carbone, C. Nobile, M. D. Giorgi, F. D. Sala, G. Morello, P. Pompa, M. Hytch, E.
Snoeck, A. Fiore, I. R. Franchini, M. Nadasan, A. F. Silvestre, L. Chiodo, S. Kudera,
R. Cingolani, R. Krahne, and L. Manna, Nano Lett. 7, 2942 (2007).
7. J. Muller, J. M. Lupton, P. G. Lagoudakis, F. Schindler, R. Koeppe, A. L. Rogach,
and J. Feldmann, Nano Lett. 5, 2044 (2005).
8. A. Sitt, F. D. Sala, G. Menagen, and U. Banin, Nano Lett. 9, 3470 (2009).
9. M. G. Lupo, F. D. Sala, L. Carbone, M. Zavelani-Rossi, A. Fiore, L. Luer, D. Polli, R.
Cingolani, L. Manna, and G. Lanzani, Nano Lett. 8, 4582 (2008).
10. G. S. He, K. T. Yong, Q. D. Zheng, Y. Sahoo, A. Baev, A. I. Ryasnyanskiy, and P. N.
Prasad, Opt. Express 15, 12818 (2007).
6