Enhanced Tunability of the Multi-photon Absorption Cross-Section 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*

1Division of Physics and Applied Physics, School of Physical and Mathematical Sciences, Nanyang Technological University, 21 Nanyang Link, Singapore 637371

2Department 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. n-Octadecylphosphonic acid (ODPA, 97%), trioctylphosphine oxide (TOPO, 99%) and n-hexylphosphonic 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 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.  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.

The size of CdSe QDs can be calculated by an empirical relationship deduced by Peng et al for the CdSe QDs in toluene: [3]

(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.

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.

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.

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