5.2.1 Characterizations of nanostructure and morphology
Among the enormous variety of nanoscale semiconductor materials, the nanostructures chosen for this study are the core/shell CdSe/CdS nanoplatelets. They are chemically syn-thesized by the research team of Benoit Dubertret, at ESPCI. These nanoplatelets consist of a CdSe core between two CdS shell layers, with different structural shapes as schematized in Figure 5.5.
The CdSe core nanoplatelets were prepared following the protocol as published in [116].
In brief, cadmium myristate is mixed with selenium powder in octadecene and heated to 240oC.
When the temperature reaches 180oC, an acetate salt is injected into the solution and CdSe nanoplatelets are formed. The thickness of these platelets is only several atomic layers and can
CdS shell CdSe core
Square Rectangle Cube
FIGURE 5.5: Schematic representations and TEM images of the square, rectangular, and cubic nanoplatelets, respectively. All the TEM measurements were performed by Benoit Dubertret0s team.
be accurately controlled. They grow in the zinc blende phase, along a [001] axis. The exposed [001] surface is atomically flat, while the other [100] and [010] directions can extend to roughly 100 nm, depending on the reaction time [161]. The shell growth is accomplished by an one-pot method [117,162]. This approach is based on the in situ generation of hydrogen sulfide by the reaction of thioacetamide with octylamine [163]. By adding cadmium source continuously, the reaction is kept performing for a few hours until nanoplatelets with a desired thickness and lateral sizes are obtained. The transmission electron microscope (TEM) images of the square, rectangular, and cubic structures are also presented in the figure 5.5.
TEM samples are prepared by drop-casting diluted nanoplatelet solutions onto carbon coated copper grids. The diluted concentration is chosen (∼ 10−8M) in order to prevent the gathering of nanoplatelets. From these TEM images, three dimensions of the square nanoplatelet is approximately estimated about (16 ± 2 nm) long x (16 ± 2 nm) wide x 2 nm thick. The rectangular nanoplatelets have the same width and thickness but they measure longer length of 20 ± 3 nm. The cubic nanoplatelets have the same length and width as the square one, but with much thicker CdS shell layers; therefore, their total thickness are briefly from 10 to 15 nm.
We assume that their two dimensions (width and length) equal to:
l1 =l0(1−∆l) (5.2)
l2 =l0(1 + ∆l) (5.3) The size factor is thus defined as:
∆l = l2−l1
l1 +l2 (5.4)
which ranges from 0 (square nanoplatelets l1 =l2) to 1 (very elongated rectangles l1 l2).
0 20 25
10 5 15
0 0.2 0.4 0.6 0.8 1
# emitters
0 4 8 12
# emitters
Δl
0 0.2 0.4 0.6 0.8 1
Δl
a) Square nanoplatelets b) Rectangular nanoplatelets
FIGURE 5.6: Histogram of the size ratio ∆l measured for (a) 69 square nanoplatelets and for (b) 88 rectangular platelets.
From the values of the width and the length of the platelets obtained from the TEM images, the size factor ∆l of 69 square nanoplatelets and 88 rectangular ones are calculated and then histogrammed in Figure 5.6 (a) and (b), respectively. For the square nanoplatelets, the ∆l factor ranges from 0 to 0.08, referring to a slightly structural elongation. On the other hand, in the case of rectanglar nanoplatelets, the deviation of ∆l is much larger (between 0.02 and 0.24), affirming their rectangular shape.
5.2.2 Optical properties of the colloidal nanoplatelets
Figure 5.7 presents the normalized absorption and photoluminescent spectra of two dif-ferent types of colloidal nanoplatelets at room temperature. Our excitation laser at 450nm is in the good range of nanoplatelet0s absorption. The emission wavelength is centered at about 650-660 nm for all types of nanoplatelets.
In both cases of square and rectangular nanoplatelets, the photoluminescent spectrum is sharp and narrow: its emission bandwidth is less than 20 nm, smaller than the typical value obtained for the spherical core/shell CdSe/CdS quantum dots (25-30 nm). It refers to minimal surface trapping, as expected in these atomically flat core/shell nanoplatelets. These narrow
300 350 400 450 500 550 600 650 700
FIGURE 5.7: Room temperature normalized absorption and photoluminescent spectra of the col-loidal square and rectangular nanoplatelets. All these measurements were performed by Benoit Dubertret0s team.
spectra with very low inhomogenous broadening also confirm that the chemical synthesis is so accurately controlled that the nanoplatelets form with no much difference in thickness.
5.2.3 Optical properties of a single nanoplatelet
We perform photoluminescence intensity and photoluminescence decay profile measure-ments for some individual square nanoplatelets deposited on the glass substrate when the oil objective is in contact with the other side of the substrate without emitters, as schematized in Figure 5.8(a). These nanoplatelets are excited by a pulsed laser at the wavelength of 450 nm and the repetition rate of 25 MHz.
Figure 5.8(b) describes the photoluminescent intensity of some square individual nanoplatelets under the constant lasing excitation energy. Their fluctuation, or blinking, confirms that the emitters are single and well isolated. Different from the typical core/shell CdSe/CdS quantum dots, the emission of the square nanoplatelets is not really stable as a dark state (no emission state) is clearly observed.
The photoluminescent decay profiles of these mentioned square nanoplatelets are shown in Figure 5.8(c). These curves fit to a biexponential function f(t) = A e−t/τ1 +B e−t/τ2 +C with time constant τ1 about 9-12 ns and τ2 about 37-40 ns. It brings up a question why we have a biexponential fitting. Since the photoluminescent decay profile reflects the contribution of non-radiative and radiative emission, there could be two hypotheses.
In the first one, the quick decay is due to non-radiative channels which accelerate the decay rate. It may result from charges or surface defects. Then the long lifetime corresponds to
0 50 100 150 200 250 300
FIGURE 5.8: (a) Schematic of experimental configuration. (b) Detected photoluminescent intensity as a function of time 5 individual square nanoplatelets on a glass substrate with the same excitation energy. (c) Their corresponding photoluminescent decay profiles in semilog scale. (d) The decay profile of emitter numbered 3 is fitted with the biexponential function f(t) =A e−t/τ1 +B e−t/τ2 +C.
the exciton emission. The other hypothesis developed in [164] is that the surface traps induce the localization of the hole wave function, resulting in a longer emission lifetime. In this case, the bandgap emission corresponds to the short decay.