book of abstracts - IM2NP

A B S T R A C T S TUESDAY, JUNE 29 N A N O S E A 2 0 1 0
Results were then discussed on the basis of theoretical calculations [8] and density of states function
measurement using resonant shell-tunneling Spectroscopy experiments [9]. Only the results related to the 7
nm are reported here.
2 - Results
2-1 Geometry, size and spacing distribution, crystal structure and relative orientation.
A typical Scanning Transmission Electron Microscope (STEM-FEG-SEM) is displayed in Figure 1. This
images show that the PbSe nanocrystals have a well-controlled size and shape and can locally form ordered
close-packed arrays as indicated from calculated Fast Fourier Transformations (FFT) presented face to face.
In the scheme of one NCs layer, hexagonal arrangement is close to be perfect while for NCs bilayers, the
defects introduce by size distribution effect, rapidly break the simple hexagonal arrangement existing locally,
to generate new geometrical stackings i.e. grids, roses. STEM-SEM images are too noisy if observed at the
magnification necessary to make accurate measurements of the nanocrystals size. In contrast, TEM images
recorded in High Resolution mode using the Cs objective corrector makes it possible, even at rather low
magnifications. In this mode, one can observe several dozen of PbSe NCs which show lattice fringes and
well-defined outlines. It is thus possible to make a rather rapid determination of the shape, mean size, size
and spacing distribution. Typical histograms of the variation of NCs size and spacing between NCs displayed
figure 2, show that NCs size distribution is very narrow. More than 85% of the nanocrystals have diameters
ranging between 6.9 and 7.1 nm, with a mean size of 7.05 nm. In contrast, NCs spacings distribution is larger
and spreads from 1 to 4 nm with less than 50% equal to 2 nm. HREM images in figure 3, show that most of
the nanoparticles appear roughly spherical, but some of them i.e. thoses with the largest size, show facets.
Therefore, due to projection effects, even spherical nanocrystals can have more or less facetted shapes. Since
some NCs are naturally in [111] or [110] zone axis, the electron diffraction patterns calculated using FFT,
allow to determine that PbSe NCs are face centered cubic (FCC) with a Fm3m space group. Using the
ASTAR software, it is possible to make mapping of the crystal structure orientation in the transmission
mode, similarly to electron backscattering mapping in the scanning mode. Figure 4 shows the result obtained
for an HRTEM image of 39.2 nm 2 scanned with a window of 5.7 nm 2 . As indicated by the colour code used
to identify the orientation within the standard triangle, preferential orientations are clearly revealed. It may
be deduced that the z axis is not random but that the particles are free to rotate around. This result is in
agreement with the observations we did locally, which have shown that in close-packed arrays, nanocrystals
tend to form chains along the direction. Such a favoured arrangement had already been predicted and
theoretically explained on the basis of dipole moment values reported by Cho et al.[4] but had never clearly
observed.
2-2 Electronic properties of single and self-assembled nanocrystals. shows the single scattering distribution
spectra (SSDS) calculated from experimental low-energy loss spectra recorded from a single 7 nm PbSe
nanocrystal and a 2D self-assembled structure composed of about fifty PbSe nanocrystals with 7 nm mean
size, deposited on a 30 nm thick Si 3 N 4 film. This spectrum shows numerous features resolved in the very low
energy region (0 - 4 eV), each one corresponding to a more or less sharp intensity jump. As expected, spectra
recorded under similar conditions on the Si 3 N 4 film show the absence of structure in the energy range below
4.5 0.1 eV. Only a very weak and uniform background, due to the Bremstralung induces by the NCs
support is observed. Thus, the first sharp intensity jump in the NCs or self-assembled structure spectra
corresponds to E g i.e. the PbSe NCs band gap energy. For both, individual and simple hexagonal selfassembled
NCs an energy equal to 0.67 eV is measured. This value is consistent to those obtained from
absorption measurements performed in parallel on the same slef-assembled samples. These data also well
agree the theoretical values calculated by different authors [5-8-10].
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