On the synchrotron radiation source they could vary the photon energy to obtain
different bulk-to-surface sensitivity ratios, bulk sensitivity being higher for higher
photon energies due to an increased mean free path of the electrons [102]. The Cd
3d 5=2 spectra recorded with highly surface sensitive X-rays (hn ¼ 445 eV) revealed
two components at 406.1 and 406.6 eV. These two Cd groups are assigned to Cd in
the bulk of the nanocrystals and Cd on the surface bonded to sulfur of the capping
agent. The S 2p spectra for all three diferent sized nanocrystals show the three
components arising from bulk, surface and capping layer sulfur species. However,
there is a fourth component in the case of the smallest sized nanocrystals which
has been attributed to the disulfide linkages formed at the surface due to bonding
between the capping agent and the surface sulfur atoms. Disulfide linkages are
very few in number and can be observed only for very small sized nanocrystals
where the surface to bulk ratio is very high. The S 2p spectra for 40 and 70 A ˚
nanocrystals also showed spectral features due to the presence of an oxidized species at a higher binding energy (168.8 eV). In smaller nanocrystals, the number of
unsaturated sulfur atoms is less and hence this oxidation is not observed. Annealing leads to minimization of various defects, though it can also give rise to structural and chemical changes. The effect of annealing in ultra-high vacuum conditions, has been closely monitored for the same sized CdS nanocrystals using
photoemission spectroscopy [101]. Thermal annealing at 240
C modifies the surface from being sulfur-terminated at room temperature to a Cd-terminated one.
This could be a result of desorption of the capping material at such high temperatures and/or segregation of Cd atoms to the surface from the interior of the
nanocrystals.
ZnS passivated InP nanocrystals have been investigated by photoelectron spectroscopy with tunable synchrotron radiation [103]. Following essentially the same
ideas presented earlier by Nanda et al. [99, 100], [103] presents a method to characterize the core–shell nature of the nanocrystals and also determine the average
thickness of the various layers. Photoelectron spectra were recorded at different
photon energies, in order to tune the surface-to-bulk sensitivity ratio via a change
in the kinetic energy of the photo-emitted electrons. Surface sensitivity is higher at
lower kinetic energies, whereas higher kinetic energies show an enhanced sensitivity towards the atoms deep inside the nanocrystals [102]. The ratio of the core
level signals, namely In 3d to Zn 2p levels, increased from 0.06 to 0.15 as the
kinetic energy of the emitted photons increased from 87 eV to 247 eV, thus indicating the presence of Zn atoms on the surface and In atoms in the core of the
nanocrystals. Using the ratios of the intensities from the In and Zn core levels and
the fact that the organic capping layer trioctylphosphine (TOP) caps about 25–50%
of the nanocrystal surface, the authors also calculated the size of the different
shells. The diameter of the inner core of InP was fixed at 19 A ˚ measured from
TEM of InP nanocrystals without the ZnS coverage. The thickness of the ZnS shell
and the outer shell of TOP were calculated to be 8.9 A ˚ and 12.3 A ˚ , respectively. A
8.9 A ˚ thick ZnS layer corresponds to 3 monolayer coverage of the InP core by ZnS.
The length of the carbon chain in TOP is about 10.5 A ˚ . Hence, this method offers a
11 Electronic Structure and Spectroscopy of Semiconductor Nanocrystals
398
different bulk-to-surface sensitivity ratios, bulk sensitivity being higher for higher
photon energies due to an increased mean free path of the electrons [102]. The Cd
3d 5=2 spectra recorded with highly surface sensitive X-rays (hn ¼ 445 eV) revealed
two components at 406.1 and 406.6 eV. These two Cd groups are assigned to Cd in
the bulk of the nanocrystals and Cd on the surface bonded to sulfur of the capping
agent. The S 2p spectra for all three diferent sized nanocrystals show the three
components arising from bulk, surface and capping layer sulfur species. However,
there is a fourth component in the case of the smallest sized nanocrystals which
has been attributed to the disulfide linkages formed at the surface due to bonding
between the capping agent and the surface sulfur atoms. Disulfide linkages are
very few in number and can be observed only for very small sized nanocrystals
where the surface to bulk ratio is very high. The S 2p spectra for 40 and 70 A ˚
nanocrystals also showed spectral features due to the presence of an oxidized species at a higher binding energy (168.8 eV). In smaller nanocrystals, the number of
unsaturated sulfur atoms is less and hence this oxidation is not observed. Annealing leads to minimization of various defects, though it can also give rise to structural and chemical changes. The effect of annealing in ultra-high vacuum conditions, has been closely monitored for the same sized CdS nanocrystals using
photoemission spectroscopy [101]. Thermal annealing at 240
C modifies the surface from being sulfur-terminated at room temperature to a Cd-terminated one.
This could be a result of desorption of the capping material at such high temperatures and/or segregation of Cd atoms to the surface from the interior of the
nanocrystals.
ZnS passivated InP nanocrystals have been investigated by photoelectron spectroscopy with tunable synchrotron radiation [103]. Following essentially the same
ideas presented earlier by Nanda et al. [99, 100], [103] presents a method to characterize the core–shell nature of the nanocrystals and also determine the average
thickness of the various layers. Photoelectron spectra were recorded at different
photon energies, in order to tune the surface-to-bulk sensitivity ratio via a change
in the kinetic energy of the photo-emitted electrons. Surface sensitivity is higher at
lower kinetic energies, whereas higher kinetic energies show an enhanced sensitivity towards the atoms deep inside the nanocrystals [102]. The ratio of the core
level signals, namely In 3d to Zn 2p levels, increased from 0.06 to 0.15 as the
kinetic energy of the emitted photons increased from 87 eV to 247 eV, thus indicating the presence of Zn atoms on the surface and In atoms in the core of the
nanocrystals. Using the ratios of the intensities from the In and Zn core levels and
the fact that the organic capping layer trioctylphosphine (TOP) caps about 25–50%
of the nanocrystal surface, the authors also calculated the size of the different
shells. The diameter of the inner core of InP was fixed at 19 A ˚ measured from
TEM of InP nanocrystals without the ZnS coverage. The thickness of the ZnS shell
and the outer shell of TOP were calculated to be 8.9 A ˚ and 12.3 A ˚ , respectively. A
8.9 A ˚ thick ZnS layer corresponds to 3 monolayer coverage of the InP core by ZnS.
The length of the carbon chain in TOP is about 10.5 A ˚ . Hence, this method offers a
11 Electronic Structure and Spectroscopy of Semiconductor Nanocrystals
398