Intrinsic optical nonlinearity in colloidal seeded grown CdSe/CdS nanostructures: Photoinduced
screening of the internal electric field
Giovanni Morello,* Fabio Della Sala, Luigi Carbone, Liberato Manna, Giuseppe Maruccio,
Roberto Cingolani, and Milena De Giorgi
National Nanotechnology Laboratory (NNL) of CNR-INFM, Distretto Tecnologico ISUFI, Università del Salento,
Via per Arnesano, 73100 Lecce, Italy
Received 10 January 2008; revised manuscript received 15 October 2008; published 13 November 2008
The assessment of the presence and the origin of an intrinsic internal electric field in novel colloidal
CdSe/CdS nanoheterostructures is of fundamental importance in order to understand their optical properties,
due to both their impact on the basic research fields, and their potential in technological applications. To this
aim, a deep study of the carrier dynamics in spherical quantum dots and rod-shaped nanorods colloidal
seeded grown CdSe/CdS nanocrystals via time-resolved photoluminescence spectroscopy has been carried out
in this report. A transient, power-dependent redshift of the spectra is observed. An optical nonlinearity is also
found by continuous-wave photoluminescence measurements on ensemble and single nanostructures, which is
attributed to a photoinduced screening of an internal field. This internal field could originate from the intrinsic
piezoelectric polarization, which is a typical effect in strained heterostructures with a lattice mismatch greater
than 3.9%. Our theoretical calculations support the experimental results.
DOI: 10.1103/PhysRevB.78.195313 PACS numbers: 73.21.La, 78.67.Hc, 77.65.Ly, 78.47.Cd
I. INTRODUCTION
In the last few years, novel colloidal synthesis techniques
have allowed nanocrystals NCs of different sizes and
shapes to grow.
1,2
Due to their peculiar optical properties,
colloidal NCs show increasing potential as active compo-
nents in several types of devices, such as lasers,
3
photovol-
taic cells,
4
and light emitting diodes.
5
Besides spherical-
shaped quantum dots QDs, other materials with more
exotic shapes and compositions have been fabricated and
studied optically.
2,6,7
One of the most intriguing nanostruc-
tures is based on the growth of a CdS rodlike shell onto a
spherical CdSe QD.
8–10
Previous studies
11,12
show that in
such a system carriers might experience different types of
localization, since the hole remains confined inside the CdSe
dot, whereas the electron is completely delocalized through-
out the CdS nanorod. However, more recent optical studies
on such core/shell dot/rod NCs Ref. 9 have showed high
photoluminescence quantum yield PLQY and radiative re-
combination occurring in the core despite carriers being gen-
erated mainly in the shell. This idea that both the electron
and the hole are confined in the CdSe core has been very
recently confirmed by scanning tunneling spectroscopy
measurements.
13
Single rod experiments have revealed a
Stark shift induced by a fluctuating charge on the CdS
surface,
14
whereas time-resolved TR measurements have
demonstrated the viability for exciton storage in such
heterostructures.
15
To exploit the full potential of these new nanomaterials, a
deeper knowledge of their properties is however required.
Among the peculiarities of such nanostructures there is, for
instance, the possible existence of an internal field that may
affect their electrical and optical properties by means of a
Stark shift of the excitonic transitions. Several experiments
have studied the permanent dipole moment in colloidal QDs
and nanorods NRs with both wurtzite and zinc-blende
structures.
16–20
The well established existence of the perma-
nent ground-state dipole moment along the
c-crystallographic axis in CdSe wurtzite NRs Ref. 17 and
QDs Ref. 19 relies on the crystallographic deviation from
the ideal wurtzite structure and it has been found to scale
with the nanostructure volume.
17
On the other hand, a per-
manent dipole moment in zinc-blende nanostructures is at-
tributed to the existence of a surface charge distribution.
18
The surface charge density found by Krishnan et al.
20
should
not be present in our rods because the rod long axis is
aligned with the c-crystallographic axis.
8–10
When heterostructures are considered, a further contribu-
tion should be taken into account, which generates an inter-
nal field, namely, the piezoelectric polarization induced by
both the high piezoelectric constants and the elastic strain
due to the lattice mismatch greater than 3.9% between the
materials constituting the heterostructures.
21–23
Many studies
of the internal piezoelectric field have been carried out on
GaN-based nanomaterials, typically QDs Refs. 24 and 25
and quantum wells QWs,
26–30
on self-assembled InAs
QDs,
31
CdSe/CdS systems,
22
and ZnO-based QWs.
32
Continuous-wave CW and time-resolved photolumines-
cence TRPL studies of nanostructures having an internal
electric field generally show a blueshift of the spectra upon
increasing carrier generation rates. In particular, time-
resolved measurements have shown a dynamical redshift of
the photoluminescence PL spectrum in the multiexciton
generation regime due to a photoinduced descreening of the
internal electric field. So far, however, there has been no
evidence of such charge-carrier screening in colloidal nano-
structures.
In this paper we show the existence of an internal field in
nanosized colloidal CdSe/CdS heterostructures both spheri-
cal and asymmetric dot/rod core/shell structures by perform-
ing TRPL measurements at the picosecond time scale and
continuous-wave PL, both on ensemble and single nano-
structures, as a function of the excitation power. The experi-
mental results have been interpreted using results from
PHYSICAL REVIEW B 78, 195313 2008
1098-0121/2008/7819/1953138 ©2008 The American Physical Society 195313-1