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Current research interests of Tze-Chien SUM and his group
are on the development and application of time-resolved and time-integrated
optical spectroscopic techniques to interrogate the fundamental and
applied properties of materials – i.e. probing the energy and charge
transfer mechanisms in semiconducting nano-heterostructures
& light harvesting systems; as well as investigating the nonlinear
optical properties of nano-scale systems. A
brief description of the various ongoing projects can be found below.
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Our Research
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Multi-Photon Absorption &
Carrier Dynamics in seeded CdSe/CdS Nanorod Heterostructures
 Over the last two decades, multi-photon absorption (MPA)
in colloidal semiconductor quantum dots (QDs) has been intensively
investigated for potential applications in bio-imaging, upconversion lasing, three dimensional data
storage and optical limiting. Increasing the MPA cross-sections of
QDs without significantly degrading its quantum yield or altering its
emission wavelength can be highly desirable for example, in
multi-photon fluorescence imaging where greater signal may be
achieved using less average incident power, thus minimizing sample
damage. While the pronounced size-dependence of the emission of
fluorescent QDs in the strong confinement regime presents a
convenient way to achieve desired emission wavelengths by simply
changing the dot size, however, it also simultaneously imposes severe
restrictions on the ability to vary the absorption cross-section
while maintaining the emission at a required wavelength. Thus from
the stand point of wavelength-specific applications, increasing the
MPA cross-section of a QD without significantly modifying its
size-dependent emission is an important and yet non-trivial challenge
to overcome.
Recently, we
presented a strategy that permits the independent tuning of the MPA
cross-section and the luminescence properties using semiconductor
core/enlarged-shell QDs. We demonstrate this with a representative CdSe/CdS nanodot/nanorod system.
The elongated CdS shell is used as a
photon-capturing “antenna”, which can greatly enhance the overall MPA
cross-section of the QD. 2PA and 3PA cross-sections 2-3 orders larger compared to
CdSe/CdS QDs can
be achieved using these seeded CdSe/CdS nanorod heterostructures.
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Furthermore,
we also proposed a simple model
(black line) which accounts for the effective 3PA absorption
cross-sections for various II-VI semiconductor quantum dots
obtained with different lasers pulse widths – i.e. comprising of
contributions from the intrinsic s3 and the excited states absorption.
Importantly, this unifying
picture provides users with a clear
basis of comparison for the 3PA
cross-sections obtained with different laser pulse widths for
various II-VI semiconductor quantum dots. This will facilitate
users to make a more judicious choice for their use in MPA
applications.
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References:
(a)
G. C. Xing, S. Chakrabortty, K. L. Chou, N. Mishra, C. H. A. Huan, Y. Chan and T. C.
Sum*, “Enhanced
Tunability of the Multi-photon Absorption Cross-Section in Seeded CdSe/CdS Nanorod Heterostructures”, Applied Physics
Letters 97 061112 (2010) - DOI:
10.1063/1.3479048 (See also Virtual Journal of Nanoscale Science
& Technology, 22,
No. 9 (2010))
(b)
G. C. Xing, S. Chakrabortty, S. W. Ngiam, Y. Chan and T. C.
Sum*,
“Three-photon absorption in Seeded CdSe/CdS Nanorod Heterostructures”, Journal of Physical
Chemistry C, 115, 17711-17716 (2011) - DOI:
10.1021/jp205238q
(c)
S. Chakrabortty, G. C. Xing, Y. Xu, S. W.
Ngiam, N. Mishra, T. C. Sum and Y.
Chan, “Engineering Fluorescence in Au-Tipped, CdSe-Seeded CdS Nanoheterostructures”, Small, 7
2847–2852 (2011) - DOI:
10.1002/smll.201100976
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The Physics of Ultrafast Saturable Absorption in Graphene
 Graphene is new class
of single atom thick materials which possesses a unique smooth-sided conical
band structure that converges to a single Dirac point. In the recent
years, graphene has been subjected to extensive studies. Among these
studies, there are several reports on the effects of weak light
interaction with graphene.
Within the
model of the non-interacting, massless
Dirac fermions, the weak light absorption is calculated to be
independent of frequency and to have a universal opacity,  (where  is the fine structure constant).
This theoretical prediction has been confirmed by recent infrared to
visible reflectivity and transmission measurements. However, studies
on the coupling between photons and fermions in graphene under high
intensity, ultrashort laser pulse
excitation are limited. In particular, the relationship between the
nonlinear optical properties of graphene and the basic fine structure
constant has never been investigated. In this work, we experimentally
characterize and theoretically model ultrafast saturable
absorption in graphene in the femtosecond time regime. This
phenomenon is well-modeled with valence
band depletion, conduction band filling and ultrafast intraband carrier thermalization.
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Figure: (a) The momentum conserved photon
absorption process in graphene (b) Instantaneous photon absorption generate nonequilibrium distributions of
electrons (blue) and holes (red),
which subsequently thermalize through
ultrafast intraband carrier-carrier and
carrier-optical phonon scattering to form Fermi-Dirac distributions
(c) Typical nonlinear transmittance of graphene layers at different
Z position (circle dots) and the theoretical predictions with Eq. (11) using different τ1.
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With our method
of z-scan measurements in conjunction with theoretical calculations,
the c-c relaxation time is determined to be ~8 (±3) fs, Such ultrafast
c-c dynamics is far beyond the time resolution of other ultrafast
techniques with typical hundred fs laser
pulses. This is much simpler technique for probing ultrafast c-c
dynamics than one employing a pump-probe setup with 5 fs pulses.
References:
(a)
G. C. Xing, H. C Guo, X. H. Zhang, T. C. Sum* and C. H. A. Huan, “The Physics of ultrafast saturable absorption
in graphene”, Optics
Express 18 4564 (2010) - DOI:
10.1364/OE.18.004564 - An OSA Spotlight on Optics article
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Charge
Transfer Dynamics in Surface Treated ZnO Nanowires
 One-dimensional
(1-D) systems and quasi 1-D systems such as nanowires and nanorods have
attracted much attention in the recent years due to their potential
as building blocks for nanoscale transistors, sensors and
optoelectronic devices. In particular, ZnO nanowires (NWs) have
received considerable attention due to their unique properties (i.e.,
wide bandgap, large exciton binding energy etc.). With a high surface
to volume ratio in NWs, surface defects, near surface traps and
surface adsorbed species, offering alternative relaxation
pathways for the dexcitation
of photo-excited carriers, will play a significant role in the carrier
relaxation dynamics of these 1-D systems. Many of the native point
defects (oxygen vacancies (VO), Zn vacancies (VZn),
oxygen antisites (OZn), Zn interstitials (Zni),
etc) are formed during the NW
growth and their concentrations are highly dependent on the growth
conditions and post-fabrication treatments. Post-fabrication thermal
annealing of ZnO NWs in an oxidizing/reducing gas ambient is commonly
used to tune the intrinsic (or native) defects, modify the surface
states and modulate the carrier concentrations; effecting changes to
the optical and electrical properties of these NWs that result in
enhanced gas sensitivity, field emission properties,
photoconductivity, and photocatalytic
activity.
 Recently,
our findings reveal an ultrafast hole-transfer process to the surface
adsorbed oxygen species (e.g., O2-) occurring
within a few hundred picoseconds (ps) in the air-annealed samples;
and an ultrafast electron-transfer process to charged oxygen
vacancies (i.e., VO2+) occurring within tens of
ps in the H2-annealed samples – see Fig above. Contrary
to the common perception that the bandedge emission (BE) dynamics are
strongly influenced by the carrier trapping to the green emission
related defect states (i.e., VZn), these above processes
compete effectively with the ZnO BE. Hole trapping by ionized VZn,
which occurs in an ultrashort sub-ps-to-ps timescale (and hence limits its
effective hole capture radius), however,
has less influence on the BE dynamics. Importantly,
our findings shed new light
on the photoinduced charge transfer processes that underpins the
novel properties of enhanced photocatalytic activity, photovoltaic
performance, and photoconductivity response of ZnO NWs; thereby
suggesting a strategy for tailoring the ultrafast carrier dynamics in
ZnO NW-based devices.
References:
(a)
M. J. Li, G. C. Xing, L. F.
N. Ah Qune, G. Z. Xing, T. Wu, C. H. A. Huan, X. H. Zhang and T. C. Sum*, “Tailoring the Charge Carrier Dynamics in
ZnO Nanowires”, Physical
Chemistry Chemical Physics, 14,
3075 - 3082 (2012) - DOI:
10.1039/C2CP23425D
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Ultrafast Charge Transfer in
Cu-doped ZnO Nanowires
Copper is one of the most pervasive and important impurities
in ZnO. Over the past four decades, there
have been extensive studies on this CuZn
defect which manifest itself as the green luminescence (GL) band
peaking at ~2.45eV. At low temperatures, the GL exhibits a distinct
phonon-related fine structure and a zero-phonon line at ~ 2.86 eV. The origin of this GL band is attributed to
the [Cu+ (d9
+ e), h] ® [Cu2+ (d9)]
+ hv
charge transfer transitions where the hole is transferred from a
level highly perturbed by the surrounding oxygen to the highly
shielded d shell of the
copper atom. In this intermediately bound exciton
model, the electron wavefunction of the
tenth electron in the Cu+ ion is delocalized due to the
hybridization of the d
states with the bottom of the conduction band, thus being depicted
as: [Cu+ (d9
+ e), h]. While there are general consensuses
that the CuZn transitions are of
the charge transfer (CT) type, direct experimental evidence of the CT
process and the CT rate between the ZnO
host and the Cu subsystem has not been reported. Investigating the
dynamics of this CT mechanism is the main objective of this work. Ultrafast optical spectroscopy
reveal the presence of an ultrafast CT process between the ZnO host and the Cu dopants
following above-bandgap photoexcitation.
An electron capture lifetime of 39 ± 9 ps
by Cu2+ (d9)
was measured.
Figure: A schematic of the phenomenological model and the
relaxation pathways in undoped ZnO (a) and CuZnO (b)
samples.
(c) PL decay of the band
edge and Cu-related SGL emission from the H-ZnCuO
sample at 10 K. Solid lines are fits to the data. Dashed line shows
the system temporal response. Inset shows the temperature dependent
plot of the band edge (UV) decay time and the rise time of the SGL
band. (d) TA decay profiles of the undoped ZnO and H-ZnCuO sample
with 325 nm pump/500 nm probe pulses at 10 K. Solid lines are fits to
the data.
References:
(a) G. Z. Xing, G. C. Xing, M. J. Li, E. J. Sie, D. D. Wang, A. Sulistio, Q. L. Ye, C. H. A. Huan, T. Wu and T. C.
Sum*, “Charge
Transfer Dynamics in Cu-doped ZnO nanowires”, Applied Physics
Letters 98 (2011) - DOI:
10.1063/1.3558912 (See also Virtual
Journal of Nanoscale Science & Technology,
23, No. 11 (2011))
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