Article pubs.acs.org/JPCC
Picosecond to Millisecond Transient Absorption Spectroscopy of Broad-Band Emitting Chiral CdSe Quantum Dots Michal Wojdyla,† Shane A. Gallagher,† Mícheál P. Moloney,† Yurii K. Gun’ko,*,† John M. Kelly,*,† Luis M. Magno,‡ Susan J. Quinn,*,‡ I. P. Clark,§ G. M. Greetham,§ and M. Towrie§ †
School of Chemistry, Trinity College Dublin, Dublin 2, Ireland School of Chemistry and Chemical Biology, University College Dublin, Dublin, Ireland § Central Laser Facility, Research Complex at Harwell, STFC, Rutherford Appleton Laboratory, Didcot, Oxon OX11 0QX, U.K. ‡
ABSTRACT: The relaxation dynamics of photoexcited carriers in watersoluble, penicillamine-capped, optically active and luminescent chiral CdSe quantum dots (QDs) were investigated. Three pump−probe techniques, namely broad-band UV−visible transient absorption (TA), picosecond time-resolved infrared (ps-TRIR) transient absorption, and nanosecond laser flash photolysis spectroscopy, were used to record transient decays from the picosecond up to the millisecond time scale. Picosecond experiments were carried out at a range of energies per unit area down to ca. 30 μJ cm−2, where only single photon excitation is expected. Whereas psUV−visible TA spectra show both bleach and transient features which are associated with depopulation of the lowest lying electron quantized state, the infrared transient bands are broad and structureless. The mid-IR transient absorption and excitonic bleach recovery is only partial, and its decay kinetics were found to be multiexponential in nature. The initial picosecond decay component is attributed to exciton-decay processes and to trapping by surface states. Nanosecond laser flash photolysis experiments have provided direct evidence for the presence of deep, long-lived states in the system. The origin of the ultrafast and long-lived species is discussed.
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INTRODUCTION Nanotechnology continues to push the boundaries of our understanding of the molecular origin of material properties, collectively invoking the efforts of researchers and engineers of various disciplines. The possibility to modulate and control photophysical properties arising due to quantum confinement effects (as the particle size decreases and becomes comparable or smaller than the Bohr exciton) opens the door to a range of designer materials with specialized properties. The recent rapid growth of interest in nanoscience is driven not only by the undoubtedly interesting properties of nanosized materials but also by the potential applications offered. Currently, quantum dots are considered as potentially exploitable in almost every modern technology including nanoelectronics, quantum computing, light energy conversion (solar cells), and display panel technology.1−18 QDs are also important candidates for medical applications in areas such as biosensing, imaging, and drug delivery.6,17,19−29 Quantum dots are usually stabilized by so-called capping ligands and other functional groups, which facilitate delivery of the particles into cells and allow the targeting of specific intracellular components.6,17,22−26,30−35 Such targeting could potentially be enhanced by using chiral substances, and remarkably it has been found possible to prepare both gold and silver nanoparticles in chiral forms.36−42 On the other hand, the area of chiral semiconductor nanoparticles is © XXXX American Chemical Society
currently in the relatively early stage of development. Indeed, the synthesis of chiral, optically active, light-emitting penicillamine (Pen)-stabilized CdS and CdSe QDs was relatively recently developed by our group,43−46 and other publications on chiral QDs are also available.47−50 Previously, we reported that our chiral quantum dots show complementary circular dichroism for the D- and L-penicillamine-stabilized CdSe, in the spectral region characteristic of the exciton bands.43,45,46 They are also luminescent with a very broad band emission. We have demonstrated that this emission is not due to intrinsic recombination of the hole−electron pairs but rather originates from surface-defect states.44 It was also found that the emission decays multiexponentially and the photoluminescence lifetime strongly depends on the wavelength of the emission. These results have suggested the presence of a wide range of emitting excited states in the electronic structure of the system.46 The defects play a crucial role in the behavior and properties of these chiral QDs because we believe that defect trap states are responsible both for CD responses and the emission properties of these QDs. Therefore, it is particularly important to study the types of electron (and hole) surface defects, their trapping mechanisms, and understand their impact on energy relaxation Received: March 9, 2012 Revised: July 4, 2012
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dx.doi.org/10.1021/jp3023088 | J. Phys. Chem. C XXXX, XXX, XXX−XXX
The Journal of Physical Chemistry C
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(model LP920) flash photolysis spectrometer. The samples were excited using a 308 nm line (pulse duration ∼20 ns, 1 Hz, 58 mJ/cm2) from a GAM excimer laser (model EX100). The pump beam from the laser was not focused, and its spot area was pinhole adjusted to about 0.6 cm2 at the sample. Transient transmittance changes were recorded using a xenon arc lamp (450 W) as a probe, working in pulsed mode and at right angles to the excitation beam. The probe beam was focused to less than about 3 mm of diameter. The lens and lens positions are chosen to achieve quasi-parallelism of the probe beam at the location of the sample. The deviation from parallelism is ±3 deg. Spectrometer was operated in either kinetic or spectral mode depending on required data. A photomultiplier R 928 and Andor ICCD (model DH501-18F-13) camera were used for signal detection. As expected, spectra obtained by data slicing were consistent with an ICCD recorded spectrum. Kinetic analysis was performed using the provided Edinburgh Instruments software.
and charge recombination processes in the chiral QDs. In this work we use UV−vis transient absorption complemented by picosecond-TRIR spectroscopy to gain insight into the ultrafast relaxation dynamics of these chiral CdSe QDs. Nanosecond flash photolysis spectroscopy has also been applied in order to probe longer lived species. Although these methods have been previously used to investigate the relaxation properties of the excitonic transitions in intrinsic CdSe nanoparticles and some core−shell QD structures,51,52 to the best of our knowledge the relaxation properties of defect dominated light-emitting chiral QDs have not previously been reported.
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EXPERIMENTAL SECTION Synthesis of CdSe QDs. Synthesis of penicillaminestabilized CdSe QDs was performed according to our previously published procedure.46 Briefly, 2 mL of an aqueous 1 × 10−2 M solution of D-, L- or rac-penicillamine was added to degassed Millipore water (40 mL) in a 100 mL flask, under a constant flow of Ar. The pH was adjusted to 11.5 by the dropwise addition of 1 M NaOH. 1 × 10−2 M Cd(ClO4)2·xH2O (2 mL) and 1 × 10−2 M of Na2SeO3 (2 mL) were then added, and the solution was stirred vigorously. The resulting homogeneous solution was then transferred to a CEM Star System 6 microwave and irradiated for 40 s at 1500 W. The resulting clear, yellow, solution was then stored in the dark for at least 1 day. The volume of the colloid was then reduced to ∼3 mL using the rotary evaporator, and propan-2-ol was added to precipitate out the nanoparticles. The particles were collected by centrifugation. The particles were washed several times with a propan-2-ol water mixture (9:1) and finally redispersed in Millipore water. UV−vis, CD, and fluorescence spectroscopy measurements were carried out on the stable suspensions in water. Ultrafast Transient Absorption and Laser Flash Photolysis Spectroscopy. The picosecond transient absorption pump−probe experiments were carried out using the highsensitivity ULTRA apparatus at the Central Laser Facility of the Science & Technology Facilities Council in the Rutherford Appleton Laboratory. Briefly, a titanium sapphire chirped pulse amplifier (custom developed by Thales Laser) is seeded with the 1.7 ms. Since deactivation of the 1S state for CdSe QDs should proceed on a time scale of tens of nanoseconds, this data suggests that below the emitting states (which have nanoseconds kinetics) there are a range of nonradiative, deep traps constituting the initial states for transient absorption (above 500 nm), and depopulation of these microsecond states occurs via new, longer lived (milliseconds range) state(s), which control the rate of slow E
dx.doi.org/10.1021/jp3023088 | J. Phys. Chem. C XXXX, XXX, XXX−XXX
The Journal of Physical Chemistry C
Article
unambiguously elucidated. Another nonlinear effect which should be considered is the possibility of biexcitonic transitions which are strongly enhanced through the presence of impurities and defects. Such transitions were predicted theoretically and experimentally observed as the induced absorption feature on the high-energy side of the bleached exciton.68,69 However, in our data there is no evidence for new absorption features (as compared to low-energy measurements) even at the highest energy used. More importantly, the shift appears to be independent of pulse energy, at least in the 5−100 nJ range, so its explanation via a Stark effect remains problematic. This shift could be due to size inhomogeneity but more probably arises from a shift in transient absorption which overlaps with the bleach. Another possible contribution to the observed shift can originate from inhomogeneity of the bleach. Overlapping with possible bleaching of higher lying interband optical transitions (which are supposed to decay faster) is expected to enhance the overall shift.67 In the microsecond time range the bleach position is invariant with time and deep levels (traps) seem to be the most likely reason for the observed transient absorption. These deeplying levels which make contribution to the extremely long bleach decay are probably associated with surface defects. The exact nature of these states and the trapping mechanisms requires further detailed study including an investigation at very low fluencies, so that one is in the single-photon excitation domain.
bleach recovery (on the nanosecond scale the bleach decays incompletely with kinetics comparable to photoluminescence). It is important to emphasize that the photoluminescence decay is essentially complete within 200 ns (depending on wavelength),46 and no luminescence signal has been detected on the micro- or millisecond time scale.
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DISCUSSION The above transient spectroscopic studies show that these CdSe penicillamine quantum dots possess complex kinetic behavior with species decaying with lifetimes ranging from subpicosecond to greater than a millisecond. This behavior is different from that of intrinsic quantum dots where complete deactivation occurs in less than 100 ns. Various processes may be responsible for the ultrafast dynamics. For colloidal, intrinsic CdSe quantum dots it is known that the ultrafast carrier dynamics are determined by Auger-type processes, which break down the expected phonon bottleneck effect. In particular, a subpicosecond intraband (1P to 1S) energy relaxation was reported even in the low pump intensity regime.62 However, quantum-confined Auger recombination (which is QD size dependent) dominates carrier dynamics in the regime of multiple e−h pair excitations. It has also been proposed to explain progressively faster interband relaxation dynamics measured for different multiple-pair states in CdSe QDs.56 In as much as direct electron intraband relaxation is an extremely fast process (
