ARTICLE pubs.acs.org/JPCC
Primary Photodynamics of Water-Solubilized Two-Dimensional CdSe Nanoribbons Arthur Thibert, F. Andrew Frame, Erik Busby, and Delmar S. Larsen* Department of Chemistry, University of California, One Shields Ave, Davis, California 95616, United States
bS Supporting Information ABSTRACT: The primary photodynamics of water-solubilized 2D CdSe nanoribbons (NRs) was characterized with femtosecond transient absorption spectroscopy and modeled using global analysis. The measured signals were decomposed into the constituent dynamics of three transient populations: hot tightly bound excitons, relaxed tightly bound excitons, and separated trapped carriers (holes and electrons). The influences of three external factors affecting the observed dynamics were explored: (1) excitation wavelength, (2) excitation fluence, and (3) presence of the hole scavenger HS. Both higher-energy excitation photons and higher-intensity excitation induce slower relaxation of charge carriers to the band edge due to the need to dissipate excess excitation energy. Nonlinear decay kinetics of the relaxed exciton population is observed and demonstrated to arise from bimolecular trapping of excitons with low-density trap sites located at CdSe NR surface sites instead of the commonly resolved multiparticle Auger recombination mechanism. This is supported by the observed linear excitation-fluence dependence of the trapped-carrier population that is numerically simulated and found to deviate from the excitation fluence dependence expected of Auger recombination kinetics. Introducing hole scavenging HS has a negligible effect on the exciton kinetics, including migration and dissociation, and instead passivates surface trap states to induce the rapid elimination of holes after exciton dissociation. This increases the lifetime of the reactive electron population and increases measured photocatalytic H2 generation activity. A broad (200 nm) and persistent (20 ps) stimulated emission observed in the tightly bound excitons suggests their potential use as broadband microlasers.
1. INTRODUCTION Quantum confined nanomaterials have garnered considerable attention in recent years due to their potential applications in optoelectronics,1,2 photovoltaics,3,4 and photocatalytic5,6 applications.712 Cadmium-based nanomaterials are particularly useful in photovoltaic applications for generating photocurrent under visible excitation,3,13,14 demonstrated recently by Kamat and coworkers who showed that CdSe quantum dot sensitized TiO2 nanoparticles exhibit size-dependent electron injection rates, where smaller diameter quantum dots increase the free energy difference and facilitate quicker charge transfer to yield higher photovoltaic efficiencies.4 Another important application of semiconductor nanomaterials is their use in photocatalysis, as originally demonstrated by Fujishima and Honda, who used TiO2 to photogenerate H2 from water without the application of electrical bias.15 Previously, photocatalysis has been demonstrated with semiconductor-based nanoparticles1618 including ZnSe nanoribbons (NRs)19 and 2D CdSe nanoribbons that exhibit photocatalytic H2 evolution activity in aqueous solution.20,21 These CdSe NRs consist of stacked 1.5 nm thick CdSe nanobelts with octylamine filling the interstitial space between layers, which form overall widths and lengths of 2040 nm and >1 μm, respectively (Figure 1B).20,21 The uniform thickness dimension (1.5 nm) of each nanobelt in a multilayer “ribbon” stack imparts r 2011 American Chemical Society
quantum confinement to the photogenerated excitons (Figure 1A) and increases the NRs bandgap (2.7 eV) over that of bulk CdSe (1.7 eV).21 This increase in bandgap (and the corresponding electron free energy) is responsible for the observed NR photoactivity, which is enhanced 116-fold by introducing Na2S:Na2SO3 (HS),20 where reactive HS serves as a sacrificial electron donor that depletes the hole population and inhibits electron/hole recombination kinetics. H2 evolution activity of CdSe NRs can be further augmented by coupling them to low concentrations of MoS2 nanoplates that decrease the electrochemical proton reduction overpotential.21 It is possible that specific surface-related sites on the NRs that are absent in bulk CdSe serve as points for adsorption and catalytic reduction, but currently such evidence has not been reported. The primary and secondary charge dynamics of electron and hole carriers strongly affect the utility and efficiency of nanoscale materials, which are sensitive to size, surface properties, and morphology.22,23 For example, charge carriers induced in multidimensional anisotropic nanoparticles (e.g., quantum rods, nanosheets) can experience directionally dependent Columbic interactions as a result of size variation in a given dimension.24 When the Received: July 17, 2011 Revised: August 24, 2011 Published: August 30, 2011 19647
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Scheme 1. Synthetic Scheme for Water-Solubilized CdSe Nanoribbons
Figure 1. (A) Structural model of 1.5 nm thick CdSe nanoribbons depicted as alternating layers of CdSe and passivating octylamine ligands. (B) TEM image of CdSe nanoribbons showing ribbons that are micrometers in length with multiple kinks and variations in width throughout the length of the ribbon.
nanoparticle dimensions are greater than three times the exciton Bohr radius (e.g., 5 nm for CdSe),25 a weak confinement is observed and photogenerated excitons exist as strongly bound electronhole pairs, whereas excitons generated in nanoparticles with dimensions smaller than the exciton Bohr radius are strongly confined.23 Strong confinement leads to weaker Columbic interactions between electronhole pairs compared with their confinement energy (band gap perturbation), thus allowing them to be treated independently in theory with the Coulombic term accounted for as a small perturbation.26 Particles with dimensions between these two extremes experience Columbic interactions on par with their confinement energy. Therefore, anisotroptic 2D NR materials are particularly interesting to explore because their photoinitiated charge carriers experience the strong confinement (thickness) necessary to increase sufficiently the free energy of conduction band electrons to photoreduce water and the weak confinement (length and width) needed to facilitate effective delocalization of longer-living excitons and separated charges.27 The primary photodynamics of semiconductor nanopartcles, including the CdSe NRs explored here, typically extend from femtoseconds to nanoseconds and include rich dynamics such as exciton formation, relaxation, migration, dissociation, and charge separation; with recombination strongly modulating the measured photocatalytic efficiencies.28 Here we investigate the primary photodynamics of octylamine-capped CdSe NRs measured with dispersed transient absorption measurements to resolve the primary processes (100 fs to 8 ns) that affect photocatalytic H2 activity. This work expands on a previous study20 by characterizing the transient signals with a multiwavelength global analysis approach and exploring the influence of three factors on the photodynamics: (1) excitation wavelength dependence (400 vs 460 nm), (2) excitation fluence dependence, and (3) the effects of added HS sacrificial electron donor. Numerical simulations are presented to address the influence of Auger recombination kinetics in the measured signals.
2. EXPERIMENTAL SECTION CdSe NR samples were prepared in the Osterloh laboratory (UC Davis) according to the following procedure. Cadmium chloride was purchased from Baker and Adamson (>99% purity), selenium powder from Aldrich (99.999% purity), hexadecylamine from Acros (>90% purity), trioctylphosphine from Aldrich
(technical grade, 90%), octylamine from Acros (>99% purity), and carbon monoxide gas from Airgas (chemically pure grade) were used without further purification. All samples were suspended in H2O that was purified by a Nanopure II system to a resistivity of >18 MΩ. The NRs were synthesized according to the published method by Joo et al.59 and our previous CdSe NR photocatalytic protocol (Scheme 1).20,21 The reaction mixture was maintained at 70 °C for 24 h, and eight washing cycles were used to remove the selenium starting material under an inert N2 atmosphere. The resulting bright-yellow CdSe NR paste was collected and stored under dark conditions in a sealed flask and was not dried before experimental studies. The ribbons were coated with octylamine and assembled into 1.5 nm nanowire-like stacks that are resolved via transmission electron microscopy (TEM) (Figure 1B). TEM measurements were conducted on a Philips CM120 TEM operating at 80 keV. Samples were deposited onto holey carbon-coated Cu grids and washed with ethanol (or chloroform) before drying at 80 °C for 0.5 h. UVvis/emission spectra were collected on Ocean Optics DH2000 light source, HR2000 CG-UVNIR spectrometer, and a Yobin Ivon Fluoromax-P fluororimeter. Water-solubilized NR samples were prepared by diluting 1 mL of stock solution (0.3% weight) with N2 purged solutions of 7 mL of Na2S:Na2SO3 (0.1 M) or 7 mL of H2O for studies conducted with and without HS, respectively. Samples were weakly sonicated for 5 min immediately prior to use and were run back-to-back for each experimental condition (i.e., excitation wavelength, excitation fluence, added HS). CdSe NRs solutions were continuously flowed through a 1 mm path-length flow cell (Starna Cells) via a peristaltic pump (Watson-Marlow 400) throughout the transient absorption experiments. The NRs tended to precipitate out of solution and adhere to the flow lines, which resulted in slowly decreasing and noisy signals as the experiment progressed. To avoid this, the sample flow lines were preconditioned prior to data collection with NRs to coat the inner walls of the flow lines completely. Data collection was performed quickly (∼10 min) by minimizing the dwell time at each data point; overlapping individual kinetic traces from different experimental runs exhibited nearidentical kinetics (not shown). The dispersed-probe transient absorption setup was constructed from an amplified Ti:sapphire laser system (Spectra Physics Spitfire Pro+Tsunami) operating at 1 kHz, which produced 2.25 mJ pulses of 800 nm fundamental output with a 40 fs (full width at halfmaximum) duration.28 The 400 nm excitation pulses were produced by frequency doubling the fundamental in a 1 mm path-length β-barium borate crystal (CasTech, Φ = 29.2°). The 460 nm excitation pulses were generated via frequency doubling the near-infrared idler (at 920 nm) from a visible-wavelength home-built noncollinear optical parametric amplifier (NOPA). The pump pulses were linearly polarized at 54.7° (magic angle) with respect to the probe pulses. Pump pulse spot diameters of 200 (460 nm initiated signals) and 360 μm (400 nm initiated signals) were estimated using a micrometer stage and razor blade; the broadband probe pulses were focused to ∼50 μm. An approximate number of excitations per NR could not be estimated because of strong inhomogeneity of particle size making it 19648
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Figure 2. UVvis absorption (black curve) and emission spectra of CdSe nanoribbons in H2O (green curve) and 0.1 M H2O/HS (red curve). The HS does not affect the absorption spectrum (not shown).
difficult to determine particle concentration. An instrument response functions of 150 and 200 fs were estimated for the 400 and 460 nm initiated signals by the rise of the stimulated Raman Stokes scattering of water and the numerical fitting of the transient data.
3. RESULTS 3.1. Static and Transient Signals. The static absorption and emission spectra of the CdSe NRs suspended in both H2O and H2O/HS solutions exhibit sharp absorption bands at 450, 425, and 370 nm corresponding to the 1B 1e, 1A 1e, and 2B 2e excitonic transitions, respectively (Figure 2). These assignments are adopted from previous reports on CdSe nanoplatelets (single-layer brick-like nanostructures of tunable thickness)29 and nanobelts (exfoliated layers of the NRs examined here)27 where A = light hole, B = heavy hole, and e = electron are designations associated with semiconductors containing hexagonal symmetry such as GaAs30 and CdSe.31 More complete discussions of the nature of light and heavy hole states on CdSe quantum dots can be found elsewhere.23,26 The CdSe NR absorption band edge lies at 460 nm (2.7 eV),21 as estimated from the half-height of the lowest energy 1B 1e transition (Figure 2, black curve). The photoluminescence (PL) spectrum exhibits a narrow peak at 455 nm in both H2O and H2O/HS solutions with a broad, weak emission band peaking at 575 nm (Figure 2, red and green curves) that is assigned to deep trap emission.3235 Introducing HS has no effect on the absorption spectra (not shown) and slightly increases the relative amplitude of the deep trap PL emission (Figure 2, red curve). For 400 nm excitation transient spectra (Figure 3), the two sharp bands in the static absorption spectra (Figure 2) are observed as negative bleaches at 455 (1B 1e) and 426 nm (1A 1e). At early times (30 ps), the stimulated emission is no longer observed and is replaced by a new weak induced absorption feature at ∼460 nm (Figure 3 insets). This spectrum is attributed to separated (trapped) charge carriers (electrons and holes) resulting from exciton dissociation (vide infra). The 445 nm induced
Figure 3. Transient spectral evolution observed for CdSe NRs excited at 400 nm in the absence (A) and presence (B) of HS at a fluence of 3.4 μJ/pulse 3 mm2. (C) Comparing the first derivative of the static absorption spectrum of CdSe NRs in H2O with their 6 ps transient absorption spectrum; the first derivative is proportional to the stark difference spectrum. Insets follows the color coding of the main panels.
absorption itself closely resembles the first derivative of the static absorption spectrum (Figure 3C), which is a measure of the Stark difference spectrum.36 Introducing HS decreases the relative amplitude of the 455 nm bleach compared with the stimulated emission and induced absorption signals (Figure 3B, black curve) in the early exciton spectrum. For the long-time trap carrier spectrum, adding HS decreases the relative amplitude of the 460 nm induced absorption compared with the 445 nm induced absorption (Figure 3B, orange curve). Select single wavelength kinetic traces contrasting the dynamics of CdSe NRs with and without HS are shown in Figure 4, which indicate that HS has a negligible effect on the exciton dynamics. Both the 428 and 455 nm bleaches exhibit slightly slower recovery kinetics at >5 ps in H2O/HS (Figure 4B,C). The 417 nm induced absorption exhibits a ∼500 fs rise time in H2O that is noticeably absent in the H2O/HS traces (Figure 4A). The 460 nm initiated signals are qualitatively similar to the 400 nm initiated signals (Figure 5) and differ only slightly within the first few picoseconds after excitation (Figure 5). The 500 fs rise time observed in the 400 nm data at 417 and 443 nm is eliminated in the 460 nm excitation data (Figure 5A,C). The stimulated emission kinetics also exhibit a similar 500 fs rise time after 400 nm excitation, which is absent after 460 nm excitation (Figure 5F). Both the 429 and 455 nm bleaches exhibit faster decays at early times with 400 nm excitation (Figure 5B,D). Deviations from the fit for the 455 nm bleach and the 460 nm 19649
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Figure 4. Select transient kinetic traces of CdSe NRs in the absence (solid circles, orange) and presence of H2O/HS (open circles, cyan) (AF). Solid lines are fits extracted from global analysis outlined in Section 3.3. Flux = 3.4 μJ/pulse 3 mm2. (C, inset) A zoomed in depiction of the signals at long time delays emphasizing that the offset observed for both bleaches is about the same. The HS signals are scaled to the H2O signals in all panels according to the values given in parentheses in the legends to aid in visual comparison.
induced absorption traces originate from interfering scatter of the 460 nm excitation pulse. 3.2. Global Analysis. In lieu of discussing individual kinetic traces or transient spectra, the multiwavelength dynamics are compared and interpreted within a global analysis formulism.37,38 Similar approaches have been used for analyzing several semiconductor nanoparticle systems including CdSe quantum dots,39 TiO2 nanoparticles,40,41 and hybrid silica-coated CdTe nanocrystals.42 This approach globally fits the data to an underlying postulated multipopulation “Target” model and estimates the concentration profiles and species associated difference spectra (SADS) of the constituent populations. This is accomplished by fitting the data with numerical solutions of linear first-order differential equations of the postulated model (eq 1) dni ¼ Ai IðtÞ þ dt
∑j Kij nj
ð1Þ
where ni represents the population of interest, AiI(t) is the pump pulse width, and Kij is the rate constant matrix describing the exponential flow of population from one compartment into another.43,44 If the underlying model accurately describes the underlying dynamics, then the extracted spectra for the populations are SADS and represent the true difference spectra of the constituent populations. However, if the model is inaccurate, then the resulting spectra from the global fit are evolutionary associated difference spectra (EADS), which are combinations of the underlying SADS that do not represent the pure spectra of the underlying populations. The integrated solutions to the linear differential equations in eq 1 are exponentially evolving
populations, and although nonexponential kinetics is common in semiconductor nanoparticle studies (and the CdSe NR signals discussed above), this approach can still be used to model the dynamics through expansion into a basis set of exponential functions, as demonstrated below.45 We refer to all estimated global analysis spectra of the CdSe NR studied here as SADS. Although a four-compartment sequential model can adequately describe the measured NR data (Figures SI3 and SI4 in the Supporting Information) over the entire spectral and temporal range a branched model is favored in the analysis (Scheme 2, Table 1). This is because the branched model addresses (phenomenlogically) the observed nonexponential kinetics and is more accurate in describing the underlying photodynamics. This model includes four populations: hExciton, cExciton1, c Exciton2, and trCarrier, in which the initial hExciton population represents unrelaxed or non-band-edge (“hot”) excitons that relax into a cExciton1 population that represents relaxed bandedge (“cool”) excitons (Figure 6A). Both cExciton1 and cExciton2 populations evolve into the trCarrier population, which corresponds to separated (and trapped) electron/hole carriers after exciton dissociation. The nonexponential evolution of the cExciton1 decay is accounted for via the second cExciton2 population, both of which generate the trCarriers population. In the 460 nm excitation signals, the SADS for the two cExciton populations are identical, indicating that cExciton2 is an artifact of fitting nonlinear kinetics with exponential basis functions and does not represent a distinct microscopic population. Although the SADS for cExciton1 and cExciton2 populations in the 460 nm excitation data are identical, in the 400 nm excitation data they differ slightly, suggesting 19650
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Figure 5. Select transient kinetic traces of CdSe NRs under 400 (solid circles, orange) and 460 nm excitation (open circles, cyan) in H2O (AF). Solid lines are fits extracted from global analysis. Scaling factors were selected to emphasize the similarities and differences at different times. Deviations from the fit at long times for the 455 nm bleach and the 460 nm induced absorption result from the overlapping pump pulse in this region of the spectrum. Flux = 3.4 and 6.2 μJ/pulse 3 mm2 for 400 and 460 nm excitation, respectively. The 460 nm excitation signals are scaled to the 400 nm signals to aid in comparison, as indicated by the scaling values in parentheses in the legends.
Table 1. Time Constants Associated with the Decay of hExciton, cExciton1, cExciton2, and trCarrier NR Populations Extracted from Global Analysis According to Scheme 2 λpump (nm)
solvent
flux (μJ/pulse 3 mm2)
h
c
exciton
exciton1
c
exciton2
tr
carrier
500 fs
22 ps (cExciton2) 8 ps (trCarrier)
20 ps
1 ps
12 ps (cExciton2) 8 ps (trCarrier)
14 ps
5 ns
3.4 1.5
500 fs 140 fs
18 ps (cExciton2) 8 ps (trCarrier) 600 fs (cExciton2) 770 fs (trCarrier)
20 ps 19 ps
8 ns 5 ns
6.2
200 fs
300 fs (cExciton2) 1.8 ps (trCarrier)
16 ps
5 ns
400
H2O
3.4
400
H2O
11.8
400 460
H2O/HS H2O
460
H2O
that a slower spectral evolution of the excitons coexists (hExciton1 f c Exciton1) with the exciton dissociation kinetics that form the tr Carrier population (cExciton1 f trCarriers). The estimated SADS for the CdSe NR dynamics in H2O after 400 and 460 nm excitation are contrasted in Figures 6 (normalized) and SI5 of the Supporting Information (nonnormalized) along with their corresponding concentration profiles. As expected from the raw spectra in Figure 3, the h Exciton and cExciton SADS are qualitatively similar and differ greatly from the trCarrier SADS. The induced absorption at 445 nm exhibits growth in the 400 nm excitation data prior to evolving into the final trCarrier population (Figure 6A). In contrast, 460 nm excitation data exhibit only decay of the
5 ns
Scheme 2. Global Analysis Modela
ah
Exciton = hot exciton, cExciton1 = cool exciton, cExciton2 = superposition of cExciton1 and trCarrier, trCarrier = trapped electron and hole carriers resulting from exciton disassociation. For 460 nm excitation, the SADS of the cExciton1 and cExciton2 populations are locked to one another because they are exactly the same. In contrast, the SADS in the 400 nm excitation data are slightly different and therefore are not locked to one another.
induced absorption at 445 nm (Figure 6B). The stimulated emission of cExciton1 also grows in under both excitation 19651
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Figure 6. Normalized SADS and concentration profiles of CdSe nanoribbons in H2O after 400 (A,C) and 460 nm (B,D) excitation calculated by fitting the raw data to eq 1 with the model in Scheme 2. The concentration profiles for the cExciton1 and cExciton2 are combined to represent the nonexponential decay kinetics.
Figure 7. (A) Normalized SADS of CdSe nanoribbons in H2O/HS normalized at the 455 nm bleach. (B) Concentration profiles of CdSe nanoribbons in H2O (solid curves) and H2O/HS (dashed curves) after 400 nm pulsed excitation. Flux was 3.4 μJ/pulse 3 mm2.
wavelength conditions, although appreciably quicker after 460 nm excitation (200 fs) than 400 nm excitation (500 fs). Individual c Exciton1 and cExciton2 concentration profiles have been combined (Figures 6C,D, and 7B) because they represent different relaxation phases of the same population (Scheme 2).
Adding HS primarily affects the SADS and only weakly affects the concentration kinetics (Figure 7) of the exciton populations. In particular, the final trCarrier population exhibits a decreased absorption at 460 nm relative to the induced absorption at 445 nm in H2O/HS (Figures 6A and 7A, blue curves). Also, the trCarrier relaxes slower (8 ns) than the same population in pure H2O (5 ns) (Figure 7B, blue curves). Individual cExciton1 and cExciton2 concentration profiles have been combined (Figures 6C,D, and 7B) because they represent different relaxation phases of the same exciton population. 3.3. Excitation Fluence Dependence. The two excitation fluence-dependent data sets contrasted in Figure 6C,D demonstrate a peculiar trend at early time, whereby higher excitation fluence slows the sub-picosecond relaxation (hExciton1 f c Exciton1) with a negligible effect on the long time decay kinetics. The slower relaxation of the hExciton population (black curves) results in the slower growth of the cExciton1 population (red curves) and trCarrier population (blue curves). The decay of the tr Carrier population is relatively unchanged with regards to excitation fluence (blue curves). The trCarrier SADS is similar to the red-shifted signals Klimov and coworkers observed in CdS nanocrystals, which were ascribed to the promotion of holes into surface traps via nonlinear Augur recombination.46,47 To evaluate whether a similar mechanism is responsible for the generation of the trCarrier population in the CdSe NRs signals, the individual 1.6 and 32 ps transient spectra were measured over a wider range of excitation intensities (Figure 8). As excitation fluence is increased, the induced absorption at 445 nm decreases, and the 460 nm induced absorption increases in the 32 ps spectrum (Figure 8B,D), which indicates that the cExciton to trCarrier transition is enhanced. At 1.6 ps, the induced absorption at 445 nm similarly decreases as excitation fluence is raised (Figure 8A,C), accompanied by a decaying stimulated emission at 460 nm (Figure 8A) that is left relatively unchanged upon the addition of HS (Figure 8C). 19652
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Figure 8. Spectral power dependence of NRs in H2O (A,B) and HS (C,D) at 1.6 and 32 ps. Arrows indicate in what direction change is being observed as pulse energy is increased. Insets represent non-normalized depictions of the data. Data labels for all panels and insets are the same as those in panel A.
Figure 9. Power-dependent decomposition of CdSe NRs at (A) 1.6 and (B) 32 ps in H2O. The spectra collected at each time point and power were fit using SADS as a basis set to determine the amplitude of each component present at each excitation intensity.
The nonlinear evolution of cExciton population into the trCarrier population suggests that a multimolecular mechanism is responsible for trCarrier generation (e.g., bimolecular or trimolecular Auger recombination). Similar Auger recombination kinetics in CdSe quantum rods (excitonexciton annihilation) and CdS quantum dots (electronholehole recombination) have been observed, which are bimolecular or trimolecular, respectively, depending on the geometry of the nanoparticle.46,47 Within these mechanisms,
increased charge carrier density resulting from increased excitation fluence yields faster population relaxation due to enhanced nonlinear multiparticle recombination kinetics.4648 To decipher the mechanism behind the excitation-dependent spectral trends in Figure 8, we decomposed the individual 1.6 and 32 ps transient spectra into contributions from each population using the SADS extracted from the global analysis of the full timeresolved data as a basis set (Figures 9 and Figures SI6 and SI7 of the Supporting Information). A similar decomposition was used to analyze the photodynamics of light-harvesting II complex from Rhodopseudomonas acidophila.50,51 The decomposition of the 1.6 ps power-dependent spectrum (Figure 9A) demonstrates that increasing incident excitation fluence increases the relative concentration of hExciton population (black squares) and to a smaller extent the trCarrier population (blue triangles). In contrast, the cExciton1 population (red circles) initially increases until reaching a threshold (28.5 μJ/pulse 3 mm2) before decreasing. This trend represents an excitation fluence-dependent shift of population from cExciton1 to hExciton, which complements the intensity-dependent kinetics in Figure 6 (Table 1). The 32 ps power-dependent spectrum is simpler to interpret because only the c Exciton and trCarrier populations (Figure 9B) are needed to fit the data. Increasing excitation fluence results in a near-linear increase in the trCarrier population with a zero y-axis intercept (blue triangles) and a rapid suppression of the cExciton population. This is the signature of an accelerated cExciton to trCarrier evolution as expected from a nonlinear mechanism (Scheme 2). This nonlinear effect was not observed in the time-resolved power-dependent transient data (Figure 6C), which exhibit near-identical cExciton decay kinetics presumably due to the small intentisy range examined (Table 1). 19653
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4. DISCUSSION Ultrafast transient absorption and fluorescence dynamics have been previously explored in several anisotropic multidimensional CdSe systems including nanowires,46,52,53 nanorods,1,54,55 and CdSSe nanobelts.56 Both nanowires57 and nanorods58 are 1-D morphologies with lengths longer than their diameters, whereas nanobelts27 are exfoliated single-layered sheets of the multilayered NRs studied here. The UVvis absorption spectra of CdSe NRs in both H2O and H2O/HS solutions (Figure 2) exhibit spectral properties similar to those observed in infinite 2D quantum wells and other anisotropic materials.20,27,29,59 Deviation from the typical atom-like labeling of quantized states commonly used with CdSe quantum dots26,36 is consistent with previous studies on multidimensional semiconductors such as CdSe nanobelts in which it is possible to resolve heavy and light hole states spectrally.27,30 The static spectra and TEM measurements verify that the NRs are confined predominantly by their uniform thickness, estimated at 1.5 nm per layer (Figure 1).20,21,27,59 The near-band-edge emission of NRs (Figure 2) is significantly narrower than CdSe quantum dot emission2,60 despite the inhomogeneous size distribution (tens to hundreds of nanometers) in the lateral dimensions further attesting to predominant quantization within the thickness dimension (Figure 1B).29,61 The 460 nm PL peak is ascribed to near-band-edge luminescence, whereas the emission peaking at 575 nm results from deep traps arising from native point defects and surface-related defect states.35,62 Alternatively, the deep traps may result from variations in size, bending, or the persistence of localized charges at surface-related states, which have been theorized to localize excitons in CdSe nanorods leading to inhomogeneous broadening.63 The trap sites are presumably located at NR surface-related states (i.e., dangling bonds, edges), and Loomis and coworkers27 previously estimated the density of edge-related trap sites of CdSe nanobelts at ∼14% via the decoration with Au nanoparticles. 4.1. Excitation Fluence Dependence. Nonexponential decay dynamics is ubiquitous in semiconductor nanoparticle studies, whereby excitons may decay from two-46 or three-particle48 Auger recombination, if more than one excitation is generated per nanoparticle. Three-particle Auger recombination was observed in materials where the confinement energy is greater than the Columbic interactions (e.g., quantum dots),47,48 whereas two-carrier Auger recombination is observed in multidimensional materials with significant Columbic interactions (e.g., quantum rods with an aspect ratio >8).46 Decay kinetics resulting from single excitations, excitonexciton annihilation, and threeparticle Auger recombination are manifested as a “disappearance” of charge-carriers as the system evolves in time.46,48 In particular, we want to evaluate if Auger recombination is responsible for the growth of the trCarrier population via the promotion of holes into deep surface traps, as Klimov and coworkers observed for CdS nanocrystals.46,47 The above decay mechanisms would exhibit different excitation fluence dependences and should show up in the excitation fluence-dependent signals in Figure 9 if present. To interpret these data in terms of potential Auger recombination dynamics of excitons, we constructed a numerical model that tracks the exciton populations in terms of the occupation of NRs with a specified number of excitons.49 In this numerical simulation, only cExciton and trCarrier populations are addressed, and the fast relaxation dynamics associated with hExciton to cExciton evolution is neglected. The NR populations with zero-excitons, single-excitons, double-
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excitons, and so on ([Excitation]i with i = 0, 1...∞) can be independently and simultaneously modeled, which allows the direct observation of state-to-state evolution and ensures the proper Poissonian statistics of excitations based on the photoexcitation fluence.50,51 The following set of coupled differential equations for the NR manifold dynamics can be constructed d½Exciton0 ¼ αIðtÞð½Exciton0 ½Exciton1 Þ þ ðk22 f 1 þ k1 Þ½Exciton1 dt
ð2aÞ d½Excitoni ¼ αIðtÞð½Excitoni1 ½Excitoni Þ dt αIðtÞð½Excitoni ½Excitoniþ1 Þ ðki2 f i 1 þ k1 Þ½Excitoni þ ðk2i þ 1 f i þ k1 Þ½Excitoniþ1 ði ¼ 1; 2:::∞Þ ð2bÞ If an Auger recombination mechanism was responsible for the growth of the trCarrier population, then its development could be tracked as a sum over all such recombination events (assuming a bimolecular mechanism with two Excitons forming one trCarrier). d½tr Carrieri ¼ dt
∞
k2i f i 1 ½Excitoni ∑ i¼2
ð2cÞ
Consecutive NR populations ([Exciton]i and [Exciton]i+1) are coupled via the electric field of the applied excitation pulse (modeled with a 150 fs Gaussian time-dependent profile, I(t)). The decay rate constants, k2 and k1, represent the decay rates due to multiexciton Auger and single-exciton recombination decays (one electron + one hole), respectively. This model assumes that the extinction coefficient, α, that couples the applied pulse to the NR is independent of number of excitons on the NR (αj = α). The total number of excitons observed in the transient signals is directly controlled by the interplay between excitation fluence and decay kinetics ½Excitontotal ¼
∑i i½Excitoni
ð3Þ
As excitation fluence increases, the total number of excitons ([Exciton]total) in the sample increases (Figure 10A,B). These excitons subsequently decay either via bimolecular Auger recombination (potentially resulting in a new trapped species) or via single-particle decay (no new population formed). The decay of the exciton population from the loss of i excitations to (i 1) excitation(s) is single exponential, and we presume that the decay from the doubly excited state ([Exciton]2 f [Exciton]1) is the predominant process describing the slowest Auger recombination decay and that relaxation from more than doubly excited NRs induces faster decay kinetics. However, these annihilation phenomena are not observed for 2D NRs (Figure 6C,D), indicating that Auger recombination is not a dominant pathway for recombination in describing the nonexponential decay kinetics. This is in contrast with the model put forth by Piotrowiak and coworkers,56 where three-carriermediated Auger recombination was implemented to interpret the dynamics of CdSSe quantum belts. Instead, the observed nonexponential dynamics here is attributed to a bimolecular trapping process, which while nonlinear in nature does not accelerate the decay kinetics of the cExciton population (Figure 6C, 19654
dx.doi.org/10.1021/jp206828y |J. Phys. Chem. C 2011, 115, 19647–19658
The Journal of Physical Chemistry C
ARTICLE
Figure 11. Illustration of the CdSe NR photodynamics.
Figure 10. Simulated excitation intensity dependence with unity extinction (α). (A) Low intensity (I = 25) kinetics and (B) high intensity (I = 250) kinetics. (C) Pulse energy-dependent exciton population before relaxation (black) exhibits a linear dependence and the simulated tr Carrier population (red) at times after exciton relaxation generated from two excitons annihilating. The exciton dependence normalized at initial slope at 3 (blue curve) and 20 ps (magenta). Dashed black line is a guide to emphasize the nonzero intercept for the trCarrier population at zero excitation. Curves are labeled in the legends. The time constants selected for the simulations are 18.5, 8, 2, and 1 ps for the 1 f 0, 2 f 1, 3 f 2, and 4 f 3 decays, respectively. Subsequent decays were set to 1 ps but were weakly populated.
D, red curves) or formation of the resulting trCarrier population (blue curves) under the excitation fluences studied. Combining the intensity, excitation wavelength, and HS dependence of the observed transient data results in the following description (Figure 11): 1) “Hot” excitons are photogenerated and relax to the band edge on a sub-picosecond time scale that depends on excitation wavelength and intensity; the greater the excitation energy or intensity, the slower this relaxation. 2) During and after this relaxation, “cool” excitons diffuse throughout the NRs on a 130 ps time scale. 3) The diffusing excitons encounter NR surface-related trap sites and dissociate into free charge carriers, one or both of which may become trapped (20 ns) electrons responsible for the observed water reduction (Figure SI9 of the Supporting Information). 5) The trap sites (presumably associated with uncoordinated Cd2+ sites) can be passivated with HS ligands, which donate sacrificial electrons to nascent separated hole carriers resulting in lifetime elongation of their electron counterparts and increase H2 generation activity. This occurs quickly after dissociation such that the decoupling of hole and electron kinetics is not significant. Because of a large degree of signal overlap in the region of the 445 nm induced absorption, it is difficult to definitively assign its origin. The first derivative of the static absorption spectrum is a measure of the Stark difference spectrum when dealing with narrow transition bands and closely resembles the transient spectra reported here for CdSe NRs (Figure 3C).36 However, the induced absorption persists for >100 ps, which is not typical of DC-Stark-induced signals in CdSe nanomaterials (
