Improving Optical Gain Performance in Semiconductor Quantum Dots

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Improving Optical Gain Performance in Semiconductor Quantum Dots via Coupled Quantum Shells Eva A. Dias, Jonathan I. Saari, Pooja Tyagi, and Patanjali Kambhampati* Department of Chemistry, McGill University, Montreal, QC H3A 2K6, Canada S Supporting Information *

ABSTRACT: Semiconductor quantum dots are of interest as optical gain media for lasing applications. Here we report on efficient, broad bandwidth optical gain in the CdSe/ZnS/CdSe quantum dot/barrier/quantum shell nanocrystal. These nanocrystals are known to support spontaneous emission from both CdSe phases, offering promise for lasing applications via wave function engineering. The CdSe/ZnS/CdSe nanocrystals were found to have enhanced optical gain characteristics relative to CdSe quantum dots, as shown using femtosecond transient absorption spectroscopy. The enhancement of gain metrics such as bandwidth and efficiency arises from stimulated emission from quantum shell-enabled excitations. These shell-enabled excitations increase gain bandwidth via emission from new transitions and increase efficiencies via tailored biexciton interactions. This unique two-color character in both spontaneous emission and optical gain is rationalized by slow exciton cooling from the core/shell states into the core localized quantum dot states.

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INTRODUCTION Semiconductor quantum dots have been under investigation due to their novel and potentially useful electronic structure. When the charge carriers are strongly confined in these materials, they exhibit size-dependent optical and electronic properties which can be exploited in light-emitting diodes,1 as labels in biological systems,2,3 and as optical gain media in lasing applications.4−7 The colloidal or nanocrystal (NC) version of quantum dots (QD) was anticipated to be ideal for optical gain since the strong confinement leads to discrete, well-separated energy levels which should translate into low excitation thresholds, thermal stability, size-dependent spectral tunability, and narrow band emission.8,9 The prototypical colloidal CdSe NCs have in fact been shown to support optical gain for a range of sizes.9−12 However, the undesired photoinduced absorption (PA) that can spectrally overlap with the desired stimulated emission has generally resulted in poor optical gain performance in standard ligand passivated (bare) CdSe NCs.12,13 This limitation has led to the study of new geometries of nanocrystals in an attempt to achieve improved gain performance in colloidal NCs, under the guise of wave function engineering. Heterostructured nanocrystals provide such a path toward controlling and ultimately improving functionality for optoelectronic applications, e.g., lasing. Changing the material parameters by adding inorganic layers of varying size, shape, and composition to colloidal nanocrystals allows the optical and electronic properties to be tuned by engineering the wave functions of charge carriers.13−21 In particular, the approach of heterostructure nanocrystals has been used to improve gain performance in semiconductor nanocrystals. Optical gain has © 2012 American Chemical Society

been observed in several heterostructures including inverted core/shell ZnSe/CdSe,14,22 type II core/shell CdS/ZnSe,13,23 “giant” CdSe/CdS,24 and CdSe/CdS quantum dot/quantum rods.19,25 Toward this goal of efficient optical gain, wave functions may be tuned in heterostructures either to control multiexcitonic interaction strengths13,26,27 or to slow multiexciton recombination rates.24 A new variant upon this layering scheme is the quantum shell, created by introducing a barrier between the quantum dot core and the outer quantum shell of a nanocrystal, e.g., the CdSe/ZnS/CdSe core/barrier/shell system (Figure 1a).28−31 The CdSe core is overcoated with a wide gap ZnS shell followed by a CdSe shell. Both nominal CdSe phases support spontaneous emission (Figure 1c), with this dual-color emission being useful for several applications, including white light emission31,32 and two-color fluorescent tags for imaging and blinking.29 Notably, all prior realizations of heterostructured nanocrystals have only shown stimulated emission from a single emitting phase. In contrast, this dual-color CdSe/ZnS/ CdSe offers particular promise for lasing applications since both CdSe phases may support optical gain. Since the emission from both CdSe phases can be tuned by changing their size,28 one immediately recognizes the potential for spectral control of stimulated emission by exploiting wave function engineering with quantum shells. Here we report on ultrafast pump−probe experiments on CdSe/ZnS/CdSe core/barrier/shell nanocrystals. Efficient Received: November 24, 2011 Revised: February 15, 2012 Published: February 15, 2012 5407

dx.doi.org/10.1021/jp211325x | J. Phys. Chem. C 2012, 116, 5407−5413

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EXPERIMENTAL METHODS

The CdSe/ZnS/CdSe heterostructures were prepared by first synthesizing CdSe cores using the inorganic precursor method developed by Peng and co-workers.34−36 The CdSe nanocrystals were capped with trioctylphosphine oxide (TOPO) and octadecylamine (ODA). The layered CdSe/ZnS/CdSe systems were prepared using the modified successive ionic layer adsorption and reaction (SILAR) method developed by Peng and co-workers.28,34,37 All CdSe/ZnS/CdSe samples were completely synthesized by the authors, including the CdSe core to which the inorganic layers were added. The bare CdSe nanocrystals used for the spectroscopic measurements (Figures 2−5) were purchased from NN laboratories with octadecylamine as the primary ligand. For the spectroscopic measurements the nanocrystals were dissolved in spectral grade toluene. Steady-state absorption measurements were performed on a Varian Cary 5000 UV/vis spectrometer. Linear absorption measurements were preformed before and after the fluencedependent gain experiments in order confirm the absence of photodegradation. The photoluminescence (PL) were measured on a Spex Fluoromax-2 spectrometer. The TEM images were taken on a Philips CM200 operating at 200 kV. The nonlinear spectroscopic measurements were made using an amplified Ti:sapphire laser system (800 nm, 70 fs, 1 kHz, 2.5 mJ; Coherent) in the pump/probe configuration. The experimental details have been described previously.10,11,38,39 The probe pulses were derived from single filament white light continuum generated in a 2 mm sapphire crystal. The pump pulses were created via optical parametric amplifiers (OPA). Spot sizes were approximately 250−400 μm for the pump pulses and 75 μm for the probe pulses with a crossing angle of 5°. The time delay between the pulses was set by a computercontrolled delay stage with 0.1 μm precision (Newport PM500). The chirp-free transient TA spectra were obtained using a scanning monochromator40 and simultaneously adjusting the relative pump−probe delay according to the calibrated chirp. Experiments were performed simultaneously at two pump wavelengths in order to maximize reproducibility of the results. Two different pump colors were specified for a given experiment and simultaneously chopped at 333 Hz with a phase shift to enable alternating collection of TA spectra on a shot-byshot level.

Figure 1. Schematic of CdSe/ZnS/CdSe core/barrier/shell nanocrystal with CdSe phases in red and ZnS phase in blue (a). A transmission electron microscopy image of the CdSe/ZnS/CdSe nanocrystals (b). Steady-state absorption (black) and photoluminescence (red) spectra of typical CdSe/ZnS/CdSe nanocrystals (c) and CdSe nanocrystals (d) dispersed in toluene. The excitonic manifold for CdSe nanocrystals is labeled (Xi) and the core/shell delocalized excitation is labeled (Yi), as discussed in the text.

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optical gain for this system was measured using ultrafast transient absorption (TA) spectroscopy and contrasted to the response of prototypical CdSe nanocrystals. The CdSe/ZnS/ CdSe nanocrystals were found to have a greatly enhanced stimulated emission bandwidth as compared to parent CdSe NC. This increase in the optical gain bandwidth is shown to arise from the CdSe quantum shell. In particular, the shell phase supports high-energy excitations that extend throughout the nanostructure, thereby enabling spontaneous and stimulated emission from these new states. These quantum shellenabled excitations weaken biexciton binding energies, thereby reducing gain thresholds and further increasing the gain bandwidths. These observations are rationalized by the excitonic state-resolved33 TA spectra. The TA spectra reveal that the electronically hot, shell-enabled excitations are surprisingly long-lived, thereby enabling the two-color emission in these materials.

RESULTS AND DISCUSSION The CdSe/ZnS/CdSe heterostructures were prepared by adding inorganic layers to CdSe cores. For this study the CdSe cores had a radius of 1.8 nm (σ < 10%)41 (Figure 1d). The CdSe cores were then overcoated with approximately two monolayers of ZnS followed by approximately two monolayers of CdSe using the SILAR method.28,34 After adding the outer shell of CdSe, a second peak in the photoluminescence spectrum was observed at higher energy due to the CdSe shell (Figure 1c). An increase in the relative absorption at higher energy was also observed after the addition of the CdSe shell (Figure 1c). There was a red shift in the lowest energy absorption and emission peaks of ∼50 meV from bare CdSe to CdSe/ZnS/CdSe due to a delocalization of charge carriers into the shell regions. An ∼20 meV red shift is expected from the ZnS shell alone, with the additional 30 meV shift coming from the outer CdSe shell.42,43 The steady-state absorption and emission spectra of CdSe/ZnS/CdSe have been described previously.28−30 5408

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Figure 2. Nonlinear absorption spectra with increasing fluence (colored lines) and linear absorption, OD0 (black line). Nonlinear absorption spectra of CdSe/ZnS/CdSe at a 2 ps time delay excited at X0 (2.1 eV) below the onset of the shell (a) and excited at Y0 (2.4 eV) above the onset of the shell (b). Inset: expanded region for OD0 + ΔOD < 0. Nonlinear absorption spectra of CdSe at a 1 ps time delay excited at X1 (2.1 eV) (c) and excited at X3 (2.5 eV) (d). For ease of comparison, the spectra are plotted such that the lowest energy peak in the linear absorption takes up the same percentage of the scale in each case.

shown here were completely consistent with previously published studies.9−11,38,39,48,49 To investigate their potential as gain media, the nonlinear absorption spectra (ODNL = OD0 + ΔOD) of CdSe/ZnS/CdSe nanocrystals were measured as a function of pump fluence and compared to CdSe (Figure 2). Optical gain corresponds to a nonlinear absorption of less than zero (ODNL < 0 or OD0 + ΔOD < 0). More simply, gain via stimulated emission surpasses loss via photoinduced absorptions. The data reveal that CdSe/ ZnS/CdSe shows optical gain upon excitation into either X0 or Y0 (Figure 2a,b). In agreement with previous studies,10,11 CdSe exhibited optical gain with pumping into both initial excitonic states (Figure 2c,d). The spectral shape and magnitude of optical gain measured in CdSe were consistent with previously published studies.9−11 With excitation below the onset of the shell (X0), CdSe/ZnS/CdSe (Figure 2a) showed optical gain in a similar spectral region and of similar shape to CdSe (Figure 2c), precisely as expected if the CdSe core phase in this heterostructure behaved like bare CdSe. In contrast, we found that upon excitation above the onset of shell emission (Y0) a dramatic increase to the optical gain bandwidth was achieved (Figure 2b). The capacity of this quantum shell approach to controlling optical gain bandwidth can be seen in the stimulated emission spectra in Figure 3. For ease of comparison, the stimulated emission region of the nonlinear absorption spectra in Figure 2 (ODNL < 0) has been normalized by the magnitude of the linear absorption at 2.1 eV for each sample and plotted along with the spontaneous PL (Figure 3). The stimulated emission spectra of bare CdSe were consistent with previously published results (Figure 3b).10,11 The stimulated emission spectrum of CdSe with excitation at higher photon energy (X3) was slightly red-shifted when compared to that with lower energy excitation (X1).

In addition to the steady-state measurements, transient absorption spectra were obtained. Ultrafast TA measures the difference in absorption of a probe pulse for an optically pumped sample and an unpumped sample, ΔOD = ODNL − OD0. ODNL is the absorption of the probe by the pumped sample, or the nonlinear absorption, and OD0 is the absorption of the probe by the unpumped sample, or the linear absorption. Specifically, we measured the ultrafast transient absorption spectra with excitonic state specificity as previously discussed (see Supporting Information for details).10,11,38,39 Spectral features, including stimulated emission and the overlapping photoinduced absorption, have been shown to be dependent on the initially prepared excitonic state.10,11,33,38,39 In this study the TA spectra for CdSe/ZnS/CdSe were taken using two different pump photon energies: 2.1 eV (X0) and 2.4 eV (Y0). In all cases the NC were dispersed in toluene, with experiments performed at 300 K. Pumping the CdSe/ZnS/ CdSe sample at X0 excited the sample below the onset of shell absorption and emission, while the pumping at Y0 excited the sample above the onset of the shell. As a point of notation, early work on this structure has suggested the high-energy features that resulted from the shell were due to shell localized excitations.28−31 An effective mass calculation (Supporting Information) suggests that the high-energy excitations can be delocalized throughout the nanocrystal rather than localized in the shell. Hence, we refer to the high-energy excitations (e.g., Y0) as shell-enabled excitations since the presence of the quantum shell creates these new spectroscopic features. Experiments were also performed on bare CdSe NCs excited at 2.1 eV (X1) and 2.5 eV(X3). The CdSe NCs used in the TA measurements were obtained from NN Laboratories and had their lowest energy absorption at the same energy (2.1 eV) as the CdSe/ZnS/CdSe samples. The electronic structure of bare CdSe has been studied in detail, and the origin of spectral features is well established.44−51 The data for the CdSe case 5409

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(Figure 3). The stimulated emission for this heterostructure has increased bandwidth in the blue by 5 meV. This small bandwidth increase arises from the 5 meV attenuation of the binding energy of the band edge biexciton by virtue of the quantum shell. This heterostructure also yields a 3-fold increase in the stimulated emission cross section upon pumping into the respective band edges. The stimulated emission cross sections were scaled to the linear absorption. We have previously shown that the stimulated emission cross sections are enhanced via ZnS passivating shells and state-resolved optical pumping.11 Similar cross sections are obtained here. Upon excitation into Y0, CdSe/ZnS/CdSe yielded a stimulated emission spectrum that is dramatically different to pumping CdSe into a hot exciton with equivalent excess electronic energy (X3) (Figure 3). The main effect of the CdSe quantum shell is to considerably enhance the optical gain bandwidth as revealed in these stimulated emission spectra. The gain bandwidths for CdSe are 94 meV (X1 pump) and 146 meV (X3 pump). In contrast, the bandwidths for CdSe/ZnS/ CdSe are 128 meV (X0 pump) and 373 meV (Y0 pump). Addition of the CdSe quantum shell has a qualitative effect of increasing the gain bandwidth by ∼3−4 times when compared to band edge excitation to either the heterostructure or the parent CdSe nanocrystals. Similar increases in gain bandwidth were achieved by Klimov using an alternative route of slowing multiexciton recombination.24 Here, the gain bandwidth increase is assigned to a new pathway based upon the creation of higher energy radiative transitions created by the quantum shell. The increase in bandwidth of the stimulated emission spectrum observed in CdSe/ZnS/CdSe with excitation at Y0 could not be obtained with CdSe/ZnS alone,11 so it must be attributed to the presence of the CdSe shell. Upon excitation into Y0, the stimulated emission cross section was also larger than for CdSe with excitation into X3. Again, similar increases in gain cross section can be achieved with ZnS passivation.10,11 The higher energy shell-enabled stimulated emission lasted for 5 ps, shorter time than that of the central core enabled region (35 ps) (data not shown). The stimulated emission lifetime is governed by the faster of either multiexciton recombination52 or surface trapping.11,38 The 35 ps decay time from the core-type region is consistent with a lifetime dictated by multiexciton recombination, whereas the 5 ps decay time from the shell-enabled excitation is consistent with surface trapping. In principle, additional shell structures may be grown31 to further optimize these processes for both efficient and long-lived optical gain. Finally, the higher energy stimulated emission region appeared at a higher fluence than in the central gain region (Figure 2b inset, Figure 3c), suggesting its multiexcitonic origin. The TA spectra give insight into the optical gain response and the electronic structure of these CdSe/ZnS/CdSe nanocrystals. TA measurements were performed at low pump fluence such that the initial fractional bleach at the band edge (2.1 eV) was ∼0.3 for both the CdSe/ZnS/CdSe and CdSe samples. Features in the TA spectra arise from three processes: ground state bleaching (ΔOD < 0) due to state filling, excited state absorption (ESA) (ΔOD > 0), and stimulated emission (SE) (ΔOD < 0).53 In this case, ESA refers to the photoinduced absorption which arises from multiexcitonic level shifting.38,39,51,54 Full transient spectra are shown to provide a coarse overview of the signals, as well as spectral projections at key points in time (Figure 4). The TA features for the CdSe/ZnS/CdSe sample exhibit a strong dependence

Figure 3. Stimulated emission region of the nonlinear spectra normalized to the linear absorption at 2.1 eV, such that positive signals correspond to stimulated emission. Comparison of steady state PL and stimulated emission spectra for CdSe/ZnS/CdSe (a) and CdSe (b). Normalized stimulated emission cross sections for CdSe/ ZnS/CdSe as a function of fluence excited at X0 and probed at 2.04 eV (red) or excited at Y0 and probed at 2.03 eV (blue) or 2.14 eV (green) (c). The probe at 2.14 eV corresponds to quantum shell-enabled stimulated emission.

Since charge carriers are confined in semiconductor nanocrystals, the charge carriers interact more strongly than they would in the bulk system. So when there are multiple excitons per nanocrystal the charge carrier interactions cause a photoinduced shift in transition energies.12,38,48,50 In the case of CdSe, binding between the multiexcitons causes the emission to be shifted to lower energy.10,11 When exciting CdSe NCs with the 2.1 eV pump, the maximum occupation is two excitations per nanocrystal since a 2-fold degenerate state is being excited.9−11,50 The binding energy of this biexciton causes a small red shift in the excited state transitions. When exciting at 2.5 eV there is a greater degeneracy, so a greater number of excitations per nanocrystal can be achieved.9−11,50 These higher order multiexcitons cause a greater red shift in the emitting transition than for the biexcitonic case. For this reason the stimulated emission spectrum for CdSe taken with the higher energy pump was red-shifted with respect to that with the lower energy pump.10,11 Upon excitation into X0, CdSe/ZnS/CdSe yielded a stimulated emission spectrum that is qualitatively similar to excitation of CdSe directly into the band edge exciton (X1) 5410

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Figure 4. Transient absorption as a function of probe photon energy and time delay (a, b) at low fluence. CdSe/ZnS/CdSe excited at X0 (2.1 eV) (a) and excited at Y0 (2.4 eV) (b). Transient absorption spectra at various time delays (colored lines) and linear absorption spectra (black line) (c−f) CdSe/ZnS/CdSe excited at X0 (2.1 eV) (c) and excited at Y0 (2.4 eV) (d). CdSe excited at X1 (2.1 eV) (e) and excited at X3 (2.5 eV) (f). Noteworthy spectral features are the presence/absence of a photoinduced absorption at 2.0 eV, the large bleaching feature at 2.4 eV with excitation at Y0, and the absence of a decay in the bleaching feature at 2.1 eV for CdSe/ZnS/CdSe excited at Y0.

on the initial excitonic state (Figure 4a,b). By comparing the spectra for CdSe/ZnS/CdSe excited at X0 and Y0 (Figure 4c,d) to CdSe excited at X1 and X3 (Figure 4e,f), the optical gain results can be explained through state filling and biexciton induced level shifting. The CdSe TA spectra reported here are consistent with prior work.11,33,38,39,48,51,54 Specifically, we observe bleaching features at 2.10, 2.22, and 2.46 eV as well as a PA at 2.35 eV with excitation at both X1 and X3 (Figure 4e,f). When CdSe was excited at X1 (2.1 eV), the TA spectrum shows an attenuated bleach on the red edge (Figure 4e). This attenuation arises from a small photoinduced absorption due to weakly bound biexcitons, a point discussed in detail elsewhere.33,38,39,48,51 For CdSe pumped at 2.5 eV, a larger amplitude photoinduced absorption at 2.04 eV was observed at early time, due to an increase in biexciton binding energy, consistent with prior work33,38,39,51 (Figure 4f). The absorptive39,51 biexciton binding energies at 300 fs were measured to be 10 meV for X1X1 (pumping into X1 and probing the absorption into X1) and 25 meV for X1X3. It has been previously shown that the higher the photon energy of the pump the greater the biexciton binding energy will be and the larger this PA will appear.11,33,38,39,51 Because of differences in surface trapping with different initial excitonic states, these features can persist longer than intraband relaxation and affect the gain measurement at 1−2 ps.11,33,38

The photoinduced absorption in CdSe/ZnS/CdSe was smaller than in CdSe. When exciting at X0 (2.1 eV), there was no PA at 2.35 eV and there was no attenuation of the lowest energy bleach (Figure 4c). Since these features can be attributed to level shifting due to biexciton interactions, this implies that CdSe/ZnS/CdSe had smaller biexciton binding energies than CdSe. This decrease in binding energy may arise from a change in the excitonic charge distribution due to the shell. The absorptive biexciton binding energies39,51 at 300 fs were