Magneto-photoluminescence Properties of Colloidal CdSe

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Magneto-photoluminescence Properties of Colloidal CdSe Nanocrystal Aggregates Daniel E. Blumling,† Takahisa Tokumoto,‡ Stephen McGill,‡ and Kenneth L. Knappenberger, Jr.*,† † ‡

Department of Chemistry and Biochemistry, Florida State University, Tallahassee, Florida 32306, United States National High Magnetic Field Laboratory, Tallahassee, Florida 32310, United States

bS Supporting Information ABSTRACT: Magnetic fields up to 10 T were used to study the relative excited-state population of exciton fine-structure states in aggregates of 0-D CdSe nanocrystals. Following excitation by 400 nm light, energy-resolved, intensity-integrated photoluminescence (PL) spectra of CdSe quantum dot aggregates were collected at 1.6 K. Temperature-dependent PL measurements revealed multiple exciton relaxation channels, which were attributed to direct and longitudinal optical phonon-assisted recombination of isolated and aggregated nanocrystals. The relative populations of the 1S(e) 1S3/2(h) exciton fine-structure levels in both isolated and aggregated nanocrystals were actively controlled by application of a magnetic field. The efficient applied magnetic field-assisted population control led to an increase in the total PL of the aggregate that resulted from a reduction in phonon-assisted recombination and an increase in direct radiative e h recombination. An increase in the relative radiative recombination yield was attributed to field-induced mixing of the “dark” and “bright” exciton states. This behavior was quantified using a novel branching ratio analysis that related the relative population of the “bright” exciton to the total excited state population. The data present promise for studying spin-dependent energy transfer in nanocrystal superstructures.

I. INTRODUCTION Quantum-confined semiconductor nanocrystals have the potential to improve a diverse range of applications including solar-toelectric energy conversion,1 3 nanoparticle-based lasers,4 quantum computing,5 fluorescence labeling,6 8 and sensor technology.9,10 Recent progress in colloidal synthesis has enabled facile fabrication of size- and shape-tunable semiconductor nanostructures with narrow size distributions. These advances facilitate the formation of “superstructure” assemblies of nanoparticles, which may demonstrate enhanced radiative properties.11 18 To advance further the potential use of nanoscale materials in energy-conversion devices, both the understanding of how energy moves within them and the control over that energy flow must be improved. The majority of applications will require organization of multiple nanocrystals into higher-order assemblies.17,19 23 In light of this, aggregates of semiconducting nanocrystals were used to investigate complex nanocrystal exciton-longitudinal-optical-phonon (LO) interactions to understand their influence on exciton dynamics in extended nanocrystal structures. These studies were performed at ultralow temperatures (1.6 K), where resolution of the LO-phonon contributions to e h recombination is possible. The exciton fine structure of semiconducting nanospheres has been described previously.24 28 Briefly, the lowest electronic energy transition (1S(e) 1S3/2(h)) is split into two multiply degenerate fine-structure levels because of electron hole exchange interactions.26,29 This degeneracy is further split into five fine-structure levels due to particle shape asymmetry and crystal field effects. The energy difference between the levels depends on nanostructure size, shape, and composition and can be calculated r 2011 American Chemical Society

using the effective-mass model.24 The 5-fold degenerate J = 2 ground state contains the 0, (1, and (2 spin states. Of these, the lowest energy state (J = (2) is dipole forbidden and thus referred to as a “dark” state; it cannot be excited by (relax via) the absorption (emission) of a photon alone. Thus, e h relaxation to the ground state proceeds mainly via the optically allowed transition from the higher energy “bright” state (J = (1), which is thermally populated at temperatures above ∼20 K.30 The e h recombination mechanisms observed for excited NCs are highly temperature dependent.31 33 At low temperatures, lattice vibrations are suppressed, reducing the thermal J = (2 to J = (1 excitation that would lead to direct radiative recombination.32 Instead, spin-flip-assisted exciton recombination proceeds from the J = (2 state with only a small (∼1 meV) activation barrier attributed to interactions between the surface-passivating ligands typically found on NCs.30,32,34 Below ∼2 K, e h recombination can proceed radiatively from the “dark” states via assistance from optical phonon modes26,29,35,36 and/or “bright” “ dark” state mixing through interaction with unpassivated surface states.30,37 Methods that increase the mixing efficiency between these levels are useful for suppressing nonradiative losses associated with colloidal semiconductor nanocrystals, as well as for analyzing the influence of nanocrystal structure on exciton dynamics. The possible recombination processes following nonresonant excitation, relevant to these studies, are shown in Figure 1: Received: March 11, 2011 Revised: June 9, 2011 Published: June 16, 2011 14517

dx.doi.org/10.1021/jp2023346 | J. Phys. Chem. C 2011, 115, 14517–14525

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Figure 1. Possible radiative relaxation channels of CdSe quantum dots at 1.6 K: (1) direct radiative recombination from the “bright” exciton, (2) field-induced and (3) surface-mediated mixing of the “bright” and “dark” excitons, and (4) indirect LO-phonon-assisted relaxation of the dark exciton.

(1) direct radiative recombination from the “bright” exciton, (2) field-induced and (3) surface-mediated mixing of the “bright” and “dark” excitons, and (4) indirect LO-phonon-assisted relaxation of the dark exciton. In mechanism 4, the momentum-conserving LO phonon satisfies the spin requirements of the optical transition from the exciton.38,39 In CdSe systems, LO phonons consume approximately 26 meV of energy.25,40 For ensemble measurements, this is typically manifested as a red shift of the global photoluminescence (PL) signal. In contrast, discrete transitions can be resolved in single-molecule experiments.34,41,42 It has been suggested that all low-temperature CdSe nanocrystal PL results from a combination of the direct and phonon-assisted processes described in Figure 1.37 In addition to temperature, applied magnetic fields can also alter the e h recombination process (mechanism 2 in Figure 1).26,37,43 48 When NCs are subjected to a large, properly oriented magnetic field, mixing of the J = (2 and J = (1 states decreases the effective energy gap between the two exciton fine-structure levels. As a result, relaxation can proceed from the “bright” J = (1 state because the spin momentum conservation contributed by the LO phonon is no longer needed. The magnetic field strength required to induce state mixing depends upon the initial bright dark splitting (ΔEbd) between the J = (1 and the (2 states, which is system- and particle size-dependent.27,35,49 In the presence of an applied magnetic field, this splitting is decreased by the exciton Zeeman energy, which is given by Ez = gexμBB, where gex is the Lande g factor, μB is the Bohr magneton, and B is the strength of the magnetic field in Tesla. When B becomes large enough, ΔEbd becomes negligible, and the total PL yield increases. For colloidal samples, we observed a further blue shift resulting from a reduction in phonon-assisted recombination events, which are detected at lower energies. The energy difference between the J = (2 and (1 levels is small—approximately 5 meV for 3.7 nm NC quantum dots.28 Although our samples were characterized by narrow particle size distributions, even this level of sample polydispersity leads to PL signal broadening and complicates resolution of such small energy differences. However, relaxation from the J = (2 and (1 levels gives rise to PL bands that are separated by more than the

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5 meV difference between these states; the peak assigned to the “dark” J = (2 state is red-shifted by ∼26 meV (the energy consumed by the LO phonon to which the radiative emission is coupled). This shift is large enough to allow for band resolution despite the size dispersity in the sample. To maximize the resolving power of the experiment, samples were examined at 1.6 K, where phonon-assisted radiative relaxation is the dominant recombination mechanism. In this work, applied magnetic fields were employed to study the optical emission properties of colloidal CdSe 0-D nanocrystal aggregates by mixing the bright and dark exciton fine-structure states. At 0 T and 1.6 K, the phonon-assisted recombination of the J = (2 dark state was resolved from the direct recombination of the J = (1 bright state in the PL spectrum. In addition, 3.1 eV photons were used to excite the sample above the bandgap, allowing observation of direct radiative as well as indirect phonon-assisted emission from two different channels. A novel branching-ratio analysis was devised to describe the controlled manipulation of exciton fine-structure level population as a function of applied magnetic field. Reduction of the ΔEbd for increasing B resulted in mixing of the J = (1 and J = (2 states, enabling relaxation via direct radiative recombination to dominate over the phonon-assisted process that is required for emission from the J = (2 state in the absence of an applied magnetic field. In the case of nanocrystal aggregates, multiple distinct radiative recombination channels were observed. Two of these transitions were assigned to direct radiative recombination from isolated and aggregated nanocrystals, while the other peaks resulted from phonon-assisted mechanisms. The increased emission efficiency led to an increase in the total PL amplitude for the e h recombination at higher applied magnetic fields.

II. EXPERIMENTAL METHODS A. Synthesis and Characterization of CdSe Quantum Dots. The synthesis of CdSe nanocrystals was carried out following established air-free protocols.50,51 First, 3.90 mmol of cadmium oxide, 5.90 mmol of trioctylphosphine oxide, 2.20 mmol of tetradecylphosphine oxide, and 23.5 mmol of hexadecylamine were added to a three-neck flask. These solid components were heated to 125 °C under nitrogen and degassed for 1 h. Next, the solution was heated to 265 °C under nitrogen, and 4.90 mmol of tributylphosphine (TBP) was injected into the stirred solution. The temperature was decreased slightly to 260 °C, and selenium TBP was injected. Nanocrystal growth was quenched after a few minutes, resulting in a sample characterized by a first exciton absorption peak at 559 nm (2.22 eV) and a PL emission centered at 574 nm (2.16 eV). The solution was cooled, and the nanocrystals were precipitated using methanol. Following centrifugation, the methanol was decanted, and the precipitate was resolvated in hexanes. The methanol-induced precipitation process was repeated several times to purify the sample further. The quantum dots were characterized at room temperature using PL (Cary Eclipse, Varian) and UV vis absorption (Lambda 950, Perkin-Elmer) spectroscopy techniques. Nanocrystal size distributions were determined by transmission electron microscopy (TEM), which was performed on a JEOL-2011 operating at 200 kV. The samples were drop coated onto Formvar/carbon-coated copper grids (200 mesh, Ted Pella, Inc.). Elemental composition was verified using energy-dispersive X-ray spectroscopy. B. Spectrally Resolved PL Measurements in Static Magnetic Fields. Magneto-optical fluorescence measurements were 14518

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Figure 3. Upper traces: raw PL data collected at 1.6 K. Lower traces: deconvoluted data obtained by fitting the raw data to a five-component Lorentzian function. In both cases, the 0 T data are portrayed as a solid line, and 10 T data are denoted by the line with the open circles. The peaks were assigned as follows: (a) single phonon-assisted PL (1PL) and (b) zero phonon-assisted PL (ZPL) from the aggregated species, (c) 2 phonon-assisted recombination (2PL), (d) 1PL, and (e) ZPL of the isolated (nonaggregated) species.

intervals. A frequency-doubled Coherent Mira Ti:Sapphire oscillator generated 20 25 nJ, 400 nm (3.1 eV) excitation pulses at 76 MHz. Exciton formation probabilities spanned from 0.005 to 0.01.52 The well-collimated laser beam entered the sample interaction region parallel to the magnetic field vector and perpendicular to the plane of the microscope slide. Polarization-resolved fluorescence emission was collected using a series of focusing lenses and passed through a quarter-wave plate combined with a Glan-laser polarizer. These photons were then transported into a McPherson spectrometer (0.75 m focal length) via a fiber optic cable, dispersed using a ruled, 600 grooves/mm grating, and detected by a liquid nitrogen-cooled Spec-10 CCD camera (Princeton Instruments). The resolution obtained under these conditions was better than 0.35 at 585 nm (0.6 meV at 2.12 eV), with a signal-to-noise ratio ranging from 35.5 to 37.3. PL data were analyzed using a five-component Lorentzian fit, yielding R2 values greater than 0.990.

III. RESULTS AND DISCUSSION Figure 2. Structural and energetic characterization data for colloidal CdSe quantum dots. (A) Room-temperature PL and UV vis absorption data. Four exciton absorption peaks are clearly resolved. (B) TEM data giving the mean NC dimensions and the size distribution. (C) X-ray elemental analysis showing that the NC are composed only of Cd and Se. The strong copper signal results from the TEM grids on which the sample was deposited.

performed at the National High Magnetic Field Laboratory in Tallahassee, FL. Samples used for magneto-PL experiments were prepared by drop-casting hexane-dispersed CdSe colloidal nanocrystals onto 0.17 mm quartz microscope slides, which were subsequently dried to create an optically clear film. The coverslips were cleaned by boiling piranha solution and rinsed repeatedly with Millipore water prior to nanoparticle deposition. Samples were mounted in a superconducting magnet (Oxford Instruments Spectromag 8), and the sample chamber was cooled to 1.6 K. The applied magnetic field was varied from 10 T to +10 T, with spectrally resolved PL data being collected at 1 T

A. Nanocrystal Characterization. Structural characterization revealed a colloidal solution of CdSe nanocrystals with a narrow size distribution, as shown in Figure 2. The room-temperature absorption spectrum (Figure 2a) included four discrete peaks superimposed on a broad extinction profile that gradually increased with the excitation energy. The first four peaks, centered at 2.22, 2.36, 2.68, and 3.15 eV, were assigned to the 1S(e)r1S3/2(h), 1S(e)r2S3/2(h), 1P(e)r1P3/2(h), and 1S(e)r3S1/2(h) transitions, respectively.53 The observed room-temperature PL (Figure 2a) was located at 2.16 ( 0.05 eV (574 ( 14 nm). Particle size analysis of the TEM data (Figure 2b) revealed a mean QD diameter of approximately 3.7 nm. These data indicated a narrow size distribution. The elemental Cd and Se composition of the QDs was confirmed by energy-dispersive X-ray analysis (Figure 2c). The additional Cu peaks observed in the spectrum arise from the copper grids on which the samples were deposited. B. Assignment of Spectroscopic Transitions. Field-free PL spectra acquired at 1.6 K were used to determine the spectroscopic 14519



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Table 1. Spectroscopic Transition Assignments, Energies, and Intensities at Zero Field peak

designation

transition

LO-phonon contribution

energy (eV)

intensity (a.u.)

fractional intensity (%)

a

1PLaggregate

1S(e) 1S3/2(h)

1PL

2.1286 ( 0.00066

163162.0 ( 3312.69

35.5

b

ZPLaggregate

1S(e) 1S3/2(h)

ZPL

2.1549 ( 0.00037

99752.6 ( 1830.77

21.7

c

2PLisolated

1S(e) 2S3/2(h)

2PL

2.1866 ( 0.00038

92334.8 ( 4183.84

20.1

d

1PLisolated

1S(e) 2S3/2(h)

1PL

2.2133 ( 0.00029

75654.4 ( 1989.18

16.5

e

ZPLisolated

1S(e) 2S3/2(h)

ZPL

2.2379 ( 0.00053

28664.6 ( 1822.48

6.2

transition assignments. Both raw and deconvoluted zero-field PL data are shown in Figure 3. A five-component Lorentzian function was used to fit the data because five peaks, labeled (a e) in Figure 3, were clearly evident in the raw data. This analysis produced random residuals and R2 values g0.990, demonstrating excellent agreement between the fit and the data. Each peak was assigned to a specific radiative relaxation pathway based on the interpeak energy separation and the expected contributions from phonon-assisted recombination phenomena. The experimental resolution was great enough to make these transition assignments unambiguous. As shown in Table 1, the energy separation between the lowest energy signal (a) and the nearest higher energy peak (b) was approximately 26 meV at zero field. This matched exactly with the predicted quanta of energy consumed nonradiatively in a LOphonon-assisted process at 1.6 K. The energy difference between peaks (c) and (d) was ∼26 meV, and the (d) to (e) gap was ∼25 meV, indicating additional phonon-assisted relaxation pathways. By contrast, the energy difference between peaks (b) and (c) was approximately 32 meV, significantly more energy than a LO phonon should consume. In light of this, peaks (a) and (b) were assigned to the LO-phonon-assisted (1PL) and direct, zerophonon-assisted (ZPL) 1S(e) 1S3/2(h) transitions, respectively. In a similar manner, peaks (c), (d), and (e) were assigned as 2PL, 1PL, and ZPL transitions, respectively. The two ZPL transitions (peaks (b) and (e)) were approximately 83 meV apart. These ZPL transitions could have been the result of 1S(e) 1S3/2(h) exciton recombination from different quantum dots in an extended nanocrystal aggregate or relaxation by high-energy excitons. We note that at 1.6 K the PL energy of peak (a) was significantly red-shifted (≈40 meV) from the room-temperature PL of the colloidal sample, inconsistent with emission from highenergy states or multiexcitons.54,55 On the basis of the theoretical and experimental work of Norris and Bawendi,53 the experimentally measured energy gap (83 meV) matched well the ∼85 meV separation expected for ∼3.7 nm CdSe quantum dots. Indirect population of the higher energy 1S(e) 2S3/2(h) state was possible via internal conversion, because of the use of above-bandgap excitation with 3.1 eV photons. In previous work,56 this transition was observed following above-bandgap excitation at low temperatures (10 K). Therefore, the observation of high-energy exciton peaks may be possible at the low temperatures used here for magneto-PL measurements. The contributions from high-energy excitons should decrease with increasing temperature.56 On the other hand, nanocrystal aggregate PL exhibits multiple red-shifted peaks even at room temperature.19 This effect is attributed to interdot energy transfer. Temperature-dependent PL measurements were carried out to identify the origin of the ZPL ((b) and (e)) peaks. Intensityintegrated PL is plotted as a function of emission energy at several temperatures in Figure 4a. Two peaks are clearly resolved at all temperatures over the range from 6 to 225 K; the

LO-phonon progression observed at 1.6 K was not observed over the 6 225 K temperature range, consistent with phononassisted models for recombination.26,30 The highest temperatures exceeded those necessary to suppress possible radiative recombination by high-energy excitons. Additionally, fitting the temperature-dependent emission energy of both peaks to the Varshni equation provided a good fit (Figure 4b), indicating semiconductor bandgap emission. Specifically, the temperaturedependent PL of the higher energy peak agreed well with expectations of the isolated colloidal sample; the fit results included a T = 0 bandgap of 2.2265 ( 0.0005 eV, a temperature coefficient of 2.9 10 4 eV/K, and a projected bandgap of 2.18 eV at 298 K. The fit results of the lower energy peak included a T = 0 bandgap of 2.1554 ( 0.0005 eV and a temperature coefficient of 3.1 10 4 eV/K. The bandgap energies that resulted from these fit results are in excellent agreement with our ZPL experimental data for both the isolated and the aggregated nanocrystals at 1.6 K (Table 1). Temperature coefficients of ∼3.0 10 4 eV/K are typical of those reported for bulk CdSe.57 Taken together, the observation of a PL red shift rather than a blue shift, along with the results from temperature-dependent measurements, indicated that the two ZPL peaks were due to exciton recombination from isolated (high energy) and aggregated (low energy) nanocrystals. We were able to exclude deep trap-state emission as a possible contribution to the ZPL peaks. Trap-state emission for these nanocrystals was expected to appear as a broad PL peak (≈0.5 eV) at energies e1.5 eV. To the contrary, our experimentally measured PL occurred at energies g2.1 eV with narrow peaks. Also, the formation of a 2D or 3D nanocrystal solid would have resulted in a maximal bandgap red shift of approximately 10 meV,13 inconsistent with the 83 meV red shift that we observed. Transition assignments for each peak in the PL spectrum shown in Figures 3 and 4a are given in Table 1, and the recombination mechanism is shown schematically in Figure 4c. The absorption of a 3.1 eV photon creates an exciton, which relaxes first by internal conversion to the 1S(e) state58 followed by either direct radiative 1S(e) 1S3/2(h) recombination by the bright exciton (mechanism 1 in Figure 1), field-mediated 1S(e) 1S3/2(h) dark exciton recombination (mechanism 2 in Figure 1), surface-induced mixing of bright and dark excitons (mechanism 3 in Figure 1), or LOphonon-assisted recombination of the dark exciton (mechanism 4 in Figure 1). Direct radiative recombination of the electron and hole would result in the highest energy peak observed for each grouping of transitions (i.e., peaks (b) and (e)). Relaxation by mechanisms 2 and 3 would produce energetically degenerate emission, red-shifted from mechanism 1 by 5 10 meV. As a result, transitions 2 3 were not distinguishable within our experimental resolution. LO-phonon-assisted transitions (1PL: peaks (a) and (d); 2PL: peak (c)) result in emitted photons with approximately 26 meV less energy (per phonon) than the photons emitted by direct, unassisted transitions. The energy of photons 14520



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Figure 5. Triangles: total sample PL intensity plotted as a function of applied magnetic field strength. Squares: sum of the PL resulting from the 1PL and ZPL relaxation of the aggregated nanocrystal species. Circles: sum of the PL arising from the 2PL, 1PL and ZPL channels of the isolated species.

Figure 4. (A) Temperature-dependent PL from isolated and aggregated CdSe QDs deposited on a substrate. (B) Temperature-dependent peak energy for each of the two features observed in panel A. The data for both transitions were fit to the Varshni equation. (C) Scheme for each of the five relaxation mechanisms that give rise to the PL peaks observed for isolated and aggregate CdSe QDs at 1.6 K.

arising from the 1PL transition originating at the J = (2 state is Ephonon ΔEbd, where hω(J=1) given as E(J=2) = hω(J=1) describes the energy of the direct relaxation from the J = (1 spin state, Ephonon is the energy consumed by the LO phonon, and ΔEbd is the difference in energy between the J = (1 and the J = (2 states. For the aggregated species, peaks (a) and (b) were 26 meV apart. Similarly, the isolated nanocrystals portrayed a 26 meV (d) (e) separation and a 51 meV (c) (e) gap. Therefore, direct

radiative recombination by the bright exciton was distinguished from indirect phonon-assisted recombination by the dark state by the experimentally resolved-phonon progression. For clarity, we refer to each of these peaks according to their recombination mechanism as well as the emissive species; for example, the lowest energy transition, the 1PL of the 1S(e) 1S3/2(h) for the aggregated species, is labeled 1PLaggregate and so on (see Table 1 for the complete listing). The deconvoluted Lorentzian peaks were integrated to determine the fraction of the total PL that could be attributed to each individual transition. These areas are summarized in Table 1 and clearly demonstrate that in the absence of an applied magnetic field, the 1PLaggregate transition was the dominant channel for e h radiative recombination at 1.6 K, contributing 35.5% of the total amplitude. Larger measured emission intensities from the aggregate were consistent with proposed energy transfer models that describe the aggregation-induced red shift.19,59 The direct recombination process terminating at the 1S3/2(h) level of the aggregates and the 2PL channel for the isolated species accounted for 21.7 and 20.1% of the intensity, respectively. The 1PL and ZPL processes for the isolated species contributed less significantly to the total PL (1PL, 16.5%; ZPL, 6.2%). It was clear that the phonon-assisted recombination processes dominated the radiative relaxation behavior of both the aggregated and the isolated CdSe QD excitons. In total, peaks corresponding to these transitions comprised over 72% of the observable emission. A similar pattern was observed for the 1S(e) 1S3/2(h) transition in resonant excitation studies on wurtzite CdSe QDs.24 To our knowledge, this study represents the first time the phononassisted transitions have been resolved for both isolated and aggregated colloidal samples of CdSe quantum dots. C. Influence of Applied Magnetic Fields on PL of Nanocrystal Aggregates. The 1S(e) 1S3/2(h) transition of isolated (nonaggregated) nanocrystals has been the sole focus of most theoretical and experimental investigations of the exciton fine structure of nanocrystals and the influences of an external field on the corresponding relaxation mechanisms.26,37,43 48 Here, we present observations regarding the direct and phonon-assisted radiative recombination of aggregated and isolated nanocrystals. In the absence of rigorous calculations on aggregates, we propose 14521



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Figure 7. Degree of circular polarization as a function of applied magnetic field strength. Relative to the other transitions, the 1PLaggregate pathway showed a shallower dependence of the spin polarization on the magnetic field strength because of the spin character of the phonon required to relax radiatively from that state.

Figure 6. (A) PL intensities for each e h recombination transition observed in these studies as a function of increasing magnetic field strength. Dramatic growth of the ZPLaggregate transition was accompanied by decay in the 1PLaggregate signal as the strength of the magnetic field was increased. (B) Normalized PL intensity plotted as a function of applied magnetic field for all five experimentally measured MPL peaks. The ZPL contribution from both the isolated and the aggregated species exhibited the most sensitive, and similar, responses to applied magnetic fields.

that the aggregate exciton fine-structure states may be treated qualitatively as similar to the well-known and characterized 1S(e) 1S3/2(h) exciton fine-structure states of isolated nanocrystals. The total PL signal resulting from the aggregate PL increased upon application of the external magnetic field (Figures 3 and 5). This increase, shown in Figure 5, was quantified by integrating the 1PL and ZPL peaks of both aggregated and isolated species and finding the sum of their intensities. Mixing of bright (J = (1) and dark (J = (2) states leads to an increase in PL intensity because of an increase in efficient direct e h recombination and a concomitant reduction in nonradiative losses.26,32 These nonradiative losses were an obligatory byproduct of the 1PL pathway because, as the name implies, interaction with a 26 meV consuming phonon is required for this process, and electron hole trapping at nanoparticle surface defects becomes more likely in this scenario.60 Subjecting the CdSe QDs to an applied magnetic field led to mixing of the J = (2 and J = (1 states and subsequent thermally driven population of the J = (1 state from the J = (2 state. As a result,

the total PL increased by approximately 18%, and the PL from the aggregated species increased by approximately 9%. The shift in the dominant relaxation channel was manifested as an increase in signal arising from the ZPL pathway and a decrease in the 1PL signal, an effect that is clearly shown in Figure 6. As the magnitude of the applied magnetic field increased, the mixing became larger, effectively eliminating ΔEbd. This allowed for the lower energy J = (2 fine-structure state to mix with the J = (1 state. As the two states mixed, the J = (1 state was thermally populated from the J = (2 state, enhancing the direct radiative relaxation and reducing the contribution from the LO-phonon-assisted mechanism.24,39 The effect of field-induced mixing is clearly shown in Figure 6a, where the ZPL of both the isolated and the aggregated species increases by a factor of ≈1.4, while the phonon-assisted channels change by factors of 0.9 1.10. The positive identification of the phonon-assisted transition 1PLaggregate was further verified upon analysis of the degree of circular polarization for each individual radiative relaxation signal. The degree of circular polarization was calculated using the intensities of the “positive” (I+) and “negative” (I ) spin polarization signals. This was accomplished by sweeping the magnetic field from 10 T to +10 T and treating the PL emitted at “negative” field as I and the PL observed at “positive” field as I+ without altering the polarization resolving optics. The degree of circular polarization was determined using the relationship CP = I+)/(I + I+). Figure 7 portrays the degree of circular (I polarization as a function of applied magnetic field strength. The degree of circular polarization evident in the PL of QDs in applied magnetic fields indicates the presence of spin-polarized excitons.43 In a complementary study, it was reported that increasing magnetic field strengths induce an initial increase of spin-polarized PL followed by a plateau at a higher magnetic field.43 We observed that the degree of circular polarization increased with the magnetic field strength for the ZPLaggregate transition (and the transitions involving the isolated species). There was no indication of a plateau at the magnetic field strengths employed here, but we expect that the effect would have reproduced at higher magnetic fields (>10 T). The experimental resolution allowed for discrimination between the 1PL 14522



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and the ZPL transitions, permitting analysis of the 1PLaggregate transition. This pathway demonstrated dramatically different behavior. Specifically, the slope of the plot seen in Figure 6 was much shallower for the 1PLaggregate transition than for the ZPLaggregate transition because coupling with the LO phonon reduced the spin polarization of the exciton. Apparently, the formation of aggregates also diminished the degree of signal spin polarization in an applied magnetic field. Controlled mixing of the J = (2 and J = (1 states was further evidenced by the energy shift of the 1PLaggregate signal as a result of increasing magnetic field. This could be seen in both the raw and the deconvoluted data shown in Figure 3 and has been quantified in Figure 8 (values are given in Tables 1 and 2). The signal assigned to the 1PLaggregate transition was blue-shifted by approximately 3.6 meV as the magnetic field strength was increased from 0 to 10 T. By comparison, no significant energy shift was observed for the ZPLaggregate peak (within error). Thus, application of the magnetic field reduced ΔEbd, leading to (a) efficient mixing between the J = (2 and the J = (1 states, (b) redistribution of PL contributions, and (c) increased total PL. The observed 3.6 meV blue shift of the 1PLaggregate peak was resolved with our experimental setup and was consistent with the ΔEbd expected for isolated CdSe nanocrystals.28 The field-dependent energy shift for this peak supports the exciton fine-structure model. We note, however, that these data do not preclude surface-mediated recombination mechanisms, simply that the current study does not allow us to address surface-mediated

mixing. Future, time-resolved measurements are planned to investigate these possible mechanisms. To quantify further the effects of the external magnetic field on the relative PL contributions from the ZPLaggregate and 1PLaggregate exciton transitions, the fractional ZPLaggregate PL contribution [ZPL/(ZPL + 1PL)] was analyzed as a function of the energy shift determined for the 1PLaggregate transition at each applied magnetic field strength. As demonstrated in Figure 9, the blue shift observed for the 1PLaggregate transition energy (resulting from the field-induced reduction of ΔEbd) was well correlated to an increase in relative intensity for the ZPLaggregate transition. This clearly indicated that application of the external magnetic field led to mixing of the J = (2 and J = (1 states. The presence of the applied magnetic field resulted in an unambiguous increase in the total direct radiative PL of the targeted system. The bandgap magneto-PL of isolated nanocrystals was also studied. Consistency in transition assignments was preserved, and the three highest energy peaks in the PL spectrum were assigned to the ZPL, 1PL, and 2PL recombinations of the isolated nanocrystal 1S(e) 1S3/2(h) exciton. ZPLisolated denotes the direct radiative recombination from the J = (1 level; 1PLisolated and 2PLisolated represent the one- and two-phonon-assisted transitions from the J = (2 fine-structure state, respectively. The PL signals for each of these transitions can be seen qualitatively in Figure 3 and quantitatively in Figure 6 (the values obtained at zero field and 10 T are given in Tables 1 and 2). The

Figure 8. PL energies for each of the five component PL peaks as a function of increasing magnetic field strength. The 1PLaggregate transition exhibited the most significant energy shift with field.

Figure 9. Fractional PL contribution from the ZPL transition of the aggregated species plotted vs the change in energy of the 1PL peak with respect to its spectral position in the 0-T data. The ZPL intensity increased for stronger magnetic fields because of field-induced mixing of the J = (2 and (1 states. This shift toward a higher percentage of relaxation occurring via direct e h recombination resulted in the net increase in PL seen in Figure 5.

Table 2. Spectroscopic Transition Assignments, Energies, and Intensities at 10 T peak

designation

energy (eV)

intensity (a.u.)

fractional intensity (%)

a

1PLaggregate

2.1322 ( 0.00114

147506.7 ( 3614.10

28.2

b c

ZPLaggregate 2PLisolated

2.1553 ( 0.00042 2.1865 ( 0.00040

138553.3 ( 2699.12 102233.3 ( 2508.00

26.4 19.5

d

1PLisolated

2.2143 ( 0.00027

94316.7 ( 2175.17

18.0

e

ZPLisolated

2.2387 ( 0.00050

41337.3 ( 1990.23

7.9

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The Journal of Physical Chemistry C observed behavior could not easily be attributed to state mixing using an analysis analogous to that used for the 1S(e) 1S3/2(h) transition because all three PL components (ZPL, 1PL, and 2PL) increased at higher magnetic field, although subjecting the sample to a 10 T applied magnetic field increased the isolated nanocrystal PL by 21% relative to that observed at 0 T. Analysis of the individual PL contributions from these transitions demonstrated that the 2PL, 1PL, and ZPL signals increased by ∼11, ∼25, and ∼44%, respectively, in the presence of a 10 T field. Thus, it was clear that although emission was increasing for all three transitions, the fractional increase seen for the direct bright state was significantly higher than that observed for the phononassisted states, suggesting that state mixing is likely occurring in this regime as well. The observed redistribution of fractional intensity indicates a field-induced mixing of the J = (1 and (2 spin states of the aggregated species. As demonstrated for the 1S(e) 1S3/2(h) transitions, the field-induced alteration in the relative free energies of the fine-structure levels within the J = (1 and (2 states (which allows for mixing) was manifested in a narrowing of the energy gap between the PL peaks arising from the ZPL and 1PL processes. More specifically, the 1PL peak was blue-shifted as the ΔEbd diminished at higher magnetic field strengths. However, as shown in Figure 8, dramatic blue shifting was not observed for either the 1PL or the 2PL peak of the isolated nanocrystals. The spectral position of the 2PLisolated signal did not change when the magnetic field was applied. Similarly, the 1PLisolated peak shifted only slightly at maximum field strength— ΔE = 1.00 meV at 10 T. Also, the photon energy released from the ZPLisolated transition remained constant in the presence of the magnetic field (ΔE = 0.73 meV at 10 T), close to the limit of detection. In the absence of shifting PL peaks, it is not possible to state definitively that the redistribution of the fractional PL intensity observed for the three isolated nanocrystal transitions upon magnetic field application arises solely from a decrease in the ΔEbd between the J = (1 and the (2 states. Future, energyresolved PL lifetime measurements on either a similar low polydispersity aggregate sample or on single aggregate systems could help to resolve this issue. The effect of the external magnetic field application on the degree of circular polarization for the ZPLisolated, 1PLisolated, and 2PLisolated photons is shown in Figure 7. As noted earlier, the degree of polarization correlated to the 1PL and ZPL recombinations of the 1S(e) 1S3/2(h) exciton responded differently to magnetic field application, with the 1PL process having a much shallower dependence upon the strength of the applied field. In contrast, the degree of circular polarization observed for the photons emitted from all three transitions possible for recombination in isolated nanocrystals increased with a similar dependence on field strength that mimicked that observed for the ZPLaggregate process. Minor differences were observed in the degree of circular polarization at 10 T (ZPL, 0.278; 1PL, 0.255; and 2PL, 0.222). These degrees of circular polarization were in good agreement with theoretical prediction for a random distribution of spherical nanocrystals at 10 T.44

IV. CONCLUSIONS Magneto-optical PL experiments were performed on wellcharacterized colloidal CdSe nanocrystal aggregates. Branchingratio analysis of the radiative recombination transitions allowed for the quantification of the influence of an applied magnetic field

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on energetics of the “dark” J = (2 spin state. Peaks corresponding to transitions originating from this state were resolvable because the sample had a narrow size distribution and because the experiments were performed at 1.6 K, where LO-phononassisted recombination was a possible relaxation channel. The narrowing of the energy gap between the 1PLaggregate and the ZPLaggregate peaks in the PL spectrum upon application of a magnetic field indicated that the field-induced mixing between the J = (2 and the (1 levels decreased the effective ΔEbd and allowed for the mixing of these two states. In addition, the presence of an applied magnetic field changed the distribution of the total PL among the 1PL and ZPL channels, with the relative amplitude of the 1PL peak decreasing and the relative amplitude of the ZPL peak increasing. This reduction in phonon-assisted recombination in favor of direct recombination led to an increase in the total measured PL amplitude because the relaxation became more efficient and less energy was consumed nonradiatively by phonon interactions. Thus, we have demonstrated the ability to increase the global PL of a semiconducting nanocrystal aggregate simply by applying an external magnetic field. In addition, this work represents the first-ever examination of the effects of an external magnetic field on the direct and LOphonon-assisted photoemission nanocrystal aggregates, which is a necessary step for understanding the properties of nanocrystal arrays. These techniques could greatly impact the future design and construction of materials with applications in solar-toelectric energy conversion, biological labeling, quantum computing, sensor technology, and nanoparticle-based lasers.

’ ASSOCIATED CONTENT

bS

Supporting Information. Room-temperature PL of colloidal and drop-cast CdSe quantum dots and verification of single-exciton formation for CdSe PL at 1.6 K. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT K.L.K. gratefully acknowledges the Air Force Office of Scientific Research for financial support through the Young Investigator Program, Grant number FA9550-10-1-0300. TEM work was carried out using the facility at the National High Magnetic Field Laboratory supported by the NSF Cooperative Agreement No. DMR-0084173 and by the State of Florida. ’ REFERENCES (1) Huynh, W. U.; Dittmer, J. J.; Libby, W. C.; Whiting, G. L.; Alivisatos, A. P. Adv. Funct. Mater. 2003, 13, 73. (2) Huynh, W. U.; Dittmer, J. J.; Teclemariam, N.; Milliron, D. J.; Alivisatos, A. P.; Barnham, K. W. J. Phys. Rev. B 2003, 67, 115326. (3) Huynh, W. U.; Dittmer, J. J.; Alivisatos, A. P. Science 2002, 295, 2425. (4) Malko, A. V.; Mikhailovsky, A. A.; Petruska, M. A.; Hollingsworth, J. A.; Htoon, H.; Bawendi, M. G.; Klimov, V. I. Appl. Phys. Lett. 2002, 81, 1303. (5) Berezovsky, J.; Mikkelsen, M. H.; Gywat, O.; Stoltz, N. G.; Coldren, L. A.; Awschalom, D. D. Science 2006, 314, 1916. (6) Bruchez, M.; Moronne, M.; Gin, P.; Weiss, S.; Alivisatos, A. P. Science 1998, 281, 2013. 14524


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Magneto-photoluminescence Properties of Colloidal CdSe

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