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Giant Excitonic Exchange Splittings at Zero Field in Single Colloidal CdSe Quantum Dots Doped With Individual Mn Impurities 2+

Rachel Fainblat, Charles J. Barrows, Eric Hopmann, Simon Siebeneicher, Vladimir A. Vlaskin, Daniel R. Gamelin, and Gerd Bacher Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.6b02775 • Publication Date (Web): 20 Sep 2016 Downloaded from http://pubs.acs.org on September 20, 2016

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Giant Excitonic Exchange Splittings at Zero Field in Single Colloidal CdSe Quantum Dots Doped With Individual Mn2+ Impurities Rachel Fainblat1,2*, Charles J. Barrows2, Eric Hopmann1, Simon Siebeneicher1, Vladmir A. Vlaskin2, Daniel R. Gamelin2 and Gerd Bacher1 1

Werkstoffe der Elektrotechnik and CENIDE, University Duisburg-Essen, Bismarckstr. 81, Duisburg, 47057 Germany

2

Department of Chemistry, University of Washington, Seattle, Washington 98195-1700, USA

ACS Paragon Plus Environment

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ABSTRACT: Replacing a single atom of a host semiconductor nanocrystal with a functional dopant can introduce completely new properties potentially valuable for "solotronic" information-processing applications. Here, we report successful doping of colloidal CdSe quantum dots with a very small number of manganese ions – down to the ultimate limit of one. Single-particle spectroscopy reveals spectral fingerprints of the spin-spin interactions between individual dopants and quantum-dot excitons. Spectrally well-resolved emission peaks are observed that can be related to the discrete spin projections of individual Mn2+ ions. In agreement with theoretical predictions, the exchange splittings are enhanced by more than an order of magnitude in these quantum dots compared to their epitaxial counterparts, opening a path for solotronic applications at elevated temperatures.

KEYWORDS: Nanocrystal, single particle, photoluminescence, solotronics, diluted magnetic semiconductors, giant Zeeman splitting


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The goal of combining magnetic and semiconductor properties in a single material was first realized experimentally in the late 1970s with the incorporation of transition-metal ions into ordinary bulk semiconductors, forming the class of multifunctional materials known as a diluted magnetic semiconductor (DMS).1,2 In recent years, the development of DMSs has experienced a renaissance due to the successful incorporation of dopants into colloidal semiconductor nanostructures.3 The strengths of the average dopant-carrier (sp-d) magnetic exchange interaction in DMS nanostructures are increased by the greater spatial overlap between the impurities and the squeezed wavefunctions of the semiconductor charge carriers, strongly enhancing their giant magneto-optical response,4–14 which manifests itself even up to room temperature.7,15,16 Up to now, DMS nanocrystals have been doped with multiple impurities, yielding magneto-optical responses from the interaction between the spins of the charge carriers and the ensemble of dopants. This collection of dopant spins exhibits paramagnetism in the presence of an external magnetic field. Enabled by advances in doping of nanoscaled materials even to a single-impurity level, the emerging research field of solotronics - optoelectronics based on the functionality of solitary dopants - has generated broad interest over the past few years.17–22 Investigation of the luminescence of individual DMS quantum dots (QDs) grown by molecular beam epitaxy provided a major step forward in the understanding of sp-d exchange interactions between charge carriers and few or even single impurities.23–25 Among other accomplishments, such materials have allowed the spins of solitary magnetic ions to be manipulated both optically26–28 and electrically29 at cryogenic temperatures. To date, however, these studies have been restricted to epitaxially grown QDs with relatively weak quantum confinement.23–27,29–31 Because the sp-d exchange interaction is expected to be inversely proportional to the QD volume,32,33 single-atom


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doping of colloidal nanostructures with even stronger quantum confinement would be of outstanding interest because this may allow solotronic applications even at elevated temperatures. Additionally, colloidal syntheses excel at nanocrystal shape, size, and composition control, and yield solution processable nanocrystals, introducing new degrees of flexibility not accessible with epitaxial growth. A key piece of evidence of the exchange interaction between an exciton and a single magnetic ion in a zero-dimensional nanostructure is the observation of multiple spectrally resolved peaks in photoluminescence (PL), representing different spin projections of the dopant interacting with the photo-generated electron-hole pair (exciton). For example, in the case of Mn2+ doping, splitting of the excitonic emission into six peaks is expected in the PL spectrum, related to the six spin projections (±5/2, ±3/2 and ±1/2) of the Mn2+ ion's S = 5/2 ground state. Theoretically, an enhancement of this splitting by up to two orders of magnitude has been predicted for colloidal QDs with strong quantum confinement compared to their self-assembled counterparts,32,34 but experimental validation is still missing, in part due to synthetic challenges. Despite recent advances in doping chemistries of nanostructures35–37 - allowing the incorporation of impurities up to >30%16,38 – much less is known about the extreme case of a few or even only one dopant per nanocrystal. Most recently, the incorporation of up to two Mn2+ ions into extremely small (CdSe)13 clusters has been demonstrated.14 In this case, however, the investigation of the exchange interaction between exciton and dopant spin states is hindered by the fact that the photoluminescence is dominated by the manganese 4T1-6A1 internal transition. No singlenanocrystal experiments on any colloidal nanocrystals doped with individual Mn2+ ions have been reported so far.


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In this paper, we report the single-nanoparticle spectroscopy of colloidal CdSe QDs containing very small numbers of magnetic impurities, including down to a single Mn2+ ion. Mn2+ ions are incorporated into high-quality pre-formed CdSe QDs by diffusion doping, as detailed previously.16,38 Ensemble magneto-optical characterization demonstrates the successful replacement of a very small number of Cd2+ ions by Mn2+. Single-particle spectroscopic studies on QDs of different nominal doping concentrations reveal signatures of sp-d exchange interactions between excitons and the spin projections of a few down to a single Mn2+ ion. A quantitative comparison between the experimentally observed energy difference between the spin-split energy states and the theoretically predicted values is presented and reveals large excitonic exchange splittings at zero applied magnetic field. Using the diffusion-doping method,16,38 Mn2+-doped CdSe QDs of different core sizes, doping concentrations,

and

shell

structure,

were

synthesized.

In

one

sample

(Cd0.9997Mn0.0003Se/CdSe/CdS), CdSe cores were doped with Mn2+ to an average concentration of 0.030 ± 0.002 % (concentration determined from the temperature dependent magneto-optical response).39 These doped NCs were then coated with ~1 monolayer of CdSe to reach a total diameter of d = 5.1 nm (Cd0.9997Mn0.0003Se/CdSe), followed by a large CdS shell (17 monolayers), thus leading to a total nanocrystal diameter of d = 16.4 nm. In a second sample (Cd0.9997Mn0.0003Se/CdSe/ZnS), similar doped cores (Cd0.9997Mn0.0003Se/CdSe) were coated with a ZnS shell (2 monolayers). In a third sample (Cd0.996Mn0.004Se/ZnSe), d = 5.4 nm CdSe cores were doped with an average Mn2+ concentration of 0.40 ± 0.04 % (concentration determined using inductive coupled plasma atomic emission spectroscopy) and then coated with a onemonolayer ZnSe shell.


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Figure 1 plots the absorption and magnetic circular dichroism (MCD) spectra of the Cd0.996Mn0.004Se cores prior to shell growth, measured at 1.8 K as a thin film prepared by dropcasting the nanoparticle dispersion onto a quartz substrate. MCD spectra were collected at various magnetic fields in the Faraday geometry using an Aviv 40DS spectropolarimeter, and data are represented following the sign convention of Piepho and Schatz.40 In the absorption spectrum, the first excitonic transition is observed at ~2.14 eV, and several excitonic features are observed to higher energy. The MCD spectra show several pronounced magneto-optically active features, whose amplitudes increase with increasing applied magnetic field, reflecting the fielddependence of the excitonic Zeeman splittings. Analysis allows the contributions of each transition to be quantified separately,9 and the magnitude of the first exciton's Zeeman splitting (∆ ) to be determined. The inset plots ∆ versus magnetic field. The data show the characteristic Brillouin-like behavior of paramagnetic Mn2+, increasing linearly with magnetic field in the low-field (Curie) regime and saturating at higher fields. A saturation value of ∆ = ~ -16 meV is obtained for the Cd0.996Mn0.004Se nanocrystals. This saturation magnetization reflects the dominance of sp-d exchange contributions to the excitonic Zeeman splitting under these conditions.


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Figure 1. Absorption and MCD spectra of Cd0.996Mn0.004Se nanocrystals measured at 1.8 K. The absorption data were collected at zero magnetic field. Inset: Extracted magnetic-field dependence of the first excitonic Zeeman splitting. Samples for single-nanocrystal PL measurements were prepared by spin-coating highly diluted suspensions of the same nanocrystals onto silicon substrates. Scanning electron microscope (SEM) images (see Supporting Information) depict a typical nanocrystal distribution of a sample after the spin-coating procedure. Note that the nanocrystal suspension used for SEM was further diluted by 250 times before spin coating for the single-QD PL measurements. Figure 2 shows a series of optical images of the luminescence of one typical single QD, recorded at 5 K with integration times of 5 s each. Each image (21x21 pixels) represents an area of ca. 3.8 x 3.8 µm2. Over the time sequence shown here, this nanocrystal alternates between on- to off-states reflecting the blinking behavior characteristic of individual colloidal nanocrystals.41


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Figure 2. Images of the 5 K emission of a single Cd0.996Mn0.004Se/ZnSe nanocrystal deposited on a silicon substrate taken with a nitrogen-cooled CCD-camera with an integration time of 5 s each. The different intensities can be taken from the colored scale on the right. Figure

3

depicts

the

photoluminescence

(PL)

spectra

of

two

selected

Cd0.9997Mn0.0003Se/CdSe/CdS nanocrystals taken at 4.7 K with an integration time of 1 s each. The bottom curve shows a spectrum with multiple features and 6 dominant maxima. We exclude the existence of biexcitons and multiexcitons, because their observation would require substantially higher excitation densities (> 3500 W/cm2)42 than what was used in our experiments (< 10 W/cm2). We also rule out the possibility that some of these lines arise from charged excitons while others arise from neutral excitons, because charged excitons in colloidal CdSe QDs of these sizes show efficient non-radiative Auger recombination.43 The fine structure splitting of the band-edge excitonic state might possibly cause multiple emission features from undoped CdSe QDs, but both ±2 and 0L states are optically forbidden,44 thus only three remaining states (±1L, 0U, ±1U) could be observed in PL, a clear mismatch to the number of peaks observed in our experimental spectra. While the intensity ratio between dark and bright excitonic emission features exhibits a strong temperature dependence,45 the intensity ratios in our samples do not show similar behavior: Increasing the temperature from 4.1 to ~ 7 K does not alter the relative intensities of the various lines (see Supporting Information). At higher


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temperatures, the spectrum broadens such that above ~ 35 K individual lines are not clearly distinguishable (see Supporting Information). Moreover, the PL spectra of undoped nanocrystals measured in this study consist only of a zero phonon line and one phonon replica, thus not exhibiting the fine structure manifold. We therefore rule out the fine structure splitting as the origin of the multiple lines in the high-energy region of our single nanocrystal spectra. As described above the observation of six spectrally resolved lines in the single QD PL spectrum is a signature of the sp-d-exchange interaction between an exciton and a single Mn2+ impurity ion. In this low doping concentration regime, the theoretical probability of finding a nanocrystal containing a single Mn2+ ion is ~ 27%, assuming a Poissonian dopant distribution and taking into account the QD Gaussian size distribution (as determined by transmission electron microscope analysis – see Supporting Information). Due to the high probability of finding a nanocrystal containing single Mn2+ impurities, and the similarity between this spectrum and experimental results reported for self-assembled QDs doped with individual Mn2+ ions,23,25 we interpret the multiple PL lines as being related to zero-field sp-d exchange in a QD containing a single Mn2+ impurity. Notably, the energy splitting between the two outer lines is ~ 35 meV, which is over an order of magnitude larger than observed in self-assembled QDs containing single Mn2+ impurities.23,46 This increase of the giant excitonic exchange splitting is discussed in detail later in this manuscript. In addition to spectra of this type, several QDs show more complex spectra. The dark blue curve in Figure 3 depicts the spectrum of another single QD from the same ensemble. Here, several more peaks are observed and the total energy scale is greater. This behavior is attributed to the presence of two or more Mn2+ dopants in this single QD, as reported for self-assembled QDs.47 We conclude the successful observation of giant excitonic exchange splittings at zero field in individual colloidal CdSe QDs with single and few Mn2+ impurities.


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Figure 3. PL spectra of two selected Cd0.9997Mn0.0003Se/CdSe/CdS nanocrystals taken at 4.7 K with an integration time of 1 s.

Because the exchange interaction between an exciton and a Mn2+ dopant depends on the exciton's confinement volume,32,33 QDs with stronger confinement than those of Figure 3 were also examined. Figure 4a shows PL spectra of several individual Cd0.9997Mn0.0003Se/CdSe/ZnS (labeled I and II) and Cd0.996Mn0.004Se/ZnSe (III, IV and V) QDs. Some spectra (e.g., I and III) are similar to previously reported pure CdSe single-particle spectra,48 exhibiting a zero phonon line at about 2.04 eV (spectrum I) or 2.05 eV (spectrum III), accompanied by an LO phonon replica about 27 meV to lower energy. The observation of undoped CdSe nanocrystals is not surprising because of the extremely low Mn2+ concentration of 0.03 % in this sample, which makes the probability of finding undoped nanocrystals within an ensemble ~ 67 %. For higher dopant concentrations (xMn = 0.4 %) the probability of finding undoped nanocrystals becomes vanishingly small (~ 0.2 %). Indeed, we found only one QD in experiments on the


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Cd0.996Mn0.004Se/ZnSe QD sample whose emission spectrum appeared undoped (III in Figure 4.a). Importantly, the PL spectra of many of the individual QDs exhibit a large number of peaks, whereby a quite general pattern can be identified. Several individual emission peaks spectrally separated by a few to tens of meV are observed in most cases across all 3 samples of nanocrystals investigated. The energy splitting between the individual peaks is largest for the Cd0.9997Mn0.0003Se/CdSe/ZnS QDs, where quantum confinement is the strongest, and it is smallest in the case of the giant-shelled Cd0.9997Mn0.0003Se/CdSe/CdS QDs, where the exciton wavefunction occupies the greatest volume. As detailed below, the large number of PL features in these single QD PL spectra arise from the combined effects of exciton-phonon coupling and sp-d exchange interactions between excitons and small numbers of Mn2+ ions.

Figure 4. (a) Low-temperature PL spectra of several single CdSe nanocrystals containing Mn2+ dopants. Spectra I and II are related to emission from single Cd0.9997Mn0.0003Se/CdSe/ZnS nanocrystals, while spectra III, IV, and V are related to Cd0.996Mn0.004Se/ZnSe nanocrystals. (b)


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Top: Time-integrated (3 to 48 s) PL-spectrum of a selected Cd0.9997Mn0.0003Se/CdSe/ZnS nanocrystal. Bottom: Emission spectra of the same Cd0.9997Mn0.0003Se/CdSe/ZnS nanocrystal depicted in the top panel collected over time (colors indicate the intensity). The integration time of each spectrum is 1 s.

One effect that could conceivably lead to the observation of several emission lines is spectral diffusion.49 To probe the time dependence of the emission of a single nanoparticle, we collected a large number of spectra with a short integration time (1 s each). Representative results for a single Cd0.9997Mn0.0003Se/CdSe/ZnS QD are presented in Figure 4b. The top spectrum shows the time-averaged PL of this nanocrystal. In the bottom two-dimensional plot, the energy of the emitted photons (x-axis) is plotted over time (y-axis), where the colors represent the PL intensities (see scale on the right side of the plot). The intensities of the emission lines from the time-averaged spectrum vary in time, but these lines exhibit neither a significant change in energy nor linewidth broadening with increasing integration time (see also Supporting Information), allowing spectral diffusion to be ruled out as the source of the multiple lines. Although the exact origin of these intensity fluctuations isn´t clear yet, a similar effect has been observed in epitaxially grown QDs and it is probably related to fluctuations of Mn2+ spin projections during the exciton lifetime.50 We thus propose that the multiple peaks observed in these PL spectra are signatures of sp-d exchange coupling between excitons and single or few Mn2+ ions with different spin projections. To investigate the origins of the complex spectral features observed in Figures 3 and 4, the 5 K PL spectra of two representative Cd0.996Mn0.004Se/ZnSe single particles with well-resolved peaks have been analyzed in detail. Figure 5a illustrates our approach for extracting the energetic


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positions of the individual PL features within a given spectrum. The experimental data are represented by the black dots, and the colored peaks reflect individual features resolved using a multi-peak fitting procedure. The black curve depicts the sum of the fitted peaks and closely matches the experimental data. In Figure 5b, the red dots represent the energies of all of the fitted features in the spectrum of Figure 4a. A similar analysis was conducted for another representative single nanocrystal (named NP VII, see Supporting Information), whose extracted energetic positions for the PL features are represented by the green dots in Figure 5b. Note that the majority of the Cd0.996Mn0.004Se/ZnSe nanocrystals do not exhibit spectrally well-resolved features due to the high average number of Mn2+-ions per QD (5 to 6 Mn2+/QD). Therefore, to analyze the interaction of an exciton with the different spin projections of one or few Mn2+ ions, the spectra reported here arise from selected particles that display well-resolved features.


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Figure 5. (a) PL spectrum of a single Cd0.996Mn0.004Se/ZnSe nanocrystal collected at 5 K. The dots depict the experimental data while the colored shaded peaks represent the fitted peaks. The sum of the fitted peaks is represented by the black line. The red curve accounts for the broad luminescence background, attributed the multiple weak phonon replicas of the various purely electronic transitions. (b) Energy position of fitted peaks for two different single nanoparticles, including the nanoparticle analyzed in panel a.

From the energy differences between neighboring features found in Figure 5b, we can distinguish between two different groups of PL peaks: one at lower energy and one at higher


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energy. The lower-energy peaks can be assigned to phonon replicas because of their characteristic energy spacing of ~ 27 meV, in accordance with literature reports.48,51 The energy differences between neighboring peaks in the higher-energy part of the spectrum are significantly smaller than 27 meV, reflecting a completely different physical origin. These splittings are associated with sp-d exchange. Each electronic origin gives rise to phonon replicas, generating the broad background intensity observed in Fig. 5a. To place this analysis on a quantitative footing, we estimated the theoretically expected energy differences arising from a single exciton-Mn2+ sp-d interaction, focusing on the splitting between the two peaks related to the Mn2+ spin projections Sz = 5/2 and Sz = -5/2. The exciton-Mn2+ sp-d exchange splitting is maximized in the ideal case of a single dopant located at the center of the QD, because this interaction is proportional to the spatial overlap between the exciton wave function and the dopant, which is greatest at the QD center. The contributions from the electronMn2+ and hole-Mn2+ interactions, represented by the exchange integrals Ie and Ih, respectively, were calculated separately according to Bhattacharjee and Pérez-Conde32 Ie (Ih) is described by the mean-field exchange constant between the electrons (holes) and the magnetic ions, N0α = +0.23 eV (N0β = -1.27 eV)52 modified by the dopant-carrier wavefunction overlap, given by the probability density of a charge carrier at the QD center. Specifically, Ie and Ih can be calculated based on the following equations:32

= 0 = | 0| =

= 0 = |0| =

= 0 = /

(1)

√

√

√$ ∙&'()*$+

(2)

!1 − &'(,√$∙*$- .

(3)


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Here, / describes the QD radius, 0(/) is a size-dependent normalization constant (equals 2.946 for a 2.7 nm radius), and 1 represents the ratio between the light-hole and heavy-hole effective masses. 21 is the first root of 3 213 4√1 ∙ 215 + 3 213 4√1 ∙ 215 = 0, where 37 8 is a spherical Bessel function of n-th order. For CdSe, 1 = 0.28,43 resulting in 21 = 4.58. For the Cd0.996Mn0.004Se/ZnSe QDs, the values Ie = 0.99 meV and Ih = -2.59 meV are calculated. The splitting between the emission peaks related to the Mn2+ spin projections Sz = 5/2 and Sz = 5/2 are then calculated using ∆Et = -∆Sz(seIe – mJ_h Ih), where se = 1/2 and mJ_h = 3/2, representing the electron spin projection and the magnetic total angular momentum of the hole, respectively. For a Cd0.996Mn0.004Se QDs containing a single Mn2+ at its center, we expect the difference in energy between the Sz = -5/2 and the Sz = +5/2 states (∆Sz = 5) to be ∆Et ≈-22 meV. Because the expected splitting ∆Et calculated above assumes the ideal location of a dopant at the QD center, in a further approach the statistical dopant distribution within the dot will be taken into account. Here, we calculate the mean value of the exchange integral between electron (hole) and dopant - represented by 9 (9 ) - by integrating the probability of finding a charge carrier over the nanoparticle and normalizing it by the QD volume :, as shown in the following equation for the case of 9 : ; ; 9 = < =| | >: = < =

;

?@A

(4)

We can assume that the measured giant Zeeman splitting in the QD ensemble using MCD (see Figure 1) is proportional to the mean sp-d exchange interaction averaged over all the QDs within the ensemble. For the Cd0.996Mn0.004Se QDs, ∆ at saturation (T = 1.8 K and B > 2 T) was found to be -16 meV. Dividing the experimental ∆ by the average number of Mn2+ per QD (6 Mn2+ ions), we can determine the average per-Mn2+ Zeeman splitting between the Sz = -5/2 and the Sz = +5/2 states, which is ~-2.7 meV/Mn2+. Using ∆ and the ratio between the ACS Paragon Plus Environment

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B

exchange integral at the QD center and the mean exchange integral ( 9C and B C

BD ), EEE BD

we calculate the

expected Zeeman splitting between the Sz = -5/2 and the Sz = +5/2 state if the QD would contain a single dopant exactly at the QD’s center:

∆F = !0.11 ∙

∆ ∆ ∙ . + !0.89 ∙ ∙ . 5 6 6 9 9 = 0.11 ∙ −2.7 OP: ∙ 6.58 + 0.89 ∙ −2.7 OP: ∙ 8.93 ≈ − 23 OP:

Here, 0.11 and 0.89 are the weighting factors describing the relative Mn2+-electron and Mn2+hole exchange contributions to the total excitonic exchange splitting. The value of -23 meV calculated based on the MCD data agrees well with the value of ~-22 meV calculated following Bhattacharjee and Pérez-Conde. These values are an order of magnitude larger than those reported for single-Mn2+-doped self-assembled QDs. Despite experimental confirmation of the anticipated increase in sp-d exchange splittings in the colloidal QDs relative to self-assembled QDs, we cannot fully explain the very large magnitudes of the spin splittings observed in Figure 5. The anticipated maximum excitonic exchange splittings (|∆Et| ~ 22 to 23 meV) are significantly smaller than the values estimated from the energy difference between extreme sp-d peaks in the two single-QD PL spectra of Figure 5 (∆Et ~ 40 and 80 meV), i.e., a factor of as much as ~3.6. To rationalize the ~3.6x larger energy scale found in the single-QD PL than anticipated by theory and MCD spectroscopy, three possible explanations have been considered: (i) the QDs in Figure 5 contain more than a single Mn2+ ion, (ii) these single QDs have significantly smaller diameters than the ensemble-average value used for the calculation of Ie and Ih based on Eq. 1 and 2, and (iii) the sp-d exchange interactions are anisotropic. An increase of ∆Et could arise from QDs containing more than one Mn2+ ion, as


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demonstrated for epitaxially grown QDs.47 Some PL spectra do indeed suggest the incorporation of more than one Mn2+ ion in the nanocrystal (see, e.g., Fig. 3, top; Fig. 4, NC V). To increase ∆Et by a factor of 3.6, however, the individual QDs must possess at least 4 Mn2+ ions, which is exceedingly unlikely at our at our lowest doping concentrations, where similarly large splittings are observed (e.g., spectrum II in Figure 3, from a Cd0.9997Mn0.0003Se/CdSe/ZnS QD, showing ∆Et ~ 90 meV). In this scenario, it is also unlikely that so few discrete PL lines would be observed. Additionally, to observe the full 3.6-fold increased energy splitting in this case, all 4 Mn2+ spin projections must be co-aligned for a significant fraction of the measurement time, a highly improbable occurrence. We therefore rule this scenario out as the only explanation for the increase of the energy splitting. For case (ii), an increase in ∆Et is indeed expected for smaller QDs because of the strong size dependence of both Ie and Ih, but decreasing the QD diameter by one standard deviation below the ensemble average (see Supporting Information) should only increase ∆Et by less than 50%, making this scenario also unlikely to explain the data. Finally, for case (iii), it is important to recognize that the above analysis has assumed perfectly spherical QDs with isotropic excitons. In actuality, wurtzite structured CdSe has an axial crystal structure that generates exciton anisotropy even in bulk, and colloidal QDs additionally have facets and low-symmetry shapes that introduce further anisotropy by giving relaxed excitons a preferred spatial orientation that does not necessarily align with crystallographic axes. Although the spherical approximation is suitable for describing ensemble-averaged properties, such as the MCD intensities of Figure 1, the single-QD PL measurements are sensitive to such anisotropies. We thus hypothesize that the large ∆Et values observed in some of our single QDs reflect anisotropic deviations from the idealized spherical QD geometry and oriented cubic lattice


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structure. This hypothesis is consistent with theoretical studies addressing anisotropic sp-d exchange arising from nanocrystal shape and crystallographic site anisotropies.34,53 To support the plausibility of this hypothesis, we compare our experimental single-QD values of ∆Et ~ 40 – 80 meV with the results from ref.

46

for an oriented non-spherical epitaxial QD

containing a single Mn2+ impurity, which shows ∆Et > 2 meV. The values of Ie = 0.99 meV and Ih = -2.59 meV determined above for the colloidal QDs are roughly 66 and 13 times greater than the analogous values reported in ref.

46

for this epitaxial QD (0.015 and -0.195 meV,

respectively). From this comparison, we can estimate ∆Et in comparably anisotropic colloidal QDs to be ~(0.11x66 + 0.89x13) = 19 times larger than in the epitaxial QD of Kobak et al.,46 reaching a value of ~ 38 meV. Although not conclusive, this comparison supports the hypothesis that anisotropy may be the origin of the greater single-QD excitonic exchange splittings observed in our colloidal QDs than predicted from theory. In summary, we have synthesized colloidal CdSe QDs doped with an extremely small number of magnetic impurities per QD. These QDs exhibit giant magneto-optical responses due to exciton-Mn2+ sp-d exchange interactions. PL experiments on single QDs reveal the spectral fingerprints of single- and few-atom doping, reflecting discrete spin projections of Mn2+ ions interacting with the optically generated excitons. The experimental trends confirm the significantly enhanced spin-spin interactions expected due to strong quantum confinement predicted more than a decade ago,32 and actually exceed theoretical predictions for spherical QDs. The very large spin splittings observed here for single QDs with multiple discrete Mn2+ spin projections may have interesting implications for future room-temperature solotronics investigations.


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Methods. Images and PL spectra of single QDs were obtained using a custom far-field epifluorescence setup in which the sample was mounted on the cold finger of a liquid helium cryostat and cooled to 5 K. The excitation source (532 nm diode-pumped solid-state laser) was reflected by a mirror at an angle of 45° and focused by a 63x magnification, long-working-distance microscope objective with a numerical aperture NA = 0.75 to a large spot (diameter > 2 µm) on the sample surface. The same microscope objective collected the emitted photons, which passed through a sharp-cutoff long-pass filter after being reflected by the same mirror used in the excitation path. Finally, the image was focused onto the monochromator entrance slit, reflected either by the chosen grating at zero-order (for images) or at first order (for spectrally resolved PL data) and detected with a liquid nitrogen-cooled charge coupled device (CCD) camera.

ASSOCIATED CONTENT Supporting

Information.

Scanning

electron

microscope

images,

single-particle

photoluminescence spectra, and transmission electron microscope images. This material is available free of charge via the Internet at http://pubs.acs.org

AUTHOR INFORMATION Corresponding Author *[email protected] Author Contributions


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The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Funding Sources Deutsche Forschungsgemeinschaft (DFG) under contract Ba 1422/13, (DAAD) under the P.R.I.M.E. program. U.S. National Science Foundation (DMR-1505901)

ACKNOWLEDGMENT We acknowledge the financial support by the Deutsche Forschungsgemeinschaft (DFG) under contract Ba 1422/13 and the German Academic Exchange Service (DAAD) under the P.R.I.M.E. program (R. F.). Support from the U.S. National Science Foundation (DMR-1505901 to D.R.G.) is gratefully acknowledged. We thank Franziska Muckel for fruitful discussions, Florian May for the assistance with the single particle photoluminescence experimental setup, and Heidi Nelson for assistance with the exchange integrals calculations. REFERENCES (1)

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