Manipulation of Carrier–Mn2+

Article pubs.acs.org/JPCC

Manipulation of Carrier−Mn2+ Exchange Interaction in CdTe/CdSe Colloidal Quantum Dots by Controlled Positioning of Mn2+ Impurities Nathan Grumbach,* Anna Rubin-Brusilovski, Georgy I. Maikov, Evgeny Tilchin, and Efrat Lifshitz* Schulich Faculty of Chemistry, Russell Berrie Nanotechnology Institute and Solid State Institute, Technion−Israel Institute of Technology, Haifa 32000, Israel ABSTRACT: Intentional inclusion of manganese ions (Mn2+) into colloidal quantum dots (CQDs) with precise positioning at a substitution lattice site provides a way to explore the interaction between photogenerated species (exciton, electron, hole) and Mn2+ spin states. We investigated the influence of Mn2+ impurity on the optical properties of CdTe/CdSe CQDs. Host CQDs exhibit a quasitype-II characteristic, with asymmetric delocalization of the carriers over the entire core/shell structure. Mn2+ ions were precisely embedded either at the core or at the shell, using controlled layer-bylayer growth. Optical measurements carried out at cryogenic temperatures revealed a change in radiative lifetime, induced circular polarization, and energy split of exciton band-edge emission in the absence of an external magnetic field, when the Mn2+ ions were situated within the core. The results suggest the existence of a significant hole−Mn2+ exchange interaction and demonstrate a way to control band-edge optical properties in confined systems. host nucleation stage from that of the cation substitution,19 aiming to achieve precise control of the shape20 and impurity positioning.21,22 For example, a recent study reported the synthesis of Mn2+-substituted core/shell CQDs,22 using a threestep approach including growth of a core, insertion of impurity, and covering by a shell. This paper describes the synthesis and optical investigation of CdTe/CdSe core/shell CQDs embedded with a low dilution of Mn2+ ions. The CdTe/CdSe CQDs were selected for the current study due to their quasi-type-II characteristic,23 providing major localization of the hole at the core regime and delocalization of the electron over the entire structure, offering selective interaction of carriers with spin states of the embedded impurities. Also, the CdTe/CdSe CQDs have previously shown a reduction of the Auger relaxation process and generation of neutral multiple excitons;24 thus, spectral stability is expected from these core/shell CQDs. A previous effort localized magnetic atoms in a core of ZnSe/CdSe core/ shell CQDs,5 while reduction of Auger relaxation was found in Mn2+-substituted CdSe CQDs.25 The current work presents an extension of previous studies in the control, concentration, and precise positioning of the impurity ions at different radial distances from the core center, ending either at the core, at the shell, or at the interface, with the optical investigations revealing strong dependence of the CQDs’ excitonic properties on the Mn2+-ion’s position.

1. INTRODUCTION Colloidal quantum dots (CQDs) have been the subject of considerable scientific interest during the past two decades, with potential implementation in various optoelectronic devices1 and biological platforms.2 Applications should benefit from high-quality CQDs with intentional inclusion of foreign impurities at substitution or interstitial lattice positions. Special attention has been given to CQDs embedded with magnetic impurities, such as a Mn2+ ion with half-filled d orbitals at the ground state (S = 5/2).3 The confinement of the impurity within a CQD induces coupling between the host carriers and the impurity atomistic states, inducing either an energy transfer or creation of a strong band-edge Mn2+ (sp−d) exchange interaction, depending on the valence and conduction bandedge energy offset with respect to the Mn2+ ground state (4T1) and the first excited state (6A1).4 For example, sp−d interaction creates in certain conditions magnetic polarons (MPs), with a giant Zeeman splitting of the valence and conduction band states5 and a large Faraday rotation.6,7 Consequently, the insertion of impurities artificially tailors the electronic and/or magnetic properties of the CQDs,8 leading to the development of novel spintronics and quantum information devices.9 The most common synthesis procedure for CQDs is related to a fast injection of precursors into a batch pot under inert conditions.10 In the past, Mn2+ inclusion in CQDs was carried out by mixing relevant precursors into the host reaction solution.11,12 This simple approach occasionally struggled with a slow substitution rate 13 that reached a substitution concentration < 1%14 or led to complete expelling of the impurities toward the CQDs’ surfaces.11,15,16 Current synthesis procedures have overcome previous difficulties, and some successes have been achieved,12,17,18 mainly by decoupling the © 2013 American Chemical Society

Received: August 28, 2013 Revised: September 15, 2013 Published: September 17, 2013 21021

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2. EXPERIMENTAL SECTION 2.1. Synthesis of CQDs. The synthesis strategy used an adapted three-step approach, when primary cores were prepared at 310 °C via a high-temperature organo-metallic reaction in an oxygen-free atmosphere, as described in ref 26. Then all cores were covered by two monolayers of CdTe and three monolayers of CdSe. A single monolayer is composed of successive cadmium and chalcogenide atomic-thick sublayers formed via consecutive instantaneous injections at 150 °C of stoichiometric amounts of the relevant precursors, continued by a pause of 15 min for completion of the reaction. The primary mother solution had a 0.25 × 10−7 M core concentration. The surplus monolayers’ cation precursors included 0.04 M Cd-acetate and 0.32 M oleylamine, and 8 × 10−4 M Mn-acetate (dissolved in octadecene (ODE)). The complementary anion precursors included 0.05 M tributylphosphine-chalcogenide (Te or Se) and diphenylphosphine with a 1:1 ratio (in ODE). It should be noted that CQD precipitation was carried out (using ethanol/acetone mixer) after the deposition of each supplementary monolayer, followed by a rinse with hexane, and redispersion in a new reaction solution (in ODE). The fresh solutions were used for characterization of the intermediate stages, and for further deposition of the successive monolayers. 2.2. Transmission Electron Microscopy (TEM). The morphology and size of the CQDs and their composition were measured via inspection of the TEM images (TEM model Tecnai T20). The CQDs were dispersed on a copper grid (300 mesh) coated with amorphous carbon film by drop-casting from a chloroform solution. The crystallographic structure of the CQDs was confirmed by a selective area electron diffraction measure, while the chemical composition was derived from the energy-dispersive X-ray analysis, as well as by X-ray photoelectron spectroscopy. 2.3. Electron Spin Resonance (ESR). ESR was used to confirm incorporation of Mn2+ ions in CQDs at selected positions. The ESR spectra were recorded on a Bruker EMX10/12 X-band (ν = 9.4 GHz) digital ESR spectrometer equipped with a Bruker N2-temperature controller. ESR spectra were recorded at a microwave power of 63 mW. Digital field resolution was 2048 points per spectrum, allowing all hyperfine splitting to be measured directly with an accuracy of better than 0.1 G. 2.4. Optical Measurements. The impact of the presence of Mn2+ on the CDQs’ physical properties was investigated by following their optical transitions. The absorption spectra of the samples were recorded on a JASCO V-570 UV−vis-NIR spectrometer. The continuous-wave photoluminescence (cwPL) or photoluminescence (PL) spectra were obtained by exciting the samples with a tunable Ar+-ion laser, a Coherent 890 (Eexc = 2.4 eV), while immersing the samples in a variabletemperature (1.4−300 K) Janis cryostat. The emission was detected with an Acton Spectrapro 2300i monochromator equipped with a photon multiplier tube, a Hamamatsu NIRPMT H10330-75. The time-resolved PL (tr-PL) curves were recorded by exciting the samples with a Nd:YAG laser, a Continum Minilite II (Eexc = 2.32 eV). The measurements were done using a laser flux < 0.1 mJ/cm2, corresponding to a photon fluence of jp ≈ 1011 photons/cm2 per pulse. Considering the absorption cross section of σ0 ≈ 1015 cm2,27 the number of photogenerated excitons is given by ⟨N0⟩ = jp × σ0, and estimated to be 10−4 ≪ 1, ensuring the generation of

single excitons. The tr-PL curves were monitored by a photon multiplier tube, a Hamamatsu NIR-PMT H10330-75. The circular polarized emissions were measured by a quarter-wave plate and linear polarizer combination. All recorded PL spectra were corrected and normalized relative to the system response at each polarization direction. The PL quantum yield at room temperature was measured using the integrating sphere technique described by Friend et al.,28 while the yield at any other temperature was calibrated with respect to that of the room temperature by the relative emission intensities gained at two different temperatures (following a procedure given in ref 28). The tr-PL curves were fitted to either single- or biexponential functions, when the extracted decay time was corrected relative to the estimated quantum yield at each temperature, supplying the radiative lifetime values. 2.5. Electronic Band Structure Calculations. The electronic band structure of the core/shell CQDs was evaluated, using a k*p model, considering specific features, related to the discontinuity of the effective mass, crystal potential, and dielectric constant at the core/shell interface. The procedure considered the variation of each physical parameter with the position (r) across the dot, with optional smooth transfer at the core/shell interface. The overall band offsets (by a step function or via gradual change) were chosen as that of the corresponding bulk CdTe and CdSe materials (where the valence band maximum of bulk CdTe lies at 0.42 eV above that of CdSe, while the conduction band minimum lies at 0.57 eV below that of CdSe29). The theoretical treatment yielded the electron and hole wave functions, as well as the approximation of the energy values of the conduction band’s and the valence band’s states.

3. RESULTS AND DISCUSSION The study included the investigation of core/shell CdTe/CdSe CQDs embedded with Mn2+ impurities at a selected position away from the core center. All CQDs have about the same total size, as well as the same core-to-shell division, although they differ by the position of the impurities. Schematic drawings of the investigated CQDs are shown in Figure 1A, where the blue area designates the core regime, the concentric black circles refer to the layer-by-layer surplus monolayers, and the red dots mark the Mn2+-ions’ positions. Substitution in an inner core layer is noted hereinafter as iMn:CdTe/CdSe, in an inner shell as CdTe/iMn:CdSe, in a boundary core as bMn:CdTe/CdSe, and in a boundary shell as CdTe/bMn:CdSe. Inclusion of Mn2+ ions in inner and boundary monolayers of a core or of a shell is given a general notation of Mn:CdTe/CdSe or CdTe/ Mn:CdSe. Exemplary TEM images of CdTe/CdSe CQDs withdrawn from the reaction solutions at intermediate stages are displayed in Figure 1A. Image I refers to the primary core, image II to CQDs after surplus coating of two CdTe monolayers, and image III after shell coating by three CdSe monolayers. The primary core diameter is 3.8 ± 0.3 nm, while the final core/shell CQDs is 9.8 ± 0.5 nm. Selective area electron diffraction pictures (not shown here)30 revealed the existence of a zinc-blende crystallographic structure of the primary core and of the entire CdTe/CdSe core/shell structures. The elemental composition, determined by an energy-dispersive analysis of X-ray that was closely confirmed by X-ray photoelectron spectroscopy, revealed the existence of one Mn2+ ion per substituted layer. For example, Table 1 lists the Mn2+/Cd2+ atomic ratio in various CQDs with the indicated sizes, embedded with two Mn2+ at separated 21022

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ejection or diffusion, enabling its incorporation at selective positions. Figure 1B shows a room-temperature continuous wave electron paramagnetic resonance (EPR) spectra of Mn:CdTe/ CdSe and CdTe/Mn:CdSe CQDs. The spectra display a dominant sextet manifold identified as the characteristic hyperfine split of the Mn2+ electron S(−1/2) ↔ S(+1/2) spin transition as a subgroup of transitions from the 5/2 nuclear spins, when the paramagnetic ions are randomly oriented with respect to the direction of an external magnetic field. Weak side bands in the spectra may be related to the forbidden transitions. The spectroscopic g-factor derived from the EPR spectra is 2.0 for Mn:CdTe/CdSe and 1.6 for CdTe/Mn:CdSe. The g-factor for Mn:CdTe/CdSe is consistent with Mn2+ in core CdTe.16 Surprisingly, the g-factor for CdTe/Mn:CdSe differs substantially from that of Mn2+ in core CdSe17 (g ∼ 2). This deviation might be related to a positioning in a strained monolayer with a modified local environment. The hyperfine isotropic constant (A) is 15.3 Gauss for Mn2+ in the CdTe core and 36 Gauss for Mn2+ in the CdSe shell, in agreement with the values found in bulk Mn:CdTe and Mn:CdSe. Figure 2A displays the absorption (solid lines) and cw-PL (dashed lines) spectra of the CQDs CdTe/CdSe, Mn:CdTe/ CdSe, and CdTe/Mn:CdSe, as indicated in the legend, when recorded at room temperature. The absorption spectra of all samples are characterized by the excitonic band-edge transitions. The cw-PL spectra consist of a single emission band centered around 1.95 eV for the pristine CdTe/CdSe, at 1.9 eV for CdTe/Mn:CdSe, and at 1.8 eV for Mn:CdTe/CdSe CQDs. As previously defined, CdTe/Mn:CdSe and Mn:CdTe/ CdSe CQDs refers to structures incorporating Mn2+ ions within two separate monolayers (inner and boundary positions). The comparison given in Figure 2A uncovers a red shift of 50−150 meV in Mn2+-incorporated structures with respect to the single emission band center of the pristine CQDs. Such an energy shift could be related to variation in the external or internal structures, strain effects,32 or signatures of exciton−Mn2+ interactions. The first alternative is excluded by the efforts to control the size dispersion and structural uniformity (see the Experimental Section), so the shift derives from a combination of the remaining factors. Previous calculations33 estimated the positioning of the Mn2+ first excited state (4T1) at −4.10 eV and the ground state (6A1) at −6.20 eV below the vacuum level in zinc-blende CQDs. Theoretical electronic band structure calculations (see the Experimental Section) of the pristine CdTe/CdSe, with total diameter and internal division as reported here, suggest a positioning of the valence band edge at −4.22 eV and that of the conduction band at −6.15 eV below the vacuum level. As a result, Mn2+ ground and excited states wrap the CQDs’ band gap edges. In such a scenario, either exciton−Mn2+ sp−d or selective s−d and p−d interactions take place, leading to the formation of a magnetic polaron (MP). A description of this occurrence was previously published by Hoffman et al.34 claiming that the exchange field is responsible for the MP formation, a process that is drastically enhanced in zero-dimensional magnetically substituted CQDs. Also, Bacher et al.7 have recently shown the alignment of the Mn2+-ion spins in the exchange field of the optically generated electron−hole pairs, whose signature can be traced at room temperature, corresponding to an absorption energy shift up to 100 meV. Figure 2B displays plots of radiative lifetime versus the measured temperature (2.2−50 K) of CQDs discussed in panel (A). As indicated in the Experimental Section, each point on

Figure 1. (A) Left: Schematic drawing of CQDs, all with the same core diameter (blue area) and shell width (black circles), when Mn2+ is incorporated. From left to right: (top) in an inner core (iMn:CdTe/ CdSe) and boundary core (bMn:CdTe/CdSe); (bottom) boundary shell (CdTe/bMn:CdSe) and inner shell (CdTe/iMn:CdSe). Right: TEM image of (I) initial CdTe core, (II) final CdTe core with two additional layers, and (III) CdTe/CdSe core/shell (inset: a zoom over a single CQD). (B) cw-EPR spectra of Mn:CdTe/CdSe (red curve) and CdTe/Mn:CdSe (blue curve).

Table 1. Size and Composition of the Mn2+-Doped CdTe/ CdSe CQDs sample

Mn2+ content (%)

total diameter (nm)

CdTe/CdSe Mn:CdTe/CdSe CdTe/Mn:CdSe

0 0.4 ± 0.08 0.5 ± 0.1

9.8 ± 0.6 9.6 ± 0.5 9.9 ± 0.4

monolayers. The strategy of successive substitution prevents conucleation of MnTe(Se) individual CQDs, while the relatively low growth temperature avoids diffusion of Mn atoms across the layers.31 Chen et al. in ref 31 revealed that cationic substitution can be sensitively controlled by varying the reaction temperature and suggested four elementary processes in substitution, namely, “surface adsorption”, “lattice incorporation”, “lattice diffusion”, and “lattice ejection”, each characterized by a critical temperature. For Mn:ZnSe CQDs, they measured one critical temperature around 40 °C, associated with the first two processes, and a second critical point around 230 °C, associated with the last two processes. In the case of Mn substitution in CdTe or CdSe, the cations’ radius differences will probably impede Mn insertion, increasing the critical temperature for adsorption and decreasing the critical temperature for ejection. Hence, a reaction procedure in the current case, at 150 °C, probably allows impurity insertion inside the CQDs and that this temperature prevents impurity 21023

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of the radiative lifetime on the position of the impurity might suggest a different degree of p−d and s−d coupling, an issue that is further confirmed in the following discussion. It is important to note that large p−d and s−d interactions are mainly pronounced at 2 K and are reduced gradually above 40 K, when the lifetime of Mn2+-embedded CQDs coincides with that of the pristine CQDs. This behavior is explained by the fact that Mn2+ preserves a high degree of spin alignment only at cryogenic temperatures in the absence of an external magnetic field,38 while its spin projections become randomly oriented at higher temperatures. Moreover, a weak circular polarization can be observed even at room temperature, despite the random orientation of the spins.39 It should be noted that a slow component might be related to a low statistical occurrence of Mn−Mn internal interaction between ions in adjacent monolayers, when such an interaction obscures the exciton− Mn2+ coupling. As shown further in the discussion, such an occurrence is excluded by examination of CQDs embedded with low dilution of Mn2+ ions within a singular monolayer (see Figure 3A). Figure 3A displays plots of radiative lifetime versus the measured temperature of the following CQDs: iMn:CdTe/ CdSe (orange), bMn:CdTe/CdSe (purple), CdTe/bMn:CdSe (blue), and CdTe/iMn:CdSe (green). This figure suggests the existence of two radiative components in the examined CQDs (the slow component is shown by the filled symbols and the fast component by the open symbols), resembling the behavior seen in Figure 2B, certainly excluding a contribution related to a Mn−Mn interaction. In particular, the curve related to CQDs with a Mn2+ ion at the inner core position has a distinctive behavior. Figure 3B,C shows the cw-PL spectra of iMn:CdTe/CdSe CQDs (B) and CdTe/iMn:CdSe CQDs (C) recorded with/ without circular polarization detection at 2.2 K. Two circular polarized components obviously exist in the spectra of iMn:CdTe/CdSe, with a mutual energy gap of ΔE σ ± = 25 meV, while the emission recombination process in CdTe/ iMn:CdSe consists of a single nonpolarized band, without any difference between the polarized signals (note that the absolute intensity of the cw-PL signal is reduced by a partial degree of light scattering when passing via the polarizer; nevertheless, the band frequency does not change). The exciton manifold in II−VI zinc-blende CQDs is split by an internal electron−hole exchange interaction and a slight shape distortion into five states, as seen in Figure 3B, inset.37 Electronic band structure calculations indicate an energy split between the two lowest excitonic states with an angular momentum of ±2 (dark) and ±1L (bright) of about 1 meV, which allows mixing of the bright−dark states, inducing in the pristine CQDs an extension of the radiative lifetime to ∼600 ns at low temperatures, as discussed above. The enhancement of the recombination time in Mn2+-embedded CQDs at 2.2 K could be related to breaking forbidden selection rules from dark states recombination due to the existence of sp−d interactions; however, it is possible to anticipate also an emission polarization, which is a different CQDs response depending on the observed light polarization if the ±1 degeneracy could be lifted, as shown by the arrows in the insets. Indeed, the observation of circular polarization properties in the emission spectrum in Figure 3B suggests the occurrence of a recombination process from the ±1L states. Moreover, one component, most likely, with the lowest energy, has a dominant intensity, associated with the relatively higher population tied to

Figure 2. (A) Normalized absorption (solid lines) and emission (dashed lines) spectra, recorded at room temperature of CQDs indicated in the legend. (B) Plots of the radiative lifetime components versus the measured temperature of CQDs shown in panel (A). Full symbols designate the fast components, and open symbols designate the slow components. The lifetime values were corrected with respect to the emission quantum efficiency at each temperature (see the Experimental Section). Inset: Representative log plot of a tr-PL decay curve of CdTe/CdSe (in black) and Mn:CdTe/CdSe (in red) CQDs, recorded at 4.2 K and fitted to a biexponent function.

the graphs was normalized with respect to the emission quantum efficiency at that temperature. Representative tr-PL curves of CdTe/CdSe and Mn:CdTe/CdSe are shown in the inset of the figure; the pristine CQDs showed a single exponent behavior, whereas all Mn2+-embedded CQDs typically show biexponential decay, with distinct slow and fast components. The slow emission process in Mn2+-embedded CQDs are represented in panel B by the filled symbols, and the fast emission processes are represented by the open symbols. The size of the symbols estimates the standard deviation of the measure. Above 50 K, the slow component coincides with the fast one. Previous publications by Chen et al.35 and Gan et al.36 suggested the association of a fast component with the exciton−Mn2+ interaction, whereas the slow component is related to a tail of the host lattice emission, associated with mixing of bright and dark exciton states.37 In the present case, the value of the fast component at the lowest temperatures (2 K) is ∼50 ns, substantially different from that of the slow component (∼600 ns). Furthermore, there is also a pronounced distinction between the lifetime of Mn:CdTe/ CdSe and CdTe/Mn:CdSe of the same Mn2+ concentration the former has the shorter lifetime. The observed dependence 21024

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interacts with the S = 5/2 d-shell moments of the Mn2+ ion. Then the exchange interaction term responsible for spin splitting is equal to ΔE = Sz N0(fe α − fh β)

where ⟨Sz⟩ is an effective spin moment of the Mn2+ (following Brillouin function, when ⟨Sz⟩ = 5/2 at 4 K5), N0 is the density of the host lattice, α arises from potential ferromagnetic s−d exchange, and β derives from kinetic-type antiferromagnetic p− d exchange, mainly approximated as −4α.41 fe and f h are characterized as the degree of spatial overlap between the Mn2+ wave function with the distribution functions of the electron and hole, respectively, and can be roughly approximated as the percentage of the carrier wave function within the core or within the shell, following the current electronic band structure calculations. α is evaluated by the expression5 α = αbulk + |Cν|2 × γ(E) × βbulk

with |Cv|2 = ΔEe/Eg, where Eg is the bulk energy gap of 1.6 eV and ΔEe is the electron energy confinement of 0.3 eV. γ(E) is estimated as γ(E) = (Eh − ε+) × (−Eh + ε−)/(Ee − ε+) × (−Ee + ε−), where Eh and Ee are the valence and conduction bandedge energy with respect to the vacuum (−4.2 and −6.1 eV, respectively); ε− amd ε+ are Mn2+ virtual d orbitals reported in CdTe-based confined system to be −1.3 and −8.3 eV, respectively (3.5 eV above and 3.5 eV under the bulk valence band edge42). Using the variables given here and in refs 5 and 42, we find N0·αbulk = 0.22 eV and βbulk = −4·αbulk,42|Cv|2 = 0.19, γ(E) = 1.30, N0·α = 0.009 eV, f h = 0.9 and fe = 0.88 in the core. ΔE is then evaluated to be 60 meV if Mn2+ is positioned in the core. Similar calculation for a Zeeman split by Mn2+ in the shell leads to a value of ΔE = 8 meV. Although the calculated values of ΔE do not coincide with the experimental estimates, they show a similar trend of a pronounced influence when the impurity is located within the core, and a smaller influence on carriers within the shell. The dominancy of the hole−Mn2+ interaction was pronounced in the variation of the radiative lifetime compared to that of the electron−Mn2+ as pronounced in the measure of radiative lifetime (see Figures 2B and 3A). The disagreement between the calculated and the experimental values might be related to incorrect approximation of one of the physical variables, which can be clarified only by additional theoretical and experimental work. In any case, the s−d coupling is smaller than that of the p−d coupling, due to a dominant contribution of a kinetic exchange interaction between p and d orbitals41 as well as the symmetry-restricted s−d hybridization. The s−d exchange interaction between impurity spins and conduction band electrons is generally weak because hybridization between the conduction band wave function at k = 0 and the transitionmetal d orbitals is formally forbidden by symmetry,41 leaving potential exchange as the main coupling mechanism. Merkulov et al.42 have suggested that quantum confinement can relax this symmetry restriction by introducing k ≠ 0 components into the conduction band-edge wave function while raising the conduction band electrons’ energy closer to resonance with the Mn2+ acceptor level. In principle, this could transform the fundamental nature of the electron−impurity s−d coupling mechanism from potential to kinetic exchange. The consequence is an increase of this s−d exchange interaction with decreasing degree of the confinement in nanostructures. Nevertheless, the p−d coupling still seems to be more

Figure 3. (A) Plots of the radiative lifetime components of the CQDs indicated in the legend. Filled symbols and open symbols designate the fast component and the slow component, respectively, color-coded with the legend. The components correspond to a biexponent fit to the measured tr-PL curves, which were corrected with respect to the emission quantum efficiency at each temperature. (B, C) Nonpolarized and circularly polarized emission spectra of iMn:CdTe/CdSe (B) and CdTe/iMn:CdSe (C) CQDs, recorded at 2.5 K. Insets: Schematic diagram of angular momentum states of an exciton. The emitted state is labeled as ±1L. The possible polarized transitions (±σ) are marked on the diagram, color-coded with the corresponding emission curves.

the Boltzmann population rules. Because of the absence of an external magnetic field, it is probable that the presence of Mn2+ impurities, in particular with their locked unidirectional orientation at 2.2 K,38 induces internal magnetization, thereby lifting the degeneracy.37,40 A Zeeman energy split can be increased in confined systems by the sp−d exchange interaction between the se = 1/2, s-like electrons and/or the jh = 3/2, p-like holes. Either of them 21025

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(2) Michalet, X.; Pinaud, F. F.; Bentolila, L. A.; Tsay, J. M.; Doose, S.; Li, J. J.; Sundaresan, G.; Wu, A. M.; Gambhir, S. S.; Weiss, S. Quantum Dots for Live Cells, In Vivo Imaging, and Diagnostics. Science 2005, 307, 538−544. (3) (a) Mayur, A. J.; Sciacca, M. D.; Kim, H. J.; Miotkowski, I.; Ramdas, A. K.; Rodriguez, S. Local and Gap Modes of Substitutional 3d Transition-Metal Ions in Zinc-Blende and Wurtzite II-VI Semiconductors. Phys. Rev. B 1996, 53, 12884−12888. (b) Bryan, J. D.; Gamelin, D. R. Doped Semiconductor Nanocrystals: Synthesis, Characterization, Physical Properties, and Applications. In Progress in Inorganic Chemistry; Karlin, K. D., Ed.; John Wiley & Sons Inc: Hoboken, NJ, 2005; Vol. 54. (4) Beaulac, R.; Archer, P. I.; Gamelin, D. R. Luminescence in

important than the s−d coupling. The correlation between the pronounced influence on the band-edge exciton radiative lifetime (see Figures 2B, 3A), as well as induced circular polarization in iMn:CdTe/CdSe CQDs (see Figure 3B,C), suggests the occurrence of a coupling between host carriers and Mn2+ impurity intentionally localized at the core, viz., a dominant p−d interaction In other words, the hole−Mn2+ dominates the sp−d interactions.

4. CONCLUSION In summary, we have presented a three-step growth approach involving layer-by-layer construction, reaching an extremely low impurity concentration inside the CQDs, while controlling the impurity location at a monolayer scale. The synthesis strategy has been applied to a particular CQD system with partial separation of the electron and the hole component of the exciton. Differences in Mn2+ position have then allowed us to separately investigate electron−Mn2+ and hole−Mn2+ interactions. Comparison to the pristine nonsubstituted CQD, the Mn-embedded CQD shows a huge red-shifted energy of the single emission band, a second faster relaxation pathway, indicating the existence of Mn−exciton interactions. Relative to the shell-substituted CQDs, the core-substituted CQDs revealed a second faster relaxation pathway and also a new circular polarization even in the absence of any external magnetic field, due to internal built-in magnetization by the Mn spins. All that leads to a p−d coupling far stronger than the s−d coupling. In other words, the hole−Mn2+ dominates the sp−d interactions Overall, the study strengthens the understanding of the magneton polaron occurrence in CQDs embedded with Mn2+ ions. The findings should be instructive in regard to future nanocrystal synthesis for spin-related applications, such as fabrication of magnetic nanodevices made of magnetic CQDs that exhibit both semiconductor and magnetic properties, or for manipulating individual spins. Finally, the synthesis strategy adopted, with precise control of impurity location and concentration, is more general and based on considerations that do not directly depend on the case of manganese insertion inside CdTe/CdSe. In consequence, it can be easily adapted to a wide variety of impurity insertions in CQDs.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Tel (Office): +972 (0)4 829-3750. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge Dr. Boris Tumansky for useful help in EPR experiments. The authors acknowledge support from the Israel Science Foundation (Project No. 985/11) and the German-Israeli Foundation for Scientific Research and Development (Project No. #I-1049-17509). Dr. Nathan Grumbach thanks the Israel Ministry of Immigrant Absorption.

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REFERENCES

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