Quantum Confinement-Controlled Exchange Coupling in

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Quantum Confinement-Controlled Exchange Coupling in Manganese(II)-Doped CdSe Two-Dimensional Quantum Well Nanoribbons Rachel Fainblat,*,† Julia Frohleiks,† Franziska Muckel,† Jung Ho Yu,‡ Jiwoong Yang,‡ Taeghwan Hyeon,‡ and Gerd Bacher† †

Werkstoffe der Elektrotechnik and CeNIDE, Universität Duisburg-Essen, Bismarckstraße 81, 47057 Duisburg, Germany School of Chemical and Biological Engineering, Seoul National University, Seoul 151-744, Korea

‡

S Supporting Information *

ABSTRACT: The impact of quantum confinement on the exchange interaction between charge carriers and magnetic dopants in semiconductor nanomaterials has been controversially discussed for more than a decade. We developed manganese-doped CdSe quantum well nanoribbons with a strong quantum confinement perpendicular to the c-axis, showing distinct heavy hole and light hole resonances up to 300 K. This allows a separate study of the s-d and the p-d exchange interactions all the way up to room temperature. Taking into account the optical selection rules and the statistical distribution of the nanoribbons orientation on the substrate, a remarkable change in particular of the s-d exchange constant with respect to bulk is indicated. Room-temperature studies revealed an unusually high effective g-factor up to ∼13 encouraging the implementation of the DMS quantum well nanoribbons for (room temperature) spintronic applications. KEYWORDS: Diluted magnetic semiconductors, nanoribbons, spintronics, sp-d exchange interaction

E

confinement is expected to significantly alter the exchange interaction between charge carriers and magnetic ion spins and thus paves the path for controlling the magneto-optical response in nanostructures by size and shape engineering. More than a decade ago, Hoffmann et al.25 predicted that quantum confinement can result in huge exchange fields up to 1000 T in magnetically doped nanoparticles, which should have a significant impact for the magneto-optical functionality at elevated temperatures. Indeed, quite recently, light-induced spontaneous magnetization has been observed in manganese doped CdSe colloidal quantum dots up to room temperature,26 which was attributed to an enhanced exchange field due to the quantum confinement. In addition, theories developed already in the late 90s predicted that, by lowering the dimensions of the nanostructure, both the s-d27 and the p-d28 exchange interactions will be changed as a consequence of the modification of the kinetic part of the exchange interaction due to quantum confinement. In manganese-doped II−VI semiconductors, such as CdSe, ZnSe, or CdTe, a strong quantum confinement might even change the ferromagnetic alignment between the spins of the electrons and the magnetic ions observed in bulk materials to an antiferromagnetic one,

ngineering size, shape, and composition of chemically prepared nanostructures has been developed to a high degree of perfection during the past decade. Quantum confinement has been observed in zero-dimensional quantum dots1−6 one-dimensional rods7 and quantum wires,8,9 and twodimensional nanobelts,10 nanoribbons,11,12 nanoplatelets,13 and nanosheets,14 most prominently reflected by a size-dependent shift of the bandgap. Besides, the dielectric confinement strongly stabilizes excitons,15 which control the optical properties in such chemically prepared nanostructures up to room temperature or even beyond. Doping these kinds of nanostructures adds an additional degree of freedom allowing the tuning of the electronic, optical, and magnetic material properties. A successful doping of chemically prepared nanostructures is still in the focus of active research efforts,16 although recent publications demonstrated significant progress.17−19 In particular doping nonmagnetic semiconductor nanostructures by transition metal ions has become an important research topic in the past few years, since diluted magnetic semiconductors (DMS) are expected to be a key element for spintronic applications, combining charge and spin degree freedom in a single material.20−24 The presence of localized magnetic ions yields to a spin exchange interaction between the dopant and the charge carriers of the host semiconductor, resulting in enhanced magneto-optical effects, such as a large Faraday rotation and a giant Zeeman splitting. Quantum © 2012 American Chemical Society

Received: July 17, 2012 Revised: August 31, 2012 Published: September 4, 2012 5311

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thus reversing the energetic order of spin-up and spin-down electron states of the Zeeman doublet. One of the main challenges for proving a quantum confined modification of both exchange constants N0α (doping ionconduction band exchange constant, also known as the s-d exchange constant) or N0β (doping ion-valence band exchange constant, known as p-d exchange constant) might be the fact that experimental techniques like magnetic circular dichroism, magneto-photoluminescence, or magneto-reflectivity quite often probe only one excitonic transition (mostly heavy hole exciton) and thus a combination of both exchange constants. In addition, the above-mentioned effects do not only depend on the value of the exchange constants but also on the exact concentration of the electronically and magnetically active magnetic dopants and their spatial distribution within the carrier wave function. By using a combination of time-resolved Kerr rotation and magneto-luminescence, respectively, a confinement-dependent exchange constant N0αhowever without a changing its signwas extracted for epitaxially grown GaMnAs quantum wells of different thickness, while an extraction of the size dependence of N0β was found to be problematic.29 Moreover, spin-flip Raman scattering indicates a 25% reduction of N0α in CdMnTe quantum wells with respect to the bulk value.27 Recent studies on magnetically doped core−shell nanoparticles30,31 report on contradictory results regarding the quantum confinement induced modification of the exchange constants. Yu et al.32 concluded that there is a modification of the s-d exchange constant due to quantum confinement in magnetically doped nanoribbonseven a negative value of N0α has been extracted from the data without being able to separately determine N0α and N0β. It should be noted that all of these studies have been restricted to temperatures far below room temperature. In this paper we report on magneto-optical measurements on manganese-doped CdSe nanoribbons with strong quantum and dielectric confinement perpendicular to the crystal c-axis for the whole temperature regime between 4.2 K up to room temperature. By a combined evaluation of heavy hole and light hole-exciton transitions taking into account the optical selection rules and the statistical orientation of the nanoribbons on the substrate, the exchange constants N0α and N0β are determined independently. A significant change of, in particular, the s-d exchange constant with respect to the bulk values is extracted from the data, which we relate to the strong quantum confinement. The giant sp-d exchange interaction combined with the strong excitonic resonances allows the observation of a pronounced magneto-optical response up to room temperature with an unusually high effective g-factor of about 13 at 300 K. The Mn2+-doped CdSe nanoribbons are synthesized from a Lewis acid−base reaction of cadmium chloride and manganese(II) chloride with octylammonium selenocarbamate at a temperature of 70 °C. More details about the sample preparation and characterization can be found in the Supporting Information. In our case, CdSe crystallizes in the wurtzite structure, as being proved by X-ray diffraction measurements.32 Figure 1a exhibits a sketch of the nanoribbon aggregates including the Miller indices. Due to the strong van der Waals force among the nanoribbons, individual layers with a thickness of about 1.4 nm, a width of 10−50 nm, and a length in the micrometer range are organized as stacks containing ∼20−30 overlaid sheets, which results in an average bundle thickness of 40−80 nm, as can be seen in the atomic force

Figure 1. (a) Schematic sketch of a stack of manganese-doped nanoribbons. The arrows represent the directions of the crystal axis. (b) Atomic force microscopy (AFM) line scan of a single nanoribbon stack. (c) Topography AFM image of a thin film of nanoribbons on a quartz substrate. (d) AFM phase image of the sample surface.

microscope (AFM) line scan in Figure 1b. The colloidal nanoribbons are dispersed in chloroform and drop-casted on a cleaned quartz substrate, whereby the concentration was optimized to achieve homogeneous layers with pronounced absorption characteristics. Figure 1c−d show AFM topography and phase scans of a typical nanoribbon thin-film layer, revealing a random orientation of the nanoribbon stacks, having their c-axis predominantly parallel to the substrate. The strong quantum confinement arising from the extremely small thickness (∼1.4 nm) 32 of a single nanoribbon perpendicular to the c-axis is revealed by the huge bandgap blueshift (∼0.9 eV) compared to the bulk bandgap of CdSe (1.751 eV).33 Three excitonic transitions can be observed in the absorption spectrum (Figure 2a and Supporting Information). Using a simplified theoretical model of a particle in an infinitely high potential well, we are able to calculate the energy of each excitonic transition and to compare the experimental data with the theoretical prediction. Due to the two-dimensional geometry based symmetry breaking, the valence band states are clearly separated into heavy hole (hh) and light hole (lh) subbands even at zero magnetic field. Moreover, we considered the energy splitting33 (Δso = 0.416 eV) between the split off (so) and the heavy-hole subbands, which arises from the spin− orbit coupling. Using an electron mass me* = 0.11m0, where m0 represents the free electron mass, and valence-band effective masses mh* of 1.09m0, 0.58m0, and 0.81m0 for the heavy-hole, light-hole, and split-off subbands, respectively,34 the calculated 1hh−1e, 1lh−1e, and 1so−1e excitonic transitions correspond to the experimental data assuming a thickness of 1.8 nm for the nanoribbons (Figure 2a). Note that the slightly enhanced nanoribbon thickness used for the calculations (1.8 nm) as compared to the value extracted from TEM images (1.4 nm)32 is most likely attributed to the fact that stacks of nanoribbons are formed and a slight extension of the wave function into the neighboring nanoribbons is expected. As the absorption spectrum does reveal at least three excitonic transitions, in 5312

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quantum well.37 Thus, it is necessary to carefully consider the orientation (that means the angle) between the quantization axis (defined as z-axis), the c-axis of the wurtzite crystal, and the polarization of the absorbed light. A detailed explanation about the impact of the sample geometry on the transition probabilities is given in the Supporting Information. Applying a magnetic field results in a lifting of the 2-fold spin degeneration of each energy level even for undoped samples with an intrinsic Zeeman splitting ΔEint according to their intrinsic g-factors (ge for the conduction band, ghh for the hh-, glh for the lh-, and gso for the so-subbands). For heavy hole excitons ΔEint is shown to be on the order of 0.1 meV/T in CdSe nanocrystals.38 Doping a semiconductor with a transition metal introduces a new term into the Zeeman splitting, ΔEsp‑d, caused by the sp-d exchange interaction. This contribution depends on the strength of the coupling between the delectrons of the doping ion and the charge carriers of the conduction band (the s-d exchange interaction) and the valence band (the p-d exchange interaction). The s-d and p-d exchange interactions are parametrized by the exchange integrals N0α and N0β, respectively, where N0 denotes the number of cations per unit volume. The total Zeeman splitting will be given by ΔEtotal = ΔEint + ΔEsp‑d. As can be seen below, ΔEint ≪ ΔEsp‑d holds for the whole temperature range studied, and thus, we do neglect the first term in the following data analysis. The giant Zeeman splitting ΔEsp‑d strongly depends on the angle between the quantization axis and the applied magnetic field.39 Let us first consider case III (see Figure 3a), where the quantization axis is parallel to the applied magnetic field. Here, ΔEsp‑d can be described for the hh-X and the lh-X transitions by eqs 1 and 2, respectively:40

Figure 2. (a) Absorption spectrum of the Cd0.92Mn0.08Se nanoribbons at 4.5 K. (b) MCD spectra of the same sample for different magnetic fields at 4.5 K.

particular strongly pronounced heavy hole and light hole excitonic resonances, it is quite powerful to investigate the magneto-optical properties using the magnetic circular dichroism (MCD). This is a very sensitive technique based on the polarization-dependent absorption of a sample placed in a magnetic field in Faraday geometry. As can be seen in Figure 2, we are able to clearly observe spectrally separated heavy hole and light hole excitonic resonances for both absorption and magnetic circular dichroism. Figure 2 compares the absorption (Figure 2a) and the MCD signal (Figure 2b) for the CdSe nanoribbons doped with 8.3(±0.5)% of manganese at different applied magnetic fields measured at 4.5 K. The amplitude of the MCD signal increases with an increasing magnetic field, and there is an evident correlation between the maxima of the absorption spectrum and the zero-crossings of the MCD spectrum for both transitions. Most interestingly, a sign reversal of the MCD signal is found between the hh- and the lh-excitonic resonances, which is related to the specific spin structure of the involved exciton states. The total measured MCD signal consists of a superposition of the MCD signal for each excitonic transition. Using MCD = [d(ln(T(E)))/dE]·(ΔE/2), where T(E) is the transmission intensity at the energy E,35 we calculated the Zeeman splitting, ΔE, for both transitions, hh-X (heavy hole exciton) and lh-X (light hole exciton). To analyze the data, we have to carefully take into account the orientation of the nanoribbons with respect to both the polarization of the exciting light and the magnetic field. According to the optical selection rules for a wurtzite structured semiconductor, it is only possible to generate a heavy hole exciton if the light polarization is perpendicular to the c-axis.36 In contrast, the generation of a light hole or a split-off exciton is allowed for light polarization perpendicular and parallel to the c-axis. Moreover, the transition probabilities according to the optical selection rules are affected by the quantum confinement in a

ΔE hh‐X_III = N0(α − β)x⟨Sz⟩

(1)

ΔE lh‐X_III = N0(α + β /3)x⟨Sz⟩

(2)

x represents the doping concentration and ⟨Sz⟩ the average spin projection along the direction of the magnetic field, which is described by a modified Brillouin function: 41 ⟨S z ⟩ = Seff·BS[gμBSB/(kB(T + TAF))]. BS is the Brillouin function depending on the magnetic field B and the temperature T, S represents the total spin of the doping ion, g is the gyromagnetic factor of the manganese atoms, and μB and kB are the Bohr magneton and the Boltzmann constant, respectively. The antiferromagnetic coupling between magnetic doping ions gives rise to the introduction of Seff and TAF. The parameters Seff and TAF represent an effective spin and an antiferromagnetic temperature, respectively, and they depend on the doping concentration of the alloy. Considering case I (see Figure 3a), where the quantization axis and applied magnetic field are perpendicular to each other, the Zeeman splitting for both heavy hole and light hole excitons changes drastically. ΔEsp‑d can be described for the hh-X and the lh-X transitions by eqs 3 and 4, respectively:39

ΔE hh‐X_I = N0αx⟨Sz⟩

(3)

ΔE lh‐X_I = N0(α + 2β /3)x⟨Sz⟩

(4)

In the case of the heavy hole excitons, only the conduction band states split, since the orbital momentum in the valence band, which is responsible for the Zeeman splitting, is oriented along the quantization axis and is therefore perpendicular to the magnetic field. Note, however, that due to the optical selection rules, the hh-exciton cannot be excited in this configuration. 5313

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for the nanoribbons doped with 8.3% of manganese. The error bars of Figure 3b−c were determined taking into account the measurement resolution. From the slope of the Zeeman splitting versus magnetic field, we obtain the effective g-factor in the small-magnetic field limit for both excitonic transitions at T = 4.5 K. The sample doped with 8.3% manganese exhibits geff_hh‑X ≈ 380 and geff_lh‑X ≈ −225, whereas the values of the 4.0% Mn2+-doped sample are geff_hh‑X ≈ 225 and geff_lh‑X ≈ −150, respectively. The parameters involved in eqs 5 and 6 are known for bulk materials. Typically, N0αbulk = 0.23 eV and the orientation averaged (since the coupling between the dopants and the valence band is anisotropic) value of N0βbulk = −1.27 eV are used in literature,42 although other values (N0αbulk = 0.26 eV and N0βbulk = −1.11 eV) are also reported.43 Using Seff,8.3% = 1.19 and TAF,8.3% = 2.24 K for the 8.3% Mn2+-doped sample and Seff,4.0% = 1.68 and TAF,4.0% = 1.77 K for the 4.0% Mn2+-doped sample from an extrapolation based on the data of Heimann et al.,44 the theoretically expected energy splitting for the different excitonic transitions was calculated and included in Figure 3b and c as dashed blue-green lines. It is obvious that the measured energy splitting cannot be described by the bulk exchange parameters. Some theoretical work has been already published regarding a quantum confinement dependence of the exchange integrals compared to the bulk parameters. Bhattacharjee presented a study28 based on the effective mass approximation for quantum confined states. Only a variation of the negative kinetic type p-d exchange parameter is taken into account. The dominant role of the kinetic interaction, which has its origin in possible transitions to virtual states in the d-shell, results in the negative sign of N0β. A confinement dependent reduction of the ratio of the excitonic Zeeman splitting to the magnetization (compared to bulk materials) by a factor ρ is obtained for both quantum wells and quantum dots. According to the calculations of Bhattacharjee, we expect for our nanoribbons a reduction of N0β by about 10−15% with respect to the bulk value N0βbulk. In bulk II−VI semiconductors the positive sign of N0α arises from the direct exchange interaction between the conduction band electrons and the d-electrons of the doping ions. Merkulov et al.27 developed a model accounting for the correlation between the kinetic exchange of N0α and the degree of quantum confinement. According to this theory, two terms contribute to a total s-d exchange: the potential type (positive, i.e. ferromagnetic) and the kinetic type (negative, i.e. antiferromagnetic). At the Brillouin zone center (Γ-point), the kinetic exchange is forbidden for the electrons due to their s-like symmetry. In this case, N0α only consists of a potential contribution, thus being positive. Away from the Γ-point, the plike valence band wave functions admix to the Bloch functions of the conduction band, allowing the hybridization of the conduction band states with the d-like orbitals of the magnetic ions. Therefore, a kinetic-type exchange is also possible for the conduction band electrons, increasing with quantum confinement. The kinetic term has a negative sign; thus its enhancement will cause a reduction of N0α. For strong quantum confinement even a sign reversal is expected; that is, the interaction between conduction band states and d-electrons of the doping atoms becomes antiferromagnetic. The total s-d exchange can be approximated by:27

Figure 3. (a) Different geometries of the nanoribbons indicating the orientation between quantization axis (z), c-axis, and applied magnetic field (B⃗ ). Zeeman splitting of the 4.0% (b) and the 8.3% Mn2+ (c) doped CdSe nanoribbons at 4.5 K for both the hh-X and lh-X excitonic transitions (symbols). The dashed blue−green curves represent the calculations of the Zeeman splitting using bulk parameters. The solid black (part b) and red (part c) curves along the measured data are fits using the exchange constants N0α and N0β as single parameters.

A quantitative analysis of the measured data requires the consideration of both the optical selection rules and the relative orientation between the magnetic field and the quantization axis (z-axis) of the nanoribbons. Taking into account that the nanoribbons have their c-axis predominantly parallel to the sample surface (i.e., perpendicular to the magnetic field, see Supporting Information), it is reasonable to assume that the angle between the magnetic field and the quantization axis may statistically vary between 0 and 90°, as shown in Figure 3a (case II). We calculate a mean Zeeman splitting of both hh and lh excitonic transitions, by integrating over the angle-dependent energy splitting weighted by the corresponding transition probability according to the optical selection rules. We finally obtain ΔE hh‐X_II = N0(α − 0.785β)x⟨Sz⟩

(5)

ΔE lh‐X_II = N0(α + 0.449β)x⟨Sz⟩

(6)

The different sign of the N0β contribution to the giant Zeeman splitting ΔEsp‑d of the heavy hole (ΔEhh‑X) and the light hole (ΔElh‑X) exciton transition gives access to an independent extraction of N0α and N0β. This should permit a direct experimental access of a possible confinement-induced variation of the exchange interaction in magnetically doped nanostructures. The measured low temperature Zeeman energy splitting for the 4.0(±0.4)% manganese-doped sample is plotted for both transitions in Figure 3b. Figure 3c depicts the equivalent data

N0αtot = N0{αpot + |Cv|2 ·[βpot + γ(Ee)βkin]} 5314

(7)

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Here, αpot, βpot, and βkin are the potential and the kinetic exchange constants representing the coupling between the doping ions and charge carriers in the conduction and the valence band, respectively. |Cv|2 ∼ ΔEe/Eg represents the contribution of the valence band states to the electron's Bloch wave function in the conduction band and depends on the ratio between the quantum confinement in the conduction band (ΔEe) and the bulk bandgap energy (Eg). The kinetic exchange coefficient γ(Ee) can be calculated in second-order perturbation theory, depending strongly on the energy difference between the confined electron (hole) energy levels, Ee (Eh), and the virtual electronic levels of the magnetic doping ions (ε+ and ε−), which can capture electrons and holes on their d shell. γ(Ee) is described by γ(Ee) = [(Eh − ε+)(ε− − Eh)]/[(Ee − ε+)(ε− − Ee)]. Neglecting the potential contribution of βpot due to βpot ≪ |βkin| and using literature values of Eg = 1.751 eV,33 N0αkin = 0.23 eV, N0βkin = −1.27 eV,42 ε+ ∼ 3.4 eV,45 and ε− ∼ 3.2 eV,46 we estimate a negative total s-d exchange energy of about N0αMerkulov ≈ −0.69 ± 0.11 eV for our nanoribbons according to this theory. Fitting the data for the hh-X and the lh-X transitions for both samples as plotted in Figure 3b and c with eqs 5 and 6 using the above-mentioned literature values of Seff and TAF and N0(α − 0.785β) and N0(α + 0.449β) as the only fitting parameters, we are able to determine both exchange constants. From the fitting procedure we obtain N0α = −0.02 ± 0.02 eV and N0β = −1.18 ± 0.2 eV (see Figures 3b−c) as the average value for the two different doping concentrations. There are different sources for the error bars given for N0α and N0β . Certainly an important one is the uncertainty in the determination of the exact doping concentration from ICP-AES measurements (4.0 ± 0.4% and 8.3 ± 0.5%, respectively), which was taken into account for the calculation of the deviation. In addition, the effective doping concentration might be influenced by the nonstatistical probability of dimer and trimer formation due to the high surface-to-volume ratio47 and by the possible dimer formation in the cation substitution of the clusters during preparation,32 the latter one being minimized by removing the surface-bound doping ions during the sample preaparation. Following the arguments of Droubay et al.,47 it is concluded that for our geometry the error in the effective Mn concentration caused by the large surface-to-volume ratio is comparable to or even smaller than the error resulting from the uncertainty of the Mnconcentration as measured by ICP-AES. Although the fitted p-d exchange constant N0β = −1.18 ± 0.2 eV seems to indicate a confinement induced variation comparable to the theoretical prediction,28 the significant error bar and the uncertainty in the bulk value suggest that a clear evidence of such a confinement induced change of N0β is tenuous. In contrast, the extracted value of N0α significantly deviates from the bulk value, although its change is less than expected according to Merkulov’s theory. Recently, Beaulac et al.48 calculated the exchange interaction between the doping ions and the conduction band electrons using a combination of density functional theory (DFT) and perturbation theory. An additional ferromagnetic interaction between the electrons of the conduction band and the 4s-electrons of the manganese ions, the so-called s-s-exchange interaction, is included, which at least partly compensates the antiferromagnetic s-d term. Using the theory presented by Beaulac et al., we calculated the exchange constants for the nanoribbons and obtain N0αBeaulac ≈

0.09 eV and N0βBeaulac ≈ −1.25 eV. In particular, the calculated value of N0α is much closer to our experimental data (N0α = −0.02 ± 0.02 eV). Importantly, both theories indicate that a confinement induced change of N0α is expected for our nanoribbon geometryin agreement with our experimental dataalthough the quantitative numbers differ. Note that the slight difference of the lattice constants and the local relaxation in nanoribbons (see Supporting Information) might in addition contribute to the variation of the exchange constant as compared to bulk probably being a source of the deviation between experiment and theory. It is instructive to compare the ratio between the Zeeman energy splitting of the hh and lh excitonic transition (ΔEhh‑X)/ (ΔElh‑X), as this ratio only depends on the exchange constants N0α and N0β. This avoids any kind of uncertainty stemming from an inaccurate determination of the doping concentration in the nanocrystals or inaccurate values of Seff and TAF, for example, due to a probability of dimer and trimer formation in the nanoribbons which is different from bulk. Figure 4 exhibits

Figure 4. Ratio (ΔEhh‑X)/(ΔElh‑X) between the Zeeman splittings of the different excitonic transitions versus magnetic field. Symbols are experimental data (black symbols represent the 4.0% and red ones the 8.3% manganese doped sample, respectively). The error bars are determined by the error bars indicated in Figure 3b−c. The horizontal lines represent calculations according to cases I (orange) and III (purple) using bulk parameters N0αbulk and N0βbulk. The green line represents the theoretical expectation taking into account the statistical variation of the nanoribbon geometry according to eqs 5 and 6 using bulk parameters. The blue line represents the same case in contrast using the fitted mean values of the exchange constants N0α = −0.02 ± 0.02 eV and N0β = −1.18 ± 0.2 eV.

the theoretical expectation of (ΔEhh‑X/ΔElh‑X) for the three different cases (I, II, and III) discussed before using bulk parameters.42 Our experimental data reveal (ΔEhh‑X)/(ΔElh‑X) ≈ −1.8 for both doping concentrations. It is clear that neither the calculated ratio in the limiting cases I (orange line) and III (purple line) nor the calculated average Zeeman splitting considering the statistical variation of the angle φ (case II, green line) can describe the experimental data if bulk parameters are used. A very good agreement between experimental data and theory for both samples is obtained, if eqs 5 and 6 are used in combination with the exchange constants N0α = −0.02 ± 0.02 eV and N0β = −1.18 ± 0.2 eV (blue line). The distinct hh-X and lh-X transitions, which can clearly be separated up to 300 K, allow the investigation of the temperature dependence of the magneto-optical properties of the nanoribbons all the way to room temperature. Figure 5a exhibits the MCD spectra measured at 1.6 T at different 5315

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Figure 5. (a) MCD spectra of the nanoribbons doped with 8.3% of Mn2+ for different temperatures from 4.5 to 300 K at 1.6 T. (b) MCD spectra of the 8.3% Mn2+ doped sample for 0.4, 1, and 1.6 T at room temperature. (c) Zeeman splitting of the hh-X and lh-X excitonic transitions of the 8.3% Mn2+ doped sample for temperatures from 4.5 to 300 K (symbols). The exchange constants extracted from the low temperature data are used for plotting the theoretically expected behavior (red line) for both excitonic transitions. (d) Zeeman splitting of the heavy hole exciton of the Cd0.92Mn0.08Se nanoribbons at 300 K for different applied magnetic fields revealing an effective g-factor of about 13 using a linear fit (solid line).

light hole exciton transitons even at 300 K, which allows an experimental access to both, N0α and N0β. Considering the geometry of the nanoribbons, which has a relevant influence on both the optical selection rules and the Zeeman splitting, we found a clear indication of a confinement-induced modification of N0α in reasonable agreement with the theory proposed by Beaulac et al.47 A confinement-induced change of N0β is consistent with our data, although the error bars do not allow a conclusive statement here. The strong exchange interaction results in a giant Zeeman effect even at room temperature with an effective g-factor of geff_hh‑X ≈ 13 for the Cd0.92Mn0.08Se nanoribbons. Thus, the magneto-optical response at room temperature is increased by about an order of magnitude with respect to the nonmagnetic counterpart, opening a pathway to room temperature spintronic applications.

temperatures. As a result of the enhanced exciton binding energy and oscillator strength due to quantum and dielectric confinement, the samples do reveal a pronounced MCD signal even at room temperature (see Figure 5b). This is an evidence of the extraordinary sp-d exchange interaction in the nanoribbons. It is known from literature49 that the intrinsic g-factor of the heavy hole exciton in undoped CdSe quantum dots is negative, thus having the opposite sign compared to the effective g-factor of the heavy hole exciton in a magnetically doped sample. Previous studies of the Zeeman splitting temperature dependence on nanocrystals50 with a low Mn concentration have revealed that above temperatures between 80 and 160 K, the sp-d exchange splitting gets smaller than the intrinsic Zeeman splitting, exhibiting a sign reversal of the MCD signal. Figure 5a reveals no sign reversal of the MCD signal over the whole temperature range. Thus, we can conclude that, even at room temperature, the sp-d exchange energy splitting term predominates over the intrinsic contribution to the total Zeeman splitting. The temperature-dependent Zeeman splitting of both hh-X and lh-X transitions is plotted in Figure 5c for the sample containing 8.3% of manganese. The red curve represents the fit according to eqs 5 and 6 using the mean value of the exchange parameters determined above. The measured data can be welldescribed over the whole temperature range for both heavy hole and light hole excitons. The Zeeman splitting investigated at room temperature for different magnetic fields is plotted in Figure 5d. At 300 K, the Zeeman splitting is expected to increase linearly with B in the magnetic field range under investigation. We obtain an effective g-factor on the order of geff_hh‑X ≈ 13 for the 8.3% manganesedoped sample at room temperature. This means that the room temperature Zeeman splitting between spin-up and spin-down exciton states has reversed its sign and is increased by roughly an order of magnitude as compared to undoped CdSe nanocrystals.38,48 For the sample doped with 4.0% of manganese, a (positive) effective g-factor on the order of geff_hh‑X ≈ 3 is obtained, dominated as well by the ΔEsp‑d term. In summary, MCD spectroscopy has been used to analyze the exchange interaction between magnetic doping ions and charge carriers of the host semiconductor in colloidal Mn2+ doped CdSe nanoribbons all the way up to room temperature. The nanoribbons show spectrally well separated heavy hole and

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ASSOCIATED CONTENT

S Supporting Information *

Sample preparation, characterization and impact of the sample geometry on the transition probabilities. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful to D. Iavarone for the sample preparation and S. Eliasson for the AFM measurements. Financial support of the DFG under contract Ba 1422/13 is gratefully acknowledged.

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