Digital Doping in Magic-Sized CdSe Clusters Franziska Muckel,†,# Jiwoong Yang,‡,§,# Severin Lorenz,† Woonhyuk Baek,‡,§ Hogeun Chang,‡,§ Taeghwan Hyeon,*,‡,§ Gerd Bacher,*,† and Rachel Fainblat†,‡,§ †
Werkstoffe der Elektrotechnik and CENIDE, University Duisburg-Essen, Bismarckstraße 81, 47057 Duisburg, Germany Center for Nanoparticle Research, Institute for Basic Science (IBS), Seoul 08826, Republic of Korea § School of Chemical and Biological Engineering and Institute of Chemical Processes, Seoul National University, Seoul 08826, Republic of Korea ‡
S Supporting Information *
ABSTRACT: Magic-sized semiconductor clusters represent an exciting class of materials located at the boundary between quantum dots and molecules. It is expected that replacing single atoms of the host crystal with individual dopants in a one-by-one fashion can lead to unique modifications of the material properties. Here, we demonstrate the dependence of the magneto-optical response of (CdSe)13 clusters on the discrete number of Mn2+ ion dopants. Using time-of-flight mass spectrometry, we are able to distinguish undoped, monodoped, and bidoped cluster species, allowing for an extraction of the relative amount of each species for a specific average doping concentration. A giant magneto-optical response is observed up to room temperature with clear evidence that exclusively monodoped clusters are magneto-optically active, whereas the Mn2+ ions in bidoped clusters couple antiferromagnetically and are magneto-optically passive. Mn2+-doped clusters therefore represent a system where magnetooptical functionality is caused by solitary dopants, which might be beneficial for future solotronic applications. KEYWORDS: single-atom doping, diluted magnetic semiconductor nanostructures, magic-sized cluster, solotronics, magneto-optics antiferromagnetic temperature (TAF).23 This description of the influence of coupled dopant spins represents a widely established model that is applied in all kinds of diluted magnetic semiconductors (DMS) of any dimensionality. In the ultimate limit of a low doping concentration, the emerging research field of solotronicsoptoelectronics based on solitary dopantshas gained broad interest during the past decade.24−28 In this low-doping limit, the exchange interaction could be demonstrated between charge carriers and a few (or even single) dopants.26,27,29 In conventional DMS quantum dots (QDs), the introduction of a single impurity corresponds to low doping concentrations of less than 0.1% and is restricted by the number of atoms contained in a single nanostructure. In contrast to this, single-impurity doping combined with an extreme reduction of the nanostructure dimensions (i.e., down to only a few dozens of atoms) is expected to result in a particularly high effective doping concentration. Magic-sized clusters, which consist of a well-defined number of atoms, represent the boundary between nanocrystals and molecules. Magic-sized clusters of various II−VI compounds have been intensively studied in terms of both chemical
D
opingthat is, the intentional incorporation of impuritiesis a key issue in the field of semiconductor technology and represents one of the most important challenges for colloidal nanocrystals.1−7 For example, the incorporation of transition metal ions into semiconductor host lattices enables the combination of semiconductor and magnetic properties in a single material.8,9 This field of research has experienced a recent boost as the concept of transition metal doping has been adopted for colloidal nanostructures.10−19 For instance, in colloidal nanocrystals doped with multiple impurities, exciting observations have been reported, such as magnetic fluctuations,13 polaron formation up to room temperature,20 and the quantumconfinement-induced modification of the exchange interaction.21 These findings are based on the sp−d exchange interaction between the spins of the host charge carriers and the magnetic moments of the transition metal ion spin ensemble. Because the mean distance between two doping atoms decreases with increasing doping concentration, the coupling between the magnetic dopants gains importance. Two impurities located on nearest-neighbor sites usually couple antiferromagnetically.22 In order to account for this, more than 30 years ago, Gaj et al. suggested a modified Brillouin function for describing the magnetization by introducing two empirical constants: an effective doping concentration (xeff) and an © 2016 American Chemical Society
Received: May 20, 2016 Accepted: July 12, 2016 Published: July 15, 2016 7135
DOI: 10.1021/acsnano.6b03348 ACS Nano 2016, 10, 7135−7141
Article
www.acsnano.org
Article
ACS Nano synthesis30−37 and theoretical investigations.38−40 In particular, the successful incorporation of magneto-optically active Mn2+ ions into (CdSe)13 clusters41 allows a direct observation of magneto-optically active and inactive fine structure states of the band-edge excitonic transitions, which was theoretically predicted two decades ago.42 Because of the small number of atoms per (CdSe)13 cluster, the replacement of a single cation by a transition metal ion results in discrete steps in the doping concentration of 7.7% per atom. This suggests that a particularly strong magneto-optical response may be generated even by single ions. When the small cluster size is taken into account, magnetic dopants in bidoped clusters will most likely interact antiferromagnetically. In that case, monodoped clusters would reveal a giant magneto-optical response, whereas undoped and bidoped species are expected to exhibit exclusively intrinsic Zeeman splitting, which is 2 orders of magnitude smaller than its giant counterpart. Thus, it can be expected that a giant magneto-optical response is generated by introducing a single dopant into a cluster, whereas it should be suppressed if two host atoms are replaced by magnetic impurities. Herein, we present magneto-optical studies of Mn2+-doped magic-sized CdSe clusters by systematically varying the nominal doping concentration. We find clear experimental evidence of a digital behavior of the magnetization per nanocluster as the number of dopants increases. Using laser desorption/ionization time-of-flight mass spectrometry (LDI-TOF MS), we can obtain relative ratios of (CdSe) 13 , Cd 12 MnSe 13 , and Cd11Mn2Se13 species among an ensemble, which follows the calculations based on a binomial probability distribution. Moreover, by using magnetic circular dichroism (MCD) spectroscopy, we monitor the magneto-optical response and extract the Zeeman splitting of the lowest excited state. Interestingly, the description of the experimental results requires a modification of the standard DMS theoretical treatment, because the established approaches, which use an effective temperature and an effective spin in considering antiferromagnetic Mn2+ coupling, are no longer applicable.23 Moreover, these clusters exhibit sp−d coupling even up to room temperature, suggesting that these materials can function as promising candidates for future solotronic applications.
Figure 1. (a) Mass spectra of the 2% Mn2+-doped CdSe clusters ionized with Cl− ions, illustrating the extraction of the ratio of each cluster species. Black curves represent high-resolution MS data, whereas colored curves depict simulated isotope distributions of (CdSe)13 (blue), Cd12MnSe13 (red), and Cd11Mn2Se13 (green), assuming the extracted relative ratio. The inset shows MS data for a given nominal Mn2+ doping concentration (xnom) that varies between 4% and 10%. (b) Comparison between simulated dopant distribution and experimentally determined ratios. Crosses represent the relative ratios of each species extracted from MS of ensembles of a given xnom value. Solid lines represent calculations of a binomial distribution that takes into account (CdSe)13 (blue), Cd12MnSe13 (red), and Cd11Mn2Se13 (green) clusters.
RESULTS AND DISCUSSION The samples were prepared by the modified method from the previous report,41 and their nominal doping concentration (xnom) was controlled from 2% to 10% (see Methods). To extract the actual population distribution of individual species in the ensemble, we conducted an LDI-TOF MS analysis (Figure 1). As shown in Figure 1a, three main peaks are observed, which can be assigned to Cd13Se13, Cd12MnSe13, and Cd11Mn2Se13 species. This suggests that our samples are Mn2+doped (CdSe)13 clusters which are composed of un-, mono-, and bidoped clusters. We could not observe a contribution from clusters with three or more Mn2+ ions, which might be due to the low formation probability originating from their high formation energy.43 A clear intensity dependence of the main peaks on xnom can be seen in Figure 1a. As xnom of the clusters increases, the relative intensity of the peak corresponding to Cd11Mn2Se13 increases, while the intensity of the peak corresponding to Cd13Se13 decreases. Because of the chemical similarity of each species, these three cluster species are expected to share a similar ionization process (i.e., similar ionization efficiency, possibility of fragmentation, etc.) under
the same measurement conditions. Consequently, from the LDI-TOF MS results we can deduce the relative amount of un-, mono-, and bidoped clusters for different xnom, although the absolute quantification by LDI-TOF MS is not easily achieved.44,45 The simulated isotope distribution for each cluster species (blue, red, and green curves in Figure 1a) based on the extracted relative ratio also matches well with the experimental patterns of the clusters with various xnom (Figure S1 in the Supporting Information), which supports that our assignments and quantification are reasonable. The relative ratios of un-, mono-, and bidoped clusters extracted from the mass spectra (crosses) are displayed in Figure 1b and Table S1 in the Supporting Information, compared with the probabilities (solid lines) that were calculated using a binomial distribution of those three species (see the Supporting Information for details on the calculation). Figure 1b depicts both simulated probabilities and extracted relative ratios from the MS spectra and they are in very good agreement, representing an additional indication of the absence of cluster species containing more than two dopant ions. The 7136
DOI: 10.1021/acsnano.6b03348 ACS Nano 2016, 10, 7135−7141
Article
ACS Nano
Figure 2. (a) Magnetic circular dichroism (MCD) spectrum and (b) absorption spectrum at 1.4 T and 5.2 K (xnom = 2%). In the absorption data, fits of magneto-optically active (filled blue) and inactive (filled red) transitions related to fine structure states are shown. The energy position of the ±1L state is marked (dashed line). (c) Extracted Zeeman splitting of the ±1L state for different xnom. The error bars for the Zeeman splitting take into account the uncertainty due to the fitting procedure. (d) Relation between the concentration of magneto-optically active Mn2+ ions and xnom for two limiting cases: Mn2+ ions in Cd11Mn2Se13 align either ferromagnetically (green continuous line) or antiferromagnetically (black line−digital doping case). The dashed green line represents the case in which only 50% of the Mn2+ ions are antiferromagnetically coupled.
A0, peaks related to the fine structure splitting are fitted to the absorption spectrum by assuming that the energy position of the magneto-optically active ±1L peak maximum corresponds to the zero-crossing in the MCD signal (dashed line in Figure 2a,b). The Zeeman splitting of the first transition extracted for clusters with different doping concentrations is shown for B = 1.43 T and T = 5.2 K in Figure 2c. The result is also summarized with corresponding effective g-factor in Table S2 in the Supporting Information. The giant Zeeman splitting in DMS materials is given by eq 2
small deviation between simulation and experimental data is systematic: while the binomial-based probability distribution slightly underestimates the proportion of un- and bidoped clusters, the number of monodoped clusters is overestimated. This might be an indication that the formation of Cd11Mn2Se13 is energetically more favorable43 than the formation of two monodoped clusters, thus inducing a small deviation from the binomial distribution. In order to probe the giant magneto-optical activity of the doping ions as evidence of an efficient sp−d exchange interaction, we performed magnetic circular dichroism (MCD) spectroscopy on clusters of different xnom. Figure 2a,b depicts the MCD signal and absorption data of Mn2+doped clusters (xnom = 2%). The MCD signal consists of several transitions, which have been identified by Yang et al. as magneto-optically active states of the degenerated 1S3/21Se band-edge exciton level.41 The observed mismatch between the energetic position of the dominant absorption peak maximum (ca. 3.7 eV) and the zero crossing in MCD (ca. 3.65 eV) reveals the existence of magneto-optically active peaks (filled blue, most likely related to ±1L and ±1U states according to the notation of a previous study42) and passive peaks (filled red, most likely related to 0L and 0U states). Here, L and U denote the lower and upper transitions, respectively.42 When the absorption line width (σ) is larger than the Zeeman splitting ΔE, the latter can be extracted according to eq 1 ⎛ 2e ⎞ ΔA |ΔE| = ⎜ ⎟·σ · ⎝ 2 ⎠ A0
with ΔA =
MCD 1.15
ΔE = C·x*·⟨Sz⟩
(2)
where ⟨Sz⟩ denotes the mean spin of the magneto-optically active dopants along the magnetic axis11 (described by a Brillouin function depending on the temperature T and the magnetic field B), whereas x* represents the concentration of magneto-optically active Mn2+ ions, and C is a scaling constant. In bulk DMS, ⟨Sz⟩ is reduced owing to the influence of distant ions (more than a unit cell apart from each other, represented by the introduction of an antiferromagnetic temperature TAF). This influence of distant ions can be excluded in the case of the cluster owing to their extremely small size. In addition, the clusters are passivated by oleylamine ligands (C-18), leading to a distinct separation between neighboring clusters, which consequently impedes the magnetic interparticle interactions. C includes both the overlap of the wave functions of the charge carriers in the host bands (electrons and holes) and the magnetic ions (represented by γ), and the strength of the spin− spin interaction between both, which are described by the exchange coupling constants N0α and N0β (i.e., C = γ (N0α − N0β), assuming an excitonic transition involving the heavy-hole
(1)
if the width (σ) and height (A0) of the corresponding absorption peak are known.46 In order to determine σ and 7137
DOI: 10.1021/acsnano.6b03348 ACS Nano 2016, 10, 7135−7141
Article
ACS Nano subband). As the influence of distant ions in the cluster can be excluded, both C and ⟨Sz⟩ do not depend on the Mn2+concentration in the cluster. Thus, ΔE is directly proportional to x* for constant values of B and T. As shown in Figure 2c, the maximum Zeeman splitting is observed for the cluster with xnom ≈ 6%. An energy splitting of about 13 meV is achieved at 1.4 T and 5.2 K, corresponding to an effective g-factor of geff = −160. Compared to conventional colloidal DMS QDs (diameter >2 nm) with similar doping concentrations,47 the ΔE value measured in this work is substantially smaller, which might be related to the modified exchange coupling constants21,43,48,49 and the impact of valence band mixing15,42 that results from the extreme quantum confinement. Moreover, it is obvious that the majority of the 13 cation lattice sites are located on the surface with nonideal overlap with the wave functions of the charge carriers, implying that γ is significantly smaller than unity. It is instructive to compare the trend in Zeeman splitting at different doping concentrations with theoretical expectations for x* in Mn2+-doped (CdSe)13 clusters for two limiting cases. In the first scenario, the Mn2+ ions in bidoped clusters are assumed to behave paramagnetically (or even ferromagnetically) in the presence of an external magnetic field. Consequently, bidoped and monodoped clusters are all magneto-optically active (x* = xnom, green continuous line, Figure 2d). In contrast, if the Mn2+ ions in bidoped clusters couple antiferromagnetically, this species becomes magnetooptically passive (except for the small intrinsic Zeeman effect). Therefore, only monodoped clusters contribute to the sp−d exchange interaction, and x* follows P1 (black line in Figure 2d). This case represents a digital change of the magnetooptical activity of the clusters mediated by the one-by-one incorporation of individual dopants into a single cluster. The green dashed line in Figure 2d is obtained for the intermediate case under the assumption that 50% of the Mn2+ ions in bidoped clusters are magneto-optically active. Comparing the dependence of the measured Zeeman splitting on xnom, it becomes obvious that the model of digital dopingwhere a maximum of ΔE is expected at xnom = 7.7% fits the experimental results quite well. Thus, one has to conclude that a high percentage of the bidoped clusters are magneto-optically passive, that is, the two Mn2+ ions inside the cluster couple antiferromagnetically. This in fact means that only the monodoped clusters generate the MCD signal. Adding an additional Mn2+ ion into the lattice switches off the giant magneto-optical response in bidoped clusters. In temperature-dependent studies (see Figure 3), giant magneto-optical responses can be traced all the way up to room temperature for clusters with different xnom values. Following the convention of previous studies,46,50 the g-factor remains negative between 5.2 and 300 K, proving that the Zeeman splitting mainly originates from the sp−d exchange interaction over the whole temperature range. It should be noted that the intrinsic Zeeman splitting exhibits a positive g-factor in bulk,51 which is known to be influenced by quantization, but remains positive even for 38 Å CdSe QDs.52 Induced by single dopants in the (CdSe)13 host, the persistence of the giant magnetooptical response at room temperature might be useful for applications in future solotronic devices. Figure 4a shows the temperature dependence of the MCD amplitude at the energetically lowest maximum and the extracted ΔE value for the clusters with an xnom value of 8%. Although we refrain from extracting the Zeeman splitting at
Figure 3. MCD spectra of samples with different xnom values (2%, 6%, and 10%) at 1.43 T for temperatures between 5.2 and 300 K. Room-temperature spectra are shown in the insets.
Figure 4. (a) Temperature-dependent behavior of the MCD amplitude (normalized to the maximum amplitude, black symbols, right axis) compared to the decrease of ΔE with increasing T at B = 1.4 T (blue symbols on left axis, xnom = 8%). The black line represents a Brillouin function with T* = 9 K. (b) Decay of the MCD amplitude with increasing temperature for different xnom values in comparison with a Brillouin function with T * = 8.4 K.
higher temperatures because of increasing absorption line width, it is obvious that the MCD amplitude is proportional to the giant Zeeman splitting. Therefore, the decline of ΔE with temperature can be deduced from the behavior of the MCD amplitude. The temperature dependence of ΔE in DMS materials is proportional to ⟨Sz⟩, which can be described by a Brillouin function (eq 3): 7138
DOI: 10.1021/acsnano.6b03348 ACS Nano 2016, 10, 7135−7141
Article
ACS Nano ⎛ gμ SB ⎞ ⟨Sz⟩ = S ·Bs ⎜ B ⎟ ⎝ kBT ⎠
with T = Tbath + T *
METHODS Cluster Synthesis and Determination of Mn2+ Content. The samples were synthesized by following our previous study.41 In a typical synthesis, cation precursor solution was prepared by heating the mixture of 1.5 mmol of CdCl2 (anhydrous, 99.999%, Sigma-Aldrich), 0.15 mmol of MnCl2 (anhydrous, 99.999%, Sigma-Aldrich), and 10 mL of noctylamine (99%, Sigma-Aldrich) at 120 °C for 2 h. In a separate batch, selenium precursor solution was prepared by reacting CO gas (99.999%, Alpha Gas) with the mixture of 4.5 mmol of elemental selenium (99.99%, Sigma-Aldrich) and 5 mL of n-octylamine. The selenium precursor solution was injected into the mixture of metal precursor solution and the reaction was kept at room temperature for ∼40 h. The final products were acquired by precipitation using ethanol containing trioctylphosphine followed by washing with ethanol. The doping concentration was precisely controlled by adjusting the amount of the MnCl2. To enhance the dispersibility and to minimize light scattering, we performed a surface modification with long-chain oleylamine (C-18).41 For the magneto-optical characterization, oleylamine capped clusters were used. The nominal doping concentration (average concentration of Mn2+ ions in an ensemble, which is calculated as the number of Mn2+ ions in an ensemble divided by the total number of Mn2+ and Cd2+ ions in an ensemble) was characterized by inductively coupled plasma-atomic emission spectrometry (ICP-AES, Shimadzu) and was varied from 2% to 10%. Different samples were used for mass spectrometry (MS) and magnetic circular dichroism (MCD) studies with slightly different Mn2+ contents (xnom = 2.2 ± 0.02%, 4 ± 0.05%, 7.4 ± 0.15%, and 9.9 ± 0.2% for MS studies and 2.30 ± 0.07%, 4.00 ± 0.25%, 6.32 ± 0.36%, 8.14 ± 0.53%, and 10.16 ± 0.63% for MCD studies). For the purpose of readability, we use integers for xnom in the text. Laser desorption/ionization time-of-flight mass spectrometry (LDITOF MS) was performed on a Voyager-DE STR Biospectrometry Workstation (Applied Biosystems Inc.) in a negative ion mode after the calibration using protein standards. Magneto-Optical Characterization. For magneto-optical measurements, the samples were prepared as a thin film between two quartz glass substrates from the dispersion. The MCD spectroscopy was carried out with a homemade setup consisting of a 75 W xenon lamp (Lot-Oriel) equipped with a monochromator (omni-λ 150, Lot-Oriel) and a photomultiplier (R928, Hamamatsu). The excitation light was modulated using a photoelastic modulator (PEM-90, Hinds Instruments). The sample was placed in a helium vapor cryostat (ST-300, Janis) between two poles of an electromagnet (EM4-HVA, Lake Shore) in Faraday geometry.
(3)
where S = 5/2 denotes the spin value and g = 2 denotes the gyromagnetic factor of the Mn2+ ions; μB, kB, and T are the Bohr magneton, Boltzmann constant, and effective temperature. In the conventional theory by Gaj et al.,23 the effective temperature T is mainly the bath temperature (Tbath) with a small correction related to the interaction between Mn2+ ions, described by T* = TAF. This empirical parameter TAF, which accounts for the influence of more distant Mn2+ neighbors (the next-nearest neighbors or higher), is proportional to xnom for small concentrations. For Mn2+-doped CdSe or CdTe, TAF usually does not exceed 2.6 K up to 10% nominal doping concentration.23,53,54 As discussed above, predominantly monodoped clusters contribute to the giant MCD signal, and thus the Mn2+ ions cannot interfere with any distant ions in neighboring clusters; accordingly, the antiferromagnetic temperature TAF should be zero. It is surprising that fitting the temperature-dependent MCD signal with a Brillouin function reveals a nonvanishing correction of the bath temperature by T* ≈ 9 K for the 8% Mn2+-doped clusters (compare Figure 4a). It was reported that photogenerated carriers can cause a heating of the Mn2+ spin system owing to a spin−spin coupling between charge carriers and dopants.22,55−58 Photogenerated charge carriers transfer energy quickly to the Mn2+ ions either indirectly via phonon coupling or directly by spin exchange scattering, including the excitation of the internal Mn2+ transition 6A1 → 4T1. This results in a spin temperature of up to 60 K in single-Mn2+doped epitaxial QDs.27 Thus, we attribute the finite value of T* to spin heating. In contrast to usual DMS materials of different dimensionality−wherein spin heating strongly depends on the nominal doping concentration − T* is found to be virtually independent of xnom in the case of the Mn2+-doped (CdSe)13 clusters. This can be seen in Figure 4b, which shows the reduction of the MCD amplitude with increasing temperature (normalized to the value at 8 K) for all samples investigated, revealing a similar behavior for all doping concentrations. The data can be fitted by T * = 8.4 ± 2.3 K independent of the doping concentration. This is consistent with our suggestion that only monodoped clusters contribute to the magnetooptical response and are probed by MCD.
CONCLUSIONS In summary, this study reveals unique magneto-optical properties in Mn2+-doped magic-sized CdSe clusters, different from those of conventional DMS QDs. Both the trend in giant Zeeman splitting with increasing doping concentration and the constant T* in the whole examined concentration range indicate that exclusively monodoped clusters are magnetooptically active, while bidoped clusters are magneto-optically inactive. This corresponds to a digital behavior of the doping in these extremely small clusters. Although our samples are composed of a statistical mixture of different cluster species, their magneto-optical response is generated by solitary dopants. As the sp−d exchange interaction persists up to room temperature, these clusters might be interesting candidates for applications in future solotronic devices.
ASSOCIATED CONTENT S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.6b03348. High-resolution MS data with simulated isotope distributions, the extracted ratios from MS data, the calculation method for the binomial distribution, and the extracted Zeeman splitting values. (PDF)
AUTHOR INFORMATION Corresponding Authors
*E-mail: (T.H.)
[email protected]. *E-mail: (G.B.)
[email protected]. 7139
DOI: 10.1021/acsnano.6b03348 ACS Nano 2016, 10, 7135−7141
Article
ACS Nano Author Contributions
Doped Inverted Core-Shell ZnSe-CdSe Nanocrystals. Nat. Mater. 2009, 8, 35−40. (17) Jing, L.; Ding, K.; Kershaw, S. V.; Kempson, I. M.; Rogach, A. L.; Gao, M. Magnetically Engineered Semiconductor Quantum Dots as Multimodal Imaging Probes. Adv. Mater. 2014, 26, 6367−6386. (18) Ithurria, S.; Guyot-Sionnest, P.; Mahler, B.; Dubertret, B. Mn2+ as a Radial Pressure Gauge in Colloidal Core/Shell Nanocrystals. Phys. Rev. Lett. 2007, 99, 265501. (19) Norris, D. J.; Yao, N.; Charnock, F. T.; Kennedy, T. A. HighQuality Manganese-Doped ZnSe Nanocrystals. Nano Lett. 2001, 1, 3− 7. (20) Beaulac, R.; Schneider, L.; Archer, P. I.; Bacher, G.; Gamelin, D. R. Light-Induced Spontaneous Magnetization in Doped Colloidal Quantum Dots. Science 2009, 325, 973−976. (21) Fainblat, R.; Frohleiks, J.; Muckel, F.; Yu, J. H.; Yang, J.; Hyeon, T.; Bacher, G. Quantum Confinement-Controlled Exchange Coupling in manganese(II)-Doped CdSe Two-Dimensional Quantum Well Nanoribbons. Nano Lett. 2012, 12, 5311−5317. (22) Gaj, J.; Kossut, J. Introduction to the Physics of Diluted Magnetic Semiconductors; Springer: Berlin/Heidelberg, 2011. (23) Gaj, J. A.; Planel, R.; Fishman, G. Relation of Magneto-Optical Properties of Free Excitons to Spin Alignment of Mn2+ Ions in Cd1−xMnxTe. Solid State Commun. 1979, 29, 435−438. (24) Koenraad, P. M.; Flatté, M. E. Single Dopants in Semiconductors. Nat. Mater. 2011, 10, 91−100. (25) Fernández Rossier, J. Single-Atom Devices: Quantum Engineering. Nat. Mater. 2013, 12, 480−481. (26) Kobak, J.; Smoleński, T.; Goryca, M.; Papaj, M.; Gietka, K.; Bogucki, A.; Koperski, M.; Rousset, J.-G.; Suffczyński, J.; Janik, E.; Nawrocki, M.; Golnik, A.; Kossacki, P.; Pacuski, W. Designing Quantum Dots for Solotronics. Nat. Commun. 2014, 5, 3191. (27) Besombes, L.; Léger, Y.; Maingault, L.; Ferrand, D.; Mariette, H.; Cibert, J. Probing the Spin State of a Single Magnetic Ion in an Individual Quantum Dot. Phys. Rev. Lett. 2004, 93, 207403. (28) Balasubramanian, G.; Neumann, P.; Twitchen, D.; Markham, M.; Kolesov, R.; Mizuochi, N.; Isoya, J.; Achard, J.; Beck, J.; Tissler, J.; Jacques, V.; Hemmer, P. R.; Jelezko, F.; Wrachtrup, J. Ultralong Spin Coherence Time in Isotopically Engineered Diamond. Nat. Mater. 2009, 8, 383−387. (29) Kudelski, A.; Lemaître, A.; Miard, A.; Voisin, P.; Graham, T. C. M.; Warburton, R. J.; Krebs, O. Optically Probing the Fine Structure of a Single Mn Atom in an InAs Quantum Dot. Phys. Rev. Lett. 2007, 99, 247209. (30) Lee, J.; Yang, J.; Kwon, S. G.; Hyeon, T. Nonclassical Nucleation and Growth of Inorganic Nanoparticles. Nat. Rev. Mater. 2016, 1, 16034. (31) Wang, Y.; Liu, Y.-H.; Zhang, Y.; Wang, F.; Kowalski, P. J.; Rohrs, H. W.; Loomis, R. A.; Gross, M. L.; Buhro, W. E. Isolation of the Magic-Size CdSe Nanoclusters [(CdSe)13(n-octylamine)13] and [(CdSe)13(oleylamine)13]. Angew. Chem., Int. Ed. 2012, 51, 6154− 6157. (32) Wang, Y.; Zhou, Y.; Zhang, Y.; Buhro, W. E. Magic-Size II−VI Nanoclusters as Synthons for Flat Colloidal Nanocrystals. Inorg. Chem. 2015, 54, 1165−1177. (33) Zanella, M.; Abbasi, A. Z.; Schaper, A. K.; Parak, W. J. Discontinuous Growth of II−VI Semiconductor Nanocrystals from Different Materials. J. Phys. Chem. C 2010, 114, 6205−6215. (34) Dagtepe, P.; Chikan, V. Effect of Cd/Te Ratio on the Formation of CdTe Magic-Sized Quantum Dots during Aggregation. J. Phys. Chem. A 2008, 112, 9304−9311. (35) Li, M.; Ouyang, J.; Ratcliffe, C. I.; Pietri, L.; Wu, X.; Leek, D. M.; Moudrakovski, I.; Lin, Q.; Yang, B.; Yu, K. CdS Magic-Sized Nanocrystals Exhibiting Bright Band Gap Photoemission via Thermodynamically Driven Formation. ACS Nano 2009, 3, 3832− 3838. (36) Groeneveld, E.; Van Berkum, S.; Meijerink, A.; de Mello Donegá, C. Growth and Stability of ZnTe Magic-Size Nanocrystals. Small 2011, 7, 1247−1256.
#
These authors (F. Muckel and J. Yang) contributed equally to this work. Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS G.B. is grateful to the Deutsche Forschungsgemeinschaft for support under contract Ba1422-13. T.H. acknowledges the financial support of the Research Center Program of Institute for Basic Science (IBS-R006-D1) in Korea. REFERENCES (1) Erwin, S. C.; Zu, L.; Haftel, M. I.; Efros, Al. L.; Kennedy, T. A.; Norris, D. J. Doping Semiconductor Nanocrystals. Nature 2005, 436, 91−94. (2) Norris, D. J.; Efros, Al. L.; Erwin, S. C. Doped Nanocrystals. Science 2008, 319, 1776−1779. (3) Shim, M.; Guyot-Sionnest, P. N-Type Colloidal Semiconductor Nanocrystals. Nature 2000, 407, 981−983. (4) Talapin, D. V.; Murray, B. PbSe Nanocrystal Solids for N- and PChannel Thin Film Field-Effect Transistors. Science 2005, 310, 86−89. (5) Buonsanti, R.; Milliron, D. J. Chemistry of Doped Colloidal Nanocrystals. Chem. Mater. 2013, 25, 1305−1317. (6) Sahu, A.; Kang, M. S.; Kompch, A.; Notthoff, C.; Wills, A. W.; Deng, D.; Winterer, M.; Frisbie, C. D.; Norris, D. J. Electronic Impurity Doping in CdSe Nanocrystals. Nano Lett. 2012, 12, 2587− 2594. (7) Kovalenko, M. V.; Manna, L.; Cabot, A.; Hens, Z.; Talapin, D. V.; Kagan, C. R.; Klimov, V. I.; Rogach, A. L.; Reiss, P.; Milliron, D. J.; Guyot-Sionnnest, P.; Konstantatos, G.; Parak, W. J.; Hyeon, T.; Korgel, B. A.; Murray, C. B.; Heiss, W. Prospects of Nanoscience with Nanocrystals. ACS Nano 2015, 9, 1012−1057. (8) Furdyna, J. K.; Kossut, J. Diluted Magnetic Semiconductors, Semiconductors and Semimetals; Academic Press, Inc.: San Diego, 1988; Vol. 25. (9) Dietl, T. A Ten-Year Perspective on Dilute Magnetic Semiconductors and Oxides. Nat. Mater. 2010, 9, 965−974. (10) Barrows, C. J.; Chakraborty, P.; Kornowske, L. M.; Gamelin, D. R. Tuning Equilibrium Compositions in Colloidal Cd1−xMnxSe Nanocrystals Using Diffusion Doping and Cation Exchange. ACS Nano 2016, 10, 910−918. (11) Nelson, H. D.; Bradshaw, L. R.; Barrows, C. J.; Vlaskin, V. A.; Gamelin, D. R. Picosecond Dynamics of Excitonic Magnetic Polarons in Colloidal Diffusion-Doped Cd1−xMnxSe Quantum Dots. ACS Nano 2015, 9, 11177−11191. (12) Grumbach, N.; Rubin-Brusilovski, A.; Maikov, G. I.; Tilchin, E.; Lifshitz, E. Manipulation of Carrier−Mn2+ Exchange Interaction in CdTe/CdSe Colloidal Quantum Dots by Controlled Positioning of Mn2+ Impurities. J. Phys. Chem. C 2013, 117, 21021−21027. (13) Rice, W. D.; Liu, W.; Baker, T. A.; Sinitsyn, N. A.; Klimov, V. I.; Crooker, S. A. Revealing Giant Internal Magnetic Fields due to Spin Fluctuations in Magnetically Doped Colloidal Nanocrystals. Nat. Nanotechnol. 2015, 11, 137−142. (14) Delikanli, S.; Akgul, M. Z.; Murphy, J. R.; Barman, B.; Tsai, Y.; Scrace, T.; Zhang, P.; Bozok, B.; Hernández-Martínez, P. L.; Christodoulides, J.; Cartwright, A. N.; Petrou, A.; Demir, H. V. Mn2+-Doped CdSe/CdS Core/Multishell Colloidal Quantum Wells Enabling Tunable Carrier−Dopant Exchange Interactions. ACS Nano 2015, 9, 12473−12479. (15) Fainblat, R.; Muckel, F.; Barrows, C. J.; Vlaskin, V. A.; Gamelin, D. R.; Bacher, G. Valence-Band Mixing Effects in the Upper-ExcitedState Magneto-Optical Responses of Colloidal Mn2+-Doped CdSe Quantum Dots. ACS Nano 2014, 8, 12669−12675. (16) Bussian, D. A.; Crooker, S. A.; Yin, M.; Brynda, M.; Efros, Al. L.; Klimov, V. I. Tunable Magnetic Exchange Interactions in Manganese7140
DOI: 10.1021/acsnano.6b03348 ACS Nano 2016, 10, 7135−7141
Article
ACS Nano
Magnetic-Semiconductor Quantum Wells. Phys. Rev. B: Condens. Matter Mater. Phys. 2006, 73, 035306. (56) Worschech, L.; Schmidt, T.; Forchel, A.; Slobodskyy, T.; Schmidt, G.; Molenkamp, L. W. Optical Polarization of Semimagnetic CdSe Quantum Dots with Low Manganese Content. Semicond. Sci. Technol. 2008, 23, 114018. (57) Keller, D.; Yakovlev, D. R.; König, B.; Ossau, W.; Gruber, T.; Waag, A.; Molenkamp, L. W.; Scherbakov, A. V. Heating of the Magnetic Ion System in (Zn, Mn)Se/(Zn, Be)Se Semimagnetic Quantum Wells by Means of Photoexcitation. Phys. Rev. B: Condens. Matter Mater. Phys. 2001, 65, 035313. (58) König, B.; Merkulov, I. A.; Yakovlev, D. R.; Ossau, W.; Ryabchenko, S. M.; Kutrowski, M.; Wojtowicz, T.; Karczewski, G.; Kossut, J. Energy Transfer from Photocarriers into the Magnetic Ion System Mediated by a Two-Dimensional Electron Gas in (Cd,Mn)Te/ (Cd, Mg)Te Quantum Wells. Phys. Rev. B: Condens. Matter Mater. Phys. 2000, 61, 16870.
(37) Eilers, J.; Groeneveld, E.; de Mello Donegá, C.; Meijerink, A. Optical Properties of Mn-Doped ZnTe Magic Size Nanocrystals. J. Phys. Chem. Lett. 2012, 3, 1663−1667. (38) Del Ben, M.; Havenith, R. W. A.; Broer, R.; Stener, M. Density Functional Study on the Morphology and Photoabsorption of CdSe Nanoclusters. J. Phys. Chem. C 2011, 115, 16782−16796. (39) Azpiroz, J. M.; Matxain, J. M.; Infante, I.; Lopez, X.; Ugalde, J. M. A DFT/TDDFT Study on the Optoelectronic Properties of the Amine-Capped Magic (CdSe)13 Nanocluster. Phys. Chem. Chem. Phys. 2013, 15, 10996−11005. (40) Botti, S.; Marques, M. A. L. Identification of Fullerene-like CdSe Nanoparticles from Optical Spectroscopy Calculations. Phys. Rev. B: Condens. Matter Mater. Phys. 2007, 75, 035311. (41) Yang, J.; Fainblat, R.; Kwon, S. G.; Muckel, F.; Yu, J. H.; Terlinden, H.; Kim, B. H.; Iavarone, D.; Choi, M. K.; Kim, I. Y.; Park, I.; Hong, H.-K.; Lee, J.; Son, J. S.; Lee, Z.; Kang, K.; Hwang, S.-J.; Bacher, G.; Hyeon, T. Route to the Smallest Doped Semiconductor: Mn2+-Doped (CdSe)13 Clusters. J. Am. Chem. Soc. 2015, 137, 12776− 12779. (42) Efros, Al. L.; Rosen, M.; Kuno, M.; Nirmal, M.; Norris, D.; Bawendi, M. Band-Edge Exciton in Quantum Dots of Semiconductors with a Degenerate Valence Band: Dark and Bright Exciton States. Phys. Rev. B: Condens. Matter Mater. Phys. 1996, 54, 4843−4856. (43) Yu, J. H.; Liu, X.; Kweon, K. E.; Joo, J.; Park, J.; Ko, K.-T.; Lee, D. W.; Shen, S.; Tivakornsasithorn, K.; Son, J. S.; Park, J.-H.; Kim, Y.W.; Hwang, G. S.; Dobrowolska, M.; Furdyna, J. K.; Hyeon, T. Giant Zeeman Splitting in Nucleation-Controlled Doped CdSe:Mn2+ Quantum Nanoribbons. Nat. Mater. 2010, 9, 47−53. (44) Park, K. M.; Bae, Y. J.; Ahn, S. H.; Kim, M. S. A Simple Method for Quantification of Peptides and Proteins by Matrix-Assisted Laser Desorption Ionization Mass Spectrometry. Anal. Chem. 2012, 84, 10332−10337. (45) Park, K. M.; Moon, J. H.; Kim, K. P.; Lee, S. H.; Kim, M. S. Relative Quantification in Imaging of a Peptide on a Mouse Brain Tissue by Matrix-Assisted Laser Desorption Ionization. Anal. Chem. 2014, 86, 5131−5135. (46) Klimov, V. Nanocrystal Quantum Dots, 2nd Edition; CRC Press: Boca Raton, FL, 2010. (47) Vlaskin, V. A.; Barrows, C. J.; Erickson, C. S.; Gamelin, D. R. Nanocrystal Diffusion Doping. J. Am. Chem. Soc. 2013, 135, 14380− 14389. (48) Bhattacharjee, A. K. Confinement-Induced Reduction of the Effective Exchange Parameters in Semimagnetic Semiconductor Nanostructures. Phys. Rev. B: Condens. Matter Mater. Phys. 1998, 58, 15660−15665. (49) Merkulov, I.; Yakovlev, D.; Keller, A.; Ossau, W.; Geurts, J.; Waag, A.; Landwehr, G.; Karczewski, G.; Wojtowicz, T.; Kossut, J. Kinetic Exchange between the Conduction Band Electrons and Magnetic Ions in Quantum-Confined Structures. Phys. Rev. Lett. 1999, 83, 1431−1434. (50) Piepho, S. B.; Schatz, P. N. Group Theory in Spectroscopy: With Applications to Magnetic Circular Dichroism; Wiley-Interscience Monographs in Chemical Physics; Wiley: New York, 1983. (51) Furdyna, J. K. Diluted Magnetic Semiconductors. J. Appl. Phys. 1988, 64, R29−R64. (52) Kuno, M.; Nirmal, M.; Bawendi, M. G.; Efros, Al. L.; Rosen, M. Magnetic Circular Dichroism Study of CdSe Quantum Dots. J. Chem. Phys. 1998, 108, 4242. (53) Heiman, D.; Shapira, Y.; Foner, S.; Khazai, B.; Kershaw, R.; Dwight, K.; Wold, A. Exchange Energy, Magnetization, and Raman Scattering of (Cd,Mn)Se. Phys. Rev. B: Condens. Matter Mater. Phys. 1984, 29, 5634. (54) Gaj, J.; Grieshaber, W.; Bodin-Deshayes, C.; Cibert, J.; Feuillet, G.; Merle d’Aubigné, Y.; Wasiela, A. Magneto-Optical Study of Interface Mixing in the CdTe-(Cd,Mn)Te System. Phys. Rev. B: Condens. Matter Mater. Phys. 1994, 50, 5512−5527. (55) Kneip, M.; Yakovlev, D.; Bayer, M.; Maksimov, A.; Tartakovskii, I.; Keller, D.; Ossau, W.; Molenkamp, L.; Waag, A. Direct Energy Transfer from Photocarriers to Mn-Ion System in II-VI Diluted7141
DOI: 10.1021/acsnano.6b03348 ACS Nano 2016, 10, 7135−7141
