Single Magnetic Impurities in Colloidal Quantum Dots and Magic-Size

Review pubs.acs.org/cm

Single Magnetic Impurities in Colloidal Quantum Dots and MagicSize Clusters Rachel Fainblat, Charles J. Barrows, and Daniel R. Gamelin* Department of Chemistry, University of Washington, Seattle, Washington 98195-1700, United States ABSTRACT: Impurity doping can be used to dramatically alter the physical properties of semiconductor nanostructures and endow them with promising new technological potential. This review summarizes recent progress toward the development of colloidal semiconductor quantum dots (QDs) doped with individual magnetic impurity ions. Such singly doped quantum dots (SDQDs), as well as related magic-sized nanoclusters, represent an exciting class of materials for cultivating unique physical effects arising from magnetic exchange coupling between delocalized charge carriers and the single impurity ions. Key exchange effects can be enhanced by carrier confinement in such small structures. The physical properties displayed by these materials may prove valuable for new technologies based on the manipulation of individual spins in semiconductor nanostructures, such as spintronics, spin-photonics, or quantum computing. Interesting chemical and analytical challenges also emerge when exploring this research frontier. Here, we describe cluster and QD results from recent literature related to these challenges. We summarize (magneto-)optical investigations of SDQDs on both the ensemble and single-particle levels. Comparisons to analogous bulk materials and self-assembled QDs are drawn and used to highlight some of the rich and unique characteristics found within this class of materials.

1. INTRODUCTION The ability to generate, manipulate, and detect spins in semiconductor nanostructures underpins many envisioned spin-based solid-state device technologies.1−3 Diluted magnetic semiconductors (DMSs) have long been viewed as promising candidates for spintronic or spin-photonic applications because they combine properties of magnets and semiconductors into a single optoelectronic material.4,5 Beyond mere superposition of these two properties, the incorporation of open-shell transitionmetal impurities into semiconductors introduces localized unpaired spins that are magnetically coupled to delocalized (free or photogenerated) charge carriers, with coupling strengths that depend on the dopant−carrier spatial overlap and on the microscopic details of the spin wave functions. These so-called sp−d exchange interactions4,5 are in many regards the defining feature of a DMS, giving rise to a suite of remarkable magneto-optical and magneto-electronic properties that have driven innovations in the fields of semiconductor spintronics and spin-photonics. Such exchange interactions can generate extraordinary magneto-optical and magneto-electronic effects such as giant band-edge Zeeman splittings and giant Faraday rotation,4,5 excitonic magnetic polaron formation,6−8 or carrier-mediated ferromagnetism,9−11 and have allowed demonstration of functional spin-LEDs,12−14 semiconductor spin filters,15,16 and Faraday optical isolators.17,18 DMSs provide a rich platform for pushing the frontiers of fundamental science toward future novel spin-based technologies. Advances in the fabrication of epitaxial doped quantum dots (QDs)5,19−22 and in the synthesis of colloidal doped QDs23−29 over the past two decades have begun to overcome the © 2017 American Chemical Society

longstanding challenges of controlled impurity doping in such nanostructures. Although a great deal of research has focused on the important goal of achieving large magnetic impurity concentrations in QDs to capitalize on the cumulative effects of multiple pairwise dopant−carrier exchange interactions, parallel efforts have sought to harness the properties of single dopants in semiconductors. There are certain advantages to materials with precisely one magnetic dopant, such as the ability to optically or electrically manipulate a single defined spin state,30−33 or long spin-coherence times,34−37 both of which are requirements for high-quality-factor spin qubits. Theoretical work has suggested that QDs with single magnetic dopants could be useful in single-electron transistors and spin filters,38−40 but such devices have not yet been demonstrated. The ability to manipulate quantum processes at single impurities forms the basis for so-called “solotronics”.22,41 Colloidal semiconductor nanocrystals offer exquisite control over carrier wave functions through synthetic engineering of sizes, shapes, compositions, and heterointerfaces.42 Recent synthetic advances have enabled unprecedented control over dopant incorporation, concentration, and spatial location, thereby tuning overall dopant-carrier exchange strengths. Because of greater quantum confinement, magneto-optical responses are enhanced in colloidal DMS nanostructures relative to their self-assembled epitaxial counterparts, allowing the observation of dopant−carrier magnetic exchange effects Received: July 28, 2017 Revised: August 28, 2017 Published: August 30, 2017 8023

DOI: 10.1021/acs.chemmater.7b03195 Chem. Mater. 2017, 29, 8023−8036

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Chemistry of Materials

Figure 1. Dopant−carrier magnetic exchange coupling in a doped quantum dot. (A) Introduction of a transition-metal (TM2+) dopant into a II−VI semiconductor quantum dot such as CdSe turns on new magnetic-exchange interactions between unpaired spins on the TM2+ ion and the unpaired spins of the charge carriers. These are referred to as s−d and p−d exchange interactions (with coupling constants α and β, respectively) for the interactions involving e−CB and h+VB, respectively. (B) Depictions of the dominant orbital exchange pathways that dictate the signs and strengths of the e−CB−Mn2+ and h+VB−Mn2+ magnetic exchange coupling. The e−CB−Mn2+ exchange energy scales with the extent of CB−Mn(4s) orbital hybridization, and can be described within perturbation theory as a ferromagnetic kinetic s−s exchange pathway. The h+VB−Mn2+ exchange energy scales with the extent of VB−Mn(3d) orbital hybridization, and is described as an antiferromagnetic kinetic-exchange process. Adapted with permission from from ref 54. Copyright 2011 American Physical Society.

even at room temperature.43−47 Similarly, molecular clusters represent chemical precursors and surface models, as well as the ultimate limit of volume confinement in these materials, but come with new challenges reflecting unknown structures, geometries, and extremely high surface-to-volume ratios. Successes in synthesis, isolation, and structural characterization of magic-sized clusters (MSCs) related to classic II−VI or III− V QDs portend exciting opportunities for bridging the molecule/materials gap in this area.48−50 Here, we review recent progress in the development and study of colloidal zero-dimensional semiconductor nanostructures with individual magnetic impurities. We start by describing the microscopic sp−d exchange interactions responsible for the giant magneto-optical responses of DMSs. We then summarize ensemble measurements of singly doped QDs (SDQDs) and related clusters. Finally, we describe the recent advance of single-particle luminescence measurements on SDQDs, showing enhanced sensitivity to dopant location.

versa.51,52 This interaction is described as an antiferromagnetic kinetic exchange whose strength is proportional to the extent of hybridization of the Mn2+ 3d orbitals with the neighboring semiconductor anion p orbitals at the valence-band edge. In Td (tetrahedral point group) symmetry, the five Mn2+ d orbitals split and transform as the well-known e and t2 sets, whereas the surrounding anion orbitals combine to form a1 and t 2 symmetry-adapted linear combinations (SALCs). The t2 anion SALCs have appropriate symmetry to interact with the t2 d orbitals of the Mn2+ ions, and this interaction at the band edge is characterized by an antibonding phase relationship between the two (Figure 1B, right). The difference in the energies of spin-up and spin-down Mn2+ 3d orbitals then causes a spin-dependent hybridization and leads to net antiferromagnetic p−d exchange coupling (β < 0). In the other case, Mn2+ exchange coupling with conductionband electrons (e−CB) at the band edge is dominated by partial spin transfer from the conduction band to the Mn2+ 4s orbital.53,54 The conduction-band edge has a significant contribution of anion p orbitals, which are arranged in a totally symmetric (a1, S-like) linear combination around the Mn2+ cation site and can thus mix with the Mn2+ 4s orbital (also a1 symmetry), again with an antibonding phase relationship between the two (Figure 1B, left). The Mn2+ 4s orbital is unoccupied, but intrasite exchange splits the spin-up and spindown 4s energies, leading to spin-dependent hybridization and a ferromagnetic kinetic exchange interaction (α > 0) when an electron is added at the conduction-band edge.53,54 The antiferromagnetic p−d exchange is typically ∼4−8 times stronger than the ferromagnetic s−d exchange that results from this s−s exchange pathway. Similar orbital pathways apply for Co2+ and for some other TM2+ ions, but for other ions with partially occupied 3d(t2) orbitals a mix of antiferromagnetic and ferromagnetic kinetic p−d exchange interactions occur, based on the specific configuration involved.52 For e−CB−Mn2+ coupling, the ferromagnetic kinetic s−s interaction dominates because the more direct antiferromagnetic kinetic s−d interaction is forbidden by symmetry, since the a1 conduction-band orbital formally has no net overlap with the spin-bearing Mn2+ d orbitals (t2 or e symmetry). It has been suggested that the symmetry forbiddenness of the kinetic s−d

2. BRIEF BACKGROUND ON DOPANT−CARRIER MAGNETIC EXCHANGE COUPLING Mn2+ is by far the most commonly studied magnetic impurity in DMSs. Because of its half-filled 3d valence shell, it has a high spin and a large energy gap between its ground state and first internal (d−d) excited state. Similarly, II−VI and III−V semiconductors have traditionally been the most common host lattices for studies of DMSs. We therefore focus primarily on Mn 2+ doped II−VI semiconductors here, but the fundamental principles we describe apply broadly across this entire class of materials. Figure 1A illustrates schematically the sp−d exchange interactions coupling a dopant to a relaxed photogenerated electron−hole pair (exciton). Coupling with the conduction-band electron and the valence-band hole of the exciton can be analyzed individually as so-called s−d and p−d exchange interactions with pairwise exchange coupling constants α and β, respectively. Figure 1B illustrates the microscopic orbital exchange pathways that dictate the signs and magnitudes of these two exchange interactions involving the relaxed band-edge carriers. Mn2+ exchange coupling with valence-band holes (h+VB) is dominated by partial spin transfer from the valence band into the Mn2+ 3d orbitals and vice 8024


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Chemistry of Materials ⎛ g μB B ⎞ 1⎡ ⟨SZ⟩ = − ⎢(2S + 1)coth⎜(2S + 1) TM ⎟ 2⎣ 2kT ⎠ ⎝

exchange interaction might be relaxed due to electron confinement even in relatively large nanostructures, resulting in overall antiferromagnetic e−CB−Mn2+ s−d coupling.55,56 It is also apparent merely from inspection that strict symmetry forbiddenness is already relaxed at any DMS size simply by displacement of the Mn2+ ion from the precise center of the e−CB wave function. Only the question of how important this kinetic s−d pathway is remains. Perturbative and ab initio calculations53,54 as well as experimentation45,57 indicate that confinement-induced antiferromagnetic kinetic s−d exchange in Cd1−xMnxSe and related DMSs is negligible relative to the dominant ferromagnetic kinetic s−s interaction until dimensions of only a few unit-cell thicknesses or smaller (diameter or thickness < ∼1.5 nm) are reached. For example, an ensemble average value of α ≈ 0 has been measured for Cd1−xMnxSe nanoribbons of thickness only ∼1.4 nm (3.9 eV are intense host-based LMCT transitions; between 2.5 and ∼3.9 eV are weaker Co2+-centered LMCT transitions; 3.8 eV) observed in the Zn2+ and Cd2+ clusters

are associated with cluster LMCT transitions and are relatively unchanged by Co2+ incorporation. Incorporation of Co2+ into the smaller [Cd4(SPh)10]2− clusters is found to be more t hermod ynamically favo rable than in the larger [Cd10S4(SPh)16]4− and [Cd17S4(SPh)28]2− clusters.77 A combination of electrospray ionization mass spectrometry (ESI-MS), 1 H NMR, and electronic absorption spectroscopies suggest a specific M-(μ-SPh)3(SPh) site within the (Cd1−xCox)10 and (Cd1−xCox)17 clusters has the lowest-energy barrier for Co2+ cation exchange. The Meijerink group has added Mn2+ to preformed ZnTe magic-sized clusters (MSCs) using cation exchange and has studied the spectroscopy of these clusters.66 MSCs with narrow absorption at ∼354 nm, identified as [(ZnTe)n(ZnLx)m] clusters having ∼45 cations total and diameters ∼1.5−1.8 nm (L = HDA or other primary amines)79 were exposed to Mn(II)-cyclohexanebutyrate at 170 °C, leading to incorporation of Mn2+ into the MSC. The resulting clusters showed photoluminescence (PL) centered at 615 nm associated with the Mn2+ 4T1→6A1 ligand-field transition, and a Mn2+ PL decay time of 45 μs, similar to the Mn2+ PL lifetime in bulk Zn1−xMnxTe. Photoluminescence excitation (PLE) spectra confirmed that Mn2+ luminescence was indeed sensitized by the ZnTe MSCs. The Feng group has reported80 diffusion of Mn2+ ions into crystallographically defined “supertetrahedral” chalcogenide clusters ( 0), indicative of a dominant intrinsic excitonic Zeeman splitting. The temperature- and fielddependence of the 1S3/21Se excitonic Zeeman splitting is plotted in Figure 8C. As the temperature increases, the field dependence of ΔEZ approaches that of the intrinsic Zeeman splittings in undoped CdSe QDs. Because of the Curie temperature dependence of the sp−d contribution and the temperature independence of the intrinsic contribution described in eqs 1 and 2, it was possible to fit the data, deconvolve the two, and reconstruct the MCD spectra of undoped, singly doped, and bidoped QDs. Even in the limit of just one Mn2+ per QD, the 1.7 K excitonic Zeeman splittings are dominated by sp−d exchange at all magnetic fields.93 This finding indicates that the sp−d exchange interaction in the average SDQD of the ensemble is sufficiently strong to dominate the material’s magneto-optical response. The temperature dependence of the sp−d exchange also leads to a unique inversion in MCD sign with increasing temperature at fixed magnetic field. As the temperature increases, the excitonic Zeeman splitting decreases in magnitude and eventually flips sign, approaching the intrinsic Zeeman splitting of undoped CdSe in the high-temperature limit. The crossover point is a function of x and has been used to quantify the effective dopant concentrations in Mn2+- and Co2+-doped CdSe QDs in the small-x limit.95 Together, these results clearly illustrate the competition between intrinsic and exchange contributions to excitonic Zeeman splittings and demonstrate the thermal and magnetic-field tunability of sp−d exchange interactions, with potentially important ramifications for the development of spin-

4. SINGLE DOPANTS IN SINGLE QUANTUM DOTS Although the synthesis of colloidal zero-dimensional nanostructures has improved tremendously over the last two decades, inhomogeneities within QD ensembles inevitably remain. Distributions in QD size, dopant concentration, and dopant location directly affect the spectroscopic properties of DMSs. Single-QD experiments can reveal effects that are obscured by such ensemble inhomogeneities. Studies of the interactions between individual magnetic dopants and charge carriers in single QDs provide insight into the photophysical properties of DMS QDs and advance the understanding of sp− d exchange on the nanoscale. 4.1. Imaging Single Dopants. Developments in microscopy now allow visualization of QD lattices with atomic resolution. Identification of single dopants inside QDs can be achieved by annular dark-field scanning transmission electron microscopy (ADF-STEM), but only when the atomic number (Z) of the dopant differs significantly from that of the host atoms.96−100 The Norris group has shown it is possible to circumvent this limitation by combining ADF-STEM with electron energy loss spectroscopy (EELS), which detects atomspecific signals regardless of Z-contrast.101 Figure 9 shows an EELS map correlated with an ADF-STEM image of a single colloidal Mn2+-doped ZnSe QD (d ∼ 3.7 nm, 6.2 ± 1.5 Mn2+/ QD on average). At specific points in the map, the characteristic Mn L2,3-edge features appeared in the EELS spectrum, identifying which atomic columns contain Mn2+. Detection of Mn2+ down to one dopant per d = 2.9 nm QD was achieved. Among other attractive possibilities, this approach could allow atomically resolved measurements of dopant diffusion fronts in QDs, perhaps in tandem with modeling and other spectroscopic data,102 to provide insight into 8030


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resolved PL lines were observed (Figure 10A), related to the six spin projections of the Mn2+ dopant (Figure 10B) onto the quantization axis defined by the large axial shape anisotropy of the self-assembled QD. Subsequent sophisticated magnetooptical studies of self-assembled SDQDs have demonstrated how dopant location,106 valence-band mixing,106,107 anisotropy,108 and carrier injection31,109 affect the zero-field excitonic exchange splittings. Additionally, optical spin orientation of a single magnetic ion in a QD has been demonstrated,110,111 as has optical spin detection.112 Coherent spin precession of an individual Mn2+ ion has been demonstrated using pump−probe techniques,113 and optical pumping and readout of a specific spin state has also been achieved.32,33,114 Although manganese is by far the most thoroughly investigated dopant for SDQD studies,31−33,110,115,116 individual cobalt,22 chromium,37 and iron117 impurities have also been examined in individual selfassembled QDs, with exciting results from each. The first photoluminescence spectra of colloidal SDQDs were reported only very recently.118 Fainblat et al. investigated the single-QD PL spectra of Cd1−xMnxSe QDs (d = 5.1−5.9 nm, x = 0.0003−0.004), each containing a very small number of Mn2+ ions. Figure 11A shows three different subsets of single QDs from one given sample: (i) undoped QDs, which showed single sharp emission features plus weaker phonon replicas, similar to literature reports,103,119 (ii) QDs containing single Mn2+ dopants, which showed 6 resolved electronic origins, and (iii) QDs containing 2 or more Mn2+ dopants, which showed a multitude of broadened features. Figure 11B shows the highly resolved spectrum of a single colloidal Cd1−xMnxSe QD containing a single Mn2+ impurity. The peak energies were analyzed and assigned as summarized in the lower panel. The low-energy side of the spectrum displays a series of peaks with an energy spacing of ∼27 meV, consistent with the exciton− phonon coupling energy in undoped CdSe.103,119 These peaks, which often appeared more pronounced in Cd1−xMnxSe QDs with thin (rather than thick) shells, were thus attributed to phonon sidebands. The high-energy part of the spectrum shows a set of excitonic electronic origins, split by sp−d exchange coupling of the exciton with Mn2+ in its various spin projections (Figure 10B). Splittings of up to ∼80 meV were observed in these colloidal SDQDs in the absence of any external magnetic field. Because the sp−d exchange coupling strength depends strongly on the spatial overlap of the excitonic and Mn2+ wave

Figure 9. (A) EELS map (shown as pixels on a grid) of the Mn L2,3edge intensity along with a corresponding ADF-STEM image of a single Mn2+-doped ZnSe QD (d = 3.7 nm, 6.2 ± 1.5 Mn2+/QD). The EELS spectrum for one of the pixels where Mn2+ was detected is included, showing the characteristic Mn L2,3-edge features. (B) Overlap of the Mn L2,3-edge intensity map and the ADF-STEM image, both shown in panel A. Adapted with permission from ref 101. Copyright 2011 American Chemical Society.

correlations between dopant location and various physical properties. 4.2. Single-Particle Spectroscopy of Singly Doped Quantum Dots. PL spectra of single undoped QDs typically exhibit a sharp emission feature stemming from exciton recombination, which in some cases is accompanied by relatively weak peaks at lower energy attributed to phonon replicas.103,104 Early spectroscopic investigations of single DMS QDs grown by molecular beam epitaxy (MBE) and containing a very small number of magnetic impurities revealed much greater PL line widths that are affected by thermal fluctuations of the dopant spins.19,105 Besombes et al. reported the first observation of spin−spin exchange interactions between a single Mn2+ dopant and the exciton of an individual selfassembled QD, in this case Cd1−xMnxTe.30 Only 6 well-

Figure 10. (A) Excitonic photoluminescence from a single self-assembled Cd1−xMnxTe QD doped with a single Mn2+ impurity ion. Adapted with permission from ref 30. Copyright 2004 American Physical Society. (B) Schematic depiction of the origins of the 6 lines seen experimentally. An exchange splitting is observed in the excitonic PL at zero applied magnetic field, arising from coupling of the photogenerated electron−hole pair (“Excited State”; mJ = ±1) with the different spin projections of a single S = 5/2 Mn2+ dopant (ms = ±5/2, ±3/2, ±1/2). The quantization axis is defined by the strong axial shape anisotropy of the self-assembled QD. 8031


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Figure 11. (A) 4.7 K PL spectra of three selected individual QDs from ensembles of Cd0.9997Mn0.0003Se QDs. The top spectrum, shows the typical features of an undoped CdSe QD, with a dominant excitonic electronic origin and a phonon sideband. The middle spectrum shows the excitonic PL split into six dominant lines, consistent with a single Mn2+ impurity in this QD. The lower spectrum shows the excitonic PL split into multiple lines, consistent with more than one Mn2+ per QD (top, thin ZnS shell; middle and bottom, thick CdS shell.) (B) PL spectrum of a single Cd0.996Mn0.004Se QD (thin ZnSe shell) collected at 5 K. The dots depict the experimental data, the shaded peaks represent the fitted peaks, and the black curve represents the sum of the fitted peaks. All major peaks have been assigned as either electronic origins or phonon replicas, as summarized in the lower panel. The red curve accounts for a broad background attributed to multiple weak and unresolved phonon replicas of the various purely electronic transitions. Adapted with permission from ref 118. Copyright 2016 American Chemical Society.

functions, the exact splitting energy reflects the dopant’s location within the QD, the largest splittings coming from dopants located closest to the QD center (Figure 2).54,120 Only the small subset of QDs for which Mn2+ resides at or near the QD center can produce exchange splittings of the magnitudes observed in Figure 11, which are ∼10−50 times larger than for Mn2+ in the average cation positions of the same QDs. These very large splittings thus also reflect the ability of single-QD measurements to selectively probe outliers within an ensemble distribution. Importantly, the very large excitonic exchange splittings observed in these colloidal SDQDs are over 1 order of magnitude larger than those observed in analogous selfassembled DMS QDs (e.g., Figure 10A),22,30 reflecting the smaller volumes of these colloidal QDs. Such extremely large exchange splittings could be useful for optical manipulation of defined spin states in single QDs. Given the widespread investigation of QDs at the single-particle level and the growing availability of methods for preparing high-quality doped colloidal QDs, we anticipate that these exciting single-QD spectroscopic results will be followed by many others from the colloidal QD community in the near future.

exchange coupling strengths because of larger dopant−carrier spatial overlap but would also yield more uniform optoelectronic properties at the ensemble level. In addition to providing unprecedented forms of matter, such experiments may provide a deeper fundamental understanding of the evolution of doped materials from clusters to nanocrystals, the chemical impact of impurities on crystal nucleation and growth, and various other aspects of the chemistries of defects in nanostructures. Following in the footsteps of the epitaxial SDQDs, it may be of interest to examine doping with impurities that have no nuclear spin, such as 52Cr2+, 56Fe2+, 56Fe3+, or 58/60Ni2+, to suppress hyperfine contributions to spectral splittings and spin relaxation. The incorporation of rare earth ions such as Eu2+ or Gd3+ into chalcogenide QDs, although synthetically challenging,121−123 could reveal interesting and unexplored magnetooptical effects in SDQDs due to the large spin−orbit coupling strengths of the lanthanides. Charge injection has already been demonstrated as an effective approach for manipulating impurity spins in ensembles of colloidal DMS QDs,43 and the effects of excess delocalized charges on Mn2+ spin relaxation have been investigated in colloidal DMS QDs,124 but it would be interesting to extend such experiments to the SDQD level. An attractive variation on this experiment would be to do so using QDs doped with redox-active impurities such as Fe3+/2+,125 which would allow redox control over the spin state of an individual dopant. Emerging materials such as haloperovskite nanocrystals are now being doped with magnetic impurities126−135 and can also be anticipated to show unique properties at the SDQD level. On the physical side, there are broad challenges to be surmounted related to the creation, manipulation, and detection of spins in colloidal QDs. Among other interesting aims will be the general goal of understanding more deeply the relationships between colloidal and self-assembled SDQDs, and exploiting the areas of contrast. Compared to self-assembled QDs, colloidal SDQDs offer unique opportunities to engineer shape anisotropies, tune compositions via postsynthetic

5. OUTLOOK Recent advances in the preparation of colloidal semiconductor quantum dots and clusters containing single magnetic impurities are beginning to open doors for investigations into this extraordinary class of compounds. Although a young pursuit, these investigations have already yielded interesting fundamental research that gives tantalizing glimpses of their potential technological promise. Building on these initial successes, further advances will be aided by improved synthetic methods for creating precisely doped QDs. Such advances may involve the use of singly doped molecular or magic-sized clusters as seeds for growth of larger nanostructures with single impurities located at or near their precise centers. Such precise synthetic control would not only enhance the dopant−carrier 8032


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Photogenerated Carriers in Magnetic III-V Semiconductor Heterostructures of (In,Mn)As/GaSb. Phys. Rev. Lett. 1997, 78, 4617−4620. (10) Beschoten, B.; Crowell, P. A.; Malajovich, I.; Awschalom, D. D.; Matsukura, F.; Shen, A.; Ohno, H. Magnetic Circular Dichroism Studies of Carrier-Induced Ferromagnetism in (Ga1‑xMnx)As. Phys. Rev. Lett. 1999, 83, 3073−3076. (11) Chiba, D.; Yamanouchi, M.; Matsukura, F.; Ohno, H. Electrical Manipulation of Magnetization Reversal in a Ferromagnetic Semiconductor. Science 2003, 301, 943−945. (12) Ohno, Y.; Young, D. K.; Beschoten, B.; Matsukura, F.; Ohno, H.; Awschalom, D. D. Electrical Spin Injection in a Ferromagnetic Semiconductor Heterostructure. Nature 1999, 402, 790−792. (13) Fiederling, R.; Keim, M.; Reuscher, G.; Ossau, W.; Schmidt, G.; Waag, A.; Molenkamp, L. W. Injection and Detection of a SpinPolarized Current in a Light-Emitting Diode. Nature 1999, 402, 787− 790. (14) Jonker, B. T.; Park, Y. D.; Bennett, B. R.; Cheong, H. D.; Kioseoglou, G.; Petrou, A. Robust Electrical Spin Injection into a Semiconductor Heterostructure. Phys. Rev. B: Condens. Matter Mater. Phys. 2000, 62, 8180−8183. (15) Rüster, C.; Borzenko, T.; Gould, C.; Schmidt, G.; Molenkamp, L. W.; Liu, X.; Wojtowicz, T. H.; Furdyna, J. K.; Yu, Z. G.; Flatté, M. E. Very Large Magnetoresistance in Lateral Ferromagnetic (Ga,Mn)As Wires with Nanoconstrictions. Phys. Rev. Lett. 2003, 91, 216602. (16) Ohno, H.; Matsukura, F.; Omiya, T.; Akiba, N. Spin-Dependent Tunneling and Properties of Ferromagnetic (Ga,Mn)As. J. Appl. Phys. 1999, 85, 4277−4282. (17) Turner, A. E.; Gunshor, R. L.; Datta, S. New class of materials for optical isolators. Appl. Opt. 1983, 22, 3152−3154. (18) Onodera, K.; Masumoto, T.; Kimura, M. 980 nm compact optical isolators using Cd1‑x‑yMnxHgyTe single crystals for high power pumping laser diodes. Electron. Lett. 1994, 30, 1954−1955. (19) Hundt, A.; Puls, J.; Henneberger, F. Spin Properties of SelfOrganized Diluted Magnetic Cd1−xMnxSe Quantum Dots. Phys. Rev. B: Condens. Matter Mater. Phys. 2004, 69, 121309. (20) Bacher, G.; Schömig, H.; Scheibner, M.; Forchel, A.; Maksimov, A. A.; Chernenko, A. V.; Dorozhkin, P. S.; Kulakovskii, V. D.; Kennedy, T.; Reinecke, T. L. Spin−Spin Interaction in Magnetic Semiconductor Quantum Dots. Phys. E 2005, 26, 37−44. (21) Quantum Dots: Optics, Electron Transport, and Future Applications; Tartakovskii, A., Ed.; Cambridge University Press: Cambridge, U. K., 2012. (22) 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. (23) Bhargava, R. N.; Gallagher, D.; Hong, X.; Nurmikko, A. Optical Properties of Manganese-Doped Nanocrystals of ZnS. Phys. Rev. Lett. 1994, 72, 416−419. (24) Hoffman, D. M.; Meyer, B. K.; Ekimov, A. I.; Merkulov, I. A.; Efros, A. L.; Rosen, M.; Couino, G.; Gacoin, T.; Boilot, J. P. Giant Internal Magnetic Fields in Mn Doped Nanocrystal Quantum Dots. Solid State Commun. 2000, 114, 547−550. (25) Norris, D. J.; Efros, A. L.; Erwin, S. C. Doped Nanocrystals. Science 2008, 319, 1776−1779. (26) Beaulac, R.; Archer, P. I.; Ochsenbein, S. T.; Gamelin, D. R. Mn2+-Doped CdSe Quantum Dots: New Inorganic Materials for SpinElectronics and Spin-Photonics. Adv. Funct. Mater. 2008, 18, 3873− 3891. (27) Beaulac, R.; Ochsenbein, S. T.; Gamelin, D. R. Colloidal Transition-Metal-Doped Quantum Dots. In Nanocrystal Quantum Dots, 2nd ed.; Klimov, V. I., Ed.; CRC Press: Boca Raton, FL, 2010; pp 397−453. (28) Buonsanti, R.; Milliron, D. J. Chemistry of Doped Colloidal Nanocrystals. Chem. Mater. 2013, 25, 1305−1317. (29) Pradhan, N.; Das Adhikari, S.; Nag, A.; Sarma, D. D. Luminescence, Plasmonic, and Magnetic Properties of Doped Semiconductor Nanocrystals. Angew. Chem., Int. Ed. 2017, 56, 7038−7054.

diffusion doping, create heterostructures, and functionalize surfaces, all of which may provide new avenues for controlling the functionally relevant physical properties of colloidal SDQDs. Elucidation of the roles of lattice and shape anisotropy, the effects of redox-active surface states, the impact of ligand or solvent vibrations, and the effects of heterostructuring on such physical properties will also pose significant challenges. Following in the footsteps of the self-assembled SDQD field, single-QD measurements in applied magnetic fields are now on the horizon and promise to yield new insights into the electronic structures and dynamical spin properties of colloidal SDQDs. Overall, the synthetic challenges and physical opportunities presented by the development of colloidal SDQDs combine to generate an exciting frontier of nanoscience research into materials that integrate electronic, photonic, magnetic, and quantum properties, with potential to drive the development of future solotronics capabilities. Further efforts to develop SDQDs and refine their unique physical properties are likely to stimulate rich advances in both fundamental and applied research.

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AUTHOR INFORMATION

Corresponding Author

*D. R. Gamelin. Email: [email protected]. ORCID

Daniel R. Gamelin: 0000-0003-2888-9916 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was funded by the US National Science Foundation (DMR-1505901 to D.R.G.). This research was also supported by the German Academic Exchange Service (DAAD) with funds from the German Federal Ministry of Education and Research (BMBF) and the People Programme (Marie Curie Actions) of the European Union’s Seventh Framework Programme (FP7/2007-2013) under REA grant agreement no. 605728 (P.R.I.M.E. − Postdoctoral Researchers International Mobility Experience to R.F.).

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Single Magnetic Impurities in Colloidal Quantum Dots and Magic-Size

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