Current-Induced Magnetic Polarons in a Colloidal Quantum-Dot

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Current-induced magnetic polarons in a colloidal quantum-dot device Franziska Muckel, Charles J. Barrows, Arthur Graf, Alexander Schmitz, Christian S. Erickson, Daniel R. Gamelin, and Gerd Bacher Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.7b01496 • Publication Date (Web): 27 Jun 2017 Downloaded from http://pubs.acs.org on June 28, 2017

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Current-induced magnetic polarons in a colloidal quantum-dot device Franziska Muckel,† Charles J. Barrows,‡ Arthur Graf, † Alexander Schmitz, † Christian S. Erickson,‡ Daniel R. Gamelin,‡ and Gerd Bacher, †,* †

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

‡

Department of Chemistry, University of Washington, Seattle, Washington, 98195-1700, United States

* Corresponding author, [email protected], phone: +49 203 379-3406, fax: +49 203 3793404

ACS Paragon Plus Environment

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Abstract Electrical spin manipulation remains a central challenge for the realization of diverse spin-based information processing technologies. Motivated by the demonstration of confinement-enhanced sp–d exchange interactions in colloidal diluted magnetic semiconductor (DMS) quantum dots (QDs), such materials are considered promising candidates for future spintronic or spin-photonic applications. Despite intense research into DMS QDs, electrical control of their magnetic and magneto-optical properties remains a daunting goal. Here, we report the first demonstration of electrically induced magnetic polaron formation in any DMS, achieved by embedding Mn2+doped CdSe/CdS core/shell QDs as the active layer in an electrical light-emitting device. Tracing the electroluminescence from cryogenic to room temperatures reveals an anomalous energy shift that reflects current-induced magnetization of the Mn2+ spin sub-lattice, i.e., excitonic magnetic polaron formation. These electrically induced magnetic polarons exhibit an energy gain comparable to their optically excited counterparts, demonstrating that magnetic polaron formation is achievable by current injection in a solid-state device.

Keywords colloidal quantum dot device, excitonic magnetic polaron, colloidal DMS, electrically induced magnetism, CdSe/CdS


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Several strategies have been explored for controlling the magnetic and magneto-optical properties of electronic materials within electronic device structures. For example, magnetic random access memories1,2 use spin-polarized currents to induce magnetization switching, and recent approaches exploit the spin-orbit torques of unpolarized currents in systems with reduced symmetry.3,4 Electric-field manipulation of magnetism5 has been developed for metallic ferromagnets,6,7 and in multiferroics the coupling between an electrical field and magnetization through electrical polarization is used.5,8 In some semiconductors doped with paramagnetic transition metal ions, magnetism can be controlled via electric-field mediated changes in chargecarrier concentrations.9,10 Despite these successes, to date these approaches have been restricted to thin-film devices prepared by vacuum growth techniques. Recent advances in the performance of solution-processed electronic and optoelectronic devices highlight the promising capabilities of nanocrystals synthesized via colloidal chemistry,11,12 but electrically induced magnetism in this class of materials has not been demonstrated. Incorporation of transition metal ions into colloidal quantum dots (QDs) introduces strong pairwise carrier–dopant magnetic exchange interactions, motivating interest in these so-called diluted magnetic semiconductor (DMS13,14) QDs as promising candidates for next-generation spin-based applications.15–19 Among the most interesting signatures of such dopant-carrier sp–d exchange in colloidal DMS QDs is the spontaneous magnetization of the spin sub-lattice by the effective magnetic exchange fields of photogenerated excitons. Because of confinementenhanced sp–d exchange, extraordinarily robust magnetic polarons have been observed,15,20,21 in some cases surviving even up to room temperature.15 Despite over 20 years of studies of this socalled excitonic magnetic polaron (EMP) phenomenon in both epitaxial22–24 and colloidal quantum dots,15,20,21 yielding a deep fundamental understanding of EMP formation and


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characteristics, EMPs have never been generated electrically in any system. Whereas optical manipulation of magnetism in colloidal DMS quantum dots15,16,25 has advantages in some contexts such as ultrafast switching, all-electrical magnetization offers compatibility with various existing electronic systems without the need to embed an additional light source, and may thus be advantageous in complementary ways, e.g., via gated spin filtering. Often motivated by applications in spin-based electronics,15,20,26,27 it has remained unclear whether EMPs can even coexist with electrical currents in active device structures, because e.g., the Stark effect due to electric fields28 or Joule heating due to current flow29 are known to influence the electronic states in quantum dot devices during electrical operation and may suppress magnetic ordering as well. Here, we use a light-emitting device structure to generate EMPs in colloidal Mn2+-doped CdSe/CdS core/shell QDs all-electrically. These results represent the first demonstration of electrically induced magnetization in any colloidal QDs, as well as the first demonstration of electrically generated EMPs in any DMS. Our approach to achieving electrically induced magnetic polarons in colloidal semiconductor nanocrystals is shown schematically in Figure 1. Magnetically doped QDs are embedded in a solid-state device, which, under applied voltage, allows for the injection of electrons and holes, forming an exciton in the nanoparticle layer. The charge carriers are exchange coupled to the ensemble of dopant spins, forcing their alignment with each other, resulting in an overall magnetization that lowers the transition energy of the exciton by the polaron energy, . The ordering force of the exciton, described by the effective magnetic exchange field , is abated by thermal energy, so that the magnetization decreases with increasing temperature. As the EMP formation happens on a timescale (~200 ps)20 much faster than the usual radiative lifetimes (10-100 ns) of excitons in QDs of this size,30 EMP formation


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can be detected and quantified via an anti-Varshni blue-shift of the emission energy with increasing temperature in the electroluminescence of this device. This anomalous temperature dependence thus provides a distinct signature of current-induced magnetization. To ensure the possibility of EMP formation, Mn2+-doped CdSe/CdS QDs (d = 13.7 ± 1.2 nm including a ~13 monolayer shell) were grown large enough that the 1S3/21Se excitonic transition lies energetically below the internal T → A Mn2+ transition (located at ~2.1 eV), allowing luminescence only from the band edge transition. Fig 2a depicts absorption and photoluminescence spectra of these QDs at room temperature. The magneto-optical response of the QDs was measured using magnetic circular dichroism (MCD) spectroscopy (Figure 2b) and exhibits a clear feature related to the 1S3/21Se band edge transition at 2.0 eV, with an enhanced effective g-factor of –7±1 at 300 K confirming successful Mn2+ doping. Assuming the exchange coupling constants for a bulk heavy hole transition appropriate for the band edge transition in QDs of this size,31 the concentration of paramagnetic Mn2+ in the exciton volume is estimated to be 1.8 ± 0.3 % (see Experimental Section for details). In a control experiment, we first demonstrate magnetic ordering after laser excitation in the device environment. Figure 2c plots the temperature dependence of the energy of the photoluminescence (PL) maximum from the Mn2+:CdSe/CdS QDs embedded within the device, along with data from analogous undoped CdSe/CdS QDs. For the Mn2+-doped QDs, an anomalous blueshift in PL energy with temperature increasing up to ~40 K is observed. In principle, a similar temperature dependence could arise from thermally induced population transfer between dark and bright excitonic states. The dark–bright splitting is less than ~2 meV for QDs of this size,32 however, and we therefore expect the bright state to be populated even at our lowest temperature. Time-resolved PL measurements at cryogenic temperatures (see


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Supporting Information for further discussion) exhibit time constants significantly shorter than the usual lifetimes of dark excitons (0.5–1 µs33), excluding dark–bright energy transfer as the origin of the anomalous temperature-dependent PL energies. Instead, this anomalous temperature dependence is attributed to exciton stabilization due to EMP formation.15,20 Figure 3a and 3b show a cartoon schematic for our device and the relative energy alignment of each layer. The device design includes hole-injection and hole-transport layers (see Experimental Section for details) to deliberately simplify the charge injection for holes. At T = 300 K, an applied voltage of ~3 V induces red electroluminescence (Figure 3c) comparable in both energy and linewidth to the photoluminescence. Although not optimized for the “turn-on” bias, brightness, or external quantum efficiency, the performance of our device is comparable to established light-emitting devices with undoped colloidal quantum dots.34 Such parameters might be improved by the inclusion of electron-injection layers but were considered beyond the scope of this investigation. Figure 4 depicts the temperature-dependent electrical behavior of the device. The device exhibits stable I–V curves over a wide range of temperatures from 7 to 250 K with a nonlinear voltage dependence that is typical for light emitting devices (Figure 4a). The turn-on voltage decreases with increasing temperatures above ~60 K which we attribute mainly to the increasing electrical conductivity of the organic layers with temperature.35 The high stability of the device across

the

whole

temperature

range

allows

collection

of

temperature-dependent

electroluminescence (EL) spectra (Figure 4b). EL intensities (see Supporting Information) measured at constant current but different temperatures do not vary by more than a factor of 2.5, indicating that the device does not degrade noticeably during the measurement with time or temperature. The EL spectra exhibit a slightly asymmetric shape, which we attribute to a slow


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component in the magnetic polaron formation, as also previously observed in the photoluminescence of colloidal DMS nanocrystals after optical excitation.15,20 As observed in the PL of these QDs (Figure 2c), the device emission exhibits an anti-Varshni temperature dependence of the emission energy up to 40 K, providing the first evidence for electrically induced EMP formation, i.e., current-induced QD magnetization. Note that an energy shift in the electroluminescence might also occur due to the quantum confined Stark effect,28 depending on the applied voltage. However, because the voltage changes less than 0.5 V at the constant measurement current of 0.5 mA over the temperature regime exhibiting the most pronounced EMP signature (7 K to 75 K), the corresponding shift of the emission energy due to the Stark effect can be estimated to be below 0.5 meV28 and is therefore negligible compared to the overall anomalous temperature shift. In a reference experiment with a device fabricated using undoped QDs (CdSe/CdS), the transition energy in electroluminescence does not show any blue shift with increasing temperature (see Supporting Information). To analyze the device performance, we fit the peak positions of the variable-temperature electroluminescence spectra measured at 0.5 mA (Figure 5a). At higher current the luminescence shifts to the red accompanied by a decrease in the EMP strength. This may be attributed either to current induced heating29 or an increasing contribution of charged exciton recombination36,37 (see Figure S3). In colloidal QDs, the emission energy tracks the absorption energy (EgAbs) but is lowered by a Stokes shift (EStokes), which is approximately temperature independent for weakly confined nanocrystals.38,39 Here, the emission energy (EEmission) is further reduced by the (temperature-dependent) polaron energy (EEMP): = − − . EgAbs(T) was determined from the 1S3/21Se absorption peak (Figure 5a) and is well described by


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Varshni’s law.40 Above 150 K, the electroluminescence and absorption exhibit a temperatureindependent Stokes shift of 32 meV, as expected for excitons in CdSe QDs of this size39 and similar to comparable Mn2+-doped CdSe nanocrystals.20 EEMP(T) was calculated from the difference between the absorption energy lowered by the Stokes shift (Figure 5a, red line) and the electroluminescence energies (black data points), and is plotted in Figure 5b. The EMP energy fits well to = ⋅

⋅!" ⋅#$ ⋅%&'( " ⋅)

, where is the maximum energy

gained in the case of completely aligning the Mn2+ ensemble with the exchange field of the current-induced charge carriers. EEMP(T) is weighted by a Brillouin function ) describing the +

interplay of the magnetic exchange field, Bexc, and the thermal energy, kBT (* = , and gMn = 2.00 are the Mn2+ spin and g-factor, and µB and kB are the Bohr magneton and Boltzmann constant, respectively). Fits to the data yield values of 14 meV for and 17 T for Bexc.

Temperature

dependent

photoluminescence

lifetime

measurements

(see

Supporting

Information for further discussion) suggest that not only neutral, but also negatively charged excitons may contribute to the luminescence. In addition, quantum dot based devices often exhibit unbalanced charge injection and transport and thus (mostly negatively) charged exciton recombination.12 In contrast to neutral excitons, where both, electron and hole contribute to EMP formation, negatively charged excitons exhibit a reduced total spin as the spins of the electrons compensate each other, and thus, only the hole triggers magnetic ordering. We therefore adapt the common assumption that the exchange field derives mostly from the hole,15,21,41 which is usually justified because the hole exchange coupling constant exceeds the electron one by a factor of 5.13 Based on the volume of our colloidal DMS QD cores, we extrapolate15 a value of ~17 T for the exchange field stemming from the hole, in good agreement with our findings.


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With this large exchange field, magnetic saturation can be achieved by electrical current injection in the low temperature limit, and the electrically induced EMP energy exceeds the thermal energy up to ~65 K. The electrically induced magnetic exchange field, although smaller than the largest exchange fields observed in colloidal15 or particular type-II self-assembled quantum dots,42 is considerably bigger than those of common type-I self-assembled quantum dots (< 3.5 T).24,41 Overall, these results demonstrate that charge-carrier-induced magnetic polaron formation can be achieved in our electrically driven devices. In conclusion, we have demonstrated electrically triggered ordering of dopant spins in a solidstate QD-based device, achieved via current-induced magnetic polaron formation in colloidal Mn2+-doped CdSe/CdS core/shell QDs. These results represent the first successful electrical generation of EMPs in any DMS, and the first demonstration of electrical magnetization in any colloidal QDs. Although Bexc solely depends on the exciton volume, i.e., the size of the quantum dot core,15 we expect that

can be tuned by varying the Mn2+ concentration in the

nanocrystals. Although this magnetization is randomly oriented among the different nanocrystals within our proof-of-concept device, this accomplishment opens the door to many interesting future experiments. For example, we have previously shown in similar QDs that even a small magnetic field (e.g., provided by a tiny permanent magnet integrated with the device) is sufficient to tip EMPs along a designated laboratory axis,20 providing a facile route to net magnetization in this device structure. Additionally, due to the long Mn2+ spin–lattice relaxation times in the absence of an exciton (~0.1 µs – ms43,44) also found in analogous colloidal Mn2+doped CdSe/CdS core/shell QDs,45 the magnetization generated here persists in the dark long after exciton recombination (< 30 ns), which could have interesting ramifications for magnetotransport applications. Finally, it is conceivable that modification of this device toward a


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memory architecture, for example by introducing a barrier on the cathode or anode side to retain charge carriers within the QD layer, could yield stable electrically gated magnetization in such QDs, a valuable functionality for many spintronics applications. Overall, our demonstration of current-induced magnetization in colloidal transition-metal-doped quantum dots highlights the unique and relatively untapped potential of this material class for applications in future spinbased technologies.

Experimental Section. Quantum dot synthesis and general characterization. Mn2+-doped CdSe quantum dots (d = 3.6 ± 0.3 nm) were prepared by diffusion doping as detailed previously.46,47 CdS shells (~13 monolayers) were grown at 300 ºC using cadmium oleate and octanethiol as precursors according to literature methods.48 Variable-temperature absorption spectra were collected using a helium cryostat (ST-300, Janis) integrated into a Shimadzu UV-2550 spectrometer. TEM samples were prepared by drop-casting 5 µL of dilute colloidal suspensions of QDs in toluene onto Cu grids (200 mesh, Ted Pella, Inc.). TEM images were obtained on an FEI TECNAI F20, 200 kV microscope at the UW Molecular Analysis Facility. QD sizes were determined from analysis of ≥100 individual QDs in TEM images using the ImageJ64 software. MCD measurements and extraction of -./00 . Room-temperature magnetic circular dichroism (MCD) spectra were collected for colloidal toluene suspensions of QDs in a 0.1 cm pathlength cuvette placed in a 1.5 T electromagnet oriented in the Faraday configuration. MCD spectra were collected using an Aviv 40DS spectropolarimeter. The differential absorption is reported as ∆A = AL–AR, where AL and AR refer to the absorption of left and right circularly polarized photons in the sign convention of Piepho and Schatz.49,50 The Zeeman splitting of the


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band-edge transition in Mn2+:CdSe QDs is given by: Δ2 = 3455 ⋅ 67 8 − 67 9 ⋅ * ⋅ :

#$ % " )

; + Δ . Here, 3455 describes the concentration of paramagnetic Mn2+ ions

weighted by 3, their spatial overlap with the exciton wavefunction. * =

+ ,

and = = 2.00

denote the Mn2+ spin and gyromagnetic factor, and μ% , B% , and are the Bohr magneton, the Boltzmann constant, and the temperature, respectively. Δ = = C% accounts for the intrinsic Zeeman splitting of the undoped host QD (~0.1 meV at 1.5 T, assuming = = +1.0 − +1.4

51

). 67 8 and 67 9 represent mean-field electron and hole exchange coupling constants,

which can be assumed to be the same as bulk values (67 8 − 67 9 = +1.5 KL) at our QD size.31 Device

fabrication

and

characterization.

Poly(3,4-ethylendioxythiophene)-

poly(styrensulfonat) (PEDOT:PSS) (Sigma Aldrich, filtered through a 0.45 µm polypropylene filter)

and

Poly[N,N’-bis(4-buthylphenyl)-N,N’-bis(phenyl)-benzidine]

(poly-TPD)

(Plasmachem) were spin coated layer-by-layer onto a tin-doped indium oxide (ITO) cover glass substrate at 2000 rpm for 60 s and 30 s, respectively. PEDOT:PSS was annealed for 20 min at 150 °C, while poly-TPD was allowed to dry under atmosphere. The active layer was treated with a crosslinking ligand (1,2-ethanedithiol, EDT) on the substrate, simultaneously removing excess ligands from the film and achieving a high particle density in order to increase robustness and stability

of

the

device,

which

is

crucial

for

conducting

temperature-dependent

electroluminescence studies. Mn2+:CdSe/CdS QDs in hexane, EDT (Sigma Aldrich, 0.1 M in acetonitrile), pure acetonitrile, and hexane (in order to remove excess EDT or ligands), were successively spin coated layer-by-layer at 2000 rpm for 60 s. During this process, the EDT solution was left on the substrate for 30 s before starting the spin coating. The four spin-coating steps were repeated three times. Finally, silver was deposited through a shadow mask in an evaporation system to serve as a cathode.


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For temperature dependent I–V and electroluminescence studies, the device was placed in a closed cycle cryostat (Oxford Instruments, see Supporting Information for details) and operated with a Keithley Sourcemeter 2601 unit. Electroluminescence and photoluminescence spectra of the QDs in the device were detected with an iHR320 monochromator and a Symphony CCD camera from Horiba Jobin Yvon. For photoluminescence measurements the 442 nm line of a Kimmon Koha He-Cd laser was used to excite the samples.


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Figure 1. Principle of electrically induced QD magnetization: In the electrical device, electrons and holes are injected into the embedded magnetically doped quantum dots under applied voltage and excitons are formed. The spins of the magnetic dopants align along the magnetic exchange field experienced due to the confined exciton and create an overall magnetic moment in the quantum dots. A photon emitted during or after the EMP formation is redshifted by the polaron formation energy , which increases with decreasing temperature, providing the opportunity to trace the EMP formation over the electroluminescence emission energy.


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Figure 2. Mn2+-doped CdSe/CdS core/shell quantum dots: (a) 300 K absorption (blue), photoluminescence (red), and (b) magnetic circular dichroism (1.5 T) spectra of Mn2+:CdSe/CdS QDs dispersed in toluene, revealing an effective room-temperature excitonic g-factor of –7±1. (c) Temperature-dependent photoluminescence (PL) energies of devices containing Mn2+-doped CdSe/CdS QDs (blue) in comparison with the PL of undoped CdSe/CdS QDs (grey). Deviation from Varshni behavior is observed in the doped QDs below ~40 K, indicating light-induced QD magnetization.


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Figure 3. Light emitting device based on colloidal Mn2+:CdSe/CdS quantum dots: (a) The lightemitting device is based on an indium tin oxide (ITO) covered glass substrate, coated with poly(3,4-ethylendioxythiophene)-poly(styrensulfonat)

(PEDOT:PSS)

and

poly[N,N’-bis(4-

buthylphenyl)-N,N’-bis(phenyl)-benzidine] (poly-TPD) as hole-injection and hole-transport layers, respectively. The active layer consisting of magnetically doped quantum dots is crosslinked with 1,2-ethanedithiol (EDT). Silver contacts evaporated on top operate as cathodes. (b) Values of energy levels in the flat-band energy level scheme taken from literature.52 (c) Room-temperature electroluminescence spectra under applied positive bias from 2.5–4.5 V.


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Figure 4. Temperature-dependent electroluminescence: (a) Voltage dependence of the current density of a device with colloidal magnetically doped quantum dots between 7 and 250 K. (b) Temperature-dependent electroluminescence spectra (normalized and offset vertically for clarity), collected at a constant current of 0.5 mA, exhibiting anomalous temperature dependence below 40 K.


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Figure 5. Determination of polaron energy and magnetic exchange field: (a) Energetic position of the electroluminescence of the device and the absorption of the excitonic transition in the QDs from 7–250 K. The absorption energies were fit to the Varshni equation = 0 − M⋅) N OP)

with 8 = 2.3 ⋅ 10R KL and 9 = 47 T. The electroluminescence energies follow the

absorption energies but are reduced by the Stokes shift and the polaron energy: = − − . The Stokes shift accounts for 32 meV, in good agreement with literature values for undoped CdSe QDs with similar exciton radii.39 (b) Polaron energy plotted vs inverse temperature, and comparison to the S = 5/2 Brillouin function for different values of the ⋅ exchange field (Bexc): =

⋅!" ⋅#$ ⋅%&'( " ⋅)

. The EMPs achieved in this device are

characterized by Bexc = 17 T and = 14 meV. For comparison, fits with the same saturation

polaron energy but different are depicted.


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ASSOCIATED CONTENT Supporting Information. The Supporting Information is included in file: SI_ Current-induced magnetic polarons in a colloidal quantum-dot device. AUTHOR INFORMATION Corresponding Author * [email protected] ACKNOWLEDGMENT We are grateful to Adam Colbert for sharing helpful details on the use of crosslinkers, Svenja Wepfer for valuable advice on device processing, and Michael De Siena for assistance with TEM. G. B. acknowledges the financial support by the Deutsche Forschungsgemeinschaft under contract BA 1422-13. D.R.G. acknowledges financial support from the U.S. National Science Foundation (DMR-1505901). Part of this work was conducted at the Molecular Analysis Facility, a National Nanotechnology Coordinated Infrastructure site at the University of Washington, which is supported in part by the National Science Foundation (grant ECC1542101), the University of Washington, the Molecular Engineering & Sciences Institute, the Clean Energy Institute, and the National Institutes of Health.

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Current-Induced Magnetic Polarons in a Colloidal Quantum-Dot

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