Aggregates of Mn2+ Ions in Mesoporous Self-Assembled Cubic ZnS

Article pubs.acs.org/JPCC

Aggregates of Mn2+ Ions in Mesoporous Self-Assembled Cubic ZnS:Mn Quantum Dots: Composition, Localization, Structure, and Magnetic Properties Sergiu V. Nistor,* Mariana Stefan, Leona C. Nistor, Victor Kuncser, Daniela Ghica, and Ioana D. Vlaicu National Institute of Materials Physics, str. Atomistilor 405A, Magurele-Ilfov 077125, Romania ABSTRACT: The source of collective magnetism in II−VI semiconductor quantum dots (QDs) doped with Mn2+ ions at high nominal impurity levels is still under debate. In the particular case of mesoporous, self-assembled cubic ZnS:Mn QDs, quantitative electron paramagnetic resonance (EPR) studies have shown that the Mn2+ ions incorporated in the core and on the surface of the QDs cannot be responsible for the observed collective magnetism because they remain in a diluted paramagnetic state up to the 50 000 ppm nominal concentration. Here we investigate the composition, localization, structure, and magnetic properties of the aggregates of Mn2+ ions incorporated in the mesoporous cZnS:Mn as a possible source of the observed collective magnetism. Samples of mesoporous cubic ZnS:Mn prepared by coprecipitation at several nominal impurity levels from 200 to 50 000 ppm are investigated by EPR, magnetometry, and analytical high resolution (scanning) transmission electron microscopy. The low temperature magnetic properties of the Mn2+ aggregates change from paramagnetic-like, for samples with nominal impurity levels up to 2000 ppm, to ones specific to larger clusters with distributed antiferromagnetic coupling at higher concentrations, behaving superparamagnetically above a certain temperature. There is also strong evidence that the Mn2+ aggregates responsible for the observed low temperature collective magnetism are incorporated as an amorphous phase of mainly Mn−Zn−O composition, localized in the interstices and pores of the mesoporous structure of the cubic ZnS:Mn QDs.

1. INTRODUCTION The magnetic properties of cubic II−VI semiconductors nanocrystals (NCs) are strongly dependent on the presence and localization of the activating transition metal ions (TMIs) in the host nanomaterial.1−3 The collective magnetism reported in such diluted magnetic semiconductors (DMS) was attributed to the incorporated TMIs,1−6 as well as to intrinsic defects7,8 and surface/interface states.3,9 In the particular case of cubic Zn1−xMnxS NCs of a few nm diameter, also called quantum dots (QDs), prepared by coprecipitation, which usually selfassemble into a mesoporous structure,10 magnetic ordering has been reported4−6,11 in samples with x ≥ 0.015 and attributed to the incorporated Mn2+ ions. There are also contradicting reports of a paramagnetic behavior in such nanocrystals at even higher concentrations of Mn2+ ions.12,13 The analysis of the electron paramagnetic resonance (EPR) spectra from Mn2+ ions incorporated in cubic ZnS QDs has revealed the presence of isolated Mn2+ ions localized at substitutional Zn2+ sites in the core and on the surface of the NCs, as well as of an aggregated phase resulting in a broad, featureless Lorentzian component line, associated with exchange coupled Mn2+ ions.14−19 In recent quantitative EPR investigations of 2.9 nm diameter cZnS:Mn QDs prepared by coprecipitation we have shown that, up to the highest nominal concentration of 50 000 ppm, the Mn2+ ions incorporated at isolated sites in the core and on the surface in a diluted © 2016 American Chemical Society

paramagnetic state characterized by magnetic dipole−dipole interactions cannot be responsible for the collective magnetism properties.20,21 Although one would expect that the aggregated Mn2+ ions could determine the observed collective magnetism at least below a certain temperature, to our knowledge this possibility has not been investigated so-far. In fact, practically nothing is known about the composition, structure, and physical properties of the phase responsible for the Lorentzian line, also seen in other II−VI semiconductor NCs at higher doping levels.1,22 It is therefore essential to determine in what manner the Mn2+ aggregated phase contributes to the reported collective magnetism properties. In this work we investigated the composition, localization, structure, and magnetic properties of the aggregates of Mn2+ impurity ions responsible for the broad single Lorentzian component line in the EPR spectra of the Mn2+ impurity ions in the cubic ZnS:Mn QDs prepared by coprecipitation with nominal impurity concentrations in the 200 to 50 000 ppm range. By correlating the EPR and magnetometry data, we show that the dominant magnetic interactions between the aggregated Mn2+ ions depend on the doping level, shifting from paramagnetic pairs of Mn2+ ions for samples prepared Received: May 13, 2016 Revised: June 13, 2016 Published: June 14, 2016 14454

DOI: 10.1021/acs.jpcc.6b04866 J. Phys. Chem. C 2016, 120, 14454−14466

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The Journal of Physical Chemistry C

Figure 1. Typical X-band EPR spectra of the investigated cZnS:Mn QDs recorded at RT. Experimental (black) and simulated (red) spectra as the sum of the contributions from core localized (green), surface localized (magenta), and aggregated (blue) Mn2+ ions are shown for the nominal concentrations of (a) 2000 ppm and (b) 20 000 ppm.

was further decanted, washed several times, and dried at 100 °C. Details concerning the preparation, structure and EPR spectra properties of the resulting nanostructured cZnS:Mn are given in refs 10, 20, and 21. The EPR investigations in the X (9.8 GHz)- and Q (34.1 GHz)-band were performed with the ELEXSYS-E580 and ELEXSYS-E500Q spectrometers from Bruker, equipped with temperature accessories for measurements from room temperature (RT) down to 10 K. Details about the EPR equipment and procedures can be found in ref 20. The magnetic measurements consisting of thermo-magnetic curves and magnetic hysteresis loops at different temperatures between 2 and 300 K have been performed with a SQUID magnetometer (7T MPMS, Quantum Design) working in the sensitive Reciprocal Space Option mode. The analytical HRTEM/STEM investigations were performed with a JEOL JEM-ARM/200F field emission atomic resolution analytical electron microscope operating at 200 kV. The microscope is equipped with a Cs probe corrector from CEOS, enabling a resolution in STEM mode of 0.08 nm. Electron energy loss spectroscopy (EELS) and spectrum imaging (EELS-SI) was performed with the Gatan GIF Quantum SE Imaging Filter/EELS spectrometer. The spatial resolution of EELS is limited by the diameter of the measuring probe, which for the instrument settings used in the present study was 0.12 nm. Compositional maps were generated via the GMS 2.0 software package using the power law background fitting at the edges of interest. The quantification of EELS signals was performed using the Hartree-Slater method for the calculation of the ionization cross sections. The specimens for HRTEM/STEM were prepared by crushing the as-grown nanopowder in ethanol, dispersing it by sonication and dropping on lacey carbon grids. It is worth mentioning that all images were obtained for regions of the specimen situated over the holes of the carbon grids. Thus, one could be sure that any observed amorphous zone belongs to the specimen and not to the carbon grid bars.

with up to 2000 ppm impurity nominal concentrations, to distributed antiferromagnetic couplings inside larger Mn2+ clusters in samples with higher nominal concentrations. The presence of Mn2+ aggregates is further confirmed by analytical high resolution transmission electron microscopy in TEM (transmission electron microscopy) and STEM (scanning transmission electron microscopy) modes (HRTEM/STEM), which evidence the formation of a manganese rich, disordered phase, localized in the interstices and pores in the mesoporous structure of the self-assembled cZnS:Mn QDs. The present investigation demonstrates that the Mn2+ ions aggregated in magnetic clusters, localized in the intergrain spaces and pores of the mesoporous self-assembled cZnS:Mn QDs, can determine in certain conditions of concentration and aggregation the observed local collective magnetic properties. To our knowledge this is the first time that the microstructural and magnetic properties of TMIs aggregated in a separate phase in a nanostructured II−VI semiconductor were investigated based on the correlation of quantitative data from EPR, magnetometry, and analytical HRTEM/STEM measurements.

2. EXPERIMENTAL SECTION The investigated samples of cZnS:Mn QDs were taken from the same batches doped with Mn2+ impurity ions in a broader nominal concentration range, previously employed in investigations concerning the doping mechanism20 as well as the distribution and interaction of the impurity ions localized in the core and on the surface of the QDs.21 The synthesis was performed in a glass reactor at room temperature (RT) by coprecipitation in bidistillated water mixed with methanol in a 10:1 ratio, at pH 5.5 (without any pH adjustments), under a 99.998% pure argon atmosphere. Solutions of zinc acetate to which manganese acetate was added in Mn2+ /Zn2+ nominal concentrations of 200, 500, 1000, 2000, 5000, 10 000, 20 000, and 50 000 ppm (104 ppm = 1 at. %) and ammonium sulfide were mixed in the presence of the Tween 20 (polyoxyethylene sorbitan monolaureate) surfactant. The resulting precipitate 14455


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3. RESULTS 3.1. Samples Structure and EPR Spectra. According to XRD measurements,20,21 all investigated samples exhibit practically identical diffraction patterns attributed to the cubic (blende) ZnS structure, consisting of cZnS QDs of d = 2.9 ± 0.2 nm average core diameter with lattice parameter a = 0.5393 ± 0.0002 nm. Neither secondary crystalline phases nor lattice parameter variation could be observed up to the highest nominal dopant concentration. These conclusions were confirmed by the HRTEM investigations, which found in all samples the same morphology, consisting of cubic ZnS NCs of a few nm size, self-assembled into a mesoporous structure with pores of similar size.20 These results underline the high structural reproducibility of our synthesis procedure,10 which was not affected by the doping with Mn2+ impurities up to the highest nominal concentration. The formation of a mesoporous structure with pores and walls build from cZnS NCs of 2.5 nm average size, as revealed by TEM images of samples prepared by the presently employed synthesis procedure, was also proved by low angle X-ray diffraction, surface area determinations by the Brunauer− Emmett−Teller (BET) method and pore size distribution measurements from the desorption branch with the Barrett− Joyner−Halenda (BJH) method.10 The investigations did indeed reveal the presence of a porous material with a narrow pore size distribution in the 1.9 to 2.5 nm range, exhibiting a relatively lower surface area of around 100 m2/g, attributed to some remaining molecules filling the smallest pores. The EPR spectrum of all samples, illustrated in Figure 1a,b for the cZnS:Mn QDs prepared with 2000 and 20 000 ppm nominal concentration of Mn2+ ions, respectively, represents a superposition of two sets of six hyperfine component lines each attributed to Mn2+ ions localized in the core and on the surface of the cZnS:Mn QDs and a broad, featureless line attributed to aggregated Mn2+ ions.20,21 The set of six narrowest lines, with the smallest hyperfine splitting, belongs to the so-called Mn2+(I) centers, consisting of Mn2+ ions incorporated in the QDs core, at Zn2+ cation sites localized next to an extended planar defect,19 a structural model confirmed and employed in further experimental and theoretical investigations.23−25 The other set of well resolved six broader lines, with a larger hyperfine splitting, belongs to isolated Mn2+ ions localized on the surface layer of the QDs.17 The broader lines reflect the increased disorder of the cZnS QD outer layer hosting the Mn2+ ion, containing in its structure either adsorbed molecule species (H2O, O2, ...), or capping molecules added to stabilize the QDs and improve their properties for various applications.26 There are similarities and differences between the properties of the core and surface incorporated Mn2+ ions, related with their incorporation and localization in the cZnS QDs. Thus, the well resolved EPR spectra of both core and surface localized Mn2+ ions observed in the whole doping range correspond to weakly interacting ions in a diluted paramagnetic state. The diluted state of both types of isolated centers was confirmed by the large average separation of the Mn2+ ions, as determined from the concentration data and linear dependence of the EPR spectra line width of the core localized Mn2+ ions vs total concentration of the isolated (core + surface localized) Mn2+ centers.21 The resulting data describe both core and surface localized Mn2+ ions which remain, up to the highest 50 000 ppm nominal concentration level, in a diluted

paramagnetic state characterized by dipolar magnetic interactions.21 Therefore, the core and surface incorporated Mn2+ ions cannot be a source of collective magnetism. According to the EPR measurements of the Mn2+ dopant concentration vs nominal impurity concentration in the synthesis process,20 there are differences in the distribution of the doped Mn2+ impurity ions in the core and on the surface of the cZnS:Mn QDs as well. Thus, while for nominal impurity concentrations of up to 200 ppm the actual concentrations of the core and surface incorporated Mn2+ ions are comparable, at higher nominal impurity levels the concentration of the surface incorporated Mn2+ ions increases much faster compared to the smaller increase in the concentration of the core incorporated impurity ions (see Figure 3 from ref 19). Therefore, at the highest 50 000 ppm (5 at. %) investigated nominal concentration level the concentration of the surface incorporated Mn2+ ions is about 5 times larger than the concentration of the core incorporated Mn2+ ions. The observed behavior is attributed to the different mechanisms of impurity incorporation in the two cases.20 Thus, the incorporation of the Mn2+ ions in the core of cZnS QDs, explained by the extended lattice defect assisted (ELDA) mechanism19,20 depends on the limited number of available trapping sites at the dislocation steps on the surface of the growing QD produced by emerging stacking planar defects, reaching saturation at higher nominal impurity concentrations. Meanwhile, the incorporation of the dopant ions in the surface layer is adsorption controlled, resulting in a dopant concentration in the surface layer proportional to the nominal concentration of impurity ions in the synthesis. The single, broad, featureless Lorentzian component line, better seen in the X-band spectra (Figure 1), with g = 2.005 ± 0.002 in the whole 10 K < T < 300 K measuring temperature range, superimposed on the spectra of the core and surface localized Mn2+ ions, was attributed to aggregated Mn2+ ions interacting through exchange interactions, very likely forming a separate phase.15,17 Our present investigation is focused on determining its localization, composition, structure, and magnetic properties. The EPR quantitation (spin counting) leads to concentrations of unpaired electron spins from specific magnetic centers. In the case of the isolated Mn2+ ions, the resulting spin concentration values equal the actual concentrations of the Mn2+ ions localized in the core and on the surface of the QDs.20,21 In the case of the aggregated Mn2+ ions, where the exchange coupling between the ions has to be considered, we can have two situations, depending on the strength of the coupling. Thus, for weakly coupled ions specific to the smallest aggregates, the spin concentration determined by EPR is equivalent with the concentration of Mn2+ ions, while for strongly interacting ions specific to larger aggregates EPR determines the concentration of coupled spins (or Mn2+ clusters) with total spin S = 5/2. According to the EPR quantitation20,21 the spin concentration of the aggregated Mn2+ ions in the whole nominal concentration range under investigation is about half of the total spin concentration of Mn2+ centers in the samples (Figure 2) the rest consisting of core and surface localized Mn2+ ions. Moreover, according to the data presented in Figure 2, for nominal concentrations of up to 5000 ppm the total incorporation rate of the Mn2+ centers exhibits an almost linear increase, at about 20%, while at higher nominal concentrations it continuously decreases, to less than 5% for the highest 50 000 ppm nominal concentration level. As will be 14456


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ΔBpp is practically constant for samples with nominal concentrations as high as 2000 ppm, it drops to less than half at higher concentrations. Meanwhile the ΔBpp values for the isolated, core, and surface localized Mn2+ ions exhibit a continuous increase with the nominal concentration increase, as expected from paramagnetic ions subjected to magnetic dipole−dipole interactions.21 The sharp decrease of ΔBpp of the aggregated phase, at nominal concentrations larger than 2000 ppm, strongly suggests a change in the properties of the magnetic interactions between the incorporated Mn2+ ions. Because the EPR intensity I obtained by double integration of the experimental EPR spectrum (representing the area under the absorption curve) is directly proportional to the magnetic susceptibility χ, its temperature dependence offers information about the nature of the magnetic interactions between the Mn2+ ions. To this purpose we recorded the EPR spectra of the samples prepared with Mn2+ ions nominal concentrations of 2000 and 20 000 ppm, from RT down to 10 K, a temperature range in which it was possible to avoid microwave saturation effects. Further on, by deconvolution of the resulting EPR spectra and double integration of the broad Lorentzian line, we obtained the I‑1(T) variation for the aggregated Mn2+ ions. In the case of the cZnS:Mn(2000) QDs sample, the results which are presented in Figure 4 describe a Curie law temperature dependence I‑1 ∼ χ−1 ∼ T, pointing to a dominant paramagnetic system.

Figure 2. Total (blue) spin concentration of the Mn2+ centers and the separate spin concentration of aggregated Mn2+ ions (red) in the mesoporous nanostructure of self-assembled cZnS:Mn QDs vs nominal concentration. (Based on data reported in ref 21.)

further shown, the actual Mn2+ ions concentration values will be reconsidered based on the presence of Mn clusters, as determined from the analysis of the EPR and magnetic data. 3.2. Magnetic Properties of the Aggregates of Mn2+ Impurity Ions Investigated by EPR. Additional information about the magnetic interactions between the aggregated Mn2+ ions is obtained from the derivative peak-to-peak line width ΔBpp variation of the broad Lorentzian line vs nominal concentration of doping ions presented in Figure 3. While

Figure 4. Variation of the inverse of the EPR integrated intensity vs temperature for the broad Lorentzian component line attributed to aggregated Mn2+ ions in cZnS:Mn(2000) QDs. The dotted line is the Curie law fit with the parameters given in the inset.

Meanwhile, the high temperature part of the I−1(T) variation for the cZnS:Mn(20000) QDs sample, presented in Figure 5, could be well fitted with a Curie−Weiss type dependence:27 I(T ) = C*(c)[T − θ(c)]−1

(1)

with C* being the Curie constant and θ being a characteristic temperature above which the magnetic coupling is destroyed. In this case, the resulting characteristic temperature θ = (−75 ± 10) K points to an antiferromagnetic coupling (θ < 0) typical for Mn2+ ions in II−VI semiconductors.28 The large margin of errors reflects the rather low accuracy of the Lorentzian line integrated intensity determinations of ±20%.20 The change in the nature of the magnetic interaction between the aggregated Mn 2+ ions with the nominal concentration increase is also reflected in the different

Figure 3. RT derivative peak-to-peak EPR line width vs nominal concentration for the component spectra of the Mn2+ ions incorporated in the core and on the surface of the cZnS:Mn QDs, as well as in the aggregated phase. The corresponding spin concentrations (in ppm) are given for each ΔBpp vs nominal concentration curve. 14457


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phase contains also weakly interacting paramagnetic Mn2+ centers. One should mention that the narrowing of the magnetic field range occupied by the EPR spectrum from isolated Mn2+ ions due to the formation of exchange coupled pairs of TMIs ions, a well-known phenomenon in EPR spectroscopy,29 is the main support for attributing the broad Lorentzian component to aggregated Mn2+ ions. The transformation of the six-line EPR spectrum of isolated Mn2+ ions into a structureless Lorentzian line with a peak-to-peak line width smaller than the extent of the isolated Mn2+ spectrum, by increasing the concentration of the doping Mn2+ ions, has been reported for host polycrystalline cubic and hexagonal ZnS31,32 and ZnO.33 A further narrowing of the Lorentzian line was observed for higher doping levels. In a theoretical investigation Ishikawa34 demonstrated that these changes in the EPR spectra reflect the initial aggregation of exchange coupled Mn2+ ions in clusters of two ions, resulting in the Lorentzian, structureless EPR line, followed at high enough concentrations by the formation of larger clusters, where the line narrowing process reaches saturation. It has been also shown that such exchange narrowing, as predicted by moments calculations,30−32 takes place for the exchange coupling constant |J | < |A| and/or |J | < |D|, where A and D are the hyperfine and anisotropic spin−spin interaction parameters, respectively. This is a weak exchange coupling case in which the spin system is characterized by the eigenstates of individual Si2 and Siz operators of each Mn2+ ions. The presence in the cZnS:Mn QDs prepared with low impurity nominal concentrations (up to 2000 ppm) of a similar aggregation process, with formation of weakly exchange coupled pairs of Mn2+ ions localized on the surface of the QDs, is reflected in the transformation of the six hyperfine components spectrum of the isolated, surface localized Mn2+ ions, extending over 45 mT (corresponding to the total hyperfine splitting ∼5A, where |A| = 9.04 mT),20 into the narrower structureless Lorentzian line with ΔBpp = 38.5 mT. One should notice that the formation of the pairs of Mn2+ ions weakly coupled by exchange interaction, evidenced by the presence of the broad Lorentzian component line, already takes place in the sample prepared with the lowest investigated nominal impurity concentration of 200 ppm. Its constant line width, accompanied by the line intensity increase for samples with nominal Mn2+ concentrations up to 2000 ppm, means that in this concentration range the dominant process consists in the formation of an increasing number of Mn2+ coupled pairs. The formation of larger clusters of Mn2+ ions occurs in samples with impurity nominal concentrations higher than 2000 ppm, a process reflected in a progressive narrowing of the Lorentzian line down to ΔBpp = 17.5 mT for the highest (50 000 ppm) nominal concentration (Figure 3). According to Ishikawa’s simulations24 the slowing down of the Lorentzian line width decrease, which occurs in samples prepared with the highest impurity nominal concentrations (Figure 3), suggests that the Mn2+ clusters consist of at least seven ions on average. One expects the Mn2+ clusters to be localized in the spaces between the cZnS:Mn QDs, a conclusion which is further confirmed by the analytical HRTEM determinations. One should mention that preliminary EPR measurements on mesoporous cZnS:Mn samples prepared with low and high manganese nominal concentrations, which have been dried in vacuum at RT or in air at 60 °C did not show significant differences in the EPR properties of the broad Lorentzian line,

Figure 5. Variation of the inverse of the EPR integrated intensity vs temperature for the broad Lorentzian component line attributed to aggregated Mn2+ ions in cZnS:Mn(20 000) QDs. The continuous line is the Curie−Weiss law fit with the parameters given in the inset.

temperature dependence of the EPR peak-to-peak line width ΔBpp at low and high nominal concentrations (Figure 6). Thus, while for the lower (2000 ppm) nominal concentration ΔBpp decreases linearly with the temperature, a behavior characteristic for paramagnetic systems reflecting the increase in the spin−lattice relaxation time,29 in the sample with high nominal concentration (20000 ppm) the line width increases with the temperature decrease. This last behavior is characteristic for

Figure 6. Variation of the peak-to-peak line width vs temperature for the low (2000 ppm) and high (20 000 ppm) nominal concentrations. The estimated maximum experimental error is ±0.5 mT. Continuous curves fit the experimental data with a linear dependence for the paramagnetic Mn2+ centers at the lower concentration and a nonlinear dependence resulting from formula 2 for the higher concentration.

magnetic clusters consisting of antiferromagnetic coupled ions27 and can be described in the high temperature range, where the Curie−Weiss law is satisfied, by a relationship of the form: ΔBpp(T ) ∼ ΔB∞(1 + |θ| /T )

(2)

where ΔB∞ is the asymptotic line width value at infinite temperature and |θ| the absolute value of the Curie−Weiss temperature as defined in formula 1.28,30 Figure 6 presents the experimental ΔBpp(T) dependence fitted in the high temperature (T > 100 K) domain by formula 2 with the θ = −75 K value. The fit by formula 2 is not perfect as the aggregated 14458


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Figure 7. Hysteresis loops collected at different temperatures from the cZnS:Mn samples prepared with (a) 2000 ppm and (b) 20 000 ppm Mn2+ nominal concentrations. The upper insets show the corrected hysteresis loops after the extraction of the paramagnetic component at low temperature (2 K) and the diamagnetic component at high temperature (300 K); the lower insets show the thermo-magnetic curves obtained under a field of 2500 Oe.

compared with the samples dried at 100 °C. The advantage of the last drying procedure was a much shorter preparation time. 3.3. Magnetic Investigations. Hysteresis loops collected at different temperatures (raw data) are presented in Figure 7, for the samples prepared with lower (2000 ppm) (a) and higher (20 000 ppm) (b) Mn2+ nominal concentrations. The loops appear at low temperature as not saturated, whereas at higher temperatures the diamagnetic character is evidenced in fields higher than 2000 Oe. A coercive field is clearly observed for the highly doped sample (of the order of 100 Oe at 2 K). A much lower one (a few tenths of Oe) is observed in the sample with 2000 ppm Mn nominal concentration level. The presence of the coercive field up to 300 K gives evidence of a magnetic ordered state, as typically observed in diluted magnetic semiconductors.35,36 The

lower insets of the loop representations at 2 K present the thermo-magnetic curves collected between 2 and 300 K while cooling the samples in a field of 2500 Oe. In both cases a rapid increase of the magnetization at low temperature specific to a paramagnetic behavior is observed, as well as the presence of a finite magnetization of the order of 0.01 emu/g, up to higher temperatures (e.g., 300 K), clearly demonstrating the superposition of different magnetic phases in the system. In order to get more information on the magnetic phases of interest, the hysteresis loops can be deeper exploited by subtracting the diamagnetic component of linear negative slope, better evidenced in the loops at 300 K. Since the diamagnetic susceptibility vs temperature is constant, the same diamagnetic contribution can be subtracted for all considered temperatures. 14459


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The Journal of Physical Chemistry C Thus, a high diamagnetic contribution χdia = (−1.1 ± 0.1) × 10−6 emu/g/Oe was obtained for the sample with higher nominal concentration and a smaller one, of only (−0.2 ± 0.1) × 10−6 emu/g/Oe, for the sample with lower nominal concentrations. Based on HRTEM investigations, further presented, the observed difference is explained by the increased degree of occupancy of the pores and intergrain spaces with amorphous material in the highest doped sample. Once the diamagnetic contribution is subtracted, a pure magnetic contribution attributed to the magnetic ordered state can be obtained at high temperature (see inset at 300 K in Figure 7b and the main graph in Figure 7a at 300 K, where the diamagnetic contribution is very low). On the other hand, the pure paramagnetic contribution can be obtained by subtracting the diamagnetic contribution from the low temperature hysteresis loops and subsequently estimating the slope in the range of linear increase of the magnetization at higher fields. A paramagnetic contribution χpara = (23 ± 1) × 10−6 emu/g/Oe was obtained for the cZnS:Mn(20000) QDs and a lower value of only (7 ± 1) × 10−6 emu/g/Oe in the case of the cZnS:Mn(2000) QDs. By further subtracting the paramagnetic contribution, specific hysteresis loops attributed to the magnetic ordered state could be plotted, as seen in the upper insets of the loops collected at 2 K. Such final loops belonging to the magnetic ordered states provide coercive fields rapidly decreasing from (110 ± 10) Oe at 2 K to (40 ± 5) Oe at 300 K in the case of the sample with 20000 ppm nominal concentration and slightly decreasing from (40 ± 5) Oe at 2 K to (30 ± 5) Oe at 300 K in the case of the sample with 2000 ppm nominal concentration. According to the saturation magnetization values at different temperatures (Figure 8), a magnetic ordered state is present up to a relatively high temperature (300 K) in both samples.

the sample with the largest doping level suggests that besides the magnetic state involving exchange interactions extending over large volumes of the sample and, therefore, over many ZnS nanocrystals, one also deals with magnetic clusters in the frozen magnetic state, behaving as an assembly of Stoner−Wohlfarth magnetic monodomain nanoparticles.37 If such magnetic clusters, most probably consisting of Mn2+ ions, are large enough, they become superparamagnetic at higher temperatures, above the blocking temperature TB < 50 K, while at T < TB they give rise to the observed increase in the saturation magnetization in the highly doped sample. Moreover, a very large dispersion in the size of such clusters is expected and, as a consequence, many of the finest clusters might have blocking temperatures below 2 K, which could therefore also contribute to the paramagnetic susceptibility. According to the EPR data, the average size of the Mn2+ clusters is expected to depend on dopant concentration for nominal impurity concentrations larger than 2000 ppm. Thus, as previously mentioned, for samples of up to 2000 ppm nominal Mn2+ concentration, the broad Lorentzian line with a constant line width value ΔBpp = 38.5 mT (Figure 3) suggests the presence of pairs of Mn2+ ions, weakly coupled by exchange interactions.34 Their presence is the reason why the EPR integrated intensity of the broad line from the aggregated Mn2+ ions respects a simple Curie law in the cZnS:Mn(2000) QDs sample. Meanwhile, the presence of larger clusters of antiferromagnetically coupled Mn2+ ions with TB < 50 K and Curie−Weiss temperature of less than 100 K explains the observed Curie−Weiss-type behavior in the cZnS:Mn(20000) QDs at higher temperatures. 3.4. Analytical HRTEM/STEM Investigations of cZnS:Mn Samples with High Concentrations of Incorporated Mn 2+ Ions. The microstructure and chemical composition down to atomic level of the cZnS samples doped with Mn2+ ions at high nominal concentrations have been investigated by HRTEM/STEM and electron energy loss spectroscopy (EELS), including quantitative elemental mapping by spectrum imaging (EELS-SI). Unlike the EPR measurements, which can determine separately the spin concentrations of Mn2+ ions incorporated in the core and on the surface of the cZnS nanocrystals, as well as of those forming aggregated phases, quantitative EELS provides only the total local concentration of Mn impurity atoms in the samples. According to the EPR results presented in Figure 2, the total Mn doping level present in these samples is rather small and does not significantly vary at high nominal concentrations. One should mention that the detection sensitivity of EELS is typically in the range of 0.1−1 atom % for thin specimens, where multiple scattering can be avoided, but depends on the particular element to be detected, on the type of other elements forming the specimen, as well as on the experimental conditions.38 Therefore, for maximum sensitivity reasons the EELS analyses were performed on the highest doped cZnS:Mn samples. Figure 9 shows HRTEM images of the cZnS samples prepared with 20 000 ppm (a) and 50 000 ppm (b) nominal concentrations of Mn impurities. As expected, considering the high structural reproducibility of the synthesis procedure, they reveal similar mesoporous morphology, with cZnS nanocrystallites in various orientations, separated by pores filled with an amorphous material, some of which are indicated by black arrows. The average dimensions of the nanocrystallites and pores are also similar in both samples. One expects the

Figure 8. Magnetization at saturation vs temperature in the two analyzed samples, after subtraction of the diamagnetic and pure paramagnetic contributions.

One finds that the magnetization at saturation increases very slightly with decreasing temperature, with almost similar values of ∼ (0.08 ± 0.01) emu/g over a large temperature interval (from 50 to 300 K), as expected in the case of a magnetic phase involving magnetic interactions extended over large volumes, which is specific for magnetic diluted systems. In these conditions, the strong increase of the saturation magnetization at T < 50 K (as well as of the coercive field) in 14460


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Figure 9. HRTEM images of mesoporous agglomerates of cZnS nanocrystals prepared with 20 000 ppm (a) and 50 000 ppm (b) nominal concentrations of Mn2+ ions. Some of the pores filled with an amorphous material are indicated with black arrows; the white arrow in (a) indicates an empty pore.

Figure 10. HAADF-STEM image of the cZnS:Mn(50000) QDs sample. The green rectangle marks the region selected for the acquisition of the EELS spectrum image; 1−3 mark pores and 4−7 nanocrystals whose composition will be further analyzed.

Mn2+ aggregates to be localized in the amorphous phase retained in the pores of the mesoporous cZnS compound. Therefore, efforts have been devoted to determine the chemical composition of the amorphous phase. Preliminary EELS composition analyses were performed on the two highly doped cZnS:Mn samples, with the electron microscope operating in the STEM mode at increasing magnifications. In these measurements the analyzed areas varied from 100 × 100 nm2, an area which contained mesoporous agglomerates of nanocrystals, to 15 × 15 nm2 which contained just a few cZnS NCs. Moreover, we analyzed only the very thinnest parts at the edges of the cZnS agglomerates to avoid multiple inelastic scattering effects.39 Due to the mesoporous nature of the specimen, one could not always avoid the superposition of the nanocrystals, resulting in thickness variations within the analyzed regions which could increase the normally 10% quantification errors. For both samples we obtained two noticeable results: first, an extremely variable content of Mn impurities from an analyzed agglomerate of nanocrystallites to another, but also between different nanocrystals inside the same agglomerate and, second, all analyzed areas exhibited an unexpectedly high content of oxygen (∼30%) and a significant content of carbon. In the next step we investigated the distribution of the Mn, O and C atoms with spatially resolved EELS by spectrum imaging (EELS-SI).40 This is a technique in which a focused electron beam scans the sample in a raster and the EELS spectrum is recorded in each position (x, y) of the scan together with the annular dark field (ADF) signal as an image reference. Thus, one obtains a complete three-dimensional data set (x, y, and energy loss E), the so-called data cube. Figure 10 shows the high angle annular dark field (HAADF) STEM image of the edge of an agglomerate from the highest doped cZnS:Mn(50000) QDs sample, presenting the mesoporous morphology of the specimen. One should mention that in the HAADF-STEM images the nanocrystals are revealed in bright contrast while the pores are dark. An area of 36 × 36 nm2 (51 × 51 pixels) has been chosen at the thin edge of the agglomerate, where the nanocrystals and pores are well-defined. This area, marked in green in Figure 10 and labeled Spectrum Image, was used for the acquisition of the EELS-SI data cube. Therefore, EELS spectra for an energy range from 100 to 1100 eV, which includes the ionization edges of S L23, C K, O K, Mn L23 and Zn L23, were acquired in each pixel of the Spectrum Image.

The following experimental conditions were used for the data cube acquisition: electron probe size 0.12 nm, convergence angle α = 17 mrad and collection angle β = 49 mrad. After standard elemental quantification procedures, elemental maps were generated by plotting the intensity of each specific ionization edge as a function of the position on the Spectrum Image. Figure 11 shows a set of images obtained by superposing over the DF reference image generated by spectrum imaging (a) the maps of the elements of interest (b−h). In this manner, the distributions of the different atoms in the analyzed area are better highlighted. We emphasize our attention on pores, where, according to Figure 9, an amorphous phase of unknown composition was observed. Figure 11b,c represents the maps of sulfur and zinc, respectively, superposed on the DF reference image. They suggest that all nanocrystallites at the top of the agglomerate in the analyzed area are sulfur deficient, but still contain zinc. Also, the presence of S and Zn atoms can be observed on the pore edges. Figure 11d reveals the Mn atoms distribution in the analyzed area. Comparing Figure 11a,d one finds that the Mn atoms are distributed not only in the pores of the cZnS mesoporous structure, but also in the nanocrystallites from the top of the agglomerate. On the other hand, oxygen atoms, as revealed in Figure 11e, are preferentially distributed at the nanocrystal edges and in the pores. Figure 11f,h presents the composite maps of Mn and O as well as of Zn, Mn and O, respectively, superposed on the DF reference image. The mixing of the colors in these two figures suggests that Mn together with O and with Zn could form oxide type compounds in the pores and at the edge of the agglomerate where sulfur is deficient. The composite maps also show that there are oxygen atoms at the agglomerate edge and surrounding the ZnS nanocrystallites, which are not linked to Zn or Mn. They probably belong to OH radicals or H2O molecules. Note that the 13 eV energy value, corresponding to the hydrogen K edge, is beyond the energy range (100−1100 eV) used in this study. Finally, one finds that the carbon (Figure 11h) is distributed mostly in the pores. To reveal the variations in the chemical composition between pores and nanocrystals, EELS spectra were extracted from the data cube for equal areas of 4 × 4 pixels containing pores (1−3) and nanocrystals (4−7), marked with red squares in the DF 14461


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Figure 11. Elemental maps (b−h) superimposed on the DF reference image (a) generated by EELS-SI. The color code is the following: S-yellow, Zn-green, Mn-red, O-blue, C-brown. In (a) the red squares 1−3 mark pores, while red squares 4−7 mark nanocrystals.

Table 1. Composition Information for Pores (1-3) and Nanocrystals (NCs) (4-7) Marked by Red Squares in Figure 11aa

a

element

pore 1% content

pore 2% content

pore 3% content

NC 4% content

NC 5% content

NC 6% content

NC 7% content

S C O Mn Zn

14.2 0.0 56.1 6.6 26.1

11.3 11.3 54.1 2.3 21.0

5.0 24.6 55.4 4.6 10.5

14.5 14.4 49.3 0.0 31.3

19.5 9.6 44.8 0.6 21.2

19.3 14.5 39.9 1.1 25.6

0.0 0.0 85.7 3.2 21.6

Note that pores 1 and 2 and NCs 4-6 are inside the agglomerate, while pore 3 and NC 7 are from the edge of the agglomerate.

4. DISCUSSION According to our previous EPR investigations,20,21 the Mn2+ ions are incorporated in all investigated mesoporous cZnS:Mn samples at isolated sites in the core and on the surface of the constituent QDs resulting in distinct six hyperfine component lines and as a separate aggregated phase of exchange coupled Mn2+ ions, reflected in the presence in the EPR spectrum of a broad, featureless Lorentzian line. It was also demonstrated21 that, due to their low incorporation rate, the isolated, core, and surface localized Mn2+ ions cannot be a source of collective magnetism. A different situation is presently reported for the Mn2+ ions incorporated as an aggregated phase, in spin concentrations comparable to those of the isolated ions. Thus, according to the EPR investigations, the aggregated Mn2+ ions in the samples prepared with nominal impurity concentrations up to 2000 ppm exhibit paramagnetic properties reflected in a Curie-like temperature variation of the magnetic susceptibility, which is proportional with the integrated intensity of the EPR line (Figure 4). On the other hand, in the sample prepared with 20 000 ppm nominal impurity concentration one finds properties associated with Mn2+ ions coupled by antiferromagnetic exchange interactions, reflected in a characteristic Curie− Weiss type temperature variation (Figure 5) of the magnetic susceptibility, with θ = −75 K. While the constant line width of the broad featureless Lorentzian component line from samples prepared with Mn2+ impurity ions nominal concentrations of up to 2000 ppm reflects the presence of pairs of weakly exchange coupled Mn2+ ions, the narrowing of the broad Lorentzian line for the samples with higher nominal concentrations (see Figure 3) corresponds to the formation of larger Mn2+ clusters.

reference image (Figure 11a) and quantified. For clarity, the analyzed NCs and pores are also marked on the HAADFSTEM image of Figure 10. The resulting chemical composition values presented in Table 1 include rather large estimated margins of errors (±30%) mainly due to the experimental errors associated with very weak and noisy extracted EELS spectra, as well as the thickness variations between the different analyzed regions, especially between pores and NCs. Consequently the data reported in Table 1 should be considered in a semiquantitative manner. Nevertheless, the data from Table 1 lead to some interesting conclusions. Thus, pores (1−3) exhibit a significantly higher content of Mn atoms than the ZnS nanocrystals (4−6). All pores contain less S but more O than the nanocrystals. Therefore, one concludes that Mn−Zn−O type oxide phases are present in the pores. Meanwhile, as expected, the nanocrystals (4−6) contain mainly Zn and S. One should mention the presence of an unexpectedly large concentration of oxygen surrounding the cZnS nanocrystals, originating, very likely, in the synthesis materials and procedure and retained especially in the amorphous material of the pores. The particular cases of the small nanocrystal NC 7 with no sulfur content and of the pore 3 with a much smaller content of S atoms, both localized at the edge of the nanocrystals agglomerate, reflect the presence of a surface oxidation process with the formation of Zn−Mn oxide phases. To sum up the results, distinct phases consisting of Mn− Zn−O based compounds are formed in the regions with amorphous material at the ZnS nanocrystallites edges and in the pores, which constitute the Mn aggregated as magnetic clusters observed by EPR and magnetic measurements. 14462


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Table 2. Spin Concentrations (in ppm) of the Isolated (Core + Surface Localized) (Cis) and Aggregated (Caggr) Mn2+ Centers Obtained by EPR Quantitation at RT and from Low Temperature Susceptibility C(χpara) and Magnetization at Saturation CS(M)a sample

Cis (S = 5/2)

Caggr (S = 5/2)

C(χpara) (μ = 5 μB)

CS(M)

Ctot(Mn)

CAA(Mn)

cZnS:Mn(2000) cZnS:Mn(20 000)

220 1100

150 1100

440 1700

150

370/440 3600/3350

455 3470

a

The total concentration of the incorporated Mn2+ ions Ctot(Mn) estimated from EPR/magnetic data (average values) is compared with the total concentration of the incorporated Mn2+ ions determined by AA spectroscopy CAA(Mn). Estimated errors are ±25% for the EPR and magnetic measurements and ±15% for the AA determination.

Additional information on the Mn2+ aggregation process is obtained from the quantitative analysis of the paramagnetic susceptibility determined from the magnetic measurements. Using the typical χpara = C/T Curie-type dependence of the paramagnetic susceptibility, with C a Curie−type constant (C = NP2/3kB, where N is the number of paramagnetic centers, P the effective magnetic moment per paramagnetic center, and kB the Boltzmann constant), one can estimate the number N of paramagnetic centers per gram of sample (noted by n), assuming a magnetic moment of 5 μB per Mn2+ center, where μB is the Bohr magneton. According to the relationship n = 3kBTχpara/P2 there are (2.6 ± 0.2) × 1018 paramagnetic centers per gram in the cZnS:Mn(2000) QDs sample and about (10.0 ± 0.2) × 1018 paramagnetic centers per gram in the cZnS:Mn(20000) QDs sample. Based on the molecular mass of ZnS of 97.4 g, the number of formula units (f.u.) of ZnS per gram is 6.1 × 1021. Hence, one obtains from the magnetic measurements the concentration of the paramagnetic centers, with respect to the number of ZnS f.u. C(χpara) = 4.4 × 10−4 (440 ppm) in the lower doping level sample and C(χpara) = 17 × 10−4 (1700 ppm) in the higher doping level sample. Considering the experimental errors involved (±25%) one finds a reasonable agreement between the spin concentration of the paramagnetic centers from magnetic measurements C(χpara) = 440 ppm and the total spin concentration Cis+ Caggr = 370 ppm provided by EPR in the case of the sample with 2000 ppm nominal concentration. These values are also in good agreement with the concentration of incorporated manganese CAA = 455 ppm determined by elemental atomic absorption (AA) analysis using a PerkinElmer atomic absorption spectrophotometer model PINAACLE 900T and standard quantitative analysis procedures. In the sample prepared with 20 000 ppm nominal Mn2+ concentration, the actual concentration of the Mn2+ ions is certainly larger compared to the spin counting determinations by EPR. Indeed, the analysis of the EPR and magnetic data reveals the presence of weakly coupled pairs and larger clusters of antiferromagnetically coupled Mn2+ ions with total spin S = 5/2. From the increment of the saturation magnetization at 2 K shown in Figure 8 (assigned to the magnetically frozen clusters) of about 0.05 emu/g, a concentration of equivalent paramagnetic centers with 5 μB magnetic moment of about CS(M) = 150 ppm was determined. CS(M) should correspond to the larger clusters of Mn2+ ions, which contribute to the Caggr = 1100 ppm spin concentration determined by EPR. Thus, one finds a good agreement between the spin concentration value Cis + Cagg − CS(M) = 1950 ppm determined from EPR measurements and the corresponding C(χpara) = 1700 ppm value determined form magnetic measurements. Both concentration values represent the sum of the isolated Mn2+ ions and the weakly coupled Mn2+ pairs. To determine the total concentration of Mn2+ ions incorporated in the investigated

cZnS:Mn(20 000) sample one should add the concentration of the strongly coupled Mn2+ ions forming large clusters, which depends on the size of the clusters. Considering an average number of 11 strongly coupled Mn2+ ions per large cluster with total spin S = 5/2, one finds total concentrations of incorporated Mn2+ ions in the higher doped sample of Ctot = 1950/1700 ppm + 11 × 150 ppm = 3600/3350 ppm. Both estimated values compare well, within the experimental errors, with the CAA = 3470 ppm value obtained by elemental AA analysis. Table 2 summarizes the quantitation of the spin concentrations obtained by EPR and magnetic measurements, the resulting total concentration of Mn2+ ions in the lower and higher doped samples and the concentration of incorporated Mn2+ ions determined by elemental AA analysis. EPR properties similar to our case were reported from investigations on Mn2+ doped ZnO thin films grown epitaxially by the metal−organic chemical-vapor deposition (MOCVD) technique,41,42 or deposited by laser ablation.43 Paramagnetism, reflected in the well resolved EPR spectra of the isolated substitutional Mn 2+ ions, was found at lower doping concentrations, while at higher doping levels the well resolved spectrum was replaced by the broad Lorentzian line exhibiting properties typical for antiferromagnetic coupled Mn2+ ions, i.e., a Curie−Weiss type temperature dependence. Larger absolute θ values at higher doping levels, in agreement with the exchange-narrowing theory developed for the CdMn(S,Se,Te) compounds, were reported.28,30 Using the linear dependence with Mn2+ concentration of the Curie parameter and Curie−Weiss temperature, valid for a randomly diluted distribution of Mn2+−Mn2+ pairs interacting via antiferromagnetic Heisenberg exchange, a value of J1/kB = −21.8 K was reported for the average exchange integral in the Zn1−xMnxO system.42 Using the same procedure as in ref 42 and the Ctot values from Table 2, J1/kB values up to 2 orders of magnitude higher are obtained. Such too large values can be explained by the nonuniform distribution of the amorphous phase containing the Mn2+ aggregates in the investigated cZnS:Mn QDs, resulting in a much higher local concentration of Mn2+ ions, and/or by a more complex local composition, in which the Mn2+ ions are very likely coordinated, besides oxygen and zinc, by other ions originating in the starting materials. The key role of the aggregated Mn2+ ions localized in the intergrain spaces of the self-assembled cZnS:Mn QDs, as the main source of collective magnetism at low temperatures, can also explain the reported12 absence of antiferromagnetic interactions in cZnS:Mn QDs, prepared with up to 5 at. % Mn2+ nominal concentration after HCl-washing of the sample. Indeed, it is very likely that such chemical washing removed the amorphous intergrain material containing Mn2+ aggregates, expected to generate antiferromagnetism. One should also discuss the presence of an ordered magnetic state extending over the whole sample volume, better observed 14463


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concentrations observed by EPR spectroscopy, offering a deeper insight into the magnetic properties of the mesoporous cZnS:Mn. Thus, besides the magnetic ordered state, with similar small magnetization values over the 50−300 K temperature range in both samples, attributed to some specific magnetic intrinsic defects in the core and/or on the surface of the cZnS:Mn QDs, we found differences in the magnetic properties attributed to doping Mn2+ ions. Thus, one could observe a sharp increase in the saturation magnetization and coercive field for T < 50 K only in the highly doped cZnS:Mn(20000) sample, its origin being attributed to the presence of large magnetic clusters in a frozen magnetic state, with a blocking temperature TB < 50 K, behaving as an assembly of Stoner−Wohlfarth magnetic monodomain nanoparticles. Such clusters may present Curie−Weiss (Néel) temperatures lower than 100 K (e.g., the ones involving Zn− Mn bonds), in agreement with the EPR measurements. Meanwhile, in the lower doped cZnS:Mn(2000) sample the paramagnetic behavior observed from RT down to 2K, is explained by the presence, besides the isolated core and surface localized Mn2+ ions, of pairs of very weakly coupled Mn2+ ions. The total concentration of the Mn2+ ions incorporated in the lower doped cZnS:Mn(2000) sample, based on the EPR and susceptibility spin concentration values of the isolated and weakly coupled pairs of Mn2+ ions, was found to be in good agreement with the corresponding value determined by AA spectroscopy. In the case of the cZnS:Mn(20 000) sample the values determined by EPR and magnetometry were found to be in good agreement with the concentration value obtained by analytical AA spectroscopy, by considering the 150 ppm concentration of clusters of 11 strongly coupled Mn2+ ions responsible for the observed low temperature magnetism. The aggregation of the Mn2+ doping ions outside the selfassembled cZnS QDs was further confirmed by analytical high resolution transmission electron microscopy, which evidence the presence of an amorphous phase in the pores and in the intergrain spaces with a dominant Zn−Mn−O-type composition, very likely hosting the clusters of Mn2+ ions responsible for the observed collective magnetism properties. In summary, we demonstrate that the collective magnetism observed at low temperatures in the mesoporous, selfassembled cZnS:Mn QDs prepared by coprecipitation, is due to the aggregation of the manganese impurities in a separate, complex amorphous phase rich in Mn, Zn, and O, localized between the cZnS QDs. This phase, which contains the Mn2+ clusters responsible for the observed low temperature collective magnetism, could be also present in other nanostructured semiconductors doped with TMIs, offering an alternative mechanism for the eventual occurrence of collective magnetism, but with no significant role in the room temperature ferromagnetism observed in DMS systems. The importance and significance of this work consists in the observation, to our knowledge for the first time, in the mesoporous structure of self-assembled cZnS:Mn QDs prepared by coprecipitation, of an amorphous phase composed mainly of Zn, Mn, and O incorporated between the cZnS:Mn QDs, which is responsible above a certain manganese concentration for the observed low temperature collective magnetism. Our research points to an alternative mechanism based on the presence of agglomerates of TMIs as a separate nanophase, which could explain the collective magnetic properties of other doped II−VI semiconductor nanocrystals

in the cZnS:Mn QDs with low doping level (Figure 7a). In this case the coercive field at 300 K was about 30 Oe, whereas the saturation magnetization was less than 0.01 emu/g. According to the value of the saturation magnetization at low temperature (0.01 emu/g), a number of 1 × 1018 μB corresponds to each gram of sample containing 6.1 × 1021 ZnS f.u. Hence, a value of 0.00016 μB might be estimated per ZnS f.u. or a magnetic moment of 0.05 μB per ZnS nanocrystal (a number of 324 ZnS molecules per 3 nm nanocrystal was calculated in ref 20). Because this collective magnetic state is almost independent of the doping level, we may assume that it is not correlated to the doping ions but to some specific magnetic lattice defects in the investigated cZnS nanocrystals.7 Finally, one should mention that because the composition, structure, and local distribution of the amorphous phase found in the intergrain spaces of the mesoporous nanomaterial, responsible for the collective magnetism, depend on the preparation procedure, a corresponding variation in the magnetic properties of self-aggregated QDs is expected. The expected sensitivity of such a magnetically active amorphous intergrain layer to thermal and mechanical treatments could also result in the observed changes in the magnetic properties of the II−VI semiconductor QDs.9

5. CONCLUSIONS The present investigation of the localization, structure, composition and magnetism of the aggregated Mn2+ ions incorporated in the self-assembled cZnS:Mn QDs with mesoporous structure and increasing doping levels in the 200 ppm (0.02 at. %) to 50 000 ppm (5 at. %) nominal concentration range is based on the ability of EPR to separate and quantify the amount of Mn2+ ions incorporated at different locations, as well as on the correlation with magnetometry, analytical high resolution TEM and elemental analysis by AA data from the same samples. To our knowledge such a correlation of the experimental data obtained by these materials characterization techniques/methods has not been reported so far. The main, most important results are the following: The observation in the EPR spectra of the mesoporous assembly of cZnS:Mn QDs of a structureless, broad Lorentzian component line attributed to aggregated Mn2+ ions, exhibiting a constant line width of 38.5 mT in the samples with nominal concentration up to 2000 ppm, with a strong progressive narrowing in samples with higher nominal concentrations (Figure 3). Based on line width simulations,34 this behavior is explained by the formation at lower nominal concentrations (up to 2000 ppm) of pairs of Mn2+ ions weakly coupled by exchange interaction, followed at higher nominal concentrations (>2000 ppm) by the formation of larger clusters of strongly exchange coupled Mn2+ ions as well. The temperature dependence of both line width and integrated intensity of the broad Lorentzian EPR line from the aggregated Mn2+ ions are characteristic at lower nominal concentrations (up to 2000 ppm) to weakly coupled pairs of Mn2+ ions with paramagnetic-like behavior, while for higher nominal concentrations (20 000 ppm) it corresponds to the additional formation of larger clusters of antiferromagnetically coupled Mn2+ ions with θ = −75 K. The analysis of magnetometry data (hysteresis loops and magnetization at saturation) confirmed the change in the magnetic properties between the samples prepared with lower (2000 ppm) and higher (20 000 ppm) Mn2+ nominal 14464


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(13) Peng, W. Q.; Qu, S. C.; Cong, G. W.; Zhang, X. Q.; Wang, Z. G. Optical and Magnetic Properties of ZnS Nanoparticles Doped with Mn2+. J. Cryst. Growth 2005, 282, 179−185. (14) Igarashi, T.; Isobe, T.; Senna, M. EPR Study of Mn2+ Electronic States for the Nanosized ZnS:Mn Powder Modified by Acrylic Acid. Phys. Rev. B: Condens. Matter Mater. Phys. 1997, 56, 6444−6445. (15) Borse, P. H.; Srinivas, D.; Shinde, R. F.; Date, S. K.; Vogel, W.; Kulkarni, S. K. Effect of Mn2+ Concentration in ZnS Nanoparticles on Photoluminescence and Electron-Spin-Resonance Spectra. Phys. Rev. B: Condens. Matter Mater. Phys. 1999, 60, 8659−8664. (16) Beermann, P. A. G.; McGarvey, B. R.; Skadtchenko, B. O.; Mulralidharan, S.; Sung, R. C. W. Cationic Substitution Sites in Mn2+Doped ZnS Nanoparticles. J. Nanopart. Res. 2006, 8, 235−241. (17) Stefan, M.; Nistor, S. V.; Ghica, D.; Mateescu, C. D.; Nikl, M.; Kucerkova, R. Substitutional and Surface Mn2+ Centers in Cubic ZnS:Mn Nanocrystals. A Correlated EPR and Photoluminescence Study. Phys. Rev. B: Condens. Matter Mater. Phys. 2011, 83, 045301. (18) Stefan, M.; Nistor, S. V.; Ghica, D. ZnS and ZnO Semiconductor Nanoparticles Doped with Mn2+ Ions. Size Effects Investigated by EPR Spectroscopy. In Size Effects in Nanostructures; Kuncser, V., Miu, L., Eds.; Springer Series in Materials Science; Springer: Berlin, 2014; Vol. 205, Part 1, pp 3−27. (19) Nistor, S. V.; Stefan, M.; Nistor, L. C.; Goovaerts, E.; Van Tendeloo, V. Incorporation and Localization of Substitutional Mn2+ Ions in Cubic ZnS Quantum Dots. Phys. Rev. B: Condens. Matter Mater. Phys. 2010, 81, 0353661. (20) Nistor, S. V.; Stefan, M.; Nistor, L. C.; Ghica, D.; Vlaicu, I. D.; Joita, A. C. Doping Ultrasmall Cubic ZnS Nanocrystals with Mn2+ Ions over a Broad Nominal Concentration Range. J. Phys. Chem. C 2015, 119, 23781−23789. (21) Nistor, S. V.; Stefan, M.; Nistor, L. C.; Ghica, D.; Vlaicu, I. D. Distribution and Interaction of Mn2+ Ions Incorporated in Cubic ZnS Quantum Dots over a Broad Concentration Range. J. Alloys Compd. 2016, 662, 193−199. (22) Bhattacharyya, S.; Zitoun, D.; Gedanken, A. Electron Paramagnetic Resonance Spectroscopic Investigation of Manganese Doping in ZnL (L = O,S,Se,Te) Nanocrystals. Nanosci. Nanotechnol. Lett. 2011, 3, 541−549. (23) Pal, S.; Ghosh, M. Influence of Pulse Shape in Modulating Excitation Kinetics of Impurity Doped Quantum Dots. Superlattices Microstruct. 2013, 55, 118−130. (24) Li, G. L.; Wu, S. Y.; Ding, C. C.; Hu, X. F.; Zhang, Z. H. Studies on the Spin Hamiltonian Parameters for Mn2+ in ZnS Nanocrystals and Bulk. Mol. Phys. 2014, 112, 3189−3194. (25) Li, Y.; Wu, Y. Spectroscopic Characteristics of (Mn2+, Nd3+) codoped Zinc Sulphide Nanocrystals. Opt. Mater. 2015, 49, 100−104. (26) Stefan, M.; Nistor, S. V.; Ghica, D. Correlation of Lattice Disorder with Crystallite Size and the Growth Kinetics of Mn2+ Doped ZnO Nanocrystals Probed by Electron Paramagnetic Resonance. Cryst. Growth Des. 2013, 13, 1350−1359. (27) Samarth, N.; Furdyna, J. K. Electron Paramagnetic Resonance in Cd1‑xMnxS, Cd1‑xMnxSe and Cd1‑xMnxTe. Phys. Rev. B: Condens. Matter Mater. Phys. 1988, 37, 9227−9239. (28) Spalek, J.; Lewicki, A.; Tarnawski, Z.; Furdyna, J. K.; Galazka, R. R.; Obuszko, Z. Magnetic Susceptibility of Semimagnetic Semiconductors: The High Temperature Regime and the Role of Superexchange. Phys. Rev. B: Condens. Matter Mater. Phys. 1986, 33, 3407−3419. (29) Abragam, A.; Bleaney, B. Electron Paramagnetic Resonance of Transition Ions; Clarendon Press: Oxford, U.K., 1970. (30) Furdyna, J. K.; Samarth, N. Static Magnetic Susceptibility of Zn1‑xMnxSe. Phys. Rev. B: Condens. Matter Mater. Phys. 1988, 37, 3707−3709. (31) Van Wierengen, J. S. Paramagnetic Resonance of Divalent Manganese Incorporated in Various Lattices. Discuss. Faraday Soc. 1955, 19, 118−126. (32) Schneider, E. E.; England, T. S. Paramagnetic Resonance at Large Magnetic Dilutions. Physica 1951, 17, 221−233.

self-assembled into mesoporous structures. It also opens a new direction of investigation concerning the influence of preparation conditions on the composition, structure, and magnetic properties of the TMIs agglomerates and how these properties determine the overall magnetic and optical properties of the nanostructured host material which could be of interest for various applications.

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AUTHOR INFORMATION

Corresponding Author

*Phone: 0040 213690185. Fax: 0040 21 3690177. E-mail: snistor@infim.ro. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Research performed in the frame of the CNCSIS-UEFISCDI projects PNII-IDEI-74/2011 and PNII-IDEI-75/2011, as well as of the Core Program PN16-480101. The authors are grateful to Dr. Mihaela Scurtu from CromatecPlus for the AA determinations and to Dan Zernescu for expert technical assistance in the EPR experiments.

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REFERENCES

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Aggregates of Mn2+ Ions in Mesoporous Self-Assembled Cubic ZnS

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