Doping Ultrasmall Cubic ZnS Nanocrystals with Mn2+ Ions over a

Article pubs.acs.org/JPCC

Doping Ultrasmall Cubic ZnS Nanocrystals with Mn2+ Ions over a Broad Nominal Concentration Range Sergiu V. Nistor,* Mariana Stefan,* Leona C. Nistor, Daniela Ghica, Ioana D. Vlaicu, and Alexandra C. Joita National Institute of Materials Physics, Street Atomistilor 105 bis, Magurele-Ilfov, 077125-Romania S Supporting Information *

ABSTRACT: Although impurity doping of nanocrystals is essential in controlling their physical properties for various applications, the doping mechanism of ultrasmall, colloidal II− VI semiconductor nanocrystals, corresponding to the initial stages of growth, is not yet understood. In this study the concentrations of Mn2+ ions in the core, on the surface, and as an agglomerated separate phase in 2.9 nm cubic ZnS nanocrystals, prepared by a surfactant-assisted liquid−liquid synthesis within 20 to 20 000 ppm nominal impurity concentration range, have been determined by quantitative multifrequency electron paramagnetic resonance. The unexpected strong decrease in the core doping efficiency with the nominal concentration increase, in contrast to the small variation of the doping efficiency for the surface-bound Mn2+ ions, and the sizable core doping efficiency observed for 1.8 nm nanocrystals were explained with the extended lattice defect assisted mechanism of incorporation. According to this mechanism, which is not size or shape limited, being active from the initial growth stages, the incorporation of Mn2+ ions takes place at surface sites with high binding energy on dislocation steps formed by the emerging stacking defects. High resolution transmission electron microscopy confirms the presence of such stacking defects in a large proportion of the investigated cubic ZnS nanocrystals, ensuring the operation of the proposed doping mechanism.

1. INTRODUCTION Doping II−VI semiconductor nanocrystals (NCs) with transition metal ions (TMIs) is essential for enhancing and diversifying the nanoscale induced changes in their physical properties,1−5 resulting in a broad range of applications, for example, light emitting devices, optoelectronics, biological labeling, solar energy harvesting, or spintronics.6−9 Contrary to bulk semiconductors which can be rather easily doped with TMIs, the chemistry to control dopant incorporation in colloidal NCs has proven to be a difficult task, requiring a better knowledge of the doping mechanisms at the atomic level.5 The standing theory regarding the incorporation of impurities in the core of semiconductor II− VI NCs prepared by colloidal synthesis at low temperature is based on the trapped dopant model.1−3,10 The model requires rather large (d > 3 nm), well faceted NCs and cannot explain the incorporation of TMIs in smaller NCs, corresponding to early growth stages. Moreover, in the case of the ultrasmall NCs of 1−3 nm size, the large surface to volume ratio results in specific properties associated with the large fraction of impurity ions localized in the surface layer, or in the shell of capping compounds added to stabilize and improve the desired properties.11−18 To understand and control the doping process it is therefore essential to quantitatively evaluate the concentration of the doping ions in the core and on the surface of NCs. © 2015 American Chemical Society

Electron paramagnetic resonance (EPR) can detect with high sensitivity the presence and amount of paramagnetic centers, including TMIs at different lattice sites in semiconductor NCs.3,19−21 Mn2+ is the best suited TMI for such studies and has been extensively investigated in NCs of ZnS, ZnSe, ZnO, CdS, and CdSe.3,7,20−22 Studies of nanocrystalline samples exhibiting well resolved EPR spectra resulted in the identification of isolated Mn2+ ions localized in the core and in the surface layer of the host NCs, as well as of agglomerates/ separate phases of Mn2+ ions observed at higher nominal concentrations.3,13,20,23 Qualitative changes in the EPR spectra of the Mn2+ impurity ions vs nominal concentration have been previously reported in NCs of cubic ZnS (cZnS),23−26 CdS,13,27 and ZnSe.1,28 Quantitative EPR has been performed for certain nominal concentrations, mainly to determine the total concentration of isolated doped ions.29−31 The resulting values generally agreed with those determined by inductively coupled plasma atomic emission (ICP-AE),1,14,32−35 optical emission,27,29 and magnetic susceptibility36 measurements. Determinations of the separate concentrations of the core-incorporated and surfaceReceived: August 20, 2015 Revised: September 24, 2015 Published: September 25, 2015 23781

DOI: 10.1021/acs.jpcc.5b08113 J. Phys. Chem. C 2015, 119, 23781−23789

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The Journal of Physical Chemistry C bound Mn2+ ions in cZnS reveal that Mn NCs have been reported only in a few cases, for particular doping levels.10,31,36 Here we report a systematic quantitative investigation of the incorporation of Mn2+ impurity ions in the core, on the surface, and in the agglomerated phase in cZnS NCs of 2.9 nm core diameter, of identical structure, morphology, and size, in the 20 to 20000 ppm nominal concentration range. To our knowledge such an investigation has not been reported so far for any TMI doped cubic II−VI semiconductor NCs. We observed a strong, unexpected difference between the variations of the core and surface doping efficiencies with the nominal concentration increase, as well as a comparable core doping efficiency for the smaller 1.8 nm size cZnS NCs. These aspects, which cannot be described with the currently accepted trapped dopant model of incorporation, valid for larger NCs,1,2,10 have been explained with the previously suggested extended lattice defect assisted (ELDA) mechanism of incorporation,37,38 which is not size or shape limited, being active even in the initial cZnS NCs growth stage.

the as-grown nanopowders in ethanol, dispersing them by sonication, and dropping on lacey carbon grids. All TEM images were obtained for regions of the specimen situated over the holes of the carbon grids, to be sure that any amorphous zones observed in the images belong to the specimen and not to the carbon grid bars. X (9.4−9.8 GHz)- and Q (34.1 GHz)-band continuous wave (CW) EPR measurements were performed with the ELEXSYSE580 and ELEXSYS-E500Q spectrometers, respectively, from Bruker. The first spectrometer, equipped with the calibrated Xband Super High QE (SHQE) cylindrical cavity resonator (ER 4123SHQE), was employed for both recording the EPR spectra and quantitative determinations of the Mn2+ ions concentration in the investigated samples with the absolute spin quantitation routine included in the XEPR software from Bruker, routine based on the double integration of each recorded spectrum. The Q-band EPR spectra were recorded at RT with the second spectrometer equipped with the cylindrical resonator ER5106QT/W. Weighted amounts of the powdery samples of nanocrystalline cZnS doped with Mn2+ ions in the investigated concentration range were inserted into 4.0 mm and 3.0 mm o.d. pure silica tubes for X- and Q-band EPR measurements, respectively, and hermetically sealed. Special care was taken, during the EPR measurements, to avoid over modulation and microwave saturation induced line broadening effects, observed at both microwave frequencies for microwave powers larger than 0.2 mW.

2. EXPERIMENTAL METHODS The preparation of the cubic ZnS nanocrystals doped with variable amounts of Mn2+ ions in standard conditions39 begins by adding manganese acetate [Mn(CH3−COO)2·4H2O] (99.98% metals basis, Alfa Aesar) in concentrations starting from 0.002% M up to 2% M to 150 mL of 0.133 M solution of zinc acetate [Zn(CH3−COO)2·2H2O] (98% purum p.a., Alfa Aesar), both in bidistillated water mixed with methanol (99.5% absolute, Alfa Aesar) in a 10:1 ratio. The resulting solution is mixed for another 30 min and further coprecipitated with 50 mL of 0.4 M solution of ammonium sulfide [(NH4)2S] (40−44% purum p.a., Alfa Aesar) in the presence of the Tween 20 (polyoxyethylene sorbitan monolaurate, Alfa Aesar) surfactant. All operations take place at room temperature (RT) under a 99.998% pure argon atmosphere. The resulting white precipitate is left for maturation for another hour under continuous stirring and afterward decanted, washed three times with bidistillated water and once with pure methanol and dried overnight at 100 °C in air. Samples with the following nominal concentrations, 20 ppm, 50 ppm, 100 ppm, 200 ppm, 500 ppm, 1000 ppm, 2000 ppm, 5000, 10 000, and 20 000 ppm, as well as an undoped reference sample, have been prepared by this procedure at pH = 5.5. Two additional samples of mesoporous cZnS have been prepared in modified (nonstandard) conditions to determine the effect of the Tween 20 additive and of a smaller NCs core size on the Mn2+ incorporation. One sample, grown with 20 000 ppm Mn2+ ions nominal concentration, was prepared without Tween 20 surfactant. Another one, grown with 2000 ppm Mn2+ nominal concentration, was prepared at an increased pH = 7.0. Details about their structure and EPR properties are presented in the Supporting Information. The structure characterization was performed by X-ray diffraction (XRD) and high resolution transmission electron microscopy (HRTEM) at low and high magnifications. XRD data were collected with a Bruker D8 Advance diffractometer (Cu anode and Ni filter) in the θ−θ geometry, from 2θ = 10° to 2θ = 110°. The lattice parameters and average crystallites size/coherence distance resulted from the Rietveld refinement of the experimental data using the Topas software. The HRTEM investigations were performed with an atomic resolution analytical JEOL JEM-ARM 200F electron microscope operating at 200 kV, on specimens prepared by crushing

3. RESULTS AND DISCUSSION 3.1. Samples Structure. The XRD measurements of all samples prepared in standard conditions with different nominal concentrations of Mn2+ ions revealed practically identical diffraction patterns (Figure 1a) from cubic (blende) ZnS (JCPDS file no. 05-0566). The values of the average coherence distance d = 2.9 ± 0.2 nm and lattice parameter a = 0.5393 ± 0.0002 nm, determined by the Rietveld analysis, were the same for all samples. No secondary crystalline phases could be observed, even for the highest dopant concentration. This does not exclude a possible presence of minority secondary phases below the XRD detection limit of 1%. HRTEM and electron diffraction (ED) determinations of the morphology and structure, respectively, of all investigated samples confirmed the conclusions of the XRD studies. Thus, according to the low magnification HRTEM and electron diffraction (ED) investigations, the sample with a nominal concentration of 5000 ppm Mn2+ ions, presented as an example, consists of cZnS NCs with cubic (blende) ZnS structure (Figure 1b) self-assembled into a mesoporous structure with pores of various sizes (Figure 1c). The rather broad diffraction rings of the ED pattern reflect the presence of cZnS nanocrystals of a few nanometers diameters, in agreement with the XRD determinations. Additional morphology and structure details were observed at higher magnifications. The HRTEM image (Figure 1d) reveals at atomic resolution a tight agglomerate of cZnS NCs in different orientations. Two of them, marked A and B, viewed along the [110] zone axis show planar defects, as stacking faults and twins.37 Because of structural reasons37 planar stacking defects can be viewed in zinc-blende structures at atomic resolution only if the crystals are oriented along the [110] zones axes. The NC marked A is placed at the edge of a large (7−8 nm) sized pore, while the one marked B is part of the tightly packed agglomerate. Black arrows indicate the steps on 23782


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the NCs surface where the planar stacking defects emerge. As suggested in refs 37 and 38 these are the places where the Mn impurity atoms can be trapped. Since the nanocrystals are tightly packed, small pores are difficult to observe at high magnifications due to overlapped images of individual NCs. Nevertheless, the white arrow indicates a small (∼1 nm) pore separating four ZnS NCs, in a very thin zone of the agglomerate. No sizable disordered layer/shell separating the cZnS NCs could be observed. The same structure and morphology, illustrated in Figure 1 for the cZnS:Mn (5000 ppm) NCs, were found for all investigated nanocrystalline cZnS:Mn samples in the whole nominal concentration range, including the undoped sample. This result underlines the high structural reproducibility of the employed synthesis procedure, which was not affected by doping with Mn2+ impurities up to the highest nominal concentration. According to the XRD and HRTEM investigations (Supporting Information) the cZnS:Mn (20000 ppm) sample prepared without surfactant also consists of cZnS NCs of 2.9 nm average diameter but self-assembled in a tighter packed structure, very likely due to the absence of the large surfactant molecules attached to the surface of the NCs. The cZnS:Mn (2000 ppm) sample prepared from solutions at high pH consists of NCs with a smaller core, of 1.8 nm average diameter, separated by layers of disordered material (Supporting Information). 3.2. The EPR Spectra. All investigated samples exhibited the well resolved narrow lines spectrum of Mn2+ ions localized substitutionally in the core of the cZnS:Mn NCs (Figure 2a,b), called Mn2+(I) centers.26,37

Figure 1. Structure of the cZnS:Mn NCs prepared in standard conditions. (a) Wide-angle XRD patterns of samples doped with increasing amounts of Mn2+ ions; (b, c, d) ED and HRTEM images of the cZnS:Mn (5000 ppm) sample illustrating the structure and morphology of the whole series of samples: (b) ED pattern, indexed with the cubic ZnS structure; (c) low magnification HRTEM image revealing the mesoporous morphology of the specimen; (d) HRTEM image showing structure details at atomic resolution.

Figure 2. Sequence of EPR spectra of 2.9 nm cZnS:Mn NCs prepared in standard conditions, with increasing Mn2+ nominal concentration. (a, b) Spectra recorded in identical conditions at RT, in the (a) Q and (b) X microwave frequency bands. (c, d) Experimental spectrum (black) and contributions of the core-incorporated (red), surface-bound (blue), and agglomerated (green) Mn2+ ions in (c) Q- and (d) X-band EPR spectra for the cZnS:Mn (5000 ppm) NCs. 23783


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Table 1. SH Parameters of the Mn2+ Ions Incorporated in the Investigated cZnS:Mn NCs, Compared with the Reference Data center/host 2+ Mn2+ core = Mn (I)/cZnS 2+ Mnsurf/cZnS NCs Mn2+ aggl/cZnS NCs 2+

Mn (A)/ZCB NCs a

NCs

g

A [10−4 cm−1]

D [10−4 cm−1]

σ(D) (%D)

ref

2.0022 ± 0.0001 2.0011 ± 0.0002 2.0024 ± 0.0006 2.0012

−63.7 ± 0.2 −84.5 ± 0.2

D = −41 D = 240a

43 43

−84.7

150−220

33 for D = 210a

this work, 26, 37 this work this work 43

Tentative value used in the simulation−deconvolution procedure.

The SH parameters of the surface-bound Mn2+ ions, called Mn2+ surf, while being the same for all cZnS:Mn NCs samples prepared in standard conditions with varying nominal concentration, are different from those previously reported for various Mn2+ surface centers in cZnS NCs.23,26 They are rather close to the SH parameters of the Mn2+(A) centers reported in nanocrystalline zinc carbonate basic (ZCB) with formula Zn5(OH)6(CO3)2, consisting of substitutional Mn2+ ions at the Zn2+ sites 6-fold coordinated by four OH− groups and two O2− ions from CO3 groups.43 This means that the surface localized Mn2+ ions in the presently investigated cZnS:Mn NCs are very likely bound to surface adsorbed H2O and CO2 molecules. The EPR line widths of the isolated core and surface localized Mn2+ ions exhibit linear variations with the total concentration, from 0.24 to 0.62 mT and from 1.8 to 4 mT, respectively, which correspond in both cases to a dominant magnetic dipole−dipole interaction. A simple calculation of the average distribution of the isolated core plus surface localized Mn2+ ions shows that even for the maximum nominal concentration of 20000 ppm one incorporated Mn2+ ion corresponds to 2.8 NCs, resulting in an average separation of 4.1 nm between the Mn2+ ions. At such diluted distributions it is unlikely that the exchange interaction between the Mn2+ ions can influence the observed spectra. The broad line, described at both microwave frequencies by a simple isotropic electron Zeeman interaction term, is attributed to an agglomerated phase of strongly interacting Mn2+ ions, called Mn2+ aggl, very likely forming a separate phase. This assumption is supported by its Lorentzian line shape, with very large and constant derivative peak-to-peak line width (ΔBpp = 38.5 mT) at low nominal concentrations, which decreases to smaller values (ΔBpp ≤ 25 mT) for nominal concentrations above 2000 ppm. An apparent different broadening and/or shift of the central and edge lines, which can be observed in the experimental spectra, especially in the X-band, is due to the overlapping with this broad line. This is demonstrated by the deconvoluted spectra, illustrated in Figures 2c,d for the particular case of the cZnS:Mn (5000 ppm) NCs. Determining the composition, structure, and magnetic properties of the agglomerated phase is beyond the scope of this investigation and will not be pursued further. However, one should mention that according to preliminary investigations the agglomerated phase is paramagnetic at RT for all doping concentrations, and therefore does not affect the results of our quantitative determinations. 3.3. The Quantitation of Mn2+ Ions in the cZnS:Mn NCs. The absolute number of Mn2+ ions in weighted amounts of cZnS:Mn NCs has been determined for each sample by the quantitation of the recorded X-band EPR spectra. The error in these determinations was estimated to decrease from ±30% at the lower nominal concentrations to ±20% for the higher ones. According to the results presented in Figure 3a the total concentration of the Mn2+ ions in the samples increases almost

The samples prepared at 100 ppm or higher nominal impurity concentration exhibited an additional Mn2+ ions spectrum, better resolved in the Q-band (Figure 2a). Its broader component lines, reflecting the increased local disorder, are attributed to Mn2+ ions localized in the surface layer of the NCs.26,40 At nominal concentrations of 200 ppm and higher a single broad, featureless line attributed to agglomerated Mn2+ ions26 could be also observed, especially in the X-band. The well resolved 6-lines EPR spectra from both core and surface localized Mn2+ impurity ions reflect their localization at isolated, weakly interacting sites in the cZnS NCs. Both spectra are described by the spin Hamiltonian (SH) with axial symmetry:41,42 ⎤ ⎡ 1 H = μB S·g ·B + S ·A ·I + D⎢Sz2 − S(S + 1)⎥ − μN gN B ·I ⎦ ⎣ 3 (1)

The axial character of the SH (1) reflects the noncubic symmetry of the local crystal field at the investigated Mn2+ ions in the cZnS:Mn NCs. It is due to the presence of a neighboring planar lattice defect, in the case of the core localized ions37 and to the local low symmetry, in the case of the surface localized impurity ions.23,26 The first two terms in eq 1 describe the electron Zeeman and hyperfine interactions of the S = 5/2 electron spin with the magnetic field B and I = 5/2 nuclear spin of the 55Mn (100% natural abundance) isotope, respectively. The next second order zero-field-splitting (ZFS) term characterizes the interaction of the electron spin with the local axial crystal field. Higher order ZFS terms were not included as their contributions to the powder-like EPR spectra are small and difficult to determine.42 The small contribution of the last nuclear Zeeman interaction term which affects the forbidden hyperfine transitions was also included. Spectra deconvolution procedures illustrated in Figure 2 panels c and d were used to separate the component spectra belonging to the Mn2+ ions with different localizations, as a first step in determining their relative concentrations. The SH parameters of the isolated core and surface localized Mn2+ ions, used in the simulation of their spectra, were obtained by line shape fitting both low (X-band) and high (Q-band) frequency spectra.42 Their values are given in Table 1, which also contains the values of the standard deviation σ(D), given in percentages of the axial parameter D value. σ(D) is associated with the Gaussian distribution of the D values and describes the EPR lines broadening due to local crystal field fluctuations, thus reflecting the degree of lattice disorder.40,42 The SH parameters of the core localized Mn2+ ions were found, within the experimental errors, to be the same for all investigated cZnS:Mn NCs samples, independent of their nominal concentration or preparation conditions. Their values (Table 1) coincide with those of the previously observed Mn2+(I) centers in NCs, which consist of Mn2+ ions substituting for Zn2+ cations localized next to a stacking defect.26,37 23784


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incorporated Mn2+ ions increases much slower, leveling out at the higher nominal concentrations. The different incorporation behavior is better observed in the evolution of the doping efficiency, defined as the percentage of incorporated/doped Mn2+ ions relative to the nominal concentration (Figure 3b). Indeed, while the surface doping efficiency is about 7% for the lower and medium nominal concentrations, that is, up to 5000 ppm, decreasing to 5% at the highest nominal concentrations, the core doping efficiency exhibits a strong decrease, from about 9% at the lowest (20 ppm) nominal concentration to less than 1% at the highest (20000 ppm) nominal concentration. Table 2 presents the concentration values of incorporated Mn2+

linearly with the nominal concentration up to 5000 ppm, slowing down at higher nominal concentrations.

Table 2. Concentration (in ppm) of the Mn2+ Ions Incorporated in cZnS NCs Prepared in Standard and Nonstandard Conditions for Certain Nominal Concentrations nominal concn Mn2+ core Mn2+ surf Mn2+ aggl

concn concn concn total concn Mn2+

20

200

2000

2000a

10000

20000

20000b

1.8

16 15.5 6 37.5

72 152 151 375

37 11 54 102

184 671 938 1793

190 915 1078 2183

240 626 854 1720

1.8

a cZnS NCs of 1.8 nm core diameter prepared at pH = 7. bcZnS NCs of 2.9 nm core diameter prepared without Tween 20 surfactant.

ions for samples prepared in standard conditions, at certain impurity nominal concentrations, as well as for the samples prepared in nonstandard conditions. Examining Table 2 one notices that a smaller, but still comparable concentration of Mn2+ ions is present in the cZnS:Mn (2000 ppm) NCs of 1.8 nm core diameter, prepared at higher pH with respect to the sample prepared in standard conditions. Meanwhile, a drastic decrease in the concentration of the surface localized Mn2+ ions is observed. In the case of the cZnS:Mn (20000 ppm) NCs prepared without the Tween 20 surfactant, we found a 25% increase in the core doping efficiency, very likely due the adverse effect of the surfactant molecules on the core incorporation of the Mn2+ ions. A reduction in the concentrations of both surface bound and agglomerated Mn2+ ions is also observed, an effect which can be attributed to the tighter aggregation of the component cZnS NCs (see Figure S2 from Supporting Information). A survey of the existing literature shows varying values of the total concentration of Mn2+ in the cZnS:Mn NCs samples, the differences seeming to be due mainly to the accuracy of the quantitation method employed. Thus, for a nominal concentration of 10000 ppm (1% mol), values as different as 4% and 40% were previously reported for the total doping efficiencies, based on ICP-AE measurements on 2.4 nm cZnS NCs34 and magnetic (Curie temperature) measurements on 3 nm cZnS NCs,36 respectively. For the same nominal concentration we found a total doping efficiency of ∼18%. Doping efficiencies ranging from 2% to 10% have been also reported from ICP-AE measurements in isostructural cZnSe NCs of 3.6−3.3 nm for Mn2+ nominal concentrations varying from 10000 to 25000 ppm.14 The separate concentration of core incorporated Mn2+ impurity ions in 3 nm size cZnS:Mn (10000 ppm) NCs was reported by Tsujii et al.36 from magnetic measurements (Curie temperature) on samples subjected to chemical etching to remove the NCs shell. The reported value of 1400 ppm,

Figure 3. Results of EPR quantitation of Mn2+ impurity ions in 2.9 nm cZnS:Mn NCs vs the nominal concentration. (a) Total concentration of Mn2+ ions in the samples, as well as their separate concentrations in the core, as Mn2+(I) centers, bound on the surface as Mn2+ surf and in an 2+ agglomerated phase as Mn2+ aggl; (b) Doping efficiency of the Mn ions in the core (solid squares) and on the surface (solid triangles). The solid lines connecting the experimental data are guide for the eye only.

The evolutions of the separate concentrations of Mn2+ ions incorporated at different sites with the nominal concentration increase, determined from the deconvoluted spectra, are also displayed in Figure 3a. While the concentration of the core localized impurity ions could be determined in the whole doping range, the concentrations of the surface-bound and agglomerated Mn2+ ions could be determined only up from nominal concentrations of 100 and 200 ppm, respectively, when the amplitude of their EPR component spectra was strong enough to be accurately measured. One should mention that the small line width (and resulting high resolution) of the EPR spectra of the isolated Mn2+ ions have been essential in determining their separate concentrations at different locations. The observed small spectra line width is attributed to the strong aggregation of the cZnS:Mn nanocrystals into a mesoporous structure which occurs at the end of the growth process.39 As previously shown44 the aggregation of the ZnS NCs reduces the internal strain, which is the main source for EPR line broadening.40 According to Figure 3a the evolution of the coreincorporated Mn2+ ions concentration with the nominal concentration is different from the evolution of the surfacebound Mn2+ ions concentration. Thus, while the concentration of the surface-bound Mn2+ ions increases continuously with the nominal concentration increase, the concentration of the core23785


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negligible intensity of the forbidden transitions.37 To explain such a preferential localization we suggested the so-called extended lattice defect assisted (ELDA) mechanism of incorporation.37,38 According to the ELDA mechanism, the incorporation of the Mn2+ impurities takes place mainly by adsorption at the dislocation steps formed by the emerging planar stacking defects at the surface of the NCs. Such dislocation steps are observed in the HRTEM images of the investigated cZnS NCs (Figure 2d and Figures S2 and S4 in Supporting Information) and indicated by black arrows. The preferential adsorption of the impurity cations at a dislocation step on the NCs surface is very likely due to a higher binding energy,10 which explained the dislocations decoration with cations in the bulk crystals.46 The ELDA mechanism is illustrated in Figure 4 on a HRTEM image of an individual cZnS NC viewed along the

attributed to core localized Mn2+ ions, is much larger than the 190 ppm value we found in NCs of comparable diameter and identical nominal concentration, and somehow smaller than the total concentration of ∼1800 ppm found in our samples (Table 2). The discrepancy seems to be due to the incomplete removal by etching of the surface-bound and agglomerated Mn2+ ions. Indeed, we found that our cZnS:Mn samples submitted to a similar etching treatment as in ref 36 still exhibited EPR lines from both surface-bound and agglomerated Mn2+ ions. In another study31 values of 121 and 5.3 ppm for the surface and core + surface incorporated Mn2+ ions, respectively, were determined by quantitative EPR in two samples of ZnS nanowires grown with 2% nominal impurity concentration. Because of the mixed cubic + wurtzite structure of the two samples, any comparison with our data is difficult to make. 3.4. The Mn2+ Ions Doping Mechanism in cZnS NCs of d < 3 nm. The observed differences in the incorporation of the Mn2+ impurity ions at isolated core and surface sites of the cZnS NCs can be explained by the action of different doping mechanisms. The standing theory regarding the incorporation of impurities in the core of semiconductor II−VI NCs prepared by colloidal synthesis at low temperatures is based on the trapped dopant model.1−3,10 In this model the impurity must be adsorbed on the NCs surface and then be covered by additional material.1,45 For this to occur, the surface must be favorable for the impurity to bind for a residence time comparable to the reciprocal growth rate. Both theoretical estimations and experimental data from cZnSe NCs with zincblende structure and well-defined facets have shown that the binding energy of the Mn2+ ions strongly depends on the facets orientation, being by far the largest for the (001) facets.1 Moreover, the resulting concentration of trapped impurity ions is expected to be proportional to their concentration in the growth solution and to depend on the nature of the surfactant.10 Another consequence of this model is that no Mn2+ ions are expected to be incorporated in NCs with 2 nm or less core diameter, and only a very small amount of dopant should enter NCs of intermediate 2−3.5 nm diameter range. Theoretical estimations have shown a similar behavior for other II−VI compounds with zinc blende structure.1 It is clear that the trapped dopant model can explain neither the presently observed strong decrease in the core doping efficiency with the nominal concentration increase, nor the presence of sizable amounts of Mn2+ ions in the core of the cZnS NCs of 1.8 nm diameter, all without clear faceting, as shown by HRTEM images. One should also mention that in the absence of the Tween 20 surfactant used in the standard synthesis, an even higher core incorporation rate of the Mn2+ in the cZnS NCs would be expected for all nominal concentrations. The observed properties can be explained based on the results of our previous multifrequency EPR investigations on cZnS:Mn NCs of 2 nm core diameter,26,37 which have shown that the Mn2+ ions are preferentially incorporated in the cZnS NCs core at cation sites next to a stacking defect. The presence of the neighboring stacking defect could explain the axial symmetry of the Mn2+(I) centers, resulting in five pairs of rather intense hyperfine forbidden transitions between the main six allowed hyperfine transitions of the Mn2+ EPR spectrum. One should remember that, according to the trapped dopant model, the impurity ions are expected to substitute the Zn2+ ions in a perfect lattice, resulting in core localized Mn2+ centers with cubic symmetry exhibiting an EPR spectrum with

Figure 4. HRTEM image of a cZnS nanocrystal containing stacking defects. The red arrow shows the presence of a dislocation step on the surface of the NC, a possible trapping site for the Mn2+ impurity, as shown in the inserted drawing.

⟨110⟩ zone axis. The NC contains three planar stacking defects crossing its entire volume. The red arrow points to one of the dislocation steps produced at its surface by an emerging stacking defect. As illustrated in the inset, such steps on the NC surface are the sites where the Mn2+ impurities can be trapped during the NC growth process. The ELDA mechanism is expected to depend very little on the size of the NCs and the concentration of the impurity cations in the precursor solutions, the incorporation being controlled mainly by the presence and density of the extended planar lattice defects emerging at the surface of the growing NCs. It should be pointed out that these stacking defects are growth defects which are formed simultaneously with the NCs. Their presence has been observed by HRTEM in all studied cZnS NCs, even in the smallest ones, including the undoped ones. Moreover, as can be seen in Figure 4, some of these defects cross the entire NC through its center. Thus, the observed higher core doping efficiency for small nominal concentrations and the lower doping efficiency at higher nominal concentrations are explained by the limited number of available trapping sites on the surface of the NC. Therefore, at higher nominal concentrations a smaller fraction of the doping ions can be incorporated during the rapid process of nucleation and growth of the NCs core, resulting in a lower doping efficiency. The remainder of the impurity ions from the solution will be either adsorbed on the NCs surface, at the end of the growth process, or form agglomerates between the NCs, very likely during their final self-assembling into the mesoporous material.39 As expected, the efficiency of the surface doping with Mn2+ impurity ions, which takes place in the final stage of the cZnS 23786


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Our investigation reveals the essential role played by the extended lattice defects in the incorporation of Mn2+ ions, and possibly of other bivalent cation impurity TMIs, in ultrasmall cubic ZnS NCs. The ELDA mechanism of doping, which is expected to be active in other cubic II−VI semiconductor NCs in the early growth stages as well, complements the trapped dopant model, valid for larger (d > 3 nm) NCs in the later growth stages.

NCs growth by a simple surface adsorption process on the whole surface of the NC, varies less strongly with the nominal concentration. The decrease in the surface doping efficiency observed at high nominal concentrations seems to be related to saturation effects. Our HRTEM observations have shown that in all investigated samples a large proportion of the cZnS:Mn NCs contained stacking defects, essential in the incorporation of the Mn2+ ions by the ELDA mechanism. For the ELDA mechanism to be active in differently prepared cZnS, or other cubic II−VI NCs, one needs the presence of the stacking defects, known to play an essential role in the formation and stability of the cubichexagonal phases.44,47,48 Despite the restrictive orientation requirements for evidencing the stacking defects in cubic II−VI NCs, their presence, even if it was not explicitly mentioned, can be observed in reported HRTEM images of many cubic NCs of various sizes, prepared by different procedures, such as CdS,49,50 ZnS,49,51−53 ZnSe.12,54 The presence of the extended lattice defects in other cubic II−VI semiconductor NCs is also supported by the observation in the reported Mn2+ ions EPR spectra of intense hyperfine forbidden transitions, resulting from the presence of a neighboring planar defect.50,54 One can therefore conclude that the ELDA mechanism of incorporation of impurities is not restricted to the presently investigated cZnS:Mn NCs, but is very likely active in other cubic II−VI semiconductor NCs as well.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b08113. Structural and EPR properties of cZnS NCs prepared without Tween 20 surfactant; structural and EPR properties of cZnS:Mn NCs prepared at higher pH = 7 (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*Tel.: 0040 213690185. Fax: 0040 21 3690177. E-mail: snistor@infim.ro. *Tel.: 0040 213 690185. Fax: 0040 21 3690177. E-mail: mstefan@infim.ro. Notes

The authors declare no competing financial interest.

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4. CONCLUSIONS Our quantitative EPR investigations of cZnS:Mn NCs of 2.9 nm average diameter, doped with Mn2+ ions in the 20 to 20000 ppm nominal concentration range, offer new insights on the dopant incorporation and distribution in ultrasmall NCs, with some surprising results. Thus, the core and surface concentrations of the Mn2+ ions follow strikingly different patterns with the nominal concentration increase, exhibiting a strong decrease from 9% to less than 1% for the core doping efficiency and a rather small variation from 7.5% to 5% in the case of the surface-bound ions. Moreover, the core incorporation is not limited by the core size, or the employed surfactant. The observed features of the Mn2+ ions incorporation into ultrasmall cZnS:Mn NCs cannot be explained by the trapped dopant model used to describe the incorporation of the impurity ions in larger (d > 3 nm) colloidal II−VI semiconductor nanocrystals. They can be understood with the ELDA mechanism of doping, which is expected to act from the early stages of the NCs growth, simultaneous with the formation of planar stacking defects emerging in dislocation steps at the NCs surface. The dislocation steps act as trapping sites for the Mn2+ ions which are incorporated in the cZnS NCs during their further growth. Such defects have been observed by HRTEM in the investigated cZnS NCs, and are quite common in NCs of II−VI compounds with zinc-blende structure. The presence of a limited number of available trapping sites/dislocation steps on the surface of the NC explains the observed high doping efficiency of the Mn2+ ions in the core of cZnS NCs at low nominal concentrations and the strong decrease at higher nominal concentrations. At higher nominal concentrations a larger percentage of the Mn2+ ions are either adsorbed on the NCs surface at the end of the growth process, or form agglomerates in the resulting mesoporous material. The ELDA doping mechanism is not size or shape dependent, therefore sizable amounts of Mn2+ ions are incorporated in the core of cZnS NCs smaller than 2 nm.

ACKNOWLEDGMENTS Research performed in the frame of the CNCSIS-UEFISCDI project PNII-IDEI-74/2011. The authors are grateful to D. Zernescu for expert technical assistance.

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Doping Ultrasmall Cubic ZnS Nanocrystals with Mn2+ Ions over a

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