Temperature- and Mn2+ Concentration-Dependent Emission

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Temperature- and Mn2+ concentration-dependent emission properties of Mn2+-doped ZnSe nanocrystals Xiaoli Yang, Chaodan Pu, Haiyan Qin, Shaojie Liu, Zhuan Xu, and Xiaogang Peng J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b08480 • Publication Date (Web): 16 Jan 2019 Downloaded from http://pubs.acs.org on January 16, 2019

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Journal of the American Chemical Society

Temperature- and Mn2+ Concentration-dependent Emission Properties of Mn2+-doped ZnSe Nanocrystals Xiaoli Yang1, Chaodan Pu1*, Haiyan Qin1, Shaojie Liu1, Zhuan Xu2, Xiaogang Peng1* 1Center for Chemistry of High-Performance & Novel Materials, Department of Chemistry, Zhejiang University,

Hangzhou 310027, P. R. China 2Department

of Physics, Zhejiang University, Hangzhou 310027, P. R. China

ABSTRACT: Mn2+-doped ZnSe nanocrystals (Mn:ZnSe d-dots) with high optical quality—high dopant emission quantum yield with mono-exponential dopant-emission decay dynamics—enable systematic and quantitative studies of temperature- and Mn2+ concentration-dependent optical properties of the dopant emission, especially its relationship with magnetic coupling. While temperature-dependent steady-state and transient dopant emission of d-dots with dilute Mn2+ concentrations originated from isolated Mn2+ ions, and can be quantitatively treated as a result of exciton-phonon coupling of isolated paramagnetic emission centers. Dopant emission of d-dots with high Mn2+ concentrations (up to 50% of Zn2+ ions being replaced by Mn2+ ions in the core nanocrystals) are found solely related to magnetically coupled Mn2+ emission. Magnetic coupling effects on steady-state dopant emission of d-dots with high Mn2+ concentrations are much stronger than those observed for doped bulk semiconductors, which is found to follow a strong and universe shell-thickness dependence for the epitaxial ZnSe and/or ZnS shells of the d-dots. By exciting the magnetically coupled Mn2+ ions directly, dopant-emission of d-dots with high Mn2+ concentrations exhibit mono-exponential decay dynamics. In addition to this emission channel, a minor channel with slightly longer decay lifetime appears when the host nanocrystals with high Mn2+ concentrations are excited, which is barely visible at room temperature and increases its fraction by decreasing temperature.

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INTRODUCTION Colloidal semiconductor nanocrystals doped with transition metal ions (d-dots) have been explored as key materials for spintronics, 1 solar concentrators, 2-3 super resolution imaging, 4 nanoscale thermometers, 5-7 and lifetime multiplexing. 8 Most of potential applications of d-dots rely on magnetic and optical properties of the transition metal ions, especially interplay of two types of properties. For example, luminescence of Mn2+ doped ZnSe nanocrystals (Mn:ZnSe d-dots) comes from the 4T1-6A1 d-d transition of a Mn2+ ion upon excitation of either the host or the Mn2+ ion.

The characteristic long luminescence decay lifetime (microsecond to

9-10

millisecond) is due to the spin-forbidden 4T1-6A1 d-d transition of Mn2+ ions.

11-12

The spin-forbidden nature

also results in neglected absorption coefficient of the dopant transition, which implies a large Stokes shift of d-dots. 13-14 With multiple dopant ions in one crystal, especially within a nanocrystal, adjacent Mn2+ ions may become magnetically coupled with each other. This results in antiferromagnetic alignment of their ground state, 15

which split their 6A1 state into six states (S = 0 to 5). Similarly, their 4T1 excited state would also split to four

states (S = 1 to 4).

16

This means that magnetic coupling between Mn2+ ions would partially relieve spin-

forbidding of the 4T1-6A1 transition,

17

transfer between coupled Mn2+ ions,

resulting in reduced luminescence decay lifetime,

18

and possible new relaxation pathways.

19

16

possible energy

This would further cause

concentration-tunable optical properties for d-dots, given magnetic coupling between Mn2+ ions within a nanocrystals being strongly distance-dependent.

Though effects of magnetic coupling on dopant-emission properties have been widely observed for both bulk and nanocrystalline semiconductors, these interesting effects have not been well understood.

20-27

For doped

bulk semiconductors, commonly applied techniques were difficult to eliminate dopant ions on the surface, heterogeneous interior distribution of dopant ions, and different bonding environments for the dopant ions. 282


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29


As a result, dopant-emission properties of doped bulk semiconductors were quite poor, 23, 30 i.e., low dopant-

emission quantum yield, non-dopant emission bands, and very complex dopant-emission decay dynamics. The poorly controlled structure and optical properties of doped bulk semiconductors made it impossible to systematically and reproducibly study the dopant-emission mechanisms, especially for the ones related to highconcentration dopants. Usually, poor dopant-emission properties were simply attributed to existence of multiple types of emitting species.

Since 1990s, semiconductor nanocrystals with dopant emission have been widely studied. 9, 12-13, 29, 31-43 Rapid development on synthetic chemistry of d-dots has greatly improved control on local structure and chemical environment of dopant ions, which is becoming superior to that in bulk single crystals. By controlled substituting-doping of Mn2+ ions in the tetrahedral lattice sites within the II-VI lattice

44-46

and eliminating

surface doping, pure and reasonably efficient dopant emission was obtained about ten years ago. Norris’

46

and Cao’s

47

14, 47

The

groups independently developed methods to place isolated Mn2+ ions at different

positions of a d-dot. In 2014, the Gamelin’s group 16 reported nearly mono-exponential dopant-emission decay dynamics for Mn:ZnSe d-dots with very low dopant concentrations, which should be the intrinsic dopantemission decay channel for substitution-doping of an isolated Mn2+ ion into the ZnSe lattice. They also confirmed that the multi-exponential photoluminescence decay dynamics of d-dots with relatively high dopant concentrations (up to 3.5% Zn2+ sites being replaced by Mn2+ ions) was a combination of coupled Mn2+-Mn2+ emission and isolated Mn2+ emission. Their results suggest that transient photoluminescence spectroscopy is a powerful tool to explore nature of the emitting species for Mn2+ doped semiconductors.

Recently, Pu et al. developed a new synthesis scheme for Mn:ZnSe and Mn:ZnSe/ZnS core/shell d-dots with 3



controllably tuned Mn2+ concentration up to 50% counted by the cations within a Mn:ZnSe d-dot.

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8

The

resulting d-dots showed high dopant-emission quantum yield (70-90%) and nearly mono-exponential dopantemission decay dynamics. Here, we shall apply this type of high quality d-dots to systematically explore relationship between dopant emission and magnetic coupling. High-quality d-dots enable us to reliably and reproducibly study steady-state dopant-emission, electron paramagnetic resonance, and transient dopantemission in the temperature range between 15 and 350 K. Controlled epitaxy of the shells makes it possible to evaluate the surface effects. High-concentration of Mn ions within a d-dot allows direct excitation of the Mn centers. All of these new studies help to quantitatively re-examine important phenomena reported literature. In addition, some new phenomena are also discovered, such as strong shell-thickness dependence of the interior dopant emission and excitation-dependent dopant-emission decay dynamics.

RESULTS Optical and magnetic properties of Mn:ZnSe/ZnS core/shell d-dots. Mn:ZnSe/ZnS core/shell d-dots with well-controlled and tunable Mn2+ concentrations are synthesized using the recently reported methods 8, 48 with some modifications. For the samples shown in Figure 1, the first step of this method yields Mn:ZnSe d-dots (4.4 nm in diameter) with homogenous dopant ion distribution (Figure S1, Supporting Information). X-ray powder diffraction patterns of Mn:ZnSe d-dots shows that all these samples are with the same crystal structure but the diffraction peaks slightly shift to the small angle direction when Mn2+ concentration increases (Figure S2, Supporting Information), consistent with the slightly increased lattice constants for zinc-blende MnSe relative to that of zinc-blende ZnSe. To remove surface dopant ions, specifically for the samples in Figure 1, the second step is to grow ~2 monolayers of pure ZnSe shells onto the d-dots from the first step. In the third step, ~7 monolayers of ZnS shells (final size around 10.5 nm) are further epitaxially grown onto the d-dots to 4


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ensure stability and durability. The final sizes of the core/shell d-dots with different Mn2+ concentrations are maintained the same at each step for the samples in Figure 1 (Figure S1, Supporting Information). In case of necessity, we may use Zn1-xMnxSe/yZnSe/zZnS to denote d-dots with the Mn to Zn ratio being x to (1-x)— defined by the first step—in the core d-dots, y monolayers of pure ZnSe shells onto the cores, and z monolayers of additional ZnS shells. To confirm the targeted Zn1-xMnxSe/2ZnSe/7ZnS core/shell/shell structure without Mn2+ ion diffusion into the shells, an etching protocol is applied to remove the shell layers controllably at room temperature using a mixture of octylphosphonic acid and hydrochloric acid. The etched d-dots with a targeted size are isolated from the etching solution and digested for Atomic Absorption measurements to determine their Zn2+ to Mn2+ mole ratio. Results in Figure S3 (Supporting Information) confirm the gradual removal of the shells from the original d-dots and all Mn2+ ions are found to be within the core nanocrystals.

Figure 1a illustrates UV-Vis absorption and steady-state photoluminescence spectra of the Zn1xMnxSe/2ZnSe/7ZnS

d-dots with x = 0.1%, 15%, and 50%. While the UV-Vis spectra are similar to each other,

with sight broadening by increasing the Mn2+ ion concentration. The broadening may be a result of perturbation of Mn2+ ions to the electronic structure of ZnSe, given nearly monodisperse size distribution of all samples (Figure S1, Supporting Information). Photoluminescence spectrum of the d-dots with 0.1% Mn2+ concentration—with less than one Mn2+ ion per dot—shows a small band-edge peak (around 430 nm) besides the dopant emission (around 600 nm). In comparison with the standard 4T1-6A1 transition in pure ZnSe lattice (around 580 nm), the dopant emission in Figure 1a shifts to low energy, which should be a result of lattice compression caused by the ZnS shells. 38 Photoluminescence excitation (PLE) spectra of dopant emission at different wavelengths are identical (Figure S4, Supporting Information), indicating homogeneous environment of dopant ions within a dot and among all d-dots in a given sample. Typical quantum yields of the dopant 5



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emission studied here are 70-90%.

Figure 1b shows that the dopant-emission decay dynamics at room temperature could be fitted with monoexponential function for all d-dots dissolved in hexane within 3-4 orders of magnitude, indicating nearly uniform decay pathway of 4T1-6A1 transition at room temperature (see more discussion later). Monoexponential decay lifetime of the d-dots with very dilute Mn2+ concentration (x = 0.1%) is ~1042 μs, which is consistent with 4T1-6A1 transition of isolated Mn2+ ion within the d-dots.

8, 16

As the Mn2+ concentration

increases, the mono-exponential lifetime decreases sharply (Figure 1b), which is consistent with dopant

Figure 1. (a) Absorption spectra (black lines) and photoluminescence (PL) spectra (red lines) of three typical samples. (b) Dopant-emission decay curves of the same samples, with their monoexponential decay lifetime as 1042 μs (x = 0.1 %), 241 μs (x = 15%), and 85 μs (x = 50%). (c, d, e) Representative EPR spectra of the three samples at different temperatures. (f) Temperaturedependent double integral intensity of the ESR signals. Inset is the close-up view at low intensities. emission from magnetically coupled Mn2+ ions. 16 Furthermore, dopant emission decay dynamics of a given sample at room temperature is found to be identical at different wavelengths.

6


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The results described in the above paragraphs are consistent with the well-controlled nanocrystal structure, doping-site homogeneity, and homogeneous distribution of dopant ions within a d-dot as well as among all ddots within a sample. 8 Specifically, when the dopant concentration is high, the dopant emission is from the coupled Mn2+ centers and, within 3-4 orders of magnitude, no isolated Mn2+ centers are observable, which offer unique probes for understanding magnetic coupling and dopant emission.

Electron paramagnetic resonance (EPR) spectroscopy is sensitive to unpaired electrons 49 and has been widely used to explore local environment of Mn2+ ions in doped semiconductors. 13, 31, 50-60 The EPR spectrum of Zn1xMnxSe/2ZnSe/7ZnS

with x = 0.1% at room temperature clearly reveals hyperfine structure of isolated Mn2+

ion in the tetrahedral crystal field (Figure 1c). 13, 31, 58 Upon decreasing temperature, profile of the EPR spectra remains the same but their intensity increases dramatically (Figure 1c), which is expected for isolated Mn ions. 61

EPR spectra change drastically upon increasing the Mn2+ concentration (compare Figures 1c-e). At room temperature, the hyperfine structure observed for isolated Mn2+ ions in tetrahedral lattice disappeared for two samples with high Mn2+ concentrations. Instead, a broad signal is observed in Figures 1d and 1e. Linewidth of the broad signal increases as Mn2+ concentration increases. In literature, such a broad signal in EPR spectra has been attributed to Mn2+-Mn2+ coupling. 62 Its linewidth increases as the ordered coupling length increases. 56

Thus, the room-temperature EPR spectra in Figures 1c-e suggest that magnetic coupling within a d-dot

appears and the coupling length increases as the Mn2+ concentration within a dot increases. Increase of the coupling order length by increasing the Mn2+ concentration is consistent with the homogeneous distribution of the Mn2+ ions within a dot. 7



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Upon decreasing temperature, EPR spectra of the d-dots with x = 15% and 50% are also found to be different from those of the d-dots with x = 0.1%. Evidently, the hyperfine structure becomes slightly visible at low temperatures for the high Mn2+ concentration samples though their intensity is very low (Figures 1d and 1e), which should be caused by a very small amount of isolated Mn2+ ions within a sample. In addition to this minor change of spectral contour, for x = 50% sample, intensity and bandwidth of the broad band decrease by decreasing the temperature, instead of increase observed for the hyperfine structure (see more discussion below).

Quantitatively, intensity of EPR signal can be obtained by integrating the original EPR spectra twice. To make comparison meaningful, the integrated intensity in Figure 1f is normalized by dividing it with the amount of Mn2+ ions per d-dot. The resulting integrated intensity per ion is proportional to the magnetic susceptibility of the sample. 61

Results for the sample with x = 0.1% in Figure 1f can be fitted with the Curie's law with a small deviation (Figure S5, Supporting Information), which indicates paramagnetism for this sample.63 Temperaturedependent magnetism with either zero field cooled or field cooled measurements overlaps with each other, further confirming that the Mn2+ ions in the x = 0.1% sample are dominated by the isolated ones (Figure S5, Supporting Information). In comparison with that for the x = 0.1% sample, Figure 1f reveals very different patterns for the doubly integrated intensity of EPR curves for the samples with x = 15% and x = 50%. At zero temperature, the integrated intensity per Mn2+ ion is proportional to the Mn2+ ions in the isolated form and has no contribution from those being in antiferromagnetic coupling. 64 Assuming 100% of Mn2+ ions in the x = 0.1% 8


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sample being paramagnetic (isolated) at 0 K, one would find that the percentage of isolated Mn2+ ions is only 0.2% for the x = 0.15% sample and 0.01% for the x = 50% sample, respectively.

As temperature increases, integrated intensity for the x = 50% sample in Figure 1f decreases slightly and then increases steadily (Figure 1f inset). As a result, the intensity is noticeably higher than that for the x = 0.1% sample at room temperature, which could not be explained by either paramagnetism or anti-ferromagnetism. Considerable amounts of reports have noted room-temperature ferromagnetism in Mn2+ doped semiconductors. Ohno et al. demonstrated that existence of holes in semiconductors gave rise to ferromagnetism for bulk Mn2+ doped GaAs and ZnTe.

65

Dzyaloshinsky et al. showed that, for bulk Mn2+ doped CdTe, the anisotropic

antiferromagnetic coupling could cause ferromagnetism, 66 whose exchange integral was about 5% compared to that of antiferromagnetic coupling. 67

In summary, regardless of temperature, the Mn2+ ions in the very diluted sample (x = 0.1%) are found to be in isolated form and exhibit paramagnetism. In the entire temperature range, magnetic coupling between Mn2+ ions is always in place for high-concentration samples (x = 15% and 50%). These results are found to be consistent with the structure and dopant-emission properties discussed above, especially the transient dopantemission in Figure 1b. Furthermore, they offer a basis for understanding temperature-dependent optical properties to be discussed below.

Temperature-dependent steady-state photoluminescence. In order to collect photoluminescence spectra at low temperatures, samples in the form of solid thin film are prepared by sandwiching the d-dots between two sapphire sheets. Absorption and steady-state photoluminescence spectra of the resulting thin films are 9



Page 10 of 38

confirmed to be the same as those for the corresponding solution samples. Dopant-emission decay dynamics remains nearly mono-exponential but the lifetime is shortened by about 1/3 in the thin films (Figure S6, Supporting Information). Shortening of dopant-emission decay lifetime is expected,

68

given the refractive

index of the thin film being significantly higher than that of the solution, i.e., 2.30-2.60 for bulk ZnSe/ZnS and 1.35 for hexane. 8, 69-71

Figures 2a-2c illustrate three series of representative steadystate photoluminescence spectra for three typical samples studied in Figure 1, i.e., the same core size and shell structure but with different Mn2+ concentrations in the core (x = 0.1%, 15%, and 50%). Details on temperature dependence of three series of dopant-emission

spectra

are

included

Figure

S7

(Supporting

in

Information).

It

Figure 2. (a-c) PL spectra at different temperatures for Zn1xMnxSe/2ZnSe/7ZnS d-dosts with x = 0.1%, x =15%, and x = 50%, respectively. (d and e) Temperature dependence of the peak position and FWHM of the three samples. ∆Emax represents the maximum peak shift relative to the room-temperature peak position and ∆FWHMmax is the maximum deviation of FWHM from the phonon-broadening. (f) The ∆Emax and ∆FWHMmax of Zn0.5Mn0.5Se/yZnSe/zZnS d-dots, with y and z being respectively the thicknesses of the ZnSe and ZnS shells and total shell thickness = y + z.

should be noted that, because absorption spectra of these nanocrystals—or photoluminescence excitation (Figure S4, Supporting Information)—would shift significantly as temperature changes, absolute intensity of the dopant emission in 10


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Figures 2a-2c and left panel of Figure S7 (Supporting Information) could not represent the real trend of temperature-dependent photoluminescence intensity. For this reason, we shall concentrate on temperature dependence of the peak position and full width at half maximum (FWHM) of three series of dopant emission. Results in Figures 2a-c show that these parameters for the samples with high Mn2+ concentrations respond to temperature change in a manner very different from that for the d-dots with 0.1% of Mn2+. It should be mentioned that evolution of the steady-state and transient photoluminescence spectra for a given sample, no matter what Mn2+ concentration is, is fully reversible during the heating-cooling cycles. This not only indicates excellent durability of the d-dots but also imply non-existence of extra carriers during our measurements. 72

Temperature-dependent photoluminescence properties of undoped ZnSe/ZnS core/shell nanocrystals are recorded as a reference. Evidently, emission properties of the bandgap emission of the ZnSe/ZnS core/shell nanocrystals (Figure S8, Supporting Information) are similar to those of intrinsic semiconductor nanocrystals 73

but qualitatively different from those of the dopant emission (Figure 2).

Figures 2d and 2e respectively summarize evolution of the peak shift and FWHM of dopant emission for the three samples by varying the temperature. For the d-dots with very low Mn2+ concentration (x = 0.1%), dopantemission peak monotonically shifts to blue for ~10 meV from 300 K to 15 K. Simultaneously, the FWHM decreases by ~90 meV steadily from 300 K to ~100 K and slightly from 100 K to 15 K. According to the magnetic measurements discussed above, the dopant emission centers—Mn2+ ions—are isolated and paramagnetic in nature for this specific sample. Thus, the slight shift of dopant emission should be a result of shrinkage of the zinc-blende lattice in the given temperature range (Figure S9, Supporting Information), 33 and the change of FWHM should be mostly due to phonon broadening

(Figure 2e, black line). 23, 74-75

11



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The drastic difference in both non-monotonous peak shift (Figure 2d) and evolution of FWHM (Figure 2e) between the samples with low and high Mn2+ concentrations implies that the exciton-phonon coupling cannot be the only dominating factor for the temperature-dependent optical properties of the d-dots with high Mn2+ concentrations. Noticeably, though qualitatively similar temperature dependence of peak shift and variation of FWHM of dopant emission was reported in literature for the bulk semiconductors doped with Mn2+ ions, 23-25, 76

the maximum peak shift relative to the room-temperature peak position (∆Emax)—the depth of the dip in

Figure 2d—was found to be much smaller for the bulk cases (40 meV for Zn0.5Mn0.5Se) 76 than what observed here (70 meV for the 15% sample and 101 meV for the 50% sample). Similarly, the maximum deviation of FWHM from the phonon-broadening (∆FWHMmax) shown in Figure 2e (10 meV for the 15% sample and 52 meV for the 50% sample) is significantly more pronounced than that for the bulk counterparts (10-20 meV for Zn0.5Mn0.5Se). 76 For the definition of ∆FWHMmax, we take the phonon-broadening function as the reference, because all three sets of data in Figure 2e possess similar values at both high- and low-temperature limits (Figure 2e).

In literature, appearance of peaks in temperature evolution for both dopant-emission peak and FWHM was considered to be associated with magnetic coupling between Mn2+ ions.

23

Consistent with the expectation,

results in Figures 2b-2e indicate that the Mn2+-Mn2+ distance is positively related to both maximum values in peak position and FWHM. For the samples studied here, distance between Mn2+ ions within a d-dot is mostly determined by their concentration within the core of a given dot.

Lattice stress

77

and surface structure

78

have also be reported to affect the Mn2+-Mn2+distance and magnetic 12


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coupling. To exclude two extrinsic effects—non-uniform lattice stress and non-ideal surface structure—from the temperature-dependent properties of the dopant emission, d-dots with identical core (50% Mn2+ ion) but different ZnSe and ZnS shell thicknesses are studied (Figure S10, Supporting Information). Both ZnS and ZnSe shell would make Mn2+ ions in the core farther away from the inorganic-organic interface, but ZnS has ~6% smaller lattice constant and hence introduce more lattice stress than ZnSe.

Typical temperature-dependent spectra of dopant emission for these samples are illustrated in Figure S11 (Supporting Information), and the corresponding temperature evolution of dopant-emission peak and FWHM is summarized in Figure S12 (Supporting Information). Evidently, all these samples are qualitatively similar to each other and different from the x = 0.1% sample. Figure 2f reveals that both ∆Emax and ∆FWHMmax of all samples—including the standard Zn0.5Mn0.5Se/2ZnSe/7ZnS d-dots—fall into the same trendlines. This means that the results in Figures 2d and 2e should not have been affected by lattice stress. Furthermore, the monotonic increase trends upon increase of the shell thickness regardless of the shell materials suggest that the distance of Mn2+ ions from the inorganic-organic interface plays a noticeable role in changing ∆Emax and ∆FWHMmax though it does not dedicate appearance of the maximums. Son et al. reported that lattice strain would influence temperature dependence of the dopant emission in d-dots.

77

It should be noted that our results may not be

considered to be against their observation. This is so because, in their work, change in lattice strain was always coupled with change of the distance of Mn2+ ions from the inorganic-organic interface.

The slopes of the curves in Figure 2f suggests strong surface effects, even for shell thickness more than 15 monolayers. Because the surface effects expand to multiple monolayers and are independent on the outer shell materials, it is unlikely due to surface disorder within the ZnSe (or ZnS) outer monolayer. It is known that the 13



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surface disorder—if any—should be limited to 1-2 monolayers from the inorganic-organic interface for high quality II-VI semiconductor nanocrystals.

79

In addition, transmission electron microscope measurements

reveal that, in comparison with the d-dots with pure ZnSe shells, there are noticeably more surface imperfections for those with ZnS outer shells (Figure S10, Supporting Information). However, their ∆Emax and ∆FWHMmax practically follow the same trends in Figure 2f, indicating insignificant contribution by the surface disorder.

Figures 2f and S10 (Supporting Information) further indicate that, by increasing the ZnSe shell thickness, the appearance temperature for ∆Emax and ∆FWHMmax changes significantly. These large surface effects could –at least partially—explain why both ∆Emax and ∆FWHMmax observed for doped bulk semiconductors are much smaller. For typical doped bulk semiconductors, distance between Mn2+ ions and the surface was usually not intentionally controlled. Noticing their poor dopant-emission properties at room temperature, such as very low emission quantum yield and complex emission decay dynamics, one would suspect that a good portion of the dopant ions were on/near the surface.

80

Consequently, the surface distance-dependent peak position should

reduce ∆Emax and ∆FWHMmax in the bulk samples.

14


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Temperature-dependent dopant-emission decay dynamics. Though steady-state dopant emission of doped bulk semiconductors is often suffered from low quantum yield and undetermined photoluminescence peaks, 76, 81

studies on its temperature dependence are still possible at a semi-quantitative level. Conversely, decay

dynamics of dopant emission for doped bulk semiconductors is too complex to be applied for significant temperature-dependent studies. 23, 25 To the best of our knowledge, systematic temperature-dependent studies using transient luminescence for d-dots with variable Mn2+ concentrations have not been reported, which should mainly be a result of complex luminescence decay dynamics for most d-dots in literature.

Figure.3. (a, b, c) Dopant-emission decay curves measured at the peak position between 15K and 350K for three typical samples of d-dots. d, Temperature-dependent decay lifetime values for samples obtained by mono-exponential (x = 0.1%). (e, f) Temperature-dependent fractions of two components for samples obtained by double-exponential fitting (x = 15% and 50%).

Figures 3a-c show representative spectra of temperature-dependent transient dopant emission for three typical samples illustrated in Figure 1. All decay curves are collected at the peak position at the corresponding temperature and the host is excited for the results in Figures 3a-c. The dopant-emission decay dynamics of the 15



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0.1% sample in the entire temperature range can be well fitted with mono-exponential, with goodness-of-fitting being ~1.90 with 5000 counts. The mono-exponential decay lifetime is 632 μs across the temperature range in the solid film (Figure 3d). This is consistent with nearly pure paramagentism of the Mn2+ ions in this sample revealed by Figure 1c in the temperature range. These results further indicate that the monotonic temperaturedependence of both emission peak position and peak width for the 0.1% sample (Figures 2d and 2e) are completely associated with the temperature-dependence of lattice constants and are not related to different dopant-emission channels of Mn2+ ion emission centers.

The dopant-emission decay dynamics of both x = 15% and x = 50% samples at room temperature in the solid films can be fitted reasonably with mono-exponential (Figure S13, Supporting Information). However, an additional component with longer lifetime appeared at low temperature. It is possible to fit all dopant-emission decay curves for a specific sample with two fixed channels, which results in graduate increase of the minorcomponent fraction as the temperature decreasing (Figures 3e and 3f).

Origin of two decay channels for the dopant emission with high Mn2+ ion concentrations. Evidently, for high-concentration samples, the lifetime for both main and minor channels at all temperatures is much shorter than that in Figure 3a. This means that either channel should not be related to isolated/paramagnetic Mn2+ ions, which is consistent with the magnetic measurements shown in Figure 1. In literature, spin-lattice coupling was often proposed to vary the lifetime of dopant-emission decay dynamics. 19 Because, in the entire temperature range, the mono-exponential decay lifetime of the main component remains the same and the minor one only increases its fraction by decreasing temperature (Figure 3), both major and minor components should not be due to temperature-dependent spin-lattice coupling. 16


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In principle, appearance of a new emission channel should be during relaxation of the exciton of the host to the 4T1 state and the subsequent 4T1-6A1 transition of a Mn2+ ion, which can be separated in three sequential steps. The first step is transfer of the photo-generated exciton in the host onto a Mn2+ ion within the given ddot, the second step is relaxation of the excited state of the Mn2+ ion into its 4T1 state, and the final step is the 4T -6A 1 1

transition with photon emission. Mathematically, if the first and second steps are the same, it can be

proven that the final step would not result in a new channel.

82

Instead, any additional pathway(s)—either

radiative or non-radiative—in the final step alone would only decrease the mono-exponential lifetime.

By directly exciting the Mn2+ ion within a d-dot, it is possible to distinguish contribution from the first and second steps discussed in the above paragraph. Though the transition between d orbitals of a Mn2+ ion is spinforbidden, it becomes observable for the d-dots with high Mn2+ concentrations (Figures S3 and S14, Supporting Information). The resulting steady-state spectra of the dopant emission in the entire temperature range are similar to those obtained by exciting the ZnSe host (Figure S15, Supporting Information).

Figures 4a-4c illustrate representative dopant-emission decay dynamics with four different excitation wavelengths (350, 465, 500, 530 nm) for the x = 50% sample. Evidently, in the entire temperature range, decay dynamics curves obtained by direct excitation of the Mn2+ ions at three wavelengths (465 nm, 500 nm, and 530 nm) always overlap with each other, each of which can be well fitted with mono-exponential function between 40-300 K and with some deviation from mono-exponential at very low temperatures (see Figure 4c). Since these three wavelengths referred to different excited states of Mn2+ ions, it is safe to conclude that relaxation from high-energy excited states of Mn2+ ions to their 4T1 state should be much faster than the 17



Page 18 of 38

microsecond time scale.

With the main channel being overlapped with the decay channel for directly exciting the Mn2+ ions, excitation at 350 nm—overwhelmingly dominated by excitation of the host—brings in a long-lifetime channel in all temperatures (Figure 4a-c). This suggests that the major decay channel of dopant emission demonstrated in Figure 3 for the samples with high Mn2+ concentrations should be equivalent to excitation of the Mn2+ ions and the minor decay channel is likely associated with excited-state transfer from the host to the dopant-emission centers.

Figure 3 reveal that, by exciting the host, lifetime values of the minor decay channels for both samples with high Mn2+ concentrations remain constant in the entire temperature range. For example, in

Figure 4. (a, b, c) Dopant-emission decay curves of Zn1xMnxSe/2ZnSe/7ZnS d-dots with excitation of the ZnSe host (350 nm) and the Mn2+ ions (465, 500, and 530 nm) respectively at the temperature of 290 K, 40 K, 15 K for x = 50%. (d) The dopant-emission decay curves of d-dots with x =50% under 350 nm excitation in hexane, toluene and film. The inset figure is the dopant-emission decay curves of samples with x = 0.1% in different media. (e) The fitting results of dopant-emission decay curves of samples with x = 50% in (d).

Figure 3c, the long-lifetime part of all decay curves is parallel to each other though the contribution of this minor channel increases somewhat by decreasing the temperature. These results could exclude energy transfer between Mn2+ ions for appearance of the minor channel observed by exciting the host of d-dots with high dopant concentrations. It was reported that such energy transfer would be sensitive to temperature.

24

In addition, energy transfer is not supported by

18


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emission-wavelength dependence of the dopant-emission decay dynamics. In principle, energy transfer would result in shortened lifetime of donor and lengthened lifetime of the corresponding acceptors, which is not observed by our measurements (Figure S16, Supporting Information). 83

Interestingly, Figures 4d and 4e reveal that the minor channel observed by exciting the host of the d-dots with high Mn2+ concentration is insensitive to the dielectric environment while the corresponding major channel is strongly responsive to the dielectric environment. To illustrate the opposite changes quantitatively, the detailed fitting results are summarized in Figure 4e. Further experiments confirm that, by exciting the host of the ddots with low Mn2+ concentration, its mono-exponential decay dynamics of dopant emission of isolated Mn2+ ions also varied significantly with the dielectric environment (Figure 4d, inset). The refractive indices of the dielectric environment applied for the measurements are varied for the experiments related to Figure 4d, namely hexane, toluene, and solid film being respectively 1.35,

71

1.50,

84

and 2.30-2.60.

69-70

In principle,

lifetime of radiative transition would increase as the reflective index of dielectric environment decreases, 68, 85 which is consistent with both major decay channel of the d-dots with high dopant concentration (Figures 4d and 4e) and the mono-exponential decay of the d-dots with low dopant concentration (Figure 4d, inset). The insensitivity of the minor channel observed by exciting the host of the d-dots with high dopant concentration suggests that its lifetime is not dominated by the radiative recombination. Instead, its long lifetime should be mostly dictated by a non-radiative process. Recently, Gamelin et al. demonstrated that exciton transfer from the host to Mn2+ ions could go through charge-separated states, such as host+-Mn+ pair. 19 Such transient states would then be transformed to Mn2+-Mn2+ excited states for generating dopant emission. This might be a plausible mechanism for the minor channel observed for exciting the host with high Mn2+ concentrations because probability on formation of host+-Mn+ pairs should increase rapidly as the Mn2+ concentration 19



Page 20 of 38

increases.

To summarize the results in this subsection, the main channel for excitation of the host is indistinguishable from the sole channel for excitation of the Mn2+ ions on the emission peak position, decay dynamics, and temperature-dependence. Thus, it should be safe to conclude that this is the intrinsic decay process of the 4T16A

1

transition of magnetically coupled Mn2+ ions within a d-dot with the given Mn2+ concentration. The minor

channel of dopant emission of the d-dots with high Mn2+ concentrations differs from the main channel in the step of energy transfer from the host to the magnetically coupled Mn2+ ions.

DISCUSSION At very low Mn2+ concentration (x = 0.1%), EPR measurements reveal that the ions are paramagnetic (Figure 1c). As a result, the dopant emission of the x = 0.1% sample is consistent with isolated-ion emission, whose emission properties can be well described by the transition between 4T1 (S = 3/2) state to 6A1 (S = 5/2) state of a paramagnetic center. The temperature-dependence of emission properties—slight decrease/narrowing of emission energy/peak width upon decreasing temperature and temperature independent dopant-emission decay dynamics—can be well accounted by simple exciton-phonon coupling (Figures 2a, 2d, 3a, and 3d). 75

20


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The optical properties of magnetically coupled Mn2+ ions are largely different from those of the isolated ones, which should be resulted from their magnetically coupled electronic structure. It is known that magnetic coupling would split both 4T1 and 6A1 electronic states of Mn2+ ions (Figure 5). Figure 5 shows that the 4T1 to 6A

1

transition would become partially allowed if the transition is between a pair of split energy levels—one

from 4T1 and the other from 6A1—with the same S. 20 As a result, the dopant-emission decay lifetime would become shortened in comparison to that from an isolated Mn2+ ion (Figure 1b). 8, 12, 16 If all Mn2+ ions in a d-dot are magnetically coupled together, one would thus observe mono-exponential decay of the dopant emission. Furthermore, magnetic coupling should increase by increasing the Mn2+ concentration, Figure 5. Schematic energy levels of Mn2+-Mn2+ dimers (the ferromagnetic coupling of the excited state and the anti-ferromagnetic coupling of the ground state). Two of four transitions between the same S are shown as vertical red arrows. Four different types of processes are shown in the plot, ① and ② different pathways for transferring excited state from the host to Mn2+ ions, ③ spin relaxation, ④ radiative relaxation.

which results in significant dependence of the dopantemission decay lifetime on the Mn2+ concentration within a d-dot (Figure 1b). 56, 86

It is well known that the magnetic coupling of 6A1 states

between

antiferromagnetic,

neighboring 87

Mn2+

ions

is

which is consistent with our

magnetic measurements shown in Figure 1. The 6A1 state would be split into 6 levels with different S and energy in a Mn2+-Mn2+ pair (the bottom section in Figure 5). Antiferromagnetic coupling implies that for the lowest split state, the spins of neighboring Mn2+ ions are anti-parallel aligned though five electrons within one Mn2+ ion are aligned parallel to each other. 15 With external magnetic field in place, each of these six levels 21



Page 22 of 38

would be further split to (2S + 1) levels, and electrons would be filled into these magnetic-field split levels according to temperature. At 0 K, electrons shall fill the lowest energy level (S = 0) and thus be fully antiferromagnetic. Above Néel temperature, a good portion of electrons shall fill into upper magnetic-field split levels following Boltzmann distribution, which results in paramagnetic response in magnetic measurements.

88

However, with/without magnetic field, antiferromagnetic coupling shown in Figure 5

(bottom section) is in place even at room temperature. This means in such samples, one should not regard Mn2+ ions in the d-dots with high Mn2+ concentrations as isolated ones at room temperature.

Nature of magnetic coupling of excited states of a Mn2+-Mn2+ pair still remains controversial. In literature, the proposed mechanism for temperature dependence of the steady-state dopant emission was usually based on the energy change between antiferromagnetically coupled Mn2+ ions at low temperature and isolated Mn2+ ions at room temperature. This picture does not match the magnetic-coupling picture in literature

23, 76

and the

magnetic properties in Figure 1. Magnetic measurements suggest that antiferromagnetic coupling is always in place in the entire temperature range tested, which would result in a constant energy gap between four sets of spin-allowed transitions and temperature-independent steady-state dopant-emission (Figure S17, Supporting Information). Alternatively, Gamelin et al. proposed ferromagnetic coupling between 4T1-6A1 states. 16 If the magnetic coupling between 4T1-6A1 states was ferromagnetic (Figure 5), the transition energy would be larger for the transitions between states with lower S. This picture is found to be consistent with the temperaturedependent absorption properties of the dopant transitions reported in literature. 25 As mentioned above, upon reducing temperature, reduced exciton-phonon coupling and changes in the crystal field experienced by the Mn2+ ions would shift the peak to red and reduce the peak width significantly for the dopant emission in all types of d-dots. 75 As discussed below, Figure 5 would offer a reasonable mechanism for the blue shift of the 22


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dopant emission of the high-concentration samples at low temperatures.

The total splitting energy for Mn2+-Mn2+ dimer is about 20-30 meV among 6A1 orbitals and 10-20 meV among 4T orbitals, and these 1

would be larger when more Mn2+ ions are coupled together. 20 This means that population

of those states in either 6A1 or 4T1 orbitals would be sensitive to the temperature change even if no magnetic field is applied. At room temperature, Boltzmann distribution shall allow significant population among all six states of 6A1. However, Boltzmann distribution would gradually shift the population to low S orbitals of 6A1 as temperature decreased. 17, 59 When excitation is directly related to Mn2+ ions, instead of the host, partially allowed transitions between a pair of energy levels with the same S shall dominate the excitation. 17 This would result in high population of low S orbitals. Below ~100K, Boltzmann distribution among four states with different S of 4T1 (Figure 5) immediately after excitation would become gradually dominated by S = 1 (Figure S18, Supporting Information). Assuming S of the excited states would retain partially before dopant emission, this would change the trend of the peak energy of the dopant emission from decrease to increase as the temperature decreases, which is observed in Figure 2. This is so because the energy gap between the pair of states with S = 1 in 4T1 and 6A1 is the largest. Simultaneously, reduction of number of pairs of states with the same S involved in dopant emission would decrease the peak width as well.

The picture illustrated in the above paragraph builds on an important assumption, that S of the excited state(s) would partially retain before dopant emission at low temperatures. Though this assumption needs to be confirmed in the future, it seems to be reasonable because of two facts. Firstly, this assumption is only needed at low temperature (below ~100 K). Secondly, the relaxation between a group of energy levels —such as the ones in 4T1 in Figure 5—requires changing of S. 89 23



Page 24 of 38

There is one more issue needs to be noted. In literature, the sharp decrease of dopant-emission energy between ~200 and ~80 K for doped bulk semiconductors was qualitatively explained by the picture outlined in the above paragraphs.

17, 25

However, results here indicate that the red-shift is too large to be accounted by the

model (Figure S19, Supporting Information). In addition, the plateau of dopant-emission peak width between ~200 and ~120 K is also difficult to be accounted by this simple model. Some other factors are needed to be understood before a fully quantitative picture can be in place. As discussed above, magnetic coupling is the basis for understanding optical properties of d-dots and doped bulk semiconductors. Results in Figure 2f, however, suggest that there are some unknown surface effects related to the distance between the magnetically coupled Mn2+ ions and inorganic-organic interface. Moreover, the current model is based on Mn2+-Mn2+ pair interaction, but, for samples with high Mn2+ concentrations, coupling based on Mn2+ clusters should be taken into account.

CONCLUSION In conclusion, high quality d-dots are excellent model systems for systematically and quantitatively studying interplay between optical and magnetic properties of transition metal doped semiconductors. While d-dots with low Mn2+ concentration exhibit emission features with paramagnetic centers, d-dots with high Mn2+ concentrations exhibit substantially different emission properties, which is elaborated by temperaturedependent measurements on magnetic properties, steady-state photoluminescence, and transient photoluminescence using high-quality d-dots. For those properties qualitatively observed for doped bulk semiconductors with high Mn2+ concentration in literature, studies on high quality d-dots offer quantitative 24


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information. In addition, new phenomena are also discovered by studying high quality d-dots. For instance, by comparing excited-state decay dynamics with excitation of the host and dopant ions, two excited-state transfer mechanisms from the host to the magnetically coupled Mn2+ emission centers at the timescale of microsecond are identified. Results included here not only are of significance for understanding and developing high-quality d-dots, but also shed new light on understanding magneto-optical interactions in dilute magnetic semiconductors in general.

EXPERIMENTAL SECTION Chemicals. 1-octadecence (ODE, 90%), oleic acid (HOl, 90%), stearic acid (HSt), tetramethylammonium hydroxide (25% w/w in methanol), zinc stearate (ZnSt2, ZnO 12.5%~14%), selenium powder (Se, 100 mesh, 99.999%), manganese chloride (MnCl2, 99.999%), zinc carbonate hydroxide (97%) were purchased from AlfaAesar. Oleylamine (NH2Ol, 70%), sulfur powder (S, 99.998%) were purchased from Sigma-Aldrich. Octylphosphonic acid was purchased from Energy Chemical. Acetone, toluene and hydrochloric acid aqueous solution were purchased from Sinopharm Chemical Reagents. All chemicals were used directly without any further purification unless otherwise stated. Manganese stearate (MnSt2) and zinc oleate (ZnOl2) were synthesized following a previous reported method. 8

Synthesis of d-dots. A series of d-dots, including three typical samples discussed here, were synthesized using a slightly modified method reported previously.

8

Zn1-xMnxSe core d-dots were prepared by injecting Se

powder suspended in ODE (Se-SUS, 0.25 mol/L) into a hot solution of ZnSt2 and MnSt2 with designated ratio in 5mL ODE at 310 oC. In order to obtain homogeneously doped nanocrystals, the amount of selenium was always in excess, and oleylamine was used as ligands to stabilize the resulting d-dots. When the total amount of cationic precursors was 0.05 mmol and the fraction of Mn precursor was 0.1%, 15%, 50%, the amount of oleylamine and selenium was (0.2 ml, 0.2 mmol), (0.4 ml, 0.35 mmol), and (0.35 ml, 0.3 mmol), respectively.

25



Page 26 of 38

Subsequently, pure ZnSe shells were grown onto the Zn1-xMnxSe core d-dots. For growth of the first two monolayers of the ZnSe shells, the reaction temperature was selected as 290 oC. Without purification, the first two monolayers of ZnSe was grown by directly introducing ZnSt2 into the reaction mixture to react with the residual selenium from the formation of the Zn1-xMnxSe core d-dots. If the ZnSe shells were more than 2 monolayers, additional Se-SUS was added before the injection of ZnSt2. Such additional growth was carried out at further reduced reaction temperature (270 oC) to avoid diffusion of the dopant ions into the pure ZnSe shells.

If ZnS shells were grown, the reaction solution was maintained at 260 °C and Zn:S stock solution was added dropwise. The Zn:S stock solution was prepared by mixing 0.1mmol ZnOl2 and 0.11mmol sulfur in 1ml ODE. The thickness of the ZnS shells was controlled by the amount of the Zn:S stock solution added.

Obtained d-dots were precipitated using acetone and re-dispersed in 4 mL toluene for further use.

Etching of d-dots. For a typical etching process, octylphosphonic acid (0.3 mmol), 0.3 mL tributylephosphine, and the purified d-dots solution (200 μL) were added into 8 mL toluene. Into the toluene solution, 1.5 μL of hydrochloric acid aqueous solution (1.2 mol/L) was added. The etching process was monitored using absorption and photoluminescence spectroscopy, which usually lasted for 15 minutes to completion. If needed, addition of the hydrochloric acid solution was repeated. When the d-dots were etched to the designated size, 16 mL methanol was added to precipitate the d-dots. Into the isolated precipitate, 700 μL aqua regia was added to digest the d-dots. Distilled water was added to adjust the concentration of the digested solution for atomic absorption spectroscopy measurements.

Optical Measurements. UV-vis absorption spectra were measured on an Agilent Jena S600 UV-vis spectrophotometer. Photoluminescence and photoluminescence excitation spectra were recorded on an Edinburgh Instruments FLS920 fluorescence spectrometer. The transient photoluminescence spectra were measured by multi-channel scaling (MCS) module of an Edinburgh Instruments FLS920 system, and the samples were excited with a µF920H Xe flash lamp at a selected wavelength and a repetition rate of 50 Hz. Solution samples were prepared by dispersing needle-tip aliquots during reaction in hexane. The absolute PL 26


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QY was measured using an Ocean Optics FOIS-1 integrating sphere coupled with a QE65000 spectrometer.

Quantum dots films were made by spin-casting the toluene solution of quantum dots onto a sapphire base, covered with another piece of sapphire. The film samples were fixed with an indium holder in a vacuum chamber equipped with closed-cycle He cryostat and a heating element. The temperature was controlled by a Lakeshore Model 330 temperature controller and detected with a calibrated silicon diode attached to the indium holder. Temperature dependent emission properties of the films were measured using the same system as above.

Transmission electron microscopy (TEM) images were collected on a Hitachi 7700 microscope at 100 kV. The specimens were prepared by dropping a mixture of toluene and hexane solution containing the d-dots onto a copper grid coated with a thin carbon support film.

X-ray powder diffraction (XRD) patterns were collected on a Rigaku Ultimate-IV X-ray diffractometer operating at 40 kV/30 mA using Cu Kαradiation (λ = 1.5418 Å). The d-dots for XRD measurements were first precipitated from reaction solution by acetone, followed by dispersing in toluene and precipitating with methanol for three times and then transferred onto a glass slide for measurements.

Electron paramagnetic resonance (EPR) spectra were collected on an X-band (9.4GHz) EMX plus spectrometer (Bruker) at the Steady High Magnetic Field Facilities, High Magnetic Field Laboratory, CAS. The temperature variation in the range of 15 k and 360 K was performed by helium gas flow technique.

Magnetic susceptibility data for Zn1-xMnxSe/2ZnSe/7ZnS nanocrystals film were collected using a superconducting quantum interference device (SQUID-VSM). All data were corrected for the diamagnetism of the substrate and sample holder. Atomic absorption spectra were collected on a thermos M6 atomic absorption spectroscopy.

AUTHOR INFORMATION Corresponding Authors 27



Page 28 of 38

[email protected] [email protected]

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work is supported by the National Key Research and Development Program of China (2016YFB0401600), Joint NSFC-ISF Research (Grant 21761142009), and China Postdoctoral Science Foundation (2016M601930). A portion of this work was performed on the Steady High Magnetic Field Facilities, High Magnetic Field Laboratory, CAS.

ASSOCIATED CONTENT Supporting Information Available: Additional optical, structural information and result of simulation. This material is available free of charge via the Internet at http://pubs.acs.org.

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Temperature- and Mn2+ Concentration-Dependent Emission

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