Effect of Different Manganese Precursors on the Doping Efficiency in

Nov 29, 2010 - Vincent C. Holmberg , Justin R. Helps , K. Andre Mkhoyan , and David J. Norris ... Andrew W. Wills , Malcolm G. Thomas , and David J. N...
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J. Phys. Chem. C 2010, 114, 21969–21975

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Effect of Different Manganese Precursors on the Doping Efficiency in ZnSe Nanocrystals Lijun Zu,†,§ Andrew W. Wills,† Thomas A. Kennedy,‡ Evan R. Glaser,‡ and David J. Norris*,†,| Department of Chemical Engineering & Materials Science, UniVersity of Minnesota, 421 Washington AVenue SE, Minneapolis, Minnesota 55455, and NaVal Research Laboratory, Washington, D.C. 20375, United States ReceiVed: July 15, 2010; ReVised Manuscript ReceiVed: October 9, 2010

We prepare ZnSe nanocrystals in the presence of several different Mn precursors to investigate the incorporation of Mn as an intentional impurity. Four different Mn sources, including dimethylmanganese, tricarbonyl methylcyclopentadienylmanganese, Mn(II) cyclohexanebutyrate, and Mn2(µ-SeMe)2(CO)8 are tested in a standard hot-injection organometallic synthesis. The resulting ZnSe nanocrystals can exhibit two optical emission peaks: one related to the incorporation of Mn at around 585 nm and one due to electron-hole recombination in the nanocrystal at around 400 nm. We use the photoluminescence intensity ratio of these two peaks to extract information about the doping efficiency. These results are further verified with inductively coupled plasma atomic emission spectroscopy (ICP) and electron paramagnetic resonance (EPR) spectroscopy. Among the four Mn sources, dimethylmanganese leads to the highest doping efficiency, tricarbonyl methylcyclopentadienymanganese and Mn2(µ-SeMe)2(CO)8 lead to poor incorporation, and Mn(II) cyclohexanebutyrate leads to weak incorporation of Mn that appears to be mostly at or near the nanocrystal surface. Introduction Doping, the intentional incorporation of atomic impurities into a solid, provides an additional means to control the optical, electrical, and magnetic properties of semiconductor nanocrystals.1-7 To understand the effect of doping on these materials, manganese has been extensively studied as a model impurity. The Mn2+ ion can substitute for the cation in the II-VI semiconductor lattice.8 When this occurs, Mn-doped nanocrystals can exhibit interesting magnetic and optical properties that may be useful for various applications, such as biomedical labeling9,10 and spintronics.11-14 Consequently, synthetic efforts to incorporate Mn impurities into a variety of semiconductor nanocrystals, including ZnS,15-23 CdS,24-26 ZnSe,27-33 CdSe,3,34-37 InAs,38 and PbSe39 have been reported. However, in most cases, when the dopant incorporation was carefully analyzed by inductively coupled plasma atomic emission spectroscopy (ICP) and electron paramagnetic resonance (EPR), the concentration of the Mn inside the nanocrystals was reported to be relatively low.3,24,27,31,37 Indeed, in some systems, the Mn was not incorporated at all and instead sat on the surface of the nanocrystals or in the surrounding matrix.34 These observations have led to interest in understanding the doping process itself. One of the key variables in the synthesis of Mn-doped nanocrystals is the Mn source that is employed. In prior work, several different molecular precursors have been used. One would expect that the choice of precursor could influence the amount of dopant that is incorporated. Consequently, this issue has been examined by several studies. For example, the doping of CdSe nanocrystals grown in the presence of several different Mn precursors was compared.34 However, in this early work, the nanocrystals were exposed to trioctylphosphine oxide (TOPO) during the synthesis, which is now known to inhibit * To whom correspondence should be addressed. E-mail: [email protected]. † University of Minnesota. ‡ Naval Research Laboratory. § Current address: 3M Center, Bldg. 0201-04-N-01, St. Paul, Minnesota 55144, United States. | Current address: ETH Zu¨rich, CH-8092 Zu¨rich, Switzerland.

Mn incorporation.3,29 In another report, the effect of the precursor was studied in Mn-doped InAs nanocrystals.38 While this work made conclusions about which Mn precursor led to the best doping, in hindsight, its EPR data suggest that the amount of Mn incorporated was probably much lower than reported. A more recent study, which used CdS nanocrystals and separated dopant incorporation from particle nucleation and growth,40,41 found that the doping efficiency varied when two different Mn precursors were employed. Manganese diethyldithiocarbamate bound more strongly to the nanocrystal surface than manganese acetate, and this enhanced doping. This confirms that the Mn precursor can indeed play an important role in the doping process. Obviously, this role should be understood if the doping efficiency is to be optimized. Here, our goal is to provide additional information about the influence of different Mn precursors when used within a standard hot-injection synthesis of nanocrystals. During the course of other experiments in our laboratory over the past dozen years, we have tested the ability of numerous Mn precursors to dope ZnSe nanocrystals within such a synthesis. Here, we collect some of these data, which we have not previously reported, and compare four different Mn sources that had been used by others to dope nanocrystals such as CdSe,34,35 ZnSe,27,28 InAs,38 and PbSe.39 We employed each precursor within a synthetic procedure that utilizes weak binding ligands,27 an approach that can incorporate Mn successfully with dimethylmanganese (MnMe2) as the dopant source. We also collected absorption, photoluminescence (PL), ICP, and EPR results to quantify the doping concentration of Mn inside the nanocrystals. However, because these results were obtained over many years, some of the experimental conditions (namely, concentrations) vary. While these variations can complicate the analysis, which we discuss further below, we believe that the comparisons are still useful. In particular, our results show significant differences between precursors, including effects on the nanocrystal growth rate, the dopant location, and the doping efficiency.

10.1021/jp106594n  2010 American Chemical Society Published on Web 11/29/2010

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Experimental Section ZnSe Nanocrystals with MnMe2. Mn-doped ZnSe nanocrystals were prepared using a published procedure.27 Briefly, 15 mL of distilled 1-hexadecylamine (HDA, Aldrich) was degassed under vacuum for 3 h and then heated to 310 °C in a reaction flask under dry nitrogen. Simultaneously, 4 mL of technical-grade trioctylphosphine (TOP, Aldrich), 2.4 mL of 1 M Se (2.4 mmol, Alfa Aesar) in TOP, 82 µL of diethylzinc (ZnEt2, 0.8 mmol, Strem), and 0.5 mL of freshly prepared 0.04 M dimethylmanganese (MnMe2, 0.02 mmol) were mixed in a 10 mL syringe in the glovebox. MnMe2 was synthesized following literature methods.42 The syringe was then removed from the glovebox and injected rapidly into the HDA, which was being vigorously stirred. Right after the injection, the reaction temperature fell to 260 °C. The reaction flask was then returned to 300 °C to promote steady growth of the nanocrystals. This involved increasing the temperature in 5-10 °C increments approximately every 10-20 min. Ultraviolet-visible (UV-vis) absorption spectra were measured on small aliquots diluted with hexane that were taken from the reaction solution at regular intervals to monitor the growth. To grow particles larger than ∼3 nm in diameter, a secondary precursor solution containing half the amount of the initial injection solution, was added dropwise at 270 °C when the first absorption peak reached 389 nm. Once the desired size was achieved, the reaction solution was cooled to 90 °C, and the particles were purified according to standard methods.43 ZnSe Nanocrystals with Tricarbonyl Methylcyclopentadienylmanganese (TCPMn). The procedure for MnMe2 was used with the following changes. A precursor mixture of TOP (4 mL), 1 M Se in TOP (2.4 mL, 2.4 mmol), ZnEt2 (82 µL, 0.8 mmol), and TCPMn (3.16 µL, 0.02 mmol) was injected rapidly into 15 mL of HDA at 310 °C. No secondary precursors were added. ZnSe Nanocrystals with Mn2(µ-SeMe)2(CO)8. The procedure for MnMe2 was used with the following changes. A precursor mixture of TOP (3 mL), 1 M Se in TOP (2.4 mL, 2.4 mmol), ZnEt2 (61.5 µL, 0.6 mmol), and Mn2(µ-SeMe)2(CO)8 (16 mg, 0.03 mmol) was injected rapidly into 15 mL of HDA at 310 °C. Mn2(µ-SeMe)2(CO)8 was synthesized according to literature methods.44 When the first absorption peak reached 384 nm, the secondary precursor was added dropwise. ZnSe Nanocrystals with Mn(II) Cyclohexanebutyrate. The procedure for MnMe2 was used with the following changes. A precursor mixture of TOP (4 mL), 1 M Se in TOP (0.6 mL, 0.6 mmol), ZnEt2 (61.5 µL, 0.6 mmol), and Mn(II) cyclohexanebutyrate (11.8 mg, 0.03 mmol, Alfa Aesar) was injected into 15 mL of HDA at 310 °C. No secondary precursors were added. This reaction had an initial Se/Zn concentration ratio of 1:1. To examine the effect of this ratio, we also show results below for a precursor mixture of TOP (4 mL), 1 M Se in TOP (2.4 mL, 2.4 mmol), ZnEt2 (61.5 µL, 0.6 mmol), and Mn(II) cyclohexanebutyrate (11.8 mg, 0.03 mmol), which represents an initial Se/Zn ratio of 4:1. This mixture was injected into 15 mL of HDA at 310 °C, and secondary precursors were not added. Characterization. Optical absorption spectra were obtained with an AIS model UV-2D lamp and an Ocean Optics SD 200 spectrometer, using 1 cm pathlength quartz cells. Photoluminescence spectra were collected with a Spex Fluorolog-2 spectrofluorometer equipped with two monochromators (doublegrating, 0.22 m, SPEX 1680) and a 450 W xenon lamp as the excitation source. For all optical characterization, 1 mL of the reaction solution was removed, precipitated with methanol, and

Zu et al. TABLE 1: Reactant Concentrations for Mn-Doped ZnSe Nanocrystals Prepared Using Different Mn Precursors Mn precursor

Se concentration (M)

Se/Zn ratio

MnMe2 TCPMn Mn2(µ-SeCH3)2(CO)8 Mn(II) cyclohexanebutyrate Mn(II) cyclohexanebutyrate

0.109 0.112 0.117 0.031 0.112

3:1 3:1 4:1 1:1 4:1

Mn/Zn ratio 2.5% 2.5% 10% 5% 5%

redispersed in hexane to obtain an optical density of around 0.2 at 300 nm. The measurements were performed at room temperature. EPR spectra were collected on a Varian E109Q spectrometer operating at 35 GHz. The magnetic field was swept from 1.2 to 1.3 T at room temperature. Samples were prepared by dispersing the nanocrystals in a poly(lauryl methacrylate) film, which was placed in a quartz tube and centered in the EPR cavity. ICP results were collected on a Perkin-Elmer Optima 3000DV after dissolving the nanocrystals in aqua regia and diluting the resulting ions to ∼10 ppm in a solution of 5% HCl and 1% HNO3. Before characterization with ICP and EPR, all samples were precipitated and repeatedly washed with either pyridine or pyridine/TOPO to remove excess Mn at the surface of the particles. Samples doped via MnMe2 and Mn cyclohexanebutyrate were dispersed in pyridine at 60 °C for one hour and then precipitated with hexanes. This cycle was repeated two more times. Additional samples doped via Mn cyclohexanebutyrate and Mn2(µ-SeMe)2(CO)8 were dispersed in pyridine at 60 °C for one hour, precipitated with hexanes, dispersed in TOPO for one hour at 60 °C, and then precipitated with methanol. This cycle was repeated two more times. Results and Discussion To compare the influence of the different Mn precursors on the doping efficiency, we analyze results from samples synthesized using each of the dopant sources. While the reactants (other than the Mn precursors) and the initial reaction temperature were the same in the different syntheses, some variations were present. For example, as in most hot-injection nanocrystal reactions, the temperature was increased manually to maintain steady growth of the particles. This introduces unavoidable differences in the time-temperature profile for each reaction. Perhaps more significantly, variations in the reactant concentrations are also present in the available data. Table 1 summarizes the Se, Zn, and Mn conditions for each of the syntheses. Two important differences should be noted. First, for most of the data, the initial Se/Zn ratio was essentially constant, either 3:1 or 4:1. However, for Mn(II) cyclohexanebutyrate, our most complete data set came from a synthesis with an initial Se/Zn ratio of 1:1. Previously, we showed that changing the Se/Zn ratio from 4:1 to 1:1 in the case of MnMe2 could decrease the doping efficiency by up to a factor of 2 in large ZnSe nanocrystals.3,29 One might then expect that the doping efficiency for Mn(II) cyclohexanebutyrate would be lower simply due to the Se/Zn ratio. To examine this question, below we also present some data for Mn(II) cyclohexanebutyrate at 4:1 and compare it directly with the 1:1 data. The second important difference seen in Table 1 is that the initial Mn/Zn ratio varied. For MnMe2, TCPMn, Mn2(µ-SeMe)2(CO)8, and Mn(II) cyclohexanebutyrate, the ratio was 2.5, 2.5, 10, and 5%, respectively. Of course, all of these variations should be kept in mind when the quantitative results are discussed.

Doping Efficiency in ZnSe Nanocrystals

Figure 1. (a) Absorption and (b) photoluminescence spectra for a growth series of Mn-doped ZnSe nanocrystals prepared with MnMe2. More reactants were added dropwise at 270 °C when the lowest energy absorption peak reached 389 nm. The photoluminescence spectra are normalized with the blue emission feature of the ZnSe. Each sample in (a) and (b) is labeled by the wavelength of its lowest energy absorption peak, which can be used to determine the nanocrystal size (see ref 27).

Figure 1 shows the absorption and photoluminescence spectra for a typical growth series of Mn-doped ZnSe nanocrystals synthesized using MnMe2. Because the lowest energy absorption peak shifts to longer wavelengths as the particles grow (Figure 1a), the absorption data give an indication of the nanocrystal size. At the same time, the photoluminescence can provide evidence about the incorporation of Mn. Undoped ZnSe nanocrystals exhibit a blue feature around 400 nm due to the recombination of the lowest energy electron-hole pair.27,28 When Mn is introduced, this feature is joined by an orange emission peak around 585 nm in the PL spectra (Figure 1b). This arises due to the internal electronic transition (4T1 f 6A1) in the Mn2+ ion. During optical excitation of the nanocrystal, energy from the electron-hole pair can be transferred to the Mn nearby, leading to orange emission. Because this process is more efficient for the Mn ions that are inside the nanocrystal than those located on the surface or in the surrounding matrix, the intensity of the Mn emission feature can provide a reasonable (although not foolproof) measure of dopant incorporation, especially if procedures to remove external Mn are utilized. Figure 1b shows PL spectra that are normalized to the intensity of the blue emission of ZnSe nanocrystals. As the blue emission peak shifts to longer wavelengths with increasing nanocrystal size, the intensity of the Mn emission increases dramatically. Therefore, the intensity ratio of the Mn emission to the blue emission increases with nanocrystal growth, suggesting a rise in the average number of Mn ions incorporated per nanocrystal.27,29

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Figure 2. (a) Absorption spectra for a growth series of Mn-doped ZnSe nanocrystals prepared with TCPMn. (b) Photoluminescence spectrum for a single size of such nanocrystals with the lowest energy absorption peak at 385 nm. The Mn emission feature is identified by the downward-pointing arrow. Each sample in (a) is labeled by the wavelength of its lowest energy absorption peak, which can be used to determine the nanocrystal size (see ref 27).

Figure 2 shows similar data for ZnSe nanocrystals synthesized with TCPMn as the dopant source. In the earliest aliquot, the lowest energy absorption peak (Figure 2a) occurred at 339 nm, which is significantly blue-shifted when compared with the MnMe2 synthesis. In other words, the nanocrystals started out smaller (i.e., they were growing more slowly) with TCPMn present. The peak at 339 nm was also narrower than the peak from the earliest samples that could be obtained using MnMe2, which indicates that the first aliquot from the TCPMn synthesis exhibited a tighter size distribution. However, as the reaction proceeded and the lowest energy absorption peak reached 364 nm, the size distribution for the TCPMn sample worsened, and the growth stalled. Because previous experiments had shown that a secondary addition of precursors did not restart the growth of particles grown in the presence of TCPMn, further precursors were not added, and nanocrystals with an absorption peak at 385 nm were the largest obtained. These results indicate that, compared to MnMe2, the presence of TCPMn in the reaction solution inhibited nanocrystal growth, presumably due to interference at the nanocrystal surface. The PL spectrum for the largest nanocrystals in Figure 2a is shown in Figure 2b. A Mn emission feature is found around 582 nm, almost the same wavelength observed for nanocrystals prepared with MnMe2. However, the intensity of the Mn emission is lower than that of the blue emission (Figure 2b). In contrast, for nanocrystals made with MnMe2 that have the same size (exhibiting a blue emission peak at 385 nm), the Mn emission is stronger than that of the blue emission (Figure 1b).

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Figure 3. (a) Absorption and (b) photoluminescence spectra for a growth series of Mn-doped ZnSe nanocrystals prepared with Mn2(µSeMe)2(CO)8, plotted as in Figure 1. More reactants were added dropwise at 270 °C when the lowest energy absorption peak reached 384 nm. The Mn emission feature is identified by the downward pointing arrow.

Specifically, the integrated intensity ratio of the Mn emission to the blue emission was 0.50 when TCPMn was used as the Mn source compared to 1.87 for MnMe2 as the precursor. This suggests that the average number of Mn incorporated per nanocrystal is reduced for TCPMn. When Mn2(µ-SeMe)2(CO)8 is used as the dopant source, growth behavior that is similar to that of MnMe2 was obtained. Figure 3 shows the resulting optical spectra. Unlike the case of TCPMn, Mn2(µ-SeMe)2(CO)8 does not have an adverse effect on the nanocrystal growth. When the lowest energy absorption peak reached 384 nm, additional precursors were added dropwise into the reaction solution, and the growth proceeded without difficulty. As in both cases above, the PL spectra of nanocrystals made with Mn2(µ-SeMe)2(CO)8 (Figure 3b) again show an orange emission peak indicating the incorporation of Mn2+ ions. However, similar to the TCPMn precursor, the intensity of the Mn emission is lower than that of the blue emission, even for the largest particles. This occurred despite the fact that the Mn concentration in the reaction had been increased by a factor of 4 compared to MnMe2 and TCPMn. In fact, one might worry that this Mn concentration is too high. If Mn incorporation is extremely efficient, the Mn emission might be weak due to Mn-Mn quenching.45 Thus, our photoluminescence results would not accurately reflect the doping efficiency for Mn2(µ-SeMe)2(CO)8. However, this is unlikely for two reasons. First, we have never been able to observe such quenching with any of our precursors in a hot-injection synthesis, presumably because even in the best case we cannot incorporate enough Mn into the nanocrystals to observe strong

Zu et al.

Figure 4. (a) Absorption and (b) photoluminescence spectra for a growth series of Mn-doped ZnSe nanocrystals prepared with Mn(II) cyclohexanebutyrate, plotted as in Figure 1. The Mn emission feature is identified by the downward pointing arrow. The initial Se/Zn concentration ratio in the reaction solution was 1:1. The initial Mn/Zn concentration was 5%.

Mn-Mn interactions. Second, and more importantly, further analysis with ICP and EPR (discussed below) confirms that the incorporation of Mn using Mn2(µ-SeMe)2(CO)8 is indeed low. Nanocrystals prepared using Mn(II) cyclohexanebutyrate (Figure 4a) grew much more rapidly than in the case of MnMe2. Consequently, the peaks in the UV-vis were broad indicating a much larger size distribution. In addition, PL spectra for nanocrystals grown with Mn(II) cyclohexanebutyrate (Figure 4b) show a yellow emission feature around 572 nm due to the Mn, instead of ∼585 nm in the three cases above. Similar PL spectra have previously been reported for Mn-doped ZnSe nanocrystals prepared with the same dopant source.28 The shorter wavelength of the emission may be because the Mn ions locate near the nanocrystal surface. Generally, the energy levels of a transition metal ion are influenced by its crystal field. We will return to this issue during the discussion of our ICP and EPR results below. The Mn(II) cyclohexanebutyrate results shown in Figure 4 come from samples grown with an initial Se/Zn concentration ratio of 1:1. As mentioned above, this ratio can lead to lower incorporation of the Mn into the nanocrystals. To quantify this effect, Figure 5 compares photoluminescence spectra obtained for two samples of nearly identically sized Mn-doped ZnSe nanocrystals. Data from Figure 4 is plotted with another sample grown with an initial Se/Zn concentration ratio of 4:1. As expected, the higher Se/Zn ratio does increase the doping efficiency; the intensity ratio of the Mn emission to the blue emission rises from 1.34 at 1:1 to 1.52 at 4:1. However, because

Doping Efficiency in ZnSe Nanocrystals

J. Phys. Chem. C, Vol. 114, No. 50, 2010 21973 TABLE 2: Se/Zn Ratio and Mn Concentration (Atomic Percent Relative to Zn) Determined by ICP for Mn-Doped ZnSe Nanocrystals Prepared Using Different Mn Precursorsa sample 1 2 3 4

Mn precursor

wash method

Dimethylmanganese (MnMe2) no wash pyridine Mn(II) cyclohexanebutyrate no wash pyridine Mn(II) cyclohexanebutyrate no wash pyridine/TOPO Mn2(µ-SeCH3)2(CO)8 no wash pyridine/TOPO

Se/Zn Mn/Zn ratio ratio 0.99 0.79 0.87 0.41 0.85 1.08 0.91 0.98

0.32% 0.50% 6.19% 6.27% 1.92% 0.16% 0.78% 0.22%

a

Both samples prepared with Mn(II) cyclohexanebutyrate utilized an initial Se/Zn concentration ratio of 1:1. Figure 5. Photoluminescence spectra for two different samples of Mndoped ZnSe nanocrystals prepared with Mn(II) cyclohexanebutyrate. The initial Se/Zn concentration ratio was 1:1 (solid line) and 4:1 (dashed line). The initial Mn/Zn concentration was 5% in both.

this effect (a 14% increase) is very small when compared to the much larger differences between MnMe2 (Figure 1) and Mn(II) cyclohexanebutyrate (Figure 4), we believe conclusions drawn from our Mn(II) cyclohexanebutyrate data set at a ratio of 1:1 are still relevant. Consequently, for the rest of our discussion below, we only consider the 1:1 data for this precursor. As discussed above, the intensity ratio of the orange emission to the blue emission (IMn/IZnSe) can provide information about the dopant concentration. If this ratio is higher in one sample compared to another of equivalent nanocrystal size, it suggests that the concentration of Mn2+ in the nanocrystals has increased. Indeed, we have previously shown that, within a single growth series using MnMe2, this ratio is completely consistent with a more detailed analysis of the Mn concentration in ZnSe nanocrystals.29 Thus, it is reasonable to use this ratio to compare the doping efficiency of different Mn precursors, at least qualitatively. For Mn-doped ZnSe nanocrystals prepared using TCPMn, Mn(II) cyclohexanebutyrate, or Mn2(µ-SeMe)2(CO)8 as the Mn source, this ratio is significantly lower than for those prepared using MnMe2 under similar conditions (see Figures 1b-4b). For Mn(II) cyclohexanebutyrate and Mn2(µSeMe)2(CO)8, this occurred even though the initial Mn concentration had been increased by 2 and 4, respectively. Therefore, the doping efficiency is apparently lower when TCPMn, Mn2(µ-SeMe)2(CO)8, and Mn(II) cyclohexanebutyrate are used, compared with MnMe2. To confirm these findings, ICP and EPR data were collected for samples from three of our precursors: MnMe2, Mn(II) cyclohexanebutyrate, and Mn2(µ-SeMe)2(CO)8. These were chosen for further testing as they all allowed growth of larger nanocrystals (>3 nm in diameter), which makes the ICP/EPR analysis easier. ICP can quantify the amount of Zn, Se, and Mn in the samples. EPR can determine the location of the Mn, either substitutional in the lattice or nonsubstitutional. In the latter case, the Mn is presumably outside the nanocrystal on its surface or in the surrounding matrix.46 Data were obtained from four samples. The first was made with MnMe2, the second and third with Mn(II) cyclohexanebutyrate (both at a Se/Zn ratio of 1:1), and the fourth with Mn2(µ-SeMe)2(CO)8. In all four, the first absorption peak occurred between 390 and 395 nm. For each sample, the nanocrystals were divided into two portions (see Table 2). One was purified with the standard precipitation procedure, and the other was washed further by

either pyridine [MnMe2 and Mn(II) cyclohexanebutyrate] or pyridine and TOPO [Mn(II) cyclohexanebutyrate and Mn2(µSeMe)2(CO)8] to remove any Mn impurities remaining on the surface or in the surrounding matrix of the nanocrystals. ICP analysis was done for both portions of each sample. Initially, we planned to also examine a sample made with MnMe2 after washing with both pyridine and TOPO, but our data showed that pyridine was already sufficient for removing Mn outside the nanocrystals for this precursor. Table 2 shows the Se/Zn ratio and the percentage of Mn (relative to Zn) in each sample before and after the pyridine or the pyridine/TOPO wash. For all samples, the Se/Zn ratio fluctuated slightly, perhaps due to the loss of Se as H2Se during the acid digestion step. This fluctuation is particularly evident for sample 2 for unknown reasons. For samples 1 and 2, the percentage of Mn (relative to Zn) increased slightly after the pyridine wash, compared to the unwashed samples. This may be explained by removal of the smaller nanocrystals from the size distribution of each sample due to unintentional sizeselective precipitation. Several studies have previously shown that smaller nanocrystals are more difficult to dope.2 Their removal from the sample distribution will increase the effective dopant concentration. Both sample 2 and sample 3 were prepared using Mn(II) cyclohexanebutyrate as the precursor. Sample 2 was washed with pyridine only, while sample 3 was washed with pyridine and TOPO. TOPO preferentially binds to the cation, for example, Zn2+ and Mn2+. Thus, pyridine/TOPO can help remove Mn bound to the surface of the nanocrystals. Indeed, the ICP data also shows that the percentage of Mn (relative to Zn) in samples 3 and 4 dramatically decreased after the pyridine/TOPO wash. EPR was then used to verify the location of the Mn. Figure 6 shows EPR spectra for the four samples after either the pyridine or the pyridine/TOPO wash. Spectrum A was taken for nanocrystals prepared with MnMe2 after pyridine exchange. It shows a well-resolved six-line pattern with a hyperfine splitting constant of 63 × 10-4 cm-1. As previously discussed,46 this value is consistent with that obtained for substitutional doping of Mn on tetrahedral sites in bulk ZnSe crystals (61.7 × 10-4 cm-1).47 Spectrum B was taken for nanocrystals prepared with Mn(II) cyclohexanebutyrate after pyridine exchange. It shows a well-resolved six-line pattern with a much larger hyperfine splitting constant of 91.7 × 10-4 cm-1, which suggests that the Mn2+ ions are mainly bound to the nanocrystal surface or in the surrounding matrix. Spectrum C was taken for nanocrystals prepared with Mn(II) cyclohexanebutyrate after a pyridine/TOPO wash. Although the ICP results show that the pyridine/TOPO removes most of the Mn outside the nanocryst-

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Figure 6. EPR spectra (35 GHz) at room temperature for Mn-doped ZnSe nanocrystals prepared using three different Mn precursors: MnMe2, Mn(II) cyclohexanebutyrate, and Mn2(µ-SeMe)2(CO)8. EPR spectra were taken after the precipitated nanocrystals were washed using either pyridine or pyridine/TOPO, as described in the text. Sample A: MnMe2, pyridine wash; Sample B: Mn(II) cyclohexanebutyrate, pyridine wash; Sample C: Mn(II) cyclohexanebutyrate, pyridine/TOPO wash; and Sample D: Mn2(µ-SeMe)2(CO)8, pyridine/TOPO wash.

als, spectrum C indicates that external Mn still dominates the EPR signal. However, the signal marked by the arrows confirms substitutional incorporation of some Mn into the ZnSe nanocrystal. The weakness of this signal indicates that the concentration of incorporated Mn is relatively low. With this information, we can revisit the photoluminescence spectrum for the Mn(II) cyclohexanebutyrate sample (Figure 4b). The wavelength of the Mn emission peak (572 nm) is shorter in this sample than that observed for Mn-doped bulk ZnSe (585 nm). Indeed, for the smallest nanocrystals (2.7 nm diameter) from this growth, this transition was at 564 nm. Combined with our EPR data (spectra B and C in Figure 6), which indicate that most of the Mn is not sitting at tetrahedral sites inside the nanocrystal, it is reasonable to conclude that this blue shift is caused by Mn that is on or near the nanocrystal surface. In general, when Mn is placed substitutionally on a cation site in a II-VI host lattice, degeneracies in its internal electronic structure are lifted by the crystal field. This produces localized levels (4T1 and 6A1) that are within the energy gap of the ZnSe nanocrystal.8 Energy transfer from the photoexcited nanocrystal to the internal 4T1 to 6A1 transition of the Mn then results in an emission feature around 585 nm. If instead, the Mn sits at or just within the nanocrystal surface, energy could still be transferred from the photoexcited nanocrystal to the Mn. In this case, the crystal field would presumably be altered. Assuming that such Mn sites could still be approximated by tetrahedral symmetry, the observed blue shift would suggest that the crystal field is lowered. The standard Tanabe-Sugano

Zu et al. diagram for a 3d5 ion in tetrahedral symmetry (which is equivalent to octahedral symmetry by group theory) shows that the 4T1-6A1 splitting increases with decreasing crystal field.45 Furthermore, this decrease in crystal field can be rationalized by changes in the nearest neighbors of the Mn, for example, from Se2- to O2- (see ref 45, p 411). However, to explain the shift definitively and quantitatively, other influences such as changes in the Racah parameters and lowering of the symmetry for specific atomic arrangements would need to be considered. Such detailed calculations are beyond the scope of this work. Spectrum D in Figure 6 is for nanocrystals prepared with Mn2(µ-SeMe)2(CO)8 after pyridine/TOPO wash. It shows two six-line patterns with hyperfine splitting constants of 63.3 × 10-4 cm-1 and 89.5 × 10-4 cm-1, respectively. This suggests that this sample contains both substitutional and external Mn. The relative amount of the latter can be approximated by taking the ratio of the signal at 1.274 T, where the external Mn dominates, and at 1.23 T, where the substitutional Mn dominates. After the pyridine/TOPO wash, ∼20% of the total Mn in the sample still remains outside the nanocrystal. The percentages of Mn relative to Zn determined by ICP (atomic percent) before and after correction by EPR to exclude external Mn are listed in Table 3 for three of our samples. The concentration of Mn incorporated into ZnSe nanocrystals prepared with MnMe2, Mn(II) cyclohexanebutyrate, and Mn2(µSeMe)2(CO)8 are 0.5,