Doping Transition Metal (Mn or Cu) Ions in Semiconductor Nanocrystals

Sep 14, 2010 - of Mn3O4 to rock salt MnS is slow at 240 °C (Figure S2 of the. Supporting .... Norris, D. J. Doping Semiconductor Nanocrystals. Nature...
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Doping Transition Metal (Mn or Cu) Ions in Semiconductor Nanocrystals )

Niladri S. Karan,† D. D. Sarma,§ R. M. Kadam, and Narayan Pradhan*,†,‡ Centre for Advanced Materials, and ‡Department of Materials Science, Indian Association for the Cultivation of Science, Kolkata, 700032 India, §Solid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore, India 560012, and Radiochemistry Division, Babha Atomic Research Centre, Mumbai, India 400085

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ABSTRACT Following growth doping strategy and using dopant oxides nanocrystals as dopant sources, we report here two different transition-metal ions doped in a variety of group II-VI semiconductor nanocrystals. Using manganese oxide and copper oxide nanocrystals as corresponding dopant sources, intense photoluminescence emission over a wide range of wavelength has been observed for different host nanocrystals. Interestingly, this single doping strategy is successful in providing such highly emissive nanocrystals considered here, in contrast with the literature reports that would suggest synthesis strategies to be highly specific to the particular dopant, host, or both. We investigate and discuss the possible mechanism of the doping process, supporting the migration of dopant ions from dopant oxide nanocrystals to host nanocrystals as the most likely scenario. SECTION Nanoparticles and Nanostructures

ZnSe) and their alloyed (ZnxCd1-xS and ZnSexS1-x) and core/ shell nanostructures (CdS/ZnS) (Table S2 of the Supporting Information), efficient emission from either Mn or Cu centers is obtained with the excitation of host nanocrystals. Therefore, irrespective of the diversity of literature reports, this synthetic technique helps to synthesize almost all colloidal dispersed doped nanocrystals emitters reported so far with quantum efficiencies comparable to the best values reported until now. Dopant oxide nanocrystals that are used as dopant sources for doping different host nanostructures are synthesized following modified literature reports, purified, and stored for doping (details in Supporting Information, Section 1). Similarly, different host semiconductor nanocrystals and their alloyed and core/shell structures are also synthesized following well-developed synthetic protocols as well as modified methods (Table S2 of the Supporting Information). Figure 1a shows a typical reaction scheme of doping Mn or Cu ions using their oxide nanocrystals as dopant source. Just after the nucleation of the host nanocrystals, dopant oxide nanocrystals are introduced to the reaction mixture at an optimized temperature and annealed. With the progress of the reaction, the dopant emission slowly appears depending on the nature of the dopant ions and host. Following this host, nanocrystals are grown further with periodical injection of the monomer to the reaction system to bury the dopant ions and to intensify the dopant emission.

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ighly luminescent nanomaterials have been attracting interest over the last two decades in view of enormous technological possibilities in various fields, particularly in the field of display or lighting devices and in biological labeling or diagnostic.1-5 Apart from the size tunable excitonic emission from different group II-VI6-9 and III-V10-13 semiconductor nanocrystals, intense, stable, and tunable dopant emissions from different transition-metal ion doped nanocrystals (d-dots) are increasingly being investigated as alternative nanocrystal emitters.14-23 Additionally, these d-dots have several advantages over quantum dots, for example, thermal and environmental stability, large Stoke shifts leading to an avoidance of self-absorption, and higher excited state lifetime.14-23 Among this category, doping of Mn and Cu ions in different zinc chalcogenides is widely studied and reported where Mn results in short-range tunable orangeyellow emission irrespective of the size and nature of hosts and Cu gives a size-tunable wide range of tunable emission depending on the size and nature of the host.14-23 On view of day-to-day literature reports,14-24 it appears that the synthesis, doping mechanism, and associated physical insights of these doped nanocrystals still need detail and further investigations. We have summarized various reports on colloidal semiconductor nanocrystals in Table S1 of the Supporting Information, and it indicates that the literature is diverse on synthesis of different doped nanocrystals following either different chemical synthetic pathway or the doping strategy. We report here a new and more versatile synthetic technique to incorporate transition-metal ions in different semiconductor hosts using their oxide nanocrystals as the dopant source. Introducing manganese and copper oxide nanocrystals during the growth of different semiconductor (ZnS and

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Received Date: August 28, 2010 Accepted Date: September 8, 2010 Published on Web Date: September 14, 2010

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precursors.25 Noncoordinating solvent 1-octadecene (ODE) has been chosen as the reaction solvent. Optimizing the reaction, Mn-doped ZnS stable emission up to 40% has been obtained. Other doped nanocrystals are synthesized following similar synthetic protocols (details in the Supporting Information). The emission from Mn centers from all types of host nanocrystals used here are characterized by the emission centered at ∼585 nm, higher excited-state lifetime of the dopant emission (in 1 to 2 ms, depending on the host), and sextuplet hyperfine splitting of their EPR (electron paramagnetic resonance) spectra. Figure 2a,b shows the representative EPR spectrum of Mn-doped ZnS and Mn-doped ZnSe nanocrystals with hyperfine splitting of 69 and 67 G, respectively, indicating the presence of Mn(II) in these host nanocrystals at the substitutional tetrahedral sites. Similarly, Cu-dopant-related emissions are evidenced by their compositiondependent tunability, higher excited-state lifetime compared with typical lifetime of excitonic and surface state emissions, broad emission spectral nature, and thermal stability (until 300 °C) (Tables S3 and S4 of the Supporting Information). Moreover, these emissions are also supported with different literature reports for different hosts and dopants and also by etching experiment (Supporting Information, Section 4.0). The TEM micrograph of Mn-doped ZnS (Figure 2c) nanocrystals shows that they are spherical in shape, and XRD (Figure 2d) supports the zinc blende crystal structure. Being a unique source of dopant, the role of dopant oxides during the synthesis process needs to be understood because it seems to be the most crucial reactant to control the generic doping process reported here. Dopant oxides Mn3O4 and CuO (supported by TEM and XRD, Figure 3a,b) are synthesized by decomposing their carboxylates in fatty amine. Control experiments showed that the key point to make these dopant oxides as ideal dopant source is the presence of additional or excess of anionic precursors in the reaction system. Hence, the doping strategy and insertion of dopants into the host mostly depend on the anionic precursors that react with these dopant oxide nanocrystals resulting in dopant sulfides (or selenides) in a slow and controlled manner. In the absence of host nanocrystals, dopant chalcogenides, formed by the aforesaid process, are indeed found to precipitate, but in the presence of host nanocrystals, such nearly molecular level

Figure 1b,c shows representative UV and PL spectra of different doped semiconductor nanocrystals that are synthesized by introducing corresponding dopant oxide nanocrystals as the dopant source. The emission spectrum is tunable only with Cu dopant with tuning the size of hosts (Figure 1c, bottom panel), whereas photoluminescence spectra from Mn-doped systems are all centered at ∼585 nm, which is similar to literature reports.14-24 However, for the case of Mn-doped ZnSe, introducing thiols or more S redshifts the emission up to 600 nm, and keeping more cation precursors during synthesis blue-shifts the emission up to 565 nm, similar to a previous report.15,16 In a typical synthesis of Mn-doped ZnS, traditional synthetic technique has been adopted for colloidal ZnS nanocrystals using zinc carboxylate for Zn and elemental sulfur as S

Figure 1. Schematic presentation of synthesis of doped nanocrystals using dopant oxides as dopant source. (b,c) UV-visible and PL spectra of different doped nanocrystals. Bottom panel of (c) shows tunable Cu dopant emission from ZnSe.

Figure 2. (a,b) EPR spectrum of Mn-doped ZnS and ZnSe, respectively. (c) Representative TEM image of Mn-doped ZnS nanocrystals. The scale bar shown is 50 nm. (d) XRD of ZnS, ZnSe, and ZnSe1-xSx host nanocrystals. All crystal phases show the zinc blende crystal structure.

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doped nanocrystals here (Table S2 of the Supporting Information), Cu can also be doped in different alloyed structure of Cd, Zn, and S or Se using copper oxides as precursors. Our preliminary results also support the fact that other transition-metal Ni may be doped in different host semiconductor nanocrystals using nickel oxide nanocrystals as dopant source.

SUPPORTING INFORMATION AVAILABLE Details of the synthetic methods, materials, supporting data, and supporting figures. This material is available free of charge via the Internet at http://pubs.acs.org. Figure 3. (a) Powder XRD pattern of the dopant sources (CuO and Mn3O4). (b) TEM micrograph of Mn3O4 nanocrystals collected after 30 min at 150 °C.

AUTHOR INFORMATION Corresponding Author: *To whom correspondence should be addressed. E-mail: camnp@ iacs.res.in.

metal chalcogenides are adsorbed on host surfaces, leading to a favorable surface adsorption (Figure S1 of the Supporting Information). Therefore, the reaction of dopant oxides to dopant chalcogenides should be targeted on host nanocrystals surfaces, this being the key step for the generic doping strategy presented here. Therefore, the synthetic strategy needs careful manipulation to allow the sulfide or selenide formation on the host surface and not to encourage formation of separate nucleation centers. For the case of Mn, the reaction of Mn3O4 to rock salt MnS is slow at 240 °C (Figure S2 of the Supporting Information), but that for CuO to CuS is much faster. Accordingly, we found that a small amount of CuO should be introduced in a stepwise manner at 180 °C for the doping process while avoiding the formation of separate CuS or CuSe nucleation. Details of the fate of dopant oxides has have been provided in Sections 5.0 and 6.0 of the Supporting Information. When molecular precursors of dopant selenides or sulfides are introduced during the growth of hosts, in some cases, doping has been found to be successful but not for all hosts we have used here (Table S2 of the Supporting Information). We report here specially for copper doping because copper sulfide and selenide tend to precipitate from the solution because their formation is very fast. The present method avoids such rapid growth of dopant chalcogenide nanocrystal, controlling its formation with control of the temperature, concentration of the anion precursor, and the concentration of the oxide nanocrystals, making use of the oxide nanocrystal as a versatile dopant source. Our intuition and experimental observations (Supporting Information, Section 5.0) suggest that even dopant molecular precursors (for example, dopant ion carboxylates) used in several doping processes may turn to their oxide forms first before they are adsorbed as sulfide on the surface of host nanocrystals during growth. In conclusion, we report here a simple and more generic doping method to dope various semiconductor nanocrystals, their alloys, and core/shell structures by using dopant oxide nanocrystals as a dopant source. In the presence of excess anionic precursors, these dopant oxide nanocrystals chemically planted the dopant ions on the surface of different host nanocrystals in a controllable manner. Further growth or tuning the composition of the host intensifies the dopant emission, leading to a variety of doped semiconductor nanocrystals with intense luminescence. Apart from the mentioned

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ACKNOWLEDGMENT N.P. and D.D.S. acknowledge Department of Science and Technology, India. N.S.K. acknowledges Council of Scientific and Industrial Research, N.P. acknowledges J. N. J. Bhilwara, and D.D.S. acknowledges JCBose for fellowships.

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