Article pubs.acs.org/JPCC
Material Diffusion and Doping of Mn in Wurtzite ZnSe Nanorods Shinjita Acharya, Suresh Sarkar, and Narayan Pradhan* Department of Materials Science and Centre for Advanced Materials, Indian Association for the Cultivation of Science, Jadavpur, Kolkata 700032, India S Supporting Information *
ABSTRACT: Light-emitting transition metal ion doped 1D nanorods can be a suitable candidate for fabrication of the advanced opto-electronic-based nanodevices. Among various doped nanocrystals, Mn doped ZnSe nanocrystals are widely studied for their intense Mn d−d emission. However, this is mostly performed in the zinc blende phase of spherical ZnSe quantum dots. But, herein we study the strategy to dope Mn ions in wurtzite phase of 1D ZnSe nanorods. To achieve this, it is essential to control the 1D crystal growth of ZnSe to facilitate the adsorption of dopants. The anisotropic 1D nanostructures are designed following thermally controlled material diffusion process rather than the most widely expected kinetically driven crystal growth protocol, and the dopants are introduced at the appropriate stage of the growth for their adsorption. Using preformed magic size wurtzite ZnSe nanowires as the source material and fragmenting them at higher reaction temperature, ZnSe nanorods with variable aspect ratios are designed. These rods follow both inter- and intrarods material diffusion and retain the wurtzite phase throughout their transformation. This helps in understanding the insertion, adsorption, and retention of dopant Mn in the wurtzite phase of the 1D nanostructure.
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CdSe21 core/shell and ZnS22 nanostructures is also reported. These reports cast confusion on the mechanistic aspect of doping of Mn2+ particularly in wurtzite nanostructures. Although each of the above reports is confined within their specified system with definite phase, shape, and composition, it instigates curiosity to indulge into the science of doping in wurtzite host. Hence, to understand the doping concept in wurtzite nanostructures, it is essential to understand the growth mechanism of different wurtzite nanostructures. Among the well-known semiconductor nanocrystals, ZnSe is one of the leading semiconductor nanomaterial and mostly studied host for Mn doping. However, in nanoscale, ZnSe is a very sensitive material and needs tedious precautions to obtain the high-quality quantum confined nanostructures.3,12,23−25 These very often also develop irregular branching, and its mechanistic aspects of crystal growth are little explored.24,26,27 Apart from the low temperature synthesis of wurtzite thin rods/ wires28 and high temperature synthesis of zinc blende spherical particles,3,12,25,26 there is no such advance in the architecture of wurtzite phase in ZnSe nanostructures. Hence, to explore the aspect of doping in wurtzite nanocrystals, it is extremely crucial to achieve the well-defined anisotropic 1D wurtzite nanorods of ZnSe, which is quite a challenging task. The major concern here is the controlled 1D growth of ZnSe nanostructures and its crystal phase. In general, the crystal growth of 1D nanostructure is predominantly observed in the
INTRODUCTION Intentional inclusion of foreign ions into a semiconductor host commonly known as doping can induce new electronic, optical, and magnetic properties.1−4 Several light-emitting doped materials and dilute magnetic semiconductors3,5−9 with various transition metal ion dopants in a wide range of semiconductor hosts manifesting the dopant induced properties are extensively studied. However, designing these materials by placing odd impurity dopant ions in the host crystal lattice has come across several difficulties, which are often attributed to various intrinsic factors as well as limitations in the chemical synthetic techniques. Moreover, doping is not a random process and is rather specific to the selection of the dopants and the host. It is even more challenging in quantum sized nanomaterials where the crystal growth is confined within a limited size regime. With the advancement of colloidal synthetic methodologies, several doped semiconductor nanomaterials have been reported, and the probable doping mechanism has been derived.3,10−17 For dopant Mn, it has been shown that the key step of doping is its adsorption on the energetically favorable facets of the host crystal.16 Accordingly, zinc blende phase of nanocrystals has been established to be most suitable to adsorb a higher amount of dopants. Further, as doping is a size, shape, and phase-based adsorption/diffusion phenomenon, it is also strongly related to the crystal growth of the host materials. A recent report further furnishes the fact that Mn2+ dopant can even alter the phase of the host from wurtzite to zinc blende18 phase, indicating its preference to retain in zinc blende phase. However, efficient doping of Mn ions (>1%) in spherical wurtzite CdSe,15,19 CdZnS20 alloy nanocrystals, and anisotropic wurtzite ZnSe/ © 2013 American Chemical Society
Received: January 15, 2013 Revised: February 25, 2013 Published: February 25, 2013 6006
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wurtzite phase where oppositely charged ions are adsorbed onto the {002} facets alternatively along the [002] polar direction during growth.22,29−34 This can be achieved usually in two pathways: one is the preferred directional growth in the presence of excess monomers, and the other is the growth of selected facets via material diffusion. The most common protocol is the first one where the size of the nanomaterials increases with the progress of the reaction.22,34,35 But, in the second case, it usually occurs in the absence of monomers or at their reduced concentration.36 According to the classical mechanistic approach, the first pathway is preferably controlled by kinetic factors, and the second pathway is thermodynamically controlled. Since the synthetic control for ZnSe 1D nanostructures following the first protocol has not yet been achieved, we have adopted here the second alternative to obtain the desired nanorods. Here, we have employed the preformed nanowires28 to obtain the nanorods of variable aspect ratio by manipulating the thermally induced material diffusion. Introduction of Mn into the nanowires leads to the Mn incorporation into the pure wurtzite nanorods through this material diffusion, a new technique of doping in anisotropic nanostructure. In addition, further annealing of these nanorods results in the rice shaped nanocrystals. This shape evolution and the aspect of Mn doping into the anisotropic nanostructures of ZnSe are investigated in this Article.
Figure 1. (a) Schematic representation of the synthetic protocol showing the fragmentation of the nanowires to form nanorods and dopant adsorption during the size focusing process. (b) Digital picture of the reaction flask under illumination, excited at the wavelength of 365 nm.
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RESULTS AND DISCUSSION To dope Mn in ZnSe anisotropic nanostructures, we have designed a phosphine free colloidal synthetic approach using polar alkylamine solvent, which results in the dimension variable wurtzite nanorods of ZnSe. Initially, magic size nanowires of fixed width of ZnSe are synthesized by a modified literature method28 by injecting Se precursor (selenourea dissolved in amine) into the preloaded Zn precursor (carboxylate salt) (Zn:Se = 1:1.5) at 150 °C in long chain fatty amine (octadecylamine, ODA). Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) images of as synthesized nanowires have been shown in the Supporting Information (Figure S1a,b). Figure S1c shows the absorption and photoluminescence (PL) spectra of the crude nanowire solution dispersed in chloroform. The dual peaks at 327 and 345 nm correspond to magic size of ZnSe, and the broad PL peak originates mostly from the surface state or defect state related emission for ZnSe nanowires. This magic size nanowire solution has been swiftly injected to a preheated alkylamine solvent at 250 °C, which transforms the wurtzite nanowires to the wurtzite nanorods. Dopant Mn precursor (MnCl2) has been introduced at 150 °C after the nanowire formation, to study the effective process of the dopant insertion. A schematic presentation of the synthesis and introduction of Mn before the fragmentation of the wires has been shown in Figure 1a. It is important to mention here that the nanowire solution along with the Mn precursor has been annealed for long time at 150 °C (∼1 h) for complete consumption of the Zn and Se precursors. Injection of this nanowire solution to the amine solvent leads to the formation of Mn doped nanorods with continuous change of their aspect ratios with the course of annealing. The digital image of the reaction flask showing the orange color emission under illumination is shown in Figure 1b. Further, it has been observed that the length of these nanorods slowly reduces and their width increases with the progress of the reaction. As a result, the average aspect ratio continuously
decreases. Figure 2 shows the representative TEM images of such rod samples collected at different time intervals during the course of the reaction after the injection, which undergo size focusing both in their lengths and in their widths. On further annealing, these rods turn to oval or rice shaped nanoparticles as shown in Figure 2d. Additional enlarged TEM images of the
Figure 2. TEM images of the samples collected at different time intervals after the injection of the nanowires to the hot alkylamine solvent. (a) TEM image of the sample collected 60 s after the injection, (b) at 2 min, (c) at 5 min, and (d) TEM image of the sample collected 8 min after the injection, respectively. 6007
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samples collected at two different time intervals are shown in Figures S2 and S3. Figure 3 represents the plot showing the
change in aspect ratio of the rods obtained in different time intervals. In some places on the TEM grid, the rods are observed to be aligned vertically on a short-range of area as shown in Figure 4a,b. Figure 4c,d presents the TEM images of the concentrated nanorod solution placed on the grid, and it has been observed that in some places on the grid the rods are aligned one over the other, linearly forming some selfassembled patterns. Further, the crystal structure of these rods has been verified using powder X-ray diffraction (XRD) and HRTEM analysis. XRD of the samples collected at the initial and final stages of the reaction has been shown in Figure 5a. All of the diffraction peaks match with the wurtzite crystal phase of ZnSe, clearly verifying the wurtzite structure. This is again supported by HRTEM analysis. Figure 5b shows the HRTEM image of rod lying on the TEM grid. The d-spacing of 0.34 nm suggest the crystal structure is wurtzite, and it is viewed along the [002] direction. FFT taken over the region marked in Figure 5b is shown in Figure 5c, clearly indicating the (002) plane of the nanorods. These rods are further characterized using optical spectroscopy. Figure 6a shows absorption spectra, Figure 6b reveals the
Figure 3. Plot shows the variation in the aspect ratio of the rods during the progress of the reaction. The plot of error range in the aspect ratio versus the time of reaction has been shown.
Figure 4. (a,b) TEM images of Mn doped ZnSe nanorods self-assembly in different resolutions. (c,d) TEM images of horizontal self-assembly obtained on the TEM grid of a highly concentrated solution of the doped rods. Inset in (c) is the HRTEM image of a single rod. 6008
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Figure 5. (a) Powder XRD patterns of the nanorods of two different stages of the reaction, collected at different time intervals showing the wurtzite crystal structure. (b) HRTEM image of the rods (rice shaped) aligned on the TEM grid, clearly showing the d-spacing of 0.34 nm, which corresponds to the (002) plane of wurtzite phase of ZnSe. (c) Selected area FFT as marked in panel (b).
Mn2+ ions.6,38 For a better scenario of the above, we have also performed the quantitative anisotropic photoluminescence measurement using polarized light in steady-state measurement.39 The anisotropy value is 0.0193 for the nanorod sample, whereas the value is just 0.0040 for a standard Mn doped ZnSe spherical dot sample (synthesized using the reported method).3 The higher value of anisotropy (calculation in the Supporting Information) in the nanorod sample further reveals that the Mn emission originates from the nanorods. Next, we investigate the mechanism of the aspect ratio variation of the nanorods and the subsequent insertion of Mn2+ ions. It can be stated here that because no additional precursors (Zn-carboxylate/selenourea) except the crude nanowire solution have been injected to obtain the quantum nanorods, the change in the dimensions of the nanorods is expected mostly due to the thermal annealing induced inter-rod material diffusion process. This change in the aspect ratio of the rods has been clearly reflected from the plot in Figure 3. The shortening of length and widening of rods in the absence of monomers support such diffusion process, which is a known phenomenon reported in the literature.36 In the classical synthetic approach, for 1D wurtzite nanostructures, the growth preferably occurs along the polar Z direction. However, in our case the rods are obtained from the fragmentation of the wires and materials mostly dissolute from the polar Z-axis and absorb onto the facets along the X and Y directions (100, 010, 11̅0 facets). Nanorods of variable sizes along length and width possess different surface potentials, and hence on annealing material transfer occurs from the rods with higher potential to those of the lower potential in the absence of excess precursors. Consequently, the dimension distribution (both length and width) gradually gets narrower until the surface potentials of the rods become equal (scheme in Figure 7a). In such a case, on further annealing, these nanorods gradually transform into oval shaped nanostructures as shown in Figure 2d. Typical HRTEM images of these rods and the subsequent oval shaped nanostructures are shown in the Supporting Information (Figure S4), respectively. This shape transformation is expected due to the intrarod material diffusion process where the potential gradient among the different facets of the rods drives
Figure 6. (a) UV−visible absorption spectra during the shape evolution from rods to rice shaped particles and corresponding (b) PL spectra (sample excitation wavelength 365 nm) showing the Mn emission of the samples collected at different stages of the reaction (mentioned in the figure). (c) EPR spectra of the three stages of the reaction showing the six hyperfine splittings and the presence of Mn2+ ions in the nanostructures.
respective photoluminescence spectra, and the appearance of the long wavelength emission at ∼580 nm further suggests that the rods are presumably doped with Mn. The maximum quantum yield has been calculated to be ∼12% as measured with respect to the standard quinine sulfate dye. The emission peaking at ∼580 nm with full width at half maxima (fwhm) value of ∼55 nm is expected from the Mn d−d transition in the Mn doped nanocrystals.6,10,14,37 The appearance of Mn emission peak indicates the successful incorporation of the Mn2+ ions into the rods. The intensity of the Mn emission in the rice shaped nanostructures (topmost PL, Figure 6b) is reduced, suggesting a decrease in the amount of Mn content in the nanostructure. This has been confirmed from the inductively coupled plasma (ICP) data (ICP results shown later), details of which are discussed in a later section. The presence of dopant Mn in these doped quantum rods is further verified from the electron paramagnetic resonance (EPR) measurements. The EPR spectra of the samples Figure 6c show six hyperfine splitting of Mn d5 electrons, and the hyperfine splitting value of 65 G suggests the tetrahedral environment of 6009
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dissolute during annealing, it is expected that the dopants have the only possibility of adsorption on (100), (010), and (11̅0) groups of wurtzite facets of the nanorods. In our synthetic protocol, even if the Mn2+ ions get adsorbed onto the (002) facets, it will be readily diffused out along with Zn and Se during the reaction. From several ICP measurements, it has been observed that the percentage of Mn remains approximately 1.1% in the nanorod stage. A schematic presentation of such adsorption and host overgrowth has been shown in Figure 7c. This also explains the reduction of the amount of Mn in the rice shaped particles. As this process is the intrarod material transfer, once the dopant Mn2+ ions are ejected from one facet, they possess less opportunity to get adsorbed again on the other facet. Here, during transformation to rice shaped particles, most of them are ejected out reducing a significant amount of the dopant Mn2+ ions. Similar observations have also been reported during ripening of Mn doped zinc blende ZnSe.40 Now, the question may arise here that as the nanowires are used in the crude form without purification, there is a possibility of the presence of excess precursors along with the nanowires that can possibly interfere with the nanorod formation by supplying additional monomers during growth. This possibility has been negated by performing the same reaction with the purified nanowires. When the thoroughly washed nanowires are injected into the hot alkylamine solvent, they undergo the same sequence of rod formation via fragmentation of wires followed by a similar change of their aspect ratios on continuous annealing. But the major observation in this scenario is the wide size distribution of the obtained nanorods as shown in Figure S5, which results from the irregular fragmentation of the wires. Hence, we assume that the unreacted selenourea present along with the crude nanowires without purification play a crucial role for the homogeneous fragmentation of the wires. The results also suggest that the higher is the amount of Se present in the reaction mixture, better control over the dimension uniformity is achieved. This has been further verified by introducing excess selenourea to purified nanowires before injecting them to amine solvent, which again leads to well-controlled nanorods similar to those obtained from crude nanowires. However, these doped rods are obtained from the fragmentation of the nanowires at the precursor ratio of 1:1.5
Figure 7. (a) Scheme showing inter-rod material transfer process; dotted arrow marks in (a) show the direction of materials transfer from the Z direction and growth along the X and Y directions of other nanorods simultaneously. (b) Scheme for intrarod material transfer; arrow marks show the direction of material diffusion from Z direction to X and Y directions within the same nanorod. (c) Scheme showing the mechanism of Mn adsorption during the inter-rod material diffusion, retention during the host overgrowth, and subsequent removal during intrarods material transfer.
the materials to diffuse from one facet of the nanorod to another (scheme in Figure 7b). The material transfer occurs from the polar (002) facet to the nonpolar (100), (010), and (11̅0) facets of the rods forming the oval shaped nanostructure.36 Further, we investigate the doping of Mn in the wurtzite nanorods. Analyzing the efficiency of dopant incorporation and the dopant emission intensity, it has been observed that continuous heating of the nanowires along with the dopant from 150 to 250 °C can incorporate maximum percentage of the dopants in the nanorods. The continuous increase of the widths of the nanowires during the size focusing of the nanorods helps for the facet-dependent adsorption of the dopant Mn and their retention during the overgrowth. This is expected as adsorption of Mn onto the nanowires provides more opportunity for their retention during the over growth. It simply follows the traditional growth doping strategy in wurtzite structures. As the materials along the (002) facets
Figure 8. TEM and HRTEM images of higher aspect ratio of ZnSe nanorods. (a,b) TEM images in different resolutions of ZnSe with Zn to Se ratio 1:0.8. (c,d) HRTEM image and FFT of the nanorods, respectively, to show the wurtzite crystal growth in these higher aspect ratio rods. 6010
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ODA solution, the nanowires (at 150 °C) solution was swiftly injected along with 0.008 g of MnCl2 (0.04 mmol) dissolved in 1 g of molten ODA. Then samples were collected periodically for microscopic and spectroscopic measurements. TEM characterization was carried out at different time intervals after purification of the sample, which showed a gradual change in the aspect ratio of the rods obtained at that temperature. Synthesis of Mn Doped Nanorods with Variation of Precursor Ratio (Zn:Se = 1:0.8). The synthetic protocol employed here was the same as that of the synthesis of rods using Zn:Se = 1:1.5, only the Se amount has been reduced. 0.24 mmol (0.0295 g) of selenourea and 3 g of ODA were loaded in a 25 mL three necked flask and degassed for 25 min under N2 flow. The temperature was then raised to 200 °C to dissolve selenourea in amine resulting in a deep red colored solution. The solution was then cooled to 150 °C and kept in molten condition. In another 25 mL flask, 0.3 mmol (0.189 g) of ZnSt2 in 3 g of ODA was degassed and heated to temperature 150 °C. The Zn solution was then swiftly injected to the selenourea solution at 150 °C. The resulting nanowires were swiftly injected into a three necked flask loaded with preheated ODA at 250 °C, leading to the formation of the nanorods of higher aspect ratio ∼30−40 nm as demonstrated by the TEM images in Figure 8. For doping, a similar protocol was followed by introducing 0.008 g of MnCl2 (0.04 mmol) dissolved in 1 g of molten ODA. Characterization. UV−vis measurements were done with an Agilent 8453 spectrophotometer. Photoluminescence spectra were collected using a Horiba Jobin Yvon Fluoromax4 spectrofluorometer. The optical measurements were done by taking a small amount of the purified sample dispersed in chloroform in a standard quartz cuvette. TEM images were taken on a JEOL-JEM 2010 electron microscope using 200 kV electron source. TEM images on STEM (HAADF) were taken on a UHR-FEG-TEM, JEOL, JEM 2100 F model using 200 kV electron source. Specimens were prepared by dropping a drop of nanocrystal solution in chloroform on a carbon coated copper grid, and the grid was dried under air. XRD of the doped sample was taken by Bruker D8 Advance powder diffractometer, using Cu Kα (λ = 1.54 Å) as the incident radiation. The EPR measurement was done using a 9.5 GHz JEOL spectrometer operated at X-band frequency. The g value (effective Zeeman factor) is dependent on the orientation of magnetic field. The dopant percentage was determined by ICP using a Perkin−Elmer Optima 2100 DV machine. At first, the nanocrystals were repeatedly purified to remove excess precursors. The purified nanocrystals then were dissolved in chloroform. The chloroform was then evaporated, and the dried nanocrystals were digested in concentrated HNO3. The nitric acid solution of the samples was diluted with double distilled water to perform the measurements.
Zn to Se, respectively. Varying this ratio to 1:0.8 with identical reaction conditions, the aspect ratio again can be altered. In this condition, rods with higher aspect ratio of length up to 30−40 nm and width up to 3.5 nm are obtained. TEM images of a typical sample collected after 1 min have been shown in Figure 8. Figure 8c and d shows the HRTEM image and the FFT of the doped rod obtained from the nanowires synthesized with 1:0.8 ratio of Zn to Se precursor, clearly demonstrating that it is grown along the [002] direction of wurtzite phase of ZnSe. The XRD plot of the rods also clearly indicates the wurtzite phase as shown in Figure S6. Although these rods are doped following a similar protocol, appreciable reduction of the length of the rods has not been observed. This can be interpreted by the higher aspect ratio of the rods, which probably hinders the intraparticle material diffusion process and the subsequent shape change process consequently.
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CONCLUSIONS In summary, we have designed here wurtzite 1D nanorods with variable aspect ratios via material diffusion and performed the Mn doping in pure wurtzite nanostructure of ZnSe. The protocol adopted here is different from the conventional nucleation and growth mechanism of colloidal synthesis. It includes controlled fragmentation of preformed ultrathin wurtzite nanowires of ZnSe with appropriate manipulation of thermal and chemical parameters and successive evolution of rods to rice shaped nanostructure. The entire process is guided by thermal diffusion phenomenon. Summarizing all of the experimental results, we can state here that this Article addresses few important issues associated with the synthesis and doping of 1D quantum confined nanorods of ZnSe nanocrystals. Additionally, the material itself has multiple functional properties that would help the community to implement them in advance technological applications particularly in opto-electronic devices in future.
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EXPERIMENTAL SECTION Materials. Zinc stearate (Zn(St)2, tech), octadecylamine (ODA, 90%), and selenourea powder were purchased from Aldrich, and manganese(II) chloride tetrahydrate (MnCl2·4H2O) was purchased from Loba Chemie Pvt. Ltd. All of the chemicals were used without further purification. Synthesis of ZnSe Nanowires Using Zn:Se = 1:1.5. Nanowires with magic size diameter (∼1.2 nm) were synthesized following a modified literature report. For selenourea solution, 0.45 mmol (0.055g) of selenourea and 3 g of octadecyalmine (ODA) were loaded in a 25 mL three necked flask and degassed for 25 min under N2 flow. The temperature then was raised to 200 °C to dissolve selenourea in amine, resulting in a deep red colored solution. The reaction was then cooled to 150 °C, which kept the solvent in molten condition. In another 25 mL flask, 0.3 mmol (0.189 g) of ZnSt2 in 3 g of ODA was degassed and heated to temperature 150 °C. The Zn solution was then swiftly injected to the selenourea solution at 150 °C. The mixture was annealed at 150 °C for 1 h, and the TEM shows the formation of long nanowires. Synthesis of Mn Doped ZnSe Nanorods of Variable Aspect Ratio. For obtaining rods with different aspect ratios, the nanowires solution was injected to hot alkylamine solvent at 250 °C, and samples were collected periodically. Typically, in a 25 mL three necked flask, 3 g of ODA was taken and degassed under N2 for 15 min and was heated to 250 °C. To this molten
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ASSOCIATED CONTENT
S Supporting Information *
Additional TEM, HRTEM images, XRD pattern, and supporting experimental technique. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. 6011
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Notes
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The authors declare no competing financial interest.
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ACKNOWLEDGMENTS CSIR and DST (Swarnajayanti, DST/SJF/CSA-01/2010-2011) of India are acknowledged for funding. The DST unit of nanoscience at IACS is acknowledged for providing the TEM facility.
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