Size-Controlled Synthesis of Bifunctional Magnetic ... - ACS Publications

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Size-Controlled Synthesis of Bifunctional Magnetic and Ultraviolet Optical Rock-Salt MnS Nanocube Superlattices Xinyi Yang,† Yingnan Wang,† Yongming Sui,† Xiaoli Huang,† Tian Cui,† Chunzhong Wang,‡ Bingbing Liu,† Guangtian Zou,† and Bo Zou*,† †

State key Laboratory of Superhard Materials, Jilin University, Changchun 130012, China Key Laboratory of Physics and Technology for Advanced Batteries (Ministry of Education), College of Physics, Jilin University, Changchun 130012, China

‡

S Supporting Information *

ABSTRACT: Wide-band-gap rock-salt (RS) MnS nanocubes were synthesized by the one-pot solvent thermal approach. The edge length of the nanocubes can be easily controlled by prolonging the reaction time (or aging time). We systematically explored the formation of RS-MnS nanocubes and found that the present synthetic method is virtually a combination of oriented aggregation and intraparticle ripening processes. Furthermore, these RS-MnS nanocubes could spontaneously assemble into ordered superlattices via the natural cooling process. The optical and magnetic properties were investigated using measured by UV−vis absorption, photoluminescence spectra, and a magnetometer. The obtained RS-MnS nanocubes exhibit good ultraviolet optical properties depending on the size of the samples. The magnetic measurements suggest that RSMnS nanocubes consist of an antiferromagnetic core and a ferromagnetic shell below the blocking temperatures. Furthermore, the hysteresis measurements indicate these RS-MnS nanocubes have large coercive fields (e.g., 1265 Oe for 40 nm nanocubes), which is attributed to the size and self-assembly of the samples.

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tor,18 solar cells,19 short-wavelength optoelectronic devices,20 and photochemical materials.21 Generally, MnS has three crystal structures: the stable rock-salt (RS), metastable zincblende (ZB), and wurtzite (WZ) structures. Both tetrahedrally coordinated ZB and WZ phases are unstable structures and easily transformed into octahedrally coordinated RS phase at high temperature or high pressure.22 Stimulated by promising applications, much attention has focused on the control of the shape and crystal structure of MnS NCs by hydrothermal, solvothermal, thermolysis, and spray-produced processes.23−26 However, to the best of our knowledge, there are few reports about MnS NCs possessing good optical and magnetic properties, which are crucial for the success of bottom-up nanodevice applications. In this work, we report on the synthesis of RS-MnS nanocubes with tunable sizes and their self-assemblies by the one-pot solvothermal method. In this process, we systematically explored the formation of RS-MnS nanocubes and found that the present synthetic method is virtually a combination of oriented aggregation and intraparticle ripening processes. First, RS-MnS nanoparticles undergo oriented aggregation to form quasi-square interstitial RS-MnS nanocubes. Second, intraparticle ripening process takes place in the shape-defined

INTRODUCTION Magnetic and optical materials are of great importance in the fields of both fundamental research and technical application. Magnetic nanocrystals (NCs) have generated great fundamental and technical interest, not only for their size-dependent magnetism but also for their many technological applications related to recording media, spintronics, medicine, and biology.1−6 In particular, antiferromagnetic transition metal chalcogenide NCs are important for data storage and spin-valve devices.7,8 The magnetic properties of such NCs usually differ from those of their bulk counterparts due to surface and size effects (e.g., small antiferromagnetic NCs should exhibit superparamagnetism and weak ferromagnetism9). On the other hand, short-wavelength ultraviolet lasers play important roles in information storage because of the shorter wavelength and the higher information storage density.10−13 Thus, excellent functionality can be introduced by combining magnetic and optical effects of magnetic NCs. Various Mn doping magneto-optical materials with short wavelength have been reported over the past few decades, which arise from a strong sp−d exchange interaction between electron/hole band states and Mn2+ 3d electron states.14−17 Manganese sulfide (MnS) is not only an antiferromagnetic transition metal chalcogenide (TN ≈ 150 K) but also a wideband-gap semiconductor (Eg ≈ 3.2 eV). Because of such dual properties, it has potential applications for short wavelength magneto-optical nanodevices,15 diluted magnetic semiconduc© 2012 American Chemical Society

Received: October 24, 2012 Revised: December 2, 2012 Published: December 4, 2012 17811

dx.doi.org/10.1021/la304228w | Langmuir 2012, 28, 17811−17816

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aggregation, resulting in single-crystalline square nanocubes. After nature cooling to room temperature, the RS-MnS nanocubes could spontaneously assemble into ordered superlattices. These RS-MnS nanocubes display size-dependent ultraviolet emission (356−373 nm). The magnetic measurements indicate the nanocubes have an antiferromagnetic core/ ferromagnetic shell structure and large coercive fields (e.g., 1265 Oe for 40 nm nanocubes).

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EXPERIMENTAL SECTION

Synthesis of RS-MnS Nanocubes and Their Superlattices. In a typical synthesis, a mixture of anhydrous MnCl2 powder (0.063 g, 0.5 mmol), thioacetamide (0.038 g, 0.5 mmol), oleic acid (OA) (1.0 mL), and oleylamine (OLA) (5.0 mL) was added to a 50 mL three-neck flask. This mixture was heated to 80 °C for 60 min under nitrogen flow. The solution was then slowly heated to 250 °C (5 °C/min) and kept for 30 min, and then it was allowed to cool down to room temperature naturally. The resultant mixture was isolated using methanol and excess acetone and centrifuging for 10 min at 10 000 rpm, and the products were collected. Subsequently, the residual samples were redispersed in toluene for characterization. Characterization. Powder X-ray diffraction (XRD) data were collected on a Bruker D8 diffractometer working with a Cu Kα target. Transmission electron microscope (TEM) images and selected area electron diffraction (SAED) were obtained with a Hitachi H-8100 transmission electron microscope operating at an acceleration voltage of 200 kV. Composition of the specimens was analyzed with an energy-dispersive X-ray (EDX) spectroscopy (INCA energy) attached to the JSM-6700F. High-resolution TEM (HRTEM), high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), and elemental mapping images were obtained on a JEM-2200FS operated at 200 kV, where the corresponding fast Fourier transform (FFT) algorithms were analyzed. Absorption and photoluminescence (PL) spectra were measured by a Shimadzu UV3150 spectrometer and a Photon Technology International PL, respectively. Magnetic measurements were carried out by a Quantum Design MPMS SQUID vibrating sample magnetometer.

Figure 1. TEM images and histograms showing the edge length distribution of the RS-MnS nanocubes obtained at 250 °C for different reaction time: (a, b) 30 min, (c, d) 60 min, and (e, f) 120 min.

checked by EDX (Figure 2b). The element proportion (atom %) of manganese to sulfide is found to be approximately 1:1 as expected (Mn: 52.16%; S: 47.84%), which is in good agreement with the stoichiometric ratio of the compound of MnS. Figure 2c shows the HRTEM image of a typical single nanocube, which illustrates that the fringe spacing is about 0.260 nm, corresponding to the (200) lattice planes of RS-MnS. The FFT pattern in the inset of Figure 2c indicates that the RS-MnS nanocube has single-crystalline structure. Furthermore, the STEM-EDS mapping images of a RS-MnS nanocube are demonstrated in Figure 2d, in which the green region is Mn elements, whereas the orange region is S elements. The result indicates that these elements are uniformly distributed. Therefore, XRD, EDX, HRTEM, and STEM-EDS mapping analyses show that pure RS-MnS nanocubes are successfully synthesized. Self-assembly of magnetic NCs, to generate two- and threedimensional superlattice structures, is very important both from the viewpoint of the fundamental interest in the collective interaction of NCs and for their potential applications in multiterabit per square inch magnetic storage media. In our experiments, the standard deviations (δ) of these NCs sizes were calculated to be 7−9%. It is well-known that the size uniformity of NCs is critical to achieve superlattice formation.27 Figure 3a shows a TEM image of typical MnS nanocubes with an average edge length of 14 nm assembling into superlattices. The SAED has the same 4-fold symmetry of the (200) and (220) reflections indicating that the nanocubes assemble into ⟨100⟩ oriented arrays (Figure 3b). This self-assembly ability was achieved via simply the natural cooling process. The orientational configuration of capped ligand chains would form with the gradual decrease of temperature, resulting in an order state of ligand chains at room temperature.28 At such a state, NCs were interacting and thus showed orderly arranged superlattice structures, which was attributed to solvent evaporation-induced ligand−ligand van der Waals (VDW) interaction and entropy driving process. Comparatively, when the crude nanocrystals were quickly quenched to room temperature, their ligand chains were captured in a disordered

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RESULTS AND DISCUSSION RS-MnS nanocubes with various edge lengths were synthesized by the one-pot solvothermal method at 250 °C. The size control of the RS-MnS nanocubes was achieved by varying the reaction time. The as-synthesized RS-MnS nanocubes could be easily dispersed in various organic solvents, such as hexane, chloroform, or toluene (Figure S1). Figure 1a shows a TEM image of typical RS-MnS nanocubes obtained for 30 min. Their average edge length is about 14 nm (Figure 1b). When prolonging the reaction time to 60 min, larger RS-MnS nanocubes were produced, which have an average edge length of about 26 nm (Figure 1c,d). Figure 1e shows a TEM image of RS-MnS nanocubes obtained for 120 min. Their average edge length is about 40 nm (Figure 1f). A low-magnification TEM image shown in Figure S2a indicates that these RS-MnS nanocubes have a high degree of monodispersity. The crystal structure and chemical composition of the asprepared samples are characterized by XRD, as shown in Figure 2a. All diffraction peaks correspond to the cubic RS crystal structure of MnS (JCPDS File No. 72-1534). The obvious diffraction peaks could be indexed to the (111), (200), (220), (311), and (222) planes of the RS crystal structure of MnS. The sharp and strong peaks also confirmed that the product is well crystallized. No characteristic peaks for impurities, such as ZB or WZ phases, are observed. In addition to XRD result, the chemical composition of the as-obtained product was further 17812

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Figure 2. Typical XRD pattern (a), EDX spectrum (b), and HRTEM image (c) of the as-prepared RS-MnS nanocubes. The insets in (b) and (c) show the table of composition and corresponding FFT patterns, respectively. (d) STEM image of a typical RS-MnS nanocube and atomic distribution of Mn and S atoms. Scale bar = 10 nm.

packing, the second layer of nanocubes is exactly stacked on top of the bottom layer by maximally contacting the (100) surfaces of two adjacent facets, leading to a minimum of total surface energy. Therefore, this type of assembly should dominate the packing structures. The formation process of the RS-MnS nanocubes was investigated in detail by characterizing the products obtained at different reaction intervals. As the reaction temperature increases, the initial mixture gradually turned green, indicative of the formation of RS-MnS NCs (Figure S3a). Figure S3b shows representative XRD patterns of the products obtained at 80, 90, 100, 150, and 200 °C in the reaction of RS-MnS nanocubes. Crystal diffraction peaks were found when the temperature was no less than 100 °C. All the diffraction peaks were labeled and could be indexed to the cubic RS crystal structure of MnS (JCPDS File No. 72-1534). At the temperature extent from 100 to 200 °C, the shape of diffraction peaks became more clean-cut and the intensities increased gradually, indicating better nanocrystalline formed. The morphology of the prepared RS-MnS NCs synthesized at different reaction durations was examined by TEM. As shown in Figure 4a, the RS-MnS obtained at 100 °C consists of monodisperse nanoparticles with mean diameters of 3 nm. When the reaction temperature was elevated to 150 °C, the TEM image clearly shows that the RS-MnS nanoparticles began to aggregate into quasi-square interstitial MnS nanocubes (Figure 4b). The mean edge length of the nanocubes is ∼32 nm. All nanoparticles aggregate into quasi-square RS-MnS nanocubes at 200 °C, but they are morphologically rough and structurally loose (Figure 4c). The mean width of the nanoparticles is ∼42 nm. Particle agglomeration is a very common phenomenon during the synthesis of NCs in solution which is avoided by researchers working on nanoparticle synthesis.33−35 However, it plays an important role in the particle ripening process through oriented attachment mode.

Figure 3. (a) TEM images of 14 nm RS-MnS nanocube superlattices. (b) SAED of (a) indexed to (200) and (220) planes of RS MnS. (c) Structure model showing multilayer simple-cubic stacking by cubic NCs. (d) HAADF-STEM image of the nanocube superlattices. The inset of (d) shows high-magnification HAADF-STEM image of the nanocube superlattices.

state.29 According to the Salem expression, when the ligand chains were in a random and disordered configuration, the interactions of ligand−ligand VDW interaction were so weak that the corresponding nanocrystals were noninteracting and randomly arranged.30 Including monolayer well-ordered structures, different concentrations of the cubic NCs could also arrange into multilayer structures with the simple-cubic packing lattice (Figure 3c). The HAADF-STEM images provide direct evidence for the formation of multilayer structures with the simple-cubic packing lattice (Figure 3d). For nanocubes, a simple-cubic packing pattern will result in a high packing density theoretically and should be energetically favorable.31 The misalignment of the planes of the RS-MnS nanocubes gives rise to forming “nanobelts”, as shown in Figure S2b. This phenomenon is attributed to shifting along one axis in the horizontal plane of simple-cubic packing pattern (i.e., 1D shifted simple-cubic packing).32 In the structure of simple-cubic 17813

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Figure 4. TEM images of the sample obtained at different reaction intervals: (a) 100, (b) 150, (c) 200, and (d) 250 °C. (e) Schematic illustration of growth and self-assembly process of RS-MnS nanocubes.

chemical environment for the configuration of the nuclei was changed.37 The successful synthesis of RS-MnS nanocubes allows us to investigate their optical and magnetic properties. The UV−vis absorbance and PL spectra of the RS-MnS nanocubes dispersed in toluene solution were measured at room temperature (Figure 5). Discernible shoulder peaks at 337, 346, and 355 nm

According to the XRD and TEM patterns, it is suggested that small RS-MnS nanoparticles aggregate into quasi-square RSMnS nanocubes with the more thermal energy supplied. The main driving force for oriented aggregation of nanoparticles generally attributed to the tendency for reducing the high surface energy through the attachment among the primary nanoparticles and the rotation of these primary nanoparticles to give coherent lattice structure at grain interfaces, i.e., oriented attachment (or oriented aggregation). With the increase of reaction temperatures, there exists a typical intraparticle ripening process, in which the materials in a NC are redistributed in the same nanocrystal during the evolution of crystal shapes. Further, annealing these loose quasi-square aggregates at elevated temperatures for a prolonged period of time would improve their crystallinity and thus eliminate their defects. With the least defects in crystallinity, solid nanostructures would finally replace their loose quasi-square aggregates morphology to terminate the shape evolution, indicating the termination of intraparticle ripening.36 It should be noted that a reaction running too long (e.g., 180 min) would cause the NC ensemble to appear polydisperse (Ostwald ripening) (Figure S4). For rock-salt structures of NCs, the growth of the higher surface energy (111) face in the ⟨111⟩ direction was faster than that of the lower surface energy (100) face in the ⟨100⟩ direction. This would favor the growth of the (111) facets, resulting in the formation of cubic-like nanostructures with the lowest total surface energy.36 In the natural cooling process, the configuration of ligand chains gradually became ordered.28 Consequently, RS-MnS nanocubes could readily self-assemble into simple-cubic stacking structures (Figure 4d). A schematic illustration of the growth and self-assembly process of the RSMnS nanocubes at different reaction intervals is given in Figure 4e. The influence of the solvent on the shape and phase of the product was also studied. If only OLA is employed as the ligand and reaction solvent, keeping the other conditions identical, the sample obtained was WZ-MnS nanorods at 250 °C for 30 min (Figure S5). This indicates that WZ-MnS nucleus seeds were formed in the early stage of nucleation because the relative

Figure 5. Optical properties of the RS-MnS nanocubes with different edge lengths: (a) UV−vis absorbance spectra; (b) photoluminescence spectra (λex = 300 nm).

were detected in the RS-MnS nanocubes with different sizes (Figure 5a), which are assigned to the optical transition of the excitonic state. Compared with that of bulk MnS of 388 nm (3.2 eV),24 the absorption peaks of the products displayed blueshifts, which can be explained as the quantum size effect originated from the electron−hole confinement in a small volume. Figure 5b shows the room temperature PL spectra of these RS-MnS nanocubes within the range 356−373 nm, which exhibit ultraviolet emission. Furthermore, the recent surge of activity in wide-band-gap semiconductors has arisen from the need for optical materials, especially emitters, which are active in the UV wavelengths.38 Therefore, we believe these nanocube superlattices can be useful for ultraviolet light-emitting and short wavelength optoelectronic devices. 17814

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RS-MnS nanocubes. It is well-known that the magnetization of ferromagnetic materials is very sensitive to the size, shape, and structure of the as-synthesized products.40 The assembly of the RS-MnS nanocubes into the superlattices results in the change of the single domain to the multidomain, leading to the higher coercivity.

The temperature dependence of magnetization measured in an applied field of 1000 Oe clearly shed light on the magnetic properties of these RS-MnS nanocube superlattices (Figure 6).

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CONCLUSION In summary, the RS-MnS nanocube with three different sizes has been successfully synthesized by one-pot solvent thermal method. We systematically explored and found that the formation of RS-MnS nanocubes is a combination of oriented aggregation and intraparticle ripening processes. In the first stage, RS-MnS nanoparticles undergo oriented aggregation to form quasi-square interstitial RS-MnS nanocubes. In the second stage, intraparticle ripening process takes place in the shapedefined aggregation, resulting in single-crystalline square nanocubes. Then, these naturally cooled nanocubes spontaneously assemble into superlattices. These RS-MnS nanocubes display size-dependent ultraviolet emission (356−373 nm). The magnetic measurements indicate the nanocubes have an antiferromagnetic core/ferromagnetic shell structure and large coercive fields (e.g., 1265 Oe for 40 nm nanocubes). This work provides a simple bottom-up approach to integrate RS-MnS nanocube superlattices with good magnetic and optical properties, which may have potential applications for the short wavelength magneto-optical nanodevices in the future.

Figure 6. Temperature dependence of FC and ZFC susceptibility for RS-MnS nanocubes of various edge lengths.

The zero-field-cooled (ZFC) and field-cooled (FC) curves of 14 nm nanocubes are smooth even if the low-temperature divergence between ZFC and FC magnetization shows that some irreversible magnetic behavior occurs below 50 K (Figure 6a). For 26 and 40 nm nanocubes, it can be seen that the ZFC curves exhibit typical blocking processes with blocking temperatures, TB, at ∼32 K (Figure 6b). Importantly, the FC curves reveal an increase of magnetization below freezing temperature (TF) ∼ 28 K, which is related with the onset of the freezing process of the surface spin-glass layer for the RS-MnS nanocubes. Considering that RS-MnS NCs are antiferromagnetic, such a spin-glass-like behavior is denoted as a surface effect, since the Mn state at the surface is not the same as that inside a NC.26 A core−shell phenomenological model was proposed, where relaxation of superexchange interaction on the surface of NCs allows the formation of ferromagnetic or spinglass shell, and the result is the generation of natural antiferromagnetic/ferromagnetic or ferromagnetic/spin-glass interfaces.39 The presence of a ferromagnetic structure at the surface of the MnS NCs is further supported by the hysteresis measured at 5 and 30 kOe, as shown in Figure 7. The

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ASSOCIATED CONTENT

S Supporting Information *

Additional digital photographs, TEM/HRTEM images, XRD and FFT patterns, SQUID magnetometry of MnS NCs. 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]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by NSFC (Nos. 91227202, 51102108, 51202086, and 51025206), the National Basic Research Program of China (No. 2011CB808200), Changjiang Scholar and Innovative Research Team in University (No. IRT1132), and Project 20121056 Supported by Graduate Innovation Fund of Jilin University.

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Figure 7. Hysteresis loops at 5 K for RS-MnS nanocubes with indicated edge lengths.

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hysteresises of the RS-MnS nanocubes show that the NCs are far from being magnetically saturated even at 30 kOe. More interestingly, it can be seen that the coercive force increases from 10 to 1265 Oe with the increase of size (Figure S6), which are relatively high compared with that of the RS-MnS nanoparticles.20 The loops indicate that the samples have ferromagnetism at low temperature, which has been found in nanostructured antiferromagnetic materials as a result of the surface spins. This result is, to the best of our knowledge, the first demonstration of a large coercive field (1265 Oe at 5 K) in 17815

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dx.doi.org/10.1021/la304228w | Langmuir 2012, 28, 17811−17816