J. Phys. Chem. C 2007, 111, 519-525
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Morphology-Tuned Growth of r-MnSe One-Dimensional Nanostructures Hye Jin Chun, Jin Young Lee, Dae Sung Kim, Sang Won Yoon, Ja Hee Kang, and Jeunghee Park* Department of Chemistry, Korea UniVersity, Jochiwon 339-700, Korea ReceiVed: September 7, 2006; In Final Form: October 22, 2006
MnSe one-dimensional nanostructures were synthesized by chemical vapor deposition and their morphology was tuned from nanowires to MnSe/silica core/shell nanocables and “peapods,” as well as nanocube-chain networks by adjusting the growth conditions. All of these MnSe nanocrystals had a single-crystalline rocksalt R-MnSe structure, grown typically with the [100] growth direction. The MnSe nanoparticles (diameter ) 40 nm) were encapsulated into the amorphous silica nanowires, forming a “peapod” structure. The MnSe nanocubes (100-200 nm) were linked in chains along their [100] and [110] directions, forming a unique network structure. We propose a growth mechanism in which the competition between the growth rates of MnSe and silica results in the formation of these different morphologies.
1. Introduction In the past few years, one-dimensional (1D) nanostructures have attracted considerable attention because of both their fundamental importance and their wide range of technological applications in nanodevices.1-3 Semiconductor nanowires are especially attractive building blocks for assembling active and integrated nanodevice systems. Manganese selenide (MnSe) is an important magnetic semiconductor material and exists in three phases, namely, R-, β-, and γ-phases, of which the R (rocksalt)- and β (zinc blende)-phases are cubic structures, while the γ-phase is hexagonal. Like other manganese chalcogenides, rock-salt-structured R-MnSe is the stable form at room temperature. The band gap energy (Eg) of the bulk phase is ∼3.0 eV at room temperature.4-8 A number of works mainly focused on their magnetic properties and reported that R-MnSe is antiferromagnetic with a N’eel temperature (TN) ≈ 150 K.8-21 The synthesis of R-MnSe quantum wires (diameter ) 3 nm) using mesoporous silica templates was reported by Chen et al.22 Recently, R-MnSe nanospheres/ nanorods have been synthesized using autoclave solvothermal/hydrothermal reactions.23,24 Its microcrystal (e.g., flakes, cubes) forms were also synthesized using such autoclave reactions.25,26 However, controlling the morphology of the MnSe 1D nanostructures still remains a significant challenge. Herein, we report, for the first time, the morphology-tuning of R-MnSe 1D nanostructures between nanowires, composite structures with silica (SiOx), and nanocrystal chains via the chemical vapor deposition (CVD) method. The present work would provide valuable information on how the morphologies of the 1D nanostructures are diverse in a distinctive way from those of their nanoparticles or bulk counterparts. 2. Experimental MnCl2 (99.99%, Aldrich) and CoSe (99%, Stream) powders, which were used as the Mn and Se sources, respectively, were loaded in a quartz boat which was placed inside a quartz tube reactor. A piece of Si substrate, acting as a Si source, was placed * Corresponding author. Tel: 82-2-3290-3993. Fax: 82-2-3290-3992. E-mail:
[email protected].
on the top of the source boat. The Si substrates coated with a 0.01 M ethanol solution of HAuCl4‚3H2O (98+%, Sigma) were positioned at a distance of 15-20 cm from the source boat. Argon gas was allowed to flow, while raising or lowering the temperature of the reactor. The temperature of the source was set at 1000-1100 °C, and that of the substrate was in the range of 800-900 °C, during the synthesis under argon ambient. The temperatures of the source and substrates were varied in order to control the morphology of the products. Scanning electron microscopy (SEM, Hitachi S-4300), field-emission transmission electron microscopy (TEM, Jeol JEM 2100F and FEI TECNAI G2 200 kV), high-voltage TEM (Jeol JEM ARM 1300S, 1.25 MV), electron diffraction (ED), and energy dispersive X-ray fluorescence (EDX) (line-scanning) were employed to examine the morphology and structure of the products. High-resolution X-ray diffraction (XRD) was performed using the 3C2 (λ ) 1.5402 Å) and 8C2 beam lines (λ ) 1.54520 Å) of the Pohang Light Source (PLS) with monochromatic radiation and the Cu KR line (λ ) 1.5406 Å) of a laboratory-based diffractometer (Philips X’Pert PRO MRD). The temperature-dependent steadystate photoluminescence (PL) measurements were carried out using an He-Cd laser (λ ) 325 nm) as the excitation source. The laser power was kept below 100 kW/cm2. The time-resolved PL spectrum was recorded using a 10-ns pulse Nd:YAG laser (λ ) 266 or 355 nm, Coherent). 3. Results and Discussion We observed that the morphology of the MnSe 1D nanostructures depends on the growth temperature (approximately 800-900 °C), which was adjusted by the position (15-20 cm) of the Au-deposited silicon (Si) substrates from the source: MnCl2 powders, CoSe powders, and a piece of Si substrates. As the growth temperature decreases from 900 to 800 °C, the morphology was tuned from nanowires to nanocables, peapods, and nanocube chains. The evaporation of the Si source takes place coincidently with that of the Mn and Se sources, resulting in the formation of amorphous SiOx outer layers and thereby producing the various morphologies of the MnSe 1D nanostructures. The structure analysis of each 1D nanostructure is described below.
10.1021/jp0658187 CCC: $37.00 © 2007 American Chemical Society Published on Web 12/14/2006
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Figure 1. (a) SEM image of the high-density MnSe nanowires grown on the substrates. (b) TEM image shows the general morphology of the MnSe nanowires (averge diameter ) 70 nm). (c) Atomic-resolved image and its corresponding FFT ED pattern (inset), revealing the singlecrystalline cubic R-MnSe nanocrystal grown with the [100] direction. (d) EDX line-scan profile of individual nanowire, showing Mn/Se ) 1:1.
(1) Various Morphological Evolutions of the MnSe 1D Nanostructures. The SEM image in Figure 1a shows the highdensity MnSe nanowires grown homogeneously on the substrates. The length of the MnSe nanowires attains 20 µm. The TEM image in Figure 1b shows the smooth morphology of the nanowires without any amorphous outer layers. Their diameter distribution is narrow, in the range of 50-80 nm, with a mean value of 70 nm. The high-resolution TEM (HRTEM) image in Figure 1c reveals that the highly crystalline R-MnSe nanowire grows along the [100] direction. Its corresponding fast Fouriertransform (FFT) ED pattern, generated from the inversion of the TEM image using DigitalMicrograph GMS1.2 software (Gatan Inc.), shows that the R-MnSe crystal has a singlecrystalline cubic structure (inset). The (020) plane fringes are separated by about 0.27 nm, which is consistent with that of the bulk R-MnSe crystal (a ) 5.462 Å, JCPDS Card No. 110683). Figure 1d shows an EDX line-scan profile of Mn and Se, revealing a 1:1 ratio of Mn/Se for the individual MnSe nanowire. No oxygen peaks were detected from the entire nanowires. Figure 2a shows a SEM image of the high-density MnSe/ SiOx core/shell nanocables. The TEM image of the nanocables shows that the average thickness of the amorphous SiOx outer layer is about 20 nm, the average diameter of the nanowire core is about 30 nm, and the outer diameter is thus approximately 70 nm (Figure 2b). The HRTEM image of the MnSe nanowire core reveals the highly crystalline (020) planes separated by about 0.27 nm (Figure 2c). The FFT ED pattern confirms that the single-crystalline cubic structure of the R-MnSe crystal
grows along the [100] direction (inset). The EDX line-scan profile reveals that the nanowire core consists of Mn/Se in ratio of 1:1, and that Si and O elements exist only in the outer layers of the nanocables (Figure 2d). Branched nanocable structures were also frequently synthesized under the same growth conditions (Figure 2e). The nanocables are decorated with a lot of shorter rodlike nanostructures with lengths of 50-300 nm (inset). Most of the branches are aligned almost vertically on the surface of the nanocable stem. The TEM image reveals that the stem and branches all have MnSe/SiOx nanocable structures (Figure 2f). In the stem, the average diameter of the MnSe core is 20 nm, and that of the SiOx outer layers is 40 nm. In the branches, the outer-layer thickness is usually smaller than that of the stem; the average diameter of the MnSe core is 20 nm, and that of the SiOx outer layers is also 20 nm. The selected-area ED (SAED) pattern shows that the MnSe nanowire in the stem and the MnSe nanorods in the branches are all single-crystalline with the [100] growth direction. As the substrates move farther from the source toward the cooling zone, nanocables with a “peapod” structure having a high density are formed. The SEM image in Figure 3a shows the peapod nanocables in which the MnSe peas are embedded in the amorphous SiOx pods. The TEM image reveals the peapod morphology in which the sphere-shaped MnSe nanocrystals are periodically implanted inside the SiOx nanowires (Figure 3b). The diameter of the spherical MnSe nanoparticles is uniformly about 40 nm, and the outer diameter of the peapod nanocables is about 60 nm. The periodic distance between the MnSe peas
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Figure 2. (a) SEM image of the nanocables homogenously grown on the substrates. (b) TEM image shows the general morphology of the MnSe/ SiOx core/shell nanocables. (c) Atomic-resolved image and its corresponding FFT ED pattern (inset), revealing the single-crystalline R-MnSe nanowire core grown uniformly with the [100] direction. (d) EDX line-scan profile of the nanocable shows Mn and Se in the core, and Si and O in the shell. (e) SEM image of the branched nanocables, and their (f) TEM image and SAED pattern (insets) showing that the MnSe nanowire (in the stem) and the MnSe nanorods (in the branches) are uniformly sheathed by the SiOx outer layers and have the [100] growth direction.
is about 100 nm. The SiOx pods contain not only the separated MnSe peas but also the not-separated longish MnSe peas (Figure 3c). All of the peas consisted of single-crystalline R-MnSe nanocrystals, whose [100] direction was aligned with the long axis of the nanocables. Figure 3d shows the atomic-resolved TEM image of the MnSe nanocrystals. The (020) plane fringes are separated by about 0.27 nm, which is consistent with that of the R-MnSe crystal. The FFT ED pattern further confirms that the [100] direction is aligned with the nanocable axis (inset). The scanning TEM (STEM) image (elemental mapping) reveals that the peas consist of Mn and Se components, while the pods
are made up of Si and O components (Figure 3e). The lefthand image shows the corresponding TEM image. In the region which has a lower temperature than that in which the peapods are formed, another unique peapod nanostructure is grown. Figure 3f shows the SEM image of the wirelike nanostructures decorated with particles. The TEM image reveals that the tiny MnSe nanocrystals (diameter < 10 nm) are encapsulated inside the tubelike amorphous SiOx layers and that the oval-shaped amorphous particles (diameter ) 300-500 nm) surround the tube layers periodically with a distance of 500 nm to 1 µm (Figure 3g). Figure 3, parts h and I, reveals their
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Figure 3. (a) SEM image showing the high-density peapod-structured nanocables. (b) TEM image reveals that the MnSe peas are encapsulated periodically in the SiOx pods. (c) The nanocrystals are frequently not separated into spherical peas. (d) Atomic-resolved image of the MnSe peas and FFT ED pattern (inset), showing the single-crystalline cubic R-MnSe nanoparticles whose [100] direction is aligned with the nanocable axis. (e) STEM elemental maps of Mn, Se, Si, and O elements for the peapod, whose TEM image is shown on the left. (f) SEM image of the wirelike nanostructures decorated with particles, and (g) TEM image showing the peapod-type structure in which the MnSe nanocrystals are encapsulated inside. (h), (i) HRTEM images revealing the ruptured holes, the bumpy amorphous particles, and the encapsulated crystalline MnSe nanoparticles. (j) EDX line-scanning showing the MnSe nanoparticles embedded in the SiOx layers surrounded by the MnOx particles.
ruptured and hollow tube inside, encapsulated crystalline MnSe nanoparticles, and bumpy amorphous surface. The EDX linescan for the bumpy part reveals the embedded MnSe nanocrystals and the SiOx layers, surrounded by the bumpy layers that are composed mainly of Mn (Figure 3j). In the lower-temperature region, “diamond necklace” morphology nanostructures, in which diamond-shaped nanocrystals are connected in a row, are grown with high density (Figure 4a). The TEM image reveals that the nanocubes are linked in chains, so as to construct a network (Figure 4b). The size of the nanocubes is in the range of 100-200 nm. These nanocubes have six equivalent sides enclosed with the (100) planes, and connected with the other nanocubes through their [110] or [100] direction to form the chain network. The connection between the nanocubes, if it is along the [110] direction, results in a straight chain. The SAED pattern confirms the [110] direction for the nanocube chain. When the link between the nanocubes takes place along the [100] direction, the chain is bent by 45 degrees. All of the nanocubes form a single-crystalline chain network. The SAED patterns confirm the [110] and [100]
directions for the nanocube chains (insets). Figure 4c shows an exclusive zigzag-surface single-crystalline nanocube chain, in which a number of nanocubes are aligned on a thick wire (diameter ) 200 nm) through their [110] direction. The TEM image shows the network between the nanocubes occurring along the diagonal direction, equivalent to [110] (Figure 4d). The atomic-resolved image for the edge part of the nanocube reveals the presence of a few defects in the single-crystalline cubic R-MnSe phase (Figure 4e). The FFT ED pattern confirms that the equilateral sides of the nanocubes are grown along the [100] direction (inset). (2) Growth Mechanism for the Morphology Evolution. From the structural information that was obtained, schematic models were constructed for these MnSe 1D nanostructures (Figure 5). As the growth temperature decreases from approximately 900 to 800 °C, the nanostructures adopt a more distinctive morphology. The growth of the MnSe 1D nanostructures is considered to follow a mechanism involving vaporliquid-solid phase changes; Mn and Se vapors diffuse into the catalytic Au nanoparticles on the substrates, saturate into the
Morphology-Tuned Growth of R-MnSe 1D Nanostructures
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Figure 4. (a) SEM image of the “diamond necklace” nanostructures. (b) TEM image and SAED pattern (inset) shows the chain structure connecting the nanocubes along the [110] and [100] directions. (c) TEM images revealing the nanocubes aligned on a thick wire through their [110] direction. (d) Typical HRTEM image for the typical connection through the diagonal [110] direction of the nanocubes. (e) The atomic-resolved image and corresponding FFT ED pattern (inset), revealing the single-crystalline cubic R-MnSe nanocrystals.
nanoparticles, and then precipitate to form the MnSe nanocrystals. The Si vapor supplied from the heated Si substrates and the O from the residual air would diffuse into the Au nanoparticles and precipitate as amorphous SiOx layers, together with the MnSe nanocrystals, following the VLS mechanism. Since few nanoparticles exist at the tip, the growth would follow a base-growth model that the nanoparticles remain at the substrates. In order to explain the formation of the various MnSe/SiOx composite nanostructures, we suggest that competition occurs between the 1D growth rates of the MnSe (VMnSe) and SiOx (VSiOx) nanocrystals. The VMnSe relative to VSiOx, depending on the growth temperature, and corresponding structures (denoted as Structures I-V) are schematically drawn in this figure. The yellow box at the bottom represents the estimated temperature range for the growth of the amorphous SiOx layers. At the highest temperature, pure MnSe nanowires (Structure I) are grown under the condition where VMnSe . VSiOx. As the temperature decreases, the growth condition is adjusted to be VMnSe ≈ VSiOx, so that the nanocable structure can be produced (Structure II). The branches can be grown on the surface of the SiOx outer layers of the nanocables, if the combined vapor pressure of MnSe and SiOx is high enough to nucleate and precipitate on it. If the growth condition comes to be VMnSe < VSiOx, where the growth of the MnSe nanowire core cannot follow the growth of the SiOx outer layers, peapod structures could be produced (Structure III). Similarly to these structures, Au, Ag, Si, and ZnSe peas encapsulated in silica pods were previously
reported.27-32 However, the formation of MnSe peas has not been reported so far. Upon further cooling of the substrates, both growth rates are slowed down, but the condition VMnSe , VSiOx is still maintained. The encapsulated MnSe nanocrystals become smaller in size and would thus easily evaporate if their size falls below a certain critical value. Such evaporation of the MnSe nanocrystals may result in the formation of the hollow and ruptured inside of the SiOx outer layers and the bumpy Mnrich oxide surface. The Mn component would remain as an amorphous phase surrounding the Si outer layers, whereas the more volatile Se component would evaporate away, thus forming the unique Structure IV. As the temperature of the substrates decreases further, the MnSe nanocube-chain networks are grown, as shown in Structure V. The deficient SiOx vapor pressure would result in the condition VSiOx ≈ 0. From the symmetry point of view, the (100), (010), and (001) surfaces of the cubic system are identical. Under the thermodynamically controlled condition, the growth of the nanocrystals can proceed along either six equivalent 〈100〉 directions; thus it is possible for the nanocube structure to be formed. The production of the nanocube chains is related to their 1D growth along either the [100] or [110] direction. In contrast to the nanowires and nanocables grown along the [100] direction, the 1D growth along the [110] direction allows the nanocubes to connect to each other through their diagonal direction. This implies that the 1D growth in the [110] direction becomes more favorable. It has been suggested that the surface energy of a cubic structure may be in the order γ{111} < γ{100} < γ{110}.33 The preferential growth direction of the MnSe
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Figure 5. Schematic diagram for the MnSe 1D nanostructures with various morphologies that are synthesized in the present work.
Figure 6. (a) XRD pattern, and (b) XPS spectrum of the R-MnSe powders and R-MnSe nanowires.
nanowires (also of the nanocables) is [100], which is probably determined by the growth kinetics at the higher temperature. As the growth temperature decreases and so falls into the thermodynamic regime, however, the low gas-phase supersaturation promotes the diffusion of Mn/Se toward these highestsurface-energy (110) planes. The facets of these nanowires grown with the [110] direction can be bounded with the lowsurface-energy planes. This high energy of the growing surface planes would determine the unique morphology of these nanocube-chain networks. However, it has to be said that these growth models may be too speculative to provide the proper growth mechanism for these MnSe nanostructures, so further extensive studies are needed to fully understand the present results. (3) XRD, XPS, and PL of R-MnSe Nanowires. The XRD pattern taken from the nanowire (Structure I) sample is displayed with that of commercial MnSe microcrystal powders (Stream Chemicals, 99.9%-Mn), confirming that only cubic R-MnSe
crystals are formed (Figure 6a). The nanowires have even higher purity than the MnSe powders containing SeO2 impurities (the peaks marked by an asterisk). The peak width of the nanowires is even narrower than that of the MnSe powders, probably due to the high degree of crystalline perfection. The same XRD pattern was measured from the other nanocable and nanocube samples (not displayed here). Figure 6b shows the XPS survey scan spectrum of the MnSe nanowire and powder samples obtained using a photon energy of 1486.6 eV. The compositions are found to be Mn/Se ) ∼1:1, and there are no other elements except for C and O contaminants. The Si peaks of the nanowires arise from the Si substrates. Figure 7 shows the PL spectra of the MnSe nanowires (Structure I) measured in the temperature range of 7-300 K. The excitation photon energy is 3.815 eV. The same PL spectrum was measured from the other samples (not displayed here). There are two broad emission bands at around 2.85 and 3.0 eV. These two spit bands were also reported for R-MnSe
Morphology-Tuned Growth of R-MnSe 1D Nanostructures
J. Phys. Chem. C, Vol. 111, No. 2, 2007 525 edge emission bands at around 2.85 and 3.0 eV and the Mn2+ emission band at 1.65 eV, which appears at temperatures below 100 K, providing the evidence for the octahedrallly coordinated Mn2+ sites of the R-MnSe structure. The present results demonstrate how widely the morphology of the R-MnSe 1D nanostructures can be tuned and provide important insight for various novel 1D nanostructures. Acknowledgment. This work is supported by KRF grants (R14-2003-033-01003-0; R02-2004-000-10025-0; 2003-015 C00265). The SEM and HVEM measurements were performed at the Korea Basic Science Institute. The experiments at the PLS were supported in part by MOST and POSTECH. References and Notes
Figure 7. Temperature-dependent PL spectrum of the MnSe nanowires measured in the temperature range of 7-300 K. The excitation wavelength is 325 nm (3.815 eV) from an He-Cd laser. The inset corresponds to the time-resolved spectrum (in log scale) of the 1.65eV emission band at 7 K, excited by an Nd:YAG 266-nm (4.66 eV) laser.
nanospheres by Lei et al.24 Since the energy positions are similar to the band gap of the bulk R-MnSe (∼3.0 eV),4,5 we assign these bands to the band-edge emission, although further studies would be necessary to clarify their origin. Notice that the other band centered at around 1.65 eV appears at temperatures below 100 K. This band originates from the decay of the excited states of the octahedrally coordinated Mn2+ ions (derived from the spin-forbidden transition 4T1g f 6A1), generated via the usual energy transfer from the band states.5 The time-resolved PL spectrum (in log scale) of the Mn2+ emission (1.65 eV) at 7 K, excited by 266-nm (4.66 eV) radiation, is displayed in the inset. The fitting of the decay parts with a single-exponential function results in an average decay time of 18 µs, where it is reasonably slow for such spin-forbidden transition. This value remains nearly the same until 100 K, thereby providing the lifetime of the Mn2+ excited states. 4. Conclusions We synthesized MnSe 1D nanostructures with various morphologies by the CVD of MnCl2/CoSe/Si. The morphology was tuned from nanowires to MnSe/SiOx core/shell nanocables and peapods, as well as nanocube-chain structures, by adjusting the growth conditions. The MnSe nanowires (average diameter ) 70 nm) consisted of single-crystalline rock-salt R-MnSe structures with the [100] growth direction. The MnSe/SiOx core/ shell nanocables (average diameter ) 70 nm) were composed of a single-crystalline rock-salt R-MnSe nanowire core grown with the [100] direction and amorphous outer layers with a thickness of 20 nm. A short-branched nanocable structure was also synthesized. The spherical MnSe nanoparticles (diameter ) 40 nm) were encapsulated into the amorphous SiOx nanowires, forming a “peapod” structure. The MnSe nanocubes (size ) 100-200 nm) were connected in a chain along the [100] and [110] directions, forming a distinctive network structure. All of these morphologies were controlled by adjusting the growth temperature. We propose a growth mechanism in which the competition between the 1D growth rates of MnSe and SiOx results in the tuning of these morphologies. The XRD pattern and XPS spectrum confirm the formation of pure R-MnSe crystals. The temperature-dependent PL spectra show two band-
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