Direct Monolithic Integration of Vertical Single Crystalline Octahedral

Article pubs.acs.org/cm

Direct Monolithic Integration of Vertical Single Crystalline Octahedral Molecular Sieve Nanowires on Silicon Adrian Carretero-Genevrier,*,†,‡,§ Judith Oro-Sole,‡ Jaume Gazquez,‡ Cesar Magén,∥ Laura Miranda,§ Teresa Puig,‡ Xavier Obradors,‡ Etienne Ferain,⊥ Clement Sanchez,§ Juan Rodriguez-Carvajal,# and Narcis Mestres*,‡ †

Institut des Nanotechnologies de Lyon (INL), UMR-CNRS 5270, Ecole Centrale de Lyon, 36 avenue Guy de Collongue, 69134 Ecully, France ‡ Institut de Ciència de Materials de Barcelona ICMAB, Consejo Superior de Investigaciones Científicas CSIC, Campus UAB, 08193 Bellaterra, Catalonia, Spain § Sorbonne Universités, UPMC Université Paris 06, CNRS, Collège de France, UMR 7574, Chimie de la Matière Condensée de Paris, F 75005, Paris, France ∥ Laboratorio de Microscopías Avanzadas LMA-Instituto de Nanociencia de Aragón INA-ARAID Universidad de Zaragoza, 50018 Zaragoza, Spain and Departamento de Física de la Materia Condensada, Universidad de Zaragoza, 50009 Zaragoza, Spain ⊥ Institute of Condensed Matter and Nanosciences, Bio & Soft Matter (IMCN/BSMA), Université Catholique de Louvain, Croix du Sud 1,1348 Louvain-la-Neuve, Belgium, and it4ip s.a., rue J. Bordet (Z.I. C), 7180 Seneffe, Belgium # Institut Laue-Langevin, 6 rue Jules Horowitz, BP 156, 38042 Grenoble Cedex 9, France S Supporting Information *

ABSTRACT: We developed an original strategy to produce vertical epitaxial single crystalline manganese oxide octahedral molecular sieve (OMS) nanowires with tunable pore sizes and compositions on silicon substrates by using a chemical solution deposition approach. The nanowire growth mechanism involves the use of track-etched nanoporous polymer templates combined with the controlled growth of quartz thin films at the silicon surface, which allowed OMS nanowires to stabilize and crystallize. α-quartz thin films were obtained after thermal activated crystallization of the native amorphous silica surface layer assisted by Sr2+- or Ba2+mediated heterogeneous catalysis in the air at 800 °C. These α-quartz thin films work as a selective template for the epitaxial growth of randomly oriented vertical OMS nanowires. Therefore, the combination of soft chemistry and epitaxial growth opens new opportunities for the effective integration of novel technological functional tunneled complex oxides nanomaterials on Si substrates. KEYWORDS: oxide nanowires integration, octahedral molecular sieves, manganese oxides, nanowires, quartz layers, soft chemistry, epitaxial growth, hollandite, strontiomelane

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INTRODUCTION Nanostructured manganese oxides are of great interest, not only from the point of view of fundamental physics and materials science but also for technological applications.1−6 In this context, OMS are getting significant attention because of their unique mixed valence character, thermodynamic stability, tunable pore size, versatile nanostructures, and their applications in catalysis, separation, ion sensing, and energy storage.7−11 Among the large repertoire of OMSs, manganese oxides displaying a tunnel structure, such as hollandites,8 have become extremely interesting due to their capability to accommodate cations within their porous architecture. Owing to their structural features, consisting of chains of edge-sharing manganese oxide (MnO6) octahedra12 which interlink through © 2013 American Chemical Society

their corners to form tunnels, a large number of OMS containing alkali and alkaline earth cations, such as Ba2+ (defined as proper hollandite), Sr2+ (strontiomelane), Pb2+ (coronadite), K+ (cryptomelane), Na+ (manjiroite), and Ag+, can be artificially synthesized.13−16 However, the presence of these cations within the structure gives rise to a charge imbalance compensated by the reduction of some Mn4+ to Mn3+ in the manganese octahedral framework. This mixed valence in the structure can be further exploited to develop materials with good semiconducting17 and magnetic properties.18−20 Received: September 13, 2013 Revised: December 13, 2013 Published: December 19, 2013 1019

dx.doi.org/10.1021/cm403064u | Chem. Mater. 2014, 26, 1019−1028

Chemistry of Materials

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Figure 1. FE-SEM images taken at interrupted intermediate temperature steps during the OMS nanowire formation on Si, together with the schematics of the grow process. (a) Cross-sectional view FE-SEM of the nanoporous polymer template (thickness 7.4 μm, pore size 200 nm) taken before infiltrating the nanopores with the precursor solution. (b) FE-SEM image taken after the sample has been heated up to 600 °C and quenched. Precursor oxide nanorods with the dimension of the original template nanopores are formed. At this temperature, the polymer template has been decomposed. (c) Cross-sectional view of the FE-SEM image of the nanowires formed on top of the Si substrate after a thermal treatment at 800 °C. (d) Schematics showing the steps depicted in the upper arrow figures from the polymer template filling with the precursor solution at RT to the nanowire formation at 800 °C. Low magnification and high magnification (in the insets) images of the final OMS nanowires obtained using. (e) La, Sr, Mn precursor salts (LaSr-2×4 OMS), (f) Ba-Mn precursors (Ba1+δMn8O16), (g) Sr-Mn precursors (Sr1+δMn8O16), and (h) Ba, Sr, Mn precursors ((BaSr)1+δMn8O16).

size on silicon wafers. This new synthesis method takes advantage of the recent development of soft-chemistry based routes to integrate epitaxial quartz films on silicon substrates.41 These α-quartz layers on single crystalline silicon substrates have been rationally exploited as a growth platform for the epitaxial stabilization of different single crystalline OMS nanowires. The generality of this original crystal growth mechanism is illustrated by the synthesis and characterization of hollandite-type OMS single crystalline nanowires, including Ba1+δMn8O16,20 Sr1+δMn8O16, (BaSr)1+δMn8O16, and the new LaSr-2×4 OMS,19 on top of (100)-silicon substrates. Importantly, LaSr-2×4 OMS nanowires display enhanced ferromagnetic properties with a Curie temperature higher than 500 K,19 and Ba1+δMn8O16 hollandite nanowires show a ferromagnetic ordering at low temperatures (∼40 K).20 This work is also of potential interest for the study of geological processes as it evidences, on a laboratory scale, of the close relation between natural quartz deposit formations and manganate oxides with alkaline earth cations existing in nature.

Moreover, nanoscale OMSs, due to their small particle size and large surface areas, are good candidates for catalysis and battery applications, as well as for selective absorption in environmental chemistry, thermoelectricity, and nanomagnetism.19,21−27 Motivated by the potentiality of these compounds, different synthetic approaches including hydrothermal treatment of a layered precursor such as birnesite;28−31 solvent free,32 sol−gel,33 or ionic liquid routes;34 and aqueous routes implying Mn2+ oxidation,35 MnO4 reduction30 or Mn2+/MnO4 comproportionation36 have been developed to obtain onedimensional (1D) OMS nanoparticles. Their typical lengths ranged between a few hundred nanometers and a few micrometers, with the diameter varying from 30 to 100 nm. Unfortunately, the reported hydrothermal and reflux routes need solvents in the reaction system and usually require long reaction times (>24 h). An additional drawback is that a mixture of 2D nanoplatelet-like and 1D-fibrous morphologies commonly appear in synthetic OMS nanostructures prepared by different methods.7,37,38 A great effort is currently being devoted to combine the functionality of oxide nanomaterials with the performances of semiconductor electronics to enable the development of novel and more efficient device applications. However, the future incorporation of functional oxide nanostructures as active materials in electronics critically depends on the ability to integrate crystalline metal oxides into silicon structures. Recent work has successfully explored ways to integrate ferroelectric,39 ferromagnetic,40 and piezoelectric thin films41 and nanowires42 on silicon substrates. Here, we present a novel chemical solution synthesis route for the direct integration of single crystalline manganese based OMS nanowires with tunable composition and microporous

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RESULTS AND DISCUSSION On the basis of the previous development of a synthesis route to grow epitaxial quartz films on silicon substrates,41 here we used (100)-silicon substrates without previous chemical etching to preserve the native oxide layer. This SiO2 native layer was first determined through ellipsometry, showing a thickness of around 3 nm. Then, nanoporous track-etched polymer membranes with a controlled cylindrical pore shape and a narrow pore size distribution coating the Si substrates were used as templates. Polymer templates displaying a 200 nm diameter and 7 μm length voids were further filled by capillary absorption with the appropriate precursor solution containing 1020



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Figure 2. Schematics of the stages of the crystallization process for single crystalline OMS nanowires on silicon. (1) Track-etched nanoporous polymer template supported on a SiO2/Si substrate filled with the chemical precursor solution allowing a homogeneous distribution of Ba2+ or Sr2+ melting agents. (2) At mild temperatures (500−600 °C), MnO2 nanoparticles are nucleated and stabilized due to the confinement in high aspect ratio nanopores. (3) Next, devitrification of the SiO2 layer and nucleation of α-quartz takes place at the interface, where epitaxial growth of OMS nanowires on the quartz is promoted at high temperatures (800 °C).

nanopores had a determinant influence on the formation of ε-MnO2 nanoparticles phases during the calcination of the precursor solution. Most specifically, nucleation and stabilization of hexagonal chemically pure ε-MnO2 phase nanoparticles at low temperatures (∼500 °C) will serve as seeds for the further growth of different manganate nanowires under high temperature treatments (i.e., 800 °C). These manganate structures need high enough temperatures to enhance atomic mobility and allow the diffusion of the alkaline earth and rare earth cations into the empty 1D channels of the ε-MnO2 structure. In our work, the polymer template nanopores are not used as mere physical spatial constraints defining the shape of the final nanostructures but rather as nanoreactors in which confined nucleation favors the stabilization of particular metastable seed nanostructures. Additionally, this mechanism takes advantage of the fact that the intimate contact of the silica native layer with alkaline-earth metal cations during the thermal treatment in oxidizing atmospheres hastens devitrification to an α-quartz layer, as recently demonstrated by the authors.41 This mechanism can be extended to any nanowire composition, as long as Ba2+ and Sr2+ are present in the initial precursor solution (or other alkaline earth cations). In our case, the homogeneous distribution of the catalyst cations required for the crystallization of quartz is provided by the confinement of the precursor solution in the template (step 1 in Figure 2). Due to the relatively low annealing temperature (800 °C), the quartz obtained on top of single crystalline Si substrates consists of an α-quartz polycrystalline layer, step 2 in Figure 2. This polycrystalline α-quartz film renders better lattice matching to the complex oxide nanostructures favoring the epitaxial growth of the different manganese oxide nanowires (step 3 in Figure 2). Since different spatial orientations of the quartz crystallites are possible, the resulting nanowires will be, as a consequence,

alkaline earth metal cations. Once fully loaded with the liquid precursor, the system was submitted to thermal treatment at high temperatures (up to 800 °C) and under static lab air conditions. General schematics of the growth process together with scanning electron microscopy (SEM) images acquired for all the different hollandite-type OMS single crystalline nanowires (NWs) at intermediate steps are shown in Figure 1. At initial stages, template nanopores (Figure 1a) are filled with the precursor solution. The corresponding field emission gun (FEG)-SEM images taken at a quenched intermediate temperature stage, i.e. 600 °C (see Figures 1b and SI-1), show that the silicon substrate was already covered with a high density of nanorods that kept the original dimensions of the polymer template voids. At this intermediate growth stage, the system has reached the polymer decomposition temperature, thus allowing the complete removal of the template. Upon higher temperature treatment (i.e., 800 °C), FEG-SEM images shown in Figure 1c and Figure SI-1 prove that the whole sample surface is already covered with both vertical and randomly inclined nanowires without any preferential direction on top of the substrate. The measured length of the resulting nanowires was between 7 and 10 μm, with diameters of about 100 ± 20 nm. This confined heteroepitaxial growth mechanism was observed for all the compositions tested, obtained from different alkaline earth precursor salts (see Figure 1e−h), thus giving rise to the formation of Ba1+δMn8O16, Sr1+δMn8O16, (BaSr)1+δMn8O16, and LaSr-2×4 single crystalline OMS nanowires on top of (100)-silicon substrates. Figure 2 shows a detailed scheme of the proposed mechanism for the nanowire formation on top of silicon substrates based on the combination of soft chemistry routes and confined epitaxial growth. In a previous work,19 we already proved that during thermal treatment, the confinement imposed by the polymer template in high aspect ratio 1021



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Figure 3. (a) Low magnification HAADF image of LaSr-2×4 nanowires epitaxially grown on an α-quartz/Si substrate (800 °C during 5 h). (b) HRTEM image of the interface between quartz layer an epitaxial LaSr-2×4 nanowires, viewed along [010]. The insets show the FFT of both phases. (c) Enlarged view of the interface showing the epitaxial relation between the LaSr-2×4 nanowires and the α-quartz according to [20−2] LaSr-2×4// [−101] α-quartz. (d) X-ray diffracted intensity recorded for single crystalline LaSr-2×4 OMS nanowires grown on an α-quartz/Si substrate by a twodimensional GADDS detector. The stronger diffraction rings at 2θ values 20.85° and 26.66° match the (100) and (101) Bragg reflections of αquartz. (e) Low magnification FEG-SEM image of LaSr-2×4 nanowires epitaxially grown on an α-quartz/Si substrate. (f) Schematics of the atomic arrangement at the interface for the orientation relationship observed for LaSr-2×4 OMS nanowires on α-quartz [20−2] LaSr-2×4//[−101] αquartz.

quartz. Additionally, the fast Fourier transform (FFT) of Figure 3b and c reveal an in-plane epitaxial relationship between LaSr2×4 NWs and quartz given by [20−2] LaSr-2×4//[−101] αquartz. The schematics of the atomic planes arrangement at the epitaxial interface between the LaSr-2×4 NW and the quartz substrate is displayed in Figure 3f. A similar analysis performed on different nanowires (see Figure SI-3) showed other possible epitaxial relationships between the LaSr-2×4 NW and α-quartz, making it evident that epitaxy is critical for the stabilization and growth of the single crystalline OMS nanowires on the α-quartz layer. In addition, in spite of the high growth temperatures used (i.e., 800−900 °C), no appreciable interdiffusion of Si into the NW bases was detected from electron energy loss spectroscopy (EELS) studies. In the case of Ba hollandite nanowires, we observed the same mechanism described above. The proof of that is displayed in Figure 4, where a side view of the FE-SEM image shows nanowires synthesized from Ba and Mn precursor salts after a thermal treatment at 800 °C for 5 h (Figure 4a). Indeed, the whole sample surface was covered with nanowires vertically grown on top of the silicon substrate with no evident predominant direction. The length of the NWs was between 5 and 7 μm with diameters of about 100 ± 20 nm. The higher magnification HAADF image of a single NW (Figure 4b) provides evidence that NWs have a smooth surface, uniform diameter, and are single crystalline as revealed by the electron diffraction pattern of a single nanowire (see inset image in Figure 4b). A detailed distribution of elements along NWs was performed using STEM in combination with electron-energyloss spectroscopy (EELS). Figure 4c displays an EELS spectrum of a single NW, showing the O K, Mn L2,3, and Ba

oriented with the same epitaxial relation but in different directions. The LaSr-2×4 nanowire system, of which structural and magnetic properties were described in detail in an early work,19,43 represents a good example of the important role played by the intermediate α-quartz layer in the OMS nanowire growth process. The composition and crystalline structure of the interfacial templating layer was elucidated by means of Xray diffraction and transmission electron microscopy (TEM; Figure 3 and Figure SI-2). Figure 3a shows a cross-sectional high-angle annular dark field image (HAADF) scanning transmission electron microscopy (STEM) image of the sample. The brighter rods correspond to the randomly oriented LaSr-2×4 nanowires, which cover the surface of the α-quartz layer. Both phases can be identified in the X-ray diffraction pattern of Figure 3d, measured with a general angle detector diffraction system (GADDS). The x axis corresponds to 2θ, and the rings correspond to angular direction χ, which varies with constant 2θ. Besides the diffraction rings associated with the LaSr-2×4 NWs phase, the stronger rings clearly identified at 2θ values of 20.85° and 26.66° match the (100) and (101) Bragg reflections of α-quartz with trigonal crystallographic structure (P3221, No. 154 or indexed according to ICDD (international center for diffraction data) entry 88−2487; see Figure SI-2). The different crystallographic orientations of quartz crystals will impose different orientations of the grown nanowires with respect to the substrate plane, as revealed by high resolution TEM (HRTEM) analysis (see Figures 3b and SI-3). Figure 3b shows a bright field HRTEM image of a LaSr-2×4 nanowire epitaxially grown on top of the α-quartz layer. A higher magnification image of the interface along the [010]-Si zone axis displayed in Figure 3c shows (010) LaSr-2×4//(010) α1022



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Figure 4. (a) Side view SEM image of BaMn8O16 nanowires grown on an α-quartz/Si substrate at 800 °C showing the regular distribution of nanowires. (b) TEM image of a single crystalline BaMn8O16 nanowire. The inset shows the electron diffraction pattern along the [001] zone axis. (c) EEL spectrum from the nanowire displayed in b, together with the elemental maps corresponding to O K, Mn L2,3, and Ba M4,5 edges. (d) High resolution HAADF image of a BaMn8O16 nanowire grown at 800 °C along the [001] zone axis. The inset shows the FFT of the image. (e) High resolution FEG-SEM image of two BaMn8O16 nanowires grown at 800 °C on the α-quartz layer. (f) Enlarged view of the nanowire quartz layer interface. The inset shows the FFT image together with the epitaxial relationship according to (001) BaMn8O16[010]//(−101) α-quartz [010]. (g) Higher magnification HAADF image of the nanowire/quartz interfacial region.

(see the inset) confirms that the BaMn8O16 nanowires are compatible with the determined monoclinic lattice. The higher magnification SEM image showed in Figure 4e evidence that NWs were anchored to the quartz substrate. Figure 4f and g present a high resolution HAADF image of the interface between a BaMn8O16 NW and the α-quartz interlayer. The FFT shows two separate sets of spots indexed considering a [001] zone axis of the monoclinic structure (white) corresponding to the BaMn8O16 NWs and a [−101] zone axis (green) corresponding to the α-quartz. Accordingly, the epitaxial relationship extracted between the BaMn8O16 NWs and α-quartz layer is (001) BaMn8O16[010]//(−101) α-quartz [010]. Moreover, in order to confirm the monoclinic unit cell of the BaMn8O16 NWs, a reciprocal space reconstruction was performed. We used a set of electron diffraction images obtained by rotating the crystal around the a* axis, as is schematically shown in Figure 5. The reconstruction revealed the following experimental real cell parameters: a = 9.75 Å, b = 2.85 Å, c = 9.9 Å, and β ≈ 89.96° with the long axis of the nanowires along the b direction. The literature parameters for this phase extracted from powder X-ray diffraction patterns

M4,5 edges (at energy losses of 510, 655, and 781 eV, respectively). Elemental maps were obtained from EELS spectrum images and show a uniform distribution of Ba, Mn, and O along the nanowire. EELS spectra of several nanowires were also used to quantify their atomic composition. The molar ratios of elements were assigned relative to the amount of oxygen, which was set to one. The resulting ratio is 100/59/11 for oxygen/manganese/barium, respectively. These ratios are in total agreement with the Ba1.2Mn8O16 composition corresponding to the Ba hollandite.20 Moreover, the absence of reactants and secondary phases in the final product, as indicated by STEM-EELS, proves that Ba hollandite is the most stable phase under the experimental conditions, this is, the confined nucleation and the epitaxial template imposed by the α-quartz layer. Figure 4d shows a high resolution HAADF image along the [001] zone axis of an individual nanowire. The distances between atomic planes observed in the image are about 9.75 Å and 2.87 Å, corresponding to the spacings of the (100) and (010) planes of the BaMn8O16 hollandite monoclinic structure, respectively.20 The diffraction pattern generated by the FFT 1023



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Figure 5. (a) Elemental mapping for Ba, O, and Si indicating the formation of the quartz layer at the nanowires−Si interface. (b) Series of electron diffraction patterns obtained when the BaMn8O16 nanowire tilted along the [100]-axis, with which the unit cell of the crystal is determined. Full black circles are in the reciprocal plane (0kl) and open circles are in the plane (1kl; for more details, see the SI).

Figure 6. (a) Cross-sectional view SEM-FEG image of SrMn8O16 nanowires grown on an α-quartz/Si substrate at 800 °C showing the regular distribution of nanowires. (b) Elemental maps corresponding to O K, Mn L2,3, and Sr L2,3 edges, showing a uniform element distribution across a single nanowire, and (c) EEL spectrum generated of a single nanowire. (d) Low resolution HAADF image of one SrMn8O16 nanowire grown at 800 °C on the α-quartz layer. The inset shows the electron diffraction pattern along the [114] zone axis. (e) High resolution HAADF image of the nanowire shown in d. The inset shows the corresponding FFT pattern. (f) Higher magnification HAADF image area highlighted in blue in e. A 3D model of the tetragonal unit cell along the [114] direction indicates the [001] direction that coincides with the longitudinal axis of SrMn8O16 nanowires.

correspond to a monoclinic cell with a = 10.052(1) Å, b = 2.8579(2) Å, c = 9.7627(10), and β = 89.96(1)°.20 Figure 5a also displays a compositional analysis conducted by energy dispersive X-ray spectroscopy (EDX) elemental mapping. This analysis shows that the NWs contain a homogeneous

composition of Ba, Mn, and O, which is in agreement with the EELS results described above, whereas silicon is only detected in the substrate. The oxygen and silicon mappings reveal the formation of a ∼250 nm thick α-quartz layer at the substrate−NWs interface. 1024



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Figure 7. (a) Series of electron diffraction patterns obtained when the SrMn8O16 nanowire is tilted along the [001] axis, with which the unit cell of the crystal and space group are determined. Full black circles are in the reciprocal plane (0kl) and open circles are in the plane (1kl), red lines define a projected part of the reciprocal space of the tetragonal reconstructed network in part b. (b) Experimental distribution of SrMn8O16 electron diffraction data along the [001] crystallographic direction. Pseudo-tetragonal unit cell can be indexed superimposed to the 3D distribution (see SI for more details). (c) Elemental mapping for Sr, O, and Si indicating the formation of the quartz layer at the nanowire−Si interface.

resolution HAADF images of a single Sr1+δMn8O16 nanowire, as shown in Figure 6d−f. This new Sr1+δMn8O16 structure proves that our synthesis method leads to new crystalline structures that might present new magnetic properties, as previously observed in LaSr-2×4 NWs.19 As a consequence, magnetic properties of the new tetragonal structure of SrMn8O16 NWs have been investigated (summarized in Figure 8). Temperature dependent magnet-

Finally, strontium based nanowires were also obtained using the same experimental procedure. Figure 6a shows a typical FESEM image of a sample grown at 800 °C over 5 h, displaying a homogeneous distribution of nanowires with diameters in the range of 80 ± 20 nm and 8−10 μm length all over the sample. The elemental composition maps extracted from the STEMEELS spectrum imaging revealed ratios of 100/64/11 for oxygen/manganese/strontium, respectively. These ratios are in agreement with the Sr1.2Mn8O16 composition. This composition would correspond to a hollandite-like structure with a 2 × 2 tunnel arrangement and with Sr2+ cations inside the tunnels instead of Ba2+.44 As published elsewhere,44 standard strontiomelane has the ideal chemical formula SrMn8O16 and P21/n symmetry, which is a subgroup of I2/m and the most common space group within hollandite-type OMS. In order to determine the unit cell of synthesized Sr1+δMn8O16 nanowires, the program RESVIS,45 within the FullProf suite,46 has been extended. This new version of the program is capable of determining the unit cell corresponding to a series of electron diffraction images containing a common reciprocal direction (see Figure 7a and the SI for a detailed explanation). The calculated unit cell of Sr1+δMn8O16 nanowires corresponds to a new pseudotetragonal (within the experimental error) body centered cell with lattice parameters a = 25.2 Å, c = 5.7 Å, and α = β = γ = 90° (see Figure 7b and SI). This unit cell had a different symmetry to that expected for strontiomelane, which is related to hollandite with the doubling of the b axis (unit cell: a = 10.05 Å, b = 5.76 Å, c = 9.88 Å, β = 90.64°). However it is possible to compare both structures since the c axis of the new tetragonal cell corresponds to the b axis of strontiomelane44 (cT ≈ bM) and the other axes are correlated by the approximate relations aT = 2(aM + cM) and bT = 2(aM − cM), if we take a shortened value aM ≈ cM ≈ 9 Å that gives aT ≈ cT ≈ 25.45 Å. This clearly indicates that the structure of Sr1.2Mn8O16 nanowires should correspond to a superstructure of the hollandite-type compounds (see SI-8). Crystalline structure was also confirmed through high

Figure 8. (a) Temperature dependence of the magnetization M(T) of Sr1+δMn8O16 nanowires for ZFC and FC under a 500 Oe field. (b) Hysteresis loop of Sr1+δMn8O16 nanowires measured at 10 K for fields parallel to the silicon substrate plane.

ization curves of the SrMn8O16 NWs on the silicon substrate were measured in zero field cooled (ZFC) and field cooled (FC) processes at an applied field of 500 Oe by using a SQUID magnetometer (see Figure 8a). The ZFC curve shows a sharp peak near 40 K and an evident separation from the FC curve below 45 K, thus indicating a magnetic ordering at low temperatures of the SrMn8O16 NWs sample. Figure 8b displays the magnetic hysteresis loop measured at 10 K displaying a coercive field of 8.0 KOe, which is a similar magnetic behavior to that reported in other hollandite nanowires. In particular, low temperature hysteresis effects have been observed for Ba1.2Mn8O16,20,47 BaMn2Ru4O12,48 and Na2−xMn8O16.49 For SrMn8O16 NWs samples, any linear field dependence of magnetization that could be ascribed to an 1025


Chemistry of Materials antiferromagnetic behavior has been suppressed by subtracting the diamagnetic signal of the silicon substrate. Likewise, our synthesis method can be used to tune the nanowires’ composition as confirmed by EELS-TEM analysis in Figure SI-7, where nanowires with two different alkaline earth cations, namely Ba2+ and Sr2+, in the tunnels of the hollandite structure, were epitaxially grown on silicon substrates. These new OMS materials might very well show different electrochemical performance, catalytic activity, and selectivity. In nature, quartz and OMS minerals are closely related and even coexist. Ba2+ or Sr2+ hollandite phases can be found forming needlelike inclusions or intergrowths in manganeseenriched quartz veins. Yet, geological deposits rich in silica and manganese bearing fluids display an interesting spectrum of trace elements such as Ba, Sr, Pb, and Zn. As a result, these elements might have promoted the devitrification and formation of post-tectonic hydrothermal quartz veins. Thereafter, or simultaneously, these trace elements will become the hollandite−strontiomelane species found in the minerals, suggesting that these two crystalline structures are connected.44,50,51 Moreover, the absence of any Mn oxide phase free of Ba, Na, Sr, etc. in natural mineral species suggests that these elements actually stabilize the hollandite−strontiomelane solid solutions encountered in nature. Our work shows that this synthesis method for OMS nanowires mimics the natural process of hollandite-type manganese compound formation on the laboratory scale.

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EXPERIMENTAL SECTION

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ASSOCIATED CONTENT

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Samples Preparation. Ba 1+δ Mn 8 O 16 , Sr 1+δ Mn 8 O 16 , and (BaSr)1+δMn8O16 OMS nanowires were prepared from either 1 M Ba(OH)2, Sr(Ac)2, or a mixed solution (1:1) of both of them in acetic solution and 1 M Mn(Ac)2 in a water solution, keeping always an excess of Ba or Sr of at least 30% respect to Mn volume to ensure the proper crystallization of the quartz film. This range of proportions made it possible to stabilize, under any of the conditions, the corresponding OMS nanowires. The final synthesis step entails mixing the previous Ba, Sr, and Mn solution into two volumes of ethanol with the objective to enhance the template filling and further optimal evaporation of the solvent (e.g., 3−5 mL of 1 M Ba(OH)2, 5 mL 1 M of Mn(Ac)2 in 10 mL of ethanol). LaSr-2×4 OMS nanowires were prepared from 1 M aqueous solutions composed of La(NO3)3·6H2O, Sr(NO3)2·4H2O, and Mn(NO3)2·4H2O in the proportion 07:0.3:1, respectively. The addition of ethylene glycol (EG) heated above 100 °C promotes polymerization of the EG in order to reach the optimum viscosity value required for the filling of the template’s pores. Common to any of the above-described synthesis methods, track etched polymer templates were provided by it4ip (Belgium)52 and prepared by irradiation of polyimide or polycarbonate directly supported on single crystalline Si substrates by using heavy ions and posterior chemical development as described elsewhere.18,19 Then, all precursor solutions were used to fill the nanopores of the polymer template. A thermal treatment of 800 °C for 5 h (ramp temperature 3 °C min−1) in static lab air was applied directly after filling a nonporous template in a tubular oven in order to obtain vertical epitaxial OMS nanowires on a silicon substrate. Samples Characterization. The OMS nanowire structure was investigated using a field emission gun scanning electron microscope (FEG-SEM), Hitachi’s SU77. Cross-sectional transmission electron microscopy (TEM) studies were performed using an FEI Titan3 operated at 300 kV and equipped with a superTwin objective lens and a CETCOR Cs objective corrector from CEOS Company. For STEMEELS spectrum imaging, a Nion Ultrastem was used, operated at 100 kV and equipped with a Nion aberration corrector and a Gatan Enfina EEL spectrometer, and an FEI Titan 60−300 microscope equipped with an X-FEG gun, a CETCOR probe corrector and a Gatan energy filter TRIDIEM 866 ERS operated in the STEM mode at 300 kV. In these microscopes, the aberration-corrected probe yields a routine spatial resolution of about 1 Å, and the high-angle annular dark field detector allows recording incoherent Z-contrast images, in which the contrast of an atomic column is approximately proportional to the square of the average atomic number (Z). Specimens for TEM observation were prepared by conventional methods, by grinding, dimpling, and Ar ion milling. Reciprocal space reconstruction and determination of the different unit cells were investigated by using a Jeol 1210 transmission electron microscope operating at 120 KV, equipped with a side-entry 60/30° double tilt GATHAN 646 analytical specimen holder and a link QX2000 XEDS element analysis system. Xray diffraction measurements were carried out using a Bruker AXS GADDS equipped with a 2D X-ray detector. Ellipsometry measurements were performed on a UV−visible (from 240 to 1000 nm) variable angle spectroscopic ellipsometer (VASE2000U Woollam), and the data analysis was performed through Wvase32 software using Cauchy models in the visible range. The chemical composition of the final solution has been investigated using inductively coupled plasmaatomic emission spectroscopy analysis on a Thermo Elemental Intrepid II XLS (Franklyn, MA, USA) spectrometer.

CONCLUSION

Our synthetic approach based on track-etched polymer templates technology combined with a chemical solution deposition route provides direct access to the monolithic integration of composition controlled vertical OMS nanowires, epitaxially grown on silicon substrates. This new synthesis method takes advantage of soft chemistry to integrate epitaxial quartz films at the silicon surface as a selective template for the stabilization of single crystalline OMS nanowires during a solid state confined reaction. The thermally activated crystallization of the silica native surface is induced by heterogeneous catalysis of Sr2+ or Ba2+ cations present in the initial precursor solution, homogeneously distributed by the track-etched polymer template. As a consequence, our mechanism can be extended to any nanowire composition including Ba1+δMn8O16, Sr1+δMn8O16, (BaSr)1+δMn8O16, and the new LaSr-2×4 OMS, as long as Sr2+ or Ba2+ are present in the initial precursor solution. Thus, the interplay between temperature, pore confinement, and epitaxial growth plays a key role for the fabrication of OMS nanowires on silicon substrates. Different compositions of elements can be further used to design new 1D manganese OMS nanowires, which may assist the oxide-semiconductor integration and the development of advanced materials and devices with unique optical, electric, or magnetic properties. Finally, this work will serve as a starting point for forthcoming studies involving the validity and generality of chemical solution deposition assisted by polymeric nanoporous templates for the generation of different OMS nanostructured systems and, more importantly, to improve the knowledge on the natural geological deposits of hollandite-type manganese compounds.

* Supporting Information S

Growth process for single crystalline OMS NWs on top of (100)-Si substrates, X-ray diffraction identification of the αquartz layer and LaSr-2×4 nanowires, HRTEM analysis of the interface between LaSr-2×4 NWs and α-quartz, FE-SEM and EDX elemental mapping of BaMn8O16 NWs, FE-SEM and 1026



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EDX elemental mapping of SrMn8O16 NWs, reciprocal space reconstruction of SrMn8O16 NWs, determination of the new unit cell of SrMn8O16 NWs using the adapted RESVIS software within the FullProf Suite, SEM and TEM characterization of BaSrMn8O16 NWs. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS A.C.G. acknowledges the financial support from Collège de France foundation, COMPHOSOL2 (Fond Unique Interministériel dans le cadre du 9eme AAP) and IMPC for use FEGSEM facilities. A.C.G. acknowledges also Collège de France and LCMCP for his Visiting Scientist position and INL-CNRS for his detachment. We acknowledge the financial support from MICINN (MAT2008-01022 and MAT2011-28874-c02-01), Consolider NANOSELECT (CSD2007-00041), Generalitat de Catalunya (2009 SGR 770 and Xarmae), and EU (HIPERCHEM, NMP4-CT2005-516858). L.M. acknowledges the National Human Resources Mobility Program of the Spanish Ministry of Economy and Competitiveness. We thank David Montero for technical support and Bernat Bozzo of ICMAB Low Temperature and Magnetometry Service for magnetic measurements. The HRTEM microscopy work was conducted in “Laboratorio de Microscopias Avanzadas” at the Instituto de Nanociencia de Aragon-Universidad de Zaragoza. The authors acknowledge the LMA-INA for offering access to their instruments and expertise. Research at ORNL was supported by the U.S. Department of Energy (DOE), Basic Energy Sciences (BES), Materials Sciences and Engineering Division, and through a user project supported by ORNL’s Shared Research Equipment (ShaRE) User Program, which is also sponsored by DOE-BES.

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Direct Monolithic Integration of Vertical Single Crystalline Octahedral

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