Article pubs.acs.org/JPCC
Mesoporous Oxide-Diluted Magnetic Semiconductors Prepared by Co Implantation in Nanocast 3D-Ordered In2O3−y Materials Eva Pellicer,† Enric Menéndez,*,‡ Jordina Fornell,† Josep Nogués,§,∥ André Vantomme,‡ Kristiaan Temst,‡ and Jordi Sort†,∥ †
Departament de Física, Facultat de Ciències, Universitat Autònoma de Barcelona, E-08193 Bellaterra, Barcelona, Spain Instituut voor Kern- en Stralingsfysica, KU Leuven, Celestijnenlaan 200 D, B-3001 Leuven, Belgium § ICN2 - Institut Catala de Nanociencia i Nanotecnologia, Campus UAB, 08193 Bellaterra, Barcelona, Spain ∥ ICREA - Institució Catalana de Recerca i Estudis Avançats, Barcelona, Spain ‡
S Supporting Information *
ABSTRACT: Mesoporous In2O3−y materials have been implanted using Co ions to induce a moderate ferromagnetic response at room temperature, forming a “mesoporous oxide-diluted magnetic semiconductor” (MODMS). X-ray photoemission spectroscopy (XPS) reveals that implantation results in up to 1 at. % Co (for 6 × 1015 ions/cm2 at 40 keV) and 15 at. % Co (for 1 × 1017 ions/cm2 at 60 keV). This is in both cases accompanied by a pronounced increase in the amount of oxygen vacancies with respect to the pristine, nonimplanted, In2O3−y. Further increase in the ion fluence (up to 2 × 1017 ions/cm2 at 60 keV) results in the collapse of the mesoporous structure, i.e., loss of the 3D-ordered porous configuration. XPS also reveals that virtually no metallic Co is formed at 40 keV, while a mixture of Co2+ and Co0 states is detected after implantation at 60 keV. Most of the Co2+ is incorporated in the bixbyite structure of the In2O3−y matrix. These results are consistent with previous models suggesting that the origin of the obtained ferromagnetic response in oxide-diluted magnetic semiconductors can be ascribed to ferromagnetic exchange interactions mediated by oxygen vacancies. This work constitutes the first report on MODMS prepared by nanocasting followed by implantation of transition metal ions.
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INTRODUCTION Extensive research in the field of porous semiconductor materials has been performed during the past decades, encouraged by their vast range of applications in diverse technological fields, such as optoelectronic devices, catalysts, gas sensors or magnetic systems, among others.1−4 Porous silicon has been perhaps the most extensively studied porous semiconductor owing to its outstanding photoluminescence properties. More recently, other semiconductor materials with controlled porosity have also been synthesized (e.g., SiC, Ge, GaAs, InP, GaN, CdS, etc.), with pore sizes ranging from a few nanometers to tens of micrometers.1−5 Introducing porosity in insulating materials is also a common strategy in semiconductor industry to decrease their dielectric constant, thereby reducing problems associated with transmission delay or cross-talk noise in electronic devices.6,7 Several techniques can be utilized to generate porosity either by removing material from the bulk (top-down approach) or joining together small building blocks while leaving empty spaces behind (bottom-up approach). Template-free electrodeposition using suitable pH conditions and additives, hydrothermal synthesis, high-energy ion irradiation, or soft- and hard-templating (nanocasting) are among the various techniques frequently used to create ordered or disordered porous materials in bulk, thin film, or particulate forms.1−5 Among them, nanocasting is a versatile and cost© 2013 American Chemical Society
effective technique able to produce ordered mesoporous structures with a precise control of the pore size in diverse three-dimensional geometrical frameworks.8 In some cases, postsynthesis physical (e.g., heat treatments, ion irradiation)9,10 and chemical (e.g., chemical or electrolytic etching) treatments2,11,12 of mesoporous or microporous materials can lead to improved material properties and enhanced performance. Among the novel types of semiconductor materials featuring additional functionalities are the so-called oxide-diluted magnetic semiconductors (ODMS), which consist of oxide semiconductors (e.g., In2O3, ZnO, SnO2, or TiO2) doped with transition metals (e.g., Co, Mn, Fe, etc.).13−17 These materials may be appealing for spintronic applications, where the electron spin is used for the processing of information. The case of In2O3 is of particular interest because of its optical transparency, which confers additional potential to this material in the field of magneto-optics.18 The origin of room temperature ferromagnetism in ODMS still remains controversial, although both the dilution of 3d elements in the bixbyite phase and the presence of structural defects (e.g., oxygen vacancies) play a crucial role in the resulting magnetic Received: May 31, 2013 Revised: July 18, 2013 Published: July 20, 2013 17084
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behavior.13−17,19−21 Most of the ODMS reported so far have been prepared as continuous thin films, bulk materials, or coarse-grained powders. However, the forthcoming integration of ODMS in electronic devices will require a precise control of the geometry and lateral dimensions of these materials at the nanoscale in order to really take advantage of the electron spin degree of freedom and to integrate these semiconductors as key components in spintronics devices. Although there are a few studies on the growth of ODMS nanoparticles and nanowires,22−24 the synthesis of three-dimensional mesoporous ODMS structures has been much less reported.4 Interestingly, porous ODMS frameworks could exhibit additional unique properties, compared to their bulk counterparts, such as quantum-confinement effects, high surface area for catalysis or gas absorption, and optical or magnetic surface effects. Ion implantation has been suggested as an effective postsynthesis physical method to prepare or enhance the properties of ODMS.25−28 Most studies have been carried out in thin films (2D structures), while the use of this technique as a means to enhance the magnetic properties of 0D and 1D ODMS has been much less exploited. In particular, In2O3 nanowires doped with rare earths have been reported to have applications as gas sensors.29 In turn, metal ion (Cr+, V+) implantation in TiO2 has been reported to enhance its photocatalytic activity.30 On the other hand, ion implantation can be employed itself as a physical method to induce porosity. In this case, heavy ions with high incident energy and relatively high fluences are used.7,31,32 Control of the pore size and pore arrangement by ion implantation is, nevertheless, not straightforward. In this work, low-energy (40−60 keV) ion implantation is used to implant Co ions at the surface of a layer of mesoporous In2O3−y particles and generate the necessary amount of oxygen vacancies to significantly enhance the otherwise rather limited ferromagnetic (FM) response in this material. The energy of the incoming ion beam and the ion fluence (ranging from 6 × 1015 to 1 × 1017 ions/cm2) are selected to induce a remarkable ferromagnetic response while preserving the ordered mesoporous structure of the pristine In2O3−y material. Recently, chemical doping has been employed to synthesize mesoporous In2O3-based ODMS.4 However, in that case, since the content of transition metals was above the solubility limits of the bixbyite phase, the presence of considerable amounts of secondary phases (transition metal oxides) induced a strong paramagnetic signal that partly masked the resulting ferromagnetic properties. Here, a ferromagnetic response is achieved with a rather small Co content, without evidence for the formation of Co oxide phases. However, beyond certain implantation conditions (e.g., 2 × 1017 ions/cm2 at 60 keV), Co clusters start to form while the mesoporous structure partially collapses due to the local annealing processes that occur at the material surface during ion implantation.
6 template (0.150 g) was put in contact with 1 mmol of In(NO3)3·xH2O (Sigma-Aldrich, 99.99% purity) in ethanol. The mixture was stirred for 30 min in a crucible and left for ethanol evaporation overnight. The crucible was then placed in a tubular furnace, and the impregnated silica was calcined. The furnace temperature was increased to 375 °C at a rate of 3 °C/ min and held at this temperature for 4 h under atmospheric conditions. At the end of this process, the furnace was slowly cooled down to room temperature. The silica host was removed with 30 mL of 2 M NaOH solution at 70 °C under stirring for 24 h. The In2O3−y replica was collected after centrifugation and decanted off the supernatant, copiously rinsed in ethanol, and finally dried. The low-angle X-ray diffraction (XRD) pattern of assynthesized In2O3‑y was acquired using a Panalytical X’Pert Pro diffractometer in the 0.5°−3° 2θ angular range operating in transmission mode using Cu Kα radiation. The wide-angle XRD pattern was collected on a Philips X’Pert diffractometer in the 20°−80° 2θ range (step size = 0.03°, step time = 10 s) using Cu Kα radiation. Brunauer−Emmet−Teller (BET) surface area analysis was performed on a Micromeritics ASAP 2020 instrument. The N2 adsorption/desorption isotherm was recorded at 77 K after degassing the powder at 175−200 °C for 10 h. The pore size was estimated applying the Barrett− Joyner−Halenda (BJH) algorithm to the desorption branch. Cobalt implantation was carried out on layers of In2O3 deposited on silicon substrates. The as-synthesized In2O3−y powder was dispersed in ethanol and deposited dropwise onto silicon substrates. A dense layer of mesoporous semiconductor material was obtained after solvent evaporation. Implantation with Co ions was carried out at room temperature, under high vacuum, using a low-energy ion implanter. Two different incident ion energies were used: 40 keV (with a fluence of 6 × 1015 ions/cm2) and 60 keV (with ion fluences of 1 × 1017 and 2 × 1017 ions/cm2). The implanted Co concentrations were modeled using Monte Carlo simulations by means of the TRIM (Transport of Ions in Matter) program included in the SRIM (Stopping Range of Ions in Matter) package.34,35 The morphology of the materials after the implantation process was investigated by transmission electron microscopy (TEM) on a Jeol-JEM 2011 system operated at 200 kV and field emission scanning electron microscopy (FE-SEM) on a Merlin Zeiss microscope operated at 15 kV. Samples for TEM observations were prepared by collecting a small amount of the Co-implanted powder in an eppendorf, ethanol was then added, and after sonication for a few minutes one or two drops of the suspension were placed dropwise onto a holey carbon supported grid. The composition of the porous layers was studied by X-ray photoelectron spectroscopy (XPS) depth profiling on a PHI ESCA-5500 using Al Kα radiation. Ar+ ion etching with energy of 3 keV beam and ion current of a few microamperes were applied at a raster area of 2 mm × 2 mm. The Co 2p peaks were fitted to a Voigt function with Lorentzian contribution of less than 10%. A Shirley background subtraction was included in the fitting procedure. To calculate the Co/In and O/(In + Co) ratios, the integral total area of cobalt, indium, and oxygen peaks, normalized according to their photoionization cross sections, was used. All spectral positions were corrected to the position of C 1s signal (285.0 eV). Magnetic hysteresis loops were recorded at room temperature using a vibrating sample magnetometer from Oxford Instruments with a maximum applied magnetic field of 1 T.
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EXPERIMENTAL METHODS Mesoporous silica KIT-6 was synthesized by dissolving Pluronic P123 copolymer (6.0 g) in diluted HCl and stirring the solution for 6 h. 1-Butanol (6 g) was then added, followed by stirring for an additional 1 h.33 Immediately after, tetraethyl orthosilicate (12.5 g) was added, and the solution was stirred for 24 h at 34 °C. The hydrothermal treatment was carried out at 90 °C in a sealed container, and the resulting white solid was filtered, copiously washed with water, and finally calcined at 550 °C for 5 h to remove the organics. The mesoporous silica KIT17085
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RESULTS AND DISCUSSION Characterization of As-Synthesized In2O3 and TRIM Simulations. The structural quality of the as-synthesized In2O3 was assessed by several techniques. TEM images of the parent silica KIT-6 template are shown in the Supporting Information (Figure S1). The low-angle XRD pattern of the assynthesized In2O3 powder features one high-intensity Bragg reflection at 0.9°, a weak shoulder peak, and a broad diffraction peak centered at 1.7° (Figure 1a). The first two can be indexed
Figure 2. TRIM simulations of the depth distribution of Co atoms for the following implantation conditions: 40 keV Co ions at 6 × 1015 ions/cm2 and 60 keV Co ions at 1 × 1017 ions/cm2. The density of the mesoporous In2O3−y estimated from BET measurements was used in the simulations.
mainly within the upper 80 and 130 nm from the surface (with maximum Co concentration around 1.5 and 16 at. % for 40 and 60 keV, respectively). However, given the porous character of the samples, these results should be taken as an average. For instance, Co concentrations may vary locally. Morphological and Structural Characterization of CoImplanted In2O3−y. Figure 3 shows TEM images of the In2O3−y material following Co ion implantation in different conditions. The typical 3D cubic Ia3d3 mesostructure of the assynthesized In2O3−y remains visible with no apparent damage of the porous network, after implantation using 40 keV and 6 × 1015 ions/cm2 (Figure 3a) or 60 keV and 1 × 1017 ions/cm2 (Figure 3b). The pore size is ca. 8 nm in both cases. However, when the fluence is further increased to 2 × 1017 ions/cm2 at 60 keV, partial collapse of the porous network and concomitant sintering of the particles are observed (Figure 3c). A FE-SEM image of the layer surface, shown as an inset, corroborates this result. Although the mesoporosity is no longer visible, the layer features a 3D network of macropores that can still be useful for some applications (e.g., gas sensing). XPS analyses were conducted in order to determine both the oxidation state and the local chemical environment of the implanted Co ions as well as the effects induced by the implantation on the In2O3 bixbyite lattice. The analyses were performed on the layers that retained the mesoporosity after the Co implantation process (i.e., 40 keV at 6 × 1015 ions/cm2 and 60 keV at 1 × 1017 ions/cm2). The thickness of the In2O3 mesoporous layer during ion implantation is estimated to be around 1−2 μm. The Co 2p core-level XPS spectra from the surface (i.e., before sputtering, tsput = 0 min) are shown in Figure 4a. Cobalt is mainly in the high divalent state (Co2+) for the 40 keV implanted sample, as expected from the relatively low Co content (around 1 at. %). At 60 keV the intensity of the Co XPS signal greatly increases due to the larger cobalt concentration (13 at. %). However, in this case two main contributions are observed: the doublet peaks at 797.0 and 781.1 eV corresponding to high spin divalent states38 and two weaker peaks located at 793.0 and 778.2 eV, indicating the presence of metallic Co clusters.39 The presence of fine Co clusters is actually anticipated because the solubility of cobalt in the In2O3 bixbyite structure is only around 7 at. %.39 Actually,
Figure 1. (a) Low-angle XRD and (b) N2 adsorption−desorption isotherm of as-synthesized In2O3−y. The pore size distribution is shown in the inset.
as (211) and (220) peaks of the mesoporous bicontinuous cubic structure (space group Ia3d).36,37 The broad peak at higher 2θ values arises from the overlapping of (321), (400), (420), and (332) reflections of the Ia3d space group. The wideangle XRD pattern is shown in Figure S2. The peaks match the bixbyite (cubic) In2O3 phase (JCPDF 65-3170), indicating that the as-synthesized powder has crystalline walls. Figure 1b shows the N2 adsorption−desorption isotherm and the corresponding BET pore size distribution in the inset, which proves that both inter- and intraparticle porosity exist. The Co implantation profiles within the deposited mesoporous In2O3‑y layers were simulated using the TRIM software.34,35 The relative density of the mesoporous In2O3‑y obtained by BET was used in the simulations taking advantage of the relationship that exists between the relative density of a powder material and its porosity volume fraction: ρporous =1−P ρbulk (1) where P=
Cpv Cpv +
1 ρbulk
(2)
Here Cpv denotes the cumulative pore volume, as determined from BET analyses, which for the investigated material is Cpv = 0.1227 cm3/g. Taking into account that the bulk density of In2O3 is 7.179 g/cm3, the relative density of the mesoporous In2O3 particles is ρporous/ρbulk = 0.532. Hence, the density of the mesoporous particles is ρporous = 3.818 g/cm3. Considering this value, Figure 2 shows the Co depth profiles for the implantations performed using 40 keV Co ions at 6 × 1015 ions/cm2 and 60 keV Co ions at 1 × 1017 ions/cm2. The simulations reveal that the ion implantation processes occur 17086
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Figure 3. TEM images of In2O3−y particles after implantation with Co ions at (a) 40 keV and 6 × 1015 ions/cm2, (b) 60 keV and 1 × 1017 ions/cm2, and (c) 60 keV and 2 × 1017 ions/cm2. The inset in (c) shows a FE-SEM image of the layer after being implanted. Scale bar in (a) and (b) is 50 nm; scale bar in (c) and inset is 100 nm.
precipitation of TM in dielectric ceramics during ion implantation has been reported for high fluences (>1016 ions/cm2).40 Clustering and nanoparticle growth are promoted upon subsequent annealing. In our case, local annealing probably accompanies the implantation process at 60 keV, as evidenced from the sintering process observed for a fluence of 2 × 1017 ions/cm2 (Figure 3c). For In2O3, a pronounced shift toward lower binding energy is observed when comparing the position of the peaks at the In 3d core-level region (Figure 4b). The 3d3/2 peak for the 40 keV implanted sample is located at 452.22 eV, whereas at 60 keV, it is centered at 451.90 eV. Hence, the shift in energy is 0.32 eV. The same holds for the 3d5/2 peak. It has been demonstrated that replacing In3+ cations by Co2+ in the In2O3 lattice causes a decrease of the binding energy of In because Co2+ has a smaller ionic radius than In3+.38 This confirms that a certain fraction of
Figure 4. (a) Co 2p and (b) In 3d XPS core-level spectra taken at the surface of the Co implanted mesoporous layers (40 keV at 6 × 1015 ions/cm2 and 60 keV at 1 × 1017 ions/cm2). (c) Co/In atomic ratio as a function of the sputtering time. The lines are a guide to the eye.
the implanted Co is substituting In sites although the presence of a CoO phase cannot be completely ruled out, particularly at 60 keV, given the large amount of Co. Indeed, coexistence of several phases was reported in Co-doped indium tin oxide (ITO) prepared by magnetron cosputtering of Co, In2O3, and ITO targets. Namely, in a film containing 12 at. % Co, it was found that 55% of Co precipitated into metallic aggregates, 30% formed CoO, and 15% Co was substituting In.39 Figure 4c depicts the Co/In atomic ratio as a function of the sputtering time (i.e., the dependence of the relative composition with depth). Assuming an etching rate of 5−7 17087
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nm/min,41 cobalt would be implanted within 120−160 nm from the surface in the 60 keV sample and within 60−90 nm for the 40 keV sample. The results are in rather good agreement with the TRIM simulations (Figure 2). These etching rates, however, have to be taken with caution due to the high porosity of the mesoporous particles, which probably tend to make the etching rates higher. Moreover, the amount of implanted Co shows a maximum as a function of sputtering time (see Table 1), as also predicted by the simulations. The
implantation of Co significantly promotes the oxygen vacancy sites up to 45% (O/(In + Co) = 0.70), a value that can be considered as very large. Similar effects have been reported for In2O3 films treated with 120 MeV Ag9+ ions, where the O/In ratio decreased from 0.98 toward 0.82 with increasing Ag9+ ions fluence (up to 1.0 × 1014 ions/cm2), suggesting high oxygen loss during irradiation.45 Note that the reduction of the oxidation state of In3+ (as a consequence of the oxygen vacancies) shifts the binding energy of the In 3d peaks toward lower values,46−48 thus causing the same effect as the replacement of In3+ cations by Co2+ in the In2O3 lattice. Hence, the observed shift (Figure 4b) has a twofold origin: the formation of oxygen-deficient, substoichiometric InxOy48 and the substitution of In3+ cations by Co2+.38 Magnetic Characterization of Co-Implanted In2O3−y. The room temperature hysteresis loops of the pristine mesoporous In2O3−y and the samples implanted at 40 keV (6 × 1015 ions/cm2) and 60 keV (1 × 1017 ions/cm2) are shown in Figure 5. The implanted materials exhibit a ferromagnetic
Table 1. Atomic percentages of In, Co, O, and O/(In + Co) Atomic Ratio in the Mesoporous Layers as a Function of the Sputtering Time (i.e., Penetration Depth) implantation conditions
sputtering time (min)
at. % In
nonimplanted
0
38
40 keV, 6 × 1015 ions/cm2
0 1 3 8 13 23
42 54 56 57 59 58
60 keV, 6 × 1017 ions/cm2
0 1 3 8 13 23
29 44 48 52 52 54
at. % Co at. % O
O/(In + Co) atomic ratio
45
1.18
