Characterization of hybrid cobalt-porous silicon systems: protective effect of the Matrix in the metal oxidation
© Muñoz-Noval et al.; licensee Springer. 2012
Received: 14 May 2012
Accepted: 14 August 2012
Published: 2 September 2012
In the present work, the characterization of cobalt-porous silicon (Co-PSi) hybrid systems is performed by a combination of magnetic, spectroscopic, and structural techniques. The Co-PSi structures are composed by a columnar matrix of PSi with Co nanoparticles embedded inside, as determined by Transmission Electron Microscopy (TEM). The oxidation state, crystalline structure, and magnetic behavior are determined by X-Ray Absorption Spectroscopy (XAS) and Alternating Gradient Field Magnetometry (AGFM). Additionally, the Co concentration profile inside the matrix has been studied by Rutherford Backscattering Spectroscopy (RBS). It is concluded that the PSi matrix can be tailored to provide the Co nanoparticles with extra protection against oxidation.
KeywordsPorous silicon Hybrid materials Metal electroinfiltration Transmission electron microscopy
The development of hybrid materials is a current topic of research with many potential applications in several fields, including optoelectronics, catalysis, and biomedicine. Concretely, the hybridization of semiconductors with ferromagnetic material such as cobalt, iron, and nickel gives the possibility to obtain materials that combine semiconducting and magnetic properties. Moreover, the continuous progress in nanotechnology during the last decades has led to a large availability of techniques for the fabrication and characterization of nanometric structures with controlled composition and dimensions, resulting in nanostructures with very specific properties and several functionalities. In fact, multifunctional metal-based nanostructures have received a great deal of attention during the past few years given their special properties and potential applications in many scientific and technologic fields, including biomedicine. In this sense, the conjugation of magnetic-semiconductor hybrid nanosystems has allowed manipulation of local spin in spintronics[4, 5] and the fabrication of high sensitive magnetic sensors. Regarding porous semiconductors such as porous silicon (PSi), they present additional advantages including high surface area and high surface reactivity.
This work aims at studying the oxidation state and crystalline structure of cobalt nanoparticles (NPs) embedded into porous silicon, resulting in Co-PSi hybrid structures. Co has been infiltrated into the PSi matrix by electrochemical techniques. The suitability of PSi to host Co NPs grown by electroinfiltration has been evaluated, and both the magnetic and structural properties of the hybrid structures have been studied. The role of the porous matrix protecting Co against oxidation has been evaluated by infiltrating the Co NPs into PSi layers with different morphologies. The chemical and structural states of the Co NPs have been determined by combining highly selective and sensitive characterization techniques such as X-Ray Absorption Spectroscopy-synchrotron (XAS-synchrotron) and Rutherford Backscattering (RBS).
Preparation of porous silicon
Electroinfiltration of Co
Field emission scanning electron microscopy (FESEM) images were obtained in a XL 30S-FEG (PHILIPS). No metallization was required to observe the samples. Samples for cross section observation were prepared according to previously optimized protocols for mechanical and ion bean milling. TEM/STEM characterization was carried out using a Jeol JEM 3000F with HAADF (High Angle Annular Dark Field) system included (300kV).
X-ray Absorption Spectroscopy (XAS)
X-Ray Absorption Near-Edge Structure (XANES) and Extended X-ray Absorption Fine Structure (EXAFS) spectroscopy measurements at the Co K-edge energy (7704.9 eV) were performed at room temperature in fluorescence mode at the BM25 Spanish CRG Beamline (SpLine) of the ESRF (European Synchrotron Radiation Facility). An INCA 13 elements X- Ray detector was used. Bulk metallic Co, plus CoO and Co3O4 powders were measured as references. Data were normalized applying the same normalization parameters for all the spectra by means of Athena Software.
Alternating gradient field magnetometry
Magnetic characterization was performed using a Micromag Model 2900 Alternating Gradient Magnetometer System (Princeton Measurements Corporation). Nine measurements were taken for each sample. Measurements were carried out applying a magnetic field from -100mT to 100mT, a time pass of 100 ms, and a field pass of 800μT.
Rutherford backscatering spectroscopy
Micro analytical techniques were used to obtain in-depth elemental information. A Cockcroft-Walton tandem accelerator located at Centro de Micro-Análisis de Materiales (CMAM, Universidad Autónoma de Madrid, Spain) was used for Rutherford Backscattering Spectroscopy (RBS). RBS was performed with a 3.050 MeV He+ beam (incidence angle was 75° with respect to the surface normal). RBS was acquired by using silicon surface barrier detectors placed at scattering angle of 170°. A 13 μm thick mylar foil was placed in front of the detector on the forward scattering angle to stop the He forward scattered particles and filter the H recoils. All RBS experiments were performed in vacuum (pressure lower than 10E-5 mbar). All spectra were simulated using the SIMNRA code to obtain the element in-depth composition.
Results and discussion
PSi single layers
From the raw spectra (Figure 6a,b) we observe a clear difference in the profiles. The feature in the higher energies region corresponding to the Co presents a localized broad peak in case of Co-PSi-p and an extended region for Co-PSi-n. In a first approximation there should exists differences in the Co distribution along the layer in both configurations. Both spectra present the Si (1600–1700 keV) and O (~1000 keV) edges with subtle differences that require finer calculations. By using the fitting software it has been determined that in Co-PSi-n structures, Co is uniformly distributed along the multilayer, and the oxygen concentrates mainly near the surface. This means that metallic Co is hosted in deeper zones of the multilayer and Co3O4 is concentrated in outer zones of the matrix. For Co-PSi-p systems, Co penetrates to less deep zones and the O concentration keeps fairly constant along the multilayer. This is due both to the presence of O bubbles inside the matrix and the coexistence of a mixture of Co oxides. Figure 6c,d summarize the elemental in-depth profiles of each systems, CoPSi-n and CoPSi-p, respectively, obtained from the simulations of the experimental spectra.
In this work the protective effect of the matrix on the oxidation of electroinfiltrated Co nanoparticles into PSi has been proved. Moreover, a selected multilayer configuration in the matrix allows minimizing the formation of Co oxides that forms an oxidation profile in the whole layer depending on the porosity gradient of the PSi template. The magnetic properties of such Co-PSi systems suggest the possibility to control the magnetization of these hybrid materials by controlling the amount of infiltrated Co. Nevertheless, the influence of the PSi porosity type and degree in the oxidation of the hosted Co need to be further characterized. Future work in this sense should be performed in order to identify the physico-chemical properties of the PSi matrix that minimizes the oxidation in such hybrid materials.
We acknowledge the European Synchrotron Radiation Facility for provision of synchrotron radiation facilities and the MEC and Consejo Superior de Investigaciones Científicas for financial support (PE-2010 6 0E 013). We would like to thank the BM25-SpLine staff for the technical support. This work has been supported by MICINN through projects FIS-2008-06249, MAT2009-14578-C03-02, MAT2008-06858-C02-01, and MAT2008-06858-C02-02, as well as Comunidad de Madrid, project NANOBIOMAGNET (S2009/MAT-1726) and European project MAGNIFYCO (Contract NMP4-SL-2009-228622).Technical support from ICTS Centro Nacional de Microscopia Electrónica (UCM, Madrid) is gratefully acknowleged.
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