Air-stable nanogranular Fe thin films formed by Chemical Vapor Deposition of triiron dodecacarbonyl as catalysts for carbon nanotube growth
Introduction
Physical Vapor Deposition (PVD) and Chemical Vapor Deposition (CVD) are two prominent techniques for synthesizing thin coatings. Both techniques are based on the deposition of vapor of a desired material as thin films on a substrate surface. The process is usually performed in high vacuum (PVD) or at controlled pressure (CVD) to avoid the interaction between the vapor and air [1], [2]. In PVD, the material to be deposited is vaporized and layered with strong directionality on the substrate surface [3]. In CVD, the material to be deposited is introduced as a precursor gas and then, still in the vapor phase, conformably coats the substrate surface [4], [5].
Nanogranular magnetic films are composed of magnetic nanoparticles embedded in a non-magnetic isolating matrix such as SiO2, Si3N4, Al2O3, BN, or C [6], [7], [8]. In these films, the non-magnetic matrix is introduced to diminish the exchange interaction between the magnetic grains [9], [10], [11]. Due to the magnetic particles being surrounded by an insulating matrix, the granular media exhibit improved properties such as oxidation resistance, corrosion resistance and wear resistance. Elemental iron is the most useful ferromagnetic element for electromagnetic interference and radio frequency interference magnetic shielding applications, as it has the highest magnetic moment at room temperature, a high Curie temperature and a fairly low coercivity [12], [13]. In addition, iron is a commonly available element, and therefore significantly cheaper than other ferromagnetic materials. Fe thin films are easily oxidized by air, which significantly affects their high magnetic moment. To prevent this phenomenon, elemental Fe should be protected by a passivation layer such as carbon [14], silica [15], and alumina [16]. Thin films of transition metals are often used in the synthesis of carbon nanotubes (CNTs) [17]. In addition to their very high aspect ratio, their potential for chemical functionalization, and their characteristic electrical, mechanical, and thermal properties, CNTs can be used in a wide range of applications including electronic devices (e.g., CNT-based field effect transistors and interconnects for microprocessors) [18], electrodes for batteries [19], [20] and supercapacitors [21], reinforced composites [22], etc. [23], [24], [25], [26]. Catalytic CVD is one of the most efficient methods to synthesize CNTs [27] for mass production [28]. Catalysts (thin films or nanoparticles) deposited on a substrate are the initiators of the CVD synthesis of CNTs. The mechanisms of CNT synthesis are multiple and complex [27], [29], [30]; they include catalysts [31], [32], substrates [33], [34], gases [35], [36], [37], and their mutual interactions. Many process parameters such as gas composition and flows, thermal processes and the duration of the various annealing, and growth times can have a significant effect on the morphology and structure of the CNTs. Typical catalysts widely employed in CNT synthesis include transition metals, and often ferromagnetic metals such as Co, Fe and Ni [38], [39]. The preparation of the catalyst can also significantly affect the CNT growth [40], [41]. In the present study, the synthesis and characterization of nanogranular magnetic conducive Fe/C thin coatings on Si wafers, by CVD of Fe3(CO)12 at low temperature in an inert environment (Ar) at a standard pressure of 100 kPa are described. These films are composed of sintered elemental Fe nanoparticles of 4.1 ± 0.7 nm diameter. These nanoparticles are stabilized against oxidation by a very thin layer of carbon coating. Since the carbon concentration in these coatings is relatively low (< 3%), for simplicity, we will refer to these coatings as “Fe coatings”. In addition, the effectiveness of these Fe coatings as catalysts for synthesizing tall carpets of crystalline and vertically aligned CNTs is also demonstrated.
Section snippets
Materials
The following analytical-grade chemicals were purchased from a commercial source and were used without further purification: ethanol and triiron dodecacarbonyl, Fe3(CO)12, protected by 1–10% methyl alcohol from Aldrich (Israel); P type, boron doped Si wafers with <100> orientation from Si-Mat, Israel; ultra-high purity precursor gases: ethylene, argon, helium, hydrogen and a mixture of argon with 1% oxygen (Ar–O2) were purchased from Maxima Air Separation Center Ltd., Israel. Alumina (Al2O3
Results and discussion
Thin Fe coatings of 10 ± 1.4, 100 ± 14 and 400 ± 46 nm thicknesses were prepared by the CVD of Fe3(CO)12 on Si wafers, as described in the Experimental details section. The thickness of the Fe based films was controlled by changing various parameters such as coating time, concentration of the precursor, and the number of repetitions of the coating process. SEM and STEM cross-section images of typical Fe thin films of 100 ± 14 and 10 ± 1.4 nm thicknesses deposited on Si wafers are shown in Figs. 2A and 3A,
Summary and conclusions
Our research demonstrates a simple CVD process for the fabrication of conductive magnetic Fe based thin films of controlled thickness on Si wafers. Films produced by this method showed metallic properties of the precursor metal. The films are composed of condensed zero-valent Fe nanoparticles and are protected against air oxidation by a thin carbon layer. These films show a non-ohmic resistivity behavior. By raising the temperature of these films from room temperature to 750 °C, the nanogranular
Acknowledgments
This study was partially supported by a Minerva grant (microscale and nanoscale particles and thin films for medical applications).
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