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Nanostructures fabricated by electron beam induced chemical vapor deposition
Superlattices and Microstructures 36 (2004) 255–264 www.elsevier.com/locate/superlattices
Nanostructures fabricated by electron beam induced chemical vapor deposition M. Takeguchia,∗, M. Shimojoa,b, K. Mitsuishia, M. Tanakaa, K. Furuyaa a High Voltage Electron Microscopy Station, National Institute for Materials Science, 3-13 Sakura,
Tsukuba 305-0003, Japan b Precision and Intelligence Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta, Midori,
Yokohama 226-8503, Japan Available online 25 September 2004
Abstract Electron beam induced chemical vapor deposition (EBICVD) with tungsten and iron carbonyl gases, W(CO)6 and Fe(CO)5 , was attempted to fabricate nanostructures. In case of the deposits from W(CO)6 gas, the structure was a mixture of nanocrystals and amorphous phases and no change was seen after a post-deposition heating treatment. On the other hand, the deposits from Fe(CO)5 gas were composed of amorphous iron surrounded by Fe oxide nanocrystals, and heating at about 600 ◦ C resulted in the formation of crystalline Fe carbide and alpha-Fe phases. Thus, it was demonstrated that EBICVD with Fe(CO)5 gas followed by a heating treatment can be used practically to fabricate nanostructures of magnetic materials. © 2004 Elsevier Ltd. All rights reserved.
1. Introduction The development of a fabrication technique for nanometer-sized structures has been becoming a key technology for producing advanced devices and new functional materials. Especially, a size and position controllable fabrication technique has been required for ∗ Corresponding author. Tel.: +81 298 59 5053; fax: +81 298 59 5054.
E-mail address: [email protected] (M. Takeguchi). 0749-6036/$ - see front matter © 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.spmi.2004.08.038
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patterning nanodots and nanowires on substrate materials. Photo-lithographic patterning combined with etching processes, which has been widely used for the production of the current generation electronic devices, cannot achieve sufficient resolution for nanostructure fabrication because of the wavelength of visible light and lens numerical aperture. Although electron-beam lithography is capable of forming a nanometer-sized probe, any producible structure has been still more than 10 nm. This is due to the resolution of resists. It has been proposed that resistless direct patterning is a promising technique for such nanofabrication. Recently, various direct nanofabrication techniques have been applied to producing nanostructures [1,2]. Beam induced chemical vapor deposition is one of the most prospective methods, in which organic or metal-organic gases are decomposed by ion [3–5] or electron beams [6–21] and thus carbon or metals are deposited at the desired position, which can be controlled by a beam position. The size of the nanostructures can also be controlled by the beam diameter. It has been reported that highly designed electron beam-focusing instruments such as a transmission electron microscope equipped with a field emission gun succeeded to fabricate nanodots and nanowires with a size less than 5 nm [15–17]. To utilize the obtained deposits to units of nanodots and nanowires in electronic devices practically, the deposits should be composed of crystalline phases rather than amorphous. However, the deposits produced at room temperature often have amorphous phase or a mixture of nanocrystals and amorphous phases. The composition and crystallinity of the nanometer-sized deposits are important issues. Furthermore, it is known that a heating treatment or heating during deposition is effective to obtain crystalline phase nanostructures [20–22], and therefore heating induced composition and phase change of the deposits are interesting. In the present work, electron beam induced chemical vapor deposition (EBICVD) using tungsten and iron carbonyls, W(CO)6 and Fe(CO)5 , was carried out and the composition and structure of the produced nanostructures were characterized before and after postdeposition heating processes. 2. Experimental EBICVD experiments were carried out in a 200 kV field emission gun transmission electron microscope (FEG-TEM), JEOL JEM-2010F, and a 30 kV field emission gun scanning electron microscope (FEG-SEM), JEOL JSM-7800. The beam position was controlled by an external deflector voltage input using a computer with digital-analog converters. A gas introduction system consisting of a nozzle (inner diameter about 0.2 mm), gas pipe line, variable leak valve and gas source reservoirs was installed to the specimen chamber of these microscopes and the nozzle tip was about 1 mm away from the electron beam position. The details of the gas introduction system were described elsewhere [17,18]. A schematic illustration of the deposition experiment is shown in Fig. 1. Tungsten and iron carbonyl gases, W(CO)6 and Fe(CO)5 , were introduced into the specimen chamber and the partial pressure of the introduced gases was controlled to be 10−5 Pa order. Nanodots and nanorods were fabricated on carbon microgrid films supported on copper meshes. The typical currents of the electron beam used in FEG-TEM and FEG-SEM were about 0.5 nA at a beam diameter 0.8 nm and 0.8 nA at a beam diameter
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Fig. 1. Schematic drawing of EBICVD experiment in a 30 kV FEG-SEM. The gas introduction nozzle tip is about 1 mm away from the center of the specimen. The electron beam position can be controlled by an external deflector voltage input using a computer with digital-analog converters.
4 nm, respectively. The irradiation time of the beam for producing the nanodots was changed from 1 to 120 s, depending on the desired size. To form free-standing nanorods, the beam position was moved from the substrate into space at speeds of 1–3 nm/s. All the deposition was performed at room temperature. The deposits were characterized with a 200 kV ultrahigh vacuum field emission gun transmission electron microscope (UHV-FEG-TEM), JEOL JEM-2000VF, equipped with an electron energy loss spectroscopy (EELS) device [23]. The heating was carried out in a pre-treatment chamber connected to the specimen chamber of the UHV microscope through a gate valve so that the specimen could be observed without exposure to air. The specimens were heated up to 600 ◦ C for 1 h. The vacuum pressure of the pre-treatment chamber was maintained at 10−7 Pa order during the heat treatments. The structure of the deposits was analyzed by transmission electron microscopy (TEM) and electron diffraction in this microscope. 3. Results and discussion Fig. 2a shows an annular dark field scanning transmission electron microscopy (ADFSTEM) image of an array of dots deposited on a carbon microgrid film with a combination of 200 kV electron beam irradiation and W(CO)6 gas introduction at room temperature in FEG-TEM, in which 3 × 4 dots can be seen as bright contrast. The size of the dots is about 15 nm in diameter, and the interval between the dots is about 180 nm. The size of the dots can be controlled by changing the beam irradiation time and beam diameter. The minimum dot size achieved by the EBICVD technique is about 3.5 nm in conventional FEG-TEM [15] and 2 nm in UHV-FEG-TEM [16]. A dynamic Monte Carlo profile simulation has been carried out to calculate the shape of the deposits for various electron energies and led to the conclusion that the shape of the deposits can roughly be estimated
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Fig. 2. ADF-STEM image of: a, an array of nanodots and b, a free-standing nanorod produced from W(CO)6 gas. They were deposited on a carbon microgrid film using a 200 keV fine electron probe with 0.8 nm diameter in an FEG-TEM. c, HRTEM image of the edge of the nanorod, in which dark contrast nanocrystals are clearly seen. d, Electron diffraction pattern taken from the nanorod.
by the trajectory of high energy primary electrons [24]. Thus, a high energy nanometersized electron beam enables the fabrication resolution to be reduced. The nanorods were fabricated by moving the beam. Fig. 2b shows an ADF-STEM image of a typical free-standing nanorod on an edge of a carbon microgrid film using W(CO)6 gas. This was made by moving the beam from the carbon film edge to space at a speed of 3 nm/s. The diameter and length of the rod are about 20 and 500 nm, respectively. With increasing the scanning speed, the self-standing nanorod became more slender. As mentioned above, the deposit shape is determined by the secondary electrons emitted from the carbon film. However, after the incident electron beam was apart from the carbon film, the self-standing nanorod could be grown by the secondary electrons generated from the
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Fig. 3. Diffraction pattern from the free-standing nanorod after a post-deposition heating treatment. A beam with about 10 nm in diameter was used. The broad rings and polycrystalline-like spots can be seen.
as-deposited nanorod itself. Thus, the self-standing nanorod maintained an almost constant width throughout its length. The deposits from W(CO)6 gas consisted of nanocrystals and amorphous phases. Fig. 2c shows an HRTEM image of the edge of the nanorod, showing dark contrast nanometer-sized particles, in which lattice fringes can be seen clearly, with an amorphous surrounding. It is found that the nanocrystals are about 2 nm in diameter. Energy dispersive X-ray spectroscopy analysis showed that the deposits were composed of carbon, oxygen and tungsten elements [19]. According to W–C and W–O binary equilibrium phase diagrams, the possible equilibrium phases at room temperature are body-centered-cubic W, hexagonal WC, primitive monoclinic WO2 and hexagonal WO3 . The diffraction pattern taken from the free-standing nanorods showed some broad diffraction rings. When the beam size was smaller than 5 nm, some faint polycrystalline-like rings appeared in addition to broad rings (similarly to Fig. 3) in the diffraction pattern. This means that the nanorods contain nanocrystals, and is consistent with the HRTEM results. A typical diffraction pattern is indicated in Fig. 2d. The radii of the 1st, 2nd and 3rd rings in Fig. 1d correspond to reciprocal distances of approximately 4.0, 7.7 and 10.9 nm−1 , respectively. A combination of W, WC, WO2 and WO3 is possible to show this kind of diffraction rings, and thereby all phases are possible to be contained in the nanorod. After heating treatment at about 600 ◦ C, no change in diffraction pattern was observed. Fig. 3 shows the diffraction pattern with a beam diameter of 5 nm, indicating the broad rings at the same positions and width as those in Fig. 2d and faint polycrystalline-like spots. In HRTEM observation, no change of the shape of the nanorod was found and the nanocrystals in the nanorod did not grow.
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Fig. 4. TEM image of: a, an array of nanodots and b, a free-standing nanorod produced from Fe(CO)5 gas. They were deposited on a carbon microgrid film using a 30 keV fine electron probe with 4 nm diameter in an FEG-SEM. c, HRTEM image of the edge of the free-standing nanorod, in which Fe oxide nanoparticles existing near the surface are clearly seen. d, Electron diffraction pattern taken from the free-standing nanorod. Diffraction indices and corresponding ring positions for alpha-Fe and Fe oxides are inserted.
Nanodots and nanorods from Fe(CO)5 were also fabricated using a 30 kV electron beam at room temperature in an FEG-SEM. Fig. 4a and b show transmission electron microscopy (TEM) images of nanodots on a carbon microgrid film and a free-standing nanorod on an edge of a carbon microgrid film. In Fig. 4a, a 3 × 3 array of nanodots can be seen as dark contrast. Each dot was made by 15 s irradiation of a beam with a diameter of 4 nm. The size of the dots is about 30 nm and the interval between the dots is about 80 nm. The size of the dots is comparable to those produced using W(CO)6 gas if the deposition beam conditions are the same. The nanorod in Fig. 4b was produced by moving the beam from the carbon
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Fig. 5. a, EELS line profiles taken across the free-standing nanorod, showing Fe, C and O distributions. The O concentration around the surface is remarkable. b, Carbon K-edges in EELS spectra before and after a postdeposition heating treatment. The spectrum from an amorphous carbon film is inserted as a reference.
film edge to space at a speed of 2 nm/s. The diameter and length of the rod are about 25 nm and 150 nm, respectively. The structure of the deposits produced from Fe(CO)5 was also a mixture of nanocrystals and amorphous phases. Fig. 4c shows an HRTEM image of the edge of the free-standing nanorod, in which nanocrystals covering a surface of the nanorod are seen. Although the nanocrystal size could not be defined clearly, it can be said that the size of the nanocrystals from Fe(CO)5 was obviously larger than that from W(CO)6 . Fig. 5a shows the result of EELS line profiles taken across the nanorod (perpendicular to the nanorod growth direction), indicating that Fe and C atoms distribute in the whole nanorod, while O atoms exist near the surface of the nanorod. EELS and HRTEM results suggest that the surface is covered by Fe oxide nanocrystals. The existence of the surface oxide may be mainly due to the exposure of the specimen to air during transportation from the FEG-SEM to the UHV-FEG-TEM. It is also found from EELS analyses that the shape of the carbon K-edge peak from the nanorod is identical to that from amorphous carbon film, whereas the carbon peak of carbide phases that appeared after the heat treatment differs from them as shown in Fig. 5b. This implies that carbon atoms contained
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Fig. 6. HRTEM image of the apex of the free-standing nanorods, of which the stoichiometry is Fe3 C, taken at [−2, 0, 1] incidence. Nanobeam electron diffraction from the nanorod is inserted in the figure.
in the as-grown nanorod were not chemically reacted with Fe at room temperature. Thus, we can deduce that the dominant phases in the nanorod could be Fe or Fe oxides. The details about EELS analyses about the deposits from Fe(CO)5 are to be reported elsewhere [25]. The possible equilibrium phases at room temperature are pure alpha-phase Fe, FeO, Fe2 O3 and Fe3 O4 . The diffraction pattern from the free-standing nanorods is shown in Fig. 4d, in which some broad diffraction rings and some faint polycrystalline-like rings can be seen. Diffraction indices and corresponding ring positions for alpha-phase Fe and Fe oxides listed above are inserted in the figure. Their positions are in agreement with the observed ring positions. It should be noted that the most intense but broad ring corresponds to a Fe {1, 1, 0} diffraction and there are polycrystalline-like spots on the rings of Fe oxides. Hence, it is concluded that Fe oxide phases are of nanometer-sized crystalline particles existing near the surface and the Fe phase is of cluster-like structure or almost amorphous. This is consistent with the HRTEM results. After a heating treatment at about 600 ◦ C, the surface oxide layers disappeared and the nanodots and free-standing nanorods transformed to single crystal or polycrystalline phases. The domain size was larger than 10 nm. Fig. 6 shows an HRTEM image of the apex of the free-standing nanorods, of which the stoichiometry is Fe3 C. The phases that appeared in the present work were alpha-Fe, Fe3 C, Fe5 C2 , Fe7 C3 , and Fe2 C. The diffraction pattern of alpha-Fe taken at the 3, 1, 1 incidence is shown in Fig. 7. Alpha-Fe was formed only in the free-standing nanorods. This may suggest that additional carbon contamination occurs from the carbon microgrid film into the dots. It has been reported
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Fig. 7. Diffraction pattern of alpha-Fe phase formed in the nanorod, taken at 3, 1, 1 incidence.
that, by heating Fe carbide up to 600 ◦ C in He gas, Fe carbide was changed to alphaFe and simultaneously CO2 was released [26]. In the case of heating the free-standing nanorod in UHV, carbon became CO2 by reacting with oxygen existing near the surface during the heat treatment and CO2 was pumped out. Carbon atoms remaining in the nanorod or carbon atoms supplied from a microgrid film could react with Fe to form Fe carbide. 4. Conclusions EBICVD was carried out using W(CO)6 and Fe(CO)5 at room temperature in field emission gun SEM and TEM to fabricate nanometer-sized dots and rods on a carbon microgrid film. The structure and composition before and after a post-deposition heat treatment at about 600 ◦ C were analyzed by TEM, HRTEM, electron diffraction and EELS. The as-deposited nanodots and free-standing nanorods from W(CO)6 contained W, O and C elements and exhibited a mixture of nanocrystals and amorphous matter. The nanodots and nanorods deposited using Fe(CO)5 contained Fe, C and O elements. Fe and C distributed in the whole volume, and Fe oxide nanocrystals existed near the surface of the deposits. Heating treatment for the nanodots and nanorods from W(CO)6 did not change their shape, structure and composition, whereas single crystal and polycrystalline phases appeared in the nanodots and nanorods from Fe(CO)5 after heating. Their stoichiometry was alpha-Fe, Fe3 C, Fe5 C2 , Fe7 C3 , and Fe2 C, all of which are magnetic materials. Thus, it was demonstrated that magnetic nanostructures can be fabricated by a combination of an EBICVD technique using Fe(CO)5 gas and a postdeposition heating treatment.
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M. Takeguchi, M. Tanaka, K. Furuya, Appl. Surf. Sci. 146 (1999) 257. K. Furuya, K. Mitsuishi, M. Shimojo, M. Takeguchi, Rev. Adv. Mater. Sci. 5 (2003) 34. A.J. DeMarco, J. Melngailis, J. Vac. Sci. Technol. B 17 (1999) 3154. D.K. Stewart, J.A. Morgan, B. Ward, J. Vac. Sci. Technol. B 9 (1991) 2670. S. Matsui, T. Kaito, J. Fujita, M. Komuro, K. Kanda, Y. Haruyama, J. Vac. Sci. Technol. B 3181 (2000) 18. I. Utke, P. Hoffmann, B. Dwir, K. Leifer, E. Kapon, P. Doppelt, J. Vac. Sci. Technol. B 18 (2000) 3168. A. Folch, J. Tejada, C.H. Peters, M.S. Wrighton, Appl. Phys. Lett. 66 (1995) 2080. K.T. Kohlmann-von Platen, L.-M. Buchmann, H.-C. Petzold, W.H. Brunger, J. Vac. Sci. Technol. B 10 (1992) 2690. K.T. Kohlmann-von Platen, J. Chiebek, M. Weiss, K. Reimer, H. Oertel, W.H. Brunger, J. Vac. Sci. Technol. B 11 (1993) 2219. H.W.P. Koops, C. Schossler, A. Kaya, M. Weber, J. Vac. Sci. Technol. B 14 (1996) 4105. H.W.P. Koops, A. Kaya, M. Weber, J. Vac. Sci. Technol. B 13 (1995) 2400. A. Ishibashi, K. Funato, Y. Mori, J. Vac. Sci. Technol. B 9 (1991) 169. S. Matsui, T. Ichihashi, Appl. Phys. Lett. 53 (1988) 842. N.A. Kislov, I.I. Khodos, E.D. Ivanov, J. Barthel, Scanning 18 (1996) 114. K. Mitsuishi, M. Shimojo, M. Han, K. Furuya, Appl. Phys. Lett. 83 (2003) 2064. M. Tanaka, M. Shimojo, K. Mitsuishi, K. Furuya, Appl. Phys. A 78 (2004) 543. M. Shimojo, K. Mitsuishi, A. Tameike, K. Furuya, J. Vac. Sci. Technol. B 22 (2004) 742. M. Shimojo, K. Mitsuishi, M. Tanaka, M. Han, K. Furuya, J. Microsc. 214 (2004) 76. M. Han, K. Mitsuishi, M. Shimojo, K. Furuya, Phil. Mag. 84 (2004) 1281. R.R. Kunz, T.M. Mayer, Appl. Phys. Lett. 50 (1987) 962. R.R. Kunz, T.M. Mayer, J. Vac. Sci. Technol. B 6 (1988) 1557. H. Siriwardane, O.A. Pringle, J.W. Newkirk, W.J. James, Thin Solid Films 287 (1996) 8. M. Takeguchi, M. Tanaka, H. Yasuda, K. Furuya, Surf. Interface Anal. 31 (2001) 68. K. Mitsuishi, Z.Q. Liu, M. Shimojo, M. Han, K. Furuya, Ultramicroscopy (in press). M. Shimojo, M. Takeguchi, M. Tanaka, K. Mitsuishi, K. Furuya, Appl. Phys. A (in press). A.R. Sethuraman, J.M. Stencel, A.M. Rubel, B. Cavin, C.R. Hubbard, J. Vac. Sci. Technol. A 12 (1994) 443.
Nanostructures fabricated by electron beam induced chemical vapor deposition M. Takeguchia,∗, M. Shimojoa,b, K. Mitsuishia, M. Tanakaa, K. Furuyaa a High Voltage Electron Microscopy Station, National Institute for Materials Science, 3-13 Sakura,
Tsukuba 305-0003, Japan b Precision and Intelligence Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta, Midori,
Yokohama 226-8503, Japan Available online 25 September 2004
Abstract Electron beam induced chemical vapor deposition (EBICVD) with tungsten and iron carbonyl gases, W(CO)6 and Fe(CO)5 , was attempted to fabricate nanostructures. In case of the deposits from W(CO)6 gas, the structure was a mixture of nanocrystals and amorphous phases and no change was seen after a post-deposition heating treatment. On the other hand, the deposits from Fe(CO)5 gas were composed of amorphous iron surrounded by Fe oxide nanocrystals, and heating at about 600 ◦ C resulted in the formation of crystalline Fe carbide and alpha-Fe phases. Thus, it was demonstrated that EBICVD with Fe(CO)5 gas followed by a heating treatment can be used practically to fabricate nanostructures of magnetic materials. © 2004 Elsevier Ltd. All rights reserved.
1. Introduction The development of a fabrication technique for nanometer-sized structures has been becoming a key technology for producing advanced devices and new functional materials. Especially, a size and position controllable fabrication technique has been required for ∗ Corresponding author. Tel.: +81 298 59 5053; fax: +81 298 59 5054.
E-mail address: [email protected] (M. Takeguchi). 0749-6036/$ - see front matter © 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.spmi.2004.08.038
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patterning nanodots and nanowires on substrate materials. Photo-lithographic patterning combined with etching processes, which has been widely used for the production of the current generation electronic devices, cannot achieve sufficient resolution for nanostructure fabrication because of the wavelength of visible light and lens numerical aperture. Although electron-beam lithography is capable of forming a nanometer-sized probe, any producible structure has been still more than 10 nm. This is due to the resolution of resists. It has been proposed that resistless direct patterning is a promising technique for such nanofabrication. Recently, various direct nanofabrication techniques have been applied to producing nanostructures [1,2]. Beam induced chemical vapor deposition is one of the most prospective methods, in which organic or metal-organic gases are decomposed by ion [3–5] or electron beams [6–21] and thus carbon or metals are deposited at the desired position, which can be controlled by a beam position. The size of the nanostructures can also be controlled by the beam diameter. It has been reported that highly designed electron beam-focusing instruments such as a transmission electron microscope equipped with a field emission gun succeeded to fabricate nanodots and nanowires with a size less than 5 nm [15–17]. To utilize the obtained deposits to units of nanodots and nanowires in electronic devices practically, the deposits should be composed of crystalline phases rather than amorphous. However, the deposits produced at room temperature often have amorphous phase or a mixture of nanocrystals and amorphous phases. The composition and crystallinity of the nanometer-sized deposits are important issues. Furthermore, it is known that a heating treatment or heating during deposition is effective to obtain crystalline phase nanostructures [20–22], and therefore heating induced composition and phase change of the deposits are interesting. In the present work, electron beam induced chemical vapor deposition (EBICVD) using tungsten and iron carbonyls, W(CO)6 and Fe(CO)5 , was carried out and the composition and structure of the produced nanostructures were characterized before and after postdeposition heating processes. 2. Experimental EBICVD experiments were carried out in a 200 kV field emission gun transmission electron microscope (FEG-TEM), JEOL JEM-2010F, and a 30 kV field emission gun scanning electron microscope (FEG-SEM), JEOL JSM-7800. The beam position was controlled by an external deflector voltage input using a computer with digital-analog converters. A gas introduction system consisting of a nozzle (inner diameter about 0.2 mm), gas pipe line, variable leak valve and gas source reservoirs was installed to the specimen chamber of these microscopes and the nozzle tip was about 1 mm away from the electron beam position. The details of the gas introduction system were described elsewhere [17,18]. A schematic illustration of the deposition experiment is shown in Fig. 1. Tungsten and iron carbonyl gases, W(CO)6 and Fe(CO)5 , were introduced into the specimen chamber and the partial pressure of the introduced gases was controlled to be 10−5 Pa order. Nanodots and nanorods were fabricated on carbon microgrid films supported on copper meshes. The typical currents of the electron beam used in FEG-TEM and FEG-SEM were about 0.5 nA at a beam diameter 0.8 nm and 0.8 nA at a beam diameter
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Fig. 1. Schematic drawing of EBICVD experiment in a 30 kV FEG-SEM. The gas introduction nozzle tip is about 1 mm away from the center of the specimen. The electron beam position can be controlled by an external deflector voltage input using a computer with digital-analog converters.
4 nm, respectively. The irradiation time of the beam for producing the nanodots was changed from 1 to 120 s, depending on the desired size. To form free-standing nanorods, the beam position was moved from the substrate into space at speeds of 1–3 nm/s. All the deposition was performed at room temperature. The deposits were characterized with a 200 kV ultrahigh vacuum field emission gun transmission electron microscope (UHV-FEG-TEM), JEOL JEM-2000VF, equipped with an electron energy loss spectroscopy (EELS) device [23]. The heating was carried out in a pre-treatment chamber connected to the specimen chamber of the UHV microscope through a gate valve so that the specimen could be observed without exposure to air. The specimens were heated up to 600 ◦ C for 1 h. The vacuum pressure of the pre-treatment chamber was maintained at 10−7 Pa order during the heat treatments. The structure of the deposits was analyzed by transmission electron microscopy (TEM) and electron diffraction in this microscope. 3. Results and discussion Fig. 2a shows an annular dark field scanning transmission electron microscopy (ADFSTEM) image of an array of dots deposited on a carbon microgrid film with a combination of 200 kV electron beam irradiation and W(CO)6 gas introduction at room temperature in FEG-TEM, in which 3 × 4 dots can be seen as bright contrast. The size of the dots is about 15 nm in diameter, and the interval between the dots is about 180 nm. The size of the dots can be controlled by changing the beam irradiation time and beam diameter. The minimum dot size achieved by the EBICVD technique is about 3.5 nm in conventional FEG-TEM [15] and 2 nm in UHV-FEG-TEM [16]. A dynamic Monte Carlo profile simulation has been carried out to calculate the shape of the deposits for various electron energies and led to the conclusion that the shape of the deposits can roughly be estimated
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Fig. 2. ADF-STEM image of: a, an array of nanodots and b, a free-standing nanorod produced from W(CO)6 gas. They were deposited on a carbon microgrid film using a 200 keV fine electron probe with 0.8 nm diameter in an FEG-TEM. c, HRTEM image of the edge of the nanorod, in which dark contrast nanocrystals are clearly seen. d, Electron diffraction pattern taken from the nanorod.
by the trajectory of high energy primary electrons [24]. Thus, a high energy nanometersized electron beam enables the fabrication resolution to be reduced. The nanorods were fabricated by moving the beam. Fig. 2b shows an ADF-STEM image of a typical free-standing nanorod on an edge of a carbon microgrid film using W(CO)6 gas. This was made by moving the beam from the carbon film edge to space at a speed of 3 nm/s. The diameter and length of the rod are about 20 and 500 nm, respectively. With increasing the scanning speed, the self-standing nanorod became more slender. As mentioned above, the deposit shape is determined by the secondary electrons emitted from the carbon film. However, after the incident electron beam was apart from the carbon film, the self-standing nanorod could be grown by the secondary electrons generated from the
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Fig. 3. Diffraction pattern from the free-standing nanorod after a post-deposition heating treatment. A beam with about 10 nm in diameter was used. The broad rings and polycrystalline-like spots can be seen.
as-deposited nanorod itself. Thus, the self-standing nanorod maintained an almost constant width throughout its length. The deposits from W(CO)6 gas consisted of nanocrystals and amorphous phases. Fig. 2c shows an HRTEM image of the edge of the nanorod, showing dark contrast nanometer-sized particles, in which lattice fringes can be seen clearly, with an amorphous surrounding. It is found that the nanocrystals are about 2 nm in diameter. Energy dispersive X-ray spectroscopy analysis showed that the deposits were composed of carbon, oxygen and tungsten elements [19]. According to W–C and W–O binary equilibrium phase diagrams, the possible equilibrium phases at room temperature are body-centered-cubic W, hexagonal WC, primitive monoclinic WO2 and hexagonal WO3 . The diffraction pattern taken from the free-standing nanorods showed some broad diffraction rings. When the beam size was smaller than 5 nm, some faint polycrystalline-like rings appeared in addition to broad rings (similarly to Fig. 3) in the diffraction pattern. This means that the nanorods contain nanocrystals, and is consistent with the HRTEM results. A typical diffraction pattern is indicated in Fig. 2d. The radii of the 1st, 2nd and 3rd rings in Fig. 1d correspond to reciprocal distances of approximately 4.0, 7.7 and 10.9 nm−1 , respectively. A combination of W, WC, WO2 and WO3 is possible to show this kind of diffraction rings, and thereby all phases are possible to be contained in the nanorod. After heating treatment at about 600 ◦ C, no change in diffraction pattern was observed. Fig. 3 shows the diffraction pattern with a beam diameter of 5 nm, indicating the broad rings at the same positions and width as those in Fig. 2d and faint polycrystalline-like spots. In HRTEM observation, no change of the shape of the nanorod was found and the nanocrystals in the nanorod did not grow.
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Fig. 4. TEM image of: a, an array of nanodots and b, a free-standing nanorod produced from Fe(CO)5 gas. They were deposited on a carbon microgrid film using a 30 keV fine electron probe with 4 nm diameter in an FEG-SEM. c, HRTEM image of the edge of the free-standing nanorod, in which Fe oxide nanoparticles existing near the surface are clearly seen. d, Electron diffraction pattern taken from the free-standing nanorod. Diffraction indices and corresponding ring positions for alpha-Fe and Fe oxides are inserted.
Nanodots and nanorods from Fe(CO)5 were also fabricated using a 30 kV electron beam at room temperature in an FEG-SEM. Fig. 4a and b show transmission electron microscopy (TEM) images of nanodots on a carbon microgrid film and a free-standing nanorod on an edge of a carbon microgrid film. In Fig. 4a, a 3 × 3 array of nanodots can be seen as dark contrast. Each dot was made by 15 s irradiation of a beam with a diameter of 4 nm. The size of the dots is about 30 nm and the interval between the dots is about 80 nm. The size of the dots is comparable to those produced using W(CO)6 gas if the deposition beam conditions are the same. The nanorod in Fig. 4b was produced by moving the beam from the carbon
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Fig. 5. a, EELS line profiles taken across the free-standing nanorod, showing Fe, C and O distributions. The O concentration around the surface is remarkable. b, Carbon K-edges in EELS spectra before and after a postdeposition heating treatment. The spectrum from an amorphous carbon film is inserted as a reference.
film edge to space at a speed of 2 nm/s. The diameter and length of the rod are about 25 nm and 150 nm, respectively. The structure of the deposits produced from Fe(CO)5 was also a mixture of nanocrystals and amorphous phases. Fig. 4c shows an HRTEM image of the edge of the free-standing nanorod, in which nanocrystals covering a surface of the nanorod are seen. Although the nanocrystal size could not be defined clearly, it can be said that the size of the nanocrystals from Fe(CO)5 was obviously larger than that from W(CO)6 . Fig. 5a shows the result of EELS line profiles taken across the nanorod (perpendicular to the nanorod growth direction), indicating that Fe and C atoms distribute in the whole nanorod, while O atoms exist near the surface of the nanorod. EELS and HRTEM results suggest that the surface is covered by Fe oxide nanocrystals. The existence of the surface oxide may be mainly due to the exposure of the specimen to air during transportation from the FEG-SEM to the UHV-FEG-TEM. It is also found from EELS analyses that the shape of the carbon K-edge peak from the nanorod is identical to that from amorphous carbon film, whereas the carbon peak of carbide phases that appeared after the heat treatment differs from them as shown in Fig. 5b. This implies that carbon atoms contained
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Fig. 6. HRTEM image of the apex of the free-standing nanorods, of which the stoichiometry is Fe3 C, taken at [−2, 0, 1] incidence. Nanobeam electron diffraction from the nanorod is inserted in the figure.
in the as-grown nanorod were not chemically reacted with Fe at room temperature. Thus, we can deduce that the dominant phases in the nanorod could be Fe or Fe oxides. The details about EELS analyses about the deposits from Fe(CO)5 are to be reported elsewhere [25]. The possible equilibrium phases at room temperature are pure alpha-phase Fe, FeO, Fe2 O3 and Fe3 O4 . The diffraction pattern from the free-standing nanorods is shown in Fig. 4d, in which some broad diffraction rings and some faint polycrystalline-like rings can be seen. Diffraction indices and corresponding ring positions for alpha-phase Fe and Fe oxides listed above are inserted in the figure. Their positions are in agreement with the observed ring positions. It should be noted that the most intense but broad ring corresponds to a Fe {1, 1, 0} diffraction and there are polycrystalline-like spots on the rings of Fe oxides. Hence, it is concluded that Fe oxide phases are of nanometer-sized crystalline particles existing near the surface and the Fe phase is of cluster-like structure or almost amorphous. This is consistent with the HRTEM results. After a heating treatment at about 600 ◦ C, the surface oxide layers disappeared and the nanodots and free-standing nanorods transformed to single crystal or polycrystalline phases. The domain size was larger than 10 nm. Fig. 6 shows an HRTEM image of the apex of the free-standing nanorods, of which the stoichiometry is Fe3 C. The phases that appeared in the present work were alpha-Fe, Fe3 C, Fe5 C2 , Fe7 C3 , and Fe2 C. The diffraction pattern of alpha-Fe taken at the 3, 1, 1 incidence is shown in Fig. 7. Alpha-Fe was formed only in the free-standing nanorods. This may suggest that additional carbon contamination occurs from the carbon microgrid film into the dots. It has been reported
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Fig. 7. Diffraction pattern of alpha-Fe phase formed in the nanorod, taken at 3, 1, 1 incidence.
that, by heating Fe carbide up to 600 ◦ C in He gas, Fe carbide was changed to alphaFe and simultaneously CO2 was released [26]. In the case of heating the free-standing nanorod in UHV, carbon became CO2 by reacting with oxygen existing near the surface during the heat treatment and CO2 was pumped out. Carbon atoms remaining in the nanorod or carbon atoms supplied from a microgrid film could react with Fe to form Fe carbide. 4. Conclusions EBICVD was carried out using W(CO)6 and Fe(CO)5 at room temperature in field emission gun SEM and TEM to fabricate nanometer-sized dots and rods on a carbon microgrid film. The structure and composition before and after a post-deposition heat treatment at about 600 ◦ C were analyzed by TEM, HRTEM, electron diffraction and EELS. The as-deposited nanodots and free-standing nanorods from W(CO)6 contained W, O and C elements and exhibited a mixture of nanocrystals and amorphous matter. The nanodots and nanorods deposited using Fe(CO)5 contained Fe, C and O elements. Fe and C distributed in the whole volume, and Fe oxide nanocrystals existed near the surface of the deposits. Heating treatment for the nanodots and nanorods from W(CO)6 did not change their shape, structure and composition, whereas single crystal and polycrystalline phases appeared in the nanodots and nanorods from Fe(CO)5 after heating. Their stoichiometry was alpha-Fe, Fe3 C, Fe5 C2 , Fe7 C3 , and Fe2 C, all of which are magnetic materials. Thus, it was demonstrated that magnetic nanostructures can be fabricated by a combination of an EBICVD technique using Fe(CO)5 gas and a postdeposition heating treatment.
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