Đây là một bài báo khoa học về dây nano silic trong lĩnh vực nghiên cứu công nghệ nano dành cho những người nghiên cứu sâu về vật lý và khoa học vật liệu.Tài liệu có thể dùng tham khảo cho sinh viên các nghành vật lý và công nghệ có đam mê về khoa học
Direct growth of amorphous silica nanowires by solid state transformation of SiO 2 films Ki-Hong Lee a, * , Hyuck Soo Yang a , Kwang Hyeon Baik a , Jungsik Bang a , Richard R. Vanfleet b , Wolfgang Sigmund a a Materials Science and Engineering Department, University of Florida, Gainesville, FL 32611, USA b Advanced Materials Processing and Analysis Center, University of Central Florida, Orlando, FL 32816, USA Received 16 September 2003; in final form 5 November 2003 Published online: 5 December 2003 Abstract Amorphous silica nanowires (a-SiONWs) were produced by direct solid state transformation from silica films. The silica nanowires grow on TiN/Ni/SiO 2 /Si substrates during the annealing in H 2 or a H 2 :CH 4 mixture at 1050 °C. Titanium nitride (TiN) films were used to induce a solid state reaction with silica (SiO 2 ) films on silicon wafers to provide silicon atoms into growing nanowires. The TiN layers induce the diffusion of silicon and oxygen to the surface by a stress gradient built inside the films. The nickel diffuses to the surface during the TiN deposition and acts as a nucleation site for the a-SiONWs. Ó 2003 Elsevier B.V. All rights reserved. 1. Introduction Formation of a liquid phase has been an essential factor for the growth of one-dimensional nanowires by the vapor–liquid–solid (VLS) [1–3] or the solid–liquid– solid (SLS) mechanism [4]. The liquid phase acts as a source for dissolution and re-precipitation of compo- nents for the growth of nanowires. Amorphous semi- conducting materials, such as Si–C–H, can be synthesized with various compositions, to manipulate the optical properties in an extremely wide range [5]. Amorphous silica is widely used in silicon based integrated devices and can also be produced as nanowires. Yu et al. [6] showed that a-SiONWs emit blue light and might hence be applied in integrated optical devices. The VLS and the SLS mechanism have been an act- ing mechanism for the growth of silica nanowires [7,8]. In this work, a novel growth mechanism for a-SiONWs is presented via direct solid state transformation from silica films. Titanium nitride (TiN) films were used to induce a solid state reaction with the silica (SiO 2 ) films on silicon wafers to provide silicon atoms into growing nanowires. The TiN layers induce the diffusion of silicon and oxygen to the surface by a stress gradient built in- side the films. The nickel diffuses to the surface during the TiN deposition and acts as a nucleation site for the a-SiONWs. 2. Experimental N-type silicon h100i wafers (3 X cm, 1 Â 1 cm) were used as substrates for the growth of SiONWs. After thermally oxidizing the Si substrates, Ni films of 5 nm were deposited on the oxide layer by e-beam evapora- tion. TiN films were deposited on the nickel films by laser ablation of a TiN target (99.9%). The ablation was carried out using a KrF excimer laser (k ¼ 248 nm) at a fluence of 1 J/cm 2 , and a repetition rate of 10 Hz. The TiN film (30–50 nm) was deposited at 650 °C under evacuated atmosphere (<8 Â 10 À6 Torr) without flowing nitrogen gas. A quartz tube furnace with diameter of 1.5 00 was used for silica nanowire synthesis. Annealing of the substrates Chemical Physics Letters 383 (2004) 380–384 www.elsevier.com/locate/cplett * Corresponding author. Fax: +13528463355. E-mail address: khonglee@ufl.edu (K H. Lee). 0009-2614/$ - see front matter Ó 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2003.11.056 was carried out in two conditions. After annealing in a H 2 :Ar (200:150 sccm) mixture for 10 min at 1050 °C, then a H 2 :CH 4 (200:10 sccm) mixture incorporated into the system for 10 min. Another approach is to flow a H 2 :Ar (200:150 sccm) mixture gas for longer times without the incorporation of CH 4 . The annealing time was increased to 80 min in this case. Ar (800 sccm) was kept flowing through the quartz tube to purge the sys- tem during the heating and cooling. A field emission scanning electron microscope (FE- SEM, JEOL 6335F) was used to investigate the growth characteristics of a-SiONWs on the substrates. A transmission electron microscope (TEM, JEOL 2010F) equipped with an energy dispersive spectroscope (EDS), was used for structure and composition analysis. Elec- tron energy loss spectroscopy (EELS, Tecnai F30) was carried out for further characterization of the nano- wires. Surface analysis of the substrates was carried out by Auger electron spectroscopy (AES, Perkin–Elmer PHI 660) at an acceleration voltage of 8 keV. The sub- strates for the AES analysis were transferred to the system after exposing to air. 3. Results and discussion A-SiONWs were synthesized by simply annealing TiN/Ni/SiO 2 /Si substrates in a H 2 :Ar or a CH 4 :H 2 mixture gas. Fig. 1 show FESEM photo graphs after annealing the substrates at 1050 °C in two gas condi- tions. Fig. 1a shows silica nanowires grown on the substrate after annealing in H 2 for 10 min followed by CH 4 :H 2 for 10 min. More process time is necessary for the synthesis of the nanowires without CH 4 . Large density of the nanowires was achieved without CH 4 with a longer process time, as shown in Fig. 1b. The seed particles are attach ed to the top of the grown nanowires as shown in the insets. The phase and the structure of the nanowires are identified by high resolution transmission electron mi- croscopy (HRTEM), electron dispersive spectroscopy (EDS), as well as electron energy loss spectroscopy (EELS). One structural difference of the nanowires be- tween the two gas mixture conditions is that an amor- phous carbon shell is formed around silica nanowires when CH 4 is incorporated into the system. Fig. 2a shows a nanowire of Fig. 1a in scanning TEM (STEM) mode used for the EELS analysis. The EELS line scanning profiles show composition changes for silicon and car- bon across the nanowire (Fig. 2b). The fine structure EELS of the silicon 2p edge from the inner phase reveals the formation of amorphous silica nanowire (Fig. 2c); Carbon 1s edge band form the outside shell shows an amorphous carbon phase (Fig. 2d). No titanium and nitrogen were detected by the EELS in the nanowires. The carbon on the shell is supplied by thermal decom- position of CH 4 by nickel. No carbon shells were ob- served from the silica nanowires grown in the Ar:H 2 gas mixture (not shown here). Titanium nitride films should play an important role on the growth of silica nanowires. No nanowires were observed without TiN films on the substrates with the same annealing processes. Titanium nitride is reduced to Ti in hydrogen atmosphere at high temperature. Tita- nium has a higher tendency to oxidize than silica films, resulting in formation of a titanium oxide (TiO x ) phase by removal of oxygen from the silica layer [9]. The re- duction of TiN seems to be a critical factor in the growth kinetics of a-SiONWs. The growth of silica nanowires was limited by introducing ammonia (NH 3 ) into the system to suppress the decomposition of TiN films. The Ni islands act as a nucleation site for the a-SiONW growth, and expedite the reduction of TiN by supply- ing extra hydrogen by thermal decomposition of Fig. 1. FESEM photographs of a-SiONWs nanowires grown on the substrates after annealing at 1050 °C in: (a) H 2 (200 sccm) for 10 min followed by CH 4 :H 2 (10:200 sccm) for 10 min; (b) H 2 :Ar (200:150 sccm) for 80 min. The arrows represent the seed particles attached to the top of nanowires. K H. Lee et al. / Chemical Physics Letters 383 (2004) 380–384 381 CH 4 , which explains the faster growth of a-SiONWs with CH 4 . Nickel atoms diffuse out to the surface during the deposition of TiN. The AES profile of the substrate surface right after the laser ablation reveals the existence of nickel on the surface, as shown in Fig. 3. Silicon at- oms diffuse out to the surface and form the nanowires during the annealing at 1050 °C. The AES of the sub- strate after annealing at 1050 °C for 20 min in Ar:H 2 shows the appearance of silicon at the surface. Silica nanowires nucleate on the nickel islands and grow on the surface by silicon diffusion from the under layer SiO 2 films. Fig. 4 shows a nanowire grown on the substrate and EDS spectra showing compositions in each layer in the structure at the same process condition with Fig. 3. As shown in the TEM photographs, the nanowires begin to grow at this stage even though they are not observed using the FESEM. The growth behavior of the nanowires and the EDS spectra of the layers in Fig. 4 show several facts which would not be observed by the VLS or the SLS mecha- nism. As shown in Fig. 4a (also can be seen in Fig. 1), the metal particles are attached to the end of the nanowires, supporting the top growth mode. Catalyst Fig. 3. AES profiles from the substrates after the deposition of TiN films, and after annealing in the H 2 :Ar (200:150 sccm) mixture for 20 min. The insets show the Ni (LMM) transition peak. Fig. 2. EELS spectrum profiles of a nanowire synthesized in the CH 4 :H 2 mixture: (a) a STEM photograph of the nanowire on a TEM grid used for the EELS analysis. The arrow shows a carbon wire grown from the surface amorphous carbon by a focused electron beam scanning cross the wire; (b) intensity profiles of Si and C cross the wire showing the composition variation cross the wire. Oxygen has a similar profile with Si (not shown here); (c) a Si 2p EELS profile from inside the nanowire indicating the formation of an amorphous silica phase; (d) a carbon 1s profile from the outside shell indicating a amorphous carbon phase. 382 K H. Lee et al. / Chemical Physics Letters 383 (2004) 380–384 materials should be at the surface of the substrate in order that source atoms dissolve and precipitate as a nanowire on a catalyst liquid droplet, called Ôbase growthÕ, as in the SLS mechanism. The existence of metal particles on the top of the nanowires illustrates that the growth is not established by the SLS mecha- nism. Silicon is present after annealing in the original TiN layer; otherwise, nickel is not present in the layer (Fig. 4d). The parti cle is composed of nickel mainly (Fig. 4b), which is unlikely to form a liquid phase at the process temperature. Silicon was not detected in the particles attached to the nanowires by the EELS (not shown here). As a result, there is little possibility to form a liquid phase either in the supporting layers (the TiN films) or in the seed particles. In addition, silicon sources were not incorporated into the system directly from vapor phase. These facts show that it is unlikely for the nanowires to grow by the VLS mechanism or the SLS. The under layer silicon oxide film is the only Si source for the growth of a-SiONWs in the system. Silicon should be supplied to growing silica nanowires by solid state diffusion through the TiN layer. The TiN layers decompose into Ti and form an oxide by the substitu- tion reaction with silica. Reduced silicon can form a nickel silicide with Ni remaining in the interface (Fig. 4e) or diffuse out to the surface to form the silica nanowires. (Fig. 4c) The Si diffusion can be derived by a stress variation built in the TiN layer during the annealing processes. The substitutional reaction initiates at the interface between the TiN and the silica, thereby, it builds a compressive stress in the interface region and a tensile stress in the surface. NH 3 gas suppresses the decomposition of TiN, as a result, it limits the growth of a-SiONWs nanowires by the mechanism. Oxygen seems Fig. 4. (a) A TEM photograph of a cross section of the substrate with the same treatment condition as Fig. 3; (b)–(f) show EDS data from each layer indicated in (a). It is difficult to define the existence of nitrogen in the TiN layer since it is close to the oxygen and the Ti(L) peak. However, nitrogen is considered to be present in the layer at this annealing stage by the AES in Fig. 3. Fig. 5. A schematic diagram showing a growth mechanism of silica nanowires by the solid–solid transformation. K H. Lee et al. / Chemical Physics Letters 383 (2004) 380–384 383 to be supplied either from the silica or the vapor phase. Fig. 5 shows a schematic diagram showing the growth mechanism of a-SiONWs in our experimental condition. 4. Conclusion In summary, silica nanowires were synthesized by solid state diffusion of silicon from the silica films. The growth mechanism could be exp lained by direct solid to solid phase transformation, so called, the SS mecha- nism. The TiN films react with the silica films to produce a silicon source for the nanowires and cause the silicon diffusion by the internal stress. Our result suggests a novel growth mechanism for growth of nanowires, and can be applied to the synthesis of other kind of nano- wires. Acknowledgements This work was supported by DARPA/Army Re- search Office under Grant No. DAAD19-00- 1-0002 through the center for materials in sensors and actuat ors (MINSA). The authors thank Kerry Siebein (of the Major Analytical Instrumentation Center at the Uni- versity of Florida) for the TEM and the EDS analysis. References [1] A. Morales, C. 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December 2003 Abstract Amorphous silica nanowires (a-SiONWs) were produced by direct solid state transformation from silica films. The silica nanowires grow on