Amorphous silica nanowires grown by the vapor–solid mechanism

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

Amorphous silica nanowires grown by the vapor–solid mechanism Ki-Hong Lee a, * , Seung-Woo Lee a , Richard R. Vanfleet b , Wolfgang Sigmund a a Materials Science and Engineering Department, University of Florida, 255 Rhines Hall, Gainesville, FL 32611, USA b Advanced Materials Processing and Analysis Center, University of Central Florida, Orlando, FL 32816, USA Received 22 May 2003; in final form 3 June 2003 Published online: 9 July 2003 Abstract Silica nanowires were synthesized by using silica nanoparticles as a growth catalyst using a gas composed of CH 4 and H 2 at 1050 °C. Silica nanoparticles provide silicon and oxygen atoms for the formation of the nanowires, as well acting as a growth site. The nanowires nucleated on graphitic carbon layers formed around the seed particles, indicating that the nanowires grow by the vapor–solid mechanism. Photoluminescence spectra of the nanowires normally showed strong blue emission peaked at 3.1 and 2.8 eV under 3.8 eV laser excitation. Post-hydrogen annealing resulted in the appearance of longer wavelength photoluminescence band. Ó 2003 Elsevier B.V. All rights reserved. 1. Introduction One-dimensional quantum nanowires are promising materials for nanoelectronic devices due to their small dimensions and unique physical characteristics. Various kinds of one-dimensional structures, such as carbon nanotubes [1,2] semi- conductor nanowires [3,4] and oxides [5], have been studied for applications in nanoelectronic devices. Wide band gap semiconductor light- emitting nanowires are useful for visible display devices and optoelectronics [6]. Amorphous semi- conducting materials, such as Si–C–H, can be synthesized with various compositions, to manip- ulate the optical properties in an extremely wide range [7]. Amorphous silica is widely used in sili- con based integrated devices and can also be produced as nanowires. Yu et al. [8] showed that amorphous silica nanowires (SiONWs) emit blue light and might hence be applied in integrated optical devices. The vapor–liquid–solid (VLS) process is a fundamental mechanism for the growth of SiONWs similar to other kinds of one- dimensional nanomaterials [9–11]. In this study, a new approach to nucleate and grow SiONWs is presented. Silica nanowires (SiONWs) with small diameter (15–40 nm) were produced using silica nanoparticles as a nucleation catalyst at 1050 °C in a highly reducing atmo- sphere. The amorphous silica nanowires are Chemical Physics Letters 376 (2003) 498–503 www.elsevier.com/locate/cplett * Corresponding author. Fax: +1-352-846-3355. E-mail address: khonglee@ufl.edu (K H. Lee). 0009-2614/03/$ - see front matter Ó 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0009-2614(03)01019-4 nucleated from graphitic carbon layers, which are formed around the seed silica nanoparticles. This fact implicates that the silica nanowires grow by vapor–solid (VS) mechanism, not by vapor– liquid–solid (VLS) mechanism. The SiONWs show strong blue emission, and a longer process time and hydrogen annealing after synthesis of the nanowires results in the emission of longer wave- length spectrum. 2. Experimental section 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; iron films of 30 nm were deposited on the oxide layer by sputtering. A droplet of aqueous 20 nm silica nanoparticles (0.1 wt%, 0.2 ml) solution was placed on a Fe/SiO 2 /Si substrate. Poor wetting of the aqueous silica solution on the substrate caused non-uniform coverage of silica nanoparticle layers. The substrates were heated up to 100 °C on a hot plate in order to expedite the drying process. After annealing in H 2 (200 sccm) for 10 min at 1050 °C, SiONWs were synthesized in a quartz tube furnace with a tube diameter of 1.5 in. in a gas mixture consisting of CH 4 (10 sccm) and H 2 (200 sccm). A constant flow of Ar (1000 sccm) through the quartz tube was performed to purge the quartz tube during heating and cooling. A field emission scanning electron microscope (FESEM, JEOL 6335F) was used to investigate the growth characteristics of SiONWs on the substrates. A transmission electron microscope (TEM, JEM 4010), equipped with energy disper- sive X-ray spectroscope (EDX), was used for structure and composition analysis. Electron en- ergy loss spectroscopy (EELS, Tecnai F30) was carried out for further characterization of the nanowires. Photoluminescence (PL) was measured at room temperature using a He–Cd laser with 325 nm excitation wavelength in the spectral range of 350–700 nm. 3. Results and discussion Due to poor wetting of aqueous silica solution on the substrate, the coverage of silica nanoparti- cles was non-uniform as described in Section 2. Low magnification FESEM studies (Fig. 1a) Fig. 1. (a) A low magnification FESEM photograph showing two distinctly different regions on the substrate after synthesis. (b) Nanowires grown in the region covered by silica nanoparticles. (c) Surface morphology in the region not covered by silica nanoparticles (no nanowire is observed). K H. Lee et al. / Chemical Physics Letters 376 (2003) 498–503 499 revealed two distinct regions on the substrate sur- face after synthesis. SiONWs grew up to tens of lm in length on the silica nanoparticles. Graphite, iron carbide, and few carbon nanotubes were found in the regions where no silica particle existed. High magnification FESEM photographs of these two regions show the difference (Figs. 1b and c). Formation of the amorphous SiONW phase could be identified by high resolution transmission electron microscopy (HRTEM), electron disper- sive spectroscopy (EDS), electron energy loss spectroscopy (EELS) as well as selected area dif- fraction (SAD) pattern analysis. A low magnifi- cation TEM photograph, as shown in Fig. 2a, shows the nanowires grown from the seed parti- cles. The diameter of the nanowires ranges be- tween 15 and 35 nm. Process time did not affect the length and the diameter of the nanowires signifi- cantly. An EELS spectrum, as shown in Fig. 2b, reveals that the nanowires have amorphous silicon oxide phase by comparing to the standard EELS spectrum for amorphous silica. Figs. 2c and d show HRTEM photographs and SAD patterns of a nanowire and a seed particle, respectively. The SAD pattern and HRTEM photographs from the seed regions indicate that crystalline graphitic layers formed around the seed particles. EDS spectra, as shown in Fig. 3, reveals different com- positions according to positions in a nanowire. In addition to silicon and oxygen, high concentration of iron and carbon was detected from seed particle regions and carbon from regions near the seed particles. Carbon which was detected from the nanowires, as shown in Fig. 3c, is negligible and seems to arise from the surface contamination due to focused electron beam. Calculated compositions of the nanowire from the EDS reveal the forma- tion of oxygen deficient silica phase (SiO 2Àx ). Silica nanoparticles act as nucleation sites, as well as they provide oxygen and silicon for the SiONW growth. Catalytic decomposition of CH 4 occurs on the iron film surface. Fig. 4a shows a Fig. 2. (a) A low magnification TEM photograph of the amorphous nanowires. (b) EELS spectrum from the amorphous nanowire. The profile resembles that of the standard EELS spectrum of amorphous silica. (c) A HRTEM photograph and a SAD pattern showing the amorphous characteristic of the nanowires. (d) A HRTEM and a SAD pattern from the seed part showing graphitic carbon layers surrounding the seed particles. 500 K H. Lee et al. / Chemical Physics Letters 376 (2003) 498–503 schematic diagram of growth mechanism of SiONWs on the agglomerated silica nanoparticles. The Fe films act as a catalyst decomposing CH 4 gas into carbon and hydrogen atoms. The carbon atoms decompose the silica nanoparticles; and SiO and carbon monoxide (CO) vapor are formed. The CO gas could be reduced to carbon and oxygen atoms by the catalytic activity of the Fe films. The oxygen and the SiO vapor diffuse up to top regions of the silica particle layers to form the nanowires. In an initial stage of the nanowire growth, iron vapor forms a Fe–Si–O phase, as shown in Fig. 3b, with silica at the surface of the silica nanoparti- cles; and the carbon atoms form graphitic layers around the nanoparticles, as shown in Fig. 2d. The existence of the graphite phase surrounding the seed particles is a reasonable indication that the nanowires grew by VS mechanism. Carbon vapor rich environment in the initial stage forms an amorphous diamond like carbon (DLC) phase in the nanowire near the seed particles, as shown in Fig. 3c. Following the initial stage, carbothermal decomposition of the silica nanoparticles by the carbon vapor produces a SiO vapor rich environ- ment to form the silica nanowires by the VS mech- anism. Increasing the process time up to 120 min does not change the diameter and the length of the silica nanowires significantly, indicating the Fig. 3. EDS spectra according to the positions of the silica nanowires: (b) from seeds; (c) from nanowires near to the seeds; (d) from nanowires. These positions are illustrated (a). (The peak at 8 eV represents Cu from a TEM grid.) Fig. 4. (a) A schematic diagram of the growth of SiONWs on the silica nanoparticle layer. (b) A FESEM photograph shows that the nanowires grow on the top side of the silica nanopar- ticle layer. K H. Lee et al. / Chemical Physics Letters 376 (2003) 498–503 501 formation of the nanowires is accomplished in a short period of after starting the synthesis. Fig. 4b shows a typical growth pattern of the silica nanowires from the agglomerated silica nanopar- ticle layers on the Fe films. PL spectra from the nanowires show a strong dependency on the processing time and post-hy- drogen annealing. Blue emission spectrum cen- tered at the wavelength of 400 nm (3.1 eV) was observed with synthesis time of 20 min, as shown in Fig. 5a. The peak intensity shifts to longer wave lengths, which has two distinguishable peaks at 415 nm (3.0 eV) and 440 nm (2.8 eV), with syn- thesizing for 60 min. The bands are analogous to the PL spectrum bands usually observed in amorphous silica nanowires. For amorphous sil- ica, the 2.7 eV band is ascribed to the neutral ox- ygen vacancy (BSi–SiB) [12], and the 3.1 eV band is due to a twofold coordinated silicon lone-pair centers (O–Si–O) [13]. These defects are clearly induced by high oxygen deficiency in silica. In case of increasing the processing time to 120 min, the PL intensity decreases and the spectrum tails to a longer wavelength. Hydrogen annealing, followed by SiONWs synthesis, causes more dramatic change in the PL band, as shown in Fig. 5b. The PL band becomes broader and the peak is centered at 560 nm (2.2 eV). As mentioned in the previous section, increased processing time and post-hy- drogen annealing does not result in a change in the size and the diameter of the nanowires. The SiO vapor supply is limited by decreased catalytic carbon formation on the Fe films. As a result, longer processing time only leads to a composition change of the nanowires. The decreased PL in- tensity with longer processing time seems to be caused by oxygen supplement, resulting in de- creasing defect density, from the source gases. The peak wavelength change of the PL by the hydro- gen annealing is not clear at this time. 4. Conclusions Silica nanowires were synthesized by VS mechanism by using silica nanoparticles as a growth catalyst. Silica nanoparticles provide sili- con and oxygen atoms for the formation of the nanowires, as well as act as a growth site. The photoluminescence spectra of the nanowires showed strong blue emission peaks at 3.1 and 2.8 eV under 3.8 eV laser excitation. Longer process time and post-hydrogen annealing resulted in the appearance of a longer wavelength photolumi- nescence band and broad PL spectra. Acknowledgements The authors thank Dr. Young-Ho Lee, Dr. Myung-Hyun Lee and Dr. Won-Seon Seo (of Advanced Materials Analysis and Evaluation Fig. 5. (a) PL spectra from the SiONWs with processing time (the peak at 650 nm shows the secondary harmonic oscillation of the laser source). (b) Hydrogen annealing effect on the PL characteristics of the SiONWs. 502 K H. Lee et al. / Chemical Physics Letters 376 (2003) 498–503 Center of the Korea Institute of Ceramic Engi- neering & Technology) for the HRTEM and the EDS analysis. This work was supported by DARPA/Army Research Office under Grant No. DAAD19-00-1-0002 through the center for mate- rials in sensors and actuators (MINSA). References [1] S. Iijima, Nature (London) 354 (1991) 56. [2] S. Fan, M.G. Chapline, N.R. Franklin, T.W. Tombler, A.M. Cassell, H. Dai, Science 283 (1999) 512. [3] A. 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Lee et al. / Chemical Physics Letters 376 (2003) 498–503 503 . around the seed silica nanoparticles. This fact implicates that the silica nanowires grow by vapor–solid (VS) mechanism, not by vapor– liquid–solid (VLS) mechanism. . the substrate after synthesis. (b) Nanowires grown in the region covered by silica nanoparticles. (c) Surface morphology in the region not covered by silica