NANO LETTERS
Precision-Cut Crystalline Silicon Nanodots and Nanorods from Nanowires and Direct Visualization of Cross Sections and Growth Orientations of Silicon Nanowires
2003 Vol. 3, No. 12 1735-1737
Boon K. Teo,*,† X. H. Sun,‡,| T. F. Hung,§ X. M. Meng,§,| N. B. Wong,*,‡,| and S. T. Lee*,§,| Department of Chemistry, UniVersity of Illinois at Chicago, Chicago, Illinois 60607, Department of Biology and Chemistry, Department of Physics and Materials Science, and Center of Super-diamond and AdVanced Films, City UniVersity of Hong Kong, Hong Kong SAR, China Received August 4, 2003
ABSTRACT An easy and convenient method, using the well-established microtome technique, has been developed to precision cut nanowires into nanodots of well-defined sizes and shapes. The technique allows the determination of the growth direction, the shape of the cross section, the internal atomic structure (including twinning or other stacking faults, if any) of single nanowires. In our experiments, both the coaxial structure of vertical cross section of normal silicon nanowires (SiNWs) and horizontal cross section of necklace-like SiNWs were obtained and examined under TEM.
Fabrication of nanomaterials of precise dimensions and welldefined shapes poses a real challenge in nanotechnology. To construct a nanodevice, one must be able to control and/or manipulate the properties, sizes, and shapes of materials or components at the nanometer level. There are, in principle, two approaches to this problem: the bottom-up strategy and the top-down approach. The bottom-up strategy seeks to synthesize large supermolecules of predetermined sizes and shapes by chemical means (usually via self-assembly).1,2 While this method has produced numerous supermolecules of a wide variety of sizes and shapes, they are, in general, limited to sizes of up to a few nanometers. Large colloidal particles, in the size range of 5-500 nm, on the other hand, can be produced rather easily. Unfortunately, precise size and shape controls are lost and “monodispersed phases” usually mean a narrow size distribution rather than “onesize” and “one-shape.” Furthermore, chemical methods are usually rather involved and materials specific (i.e., different * Corresponding author. E-mail:
[email protected]; bhnbwong@ cityu.edu.hk;
[email protected]. † University of Illinois at Chicago. ‡ Department of Biology and Chemistry, City University of Hong Kong. § Department of Physics and Materials Science, City University of Hong Kong. | Center of Super-diamond and Advanced Films, City University of Hong Kong. 10.1021/nl034603v CCC: $25.00 Published on Web 11/15/2003
© 2003 American Chemical Society
synthetic strategies need to be developed for different materials). It is therefore highly desirable to devise a simple and convenient way to produce nanoparticles of precise dimensions and well-defined shapes in the nanoregime. The approach we take is the top-down strategy. We reason that if nanodots or nanorods can be precision cut reproducibly from an oriented nanowire, a wide range of identical nanoparticles of well-defined sizes and shapes can be produced. This communication reports an easy and convenient technique for the preparation of silicon nanodots (SiNDs) or nanorods (SiNRs) precision cut from silicon nanowires (SiNWs). Silicon-based nanotechnology is of importance for the obvious reason that it is compatible with conventional silicon microtechnology. To date, prototype nanodevices3,4 such as transistors, diodes, switches, light-emitting diodes, lasers, chemical and biological sensors, etc. have been fabricated from SiNWs and SiNDs. Single-electron devices are possible due to the carrier confinement and/or Coulomb blockade phenomena exhibited by the very small sizes of SiNWs and SiNDs. Another important aspect of this work lies in the fact that it allows direct visualization of the cross sections of silicon or other nanowires or nanotubes. These materials are
Figure 1. (a) HRTEM image of the cross section of a SiNW. Note the dark and light ellipses of the top and bottom cross sections. (b) Schematic representations of the top (left) and side (right) views.
expected to play many important roles in nanotechnology (e.g., as functional components in mesoscopic electronic, optical, and magnetic devices). The direct observation under high-resolution transmission electron microscopy (HRTEM) of the cross sections of nanowires is critical in the determination of the orientation and behavior of the growth of these nanowires as well as their internal structures and atomic arrangements. This information is crucial in the understanding of their properties in the nanorealm (e.g., quantum size effects) and ultimately in the construction of nanodevices. The cutting method used here is the well-established microtome technique widely used in the preparation of microscope samples, especially of biological specimens. Cross-sectional samples of 10-50 nm in thickness suitable for TEM observation can be obtained reproducibly. The SiNWs used in the present work were synthesized by the thermal evaporation as previously described.5 As-synthesized SiNWs were long, free-standing wires of up to several microns in length. Each wire had a crystalline silicon core of 10-20 nm in diameter which was sheathed with a thick (5-10 nm) layer of amorphous silicon oxide. The SiNWs were mixed with a suitable resin (such as SPURR) and cured in air for several days. After curing, the resin-bound SiNW samples were sectioned into various thickness (10-50 nm) with a Reichert Ultracuts ultra-microtome equipped with a diamond knife. The sectioned samples were mounted onto copper grid for TEM observation. TEM and HRTEM images were recorded with Philips CM20 and Philips CM200 FEG, respectively, at 200 keV. Figure 1a shows the HRTEM image of the vertical crosssection of a SiNW. The solid-wire nature, the elliptical shape, and the perfect single crystalline structure of the cross section can be observed. The Si atomic resolution image allows us to determine the growth direction of the SiNW to be the (110) direction with the perpendicular (111) planes (d spacing 3.15 Å) meeting at the expected angle of 109.5°. It should be mentioned that, while we observed mostly (110) cross sections, other growth orientations such as (112) and (111) have also been obtained. In fact, (110) and (112) growth 1736
orientations are quite common with SiNWs prepared from the SiO thermal evaporation technique,5 while (111) growth is more common with the laser ablation method.6 The diameter of the Si core of this particular nanowire is 10.6 nm. From the HRTEM image, three parameters can be deduced: R, the radius of the Si core (which is also the short (minor) radius of the elliptical image of the cross section), r, the long (major) radius of the ellipse, and p, the distance between the centers of the two ellipses (see Figure 1b). Based on these parameters, the thickness t and the inclination angle θ of the nanocylinder can be calculated. For this particular nanocylinder, the diameter measured 2R ) 10.6 nm, with the other parameters being 2r ) 12.4 nm, the thickness t ) 9.4 nm, and the inclination angle θ ) 32°. Other sizes and shapes of nanodots can likewise be cut and measured from their HRTEM images. Figure 2a depicts the HRTEM of the cross section of another SiNW. It is obvious that this particular SiNW is twinned with the two twinning (111) planes meeting at an angle of 140°. A structure model of the twinning mechanism is portrayed in Figure 2b showing the mirror images of two twin (11h1) planes meeting at the theoretical angle of 141° at the twin boundary (1h11) plane. The diameter of the particular nanocylinder measured 2R) 8.2 nm. On the basis of the measured R, r, and p, we calculated the thickness t to be 9.4 nm and the inclination angle θ to be 20°. The fact that this nanocylinder was composed of two-half-cylinders of single-crystalline silicon is rather amazing. In addition to regular SiNWs, some necklace-like silicon nanowires5 can also be sectioned. These latter SiNWs were formed by silicon nanoparticles which are connected by a continuous outer layer of silicon dioxide. Of particular interest is the horizontal cross section of a chain-like SiNW, depicted in Figure 3. The silicon nanodots, measuring 16 nm to 20 nm in diameter, were buried in an amorphous silicon oxide nanowire of 48 nm in diameter, giving rise to the necklace-like structure. These nanodots exhibit polyhedral shapes such as hexagons and octagons in the cross-sectional view. As prepared, Si nanodots tend to adopt polyhedral Nano Lett., Vol. 3, No. 12, 2003
Figure 2. (a) HRTEM image of the cross section of a SiNW with two twinning (111) planes meeting at the angle of 140°. (b) Structure model of the twinning in (a).
section of necklace-like SiNWs were obtained and examined by HRTEM. This technique also allows the determination of the growth direction, the shape of the cross section, and the internal atomic structure (including twinning or other stacking faults, if any) of a single nanowire. This information is important in the fabrication of single-electron devices and/ or assemblage of multiwire devices. The method is rather general since it can be used to produce nanodots or nanorods of a wide variety of materials such as semiconductors, metals, alloys, superconductors, ceramics, or other functional materials. It is conceivable that nanosized rings, collars, tubes, washers, springs, screws, etc. of various lengths can be cut from a wide variety of one-dimensional nanomaterials such as nanotubes, hollow nanorods, nanocables, spiral nanowires and nanoribbons, etc. (work in progress).
Figure 3. TEM image of horizontal nanoparticle-chain silicon nanowires.
shapes in the size range of 10-50 nm (Si core diameter) while SiNWs in the similar size range tend to have circular cross sections. Beyond 50 nm, SiNWs also begin to adopt polygonal (or truncated polyhedral) shaped cross sections. Similar observations were reported by Li, et al. using a different sectioning (ion milling) technique.7 In conclusion, a convenient method, using the wellestablished microtome technique, has been developed to precision cut nanowires into nanodots of well-defined sizes and shapes. At the same time, it provides an easy way to fabricate high-quality cross-sectional TEM samples of silicon nanowires. In our experiments, both the coaxial structure of vertical cross section of normal SiNWs and horizontal cross
Nano Lett., Vol. 3, No. 12, 2003
Acknowledgment. We thank Professors R. Q. Zhang and Y. S. Lifshitz of City University of Hong Kong for helpful discussions. This work was supported by a grant from the Research Grants Council of Hong Kong (project no. CityU 3/03C). References (1) Lehn, J.-M., Supramolecular Chemistry: Concepts and PerspectiVes; VCH: Weinheim, 1995. (2) Vogtle, F., Supramolecular Chemistry: An Introduction; Wiley: Chichester, 1991. (3) Huang, Y.; Duan, X.; Cui, Y.; Lieber, C. M. Nano Lett. 2002, 2, 101. (4) Duan, X.; Huang, Y.; Lieber, C. M. Nano Lett. 2002, 2, 487. (5) Wang, N.; Tang, Y. H.; Zhang, Y. F.; Lee, C. S.; Lee, S. T. Phys. ReV. B 1998, 58, 16024. (6) Morales, A. M.; Lieber, C. M. Science 1998, 279, 208. (7) Li, C. P.; Lee, C. S.; Ma, X. L.; Wang, N.; Zhang, R. Q.; Lee, S. T. AdV. Mater. 2003, 15, 607.
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