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One-dimensional Silicon-Cadmium Selenide Heterostructures X. H. Sun,*,†,‡ T. K. Sham,*,† R. A. Rosenberg,§ and G. K. Shenoy§ Department of Chemistry, UniVersity of Western Ontario, London, Ontario, N6A 5B7, Canada, and AdVanced Photon Source, Argonne National Laboratory, Argonne, Illinois 60439 ReceiVed: March 1, 2007; In Final Form: April 11, 2007
We report the synthesis and characterization of 1D Si-CdSe heteronanostructures with different morphologies such as coaxial, biaxial, sandwiched, pattern wrapping, coiling, structures etc., via a one-step metal catalyzed thermal evaporation method. Both Si and CdSe exhibit single crystalline characteristics in the heterostructures, as revealed by scanning transmission electron microscopy. The Si nanowires formed directly from the Si substrate via the solid-liquid-solid process acts as the absorption site for CdSe deposition as well as the template for the formation of 1D Si-CdSe heterostructures. Time-resolved X-ray excited optical luminescence from the 1D Si-CdSe heteronanostructures reveals two main emission features at 530 and 637 nm with slow and fast decay lifetime, respectively. The 530 and 637 nm emission is associated with the Si and CdSe component of the heterostructures, respectively.
Introduction One-dimensional (1D) nanostructures such as wires, rods, belts, and tubes have become a focus of research not only because of their interesting electronic and optical properties intrinsically associated with their low dimensionality and the quantum confinement effect, but also because of their unique applications in mesoscopic physics and fabrication of nanoscale devices.1 For example, prototypes of diodes, transistors, sensors, memories, logic gates, and light-emitting diodes (LEDs) fabricated by nanowires have been demonstrated.2-9 Clearly, the operating principles and performance of the resulting nanodevices depend on the chemical, physical, and biological properties of the nanomaterial components, the building blocks of the nanodevices, especially at the interfaces, as well as our ability to fabricate functional heterostructures and interfaces with desirable characteristics. While heterostructures have been extensively studied and applied in micro planar devices,10 1D heteronanostructure studies have been relatively lacking due to more complicated fabrication procedures.11-15 Silicon nanowires (SiNWs) are important in nanotechnology because Si-based nanoelectronics is compatible with the Si-based microelectronics. SiNWs in the nanoscale regime exhibit quantum confinement effects and are expected to play a key role as interconnects and functional components in future nanoscale electronic and optical devices.2-9 CdSe is an important II-VI semiconductor for optoelectronics as well. With a direct band gap of 1.8 eV (689 nm), zero-dimensional CdSe quantum dots have been intensively studied and widely used for laser diodes, nanosensing, biomedical imaging, etc.16-19 Undoubtedly, Si-based 1D heteronanostructures will be the desired building blocks for the nanoscale electronics and optoelectronics. In this paper, we report the fabrication of Si-CdSe 1D heteronanostructures using a one-step thermal evaporation technique. * Corresponding authors. E-mails:
[email protected] (X.H.S.), tsham@ uwo.ca (T.K.S.). † University of Western Ontario. ‡ Present address: Center for Nanotechnology, NASA Ames Research Center, Moffett Field, CA 94035. § Argonne National Laboratory.
Different heterostructure morphologies such as coaxial, biaxial, sandwiched, pattern wrapping, coiling structures, etc., have been obtained and their structure and morphology have been characterized by scanning transmission electron microscopy (STEM). The luminescence properties of Si-CdSe 1D heteronanostructures have also been studied with time-resolved X-ray Excited Optical Luminescence (TRXEOL) in both energy and time domain. Experimentation Details The growth of 1D Si-CdSe heteronanostructures was carried out in a horizontal tube furnace inside which an alumina tube was mounted. An alumina crucible containing CdSe powder (99.995%, Alfa Aesar) was then placed in the middle of the high-temperature zone of the furnace. A strip of Si(100) wafer was used as a substrate. It was first cleaned by sonication in ethanol, acetone and deionized water, and then dipped into a 5% HF solution for 5 min to remove the native oxide layer. After HF etching, Si wafer was rinsed with deionized water then dipped into a colloidal hexane solution containing thiol-capped gold nanoparticles with an average size of 2 nm,20 which will serve as catalysts. The organic solution of the thiol-capped gold nanoparticles was selected as the catalyst because the organic solvent easily evaporated leaving behind uniform Au nanoparticles anchored on the HF-etched Si substrate without further contamination and oxidation. This catalyst is preferred over the commercial water-based Au colloid solution, which requires a buffer layer such as poly-L-lysine to facilitate adhesion of the Au nanoparticles onto the substrates.21 To keep the clean Si surface and direct contact between Au nanoparticles and clean Si surface is a key factor for the growth of 1D Si-CdSe heterostructures. Si wafer coated with Au nanoparticles was placed at a location downstream of the carrier gas. The tube was evacuated to a base pressure of 10-3 Torr prior to the experiment. Argon, the carrier gas, was introduced at one end of the tube at 50 sccm (standard cubic centimeter per minute) and at 400 Torr. The temperature of the furnace was increased to 950 °C at a rate of 15 °C/min and kept at this temperature for 60 min. The temperature at the
10.1021/jp071699z CCC: $37.00 © 2007 American Chemical Society Published on Web 05/27/2007
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Figure 1. SEM images of 1D Si-CdSe heteronanostructures. (a) 1D Si-CdSe heteronanostructures synthesized on HF-etched Si (100) substrate, in which most Si-CdSe heteronanostructures are Si-CdSe core-shell structures; (b) a single Si-CdSe heteronanostructure with the Au bead on its tip; (c) sandwiched Si-CdSe heteronanostructure; (d) partially wrapped Si-CdSe heteronanostructure.
Figure 2. XRD of as-synthesized Si-CdSe heteronanostructures.
substrate position was ∼915 °C as monitored by a thermocouple. After cooling to room temperature, a brown wool-like film was
obtained on the substrate. Two control experiments have also been carried out under similar conditions. In one experiment, only Si nanowires were found grown on a HF-etched Si wafer coated with Au nanoparticles without a CdSe source, and in another experiment, only mixtures of CdSe nanowires, nanobelt, and nanosaws were obtained on the alumina substrate (Al2O3 99.6%, Coorstek Inc.) (experimental detail in Supporting Information). The as-synthesized films were first examined with scanning electron microscope (SEM) (SEM, Hitachi S-4500 FEG), which was equipped with energy dispersive X-ray spectroscopy (EDX) for composition analysis. The crystal structure of the products was obtained by X-ray diffraction (Bruker Discover, Cu KR radiation, λ ) 1.5418 Å). The brown nanomaterial was removed from the substrate surface and dispersed onto the “holey” carbon transmission electron microscopy (TEM) grids for TEM images. Detailed morphological and structural characterizations were carried out using high-resolution scanning transmission electron microscopy (STEM, JEOL 2010F) operating at 200 kV accelerating voltage, with an EDX attachment for single nanostructure composition analyses. Time-resolved XEOL was
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Figure 3. (a) TEM image of 1D Si-CdSe heteronanostructures shows three major heterostructures, coaxial, biaxial, and sandwich structures denoted 1, 2 and 3, respectively. A partial coaxial heterostructure is circled by dash line marked as 4; (b) a coaxial heterostructure shows Moire´ fringes; (c) EDX spectrum of a Si-CdSe coaxial heterostructure.
carried out at the 4-ID-C soft X-ray SGM beamline located at the Advanced Photon Source (APS), Argonne National Lab. Results and Discussion The morphology of the as-synthesized films examined by scanning electron microscopy (SEM) is shown in Figure 1a. The SEM image shows high-yield nanowire growth. The density of the nanostructures grown on the Si substrate was determined by the concentration of Au colloid solution used. The higher concentration, the higher density of nanostructure obtained. The nanostructures are clearly visible as straight wires with diameters varying from 30 to 80 nm and lengths up to tens of micrometers. The composition of the nanostructure was confirmed by EDX, in which Si, Cd, Se, and a trace of Au and O were detected. Au nanoparticles were observed at the tip of the wires (Figure 1b), suggesting that these 1D nanostructures were grown via a similar Vapor-liquid-solid (VLS) mechanism22 which will be dis-
cussed later. In addition to normal wires, other different nanostructures such as biaxial, sandwiched (triaxial) (Figure 1c), partial wrapped (Figure 1d), coiling, etc. were also observed. Figure 2 shows the crystallinity of the nanostructures obtained by XRD. The diffraction peaks can be indexed to hexagonal CdSe (a ) 4.3 Å, c ) 7.0 Å, JCPDS No.08-459) and Si with a cubic diamond structure (a ) 5.43 Å, JCPDS No.27-1402) within experimental error. The Au nanoparticle at the tip of the wires (CdSe and Si) was found to have an fcc structure, presumably from the aggregation of the initial smaller nanoparticles. Au L3-edge XAFS, of which the details will be reported elsewhere, confirms this finding. Detailed morphological and structural characterizations of these nanostructures were performed with TEM and highresolution STEM. Figure 3a shows the low magnification TEM image of several nanostructures. There are at least three different types of structures. The first, henceforth denoted
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Figure 4. (a) TEM image of a specific coaxial structure in which the wire was partially covered by an outer layer; (b) EDX spectrum of the exposed core region marked with a black circle; (c) EDX spectrum of the wire stem marked with a white circle; (d) high-resolution TEM image of the exposed Si core; (e) high-resolution TEM image of the wire stem. Inset shows the fast Fourier transform (FFT) of the TEM image.
1, is similar to normal nanowires with an uniform contrast along the entire wire. The second (denoted 2) is a biaxial structure, which exhibits a distinct boundary between two attached, parallel wires as revealed by the light/dark contrast. The third is a sandwich structure (denoted 3) in which the light material is sandwiched between two dark (heavier mass) layers. The zoomin TEM image of an individual wire in Figure 3b shows clear Moire´ fringes, which indicate the presence of overlapping crystals with different crystalline lattices or orientations. The EDX (Figure 3c) of the individual wire reveals that it is composed of Cd, Se, and Si (Cu signal was from the TEM copper grid). Direct evidence for the structure details was captured in the TEM where the outer layer was “peeled” from a wire as marked by 4 in Figure 3a, revealing the core-shell structure. This observation immediately indicates that the wire adopts a coaxial (cable) structure or so-called radial structure. It should be noted that among all morphologies, the coaxial (core-shell) is most common followed by biaxial and sandwich (triaxial), other more exotic structures are minority. Figure 4a shows the TEM image of a specific coaxial structure in which the wire was partially covered by an outer layer. Only Si was detected in the EDX (Figure 4b) from the exposed core region marked with a black circle while Si, Cd and Se were all detected in the EDX (Figure 4c) from the wire stem region marked with a white circle in Figure 4a. No oxygen signal is detected in both EDX spectra, indicating that there is little oxide
layer between the Si core and the CdSe shell. The coaxial structure is not the same as the cable structure reported earlier,15 which has an oxide layer between the Si core and the CdSe shell and a ∼5 nm oxide layer at the outer surface of CdSe shell as well, indicating that both the CdSe and the silicon surface are oxidized in that case. Furthermore, only one cable structure was obtained in ref 15. The high-resolution TEM image of the exposed Si core (Figure 4d) shows a lattice spacing of 3.1 Å corresponding to the d-spacing of the (111) crystal planes of diamond Si, further confirming that the core is pure crystalline Si with a [111] growth direction. The high-resolution TEM image from the wire stem (Figure 4e) shows a lattice spacing of 3.7 Å corresponding to the d-spacing of the (100) crystal planes of CdSe with a hexagonal (wurtzite) crystal structure. The fast Fourier transform (FFT) of the TEM image (inset of Figure 4e) confirms that CdSe grows with a hexagonal crystal structure along the [100] direction. The absence of a Si lattice in the wire stem is most likely due to unmatched orientation relationship between the CdSe shell and the Si core. It can be concluded from these observations that the coaxial Si-CdSe structure is formed by a crystalline Si core wrapped with hexagonal crystalline CdSe nanotubes. The detailed structure analysis of the Si-CdSe biaxial structure is shown in Figure 5a,b with a low magnification and a high-resolution TEM image, respectively. The HRTEM image clearly shows both (100) crystal lattice spacing of hexagonal
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Figure 5. (a) TEM image of an individual Si-CdSe biaxial structure; (b) high-resolution TEM image of the Si-CdSe biaxial structure in panel a.
Figure 6. TEM images of 1D Si-CdSe heteronanostructures with other morphologies. (a) CdSe|Si|CdSe sandwiched structure; (b) and (c) two different pattern wrapped Si-CdSe heteronanostructures; (d) coiling Si-CdSe heteronanostructure.
CdSe and (111) crystal lattice spacing of diamond Si, which lie side by side with a blur interface of
