TY - JOUR
AU - Golberg,, Dmitri
AB - Abstract Cd4SiS6/Si composite nanowires are produced through co-thermal evaporation of CdS and Si powders with a small amount of tin sulfide as an additive. A vapor–liquid–solid growth mechanism is proposed for the anisotropic growth of the composite nanowires based on the presence of metallic tin particles at their tip-ends. Both side-by-side and core-shell composite nanowires are obtained. The product is characterized using X-ray powder diffraction and scanning electron microscopy. Detailed structural and chemical analyses of individual composite nanowires are carried out using transmission electron microscopy (TEM), high-resolution TEM (HRTEM), electron diffraction (ED) and energy-dispersive X-ray spectroscopy. Planar defects, including microtwins and stacking faults, are abundant in a Si core, as revealed by HRTEM and selected-area ED. The formation of composite nanowires is discussed in the light of thermodynamic and kinetic aspects. semiconductors, nanowires, composite, silicon, cadmium sulfide, thermal evaporation Introduction One-dimensional (1D) nanomaterials have attracted significant attention due to their potential applications as building blocks in nanoscale circuits and optoelectronic devices since carbon nanotubes were first reported in 1991 [1]. 1D nanomaterials such as rods, wires, belts, and tubes can be fabricated through a number of advanced nanolithographic techniques, solution-based methods and thermal evaporation [2–5]. Owing to the growing demands with respect to functional nanoscale electronic devices, it is crucial to assemble well-known functional materials in either radial or axial directions within a single nanoscale building block in order to form novel heterostructures. Core-shell nanostructures such as SiC/BN, Si/Ge and Ga2O3/C have indeed been produced using various approaches [6–8]. The block-by-block growth of Si/SiGe and InAs/InP semiconductor superlattice nanowires along the axial direction using Au nanoparticle catalysts has recently been reported by several groups [9–11]. Bilayered TiO2/SnO2 composite nanoribbons have been synthesized through pulsed laser ablation deposition of TiO2 on SnO2 nanoribbons resulting in the epitaxial growth [12]. Based on the crystallographic similarities, side-by-side ZnS/Si composite nanowires have been generated through chemical vapor deposition of ZnS over Si nanowires [13]. In addition to the solution-based growth, the vapor–solid (VS) or vapor–liquid–solid (VLS) growth processes are generally attributed to the anisotropic growth of 1D nanostructures [14–16]. During a VLS growth process, three growth stages are considered. First, a metal nanoparticle acts as a preferential site for the absorption of vapors to form a liquid alloy. Once the composition of an absorbed reactant is supersaturated within an alloy droplet at a given temperature, its nucleation from a supersaturated liquid droplet starts. Further condensation of the reactant vapors results in the axial growth of a 1D nanostructure. Generally, a diameter of the nanowires grown via a VLS process has an obvious correlation with that of the starting metal nanoparticles. Various 1D semiconductor nanostructures, including elemental semiconductors (Si, Ge), III–V (GaN, GaAs, InAs) semiconductors and II–IV (ZnO, ZnS, CdS) semiconductors, have widely been produced through a VLS process using metal nanoparticles as catalysts [17–21]. If the vapors of two substances are generated simultaneously, the continuous condensation of them on a single metal nanoparticle may result in the growth of composite nanowires through a VLS process. Silicon has long been considered as the prime material for electronics serving in the information technologies. It is important to produce silicon-based 1D composite nanostructures for diverse potential applications. Recently, we successfully produced ZnS/Si composite nanowires through the VLS growth using the low-melting point (232°C) tin metal as a catalyst [22]. Herein, we further demonstrate that Cd4SiS6/Si bimorph composite nanowires may be generated through the novel tin-catalyzed growth. Methods The Cd4SiS6/Si composite nanowires were synthesized in a vertical induction furnace consisting of a fused-quartz tube and an induction-heated cylinder made of high-purity graphite coated with a carbon fiber thermo-insulating layer. An inlet C pipe and an outlet C pipe on its top and base are installed in the furnace, respectively. A graphite crucible, containing a ground mixture of 1.44 g of CdS, 0.28 g of Si and 0.01 g of SnS, was placed at the center cylinder zone. After evacuation of the quartz tube to ∼10−3 Pa, a pure N2 flow was set within the carbon cylinder at a constant rate of 1000 sccm. The furnace was further heated to and kept at 1000°C for 1 h. After the reaction was terminated and the furnace cooled to the room temperature, a yellow product was collected from the outlet of a graphite induction-heated cylinder. The product was characterized using X-ray powder diffraction (XRD; RINT 2200) with CuKα radiation. The morphology of the product was observed using a JSM-6700F scanning electron microscope (SEM) operated at 10–20 kV. The product was ultrasonically dispersed in ethanol and transferred to a carbon-coated copper grid for transmission electron microscopy (TEM) observations. A field emission JEM-3000F high-resolution electron microscope that operated at 300 kV equipped with an energy-dispersive X-ray spectrometer (EDX) was employed to perform the microanalysis. Results and discussion Powder XRD pattern of a product is shown in Fig. 1a, which suggests the coexistence of diamond-like silicon and Cd4SiS6 with a monoclinic structure (a = 12.34 Å, b = 7.089 Å, c = 12.35 Å, Joint Committee on Powder Diffraction Standards (JCPDS) Card No.(21–0124). Representative SEM image of a sample is shown in Fig. 1b, which indicates that the product is composed of 1D nanostructures having a high aspect ratio. Detailed structural and chemical analyses of individual composite nanowires were further carried out using TEM, high-resolution TEM (HRTEM), electron diffraction (ED), and energy-dispersive X-ray spectroscopy (EDX). Fig. 1 Open in new tabDownload slide (a) XRD pattern of a product. (b) SEM image of the composite nanowires. Fig. 1 Open in new tabDownload slide (a) XRD pattern of a product. (b) SEM image of the composite nanowires. Figure 2a, a TEM bright-field image of a typical composite nanowire, clearly reveals that the as-grown nanowires are composed of two distinct fragments and capped with a spherical tip. An interface between the two fragments is obvious. The diameter of the composite nanowire is ∼130 nm. Energy-dispersive X-ray spectroscopic (EDX) analyses using a nanoprobe were conducted on the tip and the two distinct wire fragments, which are labeled as b, c and d in Fig. 2a, respectively. The corresponding EDX spectra are shown in Figs 2b–d. It is proved that the spherical tip is made of metallic tin. The part exhibiting a brighter contrast is composed of silicon, whereas that of a darker contrast is composed of cadmium, sulfur and silicon. Fig. 2 Open in new tabDownload slide (a) TEM image of a typical composite nanowire. (b–d) Nanoprobe EDS spectra taken in the regions labeled with b, c and d on the nanowire, respectively. Fig. 2 Open in new tabDownload slide (a) TEM image of a typical composite nanowire. (b–d) Nanoprobe EDS spectra taken in the regions labeled with b, c and d on the nanowire, respectively. Selected-area electron diffraction (SAED) pattern taken on a composite nanowire is shown in Fig. 3a. The spots on this pattern are mainly indexed as those natural for the [100] zone axis of monoclinic Cd4SiS6. The other spots can be indexed as those peculiar to the [112] zone axis of diamond-like silicon. The (220) and (111) diffraction spots of silicon are marked in Fig. 3a. The stretching of diffraction spots on the ED pattern is due to the shape effect, suggesting a rod-like morphology of the composite nanowire [23]. Figs 3b and 3c show the enlarged view of the (220) diffraction spot of silicon and the (022) diffraction spot of monoclinic Cd4SiS6. As implied by the ED pattern, the preferred growth directions of silicon and Cd4SiS6 fragments are close to the [220] and (022)* orientations, respectively. A crystallographic relationship between both fragments is also revealed by this ED pattern: (220)Si is parallel to (022)Cd4SiS6; (111)Si is parallel to (025)Cd4SiS6. High-resolution TEM image taken from a Cd4SiS6 fragment is depicted in Fig. 3d. The HRTEM image of the Cd4SiS6 fragment indicates clearly resolved fringes peculiar to the interplanar distances of 11.60 Å, corresponding to the (001) lattice spacing. In a monoclinic Cd4SiS6 crystal, both silicon and cadmium atoms are surrounded by four sulfur atoms. The SiS4 tetrahedrons are interconnected by the CdS4 tetrahedrons. Figure 4a displays the polyhedral structural model of a monoclinic Cd4SiS6 crystal viewed along the [010] axis. Figure 4b displays the atomic structural model of a monoclinic Cd4SiS6 crystal viewed along the [100] axis, which is compatible with the HRTEM image of the Cd4SiS6 fragment in Fig. 3d. Fig. 3 Open in new tabDownload slide (a) SAED pattern taken from a composite nanowire. (b and c) The enlarged view of the ED spots revealing their stretching: (b) (220) of Si; (c) (022) of Cd4SiS6. (d) High-resolution TEM image of a Cd4SiS6 nanowire fragment. Fig. 3 Open in new tabDownload slide (a) SAED pattern taken from a composite nanowire. (b and c) The enlarged view of the ED spots revealing their stretching: (b) (220) of Si; (c) (022) of Cd4SiS6. (d) High-resolution TEM image of a Cd4SiS6 nanowire fragment. Fig. 4 Open in new tabDownload slide (a) Polyhedral structural model of a monoclinic Cd4SiS6 crystal viewed along the [010] axis. This shows the connection of CdS4 and SiS4 tetrahedrons. (b) The atomic structural model of a monoclinic Cd4SiS6 crystal viewed along the [100] axis. Fig. 4 Open in new tabDownload slide (a) Polyhedral structural model of a monoclinic Cd4SiS6 crystal viewed along the [010] axis. This shows the connection of CdS4 and SiS4 tetrahedrons. (b) The atomic structural model of a monoclinic Cd4SiS6 crystal viewed along the [100] axis. In addition to the side-by-side Cd4SiS6/Si composite nanowires, some core-shell composite nanowires have also been generated. Figure 5a is a TEM image of a typical composite nanowire clearly showing the core-shell nature. EDX analyses performed on these composite nanowires reveal that it is composed of silicon, cadmium and sulfur. Its corresponding EDX spectrum is shown in Fig. 5c. When a convergent electron beam is focused within a spot of ∼50 nm in diameter, the irradiation-induced peeling of shells over a core-shell nanowire is observed. Figure 5b shows a TEM image of core-shell composite nanowires after electron beam irradiation. The bond dissociation energy of Si–Si (327 kJ mol−1) is much higher than that of Cd-S (196 kJ mol−1), thus silicon is more resistant to the electron irradiation [24]. Nanoprobe EDX analyses, conducted on the remaining fragments after electron beam irradiation, reveal that they are composed of silicon. The corresponding EDX spectrum is demonstrated in Fig. 5d. The bent contrast within the Si domain suggests its structural inhomogeneity. Figure 6 depicts the domain of a Si core within a composite nanowire. A selected-area electron diffraction pattern taken from the Si core, shown in the inset to Fig. 6, reveals the existence of a twin on the (11 1) plane. HRTEM image of the region marked with a white rectangle in Fig. 6 is demonstrated in Fig. 7. Well-defined fringes characteristic to the interplanar distances of 3.10 Å are in accordance with the {111} lattice spacing. As revealed by the HRTEM image, the top domain has a twin-like relationship with the bottom one. In the intermediate domain, there are some microtwins and stacking faults. These planar defects are parallel to the {111} crystal planes. The example of a stacking fault is denoted by the character ‘S’ in Fig. 7. Microtwins and stacking faults are standard planar defects in semiconductor nanowires [25]. The structural models of a stacking fault and a twin on the {111} crystal planes of diamond-like silicon are presented in Figs 8a and 8b. Fig. 5 Open in new tabDownload slide (a) TEM image of a single core-shell composite nanowire. (b) TEM image of a single composite nanowire after irradiation with a convergent electron beam. (c) EDX spectrum taken from the core-shell composite nanowire. (d) Nanoprobe EDX spectrum taken from the Si core of the composite nanowire after irradiation with a convergent electron beam. Fig. 5 Open in new tabDownload slide (a) TEM image of a single core-shell composite nanowire. (b) TEM image of a single composite nanowire after irradiation with a convergent electron beam. (c) EDX spectrum taken from the core-shell composite nanowire. (d) Nanoprobe EDX spectrum taken from the Si core of the composite nanowire after irradiation with a convergent electron beam. Fig. 6 Open in new tabDownload slide TEM image of the Si core of a composite nanowire clearly displaying the bent contrast. The inset is the corresponding SAED pattern. Fig. 6 Open in new tabDownload slide TEM image of the Si core of a composite nanowire clearly displaying the bent contrast. The inset is the corresponding SAED pattern. Fig. 7 Open in new tabDownload slide HRTEM image taken from a Si core region, as denoted by the white rectangle in Fig. 6. Fig. 7 Open in new tabDownload slide HRTEM image taken from a Si core region, as denoted by the white rectangle in Fig. 6. Fig. 8 Open in new tabDownload slide The structural model of planar defects in Si crystals. (a) A stacking fault. (b) A twin. Fig. 8 Open in new tabDownload slide The structural model of planar defects in Si crystals. (a) A stacking fault. (b) A twin. Based on thermodynamics [26], the chemical reactions involved in the synthetic process may be proposed as follows: Si+CdS(s)→Cd(g)+SiS(g)(ΔG°1300K=−15.056 kJ mol−1,logK1300 K=0.605) (1) CdS(s)→CdS(g)(ΔG°1300K=115.432 kJ mol−1,logK1300 K=−4.638) (2) The thermodynamic characteristics for the reactions (1) and (2) versus temperature are plotted in Figs 9a and 9b, respectively. Solid Si may react with solid CdS to produce Cd and SiS vapors. From the thermodynamic point of view, an equilibrium vapor pressure of PCd(g) and PSiS(g) existing in accordance with the reaction (1) at 1300 K is as high as 202.7 kPa. An equilibrium vapor pressure of PCds(g) resulting from the sublimation of solid CdS in accordance with the reaction (2) is ∼2.3 Pa. Reaction (1) is thermodynamically preferential. On the other hand, the reaction between solid Si and CdS must overcome a big diffusion barrier so that the sublimation of solid CdS becomes kinetically preferential. SnS melts at nearly 1155 K [27]. The diffusion energy for the reaction between solid Si and CdS can be considerably reduced by a liquid SnS, then reaction (1) becomes really preferential. From the kinetic viewpoint, SnS can act as a catalyst for the reaction (1). At a lower temperature region, Cd and SiS vapors are inclined to recombine to Cd4SiS6 and Si. Monoclinic Cd4SiS6 is a stable phase in the Cd–Si–S system [28]. In the absence of SnS, no composite nanowires can be produced. Moreover, the decomposition of SnS results in the formation of a tin vapor. The tin vapor condenses at a lower temperature region (∼600°C) to produce liquid tin droplets which then absorb reactant vapors, and catalyze the axial growth of Cd4SiS6/Si composite nanowires via a VLS growth process. During the synthetic process, SnS plays an important role in the thermal evaporation of solid-state Si and CdS to finally produce composite nanowires. Fig. 9 Open in new tabDownload slide The thermodynamic characteristics of reactions (1) and (2) versus temperature (T, K). (a) Si + CdS(s) → Cd(g) + SiS(g), (b) CdS(s) → CdS(g). Fig. 9 Open in new tabDownload slide The thermodynamic characteristics of reactions (1) and (2) versus temperature (T, K). (a) Si + CdS(s) → Cd(g) + SiS(g), (b) CdS(s) → CdS(g). Concluding remarks We have demonstrated the synthesis of Cd4SiS6/Si composite nanowires through co-thermal evaporation of CdS and Si powders with a small amount of tin sulfide as an additive. The presence of the metallic tin particles capping the composite nanowires implies that a VLS growth mechanism is attributed to their anisotropic growth. In addition to side-by-side composite nanowires, core-shell composite nanowires have also been obtained. 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TI - Synthesis and microstructure of Cd4SiS6/Si composite nanowires
JF - Journal of Electron Microscopy
DO - 10.1093/jmicro/dfi077
DA - 2005-12-01
UR - https://www.deepdyve.com/lp/oxford-university-press/synthesis-and-microstructure-of-cd4sis6-si-composite-nanowires-Z34SGaKqEU
SP - 485
EP - 491
VL - 54
IS - 6
DP - DeepDyve
ER -