N. Wang et al/Materials Science and Engineering R 60(2008)1-51 O-Kvy vire product. (b) Formation of Si nanowires from liquid clusters. (c) TEM image of Si nanowires catalyzed by metal droplets. The arrow indicates as the energetically favored reaction sites for absorption of the YBCO nanowires were structurally uniform. Their diameters reactant. They are also the nucleation sites for crystallization of range from 20 and 90 nm and their lengths are up to several the source materials when supersaturated(see Fig. 5(b). Then, micrometers. Most of the YBCO nanowires were single crystals preferential ID growth occurs in the presence of the reactant Si (an orthorhombic lattice)and their axis was along the [001 nanowires obtained by ablating a metal-containing(0.5-1%)Si direction. The growth mechanism of the YBCO nanowires is powder target are extremely long and straight. The typical not known. It might be a self-catalytic growth or the oxide- diameters of the nanowires are 10-50 nm. There is a metal assisted growth (without any metal catalysts) as discussed catalyst at the tip of each nanowire(Fig. 5(c)). During the laser below in Section 2.3 blation, the reaction is not under thermodynamic equilibrium onditions. Ultrasmall-size metal catalysts and thus very thin Si 2.3. Thermal evaporation nanowires with diameters smaller than 10 nm can be easily generated by this method. The growth rate of Si nanowires from Nanowires and some interesting morphologies of nanos- ser ablation depends on many factors, such as the power of the tructures such as nanoribbons, nano-tetrapods and comb-like laser beam, the vacuum, the carrier gasses and the temperature. structures [64, 65] can be fabricated by a simple method of A rate of 500 um/h has been observed in Si nanowire growth thermal evaporation of solid source materials. The exper assisted by laser ablation, which is much faster than that from mental setup is extremely simple as shown in Fig. 6. The the classical VLS using vapor sources temperature gradient and the vacuum conditions are two critical without adding any metal catalysts, however, nanowires parameters for the formation of nanowires by this method many other materials have been fabricated by laser ablation. Typical materials suitable for this fabrication are metal oxides, These materials include metal oxides, some semiconductors e.g, ZnO, SnO2, In2O3, VO, etc and some semiconductors and multi-component materials with rather complex stoichio- [12, 66]. The fabrication of these nanowires is simply through metries.The growth of these nanowires is called self-catalyzed evaporating commercial metal oxide powders at elevated rowth. Though no obvious catalyst is observed with these temperatures under a vacuum or in an inert gas atmosphere with nanowires, it is possible that metal elements in the source a negative pressure. Nanowire products form in the low materials may act as the catalysts. For example, the laser temperature regions where materials deposit from the vapor ablation of the ZnSe crystal surface may result in Zn clusters phase. It is believed that the nanowires are generated directly that act as the effective catalysts. Similar self-catalyst VLs from the vapor phase in the absence of a metal catalyst, and this growth has also been observed in the growth of Gan [61] and process is often called vapor-solid(VS)growth. To generate ZnO [62] nanowires. Nanowires with multi-components, for the vapor phases of the source materials, vacuum conditions are example, the yttrium-barium-copper-oxygen (YBCO)com- sometimes needed. This is because some materials may not pound, have been synthesized by laser ablation of YBa2Cu3O7 sublimate in the normal atmosphere. An effective way to (a high T-c superconductor) in an oxygen atmosphere [63]. The generate the vapor source materials in a normal atmosphere is Pressure meter A dual temperature zone furnace Nanowires Carrier gas T 600°c Seal ring I Temperature Zone Al I Temperature Zone Bl Quartz tube Fig. 6. A simple experimental setup of the thermal evaporation method for synthesizing ZnO nanostructures. The source material is Zno or a mixture of ZnO and carbon. Different forms of the Zno nanostructures, e. g, nanowires and ribbons, grow in different temperature zones
as the energetically favored reaction sites for absorption of the reactant. They are also the nucleation sites for crystallization of the source materials when supersaturated (see Fig. 5(b)). Then, preferential 1D growth occurs in the presence of the reactant. Si nanowires obtained by ablating a metal-containing (0.5–1%) Si powder target are extremely long and straight. The typical diameters of the nanowires are 10–50 nm. There is a metal catalyst at the tip of each nanowire (Fig. 5(c)). During the laser ablation, the reaction is not under thermodynamic equilibrium conditions. Ultrasmall-size metal catalysts and thus very thin Si nanowires with diameters smaller than 10 nm can be easily generated by this method. The growth rate of Si nanowires from laser ablation depends on many factors, such as the power of the laser beam, the vacuum, the carrier gasses and the temperature. A rate of 500 mm/h has been observed in Si nanowire growth assisted by laser ablation, which is much faster than that from the classical VLS using vapor sources. Without adding any metal catalysts, however, nanowires of many other materials have been fabricated by laser ablation. These materials include metal oxides, some semiconductors and multi-component materials with rather complex stoichiometries. The growth of these nanowires is called self-catalyzed growth. Though no obvious catalyst is observed with these nanowires, it is possible that metal elements in the source materials may act as the catalysts. For example, the laser ablation of the ZnSe crystal surface may result in Zn clusters that act as the effective catalysts. Similar self-catalyst VLS growth has also been observed in the growth of GaN [61] and ZnO [62] nanowires. Nanowires with multi-components, for example, the yttrium–barium–copper–oxygen (YBCO) compound, have been synthesized by laser ablation of YBa2Cu3O7 (a high T-c superconductor) in an oxygen atmosphere [63]. The YBCO nanowires were structurally uniform. Their diameters range from 20 and 90 nm and their lengths are up to several micrometers. Most of the YBCO nanowires were single crystals (an orthorhombic lattice) and their axis was along the [0 0 1] direction. The growth mechanism of the YBCO nanowires is not known. It might be a self-catalytic growth or the oxideassisted growth (without any metal catalysts) as discussed below in Section 2.3. 2.3. Thermal evaporation Nanowires and some interesting morphologies of nanostructures such as nanoribbons, nano-tetrapods and comb-like structures [64,65] can be fabricated by a simple method of thermal evaporation of solid source materials. The experimental setup is extremely simple as shown in Fig. 6. The temperature gradient and the vacuum conditions are two critical parameters for the formation of nanowires by this method. Typical materials suitable for this fabrication are metal oxides, e.g., ZnO, SnO2, In2O3, VO, etc. and some semiconductors [12,66]. The fabrication of these nanowires is simply through evaporating commercial metal oxide powders at elevated temperatures under a vacuum or in an inert gas atmosphere with a negative pressure. Nanowire products form in the lowtemperature regions where materials deposit from the vapor phase. It is believed that the nanowires are generated directly from the vapor phase in the absence of a metal catalyst, and this process is often called vapor–solid (VS) growth. To generate the vapor phases of the source materials, vacuum conditions are sometimes needed. This is because some materials may not sublimate in the normal atmosphere. An effective way to generate the vapor source materials in a normal atmosphere is Fig. 5. (a) Si nanowire product. (b) Formation of Si nanowires from liquid clusters. (c) TEM image of Si nanowires catalyzed by metal droplets. The arrow indicates the metal catalyst on the nanowire tip. Fig. 6. A simple experimental setup of the thermal evaporation method for synthesizing ZnO nanostructures. The source material is ZnO or a mixture of ZnO and carbon. Different forms of the ZnO nanostructures, e.g., nanowires and ribbons, grow in different temperature zones. 6 N. Wang et al. / Materials Science and Engineering R 60 (2008) 1–51
N. Wang et al. /Materials Science and Engineering R 60 (2008)1-51 to add additional materials to react with the source materials. temperature was about 800-1000C. A similar thermal For example, Zno powder does not sublimate in a normal evaporation experiment was actually performed in 1950 atmosphere at 1000C. By adding carbon powder to react with [55,75]. Two kinds of materials were obtained at the the ZnO source, Zn or Zn-suboxide vapor phases can be easily temperature range of 800-1000C, one was a Sio product generated at 1000C. Various forms of ZnO nanostructures and the other one was labeled as"light brown loose material ow in the low-temperature zone. In this case, vacuum The loose materials were characterized by X-ray diffraction and conditions, carrying gases and catalysts are all unnecessary. determined to be Si structures [75]. The"light brown loose The temperature is critical for the formation of different forms material"can be obtained routinely nowadays by thermal of ZnO nanostructures [67]. evaporation as Si nanowires. Unfortunately, the Si nanowires in The growth mechanisms of many nanowires from thermal the loose materials were not identified at the time of the initial evaporation (without adding metal catalysts) are poorly experiments. The advantages of the OAG technique are(1)the understood. There are some special materials containing no nanowires are highly pure since no metal catalyst is involved metal elements that can also develop into nanowires from their and (2)doping of nanowires can be easily achieved because the oxide decomposition. Wang et al. [11, 68, 69]reported that SiO2 experimental setup for OAG of Si nanowires is very similar to largely enhanced Si nanowire growth(Fig. 7(a). A model that of the laser ablation technique. Doping can be easily called oxide-assisted growth(OAG) was therefore proposed realized with the assistance of laser ablation of solid dopant with evidence from experiments not only on Si but also on Ge materials during nanowire growth. Si nanowires fabricated by [70]and Ill-V[71-73] semiconductor nanowire growth. As this method showed very uniform diameters(about 20 nm)and shown in Fig. 7(b), the presence of Sio2 in the source their lengths were over several hundred micrometers significantly increases the yield of Si nanowire product. The Si nanowire product obtained using a powder source composed of 2. 4. Metal-catalyzed molecular-beam epitaxy 50%0 SiO, and 50%o Si is 30 times larger than the amount generated by using a metal-containing target [11] Since 2000, MBE and CBe techniques have been employed The OAG reaction is special because no metal elements or to synthesize Si [15], Il-VI[14] and Ill-V[13, 16] compound catalysts are involved either in the source materials or the semiconductor nanowires based on the Vls growth mechan- nanowire itself. The starting material is oxide and the ism MBE and CBe techniques provide an ideal clean growth nanowires are in non-oxide form. In OAG using Sio, the environment, and the atomic structures, doping states and nanowires are pure Si(not Si-oxide), and Si itself does not have junctions (or heterostructures) can be well controlled. a self-catalyst effect. This means that Si nanowires are formed Combined with the VLS, these techniques are able to produce by the assistance of Si-oxide. The OAG model has been tested high-quality semiconductor nanowires. Different from other by a simple experiment [74], which was carried out by simply synthesis techniques, MBE works under ultra-high vacuum sealing highly pure Sio powder or a mixture of Si and Sio2 conditions. The mean free path of the source molecules under (: 1, Si reacts with SiO2 to form Sio or Si,o(x> 1) vapor vacuum conditions of 10 Torr is about 0. 2 m The evaporated phase)in an evacuated(vacuum <10 Torr)quartz tube and then source atoms or molecules from the effusion cells behave like a inserting the tube into a preheated furnace(1250-1300C). No beam aiming directly at the substrate(see Fig. 8). The growth, special ambient gas was needed. One end of the tube was left surface structures and contamination can be monitored in situ outside the furnace to generate a temperature gradient between by reflection high-energy electron diffraction, Auger electron the source material and the nanowire formation zone. After 20- spectroscopy and other surface probing techniques. MBE has 30 min of annealing, a high yield of sponge-like Si nanowire several advantages over other synthesis techniques: (1)the product formed on the cooler parts of the tube where the ultra-high vacuum can reduce contamination/oxidation of (b) ooE卫9 300nm SiO2(wt%) Fig. 7.(a)Si nanowires synthesized by oxide-assisted growth. (b)Yield of Si nanowires vs the percentage of Sioz in the target [ll
to add additional materials to react with the source materials. For example, ZnO powder does not sublimate in a normal atmosphere at 1000 8C. By adding carbon powder to react with the ZnO source, Zn or Zn-suboxide vapor phases can be easily generated at 1000 8C. Various forms of ZnO nanostructures grow in the low-temperature zone. In this case, vacuum conditions, carrying gases and catalysts are all unnecessary. The temperature is critical for the formation of different forms of ZnO nanostructures [67]. The growth mechanisms of many nanowires from thermal evaporation (without adding metal catalysts) are poorly understood. There are some special materials containing no metal elements that can also develop into nanowires from their oxide decomposition. Wang et al. [11,68,69] reported that SiO2 largely enhanced Si nanowire growth (Fig. 7(a)). A model called oxide-assisted growth (OAG) was therefore proposed with evidence from experiments not only on Si but also on Ge [70] and III–V [71–73] semiconductor nanowire growth. As shown in Fig. 7(b), the presence of SiO2 in the source significantly increases the yield of Si nanowire product. The Si nanowire product obtained using a powder source composed of 50% SiO2 and 50% Si is 30 times larger than the amount generated by using a metal-containing target [11]. The OAG reaction is special because no metal elements or catalysts are involved either in the source materials or the nanowire itself. The starting material is oxide and the nanowires are in non-oxide form. In OAG using SiO, the nanowires are pure Si (not Si-oxide), and Si itself does not have a self-catalyst effect. This means that Si nanowires are formed by the assistance of Si-oxide. The OAG model has been tested by a simple experiment [74], which was carried out by simply sealing highly pure SiO powder or a mixture of Si and SiO2 (1:1, Si reacts with SiO2 to form SiO or SixO (x > 1) vapor phase) in an evacuated (vacuum <10 Torr) quartz tube and then inserting the tube into a preheated furnace (1250–1300 8C). No special ambient gas was needed. One end of the tube was left outside the furnace to generate a temperature gradient between the source material and the nanowire formation zone. After 20– 30 min of annealing, a high yield of sponge-like Si nanowire product formed on the cooler parts of the tube where the temperature was about 800–1000 8C. A similar thermal evaporation experiment was actually performed in 1950 [55,75]. Two kinds of materials were obtained at the temperature range of 800–1000 8C, one was a SiO product and the other one was labeled as ‘‘light brown loose material’’. The loose materials were characterized by X-ray diffraction and determined to be Si structures [75]. The ‘‘light brown loose material’’ can be obtained routinely nowadays by thermal evaporation as Si nanowires. Unfortunately, the Si nanowires in the loose materials were not identified at the time of the initial experiments. The advantages of the OAG technique are (1) the nanowires are highly pure since no metal catalyst is involved and (2) doping of nanowires can be easily achieved because the experimental setup for OAG of Si nanowires is very similar to that of the laser ablation technique. Doping can be easily realized with the assistance of laser ablation of solid dopant materials during nanowire growth. Si nanowires fabricated by this method showed very uniform diameters (about 20 nm) and their lengths were over several hundred micrometers. 2.4. Metal-catalyzed molecular-beam epitaxy Since 2000, MBE and CBE techniques have been employed to synthesize Si [15], II–VI [14] and III–V [13,16] compound semiconductor nanowires based on the VLS growth mechanism. MBE and CBE techniques provide an ideal clean growth environment, and the atomic structures, doping states and junctions (or heterostructures) can be well controlled. Combined with the VLS, these techniques are able to produce high-quality semiconductor nanowires. Different from other synthesis techniques, MBE works under ultra-high vacuum conditions. The mean free path of the source molecules under vacuum conditions of 105 Torr is about 0.2 m. The evaporated source atoms or molecules from the effusion cells behave like a beam aiming directly at the substrate (see Fig. 8). The growth, surface structures and contamination can be monitored in situ by reflection high-energy electron diffraction, Auger electron spectroscopy and other surface probing techniques. MBE has several advantages over other synthesis techniques: (1) the ultra-high vacuum can reduce contamination/oxidation of Fig. 7. (a) Si nanowires synthesized by oxide-assisted growth. (b)Yield of Si nanowires vs. the percentage of SiO2 in the target [11]. N. Wang et al. / Materials Science and Engineering R 60 (2008) 1–51 7
N. Wang et al/Materials Science and Engineering R 60(2008)1-51 Electron Effusion Molecular RHEED Fig 8. A typical MBE growth chamber. material surfaces; (2)the low growth temperature and the temperature. In practice, Au nanoparticles are not necessarily growth rate prevent inter-diffusion in the nanostructures; (3)in molten droplets. In fact, the nanowires can grow at a situ monitoring of growth is possible; (4) since all growth temperature below the eutectic point. However, the deposition parameters can be adjusted precisely and separately, the of the source atoms on the substrate surface becomes significant intrinsic nanowire growth phenomena can be studied indivi- at a low temperature. Then, the surface diffusion becomes an dually. essential mechanism. Excess adatoms are driven to the low For a classical VLS reaction, the metal particles are essential energy state of the molten metallic particles or the molten for the catalytic decomposition of the precursors. For MBe interfaces at these particles growth, however, no molecules or precursors need to The growth temperature is a critical factor for the formation decompose. The function of the metal particles is twofold: of high-quality ZnSe nanowires. On the one hand, the (1)absorption of atoms from vapor phases or substrate surfaces. deposition of ZnSe on the substrate is restrained when the The driving force is to lower the chemical potentials of the substrate temperature is substantially higher than 300C. source atoms and(2)precipitation or crystallization of the Therefore, almost no ZnSe deposition occurs on the fresh e particle-substrate interface. The surface of the substrate(see Fig. 9(a)). On the other hand, a preparation of the substrate surface is critical for growing certain high temperature is needed in order to activate the Au- high-quality nanowires. After wet-chemical cleaning, the alloy particles on the substrate and to"catalyze"the growth of substrate has to be deoxidized. Substrate de-oxidation is the ZnSe nanowires epitaxially on the substrate. Due to the essential because the oxide layer on the substrate influences the surface melting effect, it is possible to grow ZnSe nanowires at nanowire growth direction. A poorly treated substrate results in a low temperature of about 390C. In this case, the deposition random growth directions. The deoxidation temperature of ZnSe on the substrate surface is significant(Fig 9(b)), and depends on the substrates used. For a GaP(111)substrate, the quality of the nanowires is poor compared with the quality for example, annealing at 600C is essential. For the growth of of nanowires grown at a higher temperature. These nanowires II-VI(e.g, ZnSe and ZnS) nanowires [14, the synthesis is contain high-density defects, e.g., stacking faults and twin- carried out using compound-source effusion cells at tempera- nings. However, a too-high growth temperature results in tures above 500C. According to in situ observations of the coarsening of the Au catalyst and a low growth rate and, in turn, reflection high-energy electron diffraction patterns during the leads to non-uniform diameters of the ZnSe nanowires. The growth, Au nanoparticles are in a molten state at this resulting growth rate of the nanowires is mainly determined by Fig 9. TEM images of the interface structures at the substrate. (a) No deposition of the source materials on the substrate surface under a high growth temperature. (b) Deposition of the source materials on the substrate surface under a low growth temperature
material surfaces; (2) the low growth temperature and the growth rate prevent inter-diffusion in the nanostructures; (3) in situ monitoring of growth is possible; (4) since all growth parameters can be adjusted precisely and separately, the intrinsic nanowire growth phenomena can be studied individually. For a classical VLS reaction, the metal particles are essential for the catalytic decomposition of the precursors. For MBE growth, however, no molecules or precursors need to decompose. The function of the metal particles is twofold: (1) absorption of atoms from vapor phases or substrate surfaces. The driving force is to lower the chemical potentials of the source atoms and (2) precipitation or crystallization of the source materials at the particle-substrate interface. The preparation of the substrate surface is critical for growing high-quality nanowires. After wet-chemical cleaning, the substrate has to be deoxidized. Substrate de-oxidation is essential because the oxide layer on the substrate influences the nanowire growth direction. A poorly treated substrate results in random growth directions. The deoxidation temperature depends on the substrates used. For a GaP(1 1 1) substrate, for example, annealing at 600 8C is essential. For the growth of II–VI (e.g., ZnSe and ZnS) nanowires [14], the synthesis is carried out using compound-source effusion cells at temperatures above 500 8C. According to in situ observations of the reflection high-energy electron diffraction patterns during the growth, Au nanoparticles are in a molten state at this temperature. In practice, Au nanoparticles are not necessarily molten droplets. In fact, the nanowires can grow at a temperature below the eutectic point. However, the deposition of the source atoms on the substrate surface becomes significant at a low temperature. Then, the surface diffusion becomes an essential mechanism. Excess adatoms are driven to the low energy state of the molten metallic particles or the molten interfaces at these particles. The growth temperature is a critical factor for the formation of high-quality ZnSe nanowires. On the one hand, the deposition of ZnSe on the substrate is restrained when the substrate temperature is substantially higher than 300 8C. Therefore, almost no ZnSe deposition occurs on the fresh surface of the substrate (see Fig. 9(a)). On the other hand, a certain high temperature is needed in order to activate the Aualloy particles on the substrate and to ‘‘catalyze’’ the growth of the ZnSe nanowires epitaxially on the substrate. Due to the surface melting effect, it is possible to grow ZnSe nanowires at a low temperature of about 390 8C. In this case, the deposition of ZnSe on the substrate surface is significant (Fig. 9(b)), and the quality of the nanowires is poor compared with the quality of nanowires grown at a higher temperature. These nanowires contain high-density defects, e.g., stacking faults and twinnings. However, a too-high growth temperature results in coarsening of the Au catalyst and a low growth rate and, in turn, leads to non-uniform diameters of the ZnSe nanowires. The resulting growth rate of the nanowires is mainly determined by Fig. 8. A typical MBE growth chamber. Fig. 9. TEM images of the interface structures at the substrate. (a) No deposition of the source materials on the substrate surface under a high growth temperature. (b) Deposition of the source materials on the substrate surface under a low growth temperature. 8 N. Wang et al. / Materials Science and Engineering R 60 (2008) 1–51
N. Wang et al. /Materials Science and Engineering R 60 (2008)1-51 the ZnSe flux at a fixed temperature. At 530C, the growth rate as surfactants and organic dopants and (2)"hard templates, of ZnSe nanowires is about 0. 1 nm/s [14) such as anodized alumina membranes [95-103] containing CBE is a hybrid form of molecular beam epitaxy. Different nanosized channels, track-etched polymer porous membranes, tures), gas sources are used(also called gas source molecules. and some special crystals containing nanochannels. Through MBE (using solid so r DC or AC electrochemical deposition, various materials can be beam epitaxy) CBE works at an ultrahigh vacuum condition so introduced into the nanochannels of the hard template [100- that the mean-free paths between molecular collisions become 103. In some cases, vapor molecules may selectively diffuse longer than the source inlet and the substrate. The gaseous into the channels because of special chemical properties of the ource materials are introduced(the gas transport is collision nanochannel walls [103]. without the assistance of structural free)into the reaction chamber at room temperature in the form directors, anisotropic growth of crystals induced by different of a beam. From Au-catalyzed CBe, high-quality ID surface energies can lead to the formation of elongated heterostructure nanowires(InAs/InP) with diameters of about nanocrystals. However, the differences in the surface energies 40 nm have been fabricated [16]. Very thin Inp barrier layers of most materials are not large enough to cause highly with thicknesses of 1. 5 nm and excellent interface structures anisotropic growth of long nanowires By adding surfactants to have been demonstrated by this method. The growth direction the reaction solution, some surfaces of nanocrystals can be and defect density in the nanowires grown by CBE and MBe modulated, i.e., the surfactant molecules selectively adsorb and are influenced by several factors. Twinning(see Fig 9(a)) or bind onto certain surfaces of the nanocrystals and thus reduce stacking faults are the main defects that very often occur in the growth of these surfaces. This selective capping effect thicker nanowires and cause a change of the nanowire growth induces the nanocrystal elongation along a specific direction to direction. For ZnSe nanowire growth, the growth temperature form nanowires. The selective capping mechanism has been and the ratio of the source elements are the main reasons evidenced recently in many nanomaterials such as metal causing the defects. The defect density is also dependent on the nanowires[104-109), metal oxide nanowires [110-115] and growth direction. We have observed that [00 1] growth semiconductor nanowires [116, 117. Though structural direc- nanowires contain fewer defects compared with nanowires tors are often used for the synthesis of nanowires, the actual grown in other directions, and ultra-thin nanowires(diame- growth process is poorly known. As a matter of fact, in many ter< 10 nm) generally contain few defects. The growth cases, the structural directors may not exist or the materials are directions of ultra-thin II-VI compound nanowires are mainly self-constitutive templates. The formation mechanism of determined by the diameters or the sizes of the catalysts and the nanowires in solution is complicated and the selection and growth temperature. The size-dependent and temperature- function of the structural directors require further dependent growth directions and the interface structures of l- systematic investigation Vi nanowires are discussed in Section 4.1 based on the estimation of the surface and interface energies of the nanowire 3. growth mechanisms of nanowires nuclei 3.1. Metal-catalyzed growth 25. Solution methods The most significant work on the mechanism of the The major advantages of the solution-based technique (in unidirectional growth of semiconductor whiskers grown by aqueous or non-hydrolytic media) for synthesizing nanomater- VLS was published by Wagner and Ellis in 1965 [118]. The als are high yield, low cost and easy fabrication. The solution- unidirectional growth of Si whiskers can be simply interpreted based technique has been demonstrated as a promising based on the difference of the sticking coefficients of the alternative approach for mass production of metal, semicon- impinging vapor source atoms on the liquid(the catalytic ductor and oxide nanomaterials with excellent controls of the droplet)and on solid surfaces. In principle, an ideal liquid shape and composition with high reproducibility. In particular, surface captures all impinging Si source atoms, while a solid this technique is able to assemble nanocrystals with other surface of Si rejects almost all Si source atoms if the functional materials to form hybrid nanostructures with multiple temperature is sufficiently high. This classical VLS mechanism functions with great potential for applications in nanoelectronic is still applicable to the growth of many nanoscale wires and biological systems. The nanocrystals synthesized in aqueous produced today. As schematically shown in Fig. 10(a),Au media may often suffer from poor crystallinity, but those particles deposited on the surface of an Si substrate initially ture, in general, show much better crystal quality(76,77). For the temperature of a Si-Au alloy particle is significantly decreang synthesized under nonhydrolytic conditions at a high tempera- react with Si to form active Au-Si alloy droplets. The melting formation of nanowires from solution, several routes have been once its size is in the nanometer range [119]. during the initial developed, such as metal-catalyzed solution-liquid-solid(sLs) reaction of the catalyst on a flat surface(see also Fig. 10(b)), the growth from metal seeds [78-88], self-assembly attachment shape or the contact angle (P)of the droplet is determined by growth [89-94], and anisotropic growth of crystals by the balanced forces of the surface tension and the liquid-solid thermodynamic or kinetic control (LS)interface tension. The droplet has a radius, R, which can be Many nanowires grown from solution methods largely rely described by R=o/sin(Bo)(ro is the radius of the contact area) on"structural directors", including(1)"soft templates, such [ 120, 121]. The contact angle is related to the surface tension
the ZnSe flux at a fixed temperature. At 530 8C, the growth rate of ZnSe nanowires is about 0.1 nm/s [14]. CBE is a hybrid form of molecular beam epitaxy. Different from MBE (using solid sources evaporated at high temperatures), gas sources are used (also called gas source molecular beam epitaxy). CBE works at an ultrahigh vacuum condition so that the mean-free paths between molecular collisions become longer than the source inlet and the substrate. The gaseous source materials are introduced (the gas transport is collision free) into the reaction chamber at room temperature in the form of a beam. From Au-catalyzed CBE, high-quality 1D heterostructure nanowires (InAs/InP) with diameters of about 40 nm have been fabricated [16]. Very thin InP barrier layers with thicknesses of 1.5 nm and excellent interface structures have been demonstrated by this method. The growth direction and defect density in the nanowires grown by CBE and MBE are influenced by several factors. Twinning (see Fig. 9(a)) or stacking faults are the main defects that very often occur in thicker nanowires and cause a change of the nanowire growth direction. For ZnSe nanowire growth, the growth temperature and the ratio of the source elements are the main reasons causing the defects. The defect density is also dependent on the growth direction. We have observed that [0 0 1] growth nanowires contain fewer defects compared with nanowires grown in other directions, and ultra-thin nanowires (diameter < 10 nm) generally contain few defects. The growth directions of ultra-thin II–VI compound nanowires are mainly determined by the diameters or the sizes of the catalysts and the growth temperature. The size-dependent and temperaturedependent growth directions and the interface structures of II– VI nanowires are discussed in Section 4.1 based on the estimation of the surface and interface energies of the nanowire nuclei. 2.5. Solution methods The major advantages of the solution-based technique (in aqueous or non-hydrolytic media) for synthesizing nanomaterials are high yield, low cost and easy fabrication. The solutionbased technique has been demonstrated as a promising alternative approach for mass production of metal, semiconductor and oxide nanomaterials with excellent controls of the shape and composition with high reproducibility. In particular, this technique is able to assemble nanocrystals with other functional materials to form hybrid nanostructures with multiple functions with great potential for applications in nanoelectronic and biological systems. The nanocrystals synthesized in aqueous media may often suffer from poor crystallinity, but those synthesized under nonhydrolytic conditions at a high temperature, in general, show much better crystal quality [76,77]. For the formation of nanowires from solution, several routes have been developed, such as metal-catalyzed solution-liquid-solid (SLS) growth from metal seeds [78–88], self-assembly attachment growth [89–94], and anisotropic growth of crystals by thermodynamic or kinetic control. Many nanowires grown from solution methods largely rely on ‘‘structural directors’’, including (1) ‘‘soft templates,’’ such as surfactants and organic dopants and (2) ‘‘hard templates,’’ such as anodized alumina membranes [95–103] containing nanosized channels, track-etched polymer porous membranes, and some special crystals containing nanochannels. Through DC or AC electrochemical deposition, various materials can be introduced into the nanochannels of the hard template [100– 103]. In some cases, vapor molecules may selectively diffuse into the channels because of special chemical properties of the nanochannel walls [103]. Without the assistance of structural directors, anisotropic growth of crystals induced by different surface energies can lead to the formation of elongated nanocrystals. However, the differences in the surface energies of most materials are not large enough to cause highly anisotropic growth of long nanowires. By adding surfactants to the reaction solution, some surfaces of nanocrystals can be modulated, i.e., the surfactant molecules selectively adsorb and bind onto certain surfaces of the nanocrystals and thus reduce the growth of these surfaces. This selective capping effect induces the nanocrystal elongation along a specific direction to form nanowires. The selective capping mechanism has been evidenced recently in many nanomaterials such as metal nanowires [104–109], metal oxide nanowires [110–115] and semiconductor nanowires [116,117]. Though structural directors are often used for the synthesis of nanowires, the actual growth process is poorly known. As a matter of fact, in many cases, the structural directors may not exist or the materials are self-constitutive templates. The formation mechanism of nanowires in solution is complicated and the selection and function of the structural directors require further and systematic investigation. 3. Growth mechanisms of nanowires 3.1. Metal-catalyzed growth The most significant work on the mechanism of the unidirectional growth of semiconductor whiskers grown by VLS was published by Wagner and Ellis in 1965 [118]. The unidirectional growth of Si whiskers can be simply interpreted based on the difference of the sticking coefficients of the impinging vapor source atoms on the liquid (the catalytic droplet) and on solid surfaces. In principle, an ideal liquid surface captures all impinging Si source atoms, while a solid surface of Si rejects almost all Si source atoms if the temperature is sufficiently high. This classical VLS mechanism is still applicable to the growth of many nanoscale wires produced today. As schematically shown in Fig. 10(a), Au particles deposited on the surface of an Si substrate initially react with Si to form active Au–Si alloy droplets. The melting temperature of a Si–Au alloy particle is significantly decreased once its size is in the nanometer range [119]. During the initial reaction of the catalyst on a flat surface (see also Fig. 10(b)), the shape or the contact angle (bo) of the droplet is determined by the balanced forces of the surface tension and the liquid–solid (LS) interface tension. The droplet has a radius, R, which can be described by R=r0/sin(b0) (r0 is the radius of the contact area) [120,121]. The contact angle is related to the surface tension N. Wang et al. / Materials Science and Engineering R 60 (2008) 1–51 9
N. Wang et al/Materials Science and Engineering R 60(2008)1-51 Vapor phases Si+M Liquid (Si+M OLs o Fig. 10. Schematic of Au-Si droplets(a) formed on the substrate (b)Initial growth of the nanowire. (c) The hillock shape of the nanowire root(from Ref. [121] courtesy of Prof. T Y. Tan and the line tension, t, by a modified(a line tension is added) Gibbs free energy, Li et al. obtained [120, 121]: Youngs equation [122] OVL COS Bo=OVS --OLS (3.1.3) O1 cos( Bo)=os-OIs (3.1.1 For a droplet of macroscopic size, the effect of the line tension LSo 221, can be ignored. For a nanosized droplet, the line tension should where ois is the effective surface tension, t' the effective be considered. At the initial growth, when the nanowire's chemical tension, Io the elementary thickness and n is the vapor length, dh, increases, the radius of the contact area. source of the actual-to-equilibrium-pressure ratio. The cher decreases. The inclination angle, a, of the nanowire flanks will cal tensional is defined as: o=ois +(r/r). Then, the increase(a=0 before growth). The inclination angle can be general equation for a wire already grown to some length is OVL COS Bo=Ovs -OLS -o. The equilibrium condition of the VlS reaction is the balance among the various static factors in a1 cos(B)=Os cos(ax)-a1 (3.1.2) the system, the surface energies, the dynamic factors due to the growth of a crystal layer, and the chemical tension. The shape of An increase in a is accompanied by an increase in B. The an initially grown Si nanowire(due to the line-tension)is shown droplet will approach a spherical section. Since the contact area in the TEM image in Fig. 11(a). Based on the chemical-tension decreases with an increase in the nanowire length, the final model, Li et al. predicted that different line-tension values can radius of the nanowire should be smaller than the initial radius, result in nanowire or nanohillock growth as shown in Fig. 11(b) 10(c)). The line tension ult to determine For Si whisker growth, a typical kinetic experimental result experimentally)strongly influences the catalyst contact area. a is the growth rate dependence on the whisker diameter. The large line tension can result in hillock growth and thus stop the larger the whisker diameter, the faster is its growth rate growth [120]. Using the minimization method of the systems This growth phenomenon is attributed to the well-known =1.46 T'=0.04 k=0.73 4+℃=008 0.12 r(。) Fig. Il.(a) The shape of the initial growth of Si nanowires due to the line-tension(b) Prediction of Si nanowire and nanohillock growth by the chemical-tension model for various line-tension values(from Ref [ 120]: reproduced with permission from Springer Science)
and the line tension, t, by a modified (a line tension is added) Young’s equation [122]: s1 cosðb0Þ ¼ ss sls t r0 : (3.1.1) For a droplet of macroscopic size, the effect of the line tension can be ignored. For a nanosized droplet, the line tension should be considered. At the initial growth, when the nanowire’s length, dh, increases, the radius of the contact area, dr, decreases. The inclination angle, a, of the nanowire flanks will increase (a = 0 before growth). The inclination angle can be expressed as s1 cosðbÞ ¼ ss cosðaÞ sls t r0 : (3.1.2) An increase in a is accompanied by an increase in b. The droplet will approach a spherical section. Since the contact area decreases with an increase in the nanowire length, the final radius of the nanowire should be smaller than the initial radius, r0 (see Fig. 10(c)). The line tension (difficult to determine experimentally) strongly influences the catalyst contact area. A large line tension can result in hillock growth and thus stop the growth [120].Using the minimization method of the system’s Gibbs free energy, Li et al. obtained [120,121]: sVL cos b0 ¼ sVS sLS sc LS tc ro ; (3.1.3) sc LS ¼ lo kBT V ln h; tc ¼ losVS; (3.1.4) where sc LS is the effective surface tension, tc the effective chemical tension, lo the elementary thickness and h is the vapor source of the actual-to-equilibrium-pressure ratio. The chemical tensional is defined as: sc ¼ sc LS þ ðtc=roÞ. Then, the general equation for a wire already grown to some length is sVL cos b0 = sVS sLS sc . The equilibrium condition of the VLS reaction is the balance among the various static factors in the system, the surface energies, the dynamic factors due to the growth of a crystal layer, and the chemical tension. The shape of an initially grown Si nanowire (due to the line-tension) is shown in the TEM image in Fig. 11(a). Based on the chemical-tension model, Li et al. predicted that different line-tension values can result in nanowire or nanohillock growth as shown in Fig. 11(b). For Si whisker growth, a typical kinetic experimental result is the growth rate dependence on the whisker diameter. The larger the whisker diameter, the faster is its growth rate. This growth phenomenon is attributed to the well-known Fig. 11. (a) The shape of the initial growth of Si nanowires due to the line-tension. (b) Prediction of Si nanowire and nanohillock growth by the chemical-tension model for various line-tension values (from Ref. [120]; reproduced with permission from Springer Science). Fig. 10. Schematic of Au–Si droplets (a) formed on the substrate. (b) Initial growth of the nanowire. (c) The hillock shape of the nanowire root (from Ref. [121]; courtesy of Prof. T.Y. Tan). 10 N. Wang et al. / Materials Science and Engineering R 60 (2008) 1–51