《复合材料 Composites》课程教学资源（学习资料）第二章 增强体_Catalytic growth of nanowires：Vapor–liquid–solid, vapor–solid–solid, solution–liquid–solid and solid–liquid–solid growth

Science Direct Current Opinion in Solid state Materials science ELSEVIER Current Opinion in Solid State and Materials Science 10(2006)182-191 Catalytic growth of nanowires: Vapor-liquid-solid vapor-solid-solid, solution-liquid-solid and solid-liquid-solid growth Kurt w. Kolasinski Department of Chemistry, West Chester Unirersity, West Chester, PA 19383, United States Received 12 March 2007: accepted 12 March 2007 Abstract Catalytic growth is a powerful tool to form a variety of wire( whisker) like structures with diameters ranging from just a few nano- metres to the millimetre range. A range of phases(gas, solid, liquid, solution and supercritical fluid) have been used for the feeder phase. i.e. the source of material to be incorporated into the nanowire Solid, liquid, eutectic, alloy and metastable phases have all been invoked to explain the structure of the catalytic particle. Rather than focussing on the differences that lead to the proliferation of an alphabet soup of names for the various growth techniques, this review attempts to focus on the similarities between all of these catalytic growth pro- ses in an attempt to help stimulate a more universal understanding of the phenomenon. The review begins with a precis of the mate- rials from which nanowires have been formed and then proceeds to a discussion of mechanistic aspects e 2007 Elsevier Ltd. All rights reserved 1. Introduction tors have also been gre ImaI nanowires, and are prized for their potential in electronic As a controlled means of growing whiskers and more optoelectronic and sensing applications[6-8]. For more recently nanowires, catalytic growth of solid structures on the potential of these nanostructures in applications traces back to the discovery of Wagner and Ellis [1] that the reader is referred to these recent reviews. Buhro and Si whiskers could be grown by heating a Si substrate in a co-workers [9] have reviewed the formation of semicon- mixture of SiCl4 and H2 with their diameters determined ductor nanowires from solutions and supercritical fluids by the size of Au particles that had been placed on the sur- Here I concentrate on the production of ID nanostructures face prior to growth. Of course, the catalytic growth of car- with the use of vapor phase transport and surface diffusion. bon fibres has long been a recognized problem in the field In the literature we might variously encounter nanowires of catalysis [2]. In this case, such growth must be avoided, (solid core structures with diameters below 100 nm), for instance, in the steam reforming of CH, over Ni cata- nanotubes(single or multi-walled hollow core structures ts, which is the primary industrial source of H with diameters below M100 nm) and whiskers(larger solid The poster child of one-dimensional(ID)nanostruc- core structures). For simplicity, I will use the term nano- tures is the carbon nanotube(CNT) either in single-walled wire generically to describe the structures formed by cata- (SW-CNT) or multiwalled variants (MW-CNT). They are lytic growth unless I specifically want to call attention to valued for a wide range of extreme properties for electronic nanotubes or whiskers. This review does not attempt to applications, for their high thermal conductivity and for be exhaustive. Rather it looks first at a number of materials their high strength [2-5]. A number of other semiconduc- systems that have been grown catalytically in the form of nanowires, nanotubes or whiskers in the past year or two. Then a review of the mechanistic aspects of catalytic E-inail address: kkolasinski(@wcupaedt nanowire growth is made 1359-0286/- see front matter 2007 Elsevier Ltd. All rights reserved doi:10.1016 cossms.2007.03.002

Catalytic growth of nanowires: Vapor–liquid–solid, vapor–solid–solid, solution–liquid–solid and solid–liquid–solid growth Kurt W. Kolasinski Department of Chemistry, West Chester University, West Chester, PA 19383, United States Received 12 March 2007; accepted 12 March 2007 Abstract Catalytic growth is a powerful tool to form a variety of wire (whisker) like structures with diameters ranging from just a few nano￾metres to the millimetre range. A range of phases (gas, solid, liquid, solution and supercritical fluid) have been used for the feeder phase, i.e. the source of material to be incorporated into the nanowire. Solid, liquid, eutectic, alloy and metastable phases have all been invoked to explain the structure of the catalytic particle. Rather than focussing on the differences that lead to the proliferation of an alphabet soup of names for the various growth techniques, this review attempts to focus on the similarities between all of these catalytic growth pro￾cesses in an attempt to help stimulate a more universal understanding of the phenomenon. The review begins with a pre´cis of the mate￾rials from which nanowires have been formed and then proceeds to a discussion of mechanistic aspects. 2007 Elsevier Ltd. All rights reserved. 1. Introduction As a controlled means of growing whiskers and more recently nanowires, catalytic growth of solid structures traces back to the discovery of Wagner and Ellis [1] that Si whiskers could be grown by heating a Si substrate in a mixture of SiCl4 and H2 with their diameters determined by the size of Au particles that had been placed on the sur￾face prior to growth. Of course, the catalytic growth of car￾bon fibres has long been a recognized problem in the field of catalysis [2]. In this case, such growth must be avoided, for instance, in the steam reforming of CH4 over Ni cata￾lysts, which is the primary industrial source of H2. The poster child of one-dimensional (1D) nanostruc￾tures is the carbon nanotube (CNT) either in single-walled (SW-CNT) or multiwalled variants (MW-CNT). They are valued for a wide range of extreme properties for electronic applications, for their high thermal conductivity and for their high strength [2–5]. A number of other semiconduc￾tors have also been grown in 1D structures, primarily nanowires, and are prized for their potential in electronic, optoelectronic and sensing applications [*6–*8]. For more on the potential of these nanostructures in applications, the reader is referred to these recent reviews. Buhro and co-workers [*9] have reviewed the formation of semicon￾ductor nanowires from solutions and supercritical fluids. Here I concentrate on the production of 1D nanostructures with the use of vapor phase transport and surface diffusion. In the literature we might variously encounter nanowires (solid core structures with diameters below 100 nm), nanotubes (single or multi-walled hollow core structures with diameters below 100 nm) and whiskers (larger solid core structures). For simplicity, I will use the term nano￾wire generically to describe the structures formed by cata￾lytic growth unless I specifically want to call attention to nanotubes or whiskers. This review does not attempt to be exhaustive. Rather it looks first at a number of materials systems that have been grown catalytically in the form of nanowires, nanotubes or whiskers in the past year or two. Then a review of the mechanistic aspects of catalytic nanowire growth is made. 1359-0286/$ - see front matter 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.cossms.2007.03.002 E-mail address: kkolasinski@wcupa.edu Current Opinion in Solid State and Materials Science 10 (2006) 182–191

K w. Kolasinski Current Opinion in Solid State and Materials Science 10(2006 )182-191 183 2. Materials systems upper limit on the Ge fraction that can be obtained in this mDe Silicon was the system first investigated by Wagner and Silanes do not have to be used as the source material for Ellis [1] and it remains one of the most intensively studied the growth of SiNW and Si- Gex nanowires. Dujardin systems [7, 10-14, 15, 16-19]. Lieber's group["8] has stud- et al. [10] have used a molecular beam epitaxy(MBe) ied silicon extensively including the formation of branched source and a substrate coated with a thin film of Au for this Si nanowires(SINW)[20]. Carbon nanotubes [2, 21, 22]and purpose. The use of an MBE source is quite significant carbon nanofibers [23] are also produced by catalytic because of a basic difference in the dynamics of the interac growth [24, 25]. Heterojunctions between SiNW and CNT tion of an atomic vapor of Si as compared to SiH4 have been formed [26]. Other materials that exhibit cata- Whereas the sticking coefficient of SiH4 is low on a Si lytic growth of nanowires include Sio(a substoichiomet- and practically zero on a H-terminated Si surface, it is ric silicon oxide)[27]: SiO2 [28, 29] Sil- Gex [10, 30] Ge much higher on the surface of a metal catalyst. In contrast, 61, 323: AIN [33] r-Al2O3 [34]: oxide-coated B[35]; the sticking coefficient of Si atoms is unity on a clean or H- CNx [36]: Cdo [37]: Cds [38]: CaSe [9] CdTe [9]: a- terminated Si surface as well as on a metal catalyst particle Fe2O3(hematite), &-Fe2O3 and Fe3 O4(magnetite)[39] Sun et al. [31] have created Ge nanowires( GeNw) by GaAs [15,40, *41, 42, 43, 44]: Gan [18: Ga2O3 [18, 45]; evaporation of Ge powder in flowing Ar at 600C.Au GaP [40, *41, 46] InAs ["41, 471: InN (hexangular struc- nanoparticles act as catalysts and are supplied from a col- tures)[48] InP [9, 41,42]: In2O3 [45]: In2 Se3 [49] LiF lodal solution of thiol-capped Au. The nanowires have [50] SnO2[45, 51, * 52: ZnO nanowires ["7, 8,53and nano- uniform diameters of 30 nm, are up to tens of microme- plates [53]; ZnS [54]: ZnSe [55]: Mn doped Zn2SO4 [56]; and tres in length and have a Au particle on their tip. The ori- ZnTe [57]. Let us now look at the conditions under which ginal size of the colloidal Au particles was 2 nm; hence, catalytic growth has been used to create nanostructures significant aggregation must have occurred. Here we have that we can better understand the range of growth condi- vapor phase transport of atomic Ge to already formed tions that have been used, as well as the similarities and dif- Au catalytic particles ferences in growth characteristics that have been observed Chandrasekaran et al.[32] use a solid-liquid-solid so that we may generalize about some of the important method to produce GeNW. Many um long GeNW are mechanistic characteristics observed to emanate from the same mm diameter Ga cat- An exciting development in the growth of single-walled alyst particle(multiprong root growth, as defined in the carbon nanotubes(SW-CNT)was reported by Takagi et al. next section). In and Sn are also suitable catalysts. A thin [58]. A range of metal catalysts have been shown previ- film of the molten metal is spread on a Ge(1 00) wafer, ously to work for the synthesis of carbon fibres and CNTs which is then exposed to a microwave plasma struck in [2]. Takagi et al. have shown that pyrolysis of ethanol can H2/N2. Ground NaCl is placed around the substrate. This be used in the presence not only of Fe, Co or Ni(the most acts as a source of Cl, which in combination with the common catalysts) or Pt and Pd(which had been previ- plasma acts to transport the Ge. Alternatively, a quartz ously reported) but also the coinage metals(Cu, Ag and substrate can be coated with the metal, and a powdered Au). For the latter three metals to work not only do they mixture of NaCl and Ge is placed around the substrate have to be clean to start with, they must also be smaller Excitation involving a plasma is again used to elicit trans- than 5 nm in diameter for growth to be efficient. Their port and engender GeNw growth explanation is that the metal particles are in a cluster-like A Si wafer rather than a powdered sample has been used structure rather than a crystalline state, and C atoms are as the source material for simultaneous growth of Sio soluble in these clusters. Then, C atoms might precipitate nanowires and SnO, nanobelts by Zhang et al. [27]. SnCl2 formation of a hemispherical cap with a graphitic structure Ar t O2 to 950C for 2 h The Sn catalyst particles, which as the precursor of Sw-CNT growth. They propose that also contain several percent Si and O, form at the base of the essential role of metal particles is to provide a platform the SiOx nanowires rather than the tips. Both the SiOx on which carbon atoms can form a hemispherical cap from nanowires and the SnO2 nanobelts experience multiprong which SW-CNT grow in a self-assembled fashion. root growth Si_ Gex nanowires can be grown with a Au catalyst Zhang et al. [38] also used catalysis over Sn nanoparti much as SiNW can be grown as shown by Lew et al. cles to form branched CdS nanowires. They mixed Cds [30]. Growth was carried out in an isothermal quartz tube SnO2 and graphite powders (in a 1: 1: I ratio) and heated reactor at 325-525C with a total reactor pressure of them to 1200C for 2 h under a constant flow of Ar In this 13 Torr composed of a 10% mixture of SiH4 in H2 and case the Sn particle at the end of the nanowire is signifi- either a 1% or 2% mixture of GeH in H, as source gases. cantly larger than the diameter of the wire that grows from Higher temperatures(>375C) favour the growth of Si rich it. In both of the systems investigated by Zhang et al (x<0.5)Sil_Gex nanowires; however, Ge thin film depo- vapor phase transport is used not only to supply the sition on the outer surface of the wire was observed at growth material, but also to supply the material that forms increased GeH/(SiH4+ GeH,)inlet gas ratios setting an the catalytic particle

2. Materials systems Silicon was the system first investigated by Wagner and Ellis [1] and it remains one of the most intensively studied systems [7,10–14,*15,16–19]. Lieber’s group [*8] has stud￾ied silicon extensively including the formation of branched Si nanowires (SiNW) [20]. Carbon nanotubes [2,21,22] and carbon nanofibers [23] are also produced by catalytic growth [24,25]. Heterojunctions between SiNW and CNT have been formed [26]. Other materials that exhibit cata￾lytic growth of nanowires include SiOx (a substoichiomet￾ric silicon oxide) [27]; SiO2 [28,29]; Si1xGex [10,30]; Ge [31,*32]; AlN [33]; c-Al2O3 [34]; oxide-coated B [*35]; CNx [36]; CdO [37]; CdS [38]; CdSe [*9]; CdTe [*9]; a￾Fe2O3 (hematite), e-Fe2O3 and Fe3O4 (magnetite) [39]; GaAs [15,40,*41,42,*43,44]; GaN [18]; Ga2O3 [18,45]; GaP [40,*41,*46]; InAs [*41,*47]; InN (hexangular struc￾tures) [48]; InP [*9,*41,42]; In2O3 [45]; In2Se3 [49]; LiF [50]; SnO2 [45,51,*52]; ZnO nanowires [*7,*8,53] and nano￾plates [53]; ZnS [54]; ZnSe [55]; Mn doped Zn2SO4 [56]; and ZnTe [57]. Let us now look at the conditions under which catalytic growth has been used to create nanostructures so that we can better understand the range of growth condi￾tions that have been used, as well as the similarities and dif￾ferences in growth characteristics that have been observed so that we may generalize about some of the important mechanistic characteristics. An exciting development in the growth of single-walled carbon nanotubes (SW-CNT) was reported by Takagi et al. [*58]. A range of metal catalysts have been shown previ￾ously to work for the synthesis of carbon fibres and CNTs [2]. Takagi et al. have shown that pyrolysis of ethanol can be used in the presence not only of Fe, Co or Ni (the most common catalysts) or Pt and Pd (which had been previ￾ously reported) but also the coinage metals (Cu, Ag and Au). For the latter three metals to work not only do they have to be clean to start with, they must also be smaller than 5 nm in diameter for growth to be efficient. Their explanation is that the metal particles are in a cluster-like structure rather than a crystalline state, and C atoms are soluble in these clusters. Then, C atoms might precipitate to cover the surface of the nanoparticles, resulting in the formation of a hemispherical cap with a graphitic structure as the precursor of SW-CNT growth. They propose that the essential role of metal particles is to provide a platform on which carbon atoms can form a hemispherical cap from which SW-CNT grow in a self-assembled fashion. Si1xGex nanowires can be grown with a Au catalyst much as SiNW can be grown as shown by Lew et al. [30]. Growth was carried out in an isothermal quartz tube reactor at 325–525 C with a total reactor pressure of 13 Torr composed of a 10% mixture of SiH4 in H2 and either a 1% or 2% mixture of GeH4 in H2 as source gases. Higher temperatures (>375 C) favour the growth of Si rich (x < 0.5) Si1xGex nanowires; however, Ge thin film depo￾sition on the outer surface of the wire was observed at increased GeH4/(SiH4 + GeH4) inlet gas ratios setting an upper limit on the Ge fraction that can be obtained in this temperature range. Silanes do not have to be used as the source material for the growth of SiNW and Si1xGex nanowires. Dujardin et al. [10] have used a molecular beam epitaxy (MBE) source and a substrate coated with a thin film of Au for this purpose. The use of an MBE source is quite significant because of a basic difference in the dynamics of the interac￾tion of an atomic vapor of Si as compared to SiH4. Whereas the sticking coefficient of SiH4 is low on a Si and practically zero on a H-terminated Si surface, it is much higher on the surface of a metal catalyst. In contrast, the sticking coefficient of Si atoms is unity on a clean or H￾terminated Si surface as well as on a metal catalyst particle. Sun et al. [31] have created Ge nanowires (GeNW) by evaporation of Ge powder in flowing Ar at 600 C. Au nanoparticles act as catalysts and are supplied from a col￾loidal solution of thiol-capped Au. The nanowires have uniform diameters of 30 nm, are up to tens of microme￾tres in length and have a Au particle on their tip. The ori￾ginal size of the colloidal Au particles was 2 nm; hence, significant aggregation must have occurred. Here we have vapor phase transport of atomic Ge to already formed Au catalytic particles. Chandrasekaran et al. [*32] use a solid–liquid–solid method to produce GeNW. Many lm long GeNW are observed to emanate from the same mm diameter Ga cat￾alyst particle (multiprong root growth, as defined in the next section). In and Sn are also suitable catalysts. A thin film of the molten metal is spread on a Ge(1 0 0) wafer, which is then exposed to a microwave plasma struck in H2/N2. Ground NaCl is placed around the substrate. This acts as a source of Cl, which in combination with the plasma acts to transport the Ge. Alternatively, a quartz substrate can be coated with the metal, and a powdered mixture of NaCl and Ge is placed around the substrate. Excitation involving a plasma is again used to elicit trans￾port and engender GeNW growth. A Si wafer rather than a powdered sample has been used as the source material for simultaneous growth of SiOx nanowires and SnO2 nanobelts by Zhang et al. [27]. SnCl2 powder was placed upstream from a Si wafer and heated in Ar + O2 to 950 C for 2 h. The Sn catalyst particles, which also contain several percent Si and O, form at the base of the SiOx nanowires rather than the tips. Both the SiOx nanowires and the SnO2 nanobelts experience multiprong root growth. Zhang et al. [38] also used catalysis over Sn nanoparti￾cles to form branched CdS nanowires. They mixed CdS, SnO2 and graphite powders (in a 1:1:1 ratio) and heated them to 1200 C for 2 h under a constant flow of Ar. In this case the Sn particle at the end of the nanowire is signifi- cantly larger than the diameter of the wire that grows from it. In both of the systems investigated by Zhang et al., vapor phase transport is used not only to supply the growth material, but also to supply the material that forms the catalytic particle. K.W. Kolasinski / Current Opinion in Solid State and Materials Science 10 (2006) 182–191 183

K.w. Kolasinski/ Current Opinion in Solid State and Materials Science 10(2006)182-191 Carbothermal reduction, vapor phase transport of in contrast to, for instance, Au catalyzed growth of Si and growth material and reaction with trace amounts of oxygen SiGe nanowires. Also in this system, both the growth and have been utilized by Kuo and Huang in the growth of catalyst phases are transported via the vapor phase Taper Cdo nanowires that are 40-80 nm in diameter and 30- ing to larger diameters is observed in this system, most 50 um in length. Cdo and graphite powders are mixed in likely because the Sn nanoparticles are growing during a 4: 1 ratio. The silicon substrate is coated with a I nm growth. Au film and then placed 25 cm downstream from the reac Xu et al. [53] formed Zno nanowires with a hexagonal tant mixture. The reactants are held at 500C and the cross section. Interestingly no catalytic particle is found substrate at 400C. The Au particle at the end of the at the tip. Rather the catalytic action is provided by a nanowire has a diameter slightly smaller than the nano- ZnBil thin film(a combination of tetragonal ZnI, and wire. Most of the nanowires have a smooth surface but hexagonal Bil3) Mixtures of either Bil3 and Zn powder I an area of the reactor where there was probably a greater or Bi and Zn powders were heated in flowing Ar to 250- amount of oxygen available a jagged necklace structure 300C. The presence of I changes the growth direction of formed by a lateral growth of the rhombohedral Cdo the nanowires. The O can be supplied by an impurity in nanocrystals over the smooth nanowires. the Ar but no growth is observed in the absence of Bi when Pulsed laser deposition has been used by Morber et al. only Zn or Znl2 powders are used [39]to synthesize FeOy and Mg doped a-Fe2O3 nanorods, Carbothermal reduction can be used to produce ZnO nanowires and nanobelts. The ablation target was a pressed nanowires on a silicon substrate. Moreover, what Yang powder of magnetite(Fe3 O4), which was placed next to a et al. have shown is that growth can be switch away from quartz boat containing polycrystalline alumina wafer sub- Zno nanowires to Mn doped ZnSiO4 if MnCl2 4H2O is strates coated with a 2 nm Au film. Au particles are found added to the reaction mixture. The furnace system was at the ends of the nanowires, therefore it appears that the flushed with high-purity Ar gas to eliminate O2 and heated u film spontaneously breaks up to form the catalytic to 1 100C under a constant flow of Then. th nanoparticles quartz boat was placed in the centre of the furnace and An arc discharge has been used by Li et al. [34] to form held at 1100C under the same Ar flow. After reaction ubic 7-Al2O3 nanorods. The source material is a pressed for 50-60 min at 800-900C, the Si wafers, which are situ- powder mixture of Fe and Al, surprisingly in a 60: 40 ratio. ated downstream of the reaction mixture, were coated with Fe catalyst particles are found at the ends of the nanorods. a layer of nanowires. The majority of this material is com- The discharge is run in a mixture of 0.018 MPa Ar (99.9% posed of a willemite phase(a-Zn2SiO4 with rhombohedral purity) and 0.008 MPa H2(99.99% purity ). The growth structure) forming well-aligned nanorods with lengths of 2- conditions are highly non-equilibrium with the o being 4 um and diameters of 70-150 nm. The Zn catalyst is con- pplied at the trace level as an impurity in the process sumed so there is a severe taper at the end of the wires and gases. The diameters of the 7-Al2O3 nanorods are relatively no metal particle is found uniform, ranging from 20 to 30 nm Simultaneous growth of ZnO and LiF nanowires has Carbothermal reduction of a mixture of an Al com- been reported by Jiang et al. [50]. Zn acts as the cata olex with Fe powder has been used by Jung and Joo [33] ly hen LiF t Zno powders are heated in Ar to to create AIn whiskers. The mixture of the Al complex, 750-850C. The downstream deposition region has a Fe and graphite was calcined at 1200-1500C for 5 h in temperature in the range of 400-500C. Cubic-structured flowing N2. The whiskers often show modulations in their single-crystalline LiF nanowires grew along the (001) diameters along the lengths of the whisker and the shapes and(110) crystallographic directions with diameters of obtained depend on the growth temperature. This system 100-500 nm and lengths of tens of microns. The authors exhibits rather complex chemistry on account of the carbo- propose that there must be a barrier to the incorporation thermal reduction to produce Al, dissociative adsorption of of Lif into the lattice and the Zn particle acts to lower this N2, presumably on the Fe particles, and the formation of barrier and cause growth to be preferential at its ba Fe/Al/N alloy particles of the appropriate size. Very similar conditions can be used to grow Dendritic ZnO nanowires can be grown from Sn catalyst Ga2O3 and SnO, nanowires. Indeed, In,O3 and particles as shown by Gao et al. [59]. They heated ZnO and nanowires can be grown simultaneously without cross con- SnO, powders(1: I ratio) for I h under a pressure of 300- tamination or doping as demonstrated by Johnson et al 400 Torr of Ar carrier gas. The nanostructures grew on [45]. Fifty nanometer Au particles were deposited from the top of the inner alumina wall of the tube furnace in a solution onto the si substrate placed downstream from region located downstream M15 cm away from the source high-purity(6 N)metal reactant species (In, Ga, or Sn) material, which was located in the middle of the furnace placed separately in a quartz boat. The furnace was heated at 1300C, and the local growth temperature was in the to 800-1000C under flowing N2. No oxygen was supplied range of 700-800C. The Sn particle is significantly larger tly to the system other than as an impurity in the n than the Nw diameter. Note that in this system, as in sev- or that which desorbs from the surfaces inside the furnace. eral others, there is a high proportion of the catalyst start- Rectangular In]O3 rods are capped with a rectangular Au ing material compared to the growth material. This stands particle that is slightly smaller than the rod, whereas

Carbothermal reduction, vapor phase transport of growth material and reaction with trace amounts of oxygen have been utilized by Kuo and Huang in the growth of CdO nanowires that are 40–80 nm in diameter and 30– 50 lm in length. CdO and graphite powders are mixed in a 4:1 ratio. The silicon substrate is coated with a 1 nm Au film and then placed 25 cm downstream from the reac￾tant mixture. The reactants are held at 500 C and the substrate at 400 C. The Au particle at the end of the nanowire has a diameter slightly smaller than the nano￾wire. Most of the nanowires have a smooth surface but in an area of the reactor where there was probably a greater amount of oxygen available, a jagged necklace structure formed by a lateral growth of the rhombohedral CdO nanocrystals over the smooth nanowires. Pulsed laser deposition has been used by Morber et al. [39] to synthesize FexOy and Mg doped e-Fe2O3 nanorods, nanowires and nanobelts. The ablation target was a pressed powder of magnetite (Fe3O4), which was placed next to a quartz boat containing polycrystalline alumina wafer sub￾strates coated with a 2 nm Au film. Au particles are found at the ends of the nanowires, therefore it appears that the Au film spontaneously breaks up to form the catalytic nanoparticles. An arc discharge has been used by Li et al. [34] to form cubic c-Al2O3 nanorods. The source material is a pressed powder mixture of Fe and Al, surprisingly in a 60:40 ratio. Fe catalyst particles are found at the ends of the nanorods. The discharge is run in a mixture of 0.018 MPa Ar (99.9% purity) and 0.008 MPa H2 (99.99% purity). The growth conditions are highly non-equilibrium with the O being supplied at the trace level as an impurity in the process gases. The diameters of the c-Al2O3 nanorods are relatively uniform, ranging from 20 to 30 nm. Carbothermal reduction of a mixture of an Al3+ com￾plex with Fe powder has been used by Jung and Joo [33] to create AlN whiskers. The mixture of the Al complex, Fe and graphite was calcined at 1200–1500 C for 5 h in flowing N2. The whiskers often show modulations in their diameters along the lengths of the whisker and the shapes obtained depend on the growth temperature. This system exhibits rather complex chemistry on account of the carbo￾thermal reduction to produce Al, dissociative adsorption of N2, presumably on the Fe particles, and the formation of Fe/Al/N alloy particles of the appropriate size. Dendritic ZnO nanowires can be grown from Sn catalyst particles as shown by Gao et al. [59]. They heated ZnO and SnO2 powders (1:1 ratio) for 1 h under a pressure of 300– 400 Torr of Ar carrier gas. The nanostructures grew on the top of the inner alumina wall of the tube furnace in a region located downstream 15 cm away from the source material, which was located in the middle of the furnace at 1300 C, and the local growth temperature was in the range of 700–800 C. The Sn particle is significantly larger than the NW diameter. Note that in this system, as in sev￾eral others, there is a high proportion of the catalyst start￾ing material compared to the growth material. This stands in contrast to, for instance, Au catalyzed growth of Si and SiGe nanowires. Also in this system, both the growth and catalyst phases are transported via the vapor phase. Taper￾ing to larger diameters is observed in this system, most likely because the Sn nanoparticles are growing during growth. Xu et al. [53] formed ZnO nanowires with a hexagonal cross section. Interestingly no catalytic particle is found at the tip. Rather the catalytic action is provided by a ZnBiIx thin film (a combination of tetragonal ZnI2 and hexagonal BiI3). Mixtures of either BiI3 and Zn powder or Bi and Zn powders were heated in flowing Ar to 250– 300 C. The presence of I changes the growth direction of the nanowires. The O can be supplied by an impurity in the Ar but no growth is observed in the absence of Bi when only Zn or ZnI2 powders are used. Carbothermal reduction can be used to produce ZnO nanowires on a silicon substrate. Moreover, what Yang et al. have shown is that growth can be switch away from ZnO nanowires to Mn doped ZnSiO4 if MnCl2 Æ 4H2O is added to the reaction mixture. The furnace system was flushed with high-purity Ar gas to eliminate O2 and heated to 1100 C under a constant flow of Ar gas. Then, the quartz boat was placed in the centre of the furnace and held at 1100 C under the same Ar flow. After reaction for 50–60 min at 800–900 C, the Si wafers, which are situ￾ated downstream of the reaction mixture, were coated with a layer of nanowires. The majority of this material is com￾posed of a willemite phase (a-Zn2SiO4 with rhombohedral structure) forming well-aligned nanorods with lengths of 2– 4 lm and diameters of 70–150 nm. The Zn catalyst is con￾sumed so there is a severe taper at the end of the wires and no metal particle is found. Simultaneous growth of ZnO and LiF nanowires has been reported by Jiang et al. [50]. Zn acts as the cata￾lyst when LiF + ZnO powders are heated in Ar to 750–850 C. The downstream deposition region has a temperature in the range of 400–500 C. Cubic-structured single-crystalline LiF nanowires grew along the h001i and h110i crystallographic directions with diameters of 100–500 nm and lengths of tens of microns. The authors propose that there must be a barrier to the incorporation of LiF into the lattice and the Zn particle acts to lower this barrier and cause growth to be preferential at its base. Very similar conditions can be used to grow In2O3, Ga2O3 and SnO2 nanowires. Indeed, In2O3 and SnO2 nanowires can be grown simultaneously without cross con￾tamination or doping as demonstrated by Johnson et al. [45]. Fifty nanometer Au particles were deposited from solution onto the Si substrate placed downstream from high-purity (6 N) metal reactant species (In, Ga, or Sn) placed separately in a quartz boat. The furnace was heated to 800–1000 C under flowing N2. No oxygen was supplied directly to the system other than as an impurity in the N2 or that which desorbs from the surfaces inside the furnace. Rectangular In2O3 rods are capped with a rectangular Au particle that is slightly smaller than the rod, whereas 184 K.W. Kolasinski / Current Opinion in Solid State and Materials Science 10 (2006) 182–191

K w. Kolasinski/ Current Opinion in Solid State and Materials Science 10(2006)182-191 185 Ga2O3 nanowires appear to have a nearly spherical particle In, no detectable Se and is slightly larger than the nano- at their wire. When a 30 nm In film is used for catalysis, no In par Mohammad [60] has grown a variety of Ga and In con- ticle is found at the end of grown nanowires. At the le growth taining nanowires including GaN, InAs, InN, In GaAs, temperature either Au or In would be liquids In GaN, In gaasn using self-catalyzed growth involving Chen et al. [36]have grown CsN nanotubes from pyri either liquid Ga or liquid In droplets. Depending on the dine over a Fe-Co catalyst deposited on y-Al2O3. A mix- conditions either multipronged root growth or single- ture of N2 and pyridine passes over the catalyst while pronged float growth (defined in the next section) are heated to 550-950oC. In this case, the catalyst particles observed. A combination of carrier gas(N2 or H2) and have an unusual conical shape. Not only do the appear NH3, if required, flows over the metallic reagents placed to poke into the growing nanotube, they also are attached in Bn boats. One of the reactant metals may also be placed to the substrate rather than floating to the top of the nano- on the substrate. The distance between boats and the sub- tube even though only one tube grows from one particle. A strate, the flow rates and the temperatures of the boats and tapered particle that pokes into the core of a Cnt has also the substrate are all important variables that affect the been noted by Hofmann et al. [22] during plasma enhanced omposition and characteristics of the nanowires. Autocat- growth from C2H, or CH alytic growth shares much in common with and may actu ally be the mechanism behind what is called vapor-solid (VS)growth, in which no catalyst is intentionally added 3. Mechanisms to the system. Nonetheless, if one of the components is a low melting point metal such as Ga, In or Zn, a liquid cat Let us first start out with some generalities. The singular alyst particle may result during growth even if now none is attribute that usually leads to the conclusion that VLS intentionally added growth has occurred is that a metal particle of roughly Yun et al. [35] have heated a mixture of B+40 wt% the same diameter as the nanowire is found at the end of 9203 (which is a liquid at the growth temperature)in vac- the wire. This, of course, does not determine the phase uum over a 5-20 nm film of Au on Si to 600-950C for of the particle during growth and this controversial aspect 30 min B nanowires coated with an oxide layer are formed. will be dealt with further below Interestingly, a root growth mechanism, in which several Nanowires produced by catalytic growth are often nanowires emanate from a single catalytic particle, occurs found to have a uniform diameter. The wires are not at low temperature and float growth, in which one catalyst always round but might also exhibit other crystallograph particle sits atop each nanowire, occurs at high tempera- cally defined shapes, such as hexagonal ZnO or rectangular ture. The catalyst particle is a Au-B eutectic and dissolu- In,O3. Under some conditions, particularly for long tion of not only B into the Au particles but also the growth times, tapering of diameter to smaller (or less com- interaction of the eutectic with liquid B2O3 may be impor- monly larger) values is found. Often growth requires a bit tant in describing the growth dynamics of bundles of nano- of an induction period before uniform nanowires begin to bes and the switching between root growth and float grow. These considerations are represented schematically growth Laser ablation has been used by Jia et al. [55]to produce The initial period before uniform growth commences is ZnSe nanowires. Contrary to previous experiments involv- associated with any of a number of processes. In some g nanowire growth during laser ablation [61-63], the cases, the catalytic particles must be formed by vapor phase wafer target was not placed in a furnace. The evidence and/or surface diffusion transport or else their surfaces for self-catalytic VLS growth, as suggested by the authors, have to be cleansed of impurities(oxides or terminating thi- is not conclusive ols). The particles may be deposited directly, for instance, ZnTe nanowires with an average diameter of 30 nm from the evaporation of a colloidal solution with a well- and lengths >l um can be grown under the influence of a defined size. Alternatively, a thin film of metal can be evap- Au catalyst. Janik et al. [57] used a MBE source and a 3- orated directly onto a substrate and if the metal does not 20 A film of Au coated onto a GaAs substrate. The nano- wet the substrate, it will ball up into islands either immedi- wires, which are inclined about 55 to the(100)substrate ately as the result of Volmer-Weber growth [64] or else normal, have a zincblende crystal structure and their subsequently when the system is annealed, the onset of ost- growth axis is ( 111. The growth temperature is at slightly wald ripening [65] will lead to a distribution of island sizes above350°C The particles might also result from an evaporation and In2Se] nanowires have been grown either with an Au growth process that occurs during the initial stage, such talyst or with In acting as a self-catalyst by Sun et al. as when carbothermal reduction is used to generate a vol [49]. In2Se3 powder was held at 900-950C and placed atile metal that proceeds to condense elsewhere in the reac- upstream in flowing Ar from a Au or In coated silicon tor. Indeed, catalytic particles form readily under a variety wafer held at 650-700C. The nanowires are 40-80 nm in of conditions from any number of high vapor pressure, low diameter and up to 100 um in length. The spherical Au par- melting point metals. Instead of being the exception, ticle found at the tip of the nanowire contains less than 5% rather seems to be the case that the onus is on an investiga

Ga2O3 nanowires appear to have a nearly spherical particle at their tip. Mohammad [60] has grown a variety of Ga and In con￾taining nanowires including GaN, InAs, InN, InGaAs, InGaN, InGaAsN using self-catalyzed growth involving either liquid Ga or liquid In droplets. Depending on the conditions either multipronged root growth or single￾pronged float growth (defined in the next section) are observed. A combination of carrier gas (N2 or H2) and NH3, if required, flows over the metallic reagents placed in BN boats. One of the reactant metals may also be placed on the substrate. The distance between boats and the sub￾strate, the flow rates and the temperatures of the boats and the substrate are all important variables that affect the composition and characteristics of the nanowires. Autocat￾alytic growth shares much in common with and may actu￾ally be the mechanism behind what is called vapor–solid (VS) growth, in which no catalyst is intentionally added to the system. Nonetheless, if one of the components is a low melting point metal such as Ga, In or Zn, a liquid cat￾alyst particle may result during growth even if now none is intentionally added. Yun et al. [*35] have heated a mixture of B + 40 wt% B2O3 (which is a liquid at the growth temperature) in vac￾uum over a 5–20 nm film of Au on Si to 600–950 C for 30 min. B nanowires coated with an oxide layer are formed. Interestingly, a root growth mechanism, in which several nanowires emanate from a single catalytic particle, occurs at low temperature and float growth, in which one catalyst particle sits atop each nanowire, occurs at high tempera￾ture. The catalyst particle is a Au–B eutectic and dissolu￾tion of not only B into the Au particles but also the interaction of the eutectic with liquid B2O3 may be impor￾tant in describing the growth dynamics of bundles of nano￾tubes and the switching between root growth and float growth. Laser ablation has been used by Jia et al. [55] to produce ZnSe nanowires. Contrary to previous experiments involv￾ing nanowire growth during laser ablation [61–63], the wafer target was not placed in a furnace. The evidence for self-catalytic VLS growth, as suggested by the authors, is not conclusive. ZnTe nanowires with an average diameter of 30 nm and lengths >1 lm can be grown under the influence of a Au catalyst. Janik et al. [57] used a MBE source and a 3– 20 A˚ film of Au coated onto a GaAs substrate. The nano￾wires, which are inclined about 55 to the (1 0 0) substrate normal, have a zincblende crystal structure and their growth axis is h111i. The growth temperature is at slightly above 350 C. In2Se3 nanowires have been grown either with an Au catalyst or with In acting as a self-catalyst by Sun et al. [49]. In2Se3 powder was held at 900–950 C and placed upstream in flowing Ar from a Au or In coated silicon wafer held at 650–700 C. The nanowires are 40–80 nm in diameter and up to 100 lm in length. The spherical Au par￾ticle found at the tip of the nanowire contains less than 5% In, no detectable Se and is slightly larger than the nano￾wire. When a 30 nm In film is used for catalysis, no In par￾ticle is found at the end of grown nanowires. At the growth temperature either Au or In would be liquids. Chen et al. [36] have grown C5N nanotubes from pyri￾dine over a Fe–Co catalyst deposited on c-Al2O3. A mix￾ture of N2 and pyridine passes over the catalyst while heated to 550–950C. In this case, the catalyst particles have an unusual conical shape. Not only do the appear to poke into the growing nanotube, they also are attached to the substrate rather than floating to the top of the nano￾tube even though only one tube grows from one particle. A tapered particle that pokes into the core of a CNT has also been noted by Hofmann et al. [22] during plasma enhanced growth from C2H2 or CH4. 3. Mechanisms Let us first start out with some generalities. The singular attribute that usually leads to the conclusion that VLS growth has occurred is that a metal particle of roughly the same diameter as the nanowire is found at the end of the wire. This, of course, does not determine the phase of the particle during growth and this controversial aspect will be dealt with further below. Nanowires produced by catalytic growth are often found to have a uniform diameter. The wires are not always round but might also exhibit other crystallographi￾cally defined shapes, such as hexagonal ZnO or rectangular In2O3. Under some conditions, particularly for long growth times, tapering of diameter to smaller (or less com￾monly larger) values is found. Often growth requires a bit of an induction period before uniform nanowires begin to grow. These considerations are represented schematically in Fig. 1. The initial period before uniform growth commences is associated with any of a number of processes. In some cases, the catalytic particles must be formed by vapor phase and/or surface diffusion transport or else their surfaces have to be cleansed of impurities (oxides or terminating thi￾ols). The particles may be deposited directly, for instance, from the evaporation of a colloidal solution with a well￾defined size. Alternatively, a thin film of metal can be evap￾orated directly onto a substrate and if the metal does not wet the substrate, it will ball up into islands either immedi￾ately as the result of Volmer–Weber growth [64] or else subsequently when the system is annealed, the onset of Ost￾wald ripening [65] will lead to a distribution of island sizes. The particles might also result from an evaporation and growth process that occurs during the initial stage, such as when carbothermal reduction is used to generate a vol￾atile metal that proceeds to condense elsewhere in the reac￾tor. Indeed, catalytic particles form readily under a variety of conditions from any number of high vapor pressure, low melting point metals. Instead of being the exception, it rather seems to be the case that the onus is on an investiga￾K.W. Kolasinski / Current Opinion in Solid State and Materials Science 10 (2006) 182–191 185

K.w. Kolasinski/ Current Opinion in Solid State and Materials Science 10(2006)182-191 Fig. 2 illustrates several other aspects of growth that Initiation Steady State Termination must be considered. Will the nanowire be produced from deposition I transport to drdt0 of sidewalls ple prong growth ensue, Fig. 2c, in which more than one dndt=o nanowire emanates from each particle or will single-prong growth occur, Fig. 2d? A rather widely reported misunderstanding [1, 66, 67] is that VLS growth occurs because the sticking coeffic Fig. 1. General considerations on the different regimes that occur during on a liquid is unity and must be higher than the sticking talytic growth of es and nanotubes coefficient on the solid. This has also been mistakenly repeated in other fields of structure formation involving growth [68]. For growth of SiNWs from silanes, which tor to show that catalytic particle are not involved in the have a much higher dissociative sticking coefficient on growth process the particle than on the substrate or sidewalls, the reason Once the catalytic particles are formed or deposited, for the greater rate of dissociation is because of the cata- they may still need to be primed for the growth of nano- lytic action of the metal in the particle not the fact that it wires. For instance, the pure metal catalytic particle might is liquid. There is no general evidence for the assertion not be that active for nanowire formation. Instead, an that the sticking coefficient must be larger on a liquid than admixture of the growth compound and the metal might a metal. Second, the assertion does not even apply gener be required to form an (unstable or stable)alloy, a true ally to VLS growth. The success of mBe in semiconduc- eutectic or some other solid/liquid solution. In this case, tor processing relies largely on the fact that the sticking saturation of the catalytic particle with the growth material coefficient of numerous evaporated semiconductor materi or the formation of the proper composition may lead to an als is virtually unity on a solid substrate regardless of its induction period before growth. Note that the incorpora- composition. VLS growth in a MBE configuration, in tion of a significant amount of growth material into the which the growth material is supplied by evaporation catalytic particle is expected to change the volume and, from a crucible unto the substrate, has been demonstrated therefore, the diameter of the particle from its initial value. for nanowires composed of, e.g. Si, SiGe and Ill-V com- Hence Ostwald ripening and incorporation of growth pounds. a sticking coefficient difference alone cannot material can both conspire to change the size of the cata- account for the formation of nanowires. Other factors lytic particles must also be considered that allow the nanowire/catalytic The growth of nanowires with a uniform radius is asso- particle interface to act as a sink for the incorporation of ciated with a steady state growth in which material is trans- new material into the nanowire at a greater rate than the ported to the particle/nanowire interface. If the nanowire growth of the sidewalls or the thickness of the substrate radius is related to the particle radius and the particle in between particle sites. For instance, the catalytic parti radius is constant, a natural explanation for a uniform cle can lower the barrier that is present for the incorpora nanowire diameter is that the particles have reached a tion of new material at the growth interface as compared steady state and their diameter is not evolving in this to the nucleation of an island on a sidewall or the region. If the incorporation of material is directed solely substrate. y the particle and incorporation directly into the sidewalls Fig. 2a and b also illustrate several dynamical process is suppressed, a constant particle diameter leads to a con- that can affect growth. Adsorption occurs from the fluid stant nanowire diameter. The particle might be able to (whether gaseous, liquid or supercritical) phase. Adsorp- affect the size of the nanowire either by direct matching tion might be molecular or dissociative and may either of the size of the nanowire to the size of the particle or else occur (vi) on the nanowire(viii) on the particle, (ix)on by some mechanism involving the curvature of the particle the substrate. A natural way for the catalytic particle to in which strain and lattice matching play a role. direct material to the growth interface is if the sticking Finally, a tapering to smaller diameters and cessation of coefficient(probability of adsorption) is higher on the par growth will occur if either the particle enters a phase in ticle and vanishingly small elsewhere. Diffusion of adatoms which it is consumed, if the growth material is no longer will occur (i)across the substrate(if the sticking probability supplied to the system, or if the temperature is reduced is not negligible), (ii) across the particle and (iii) along the below a critical value. The temperature will play a role in sidewalls. Diffusion across the substrate and along the side- umerous processes, e.g. dissociative adsorption, surface walls must be rapid and cannot lead to nucleation events diffusion, bulk diffusion through the particle, in determin- Nucleation of the nanowire anywhere other than on the ng the composition of the particle by affecting solubilities particle must be suppressed so that growth only occurs at and thermodynamic stability of certain phases as well as the particle/ nanowire interface and so that sidewalls do diffusion of metal atoms away from the particle not grow independently of the axial growth. There may

tor to show that catalytic particle are not involved in the growth process. Once the catalytic particles are formed or deposited, they may still need to be primed for the growth of nano￾wires. For instance, the pure metal catalytic particle might not be that active for nanowire formation. Instead, an admixture of the growth compound and the metal might be required to form an (unstable or stable) alloy, a true eutectic or some other solid/liquid solution. In this case, saturation of the catalytic particle with the growth material or the formation of the proper composition may lead to an induction period before growth. Note that the incorpora￾tion of a significant amount of growth material into the catalytic particle is expected to change the volume and, therefore, the diameter of the particle from its initial value. Hence Ostwald ripening and incorporation of growth material can both conspire to change the size of the cata￾lytic particles. The growth of nanowires with a uniform radius is asso￾ciated with a steady state growth in which material is trans￾ported to the particle/nanowire interface. If the nanowire radius is related to the particle radius and the particle radius is constant, a natural explanation for a uniform nanowire diameter is that the particles have reached a steady state and their diameter is not evolving in this region. If the incorporation of material is directed solely by the particle and incorporation directly into the sidewalls is suppressed, a constant particle diameter leads to a con￾stant nanowire diameter. The particle might be able to affect the size of the nanowire either by direct matching of the size of the nanowire to the size of the particle or else by some mechanism involving the curvature of the particle in which strain and lattice matching play a role. Finally, a tapering to smaller diameters and cessation of growth will occur if either the particle enters a phase in which it is consumed, if the growth material is no longer supplied to the system, or if the temperature is reduced below a critical value. The temperature will play a role in numerous processes, e.g. dissociative adsorption, surface diffusion, bulk diffusion through the particle, in determin￾ing the composition of the particle by affecting solubilities and thermodynamic stability of certain phases as well as diffusion of metal atoms away from the particle. Fig. 2 illustrates several other aspects of growth that must be considered. Will the nanowire be produced from root growth in which the catalyst particle is found at the base of the nanowire, Fig. 2a, or by float growth, in which the particle is located at the tip of the nanowire? Will multi￾ple prong growth ensue, Fig. 2c, in which more than one nanowire emanates from each particle or will single-prong growth occur, Fig. 2d? A rather widely reported misunderstanding [1,66,67] is that VLS growth occurs because the sticking coefficient on a liquid is unity and must be higher than the sticking coefficient on the solid. This has also been mistakenly repeated in other fields of structure formation involving growth [68]. For growth of SiNWs from silanes, which have a much higher dissociative sticking coefficient on the particle than on the substrate or sidewalls, the reason for the greater rate of dissociation is because of the cata￾lytic action of the metal in the particle not the fact that it is liquid. There is no general evidence for the assertion that the sticking coefficient must be larger on a liquid than a metal. Second, the assertion does not even apply gener￾ally to VLS growth. The success of MBE in semiconduc￾tor processing relies largely on the fact that the sticking coefficient of numerous evaporated semiconductor materi￾als is virtually unity on a solid substrate regardless of its composition. VLS growth in a MBE configuration, in which the growth material is supplied by evaporation from a crucible unto the substrate, has been demonstrated for nanowires composed of, e.g. Si, SiGe and III–V com￾pounds. A sticking coefficient difference alone cannot account for the formation of nanowires. Other factors must also be considered that allow the nanowire/catalytic particle interface to act as a sink for the incorporation of new material into the nanowire at a greater rate than the growth of the sidewalls or the thickness of the substrate in between particle sites. For instance, the catalytic parti￾cle can lower the barrier that is present for the incorpora￾tion of new material at the growth interface as compared to the nucleation of an island on a sidewall or the substrate. Fig. 2a and b also illustrate several dynamical process that can affect growth. Adsorption occurs from the fluid (whether gaseous, liquid or supercritical) phase. Adsorp￾tion might be molecular or dissociative and may either occur (vii) on the nanowire (viii) on the particle, (ix) on the substrate. A natural way for the catalytic particle to direct material to the growth interface is if the sticking coefficient (probability of adsorption) is higher on the par￾ticle and vanishingly small elsewhere. Diffusion of adatoms will occur (i) across the substrate (if the sticking probability is not negligible), (ii) across the particle and (iii) along the sidewalls. Diffusion across the substrate and along the side￾walls must be rapid and cannot lead to nucleation events. Nucleation of the nanowire anywhere other than on the particle must be suppressed so that growth only occurs at the particle/nanowire interface and so that sidewalls do not grow independently of the axial growth. There may time Initiation deposition nucleation saturation dr/dt > 0 Steady State transport to growth interface passivation of sidewalls dr/dt = 0 Termination dr/dt < 0 Fig. 1. General considerations on the different regimes that occur during catalytic growth of nanowires and nanotubes. 186 K.W. Kolasinski / Current Opinion in Solid State and Materials Science 10 (2006) 182–191

KW. Kolasinski Current Opinion in Solid State and Materials Science 10(2006)182-191 b root growth float growth d multiprong single prong Fig. 2. The processes that occur during catalytic growth.() In root growth, the particle stays at the bottom of the nanowire (b) In float growth, the rticle remains at the top of the nanowire. (c) In multiple prong growth, more than one nanowire grows from one particle and the nanowires must cessarily have a smaller radius than the particle. (d) In single-prong growth, one nanowire corresponds to one particle. One of the surest signs of this lode is that the particle and na have very similar radii. be(vi)diffusion of material through the catalytic particle in example, there have been several reports on the growth addition to(ii) diffusion along its surface. Substrate atoms of Ill-V nanowires -such as GaAs, GaP, InAs and might also be mobile. They might (v) enter the particle InP-that have invoked either a solid [41, 42, 44, 69, 70 directly or else (iv) surface diffusion along the substrate or liquid [43, 47, 71] catalyst particle. Similarly, during can deliver them to the surface of the particle. Not shown the growth of carbon nanotubes or fibres both liquid [21 in the diagram is that atoms from the catalytic particle and solid [23] catalyst particles have been reported. What might also be mobile and diffuse along the sidewalls and is clear from these reports is that there are circumstances the substrate under which the catalyst can be liquid and others in which a Four major distinctions in the growth process are illus- it can be solid and the nanowires that result do not appear ted in Fig. 2: root vs. float growth and multiprong vs. to be materially affected. In other words, while it is impor single-prong growth. The particle may either end up at tant for characterizing the growth mechanism, as far as the the bottom(root growth) or top(float growth)of the nano- dynamics of nanowire and nanotube formation are con- wire. In multiprong growth, Fig 2c, more than one nano- cerned, it does not appear to matter whether the catalytic wire grows from a single particle. In this case, the radius of particle is liquid or solid. Therefore, any mechanism that the nanowire rw must be less than the radius of the catalytic relies upon a particular phase for the catalyst cannot be gen- particle rp. In single-prong growth there is a one-to-one erally true. correspondence between particles and nanowires. A natu Likewise, the phase from which the growth material is ral means to exercise control over the nanowire diameter taken is of little consequence. The growth material may in single-prong growth would be if the nanowire radius come from a gas that is unreactive on the substrate and determines this value and w A Tp. Here is should be men- only reactive on the catalyst surface, such as in the case tioned that in single-prong growth, IwArp is usually of SiNW growth from silane. It may come from an atomic observed but that the catalyst particle sometimes is signif- vapor that has unit sticking probability on both the sub- icantly larger and occasionally is somewhat smaller than strate and the catalyst, as in mBe growth of SiNW or the nanowire radius. In multiprong growth rw is not deter- III-V compounds. It may come from a plasma, a solution mined directly by rp but must be related to other structural or even supercritical fluids. Hence, any model that requires factors such as the curvature of the growth interface and a particular phase for the growth material cannot be gener- lattice matching between the catalytic particle and the ally valid. What is required is that the growth material is nanowire mobile and can readily reach the growth interface with a Quite a bit of discussion has revolved around whether low probability of nucleating a crystallite(alternate growth he catalyst particle is liquid or solid. In other front)anywhere other than at the nanowire/catalyst whether the mechanism of growth is VlS or VSS. As an interface

be (vi) diffusion of material through the catalytic particle in addition to (ii) diffusion along its surface. Substrate atoms might also be mobile. They might (v) enter the particle directly or else (iv) surface diffusion along the substrate can deliver them to the surface of the particle. Not shown in the diagram is that atoms from the catalytic particle might also be mobile and diffuse along the sidewalls and the substrate. Four major distinctions in the growth process are illus￾trated in Fig. 2: root vs. float growth and multiprong vs. single-prong growth. The particle may either end up at the bottom (root growth) or top (float growth) of the nano￾wire. In multiprong growth, Fig. 2c, more than one nano￾wire grows from a single particle. In this case, the radius of the nanowire rw must be less than the radius of the catalytic particle rp. In single-prong growth there is a one-to-one correspondence between particles and nanowires. A natu￾ral means to exercise control over the nanowire diameter in single-prong growth would be if the nanowire radius determines this value and rw  rp. Here is should be men￾tioned that in single-prong growth, rw  rp is usually observed but that the catalyst particle sometimes is signif￾icantly larger and occasionally is somewhat smaller than the nanowire radius. In multiprong growth rw is not deter￾mined directly by rp but must be related to other structural factors such as the curvature of the growth interface and lattice matching between the catalytic particle and the nanowire. Quite a bit of discussion has revolved around whether the catalyst particle is liquid or solid. In other words, whether the mechanism of growth is VLS or VSS. As an example, there have been several reports on the growth of III–V nanowires – such as GaAs, GaP, InAs and InP – that have invoked either a solid [*41,42,44,69,70] or liquid [*43,*47,71] catalyst particle. Similarly, during the growth of carbon nanotubes or fibres both liquid [21] and solid [23] catalyst particles have been reported. What is clear from these reports is that there are circumstances under which the catalyst can be liquid and others in which it can be solid and the nanowires that result do not appear to be materially affected. In other words, while it is impor￾tant for characterizing the growth mechanism, as far as the dynamics of nanowire and nanotube formation are con￾cerned, it does not appear to matter whether the catalytic particle is liquid or solid. Therefore, any mechanism that relies upon a particular phase for the catalyst cannot be gen￾erally true. Likewise, the phase from which the growth material is taken is of little consequence. The growth material may come from a gas that is unreactive on the substrate and only reactive on the catalyst surface, such as in the case of SiNW growth from silane. It may come from an atomic vapor that has unit sticking probability on both the sub￾strate and the catalyst, as in MBE growth of SiNW or III–V compounds. It may come from a plasma, a solution or even supercritical fluids. Hence, any model that requires a particular phase for the growth material cannot be gener￾ally valid. What is required is that the growth material is mobile and can readily reach the growth interface with a low probability of nucleating a crystallite (alternate growth front) anywhere other than at the nanowire/catalyst interface. Fig. 2. The processes that occur during catalytic growth. (a) In root growth, the particle stays at the bottom of the nanowire. (b) In float growth, the particle remains at the top of the nanowire. (c) In multiple prong growth, more than one nanowire grows from one particle and the nanowires must necessarily have a smaller radius than the particle. (d) In single-prong growth, one nanowire corresponds to one particle. One of the surest signs of this mode is that the particle and nanowire have very similar radii. K.W. Kolasinski / Current Opinion in Solid State and Materials Science 10 (2006) 182–191 187

K.w. Kolasinski/ Current Opinion in Solid State and Materials Science 10(2006)182-191 leads us to contemplate what is the nature of the on the growth parameters(pressure and temperature)but is. Originally, Wagner and Ellis figured that the also the diameter of the catalyst particle. It is often found s was of an ordinary variety. The metal particle cat- that, the growth rate should decrease with decreasing dia alyzed the decomposition of the molecule that supplied the meter [67, 76]. However, this conclusion depends on the growth material. But we see that this does not have to be growth conditions [46]since the extent of supersaturation the case for catalytic nanowire growth to proceed. The acti- within the catalyst depends on the temperature and gas- vation energy of growth can be associated with activated phase composition. A transition from smaller diameters adsorption or with surface or bulk diffusion. However, having lower growth rates to smaller diameter having the essential role of the catalyst appears to be in lowering higher growth rates can occur as temperature and gas- the activation energy of nucleation at the axial growth inter- phase composition are changed face. There is a substantial barrier associated with the for Gosele and co-workers [66, 77, "78] have examined mation of the critical nucleation cluster at a random whether there is a thermodynamically determined mini- position on the substrate or nanowire according to classical mum size and the parameters that affect not only the nano- nucleation theory. If the catalyst can lower the nucleation wire size but also the size of the catalytic particle. During barrier at the particle/nanowire interface, then growth only SiNw growth in the presence of a Au catalyst, the catalyst occurs there. Any of a number of processes may be the rate minimum size is determined by the vapor pressures of si determining step depending on the exact conditions, but and the metal. The SiNW minimum determined by the most important role of the catalyst particle is to ensure the catalyst composition and its size. They arrive at the that material is preferentially incorporated at the growth conclusion that there is no thermodynamically determined interface minimum size. rather that the minimum size is determined Virtually all of the theoretical work on catalytic growth by kinetic limitations. In the case of SiNWs, Tan et al. [77] has treated the VLS mechanism explicitly. Therefore, there predict that the catalyst should be larger than the nano- is little information on why root growth and multiprong wire, but this is clearly not universally true for all materials. growth occur under some circumstances. We do not know Chandrasekaran et al. [32] have attacked the problem why a system such as the growth of B nanowires studied by of trying to explain the nucleation of uniformly sized nano- Yun et al. [35]changes from multipronged root growth at wires in multipronged root growth. They specifically trea low temperatures to single-pronged float growth at high ted the case of GeNw growth from Ga particles and temperatures. Root growth is also observed by Chen developed an expression for the nanowire diameter et al. [36] for CsN nanotubes grown from an Fe-Co cata- lyst but in this case the growth is single-pronged. Zno do can exhibit root growth as shown by Xu et al. [53] for a Zn-B-I thin film catalyst though it is more conventionally where dc is the diameter of the resulting nanowire, Vm is the grown in a float growth mode with, for instance, a Sn cat- molar volume, o is the interfacial energy, and XGe/XG is alyst [59]. Zhang et al. [27]report that both SiOx nanowires the ratio of solute concentration at the point of instability and SnO2 nanobelts form via multiprong root growth from to the corresponding equilibrium solubility at a given tem- an Sn/Si/o alloy catalyst whereas Al, Ni, Cu, Pd, Ag and perature, T. The agreement with experiment is reasonable Au can all be used for float growth of SnO2 nanowires as and predicts that the nanowire diameter should depend shown by Nguyen, Ng and Meyyappan[52]. on the temperature and the metal used for catalysis Theoretical approaches to nanowire growth can be sep important conclusion of th arated into at least three different categories: molecular studies of Ding et al. [24, 72, 73] is that a thermal gradient dynamics, thermodynamics and kinetics. Molecular is not required for the growth of single-walled carbon dynamics has been used, for instance, by Rosen and co- nanotubes. Their study shows that a concentration gradi workers [24, 72, 73] to examine the growth of CNTs Kinet- ent is more important than a thermal gradient for the ics(mass-transport) based models have been considered by growth of Sw-CNTs on small metal particles. Further Verheijen et al. [40]. Dubrovskii et al. [15]. Tersoff and co- more, SW-CNTs growth can occur in the presence of an workers [14, 16], Persson et al. [44]and Johansson et al. opposing thermal gradient, ie, the SW-CNTs grow from [41, 46]. Thermodynamic approaches trace back to Blak- the hot region of the catalyst particle ely and Jackson [74] as well as Givargizov [67] and more Experimental studies of growth kinetics are not numer- recently have been treated by Kwon and Park [75]. Wang ous. Verheijen et al. [40] have studied the growth of GaP et al. [12]. Chandrasekaran et al. [ 32] Chen and Cao and GaAs in heterostructured GaP-GaAs nanowires [76]. Mohammad [60], and Tan, Li and Gosele [66,77, 781. GaAsNWs exhibit diffusion-limited growth wherein the The Gibbs-Thomson effect expresses how a curved rate is determined by the partial pressure of both reagents interface affects the chemical potential of a body. This (trimethyl gallium or AsH3)but has little dependence on causes the vapor pressure and solubilities to become depen- the temperature. In what might be called classical VLS dent on the size of a catalyst particle. Thermodynamic behaviour, they show that activated PH3 dissociation on treatments are then able to show how the Gibbs-Thomson the Au catalyst following Langmuir-Hinshelwood kinetics effect leads to nanowire growth rates that depend not only is the rate determining step for GaP growth. Sidewall

This leads us to contemplate what is the nature of the catalysis. Originally, Wagner and Ellis figured that the catalysis was of an ordinary variety. The metal particle cat￾alyzed the decomposition of the molecule that supplied the growth material. But we see that this does not have to be the case for catalytic nanowire growth to proceed. The acti￾vation energy of growth can be associated with activated adsorption or with surface or bulk diffusion. However, the essential role of the catalyst appears to be in lowering the activation energy of nucleation at the axial growth inter￾face. There is a substantial barrier associated with the for￾mation of the critical nucleation cluster at a random position on the substrate or nanowire according to classical nucleation theory. If the catalyst can lower the nucleation barrier at the particle/nanowire interface, then growth only occurs there. Any of a number of processes may be the rate determining step depending on the exact conditions, but the most important role of the catalyst particle is to ensure that material is preferentially incorporated at the growth interface. Virtually all of the theoretical work on catalytic growth has treated the VLS mechanism explicitly. Therefore, there is little information on why root growth and multiprong growth occur under some circumstances. We do not know why a system such as the growth of B nanowires studied by Yun et al. [*35] changes from multipronged root growth at low temperatures to single-pronged float growth at high temperatures. Root growth is also observed by Chen et al. [36] for C5N nanotubes grown from an Fe–Co cata￾lyst but in this case the growth is single-pronged. ZnO can exhibit root growth as shown by Xu et al. [53] for a Zn-B-I thin film catalyst though it is more conventionally grown in a float growth mode with, for instance, a Sn cat￾alyst [59]. Zhang et al. [27] report that both SiOx nanowires and SnO2 nanobelts form via multiprong root growth from an Sn/Si/O alloy catalyst whereas Al, Ni, Cu, Pd, Ag and Au can all be used for float growth of SnO2 nanowires as shown by Nguyen, Ng and Meyyappan [*52]. Theoretical approaches to nanowire growth can be sep￾arated into at least three different categories: molecular dynamics, thermodynamics and kinetics. Molecular dynamics has been used, for instance, by Rose´n and co￾workers [24,72,73] to examine the growth of CNTs. Kinet￾ics (mass-transport) based models have been considered by Verheijen et al. [40], Dubrovskii et al. [*15], Tersoff and co￾workers [14,16], Persson et al. [44] and Johansson et al. [*41,*46]. Thermodynamic approaches trace back to Blak￾ely and Jackson [74] as well as Givargizov [67] and more recently have been treated by Kwon and Park [75], Wang et al. [12], Chandrasekaran et al. [*32], Chen and Cao [76], Mohammad [60], and Tan, Li and Go¨sele [66,77,*78]. The Gibbs–Thomson effect expresses how a curved interface affects the chemical potential of a body. This causes the vapor pressure and solubilities to become depen￾dent on the size of a catalyst particle. Thermodynamic treatments are then able to show how the Gibbs–Thomson effect leads to nanowire growth rates that depend not only on the growth parameters (pressure and temperature) but also the diameter of the catalyst particle. It is often found that, the growth rate should decrease with decreasing dia￾meter [67,76]. However, this conclusion depends on the growth conditions [*46] since the extent of supersaturation within the catalyst depends on the temperature and gas￾phase composition. A transition from smaller diameters having lower growth rates to smaller diameter having higher growth rates can occur as temperature and gas￾phase composition are changed. Go¨sele and co-workers [66,77,*78] have examined whether there is a thermodynamically determined mini￾mum size and the parameters that affect not only the nano￾wire size but also the size of the catalytic particle. During SiNW growth in the presence of a Au catalyst, the catalyst minimum size is determined by the vapor pressures of Si and the metal. The SiNW minimum size is determined by the catalyst composition and its size. They arrive at the conclusion that there is no thermodynamically determined minimum size, rather, that the minimum size is determined by kinetic limitations. In the case of SiNWs, Tan et al. [77] predict that the catalyst should be larger than the nano￾wire, but this is clearly not universally true for all materials. Chandrasekaran et al. [*32] have attacked the problem of trying to explain the nucleation of uniformly sized nano￾wires in multipronged root growth. They specifically trea￾ted the case of GeNW growth from Ga particles and developed an expression for the nanowire diameter dc ¼ 4V mr RT lnðX Ge=Xl GeÞ where dc is the diameter of the resulting nanowire, Vm is the molar volume, r is the interfacial energy, and X Ge=Xl Ge is the ratio of solute concentration at the point of instability to the corresponding equilibrium solubility at a given tem￾perature, T. The agreement with experiment is reasonable and predicts that the nanowire diameter should depend on the temperature and the metal used for catalysis. An important conclusion of the molecular dynamics studies of Ding et al. [24,72,73] is that a thermal gradient is not required for the growth of single-walled carbon nanotubes. Their study shows that a concentration gradi￾ent is more important than a thermal gradient for the growth of SW-CNTs on small metal particles. Further￾more, SW-CNTs growth can occur in the presence of an opposing thermal gradient, ie, the SW-CNTs grow from the hot region of the catalyst particle. Experimental studies of growth kinetics are not numer￾ous. Verheijen et al. [40] have studied the growth of GaP and GaAs in heterostructured GaP–GaAs nanowires. GaAsNWs exhibit diffusion-limited growth wherein the rate is determined by the partial pressure of both reagents (trimethyl gallium or AsH3) but has little dependence on the temperature. In what might be called classical VLS behaviour, they show that activated PH3 dissociation on the Au catalyst following Langmuir–Hinshelwood kinetics is the rate determining step for GaP growth. Sidewall 188 K.W. Kolasinski / Current Opinion in Solid State and Materials Science 10 (2006) 182–191

K w. Kolasinski/ Current Opinion in Solid State and Materials Science 10(2006)182-191 growth and axial growth can exhibit different temperature the rate of adsorption, the Gibbs-Thomson correction for dependencies and an observed decrease in axial growth rate the finite curvature of the surface, diffusion-induced Nw for both Gap above 500C and GaAs above 440C is growth, and the nucleation-mediated growth at the related to increased sidewall growth. Sidewall growth is liquid-solid interface with allowances for tapering of the also involved in producing tapered nanowires. The sub- nanowires. This enables them to make very specific predic strate can play an active role in the growth kinetics. Here tions. For example, the tapering of GaasNw grown by the Sioz substrate suppresses epitaxial growth, whereas MBE on the GaAs(1 l l)B surface at T=590C can be epitaxial growth is competitive with nanowire growth on explained by the reduction in the Ga flux to the catalyst III-V substrates caused by the high rate of desorption of Ga atoms from Extensive studies of growth kinetics and the develop- the sidewalls. Tapering due to the nucleation and growth ment of a mass-transport model have also been performed of islands on the gaAs(1 10) sidewalls is not thought to in Lund [41, 44, * 46,70]. Seifert and co-workers [46] have be significant since the activation barrier for nucleation is developed a model of a nanowire growing from a metal too high. They also are able to reproduce well growth rates particle which need not be liquid. They make the following as a function of nanowire diameter so that they can fit data assumptions:(i) the metal particle is assumed to be hemi- regarding the dependence of nanowire diameter on nano- spherical, (ii) there is steady state adatom diffusion on the wire length and the nanowire length(for a given diameter) substrate and nanowire sides toward the metal particle, as a function of growth temperature 11) the processes within the metal particle(diffusion) as well as at the metal-semiconductor interface (nucleation) 4. Conclusion need not be considered in detail, and (iv) the interwire sep- aration is fairly large. The authors note that assumption Characterization studies of catalytic growth are demon- (iii) limits the generality of the model because there are rating an ever larger array of systems that exhibit the cases in which these processes are rate determining. None- growth of nanowires and nanotubes. Our understanding theless, for the specific case of lll/V nanowire growth, of catalytic growth, regardless of whether it is labelled experimental growth conditions can easily be set to validate VLS, VSS, SLS, SCLS or otherwise, is becoming increas- these assumptions ingly sophisticated. However, may questions still remain Their model describes a mass-transport-limited system unanswered particularly regarding the size and position in which deposition occurs on the substrate as well as of the catalyst relative to the nanowire, in other words, on the nanowire walls and the metal particle. Growth is regarding aspects of float versus root growth and single favoured at the metal/ semiconductor interface, which acts prong versus multiprong growth and why the particle can as a sink, and is kinetically hindered on the substrate and be sometimes larger and sometimes smaller than the nano- walls. Transport via diffusion across the sub- wire. It is now clear that growth can occur by autocatalysis strate and up the walls plays a vital role. The microscopic as well as by catalysis due to a foreign metal/alloy particle details of this model are sketchy and the explanation for The catalytic particle can be either liquid or solid but this why incorporation does not occur on the substrate and does not really change the dynamics of growth. The result- walls is vague, nonetheless its phenomenological descrip- ing nanowires can be unitary, binary, ternary or even qua- tion of length versus radius for various temperatures is ternary compounds and they have been produced either pure or in doped forms. In some cases, more than one type Tersoff and co-workers have investigated the Au cata- of nanowire can be grown simultaneously. Catalytic lyzed VLS growth of SiNWs [14, 16]. They use disilane growth represents a powerful method for producing ID (Si2H6) as the source gas at 10-10 Torr. A thin film nanostructures and there certainly are many more surprises Au(2-3 nm) is deposited on a Si(ll 1)substrate, which to be discovered in this area of research is then heated to 500-650C. No detectable dependence of growth rate on the wire diameter is observed. They ascribe this rather unexpected result to the irreversible character of References dissociative adsorption on the catalyst surface, which is the rate determining step for these conditions. They have also The papers of particular interest have been highlighted observed periodic sawtooth faceting of SiNW walls during growth. Growth occurs at the(111)facet at the end of the of special interest. wire, however, the size and shape of this facet oscillates [ u Wagner RS, Ellis WC. Vapor-liquid-solid mechanism of single crystal growth. Appl Phys Lett 1964; 4: 89-90 forces generated by the surface energies of the wire and [2] De Jong KP, Geus JW. Carbon nanofibers: catalytic synthesis and the droplet. They also predict that this will be observed applications. Catal Rev 2000: 42: 481-510 in any system in which the orientations parallel to the [] Anantram MP, Leonard F. Physics of carbon nanotube electronic growth direction are not stable devices. Rep Prog Phys 2006: 69: 507-61 4 Ajayan PM. Nanotubes from carbon Chem Rev 1999: 99: 1787-99 Perhaps the most general scheme to explain nanowire [5] Dai H. Carbon nanotubes: opportunities and challenges. Surf Sci growth is that of Dubrovskii et al. [15]. They incorporate

growth and axial growth can exhibit different temperature dependencies and an observed decrease in axial growth rate for both GaP above 500 C and GaAs above 440 C is related to increased sidewall growth. Sidewall growth is also involved in producing tapered nanowires. The sub￾strate can play an active role in the growth kinetics. Here the SiO2 substrate suppresses epitaxial growth, whereas epitaxial growth is competitive with nanowire growth on III–V substrates. Extensive studies of growth kinetics and the develop￾ment of a mass-transport model have also been performed in Lund [*41,44,*46,70]. Seifert and co-workers [*46] have developed a model of a nanowire growing from a metal particle which need not be liquid. They make the following assumptions: (i) the metal particle is assumed to be hemi￾spherical, (ii) there is steady state adatom diffusion on the substrate and nanowire sides toward the metal particle, (iii) the processes within the metal particle (diffusion) as well as at the metal-semiconductor interface (nucleation) need not be considered in detail, and (iv) the interwire sep￾aration is fairly large. The authors note that assumption (iii) limits the generality of the model because there are cases in which these processes are rate determining. None￾theless, for the specific case of III/V nanowire growth, experimental growth conditions can easily be set to validate these assumptions. Their model describes a mass-transport-limited system in which deposition occurs on the substrate as well as on the nanowire walls and the metal particle. Growth is favoured at the metal/semiconductor interface, which acts as a sink, and is kinetically hindered on the substrate and nanowire walls. Transport via diffusion across the sub￾strate and up the walls plays a vital role. The microscopic details of this model are sketchy and the explanation for why incorporation does not occur on the substrate and walls is vague, nonetheless its phenomenological descrip￾tion of length versus radius for various temperatures is quite good. Tersoff and co-workers have investigated the Au cata￾lyzed VLS growth of SiNWs [14,16]. They use disilane (Si2H6) as the source gas at 108 –105 Torr. A thin film of Au (2–3 nm) is deposited on a Si(1 1 1) substrate, which is then heated to 500–650 C. No detectable dependence of growth rate on the wire diameter is observed. They ascribe this rather unexpected result to the irreversible character of dissociative adsorption on the catalyst surface, which is the rate determining step for these conditions. They have also observed periodic sawtooth faceting of SiNW walls during growth. Growth occurs at the (1 1 1) facet at the end of the wire; however, the size and shape of this facet oscillates periodically. They explain this based on the balance of forces generated by the surface energies of the wire and the droplet. They also predict that this will be observed in any system in which the orientations parallel to the growth direction are not stable. Perhaps the most general scheme to explain nanowire growth is that of Dubrovskii et al. [*15]. They incorporate the rate of adsorption, the Gibbs–Thomson correction for the finite curvature of the surface, diffusion-induced NW growth, and the nucleation-mediated growth at the liquid–solid interface with allowances for tapering of the nanowires. This enables them to make very specific predic￾tions. For example, the tapering of GaAsNW grown by MBE on the GaAs(1 1 1)B surface at T = 590 C can be explained by the reduction in the Ga flux to the catalyst caused by the high rate of desorption of Ga atoms from the sidewalls. Tapering due to the nucleation and growth of islands on the GaAs(1 1 0) sidewalls is not thought to be significant since the activation barrier for nucleation is too high. They also are able to reproduce well growth rates as a function of nanowire diameter so that they can fit data regarding the dependence of nanowire diameter on nano￾wire length and the nanowire length (for a given diameter) as a function of growth temperature. 4. Conclusion Characterization studies of catalytic growth are demon￾strating an ever larger array of systems that exhibit the growth of nanowires and nanotubes. Our understanding of catalytic growth, regardless of whether it is labelled VLS, VSS, SLS, SCLS or otherwise, is becoming increas￾ingly sophisticated. However, may questions still remain unanswered particularly regarding the size and position of the catalyst relative to the nanowire, in other words, regarding aspects of float versus root growth and single￾prong versus multiprong growth and why the particle can be sometimes larger and sometimes smaller than the nano￾wire. It is now clear that growth can occur by autocatalysis as well as by catalysis due to a foreign metal/alloy particle. The catalytic particle can be either liquid or solid but this does not really change the dynamics of growth. The result￾ing nanowires can be unitary, binary, ternary or even qua￾ternary compounds and they have been produced either pure or in doped forms. In some cases, more than one type of nanowire can be grown simultaneously. Catalytic growth represents a powerful method for producing 1D nanostructures and there certainly are many more surprises to be discovered in this area of research. 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