1-Dimensional Nanostructures:Nanowires and Nanorods

1-dimensional nanostructures are called with many names: whiskers,fibers or fibrils, nanowires and nanorods. Also nanotubules, nanotubesand nanocables are considered 1-dimensional structures. 1-dimensionalnanostructures of variuos materials are commonly produced andstudied such as metals, semiconductors, polymers and insulatingoxides.

Many techniques have been developed in the synthesis andformation of one-dimensional nanostructured materials:

(1) Spontaneous growth:(a) Evaporation (or dissolution)-condensation (VS)(b) Vapor (or solution)-liquid-solid (VLS or SLS) growth(c) Stress-induced recrystallization

(2) Template-based synthesis:(a) Electroplating and electrophoretic deposition(b) Colloid dispersion, melt, or solution filling(c) Conversion with chemical reaction

(3) Electrospinning

(4) Lithography

Bottom-up

Top-down

Spontaneous growth

Spontaneous growth is a process driven by the reduction of Gibbsfree energy or chemical potential. The reduction of Gibbs free energyis commonly realized by phase transformation or chemical reactionor the release of stress.For the formation of nanowires or nanorods, anisotropic growth isrequired which means that the crystal grows along a certainorientation faster than other directions.Defects and impurities on the growth surfaces can play a significantrole in determining the morphology of the final products.

The main type of spontaneous growth are:

A) Evaporation-Condensation (Vapor-Solid process (VS))

B) Vapor-Liquid-Solid (VLS)

Crystal growthCrystal growth can be generally considered as a heterogeneousreaction. The main steps are:

(1) Diffusion of growth species from the bulk to the growing surface.(2) Adsorption and desorption of growth species onto and from thegrowing surface.(3) Surface diffusion of adsorbed growth species.(4) Surface growth by irreversibly incorporating the adsorbed growthspecies into the crystal structure.(5) Desorption of reaction by-products from the growth surface.(6) By-product chemicals diffuse away from the surface.

For most crystal growth, rate-limiting step is either adsorption-desorption of growth species on the growth surface (step 2) or surfacegrowth (step 4).

Adsorption-desorption of growth speciesThe growth rate is determined by condensation rate, J (atoms/cm2sec),which depends on the number of growth species adsorbed onto thesurface, which is directly proportional to the vapor pressure orconcentration, P of the growth species in the vapor:

is the accommodation coefficient, = (P-P0)/P0 is the supersaturationof the growth species in the vapor, P0 is the equilibrium vapor pressureof the crystal at temperature T, m is the atomic weight of the growthspecies and k is Boltzmann constant. Growth rate increases linearlywith the increase in the concentration of growth species.

is the fraction of impinging growth species that becomesaccommodated on the growing surface, and is a surface specificproperty. A surface with a high accommodation coefficient will havea high growth rate.

Surface growthIf a high concentration of growth species is available then the surfacegrowth becomes a limiting step, the growth rate becomes independentof the concentration of growth species.The high concentration may result in a secondary nucleation on thegrowth surface or even homogeneous nucleation, which can effectivelyterminate the epitaxial or single crystal growth.An impinging growth species onto thegrowth surface can be described interms of the residence time and/ordiffusion distance before escapingback to the vapor phase:

s is the residence time, Ds isthe surface diffusioncoefficient, is the vibrationalfrequency of the adatom (10-12s),Edes is the desorption energy toescape back to the vapor, a0 isthe size of the growth species andEs is the activation energy forsurface diffusion.

The mean diffusion distance, X for a growth species from the siteof incidence:

On a crystal surface if the mean diffusion distance is far longerthan the distance between two growth sites (kinks or ledges), alladsorbed growth species will be incorporated into the crystalstructure and the accommodation coefficient () would be unity.If the mean diffusion distance is far shorter than the distancebetween growth sites, all adatoms will escape back to the vapor andthe accommodation coefficient will be zero.

KSV theoryFor a flat surface, the classic step-growth theory was developed by Kossel,Stranski and Volmer which is known as the KSV theory.Crystal surface, on the atomic scale, is not smooth, flat or continuous,and discontinuities are responsible for the crystal growth.An atom adsorbed onto the surface diffuses randomly and if itencounters an energetically favorable site, it will be irreversiblyincorporated into the crystal structure, resulting in the growth of thesurface.If the atom (adatom) is on a terrace it forms one chemical bond, this is athermodynamically unstable state.In a ledge site, two chemical bonds.In a ledge-kink site, three chemical bonds. In a kink site, four chemical bonds.

Ledge, ledge-kink and kink sites are all good growth sites; incorporationof atoms into these sites is irreversible and results in growth of the surface.

The growth of a flat surface is due to the advancement of the steps (orledges). An increased step density favors the crystal growth.Screw dislocations serve as continuous sources to generate growthsites so that the stepped growth would continue.The crystal growth proceeds in a spiral growth, and this mechanism is known as BCF (Burton, Cabrera and Frank) theory.

For crystals the different facets can have a significantly differentability to accommodate dislocations. The presence of dislocations ona certain facet can result in anisotropic growth, leading to theformation of nanowires or nanorods.

PBC theoryAccording to Periodic Bond Chain (PBC) theory all crystal facets can becategorized into three groups based on the number of broken periodicbond chains on a given facet: flat surface, stepped surface and kinkedsurface.For a simple cubic crystal:{100} faces are flat surfaces (denoted as F-face) with one PBC running through one such surface{110} are stepped surfaces (S-face) that have two PBCs{111} are kinked surfaces (K-face) that have three PBCs.Both {110} and {111} faces have faster growth rate than that of {100} surface in a simple cubic crystal.In a general term, S-faces and K-faces have a higher growth rate than F-faces.Different growth rates of various facets is not the only mechanism. Defect-induced growth and impurity-inhibited growthmechanisms are usuallyinvolved in nanorods growth.

Evaporation-Condensation growth

Different types of twinned microstructures observed in goldnanowires grown by wet chemistry. (a) Defect‐free circularnanowire. (b) Periodically‐twinned circular nanowire withconstant twin boundary spacing (TBS). (c) Periodically‐twinned nanowire with zigzag surface morphology made of{111} facets and constant TBS.

Stacking faults are planar defects formedwhen regular sequence of stacking isdisturbed

Many processes are involved in the anisotropic growth of nanorods byevaporation-condensation method. In general the impurities havedifferential adsorption on various crystal facets of a crystal and theadsorption of impurity can retard the growth process. In particular:-Presence of axial screw dislocations.-Micro-twins and stacking faults.-Dislocation-diffusion (growth rate exceeds the condensation rate on aflat surface).

No nanorods have been grown byvapor condensation methods basedon impurity poisoning by design.

Growth of single crystal nanobelts of varioussemiconducting oxides (ZnO, SnO2, In2O3, CdO)by evaporating metal oxides at hightemperatures under a vacuum of 300 torr andcondensing on an alumina substrate, placedinside the same alumina tube furnace, at lowtemperatures.

ZnO

Typical thickness and width-to-thickness ratios of the ZnO nanobeltsare in the range of 10 to 30nm and 5to 10, respectively. Two growthdirections were observed: [0001] and[01-10]. No screw dislocation wasfound throughout the entire length ofthe nanobelt, except a single stackingfault parallel to the growth axis in thenanobelts grown along [01-10]direction. The surfaces of thenanobelts are clean, atomically sharpand free of any sheathed amorphousphase.

By controlling growth kinetics, left-handed helical nanostructures andnano-rings can be formed byrolling up single crystal ZnOnanobelts. This phenomenon isattributed to a consequence ofminimizing the total energyattributed by spontaneouspolarization and elasticity. Thespontaneous polarization resultsfrom the noncentrosymmetric ZnOcrystal structure. In (0001) facet-dominated single crystal nanobelts,positive and negative ionic chargesare spontaneously established onthe zinc- and oxygen-terminated (0001) surfaces, respectively.

“Hydrothermal growth of ZnO nanorods on Zn substrates and their application in degradation of azo dyes under ambient conditions” X. Cai et al., Cryst Eng Comm 2014

Schematic illustration of the possible growth mechanism for theformation of different ZnO arrays grown under different reactionconditions.

SEM images of ZnO nanorods without (a), (b), andwith (c), (d) TiO2 nanoparticles.

ZnO nanorods

Synthesis of nanorods by converting nanoparticles at elevatedtemperatures (several hundreds degree) is another growth process.When SnO2 amorphous nanoparticles are heated to temperaturesranging from 780°C to 820°C in air, single crystal Sn02 nanorods withrutile structure were formed. Nanorods are straight and have uniformdiameters ranging from 20 to 90 nm and lengths from 5 to 10 m,depending on annealing temperature and time.Various oxide nanowires, such as ZnO, Ga203 and MgO, and CuOwere synthesized by such evaporation-condensation.

Dissolution-Condensation growthIn dissolution-condensation process, the growth species first dissolveinto a solvent or a solution, and then diffuse through the solvent orsolution and deposit onto the surface resulting in the growth ofnanorods or nanowires.

Some examples are:

Single crystal nanowires of selenium starting from spherical colloidparticles of amorphous selenium in acqueous solution. Somenanocrystalline selenium with trigonal structure precipitate and withthe aging at RT in the dark selenium crystallites start to grow. In thissolid-solution-solid transformation, the morphology of the crystallineselenium products was determined by the anisotropic growth, which isattributed to the one-dimensional characteristics of helical chains ofselenium in the trigonal structure.

Single crystal ZnTe nanorods with diameters of 30-100 nm and lengthsof 500-1200 nm were synthesized using Zn and Te metal powders asreactants and hydrazine hydrate as solvent by a solvothermal process.Hydrazine can promote anisotropic growth in addition to its role as areduction agent.

Nanowires can grow on alien crystal nanoparticles, which serve asseeds for heteroepitaxial growth, by solution processing. Oneexample is the synthesis of crystalline silver nanowires of 30-40nm in diameter and 50 m in length using platinumnanoparticles as growth seeds.The growth species of Ag is generated by the reduction of AgN03 withethylene glycol, whereas the anisotropic growth was achieved byintroducing surfactants such as polyvinyl pyrrolidone (PVP) in thesolution.Polymer surfactantsadsorbed on somegrowth surfaces, sothat kineticallyblocked (or poisoning)the growth, resultingin the formation ofuniform crystallinesilver nanowires.

Nanowires or nanorodsby the evaporation(dissolution)-condensation depositionmost likely have facetedmorphology and aregenerally short in lengthwith relatively smallaspect ratios,particularly when grownin liquid medium.However, anisotropicgrowth induced by axialimperfections, such asscrew dislocations, microtwins and stacking faults, or by impuritypoisoning, can result in the growth of nanowires with very largeaspect ratios.

Nanowires can also be grown by decomposing of organometalliccompounds in the presence of coordinating organics.One example is the synthesis of single crystal BaTiO3 nanowires withdiameters ranging from 5 to 70 nm and lengths up to > 10 m bysolution-phase decomposition of barium titanium isopropoxide.

Vapor-Liquid-Solid growth (VLS)In the VLS growth, a second phase material, catalyst or impurity,is introduced to direct and confine the crystal growth on to aspecific orientation and within a confined area.

The main requirements for VLS growth are:

(1) The catalyst must form a liquid solution with the crystallinematerial to be grown at the deposition temperature(2) The equilibrium vapor pressure of the catalyst over the liquiddroplet must be very small. The evaporation reduces the total volume ofthe liquid droplet.(3) The catalyst must be inert chemically.(4) The interfacial energy plays a very important role. The wettingcharacteristics influence the diameter of the grown nanowire. A smallwetting angle results in a large growth area, leading to large diameternanowires.(5) For a compound nanowire growth, one of the constituents can serveas the catalyst.(6) For controlled unidirectional growth, the solid-liquid interface mustbe well defined crystallographically. One of the simplest methods is tochoose a single crystal substrate with desired crystal orientation.

The growth species isevaporated first, and thendiffuses and dissolves into aliquid droplet.

The surface of the liquidhas a large accommodationcoefficient, and is thereforea preferred site fordeposition.

Saturated growth speciesin the liquid droplet willdiffuse to and precipitateat the interface betweenthe substrate and theliquid. The precipitationwill first follow nucleationand then crystal growth.

Continued precipitation orgrowth will separate thesubstrate and the liquiddroplet, resulting in thegrowth of nanowires.

Growth of Silicon nanowiresGrowth of silicon nanowires with gold as a catalyst. Thin layer of gold issputtered on a silicon substrate and annealed at elevated temperature(above the eutectic point of the silicon-gold system), which is typicallythe same as the growth temperature. During the annealing, silicon andgold react and form a liquid mixture, which forms a droplet on the siliconsubstrate surface. Silicon species preferentially condensed at the surfaceof the liquid droplet, the liquid droplet will become supersaturated withsilicon. Subsequently, the supersaturatedsilicon will diffuse from the liquid-vaporinterface and precipitate at the solid-liquid interface resulting in the growthof silicon. The growth will proceedunidirectionally perpendicular to thesolid-liquid interface.Crystalline defects are not essentialfor VLS growth. However, defectspresent at the interface may promotethe growth and lower the requiredsupersaturation. The nanowires can besingle crystal, polycrystalline oramorphous depending on thesubstrates and growth conditions.

The preferential adsorption ofgrowth species onto the liquiddroplet surface can beunderstood considering that aliquid surface is distinctlydifferent from a perfect orimperfect crystal surface, andcan be considered as a “rough”surface. Rough surface iscomposed of only ledge, ledge-kink, or kink sites. That is everysite over the entire surface is totrap the impinging growthspecies. The accommodationcoefficient is unit. For examplegrowth rate of silicon nanowiresusing a liquid Pt-Si alloy is about60 times higher than directly onthe silicon substrate at 900°C.

The equilibrium vapor pressure or solubility is dependent on the surface energy and radius (or curvature of a surface):

Kelvin equation

P∞ the equilibrium vapor pressure of flat solid surface, Pc is theequilibrium vapor pressure of the curved solid surface, the atomicvolume, surface energy, R surface radius.

If nanowires were cylindrical in shape, the lateral growth rate wouldbe significantly smaller than the longitudinal one, assuming allsurfaces have the same surface energy. Convex surface (the sidesurface) with very small radius (<100 nm) would have asignificantly higher vapor pressure as compared with that of aflat growing surface. A supersaturated vapor pressure orconcentration of the growth species for the growing surface may bewell below the equilibrium vapor pressure of the convex surface ofthin nanowires.

Nanowire shape

Lateral and substrate growthrates are essentially the same,whereas the axial rate by the VLSprocess are approximately twoorders of magnitude higher thanthe VS growth rates under thesame conditions.The enhanced growth rate can bepartly due to the condensationsurface area that for the growthspecies in the VLS growth is largerthan the surface area of the crystalgrowth. While the growth surface isthe interface between the liquiddroplet and the solid surface, thecondensation surface is theinterface between the liquiddroplet and vapor phase.Depending on the contact angle, theliquid surface area can be severaltimes of the growth surface.

Axial, lateral and substrate growth

Growth of silicon nanowires with the VLS method exploits mainly goldas catalyst, however also other catalysts have been found to be effectivesuch as Fe using high growth temperature (1200°C). Nanowires of thistype have nominal diameters of ~15 nm and a length varying from a fewtens to several hundreds micrometers. An amorphous layer of siliconoxide of about 2nm in thickness over coated the outside of siliconnanowires.

Nanowires of compound materials can also grow using VLS method.Some examples are semiconductor nanowires of the III-V materialsGaAs, GaP, GaAsP, InAs, InP, InAsP, the II-VI materials ZnS, ZnSe,CdS, CdSe and IV-IV alloys of SiGe.Catalysts can be chosen by identifying metals in which the nanowirecomponent elements are soluble in the liquid phase but that do notform solid compounds more stable than the desired nanowire phase; i.e.the ideal metal catalyst should be physically active but chemicallystable or inert.