Well-Aligned IrO 2 Nanocrystals

1 Department of Electronic Engineering, National Taiwan University of Science and Technology, Taipei 106, Taiwan 2 Biomedical NMR Laboratory, Howard University, Washington, DC 20059, USA 3 Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei 106, Taiwan 4 Department of Chemical Engineering, National Taiwan University of Science and Technology, Taipei 106, Taiwan 5 Department of Electrical Engineering, National Taiwan Ocean University, Keelung 202, Taiwan


INTRODUCTION
Recently, one-dimensional (1D) nanoscaled materials in the form of wires, rods, belts, and tubes have become the focus of intensive research owing to their fundamental interests in science and potential in fabrication nanodevices [1][2][3].The development of nanodevices should benefit from the unique morphology, huge surface area, and high aspect ratio of nanocrystals (NCs).A wide range of the nanosized oxide materials is currently the focus of a rapidly growing scientific community.The electrically insulating and/or semiconducting oxides of nanostructured SiO 2 [4], TiO 2 [5], SnO 2 [6], GeO 2 [7], Ga 2 O 3 [8], and VO x [9] have been synthesized and studied.Among the numerous metallic oxides, the electrically conducting iridium dioxide (IrO 2 ) belongs to a class of materials with unique properties [10], whose nanophase are not well cultivated and required extensive investigation.
As a result of these diverse applications, there is a growing need to develop simple and reliable methods for synthesizing different IrO 2 phases in micro-or nanophase forms.Various methods such as reactive magnetron sputtering [13,14,31], pulsed laser deposition [32,33], solution growth [21,34], thermal preparation [35,36], and metal-organic chemical vapor deposition (MOCVD) [37,38] have been employed for this purpose.Recently, MOCVD have been successfully implemented for the growth of IrO 2 one-dimensional nanostructures on different substrates by Chen et al. [39][40][41].However, MOCVD generally requires multiple processing steps to fabricate nanostructures.It is difficult to have proper control of these processes, for example, the properties of the precursor might change during deposition or after a few runs of the growth process.On the other hand, the method of reactive radio frequency magnetron sputtering (RFMS) has demonstrated its potential applicable for the synthesis of nanostructured materials [42,43] as it possesses the advantage of being a single-step fine control growth conditions technique.
In this article, we review the efforts to develop RFMS and MOCVD techniques for deposition of nanostructural IrO 2 during the past few years.A strong substrate effect on the alignment of the IrO 2 NCs deposition has been demonstrated.The roles of different substrates for the formation of various textures of nanocrystalline IrO 2 are reported and the probable mechanisms for the formation of these NCs are discussed.Section 2 reviews the results of the deposition of IrO 2 NCs on different substrates by RFMS using Ir metal target.A substrate effect on the alignment of the IrO 2 NCs has been discussed, and the possible explanation for the formation of oriented IrO 2 nanostructures has been provided.Section 3 reviews the results of synthesis of IrO 2 NCs on different substrates via MOCVD using (MeCp)(COD)Ir as the source reagent.The successful growth of vertically aligned IrO 2 nanotubes (NTs) on α-Al 2 O 3 (100) [sapphire(100)] (SA(100)) and LiNbO 3 (100) (LNO(100)) substrates are presented in Section 3.1.An interesting tilted growth of well-aligned IrO 2 NTs on the LiTaO 3 (012) (LTO(012)) and SA(012) substrates is shown in Section 3.2.In Section 3.3, a morphological study showing the formation conditions and mechanism for various 1D nanostructures of IrO 2 including NRs and NTs is presented.In Section 3.4, area-selective growth of IrO 2 NRs has been demonstrated on SA(012) and SA(100) substrates, which consist of patterned SiO 2 as the nongrowth surface.The study of the initial growth of IrO 2 nuclei is also presented.Section 4 is the summary.

DEPOSITION OF WELL-ALIGNED IrO 2 NANOCRYSTALS VIA RFMS
In this section, we review the growth of well-aligned IrO 2 NCs via RFMS on different oriented single-crystal oxide substrates [42,43].Reactive sputtering was carried out using a high vacuum RFMS system in a mixture of argon (5 sccm) and oxygen (2.5 sccm) gases.A schematic diagram of the system is shown in Figure 1.The sputtering target was a 1inch Ir (99.95%) metal.A working pressure of ∼ 5 × 10 −2 Torr, power of the RF generator at 65 W, substrate temperature T s at 300-350 • C, and deposition time of 60-90 minutes were used in the deposition process.The surface morphology and structural properties of the as-deposited NCs were characterized.The growth behavior of IrO 2 NCs is highly correlated with the growth conditions and orientations of the substrates.A strong substrate effect on the alignment of the NCs has been observed, and the possible explanation for the NCs' structure formation has also been given.RFMS has been demonstrated to be a simple method to fabricate largearea structures, which has several advantages including better control of the growth conditions and a single deposition step to obtain the nanostructures of IrO 2 .For the IrO 2 NCs on LNO(100) substrate, a weak diffraction signal indexable to the IrO 2 (301) plane (2θ ∼69.3 • ) can be distinguished.This fact proves the predominantly (001) oriented growth on SA(100) and LNO(100) with a small presence of (301) growth orientation on LNO(100) substrates, which can be predicted according to lattice relationship of substrates and nanostructures interfaces.The preferable oriented growth of IrO 2 (001) along [001] can be explained by examining and correlating the epitaxial relation between the rutile lattice IrO 2 and the underlying SA(100)/LNO(100) planar structures at the atomic level.

Deposition of vertically aligned
The main assumption is that the SA(100) and LNO(100) surfaces are terminated by dislocated oxygen atoms as in the single crystals.The schematic diagrams illustrated in Figure 3 show the atomic arrangements of IrO 2 (001) on SA(100) and LNO( 100      (Reprinted from [43] with permission from Institute of Physics Publishing.) below.The FESEM images reveal that IrO 2 NCs have an average diameter and length of 40 ± 5 nm and 400 ± 40 nm, respectively.Figure 4(c) shows the typical XRD patterns of the regularly tilted IrO 2 NCs deposited on SA(012) and SA(110).Two peaks can be indexed as (101) and (202) diffraction planes at 2θ ∼ 34.7 • and ∼ 73.2 • , respectively, indicating parallel inplane IrO 2 (101) orientation.Here, we observe anisotropic growth and as a result, film formation is restricted by the in-plane mismatch.Thus, the deposited Ir and O atoms are stacked into a 1D nanostructure in c-direction with IrO 2 plane formation following the substrate orientation.The probable allowed orientations of the NCs to the substrate interfaces are IrO 2 (101) // SA(012) or SA(110).
To determine the directions of planar deposition, we have to examine the atomic arrangements of the appropriate surfaces.Figure 5 illustrates the schematic plots of the atoms arrangements and lattice relationships between IrO 2 and SA(012), SA(110) surfaces.According to the argument on minimization of the oxide sublattice structural mismatch, the possible NCs-substrates alignment can be described as IrO NCs.The substrate orientation combining with the temperature of substrate can also influence the internal factor such as energetically favorable surface for the incoming atoms (c-directional growth mechanism) and initiate the preferable plane orientation of IrO 2 NCs whereby the incoming atoms will stick onto the lower energy sites.The c-directional growth mechanism comes from the anisotropy of the crystal structure that results in different growth rate for the different directions of NCs.  Figure 7(c), which illustrates the growth of IrO 2 (100) NWs on SA(001).In other words, the mismatch is not as energetically favorable along the IrO 2 [001].The experimental observation that the IrO 2 NWs take on a mosaic structure consisting of three equivalent domains would be consistent with an epitaxy structure in which IrO 2 unit cells are distributed on the SA(001) surface such that the match along IrO 2 [010] is maximized, while that along IrO 2 [001] is minimized.Schematically, this is illustrated by the three equivalent unit cells of IrO 2 , rotated 120 • from each other, as depicted in Figure 7(c).The maximized/minimized match of IrO 2 along [010]/[001] can explain the reason of producing discontinuous growth and forming the nanowalls-like structure instead of an epitaxial film.The c-directional growth behavior aligns NWs along [001] direction whereas a higher degree of lattice mismatch along the c-direction tends to impede the growth process of producing a structure with shorter NWs.

GROWTH OF IrO 2 NANOCRYSTALS BY MOCVD
In this section, we review the growth of IrO 2 nanocrystals utilizing a vertical-flow cold-wall MOCVD system.A schematic diagram of the system is shown in Figure 8.The low-melting and highly volatile iridium precursor (methylcyclopentadienyl) (1,5-cyclooctadiene) iridium (I), (MeCp)(COD)Ir, was used for the chemical vapor deposition of IrO 2 samples [37].

Growth of vertically aligned IrO 2 nanotubes on α-Al 2 O 3 (100) and LiNbO 3 (100) substrates
In this section, growth and a detailed characterization of vertically aligned single-crystalline IrO 2 NTs on (SA)(100) and (LNO)(100) substrates [10] via MOCVD will be presented.The synthesis parameters for the growth of vertically aligned IrO 2 nanotubes are as follows: both the temperatures of transfer line T tl and the precursor reservoir T pr were kept at a constant temperature of 100-110 • C, high-purity oxygen was used as both carrier and reactive gas with a flow rate of 100 sccm, the substrate temperature and pressure of the CVD chamber were at 350 • C and 10-50 Torr, respectively.The FESEM images illustrated in Figures 9(a), 9(b) show that most of the IrO 2 crystals grown on SA(100) substrates reveal hollow square cross-section and exhibit vertically aligned growth.The estimated edge size and tube length of the nanotubes (NTs) are around 40-100 nm and 0.2-2.0μm, respectively.Similar results were also found in the growth of the IrO 2 NTs using LNO(100) substrate as shown in Figures 9(c), 9(d).The vertically aligned tubes grown on LNO(100) have edge size and length around 50-100 nm and 0.5-1.0μm, respectively.Nonetheless, some differences in the uniformity of growth alignment and orientation between the samples on SA(100) and LNO(100) can still be observed.The topview images of the overall tubules on SA(100) (Figure 9(a)) are clear open squares with the edges parallel to each other.This result indicates that the tubules standing on substrate are perfectly vertical and follow the same in-plane orientation.On the other hand, the top-view image for the tubules on LNO(100) (Figure 9(c)) shows some degrees of deviation as compared to that on SA(100), indicating the probable occurrence of other growth planes.
The TEM images, depicted in Figures 10(a  on the lattices relationship as described in Section 2.1.Lattice misfit at interface produces strain energy when IrO 2 is nucleated.The orientation that minimizes the lattice misfit and produces the smallest strain energy will be preferred.The overall orientation relationship between the nanotubes and substrates can be described as: IrO 2 (001) [100] // SA(100) [010] and IrO 2 (001) [100] // LNO(100) [010].A higher degree of lattice mismatch for IrO 2 grown on LNO(100) is probably the reason for the generation of the two preferential orientations of (001) and (301), with the former being the dominant.The higher lattice mismatch and formation of the (301) plane also explain why the IrO 2 NTs on LNO(100) substrate grow with lesser uniformity in alignment.

Tilted growth of the well-aligned IrO 2 nanotubes on LiTaO 3 (012) and α-Al 2 O 3 (012) substrates
In this section, growth and characterization of the wellaligned IrO 2 NTs on the (LTO)(012) [40] and SA(012) [44] substrates with a tilt angle of 35 • will be presented.Detailed synthesis parameters for the growth of tilted IrO 2 nanotubes are as follows: the temperatures of transfer line T tl and the precursor reservoir T pr were kept at a constant temperature of 100-110 • C, oxygen flow rate at 100 sccm, substrate temperature at 350 • C, and the chamber pressure in the range of 10-30 Torr.The deposition rate of the 1D crystal with tubular morphology was estimated to be 5-10 nm/min.As illustrated in Figure 12, the FESEM images show high density and well-aligned IrO 2 NTs grown on a LTO(012) substrate.The self-assembled NTs were grown with an identical tilt angle from the normal to the substrate.Unlike the cylindrical symmetry of most of the NTs reported so far, the IrO 2 tubes show open ends with square cross-section.The estimated edge size, length, and packing density are 50−80 nm, 1.0−1.5 μm, and 75 ± 5 μm −2 , respectively.Energy dispersive X-ray spectroscopy EDS measurements indicate that the tubules have an average atomic ratio of Ir to O of 1 : 2.
The cross-sectional TEM image in Figure 13(a) shows that all of the IrO 2 NTs grow with a tilt angle of ∼35 • from the normal to the substrate surface.By separately focusing on the NTs and substrate, the tetragonal IrO 2 [ 111] and rhombohedral LTO [2 21] zone patterns are obtained and shown in Figures 13(b) and 13(c), respectively.Furthermore, a mixed SAD pattern at the interface region, depicted in Figure 13(d), indicates that the IrO 2 (101) layers are heteroepitaxially deposited on the LTO(012) substrate.This result is further confirmed by XRD measurements.Figure 14 shows a typical XRD pattern of the well-aligned IrO 2 NTs grown on LTO(012) substrate.Two peaks at around 35 • and 73 • are indexed as (101) and (202), respectively, of rutile IrO 2 , indicating that all the IrO 2 (101) planes are parallel to the substrate plane.In addition, these results also provide a reasonable explanation of the substrate effect on the tilted growth of the IrO 2 NTs.Initially, the deposition of IrO 2 starts from the epitaxy of the {101} planes on the LTO(012) surface.Since the long axis of NT is along the [001] direction, the growth rate of (00l) planes should be the highest in this case.Then the tilted growth occurs along the [001] direction which is ∼35 • from the normal to the LTO(012) substrate or IrO 2 (101) plane.Figure 13(e) illustrates the schematic diagram of the orientation relationship between IrO 2 NTs and the LTO(012) substrate.The allowed probable orientation of the nanotube to substrate interface is IrO 2 (101)[010] // LTO(012) [100].Similar results were also found in growth of the IrO 2 NTs using SA(012) substrates [44] (see Figure 15).
The obtained heteroepitaxy could be interpreted by examining the planar atomic arrangement of the IrO 2 (101) and LTO(012)/SA(012) planes [44] (Figure 16).The crystal formation follows the substrate orientationat conditions when the surface mobility of the oxygen atoms constructing IrO 2 is just sufficient to maintain and sustain the formation of the plane with lowest energy.The orientation that minimizes the lattice misfit and produces the smallest strain energy will be preferred.In accordance with the argument of the minimization of the oxide sublattice structural mismatch mechanism, the best match of IrO

Morphological evolution of IrO 2 one-dimensional nanocrystals
In this section, we present the results of direct observation of the morphological evolution from solid-triangle NRs via hollow-square NTs to solid-square NRs for a tetragonal rutile material of IrO 2 by precisely controlling the growth rate of these 1D nanocrystals via MOCVD [41].For a CVD process, the surface morphology of the asdeposited structures is determined by the complex interplay between mass transport and surface kinetics of the system,  (Reprinted from [44] with permission from Institute of Physics Publishing.) which is critically dependent on the temperature of precursor reservoir T pr , the substrate temperature T s , the flow rate of the carrier gas J 0 , and chamber pressure P c , and so forth.Usually, T s is chosen to be higher than the pyrolysis temperature of the reactants to ensure their rapid decomposition and heterogeneous reaction at the growth interface.In addition, T s also plays an important role in surface kinetics and strongly influences the surface morphology.To study the growth kinetics, a surface morphology diagram in terms of the degree of supersaturation Δμ versus 1/T s can be used to interpret the morphological evolution [45].From a crystal grower's point of view, a larger Δμ and a lower T s can result in an instability of surface morphology of the as-deposited structures.
At first, by fixing all other parameters (T s = 350 • C, J 0 = 100 sccm, and P c = 15-20 Torr) and by only adjusting T pr from 70 to 140 • C, the partial pressure of the incoming source vapor (MeCp)(COD)Ir is changed.Because the vapor pressure of (MeCp)(COD)Ir near the growth interface is not available, the actual Δμ of the corresponding CVD system cannot be determined.Hence, the notation of Δμ(T pr ) at various T pr is then taken as a reference for our discussion.Different values of Δμ would result in different morphologies and lead to different growth rates of the IrO 2 1D nanostructures.The growth rate R is defined as the increase in length of the long axis per unit growth time for the 1D crystal.All the samples in this study were grown on SA(100) substrates to make the IrO 2 crystals uniformly arrange in vertically aligned arrays [39].
In the largest Δμ region (T pr = 125-140 • C) and R(=18-40 nm/min), the IrO 2 NRs with nearly triangular (Figures 17(a)-17(c)) and wedge-like (Figures 17(d)-17(f)) crosssections are grown accompanying the formation of selfassembled sharp tips [38].Usually, the former is preferred at larger Δμ than the latter.While reducing Δμ(T pr = 110-125 • C) and R(15-22 nm/min), the first morphological evolution occurs.The wedged rods evolve new walls and tend to complete a square loop.However, under this condition, IrO 2 crystals always evolve into incompletely enclosed tubes (Figures 18(a)-18(c)) and the scrolled tubes (Figures 18(d)-18(f)).The second stage of evolution is from the incomplete and scrolled tubes to the square NTs.Figures 19(a)-19(c) show that further reducing Δμ(T pr = 100-110 • C) and R(=8-17 nm/min) could make the wedged NR enclose a perfect square loop and evolve into the NT rather than its incomplete counterparts (Figure 18).The edge sizes of these NTs on the SA(100) substrate are around 50-100 nm.The tube walls of the square NTs will become thickened and be filled inside upon further decreasing Δμ or R as depicted in In addition to the evolution between the 1D nanocrystals, a transformation from anisotropic 1D to isotropic 3D growth was also observed.Figure 20(a) shows that in the lowest Δμ(T pr = 70-80 • C) region the as-grown IrO 2 mixture is comprised of continuous grains and a few short rods protruding from the film surface.The growth rate range of this sample estimated from the thickness of film layer and length of the rod is around 1-2 nm/min which is also the lowest R in this study.Above results suggest that the self-mediated 1D growth habit of IrO 2 could be gradually retarded by reducing Δμ.
From a surface kinetics point of view, a lower value of Δμ or R means that the adhered surface atoms have sufficient time to make the surface diffusion.Thus, the morphology of the as-grown structures becomes more stable.Similarly, a higher value of T s , which provides sufficient surface diffusion energy for the adhered surface atoms, also plays an important role in determining the shape of the as-grown structures.Accordingly, as the second part of this work, T s is varied from 350 to 500 • C. The study of the morphological evolution can then be carried out by adjusting T pr from 70-140 • C to change Δμ of the corresponding MOCVD system.The morphology distribution of IrO 2 in terms of Δμ and T s is schematically illustrated in Figure 21(a).Overall, the NRs and NTs with square cross-section are more energetically favorable among these 1D nanostructures.The film, composed of continuous 3D grains (Figure 20(c)), is formed under the highest T s and the lowest Δμ condition, which is the most morphologically stable condition.confirmed using other substrates including LiNbO 3 (100) and LiTaO 3 (012).Similar phenomena have been observed in a solution-phase growth for the transition of nanorods to nanotubes with respect to different solute concentrations (i.e., different values of Δμ) [46] and in thermal evaporation method for the size variation of box-beams with respect to substrate temperatures [47].
For the four typical IrO 2 1D nanostructures and a thin film grown on sapphire (100) substrates, their corresponding XRD patterns (Figure 22) show the nearly single-crystalline quality and the same [001] long-axis directions for the vertically aligned NRs and NTs [38,39] according to the unique (002) diffraction signal.The results also suggest that the orientation and crystallinity of the as-grown IrO 2 samples are not influenced by varying T pr and T s , while the morphology is highly dependent on the variations of CVD conditions.Therefore, the evolution begins from a spiral growth mode on the plane perpendicular to the [001] long-axis directions with wedged NRs as embryos.Under the condition of highest morphological instability, these embryos grow and persistently remain as shown in Figures 18(a)-18(c).By reducing the degree of morphological instability, via the growth mode, wedged NRs composed of two side walls can evolve new walls and encloses spirally into various tubular structures.Square NTs formed under lower Δμ and higher T s show more energetically favorable than the incomplete and scrolled ones.The most energetically stable 1D structure is the solid-square rods.These results can be explained as follows: the tetragonal rutile IrO 2 has the relationship of the lattice constants a = b > c.In term of crystallography, the crystal morphology with square cross-section should be the most stable rather than the triangular, wedged, scrolled, spiral, and any other forms.With the implication of diffusion-limited aggregation (DLA) model [45], the protruding part of the as-grown structure can easily capture the vaporized reactants and can grow faster leaving the inner growth sites (shielded by the outer branches) vacant.Naturally, by increasing the degree of interface instability (i.e., increasing Δμ and reducing T s ), since most of the source atoms are enforced to elongate along the longitudinal length, the corresponding R increases.Triangular and wedged rods with the fastest growth rate are grown at the highest Δμ and the lowest T s .Under this condition, because the adhered atoms prefer to stack along [001] directions, no sufficient atoms can build a new wall from the rod edges, and so the spiral growth will not occur to complete a square circumference which is energetically more favorable.After the square tubes are formed, further lowering Δμ and increasing T s will provide surface atoms sufficient time and energy to arrange and diffuse into the center of the hollow structure, resulting in the thickening of the tube walls (Figures 19(d)-19(f)).The hollow structure will be filled up and solid structure will form, instead (Figures 19(g)-19(i)) when the deposition and diffusion conditions are suitable.(Reprinted from [48] with permission from The Royal Society of Chemistry).In this section, area-selective growth of IrO 2 NRs will be demonstrated on SA(012) and SA(100) substrates which consist of patterned SiO 2 as the nongrowth surface [48].The optimal substrate temperature for selective growth is 450 • C at a chamber pressure of ∼20 Torr.The two crystal planes are chosen to align the nanorods in a specific orientation.Origin of the selectivity along with other 1D morphological features are traced back to nucleation in its initial growth period.Photolithography was employed in patterning a silicon thin layer on sapphire.It began with standard wafer cleaning, and followed by sputtering a 20 nm thick Si thin film on SA(012) and SA(100) substrates.Si patterns of stripe and square window were created by spin-coating a photoresist, exposure, and wet etching.After removing the photoresist, the substrate was transferred to an MOCVD chamber and heated in flowing oxygen at 480 • C for 25 minutes so that the patterned Si thin film was oxidized.The area covered by noncrystalline SiO 2 thin film was always designated as the nongrowth region and the exposed sapphire area was the growth region for iridium dioxide nanorods.

Area-selective growth of
The starting point of IrO 2 selective growth on these two patterned sapphire substrates is the nucleation energy barrier difference between sapphire and noncrystalline silica surfaces.Figure 23 compares kinetics of IrO 2 initial growth on SA(012), SA(100), and noncrystalline silica surfaces at T s = 450 • C. The low-energy barrier of IrO 2 nucleation on sapphire is manifested by its short incubation time, approximately 19 seconds on SA(012) surface and 9 seconds on SA(100) surface.On the contrary, the incubation time on the noncrystalline silica surface is much longer at 369 seconds.Here the incubation time is defined as the intercept extrapolated from the linear plot of surface area covered by IrO 2 nuclei versus growth time.
The upper inset of Figure 23 shows that the number density of nuclei increases rapidly on both sapphire surfaces, while the number density increases on the glassy silica surface much more slowly.The lower inset of Figure 23 is a plot of average nucleus size versus growth time.These two insets indicate that the number density of nuclei on SA(012) and SA(100) is large, but its average size is small and increases quickly.On the other hand, the number density of nuclei on glassy silica surface is zero before incubation or small after incubation; and once IrO 2 nuclei appear on the silica surface, their sizes are comparatively large.Observation of a few large nuclei after considerably long incubation suggests that establishment of an IrO 2 nucleus on the glassy silica surface needs to collect a sufficient number of atoms.A nucleus of insufficient size tends to dissipate, diffuse or evaporate away.Some morphological features of the IrO 2 nanorods can find their roots in the nucleation behavior on sapphire.Figure 24 illustrates IrO 2 nuclei at growth time 30 and 60 seconds on SA(012).The nuclei in Figure 24(a) appear to be lined up and develop several dotted lines along the diagonal direction.The distance between two dotted lines is approximately 100-130 nm.A few IrO 2 nuclei are present in the interval between two dotted lines.Lining up of these nuclei seems to be the consequence of preferential nucleation sites on sapphire at certain surface defects, such as steps and kinks.Figure 24(b) indicates that these earlier nuclei are evolving into short rods and simultaneously most of the gap is being filled by new nuclei resulting in a reduced nucleus free area.Even in an initial stage as shown in Figure 24(b), some nuclei have turned into short rods.These short rods are clearly evolved from the older nuclei.The head start of those rods persists throughout the growth period and develops a height advantage since a taller rod is in a superior position to receive more growth species in the gas phase.
Orientation of IrO 2 nanorods is a morphological feature that is easily affected by the pattern resolution at the border between sapphire and glassy silica regions.Figures 25(a We would like to emphasize that a border of less-populated nanorods, such as Figure 25(d), is uncommon.Nevertheless, such an image allows us to see the boundary between sapphire and silica thin film clearly, which is generally hidden in the nanorods.Figure 25(c) illustrates a typical border image, in which rods are aligned along the border line.Occasionally, there are toppled rods sticking out from the nanorods forest.The toppled rods stem from nuclei whose growth are influenced by both sapphire and silica surfaces.der the CVD condition.The nuclei with their (001) planes parallel with the sapphire (100) plane will grow into vertical nanorods and stand out since they are in a favorite position to receive growth species from the gas phase.The nuclei that do not satisfy the epitaxial relation also grow, but their sizes are limited.As deposition proceeds, they turn into grains surrounding the roots of vertical rods.The size limitation occurs since most of the growth species are intercepted by the vertical rods.The IrO 2 crystals which satisfy the epitaxial relation display the 1D growth feature.The morphological results indicate that the SA(012) surface exerts a tighter control on IrO 2 nucleation than the SA(100) surface.

SUMMARY
We review the results of the synthesis of well-aligned 1D IrO 2 nanocrystals on different substrates via reactive radio frequency magnetron sputtering and metal-organic chemical vapor deposition.The 1D growth behavior of IrO 2 is found to be highly correlated to the oxygen-rich ambient, substrate temperature, and the crystal structure of substrates.A strong substrate effect on the alignment of the IrO 2 NCs has been discussed, and the possible explanation for the formation of the oriented IrO 2 NCs structure has been provided.
By designing a series of MOCVD experiments, a morphological evolution of IrO 2 1D nanocrystals has been studied.The as-grown 1D nanostructures have their origin from the interface instability driven by increasing the degree of Journal of Nanomaterials supersaturation Δμ and/or reducing substrate temperature T s .By decreasing the degree of interface instability, the 1D nanostructures evolve from triangular/wedged NRs via incomplete/scrolled NTs to square NTs and square NRs according to their morphological stability.The results show that the 3D grains composing film belong to the most stable form as compared to the 1D nanocrystals and the sequential shape evolution has been found to be highly correlated to a morphological phase diagram based on the growth kinetics.The results could help material scientists and chemists to understand the mechanisms and to control the anisotropic 1D growth for solid and hollow nanostructures from bulk materials.
In addition, area-selective growth of IrO 2 nanorods have been demonstrated on silica-patterned SA(012) and SA(100) substrates by MOCVD.Area-selective chemical deposition is known as a chemical technique to realize patterned thin films, which are essential for many electronic and miniaturized electrical devices.The area-selective growth takes advantage of the nucleation barrier difference between glassy silica surface and sapphire surface.The glassy silica surface serves as the nongrowth surface and the sapphire surface as the growth surface in the selective growth process.The IrO 2 NCs which satisfy the epitaxial relation display the 1D growth feature.The morphological results indicate that the SA(012) surface exerts a tighter control on IrO 2 nucleation than the SA(100) surface.

Figure 1 :
Figure 1: Schematic diagram of the RF magnetron sputtering system.

Figure 8 :
Figure 8: Schematic diagram of a vertical-flow cold-wall MOCVD system.

Figure 12 :
Figure 12: FESEM images of the well-aligned IrO 2 nanotubes grown on the LiTaO 3 (012) substrate by MOCVD: (a) and (b) top view; (c) cross-sectional view; (d) focus on a typical IrO 2 nanotube.(Reprinted from[40] with permission from The American Chemical Society).

Figure 13 :Figure 14 :Figure 15 :
Figure 13: (a) The cross-sectional TEM image of the IrO 2 nanotubes on LiTaO 3 (012) substrate and its corresponding SAD patterns taken separately from the regions of (b) IrO 2 nanotubes, (c) LiTaO 3 substrate, and (d) interface along the zone axes of IrO 2 111] and LiTaO 3 [2 21].(e) The schematic diagram of the orientation relationship between the nanotube and substrate.(Reprinted from[40] with permission from The American Chemical Society).

Figure 18 :Figure 19 :Figure 20 :Figure 21 :Figure 22 :
Figure 18: The top and 30 • perspective view FESEM micrographs and the corresponding schematic plots for (a)-(c) the incomplete nanotubes and (d)-(f) the scrolled nanotubes of IrO 2 .(Reprinted from[41] with permission from Institute of Physics Publishing.)

Figure 23 :Figure 24 :
Figure 23: Variation of the surface area being covered, the number density (inset), and the average size (inset) of IrO 2 nuclei with growth time in the initial growth stage.The growth temperature is 450 • C. (Reprinted from[48] with permission from The Royal Society of Chemistry).

Figure 25 :
Figure 25: IrO 2 nanorods on SA(012) patterned by the photolithographic method and selectively grown at 450 • C; (a) a stripe pattern, (b) a corner of square patch, (c) a border of populated nanorods, and (d) a border of less-populated nanorods.(Reprinted from[48] with permission from The Royal Society of Chemistry).
) and 25(b) illustrate a stripe pattern and a square corner of tilted IrO 2 nanorods on SA(012) substrate, respectively.A sharp boundary in Figure25(d) delineates the upper half (growth region) and lower half (nongrowth region) of micrograph.

Figures 26 (
a) and 26(b) show well-defined images of four nongrowth squares and a corner of vertically aligned IrO 2 nanorods on SA(100) substrate.Although nanorods in Figures 25 and 26 are grown under the same CVD condition and patterned by the same procedure, morphological influence of SA(012) and SA(100) goes beyond rod orientation.There are more grains surrounding the roots of IrO 2 rods illustrated in Figures26(c) and 26(d) compared with those in Figures25(c) and25(d).Those surrounding grains can be understood from the difference in nucleation behavior on SA(100) and SA(012) surfaces.Compared with SA(012), the incubation time is shorter and the nucleation rate is higher on SA(100).Not all nuclei on SA(100) are well oriented un-

Figure 26 :
Figure 26: IrO 2 nanorods on SA(100) patterned by the photolithographic method and selectively grown at 450 • C; (a) a pattern of four-square nongrowth patches, (b) a corner of square patch, along with images showing the nanorods and their surrounding grains at, and (c) a corner, and (d) a border line.(Reprinted from [48] with permission from The Royal Society of Chemistry).