Increased Yield and Uniformity of Vanadium Dioxide Nanobeam

Jan 17, 2012 - Synopsis. Three distinct morphologies, nanosheet (NS), nanowire (NW), and nanoparticle (NP), of VO2 were observed within specific regio...
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Increased Yield and Uniformity of Vanadium Dioxide Nanobeam Growth via Two-Step Physical Vapor Transport Process In Soo Kim and Lincoln J. Lauhon* Department of Materials Science and Engineering, Northwestern University, Evanston, Illinois 60208, United States ABSTRACT: Single-crystal vanadium dioxide (VO2) nanobeams are attractive materials to investigate the influence of dimensions on the metal−insulator transition, so simple strategies to control yield and morphology are desirable. In a physical vapor transport process, three distinctive morphologies of VO2 nanostructures, nanoparticles, nanowires, and nanosheets, were observed depending on local source supersaturation and temperature. On the basis of these observations, a practical two-step synthetic approach was devised to modify the supersaturation during growth, separately optimizing nucleation density and nanobeam aspect ratio. Increased yield and uniformity in length associated with simultaneous nucleation could be achieved with modulation of oxygen flow or seeding the substrate with nuclei. The ability to lower the supersaturation while maintaining high densities of nanobeams also improved control over the morphology. a nanobeam in the VO2 phase through reduction.7b In-situ GISAXS experiments,7a on the other hand, did not reveal such remelting effects in the intermediate stages of the growth process, perhaps due to the different synthesis conditions. Epitaxial growth of VO2 nanobeams, both in-plane and out-ofplane, on Al2O3 and TiO2 substrates has also been reported,7b,9 as well as the thickness dependence of VO2 nanobeams on the temperature of the reactor.7a,10 Despite these advances in understanding of growth mechanisms that are active under particular conditions, simple strategies to achieve control over aspect ratio and yield of VO2 nanobeams have not yet been reported. A particular challenge for physical vapor transport approaches is that the ideal conditions for nucleation, to control density, and growth, to control aspect ratio, may be different. Consideration of the advances in understanding reported in refs 7 and 8, together with our experience with the variability in product yield upon changes in powder sources, led us to investigate the influence of oxygen partial pressure on nucleation and growth of VO2 nanobeams. We hypothesized that small amounts of oxygen promoted the sublimation of the source powder and therefore the supersaturation of the vapor, suggesting that oxygen partial pressure could provide a means to vary supersaturation during growth. Here, we introduce a practical, two-step vapor transport synthesis to control morphology, density, and site-specificity of VO2 nanobeams. Nucleation of nanobeams is controlled in one of two ways: by the introduction of small partial pressures of O2 in the beginning of growth or by using a “local” source of VOx. Significantly improved control of aspect ratio and density was

1. INTRODUCTION The interplay between electronic and lattice degrees of freedom in metal oxides lead to a wide range of intriguing properties1 with controlled variations in stoichiometry and/or doping. Vanadium dioxide (VO2), for example, exhibits a reversible metal−insulator transition (MIT) that can be tuned widely by doping.1e The transition is accompanied by drastic changes in the electrical and optical properties: the electrical resistivity decreases by 5 orders of magnitude,2 whereas the optical reflectivity in the near-infrared region increases upon transition from the insulating state to the metallic state.3 The switching behavior could be useful for devices such as field effect transistors and optical switches as well as strain and gas sensors,4 but there remain basic challenges to the reliable engineering of switching. In particular, strain associated with grain boundaries, domain boundaries, and substrate interfaces broadens the phase transition. Single-crystal nanowires offer an appealing alternative to thin films because they tend to maintain single domains across their width,5 and therefore provide a model system to study the characteristics of the metal−insulator transition with improved control over the boundary conditions that influence structural domain formation. The synthesis of VO2 nanobeams, or nanowires with an approximately rectangular cross-section, was first reported by the Park group using the vapor transport process.6 Despite the increasing interest in VO2 nanostructures, there is not yet a clear consensus on the growth mechanism(s). Exsitu spectroscopic studies on the initial stages of growth7 have been interpreted to suggest that liquid droplets of V2O5 nucleate prior to VO2 growth, and these droplets may coexist with VO2 nanobeams at the growth front.8 Recent in situ monitoring of VO2 nanobeam growth using optical microscopy revealed that V2O5 droplets transform into V6O13 nanowires, which in turn melt back into a droplet before finally producing © 2012 American Chemical Society

Received: November 10, 2011 Revised: December 22, 2011 Published: January 17, 2012 1383

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Figure 1. (a) Schematic of the tube furnace reactor (not to scale). Temperature of the reactor is illustrated by red (hot) and blue (cold) colors. (b) Raman spectrum of the nucleus exhibiting characteristic spectrum of V2O5. In the inset is an optical image of nuclei on Si substrate. Cross-hair indicates the position where the measurement was taken. Representative morphologies of the growth product observed in accordance with source vapor pressure include (c) nanosheets, (d) nanowires, and (e) nanoparticles.

to produce VO2 nuclei across the entire growth wafer. The spatially varying supersaturation and temperature during stage II (without O2) explain the observed variation in nanostructure morphology with position. For high aspect ratio nanobeam growth, the reduced supersaturation in stage II limits the rate of nucleation on the low index (110) planes that compose the sides of the nanobeam. Furthermore, diffusion is sufficiently fast to transport adatoms on the side facets to the growth tip, which exhibits faceted and/or curved surfaces with higher step densities and rates of adatom incorporation (Figure 3d) as explained previously.9 In contrast, the higher supersaturation in the nanosheet region upstream promotes growth on the sidefacets, leading to a two-dimensional morphology. Here, the transition in morphology can semiquantitatively be described by the probability of nucleation (PN), which can be expressed as a function of supersaturation (α), temperature (T), and surface energy (σ):12

achieved using the two-step procedure involving introduction of O2.

2. EXPERIMENTAL SECTION Synthesis of VO2 nanobeams was carried out via vapor phase transport in a three-zone standard tube furnace, a schematic of which is shown in Figure 1a. VO2 powder (Sigma-Aldrich, 99.9% trace metals basis) was used as the source material, which was placed in an alumina combustion boat, upstream from Si (with native oxide) substrates. The quartz tube was then evacuated to ∼10 mTorr with a rotary vane pump. He (Airgas, 99.999%) carrier gas was introduced at the flow rate of 2 sccm to stabilize the total pressure at ∼4.0 Torr, at which point the furnace was ramped up to 820 °C at the rate of 50 °C/min (stage I) and kept constant at 820 °C for 0−90 min (stage II). The furnace was left to cool for ∼3 h in flowing He after growth. For growths using a process we refer to as method A, 0.1 sccm of O2 (Airgas, 99.994%) was maintained during stage I. In method B, growth substrates were patterned with sol−gel derived V2O5 or pretreated with ground VO2 source powder. V2O5 sol was prepared from vanadium tri-isopropoxide [VO(i-OC2H5)3] (Alfa Aesar, 95−99%), ethanol [C2H5OH] (Sigma-Aldrich, 200 proof spectrophotometric grade), and water, following the recipe reported previously.11 Electron beam lithography of a PMMA/MMA bilayer was carried out to define patterns for sol−gel deposition. After being developed, the V2O5 sol was deposited by spin-coating at 3000 rpm for 30 s. The substrates were then preheated at 150 °C for 10 min before lift-off in acetone. In a separate approach, VO2 powder was ground and dispersed in ethanol to make a 1:10 mg/mL solution, which was spun on Si substrates. Morphological analysis of VO2 was carried out using scanning electron microscopy (Hitachi S4800-II). A transmission electron microscope (JEOL 2100F) was employed to characterize crystal structure, and phase identification was confirmed by single nanowire confocal Raman spectroscopy (WiTec Alpha300) using an excitation wavelength of 532 nm.

⎛ σ 2π ⎞ ⎟⎟ PN = B exp⎜⎜ − 2 2 ⎝ kBT lnα ⎠ where B is a constant, and kB is Boltzmann’s constant. The irregular polygonal shapes observed for some platelets are likely due to agglomeration of neighboring platelets. The high density of nanoparticles downstream is attributed to the decrease in diffusivity of adatoms at the lower local temperature. The temperature reduction also increases the supersaturation of the remaining vapor, so growth still occurs, but on multiple facets instead of selectively at one end of the nanocrystal. The changes in product morphology observed in this experiment are analogous to the morphological evolution of ZnO with varying supersaturation observed in screw-dislocation driven growth of ZnO nanowires employing a continuous flow reactor (CFR).13 As the authors point out, one should keep in mind that the kinetics and resulting product morphology may not be dictated entirely by the supersaturation if a defect such as a screw dislocation is present. No evidence was found for such a dislocation in these experiments. 3.2. Controlled Nucleation and Growth via Modulated Supersaturation. Ex-situ studies of the early stages of growth

3. RESULTS AND DISCUSSION 3.1. Influence of Supersaturation on Morphology. Growths using method A produced three distinct nanostructure morphologies, nanosheet (NS), nanowire (NW), and nanoparticle (NP), within distinct regions of the growth substrate as shown in Figure 1. Evidently, the vapor phase supersaturation was sufficiently high during stage I (ramp up with O2 flowing) 1384

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growth analysis by Raman spectroscopy determined that the nuclei produced by the oxygen flow exist in the V2O5 phase (Figure 1b). Figure 3 presents a detailed examination of method A nanobeams that exhibit uniform areal distribution and size; the SEM images are representative of densities observed over the entire wafer for the indicated growth region. The uniform density is attributed to the promotion of uniform heterogeneous (surface) nucleation at high source vapor pressures, and the improved uniformity in size follows from the contemporaneous nucleation at the beginning of the growth phase, rather than temporally distributed nucleation events throughout the growth phase. We further note a decrease in aspect ratio moving from region I to region III (Figure 3e). As noted above, both the source vapor concentration and the temperature decrease moving from region I to region III. The temperature is high enough in regions I and II for surface diffusion to lead to one-dimensional growth; the supersaturation is high enough that long nanobeams are produced, but not so high that nanosheet growth dominates. There are several factors that can contribute to the short nanobeams observed in region III, the lower temperature region. First, adatom diffusion is reduced, reducing material exchange between nanobeams and inhibiting one-dimensional growth on any given nanobeam. Second, the source vapor concentration is reduced, limiting the growth of the nanoparticles. Finally, if the nanobeam growth is entirely mediated by a liquid droplet of V2O5 as has been shown in some measurements,7 the low supersaturation and temperature in region III could be insufficient to maintain the liquid state, diminishing the preference for the end-facet growth. To verify general applicability of the two-step synthesis process (method A), growths were carried out on various substrates including Si (with native oxide), thick SiO2 on Si, Si3N4 on Si, and Al2O3 substrates with different orientations. On all substrates, higher uniformity and density of nanowires was achieved. Further, we were able to induce epitaxial growth of VO2 nanowires on sapphire substrates similar to previous reports,7a,9 but with improved control over yield and uniformity (Figure 4). 3.2.2. Method B: Pretreatment of Substrates. Substrates were pretreated with sol−gel derived V2O5 patterns or ground VO2 source powder7b prior to the standard growth to serve as pre-existing nuclei. Annealing of the sol−gel derived films, without subsequent nanobeam growth, resulted in the formation of VOx nanoparticles; such particles are expected to form during the normal growth process. Although method B did not involve oxygen during the temperature ramp, it is a two-step approach in that a high density of nuclei were generated before achieving the steady-state growth conditions. Improvements in density and aspect ratio were observed in regions where sol−gel derived V2O5 patterns were deposited prior to growth (Figure 5), leading to nanobeam densities as high as 3 nanobeams/μm2. The patterned growth also demonstrates the realization of site-specific growth, although some nucleation did occur in regions without patterns. Growth mostly occurs on predeposited nuclei as negligible nucleation occurs during the synthetic process, thereby increasing the aspect ratio of VO2 nanobeams. The increased yield is associated with the dramatically increased nuclei density in the patterned regions. Seeding substrates with ground VO2 powder dispersed in ethanol prior also resulted in improved nanobeam density (∼2.2 nanobeams/μm2). Aspect ratios were also very high (as high as ∼50), although there was a large variation within each region. The ground VO2 powder in this

suggest that nucleation of VO2 nanobeams is preceded by formation of V2O5 nuclei,7,8 which one might expect to form if oxygen is introduced into the reactor. In-situ grazing incidence small-angle X-ray scattering (GISAXS),7a as well as optical microscopy monitoring,7b suggest that under some conditions, growth is mediated by liquid V2O5,7 as well as V6O13.7b Consideration of these prior studies, together with the data of Figure 1, suggest why the manipulation of oxygen partial pressure is a useful approach to improving uniformity and yield in VO2 nanobeam growth: a high initial oxygen partial pressure should promote V2O5 nucleation, and a subsequent reduction in oxygen partial pressure (and therefore source supersaturation) will promote 1D growth over 2D or thin film growth. In addition, modulation of oxygen partial pressure is faster and simpler than direct modulation of source powder temperature. Method A, described above, was devised on the basis of these principles; further evidence of its efficacy is given below. Method B is an alternative approach to achieve similar ends. We first describe the application of method A followed by method B. 3.2.1. Method A: Introduction of O2. Oxygen gas was introduced during the ramp up to growth temperature (stage I) to favor formation of V2O5 droplets, which in turn leads to the nucleation of VO2 nanocrystals. Upon reaching the target temperature for growth (stage II), O2 flow was terminated to favor nanobeam growth. As compared to growths without an O2 coflow, this two-step approach increased the density of nanobeams from ∼0.05 to ∼0.6 nanobeams/μm2, increased their aspect ratios, and improved the uniformity in distribution and size (Figure 2). To verify that the increased density is

Figure 2. (a) SEM image of VO2 nanobeams grown without an oxygen coflow. Heterogeneous nucleation is associated with impurities left by tweezer tips (above the dashed line). (b) SEM image of VO2 nanobeams grown with an oxygen coflow during step I (nucleation).

induced by the introduction of oxygen during the ramp up (stage I), two sets of growth substrates were cooled immediately following stage I. A significantly increased density of nuclei was observed in the substrates exposed to oxygen. Introduction of O2 causes a disproportionation reaction of VO2 source powder, resulting in the formation of lower melting point V2O5 (mp 963 K).14 This in turn increases the source vapor supersaturation and the probability of nucleation on growth substrate. Consistent with previous reports,7 post 1385

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Figure 3. (a−c) SEM images of VO2 nanobeams observed in the regions I, II, and III, respectively, as indicated in Figure 1a. (d) TEM image of a VO2 nanobeam grown via two-step synthesis procedure with the introduction of O2. (e) Aspect ratios of nanowires in (a) red, (b) purple, and (c) blue. (f) SAED pattern taken from the [122] zone axis indicating single-crystalline nature of the VO2 nanobeam with monoclinic (M1) structure as confirmed by Raman spectroscopy.

Figure 5. (a,b) Preferential growth of VO2 nanobeams on sol−gel derived VOx patterns after growth for 3 min and 2 h, respectively. (c) Formation of VOx nanoparticles upon annealing of sol−gel derived thin film (dashed lines are a guide to the eye indicating location of the VOx pattern) prior to growth. (d) Higher magnification image of (b) taken at the bottom right corner.

Figure 4. (a,b) Optical images of VO2 nanobeams grown on c-cut sapphire substrates with low and high oxygen partial pressure, respectively. Significantly improved density and uniformity of VO2 nanobeams is clearly seen, illustrating the importance of oxygen.

4. CONCLUSIONS The influence of local reactor conditions on VO2 nanocrystal morphology was described for a vapor transport growth process. The morphology evolved from nanoparticles to nanobeams to nanosheets with increasing supersaturation and temperature. On the basis of the observed relationships, we conclude that a two-step synthesis to first induce nucleation and then promote high aspect ratio nanobeam growth provides improved control over yield and morphology. The deliberate modulation of oxygen partial pressure was shown to be a facile means to modulate source vapor partial pressure. Several related approaches were observed to promote nucleation, including introduction of oxygen during the temperature ramp,

case appears to act as local source,7b enhancing the local partial pressure of the source vapor at the location of seeds, which in turn increases the density of nuclei near seeds. We note that similar results have been reported7b using V2O5 powder, possibly due to local V2O5 powder acting as the source of nuclei. The novel aspect of the present work lies in the patterning of the nanostructure growth, which also enables a qualitative perspective on the extent to which the patterns themselves act as sources for nanostructure growth outside their boundaries. 1386

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fabricating oxide patterns on substrates prior to growth, and seeding the substrate with source powder. Site-specific growth of VO2 nanobeams was also demonstrated. In light of our recent study demonstrating the unusually strong influence of stoichiometry on the metal−insulator transition in VO2 nanobeams,15 this work demonstrates the importance and utility of controlling oxygen partial pressures at all stages of growth.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected].

ACKNOWLEDGMENTS We thank the Office of Naval Research (N00014-09-0182) and the National Science Foundation (DMR-1006069) for support. We acknowledge use of the NUANCE facility at Northwestern University.

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REFERENCES

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