Synthesis, Spectroscopic Characterization, and Observation of

May 28, 2010 - Synthesis, Spectroscopic Characterization, and Observation of Massive Metal—Insulator Transitions in Nanowires of a Nonstoichiometric...
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Synthesis, Spectroscopic Characterization, and Observation of Massive MetalsInsulator Transitions in Nanowires of a Nonstoichiometric Vanadium Oxide Bronze Christopher J. Patridge,†,# Tai-Lung Wu,‡,# Cherno Jaye,§ Bruce Ravel,§ Esther S. Takeuchi,|,⊥ Daniel A. Fischer,§ G. Sambandamurthy,*,‡ and Sarbajit Banerjee*,† †

Department of Chemistry, University at Buffalo, Buffalo, New York 14260-3000, ‡ Department of Physics, University at Buffalo, Buffalo, New York 14260, § Materials Science and Engineering Laboratory, National Institute of Standards and Technology, Gaithersburg, Maryland 20899, | Department of Electrical Engineering, University at Buffalo, Buffalo, New York 14260, and ⊥ Department of Chemical and Biological Engineering, University at Buffalo, Buffalo, New York 14260 ABSTRACT Metalsinsulator transitions in strongly correlated materials, induced by varying either temperature or dopant concentration, remain a topic of enduring interest in solid-state chemistry and physics owing to their fundamental importance in answering longstanding questions regarding correlation effects. We note here the unprecedented observation of a four-orders-of-magnitude metalsinsulator transition in single nanowires of δ-KxV2O5, when temperature is varied, which thus represents a rare new addition to the pantheon of materials exhibiting pronounced metalsinsulator transitions in proximity to room temperature. KEYWORDS Nanowires, X-ray absorption spectroscopy, solvothermal synthesis, vanadium oxides, metal-insulator transition

+5), varying coordination environments (octahedral, square pyramidal, and tetrahedral), different possible connectivities/ distortions of vanadium-centered polyhedra, and the available recourse to crystallographic shear and slips to accommodate vacancies and nonstoichiometry.10–12 The open framework adopted by V2O5 is notable for enabling the intercalation of various alkali- and alkaline-earth metal ions, yielding nonstoichiometric bronzes with the general formula MxV2O5.12,13 The intercalated ions typically alter the electronic and geometric structure of the resulting frameworks and give rise to a myriad of interesting properties including charge ordering, superconductivity at high pressures, metalinsulator transitions, and low-dimensional quantum spin phenomena.14–17 Nevertheless, of the MxV2O5 bronzes studied thus far, none are comparable to their more famous binary oxide kin VO2 and V2O3 in terms of magnitude of the metal-insulator transition and proximity of the thermal transition to room temperature.14,18 Notably, however, several studies suggest that metal-insulator transitions in quasi-1D bronzes exhibit a fine dependence on the precise nonstoichiometry of the intercalating cations, and in particular significant degradation of metallic behavior is evidenced even at low defect densities.14,18,19 Measurements of single nanostructures of vanadium oxide bronzes are thus potentially of interest to obtain fundamental insight into the intrinsic conductivity aspects of these materials that are reflective of true changes in electronic structure and electron correlation effects without obscuration from extraneous

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he underlying operating principles for proposed technological applications ranging from switching elements and thermochromic coatings to infrared modulators essentially involve the thermal or voltage-induced induction of an insulator-metal transition.1–4 Recently, there has been significant interest in understanding the influence of finite size on phenomena influenced by correlation effects, such as near the transition to the metallic state, since scaling materials to nanoscale dimensions offers the possibility of enabling the observation of single-domain phenomena that are otherwise obscured in bulk (even singlecrystalline) samples.6–9 Finite size can alter the transition thermodynamics via strain effects or by modifying the density (and overall number) of possible nucleation sites. The binary vanadium oxides VO2 and V2O3 are textbook examples of correlated materials exhibiting pronounced thermally activated alterations in spin, lattice, and orbital degrees of freedom near the metal-insulator transition temperature.3,5 Binary vanadium oxides and ternary vanadium oxide bronze phases derived from the intercalation of metal ions within vanadium oxide frameworks can adopt various structures because of the ready accessibility of different vanadium oxidation states (ranging from -1 to

* To whom correspondence should be addressed, [email protected] and [email protected]. # These authors contributed equally to this work. Received for review: 02/22/2010 Published on Web: 05/28/2010

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defects that are inevitably present in bulk samples. We demonstrate here the observation of an unprecedented 4-orders-of-magnitude metal-insulator transition in nanowires of δ-KxV2O5 prepared by a facile hydrothermal synthesis approach. Experimental Section. Synthesis of KxV2O5 Nanowires. NaNO3 (0.095 g) and V2O5 (0.205 g) were placed in a Teflon cup along with 14 mL of H2O (F ) 18 MΩ cm) and 1 mL of 0.61 M KOH. The mixture was sealed in an acid digestion bomb (Parr) and maintained at 210 °C for 96 h. Upon cooling, the solid was vacuum-filtered and washed with deionized water. Recovered solid was dried at 110 °C overnight. The material was then heated to 250 °C for 12 h to remove any bound water. Characterization. The prepared nanowires were characterized by scanning electron microscopy (SEM, JSM5610LV and Hitachi SU-70 operated at 20 kV equipped with an EDX detector) and transmission electron microscopy (TEM, JEOL 2010 operated at 200 kV). Selected area electron diffraction data were acquired on isolated nanowires using an aperture of 50 µm. The geometric and electronic structure of the δ-KxV2O5 nanowires was studied using V K-edge, K L-edge, V L-edge, and O K-edge NEXAFS measurements performed on the National Institute of Standards and Technology (NIST) beamlines U7A and X23A2 at the National Synchrotron Light Source (NSLS) of Brookhaven National Laboratory. (See Supporting Information for more details of the experimental protocols.) Single-Nanowire Electrical Measurements. As-prepared δ-KxV2O5 nanowires were dispersed in 2-propanol by mild ultrasonication to break up agglomerates of nanowires. After the solution was allowed to settle for several hours, a small amount of the solution containing individual nanowires was taken from the top after allowing the larger bundles to settle to the bottom. The mostly individual nanowires were dispersed onto Si/SiO2 substrates patterned with predeposited gold position markers. Optical microscopy was then used to locate the position of individual nanowires, and gold electrodes (10 nm of Ti/Cr and 60 nm of Au) were subsequently deposited onto the nanowires using standard photolithography or electron-beam lithography and metallization techniques. The electrical transport properties of the nanowires were measured using low-frequency (f < 100 Hz) lock-in techniques with ac excitation currents of ∼10 nA. Room temperature commercial preamplifiers were used for low resistance values in the metallic phase. The measurements were carried out in a Quantum Design physical property measurement system (PPMS). Results and Discussion. Figure 1A shows SEM images of novel δ-KxV2O5 nanowires prepared by the hydrothermal intercalation of potassium ions between the layers of V2O5. The use of excess KOH precursor and long reaction times (96 h) ensures the formation of vanadium bronze structures that are near the homogeneity limit of x ) 0.50 for the monoclinic double-layered δ-phase. The TEM images in © 2010 American Chemical Society

FIGURE 1. (A) SEM image indicating the high yield and purity of the hydrothermal synthesis process yielding δ-KxV2O5 nanowires. (B) TEM image showing several nanowires; the inset depicts the diameter distribution deduced from the TEM images. (C) HRTEM image of an individual δ-KxV2O5 nanowire evidencing the single crystalline nature of the nanowires and clearly depicting the nanowire growth direction. The inset (C-1) shows a lower-magnification image of this nanowire. (D) Selected area electron diffraction pattern acquired for the individual nanowire shown in (C).

panels B and C of Figure 1 indicate that the solid nanowires have an approximate diameter of 180 nm (with surprisingly good monodispersity for hydrothermal synthesis) with lengths ranging up to several hundreds of micrometers. The latticeresolved high-resolution TEM image and selected area electron diffraction pattern demonstrate that the nanowires are solid and single crystalline with no cavities and that the growth direction is perpendicular to the (001) planes. Figure 2B shows the X-ray diffraction pattern of the obtained nanowires, which can be readily indexed to monoclinic δ-K0.486V2O5 (a ) 11.688 Å, b ) 3.668 Å, c ) 9.505 Å, β ) 92.24°; Joint Committee on Powder Diffraction Standards (JCPDS# 86-0347)).20 The specific geometry adopted upon the intercalation of cations within the layered V2O5 framework depends upon the stacking distance and crystallinity of the host framework and the size, charge, concentration, and polarizability of the inserted ions.10,12,13 For larger ions such as Ag+, Tl+, and K+, double-layered bronzes are strongly preferred due to the significant lattice distortions induced upon their intercalation, as shown in Figure 2A. The resulting structure can be conceptually derived from the layered V2O5 framework based on cation insertion in every other layer with inversion of some VO5 square pyramids and slipping of adjacent layers.12 In the δ-MxV2O5 phase, single sheets of VO6 polyhedra with vanadyl bonds alternately facing up and down are stitched together by edge sharing along the weak V-O bond direction to yield the double-layered structure depicted in Figure 2A, thus leaving all the vanadyl bonds terminal along the infinite chains in the ab plane.10,12 The intercalated potassium ions are surrounded by seven oxygen atoms 2449

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yielding quasi-1D conduction paths separated by the interstitial K+ ions. Further evidence for the said structure and stoichiometry is derived from the X-ray absorption spectroscopy measurements shown in Figure 2C. Upon intercalation, the potassium atoms are appreciably ionized, as observed with other alkali-metal bronzes,14,17,18,21 resulting in reduction of onefourth of the framework vanadium atoms from V5+ to V4+. Indeed, while the nominal V4+/V5+ ratio of ∼0.25 is particularly attractive for triggering electronic instabilities at the Fermi level to induce electronic phase transitions,22 previous attempts to evidence such a phase transition in δ-phase bronzes have been unsuccessful although a small change in the activation energy for electronic transport was observed for Tl0.48V2O5 at 655 K.17 In this context, anisotropic transport with a modest transition around 340 K was also noted for a β-phase with an approximate stoichiometry of K0.2V2O5 (and a very different crystal structure from the δ-phase) after thermally annealing the sample at 620 K.23 Large shifts of the 2p3/2 and 2p1/2 K L-edge peaks from typical values of 294.6 and 297.3 eV to higher energies (299.3 and 301.9 eV, respectively), evidenced in the near-edge X-ray absorption fine structure (NEXAFS) data in Figure 2C(i), indicate the strong binding of core p electrons and are suggestive of significant ionization of the intercalated potassium atoms.24,25 Figure 2C(iii) shows V K-edge XANES data. Specifically, the threshold absorption and pre-edge features observed at the V K-edge for vanadium oxides show a monotonic dependence of their peak positions with oxidation state according to Kunzl’s law.26 On the basis of measurements of different vanadium standards, an average oxidation state intermediate between +4 and +5 and a distorted octahedral geometry (distorted toward being square pyramidal) is deduced for V atoms in the δ-KxV2O5 nanowires from the position and intensity of the pre-edge feature at the V K-edge (Figure 2C(iii)),27 as also shown in the Supporting Information. The V LIII-edge and O K-edge NEXAFS data (Figure 2C(ii)) suggest the presence of charge disproportionation at room temperature since the splitting observed at the V LIII-edge (∆1 ) 2.0 eV) varies from the splitting at the O K-edge (∆2 ) 2.2 eV). Ma et al. have used density functional theory modeling of the unoccupied density of states to demonstrate that crystallographically inequivalent vanadium atoms in the distorted VO6 polyhedra of vanadium bronzes have very similar local geometries and thus the observation of pronounced splitting at the V LIII-edge cannot be attributed to contributions from different vanadium sites.28 The measured splitting consequently implies the presence of localized V4+ and V5+ sites, characterized by different binding energies of core 2p electrons (due to the different effective nuclear charges), as previously noted for β′-Cu0.65V2O5.28,29 Unlike the characteristic behavior observed for related tungsten (MxWO3) and molybdenum (MxMoO3) bronzes wherein ionization of the intercalating species results in electron occupancy of delocalized bands engendering metal-

FIGURE 2. (A) Crystal structure of δ-KxV2O5 indicating the crystallographically inequivalent vanadium atoms V(1) and V(2). The pink spheres denote the intercalated potassium ions. (B) Powder X-ray diffraction pattern acquired for the nanowires. (C) X-ray absorption near edge structure (XANES) data acquired at (i) K L-edge, (ii) V K-edge, and (iii) V L-edge and O K-edge. In (ii), the splittings ∆1 ) 2.0 eV and ∆2 ) 2.2 eV at the V L-edge and O K-edge, respectively, are indicated on the figure. The top red curve in (iii) shows the derivative of the spectra acquired for the δ-KxV2O5 nanowires at the V K-edge. The arrows and adjacent numerical values are assigned to spectral transitions at the V K-edge as follows: the 5.3 and 28.3 eV peaks indicated in the black curve for δ-KxV2O5 correspond to the pre-edge feature and the 1s f 4p transition, respectively; the 4.2 and 14.7 eV transitions observed in the derivative spectra correspond to the threshold absorption and the edge-jump features, respectively. The intensity of the pre-edge feature in binary and ternary vanadium oxides is known to depend strongly on the “cage” of nearest neighbor atoms and thus provides a good approximation of the local coordination geometry upon locating the measured features within the Chaurand plot depicted in the Supporting Information.26

forming a monocapped trigonal prism.20 Notably, the K-K distance is 2.25 Å in this structure, indicating a maximum value of x ) 0.5 for δ-KxV2O5. Inductively coupled plasma optical emission spectroscopy and energy dispersive X-ray (EDX) analysis indicate potassium stoichiometries very close to this limit for the obtained nanowires. The nanowire growth direction evident in the high-resolution TEM (HRTEM) image of Figure 1C indicates that the double layers of V2O5 are arranged parallel to the long axis of the nanowire, © 2010 American Chemical Society

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lic character, in MxV2O5 structures, the electrons remain localized at specific vanadium sites but are still mobile through a thermally activated small polaron hopping mechanism.17,21 An alternative view would be to consider the bronzes to lie on the insulator side of the Mott transition with Coulomb repulsions causing electron localization upon intercalation. Consistent with this view, the nearest V-V (between V(1) and V(2) sites in Figure 2A) distance in this structure is 2.991 Å,20 which is slightly longer than the critical cationscation separation of 2.94 Å deduced using a semiempirical model by Goodenough for the development of a collective electron band structure based on the direct overlap of d orbitals.21 In the main panel of Figure 3A, we present temperature (T)-dependent electrical resistance data obtained from a single δ-KxV2O5 nanowire aligned between gold electrodes. Optical microscopic and SEM images of one of our device structures are shown in Figure 3C. As shown in Figure 3A, with increasing T above 300 K, the two-terminal resistance of the nanowire decreases in an exponential fashion until it reaches T ∼385 K, at which point the resistance drops by more than 4 orders of magnitude. Upon cooling the wire from 400 K, a reverse discontinuous jump to the insulating phase is seen albeit at a lower T ∼ 366 K. In the insulating phase, the resistance behavior clearly follows R ) Roe(-Ea/kBT) (Figure 3A, inset), consistent with the thermal activation of only the mobility with an activation energy of Ea. The Ea value is 100 ( 10 meV for all the nanowires measured (∼10) in this study. This can be contrasted with the recent single-beam transport measurements in VO2 nanobeams that have yielded an activation energy on the order of 300 meV.6 The small polaron hopping model, often used to explain transport on the insulating side of the metal-insulator transition for vanadium bronzes,21,30 essentially predicts that the carrier density is defined by the stoichiometry of intercalating ions and remains constant over a large temperature range and that only the diffusion constant for the intersite excitation/jumping of electrons is thermally activated with a distinct exponential behavior observed in electrical transport data.21,30 The magnitude of the phase transition noted here, an unprecedented transition for a vanadium bronze (or for that matter any material other than binary vanadium oxides), possibly implies switching from small polaron hopping to collective electron motion through the V2O5 slabs parallel to the nanowire growth direction. Given the proximity of V(1)-V(2) vanadium sites within the δ-KxV2O5 structure, a subtle structural transition may well enable sufficient overlap of the d orbitals, enabling electron delocalization and development of a realistic band structure, giving rise to metallic behavior. Figure 3B shows current-voltage (I-V) plots acquired for the δ-KxV2O5 nanowires at two different temperatures. In the insulating phase (T ) 296 K), when the voltage across the nanowires reaches ∼3.2 V, a discontinuous jump in the current through the wires is seen, clearly indicating a voltage© 2010 American Chemical Society

FIGURE 3. (A) Electrical transport data from a single nanowire. A pronounced hysteretic insulator-metal transition spanning >4 orders of magnitude in resistance is seen. Inset: resistance in the insulating phase follows simple activated behavior. (B) Currents voltage (I-V) curves acquired for the KxV2O5 nanowires indicate a sharp voltage-induced insulator-metal transition. In the insulating phase (T ∼ 296 K) a discontinuous jump to the metallic state is seen ∼3.2 V. The current through the sample is limited at 250 mA in our measurements to avoid damage to the wires. (C) SEM image of the device structure. The inset shows an optical microscopy image of the device. (D) High-magnification SEM image of a nanowire within the device geometry. The inset shows an EDX spectrum acquired for the nanowire, verifying the δ-KxV2O5 stoichiometry. 2451

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induced insulator-metal transition. Going to a higher temperature of 380 K, the wire is now in the metallic phase, and consequently the I-V curve is primarily ohmic (at low bias) and does not show any discontinuous jump. Voltage-induced insulator-metal transitions have previously been demonstrated in VO2 thin films and single crystals, and our data show that δ-KxV2O5 constitutes a new addition to the materials showing both thermally induced and voltage-driven transitions. Notably, no discontinuous phase transitions are observed in measurements of bulk δ-KxV2O5 (Figure S2, Supporting Information), underlining the importance of scaling to single-domain limits. A number of examples of metallic oxides are known for the vanadium bronze family, and there has been longstanding interest in first- and second-order transitions in these materials that yield either charge-ordered insulating phases at ambient pressure or superconducting phases at highpressures upon suppression of the metal-insulator transition.14,18 The metal-insulator transition in these materials typically lies much below room temperature with some of the most relevant values being located at 180 K for β-Li0.33V2O5 and 165 K for β-Sr0.33V2O5.14,18 Further, as noted above, the phase transitions tend to span only about 1 order of magnitude and are not comparable to the massive transitions observed for VO2 and V2O3. The unprecedented magnitude and location of the metal-insulator transition in nanowires of δ-KxV2O5 thus represent an exciting new frontier for the study of electron correlation and novel transport phenomena for materials structurally and electronically approaching single domain limits. Interestingly, a pronounced hysteresis is observed in Figure 3A, ranging up to 40 K in different wires. This scenario appears to be analogous to the stabilization of the supercooled metallic phase for nanobeams and nanobelts of VO2 as evidenced by single-nanowire electrical transport and differential scanning calorimetry measurements.6,8 The observed metalinsulator transition temperature shows variations within ∼30 K for the prepared nanowires and is quite sensitive to the stoichiometry of the M-cation, as has been noted previously for related β-phase MxV2O5 structures.19 In conclusion, single-nanowire measurements of solutiongrown δ-KxV2O5 (x ∼ 0.50) nanostructures evidence massive metal-insulator transitions reminiscent of VO2 and V2O3 in the vicinity of 380 K, representing a valuable new addition to materials exhibiting pronounced metal-insulator transitions above room temperature. On the basis of our preliminary results, analogous to VO2, the transition temperature and hysteresis may be amenable to tuning using strain, substitutional doping, or scaling to yet smaller dimensions. The measurements demonstrate in particular the utility of systematic single-nanowire measurements to uncover phenomena that are obscured in bulk samples.

We are grateful to Professor Hao Zeng for allowing us access to his PPMS system. Certain commercial equipment, instruments, or materials are identified in this article in order to specify the experimental procedure adequately. Such identification is not intended to imply recommendation or endorsement by the National Institute of Standards and Technology, nor is it intended to imply that the materials or equipment are necessarily the best available for this purpose. Supporting Information Available. Experimental protocol for the acquisition of X-ray absorption data, Chaurand plot constructed using V K-edge data and plots of potassium bronze and vanadium standards, and currentsvoltage measurements of bulk δ-KxV2O5 compared to measurements acquired for δ-KxV2O5 nanowires. This material is available free of charge via the Internet at http://pubs.acs.org. REFERENCES AND NOTES (1) (2) (3)

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Acknowledgment. This work was primarily supported by the National Science Foundation under DMR0847169. © 2010 American Chemical Society

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