The roster of materials exhibiting metal–insulator transitions with sharply discontinuous switching of electrical conductivity close to room temperature remains rather sparse, despite the fundamental interest in the electronic instabilities manifested in such materials and the plethora of potential technological applications ranging from frequency‐agile metamaterials to electrochromic coatings and Mott field‐effect transistors. Here, unprecedented, pronounced metal‐insulator transitions induced by application of a voltage are demonstrated for nanowires of a vanadium oxide bronze with intercalated divalent cations, β‐PbxV2O5 (x ≈ 0.33). The induction of the phase transition through application of an electric field at room temperature makes this system particularly attractive and viable for technological applications. A mechanistic basis for the phase transition is proposed based on charge disproportionation evidenced at room temperature in near‐edge X‐ray absorption fine structure (NEXAFS) spectroscopy measurements, ab initio density functional theory calculations of the band structure, and electrical transport data, suggesting that transformation to the metallic state is induced by melting of specific charge localization and ordering motifs extant in these materials.

VO2 nanostructures derived from solution-phase methods are often plagued by broadened and relatively diminished metal-insulator transitions and adventitious doping due to imperfect control of stoichiometry. Here, we demonstrate a stepwise scalable hydrothermal and annealing route for obtaining VO2 nanowires exhibiting almost 4 orders of magnitude abrupt (within 1 °C) metal-insulator transitions. The prepared nanowires have been characterized across their structural and electronic phase transitions using single-nanowire Raman microprobe analysis, ensemble differential scanning calorimetry, and single-nanowire electrical transport measurements. The electrical band gap is determined to be 600 meV and is consistent with the optical band gap of VO2, and the narrowness of differential scanning calorimetry profiles indicates homogeneity of stoichiometry. The preparation of high-quality free-standing nanowires exhibiting pronounced metal-insulator transitions by a solution-phase process allows for scalability, further solution-phase processing, incorporation within nanocomposites, and integration onto arbitrary substrates.

There has been long-standing interest in tuning the metal-insulator phase transition in vanadium dioxide (VO2) via the addition of chemical dopants. However, the underlying mechanisms by which doping elements regulate the phase transition in VO2 are poorly understood. Taking advantage of aberration-corrected scanning transmission electron microscopy, we reveal the atomistic origins by which tungsten (W) dopants influence the phase transition in single crystalline WxV1-xO2 nanowires. Our atomically resolved strain maps clearly show the localized strain normal to the (122̅) lattice planes of the low W-doped monoclinic structure (insulator). These strain maps demonstrate how anisotropic localized stress created by dopants in the monoclinic structure accelerates the phase transition and lead to relaxation of structure in tetragonal form. In contrast, the strain distribution in the high W-doped VO2 structure is relatively uniform as a result of transition to tetragonal (metallic) phase. The directional strain gradients are furthermore corroborated by density functional theory calculations that show the energetic consequences of distortions to the local structure. These findings pave the roadmap for lattice-stress engineering of the MIT behavior in strongly correlated materials for specific applications such as ultrafast electronic switches and electro-optical sensors.

Vanadium oxide bronzes, with the general formula M x V 2 O 5 , provide a wealth of compositions and frameworks where strong electron correlation can be systematically (albeit thus far only empirically) tuned. In this work, we report the synthesis of single-crystalline δ-Ag 0.88 V 2 O 5 nanowires and unravel pronounced electronic phase transitions induced in response to temperature and applied electric field. Specifically, a pronounced semiconductor−semiconductor transition is evidenced for these materials at ca. 150 K upon heating, and a distinctive insulator−conductor transition is observed upon application of an in-plane voltage. An orbital-specific picture of the mechanistic basis of the phase transitions is proposed using a combination of density functional theory (DFT) and near-edge X-ray absorption fine structure (NEXAFS) spectroscopy. Structural refinements above and below the transition temperature, angle-resolved O K-edge NEXAFS spectra, and DFT calculations suggest that the electronic phase transitions in these 2D frameworks are mediated by a change in the overlap of d xy orbitals. ■ INTRODUCTIONElectronic phase transitions accompanied by dramatic changes of electrical conductivity are of great fundamental interest, and while defining design principles remain to be elucidated, such phenomena are often manifested in materials characterized by structural or electronic instabilities. 1−4 Beyond the fundamental allure of developing a mechanistic description of abrupt changes in physical properties, such phase transitions are also of great practical interest for designing new computing vectors (such as Mott field-effect transistors) and for applications spanning the range from memristors, electromagnetic modulators, and thermal switches to neural networks and electrochromic/thermochromic coatings. 1,3 Electronic phase transitions induced in materials as a result of thermal, electrical, mechanical, or magnetic stimuli can be underpinned by a wide range of mechanisms, such as electron−electron correlation (the Mott−Hubbard picture), electron−phonon coupling (such as Peierl's distortion of the atomistic structure), and disorder (Anderson's localization). 3,5 While considerable interest has focused on the canonical metal−insulator transition material VO 2 , the relatively large structural transformation (and concomitant elastic and strain effects), sluggish relaxation dynamics of the metal → insulator transition, and impediments to decoupling the structural progression from the electronic transition in VO 2 have spurred increasing interest in the discovery of other materials exhibiting pronounced electronic phase transitions at relatively high temperatures. Vanadium oxide bronzes, with the general formula M x V 2 O 5 , provide a richly diversified set of compositions and compounds, where strong electron correlation can be systematically (albeit thus far only empirically) tuned. 6−13 In this work, we report the synthesis of single-crystalline δ-Ag 0.88 V 2 O 5 nanowires, examine the electronic structure of this m...

We demonstrate through electrolyte gating measurements of a single nanobeam that the rultile phase of VO 2 is electrochemically more reactive than the monoclinic phase. Our results show that the complete suppression of the metal to insulator transition and stabilization of the metallic phase is possible when gate voltage is applied in the rutile metallic phase. The results are discussed based on the formation of oxygen vacancies wherein accommodation of a high concentration of vacancies in the rutile phase selectively stabilizes it by disrupting dimerization of adjacent V-V pairs required for transition to the monoclinic phase. The creation of oxygen vacancies is proposed to proceed through the oxidation of the electrolyte. Raman spectroscopy data suggest surface metallization upon electrolyte gating with initial coexistence of insulating monoclinic and metallic domains. The selective electrochemical reactivity of the rutile phase and the resulting defect-induced stabilization of this phase across a vastly expanded temperature window suggest a facile defect engineering route to tune electronic phase transitions.