Electronic Phase Transitions of δ-Ag_xV₂O₅ Nanowires: Interplay between Geometric and Electronic Structures

Abstract

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...

Mapping polaronic states and lithiation gradients in individual V2O5 nanowires

“…1e), the integrated element-specific spectrum in Fig. 2b suggests that the electrons donated by the inserted Li ions have been transferred and reside on the V 2 O 5 framework but are localized on the lowest-lying V=O 3 d xz/yz –O 2 p hybridized states of the conduction band, which have been substantially distorted as a result of lithiation17394041. Interestingly, on delithiation by immersion in Br 2 solution the electronic structure of V 2 O 5 is recovered in its entirety (Fig.…”

Emptying and filling a tunnel bronze

“… 16 , 17 Cation intercalation and concomitant partial reduction of the V 2 O 5 frameworks yields charge ordered networks that extend along the length of the vanadium oxide chains, thereby affording excellent model systems for examination of strong electron correlation. 18 The size, polarizability, extent of covalency of cation–oxygen interactions, and cation concentration dictates the structure adopted by the ternary and quaternary vanadium oxides with single-layered, highly puckered layered, double-layered, and tunnel frameworks being some of the more commonly adopted frameworks. 2a , c , 16c , 19 , 20 Indeed, the ability to fill empty tunnels of the novel ζ-V 2 O 5 phase with other cations as noted below provides a versatile synthetic route for obtaining intercalated ternary vanadium oxide bronzes and for systematically tuning electron correlation.…”

Structure‐Induced Switching of the Band Gap, Charge Order, and Correlation Strength in Ternary Vanadium Oxide Bronzes

“…At the same time, this can lead to the creation of many unique V 2 O 5 bronzes (M x V 2 O 5 , M=Ca, Sr, K, Pb. ), which exhibit multiple charge‐ordered states, electronic correlation effects, and colossal metal to insulator transitions . The accessibility of multiple oxidation states of the V ions also makes these materials of significant interest in the creation of supercapacitors .…”

“…), whiche xhibit multiple charge-ordered states, electronic correlation effects, and colossalm etal to insulator transitions. [11][12][13][14][15][16][17][18][19] The accessibility of multiple oxidation states of the Vions also makes these materials of significant interest in the creation of supercapacitors. [20,21] As such,t hese materials are of interest from the perspective of fundamental materials science,a sw ell as for their potential applications at the frontiers of electronic devicee ngineering, such as neuromorphic circuitry,M ott field-effect transistors,a nd electrochromic devices.…”

“…[22][23][24][25] The electronic phase diagrams of M x V 2 O 5 bronzes (whereM is an intercalated cation and x is its stoichiometry) are highly sensitivet ot he type and concentration of cation, temperature, and applied voltage, and are closely linked to the crystal structure. [11,12,[14][15][16] This leads to several methodsf or tuning their electronic properties through alteration of the precise pattern of chargeo rdering, the overlap of the Vd -states,a nd the band gap, which ultimately controlt he conductivity.…”