Coherently coupled ZnO and VO2 interface studied by photoluminescence and electrical transport across a phase transition
TL;DR: In this paper, the photoluminescence and electrical properties of a coherently coupled interface consisting of a ZnO layer grown on top of an oriented VO2 layer on sapphire across the phase transition of VO2 were investigated.
Abstract: We have investigated the photoluminescence and electrical properties of a coherently coupled interface consisting of a ZnO layer grown on top of an oriented VO2 layer on sapphire across the phase transition of VO2. The band edge and defect luminescence of the ZnO overlayer exhibit hysteresis in opposite directions induced by the phase transition of VO2. Concomitantly the phase transition of VO2 was seen to induce defects in the ZnO layer. Such coherently coupled interfaces could be of use in characterizing the stability of a variety of interfaces in situ and also for novel device application.
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TL;DR: This work reveals the feasibility to realize tunable/active and extremely low-profile polarization manipulation devices in the terahertz regime through the incorporation of such phase-change metasurfaces, enabling novel applications of ultrathin terAhertz meta- devices.
Abstract: Metamaterials open up various exotic means to control electromagnetic waves and among them polarization manipulations with metamaterials have attracted intense attention. As of today, static responses of resonators in metamaterials lead to a narrow-band and single-function operation. Extension of the working frequency relies on multilayer metamaterials or different unit cells, which hinder the development of ultra-compact optical systems. In this work, we demonstrate a switchable ultrathin terahertz quarter-wave plate by hybridizing a phase change material, vanadium dioxide (VO2), with a metasurface. Before the phase transition, VO2 behaves as a semiconductor and the metasurface operates as a quarter-wave plate at 0.468 THz. After the transition to metal phase, the quarter-wave plate operates at 0.502 THz. At the corresponding operating frequencies, the metasurface converts a linearly polarized light into a circularly polarized light. This work reveals the feasibility to realize tunable/active and extremely low-profile polarization manipulation devices in the terahertz regime through the incorporation of such phase-change metasurfaces, enabling novel applications of ultrathin terahertz meta-devices.
246 citations
Journal Article•
TL;DR: In this paper, electron transport data was correlated with energy band structure measurements in vanadium oxide thin films with varying V-O stoichiometry across the VO2 metal-insulator transition.
Abstract: We correlate electron transport data directly with energy band structure measurements in vanadium oxide thin films with varying V-O stoichiometry across the VO2 metal-insulator transition. A set of vanadium oxide thin films were prepared by reactive dc sputtering from a V target at various oxygen partial pressures (O2 p.p.). Metal-insulator transition (MIT) characteristic to VO2 can be seen from the temperature dependence of electrical resistance of the films sputtered at optimal O2 p.p. Lower and higher O2 p.p. result in disappearance of the MIT. The results of the near edge x-ray absorption fine structure spectroscopy of the O K edge in identical VO films are presented. Redistribution of the spectral weight from {sigma}* to {pi}* bands is found in the vanadium oxide films exhibiting stronger VO2 MIT. This is taken as evidence of the strengthening of the metal-metal ion interaction with respect to the metal-ligand and indirect V-O-V interaction in vanadium oxide films featuring sharp MIT. We also observe a clear correlation between MIT and the width and area of the lower {pi}* band, which is likely to be due to the emergence of the d|| band overlapping with {pi}*. The strengthening of this d|| band near the Fermi levelmore » only in the vanadium oxide compounds displaying the MIT points out the importance of the role of the d|| band and electron correlations in the phase transition.« less
85 citations
TL;DR: The high-yield fabrication of a series of functionalized V2O5 nanoassemblies through a facile polyol approach with specific varieties of polyvinylpyrrolidone exhibit distinct hemispherical or spherical hollow morphologies and operate as room-temperature hydrogen sensors with remarkable sensitivities and responses.
Abstract: Nanostructured oxides with characteristic morphologies are essential building blocks for high-performance gas-sensing devices. We describe the high-yield fabrication of a series of functionalized V2O5 nanoassemblies through a facile polyol approach with specific varieties of polyvinylpyrrolidone. The synthesized V2O5 nanoassemblies consisting of tiny one-dimensional nanoblocks with the absence of any extrinsic catalysts exhibit distinct hemispherical or spherical hollow morphologies and operate as room-temperature hydrogen sensors with remarkable sensitivities and responses.
56 citations
TL;DR: Time-resolved photoluminescence study did not reveal any significant change of luminescence lifetime of deposited quantum dots under metal to insulator transition, and there is a strong argument in favor of the proposed explanation based on the reflectance data.
Abstract: We have proposed a method to probe metal to insulator transition in VO2 measuring photoluminescence response of colloidal quantum dots deposited on the VO2 film. In addition to linear luminescence intensity decrease with temperature that is well known for quantum dots, temperature ranges with enhanced photoluminescence changes have been found during phase transition in the oxide. Corresponding temperature derived from luminescence dependence on temperature closely correlates with that from resistance measurement during heating. The supporting reflectance data point out that photoluminescence response mimics a reflectance change in VO2 across metal to insulator transition. Time-resolved photoluminescence study did not reveal any significant change of luminescence lifetime of deposited quantum dots under metal to insulator transition. It is a strong argument in favor of the proposed explanation based on the reflectance data.
50 citations
Cites background from "Coherently coupled ZnO and VO2 inte..."
...In contrast to [4], a deposition of self-assembled QD layer on a VO2 film does not require a high-temperature treatment; therefore, the intrinsic properties of vanadium...
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...In the bilayer structure of ZnO/VO2, MIT in VO2 was shown to control a photoluminescence response of zinc oxide [4]....
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TL;DR: In this article, SiO2 coated VO2 films with clearly enhanced visible transmittance by introducing antireflection coatings (ARCs) and excellent thermochromic performance were present.
33 citations
References
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TL;DR: The photoluminescence (PL) spectra of undoped ZnO films deposited on Si substrates by dc reactive sputtering have been studied in this paper, where two emission peaks, centered at 3.18 eV and 2.38 eV, were found to correspond to oxide antisite defect OZn rather than oxygen vacancy VO, zinc vacancy VZn, interstitial zinc Zni, and interstitial oxygen Oi.
Abstract: The photoluminescence (PL) spectra of the undoped ZnO films deposited on Si substrates by dc reactive sputtering have been studied. There are two emission peaks, centered at 3.18 eV (UV) and 2.38 eV (green). The variation of these peak intensities and that of the I–V properties of the ZnO/Si heterojunctions were investigated at different annealing temperatures and atmospheres. The defect levels in ZnO films were also calculated using the method of full-potential linear muffin-tin orbital. It is concluded that the green emission corresponds to the local level composed by oxide antisite defect OZn rather than oxygen vacancy VO, zinc vacancy VZn, interstitial zinc Zni, and interstitial oxygen Oi.
1,923 citations
TL;DR: A low-temperature, large-scale, and versatile synthetic process is needed before ZnO nanowire arrays find realistic applications in solar energy conversion, light emission, and other promising areas, and the ease of commercial scale-up is presented.
Abstract: Since the first report of ultraviolet lasing from ZnO nanowires, substantial effort has been devoted to the development of synthetic methodologies for one-dimensional ZnO nanostructures. Among the various techniques described in the literature, evaporation and condensation processes are favored for their simplicity and high-quality products, but these gas-phase approaches generally require economically prohibitive temperatures of 800–900 8C. Despite recent MOCVD schemes that reduced the deposition temperature to 450 8C by using organometallic zinc precursors, the commercial potential of gas-phase-grown ZnO nanowires remains constrained by the expensive and/or insulating (for example, Al2O3) substrates required for oriented growth, as well as the size and cost of the vapor deposition systems. A low-temperature, large-scale, and versatile synthetic process is needed before ZnO nanowire arrays find realistic applications in solar energy conversion, light emission, and other promising areas. Solution approaches to ZnO nanowires are appealing because of their low growth temperatures and good potential for scale-up. In this regard, Vayssieres et al. developed a hydrothermal process for producing arrays of ZnO microrods and nanorods on conducting glass substrates at 95 8C. Recently, a seeded growth process was used to make helical ZnO rods and columns at a similar temperature. Here we expand on these synthetic methods to produce homogeneous and dense arrays of ZnO nanowires that can be grown on arbitrary substrates under mild aqueous conditions. We present data for arrays on four-inch (ca. 10 cm) silicon wafers and two-inch plastic substrates, which demonstrate the ease of commercial scale-up. The simple two-step procedure yields oriented nanowire films with the largest surface area yet reported for nanowire arrays. The growth process ensures that a majority of the nanowires in the array are in direct contact with the substrate and provide a continuous pathway for carrier transport, an important feature for future electronic devices based on these materials. Well-aligned ZnO nanowire arrays were grown using a simple two-step process. In the first step, ZnO nanocrystals (5–10 nm in diameter) were spin-cast several times onto a four-inch Si(100) wafer to form a 50–200-nm thick film of crystal seeds. Between coatings, the wafer was annealed at 150 8C to ensure particle adhesion to the wafer surface. The ZnO nanocrystals were prepared according to the method of Pacholski. A NaOH solution in methanol (0.03m) was added slowly to a solution of zinc acetate dihydrate (0.01m) in methanol at 60 8C and stirred for two hours. The resulting nanoparticles are spherical and stable for at least two weeks in solution. After uniformly coating the silicon wafer with ZnO nanocrystals, hydrothermal ZnO growth was carried out by suspending the wafer upside-down in an open crystallizing dish filled with an aqueous solution of zinc nitrate hydrate (0.025m) and methenamine or diethylenetriamine (0.025m) at 90 8C. Reaction times spanned from 0.5 to 6 h. The wafer was then removed from solution, rinsed with deionized water, and dried. A field-emission scanning electron microscope (FESEM) was used to examine the morphology of the nanowire array across the entire wafer, while single nanowires were characterized by transmission electron microscopy (TEM). Nanowire crystallinity and growth direction were analyzed by X-ray diffraction and electron diffraction techniques. SEM images taken of several four-inch samples showed that the entire wafer was coated with a highly uniform and densely packed array of ZnO nanowires (Figure 1). X-ray diffraction (not shown) gave a wurtzite ZnO pattern with an enhanced (002) peak resulting from the vertical orientation of the nanowires. A typical synthesis (1.5 h) yielded wires with diameters ranging between 40–80 nm and lengths of 1.5–2 mm.
1,676 citations
TL;DR: In this paper, the optical constants of V${\mathrm{O}}_{2} have been determined between 0.25 and 5 eV both below and above the semiconductor-metal transition temperature.
Abstract: The optical constants of V${\mathrm{O}}_{2}$ have been determined between 0.25 and 5 eV both below and above the semiconductor-metal transition temperature ${T}_{t}=340\ifmmode^\circ\else\textdegree\fi{}$K. Reflectivity and transmission spectra have been measured on both single crystals and than films. The reflectivity spectra of the bulk crystals were measured with E \ensuremath{\perp} ($c$ axis) in the tetragonal phase [or \ensuremath{\perp} ($a$ axis) in the monoclinic phase], and with E parallel to these axes. While there are some differences in magnitude between the dielectric constants obtained from thin-film and single-crystal measurements, the structural features are in good agreement. Below ${T}_{t}$ there are four prominent absorption peaks centered near photon energies of 0.85, 1.3, 2.8, and 3.6 eV. Above ${T}_{t}$, metallic free-carrier absorption is observed below 2.0 eV, but the same two absorption peaks near 3 and 4 eV are present. The energy location and polarization dependence of these two higher energy peaks can be related to similar absorption peaks in rutile, and are interpreted using the rutile band structure. The results are consistent with a picture in which filled bands arising primarily from oxygen $2p$ orbitals are separated by approximately 2.5 eV from partially filled bands arising primarily from vanadium $3d$ orbitals. Transitions from the filled $2p$ bands are responsible for the high-energy peaks in the optical absorption in both the high- and low-temperature phases. In the high-temperature metallic phase, there is evidence that there is overlap among the $3d$ bands such that at least two bands are partially occupied by the extra $d$ electron per vanadium ion. In the low-temperature semiconductor phase, a band gap of approximately 0.6 eV opens up within the $3d$ bands, separating two filled bands from higher-lying empty bands. The two absorption peaks at 0.85 and 1.3 eV are due to transitions from these two filled bands.
509 citations
TL;DR: In this article, the influence of oxygen pressure on the epitaxy, surface morphology, and optoelectronic properties has been studied in the case of ZnO thin films grown on sapphire (0001) by pulsed-laser deposition.
Abstract: Influence of oxygen pressure on the epitaxy, surface morphology, and optoelectronic properties has been studied in the case of ZnO thin films grown on sapphire (0001) by pulsed-laser deposition. Results of Rutherford backscattering and ion channeling in conjunction with atomic force microscopy clearly indicate that the growth mode, degree of epitaxy, and the defect density strongly depend on the oxygen background pressure during growth. It is also found that the growth mode and the defects strongly influence the electron mobility, free-electron concentration, and the luminescence properties of the ZnO films. By tuning the oxygen pressure during the initial and the final growth stages, smooth and epitaxial ZnO films with high optical quality, high electron mobility, and low background carrier concentration have been obtained. The implication of these results towards the fabrication of superlattices and controlled n- and p-type doping is discussed.
318 citations
TL;DR: The first Raman scattering measurements on nanoparticulate vanadium dioxide (VO(2), as well as the first observations of the temperature-induced phase transition in individual VO(2) nanoparticles (NPs), are presented.
Abstract: We present the first Raman scattering measurements on nanoparticulate vanadium dioxide (VO2), as well as the first observations of the temperature-induced phase transition in individual VO2 nanoparticles (NPs). We compare the Raman response of two VO2 NPs and a companion VO2 film undergoing their monoclinic-tetragonal-monoclinic transformations and offer qualitative explanations for the large observed differences in hysteresis width. While bulk crystals and contiguous films contain numerous nucleation sites, individual NPs likely harbor only a few, which may make it possible to correlate detectable defects (e.g., grain boundaries and dislocations) with the “ease” of switching phases, as quantified by the width of the thermal hysteresis.
115 citations