One route to create tunable metamaterials is through integration with "on-demand" dynamic quantum materials, such as vanadium dioxide (VO2). This enables modalities to create high-performance devices for historically challenging applications. Indeed, dynamic materials have often been integrated with meta -materials to imbue artificial structures with some degree of tunability. Conversely, metamaterials can be used to enhance and extend the natural tuning range of dynamic materials. Utilizing a complementary split -ring resonator array deposited on a VO2 film, we demonstrate enhanced terahertz transmission modulation upon traversing the insulator-to-metal transition (IMT) at approximately 340 K. Our complementary meta -material increases the modulation amplitude of the original VO2 film from 0.42 to 0.68 at 0.47 THz upon crossing the IMT, corresponding to an enhancement of 62%. Moreover, temperature-dependent transmis-sion measurements reveal a significant redshift of the resonant frequency in a narrow temperature range where phase coexistence is known to occur. Neither Maxwell-Garnett nor Bruggeman effective medium theory adequately describes the observed frequency shift and amplitude decrease. However, a Drude model incorporating a significant increase of the effective permittivity does describe the experimentally observed redshift. Our results highlight that symbiotic integration of metamaterial arrays with quantum materials provides a powerful approach to engineer emergent functionality.

The transmission of terahertz (THz) waves in vanadium dioxide (VO2) films decreases with the decrease in resistivity caused by the insulator-to-metal transition. Doping of VO2 films with Cu leads to a narrowing of the hysteresis width and a decrease in the transition temperature. However, Cu doping affects both electrical properties and THz optical properties in such a way that for films with different doping ratios it is not possible to obtain similar resistivities and similar THz transmissions simultaneously. The investigations reported here reveal both the substitutional and interstitial roles of Cu doping, with the latter, in particular, leading to Cu separation. Further analysis shows that the conditions at grain boundaries mainly influence resistivity, while crystal quality significantly affects THz optical properties. Consequently, although the variations in resistivity and optical properties show similar trends, the ranges of variation are different. This can be attributed to the fact that, according to the Drude model, the relaxation time and static conductivity depend on the conditions at grain boundaries. The results are used to explore the possibility of balancing the electrical and THz optical performances of THz devices.

Vanadium dioxide (VO2) is an ideal material for smart windows, which can initiate an automatic reversible metal-to-insulator transition from tetragonal to monoclinic structure at the transition temperature (Tc) of 68 °C, resulting in a large difference in near-infrared transmittance, but its application is limited by the poor luminous transmittance (Tlum) and low solar modulation ability (ΔTsol). Besides, metamaterials have shown their superiority as one of the strong competitors and candidates according to previous researches. In this paper, a VO2-based metamaterial structure (VO2(ms), where “ms” is an abbreviation for metamaterial structure) uniformly distributed with round holes was introduced to form a VO2(ms)/TiO2/VO2 multilayer structure, which is designed and produced by simulation and polystyrene microsphere-assisted preparation for enhanced performance. The simulation results obtained a considerable Tlum up to 50.4% and an ultra-high ΔTsol of 22.8%; this is due to the introduction of VO2(ms) which causes the multilayer structure to form metal–insulator–metal cavities at high temperatures and produces a resonance absorption effect. Meanwhile, it is found that the spacing of the round holes (D) has a great influence on the performance of the structure; specifically, an increase in D results in a slight decrease in both ΔTsol and Tlum. Moreover, the experimentally prepared sample demonstrated a ΔTsol of 20.4% and a Tlum of 42.6%, slightly lower than simulation because it has a larger spacing D, which is consistent with the analysis. In summary, both simulation and experiment can get ultra-high ΔTsol while guaranteeing a high Tlum. Such enhanced performance will benefit the application for VO2-based smart windows.

Owing to its outstanding metal–insulator transition, vanadium dioxide is considered as an attractive material used for optical modulation devices at far-IR bands. In this work, vanadium dioxide films were successfully prepared on (111) directional silicon substrates using Al2O3 as buffer layer by direct current magnetron sputtering. It is worth noting that, Al2O3 buffer layer significantly enhances the amplitude modulation at far-IR bands. Particularly, when Al2O3 buffer layer’s thickness increases to ~ 40 nm and VO2 film is ~ 300 nm, the amplitude modulation is largely increased from 51.2 to 71.3% at 395 cm−1 and from 18.0 to 40.2% at 280 cm−1. According to XRD, XPS and SEM tests, it can be found that Al2O3 buffer layer affects the composition, crystal structure and metal–insulator transition properties of the films. Moreover, compared with VO2 films deposited on silicon substrates, VO2 films using Al2O3 as buffer layer exhibit better metal–insulator phase transition properties with narrower hysteresis width (11.8 °C, 6.7 °C narrower than former films). Such excellent metal–insulator transition properties indicate that VO2 films using Al2O3 as buffer layer have great potential for far-IR bands modulation applications.

In this work, we investigate the terahertz transmittance and metal-insulator phase transition properties of M2 phase VO2 films induced by Cr doping. Firstly, Cr-doped VO2 films were successfully produced on silicon substrates using DC reactive magnetron sputtering and then fully characterized by XRD, XPS, Raman shift and SEM tests. These results demonstrate the formation of M2 phase and the effect of Cr doping on composition, crystal structure and surface morphology of VO2 films. Compared with undoped VO2, Cr doping significantly enhances the amplitude modulation in the frequency of phonon-absorption peaks by the suppression of infrared-active phonon modes. More concretely, the amplitude modulation is largely increased from 29.6% to 39.0% at 8.25 THz and from 24.1% to 37.1% at 9.33 THz, respectively. This enhancement strongly contributes to the development of VO2 -based devices in high THz range. In addition, Cr doped VO2 films exhibit outstanding metal-insulator phase transition properties with very narrower hysteresis width (8.0 degrees C) as well as smaller transition sharpness (3.4 degrees C) than undoped film, which is ascribed to the increase of heterogeneous nucleation site density and the transformation of crystal structure from Ml phase to M2 phase, respectively. This work indicates that VO2 films with suitable Cr doping concentration have great potential for THz modulation applications.

In the pursuit of energy efficient materials, vanadium dioxide (VO2) based smart coatings have gained much attention in recent years. In this paper, we investigate Al-doped VO2 films as thermochromic coatings on glass substrates by DC magnetron sputtering. It is found that adding Al3+ ions into VO2 films can generate groups of polygonal grains and nanowire clusters, apart from routinely reducing the valence and decreasing the grain size, through XRD, XPS, Raman shift and SEM results. For optical properties of Al doped films, a blue-shift of absorption edge in transmittance spectra has been introduced, which can significantly improve the luminous transmittance. Further, largely reducing phase transition temperature and enhancing solar modulation ability are also achieved. After carefully analyzing the relationship between the film characterizations and optical performance, we attribute the advanced theromochromic properties to the generation of nanowire clusters by the depositing conditions and the transformation effect of Al dopants. Such good thermochromic performances obtained by Al doping obviously overcome the drawbacks of undoped VO2 films for practical application. This work provides a considerable and new method of optimizing thermochromic properties of VO2 films as smart window coatings.

A new method, Fe/Mg co-doping, is proposed for the first time to optimize thermochromic VO2 and the promising performance of VO2-based smart windows for practical applications is successfully achieved.

A low TC and small ΔH were successfully achieved without significantly impairing THz MD by doping through an appropriate annealing process.

The metal-insulator phase transition in vanadium oxide makes it an attractive material for developing reconfigurable infrared optoelectronic components. In this paper, we present a tunable mid-infrared plasmonic patch antenna array based on vanadium oxide. The antennas consist of a circular gold patch array separated from a metallic ground plane by a film of vanadium dioxide. As the insulator-to-metal phase transition is thermally triggered, the resonances of the antenna array redshift with reduced absorbance before they are eventually switched off. The measured tuning range is about 10% of the resonant frequency, and the modulation depth in reflection is as high as 50%. A hysteresis loop in the tuning behavior is also observed. The XRD and XPS characterizations reveal a polycrystalline and multi-phase vanadium oxide. Our demonstrated tunable patch antennas hold promise for optical switching and modulating in mid-infrared applications.

The distinctive dispersion of composite right/left-handed transmission-line metamaterial offers a unique way of manipulating electromagnetic waves across a wide spectral range from microwave to the infrared. In this paper, we present a tunable mid-infrared composite right/left-handed metasurface based on the phase-change material of vanadium dioxide. The metasurface consists of an array of ‘H’-shaped gold pads separated from a metallic ground plane by a film of vanadium dioxide. As the insulator-to-metal phase transition is thermally triggered, both right-handed and left-handed metasurface modes redshift with reduced absorbance before they are eventually switched off. The tunabilities of right-handed mode frequency and left-handed mode frequency are measured to be approximately 3.6% and 2.7%, respectively. Our demonstrated metasurface with tunable composite right/left-handed dispersion could be useful for beam scanning for a fixed frequency in mid-infrared applications.

Silicon doped vanadium dioxide (VO2) films were successfully prepared on high purity Si(111) substrate. Confirmed by X-ray diffraction, all samples showed a preference orientation of (011) direction. Introducing silicon led grain sizes decreasing comparing to undoped VO2 film, and this result induced a narrow hysteresis width in MIT performance. Furthermore, silicon doped VO2 films annealing in different temperature presented different phase transition properties. In the electrical, a higher annealing temperature resulted in a decrease of sheet resistance and lowering the transition temperature. In terahertz optical transmittance, silicon doped VO2 films keep an excellent modulation ratio, indicating a great potential in the application of terahertz modulator devices.

Vanadium dioxide (VO2) films have great potential applications in photoelectric switching, storage devices, terahertz modulators and smart windows, due to the abruptly insulator-metal phase transition (IMT) near room temperature. In this research, vanadium oxide films were deposited by DC reactive magnetron sputtering in different annealing time of 450°C on glass substrates. As for electrical properties, the increasing of annealing time turns out sheet resistance increases at first, and then decreases in insulating phase, vice versa in metallic phase. In optical properties, the visible transmittance of VO2 films initially drops with annealing time prolonging, afterwards the transmittance slightly recovers. Differences between the electrical and optical are due to the grain size. Moreover, VO2 film annealing 15 min presents excellent visible transmittance, highly near-IR modulation efficiency (about 92% at a wavelength of 1100nm) and the lowest phase transition temperature (55.7°C). This result indicates that an appropriate annealing ambient can facilitate the application of VO2 film in smart windows.

Some infrared-active phonons in VO2 films suppress their modulation performance in the infrared region. Al-doped VO2 films, due to transforming VO2 crystal into the M2 phase, promptly eliminate absorption peaks in far-IR/THz bands and present widely modulating properties. Furthermore, we found high-frequency shifts of phonon vibration modes in Raman spectra by Al doping, indicating the stronger V-O bonds as the evidence of VO2 crystalline modification. However, although the high-frequency shifts and peak broadening were observed in V-O-V bending modes, mid-infrared spectra as the other phonon characterization show that its resonances are less involved, which is different from the remarkable variation of THz phonons. We attribute the difference to the distinct origins of phonon vibrations. As Al doped into films, the group-rotational peaks were rapidly erased with crystalline deformation whereas the high-frequency bending modes only slightly changed.

A promising approach was proposed to enhance the responsivity of a ZnO nanowires array based photodetector with metal-semiconductor-metal structure through piezo-phototronic effect. The ZnO nanowires arrays were synthesized via hydrothermal method to function as both piezoelectric and light-trapping layer. The photoresponses of the ZnO nanowires based photodetector were tuned by the modulation of local Schottky barrier heights (SBH) at the metal-ZnO interfaces as a result of strain-induced polarization charges. Subjected to a -0.62% compressive strain, the responsivity of the prepared photodetector was increased by as much as 176%. Furthermore, the devices hydrothermally synthesized at 0.10 M have a larger photoresponse and higher stress sensitivity compared to the ones prepared at 0.05 M and 0.15 M. This work not only strengthens the fundamental understanding of piezo-phototronic effect on photodetectors but also provides an efficient means for optimization/improvement of piezoelectric semiconductor nanowires based optoelectronic devices. (C) 2016 Elsevier B.V. All rights reserved.

Vanadium dioxide is a promising material for THz modulations due to its remarkable insulator-to-metal transition (IMT) properties. Silicon-doped VO2 films, exhibiting excellent IMT properties with giant modulation amplitude and tunable phase transition temperature, greatly adapt in this area. In this paper, we report on a rebound effect of the IMT in Si-doped VO2 films. As the silicon dopants are increasingly introduced into VO2 films, the IMT is first tuned to lower temperature and then is anomalously shifted to higher temperature. This rebound effect is confirmed by crystal structure, valence concentration, and surface morphology. We attribute this rebound behavior to the interstitial and substitutive doping of Si atoms. Due to their distinct impactions on the crystallite, IMT properties of the VO2 films are depressed initially and recovered later.

Due to the insulator-metal transition (IMT) performance covering the full terahertz (THz) band, VO2 films were extensively investigated as an excellent candidate for modulating, switching, and memory devices. However, some remarkable absorption peaks owing to the infrared-active phonon modes suppressed the films' modulation ability and restricted the films' application in high THz frequency. Here we prepared Al-doped VO2 films on (111) directional silicon substrate, which rapidly counteracted the absorption peak and exhibited widely modulating properties. Al dopants introduced into the films brought a significant shift to high frequency in Raman spectra. The result was attributed to the effect of modifying VO2 crystal, leading the V-O bond to be strained more intensively, contracting the distance of the V-V dimers. All the Raman results indicated an oxidation effect by Al doping. However, the XPS results showed a valence reduction of the vanadium element, which was caused by the valence difference between V and Al atoms. In addition to the surface morphology characterization, the IMT properties of the shrinkage of hysteresis width and resistance variations in both electrical and THz optical aspects have been systemically analyzed. An additional difference is that the temperature of the optical transition behaves lower than the electrical transition observed, which resulted from the mechanism of transition propagation and boundary barriers.

Research on high performance terahertz (THz) detector is essential for promoting the application of THz science and technology. Lithium tantalate crystal (LiTaO 3 ) was used to fabricate the THz detector in this paper. Polishing process were used to reduce the thickness of LiTaO 3 crystal slice obtained the area of 2mm×2mm×10μm LiTaO 3 wafer pyroelectric coefficient of 4.7×10 −4 Cm −2 K −1 by chemical mechanical polishing techniques. The THz responsivity for detector tested by lock in amplifier reaches 8.38×10 4 V/W and the lowest noise equivalent power value(NEP) reaches 3.25×10 −12 W at 20Hz operating frequency use 2.52THz radiation, which is suitable for THz imaging application. Meanwhile it provides a feasible approach for fabricating high responsivity THz detector.

•Si-doped VO2 films were prepared on high-purity single-crystal Si substrates.•The surface morphology and phase transition properties depend on the Si content.•All the films show a high transmission modulation ratio at terahertz range.•Small hysteresis width of 4.5°C and large terahertz modulation ratio of about 82%.

Silicon doped vanadium dioxide films were successfully prepared on indium tin oxide coated glass substrates at 255 °C annealing temperature by direct current magnetron sputtering. Maintaining spheroidal grains, the size of VO2 films nanoparticles decreased with silicon doping ratios increasing. Transmittance spectra indicated a 46 nm blue-shift of absorption edge and improved the samples' integrated luminous transmittances (from 28.4% at Si/V = 0 to 36.1% at Si/V = 0.17) with similar thickness of 90 nm. The samples presented excellent thermochromic properties that possessed higher than 40% near-infrared switching efficiency at 2000 nm and showed at least 9.2% solar modulation efficiency. Moreover, the metal–semiconductor phase transition temperature for heavily doped VO2 (Si/V = 0.17) decreased to 46.1 °C (22 °C lower than bulk). These features suggest the practical application of VO2 to smart windows.

Nanostructured zirconium (Zr)-doped vanadium oxide (VOx) films were prepared at low temperature on glass substrates by reactive direct current magnetron sputtering followed by in-situ annealing process. The effect of Zr content on the chemical composition, structure, morphology and metal–semiconductor transition properties of the deposited films was investigated systematically. It was found that Zr doping significantly reduced the grain size of VOx films due to increased density of nucleation centers, but was found almost not to influence the structure and the +4 valence vanadium concentration. Interestingly, the hysteresis width was hugely modulated from 30.0 to 5.8°C while the phase transition temperature upon cooling increases from 42.0 to 56.7°C as the Zr/V atomic ratio in the deposited film increases from 0 to 13.9%. However, the phase transition temperature upon heating initially decreases with Zr/V atomic ratio, attains a minimum of 53.8°C at a Zr/V atomic ratio of 8.5%, and then increases with increase in Zr/V atomic ratio. The decrease in hysteresis width with the increase in Zr contents can be attributed to the increased density of nucleation defects introduced by Zr doping. This work shows that Zr doping can effectively regulate the morphology and the phase transition characteristics of VOx films.