Tuning the Metal–Insulator Transition in TiO₂/VO₂ Superlattices by Modifying the Layer Thickness or Inducing Defects

Abstract

Vanadium dioxide (VO2) is a thermochromic material that can be used in advanced applications such as smart energy-saving windows and other smart optical/electronic devices. However, obtaining a comfortable metal–insulator transition temperature while improving solar utilization in VO2 remains an unresolved question at both the fundamental and application levels of research. Although studies on designing TiO2/VO2 multilayers to address the above issues have been widely reported, the nature of the metal–insulator transition and how thickness and defects affect phase transition behaviors are still subjects of ongoing debate. Herein, by varying the VO2 or TiO2 layer thicknesses or inducing defects such as oxygen vacancies and interstitial Ti/V atoms, the metal–insulator behavior including the atomic and electronic structures of TiO2/VO2 superlattices was systematically investigated. Our results show that the V–V distances in (m + n)­TiO2/VO2(001) superlattices exhibit discontinuous dimerization characteristic and the superlattices exhibit alternating metal–insulator transition characteristics as the layer thickness m increases from 0 to 10. When 0 < m = n < 10, the band gaps for (m + n)­TiO2/VO2(001) superlattices exhibit a downward-opening parabola. However, when 0 < m < 10 and m + n = 10, the band gaps fluctuate around 0.4 eV. Additionally, defects such as oxygen vacancies or cationic interstitial Ti/V atoms have a great impact on the metal–insulator transition in (m + n)­TiO2/VO2(001) superlattices. Oxygen vacancies are preferentially located in the VO2 layer. When oxygen vacancies are present in the TiO2 layer, they migrate across the interface into the VO2 layers, indicating that there is considerable interdiffusion of V/Ti interstitial atoms across the interface. The interstitial V atoms diffuse more easily into the VO2 layer than interstitial Ti atoms. The current findings may be useful in understanding the metal–insulator behavior of VO2/TiO2 superlattices by varying the layer thickness or inducing defects, thereby providing a new approach for designing VO2-based heterostructures for smart energy-saving windows or other smart optical/electronic devices.

Strong Substrate Influence on Atomic Structure and Properties of Epitaxial VO2 Thin Films

“…So, here we have the So, when viewing along the [110] R zone-axis, we see a structure that is completely epitaxial and homogenous across the lamella, but, when viewing along [100/010] R , we always see different ori-entations of VO 2 on the two sides of the cracks. These observations validate previous studies of the cracks developing as a stress relaxation mechanism [44,45] -all these grains have exactly the same rutile structure above the transition temperature, but as the thin film cools down, M1 VO 2 grains with different orientations nucleate and propagate until they meet resulting in stress which is only able to relax due to the formation of cracks. Overall, the presence of deep cracks in samples on (001) substrate results in relatively poor MIT characteristics, and thus VO 2 on this substrate is not suitable for electronic applications unless the stress can be tuned by external mechanisms like doping, or caused to relax with features other than cracks.…”

“…Further studies are required to study the properties of these heterostructures since the MIT characteristics should be affected by this intermediate intermixing layer. [ 45 ]…”

“…Further studies are required to study the properties of these heterostructures since the MIT characteristics should be affected by this intermediate intermixing layer. [45] From a sample grown at 375 °C we produced multiple lamellas from areas with different domain widths. In Figure 6a, we show an overview drift-corrected frame integration (DCFI) image of a lamella taken with a camera length of 190 mm for better contrast.…”

“…[28][29][30][31][32] For such epitaxially grown films, the nature of the microstr ucture, including distribution of differently oriented grains and their grain boundaries, strongly affect the transition properties. [33][34][35] Such studies have been performed individually on the most common substrates used for growing VO 2sapphire [36][37][38][39][40][41][42] and TiO 2 , [43][44][45][46][47] but we could not find any studies of such films and their microstructure across substrates using very similar growth conditions to maximize their comparison. Furthermore, the microstructure has been commonly studied using diffraction and in fewer cases with direct imaging using scanning transmission electron microscopy (STEM).…”

Highly Reduced Phase Transition Hysteresis of Vanadium Dioxide Thin Films in Multilayer Structure with Titanium Dioxide

“…Therefore, the effective design of buffer materials and the integration of VO 2 into multilayer structures prepared on affordable commercial substrates must be studied. Titanium dioxide (TiO 2 ) has recently emerged as a promising candidate because of its rutile-phase crystal structure with small lattice mismatch and high visible–IR transmittance. − Zhang et al found that using TiO 2 as a buffer layer improves the phase transition performance and IR solar modulation ability from 13 to 23% for VO 2 thin films deposited on glass substrates. However, even with the introduction of the TiO 2 buffer layer, a narrow thermal hysteresis loop ( H LW ) of the electro-optic thermochromism of the VO 2 thin film on the glass substrate was still unattainable, which is essential for high-speed optical switching device applications.…”