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Nano-optical investigations of the metal-insulator phase behavior of individual VO_2 microcrystals

机译：单个VO_2微晶的金属-绝缘体相行为的纳米光学研究

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Despite the relatively simple stoichiometry and structure of VO _2, many questions regarding the nature of its famous metal-insulator transition (MIT) remain unresolved. This is in part due to the prevailing use of polycrystalline film samples and the limited spatial resolution in most studies, hindering access to and control of the complex phase behavior and its inevitable spatial inhomogeneities. Here, we investigate the MIT and associated nanodomain formation in individual VO_2 microcrystals subject to substrate stress. We employ symmetry-selective polarization Raman spectroscopy to identify crystals that are strain-stabilized in either the monoclinic M1 or M2 insulating phase at room-temperature. Raman measurements are further used to characterize the phase dependence on temperature, identifying the appearance of the M2 phase during the MIT. The associated formation and spatial evolution of rutile (R) metallic domains is studied with nanometer-scale spatial resolution using infrared scattering-scanning near-field optical microscopy (s-SNOM). We deduce that even for small crystals of VO_2, the MIT is influenced by the competition between the R, M1, and M2 crystal phases with their different lattice constants subjected to the external substrate-induced stress. The results have important implications for the interpretation of the investigations of conventional polycrystalline thin films where the mutual interaction of constituent crystallites may affect the nature of the MIT in VO_2.
机译：尽管VO _2的化学计量和结构相对简单，但有关其著名的金属-绝缘体转变（MIT）性质的许多问题仍未解决。这部分是由于在大多数研究中普遍使用多晶膜样品和有限的空间分辨率，这阻碍了对复杂相行为及其不可避免的空间不均匀性的获取和控制。在这里，我们研究了受衬底应力作用的单个VO_2微晶中的MIT及其相关的纳米域形成。我们采用对称选择性极化拉曼光谱法来鉴定在室温下在单斜M1或M2绝缘相中应变稳定的晶体。拉曼测量进一步用于表征相位对温度的依赖性，从而确定MIT期间M2相的出现。使用红外散射扫描近场光学显微镜（s-SNOM），以纳米尺度的空间分辨率研究了金红石（R）金属域的相关形成和空间演化。我们推论，即使对于VO_2的小晶体，MIT也受R，M1和M2晶相之间竞争的影响，它们的晶格常数不同，它们受到外部衬底诱导的应力。该结果对解释常规多晶薄膜的研究具有重要意义，在常规多晶薄膜中，组成微晶的相互作用可能会影响VO_2中MIT的性质。

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