摘要
二氧化钒(VO2)由于在68℃附近具有显著且可逆的金属-绝缘体相变特性而备受关注。这种相变将导致VO2的电、光和磁学性质发生突变,例如在相变过程中其电阻率和红外透射率的突变。因此,VO2在很多领域具有潜在的应用前景。
本论文以VO2为研究对象,采用溶胶-凝胶法和真空退火处理制备V02薄膜,并对VO2薄膜的相结构和相变特性进行了深入的研究。此外,采用水热法和后期退火处理制备掺杂VO2粉体材料,通过掺杂不同价态和不同半径的离子来调控VO2的相变温度,并系统研究了掺杂离子对掺杂体系结构、相变特性的影响。本论文的主要研究内容和成果如下:
(1)采用溶胶-凝胶法和简单的真空退火处理,在Al2O3(0001)和Al2O3(1010)衬底上制备出择优取向生长的VO2薄膜。电学特性表征结果显示,薄膜具有优异的金属-绝缘体相变性能,相变前后电阻达到4个量级的突变。这种制备高质量VO2薄膜的简单方法,有利于推动VO2材料在智能窗领域的应用。此外,我们通过X射线衍射(XRD)、拉曼谱和X射线吸收精细结构(X-ray Absorption Fine-Structure,XAFS)谱对薄膜的相结构进行综合的研究,结果证明沉积在Al2O3(1010)衬底上取向生长的VO2薄膜也是M1相结构,而不是以往报道认为的M2相结构,且相结构与制备方法无关。结合本文中VO2薄膜的升温拉曼谱和文献调研结果,我们对VO2薄膜中存在M2相结构的可行性及形成机理进行了深入研究,结果表明:仅受膜内应力的作用,单一的M2相结构很难在无掺杂VO2薄膜中稳定存在。而在相变临界温度附近,高取向VO2薄膜仍处于较强的应力作用状态是混杂形成中间M2相的条件。
(2)离子掺杂可以有效地调控VO2的相变温度,但调控机理尚不明确。因此,对于调控机理的研究已逐渐成为VO2研究的热点。在本文中,我们选择WxV1-xO2体系,对W掺杂导致VO2相变温度降低的微观机理进行研究。为了阐明这个机理,我们利用同步辐射XAFS谱技术,获得W和V原子的化学态和局域结构信息,从原子尺度上分析了W掺杂导致VO2的电子能态和原子结构上的变化。结果显示:掺杂W6+导致WxV1-xO2中少量的V4+被还原为V3+,从实验上证实了掺杂W6+会贡献电子给周围V4+,而这种电子掺杂会增加绝缘相的载流子浓度并破坏二聚化的V-V对;同时,W掺杂原子周围形成的局域四方相结构,成为了金红石结构金属相的成核位点,这有助于绝缘相→金属相的转变;另外,随着掺杂浓度增加,W局域结构出现膨胀,进而产生的应力扭曲了邻近的单斜V02晶格,导致V d‖轨道的相互作用减弱和V3d-O2p轨道的杂化交叠减小,绝缘相的能带结构发生变化,引起WxV1-xO2的能隙变小。以上这些结果的协同作用导致了W掺杂的VO2在绝缘-金属相变过程所需克服的能垒降低,因此宏观上表现为相变温度降低。
(3)在Al掺杂调控VO2相变温度研究中,我们发展了一种简单有效的可调控AlxV1-xO2相结构的方法,即经过水热合成后,通过调节后续退火过程中的氩气流量来调控AlxV1-xO2样品的相结构。这种简单的方法对于制备特定相结构的钒氧化物或其他多价态的金属氧化物具有重要的指导意义。在合适的氩气流量范围内,通过调节氩气流量可以调控AlxV1-xO2样品形成M1或M2相结构,即使在低Al掺杂浓度下,也能形成稳定的室温M2相结构。研究还发现,只有在掺杂A13+离子的前提下,才可能通过调节氩气流量而得到M2相结构,而无掺杂V02经过退火处理却很难得到M2相结构。这些研究结果表明除了退火过程中的氩气流量,AP+掺杂离子对M2相结构的形成也起到极其重要的作用。DSC表征相变特性的结果显示,AlxV1-xO2样品形成M1相结构,其相变温度几乎不变(-67℃C),和无掺杂VO2(M1)样品的差不多相等;AlxV1-xO2样品只有形成M2相结构,其相变温度才会随着A1掺杂浓度的增加而显著地提高,最高可到87.5℃。本研究结果证明了掺杂体系的相结构在调控相变温度过程中扮演着重要的角色。
(4)为了排除掺杂离子的价态对相变温度调控的影响,我们选择TixV1-xO2掺杂体系并通过水热法和后期退火处理获得TixV1-xO2粉体样品,研究了掺杂Ti4+离子对VO2晶格结构的影响,并单纯从晶格结构变化角度分析掺杂离子对相变特性的影响。对于TixV1-xO2体系,在氩气退火处理过程并没有像AlxV1-xO2体系那样出现可调控的相结构。DSC表征相变特性的结果显示,随着Ti掺杂浓度的增加,TixV1-xO2的相变温度先稍微降低再逐渐升高。利用XAFS谱分析TixV1-xO2样品中Ti和V原子局域结构的演化过程,结果显示随着Ti掺杂浓度的增加,Ti原子周围形成类似于局域TiO2(Anatase)结构到局域TiO2(Rutile)结构的变化,而V原子局域结构则出现了两种变化趋势,对应于相变温度的两种变化趋势,这说明了相变温度的变化与掺杂诱导的结构变化有直接的关系,证实了掺杂诱导的结构变化对相变温度调控的作用。
Vanadium dioxide (VO2) has attracted much attention due to its distinct and reversible Metal-Insulator Transition (MIT) occurring around68℃. The MIT leads to the large changes in electrical, optical and magnetic properties of the VO2, e.g., to large electrical resistivity and infrared reflectivity/transmission changes. Therefore, VO2is a peculiar system with potential applications in many different fields.
In this thesis, we take the VO2as the research object. The VO2thin films were prepared by the sol-gel method and the subsequent vacuum annealing treatment A systematic analysis ofphase structure and phase transition characteristics of these thin films was performed. In addition, the ions-doped VO2powders were prepared by the hydrothermal method and the subsequent annealing treatment. The different valence and radius of the doped ions were used to regulate the MIT critical temperature (Tc). And the effects of the doped ions on the structure and the phase transition characteristics were systematically investigated. Research work has been done as the following parts in detail:
(1) Highly oriented VO2thin films were prepared on Al2O3(0001) and Al2O3(1010) substrates by a sol-gel method and subsequent vacuum annealing treatment. Electrical properties characterizations indicated that the obtained VO2thin films exhibited excellent MIT characteristics, i.e., the resistance change across the MIT up to four orders of magnitude. This simple preparation method of high quality VO2thin films will be conducive to the smart windows application. In addition, the X-ray diffraction (XRD), Raman spectra and X-ray Absorption Fine Structure (XAFS) spectra were used to systematically investigate the phase structure of these films. Results indicate that the highly oriented VO2film on Al2O3(1010) substrate grows with the Ml structure instead of the M2structure as reported in previous studies. And the phase structure is independent by the preparation methods. Moreover, combining with temperature-dependent Raman spectra and the literature survey, we analyzed the possible presence of M2phase structure in VO2films and its formation mechanism. The analysis points out that the pure M2phase of the undoped VO2films is difficult to exist only subjecting to the internal stress. While near the Tc, the high-strained structure of oriented VO2films can be a mechanism for the formation of the intermediate M2phase.
(2) Since doping ions can effectively regulate the Tc of VO2, the mechanism for the regulating Tc by doping ions is a fundamental issue. Here, to clarify the mechanism of decreasing Tc by tungsten (W) doping, we systematically investigated by means of the XAFS spectroscopy the chemical states and local geometrical structures of W and V atoms in WxV1-xO2. On the atomic scale we analyzed the change in the electronic energy state and atomic structure of VO2due to the W doping. Based on the XAFS analysis, the evidence of electron doping in VO2by W6+ions was obtained from the detection of reduction of V4+to V3+ions. Actually, in the monoclinic phase, the electron doping results in an increased conductivity and in the breakdown of dimerizated V-V pairs. Local rutile domains around the W dopants in the host monoclinic VO2matrix are formed as the initial nucleation sites of rutile metallic phase, which facilitates the insulator to metal transition. With the increase of doping concentration, the expansion of the local rutile structure around W dopants induces additional internal stresses, thus yielding to the detwisting of the nearby monoclinic VO2lattice. The occurring atomic change will decrease the overlap of V3d-O2P orbitals and weaken the localized dinteractions, resulting in the decreased band gaps of WxV1-xO2samples. All of these results were cooperated together to reduce the potential energy barrier for phase transition, thus lowering the Tc.
(3) In the study of regulating Tc via Al doping, we developed an effective and economical method to control the different phase structures of AlxV1-xO2powders, i.e., after the hydro thermal synthesis, different phases of AlxV1-xO2were obtained by controlling the Ar flux rates during the subsequent annealing process. This method has a relevant intrinsic importance in order to prepare target structures of vanadium oxide or other multivalent metal oxides. Within the suitable range of Ar flux rates, the Ml and M2phase of AlxV1-xO2were successfully achieved by changing the Ar flux rates. In particular, the room-temperature M2phase of AlxV1-xO2was achieved even at low Al-doping concentration. Experiments also showed that M2phase was difficult to be achieved for undoped VO2, while the M2phase could be formed on the premise of Al doping, showing that not only the Ar flux rates but also the Al3+dopants played the important roles in the formation of stable M2phase. The phase transition property measured by DSC revealed that the Tc was almost constant for Ml phase of AlxV1-xO2, close to the value of~67℃of undoped VO2(M1). Only the formation of M2phase, the Tc increased up to87.5℃by Al doping. This research clearly showed the important role of the phase structure in regulating Tc.
(4) To rule out the influence of the valence state of doped ions on the Tc, we chose TixV1-xO2system which were prepared by a hydrothermal method and subsequent annealing treatment, and investigated the influence of the doped Ti4+ions on the VO2lattice structure and on phase transition characteristic from the perspective of lattice structure change. Different from the AlxV1-xO2system, in the case of the TixV1-x02system, no controllable phase structure was achieved through the Ar annealing treatment. The phase transition property measured by DSC revealed that the Tc of TixV1-xO2samples initially slightly decreased and then increased with the Ti concentration. The variation in the local geometrical structures of Ti and V atoms in TixV1-xO2were systematically investigated by XAFS technique. The results showed the change of local structure around Ti dopants analogously from that of TiO2(Anatase) to TiO2(Rutile) with the increase of Ti concentration, However, two change trends of the local structure around V atoms were observed, corresponding to the two change trends of Tc. The behavior points out that the changes of Tc are directly correlated with the structural changes induced by the Ti doping, i.e., confirming the effects of structure change induced by ions doping on regulating the Tc.
引文
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