Department of Mechanical Engineering, University of Houston, Houston, TX 77204, USA.
Nanoscale. 2018 Feb 22;10(8):3863-3870. doi: 10.1039/c7nr07951f.
Manipulating the transport properties of titania nanotubes (NTs) is paramount in guaranteeing the material's successful implementation in various solid state applications. Here we present the unique semiconducting properties of individual titania NTs as revealed from thermoelectric and structural studies performed on the same individual NTs. The NTs were in the anatase phase fabricated by anodic oxidation and doped with intrinsic defects created by reducing the lattice thermally. Despite their polycrystalline nature and nanoscale walls, the doped NTs were found to be 4-5 orders of magnitude more electrically conducting than TiO nanowires and thin films, with values approaching the bulk single crystal conductivity. The reason for the high conductivity was found to be the high carrier concentration on the order of 10 cm, which counteracted the low mobility values ∼0.006 cm V s. Furthermore, this high level of carrier concentration transitioned the NTs to a degenerate state, which is the first such example in thermally doped titania NTs. More importantly, our study showed the creation of acceptor states along with donor states in individual nanotubes upon lattice reduction. These acceptor levels were found to be active at low temperatures when donor states were not ionized, shifting the Fermi level (E) from the conduction band to the valence band.
操控二氧化钛纳米管（NTs）的输运性质对于保证该材料在各种固态应用中的成功实现至关重要。在这里，我们展示了单个二氧化钛 NTs 的独特半导体性质，这些性质是通过对相同的单个 NTs 进行热电和结构研究揭示的。NTs 采用阳极氧化法制备，处于锐钛矿相，并通过热还原晶格形成本征缺陷掺杂。尽管它们具有多晶性质和纳米级壁，但掺杂 NTs 的电导率比 TiO 纳米线和薄膜高出 4-5 个数量级，接近体单晶的电导率。高电导率的原因是载流子浓度高达 10 cm，这抵消了低迁移率值∼0.006 cm V s。此外，这种高载流子浓度使 NTs 转变为简并态，这是热掺杂二氧化钛 NTs 中的首例。更重要的是，我们的研究表明，在晶格还原过程中，单个纳米管中同时形成了受主态和施主态。这些受主能级在低温下是活跃的，当施主态未被离子化时，费米能级（E）从导带转移到价带。
