Department of Chemistry, University of Oregon, Eugene, Oregon, USA.
Nanoscale. 2016 Aug 14;8(30):14665-72. doi: 10.1039/c6nr04274k. Epub 2016 Jul 20.
A homologous series of quasi-2D ([PbSe]1+δ)m(TiSe2)m nanolayered heterostructures are prepared via self-assembly of designed precursors with 1 ≤m≤ 4 and their structures and properties investigated. All heterostructures have the same global composition but vary in their interface density. X-ray diffraction and electron microscopy studies show that the structures consist of rock salt structured PbSe layers alternating with TiSe2 layers, and that grain size increases with m. The compounds are all metallic with upturns in resistivity at low temperature suggesting electron localization, with room temperature resistivity of 1-3 10(-5)Ω m, negative Hall coefficients and Seebeck coefficients between -50 and -100 μV K(-1). A decrease in the mobile carrier concentration with temperature is observed for all m and the rate increases with increasing low-dimensionality. Decreasing the interface density also decreases the average carrier concentration while increasing the electron mobility. The Seebeck coefficients systematically increase in magnitude as m is increased, but the net effect to the power factor is small due to a compensating increase in resistivity. The observed transport behavior is not described by the simple rigid band models with charge transfer between constituents used previously. Charge exchange between constituents stabilizes the intergrowth, but also introduces mobile carriers and interfacial band bending that must play a role in the transport behavior of the heterostructures. As chemical potentials equilibrate in high m heterostructures there is a decrease in total coulombic stabilization as there are fewer interfaces, so m = 1 is likely to be most stable. This rationalizes why the structurally similar misfit layer compounds with m = 1 are often the only intergrowths that can be prepared. Charge transfer and band bending at interfaces should occur in other heterostructures with similar type II broken-gap band alignments and are important considerations regarding both their stability and transport properties.
通过自组装具有 1≤m≤4 的设计前体，制备了一系列同源的准二维([PbSe]1+δ)m(TiSe2)m 纳米层状异质结构，并研究了它们的结构和性能。所有的异质结构都具有相同的全局组成，但界面密度不同。X 射线衍射和电子显微镜研究表明，这些结构由岩盐结构的 PbSe 层与 TiSe2 层交替组成，晶粒尺寸随 m 的增加而增大。这些化合物都是金属的，低温下电阻率上升表明电子局域化，室温电阻率为 1-3×10-5Ω·m，负霍尔系数和塞贝克系数在-50 到-100μVK-1之间。所有 m 的载流子浓度随温度降低而降低，且低温下的载流子浓度随维度降低而增加。随着界面密度的降低，平均载流子浓度降低，电子迁移率增加。随着 m 的增加，塞贝克系数的绝对值呈系统性增大，但由于电阻率的补偿性增加，对功率因子的净效应较小。观察到的输运行为不能用以前使用的组分间电荷转移的简单刚性带模型来描述。组分间的电荷交换稳定了共生长，但也引入了可移动载流子和界面能带弯曲，这些因素在异质结构的输运行为中必须发挥作用。在高 m 异质结构中，化学势平衡，由于界面减少，总库仑稳定化作用降低，因此 m=1 可能最稳定。这解释了为什么结构相似的失配层化合物 m=1 通常是唯一可以制备的共生长物。界面处的电荷转移和能带弯曲应该发生在具有类似 II 型带隙不连续性能带排列的其他异质结构中，这对于它们的稳定性和输运性质都是重要的考虑因素。
