We report on a theoretical derivation of the electronic dielectric response of semiconductor nanocrystals using a tight-binding framework. Extending to the nanoscale the Hanke and Sham approach [Phys. Rev. B 12, 4501 (1975)] developed for bulk semiconductors, we show how local field effects can be included in the study of confined systems. A great advantage of this scheme is that of being formulated in terms of localized orbitals and thus it requires very few computational resources and times. Applications to the optical and screening properties of semiconductor nanocrystals are presented here and discussed. Results concerning the absorption cross section, the static polarizability and the screening function of InAs (direct gap) and Si (indirect gap) nanocrystals compare well to both first principles results and experimental data. We also show that the present scheme allows us to easily go beyond the continuum dielectric model, based on the Clausius-Mossotti equation, which is frequently used to include the nanocrystal surface polarization. Our calculations indicate that the continuum dielectric model, used in conjunction with a size dependent dielectric constant, underestimates the nanocrystal polarizability, leading to exceedingly strong surface polarization fields.
Intrinsic Dielectric Response in Ferroelectric Nanocrystals
对厚度低至75 nm的“自立式”单晶钛酸钡电容器的测量显示出典型的大单晶而不是传统薄膜的介电响应。介电峰或损耗角正切明显没有任何加宽或温度漂移。钙的峰值介电常数。25,000,Curie-Weiss分析表明一阶相变行为。这与常规沉积的薄膜异质结构上的结果形成令人惊讶的对比,常规沉积的异质结构显示出大的介电峰展宽和温度漂移[例如,C. B. Parker,J-P. Maria和A. I. Kingon,应用物理学快报,81,340(2002)],以及顺电-铁电转变的性质从1阶到2阶的明显变化。我们的数据是兼容的最近的模型由Bratkovsky和Levanyuk,属性介电峰展宽的梯度条款,将存在于任何薄膜电容器异质结构,无论是通过在生长过程中引入的缺陷配置文件,或通过顶部和底部电极之间的微妙的不对称性。观察到的一阶转变行为的恢复与我们的实验中不存在显著的衬底夹持一致,如Pertsev等人在Phys. Rev. Lett.,80,1988(1998),并且说明了在常规沉积的薄膜中看到的二阶行为不能归因于系统中降低维度的效应,也不能归因于与电极-铁电界面相关的本征通用界面电容的影响。
Size-dependent Dielectric Permittivity of Perovskite Nanocrystals
Donostia International Physics Center (DIPC)(多诺斯蒂亚国际物理中心);Centro de Física de Materiales (CFM-MPC)(材料物理中心);University of the Basque Country (UPV/EHU)(巴斯克大学);Uppsala University(乌普萨拉大学)
Two-dimensional halide perovskite nanoplatelets (NPLs) have exceptional light-emitting properties, including wide spectral tunability, ultrafast radiative decays, high quantum yields (QY), and oriented emission. To realize efficient devices, it is imperative to understand how exciton transport progresses in NPL thin films. Due to the high binding energies of electron-hole pairs, excitons are generally considered the dominant species responsible for carrier transfer. We employ spatially and temporally resolved optical microscopy to map exciton diffusion in perovskite nanocrystal (NC) thin films…
Rock-salt lead selenide nanocrystals can be used as building blocks for large scale square superlattices via two-dimensional assembly of nanocrystals at a liquid-air interface followed by oriented attachment. Here we report measurements of the local density of states of an atomically coherent superlattice with square geometry made from PbSe nanocrystals. Controlled annealing of the sample permits the imaging of a clean structure and to reproducibly probe the band gap and the valence hole and conduction electron states. The measured band gap and peak positions are compared to the results of opt…
Atomistically detailed computational studies of nanocrystals, such as those derived from the promising lead-halide perovskites, are challenging due to the large number of atoms and lack of symmetries to exploit. Here, focusing on methylammonium lead iodide nanocrystals, we combine a real-space tight binding model with the GW approximation to the self-energy and obtain exciton wavefunctions and absorption spectra via solutions of the associated Bethe-Salpeter equation. We find that the size dependence of carrier confinement, dielectric contrast, electron-hole exchange, and exciton binding energ…
Engineering of the dielectric environment represents a powerful strategy to control the electronic and optical properties of two-dimensional (2D) materials without compromising their structural integrity. Here we show that the recent development of high-$κ$ 2D materials present new opportunities for dielectric engineering. By solving a 2D Mott-Wannier exciton model for WSe$_2$ on different substrates using a screened electron-hole interaction obtained from first principles, we demonstrate that the exciton Rydberg series changes qualitatively when the dielectric screening within the 2D semicond…