N. Shi

First principles study of CdSe quantum dots: Stability, surface unsaturations, and experimental validation

Ab initio computational studies were performed for CdSe nanocrystals over a wide range of sizes and topologies. Substantial relaxations and coordination of surface atoms were found to play a crucial role in determining the nanocrystal stability and optical properties. While optimally (threefold) coordinated surface atoms resulted in stable closed-shell structures with large optical gaps, suboptimal coordination gave rise to lower stability and negligible optical gaps. These computations are in qualitative agreement with recent chemical etching experiments suggesting that closed-shell nanocrystals contribute strongly to photoluminescence quantum yield while clusters with nonoptimal surface coordination do not.Ab initio computational studies were performed for CdSe nanocrystals over a wide range of sizes and topologies. Substantial relaxations and coordination of surface atoms were found to play a crucial role in determining the nanocrystal stability and optical properties. While optimally (threefold) coordinated surface atoms resulted in stable closed-shell structures with large optical gaps, suboptimal coordination gave rise to lower stability and negligible optical gaps. These computations are in qualitative agreement with recent chemical etching experiments suggesting that closed-shell nanocrystals contribute strongly to photoluminescence quantum yield while clusters with nonoptimal surface coordination do not.

Local properties at interfaces in nanodielectrics: An ab initio computational study

First-principles computational methodologies are presented to study the impact of surfaces and interfaces on the dielectric and electronic properties of emerging technologically important systems over length scales of the order of inter-atomic distances. The variation of dielectric constant across Si-SiO2, Si-HfO2 and SiO2-polymer interfaces has been correlated to interfacial chemical bonding environments, using the theory of the local dielectric permittivity. The local electronic structure variation across Si-HfO2 and SiO2-polymer interfaces, including band bending, band offsets and the creation of interfacial trap states have been investigated using a layer-decomposed density of states analysis. These computational methods form the groundwork for a more thorough analysis of the impact of surfaces, interfaces, and atomic level defects on dielectric and electronic properties of a wide variety of nano-structured systems.

Scalability of phononic crystal heterostructures

Phononic (or acoustic) band structure calculations have been performed for a nanoscale HfO2–ZrO2 multilayer stack using first-principles methods at the atomistic level and by solving the acoustic wave equation at the continuum level, as a first step toward determining the length scales when conventional continuum acoustic band-gap treatments become inadequate. Transverse acoustic waves are the focus of this study. The material parameters that continuum acoustic band gap methods require, such as the mass density and transverse wave velocity of the components of the acoustic crystal (i.e., for HfO2 and ZrO2), were determined using separate phonon calculations of the corresponding bulk materials. Comparison of the phononic band structure for a nanoscale HfO2–ZrO2 multilayer stack calculated using first-principles and continuum methods indicates the need for careful treatments of wave propagation properties at these length scales.

Dielectric properties of Cu-phthalocyanine systems from first principles

The authors present a first principles approach for investigating the dielectric properties of Cu-phthalocyanine (CuPc). The local position-dependent dielectric constant of CuPc oligomers is determined from the charge density induced by an external finite electric field. The dielectric constants of a CuPc monomer along and perpendicular to its plane are extracted from appropriately chosen periodic arrangements of CuPc oligomers. The authors obtain dielectric constant values of about 15 along the CuPc plane and about 3.4 perpendicular to the plane.

Dielectric properties of ultrathin SiO2 slabs

First-principles total energy calculations have been performed to determine the extent to which surfaces impact the dielectric properties of ultrathin dielectric materials. SiO2 (0001) slabs in α-quartz phase with various thicknesses were considered in this study, using a new method that allows for the partitioning of the surface and bulk contributions to the total field-induced polarization. It was found that the bulk polarization and the dielectric constant can be determined even from ultrathin films terminated with Si atoms, and that surface effects do not significantly impact the dielectric properties of (0001) α-quartz slabs.

Modeling the Physics and Chemistry of Interfaces in Nanodielectrics

The properties of nanodielectrics can be dominated by interfacial phenomena. This chapter reviews recent work performed using ab initio density functional theory (DFT) aimed at interfacial properties pertinent to dielectrics applications. We begin by providing an overview of the predictive power of modern DFT computations, followed by specific applications of these methods that could provide insights into the role played by interface chemistry at the atomic level. The electronic structure and dielectric constant across interfaces with atomic level resolution, electron-phonon interactions, stability of interfaces, and impurity segregation to interfaces are discussed.

Local dielectric permittivity of HfO2 based slabs and stacks: A first principles study

A recently developed theory of atomic-scale local dielectric permittivity has been used to determine the position dependent optical and static dielectric permittivity profiles of a few nanoscale HfO2 and Si–HfO2 heterojunction slabs. The dielectric constants at the interior regions of each component recovered their respective bulk values. Enhancement of the dielectric constant at the free surfaces and its variations at the Si–HfO2 interface could be correlated to the corresponding surface and interfacial chemistry.