Surface States in Passivated, Unpassivated and Core/Shell Nanocrystals: Electronic Structure and Optical Properties
- PDF / 378,257 Bytes
- 6 Pages / 612 x 792 pts (letter) Page_size
- 52 Downloads / 195 Views
N13.2.1
Surface States in Passivated, Unpassivated and Core/Shell Nanocrystals: Electronic Structure and Optical Properties Garnett W. Bryant1 and W. Jaskolski2 1 Atomic Physics Division, National Institute of Standards and Technology, Gaithersburg, MD 20899-8423 2 Instytut Fizyki, UMK, Grudziadzka 5, 87-100 Torun, Poland ABSTRACT Surface effects significantly influence the functionality of semiconductor nanocrystals. A theoretical understanding of these surface effects requires models capable of describing surface details at an atomic scale, passivation with molecular ligands, and few-monolayer capping shells. We present an atomistic tight-binding theory of the electronic structure and optical properties of passivated, unpassivated and core/shell nanocrystals to study these surface effects. INTRODUCTION Quantum dots and nanocrystals have been studied intensely due to their enticing possibilities as artificial atoms with enhanced optical properties. Nanocrystals are being used now as building blocks for bio/nanohybrids for use in biological systems as biomarkers [1-3] and resonant energy-transfer sensors [4]. Construction of these bio/nanohybrids is a challenging, ongoing problem [3,4]. An understanding of how dots function in these structures is needed so that tailored nanooptics with these systems can be exploited. Surface effects significantly influence the functionality of semiconductor nanocrystals. Passivation with ligands or high bandgap semiconductor shells is necessary to reduce surface trap densities, enhance quantum yield and increase photostability [5-8]. Continuum effective mass theory [9,10] can provide an understanding of internal confined states in semiconductor nanocrystals, although atomistic theories are needed for a more complete understanding of these confined states [11-16]. However, a full theoretical understanding of surface effects requires atomistic models capable of describing surface faceting and relaxation; site-dependent, partial and random passivation; the molecular structure of passivants; and few-monolayer shells. We present an atomistic tightbinding theory of the electronic structure and optical properties of passivated, unpassivated and core/shell nanocrystals to study these surface effects. THEORY We calculate electron and hole states in nanocrystals by use of the empirical tight-binding (ETB) method [12-16]. The ETB approach is an atomistic approach well suited for calculating the electronic states of nanosystems with atomic-scale interfaces, surface structure and variations in composition and shape. In this paper, we consider zinc-blende CdS and CdS/ZnS nanocrystals. We assume that all atoms occupy a common, zinc-blende lattice with no relaxation in the bulk, at interfaces or surfaces. To date, lattice relaxation and surface reconstruction have been considered only for very small nanocrystals with a few hundred atoms [17-19]. Here, we consider a full
N13.2.2
range of nanocrystal sizes, including core/shell structures with up to 25,000 atoms. It is extremely time-consuming to
Data Loading...
