Confinement effects on optical phonons in spherical, rod-, and tetrapod-shaped nanocrystals detected by Raman spectroscopy

Spherical, rod- and tetrapod shaped CdSe nanocrystals are investigated by Raman spectroscopy and the longitudinal-optical and surface optical phonons are observed. We find that the position of the longitudinal-optical phonon slightly red-shifts with decreasing diameter, whereas the position of the surface optical phonon depends significantly on diameter and length of the rods or the tetrapod arms

Confined optical phonon modes in polar tetrapod nanocrystals detected by resonant inelastic light scattering

We investigated CdTe nanocrystal tetrapods of different sizes by resonant inelastic light scattering at room temperature and under cryogenic conditions. We observe a strongly resonant behavior of the phonon scattering with the excitonic structure of the tetrapods. Under resonant conditions we detect a set of phonon modes that can be understood as confined longitudinal-optical phonons, surface-optical phonons, and transverse-optical phonons in a nanowire picture.Comment: 12 pages, 4 figure

Chemically induced self-assembly of spherical and anisotropic inorganic nanocrystals

The self-assembly of inorganic nanoparticles is a research area of great interest aiming at the fabricationof unique mesostructured materials with intrinsic properties. Although many assembly strategies have been reportedover the years, chemically induced self-assembly remains one of the dominant approaches to achieve a high levelof nanoparticle organization. In this feature article we review the latest developments in assembly drivenby the active manipulation of nanoparticle surface

Optical properties of tetrapod-shaped CdTe nanocrystals

We studied the carrier confinement in tetrapod-shaped colloidal CdTe nanocrystals by means of absorption, photoluminescence, and photoluminescence excitation spectroscopy at room and cryogenic temperatures. The spectra show features characteristic of the tetrapod shape together with a clear dependence on the dominant confinement parameter, i.e., the diameter of the tetrapod arm. Theoretical calculations based on an envelope-function approximation and using the exact tetrapod shape have been performed to assign the observed spectral features. Oscillator strength and size dependence of the transitions energy have been calculated showing a direct correlation between the oscillator strength and the nanocrystal shape

Synthesis and perspectives of complex crystalline nano‐structures

Research on inorganic colloidal nanocrystals has moved from the synthesis of simple structures, such as spherical nanoparticles, to more elaborate particles with shapes such as rods, stars, discs, and branched nanocrystals, and recently to nanoparticles that are composed out of sections of different materials. Nanocrystal heterostructures represent a convenient approach to the development of nanoscale building blocks, as they merge sections with different functionality in the same particle, without the need of inorganic cross-linkers. The present article gives an overview of synthesis strategies to complex nanocrystals and will highlight their structural properties, as well as discuss some envisaged applications

Formation of Colloidal Semiconductor Nanocrystals

The present work describes different techniques to control some ma jor parameters of colloidal nanocrystals. The individual techniques rely on the manipulation of the nucleation event. The sensitive control of the nanocrystals’ size and shape is discussed. Furthermore the formation of hybrid nanocrystals composed of different materials is presented. The synthesis technique for the production of the different samples involves organic solvents and surfactants and reactions at elevated temperatures. The presence of magic size clusters offers a possibility to control the size of the nanocrystals even at very small dimensions. The clusters produced comprise ca. 100 atoms. In the case of CdSe, nanocrystals of this size emit a blue fluorescence and therefore extend the routinely accessible spectrum for this material over the whole visible range. Samples fluorescing in the spectral range from green to red are produced with standard recipes. In this work a reaction scheme for magic size clusters is presented and a theoretical model to explain the particular behaviour of their growth dynamics is discussed. The samples are investigated by optical spectroscopy, transmission electron microscopy, X-ray diffraction and elemental analysis. Shape controlled nanocrystals might be of interest for a variety of applications. The size dependent properties of nanocrystals are dominated by their smallest dimension. Therefore anisotropically shaped nanocrystals exhibit similar optical and electronic properties as spherical nanocrystals with a compatible diameter. This makes nanorods and nanowires an appealing object for electronics. Another possible application for these materials is to incorporate them into synthetic materials to influence their mechanical stability. Here, a method to form branched nanocrystals is discussed. It turned out that the presence of small impurities in the reaction vessel triggers the formation of branching points. Furthermore this synthesis technique offers some insights into the architecture of the branching point. The branching point is analysed by high resolution transmission electron microscopy and proves for the occurrence of a multiple twinned structure are strengthened by simulation of the observed patterns. Incorporation of a second material into a nanocrystal adds different functionality to the entire ob ject. Ideally both materials contribute with their own functionality and they are not affected by the presence of the other material. Two different techniques to generate nanocrystals of this type are presented. The first relies on a seeded growth approach in which the nucleation of the second material is allowed only on defined sites of the seeds. Anisotropic nanorods show a reactivity that varies for the individual facets. Using such nanorods as seeds dumbbell structures are formed. The second technique uses the tips of pre-formed nano-dumbbells as sacrificial domains. The material on the tips is replaced by gold. In any of the processes a different aspect of the nucleation event or the earliest stage of the growth is of relevance. In the growth of the magic size clusters the nucleation event itself is slowed down to a pace at which the experimenter can follow any step. The occurrence of branching can be traced down to the emergence of defects in the crystalline structure in the earliest stage of the growth. Hybrid materials are formed by a seeded-growth mechanism. Pre-formed nanocrystals provide the nucleation sites for the second material

Doctor of Philosophy

dissertationThe optoelectronic properties of nanoscale metal and semiconductor material systems are notably sensitive to their corresponding physical structure. Contemporary synthesis techniques enable careful control of nanoparticle con figurations and therefore provide a wide array of systems where the eff ects of physical morphology on the interaction between nanoscale materials and light can be carefully probed. The investigated properties are immediately relevant to light-harvesting and ultra-sensitive trace-analysis and sensing applications. In this work, the structure-property relationships of both individual semiconductor nanocrystal heterostructures and aggregates of plasmonic silver nanoparticles in rough metal fi lms are probed. The semiconductor heterostructures behave as model light-harvesting systems where optical energy absorbed by one portion of the structure is funneled, on the nanoscale, to a model light-harvesting center, in analogy to photosynthesis. In the plasmonic silver nanostructures, collective optical excitation of the conduction electrons - plasmons - con es electromagnetic radiation to well beyond the traditional diraction limit of light in nanoscale regions called "hot spots." Within these hot spots, light-matter interactions are greatly enhanced and thus enable trace-sensing applications such as Raman scattering from a single molecule. Thorough application of relatively simple single particle spectroscopy techniques is combined with high resolution electron microscopy to elucidate the subtle details on how physical structure controls the optical properties of both material systems. There are four main results of this work. (1) The linear and nonlinear optical response of rough silver fi lms is shown to be enhanced by the excitation of surface plasmon polaritons. (2) The enhanced nonlinear response of rough metal films is conjectured to originate from metal clusters, and the observation of stark fluctuations in their efficiency of second-harmonic generation is reported for the fi rst time. (3) The presence of and enhanced emission from silver clusters of only a few atoms plays an important role in the intrinsic optical response of the silver films with considerable implications for surface-enhanced Raman scattering. (4) The e ffects of physical anisotropy on the electronic states of semiconductor nanocrystals are explicitly identifi ed through correlated optical and electron microscopy of single particles. These eff ects are shown to have important rami cations in the internal energy-transfer process of single nanocrystals