The stabilization of a single domain in free-standing ferroelectric nanocrystals.

High resolution electron microscopy, electron diffraction and electron holography were used to study individual free-standing ∼ 30 nm barium titanate nanocrystals. Large unidirectional variations in the tetragonal distortion were mapped across the smaller nanocrystals, peaking to anomalously large values of up to 4% at the centers of the nanocrystals. This indicated that the nanocrystals consist of highly strained single ferroelectric domains. Simulations using an effective Hamiltonian for modeling a nanocrystal under a small depolarizing field and negative pressure qualitatively confirm this picture. These simulations, along with the development of a phenomenological model, show that the tetragonal distortion variation is a combined effect of: (i) electrostrictive coupling between the spontaneous polarization and strain inside the nanocrystal, and (ii) a surface-induced effective stress existing inside the nanodot. As a result, a 'strain skin layer', having a smaller tetragonal distortion relative to the core of the nanocrystal, is created.

Size dependence of chiroptical activity in colloidal quantum dots.

The synthesis of chiral penicillamine-capped CdS and CdSe quantum dots (QDs) was adjusted to control the size of the nanoparticles. This, together with size separation, allowed for simultaneous tuning of absorption, circular dichroism (CD), and fluorescence on a wide wavelength range. Band edge transitions were accompanied by circular dichroism peaks which red-shifted together with the increase in particle size. The clear correlation between absorption and CD bands as well as between absorption bands and size in semiconductor QDs was used to derive an experimental scaling law for optical activity. The decrease in the intensity of circular dichroism-to-absorption ratio (dissymmetry) with the increase in particle size was stronger than linear, probably exponential. In addition, strong material type dependence was observed. The CD line shape appeared to be sensitive to the nature of the transition and may thus serve as a tool for sorting out the electronic states of the QDs. Fluorescence-detected circular dichroism (FDCD) was introduced as a new probe of optically active fluorescent nanoparticles. The analysis of the size and material dependence of the chiroptical induction effect leads to the conclusion that it is primarily an electronic interaction effect between the adsorbed chiral molecules and the electron-hole states.

The size-dependent ferroelectric phase transition in BaTiO3 nanocrystals probed by surface plasmons.

A new technique for probing the temperature dependence of the dielectric constant of ferroelectric nanocrystals (NCs) using shifts in the localized surface plasmon resonance (LSPR) wavelength of gold nanoparticles attached to the surface of the ferroelectric NCs is demonstrated. This technique can selectively probe the surface of the NCs and was used to study the ferroelectric-to-paraelectric phase transition of barium titanate (BTO) nanocubes in three size regimes of 16 ± 4, 47 ± 11, and 220 ± 140 nm. Temperature-dependent Raman spectroscopy was also applied to probe the whole volume of the NCs. The LSPR-based technique revealed that the ∼16 nm BTO NCs were dominated by surface effects, and as the NC size increased bulk BTO behavior governed. This supports recent propositions about the lack of intrinsic size dependence of the transition temperature. Therefore, the surface chemistry/structure probably affected the ferroelectric behavior rather than finite size effects. A distinct phase transition at the surface characterized by a very long relaxation time was detected by the LSPR-based technique.

Growth of Au/Ag nanowires in thin surfactant solution films: an electron microscopy study.

The surfactant templated gold-silver nanowire growth process in a thin solution film was probed by cryo-transmission electron microscopy (cryo-TEM). The increasing surfactant concentration upon film drying appears to induce phase transformations in the film and form a liquid crystalline template for the nanowires growth. High-resolution transmission electron microscopy (HRTEM) and electron holography revealed that the nanowires were polycrystalline with some preferred crystallite orientations and had a roughly cylindrical cross-section. Further improvement of the technique may lead to highly ordered metal nanowire arrays within the surfactant matrix similar to the closely related mesoporous materials.