Low-temperature polarized and time-resolved photoluminescence spectroscopy in high magnetic fields (up to 30 T) has been used to study the spin-polarization, spin-relaxation, and radiative lifetimes of excitons in wurtzite semiconductor (e.g., CdSe) colloidal nanocrystals. The applied magnetic field leads to a significant degree of circular polarization of the exciton photoluminescence, accompanied by a reduction in the photoluminescence decay time. The circular polarization arises from the Zeeman splitting of exciton levels, whereas lifetime reduction results from a polarization-preserving field-induced mixing of exciton levels. We analyze these experimental findings in terms of a simple model that combines both Zeeman effect and exciton-level mixing, as a function of the relative orientation of the nanocrystal c-axis and the magnetic field. This model is able to simultaneously describe the degree of circular polarization and lifetime reduction of the exciton photoluminescence, permitting us to quantify the exciton, electron, and hole g-factors.

Wave function engineering has become a powerful tool to tailor the optical properties of semiconductor colloidal nanocrystals. Core–shell systems allow to design the spatial extent of the electron (e) and hole (h) wave functions in the conduction- and valence bands, respectively. However, tuning the overlap between the e- and h-wave functions not only affects the oscillator strength of the coupled e–h pairs (excitons) that are responsible for the light emission, but also modifies the e–h exchange interaction, leading to an altered excitonic energy spectrum. Here, we present exciton lifetime measurements in a strong magnetic field to determine the strength of the e–h exchange interaction, independently of the e–h overlap that is deduced from lifetime measurements at room temperature. We use a set of CdTe/CdSe core/shell heteronanocrystals in which the electron–hole separation is systematically varied. We are able to unravel the separate effects of e–h overlap and e–h exchange on the exciton lifetimes, and we present a simple model that fully describes the recombination lifetimes of heteronanostructures (HNCs) as a function of core volume, shell volume, temperature, and magnetic fields.Keywords: core−shell heterostructure; electron−hole exchange; electron−hole overlap; excitons; magnetic field; nanocrystals

In this review we will focus on how magnetic fields can be used to manipulate the motion of various micro- and nanostructures in solution. We will distinguish between ferromagnetic, paramagnetic and diamagnetic materials. Furthermore, the use of various kinds of magnetic fields, such as homogeneous, inhomogeneous and rotating magnetic fields, is discussed. To date most research has focused on the use of ferro- and paramagnetic materials, but here we also describe the possibilities of magnetic manipulation of diamagnetic materials. Since the vast majority of soft matter is diamagnetic, this paves the way for many new applications to manipulate the motion of micro- and nanostructures.

Light emission of semiconductor nanocrystals is a complex process, depending on many factors, among which are the quantum mechanical size confinement of excitons (coupled electron–hole pairs) and the influence of confined phonon modes and the nanocrystal surface. Despite years of research, the nature of nanocrystal emission at low temperatures is still under debate. Here we unravel the different optical recombination pathways of CdSe/CdS dot-in-rod systems that show an unprecedented number of narrow emission lines upon resonant laser excitation. By using self-assembled, vertically aligned rods and application of crystallographically oriented high magnetic fields, the origin of all these peaks is established. We observe a clear signature of an acoustic-phonon assisted transition, separated from the zero-phonon emission and optical-phonon replica, proving that nanocrystal light emission results from an intricate interplay between bright (optically allowed) and dark (optically forbidden) exciton states, coupled to both acoustic and optical phonon modes.Keywords: acoustic phonons; core−shell heterostructure; excitons; fluorescence line-narrowing; magnetic fields; nanocrystals; optical phonons

We have used magnetic-field-induced birefringence as a new sensitive technique to probe the aggregation kinetics of macrocyclic molecules in solution. We have found three consecutive aggregation stages: disordered objects, ordered fibers, and a network. The transition from disordered objects to ordered fibers is found to be slow, taking days or weeks to complete. We attribute this to the molecular tails of the macrocycles, which hamper fiber formation. We anticipate that linking aggregation kinetics to molecular properties will lead to a better understanding of the mechanisms by which molecules self-assemble, allowing for a more rational design of the molecular building blocks.

We have determined the internal organization of elongated sexithiophene aggregates in solution by combining small-angle X-ray scattering and magnetic birefringence experiments. The different aggregate axes can be probed independently by performing the experiments on magnetically aligned aggregates. We have found multiwalled cylindrical aggregates consisting of radially oriented sexithiophene molecules with π−π-stacking in the tangential direction, a structure that is considerably different from those previously found in other solvents. The aggregate morphology of this semiconducting material can thus be tuned by using different solvents, which offers the attractive perspective to steer chemical self-assembly toward nanostructures with desired functionalities, especially in combination with the alignment in a magnetic field.

We review the recent literature on the use of optical spectroscopy of semiconductor quantum dots in high magnetic fields. We address both self-assembled epitaxial dots and colloidal nanocrystal quantum dots, each of which has its own characteristic optical response. Combining simple theoretical models for quantum confinement with the effect of high magnetic fields we describe the basic optically allowed transitions expected for epitaxial and colloidal quantum dots. Within these models we discuss the effects of quantum confinement and orbital and spin Zeeman effects on the optical spectra, illustrated by experimental examples. Finally, effects of electron–electron and exchange interactions are addressed.Nous discutons lʼapplication des méthodes de spectroscopie optique en champs magnétiques intenses aux études des boîtes quantiques semiconductrices, telles quʼelles sont présentées dans la littérature la plus récente. Nous traitons à la fois le cas des boîtes épitaxies, auto-assemblées, et le cas des boîtes quantiques obtenues à partir de nano-cristaux colloïdaux. En combinant des modèles théoriques simples, concernant le confinement quantique, avec les effets de champ magnétique forte, nous décrivons les transitions optiques fondamentales qui sont attendues dans le cas de boîtes quantiques épitaxies et colloïdales. Dans le cadre de ces modèles, nous discutons des conséquences sur le spectre optique des effets de confinement quantique et des effets Zeeman orbitaux et de spins, ce que nous illustrons à lʼaide des exemples expérimentaux. Enfin, nous terminons en traitant le problème des effets électrons–électrons et dʼinteraction dʼéchange.