A tight-binding model for illustrating exciton confinement in semiconductor nanocrystals.

The Brus equation describes the relation between the lowest energy of an electron-hole pair and the size of a semiconductor crystallite. However, taking the strong confinement regime as a starting point, the equation does not cover the transition from weak to strong confinement, the accompanying phenomenon of charge-carrier delocalization, or the change in the transition dipole moment of the electron-hole pair state. Here, we use a one-dimensional, two-particle Hubbard model for interacting electron-hole pairs that extends the well-known tight-binding approach through a point-like electron-hole interaction. On infinite chains, the resulting exciton states exhibit the known relation between the Bohr radius, the exciton binding energy, and the effective mass of the charge carriers. Moreover, by introducing infinite-well boundary conditions, the model enables the transition of the exciton states from weak to strong confinement to be tracked, while straightforward adaptations provide insights into the relation between defects, exciton localization, and confinement. In addition, by introducing the dipole operator, the variation of the transition dipole moment can be mapped when shifting from electron-hole pairs in strong confinement to delocalized and localized excitons in weak confinement. The proposed model system can be readily implemented and extended to different multi-carrier states, thus providing researchers a tool for exploring, understanding, and teaching confinement effects in semiconductor nanocrystals under different conditions.

From ligands to binding motifs and beyond; the enhanced versatility of nanocrystal surfaces.

Surface chemistry bridges the gap between nanocrystal synthesis and their applications. In this respect, the discovery of complex ligand binding motifs on semiconductor quantum dots and metal oxide nanocrystals opens a gateway to new areas of research. The implications are far-reaching, from catalytic model systems to the performance of solar cells.

Near-field resonance at far-field-induced transparency in diffractive arrays of plasmonic nanorods.

We numerically demonstrate that a periodic array of metallic nanorods sustains a maximum near-field enhancement and a far field (FF)-induced transparency at the same energy and in-plane momentum. The coupling of bright and dark plasmonic lattice resonances, and electromagnetic retardation along the nanorod length, are responsible for this effect. A standing wave with a quadrupolar field distribution is formed, giving rise to a collective suppression of FF scattering and simultaneously enhanced local fields.

Quantum dot based rapid tests for zearalenone detection.

Three different kinds of immunosorbent assays with luminescence detection were developed for the determination of zearalenone (ZEN), a secondary toxic metabolite of Fusarium fungi. CdSe/ZnS core/shell quantum dots (QDs) were used as a label in quantitative micro-well plate immunoassays (fluorescent-labeled immunosorbent assay, FLISA) and in qualitative column test methods. As carriers for QD-based column tests, sepharose gel (for covalent binding of antibody) and polyethylene frits (for physical absorption of antibody) were used and compared. The application of QDs as a label resulted in a fourfold decrease in the IC(50) value with FLISA (0.1 ng mL(-1)) with a detection limit of 0.03 ng mL(-1) when compared with the traditional immunosorbent assay which makes use of horseradish peroxidase as the enzyme label. The cutoff levels for both qualitative column test methods were selected based on the maximum level for ZEN in unprocessed cereals established by the European Commission (100 µg kg(-1)) as 5 ng mL(-1) taking into account extraction and dilution. The different developed immumoassays were tested for ZEN determination in raw wheat samples. As a confirmatory method, liquid chromatography coupled to tandem mass spectrometry was used. The obtained results allow using FLISA and both qualitative column test methods for the analysis of analytes with very low established maximum limits, even in very complicated food matrices, owing to the high dilution of the sample extract.

Quantum dot micropatterning on si.

Using InP and PbSe quantum dots, we demonstrate that the Langmuir-Blodgett technique is well-suited to coat nonflat surfaces with quantum dot monolayers. This allows deposition on silicon substrates covered by a developed patterned resist, which results in monolayer patterns with micrometer resolution. Atomic force microscopy and scanning electron microscopy reveal the formation of a densely packed monolayer that replicates predefined structures with high selectivity after photoresist removal. A large variety of shapes can be reproduced and, due to the excellent adhesion of the quantum dots to the substrate, the hybrid approach can be repeated on the same substrate. This final possibility leads to complex, large-area quantum dot monolayer structures with micrometer spatial resolution that may combine different types of quantum dots.

Electrophoretic deposition of ZnO nanoparticles, from micropatterns to substrate coverage.

We report on an electrophoretic deposition (EPD) method that is suited for the preparation of both ZnO thin films and micropatterns. By applying small DC voltages between a Cu electrode and a conductive Si substrate, submersed in a suspension of ZnO quantum dots, we can cover entire substrates with ZnO layers of a tuneable thickness ranging from a few monolayers to 200 nm. The deposition occurs selectively at the cathode, which indicates that the ZnO particles have a positive charge. Atomic force microscopy was used to study the influence of the deposition voltage, time, and the quantum dot concentration on the final layer thickness. By using lithographically patterned Si substrates, the same technique enables the formation of ZnO micropatterns of variable thickness with dimensions down to 5 µm. This is done by depositing a ZnO layer on a Si substrate that is covered with a patterned, developed photoresist. After EPD, the resist is removed by submersing the substrate in the appropriate solvent without damaging the ZnO deposit. This illustrates the robustness of the layers obtained by EPD.

Langmuir-Blodgett monolayers of InP quantum dots with short chain ligands.

We demonstrate the organization of nearly monodisperse colloidal InP quantum dots at the air/water interface in Langmuir monolayers. The organization of the particles is monitored in situ by surface pressure-surface area measurements and ex situ by AFM measurements on films transferred to mica by Langmuir-Blodgett deposition. The influence of different ligands on the quality of the monolayer formed has been studied. We show that densely packed monolayers with little holes can be formed using short chain ligands like pyridine and pentamethylene sulfide. The advantage of using short chain ligands for electron tunneling to or from the quantum dots is demonstrated using scanning tunneling spectroscopy.

Effect of quantum confinement on the dielectric function of PbSe.

Monolayers of lead selenide nanocrystals of a few nanometers in height have been made by electrodeposition on a Au(111) substrate. These layers show a thickness-dependent dielectric function, which was determined using spectroscopic ellipsometry. The experimental results are compared with electronic structure calculations of the imaginary part of the dielectric function of PbSe nanocrystals. We demonstrate that the size-dependent variation of the dielectric function is affected by quantum confinement at well-identifiable points in the Brillouin zone, different from the position of the band-gap transition.

Effects of crystal shape on the energy levels of zero-dimensional PbS quantum dots.

Nanometer-size PbS quantum dots have been made by electrodeposition on a Au(111) substrate. The deposited nanocrystals have a flattened cubic shape. We probed the single-electron energy-level spectrum of individual quantum dots by scanning tunneling spectroscopy and found that it deviates strongly from that of spherical PbS quantum dots. The measured energy-level spectrum is successfully explained by considering strong confinement in a flattened cubic box.