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Peter Liljeroth

Publications and source records attributed to Peter Liljeroth.

15 recordsLinked to original sources
PubMed2006

Variable orbital coupling in a two-dimensional quantum-dot solid probed on a local scale.

The optoelectronic properties of semiconductor quantum-dot (QD) solids depend on the electronic structure of the building blocks and their interactions. Disorder may affect the coupling on a local scale. We have measured the density of states of 2D arrays of PbSe QDs site by site using scanning tunneling spectroscopy. It markedly differs from that of isolated QDs due to electronic coupling in the array. We observe strong local variations in the coupling strength with two prototypical cases: delocalization of the conduction electrons only, and full coupling with both hole and electron delocalization over the QD sites in the array.

Journal Article↗
PubMed2006

Electronic coupling and exciton energy transfer in CdTe quantum-dot molecules.

Stable dispersions of molecularlike aggregates of CdTe quantum dots are prepared by chemical cross-linking. Cryo-TEM images confirm the presence of cross-linked quantum dots and show that the size of the small aggregates can be controlled by the amount of cross-linker added. Optical measurements reveal two types of interdot interactions within these quantum-dot molecules: exciton energy transfer and electronic coupling. Quantitative information on the energy transfer rates in quantum-dot molecules is obtained by photoluminescence lifetime measurements. The degree of electronic coupling is dependent on the size of the quantum dots, which is supported by quantum mechanical calculations.

Journal Article↗
PubMed2006

Resolving electron transfer kinetics at the nanocrystal/solution interface.

The kinetics of electron transfer between individual gold nanocrystals and a solution redox species is quantified. The observed rate is dependent on the extent of electronic coupling between nanocrystals in the monolayer indicating the effect of Coulomb blockade on electrochemical kinetics.

Journal Article↗
PubMed2005

Mixed self-assembled monolayers of semirigid tetrahydro-4H-thiopyran end-capped oligo(cyclohexylidenes).

Single-component and mixed self-assembled monolayers (SAMs) of one- and three-ring semirigid tetrahydro-4H-thiopyran end-capped oligo(cyclohexylidenes)-that is, thiopyran (1), 4-(4-cyclohexylidene-cyclohexylidene)tetrahydro-4H-thiopyran (2), and 4-(tetrahydro-4H-thiopyran-4-cyclohexylidene-4'-ylidene)tetrahydro-4H-thiopyran (3)--on Au(111) substrates have been prepared and studied by cyclic voltammetry (CV), atomic force microscopy (AFM), and scanning tunneling microscopy (STM). It was found that the shortest adsorbate 1 more readily forms a SAM than 2 or 3. Notwithstanding, the SAMs of 2 or 3 are thermodynamically more stable due to favorable intermolecular attractions. Holes were made with the AFM tip establishing tilt angles of 30-50 degrees with respect to the surface normal for all SAMs. STM imaging showed well-ordered, line-shaped packing patterns with molecular resolution for the SAM of 2. Similar patterned structures were not observed for 1 and 3. Mixed SAMs were prepared by exposing a SAM of 1 to ethanol solutions of either 2 or 3. STM imaging revealed that domains of molecules of 2 or 3 amidst a monolayer of 1 are formed in both cases. Whereas in the mixed SAM of 1 and 2 the domains are irregularly shaped, circular islands of uniform size are found in the mixed SAM of 1 and 3.

Journal Article↗
PubMed2005

Channel flow configuration for studying the kinetics of surfactant-polyelectrolyte binding.

A novel channel flow configuration was developed and utilized for studying the poly(styrenesulfonate)-cetylpyridinium ion interaction kinetics. The surfactant solution was continuously injected into a flow of polyelectrolyte solution, and the extent of the association reaction was probed at an ion-selective detector electrode. The system was modeled within an analytical approximation, which was tested by a finite-element simulation of the full convective mass transport problem including the homogeneous complexation reaction. The results show that association kinetics can be resolved and that the initial steps of the reaction are not influenced by intermolecular interactions between the bound surfactants. The presented methodology is general, and further development should enable the study of complex cooperative kinetics of surfactant-polyelectrolyte systems.

Cetylpyridinium↗
PubMed2005

Density of states measured by scanning-tunneling spectroscopy sheds new light on the optical transitions in PbSe nanocrystals.

The density-of-states function of individual colloidal PbSe nanocrystals varying in diameter between 3 and 7 nm is measured by resonant tunneling spectroscopy. It is in semiquantitative agreement with tight-binding calculations, but the energy separation between electron (hole) levels of S and P symmetry is systematically smaller than predicted by the theory. These results provide an explanation for the second and third excitonic optical transitions, which have been debated for a long time.

Journal Article↗
PubMed2005

Chemisorption determines the photovoltage of a Ti/TiO2/Au/dye internal electron emission photovoltaic cell.

The Ti/TiO2/Au junction forms the basis of a promising new type of photovoltaic cell, provided that a light-harvesting antenna layer can be deposited on the thin gold film. We report that the electrical diode characteristics of the TiO2/Au Schottky barrier deteriorate by deposition of a hydrophobic quantum dot film, Merbromin dye adsorption, or electron-hole photogeneration in TiO2 under inert conditions. In the presence of oxygen and water vapor, the Schottky barrier characteristics and high photovoltage are recovered. The strong influence of the TiO2 surface chemistry on the (photo)electrical characteristics of the solar cell is discussed on the basis of an existing microscopic model.

Journal Article↗
PubMed2005

Electron-conducting quantum dot solids: novel materials based on colloidal semiconductor nanocrystals.

We review the optical and electrical properties of solids that are composed of semiconductor nanocrystals. Crystals, with dimensions in the nanometre range, of II-VI, IV-VI and III-V compound semiconductors, can be prepared by wet-chemical methods with a remarkable control of their size and shape, and surface chemistry. In the uncharged ground state, such nanocrystals are insulators. Electrons can be added, one by one, to the conduction orbitals, forming artificial atoms strongly confined in the nanocrystal. Semiconductor nanocrystals form the building blocks for larger architectures, which self-assemble due to van der Waals interactions. The electronic structure of the quantum dot solids prepared in such a way is determined by the orbital set of the nanocrystal building blocks and the electronic coupling between them. The opto-electronic properties are dramatically altered by electron injection into the orbitals. We discuss the optical and electrical properties of quantum dot solids in which the electron occupation of the orbitals is controlled by the electrochemical potential.

Colloids↗
PubMed2005

Physicochemical evaluation of the hot-injection method, a synthesis route for monodisperse nanocrystals.

The quintessence of the hot-injection method, a synthesis route for monodisperse, highly luminescent semiconductor nanocrystals, is reviewed. The separate stages of nucleation and growth of the nanocrystals are discussed in the framework of classical nucleation theory and an equilibrium model proposed by Debye. We also review the numerous adaptations of the original synthesis that currently provide colloidal nanocrystals with well-defined, size-dependent optical, electrical, and magnetic properties. The availability of these remarkable materials is one of the most promising developments in nanoscience and nanotechnology.

Cadmium Compounds↗
PubMed2004

Electron transport in two-dimensional arrays of gold nanocrystals investigated by scanning electrochemical microscopy.

This article reports the use of the scanning electrochemical microscope (SECM) to investigate the electronic properties of Langmuir monolayers of alkane thiol protected gold nanocrystals (NCs). A substantial increase in monolayer conductivity upon mechanical compression of the Au NC monolayer is reported for the first time. This may be the room temperature signature of the insulator to metal transition previously reported for comparable silver NC monolayers. Factors influencing the conductivity of the monolayer NC array are discussed.

Journal Article↗
PubMed2003

Membrane activity of biotechnological peptide drugs.

Charged Langmuir-Blodgett monolayers deposited at an immobilised liquid-liquid interface have been used as a simple model for a biological membrane to investigate the membrane activity of biotechnological oligopeptide drugs.

Biotechnology↗
PubMed2003

Dissolution testing of acetylsalicylic acid by a channel flow method-correlation to USP basket and intrinsic dissolution methods.

A new modification of the channel flow dissolution method is introduced together with the theoretical basis to extract the solubility and mass transfer parameters from the dissolution experiments. Correlation of drug dissolution profiles in the channel flow apparatus was evaluated with respect to USP basket and intrinsic dissolution methods at pH 1.2 or 6.8. Acetylsalicylic acid (ASA) was studied as a pure drug substance and as three simple tablet compositions with microcrystalline cellulose (MCC) and/or lactose as excipients. The channel flow measurements of 100% ASA tablets correlated well with the results of intrinsic dissolution tests. In the channel flow method as well as in the USP basket method the release of ASA was fastest from the tablet compositions containing lactose, while the slowest dissolution rate was observed with the composition containing MCC as the only excipient. As presumed, the dissolution rate of the weak acid was decreased as the pH of the medium was lowered, which was clearly confirmed also by the three dissolution methods. MCC forms matrix tablets and in the USP basket method the dissolution profiles followed square root of time kinetics indicating that diffusion was the rate-controlling step of ASA dissolution. Also the channel flow results indicated that the dissolution of ASA was controlled by mass transfer. The swelling behaviour of the tablets is different in the channel flow method as compared to the basket method: only one tablet surface is exposed to the dissolution medium in the channel flow system. The contact between the tablet surface and the dissolution medium is more similar between the channel flow and intrinsic dissolution methods.

Algorithms↗
PubMed2002

Interfacial reactivity of monolayer-protected clusters studied by scanning electrochemical microscopy.

Solutions of monodisperse monolayer-protected clusters (MPCs) of gold can be used as multivalent redox mediators in electrochemical experiments due to their quantized double-layer charging properties. We demonstrate their use in scanning electrochemical microscopy (SECM) experiments wherein the species of interest (up to 2-electron reduction or 4-electron oxidation from the native charge-state of the MPCs) is generated at the tip electrode, providing a simple means to adjust the driving force of the electron transfer (ET). Approach curves to perfectly insulating (Teflon) and conducting (Pt) substrates are obtained. Subsequently, heterogeneous ET between MPCs in 1,2-dichloroethane and an aqueous redox couple (Ce(IV), Fe(CN)63-/4-, Ru(NH3)63+, and Ru(CN)64-) is probed with both feedback and potentiometric mode of SECM operation. Depending on the charge-state of the MPCs, they can accept/donate charge heterogeneously at the liquid-liquid interface. However, this reaction is very slow in contrast to ET involving MPCs at the metal-electrolyte interface.

Journal Article↗
PubMed2002

Disk-generation/ring-collection scanning electrochemical microscopy: theory and application.

The potential of ring-disk ultramicroelectrodes (RD UMEs) as probes for scanning electrochemical microscopy (SECM) was investigated both theoretically and experimentally. In particular, the disk-generation/ring-collection (DG/RC) mode of operation was considered. In this case, the interaction of two species with the substrate under investigation can be followed simultaneously from single tip current-distance measurement (approach curve) to the substrate. Theoretical approach curves for DG/RC were calculated by numerical methods. Such approach curves to both insulating and conducting substrates indicate a strong tip response dependence on the ring radius while the response was relatively insensitive to ring thickness and overall tip radius. The RD tip was characterized by fitting experimental approach curves recorded at insulating and conducting substrates to simulated curves for a given tip geometry. DG/RC SECM was then applied to investigate the partitioning of iodine across a liquid-liquid interface.

Journal Article↗