Karin Overgaag
Utrecht University
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Publication
Featured researches published by Karin Overgaag.
Nano Letters | 2009
Heiner Friedrich; Cédric Gommes; Karin Overgaag; Johannes D. Meeldijk; Wiel H. Evers; Bart de Nijs; Mark P. Boneschanscher; Petra E. de Jongh; Arie J. Verkleij; Krijn P. de Jong; Alfons van Blaaderen; Daniel Vanmaekelbergh
Binary nanocrystal superlattices, that is, ordered structures of two sorts of nanocolloids, hold promise for a series of functional materials with novel collective properties. Here we show that based on electron tomography a comprehensive, quantitative, three-dimensional characterization of these systems down to the single nanocrystal level can be achieved, which is key in understanding the emerging materials properties. On four binary lattices composed of PbSe, CdSe, and Au nanocrystals, we illustrate that ambiguous interpretations based on two-dimensional transmission electron microscopy can be prevented, nanocrystal sizes and superlattice parameters accurately determined, individual crystallographic point and plane defects studied, and the order/disorder at the top and bottom surfaces imaged. Furthermore, our results suggest that superlattice nucleation and growth occurred at the suspension/air interface and that the unit cells of some lattices are anisotropically deformed upon drying.
Journal of the American Chemical Society | 2008
Karin Overgaag; Wiel H. Evers; Bart de Nijs; Rolf Koole; Johannes D. Meeldijk; Daniel Vanmaekelbergh
In this paper we show that self-organization of colloidal PbSe and CdSe semiconductor nanocrystals with a size ratio of 0.57 leads to binary structures with a AB2 or a cuboctahedral AB13 lattice. The type of superlattice formed can be regulated by the relative concentration of both nanocrystals in the suspension.
ACS Nano | 2008
Karin Overgaag; Peter Liljeroth; B. Grandidier; Daniel Vanmaekelbergh
The electronic local density of states (LDOS) of single PbSe quantum dots (QDs) and QD molecules is explored using low-temperature scanning tunneling microscopy (STM) and spectroscopy (STS). Both individual PbSe QDs and molecular aggregates of PbSe QDs (dimers, trimers, etc.) are mechanically stabilized in a two-dimensional superlattice of wide band gap CdSe QDs acting as an inert matrix. The LDOS measured at individual QDs dispersed in the matrix is identical to that of single isolated QDs chemically linked to a substrate. We investigate the degree of quantum mechanical coupling between the PbSe QDs in molecular aggregates by comparing the LDOS measured at each site in the aggregates to that of an individual PbSe QD. We observe a variable broadening of the resonances indicating a spatially dependent degree of electron delocalization in the molecular aggregates.
Physical Chemistry Chemical Physics | 2006
Peter Liljeroth; Lucian Jdira; Karin Overgaag; B. Grandidier; S. Speller; Daniel Vanmaekelbergh
Molecules, supramolecular structures and semiconductor nanocrystals are increasingly used as the active components in prototype opto-electrical devices with miniaturized dimensions and novel functions. Therefore, there is a strong need to measure the electronic structure of such single, individual nano-objects. Here, we explore the potential of scanning tunnelling spectroscopy to obtain quantitative information on the energy levels and Coulomb interactions of semiconductor quantum dots. We discuss the conditions under which shell-tunnelling, shell-filling and bipolar spectroscopy can be performed, and illustrate this with spectra acquired on individual CdSe and PbSe quantum dots. We conclude that quantitative information on the energy levels and Coulomb interactions can be obtained if the physics of the tip/quantum dot/substrate double-barrier tunnel junction is well understood.
Nano Letters | 2008
Lucian Jdira; Karin Overgaag; J.W. Gerritsen; Daniël Vanmaekelbergh; Peter Liljeroth; S. Speller
We use scanning tunnelling microscopy (STM) to controllably contact individual CdSe quantum dots (QDs) in a multilayer array to study electrical contacts to a model QD solid. The probability of electron injection into the QD array depends strongly on the symmetry of the QD wave functions and their response to the local electric field. Quantitative spectroscopy of the QD energy levels is possible if the potential distribution in the STM tip-QD array-substrate system is taken into account.
Journal of Chemical Physics | 2009
Karin Overgaag; Daniel Vanmaekelbergh; Peter Liljeroth; G. Mahieu; B. Grandidier; C. Delerue; G. Allan
The linewidth of the resonances in the single-electron tunneling spectra has been investigated for PbSe semiconductor nanocrystals (NCs) with scanning tunneling spectroscopy at low temperature. The linewidth of the resonances corresponding to tunneling through the first conduction and valence levels is found to increase with decreasing size of the NCs. Based on theoretical calculations, this broadening is mainly induced by the coupling between the tunneling electrons and the longitudinal optical phonon mode of the NC, and by the splitting of the degenerate electronic levels between the different L-valleys in the Brillouin zone. For the smallest sizes, it is shown that the intervalley splitting is the major source of broadening.
MRS Proceedings | 2001
Dirk L. J. Vossen; Jacob P. Hoogenboom; Karin Overgaag; Alfons van Blaaderen
We describe a method for patterning substrates with colloidal particles in any designed twodimensional structure. By using optical tweezers particles are brought from a reservoir to a surface that carries a surface charge opposite to that of the particles. Using this technique large, two-dimensional patterns can be created, where the pattern can be manipulated on a single particle level. We show that these structures can be dried using critical point drying thus preventing distortions due to surface tension forces. After drying patterned surfaces can be used for further processing, which includes repeating the procedure of patterning. We show some first results of three-dimensional structures created using this layer-by-layer method. The method is generally applicable and has been demonstrated for a variety of (core-shell) colloidal particles including particles that are interesting for photonic applications like high-refractive index (ZnS)core – silica shell particles, metallodielectric (gold)-core – silica-shell particles, fluorescently labeled particles and small (several nanometers large) gold particles. Particle sizes used range from a few nanometers to several micrometers.
Nature | 2004
Peter Lodahl; A. Floris van Driel; Ivan S. Nikolaev; A. Irman; Karin Overgaag; Daniel Vanmaekelbergh; Willem L. Vos
Nano Letters | 2008
Marijn A. van Huis; Lucas T. Kunneman; Karin Overgaag; Qiang Xu; G. Pandraud; H.W. Zandbergen; Daniël Vanmaekelbergh
Journal of Materials Chemistry | 2009
Milena Rosso-Vasic; Evan Spruijt; Zoran Popović; Karin Overgaag; Barend van Lagen; B. Grandidier; Daniel Vanmaekelbergh; David Domínguez-Gutiérrez; Luisa De Cola; Han Zuilhof