Papers by Marijn Van Huis
Physical Review B, 2002
Quantum-confined positrons are sensitive probes for determining the electronic structure of nanoc... more Quantum-confined positrons are sensitive probes for determining the electronic structure of nanoclusters embedded in materials. In this work, a depth-selective positron annihilation 2D-ACAR ͑two-dimensional angular correlation of annihilation radiation͒ method is used to determine the electronic structure of Li nanoclusters formed by implantation of 10 16 -cm Ϫ2 30-keV 6 Li ions in MgO ͑100͒ and ͑110͒ crystals and by subsequent annealing at 950 K. Owing to the difference between the positron affinities of lithium and MgO, the Li nanoclusters act as quantum dots for positrons. 2D-ACAR distributions for different projections reveal a semicoherent fitting of the embedded metallic Li nanoclusters to the host MgO lattice. Ab initio Korringa-Kohn-Rostoker calculations of the momentum density show that the anisotropies of the experimental distributions are consistent with an fcc crystal structure of the Li nanoclusters. The observed reduction of the width of the experimental 2D-ACAR distribution is attributed to positron trapping in vacancies associated with Li clusters. This work proposes a method for studying the electronic structure of metallic quantum dots embedded in an insulating material.
CdSe nanorods are colloidal nanoparticles having organic capping molecules for stabilization and ... more CdSe nanorods are colloidal nanoparticles having organic capping molecules for stabilization and separation. CdSe is a II-VI type semiconductor with a bulk direct band gap of 1.7 eV . Band gap increase in CdSe nanorods is observed through the blue shift in the absorbance and luminescence spectra when the width or the length of the nanorod decreases, particularly when the nanorod width is below the bulk exciton Bohr radius (6 nm for CdSe) . This behaviour is attributed as the quantum confinement regime. The size-tunable optical properties of CdSe nanocrystals is of importance in numerous applications such as light emitting diodes (LEDs), lasers and solar cells .
Journal of Physics: Condensed Matter, 2015
Intrinsic defects are of central importance to many physical and chemical processes taking place ... more Intrinsic defects are of central importance to many physical and chemical processes taking place in compound nanomaterials, such as photoluminescence, accommodation of off-stoichiometry and cation exchange. Here, the role of intrinsic defects in the above mentioned processes inside rock salt (RS) lead chalcogenide systems PbS, PbSe and PbTe (PbX) was studied systematically using first principles density functional theory. Vacancy, interstitial, Schottky and Frenkel defects were considered. Rock salt PbO was included for comparison. The studied physical properties include defect formation energy, local geometry relaxation, Bader charge analysis, and electronic structure. The defect formation energies show that monovacancy defects and Schottky defects are favoured over interstitial and Frenkel defects. Schottky dimers, where the cation vacancy and anion vacancy are adjacent to each other, have the lowest defect formation energies at 1.27 eV, 1.29 eV and 1.21 eV for PbS, PbSe and PbTe, respectively. Our results predict that a Pb monovacancy gives rise to a shallow acceptor state, while an X vacancy generates a deep donor state, and Schottky defects create donor-acceptor pairs inside the band gap. The surprisingly low formation energy of Schottky dimers suggests that they may play an important role in cation exchange processes, in contrast to the current notion that only single point defects migrate during cation exchange.
Nature communications, 2016
Cation exchange is a powerful tool for the synthesis of nanostructures such as core-shell nanocry... more Cation exchange is a powerful tool for the synthesis of nanostructures such as core-shell nanocrystals, however, the underlying mechanism is poorly understood. Interactions of cations with ligands and solvent molecules are systematically ignored in simulations. Here, we introduce the concept of pseudoligands to incorporate cation-ligand-solvent interactions in molecular dynamics. This leads to excellent agreement with experimental data on cation exchange of PbS nanocrystals, whereby Pb ions are partially replaced by Cd ions from solution. The temperature and the ligand-type control the exchange rate and equilibrium composition of cations in the nanocrystal. Our simulations reveal that Pb ions are kicked out by exchanged Cd interstitials and migrate through interstitial sites, aided by local relaxations at core-shell interfaces and point defects. We also predict that high-pressure conditions facilitate strongly enhanced cation exchange reactions at elevated temperatures. Our approach...
Gold, silver and lithium nanoclusters (NCs) embedded in MgO were created by means of ion implanta... more Gold, silver and lithium nanoclusters (NCs) embedded in MgO were created by means of ion implantation of Au, Ag and Li into single crystals of MgO(100) and subsequent thermal annealing. Optical, electrical and structural properties of the NCs were investigated using optical absorption spectroscopy (OAS), dc I-V measurements, high-resolution X-ray diffraction (XRD) and crosssectional transmission electron microscopy (XTEM). The mean nanocluster size is estimated from the broadening of the Mie plasmon optical absorption bands using the Doyle formula. These results are compared with the NC size as obtained from XRD (using the Scherrer formula) and from direct XTEM observations. The three methods are found to be in reasonable agreement with a mean size of 4.0 and 10 nm found for the Au and Ag clusters, respectively. Using TEM observations, the relative {100} and {110} interface energies of MgO//Au and MgO//Ag interfaces are also determined are found to be not in agreement with theoretical predictions in the literature. The I-V measurements show that the conductivity in MgO containing NCs is a few orders of magnitude higher than that of reference MgO.
In the literature, several approaches have been taken to deal with the thermal drift problem duri... more In the literature, several approaches have been taken to deal with the thermal drift problem during insitu heating in the TEM. Kamino et al. used a design based on thin films wrapped around a spirally wound tungsten wire and reported a resolution of 0.24 nm at 770 K without specification of the thermal drift. Other designs are based on a micro-hotplate configuration made with semiconductorbased MEMS technology. [2] This approach has the advantage that by miniaturization, power consumption can be drastically reduced, which minimizes thermal expansion and drift. Damiano et al. reported high-resolution imaging at 1300 K but did not specify drift or resolution figures. [2d] Under normal (bright field) imaging conditions and using an in-situ TEM nanoreactor a temperature of 800 K, Creemer et al. obtained a resolution of 180 pm with a drift of 0.04 nm s -1 after 10 min. [2b] These results were obtained in an ambient of 1.2 bar of hydrogen and using a CM300-UT microscope.
The Journal of Physical Chemistry C, 2015
Two-dimensional (2D) hydrous silica sheets (HSSs) 8 and hydrous silica nanotubes (HSNTs) have man... more Two-dimensional (2D) hydrous silica sheets (HSSs) 8 and hydrous silica nanotubes (HSNTs) have many unique 9 properties and potential applications. Although preparation of 2D 10 HSSs was patented already about half a century ago, very little is 11 known about their structure and physical properties. Here we 12 predict formations of various kinds of 2D structures. For this 13 purpose, a first-principles study was performed using density-14 functional theory (DFT) with van der Waals dispersion interaction 15 corrections (optB88-vdW). The uneven hydrous silica sheets and 16 nanotubes have a high stability and are composed of hexagonal 17 rings. The calculations also showed that a bilayer of anhydrous silica 18 sheets is highly stable. Furthermore, the formation of defects which 19 can induce a transition to glassy silica was investigated. The 20 predicted high stability and versatility of these 2D materials offer 21 many opportunities for more extensive developments, including doping with extrinsic elements to functionalize the nanosheets 22 and nanotubes. The present simulation findings pose a challenge to experimentalists for finding useful synthesis routes to access 23 these novel 2D materials.
The Journal of Physical Chemistry C, 2015
Whereas bulk zinc oxide (ZnO) exhibits the wurtzite crystal structure, 8 nanoscale ZnO was recent... more Whereas bulk zinc oxide (ZnO) exhibits the wurtzite crystal structure, 8 nanoscale ZnO was recently synthesized in the rock salt structure by addition of Mg. Using 9 first-principles methods, we investigated two stabilization routes for accessing rock salt 10 ZnO. The first route is stabilization by Mg addition, which was investigated by considering 11 ZnO−MgO mixed phases. The second route is through size effects, as surface energies 12 become dominant for small nanocrystal sizes. We discovered that the surface energy of 13 rock salt ZnO is surprisingly low at 0.63 J m −2 , which is lower than those of wurtzite and 14 zinc blende ZnO and lower than that of rock salt MgO. We predict that pure ZnO is stable 15 for nanocrystals smaller than 1.6 nm, although Mg additions can greatly extend the size 16 range in which the rock salt phase is stable. Both mixed-phase and core−shell models were 17 considered in the calculations. The present approach could be applied to predict the 18 stabilization of many other nanocrystal phases in deviating crystal structures.
RSC Advances, 2014
relationships, crystal structures, and electronic and magnetic properties. Here we present a syst... more relationships, crystal structures, and electronic and magnetic properties. Here we present a systematic density functional theory (DFT) study on these transition metal nitrides, assessing both the I-M 4 N phase and the II-M 4 N phase, which differ in ordering of the N atoms within the face-centered cubic (FCC) framework of metal atoms. The calculations showed that for M ¼ Mn, Fe, Co and Cu, the I-M 4 N phases with perfect metal sub-lattices are favored, while for M ¼ Sc-Cr, and Ni, the II-M 4 N phases with distorted metal sub-lattices are favored. We predict that several currently not existing II-M 4 N phases may be synthesized experimentally as metastable phases. From Bader charge analysis the M 4 N phases are found to be ionic with significant metal-metal bonding. I-M 4 N with M ¼ Cr to Ni are magnetic, while II-M 4 N with M ¼ Cr and Ni are non-magnetic. The calculations revealed unusually high local magnetic moments and high spin-polarization ratios of the M1 atoms in I-M 4 N (M ¼ Cr to Ni). The origin of magnetism and lattice distortion of the M 4 N phases is addressed with the Stoner criterion. Detailed information about the relative stability, structures, chemical bonding, as well as the electronic and magnetic properties of the phases are of interest to a wide variety of fields, such as chemical synthesis, catalysis, spintronics, coating technology, and steel manufacturing.
The Journal of Physical Chemistry C, 2015
We present a new semiempirical potential for graphene, which includes also an out-of-plane energy... more We present a new semiempirical potential for graphene, which includes also an out-of-plane energy term. This novel potential is developed from density functional theory (DFT) calculations for small numbers of atoms and can be used for configurations with millions of atoms. Our simulations show that buckling caused by typical defects such as the Stone−Wales (SW) defect extends to hundreds of nanometers. Surprisingly, this long-range relaxation lowers the defect formation energy dramaticallyby a factor of 2 or 3implying that previously published DFT-calculated defect formation energies suffer from large systematic errors. We also show the applicability of the novel potential to other long-range defects including line dislocations and grain boundaries, all of which exhibit pronounced out-of-plane relaxations. We show that the energy as a function of dislocation separation diverges logarithmically for flat graphene but converges to a constant for freestanding buckled graphene. A potential in which the atoms are attracted to the 2D plane restores the logarithmic behavior of the energy. Future simulations employing this potential will elucidate the influence of the typical long-range buckling and rippling on the physical properties of graphene.
Chemistry of Materials, 2015
ABSTRACT Ultrathin two-dimensional (2D) nanosheets (NSs) possess extraordinary properties that ar... more ABSTRACT Ultrathin two-dimensional (2D) nanosheets (NSs) possess extraordinary properties that are attractive for both fundamental studies and technological devices. Solution-based bottom-up methods are emerging as promising routes to produce free-standing NSs, but the synthesis of colloidal NSs with well-defined size and shape has remained a major challenge. In this work, we report a novel method that yields 2 nm thick colloidal Cu2-xS NSs with well-defined shape (triangular or hexagonal) and size (100 nm to 3 mu m). The key feature of our approach is the use of a synergistic interaction between halides (Br or Cl) and copper-thiolate metal-organic frameworks to create a template that imposes 2D constraints on the Cu-catalyzed C-S thermolysis, resulting in nucleation and growth of colloidal 2D Cu2-xS NSs. Moreover, the NS composition can be postsynthetically tailored by exploiting topotactic cation exchange reactions. This is illustrated by converting the Cu2-xS NSs into ZnS and CdS NSs while preserving their size and shape. The method presented here thus holds great promise as a route to solution-processable compositionally diverse ultrathin colloidal NSs with well-defined shape and size.
The Journal of chemical physics, Jan 28, 2014
A transferable force field for the PbSe-CdSe solid system using the partially charged rigid ion m... more A transferable force field for the PbSe-CdSe solid system using the partially charged rigid ion model has been successfully developed and was used to study the cation exchange in PbSe-CdSe heteronanocrystals [A. O. Yalcin et al., "Atomic resolution monitoring of cation exchange in CdSe-PbSe heteronanocrystals during epitaxial solid-solid-vapor growth," Nano Lett. 14, 3661-3667 (2014)]. In this work, we extend this force field by including another two important binary semiconductors, PbS and CdS, and provide detailed information on the validation of this force field. The parameterization combines Bader charge analysis, empirical fitting, and ab initio energy surface fitting. When compared with experimental data and density functional theory calculations, it is shown that a wide range of physical properties of bulk PbS, PbSe, CdS, CdSe, and their mixed phases can be accurately reproduced using this force field. The choice of functional forms and parameterization strategy is ...
Materials Science Forum, 2004
We present a depth-selective 2D-ACAR and two-detector Doppler broadening study on Au nanocrystals... more We present a depth-selective 2D-ACAR and two-detector Doppler broadening study on Au nanocrystals in monocrystalline MgO(100), produced in sub-surface layers by ion implantation and subsequent thermal annealing to temperatures beyond the stability range of vacancy clusters in MgO. In contrast to the case of Li nanocrystals, it was found that positrons do not trap inside the Au nanocrystals, but only in defects at the nanocrystal-to-host interface (attached vacancy clusters). This is interpreted in terms of the positron affinity of Au, MgO and the defects.
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Papers by Marijn Van Huis