Search results for "Clusters"
showing 10 items of 1274 documents
Stability, electronic structure, and optical properties of protected gold-doped silver Ag29-xAux (x = 0-5) nanoclusters
2017
In this work, we used density functional theory (DFT) and linear response time-dependent DFT (LR-TDDFT) to investigate the stability, electronic structure, and optical properties of Au-doped [Ag29−xAux(BDT)12(TPP)4]3− nanoclusters (BDT: 1,3-benzenedithiol; TPP triphenylphosphine) with x = 0–5. The aim of this work is to shed light on the most favorable doped structures by comparing our results with previously published experimental data. The calculated relative energies, ranging between 0.8 and 10 meV per atom, indicate that several doped Ag29−xAux nanoclusters are likely to co-exist at room temperature. However, only the Au-doped [Ag29−xAux(BDT)12(TPP)4]3− nanoclusters that have direct bon…
Ligand mediated evolution of size dependent magnetism in cobalt nanoclusters.
2018
We use density functional theory to model the impact of a ligand shell on the magnetic properties of CoN (15 ≤ N ≤ 55) nanoclusters. We study three different ligand shells on each nanocluster core size, each known to have different electronic interactions with the surface: pure Cl ligand shells (X-type), pure PH3 ligand shells (L-type), and two component ligand shells with mixtures of Cl and PH3 ligands. The simulations show that the identity, arrangement, and total coverage of the ligand shell controls the distribution of local magnetic moments across the CoN core. On the surface of an unpassivated CoN nanocluster, the Co-Co coordination number (CN) is known to determine the local magnetic…
Model selection procedure for mixture hidden Markov models
2021
This paper proposes a model selection procedure to identify the number of clusters and hidden states in discrete Mixture Hidden Markov models (MHMMs). The model selection is based on a step-wise approach that uses, as score, information criteria and an entropy criterion. By means of a simulation study, we show that our procedure performs better than classical model selection methods in identifying the correct number of clusters and hidden states or an approximation of them
A magnetic look into the protecting layer of Au25 clusters
2016
The field of molecular metal clusters protected by organothiolates is experiencing a very rapid growth. So far, however, a clear understanding of the fine interactions between the cluster core and the capping monolayer has remained elusive, despite the importance of the latter in interfacing the former to the surrounding medium. Here, we describe a very sensitive methodology that enables comprehensive assessment of these interactions. Pulse electron nuclear double resonance (ENDOR) was employed to study the interaction of the unpaired electron with the protons of the alkanethiolate ligands in four structurally related paramagnetic Au25(SR)0 18 clusters (R ¼ ethyl, propyl, butyl, 2-methylpro…
Surface modification of gold nanoparticles and nanoclusters
2016
Gold nanoparticles are used in many beneficial technological applications in biochemistry, medicine and electronics. Among them, monolayer protected gold nanoclusters (MPCs) have received a significant attention in the scientific community due to their well-defined atomic structure, which is important for fundamental studies of nanoparticles properties and their functionalization. These particles, with a precise number of atoms, exhibit size-dependent optical, chemical and electronic properties. The thesis focuses on the structure, preparation, characterization, and properties of MPCs. For multifunctional applications, gold nanoparticles are an ideal class of compounds for surface functiona…
Multi-word Clusters, Keywords and Key Terms in English Car Reviews: a Preliminary Corpus Linguistic Study
2018
Classroom use of authentic texts used in everyday communication has not always been an inseparable part of foreign language teaching. According to Grigaliuniene (2013, p. 17), “the structures many current textbooks teach for certain functions are either never used or used infrequently while quite unexpected structures are the ones that actually occur”. Despite the awareness of the importance of authentic patterns of language use, students often learn structures (lexical or grammatical) which are rarely used in actual communication. Teaching authentic phrases is even more important in English for Specific Purposes (ESP) where learners’ specific needs impact the design of a course (Dudley-Eva…
Covalently linked multimers of gold nanoclusters Au102(p-MBA)44 and Au∼250(p-MBA)n
2016
We present the synthesis, separation, and characterization of covalently-bound multimers of para-mercaptobenzoic acid (p-MBA) protected gold nanoclusters. The multimers were synthesized by performing a ligand-exchange reaction of a pre-characterized Au102(p-MBA)44 nanocluster with biphenyl-4,4′-dithiol (BPDT). The reaction products were separated using gel electrophoresis yielding several distinct bands. The bands were analyzed by transmission electron microscopy (TEM) revealing monomer, dimer, and trimer fractions of the nanocluster. TEM analysis of dimers in combination with molecular dynamics simulations suggest that the nanoclusters are covalently bound via a disulfide bridge between BP…
From Symmetry Breaking to Unraveling the Origin of the Chirality of Ligated Au13Cu2 Nanoclusters
2018
A general method, using mixed ligands (here diphosphines and thiolates) is devised to turn an achiral metal cluster, Au13Cu2, into an enantiomeric pair by breaking (lowering) the overall molecular symmetry with the ligands. Using an achiral diphosphine, a racemic [Au13Cu2(DPPP)3(SPy)6]+ was prepared which crystallizes in centrosymmetric space groups. Using chiral diphosphines, enantioselective synthesis of an optically pure, enantiomeric pair of [Au13Cu2((2r,4r)/(2s,4s)‐BDPP)3(SPy)6]+ was achieved in one pot. Their circular dichroism (CD) spectra give perfect mirror images in the range of 250–500 nm with maximum anisotropy factors of 1.2×10−3. DFT calculations provided good correlations wit…
The Role of Donor and Acceptor Substituents on the Nonlinear Optical Properties of Gold Nanoclusters
2018
In recent years, a large number of monolayer-protected clusters (MPCs) have been studied by means of single crystal structure characterization. A central aspect of research on MPCs is the correlation of their interesting optical properties with structural features and the formulation of a theoretical framework that allows interpretation of their unique properties. For this, superatom and jellium models have been proven successful. Little attention, however, has been paid to the influence of the protecting ligands. Here, we investigate the effect of changes in [Au25(SR)18-x(SR′)x]−, where SR′ represents a para-substituted thiophenolate derivative (SPh-4-X). We computed the first hyperpolariz…
Robust, Highly Luminescent Au13 Superatoms Protected by N-Heterocyclic Carbenes
2019
Gold superatom nanoclusters stabilized entirely by N-heterocyclic carbenes (NHCs) are reported. The reduction of well-defined NHC–Au–Cl complexes produces clusters com-prised of an icosahedral Au13 core surrounded by a symmetrical arrangement of 9 NHCs and 3 chlorides. X-ray crystallography shows that the clusters are characterized by multiple CH–π and π–π interactions, which rigidify the ligand and likely con-tribute to the exceptionally high photoluminescent quantum yields observed, up to 16.0 %, which is significantly greater than the most luminescent ligand-protected Au13 superatom cluster. Density functional theory analysis suggests that clus-ters are 8-electron superatoms with a wide …