Odd-numbered Clusters Research Articles

Decoration of atomic PdxAuy (1 ≤ x + y ≤ 4) clusters on polyaniline in electrochemical sensing of 1-propanol

Atomic PdxAuy (1 ≤ x + y ≤ 4) clusters are decorated on polyaniline to be used as the catalytic electrode in electrochemical sensors. The current responses in the oxidation of 1-propanol are used to reveal the catalytic activity. Effects of the size, decoration sequence, and composition of the atomic metal clusters decorated on the PANI on the catalytic activity are evaluated. Regarding the size, an odd-even size effect is observed in the atomic pure metal clusters, which the even-numbered pure metal clusters show a higher catalytic activity than the odd-numbered pure metal clusters. For the decoration sequence, the catalytic activity is high when the same metal is decorated in the first two cycles of the cyclic atomic electrodeposition process. For the composition effect, the atomic metal clusters show a high catalytic activity in the oxidation, in the region of second oxidation peak, when there are more Au in the atomic metal clusters.

Orchestration of ferro- and anti-ferromagnetic ordering in gold nanoclusters.

The unpaired electron in the gold clusters (Aun, n = no. of Au atoms) with an odd number of total electrons is solely responsible for the magnetic properties in the small-sized Au nano-clusters. However, no such unpaired electron is available due to pairing in the even number of atom gold clusters and behaving as a diamagnetic entity similar to bulk gold. In this work, we unveiled the spin-density distribution of odd Aun clusters with n = 1 to 19 that reveals that a single unpaired electron gets distributed non-uniformly among all Au-atoms depending on the cluster size and morphology. The delocalization of the unpaired electron leads to the spin dilution approaching a value of ∼1/n spin moments on each atom for the higher clusters. Interestingly, small odd-numbered gold clusters possess spin-magnetic moments similar to the delocalized spin moments as of organic radicals. Can cooperative magnetic properties be obtained by coupling these individual magnetic gold nanoparticles? In this work, by applying state-of-the-art computational methodologies, we have demonstrated ferromagnetic or anti-ferromagnetic couplings between such magnetic nanoclusters upon designing suitable organic spacers. These findings will open up a new avenue of nanoscale magnetic materials combining organic spacers and odd-electron nano-clusters.

Formation Mechanism of Odd- and Even-Numbered Hydrogen Cluster Cations Using the Direct Ab Initio Molecular Dynamics Approach.

The photoreaction of hydrogen clusters caused by cosmic-ray irradiations plays a crucial role in interstellar molecular clouds because the reaction of molecules takes place in these surrounding clusters. Previous gas-phase experiments introduced the reaction products after the ionization of the neutral hydrogen cluster. In the experiments, the odd-numbered hydrogen cluster cation, H+(H2)m, was the main product, while the even-numbered cluster cation, (H2)n+, was the minor product. However, the formation yield of (H2)n+ was significantly low. Despite many theoretical calculations and experiments, the formation mechanism of odd- and even-numbered cluster ions from ionized hydrogen clusters is still unclear. In this study, the direct ab initio molecular dynamics (AIMD) method was applied to the reaction of the hydrogen cluster cation (H2)n+ to understand the abovementioned formation mechanism. The trajectories of (H2)n+ following the vertical ionization of the neutral cluster were calculated. Direct AIMD calculations indicated that odd-numbered cluster cations, H3+(H2)n-2 + H (dissociation), were formed directly as the main product after the ionization of (H2)n. An even-numbered cluster, (H2)n+, was temporally formed when an H-shaped complex composed of three H2 molecules existed within a neutral cluster. However, it was rapidly dissociated to an odd-numbered cluster. Static ab initio calculations suggested that the odd-numbered cluster complex, H3+(H2)n-2-H, could be transferred to even-numbered ions via a transition state with a low energy barrier. The even-numbered cluster cation was formed stepwise from the odd-numbered cluster cation. The formation mechanism of odd- and even-numbered cluster cations is discussed based on these results.

Computational Appraisal of Silver Nanocluster Evolution on Epitaxial Graphene: Implications for CO Sensing.

Early stages of silver nucleation on a two-dimensional (2D) substrate, here, monolayer epitaxial graphene (MEG) on SiC, play a critical role in the formation of application-specific Ag nanostructures. Therefore, it is of both fundamental and practical importance to investigate the growth steps when Ag adatoms start to form a new phase. In this work, we exploit density functional theory to study the kinetics of early-stage nuclei Agn (n = 1–9) assembly of Ag nanoparticles on MEG. We find that the Ag1 monomer tends to occupy hollow site positions of MEG and interacts with the surface mainly through weak dispersion forces. The pseudoepitaxial growth regime is revealed to dominate the formation of the planar silver clusters. The adsorption and nucleation energies of Agn clusters exhibit evident odd–even oscillations with cluster size, pointing out the preferable adsorption and nucleation of odd-numbered clusters on MEG. The character of the interaction between a chemisorbed Ag3 cluster and MEG makes it possible to consider this trimer as the most stable nucleus for the subsequent growth of Ag nanoparticles. We reveal the general correlation between Ag/MEG interaction and Ag–Ag interaction: with increasing cluster size, the interaction between Ag adatoms increases, while the Ag/MEG interaction decreases. The general trend is also supported by the results of charge population analysis, according to which the average charge per Ag adatom in a Agn cluster demonstrates a drastic decrement with cluster size increase. 2D–3D structural transition in Agn clusters was investigated. We anticipate that the present investigation is beneficial by providing a better understanding of the early-stage nucleation of Ag nanoparticles on MEG at the atomic scale. Specific interaction between odd-numbered Ag clusters preadsorbed onto the MEG surface and carbon monoxide (CO) as well as clusters’ stability at 300 K is discussed in terms of sensing applications.

Towards understanding the trapping, migration and clustering of He atoms in W–Ta alloy

We investigated the properties of He defects, the migration of a single He atom and the formation of He clusters in W–Ta (atomic ratio 1:1) alloy via first-principles density functional theory calculations. Results showed that He most preferred to occupy the substitutional site first and then the tetrahedral and octahedral interstitial sites . The formation energies of interstitial He in W–Ta alloy are between in bulk W and Ta. While the formation energies of substitutional He in W–Ta alloy were higher than those in bulk W and Ta. Structural relaxation and energy calculations revealed that the formation energies of He clusters linearly increased with cluster size. The binding energies of even-numbered He clusters were lower than those of their neighbouring odd-numbered He clusters. The migration calculation showed that the energy of Sub Ta was lower, and it had stronger attraction to He atom. Density of state and electronic structure were analysed to reveal the underlying mechanisms governing cluster formation. Results demonstrated that the hybridizations of p -projected DOS of He and its neighbouring metal atoms were responsible for the site-preference order of He in DOS. Moreover, the differences in charge density between He and W(Ta) were negative under all conditions, suggesting the repulsive interaction of He and W(Ta) in W–Ta alloy. This study provides a good reference for evaluating the influence of He on the performance of W–Ta alloy for plasma-facing materials.

Ground and excited state geometrical and optical properties of Aun (n = 2–13) nanoclusters: A first-principles study

The geometrical and optical properties of small gold nanoclusters Aun (n = 2–13) in the ground and first excited state were studied by combined artificial bee colony algorithm and first-principles calculations. The global minima for all of the clusters are planar in the ground state, which are also preserved in the first excited state. The electronic transition properties obtained at the first excited state indicated that all of the odd-numbered clusters are weakly emissive with tunable excitation energies. In contrast, only two of the even-numbered clusters, Au4 and Au6, are emissive with the respective excitation energies of 1.61 eV and 2.35 eV.

Quantum Chemical View on the Growth Mechanisms of Odd-Sized Nitrogen Cluster Anions.

The stable structures of odd-numbered anionic nitrogen clusters, N2 n+3-, have been theoretically investigated in the size range n = 1-9 using a variety of quantum chemistry methods that include perturbation theory, coupled cluster, and density-functional theory with different exchange-correlation functionals. We generally find that the clusters are composed of an azide chromophore N3- surrounded by essentially neutral nitrogen molecules. The growth initially proceeds by placing the neutral molecules parallel to the azide anion, completing a first shell at N13-, above which the extra molecules arrange on the side but with a significantly lower binding energy. Comparison with the cyclic N5- anionic core shows that the latter is unfavorable, the spectral signatures of both N5- and N2N3- being provided in both the infrared and ultraviolet ranges. The trend of these clusters to be highly stable as (N2) nN3- agrees with recent mass spectrometry experiments under the cryogenic environment of helium droplets. The issues associated with the successful development of a nonreactive force field for such clusters are also highlighted.

Formation of positive and negative clusters of gold atoms inside helium nanodroplets close to zero K

We report the mass spectrometric detection of positive and negative cluster ions of gold atoms formed by the pickup of gold by helium nanodroplets (HNDs) near zero K and subsequent exposure to electron impact. Both singly and doubly charged positive cluster ions were observed. Ionization is initiated by the helium ions He+ and He*−, metastable helium atoms He* as well as electron attachment. We focus on the size distributions of the ions, electron energy dependence of ion yield, patterns of stability and the presence of “magic” clusters. As reported previously by others using a variety of other methodologies, a clear oscillation in the intensity of (Au)n+/- is seen, especially for n <21, with odd-numbered clusters being more intense than even-numbered clusters. The distributions peak early around n = 8. There is a suggestion of magic numbers at n = 35 and 58. Efficiencies of formation of the gold cluster anions were monitored as a function of the energy of the electrons; the measured intensity distributions exhibit a shift to higher energies with increasing cluster size that is attributed to the need to evaporate helium from the clusters for their observation in the nascent state.

Structural, optical, electronic, and magnetic properties of Ag-Cu bimetallic clusters: a density functional theory study

In this study, the structural, optical, electronic, and magnetic properties of AgmCun (m + n = 3 to 6) bimetallic clusters were systematically investigated by density functional theory in the theoretical framework of the generalized gradient approximation exchange-correlation functional. The results show that the ground state structures of these clusters are planar structures, with triangular geometries for three-atom Ag-Cu clusters, rhombic geometries for four-atom Ag-Cu clusters, trapezoids for five-atom Ag-Cu clusters, and triangular geometries for six-atom Ag-Cu clusters. The Ag2Cu2, Ag2Cu3, and Ag3Cu3 clusters are the geometric magic clusters for four-, five-, and six-atom Ag-Cu clusters, respectively. As the number of Cu atoms increases, the vertical ionization potential values of the four- to six-atom Ag-Cu clusters increase, while the vertical electron affinity values of the three- to five-atom Ag-Cu clusters decrease. Compared to pure Ag clusters, the main absorption peaks of the Ag-Cu clusters of the same number of atoms appear to blueshift. The even-numbered clusters exhibit no magnetic moments, while the odd-numbered clusters exhibit large magnetic moments of 1.00 μB. The magnetic moments of these Ag-Cu clusters are believed to be related to the atom sites.

Computational investigation on the structures and electronic properties of the nanosized rhenium clusters

The stable equilibrium geometries, relative stabilities, and electronic and magnetic characteristics of Ren (n=2–16) clusters were investigated by density functional theory method. The calculated fragmentation energies and second-order differences of energies exhibited interestingly that the stabilities of Ren (n=2–16) clusters show a dramatic odd-even alternative behavior of the cluster size n: with the even-numbered Ren clusters being obviously more stable than their neighboring odd-numbered Ren clusters (beside n=11). Simultaneously, the calculated HOMO-LUMO gaps of Ren (n=6–16) display an oscillatory feature at large-sized Ren clusters. From the calculated magnetic moments and growth behaviors of Rhenium clusters, the magnetic Re6 unit can be seen as the building block for the novel magnetic cluster-assembled nanomaterial. Such calculated results are in good agreement with the available experimental measurements.

Three Giant Lanthanide Clusters Ln37 (Ln = Gd, Tb, and Eu) Featuring A Double-Cage Structure.

Three homometallic high-nuclearity clusters, formulated as [(CO3)2@Ln37(LH3)8(CH3COO)21(CO3)12(μ3-OH)41(μ2-H2O)5(H2O)40]·(ClO4)21·(H2O)100 (abbreviated as Ln37, Ln = Gd (1); Tb (2); Eu (3), LH3 = 1,2,3-cyclohexanetriol) and featuring a double cage-like structure, were obtained through the reaction of 1,2,3-cyclohexanetriol, acetate ligand, and Ln(ClO4)3. The largest odd-numbered lanthanide cluster Gd37 exhibits an entropy change (-ΔSm) of 38.7 J kg-1 K-1.

Geometry and electronic structure of ultrafine/nanoparticle chromium clusters (Crn, n = 2–5) and their interaction with oxygen (triplet) and ethylene molecules: A DFT–NBO study

The present research is divided into two parts: first, detailed analysis of geometry and electronic structure of small naked clusters Crn (n=2–5) as nanoparticles have been investigated. According to our results, dimer-growth character is suggested for ground state geometries. Transition from diamagnetic even-numbered cluster to paramagnetic odd-numbered cluster was obtained. HOMO–LUMO energy gap of these small metal clusters (calculated from NBOs, 1.42–2.15eV) falls in the range of semiconductors band gap (0.5–3.5eV). Surface area to volume ratio is larger than 11nm−1 for all these ultrafine Cr nanoparticles and it is decreased as the cluster size is increased. Natural Bond Orbital (NBO) analysis confirms the presence of maximum bond multiplicity (6 and 5) for these small Cr clusters. Second, we have studied the interaction of O2 (triplet) and C2H4 molecules with mentioned Cr clusters considering different coordination modes. We have shown that O2 adsorption on Cr cluster can possess both dissociative and molecular mechanism, although the dissociative one is preferred due to more reliable values of binding energies and ΔGs. For C2H4 adsorption, only molecular mechanism (weaker in comparison with O2 adsorption) was obtained which is accompanied by the activation of C–C double bond, except for Cr2 dimer. Density Functional Theory (DFT), BLYP exchange–correlation functional was applied which is picked out through the DFT calibration.

Mixed-anion templated cage-like lanthanide clusters: Gd27 and Dy27

Two record-large odd-numbered Ln27 lanthanide clusters, each one featuring one CO32− and eight ClO4− groups as mixed anion templates, were obtained by controlling the hydrolysis of the Ln3+ ions in the presence of simple propionate ligands.

Cyclic single-molecule magnets: from the odd-numbered heptanuclear to a dimer of heptanuclear dysprosium clusters.

A heptanuclear and a dimer of heptanuclear dysprosium clusters (Dy7 and Dy14) have been successfully synthesized by ingenious coalescence of the single and double pyrazinyl hydrazone as well as phosphonate ligands. The complexes feature the largest odd-numbered cyclic lanthanide clusters reported thus far. Both exhibit single molecule magnet behaviors at low temperature.

Geometry, chemical reactivity and Raman spectra of gold clusters

AbstractStructures, stability, and chemical reactivity of Aun (n = 2-10) clusters are investigated using density functional theory (DFT). We have studied the reactivity parameters of the clusters in terms of relevant electronic structure principles. It is observed that stability and properties are strongly dependent on the cluster size. Clusters with an even number of atoms are found to be energetically and chemically more stable than odd-numbered clusters. Electronic structure of clusters has been investigated using partial density of states (PDOS). PDOS analysis clearly shows that energy states of highest occupied molecular orbital and lowest unoccupied molecular orbital are predominantly contributed by s orbital. From time-dependent DFT calculations, it is shown that absorption spectra of even-numbered clusters are more intense and are observed at lower wavelength region than the odd-sized gold clusters.

Geometrical and Electronic Characteristics of AunO2– (n = 2–7)

Most density functionals do not properly describe the characteristics of superoxide (O2–) (i.e., first two vertical electron detachment energies and the excitation energies of neutralized singlet state) of small even-numbered AunO2– clusters. However, the second-order Møller–Plesset theory (MP2) shows significant charge transfer from Au cluster anions to oxygen molecule and so provides proper electronic characteristics of superoxide of small even-numbered AunO2– clusters. This has allowed us to properly describe the properties of even-numbered AunO2– clusters. Even in the case of odd-numbered AunO2– clusters, we find that Au5– is a chemically O2-adsorbed singlet state at 0 K, against a commonly accepted physisorbed triplet state. This is further evidenced by our extensive coupled cluster with single, double, and perturbative triple excitations [CCSD(T)] calculations, including the relativistic effect. However, the entropy effect makes the physisorbed triplet state more stable than the chemisorbed singlet state at higher temperatures, consistent with the experiment. The weak O2 binding by odd-numbered cluster anions (n = 3, 5, and 7) could be further weakened by the entropic effect, which results in van der Waals complexes at high temperatures. The present study reports the geometrical and electronic characteristics of small AunO2– (n = 2–7) clusters including isomers, which match the corresponding photoelectron spectra (PES).

The nature and role of the gold-krypton interactions in small neutral gold clusters.

We investigate the nature and role of krypton embedding in small neutral gold clusters. For some of these clusters, we observe a particular site-dependent character of the Kr binding that does not completely follow the criterion of binding at low-coordinated sites, widely accepted for interaction of a noble gas with closed-shell metal systems such as metal surfaces. We aim at understanding the effect of low dimensionality and open-shell electronic structure of the odd-numbered clusters on the noble gas-metal cluster interaction. First, we investigate the role of attractive and repulsive forces, and the frontier molecular orbitals. Second, we investigate the Au-Kr interaction in terms of reactivity and bonding character. We use a reactivity index derived from Fukui formalism, and criteria provided by the electron localization function (ELF), in order to classify the type of bonding. We carry out this study on the minimum energy structures of neutral gold clusters, as obtained using pseudo potential plane-wave density functional theory (DFT). A model is proposed that includes the effect of attractive electrostatic, van der Waals and repulsive forces, together with effects originating from orbital overlap. This satisfactorily explains minimum configurations of the noble gas-gold cluster systems, the site preference of the noble gas atoms, and changes in electronic properties.

All-Electron Scalar Relativistic Calculations on the Adsorption of Small Gold Clusters Toward Methanol Molecule.

Under the framework of DFT, an all-electron scalar relativistic calculation on the adsorption of Aun (n = 1-13) clusters toward methanol molecule has been performed with the generalized gradient approximation at PW91 level. Our calculation results reveal that the small gold cluster would like to bond with oxygen of methanol molecule at the edge of gold cluster plane. After adsorption, the chemical activities of hydroxyl group and methyl group are enhanced to some extent. The even-numbered AunCH3OH cluster with closed-shell electronic configuration is relatively more stable than the neighboring odd-numbered AunCH3OH cluster with open-shell electronic configuration. All the AunCH3OH clusters prefer low spin multiplicity (M = 1 for even-numbered AuNCH3OH clusters, M = 2 for odd-numbered AunCH3OH clusters) and the magnetic moments are mainly contributed by gold atoms. The odd-even alterations of magnetic moments and electronic configurations can be observed clearly and may be simply understood in terms of the electron pairing effect.

Thermally accessible triplet state of π-nucleophiles does exist. Evidence from first principles study of ethylene interaction with copper species

Three different models of ethylene interaction with copper species, namely, the Cu(100) surface, odd-numbered copper clusters C2H4/Cun(wheren= 3, 7, 11, 15, 17, 19, 21, 25 and 27) and atomic copper C2H4/Cu were studied theoretically.

Size-resolved infrared spectroscopic study of structural transitions in sodium-doped (H₂O)n clusters containing 10-100 water molecules.

In water clusters containing 10-100 water molecules the structural transition takes place between "all surface" structures without internally solvated water molecules to amorphous water clusters with a three dimensionally structured interior. This structural evolution is explored with rigorous size selection by IR excitation modulated photoionization spectroscopy of sodium-doped (H2O)n clusters. The emergence of fully coordinated interior water molecules is observed by an increased relative absorption from 3200 to 3400 cm(-1) in agreement with theoretical predictions and earlier experimental studies. The analysis has also shown that the intermediate-sized water clusters (n = 40-65) do not smoothly link the structures in the largest and smallest analyzed size regions (n = 15-35 and n = 100-150) in line with previous reports suggesting the appearance of exceptionally stable water cluster isomers at n = 51, 53, 55, and 57. In the size range from n = 49 to n = 55 a reduced ion yield, a plateau in the total IR signal gain and signatures in the distribution of free OH stretch oscillator absorption have been observed. Recently reported putative global minima structures for n = 51 and n = 54 point to the presence of periplanar interior rings in odd-numbered clusters in this size range, which may affect cluster (surface) stability and the shape of the free OH stretch absorption peak. Potential links between pure and sodium-doped water cluster structures and the signatures of solvated electrons in photoelectron spectra of anionic water clusters are discussed.