Doped Clusters Research Articles

Effects of single atom doping on the ultrafast electron dynamics of M1Au24(SR)18 (M = Pd, Pt) nanoclusters.

Atomically precise, doped metal clusters are receiving wide research interest due to their synergistic properties dependent on the metal composition. To understand the electronic properties of doped clusters, it is highly desirable to probe the excited state behavior. Here, we report the ultrafast relaxation dynamics of doped M1@Au24(SR)18 (M = Pd, Pt; R = CH2CH2Ph) clusters using femtosecond visible and near infrared transient absorption spectroscopy. Three relaxation components are identified for both mono-doped clusters: (1) sub-picosecond relaxation within the M1Au12 core states; (2) core to shell relaxation in a few picoseconds; and (3) relaxation back to the ground state in more than one nanosecond. Despite similar relaxation pathways for the two doped nanoclusters, the coupling between the metal core and surface ligands is accelerated by over 30% in the case of the Pt dopant compared with the Pd dopant. Compared to Pd doping, the case of Pt doping leads to much more drastic changes in the steady state and transient absorption of the clusters, which indicates that the 5d orbitals of the Pt atom are more strongly mixed with Au 5d and 6s orbitals than the 4d orbitals of the Pd dopant. These results demonstrate that a single foreign atom can lead to entirely different excited state spectral features of the whole cluster compared to the parent Au25(SR)18 cluster. The detailed excited state dynamics of atomically precise Pd/Pt doped gold clusters help further understand their properties and benefit the development of energy-related applications.

Dopant induced modulation in the structure and electronic properties of Au10 cluster

DFT calculations at PBE0/SDD ∪ 6-31++G(d,p) level suggest that doped Au10 clusters (with alkali and alkaline earth metals as dopants) are better potential candidates for use in heterogeneous catalysis.

Structural determination of niobium-doped silicon clusters by far-infrared spectroscopy and theory.

In this work, the structures of cationic SinNb(+) (n = 4-12) clusters are determined using the combination of infrared multiple photon dissociation (IR-MPD) and density functional theory (DFT) calculations. The experimental IR-MPD spectra of the argon complexes of SinNb(+) are assigned by comparison to the calculated IR spectra of low-energy structures of SinNb(+) that are identified using the stochastic 'random kick' algorithm in conjunction with the BP86 GGA functional. It is found that the Nb dopant tends to bind in an apex position of the Sin framework for n = 4-9 and in surface positions with high coordination numbers for n = 10-12. For the larger doped clusters, it is suggested that multiple isomers coexist and contribute to the experimental spectra. The structural evolution of SinNb(+) clusters is similar to V-doped silicon clusters (J. Am. Chem. Soc., 2010, 132, 15589-15602), except for the largest size investigated (n = 12), since V takes an endohedral position in Si12V(+). The interaction with a Nb atom, with its partially unfilled 4d orbitals leads to a significant stability enhancement of the Sin framework as reflected, e.g. by high binding energies and large HOMO-LUMO gaps.

Monodisperse and polydisperse platinum nanoclusters supported over TiO2 anatase as catalysts for catalytic oxidation of styrene

Very small platinum nanoclusters have been synthesized by reduction of H2PtCl6·6H2O salt with NaBH4 in the presence and absence of water-soluble l-cysteine ligand. The synthesized platinum clusters were supported over anatase titanium dioxide (TiO2) by 1% wt/wt. The spectroscopic properties of Ptn(l-Cys)m clusters were studied by UV–vis spectroscopy and Fourier transform infrared spectroscopy (FTIR) and its chemical structure were determined by thermogravimetric analysis (TGA) and elemental analysis. The particle sizes of supported and non-supported platinum clusters were characterized by transmission electron microscopy (TEM) and powder X-ray diffraction analysis. As well as their specific surface areas were determined using nitrogen adsorption/desorption measurements, pore volume and average pore diameter were also calculated. Finally, oxidation of styrene is used as a simple model reaction to demonstrate that these systems exhibit the potential to be used as heterogeneous catalysts. The doped monodisperse platinum clusters (1% Ptn(l-Cys)m/TiO2) catalyst exhibited the highest catalytic activity and selectivity (100% benzaldehyde) in oxidation of styrene. The non-doped platinum clusters (Ptn(l-Cys)m) showed more catalytic activity than doped polydisperse platinum clusters (1% Pt/TiO2). Hydrogen peroxide was used as oxidizing agent, which exhibited stronger oxidation potential than oxygen gas. The unique atom-packing structure and electronic properties of Ptn(l-Cys)m nanoclusters (∼1nm) are rationalized to be responsible for their extraordinary catalytic activity observed in oxidation of styrene.

Route to the Smallest Doped Semiconductor: Mn(2+)-Doped (CdSe)13 Clusters.

Doping semiconductor nanocrystals with magnetic transition-metal ions has attracted fundamental interest to obtain a nanoscale dilute magnetic semiconductor, which has unique spin exchange interaction between magnetic spin and exciton. So far, the study on the doped semiconductor NCs has usually been conducted with NCs with larger than 2 nm because of synthetic challenges. Herein, we report the synthesis and characterization of Mn(2+)-doped (CdSe)13 clusters, the smallest doped semiconductors. In this study, single-sized doped clusters are produced in large scale. Despite their small size, these clusters have semiconductor band structure instead of that of molecules. Surprisingly, the clusters show multiple excitonic transitions with different magneto-optical activities, which can be attributed to the fine structure splitting. Magneto-optically active states exhibit giant Zeeman splittings up to elevated temperatures (128 K) with large g-factors of 81(±8) at 4 K. Our results present a new synthetic method for doped clusters and facilitate the understanding of doped semiconductor at the boundary of molecules and quantum nanostructure.

Magnetic mediation effect of a C impurity in a Mn-doped Zn12O12 nanocluster: a case of multiple exchange interactions

The stability and exchange interaction mechanism of a doped Zn12O12 cluster with Mn and C atoms were investigated by first-principles calculations. For the Mn-doped Zn12O12 cluster, it is identified that a superexchange interaction deriving the hybridization between the Mn and O 2pxy orbitals dominates the Mn(↑)–Mn(↓) antiferromagnetic coupling, although a direct exchange interaction deriving the Mn–Mn bonding is also found. In order to turn the Mn spin state in the Mn-doped Zn12O12 cluster, C doping is undertaken to change the magnetic interactions of these impurities. It is proved that the C incorporation into the Mn-doped Zn12O12 cluster destroys the short-ranged antiferromagnetic coupling, where multiple exchange interactions take over, including the direct exchange interaction and the kinetic p-d exchange interaction partially due to the geometric distortion and surface effect with dangling bonds (sp2-like hybrids). It is concluded that the kinetic p-d exchange interaction plays a dominant role in Mn/C-doped Zn12O12 clusters.

Catalytic CO Oxidation on Single Pt-Atom Doped Aluminum Oxide Clusters: Electronegativity-Ladder Effect

Single platinum-atom catalysts exhibit extraordinary activity in a large number of reactions. However, a consensus regarding the molecular origin of Pt catalysis is far from being reached. Here, benefiting from the study of atomic clusters, we propose the Electronegativity-Ladder (E-Ladder) effect to account for the origin of Pt catalysis. The concept was obtained from the study of single Pt-atom doped aluminum oxide clusters PtAl3O5-7-, which are catalytically active in CO oxidation by molecular O-2. The undoped aluminum oxide clusters, however, cannot drive such a catalytic cycle. The reactions have been identified by mass spectrometry and density functional theory calculations. The key to drive the cycle lies in the unique structure of PtAl3O6-, in which the Pt atom that is not fully oxidized can coexist with the highly oxidative oxygen-centered radical (O-center dot). After the oxidation of one CO by PtAl3O7-, the resulting PtAl3O6- can also oxidize a second CO. The E-Ladder effect originates from the well-fitting electronegativity of the Pt atom (2.28) in between that of the Al atom (1.61) and the O atom (3.44), and this effect promotes the generation of an unpaired electron localized O-center dot radical, which results in the oxidative nature of PtAl3O6- toward CO. Thus, the large enthalpy in the catalytic reaction (2CO+ O-2 -> 2CO(2)) can be distributed much more evenly into several elementary reactions in the Pt-Al-O system than in the pure Al-O system.

Ab Initio Search for Global Minimum Structures of Pure and Boron Doped Silver Clusters.

The global minimum structures of pure and boron doped silver clusters up to 16 atoms are determined through ab initio calculations and unbiased structure searching methods. The structural and electronic properties of neutral, anionic, and cationic Ag(n)B (n ≤ 15) and Ag(n)B2 (n ≤ 14) clusters are much distinct from those of the corresponding pure silver. Considering that Ag and B possess one and three valence electrons, respectively, both the single and the double boron-atom doped silver clusters with even number of valence electrons are more stable than those with odd number of electrons, a feature also observed in the pure silver clusters. We demonstrate that the species with a valence count of 8 and 14 appear to be magic numbers with enhanced stability irrespective of component or the charged state. A new putative global minimum structure of Ag13(-) cluster, with high symmetry of C(2v), is unexpectedly observed as the ground state, which is lower in energy than the previous suggested bilayer structure.

Activation of Methane and Carbon Dioxide Mediated by Transition-Metal Doped Magnesium Oxide Clusters [MMgO](+/0/-) (M=Sc-Zn).

Mission: impossible? DFT calculations show that the trends in the thermochemistry are very different for the activation of CO2 and CH4 mediated by transition-metal doped magnesium oxide clusters [MMgO](+/0/-) (M=Sc-Zn). Thus, seeking a "simple" reagent to simultaneously mediate activation and coupling of CH4 and CO2 with high efficiency seems extremely daunting, if not impossible.

Generation of free hydrogen atoms in thermal reaction of ethane with AuNbO3+ cluster cations

Single gold-atom doped niobium oxide cluster AuNbO3+ has been prepared and mass-selected to react with C2H6 in an ion trap reactor under thermal collision conditions. The reaction has been characterized by mass spectrometry with isotopic substitution (C2H6→CH3CD3 and C2D6) and density functional theory calculations. In addition to hydrogen atom abstraction channels identified in a previous fast-flow reactor experiment, a rather unexpected channel of generating the free H atom was observed. The computational study revealed that the gold atom plays an important role in the reaction and the conversion of one AuO bond to one AuNb bond drives the generation of the free H atom. This study enriches the chemistry of gold atoms doped in atomic clusters and provides an ideal model reaction to generate the free H atom that can be a very important intermediate in hydrocarbon transformation.

Structures, stabilities and electronic properties of CunNa (n = 1–8) clusters

The structures, stabilities and electronic properties of CunNa (n=1–8) clusters have been systematically investigated by using density-functional approach. According to optimized doped clusters, growth behaviors are organized as follows: Na added Cun clusters or Na substituted Cun+1 clusters as well as Cu added Cun−1Na clusters. The calculated averaged binding energies, fragmentation energies, and second-order difference of energies exhibit a pronounced even–odd alternation phenomenon as a function of the cluster size. Electronic properties of doped clusters are studied by calculating the HOMO–LUMO gaps, the contour maps of HOMOS and electrostatic potential. At last, the molecular orbitals, VIP, VEA and chemical hardness are also analyzed.

Computational investigation on the catalytic activity of Rh6 and Rh4Ru2 clusters towards methanol activation

Catalysis of molecular activation of small molecules through scission of strong chemical bonds is one of the major challenges faced by chemists. More specifically, activation of the strong C–H and O–H bonds of various alcohols, especially methanol, is one of the various important intermediate steps of key organic reactions. Our present work explores a suitable metal cluster catalyst towards methanol dissociation. In particular, we have examined the effect of ruthenium doping (Rh:Ru = 2:1) on the catalytic activity of Rh6 cluster towards methanol dissociation. Density functional theory-based calculations illustrate two competitive pathways for methanol dissociation, which are via O–H and C–H bond breaking. Both the pathways are found to be energetically favourable in the presence of bimetallic and mono-metallic clusters. Importantly, energy barrier for O–H bond dissociation reduces considerably in doped cluster as compared to pure Rh6 cluster and is smaller than the values reported for a number of other small metallic clusters.

A comparative theoretical study on the structural, electronic and nonlinear optical features of B12N12 and Al12N12 nanoclusters with the groups III, IV and V dopants

The structural, electronic and nonlinear optical properties of the two important fullerene-like cages of B12N12 and Al12N12 nanostructures with the groups III, IV and V dopants are investigated through density functional theory (DFT) calculations. It has been found that doping process induces local deformation at bond lengths near the doping site. Natural bond orbital (NBO) analyses are also performed for scrutinizing the structural properties of the considered nanoclusters. The results indicate that the groups III, IV and V dopants remarkably narrow the energy gap of the B12N12 nanocluster. On the other hand, although the energy gap of Al12N12 nanocluster is insensitive to groups III and V dopants; the carbon, silicon and germanium dopants extremely reduce the energy gap of this cluster. It seems that the electronic character of the B12N12 and Al12N12 nanocluster is sensitive to the dopants and it could be adjusted by particular impurity. Moreover the considered dopants induce hyperpolarizability in both of the considered nanoclusters. Interestingly, the replacing aluminum atom by carbon one in Al12N12 nanocluster (CAl11N12) leads to an extremely large hyperpolarizability value of 4358.77a.u., which is the largest one among the considered doped clusters. It shows that the doping process plays an important role in enhancing the first hyperpolarizability of the B12N12 and Al12N12 nanoclusters.

Tin content effect on the structural and energetic properties of lead telluride clusters

Density functional theory calculations are performed to investigate tin content effect on the structural and energetic properties of lead telluride clusters. Tin atoms prefer to be doped on the exterior sites rather than the interior sites of lead telluride clusters. The geometry of the clusters is not affected due to the amount of tin content except for a slight distortion observed by changes in bond length. A considerable change has been observed in the HOMO–LUMO gap of the clusters with increasing number of tin atoms for corresponding size, but the band inversion which is observed in the bulk is not observed on the clusters in these size ranges. The change in binding energy between doped clusters is almost negligible for the same number of tellurium atoms, it is observed to be constant or there is a change of up to 0.02 eV with each successive doping of tin atoms. Both adiabatic and vertical detachment energies are observed to increase with increasing number of doping tin atoms. The fragmentation energy of losing PbTe dimer changes with an even–odd alternation, clusters with even number of Te atoms are more stable because they need higher energy to dissociate as compared to the other clusters. Some clusters are found to be magic (more stable) because of their enhanced binding, HOMO–LUMO gap, and fragmentation energies, typical examples are Pb4−mSnmTe4 (m = 1–4) clusters.

Evolution of the vibrational spectra of doped hydrogen clusters with pressure.

The evolution of the vibrational spectra of the isoelectronic hydrogen clusters H26, H24He, and H24Li(+) is determined with pressure. We establish the vibrational modes with collective character common to the clusters, identify their individual vibrational fingerprints and discuss frequency shifts in the giga-Pascal pressure region. The results are of interest for the identification of doping elements such as inert He and ionic Li(+) in hydrogen under confinement or, conversely, establish the pressure of doped hydrogen when the vibrational spectrum is known. At high pressure, the spectra of the nanoclusters resemble the spectrum of a solid, and the nanoclusters may be considered crystals of nanometer scale. The computations are performed at the gradient-corrected level of density functional theory. The investigation is the first of its kind.

Titanium embedded cage structure formation in Al(n)Ti+ clusters and their interaction with Ar.

Recently, Ar physisorption was used as a structural probe for the location of the Ti dopant atom in aluminium cluster cations, Al(n)Ti(+) [Lang et al., J. Am. Soc. Mass Spectrom. 22, 1508 (2011)]. As an experiment result, the lack of Ar complexes for n > nc determines the cluster size for which the Ti atom is located inside of an Al cage. To elucidate the decisive factors for the formation of endohedrally Al(n)Ti(+), experimentalists proposed detailed computational studies as indispensable. In this work, we investigated, using the density functional theory, the structural and electronic properties of singly titanium doped cationic clusters, Al(n)Ti(+) (n = 16-21) as well as the adsorption of an Ar atom on them. The first endohedral doped cluster, with Ti encapsulated in a fcc-like cage skeleton, appears at nc = 21, which is the critical number consistent with the exohedral-endohedral transition experimentally observed. At this critical size the non-crystalline icosahedral growth pattern, related to the pure aluminium clusters, with the Ti atom in the surface, changes into a endohedral fcc-like pattern. The map of structural isomers, relative energy differences, second energy differences, and structural parameters were determined and analyzed. Moreover, we show the critical size depends on the net charge of the cluster, being different for the cationic clusters (nc = 21) and their neutral counterparts (nc = 20). For the Al(n)Ti(+) · Ar complexes, and for n < 21, the preferred Ar adsorption site is on top of the exohedral Ti atom, with adsorption energy in very good agreement with the experimental value. Instead, for n = 21, the Ar adsorption occurs on the top an Al atom with very low absorption energy. For all sizes the geometry of the Al(n)Ti(+) clusters keeps unaltered in the Ar-cluster complexes. This fact indicates that Ar adsorption does not influence the cluster structure, providing support to the experimental technique used. For nc = 21, the smallest size of endohedral Ti doped cationic clusters, the Ar binding energy decreases drastically, whereas the Ar-cluster distance increases substantially, point to Ar physisorption, as assumed by the experimentalists. Calculated Ar adsorption energies agree well with available experimental binding energies.

Investigations of Al:CdS/PVA nanocomposites: A joint theoretical and experimental approach

In the present work we investigate the aluminium doped cadmium sulphide (Al:CdS) nanoparticles embedded in polyvinyl alcohol (PVA) matrix by chemical route and density functional theory (DFT) based simulations. Supertetrahedron (Tn) cluster models are considered for the simulation of CdS nanoparticles. Using DFT simulations on Tn clusters, we observe that band gap of ligated clusters is slightly more as compare to bare clusters. This indicates the ability of organic ligands (PVA) to open the band gap of inorganic CdS nanoclusters. Negative value of binding energy indicates the stability of the inorganic–organic hybrid system. Frontier molecular orbitals (FMOs) indicate the charge transfer between organic and inorganic moieties which provides stability and longevity to nanoparticles, a prime function of ligands in nanocomposites. Absorption spectra of pure and doped clusters are calculated using time dependent density functional theory (TDDFT). CdS/PVA and Al:CdS/PVA samples are synthesized at room temperature by chemical method. Their structure, size and band gap is characterized by XRD, TEM, FTIR and UV spectroscopy. Optical band gap values as observed experimentally are in agreement with simulated TDDFT results.

Effect of Silicon Doping on the Reactivity and Catalytic Activity of Gold Clusters

Doping is known to be an excellent and simple way of catalyst design. Although notable progress has been made in understanding the reactivity and catalytic activity of gas-phase and supported gold clusters, very few studies have been carried out on the doped gold clusters. In the present work, we have carried out density functional theory calculations to investigate the effect of silicon doping on the reactivity and catalytic activity of gold nanoclusters. The present work particularly focuses on the adsorption and activation of molecular oxygen on the pristine and silicon-doped gold clusters. The results confirm that the silicon-doped Au7Si cluster shows considerable binding and activation of the O2 molecule in comparison to the pristine Au8 cluster as reflected in the relevant geometrical parameters (O–O and Au–O bond lengths) and O–O stretching frequency. However, silicon doping has no contrasting effect on the reactivity and catalytic activity of the Au7 cluster. In addition to the stronger binding an...

Path integral Monte Carlo calculations of calcium‐doped 4He clusters

The energetics and structures of HeNCa clusters have been studied by means of path integral Monte Carlo calculations. Sizes ranging between N = 10 and 40 helium atoms were considered at T = 1, 1.5, and 2 K. Radial and angular distributions have been analyzed in detail to investigate the geometry of the bound systems. The comparison of the results obtained with two current He–Ca interactions (Kleinekathöfer, Chem. Phys. Lett. 2000, 324, 40, and Lovallo and Klobukowski J. Chem. Phys. 2004, 120, 246) reveals substantial differences regarding the precise location of the Ca impurity with respect to the helium droplet. Whereas the use of the first potential yields a doped cluster in which the Ca atom is solvated inside a helium cage, predictions with the much weaker HeCa potential by Lovallo and Klobukowski correspond to the formation of a dimple at the surface of the outer He atoms to host the Ca atom, a situation which is consistent with the experimental findings for the system. © 2014 Wiley Periodicals, Inc.

Density functional theory study of structure characteristics and stabilities of Al2Sn(n=2-10) clusters

The geometric configurations and electronic structures of the Al2Sn (n=2-10) clusters are calculated by the B3LYP (density functional theory) method at 6-311G** level. The variations of the ground state structure, charge transfer and bonding characteristic of the aluminum-sulfur doped clusters are discussed in detail. The results show that the structures of pure Sn clusters are fundamentally changed due to the doping of the Al atoms. Our work exhibits that the all the ground states of Al2Sn clusters share the same four-member Al2S2 ring, and from single ring to three rings plane and three-dimensional structure structures are formed by inserting one S atom or S clusters. The stability of cluster structure is influenced by both the chemical bond structure and number. The stability information of Al2Sn clusters is obtained by analyzing the dissociation energies and the second-order difference energies of the ground state structures.