Cluster generation under pulsed laser ablation of zinc oxide

Neutral and cationic Zn _n O _m clusters of various stoichiometry have been produced by nanosecond laser ablation of ZnO in vacuum and investigated by time-of-flight mass spectrometry. Particular attention was paid to the effect of laser wavelength (in the range from near-IR to UV) on cluster composition. Under 193-nm laser ablation, the charged clusters are essentially substoichiometric with Zn_nO_n-1⁺\mathrm{Zn}_{n}\mathrm{O}_{n-1}^{+} and Zn_nO_n-3⁺\mathrm{Zn}_{n}\mathrm{O}_{n-3}^{+} being the most abundant series. Both sub- and stoichiometric cationic clusters are generated in abundance at 532- and 1064-nm ablation whose composition depends on the cluster size. The reactivity of small stoichiometric Zn_nO_n⁺\mathrm{Zn}_{n}\mathrm{O}_{n}^{+} clusters (n<11) toward hydrogen is found to be high, while oxygen-deficient species are less reactive. The neutral plume particles are mainly stoichiometric with Zn₄O₄ tetramer being a magic cluster. It is suggested that the Zn₄O₄ loss is the dominant fragmentation channel of large zinc oxide clusters upon electron impact. Plume expansion conditions under ZnO ablation with visible and IR laser pulses are shown to be favorable for stoichiometric cluster formation.     

Electrical and optical spectroscopy on ZnO:Co thin films

Magnetic oxide semiconductors, for example the highly transparent and intrinsically n-type conducting zinc oxide doped with the 3d transition metal Co (ZnO:Co), are promising for the emerging field of spintronics [1]. We investigated n-conducting ZnO:Co thin films with a Co content of nominal 0.02, 0.20, or 2.00 at. %. The substitution of Co cations in the tetrahedral sites of wurtzite ZnO with Zn was confirmed at low temperature by the 1.877 eV photoluminescence between crystal field split d-levels of Co²⁺ (d⁷) ions. Based on theoretical studies, it is predicted that the formation of electron levels with zinc interstitials (I_Zn) or hole levels with zinc vacancies (V_Zn) is necessary to induce ferromagnetism, whereas the formation of electron levels with oxygen vacancies (V_O) is detrimental for ferromagnetism in ZnO:Co [2]. Cobalt generates a hole level in ZnO [3]. We investigated the generation of electron levels in n-conducting ZnO:Co in dependence on the Co content by means of deep level transient spectroscopy (DLTS). However, because of the ambiguous categorization of deep defects in n-conducting ZnO (V_O, I_Zn), an optimization of defect-related ferromagnetism in ZnO:Co is not possible at the moment. PACS 78.30.Fs; 91.60.Ed; 91.60.Mk     

DFT study on size-dependent geometries, stabilities, and electronic properties of AunM2 (M = Si, P; n = 1–8) clusters

The ab initio method based on density functional theory at the PW91PW91 level has been employed to systematically study the structures, stabilities, electronic, and magnetic properties of gold clusters with or without silicon/phosphorus doping. The optimized geometries show that the most stable isomers for Au _n Si₂ and Au _n P₂ (n = 1–8) clusters prefer a three-dimensional structure when n = 2 and n = 3 upwards, respectively, and they can be viewed as grown from the already observed Au _n−1M₂ (M = Si, P). The relative stabilities of calculated Au _n M₂ (M = Si, P) clusters have been analyzed through the atomic average binding energy, fragmentation energy, second-order difference of energy, and HOMO-LUMO gap. A pronounced odd-even alternative phenomenon indicates that the clusters with even-numbered valence electrons possess a higher stability than their neighboring ones. For both systems, natural population analysis reveals that electronic properties of dopant atoms in the corresponding configuration are mainly related to s and p states. We also investigated magnetic effects of clusters as a function of cluster size, however, their oscillatory magnetic moments were found to vary inversely to the fragmentation energy, second-order difference of energy, and HOMO-LUMO gap.     

Fragmentation of sputtered Si<Subscript><Emphasis Type="Italic">n</Emphasis></Subscript>O<Stack><Subscript><Emphasis Type="Italic">m</Emphasis></Subscript><Superscript>+</Superscript></Stack> clusters: Release kinetic and dissociation energies

The spectra of kinetic energies of positive Si _n O _m ⁺ cluster ions (n = 2–5, m = 2–7) have been measured using a double focusing ion microanalyzer with reverse geometry at instants 10⁻⁵ to 10⁻⁴ s after emission. The dissociation energies have been determined within the evaporative ensemble model and the theory of unimolecular decay reactions. The results obtained are compared with the binding energies of neutral Si _n O _m clusters.     

Secondary ion emission from bismuth under bombardment with Bi m ? and Au m ? cluster ions

The mass spectra of secondary cluster ions Bi _n ⁺ (n = 1–9) from bismuth sputtered by (6–21) keV Au _m ⁻ (m = 1–9) and Bi _m ⁻ (m = 1–5) cluster ions have been investigated. New features of manifestation of dense nonlinear cascades and thermal spikes have been revealed. It is concluded that the thermal spike regime makes a significant contribution to the mechanism of formation of small cluster ions, containing up to seven atoms for a bismuth target.     

Rings, towers, cages of ZnO

Theoretical electronic structure studies on (ZnO)_n (n= 2–18, 21) have been carried out to show that the transition from an elementary ZnO molecule to the bulk wurtzite ZnO proceeds via hollow rings, towers, and cages. Our first principles electronic structure calculations carried out within a gradient corrected density functional framework show that small Zn_nO_n (n=2–7) clusters form single, highly stable rings. Zn₃O₃ and the symmetric cage Zn₁₂O₁₂ are shown to be particularly stable clusters. Among larger clusters, the most stable are oblong cages, Zn₁₅O₁₅, Zn₁₈O₁₈, and Zn₂₁O₂₁, which are reminiscent of nanotubes.     

Structural and magnetic properties of bimetallic Con ? 1Cr clusters with density functional theory

We have investigated the structural and magnetic properties of small bimetallic Co _{n − 1}Cr (n = 2 − 9) clusters by means of spin-polarized density functional theory approach. We found the ground state structures of Co _{n − 1}Cr were very similar to those of pure Co _n except n = 3,6. The clusters were of high stability at n = 6. Magnetic moments showed an interesting size-dependent variation: the magnetic moments of Co _{n − 1}Cr can be obtained by minus 7μB for n = 2, 4, 6–9 or by plus 1μB for n = 5 from those of the Co _n counterparts. The different magnetic behavior stems from ferromagnetic alignment or ferrimagnetic alignment of Co and Cr atoms in Co _{n − 1}Cr, associated with their longer or shorter interatomic distances.      

Carbon clusters for absolute mass measurements at ISOLTRAP

The cyclotron frequencies of singly charged carbon clusters C_n ⁺ (n ≥ 2) were measured with the Penning-trap mass spectrometer ISOLTRAP at ISOLDE/CERN. The present limit of mass accuracy δm/m = 1.2 ^. 10^-8 and the extent of the mass-dependent systematic shift (δm/m)_sys = 1.7(0.6) ^. 10^-10/u ^. (m - m _ref) of the setup were investigated for the first time. In addition, absolute mass measurements by use of pure clusters of the most abundant carbon isotope ¹²C are now possible at ISOLTRAP. Received: 21 March 2002 / Accepted: 16 May 2002 / Published online: 31 October 2002 RID="a" ID="a"Present address: CERN, CH-1211 Geneva 23, Switzerland; e-mail: klaus.blaum@cern.ch     

Effects of preannealing temperature of ZnO thin films on the performance of dye-sensitized solar cells

The preferred (002) orientation zinc oxide (ZnO) nanocrystalline thin films have been deposited on FTO-coated glass substrates by sol–gel spin-coating technology and rapid thermal annealing for use in dye-sensitized solar cells (DSSC). The effects of preannealing temperature (100 and 300°C) on the microstructure, morphology and optical properties of ZnO thin films were studied. The ZnO thin films were characterized by X-ray diffraction (XRD), scanning electron microscopic (SEM) and Brunauer–Emmett–Teller (BET) analysis. The photoelectric performance of DSSC was studied by I–V curve and the incident photon-to-current conversion efficiency (IPCE), respectively. From the results, the intensities of (002) peaks of ZnO thin films increases with increasing preannealing temperature from 100°C to 300°C. The increase in pore size and surface area of ZnO films crystallized at the increased preannealing temperature contributed to the improvement on the absorption of N3 dye onto the films, the short-circuit photocurrent (J _sc) and open-circuit voltage (V _oc) of DSSC. The higher efficiency (η) of 2.5% with J _sc and V _oc of 8.2 mA/cm² and 0.64 V, respectively, was obtained by the ZnO film preannealed at 300°C.     

Metastable fragmentation of silver bromide clusters

The abundance spectra and the fragmentation channels of silver bromide clusters have been measured and analyzed. The most abundant species are Ag_nBr_{n - 1} ⁺ and Ag_nBr_{n + 1} ^- and Ag₁₄Br₁₃ ⁺ is a magic number, revealing their ionic nature. However, some features depart from what is generally observed for alkali-halide ionic clusters. From a certain size, Ag_nBr_{n - 1} ⁺ is no more the main series, and Ag_nBr _{n - 2, 3} ⁺ series become almost as important. The fast fragmentation induced by a UV laser makes the cations lose more bromine than silver ions and lead to more silver-rich clusters. Negative ions mass spectra contain also species with more silver atoms than required by stoichiometry. We have investigated the metastable fragmentation of the cations using a new experimental method. The large majority of the cations release mainly a neutral Ag₃Br₃ cluster. These decay channels are in full agreement with our recent ab initio DFT calculations, which show that Ag⁺-Ag⁺ repulsion is reduced due to a globally attractive interaction of their d orbitals. This effect leads to a particularly stable trimer (AgBr)₃ and to quasi-planar cyclic structures of (AgBr)_n clusters up to n = 6. We have shown that these two features may be extended to other silver halides, to silver hydroxides (AgOH)_n, and to cuprous halide compounds. Received 9 November 2000 and Received in final form 25 January 2001     

ConCm±(n=1-5; m=1,2)团簇的密度泛函理论研究

本文采用基于自旋极化的密度泛函理论系统研究了Co_nC_m^± (n=1-5; m=1,2)团簇的几何结构和电子结构特性. 将Co_nC^± (n=2-5)团簇中的一个Co替换为C原子, 个体的基态几何结构发生明显变化; 在Co_nC₂^± (n=1-5)团簇的生长序列中, 发现从n=3开始团簇中的两个C原子有彼此分离分布的趋势, 我们分析, 这是Co金属能够维持单壁碳纳米管(SCNTs)保持开口生长, 成为非常有效的一种催化剂的重要原因. 同时, 将Co_nC^± (n=2-5)团簇中添加一个Co原子后系统的总磁矩出现大幅下降的趋势, 但仍保持奇偶交替的规律. 通过比较中性及带电的Co_nC以及Co_nC₂ (n=1-5)团簇的碎裂能, 本工作发现: 由实验获取的SCNTs应均为带正电的体系, 这一结论与已有的实验模型拟合得很好.     

Density functional theory study on the structural and electronic properties of Ag-adsorbed (MgO)<Subscript><Emphasis Type="Italic">n</Emphasis></Subscript> clusters

Equilibrium geometries, charge distributions, stabilities and electronic properties of the Ag-adsorbed (MgO) _n (n = 1–8) clusters have been investigated by density functional theory (DFT) with generalized gradient approximation (GGA) for exchange-correlation functional. The results show that hollow site is energetically preferred for n⩾4, and the incoming Ag atoms tend to cluster on the existing Ag cluster. The Mulliken populations indicate that the interaction between the Ag atom and Magnesia clusters is mainly induced by a weak atomic polarization. The adsorbed Ag atom only causes charge redistributions of the atoms near itself. The effect of the adsorbed Ag atom on the bonding natures and structural features of Magnesia clusters is minor. Furthermore, the investigations on the first energy difference, fragmentation energies and electron affinities show that the Ag(MgO)₄ and Ag(MgO)₆ are the most stable among studied clusters. Supported by the Startup Fund of High-level Personal for Shihezi University (Grant No. RCZX200747)     

Charge transfer between alkali cluster ions and atoms in the 1 to 10 keV collisional energy range

The cross-sections for collisional charge transfer between singly charged free clusters M _n ⁺ (M = Li, Na; n=1...50) and atomic targets A (cesium, potassium) have been measured as a function of collisional relative velocity in laboratory energy range 1–10 keV. For each cluster size, the experimental values of the charge transfer cross-section are fitted with an universal parametric curve with two independent parameters and v_m, the maximum cross-section and the corresponding velocity. For small size clusters (), the characteristic parameters show strong variations with the number of atoms in the cluster. Abrupt dips observed for n=10 and n=22 are attributed to electronic properties. Charge transfer patterns observed for various collisional systems present similarities, which appear more sensitive to cluster quantum size effects than to collision energy defects. In their whole, the and v_m parameters show differences in both their size evolution and their absolute values discussed in term of projectile and target electronic structures. Received 13 April 2000 and Received in final form 29 June 2000     

Comparative ab initio studies of small tin and lead clusters

We have performed ab initio total-energy pseudopotential calculations on neutral and negatively charged Snn and Pb_n (n = 3 ? 10) clusters. The lowest energy structures have been determined for all clusters, and the stabilities of neutral clusters were investigated by comparing their evaporation energies and stability functions. Clusters with n = 7,10 were found to be most stable while the clusters with n = 8 and Pb_s were much less stable, in agreement with features of the observed mass spectra. Calculations on Sn^?_n and Pb^?_n show that both atomic and electronic structures of a neutral cluster change substantially upon charging. The densities of states of Sn^?_n clusters reproduce the main features of the experimental photoelectron spectra. The agreement is poorer for Pb^?_n clusters where the calculations underestimate the separation between energy levels which we think is due to the larger spin-orbit splitting in Pb, which was neglected in the calculations. We found that the differences between Sn and Pb clusters cannot be completely addressed without a more complete accounting of relativistic effects. The electron affinities of Sn_n and Pb_n clusters have also been calculated and the results agree fairly well with experimental values. Finally we considered Sn^2?₄ and Pb^2?₄ clusters and related the results to the formation of Zintl anions in liquid alkali-Sn and alkali-Pb alloys.     

Femtosecond ionization and fragmentation of free silicon nanoparticles

Femtosecond-laser spectroscopy is used to study the photoionization and photofragmentation of large neutral silicon clusters in a beam. Silicon clusters Si_n with sizes up to n≈6000, corresponding to nanoparticles with diameters up to 6 nm, are generated in a laser vaporization source. Nanosecond- and femtosecond-laser ionization are employed to characterize the free silicon nanoparticles. Excitation with intense femtosecond-laser pulses leads to prompt formation of doubly and triply charged Si_n clusters. Additionally, strong fragmentation of charged clusters occurs by Coulomb explosion, resulting in high released kinetic energies. Multiply charged atoms up to Si⁴⁺ with initial kinetic energies in the range of 500 eV are observed for laser intensities of about 10¹³ W/cm². Pump–probe spectroscopy yields decay times of the intermediate resonances of the nanoparticles. Received: 22 January 2000 / Published online: 7 August 2000     

Evolution of structures,stabilities, and electronic properties of anionic [AunRb]− (n = 1–10) clusters: comparison with pure gold clusters

The geometric structures, stabilities, and electronic properties of small size anionic [Au_nRb]^? and Au_n+1^? (n = 1–10) clusters have been systematically investigated by using density functional theory. The optimised geometries show that the structures of [Au_nRb]^? clusters favour the three-dimensional structure at n ≥ 8. The Rb atoms tend to occupy the most highly coordinated position and form the largest probable number of bonds with gold atoms. One Au atom capped on [Au_n-1Rb]^? structures is the dominant growth pattern for n = 2–8 and Rb atom capped on Au_n^? structures for n = 9–10. The averaged atomic bonding energies, fragmentation energies, second-order difference of energies, and highest occupied molecular orbital–lowest unoccupied molecular orbital gaps exhibit a pronounced even–odd alternations phenomenon. The charges in [Au_nRb]^? clusters transfer from the Rb atoms to Au_n host. In addition, it is found that the most favourable dissociation channel of the [Au_nRb]^? clusters is to eject a Rb atom and the highest energy dissociation path is Rb^? anion ejection.     

High energy collisions of protonated water clusters

We have measured attenuation cross sections and fragmentation cross sections for protonated water clusters H(H₂O)_n ⁺ (n = 1 to 100) colliding with noble gas atoms (He and Xe) at a laboratory energy of 50 keV. In collisions with He, a transparency effect in the attenuation cross section was observed. For the case of fragmentation in collisions with Xe, a strong enhancement of small clusters was observed which we attribute to multifragmentation. Received 30 November 2000     

An ab initio study of the electronic and geometric structures of SimCn + cationic nanoclusters

The theoretical formalism of local density approximation (LDA) to density functional theory (DFT) has been used to study the electronic and geometric structures of Si_mC_n⁺ (1≤m, n ≤4) cationic clusters. An all electron 6-311++G^** basis set has been used and complete geometry optimizations of different possible structures for a specific cluster have been carried out. Binding energies, relative energies, fragmentation energies, vibrational frequencies, and adiabatic ionization potentials of the optimized clusters have been investigated and reported in detail. Results have been compared with other experimental and theoretical results available in the literature.     

Structural,energetic, and electronic properties of hydrogenated aluminum arsenide clusters

The chemisorptions of hydrogen on aluminum arsenide clusters are studied with density functional theory (DFT). The on-top site is identified to be the most favorable chemisorptions site for hydrogen. And the Al-top site is the preferred one in the most cases for one hydrogen adsorption in (AlAs) _n (n = 2, 5, 6, 8–15) clusters. Top on the neighboring Al and As atoms ground-state structures are found for two hydrogen adsorption on (AlAs) _n except for (AlAs)₂ cluster. The Al–As bond lengths decrease generally as the size of the cluster increases. And there is a slight increase in the mean Al–As bond lengths after H adsorption on the lowest-energy sites of the most AlAs clusters. In general, the binding energy of H and 2H are both found to decrease with an increase in the cluster size. And the result shows that large binding energies (BE) of a single hydrogen atom on small AlAs clusters and large highest occupied and lowest unoccupied molecular-orbital gaps for (AlAs)H and (AlAs)₃H make these species behaving like magic clusters. Calculations on two hydrogen atoms on (AlAs) _n clusters show large BE for (AlAs) _n H₂ with an odd number of n. The stability of these complexes is further studied from the fragmentation energies. (AlAs)₇H₂ and (AlAs)₉H₂ clusters are again suggested to be the stable clusters. On the other hand both the fragmentation energy and the binding energy for (AlAs)₁₃H are close to the lowest values.