Yufen Li

Linear and ultrafast nonlinear optical response of Ag:Bi2O3 composite films

Silver/bismuth oxide composite thin films with Ag concentration varying from 13.2% to 59.3% were prepared by cosputtering in a multitarget magnetron sputtering system. The absorption spectra of these thin films were measured, and the absorption peaks due to surface plasmon resonance of Ag particles showed a redshift behavior with increasing Ag concentration. By using the femtosecond time-resolved optical Kerr effect technique under 800 nm wavelength, the nonlinear optical properties of these films were investigated. The Kerr signals showed an ultrafast rise, suggesting that the main physical mechanism involved in the optical nonlinearity of Ag:Bi2O3 thin films on the femtosecond scale was the pure electronic contributions. The maximum value of the third-order nonlinear optical susceptibility of Ag:Bi2O3 thin films was 4.1×10−10 esu, and occurred at a Ag concentration of about 35.7%.

Structural and optical characteristics of fullerenes incorporated inside porous silica aerogel

Abstract The successful synthesis of fullerene-doped silica aerogel by the sol-gel process followed by the supercritical drying technique is reported. The fullerenes (C 60 /C 70 ) can be embedded inside micropores linked by the silica skeleton and the fullerene structures were retained in the fabrication process as seen using mass spectroscopy, Fourier transform infrared spectroscopy and transmission electron microscopy. The fulleren-doped composites were found to be able to emit intense visible light (2.26 eV) under Ar + laser (488 nm) excitation at room temperature. The luminescent intensity increased strongly with low doping of fullerenes (0.05 mol%), then decreased while the peak was red-shifted and finally quenched upon heavy doping (2.5 mol%). This phenomenon is explained as a result of the quantum confinement effect for fullerenes which produces emissions far above the inter-band gap of C 60 /C 70 .

Intense visible light emission from fullerene‐doped silica aerogel

We report that fullerenes (C60/C70) can be uniformly encased in silica aerogel by sol‐gel process together with supercritical drying technique. Intense visible light emission (2.26 eV) is observable with the naked eye from the composite under an Ar+ laser (488 nm) excitation at room temperature. The luminescent intensity is found to increase strongly with low doping of fullerenes (0.05 mol %) and then to decrease while the peak is red shifted, and finally quenched upon heavy doping (up to 2.5 mol %). This effect is consistent with the behavior of the fullerenes entrapped in the micropores linked by the silica networks resulting in the quantum confinement effect which then produce emission far above the interband gap of C60/C70.

Mechanisms for adatoms diffusing on metal fcc(111) surfaces

Abstract On a series of fcc(111) surfaces modeled by the embedded atom method, adatom diffusion mechanisms are studied systematically by molecular dynamics. On most surfaces, as expected, the diffusion mechanism is only hopping. For some other surfaces, however, we find that the exchange mechanism is possible besides the well-known hopping mechanism. More interesting is that the hcp and fcc sites are now different for adatom diffusion: the exchange diffusion process only occurs between the hcp sites.

Formation of various types of metallofullerenes by laser ablation of externally doped fullerenes C60Mx (M=Sm, Pt, Ni, La, Y, and Rh)

A photofragmentation study of metal fullerides C60Mx (M=Sm, Pt, Ni, La, Y, and Rh) by excimer laser ablation-TOF mass spectrometry shows that many kinds of metallofullerenes have been observed in both the positive and negative ionic modes. For C60Mx (M=Sm, Pt, and Ni), the metal atom is incorporated into the network of the fullerene cage to replace one carbon atom of the cage forming substitutional metallofullerene. While in the case of metal fullerides C60Mx (M=La, Y), evidence of the encapsulation of La and Y atoms in fullerene cages forming endohedral fullerenes has been observed. Different from the above two cases, the laser ablation of rhodium fulleride C60Rhx(x≈1) produces two sequences of rhodium-containing fullerene clusters C2nRh and C2n+1Rh(2n=50–62). Odd-numbered all-carbon clusters in the fullerene regime are observed in our laser ablation study of all the metal fullerides in the negative ion channel. The structures of metallofullerenes C2n+1M and C2nM with an even and odd number of total atom...

Mass spectrometric investigation of the aggregation and coalescence of fullerenes in C60-Modified poly (N-vinylcarbazole) by UV-laser ablation

The phenomena of aggregation and coalescence of fullerenes in the UV-laser ablation time-of-flight mass spectrometric investigation of C60-modified poly(N-vinylcarbazole) both in the positive and in the negative ion channels have been observed. The results indicate that in C60 chemically modified PVK (C60–PVK) copolymer the nascent fullerene fragments ruptured from main chain can easily coalesce into large fullerenes through collisions, whereas in the C60-doped PVK the aggregation and coalescence of C60 were relative weak due to nonbounding action and incomplete charge transfer behavior between C60 and PVK. Furthermore, the photoinduced electron transfer behavior between C60 and carbazole units in the C60 chemically modified poly(N-vinylcarbazole) in benzonitrile by laser flash photolysis at 355 nm has also been investigated. Efficiency of the anion radical of C60 in copolymer at 1080 nm is higher than that of the C60-doped poly(N-vinylcarbazole) polymers. The formation of a C60 radical anion may be ascribed to photoinduced electron transfer between C60 pendanted on the main chain backbone and the inter-, and intrachain carbazole units in the copolymer.

Laser-induced clustering dynamics of [(SiO2)nX]− from porous silica powders

Abstract The clustering dynamics of SiO 2 -containing negative ion clusters generated from ultrafine amorphous SiO 2 powders with high specific surface area is investigated by XeCl excimer laser ablation time-of-flight mass spectrometry (TOF-MS). From the evolution of the mass spectra with the delay time and the number of laser irradiation shots, the key roles played by OH and OH − fragments from the surface silanol groups in the formation of SiO 2 -containing negative ion clusters are discussed.

Reliable lateral and vertical manipulations of a single Cu adatom on a Cu(111) surface with multi-atom apex tip: semiempirical and first-principles simulations

We study the reliability of the lateral manipulation of a single Cu adatom on a Cu(111) surface with single-atom, dimer and trimer apex tips using both semiempirical and first-principles simulations. The dependence of the manipulation reliability on tip height is investigated. For the single-atom apex tip the manipulation reliability increases monotonically with decreasing tip height. For the dimer and trimer apex tips the manipulation reliability is greatly improved compared to that for the single-atom apex tip over a certain tip-height range. Two kinds of mechanism are found responsible for this improvement. One is the so-called enhanced interaction mechanism in which the lateral tip-adatom interaction in the manipulation direction is improved. The other is the suspended atom mechanism in which the relative lateral trapping ability of the tip is improved due to the strong vertical attraction of the tip on the adatom. Both mechanisms occur in the manipulations with the trimer apex tip, while in those with the dimer apex tip only the former is effective. Moreover, we present a method to realize reversible vertical manipulation of a single atom on a Cu(111) surface with the trimer apex tip, based on its strong vertical and lateral attraction on the adatom.

Formation of odd-numbered fullerene-related species and its relation to the formation of metallofullerenes

Abstract Odd-numbered carbon clusters in the fullerene regime are formed through laser ablation of various fullerene-containing materials: organic fullerene derivatives, metal fullerides, and guest-host fullerene materials. The formation mechanism of odd-numbered “fullerene” species is briefly discussed. The structural properties are calculated with genetic algorithm and ab initio methods, and the results show that the odd-numbered fullerene species are pseudo-open-caged fullerenes. In the laser ablation of metal fullerides, substitutional or endohedral metallofullerenes are observed. A new way for metallofullerene preparation through the odd-numbered fullerene intermediates is suggested and realized experimentally.

Cluster structure of silica aerogel investigated by laser ablation

Abstract Silica clusters were obtained by laser ablation time-of-flight mass spectrometry from fullerene-doped and carbon-doped silica aerogels, which can be explained by cluster assembly structure of silica aerogel and the doping conditions. The results reported here not only provide structural evidence for the proposed structure of silica aerogel, but also give a novel method to detect the cluster structures of aerogels.