Electron-impact induced fragmentation of fullerene ions

The C2 fragmentation of fullerene ions C(q+)(60) (q = 1,2,3) induced by electron impact was studied for the first time. The cross sections for the loss of a C2 fragment indicate the presence of two different processes. At low electron energies the projectile electron leads to the direct excitation of the giant plasmon resonance. At electron energies larger than 100 eV the fragmentation of the fullerene ions can be described as an unsuccessful ionization. Only this second part of the cross section shows a dependence on the charge state q of the precursor ion.     

一维原子链局域等离激元的非线性激发

本文基于含时密度泛函理论, 研究了不同频率光脉冲场作用下一维钠原子链中电子的输运与等离激元共振之间的耦合规律. 在等离激元共振点附近约0.8 eV频率范围内的光脉冲场, 都可以激发体系的等离激元共振. 这些不同频率外场激发的等离激元共振强度大小在一个数量级. 外场频率越接近等离激元共振频率, 外场激起的等离激元振动的振幅越大. 对于线性原子链等离激元的非线性激发现象, 本文用经典谐振子模型给出了定性解释.     

一组线性稠环芳香烃的等离激元激发

采用含时密度泛函理论,研究了一组线性稠环芳香烃量子点的等离激元激发.沿着线性稠环芳香烃所在的平面方向,体系中有两个主要的等离激元共振带:一个位于3.9 e V附近;另一个位于16 e V附近.随着线性稠环芳香烃长度的增加,在低能共振区,沿不同方向激发时,体系的吸收峰分别发生了蓝移和红移.此外,与并五苯相比,在低能共振区,一氯并五苯的吸收光谱的吸收峰发生了红移和展宽,其中氯原子位于并五苯的两端时对一氯并五苯等离激元激发影响较大.在低能共振区,线性稠环芳香烃的等离激元共振模式与石墨烯量子点相似.     

Collective electron excitations in a spherical potential well and the polarizability of the fullerene molecule

The collective motion of the electron system of the C60 molecule was investigated within the framework of the linear response theory. The model originally proposed by Mukhopadhyay and Lundqvist [1, 2, 3] and applied for C60 by Östling et al [4] was used with an electron density calculated from a spherical well potential. From the calculated dynamic polarizability a main plasmon peak at 14.4 eV in the photoabsorption cross section of the C60 molecule and a broad double peaked structure around 25 eV in the transmission electron energy loss spectrum of the solid C60 can be derived.     

Spectral degeneracy breaking of the plasmon resonance of single metal nanoparticles by nanoscale near-field photopolymerization

We report on controlled nanoscale photopolymerization triggered by enhanced near fields of silver nanoparticles excited close to their dipolar plasmon resonance. By anisotropic polymerization, symmetry of the refractive index of the surrounding medium was broken: C infinity v symmetry turned to C2v symmetry. This allowed for spectral degeneracy breaking in particles plasmon resonance whose apparent peak became continuously tunable with the incident polarization. From the spectral peak, we deduced the refractive-index ellipsoid fabricated around the particles. In addition to this control of optical properties of metal nanoparticles, this method opens new routes for nanoscale photochemistry and provides a new way of quantification of the magnitude of near fields of localized surface plasmons.     

Tuning the plasmon resonance of metallic tin nanocrystals in Si-based materials

The optical properties of metallic tin nanoparticles embedded in silicon-based host materials were studied. Thin films containing the nanoparticles were produced using RF magnetron sputtering followed by ex situ heat treatment. Transmission electron microscopy was used to determine the nanoparticle shape and size distribution; spherical, metallic tin nanoparticles were always found. The presence of a localized surface plasmon resonance in the nanoparticles was observed when SiO₂ and amorphous silicon were the host materials. Optical spectroscopy revealed that the localized surface plasmon resonance is at approximately 5.5 eV for tin nanoparticles in SiO₂, and at approximately 2.5 eV in amorphous silicon. The size of the tin nanoparticles in SiO₂ can be varied by changing the tin content of the films; this was used to tune the localized surface plasmon resonance.     

Probing the electronic structure of ZnO nanowires by valence electron energy loss spectroscopy

Valence electron energy loss spectroscopy in a transmission electron microscope is employed to investigate the electronic structure of ZnO nanowires with diameter ranging from 20 to 100 nm. Its excellent spatial resolution enables this technique to explore the electronic states of a single nanowire. We found that all of the basic electronic structure characteristics of the ZnO nanowires, including the 3.3 eV band gap, the single electron interband transitions at approximately = 9.5, approximately = 13.5,and approximately = 21.8 eV, and the bulk plasmon oscillation at approximately 18.8 eV, resemble those of the bulk ZnO. Momentum transfer resolved energy loss spectra suggest that the 13.5 eV excitation is actually consisted of two weak excitations at approximately = 12.8 and approximately = 14.8 eV, which originate from transitions of two groups of the Zn 3d electrons to the empty density of states in the conduction band, with a dipole-forbidden nature. The energy loss spectra taken from single nanowires of different diameters show several size-dependent features, including an increase in the oscillator strength of the surface plasmon resonance at approximately = 11.5 eV, a broadening of the bulk plasmon peak, and splitting of the O 2s transition at approximately = 21.8 eV into two peaks, which coincides with a redshift of the bulk plasmon peak, when the nanowire diameter decreases. All these observations can be well explained by the increased surface/volume ratio in nanowires of small diameter.     

Observed radiation from surface plasmons in aluminum

Plasma radiation from thin films of aluminum bombarded by non-normally incident 80 keV electrons has been investigated. Radiative decay of both volume plasmons and surface plasmons was observed with the surface plasmon radiation being very sensitive to contamination of the film surface by an oxide layer. The surface resonance occurs at 10.2 eV, whereas the volume resonance is at 15 eV. These values are in accord with electron energy-loss experiments.     

FeN_4掺杂对富勒烯催化特性调制的第一性原理研究

因其较好的稳定性和催化活性,非金属N与金属共掺杂的富勒烯(C60)作为新型氧化还原反应(ORR)催化剂受到了人们的广泛关注.采用基于密度泛函理论的第一性原理方法系统地研究了Fe N4掺杂对C60催化特性的调制规律,揭示了O_2在Fe N4掺杂的C60上的吸附和氢化特性.结果表明:(1)O_2倾向于以side-on模式吸附在Fe的顶位上,O-O键平行于C60的球切面,与Fe形成O-Fe-O三元环结构,对应的吸附能为1.48 e V.(2)O_2的氢化反应路径可以分为两条:(i)O_2先解离为O+O,然后氢化为O+OH.O_2的解离为反应的速控步,势垒为2.82 e V.(ii)O_2先氢化形成OOH结构,然后解离.氢化为反应的速控步,势垒为2.83 e V.     

FeN4掺杂对富勒烯催化特性调制的第一性原理研究

因其较好的稳定性和催化活性,非金属N与金属共掺杂的富勒烯（C60）作为新型氧化还原反应（ORR）催化剂受到了人们的广泛关注。采用基于密度泛函理论的第一性原理方法系统地研究了FeN4掺杂对C60催化特性的调制规律,揭示了O2在FeN4掺杂的C60上的吸附和氢化特性。结果表明：（1）O2倾向于以side-on模式吸附在Fe的顶位上,O-O键平行于C60的球切面,与Fe形成O-Fe-O三元环结构,对应的吸附能为1.48 eV。（2）O2的氢化反应路径可以分为两条：（i）O2先解离为O + O,然后氢化为O + OH。O2的解离为反应的速控步,势垒为2.82 eV。（ii）O2先氢化形成OOH结构,然后解离。氢化为反应的速控步,势垒为2.83 eV。     

Cavity ring-down spectroscopy of metallic gold nanoparticles

The optical properties of supported gold nanoparticles with sizes of 1.3 nm, 1.6 nm, 2.5 nm, and 2.9 nm have been studies by using cavity ring-down spectroscopy in the photon energy range between 1.8 eV and 3.0 eV. The obtained results show the possibility to obtain optical information of nanoassembled materials with high sensitivity. The experimental findings are compared to calculations using Mie-Drude theory. Whereas the broadening of the surface plasmon resonance with decreasing size is well described by this model, the observed blue-shift of the surface plasmon resonance contradicts the predictions of the Mie-Drude theory. The latter effect can be explained by the presence of a skin region with decreased polarizability typical for coinage metal particles. Furthermore, it is found that the supported gold nanoparticles are robust under ambient conditions, an important issue when using these materials for optical applications.     

Electron energy-loss spectra of clean and gas-covered Ni(100) surfaces

Electron energy-loss spectra have been measured on Ni(100) surfaces, clean and following oxygen and carbon monoxide adsorption, at primary energies of 40–300 eV. The observed peaks at 9.1, 14 and 19 eV in the clean-surface spectrum are ascribed to the bulk plasmon of the 4s electrons, the surface plasmon, and the bulk plasmon of the coupled 3d + 4s electron, respectively, and the weak but sharp peak at 33 eV is tentatively attributed to the localized many-body effect in the final state. Assignments of the loss structures on the gas-covered surfaces have been attempted.     

Manganese doping influence on the plasmon energy of nickel films

Manganese doping in nickel films capped with copper have been prepared by evaporation in vacuum. The films are composed of grains with an average diameter of ~ 20 nm from scanning electron microscope scans. Optical absorption is measured over a wavelength range of 190–450 nm. Two plasmon peaks are observed at 3.30 eV and 4.45 eV for a range of concentrations of films. The 4.45 eV peak is a bulk plasmon peak that is enhanced by increasing the manganese in nickel. The 3.30 eV peak is a surface plasmon peak that increases in width or strength of plasmon resonance with increasing concentration of manganese. This may be a combination effect of charge carrier concentration and dielectric screening from the reformed electronic band structure caused by manganese doping. By adding manganese into nickel, the ferromagnetic order is further destroyed as a transition into a spin glass occurs. This spin glass behavior is seen in a coercivity measurement at 4 K where the coercivity drops precipitously as the doping concentration increases.     

Angular dependence of the electron energy loss spectra from a clean and adsorbate covered W(001) surface

A dispersion analysis of the EELS from a W(001) surface in the range 1 < ΔE < 35 eV has been performed and compared with recent and complete optical data for tungsten. The non-dispersive (k ～ 0) EELS correlated well with a combination of the surface and bulk loss functions calculated from the optical data. Losses at 1–5 eV and a pair at 32 and 34.5 eV were assigned to interband and N_6,7 core ionization excitations respectively. The principal bulk and surface plasmon losses were identified at 24.0 and 20.3 eV respectively. Two further losses at 14.0 and 9.6 eV were also observed and assigned to subsidiary plasmon losses. All four plasmon losses showed only minimal energy dispersion, never exceeding 1.5 eV. A momentum selectivity for separating bulk and surface interband losses was demonstrated with the non-dispersive losses arising from excitations within the bulk even with incident energies as low as 88 eV, whereas their dispersive counterparts were extremely sensitive to the chemical state of the surface. New adsorbate derived losses which develop during adsorption were associated with excitations from the new deep lying adsorbate levels to final state levels at or near the Fermi level. It was concluded that this final state was also responsible for the N_6,7 ionization losses.     

功能光学纳米Ag薄膜的制备及其光学特性研究

对以热蒸发法制备的超薄Ag薄膜,扫描电子显微镜结果显示其呈纳米尺度的颗粒状,由透射谱测量发现其具有表面等离子体激元特征.检测到不同条件制备纳米Ag薄膜的表面等离子体共振吸收峰的位移规律,且纳米Ag材料具有选择性的透过、反射特性.通过不同的制备条件,得到了在长波范围内透过率超过90％、在表面等离子体共振峰值位置处反射率接近50％且峰位可调的光学薄膜材料.这种薄膜材料有望成为应用在薄膜太阳电池中间层中具有潜在性光管理功能的光学薄膜材料. 关键词： 热蒸发 纳米Ag薄膜 表面等离子体激元 光管理     

Method for calculating photoionization cross sections of fullerenes in the local density and random phase approximations

A method for calculating the photoionization cross sections of fullerenes taking into account many-electron correlations on the basis of the local density and random phase approximations is proposed and implemented. Calculations are made specifically for fullerenes C₆₀ and C₂₀. It is shown that the photoionization spectrum of C₆₀ acquires a plasmon resonance whose position and magnitude are in good agreement with experimental results [I.V. Hertel, H. Steger, J. de Vries et al., Phys. Rev. Lett. 68, 784 (1992)] and with the results of other calculations [M.J. Puska and R.N. Nieminen, Phys. Rev. A47, 1181 (1993)]. The emergence of a giant resonance is predicted in the photoionization spectrum of fullerene C₂₀ with the center at a photon energy on the order of 27 eV, which corresponds to the frequency of resonant surface plasmon oscillations in a conducting sphere.     

Large optical-frequency shift of molecular radiation via coherent coupling to an off-resonance plasmon

We demonstrate coherent optical coupling between molecular and plasmon resonances that are well separated in energy. In the presence of metallic nanoparticles, the second harmonic spectrum of organic dyes no longer peaks at the absorption wavelength but is instead blueshifted by 25 nm towards the localized plasmon resonance. The phase of the light generated by the dyes displays a large modulation across the plasmon resonance and no change across the molecular one. The second harmonic signal contributed by the nanoparticles, which is peaked at the plasmon frequency when no molecules are present, similarly displays a shift towards the molecular resonance in their presence. A model based on the interplay of the nonlinear optical near fields is able to account for these observations.     

First-principles study of the optical properties of PbTiO<Subscript>3</Subscript>

The optical properties of PbTiO₃ were studied from first principles using the density functional theory. The dielectric functions and optical constants are calculated using the full potential–linearized augmented plane wave (FP-LAPW) method with the generalized gradient approximation (GGA). The theoretical calculated optical properties and energy loss (EEL) spectrum yield a static refractive index of 2.83 and a plasmon energy of 23.1 eV for cubic phase. The effective electron number at low energy saturates near 20 eV with the value of 18.1 for the effective electron number. In the tetragonal phase the static refractive index decreases to 2.59 and yields a plasmon energy of 22.7 eV.     

High-transmission hybrid-effect-assisted nanoaperture

We present a C-aperture encircled by a groove that makes possible the hybrid effect of coupling surface plasmon resonance to propagating waves. Compared to a single C aperture, the groove-encircled aperture can increase transmission by a factor of 2.45 in the near field and by a factor of 1.88 in the far field, showing good agreement with our theoretical calculation.     

Electron energy loss spectroscopic investigation of palladium metal and palladium(II) oxide

Electron energy loss spectra (ELS) obtained from polycrystalline Pd metal and PdO powder using primary electron energies ranging from 100 to 1150 eV have been obtained and examined in an attempt to gain a better understanding of the origins of the loss features and to assess the utility of ELS in investigations of Pd catalysts. The two sets of ELS spectra differ significantly. The ELS spectra from Pd metal exhibit a predominant peak at 6.5 eV, shown to arise from a surface plasmon excitation, and two broad features at 25.1 and 31.9 eV, which originate from bulk loss processes. The broad features consist of several overlapping losses due mainly to interband transitions from the d-band, though a bulk plasmon excitation is believed to produce a feature near 24 eV. Two distinct peaks are present at 3.7 and 7.6 eV in the ELS spectra obtained from PdO, while a broad region of intensity appears over the range from 20 to 40 eV. The peak at 3.7 eV is attributed to a transition between the top of the valence band and the bottom of the conduction band. The feature at 7.6 eV is broad and arises from several overlapping features that are most likely caused by interband transitions rather than collective excitations. Furthermore, the ELS spectra obtained from PdO and oxidized Pd are also quite different indicating that ELS can provide useful information for determining the bonding states of oxygen on Pd-containing catalysts.