Surface plasmon assisted photoemission from Au nanoparticles on graphite

The resonant multiple excitation of collective modes in metallic nanoparticles using ultrashort laser pulses leads to an enhanced multiphoton photoemission from the particles. This effect is here demonstrated for the surface-plasmon resonance of Au nanoparticles on graphite. The shape of the photoemission spectra is explained by multiphoton photo-assisted thermionic emission from the nanoparticles and resonant emission via the image-potential state on graphite. Tuning the photon energy between 1.7 eV and 3.2 eV allows the identification of an enhancement of the photoemission yield at 2.1±0.1-eV photon energy that is attributed to the resonant excitation of the surface plasmon in the Au nanoparticles. This identification of the surface-plasmon excitation in this energy range is also supported by electron energy loss spectroscopy. Received: 8 August 2001 / Revised version: 13 September 2001 / Published online: 10 October 2001     

Photoemission from multiply excited surface plasmons in Ag nanoparticles

Two-photon photoemission spectroscopy using femtosecond laser pulses is used to investigate the excitation and decay mechanisms of the surface plasmon resonance in Ag nanoparticles grown on graphite. The resonant excitation of this collective excitation leads to a two-orders-of-magnitude-enhanced two-photon photoemission yield from a graphite surface with Ag nanoparticles compared to the yield from pure graphite. From the shape of the photoemission spectra, the polarization dependence of the photoemission yield and the excitation probabilities for different excitation pathways we conclude that excitation with 400-nm femtosecond laser pulses leads to the coherent multiple excitation of the surface plasmon in the Ag nanoparticles. This multiply excited plasmon mode can decay via the coupling to a single-particle excitation leading to the emission of an electron if its final state is located in the continuum. The surface plasmon in metallic nanoparticles is a model system to investigate collective excitations in multiphoton processes. Received: 26 June 2000 / Accepted: 2 September 2000 / Published online: 12 October 2000     

Theory of multiphoton photoemission disclosing excited states in conduction band of individual TiO2 nanoparticles

A theory of multiphoton photoemission is derived to explain the experimentally observed monotonic decrease with the wavelength in the electron yield of TiO₂ nanoparticles (NPs) by as large as four orders of magnitude. It is found that the fitting parameter corresponds to the energy position of Ti3d e_g and t_2g states, and the derived theory is a novel diagnostic of excited states in the conduction band, very importantly, applicable to individual NPs. The difference between four-photon slope NPs and three-photon slope NPs is attributed to the difference in defect density. The success of the theory in solving the puzzling result shows that thermal emission from high-lying levels may dominate over direct multiphoton ionization in solids when the photon number larger than four is required.     

Charge transfer between sensing and targeted metal nanoparticles in indirect nanoplasmonic sensors

In indirect nanoplasmonic sensors, the plasmonic metal nanoparticles are adjacent to the material of interest, and the material-related changes of their optical properties are used to probe that material. If the latter itself represents another metal in the form of nanoparticles, its deposition is accompanied by charge transfer to or from the plasmonic nanoparticles in order to equalize the Fermi levels. We estimate the value of the transferred charge and show on the two examples, nanoparticle sintering and hydride formation, that the charge transfer has negligible influence on the probed processes, because the effect of charge transfer is less important than that of nanoparticle surface energy. This further corroborates the non-invasive nature of nanoplasmonic sensors.     

Adhesion of silver nanoparticles on the montmorillonite surface

Adhesion of silver nanoparticles on the montmorillonite substrate was investigated using molecular modeling (force field calculations) and experiment (infrared spectroscopy, high-resolution transmission electron microscopy). Modeling revealed the preferred orientation of silver nanoparticles on the silicate substrate and showed the strong dependence of total energy and stability of nanocomposite structure on two factors: (1) the mutual crystallographic orientation of nanoparticle and substrate structure and (2) the size and thickness of the nanoparticle. The size of silver single crystalline domains calculated by modeling was in good agreement with the experimental observations. Molecular modeling in confrontation with high-resolution transmission electron microscopy analysis showed the prediction possibility as to the nanoparticles structure and stability of nanocomposite.     

X-RAY PHOTOELECTRON EMISSION STRUCTURES OF CuO NANOPARTICLES

In this paper we report the X-ray photoemission characteristics of CuO nanoparticles with surface modifications. We found the enhancement of interatomic resonant charge transfer and multi-electron relaxation processes in the coated CuO nanoparticles, which were in agree-ment with the conclusions of polaron-related structural transition appearing under quantum confinement and dielectric confinement effects, as observed in the optical absorption and infrared vibration spectra of this system.     

Femtosecond optical investigation of electron–lattice interactions in an ensemble and a single metal nanoparticle

Electron–lattice energy exchange is investigated in an ensemble of silver nanoparticles of mean diameter 9 nm and in a single 30-nm particle using a femtosecond pump–probe technique. The dependences of the measured transient transmission change and of the electron energy loss kinetics on the excitation amplitude are compared to the results of numerical simulations of nonequilibrium electron relaxation and of the two-temperature model. The good agreement between the theoretical and experimental data indicates that, for the studied low particle density samples, hot-electron cooling is dominated by electron–lattice coupling in a nanoparticle both for weak and large electron heating with a minor influence of their surrounding environment (glass or polymer matrix). PACS 78.47.+p; 42.65.-k; 73.20.Mf     

Phase propagation of localized surface plasmons probed by time-resolved photoemission electron microscopy

In combining time-resolved two-photon photoemission (TR-2PPE) and photoemission electron microscopy (PEEM) the ultra-fast dynamics of collective electron excitations in silver nanoparticles (localized surface plasmons – LSPs) is probed at fs and nm resolution. Here we demonstrate that the sampling of the LSP dynamics by means of time-resolved PEEM enables detailed insight into the propagation processes associated with these excitations. In phase-integrated as well as phase-resolved measurements we observe spatio-temporal modulations in the photoemission yield from a single nanoparticle. These modulations are assigned to local variations in the electric near field as a result of the phase propagation of a plasmonic excitation through the particle. Furthermore, the control of the phase between the fs pump and probe laser pulses used for these experiments can be utilized for an external manipulation of the nanoscale electric near-field distribution at these particles. PACS 78.47.+p; 78.67.Bf; 79.60.-i; 73.20.Mf     

Excited state dynamics in silver nanoparticles embedded in phosphate glass

Excited state dynamics in silver nanoparticles embedded in aluminophosphate glass was studied by ultrafast optical pump–probe technique. The absorption process of pump radiation and the electron–phonon relaxation on the 10^?13–10^?11 s scale were analyzed in the framework of two-temperature model. The time evolution of the light-induced transient diffraction grating shows an uncommon relaxation on the nanosecond time scale. This relaxation is assigned to phonon–phonon scattering process as well as to the energy transfer from photoexcited electronic states in glass matrix to silver nanoparticles.     

Chemically modified semiconductor surfaces: 1,4-phenylenediamine on Si(100)

The formation of inorganic/organic hybrid systems in ultra-high-vacuum (UHV) is studied for the adsorption of 1,4-phenylenediamine (PDA) and aniline on Si(100)(2 × 1) single crystal surfaces. X-ray photoemission spectra (XPS) of PDA and aniline adsorbed on silicon are interpreted in terms of an adsorbate configuration involving two covalent bonds between one amino group per molecule and the dangling bonds of the clean silicon surface. Vibrational high resolution electron energy loss spectroscopy (HREELS) allows us to identify the remaining amino groups in the PDA-adsorbate. Ultraviolet photoemission spectroscopy (UPS) and electron energy loss (EELS) data in the valence band range indicate that the aromatic ring stays intact after the chemisorption of PDA. Changes in work function and energetic shifts of the electronic transitions characterize the charge transfer between adsorbate and substrate. The experimental results are finally discussed in the framework of semi-empirical cluster calculations using MNDO-PM3.     

Experimental and Theoretical Studies on the Interaction between Isonicotinic Acid Molecules and Silver Nanoclusters

The present paper describes the studies on the adsorption behavior and charge transfer from isonicotinic acid to silver nanoparticles with experiment and theory. Compared with UV–Visible adsorption spectrum, the adsorption spectrum of Ih-Ag₁₃ cluster was quite good agreed with that of silver colloidal nanoparticle. So that one Ag₁₃ cluster as a substrate was used to simulate Raman frequencies of the adsorption configuration. Here, it is demonstrated the calculated Raman spectra are in good agreement with experimental results. The analysis of Mulliken charge was obtained by density functional theory, which indicated the charge characteristics of Ag₁₃ nanoparticle. Once isonicotinic acid molecules were adsorbed on sliver clusters, the charges transfer from isonicotinic acid to silver clusters, so that the surface charges of silver clusters are uneven.     

Cs adsorption on a polar NbC(111) surface: photoemission and auger electron spectroscopy studies

The Cs adsorption process on a NbC(111) surface has been studied with core-level photoemission spectroscopy (PES) and Auger electron spectroscopy (AES). Coverage-dependent Cs 4d core-level PES shows that the polarization-depolarization transition of the Cs overlayer occurs in the coverage region of 0.5 ≤ θ ≤ 0.8 ML where the work function shows a minimum value. The charge transfer in the initial stage of adsorption is investigated using valence-related AES, and it is found that the transfer of the Cs 6s charge to the substrate occurs in the polarized phase.     

Electron and hole doping in NiO

We have investigated hole doped (by lithium) and electron-doped (by nickel metal) NiO with photoemission (PES), inverse photoemission (IPES) and low and high energy electron energy loss spectroscopy (EELS). Both types of doping create empty states approximately in the middle of the charge transfer gap of undoped NiO.     

A novel femtosecond-laser formation of CdS nanocrystallites in zirconia matrices

A novel method for direct laser writing of two-dimensional cadmium sulfide (CdS) semiconductor nanoparticle microstructures is reported. A two photon or a higher-order multiphoton absorption process, originating from femtosecond laser pulses, was used to decompose CdS precursors dispersed in a zirconia thin film previously dip-coated on a glass substrate. The kinetics of nanoparticle formation as a function of laser power were monitored in situ by photoluminescence spectroscopy. Raman spectroscopy was also performed to characterize the structural changes of the zirconia matrix under irradiation and to verify the formation of CdS nanoparticles. Results show that CdS nanoparticles were formed by two-photon absorption (TPA) with or without the help of an additional carbazole photoinitiator.     

Charging mechanisms of trapped element-selectively excited nanoparticles exposed to soft x rays

Charging mechanisms of trapped, element-selectively excited free SiO2 nanoparticles by soft x rays are reported. The absolute charge state of the particles is measured and the electron emission probability is derived. Changes in electron emission processes as a function of photon energy and particle charge are obtained from the charging current. This allows us to distinguish contributions from primary photoelectrons, Auger electrons, and secondary electrons. Processes leading to no change in charge state after absorption of x-ray photons are identified. O 1s-excited SiO2 particles of low charge state indicate that the charging current follows the inner-shell absorption. In contrast, highly charged SiO2 nanoparticles are efficiently charged by resonant Auger processes, whereas direct photoemission and normal Auger processes do not contribute to changes in particle charge. These results are discussed in terms of an electrostatic model.     

Charge transfer and polarization screening in organic thin films: phthalocyanines on Au(100)

Core hole screening effects at organic/metal interfaces were studied by core level X-ray photoemission spectroscopy (XPS), X-ray excited Auger electron spectroscopy (XAES), and valence band ultraviolet photoemission spectroscopy (UPS). The comparison of energetic shifts in XPS and XAES enables the estimation of electronic relaxation energy (screening ability). Magnesium phthalocyanine (MgPc) and zinc phthalocyanine (ZnPc) evaporated on single crystalline Au(100) were used as model molecules. Two different features in the Mg KLL spectra can be clearly separated for (sub-)monolayer coverages, while only minor changes of the shape of Mg 1s are observed. Applying a dielectric continuum model, the major screening mechanism cannot be described sufficiently by polarization screening due to mirror charges, significant contributions by charge transfer screening have to be considered. In contrast, small screening effects in the bulk material can be explained by surface polarization.     

Modeling of X-ray excited luminescence intensity dependence on the nanoparticle size

The thermalization length distribution of electrons over their kinetic energy in a conduction band is calculated on the basis of the data on the electron effective mass, density of states in conduction band, dielectric permittivity and energy of longitudinal optical phonons. The method of modeling of a recombinational luminescence intensity dependence on the nanoparticle size is proposed on the basis of the assumption that the contribution to a recombinational luminescence gives only those charge carriers which in the result of thermalization did not reach a near-surface layer of nanoparticles. Using such the approach the theoretical dependence of recombinational luminescence intensity on the nanoparticle size for LaPO₄ and LuPO₄ are calculated. The revealed correlation of experimental and theoretical dependences confirms that the commensurability of electron thermalization length with nanoparticle size is the main reason of the sharp decrease of X-ray excited luminescence intensity when the nanoparticle size decreases.     

Electron‐transfer transparency of graphene: Fast reduction of metal ions on graphene‐covered donor surfaces

The mechanism of charge transfer through nanomaterials such as graphene remains unclear, and the amount of charge that can be transferred from/to graphene without damaging its structural integrity is unknown. In this communication, we show that metallic nanoparticles can be decorated onto graphene surfaces as a result of charge transfer from the supporting substrate to an adjoining solution containing metal ions. Au or Pt nanoparticles were formed with relatively high yield on graphene‐coated substrates that can reduce these metal ions, such as Ge, Si, GaAs, Al, and Cu. However, metal ions were not reduced on graphene surfaces coated onto non‐reducing substrates such as SiO₂ or ZnO. These results confirm that graphene can be doped by exploiting charge transfer from the underlying substrate; thus graphene is not only transparent with respect to visible light, but also with respect to the charge transfer. (© 2015 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

Local 2PPE-yield enhancement in a defined periodic silver nanodisk array

Well-prepared periodic arrays of silver nanoparticles are investigated by means of linear and non-linear photoemission electron microscopy. The structures show homogeneous photoemission for UV excitation in the linear photoemission regime whereas striking inhomogeneities are mapped in the case of the nonlinear (2 photon) excitation using ultrashort 400 nm laser pulses. A detailed analysis enables to assign these inhomogeneities to defect induced electron momentum transfer processes only effective for the 2 photon excitation process. We propose this mechanism to be of relevance for the appearance of so-called hot spots in nonlinear photoemission as identified in other 2PPE studies in the past. Furthermore, the complementarity between all-optical studies and nonlinear photoemission studies of localized surface plasmons in nanoparticles is discussed.     

Esca. results versus other physical and chemical data

X-ray photoemission spectra yield quantities of very direct interest in physics and chemistry. In this paper the relations of these spectra to other data and concepts are discussed. Both initial-state and final-state properties may be studied: the former are treated first. Charge distributions in molecules alter the effective (Coulomb plus exchange) potential experienced by core electrons in molecular ground states, there by shifting their binding energies. The shifts can be calculated by $\text{ab}$$\text{initio}$ methods or more directly by using potential models based on intermediate-level molecular-orbital theories such as INDO. One version, the ground-state potential model (GPM) yields good predictions of core-level shifts among atoms in similar environments. Alternatively, the measured shifts may be used to derive charges on individual atoms in molecules. It is more difficult to derive charges in solids in this way, but a characteristic splitting in the more tightly-bound valence bands yields a direct measure of ionicity in simple binary compounds of the zinc-blende and rocksalt structures. Atomic orbital composition of molecular orbitals can be deduced from photoemission spectra. In solids such as diamond and graphite comparison of photoemission spectra with x-ray emission spectra yields the atomic-orbital composition of the valence bands. Turning to final-state properties, the spectra are dominated by relaxation effects. Again a simple approach—the relaxation potential model (RPM)—predicts core-level shifts well for cases in which the atomic environments are varied substantially. Among ammonia and the methylamines, for example, the N(ls) shifts are predicted correctly by RPM, while GPM reverses the order. For paramagnetic molecules RPM predicts electron charge transfer toward the positive hole but usually spin transfer away, in agreement with experiment. Extra-atomic relaxation in metals, a many-body effect, is manifest both as a contribution to the binding energy and as line-shape asymmetry. Delocalized valence electrons also show relaxation shifts that can be understood as polari zation of the electron gas toward the “Coulomb hole”. Auger lines show larger relaxation shifts. Comparison of core-level or Auger shifts in nonmetallic solids separately is questionable because there is no reference level, but intercomparison of the two is meaningful. Finally, core-level binding-energy trends in series of simple alcohols, etc., agree quantitatively with proton affinities and core-level shifts in other functional groups. This suggests extending the concept of Lewis basicity to include lone pairs of core electrons. Thus, core-level shifts measure the chemical reactivity—a quantity of great chemical importance that depends on both initial- and final-state properties—rather directly. Rela xation energies are shown to be the dominant cause of trends in the lowest ionization potentials of simple alcohols and amines.