Photoemission time-of-flight spectromicroscopy of Ag nanoparticle films on Si(111)

Cinchetti, Mirko; Valdaitsev, D. A.; Gloskovskii, A.; Oelsner, A.; Nepijko, S. A.; Schönhense, G.

doi:10.1016/j.elspec.2004.02.052

Cited by 27 publications

(26 citation statements)

References 32 publications

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“…21 (The model holds for Ag NPs until they are smaller than 1.5 nm, where a steep increase in WF arises from quantum size effects. 18,25 ) In our case, the ligands capping our colloidal NPs appear to reverse and magnify the size trend observed in bare Ag NPs. Though our WF curve looks like there is an exponential increase, the WF is not expected to increase much more than the value for the 75 nm batch.…”

Section: ■ Results and Discussionmentioning

confidence: 64%

Energy States of Ligand Capped Ag Nanoparticles: Relating Surface Plasmon Resonance to Work Function

et al. 2014

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The work function (WF) and surface plasmon resonance (SPR) of organic ligand capped Ag nanoparticles (NPs) have been studied experimentally and computationally. Experimental observations reveal a significant increase in WF as the size of ligand-capped Ag NPs increases, a trend contrary to that previously observed for bare Ag NPs. Computational results confirm the effect on the WF from simplified ligand molecules and relate it to charge transfer between the Ag core and surrounding ligands. We also observe a possible coupling between increases in WF and decreases in SPR transition energy, supported by computational results and attributed to the interplay between the 4d and 5s electron states of the system. These results, along with our observations of WF dependence on ligand choice, indicate the ability to strongly engineer the electronic structure of metal NPs through size and ligand control. ■ INTRODUCTIONMetal nanoparticles (NPs) have recently attracted significant attention due to their promise for a growing number of applications, including biomedicine, 1 catalysis, 2 optical sensors, 3,4 and surface-enhanced Raman spectroscopy. 5 However, better understanding and control of the electron energy structure of these particles are needed to realize this promise. For example, in electrical devices correct band structure is essential for efficient charge transport, and in electro-catalysis the ease of charge extraction from surfaces is a major determiner of reaction rates. These properties are reflected in the work function (WF) of the material system employed. In many frontier applications, such as LEDs, PVs, and FETs, the NPs used are typically capped by ligands, which modify the electron energy levels at the metal−organic interface. 6,7 Better understanding of the effect of capping ligands is needed in order to exploit the benefits offered by NPs. Additionally, the burgeoning field of plasmonics depends on the nature of the surface plasmon resonances (SPRs) supported by NPs. SPRs stem from the physical interaction between light and metal NPs, which induces a collective oscillation of the conduction electrons due to the presence of an electromagnetic field. 8−10 This interaction is strongly affected by the presence of capping ligands, and much remains to be learned about the nature of their influence.In this paper, we report observations of trends in the WF and SPR of Ag NPs capped by different ligands as a function of NP size and ligand type. WF measurements were taken using ultraviolet photoelectron spectroscopy (UPS), and SPR measurements were taken using ultraviolet−visible spectroscopy (UV−vis) and electron energy loss spectroscopy (EELS) by scanning transmission electron microscopy (STEM). Additionally, we present the results of a simplified ab initio quantum model of the systems under study, gleaning insight into the trends observed. Our experimental and simulated results show consistent and correlated trends in the WF and SPR of Ag NPs. ■ EXPERIMENTAL METHODSWF and SPR Measurements. WF and SPR measurements...

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Section: ■ Results and Discussionmentioning

confidence: 64%

Energy States of Ligand Capped Ag Nanoparticles: Relating Surface Plasmon Resonance to Work Function

et al. 2014

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“…Using the same experimental technique, we have been previously able to localize regions on inhomogeneous Ag and Cu surfaces where laser illumination excites collective electron modes, or localized surface plasmons (LSP). Laterally resolved electron energy distribution spectra have shown that the LSP-induced enhanced near field affects the photoemission and its dynamics in a crucial way [25,26]. In these experiments the structural features giving rise to the locally enhanced photoemission yield were not known and remained speculative.…”

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confidence: 99%

Photoemission Electron Microscopy as a Tool for the Investigation of Optical Near Fields

et al. 2005

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Photoemission electron microscopy was used to image the electrons photoemitted from specially tailored Ag nanoparticles deposited on a Si substrate (with its native oxide SiO x ). Photoemission was induced by illumination with a Hg UV-lamp (photon energy cutoffhω U V = 5.0 eV, wavelength λ U V = 250 nm) and with a Ti:Sapphire femtosecond laser (hω l = 3.1 eV, λ l = 400 nm, pulse width below 200 fs), respectively. While homogeneous photoelectron emission from the metal is observed upon illumination at energies above the silver plasmon frequency, at lower photon energies the emission is localized at tips of the structure. This is interpreted as a signature of the local electrical field therefore providing a tool to map the optical near field with the resolution of emission electron microscopy.

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“…It is characterized by a nonmonotonic size dependence [22][23][24]. Figure 6 shows the electron emission current from a silver film with a wedge-shaped thickness profile excited by a high-pressure mercury lamp (photon energy cutoff hn = 5.2 eV corresponding to wavelength l = 250 nm) [22].…”

Section: Photon Excitationmentioning

confidence: 99%

“…10 is the brightest as well). The spectral dependence is determined by parameters of the nanoparticle film and of the excitation [22][23][24]. Depending on the excitation mechanism, the intensity of photoemission may be several orders of magnitude larger (see Figs.…”

Section: Photon Excitationmentioning

confidence: 99%

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Emission Properties of Metal Nanoparticles

Nepijko¹,

Elmers²,

Schönhense³

2015

Handbook of Nanoparticles

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Nanoparticles emit electrons and photons when they are excited by electron injection via electric current; electromagnetic radiation via microwave fields; laser radiation in infrared, visible and synchrotron (X-ray) ranges; and electron and ion bombardment. In each case, the emission mechanism depends on characteristic length scales of the nanoparticle. If the particle size is commensurate with it or is smaller, a size dependence of emission is observed [1][2][3]. Also, it is interesting that nanoparticles may demonstrate properties absent in bulk material. Electron Emission from Metal Nanoparticles Excitation by Electron InjectionExcitation by electron injection into nanoparticles via electron tunneling (electric current flow through an ensemble of tunnel-coupled metal nanoparticles on a dielectric substrate) is accompanied by electron emission [1][2][3][4]. Electron emission is observed as soon as the conductivity deviates from an ohmic behavior. Corresponding experiments are realized "in situ", e. g., when an ensemble of gold nanoparticles is deposited onto an insulating substrate directly in the column of a transmission electron microscope [5]. The main discussion in the literature concerning the mechanism of electron emission is connected with field emission [6] and electron gas heating [7]. In the first case, it is supposed that an electric field is strongly enhanced near nanoparticles and thus sufficient for field emission because of the small radius of curvature of nanoparticles. In the second case, the electron gas heating results from an attenuation of the electron-phonon interaction in nanoparticles. If the size is much smaller than the electron mean free path, the electrons execute a periodic motion within the particle without volume scattering and are reflected specularly from the boundaries. This oscillation has the frequency o ¼ ). The larger the frequency difference for electron and phonon subsystems, the less they exchange energy with one another. Experimental studies show that the electron-phonon interaction constant observed in ten nanometer-sized gold particles is several orders of magnitude lower than in the case of bulk material [8]. The energy fed into a nanoparticle under current excitation is absorbed by its electron subsystem. If the electron-phonon interaction is sufficiently attenuated, the electron subsystem temperature increases and electrons become heated, whereas the lattice remains relatively cold. The electron emission takes place under such conditions. Electron emission provoked by electron transport has been visualized using emission electron microscopy (EEM) [9][10][11]. Figure 1a-d shows a series of EEM images of a silver nanoparticle film (caesium was used to reduce the work function). The images have been acquired at different voltages applied between the contacts U f = 0 (a), 8 (b), 10 (c), and 11 V (d). The gap with a silver nanoparticle film between the silver contacts is 5 mm. Nanoparticle emissivity considerably differs because of the spread of the particle

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Photoemission time-of-flight spectromicroscopy of Ag nanoparticle films on Si(111)

Cited by 27 publications

References 32 publications

Energy States of Ligand Capped Ag Nanoparticles: Relating Surface Plasmon Resonance to Work Function

Energy States of Ligand Capped Ag Nanoparticles: Relating Surface Plasmon Resonance to Work Function

Photoemission Electron Microscopy as a Tool for the Investigation of Optical Near Fields

Emission Properties of Metal Nanoparticles

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