Image potential states at metal-dielectric interfaces

Merry, Jr

doi:10.2172/7069868

Cited by 3 publications

(3 citation statements)

References 36 publications

(47 reference statements)

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“…The original design of the system UHV systems is explained in more detail in the thesis of Walt Merry 42 . The photoelectron detection system is described most thoroughly in Jason McNeil's thesis 43 .…”

Section: Experimental Apparatusmentioning

confidence: 99%

Watching Electrons Transfer from Metals to Insulators using Two Photon Photoemission

Johns

2010

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Ultrafast angle-resolved two photon photoemission was used to study the dynamics and interfacial band structure of ultrathin films adsorbed onto Ag(111). Studies focused on the image potential state (IPS) in each system as a probe for measuring changes in electronic behavior in differing environments.The energetics and dynamics of the IPS at the toluene/Ag(111) interface are strongly dependent upon coverage. For a single monolayer, the first IPS is bound by 0.81 eV below the vacuum level and has a lifetime of 50 femtoseconds (fs). Further adsorption of toluene creates islands of toluene with an exposed wetting layer underneath. The IPS is then split into two peaks, one corresponding to the islands and one corresponding to the monolayer. The wetting layer IPS shows the same dynamics as the monolayer, while the lifetime of the islands increases exponentially with increasing thickness. Furthermore, the island IPS transitions from delocalized to localized within 500 fs, and electrons with larger parallel momenta decay much faster. Attempts were made using a stochastic model to extract the rates of localization and intraband cooling at differing momenta.In sexithiophene (6T) and dihexyl-sexithiophene (DH6T), the IPS was used as a probe to see if the nuclear motion of spectating side chains can interfere with molecular conduction. The energy and band mass of the IPS was measured for 6T and two geometries of DH6T on Ag(111). Electrons injected into the thicker coverages of DH6T grew exponentially heavier until they were completely localized by 230 fs, while those injected into 6T remained nearly free electron like. Based off of lifetime arguments and the density of defects, the most likely cause for the mass enhancement of the IPS in this system is small polaron formation caused by coupling of the electron to vibrations of the alkyl substituents. The energetic relaxation of the molecular adsorbate was also measured to be 20 meV/100 fs for the DH6T, and 0 meV/100 fs for the 6T. This relaxation is consistent with the localization of the charge creating a barrier for it moving from one lattice site to a neighboring one.Finally, the IPS was used to study the evolution of the surface band gap at the Mg/Ag(111) interface. The Mg(0001) surface band gap lies 1.6 eV below the Fermi level, and consequently shows no peak in the projected density of states for the IPS. A method for creating layer by layer growth of Mg on Ag(111) was determined using Auger Spectroscopy and low energy electron diffraction. By monitoring the decay of the 1 2 intensity of the IPS versus coverage, it was determined that four layers of magnesium on Ag(111) is sufficient to completely eliminate the surface band gap

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Section: Experimental Apparatusmentioning

confidence: 99%

Watching Electrons Transfer from Metals to Insulators using Two Photon Photoemission

Johns

2010

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“…The cover limited the photoelectron angular acceptance range. Further information about the construction of the detector may be found in Walt Merry's thesis [30]. Replacing the MCPs should only be accomplished with Walt Merry's 'jig,' which substantially reduces the peril of breaking these fragile wafers.…”

Section: Ultra-high Vacuum Chambermentioning

confidence: 99%

“…While the most authoritative treatments of the detection electronics remain Walt Merry's and Jason McNeill's theses [30,34], the unsettling spate of troubleshooting that has oc- density is more expensive, but would allow higher voltages to be applied and presumably more signal to be collected. The MCPs are advertised as suitable for mass spectrometery at high vacuum, but several factors will degrade their performance.…”

Section: Detection Electronicsmentioning

confidence: 99%

Two-photon Photoemission of Organic Semiconductor Molecules on Ag(111)

Yang

2008

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Angle-and time-resolved two-photon photoemission (2PPE) was used to study systems of organic semiconductors on Ag(111). The 2PPE studies focused on electronic behavior specific to interfaces and ultrathin films. Electron time dynamics and band dispersions were characterized for ultrathin films of a prototypical n-type planar aromatic hydrocarbon, PTCDA, and representatives from a family of p-type oligothiophenes.In PTCDA, electronic behavior was correlated with film morphology and growth modes.Within a few monolayers of the interface, image potential states and a LUMO+1 state were detected. The degree to which the LUMO+1 state exhibited a band mass less than a free electron mass depended on the crystallinity of the layer. Similarly, image potential states were measured to have free electron-like effective masses on ordered surfaces, and the effective masses increased with disorder within the thin film. Electron lifetimes were correlated with film growth modes, such that the lifetimes of electrons excited into systems created by layer-by-layer, amorphous film growth increased by orders of magnitude by only a few monolayers from the surface. Conversely, the decay dynamics of electrons in Stranski-Krastanov systems were limited by interaction with the exposed wetting layer, which limited the barrier to decay back into the metal.Oligothiophenes including monothiophene, quaterthiophene, and sexithiophene were deposited on Ag(111), and their electronic energy levels and effective masses were studied as a function of oligothiophene length. The energy gap between HOMO and LUMO decreased with increasing chain length, but effective mass was found to depend on domains 2 from high-or low-temperature growth conditions rather than chain length. In addition, the geometry of the molecule on the surface, e.g., tilted or planar, substantially affected the electronic structure.

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