Two-Photon Photoemission of Ultrathin Film PTCDA Morphologies on Ag(111)

Yang, Aram; Shipman, Steven T.; Garrett-Roe, Sean; Johns, James E.; Strader, Matt; Szymanski, Paul; Muller, Eric A.; Harris, Charles B.

doi:10.1021/jp076632q

Cited by 36 publications

(39 citation statements)

References 42 publications

Supporting

Mentioning

Contrasting

Order By: Relevance

“…Energetic ordering of the states was determined by changing the wavelength of the probe pulse and measuring the change in photoelectron energy. The increased binding of the n=1 image potential state at the 6T/Ag(111) interface compared to the clean silver binding energy of -.77 eV is consistent with other organic semiconductor metal interfaces that have a high electron affinity (> 2 eV) 30,63,64 . Increased coverage had a negligible affect on the workfunction or binding energy of the n=1 peak.…”

Section: Resultssupporting

confidence: 79%

Watching Electrons Transfer from Metals to Insulators using Two Photon Photoemission

Johns

2010

View full text Add to dashboard Cite

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

show abstract

Section: Resultssupporting

confidence: 79%

Watching Electrons Transfer from Metals to Insulators using Two Photon Photoemission

Johns

2010

View full text Add to dashboard Cite

show abstract

“…10,11 It originates from the Shockley state of the Ag(111) surface which is upshifted from below the metallic Fermi level by as much as 0.7 eV. The initially partially occupied state becomes unoccupied and approaches the conduction band of the organic semiconductor.…”

mentioning

confidence: 99%

Energy shift and wave function overlap of metal-organic interface states

et al. 2011

View full text Add to dashboard Cite

The properties of Shockley-type interface states between π -conjugated organic molecular layers and metal surfaces are investigated by time-resolved two-photon photoemission experiments and density functional theory. For perylene-and naphthalene-tetracarboxylic acid dianhydride (PTCDA and NTCDA) adsorbed on Ag(111), a common mechanism of formation of the interface state from the partly occupied surface state of the bare Ag (111) is revealed. The energy position is found to be strongly dependent on the distance of the molecular carbon rings from the metal and their surface density. Bending of the carboxyl groups enhances the molecular overlap of the interface state. The energetic position and wave function overlap of electronic states at the interface between layers of organic molecules and metals is of fundamental interest for the design of organic semiconductor devices and for future applications of molecular electronics. Previous studies, both experimentally and theoretically, concentrated on those electronic states that either result directly from the chemical bonding at the interface or from the shift and broadening of localized molecular orbitals upon interaction with the metal substrate.1-8 However, also states intrinsic to metal surfaces are affected by adsorption and can become an important factor for the electronic coupling between metal and organic molecules. This has recently become apparent for 3,4,9,10-perylene-tetracarboxylicacid-dianhydride (PTCDA) on Ag(111), a structurally very well characterized model system for the interface between π -conjugated organic molecular layers and metals.4 A dispersing, free-electron-like electronic state located 0.6 eV above the Fermi level was observed by tunneling spectroscopy 9 and by two-photon photoemission (2PPE). 10,11 It originates from the Shockley state of the Ag(111) surface which is upshifted from below the metallic Fermi level by as much as 0.7 eV. The initially partially occupied state becomes unoccupied and approaches the conduction band of the organic semiconductor. [11][12][13] In this Rapid Communication we address the question which properties of the molecule-surface interaction determine the energetic position of such Shockley-type interface states (IS) and which factors facilitate a large overlap of the state with the molecular layer in order to tailor the degree of electronic coupling. For rare-gas adsorbates on noble metals it is well established that their adsorption also leads to an upshift of the Shockley state, albeit by much smaller values, and that the shift increases systematically from the weakly bound Ar to the more strongly interacting Xe.14,15 The comparison of our 2PPE experiments for 1,4,5,8-naphthalene-tetracarboxylic acid dianhydride (NTCDA) on Ag(111) with PTCDA/Ag(111) shows an analogous trend. The structurally similar, but smaller NTCDA molecules cause a smaller IS upshift than the larger PTCDA molecules. The time-resolved 2PPE data indicate a similar overlap of the wave function with the metal in both cases. Density functio...

show abstract

“…They reported that its effective mass depends on specific preparation conditions and interpreted this in terms of a varying overlap of the LUMO þ 1 level of neighboring molecules [7].…”

mentioning

confidence: 99%

Electron Lifetime in a Shockley-Type Metal-Organic Interface State

et al. 2008

View full text Add to dashboard Cite

The lifetimes of electrons at the interface between 3,4,9,10-perylene-tetracarboxylic acid dianhydride (PTCDA) and Ag(111) have been studied by means of time- and angle-resolved two-photon photoemission. We observe a dispersing unoccupied state 0.6 eV above the Fermi level with an effective electron mass of 0.39m{e} at the Gamma[over ] point. The short lifetime of 54 fs is indicative of a large penetration of the wave function into the metal. Supported by model calculations this interface state is interpreted as predominantly arising from an upshift of the occupied Shockley surface state of the clean metal due to the interaction with the PTCDA overlayer.

show abstract

Two-Photon Photoemission of Ultrathin Film PTCDA Morphologies on Ag(111)

Cited by 36 publications

References 42 publications

Watching Electrons Transfer from Metals to Insulators using Two Photon Photoemission

Watching Electrons Transfer from Metals to Insulators using Two Photon Photoemission

Energy shift and wave function overlap of metal-organic interface states

Electron Lifetime in a Shockley-Type Metal-Organic Interface State

Contact Info

Product

Resources

About