Delocalized π state between molecules through a surface confined pseudodihydrogen bond

When 3,4,9,10-perylene-tetracarboxylic-dianhydride (PTCDA) and coronene molecules coadsorb on the Ag(111) surface, one-dimensional PTCDA molecular oligomers with efficient electronic connection via noncovalent bonds are observed by low temperature scanning tunneling microscopy. Density functional theory calculations indicate the neighboring PTCDA molecules form oligomers due to strong PTCDA-metal interactions, which result in overlapping of π orbitals and pseudodihydrogen surface bonds between molecules. Our results provide a potential approach for electron transport from molecule to molecule directly through noncovalent bond.     

Characterization of ultra-thin organic hetero-interfaces — SnPc/PTCDA/Ag(111)

Ultra-thin organic hetero-layers consisting of tin-phthalocyanine (SnPc) and 1,3,4,8-perylen-tetracarboxylicacid dianhydride (PTCDA) adsorbed on a Ag(111) surface are characterized with photoelectron spectroscopy (PES) and near edge X-ray absorption fine structure (NEXAFS) spectroscopy. If SnPc is deposited on the Ag(111) substrate, which is precovered with one monolayer of PTCDA, a well defined interface is formed with a closed SnPc wetting layer as can be derived from angle dependent core level and from valence photoelectron spectra. Moreover, X-ray absorption data show that the molecules are lying flat and that the bonding at the SnPc/PTCDA interface is weak.     

Supramolecular architecture of organic molecules: PTCDA and CuPc on a Cu(1 1 1) substrate

We report on the molecular self-assembly of a highly ordered mixed molecular structure of copper-phthalocyanine (CuPc) and 3,4,9,10-perylene-tetracarboxylic-dianhydride (PTCDA) on Cu(1 1 1) starting from dense PTCDA domains. Low temperature scanning tunneling microscopy (LT-STM) data reveal the surface structure as the CuPc coverage is stepwise increased. A highly ordered mixed phase as well as a disordered mixed phase can be found, where the highly ordered mixed phase seems to be thermodynamically favored. Low energy electron diffraction (LEED) measurements aid the STM analysis. Tentative models for the formation of the highly ordered mixed phase are given.     

Organic Light Emitting Diodes with an Organic Acceptor/Donor Interface Involved in Hole Injection

Organic light emitting diodes with an interface of organic acceptor 3-, 4-, 9-,10-perylenetetracarboxylic dianhydride （PTCDA） and donor copper phthalocyanine （CuPc） involved in hole injection are fabricated. As compared to the conventional device using a 5 nm CuPc hole injection layer, the device using an interface of 10nm PTCDA and 5 nm CuPc layers shows much lower operating voltage with an increase of about 46% in the maximum power efficiency. The enhanced device performance is attributed to the efficient hole generation at the PTCDA/CuPc interface. This study provides a new way of designing hole injection.     

Change in surface states of Ag(111) thin films upon adsorption of a monolayer of PTCDA organic molecules

The change in the electronic structure of silver thin films of different thicknesses with the Ag( 111) orientation due to the interaction with an adsorbed monolayer of ordered organic molecules of 3,4,9,10-perylene-tetracarboxylic acid dianhydride (PTCDA) has been investigated in terms of density functional theory. It has been shown that one of the two surface states of the pure films transforms into an unocc upied interface state due to the interaction so that all the main features of the initial state are retained. The relation of the resulting state to the unoccupied state experimentally observed in the PTCDA/Ag( 111 ) system by scanning tunneling and two-photon photoemi ssion spectroscopy has been discussed.     

Time-resolved measurements of electron transfer processes at the PTCDA/Ag(111) interface

The electron transfer processes at the interface between 3,4,9,10-perylene-tetracarboxylic acid dianhydride (PTCDA) and Ag(111) have been studied using time- and angle-resolved two-photon photo-emission (2PPE). For this system a dispersing unoccupied interface state can be identified that is located 0.6 eV above the Fermi level with an effective electron mass of 0.39 m_e at the [`(G)]\overline{\Gamma}-point. The lifetime of 54 fs for the interface state is relatively short indicating a large penetration of the wavefunction 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 substrate due to the interaction with the PTCDA overlayer. Coverage dependent measurements show a second long-lived component in the time-resolved measurements for higher PTCDA coverages that can be associated to charge transfer processes from the PTCDA multilayers into the metal substrate. Additionally the influence of the PTCDA adlayers on the image-potential states is studied indicating that the n = 1 image-potential state is localized in the first two monolayers of the PTCDA film.     

IR and SFM study of PTCDA thin films on different substrates

FT-IR spectroscopy and SFM were used to investigate the growth of thin films of the organic semiconductor 3,4,9,10-perylenetetracarboxylicdianhydride (PTCDA) deposited by vacuum sublimation onto various substrates, i.e. Ag(111) layers on mica, KBr(100), mica, oxidized Si, and TiO₂ nanoparticles on Si. Layer thicknesses of PTCDA varied from 10 to 1500 nm.The anhydride vibrations of PTCDA differ for the used substrates, which can be connected to the orientation of the molecules relative to the substrate surface and the film morphology as detected in the SFM pictures.     

Electron lifetime in a Shockley-type metal-organic interface state

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.     

Chemical bonding of PTCDA on Ag surfaces and the formation of interface states

We present a detailed investigation of the interface bonding of 3,4,9,10-perylene-tetracarboxylic acid dianhydride (PTCDA) on Ag(1 1 1) and Ag(1 1 0) surfaces by a combination of structural and electronic techniques (SPA-LEED, STM, TPD, UPS, HR-XPS, and NEXAFS) thus obtaining a consistent picture of the adsorption behaviour of PTCDA/Ag in the monolayer regime. The interaction with silver is strong and leads to the formation of new common hybrid orbitals in the monolayer, which are interface states for PTCDA films on Ag, involving at least LUMO, HOMO, and HOMO-1, and the Ag 5s- and 4d-states. This chemisorption is based on a covalent interaction between metal and molecular states, and can unambiguously be distinguished from mere van-der-Waals bonding.     

Formation of sharp metal-organic semiconductor interfaces: Ag and Sn on CuPc

A detailed investigation of the chemistry and electronic structure during the formation of the interfaces between thin films of the archetypal organic molecular semiconductor copper phthalocyanine (CuPc) and Ag or Sn deposited on it was performed using photoemission and near-edge X-ray absorption spectroscopies with synchrotron light. Our study demonstrates the formation of sharp, abrupt interfaces, a behavior which is of particular importance for applications in organic devices. Moreover, for Ag on CuPc we demonstrate that this interface is free from any reaction, whereas there is slight interface reaction for Sn/CuPc.     

Scanning tunneling microscopy observation of copper-phthalocyanine molecules on Si(100) and Si(111) surfaces

Molecules of Cu-phthalocyanine (CuPc) deposited on Si(100) and Si(111) surfaces have been observed by an ultra high vacuum field ion scanning tunneling microscope (FI-STM). On a Si(100) surface, STM images with four-fold symmetry are observed, which reflect the shape of the CuPc molecule. The STM pictures show that CuPc molecules are deposited with the molecular plane parallel to the substrate surface and have three kinds of adsorption configurations on the dimer-row of Si(100). The images of the CuPc are modified by the electronic state of the Si(100) surface. This behavior suggests strong interaction between the molecule and the substrate. The molecular images on the Si(111) surface have a unique bias-voltage dependence. At a sample bias of 1.6 V, the molecule looks transparent by STM, and becomes dark like a vacancy at 1.2 V. From the bias dependence, the electronic interaction between the CuPc molecule and the Si surface is discussed.     

Interaction of metals with an organic semiconductor: Ag and In on PTCDA

The interaction of Ag and In with a thin film of 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA) was studied by near-edge X-ray absorption fine structure (NEXAFS). Upon Ag deposition on a PTCDA film of 20 nm thickness the relative intensities and lineshapes, as well as the angular dependence of the spectra remains unchanged, illustrating the formation of a chemically unreactive Ag/PTCDA interface. On the other hand, the adsorption of 0.3 nm In strongly decreases the intensity of the π^* resonances in C and O K-edge NEXAFS spectra. This is attributed to a strong charge transfer between In and PTCDA, leading to a redistribution of the charge in the molecule. However, the absence of a strong shift or new features and negligible dependence of peak intensities corresponding to π^* resonances on the In thickness indicate that the interaction between In and PTCDA is not accompanied by a covalent bond formation.     

The growth and chemisorptive propertles of Ag and Au monolayers on platinum single crystal surfaces: An AES,TDS and LEED study

The growth and chemisorptive properties of monolayer films of Ag and Au deposited on both the Pt(111) and the stepped Pt(553) surfaces were studied using Auger electron spectroscopy (AES), thermal desorption spectroscopy (TDS), and low energy electron diffraction (LEED). AES studies indicate that the growth of Au on Pt(111) and Pt(553) and Ag on Pt(111) proceeds via a Stranski-Krastanov mechanism, whereas the growth of Ag on the Pt(553) surface follows a Volmer-Weber mechanism. Au dissolves into the Pt crystal bulk at temperatures > 800 K, whereas Ag desorbs at temperatures > 900 K. TDS studies of Ag-covered Pt surfaces indicate that the AgPt bond (283 kJ mol^?1) is ～25 kJ mol^?1 stronger than the AgAg bond (254 kJ mol^?1). On the Pt(553) surface the Au atoms are uniformly distributed between terrace and step sites, but Ag preferentially segregates to the terraces. The decrease in CO adsorption on the Pt crystal surfaces is in direct proportion to the Ag or Au coverage. No CO adsorption could be detected for Ag or Au coverages above one monolayer at 300 K and 10^?8 Torr. The heat of adsorption of CO on Pt is unaltered by the presence of Ag or Au.     

Modification of the electronic properties of the TiO2 (1 1 0) surface upon deposition of the ultrathin conjugated organic layers

A few nm thick 3,4,9,10-perylenetetracarboxylic acid dianhydride (PTCDA) and Cu-phthalocyanine (CuPc) overlayers were thermally deposited in situ in UHV onto TiO₂ (1 1 0) surface. Atomic composition of the surfaces under study was monitored using Auger electron spectroscopy (AES). The formation of the interfacial potential barrier and the structure of the unoccupied electronic states located 5-25 eV above the Fermi level (E_F) was monitored using a probing beam of low-energy electrons according to the total current electron spectroscopy (TCS) method. The work function values upon the overlayer deposition changed from 4.6 to 4.9 eV at the PTCDA/TiO₂ (1 1 0) interface and from 4.6 to 4.3 eV at the CuPc/TiO₂ (1 1 0) interface. Band bending in the TiO₂ substrate, molecular polarization in the organic film and changes in the work function due to the change in the surface composition were found to contribute to the formation of the interfacial potential barriers. Oxygen admixture related peaks were observed in the AES and in the TCS spectra of the CuPc overlayers. A mechanism of the transformations in the PTCDA and CuPc overlayers on the TiO₂ (1 1 0) upon elevating temperature from 25 to 400 °C was suggested.     

Barrier height engineering of Ag/GaAs(100) Schottky contacts by a thin organic interlayer

Thin films of 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA) were used as an interlayer for the electronic modification of Ag/n-GaAs(100) Schottky contacts. The electronic properties were investigated recording in situ current–voltage (I–V) and capacitance–voltage (C–V) characteristics. For H-plasma treated substrates the effective barrier height decreases from 0.81 to 0.64 eV as a function of the PTCDA layer thickness (d_PTCDA). In the case of the sulphur passivated GaAs the effective barrier height first increases and then decreases, the overall range being 0.54–0.73 eV. The substrate treatment leads to a different alignment between the band edges of the GaAs and the molecular orbitals of the PTCDA, making it possible to determine the energy position of the LUMO transport level.     

Organic light emitting diodes using magnesium doped organic acceptor as electron injection layer and silver as cathode

Organic light emitting diodes employing magnesium doped electron acceptor 3, 4, 9, 10 perylenetetracarboxylic dianhydride (Mg:PTCDA) as electron injection layer and silver as cathode were demonstrated. As compared to Mg:Ag cathode, the combination of the Mg:PTCDA layer and silver provided enhanced electron injection into tris (8-quinolinolato) aluminium. The device with 1:2 Mg:PTCDA and Ag showed an increase of about 12% in the maximum current efficiency, mainly due to the improved hole-electron balance, and an increase of about 28% in the maximum power efficiency, as compared to the control device using Mg:Ag cathode. The properties of Mg:PTCDA composites were studied as well.     

Ag(110)表面吸附酞菁铜分子的紫外光电子谱研究

用紫外光电子能谱(UPS)研究了酞菁铜分子在Ag(110)单晶表面上的吸附,随着酞菁铜分子覆盖度增加,衬底Ag的3d电子信号逐渐减弱,在此能带区域出现两个新的谱峰,这两个与吸附有机分子轨道有关的谱峰的束缚能分别为4.45 和6.36 eV.随着覆盖度的增加,在结合能为1.51和9.20 eV处又出现了两个谱峰,它们同样来自吸附有机分子的轨道.随着覆盖度的继续增加,上述四个谱峰的强度逐渐增加,其能量位置均发生了明显的偏移.根据角分辨光电子能谱的实验结果,酞菁铜分子的分子平面基本与衬底表面平行.密度泛函理论计 关键词： 酞菁铜 紫外光电子谱 吸附电子态 密度泛函理论     

Role of intermolecular interactions on the electronic and geometric structure of a large pi-conjugated molecule adsorbed on a metal surface

The organic semiconductor molecule 3,4,9,10-perylene-tetracarboxylic-dianhydride (PTCDA) exhibits two adsorption states on the Ag(111) surface: one in a metastable disordered phase, prepared at low temperatures, the other in the long-range ordered monolayer phase obtained at room temperature. Notably, the two states differ substantial in their vertical bonding distances, intramolecular distortions, and electronic structures. The difference is explained by intermolecular interactions, which are particularly relevant for the long-range ordered phase, and which hence require attention.     

Substrate-interaction,long-range order,and epitaxy of large organic adsorbates

Large and symmetric organic molecules (>200 amu) can form highly-ordered adsorbate layers and thin films when they are deposited by vacuum sublimation on clean reactive surfaces. In such cases covalent bonding often occurs via the molecular -system leading to a parallel orientation of the adsorbate as shown for oligothiophenes and PTCDA on Ag(1 1 1). A proper choice of the substrate and/or a preadsorbate may also cause an upright orientation with bonding via a reactive group of the molecule (example: NDCA/Ni(1 11)). Most of the used molecules yield long-range ordered monolayers with large, almost defect-free domains. The stronger the bonding and the smaller the molecule the more likely is the formation of commensurate superstructures which indicate site-specific adsorption even for such large molecules as PTCDA or EC4T. Organic epitaxy is discussed and shown for a particular system, PTCDA on Ag(1 1 1), for which the structure of the monolayer is nearly identical to that of the-modification of PTCDA crystals, whereas on other substrates (e.g. Si(1 1 1), Ge(1 0 0)) a disordered interface and hence no true epitaxy is found.