Post deposition purification of PTCDA thin films

The decomposition of perylene-3,4,9,10-tetracarboxylic dianhydride (PTCDA) molecules during evaporation of unpurified raw material in ultra high vacuum was studied. The fragments were identified by mass spectrometry and the influence of these fragments and further contaminations of the raw material on the electronic structure of PTCDA thin films was measured by photoemission spectroscopy. Annealing of contaminated PTCDA films was tested as cheap and easy to perform method for (partial) post deposition purification of the contaminated films.     

Commensurate registry and chemisorption at a hetero-organic interface

We present evidence for a partly chemisorptive bonding between single monolayers of copper-II-phthalocyanine (CuPc) and 3,4,9,10-perylene-tetracarboxylic-dianhydride (PTCDA) that are stacked on Ag(111). A commensurate registry between the two molecular layers and the substrate, i.e., a common crystallographic lattice for CuPc and PTCDA films as well as for the Ag(111) surface, indicates that the growth of the upper layer is dominated by the structure of the lower. Photoemission spectroscopy clearly reveals a gradual filling of the lowest unoccupied molecular orbital of PTCDA due to CuPc adsorption, which proves the chemisorptive character.     

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.     

Optical constants of 3,4,9,10-perylenetetracarboxylic dianhydride films on silicon and gallium arsenide studied by spectroscopic ellipsometry

The optical constants of 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA) films grown by organic molecular beam deposition on Si and GaAs substrates were determined in the spectral range from 300 nm to 1700 nm. All PTCDA layers deposited at room temperature with a low deposition rate of about 0.2 nm/min are uniaxial and strongly optically anisotropic. For the layers on Si a refractive index of 2.21 is derived in the substrate plane at 830 nm. The out-of-plane refractive index has a much lower value of 1.58. A similar anisotropy is observed for PTCDA layers on GaAs. The altogether lower refractive indices of 2.03 and 1.54, however, indicate a lower density of the films, which can be explained by the film structure. Received: 6 November 2000 / Accepted: 10 August 2001 / Published online: 17 October 2001     

Structural analysis of PTCDA monolayers on epitaxial graphene with ultra-high vacuum scanning tunneling microscopy and high-resolution X-ray reflectivity

Epitaxial graphene, grown by thermal decomposition of the SiC (0001) surface, is a promising material for future applications due to its unique and superlative electronic properties. However, the innate chemical passivity of graphene presents challenges for integration with other materials for device applications. Here, we present structural characterization of epitaxial graphene functionalized by the organic semiconductor perylene-3,4,9,10-tetracarboxylic dianhydride (PTCDA). A combination of ultra-high vacuum scanning tunneling microscopy (STM) and high-resolution X-ray reflectivity (XRR) is used to extract lateral and vertical structures of 0, 1, and 2 monolayer (ML) PTCDA on epitaxial graphene. Both Fienup-based phase-retrieval algorithms and model-based least-squares analyses of the XRR data are used to extract an electron density profile that is interpreted in terms of a stacking sequence of molecular layers with specific interlayer spacings. Features in the STM and XRR analysis indicate long-range molecular ordering and weak π–π* interactions binding PTCDA molecules to the graphene surface. The high degree of both lateral and vertical ordering of the self-assembled film demonstrates PTCDA functionalization as a viable route for templating graphene for the growth and deposition of additional materials required for next-generation electronics and sensors.     

衬底对PTCDA薄膜结构与电荷输运特性的影响

在不同导电衬底(Au,Al和ITO)上制备了PTCDA薄膜,用XRD和AFM技术研究了PTCDA薄膜的结构和表面形貌。结果表明,薄膜中的大部分PTCDA分子平面与衬底不平行,这表明薄膜垂直方向的电流传导将以电子传输为主;在ITO和Au衬底上生长的PTCDA薄膜晶粒排列规则,在薄膜垂直方向呈现出较好的电子传输性能;而在Al衬底上生长的PTCDA薄膜晶粒排列无序,电子传输性能差。通过制备单层结构有机薄膜器件,研究了PTCDA薄膜垂直方向的电子迁移率。综合应用金属-有机界面的热电子发射理论和有机层体内空间电荷限制传导理论,并考虑电场强度对迁移率变化的影响,对ITO/PTCDA/Al器件的电流密度-电压曲线进行拟合,得到ITO衬底上生长的PTCDA薄膜在垂直方向随电场强度变化的电子迁移率数值。     

Electronic structure and work function of potassium-doped PTCDA thin films

The electronic properties of thin films of potassium-doped 3,4:9,10-perylenetetracarboxylic dianhydride (PTCDA) have been studied using photoemission spectroscopy. Potassium addition results in a charge transfer to PTCDA molecules as signaled by the evolution of the valence band photoemission data, and the potassium ions mainly interact with the carbonyl end groups of the PTCDA molecules. None of the potassium-doped PTCDA films prepared in our studies is metallic, which is attributed to the effect of electronic correlations. PACS 71.20.Rv; 79.60.-i     

Quasi-one-dimensional K-O chain in PTCDA thin films: evidence from first-principles calculations

Using density functional theory calculations we have found that K atoms in a PTCDA (3,4:9,10-perylenetetracarboxylic dianhydride) crystal form a quasi-one-dimensional (1D) K-O chain interacting with carboxylic oxygen of the terminal anhydride groups of PTCDA. The K-K distance in the chain (3.72 Angstrom) is commensurate to the periodicity of the organic semiconductor. We obtain that the K-O structure is stabilized by charge transfer from K to PTCDA molecules, forming prevalently ionic bonds: the electronic density of the chemistry induced gap states is essentially delocalized on the perylene core of PTCDA, while potassium appears spoiled of its charge. Band dispersion along the direction of molecular stack is evaluated to be 0.2 eV in pure PTCDA crystal and 0.5 eV in the K-doped system, confirming that the interaction occurs between different molecular planes.     

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.     

苝四甲酸二酐的真空升华提纯及其光谱测试与分析

纯度为9.75%的有机半导体材料苝四甲酸二酐(PTCDA),在其升华点进行了真空升华提纯,其纯度可达99.9%。利用质谱、红外光谱及X光电子能谱对这种高纯材料进行了测试并详细分析了其分子结构、化学键的形成、原子在晶格平衡位置的振动模式、电子的组态和原子的结合能的变化。由红外光谱分析得出,苝四甲酸二酐的分子结构是中央5个C构所组成的苝核基团及位于苝核两端的两个酸酐组成,它们主要以共价键结合。晶格上的C原子在其平衡位置主要以伸缩振动为主。其分子中有大量可以自由移动的π电子;分子间离域大π键的交叠决定了苝四甲酸二酐的导电性能。由XPS谱分析得出,高纯度的苝四甲酸二酐中有结合能不同的两种C原子,结合能分别为：285.3和288.7 eV。它们对应于苝环及酸酐上的C原子。另外,有两种类型的O原子,即CO和C—O—C,其结合能分别为531.3和533.1 eV。     

Thin PTCDA films on Si(0 0 1): 2. Electronic structure

We have studied the thin film formation and the electronic structure of the organic molecular semiconductor 3,4,9,10 perylene tetracarboxylic dianhydride (PTCDA), on clean and on hydrogen-passivated Si(0 0 1) surfaces. The studies were made by means of high resolution X-ray photoelectron spectroscopy (HRXPS), angle-resolved photoelectron spectroscopy (ARPES), near edge X-ray absorption fine structure (NEXAFS) and low energy electron diffraction (LEED). On the H passivated surface the changes in the electronic structure of the substrate and the molecules with increasing film thickness are very small. The molecular orbitals show a dispersive behavior, indicating that the PTCDA layers are ordered. On the reactive clean surface the anhydride groups of the molecule interact with the substrate as indicated by changes in the core level binding energies. This results in a much lower ordering in the film compared to PTCDA on a passivated silicon surface. There is no sign of decomposition of the molecule because of the more reactive substrate.     

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.     

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.     

水溶性苝系衍生物荧光探针识别金属离子的光物理机制计算研究

随着人们对荧光化合物电子光谱和光物理行为的深入研究,在利用荧光分子作为探针,检测各种不同体系的状态及其变化等方面都有了巨大的进展。其中,N,N′-二天冬氨酸铵盐-3,4,9,10-四羧酸二酰亚胺(PTCDA)是一种水体环境中选择性好灵敏度高的典型荧光分子探针。本文用密度泛函理论对PTCDA的光物理机制进行研究。计算了PTCDA分子在理想状态下的最优构型,电荷布居和激发光谱。根据计算结果,拟合此苝系衍生物激发态与Cu2+结合前后的吸收光谱,与Cu2+结合前后,吸收光谱峰形相似,加铜后整体吸收峰位发生了红移,有猝灭变色现象。通过与实验值的对比,计算所得分子构型合理有效,激发光谱谱峰位置切合实际。分析得出：PTCDA分子对二价铜离子有较好荧光探测活性,其光信号响应机制属于分子内电荷转移(ICT)机制。当分子接收二价铜离子时,吸收光谱谱峰位置红移,分子内电荷转移方向和强度均发生变化,既有猝灭信号,也有光的颜色变化信号,是一种具有猝灭与变色双信号的荧光探针材料,具有很大的开发潜力。所做工作只是用量子化学计算方法在分子荧光探针领域进行光物理响应机制分析的初步探索,可以为该领域提供系统而有价值的理论参考。     

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.     

Organic heterojunctions of layered perylene and phthalocyanine dyes: characterization with UV-photoelectron spectroscopy and luminescence quenching

We present here the characterization of organic/organic′ heterojunctions created from either of two perylene dyes, perylenetetracarboxylicdianhydride (PTCDA) or the bisimide derivative perylenetetracarboxylicdianhydride-N,N′-bis (butyl)imide (C4-PTCDI), and two chloro-metallated donor phthalocyanines (ClAlPc or ClInPc). The perylene dyes were selected to create thin films with the core of the perylene dye parallel to the substrate plane (PTCDA) or nearly vertical to the substrate plane, with layer planes defined by the butyl substituents (C4-PTCDI). We compare the frontier orbital offsets revealed by UV-photoelectron spectroscopy, and quenching of luminescence of the perylene dyes, as a function of Pc coverage. The ionization potentials (IPs) of the Pc layers, the degree to which interface dipoles are formed at the Pc/perylene dye interface, and the degree of quenching of the perylene luminescence are affected by the structure of the Pc/perylene interface. Pc/PTCDA heterojunctions show significant interface dipoles and higher IPs for the first-deposited Pc layers compared to Pc/C4-PTCDI heterojunctions, which show negligible interface dipoles and lower overall IP values for initial Pc layers. Luminescence of the selectively excited perylene layers is quenched by the addition of even submonolayer coverages of Pc. This quenching process occurs as a result of both energy transfer (perylene to Pc) and charge transfer (Pc to perylene). Luminescence from monomeric and aggregated ClAlPc and ClInPc monolayers is seen on C4-PTCDI films, whereas only luminescence from the aggregated forms of these Pcs is seen on PTCDA films. These studies reveal aspects of organic heterojunction energetics which may have important implications for organic solar cell design.     

Growth and characterization of thin PTCDA films on 3C-SiC(0 0 1)c(2 × 2)

The growth of thin 3,4,9,10-perylene tetracarboxylic dianhydride (PTCDA) films on a 3C-SiC(0 0 1)c(2 × 2) substrate has been studied by means of photoelectron spectroscopy (PES) and atomic force microscopy (AFM). In the first monolayer the molecules interact with the substrate mainly through the O atoms in the end groups of the molecule. The O atoms have a higher binding energy in the first molecular layer compared to the following layers. No chemical shifts are observed in the Si 2p spectra or in the C 1s spectra from the perylene core of the molecules. From the VB spectra and LEED pattern we conclude that the substrate remains in the c(2 × 2) reconstruction after PTCDA deposition. For thicker films a Stranski-Krastanov film growth was observed with flat lying molecules relative to the substrate.     

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.