STM characterisation of organic molecules on H-terminated Si(111)

We present the first high resolution STM images of organic molecules on the technological important hydrogen terminated silicon surface. Ordered layers of PTCDA and PTCDI were prepared on this surface by organic molecular beam epitaxy. The submolecular contrast of these molecules on Si(111)/H obtained in the high resolution images agrees with the corresponding images on HOPG and MoS₂ substrates.     

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     

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.     

有机分子PTCDA在P型Si单晶(100)晶面的生长机理

对PTCDA的分子结构及其化学键的形成进行了分析,并讨论了晶面指数（100）Si单晶的晶格结构。在此基础上,评述了PTCDA分子在P-Si单晶（100）晶面上生长的机理,并制备了样品PTCDA/P-Si（100）。利用XRD对样品测试得出,在P-Si（100）晶面上沉积的PTCDA薄膜中仅存在α物相。利用XPS对样品测试得出,在其界面层中PTCDA酸酐中的4个羟基O原子与C原子结合,其结合能为532.4 eV;苝核基团外围的8个C、H原子以共价键结合,其结合能为289.0 eV;在界面处,悬挂键上的Si原子与PTCDA酸酐中的C、O原子结合,形成C-Si-O键及C-Si键,构成了界面层的稳定结构。     

Self-assembly of trimetallic nitride template fullerenes on surfaces studied by STM

Trimetallic nitride template fullerenes have been deposited onto a variety of substrates in order to elucidate the substrate-fullerene interactions. We have investigated self-assembled island formation and molecular detail of Er₃N@C₈₀ and Sc₃N@C₈₀ on Ag/Si(1 1 1), Au(1 1 1)/mica, Si(1 1 1), and Si(0 0 1) using variable temperature scanning tunnelling microscopy (STM). At room temperature, the fullerenes self-assemble into monolayer-high hexagonal close-packed islands on Ag-passivated Si(1 1 1) whereas annealing at elevated temperatures (250-300 °C) is necessary for the self-assembly of close-packed islands on Au(1 1 1). Intra-molecular resolution of the fullerenes has been achieved at liquid nitrogen temperature on Ag/Si(1 1 1) and already at room temperature on Si(0 0 1), when the rotation of the fullerenes is frozen. Whereas the bonding between the fullerenes and Si surfaces is mainly covalent, it appears to be mainly van-der-Waals on the other surfaces.     

Elastic instability of the nano-structured state as an intrinsic probe to study the early formation stages of sol–gel derived (Pb<Subscript>1-x</Subscript>Ca<Subscript>x</Subscript>)TiO<Subscript>3</Subscript> thin films

A novel method to investigate the early formation stages of polycrystalline (Pb_1-xCa_x)TiO₃ (PCT) perovskite films by means of traditional Brillouin and micro-Brillouin spectroscopy (BS, mBS) is described in the present work. The films were prepared by chemical solution deposition (CSD) onto oxidized (100)Si substrates and treated at temperatures between 350–650 °C by rapid thermal processing (RTP). The elastic instability observed by Brillouin spectroscopy at the nano-structured state of the PCT films was used here to determine their crystallization temperatures. Coexistence of different nanocrystalline phases (e.g., pyrochlore, perovskite) in the films could also be detected by this technique. The reliability of these results is demonstrated by complementary information obtained by X-ray diffraction (XRD) and scanning force microscopy (SFM). The effects of the annealing temperature and of the Ca²⁺ content on the crystallization process of these films are also discussed. PACS 78.35.+c; 77.84.-s; 61.82.Rx     

Characterization of silicon nanowires grown on silicon, stainless steel and indium tin oxide substrates

Silicon nanowires (SiNWs) have been grown on crystalline silicon (Si), indium tin oxide (ITO) and stainless steel (SS) substrates using a gold catalyst coating with a thickness of 200 nm via pulsed plasma-enhanced chemical vapor deposition (PPECVD). Their morphological, mineralogical and surface characteristics have been investigated using scanning electron microscopy (SEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and Raman analysis. SiNWs growth is accompanied by oxidation, thus yielding partially (SiO _x ) and fully oxidized (SiO₂) Si sheaths. The mean diameters of these SiNWs range from 140 to 185 nm. Si with (111) and (220) planes exists in SiNWs grown on all three substrates while Si with a (311) plane is detected only for Si and ITO substrates. Computational simulation using density functional theory (DFT) has also been conducted to supplement the experimental Raman analyses for crystalline Si and SiO₂. XPS results reveal that ca. 30 % of the SiNWs have been oxidized for all substrates. The results presented in this paper can be used to aid selection of appropriate substrates for SiNW growth, depending on specific applications.     

Structure of vacant electronic states of an oxidized germanium surface upon deposition of perylene tetracarboxylic dianhydride films

This paper presents the results of the investigation of the interface potential barrier and vacant electronic states in the energy range of 5 to 20 eV above the Fermi level (E_F) in the deposition of perylene tetracarboxylic dianhydride (PTCDA) films on the oxidized germanium surface ((GeO₂)Ge). The concentration of oxide on the (GeO₂)Ge surface was determined by X-ray photoelectron spectroscopy. In the experiments, we used the recording of the reflection of a test low-energy electron beam from the surface, implemented in the mode of total current spectroscopy. The theoretical analysis involves the calculation of the energy and spatial distribution of the orbitals of PTCDA molecules by the density functional theory (DFT) using B3LYP functional with the basis 6-31G(d), followed by the scaling of the calculated values of the orbital energy according to the procedure well-proven in the studies of small organic conjugated molecules. The pattern of changes in the fine structure of the total current spectra with increasing thickness of the PTCDA coating on the (GeO₂)Ge surface to 6 nm was studied. At energies below 9 eV above E_F, there is a maximum of the density of unoccupied electron states in the PTCDA film, formed mainly by π* molecular orbitals. The higher density maxima of unoccupied states are of σ* nature. The formation of the interface potential barrier in the deposition of PTCDA at the (GeO₂)Ge surface is accompanied by an increase in the work function of the surface, E_vac–E_F, from 4.6 ± 0.1 to 4.9 ± 0.1 eV. This occurs when the PTCDA coating thickness increases to 3 nm, and upon further deposition of PTCDA, the work function of the surface does not change, which corresponds to the model of formation of a limited polarization layer in the deposited organic film.     

含偶氮两亲分子自组装单层膜的扫描力显微镜和共聚焦拉曼光谱研究

两亲性分子在液 /固界面组装成二维有序的结构是目前超分子科学研究的热点。含有偶氮苯单链季铵盐化合物 4-[(4’ -十二烷氧基 )偶氮苯氧基 ]己基三乙基溴化铵 (简写为C12 AzoC6 N+)在水溶液中能聚集成胶束 ,当它自组织到新鲜的云母片上时 ,则形成二维有序的纳米尺寸的树枝状条带结构。为了进一步系统的研究该自组织树枝状条带结构单层膜的性质 ,本文利用扫描力显微镜 (SFM)研究了该化合物在大于其临界胶束浓度时在新鲜的云母片上自组织成树枝状条带结构的行为 ,并通过共聚焦拉曼光谱对该自组织树枝状条带结构单层膜进行了测试 ,指认了固体和自组织单层膜的拉曼位移。通过比较 ,我们进一步确认了在云母片上的自组织的二维有序的纳米尺寸树枝状条带单层膜的结构。这将为这种二维有序的纳米尺寸树枝状条带结构将来的应用奠定基础。     

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。     

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.     

STM investigations of PTCDA and PTCDI on graphite and MoS2. A systematic study of epitaxy and STM image contrast

Monolayers of the organic molecules perylene-3,4,9,10-tetra-carboxylic-dianhydride (PTCDA) and diimide (PTCDI) on graphite and MoS₂ have been imaged with scanning tunneling microscopy. The epitaxial growth of the two molecules is determined by the intermolecular interaction but nearly independent of the substrate. On both substrates the STM image contrast in the submolecularly resolved images is dominated by the aromatic perylene system whereas the polar oxygen and nitrogen groups are invisible. The correlation of the observed inner structure of the molecules to their molecular structure allows us to compare our results with theoretical considerations.     

Solvent-Induced Crystallization of PS-b-PEO-b-PS Block Copolymer Films

Solvent-induced crystallization and dewetting behaviors of polystyrene-b-poly (ethylene oxide)-b-polystyrene (PS-b-PEO-b-PS) block copolymer films deposited on three different substrates, silicon, mica, and carbon-coated mica have been studied by atomic force microscope (AFM). When the common solvent dichloromethane was used for annealing, the films on all three of the substrates exhibited dewetting behavior; the dendritic crystallization patterns were found in the dewetted regions for the films on silicon and mica substrates, while for the films on carbon-coated mica, no crystallization pattern appeared. According to the observation of the crystallization patterns formed in the films with different initial thicknesses, it is shown that the width of the dendritic branches decreases with the increase of film thickness and solvent annealing time. When the PS-selective solvent toluene was used for annealing, the dewetting process was absent, and the crystallization patterns were observed on the surface of the films on all three kinds of substrates.     

Epitaxial lithium fluoride films grown by pulsed laser deposition

Lithium fluoride (LiF) films have been prepared on LaAlO₃ (LAO), MgO, Si and TiN buffered Si substrates using a pulsed laser deposition (PLD) technique. Their surface morphology and structural qualities were studied by using scanning electron microscopy (SEM) and X-ray diffractometry (XRD). Those films deposited on (100)MgO and (100)TiN buffered Si exhibited (200)_LiF||(200)_MgOand(200)_LiF||(200)_TiN||(200)_Si epitaxial relationships, respectively. These results suggest that LiF films can be epitaxially grown on lattice match substrates and, for the first time, on (100)Si via a buffer layer. LiF grown on bare (100)Si and (100)LAO substrates, however, yielded films of (100) preferred orientation only. Lattice mismatch and thermal effects are invoked to explain these observations. The surfaces of the LiF films are very rough and covered with large globules due to splashing of molten droplets from the target. In order to remedy such deficiencies we used a shadow mask to reduce both the size and the number of these globules while maintaining the heteroepitaxial properties. As a result optically smooth LiF films of excellent structural quality are produced. PACS 78.55.FV; 81.15.FG; 68.55.JK     

Influence of Si substrate preparation on surface chemistry and morphology of L-CVD SnO2 thin films studied by XPS and AFM

Results of experimental studies of the influence of substrate preparation on the surface chemistry and surface morphology of the laser-assisted chemical vapour deposition (L-CVD) SnO₂ thin films are presented in this paper. The native Si(1 0 0) substrate cleaned by UHV thermal annealing (TA) as well as thermally oxidized Si(1 0 0) substrate cleaned by ion bombardment (IBA) have been used as the substrates. X-ray photoemission spectroscopy (XPS) has been used for the control of surface chemistry of the substrates as well as of deposited films. Atomic force microscopy (AFM) has been used to control the surface morphology of the L-CVD SnO₂ thin films deposited on differently prepared substrates. Our XPS shows that the L-CVD SnO₂ thin films deposited on thermally oxidized Si(1 0 0) substrate after cleaning with ion bombardment exhibit the same stoichiometry, i.e. ratio [O]/[Sn] = 1.30 as that of the layers deposited on Si(1 0 0) substrate previously cleaned by UHV prolonged heating. AFM shows that L-CVD SnO₂ thin films deposited on thermally oxidized Si(1 0 0) substrate after cleaning with ion bombardment exhibit evidently increasing rough surface topography with respect to roughness, grain size range and maximum grain height as the L-CVD SnO₂ thin films deposited on atomically clean Si substrate at the same surface chemistry (nonstoichiometry) reflect the higher substrate roughness after cleaning with ion bombardment.     

Solid surface dependent layering of self-arranged structures with fibril-like assemblies of alpha-synuclein

In present work the formation of hybrid constructions composed of alpha-synuclein-based colloidal solutions on various solid surfaces (silica coated Si, mica, CaF₂ and KBr) is investigated by scanning probe microscopy, spectrocopic ellipsometry, Fourier transformed infrared spectroscopy and vibrational circular dichroism. Prior to the modification of the solids, the proteins were intentionally fibrilled under special conditions. It is proved that the multi-component coatings are self-arranged on the solid substrates. Depending on the substrate material, the interface films consisting of individual biomolecules can be detected on the solid surfaces. The coatings with fibril-like alpha-synuclein objects can be obtained on solid surfaces with negligible or comparatively thick interface films. The results are interpreted in terms of the charged surface-controlled electrostatic interaction between the substrate and the biomolecules. Solubility of solids is considered in this interpretation.     

Tandem organic light-emitting diodes with an effective charge-generation connection structure

The tandem organic light-emitting diodes (OLEDs) with an effective charge-generation connection structure of Mg-doped tris(8-hydroxyquinoline) aluminum (Alq₃)/Molybdenum oxide (MoO₃)-doped 3, 4, 9, 10-perylenetetracarboxylic dianhydride (PTCDA) were presented. At a current density of 50 mA/cm², the current efficiency of the tandem OLED with two standard NPB/Alq₃ emitting units is 4.2 cd/A, which is 1.7 times greater than that of the single EL device. The tandem OLED with the similar connection structure of Mg-doped PTCDA/ MoO₃-doped PTCDA was also fabricated and the influences of the different connection units on the current efficiency of the tandem OLED were discussed as well.     

Adsorption geometry of PTCDA on 2H-NbSe2

Low-temperature scanning tunnelling microscopy of submonolayers of PTCDA adsorbed on 2H-NbSe₂ reveals an adsorbate layer that is neither commensurate with the substrate lattice nor with the charge density wave periodicity. The observed smallness of deviations from the PTCDA bulk-state unit-cell dimensions indicates that PTCDA film formation is dominated by intermolecular interactions rather than by interaction between PTCDA and 2H-NbSe₂. PACS 61.66.Hq; 68.35.Bs; 68.37.Ef; 68.43.Fg     

Spectroscopic ellipsometry of 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA)

The optical functions of PTCDA have been determined by spectroscopic ellipsometry. The samples were prepared by thermal evaporation of PTCDA on glass substrates. The influence of the substrate temperature on the properties of PTCDA films has also been investigated. The lowest substrate temperature investigated here was room temperature. At room temperature and higher substrate temperatures, we have not observed ordering of the evaporated films, which would result in high anisotropy, as reported for PTCDA films grown by organic molecular-beam deposition at very low temperatures (so-called quasi-epitaxial growth). Therefore, the samples have been assumed to be isotropic. The obtained imaginary part of the dielectric function exhibits the expected features: a low-magnitude peak around 370 nm and a higher magnitude, two-peak structure in the 400–600 nm spectral region. PACS 78.20.Ci; 78.66.Qn     

Low temperature formation of multi-layered structures of ferromagnetic silicide Fe3Si and Ge

Low-temperature (<300 °C) molecular beam epitaxy of Fe₃Si/Ge was investigated. By optimizing growth conditions, Fe₃Si layers with a flat interface and good crystallinity were epitaxially grown on Ge(1 1 1) substrates. In addition, double heteroepitaxial growth of Fe₃Si/Ge on high quality Fe₃Si/Ge substrates was investigated. Reflective high-energy electron diffraction measurements suggested Fe₃Si and Ge layers were epitaxially grown on Fe₃Si/Ge substrates. However, transmission electron microscopy measurements indicated stacking faults formed in the intermediate Ge and top Fe₃Si layers. Improved crystallinity of the intermediate Ge layer is essential to realize high quality [Fe₃Si/Ge]₂ multi-layered structures.