苝四甲酸二酐在Au(111)表面的取向生长及电子结构研究

利用X射线光电子能谱(XPS),同步辐射紫外光电子能谱(SRUPS),近边X射线吸收精细结构(NEXAFS)以及原子力显微镜(AFM)等技术研究了苝四甲酸二酐(PTCDA)与Au(111)的界面电子结构、PTCDA分子取向及有机薄膜的表面形貌.SRUPS价带谱显示,伴随PTCDA分子的微量沉积(0·5ML),位于费米能级附近Au的表面电子态迅速消失,但却观察不到明显的界面杂化态,这说明PTCDA分子和Au(111)界面间存在弱电子传输过程,但并没有发生明显的化学反应.角分辨NEXAFS以及SRUPS结果证明PTCDA分子是平铺在衬底表面.根据Au4f7/2和C1s峰积分强度随薄膜厚度的变化以及AFM图像可知,PTCDA分子在Au(111)表面是一种典型的Stranski-Krastanov生长模式,即先层状生长,再岛状生长,并且在层状生长到岛状生长的转变过程中,存在有机分子的去润湿过程.     

In-situ formation and detailed analysis of imine bonds for the construction of conjugated aromatic monolayers on Au(111)

We present the synthesis of 4′-amino-4-mercaptobiphenyl (AMB) and its deposition from solution onto Au(111) substrates. The resulting organic thin films were characterized by contact angle, infrared reflection absorption spectroscopy (IRRAS), X-ray photoelectron spectroscopy (XPS), and near edge X-ray absorption fine structure (NEXAFS) measurements. It is demonstrated that the majority of AMB molecules are coupled to the gold surface via S–Au covalent bonds, although only little orientational order of the AMB layer could be detected by NEXAFS. Furthermore, aromatic imine bonds between AMB and 4-hydroxybenzaldehyde (HB), 4-carboxybenzaldehyde (CB), 4-methylbenzaldehyde (MB), or 4-(trifluoromethyl) benzaldehyde (TMB) have been successfully formed. As a result of the limited order, this coupling reaction was incomplete. Nevertheless, the experimental results confirmed the formation of conjugated aromatic imine bonds.     

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.     

Molecular adsorption and growth of naphthalene films on Ag(1 0 0)

The adsorption of naphthalene, vacuum deposited on a Ag(1 0 0) surface, was comprehensively investigated by means of low-energy electron diffraction (LEED), temperature-programmed thermal desorption (TPD) spectroscopy, X-ray photoelectron spectroscopy (XPS), and polarization-dependent near-edge X-ray absorption fine structure (NEXAFS) spectroscopy in the mono- and multilayer regime. A growth of long-range ordered monolayer at 140 K is observed with LEED. The polarization-dependent C 1s NEXAFS shows that the naphthalene molecules in the monolayer lie almost parallel to the Ag(1 0 0) surface. With increasing film thickness, the molecular orientation turns to upright position. Furthermore, NEXAFS measurements show that in the multilayer regime the molecular orientation depends on the substrate temperature during deposition.     

NEXAFS characterization of DNA components and molecular-orientation of surface-bound DNA oligomers

Single stranded DNA oligomers (ssDNA) immobilized onto solid surfaces forms the basis for several biotechnological applications such as DNA microarrays, affinity separations, and biosensors. The surface structure of the surface-bound oligomers is expected to significantly influence their biological activity and interactions with the environment. In this study near-edge X-ray absorption fine structure spectroscopy (NEXAFS) is used to characterize the components of DNA (nucleobases, nucleotides and nucleosides) and the orientation information of surface-bound ssDNA. The K-edges of carbon, nitrogen and oxygen have spectra with features that are characteristic of the different chemical species present in the nucleobases of DNA. The effect of addition of the DNA sugar and phosphate components on the NEXAFS K-edge spectra was also investigated. The polarization-dependent nitrogen K-edge NEXAFS data show significant changes for different orientations of surface bound ssDNA. These results establish NEXAFS as a powerful technique for chemical and structural characterization of surface-bound DNA oligomers.     

Determination of molecular orientation in ultrathin liquid crystal. films on solid substrates using X-ray absorption spectroscopy

42 H₆₀O₆, HBT) deposited on a (111)-oriented Au single crystal and a polycrystalline indium tin oxide (ITO) substrate has been carried out using X-ray absorption spectroscopy. Films of thicknesses between 2 nm and 15 nm were prepared in UHV by evaporation of HBT [which exhibits a discotic liquid crystalline (LC) bulk phase] from a Knudsen cell. Thickness and composition of the HBT films were determined using X-ray photoelectron spectroscopy (XPS). For the thinnest films with thicknesses in the monolayer regime, the orientational analysis reveals a pronounced orientation of the disc-shaped HBT molecules parallel to the Au surface. For thicker films, a significantly reduced anisotropy is observed with the molecular plane oriented more normal to the Au surface. In the case of the ITO-substrates, no significant differences were observed between different thickn esses and the average orientation of the molecular planes was predominantly normal to the surfaces, as for the thicker films on the Au substrate. Received: 1 March 1997/Accepted: 24 April 1997     

Phenylacetylene chemisorbed on Pt (111), reactivity and molecular orientation as probed by NEXAFS Comparison with condensed multilayer and polyphenylacetylene

Near-edge X-ray absorption spectroscopy (NEXAFS) has been used to investigate the reactivity of phenylacetylene (PA) towards a Pt(111) surface and its molecular orientation. Spectra have been taken in the photon energy region 280–320 eV at both grazing and normal X-ray beam incidence angles with respect to the sample surface. NEXAFS K-edge spectra have been recorded for PA adsorbed in the monolayer as well as in the multilayer regime and, for comparison, the spectrum of the PA related polymer, polyphenylacetylene (PPA), has also been measured. The assignment of the π* and σ* resonances has been made on the basis of the chemical structure and interpreted with the help of ab initio calculations. Both the ionisation potentials for the C 1s electrons and the theoretical C K-edge absorption spectra have been calculated for all the six inequivalent carbon atoms of PA and of phenylethylene (PE) which is a model molecule for PPA. From the analysis of the data we have indication for an interaction involving mainly the CC triple bond of the acetylenic functional group of PA that undergoes opening and leads to the formation of a di-sigma bond with Pt, while the benzene CC bonds seem not affected. A polarization dependence has been evidenced for the π* resonances of PA adsorbed in monolayer condition indicating a molecular orientation of the phenyl ring π* orbitals between 37° and 34° relative to the normal to the crystal surface.     

The interaction between hexahydroxytriphenylene and the rutile TiO2(110)-(1 × 1) surface at UHV conditions

The interaction between 2,3,6,7,10,11-hexahydroxytriphenylene (HHTP) and the rutile TiO₂(110)–(1 × 1) surface under ultrahigh vacuum (UHV) conditions was investigated using X-ray photoemission spectroscopy (XPS), ultraviolet photoemission spectroscopy (UPS), near-edge X-ray absorption fine structure (NEXAFS) spectroscopy, and density functional theory (DFT) calculations. The NEXAFS results showed that HHTP molecules formed a submonolayer and a monolayer that aligned along the [001]-direction with, respectively, a more or less flat downward orientation and a more upright orientation to the TiO₂ surface. The HHTP molecules that aligned along the [001]-direction were most likely grafted onto the TiO₂(110) surface by a bidentate bridge between each of the oxygen atoms of one of the catechol units within the HHTP molecule and two adjacent Ti(5f)⁴⁺ ions on the TiO₂(110) surface. The coordination is non-dissociative in the case of the submonolayer, but dissociative in the monolayer, according to the analysis of the C1s XPS, UPS, C1s NEXAFS data and complementary DFT calculations.     

Determination of molecular orientation in ultrathin films of aminophenylthiolate on Cu (100) prepared by vapour phase deposition

Ultrathin layers of aminophenylthiol on Cu(100) were prepared by vapour phase deposition of 4,4'-diaminodiphenyldisulphide (DAPS). Composition and molecular orientation in the films were determined by X-ray photoelectron spectroscopy (XPS) and near edge X-ray absorption fine structure (NEXAFS) spectroscopy. DAPS molecules adsorb on Cu(100) by dissociation of the intramolecular S-S bond and the formation of aminophenylthiolate at the copper surface. NEXAFS spectra indicate the presence of well-ordered monolayers with the plane of the phenyl ring tilted by 24° with respect to the surface normal. The same tilt-angle is found in the submonolayer regime.     

The adsorption of adenine on mineral surfaces: Iron pyrite and silicon dioxide

The adsorption of the nucleobase adenine on surfaces of the minerals iron pyrite and silica has been studied by photoemission and soft X-ray photoabsorption spectroscopy. Pyrite samples were prepared by fracture under nitrogen followed by transfer to ultrahigh vacuum, or by cleavage in vacuum. By comparing data with multilayer spectra, adenine was found to chemisorb on pyrite, with small changes in the valence band spectrum, and stronger changes in the NEXAFS spectrum. The molecules were bonded with the molecular plane at a steep angle to the surface plane. On silica the molecule was found to adsorb at a reduced angle to the surface. The C and N 1s photoemission spectra on this surface suggest chemisorption, although the nitrogen NEXAFS spectra are similar to multilayer spectra.     

Structural, magnetic and electronic structure studies of Mn doped TiO2 thin films

We report structural, magnetic and electronic structure study of Mn doped TiO₂ thin films grown using pulsed laser deposition method. The films were characterized using X-ray diffraction (XRD), dc magnetization, X-ray magnetic circular dichroism (XMCD) and near edge X-ray absorption fine structure (NEXAFS) spectroscopy measurements. XRD results indicate that films exhibit single phase nature with rutile structure and exclude the secondary phase related to Mn metal cluster or any oxide phase of Mn. Magnetization studies reveal that both the films (3% and 5% Mn doped TiO₂) exhibit room temperature ferromagnetism and saturation magnetization increases with increase in concentration of Mn doping. The spectral features of XMCD at Mn L_3,2 edge show that Mn²⁺ ions contribute to the ferromagnetism. NEXAFS spectra measured at O K edge show a strong hybridization between Mn, Ti 3d and O 2p orbitals. NEXAFS spectra measured at Mn and Ti L_3,2 edge show that Mn exist in +2 valence state, whereas, Ti is in +4 state in Mn doped TiO₂ films.     

A NEXAFS study on the adsorption of ammonia on clean and potassium-promoted iron

The adsorption of ammonia on thin films of iron with and without potassium has been investigated with near-edge X-ray absorption fine-structure (NEXAFS) and X-ray photoelectron spectroscopy (XPS). The NEXAFS results suggest that for low coverages of chemisorbed ammonia on iron there is a dilation of the NH bond length with respect to solid ammonia. At higher ammonia coverages there is less perturbation of NH₃, as inferred from NEXAFS and XPS experiments, suggesting that the adsorption energy of the ammonia is a strong function of surface coverage. The coadsorption of potassium metal is found to prevent the ammonia NH bond dilation observed over clean iron.     

Synchrotron radiation studies of the orientation of thin silicon phthalocyanine dichloride film on HOPG substrate

Thin silicon Phthalocyanine dichloride films on HOPG were prepared and the sample was heated in the vacuum with laser.The thickness of the thin sample on HOPG was checked by X-ray photoemission spectroscopy.The orientation of the molecules in respect to the substrate plane Was investigated by measuring the silicon K-edge near edge X-ray absorption fine structure(NEXAFS).In the NEXAFS spectra of the thin sample,two clear peaks which were assigned to 1s→σ*Si-N and 1s→σ*Si-C1 appeared around 1847.2 eV and 1843.1 eV respectively.The intensities of the resonance peaks showed strong polarization dependence.A quantitative analysis of the polarization dependence revealed that the Si-N bond tended to lie down while the Si-C1 bond was out of the molecular plane.     

Chemical states of nitrogen in ZnO studied by near-edge X-ray absorption fine structure and core-level photoemission spectroscopies

Interaction of low-energy nitrogen ions with ZnO surface has been studied by photoemission spectroscopy (PES) around N 1s core-level and near-edge X-ray absorption fine structure (NEXAFS) around N K-edge. Nitrogen can break Zn-O bonds at the surface and form N-O, Zn-N or Zn-N-O bonds, characterised by specific chemical shifts in PES or absorption peaks in NEXAFS. A distinctive signal from molecular nitrogen has also been observed in ion-bombarded samples in both NEXAFS and PES.     

Study on selective adsorption of deuterium on boron nitride using photon-stimulated ion-desorption

Adsorption behavior of atomic deuterium on a hexagonal boron nitride (h-BN) thin film is studied by photon-stimulated ion desorption (PSID) of D⁺ and near edge X-ray absorption fine structure (NEXAFS) at the B and N K-edges. After the adsorption of atomic deuterium, D⁺ desorption yield η(hν) shows clear enhancement at the B K-edge and almost no enhancement at the N K-edge. NEXAFS spectra show a large change in the B K-edge and a small change in the N K-edge after the adsorption. We propose selective adsorption of atomic deuterium on the h-BN thin film based on the experimental results, and mention the effectiveness of applying the PSID method with X-ray to study hydrogen storage materials.     

Controllable oxidation of h-BN monolayer on Ir(111) studied by core-level spectroscopies

The effect of atomic oxygen adsorption on the structure and electronic properties of monolayer hexagonal boron nitride (h-BN) grown on Ir(111) has been studied using near edge X-ray absorption fine structure spectroscopy (NEXAFS), photoelectron spectroscopy (PES), and low-energy electron diffraction (LEED). It has been shown that the oxidation of the h-BN monolayer occurs through a gradual substitution of N by O in the h-BN lattice. This process leads to the formation of defect sites corresponding to three different types of the B atom environment (BN_3 ? xO_x with x = 1,2,3). The oxidation of the h-BN monolayer is very different from the case of graphene on Ir(111), where adsorption of atomic oxygen results mainly in the formation of epoxy groups [J. Phys. Chem. C. 115, 9568 (2011)]. A post-annealing of the h-BN monolayer after oxygen exposure results in further destruction of the B–N bonds and formation of a B₂O₃-like structure.     

吡嗪分子在硅(100)面吸附构型的XPS和NEXAFS谱理论研究

本文利用X射线谱研究了吡嗪(C₄H₄N₂)分子共价吸附于硅(100)面的几种吸附构型的几何结构和电子结构. 利用密度泛函理论结合团簇模型,对预测的吸附结构的碳K壳层(1s)X射线光电子能谱(XPS)和近边X射线吸收精细结构(NEXAFS)谱进行了模拟. 计算结果阐明了XPS和NEXAFS谱与不同吸附构型的对应关系. 与XPS谱相比,NEXAFS谱对所研究的吡嗪/硅(100)体系的结构有明显的依赖性,可以很好地用于结构鉴定. 根据碳原子的分类,研究了在NEXAFS光谱中不同化学环境下碳原子的光谱成分.     

Adsorption of carboxymethylester-azobenzene on copper and?gold?single crystal surfaces

The adsorption of 3,3′-di(methoxycarbonyl)azobenzene (CMA) on Au(111) and on Cu(001) substrates was studied by X-ray absorption spectroscopy measurements at the C, N, and O K edges. We find the molecules physisorbed in a planar conformation flat on the Au(111) surface. At higher coverages, a molecular crystal is formed wherein the molecules have the same flat geometry. On Cu(001), additional chemical bonds are formed between the molecules and the surface via the nitrogen atoms. Here the methyl benzoate moieties are tilted out of the surface plane.     

氮气含量对硅基AlN薄膜结构及性能的影响

研究了直流磁控溅射法沉积AlN薄膜过程中氮气含量对AlN薄膜结构及性能的影响.实验结果表明:当真空腔的氩气含量较低时,薄膜呈非晶态,在红外波段的傅里叶变换光谱中没有明显的吸收峰,当氮气流量为75%时薄膜中出现明显的六方AlN(100)和AlN(110)衍射峰,在波数为670～700cm-1处有强烈的吸收峰;增加氮气含量,薄膜又呈现出非晶状态.薄膜的表面粗糙度和颗粒大小都随氮气含量的增加先增大后减小..     

Experimentally observed orientation of C60F18 molecules on the nickel single crystal (100) surface

The angular dependence of near edge X-ray absorption fine structure (NEXAFS) spectra has been obtained in the vicinity of carbon and fluorine 1s absorption edges in a monolayer film of polar fullerene fluoride (C₆₀F₁₈) molecules on a Ni(100) substrate. The fine structure of the spectra has been identified according to experimental data via calculations based on the density functional theory, and the angular dependence of the spectra has been explained. The orientations of structural molecular fragments are determined from the angular dependence of the NEXAFS spectra. It is demonstrated that the electric dipole moment of a C₆₀F₁₈ molecule is oriented along the normal to the substrate surface with an accuracy of 5°.