Electronic properties of the organic semiconductor hetero-interface CuPc/C60

We present a study of the electronic properties of the interface between the well-established molecular organic semiconductor copper phthalocyanine (CuPc) and the fullerite C₆₀ using photoelectron spectroscopy and the Kelvin-probe (KP) method. Upon deposition of CuPc on C₆₀, we found interfacial shifts of the vacuum level indicating the formation of a dipole layer, while band bending is found to be negligible. The interface dipole of 0.5 eV measured with KP is close to the difference between the work functions of bulk CuPc and C₆₀. No evidence for a chemical interaction at the interface is concluded from the absence of additional features in the core-level spectra at the earliest stages of deposition. The energy-level alignment diagram at the CuPc/C₆₀ interface is derived.     

Low-temperature scanning tunneling microscopy and near-edge X-ray absorption fine structure investigation of epitaxial growth of F<Subscript>16</Subscript>CuPc thin films on graphite

Low-temperature scanning tunneling microscopy (LT-STM) and near-edge X-ray absorption fine structure (NEXAFS) measurements are used to study the epitaxial growth and molecular orientation of organic thin films of copper hexadecafluorophthalocyanine (F₁₆CuPc) on highly oriented pyrolytic graphite (HOPG). Our results show that F₁₆CuPc molecules lie flat on HOPG up to 5 nm thickness, stabilized by interfacial and interlayer π–π interactions. LT-STM experiments reveal the coexistence of two different in-plane orientations of the F₁₆CuPc monolayer on HOPG. On the second layer of F₁₆CuPc on HOPG, however, all F₁₆CuPc molecules possess the same in-plane orientation.     

Probing the interaction at the C60-SiC nanomesh interface

Synchrotron-based high-resolution photoemission spectroscopy (PES) and in situ scanning tunneling microscopy (STM) are used to investigate the interaction at the C₆₀-SiC nanomesh interface during the sequential deposition and subsequent desorption of C₆₀ molecules. A weak charge transfer occurs at the C₆₀-nanomesh interface, involving electrons transferring from nanomesh to C₆₀ overlayer. The interface interaction originated from the weak charge transfer at the C₆₀-nanomesh interface is stronger than C₆₀ intermolecular interaction (e.g., van-der-Waals force), facilitating the layer-by-layer growth for the first two layers of C₆₀ on SiC nanomesh. The highly corrugated nanomesh surface results in an anisotropic diffusion and high diffusion barrier of C₆₀ on top, and thereby leads to the formation of irregularly shaped C₆₀ islands under submonolayer condition. In contrast, C₆₀ diffusion on HOPG and Ag(1 1 1) surfaces is rather isotropic, resulting in the formation of hexagonally shaped C₆₀ islands with smooth domain boundaries. STM results show the partial desorption of C₆₀ molecules from the SiC nanomesh surface after annealing the 1 ML C₆₀ sample (complete wetting layer of C₆₀ on SiC nanomesh) at around 150 °C for 20 min. Thorough desorption of C₆₀ molecules and full recovery of the clean SiC nanomesh are observed after annealing at around 200 °C for 20 min. In situ PES and STM experiments clearly demonstrate that C₆₀ adsorption and desorption processes do not affect the underlying SiC nanomesh structure, revealing its thermal stability and chemical inertness to C₆₀ molecules.     

Determination of electron affinity of electron accepting molecules

The unoccupied π ^* states of the solid film of electron accepting organic molecules, 7,7,8,8-tetracyanoquinodimethane (TCNQ), fluorinated TCNQ derivatives, 11,11,12,12-tetracyanonaphtho-2,6-quinodimethane (TNAP), C₆₀, and 6,6-phenyl-C₆₁-butyric acid methyl ester (PCBM) have been studied by inverse photoemission spectroscopy. The assignment of the π ^* affinity levels of these typical electron accepting molecules provides the basic information for the organic electronics and the new electronic functional molecular design. The comparison with density functional theory calculations enables understanding how the electron affinity evolves in terms of molecular orbitals. The correlation between the film morphology and the irradiation damage on the TCNQ derivative samples by electron impact during the inverse photoemission measurements is also discussed.     

对以并五苯和酞菁铜为不同有源层的有机薄膜晶体管特性研究

通过扫描电镜和X射线衍射对SiO₂衬底上生长并五苯和酞菁铜薄膜的表面形貌进行表征,并得到在SiO₂衬底上生长的并五苯薄膜是以岛状结构生长,其大小约为100nm,且薄膜有较好的结晶取向,呈多晶态存在. 酞菁铜薄膜则没有表现出明显的生长机理,其呈非晶态存在. 还对通过掩膜的方法制作得以酞菁铜和并五苯为有源层的顶栅极有机薄膜晶体管的特性进行了研究. 有源层的厚度为40nm,绝缘层SiO₂的厚度为250nm,器件的沟道宽长比(W/关键词： 有机薄膜晶体管 并五苯薄膜 酞菁铜薄膜 μ_EF)')" href="#">场效应迁移率(μ_EF)     

Controlling geometric and electronic properties of highly ordered CuPc thin films

Geometric and electronic properties of ordered copper phthalocyanine (CuPc) thin films grown on hydrogen- and antimony-passivated Si(1 1 1) surfaces have been studied using near edge X-ray absorption fine structure (NEXAFS) and photoemission spectroscopy. The H- and Sb-passivations of vicinal Si surfaces resulted in different molecular orientations in thick films, namely upright and near lying molecules, respectively. In the absence of the vicinality, the molecules on the Sb-passivated surface changed towards upright orientation. The work function of the films was monitored during the growth and correlated with the molecular orientation.     

Radiation-stimulated modification of C60 films on Si-oxide surfaces

The electronic structure of C₆₀/SiO_x/Si(1 0 0) interface was studied by photoemission (valence-bands, C 1s, and Si 2p core levels) and near-edge X-ray absorption fine structure (C 1s threshold) spectroscopies. It was concluded that the SiO_x/Si surface is non-reactive with respect to the interaction with C₆₀. The exposure of the C₆₀/SiO_x/Si system under non-monochromatic synchrotron radiation causes modification of the electronic structure of this system. It is explained by polymerization of the C₆₀ molecules and arising a strong ionic-like interaction of the polymerized C₆₀ with the SiO_x surface. Annealing of this system up to temperatures of 550–625 °C leads to complete desorption of the C₆₀ molecules from the non-irradiated sample areas while the modified by radiation fullerenes remain attached to the substrate.     

Surface analysis for LiBq4 growing on ITO and CuPc film using atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS)

We have investigated the morphology and surface electron states of LiBq₄ deposited on ITO and CuPc/ITO, using atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS). The AFM observations indicate that LiBq₄ can form a much more uniform film on CuPc than that on ITO. Furthermore, X-ray photoelectron spectroscopy (XPS) is utilized to further demonstrate the AFM results. From the analysis of XPS, we found that LiBq₄ molecules have poor thermal stability, they are seriously oxidized during depositing; but when a CuPc layer is inserted between LiBq₄ and ITO film, the oxidation and surface contamination of LiBq₄ are significantly reduced. It is then concluded that the introduction of a CuPc buffer layer under the LiBq₄ film can improve the film quality of LiBq₄.The XPS results also testified the fact that no coordination bonds between N atoms and B atoms are formed in LiBq₄ molecules, which make LiBq₄ to be potential blue organic light-emitting material.     

UV photoemission study on fullerene molecules deposited on defect-rich carbon surfaces

Submonolayers of C₆₀ molecules have been deposited on different carbon substrates (pristine HOPG, argon-ion irradiated HOPG and amorphous carbon) and investigated by means of Ultraviolet Photoelectron Spectroscopy. The desorption behavior and spectral changes in the valence band were examined as a function of the sample temperature. Strong fullerene-substrate interaction (chemisorption) was observed on defect-rich surfaces (irradiated HOPG, a-C), indicated by binding energy shifts and broadening of C₆₀ VB features. These chemisorbed species proved to be more resistant against temperature-induced desorption than usually observed for physisorbed molecules on pristine HOPG. The results presented here suggest that deposition of fullerenes on heated substrates might be a feasible method of surface nanostructuring by preferential chemisorption on defects.     

原子力显微镜与x射线光电子能谱对LiBq4/ITO和LiBq4/CuPc/ITO的表面分析

利用原子力显微镜对8-羟基喹啉硼化锂(LiBq₄)/铟锡氧化物和8-羟基喹啉硼化锂/酞菁铜(CuPc)/铟锡氧化物表面分别进行了扫描，显示了LiBq₄在不同衬底上的形貌差异，并进一步利用样品表面的x射线光电子能谱图验证了这一差异.实验表明，CuPc层的加入改善了LiBq₄的成膜质量，并将这种改善归因于分子构型与电子亲和势的不同. 关键词： 原子力显微镜 x射线光电子能谱 电子亲和势     

Influence of the substrate temperature during deposition on film characteristics of copper phthalocyanine and field-effect transistor properties

In this paper, we employ different substrate temperatures during the deposition process and observe a highly ordered structure and strong orientation of copper phthalocyanine (CuPc) molecules on Si/SiO₂ by using X-ray-diffraction and transmission electron microscopy analysis. The results show the effect of CuPc morphology at different substrate temperatures on the organic field-effect-transistor performance. When the substrate temperature for deposition of CuPc is 120 °C, a mobility of 3.75×10^-3 cm²/V s can be obtained. PACS 73.61.Ph; 85.30.Tv; 78.66.Tr     

Self-assembly of C60 into highly ordered chain-like structures on HOPG observed at ambient conditions

The observation of chain-like structures of self-assembled C₆₀ molecules on HOPG surfaces at room temperature in aerial atmosphere by means of scanning tunneling microscopy is reported. The ca. 2.5 nm center-to-center distance between two fullerene molecules is much larger than in the close-packed layered or film structures of C₆₀ usually found on HOPG surfaces.     

酞菁铜与MoS_2(0001)范德瓦耳斯异质结研究

利用光电子能谱、原子力显微镜以及低能电子衍射等表面研究手段系统研究了真空沉积生长的酞菁铜薄膜与衬底MoS2(0001)之间的范德瓦耳斯异质结界面电子结构和几何结构.角分辨光电子能谱清楚地再现了MoS2(0001)衬底在Γ点附近的能带结构.低能电子衍射结果表明,CuPc薄膜在MoS2(0001)表面沿着衬底表面[11ˉ20],[1ˉ210]和[ˉ2110]三个晶向有序生长,反映了衬底对CuPc的影响.原子力显微镜结果表明,CuPc在MoS2衬底上遵循层状-岛状生长模式:在低生长厚度下(单层薄膜厚度约为0.3 nm),CuPc分子平面平行于MoS2表面上形成均匀连续的薄膜;在较高的沉积厚度下,CuPc沿衬底晶向形成棒状晶粒,表现出明显的各向异性.光电子能谱显示界面偶极层为0.07 eV,而且能谱在膜厚1.2 nm饱和,揭示了酞菁铜与MoS2(0001)范德瓦耳斯异质结的能级结构.     

C60 adsorption on Cu(110) studied by optical spectroscopy

The formation of the interface between C₆₀ thin films and the Cu(110) surface has been investigated in situ using reflectance difference spectroscopy (RDS). The electronic interaction between C₆₀ molecules in the first monolayer and the substrate inhibits low‐energy intramolecular transitions, whereas the C₆₀ molecules above the first monolayer are effectively decoupled from the substrate. The morphology of C₆₀ thin films prepared at room temperature is thermally stable up to 500 K. Above this threshold, optical spectroscopy and low energy electron diffraction (LEED) indicate the formation of rather large three dimensional C₆₀ islands on a one monolayer thick wetting layer. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Interaction of ranked copper phthalocyanine films with dry nitrogen dioxide on the basis of ESR data

The calculation procedure of the orientation distribution of molecules by the angular dependence of electron spin resonance (ESR) spectra was used to study copper(II) phthalocyanine (CuPc) films of different thickness, obtained by the molecular complex deposition on flat quartz plates. In the first stage of the deposition, the formation of the layer of the α-phase of CuPc with the predominant orientation of molecular stacks (theb-axis of the crystalline lattice) along the plate surface was formed. In the second stage, the layer with the normal disposition of molecular stacks is condensed over the first layer. The copper binuclear associates were formed during the reaction of CuPc with NO₂. The ESR spectra analysis allows us to make an assumption about the construction of associates and to calculate the distance between paramagnetic copper(II) ions. It was shown that the orientation distribution of axially symmetric CuPc associates in the film depends both on the initial film order and the conditions under which it was treated.     

Band bending variation of the Si(111) surface during its thermal oxidation

During the thermal oxidation of Si(111) surfaces performed under ultra high vacuum conditions and investigated with various surface techniques such as Auger electron spectroscopy, low energy electron diffraction, ultra violet and X-ray induced photoemission spectroscopy, the Si 2p core level binding energy was measured and provided directly the band bending variation of the Si surface. A net interface charge then could be deduced. This is an elegant in situ method to have access to the electrical characteristics of the SiSiO₂ interface during its formation.     

Charge-transfer at silver/phthalocyanines interfaces

Valence band photoemission spectroscopy (VB-PES) and inverse photoemission spectroscopy (IPES) were employed to determine the occupied and unoccupied density of states upon silver deposition onto layers of two phthalocyanines (H₂Pc and CuPc). The two different Pc molecules give rise to very distinct behaviour already during the initial stage of silver deposition. While in the CuPc case no shift occurs in the energy levels, the H₂Pc highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) are shifting simultaneously by 0.3 eV, i.e., the HOMO shifts away from the Fermi level while LUMO shifts towards the Fermi level. As the silver quantity increases the HOMO levels of both Pcs are shifting towards the Fermi level. When the Fermi level is resolved in the VB spectra, the characteristic features of H₂Pc and CuPc are smeared out to some extent. Shifts in HOMO and LUMO energy positions as well as changes in line shapes are discussed in terms of charge-transfer and chemical reactions at the interfaces.     

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     

Electronic properties of ultrathin films based on pyrrolofullerene molecules on the surface of oxidized silicon

Results of the investigation into the interface formation during the deposition of the films based on aziridinylphenylpyrrolofullerene (APP-C₆₀) up to 8 nm thick on the surface of the oxidized silicon substrate are presented. The procedure of detecting reflection of testing low-energy electron beam from the surface implemented in the total current spectroscopy mode with a change in the incident electron energy from 0 to 25 eV is used. The structure of maxima in the total current spectra induced by the APP-C₆₀ deposited film is established, and the character of interrelation of these maxima with π* and σ* energy bands in the studied materials is determined. It is revealed due to analyzing the variation in intensities of the total current spectra of the deposited APP-C₆₀ film and the (SiO₂)n-Si substrate that the APP-C₆₀ film is formed at the early deposition stage with the coating thickness thinner than one monolayer without the formation of the intermediate modified organic layer. As the APP-C₆₀/(SiO₂)n-Si interface is formed, the work function of the surface increases by 0.7 eV, which corresponds to the transfer of the electron density from substrate (SiO₂)n-Si toward the film APP-C₆₀. The optical absorption spectra of the APP-C₆₀ films are measured and compared with the spectra of films of unsubstituted C₆₀.     

Enhanced photosensitivity of InGaZnO-TFT with a CuPc light absorption layer

A copper phthalocyanine (CuPc) organic semiconductor is capped onto an amorphous indium–gallium–zinc-oxide (InGaZnO) thin film transistor (TFT) to enhance the photosensitivity of InGaZnO-TFT. The CuPc organic semiconductor is served as a light absorption layer and forms a p–n junction with the InGaZnO film. After 60 s white light illumination, light responsivity (R) of InGaZnO-TFT with a CuPc light absorption layer reaches a value of 148.5 A/W at a gate-source voltage (V_GS) of 20 V, which is much larger than that (31.2 A/W) of the conventional InGaZnO-TFT. The results are attributed to the following mechanism. First, a CuPc layer is employed as the light absorption layer. Second, CuPc/InGaZnO p–n junction enables the injection of electron into InGaZnO film. Our results indicate that using CuPc as light absorption layer is an effective approach to improve the photosensitivity of InGaZnO-TFT.