Effect of Gd doping on the magnetism and work function of Fe1-xGdx/Fe（001）

The magnetism and work function Ф of Fe1-xGdx/Fe （001） films have been investigated using first-principles methods based on the density functional theory. The calculated results reveal that Gd doping on the Fe （001） surface would greatly affect the geometrical structure of the system. The restruction of the surface atoms leads to the transition of magnetic coupling between Gd and Fe atoms from ferromagnetic （FM） for 0.5 ≤x ≤ 0.75 to antiferromagnetic （AFM） for x = 1.0. For Fe1-xGdx/Fe （001） （x = 0.25, 0.5, 0.75, 1.0）, the charge transfer from Gd to Fe leads to a positive dipole formed on the surface, which is responsible for the decrease of the work function. Moreover, it is found that the magnetic moments of Fe and Gd on the surface layer can be strongly influenced by Gd doping. The changes of the work function and magnetism for Fe1-xGdx/Fe （001） can be explained by the electron transfer, the magnetic coupling interaction between Gd and Fe atoms, and the complex surface restruction. Our work strongly suggests that the doping of the metal with a low work function is a promising way for modulating the work function of the magnetic metal gate.     

Thickness dependence of the work function in double-layer metallic films

The work function of metallic thin films limited by symmetric surfaces is expected to be thickness dependent at a level of 0.1 eV and a thickness range of about 5 nm. Recent experiments, however, demonstrated that Cu films on glass or Ni substrates show a long ranging (10–20 nm) increase of the work function with increasing film thickness [1]. This effect was attributed to a violation of local charge neutrality in films with unlike surfaces. In this paper we show that the barrier height of thin film diodes like metal-insulator-metal (MIM)-, metal-semiconductor (Schottky contacts)-and metal-vacuum-metal (Kelvin capacitors) structures decreases with increasing thickness of one metal electrode. This metal electrode consists of a double layer whose single layer thicknesses are of the order of few tens of nm. The observed effect can be attributed to a decrease of the work function at the counter limiting interface not exposed to the evaporation beam. A possible explanation can be found again in the violation of the local charge neutrality in films with unlike surfaces.     

Macroscopic theory of the electron work function in solids

The approach is based on the dielectric formalism theory. The relations for calculation of the electron work function in solids – metals, semiconductors and dielectrics, were obtained. The calculated results are in an agreement with experimental ones. This approach can be used for computation of work function values of numerous materials.     

氯化铯甲醇溶液对有机聚合物太阳能电池的影响

研究了氯化铯的甲醇溶液作为阴极修饰层,来提高传统有机聚合物太阳能电池器件性能。通过电容-电压(C-V)测试分析了铝电极和PTB7/PC_(70)BM之间的界面电荷积累情况,同时测试了紫外光电子能谱(UPS),对铝的功函数改变作了研究。结果表明,采用氯化铯的甲醇溶液作阴极修饰层的器件,其短路电流(J_(sc))、开路电压(V_(oc))、填充因子(FF)都有所提高,光电转化效率达到6.36%,与仅用甲醇处理过的器件相比,光电转化效率提高了11%;与未经甲醇处理的器件相比,光电转化效率提高了42.6%。这种一步溶液处理法能够减少电荷积累,同时降低铝电极的功函数,利于电子收集,进而提高器件性能。     

Effects of electron beam irradiation on the friction and work function of the wrinkled graphene

The ability to control the tribological and electrical properties of graphene is critical to the fabrication of micro- and nanoelectromechanical systems (MEMS/NEMS) devices. Due to its high energy, electron beam irradiation has been widely used to adjust the local electrical properties of the graphene, such as inducing local defects or n-type doping. However, whether electron beam irradiation can affect the local tribological properties of wrinkled graphene has not been investigated yet. In this research, we demonstrated that the lateral force signal and the work function of the wrinkled monolayer graphene were affected by the electron beam irradiation.By using Kelvin-probe force microscopy (KPFM) and Raman spectroscopy, we measured the local electrical properties of the wrinkled monolayer graphene and confirmed that the electron-beam exposed area was changed as n-doped graphene. We compared the lateral force signal with surface potential data and concluded that the n-type doping induced by electron beam affected the tribological characteristics. Characterization of the electron-beam exposed wrinkled graphene provides a physical insight that the electrical and tribological characteristics of wrinkled graphene are correlated.     

Surface work function of chemically derived graphene: A first-principles study

Using first-principles calculations within the framework of density-functional theory, we systematically study the modulation effect of chemical decoration including hydrogenation, fluorination, and oxidization on the surface work function of graphene. The chemical decoration is effective approach to modulate the surface work function, which expands the space to design diverse nano-devices based on graphene. Moreover, we also find some un-expectation chemically decorated cases which do not follow the traditional rule of “electronegative (electropositive) adsorbates, which increase (decrease) the work function of the surface”. Such a phenomenon is mainly derived from the charge redistribution induced by the bonding process between adsorbates and carbon atoms along with the chemical decoration.     

Temperature dependence of the work function of free nanoparticle agglomerates

Starting point for this study was the experimentally found unusual high charge state of free submicron particles at elevated temperatures. By employing an aerosol setup with single electron counting efficiency it is shown for Pt and TiO₂ particles that the work function decreases with increasing temperature. Sign and amount of the work function decrease agree with the results of other experiments on extended surfaces, single crystals and clusters. On the basis of the reduced work function the observed high particle charge states are quantitatively understood.     

Effect of branching in base polymer on ionic conductivity in hyperbranched polymer electrolytes

Novel hyperbranched polymer, poly[bis(diethylene glycol)benzoate] capped with a 3,5-bis[(3′,6′,9′-trioxodecyl)oxy]benzoyl group (poly-Bz1a), was prepared, and its polymer electrolyte with LiN(CF₃SO₂)₂, poly-Bz1a/LiN(CF₃SO₂)₂ electrolyte, was all evaluated in thermal properties, ionic conductivity, and electrochemical stability window. The poly-Bz1a/LiN(CF₃SO₂)₂ electrolyte exhibited higher ionic conductivity compared with a polymer electrolyte based on poly[bis(diethylene glycol)benzoate] capped with an acetyl group (poly-Ac1a), and the ionic conductivity of poly-Bz1a/LiN(CF₃SO₂)₂ electrolyte was to be 7×10⁻⁴ S cm⁻¹ at 80 °C and 1×10⁻⁶ S cm⁻¹ at 30 °C, respectively. The existence of a 3,5-bis[(3′,6′,9′-trioxodecyl)oxy]benzoyl group as a branching unit present at ends in the base polymer improved significantly ionic conductivity of the hyperbranched polymer electrolytes. The polymer electrolyte exhibited the electrochemical stability window of 4.2 V at 70 °C and was stable until 300 °C.     

Increased work function in PEDOT:PSS film under ultraviolet irradiation

An increase of work function （0.3 eV） is achieved by irradiating poly（3,4-ethylenedioxythiophene）：poly（styrene sul- fonate） （PEDOT：PSS） film in vacuum with 254-nm ultraviolet （UV） light. The mechanism for such an improvement is investigated by photoelectron yield spectroscopy, X-ray photo electron energy spectrum, and field emission technique. Sur- face oxidation and composition change are found as the reasons for work function increase. The UV-treated PEDOT：PSS film is used as the hole injection layer in a hole-only device. Hole injection is improved by UV-treated PEDOT：PSS film without baring the enlargement of film resistance. Our result demonstrates that UV treatment is more suitable for modifying the injection barrier than UV ozone exposure.     

Bathocuproine/Ag复合电极对于聚合物光伏器件效率和稳定性的影响

采用Bathocuproine/Ag (BCP/Ag)复合电极代替Ca/Al复合电极, 制备PTB7:PC71BM 作为光敏层的聚合物光伏器件, 并通过改变BCP薄膜厚度来研究BCP/Ag复合电极对于器件光电转换器和稳定性的影响. 研究发现: 在光敏层和金属电极之间插入BCP修饰层后, 器件性能得到了显著的改善, 在BCP厚度为5 nm时, 器件的效率达到了6.82%, 且略高于Ca/Al复合电极的器件效率; 相比于采用Ca/Al复合电极的器件, BCP/Ag复合电极增大了器件的短路电流和外量子效率, 使器件效率得到提高; 同时器件的稳定性得到了显著的改善, BCP/Ag 复合电极器件的衰减速率几乎和未插入BCP的器件衰减速率相同, 相对于Ca/Al复合电极器件大幅提高.     

The work function of hydrothermally synthesized UO2 and the implications for semiconductor device fabrication

The photoelectric work function of nearly stoichiometric (111) and (100) hydrothermally grown UO₂ was measured to be 6.28 ± 0.36 eV and 5.80 ± 0.36 eV, respectively. Candidate metals for electrical contacts are identified for both rectifying and non‐rectifying contacts based on work function, lattice compatibility, and electrical conductivity.     

Molecular-dynamics simulation of a glassy polymer melt: Incoherent scattering function

We present simulation results for a model polymer melt, consisting of short, nonentangled chains, in the supercooled state. The analysis focuses on the monomer dynamics, which is monitored by the incoherent intermediate scattering function. The scattering function is recorded over six decades in time and for many different wave-vectors which range from the size of a chain to about three times the maximum position of the static structure factor. The lowest temperatures studied are slightly above , the critical temperature of mode-coupling theory (MCT), where was determined from a quantitative analysis of the - and -relaxations. We find evidence for the space-time factorization theorem in the -relaxation regime, and for the time-temperature superposition principle in the -regime, if the temperature is not too close to . The wave-vector (q-) dependence of the nonergodicity parameter, of the critical amplitude, and the -relaxation time are in qualitative agreement with calculations for hard spheres. For q larger than the maximum of the structure factor the -relaxation time already agrees fairly well with the asymptotic MCT-prediction . The behavior of the relaxation time at small q can be rationalized by the validity of the Gaussian approximation and the value of the Kohlrausch stretching exponent, as suggested in neutron-scattering experiments. Received 30 October 1998     

Study of dynamic behavior of trapped charge in the insulating materials by using a new experimental approach

A new experimental procedure is proposed which uses the Scanning Electron Microscope Mirror Effect (SEMME) for the dynamic investigation of charge trapping in insulating materials. This experimental procedure allows to the mirror method which is usually described as static to be considered as dynamic. The dynamic trapping properties of the polymethyl-methacrylate (PMMA) polymer material are investigated and the time constants of charge processes are evaluated for various accelerating voltages. The experimental results reveal that the values of trapped charge at steady state obtained by the proposed procedure are in good agreement with these obtained using the electrostatic influence method (EIM) under the identical experimental conditions of electron irradiation.     

载流子复合及能量无序对聚合物太阳电池开路电压的影响

聚合物太阳电池中载流子的复合与能量无序对器件的开路电压有着深刻的影响.本文同时研究了基于传统富勒烯(PC71BM)和非富勒烯(O-IDTBR)电子受体的聚合物太阳电池.通过交流阻抗谱、低温电流密度-电压谱、瞬态光电压以及电致发光光谱等手段重点研究了载流子复合及能量无序对电池器件开路电压的影响.具体地,交流阻抗谱和瞬态光电压测试结果表明,富勒烯体系载流子复合损失较为严重.电致发光光谱研究显示,PC71BM器件的发光峰随着注入电流的增加不断向短波长处移动,而O-IDTBR体系发光峰位置基本不变,该结果证明PC71BM体系中能量无序度更高.载流子复合严重及能量无序度更高共同作用导致了富勒烯器件开路电压的降低.     

第一性原理研究氧在Ni(111)表面上的吸附能及功函数

采用基于密度泛函理论（DFT）广义梯度近似（GGA）下的第一性原理方法系统地研究了不同覆盖度下O在Ni（111）表面的吸附特性.计算结果表明,O在Ni（111）表面的稳定吸附位为三重面心立方（fcc）洞位,吸附能随着覆盖度的增加而减小,O诱导Ni（111）表面功函数的变化量与覆盖度成近线性关系,并随着覆盖度的增加而增大.同时,通过对电子密度和分波态密度的分析发现：O在Ni（111）表面的吸附使得Ni表面电子向O原子转移,形成表面偶极矩,导致功函数增加;表面Ni原子的3d轨道和O的2p轨道通过耦合、杂化作用形成成键态和反键态,而反键态几乎不被占据,因而O—Ni键相互作用比较强,吸附能较大. 关键词： 表面吸附 密度泛函理论 吸附能 功函数     

局域功函数图像及其在Cu(111)-Au/Pd表面的应用

用扫描隧道显微镜（STM）对Cu(111)-Au和Cu(111)-Pd表面的局域功函数进行了研究.通过 测量隧道电流对针尖样品间距的响应，得到了与STM形貌图一一对应的表面局域功函数图像. 实验发现，Au/Pd覆盖层和Cu衬底间的功函数有明显的不同.Pd薄膜的功函数甚至超过了其体 本征值，且功函数在台阶处变小.用偶极子的形成解释了台阶处功函数的降低.这一工作表明 ，用测量局域功函数的方法容易区分表面上不同的元素，并具有纳米尺度的空间分辨率. 关键词： 扫描隧道显微镜 局域功函数 台阶     

Mapping of the electronic work function anisotropy of RF sputtered molybdenum thin film electrodes for piezoelectric devices

Amplitude modulated Kelvin probe force microscopy was performed on molybdenum (Mo)-thin films deposited on Si(001) substrates by RF magnetron sputtering. Evolution of film microstructure from amorphous to crystalline was observed with increasing RF power from 25 W to 200 W. Spatial mapping of work function across the film surface revealed that the Mo-thin film deposited at 200 W possesses an average work function ~4.94 ± 0.06 eV while higher values were observed at lower RF powers. The genesis of distinct periodic changes in work function is attributed to the formation of the surface dipole layer associated with the adsorbates of different polarities (O₂⁻/OH⁻ or H⁺). A phenomenological model is also presented to elucidate their effect.     

Temperature dependence of the work function of hafnium islands on tungsten

Two-dimensional islands of hafnium near {100{ faces of tungsten and their terraces were examined by field electron microscopy. The two-dimensional sublimation energy of the islands was found to be 6.1±0.5 eV. The field electron emission from the islands grows with temperature, the effect being considerably greater than theoretically expected. The negative temperature coefficient of ϕ, α_T, for the islands was estimated at (4–6)×10⁻⁴ eV grad⁻¹ or more, whereas for pure tungsten α_T was found to be 2×10⁻⁵ eV grad⁻¹.     

Enhancing the performance of polymer light-emitting diode via methanol treatment

Enhanced performance of polymer light-emitting diodes (PLEDs) was realized by performing methanol treatment on the emissive layer. Experimental results show that with methanol treatment the electron current increased owing to the rougher surface topography of the emissive layer. Therefore, the balance between electron and hole current was improved. The maximum luminous efficiency and external quantum efficiency of PLEDs increased from 19.7 cd/A and 6.89% to 22.5 cd/A and 7.87%, respectively.     

The timescale of spinodal dewetting at a polymer/polymer interface

We investigate the dynamics of spinodal dewetting in liquid-liquid polymer systems. Dewetting of poly(methyl-methacrylate) (PMMA) thin films on polystyrene (PS) “substrates” is followed in situ using neutron reflectivity. By following the development of roughness at the PS/PMMA interface and the PMMA surface we extract characteristic growth times for the dewetting process. These characteristic growth times are measured as a function of the molecular weight of the two polymers. By also carrying out experiments in the regime where the dynamics are independent of the PS molecular weight, we are able to use dewetting to probe the scaling of the PMMA thin film viscosity with temperature and molecular weight. We find that this scaling reflects bulk behaviour. However, absolute values are low compared to bulk viscosities, which we suggest may be due in part to slippage at the polymer/polymer interface. Received 25 June 2001 and Received in final form 5 December 2001