Dielectric spectroscopy of ultrathin ferroelectric polymer films

The dielectric dispersion of poly(vinylidene fluoride-trifluoroethylene) copolymer films prepared using the Langmuir-Blodgett technique is studied. All films of different compositions and thicknesses undergo a Debye-type dielectric relaxation. The phenomenon of critical slowing down is investigated. It is found that the relaxation time τ and the kinetic coefficient ξ depend on the thickness of the film.     

有机功能超薄膜铁电性研究进展

扼要叙述了有机铁电LB膜的制备方法，着重介绍了近几年来有机LB膜的铁电性应用与研究的进展．     

Stress and magnetic properties of surfaces and ultrathin films

Stress measurements with sub-monolayer sensitivity are performed to investigate the correlation between mechanical film stress and magneto-elastic anisotropy in epitaxial ferromagnetic monolayers. The magneto-elastic coupling B₁ of Fe(1 0 0) films is measured directly. Magnitude and sign of B₁ deviate from the respective bulk value. A strain-dependent correction of the magneto-elastic coupling coefficient B₁ describes the apparent thickness dependence of B₁ for film thicker than 10 nm. For thinner films, the possible contribution of surface corrections is discussed to explain the almost constant B₁. The implications of a modified magneto-elastic coupling for the anisotropy of ultrathin films is elucidated.     

Thermodesorption of ultrathin organic films studied by reflection

Optical reflection measurements are described as a new and simple technique to measure thicknesses and thickness changes of ultrathin organic overlayers on inorganic support. The prime advantages are high sensitivity in thickness measurements (0.1–1 Å), applicability under varying environmental conditions, nondestructiveness, applicability on different supports and high time resolution. These features are assessed theoretically and experimentally studying Langmuir-Blodgett films on SiO₂ and Au supports. These films, in this work prepared with the model compound arachidic acid, provide the salient feature of well-defined thicknesses in integer multiples of 26.7 Å. Application of the technique in thermodesorption experiments reveals distinguished binding states with binding energies (73–110 kJ/mol) depending on the counterion (Mg²⁺ or Cd²⁺) used in preparing the films and ratios of different states depending on thickness. The binding energy for the main component of multilayers (73 and 81 kJ/mol) with thickness larger than 100 Å is comparable to the heat of evaporation of bulk fatty acids.In addition an analytic expression between layer thicknesses and reflection is derived from the Fresnel equation.     

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     

Growth of ultrathin Pd films on Al(001) surfaces

High-energy ion backscattering spectroscopy (HEIS) and X-ray photoelectron spectroscopy (XPS) were used to determine the growth mode and the interface structure of ultrathin Pd films deposited on Al(001) surfaces at room temperature. Measured Al and Pd surface peak areas for MeV He⁺ ions incident normal to the surface show that Pd atoms intermix with and displace Al substrate atoms. The mixing continues for Pd coverages from 0–5 monolayers, at which point a Pd metal film begins to grow on the alloy surface. XPS measurements of the Pd 3d photopeaks show a chemical shift that is consistent with the formation of an AlPd-like compound during the mixing phase, and Pd metal thereafter. HEIS results further reveal that the alloyed overlayer as well as the Pd metal film have some degree of axial alignment with respect to the Al substrate. The XPS intensity measurements are consistent with this two-stage growth model.     

Conducting-AFM spectroscopy on ultrathin SiO2 films

We study the dielectric degradation phenomena of ultrathin SiO₂ films using conducting-AFM spectroscopy in a vacuum (1᎒^-5 Pa). In successive current-voltage characteristics, a change of the carrier transport (from Fowler-Nordheim tunneling to direct tunneling) and current instability (which is due to a change in conductance of leakage path by isolated traps) are observed before 'breakdown'. As an example of a possible application of conducting-AFM spectroscopy, we investigate a change in conductivity at a 'breakdown' area.     

Dielectric properties of ultrathin films of molecular crystals

Dielectric properties of thin films (TF) of molecular crystals, including the effect of size and the boundary surfaces were analysed using Green's function method. Polarisability of molecules in various film layers and dielectric susceptibility of TF were calculated. A comparison with crystal bulk has shown that dielectric properties of TF are strongly influenced both by the sample dimensions and by the boundary conditions.The frequency dependence of the dielectric susceptibility has also been derived. One obtains the monotonous variation for the frequencies above and below exciton band. However, for the frequencies within the exciton band there appears to be complicated, non-monotonous dependence. The relationship between the dielectric susceptibility and the film width for the same frequency shows a complex, oscillatory behaviour. Furthermore, the amplitude of these oscillations increases with increasing film width, demanding the introduction of a damping factor. Finally, the thickness dependence of dielectric susceptibility was analysed in the Cole-Cole plot.     

Structural manipulation of ultrathin organic films on metal surfaces: the case of decane thiol/Au(111)

3 (CH₂)₉]SH) have been adsorbed on Au(111) single crystals both via vacuum deposition and from an ethanolic solution. The epitaxial structure of the ultrathin organic films has been identified at room temperature via low-energy electron diffraction to be c(4×2)R30° for the solution grown film and rectangular c(23×) for the vacuum deposited film. These structures correspond to molecules adsorbed on the surface with their carbon chains flat on the surface (vacuum deposited) and nearly perpendicular (solution grown). It is demonstrated that this orientation can be changed reversibly in vacuum via either annealing the films or exposing them to additional gas. Received: 7 February 1997/Accepted: 27 May 1997     

有序组装超薄膜热释电性能的优化研究

报道两亲性染料半花菁(DAEP)与隔层材料氮冠醚(NC)交替LB膜的热释电效应以及掺杂金属离子（Ba2+）对LB膜热释电性能的影响.发现所测样品的热释电系数p高达58μCm-2K-1；在频率为1kHz—100kHz的范围内，其εr和tanδ的数值分别为2.34—1.96和0.08—0.04.并讨论了不同成膜方式和掺杂金属离子Ba2+对LB膜热释电性能影响的物理机理.     

Spectral investigations of surface ordering in ultrathin molecular films

Surface molecular ordering in ultrathin molecular films is investigated. The optical transmission spectra of molecular films ranging in thickness from 2 to 13 smectic layers (6.7–43 nm) in the region of the electronic absorption bands in the smectic A phase of cyanobiphenyl CB9 are measured. The thickness and temperature dependences of the permittivity are determined. It is found that the orientational ordering of the molecules depends on the film thickness. The penetration depth of the surface molecular orientational order does not exceed two smectic layers (<7 nm). Zh. éksp. Teor. Fiz. 115, 1833–1842 (May 1999)     

Ferromagnetic order and critical behavior at surfaces of ultrathin epitaxial films

The critical behavior, ferromagnetic order and magnetic anisotropies of ultrathin, epitaxial, magnetic films is studied using electron capture spectroscopy (ECS), which is capable of probing the long-ranged and short-ranged electron spin polarization (ESP) at the topmost surface layer of uncoated and coated magnetic structures. For all systems [Ni(100)/Cu(100), Ni(100)/NaCl(100), fcc Fe(111)/Cu(111), Fe(100)/Ag(100), Tb/Fe(100)/Ag(100), Fe(100)/Au(100), hcp Tb(0001)/W(110), Fe(110)/W(110), V(100)/Ag(100), Pd(100)/Ag(100), Pd/W(110)] investigated so far, ferromagnetic order is detected. It is found that the surface Curie temperatureT _Cs depends on film thicknessd. ECS data obtained at the surface of various systems reveal the existence ofT- andd-dependent magnetic anisotropies. Although for V(100)/Ag(100) the measured critical exponent=0.128 agrees very well with=1/8 predicted for the two-dimensional Ising model, for other systems, such as Fe(100)/Au(100), the measured value (0.25) is in disagreement with theoretical predictions. The experimental results are discussed within the framework of presently available experimental and theoretical data.     

Characterization of the photo-irradiation effects on polystyrene ultrathin films with ultraviolet photoemission spectroscopy

The synchrotron-radiation-induced structural degradation in polystyrene thin films was analyzed by comparing measured ultraviolet photoemission spectroscopy (UPS) spectra with model molecular orbital calculations. The analysis confirms degradation of the phenyl pendant groups and the generation of conjugated double-bonds in the degraded polymer. The observed degradation in polystyrene is found to be very similar to one from polyethylene previously investigated.     

Accelerated aging in ultrathin films of a molecular glass former

We report the thermodynamic measurement of the enthalpy released during the aging of supported films of a molecular glass former, toluene, at temperatures well below the glass transition temperature. By using microfabricated devices with very short equilibration times (below 1 s), we evidence a remarkable variation of the relaxation rate on decreasing film thickness from 100 nm down to a 7 nm thick film. Our results demonstrate that surface atoms are more efficient than bulk atoms in attaining low energy configurations within the potential energy landscape.     

Vacuum ultraviolet ellipsometry investigation of ultrathin organic films and their heterostructures

Thin layers and multilayer structures of the DNA bases guanine and cytosine on ZnO substrates were prepared by organic molecular beam deposition under ultra-high vacuum conditions and measured in situ by means of vacuum ultraviolet spectroscopic ellipsometry at the synchrotron source BESSY. Using the dielectric function of the individual layers the optical response of the G/C and C/G heterostructures was modeled. Deviations between simulated and experimental data were mainly attributed to the ordering of cytosine over guanine in the G/C and C/G structures and to the influence of the substrate.     

Structural and magnetic properties of ultrathin films by Mössbauer spectroscopy

The magnetic properties of ultrathin films are determined first of all by their structure. The experimental technique giving simultaneously information about the structure and the magnetic behaviour of the film would be optimum. To follow this idea the information about the film structure, obtained from Mossbauer spectra, are discussed.