Molecular Thin Films Research Articles

Study on Anomalous Scaling Exponents for Molecular Thin Film Growth Using Surface Lateral Diffusion Model

Anomalous scaling behaviors such as significantly large growth exponent (<TEX>${\beta}$</TEX>) and small reciprocal of dynamic exponent (1/z) values for many molecular crystalline thin films have been reported. In this study, the variation of scaling exponent values and consequent growth behaviors of molecular thin films were more quantitatively analysed using a (1+1)-dimensional surface lateral diffusion model. From these simulations, influence of step edge barriers and grain boundaries of molecular thin films on the various scaling exponent values were elucidated. The simulation results for the scaling exponents were also well consistent with the experimental data for previously reported molecular thin film systems.

A Comparison Study of the Organic Small Molecular Thin Films Prepared by Solution Process and Vacuum Deposition: Roughness, Hydrophilicity, Absorption, Photoluminescence, Density, Mobility, and Electroluminescence

In this work, we report a systematic study of the properties of the solution-processed organic small molecular thin films in order to get an in-depth understanding of the distinctive nature of solution-processed thin films. N,N′-Di(3-methylphenyl)-N,N′-diphenyl-(1,1′-biphenyl)-4,4-diamine (TPD), a typical hole-transporting compound, was chosen to fabricate films by both solution process and vacuum deposition. The comparison study between the two kinds of TPD films shows that the surface of the solution-processed TPD films is more smooth and hydrophilic than that of the vacuum-deposited film. In addition, the density of the TPD films was measured by two different methods, and an increase in density was observed in solution-processed films in comparison with the vacuum-deposited film. The space-charge-limited current (SCLC) measurements indicate an increased hole transport mobility of the solution-processed TPD films over that of the vacuum-deposited film, which may result from the more compact intermolecul...

Measuring Photochemical Kinetics in Submonolayer Films by Transient ATR Spectroscopy on a Multimode Planar Waveguide

Understanding the kinetics of reactions in molecular thin films can aid in the molecular engineering of organic photovoltaics and biosensors. We have coupled two analytical methods, transient absorbance spectroscopy (TAS) and attenuated total reflectance (ATR), in a relatively simple arrangement when compared with previous TAS/ATR instruments to interrogate molecular structure and photochemistry at interfaces. The multimode planar waveguide geometry provides a significant path length enhancement relative to a conventional transmission geometry, making it feasible to perform measurements on low-surface-coverage films. The performance of the instrument was assessed using a thin film composed of purple membrane (PM) fragments containing bacteriorhodopsin deposited onto PDAC, a positively charged polymer. The surface coverage of retinal chromophore in this film is ∼0.1 monolayer and its orientation distribution is anisotropic, with a mean tilt angle of 68° from surface normal. After photoinduced formation of the transient M state, the chromophore decays to the ground state in 4.4 ± 0.6 ms, equivalent to the decay of suspended PM fragments, which shows that deposition on PDAC does not alter M-state photokinetics. The surface coverage of the M state is calculated to be 2 pmol/cm(2), which is ∼1% of a close-packed monolayer. This work demonstrates that TAS/ATR can be used to probe structure and photochemical kinetics in molecular films at extremely low surface coverages.

Effect of molecular film thickness on thermal conduction across solid-film interfaces

The Brownian motion and aggregation of particles in nanofluids often lead to the formation of solid-film-solid structures. The molecular thin film confined between nanoparticles may have non-negligible effects on thermal conduction among nanoparticles. Using nonequilibrium molecular dynamics simulations, we study thermal conduction across the Ag particle-Ar thin-film interface. If the film contains only one molecular layer, we find that the solid-film interfacial thermal resistance R(SF) is about 1 order of magnitude smaller than the solid-liquid (bulk) interfacial thermal resistance R(SL). If there are two or more molecular layers in the film, it is shown that R(SF) increases rapidly toward R(SL) as film thickness increases. By comparing the vibrational density of states of Ag atoms and Ar molecules in the film, we demonstrate that the low thermal resistance in the monolayer film case is caused by the resonant thermal transport between Ag particles and Ar thin films.

Organic spin valves with inelastic tunneling characteristics

Electrons may experience inelastic coupling with the organic spacer layer during tunneling between two ferromagnetic electrodes. To probe the transport behavior of spin-polarized electrons in organic materials, organic spin valves were fabricated utilizing a relatively thin organic barrier of 3,4,9,10-perylene-teracarboxylic dianhydride (PTCDA) dusted with alumina at the organic/ferromagnetic interfaces. These structures, with an organic barrier layer, exhibited magnetoresistance up to 12% at room temperature. In studies of the inelastic tunneling spectrum, the observed characteristic peak of the organic layer provides direct evidence of the interplay between the spin-polarized electrons and the organic molecules. Combining the inelastic tunneling results with a simple molecular vibration calculation yields further information on the configuration of the molecular thin film and the possible tunneling states of the spin-polarized electrons. Such interplay indicates a true transport of spin-polarized electrons through organic material rather than through defects or interdiffusion compounds formed at the interfaces within the organic spin valve.

Performance Comparison of Different Organic Molecular Floating-Gate Memories

In this paper, nanosegmented floating-gate memories consisting of a uniform set of identical organic dye molecules were fabricated and evaluated for potential use as programmable charge storage and charge retention elements in a future flash-memory technology. Viability of molecular thin films to serve as an energetically uniform set of ~1 nm in size charge-retaining sites is tested on a series of molecular materials, the best performing of which are thermally evaporated thin films of 3,4,9,10- perylenetetracarboxylic bis-benzimidazole. The initial results show device durability over 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">5</sup> program/erase cycles, with hysteresis window of up to 3.3 V, corresponding to charge-storage density as high as 5 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">12</sup> cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-2</sup> . Data shows that charge retention is improved for molecular films with lower carrier mobility, which for the first time experimentally confirms in a coherent material set that inhibiting charge transport by nanosegmented floating-gate structures benefits the memory retention.

Quantum confinement and anisotropy in thin-film molecular semiconductors

Unoccupied electronic states are probed in epitaxial films of metal phthalocyanines on Ag(111) using angle-resolved two-photon photoemission spectroscopy. Quantum confinement of an image state wave function results in band folding and the opening of a band gap. Angle-resolved photoemission intensity measurements resolve the probability density of the wave function, and are able to identify the symmetry of each band. These results are compared with predictions from a two-dimensional scattering model, and they point to the importance of the spatial extent and symmetry of functional groups in predicting and controlling electronic structure on surfaces and in molecular thin films.

Electro-optic effect in crystalline films of transverse planar octupolar symmetry

We present theoretical and experimental demonstrations of the electro-optic activity in crystalline molecular thin films with octupolar D(3h) symmetry. Applying a longitudinal electric field modulation within the molecular plane, we analyze the induced refractive index change relative to the orientation of the octupoles in their plane, and show that a maximum value is reached when one octupolar branch lies along the direction of the modulating field. These characteristics, as well as their electric field dependence, are drastically different from more traditional one-dimensional symmetry samples, bringing additional advantages related to electro-optic coupling possibilities.

Roles of 2D Liquid in Reduction of the Glass-Transition Temperature of Thin Molecular Solid Films

To clarify the properties of a liquidlike phase formed on the surface, self-diffusion of molecules in monolayer and multilayer films has been investigated by measuring their uptake behaviors in porous Si substrates using time-of-flight secondary ion mass spectrometry and temperature programmed desorption. It is found that self-diffusion of water, ethanol, and 3-methylpentane in the first monolayer commences at temperatures of ca. 110, 85, and 50 K, respectively, which are considerably lower than the corresponding glass-transition temperatures (Tg) in the bulk. The surface mobility of water appears to be not influenced by hydrogen bonds with substrates: The molecules form droplets on the H-terminated Si substrate at 110 K, whereas they start to diffuse into pores of the OH-terminated Si substrate at the same temperature. The ethanol and 3-methylpentane molecules tend to diffuse into pores of both hydrophilic and hydrophobic Si substrates, although a small number of much slower diffusers remain on the surface. Multilayer films are also incorporated into pores at temperatures well below Tg because of sequential diffusion of the topmost-layer molecules. The origins of nanoconfinement effects on reduction of Tg are discussed based on these findings.

Surface-Driven Porphyrin Self-Assembly on Pre-Activated Si Substrates

Self-assembled molecular thin films of melamine-bridged bis-porphyrin dyad, both in free form, P(H2)P(H2), or as Zn(II) metallated complex, P(Zn)P(Zn), were deposited on crystalline Si(100) by soaking or drop-casting techniques. The influence of the adopted preparation methodology, the substrate surface pre-activation procedure and the used solvent (THF or CHCl3) on the morphology and surface coverage of the resulting films was investigated by FE-SEM (Field Emission-Scanning Electron Microscopy) and AFM (Atomic Force Microscopy). The chemical composition and electronic structure of the most promising systems were also addressed by XPS (X-ray Photoelectron Spectroscopy). The results pointed out that an accurate tuning of porphyrin-porphyrin, porphyrin-substrate and porphyrin-solvent molecular interactions allow to tailor the morphogenesis and chemico-physical properties of the final self-assembled films. In addition, preliminary gas sensing tests evidenced the potential of the present porphyrin-based films for the development of new molecular sensing devices.

Very Large Silacylic Substituent Effects on Response in Silole-Based Polymer Transistors

Understanding the interrelationships between molecular structure and organic thin film transistor performance is key to the realization of novel organic semiconductors achieving superior device characteristics. Herein we report the synthesis, characterization, and charge-transporting properties in organic field-effect transistors (OFETs) of dithieno silole-based oligomers and copolymers having silacycloalkyl substituents. Silacyclization of the alkyl substituents on the silole silicon atom reduces steric encumbrance, contracts solid state intermolecular π−π contacts, and enhances the charge-transport capacity of the oligomers. Oligomer 3,3′-dihexylsilylene-2,2′:5,2′′:5′,2′′′:5′′,2′′′′:5′′′,2′′′′′-sexithiophene (SM5) with two Si-n-hexyl substituents is not FET-active, while the mobilities of 3,3′-cyclopentanylsilylene-2,2′:5,2′′:5′,2′′′:5′′,2′′′′:5′′′,2′′′′′′-sexithiophene (SM4) and 3,3′-cyclobutysilylene-2,2′:5,2′′:5′,2′′′:5′′,2′′′′:5′′′,2′′′′′-sexithiophene (SM3) FETs are 2.6 × 10−4 and 3.4 × 10−4 cm2/(V...

Study on Electronic Absorption and Surface Morphology of Double Layer Thin Films of Phthalocyanines

The electronic absorption and surface morphology evolution of two types of molecular double layer thin films, copper phthalocyanine (CuPc) layer deposited on chloro[subphthalocyaninato]boron(III) (SubPc) layer, denoted as SubPc/CuPc, and vice versa, with various thicknesses were investigated using ultraviolet (UV)-visible spectroscopy and atomic force microscopy (AFM). Both types of double layer structures showed similar broadened absorption patterns in the UV-visible region that were consistent with the fitted spectra following simple linear combination of the single layer absorption spectra of the two materials. In contrast, the surface morphology of double layer structures was dependent on the order of deposition. For the CuPc/SubPc structures, surface morphology was characterized by elongated grains, which are characteristic of SubPc thin films, indicating that the morphological influence of the underlying CuPc layer on the subsequent SubPc layer was not large. For the SubPc/CuPc structures, however, the underlying SubPc layer acted as a morphological template for the subsequently deposited CuPc layer. It was also observed that the grain size of the CuPc layer varied according to the thickness of the underlying SubPc layer.

Structural templating and growth behavior of copper phthalocyanine thin films deposited on a polycrystalline perylenetetracarboxylic dianhydride layer

Structural templating and the growth behavior of copper phthalocyanine (CuPc) thin films deposited on a polycrystalline 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA) layer were examined using x-ray diffraction (XRD) and atomic force microscopy. The templated CuPc films were found to grow on both the (110) and (102) plane of the α-PTCDA layer, indicating the intermolecular π-π interaction is strong enough to induce templating even on the underlying layer which is tilted at an angle of 25° from the substrate surface as well as on the surface-parallel layer. In contrast to the large growth exponent (β) values for other single layer molecular thin films, a significantly small β value of 0.17 ± 0.06 was obtained on the PTCDA/CuPc heterolayer thin film system. The XRD and scaling behavior studies suggest that this relatively slow surface roughening can be rationalized by the lack of surface parallel crystalline ordering at the initial stage of CuPc film growth on the PTCDA layer.

Study of a hydrogen-bombardment process for molecular cross-linking within thin films

A low-energy hydrogen bombardment method, without using any chemical additives, has been designed for fine tuning both physical and chemical properties of molecular thin films through selectively cleaving C-H bonds and keeping other bonds intact. In the hydrogen bombardment process, carbon radicals are generated during collisions between C-H bonds and hydrogen molecules carrying ∼10 eV kinetic energy. These carbon radicals induce cross-linking of neighboring molecular chains. In this work, we focus on the effect of hydrogen bombardment on dotriacontane (C(32)H(66)) thin films as growing on native SiO(2) surfaces. After the hydrogen bombardment, XPS results indirectly explain that cross-linking has occurred among C(32)H(66) molecules, where the major chemical elements have been preserved even though the bombarded thin film is washed by organic solution such as hexane. AFM results show the height of the perpendicular phase in the thin film decreases due to the bombardment. Intriguingly, Young's modulus of the bombarded thin films can be increased up to ∼6.5 GPa, about five times of elasticity of the virgin films. The surface roughness of the thin films can be kept as smooth as the virgin film surface after thorough bombardment. Therefore, the hydrogen bombardment method shows a great potential in the modification of morphological, mechanical, and tribological properties of organic thin films for a broad range of applications, especially in an aggressive environment.

Predicting organic thin-film transistor carrier type from single molecule calculations

The hole- or electron-conducting ability of functionalized acene-derivative organic molecular semiconductor thin-films can be predicted by electrochemical measurements of the frontier molecular orbital energies. We find that PM6 calculations using a Conductor-like Polarizable Continuum Model (CPCM) solvent are in complete qualitative agreement with the electrochemical results, and B3LYP/6-311+G(d,p)//B3LYP/6-31G(d) CPCM calculations are in quantitative agreement with the electrochemical results, given a suitable linear scaling function presented in this paper. These calculated electrochemical results can thus be used to predict the charge-carrier type of the resulting thin-film organic semiconductor device, enabling rapid computational screening of new materials. We also examine the role of internal reorganization energy and dielectric medium shifts to modify carrier conduction. Neither effect is large enough to explain the experimental results, demonstrating the importance of the relative orientations and packing of the molecules in the thin-film.

Study of the charge transport characteristics of dendrimer molecular thin films

In this work, we systematically studied the electrical characteristics of two types of dendritic arylamine thin film devices. We observed that, for devices with different interfacial structures, their charge injection barriers and transport properties are obviously different. The smallest charge injection barrier is observed in dendrimer devices without charge-transfer interfacial layers. The Richardson–Schottky thermionic emission model can be well used to fit the experimental current–voltage characteristics at a lower voltage region. The charge injection barrier increases about 0.4 eV and 0.5 eV when a 1-decanethiol self-assembly layer and -CN terminated dendrimer thin films are inserted as the interfacial layer, respectively. It is shown that the molecule/electrode charge-transfer interfaces can largely affect the device charge injection/transport process and consequently change the device performance. In this case, the space charge limited conduction theory is more applicable to simulate the device conduction mechanism. Owing to its ultra-thin thickness, the self-assembly monolayer technique is proved to be an efficient approach in engineering the interfacial electronic structures of dendrimer thin film devices.

An Observation of Diamond-Shaped Particle Structure in a Soya Phosphatidylcohline and Bacteriorhodopsin Composite Langmuir Blodgett Film Fabricated by Multilayer Molecular Thin Film Method

A composite film of soya phosphatidylcohline (soya PC) and bacteriorhodopsin (BR) was fabricated by the multilayer molecular thin film method using fatty acid and lipid on a quartz substrate. Direct Force Microscopy (DFM), UV absorption spectra and IR absorption spectra of the film were characterized on the detail of surface structure of the film. The DFM data revealed that many rhombus (diamond-shaped) particles were observed in the film. The spectroscopic data exhibited the yield of M-intermediate of BR in the film. On our modelling of molecular configuration indicate that the coexistence of the strong inter-molecular interaction and the strong inter-molecular interaction between BR trimmers attributed to form the particles.

Tailoring the Morphology and Dewetting of an Organic Thin Film

Methods of tailoring molecular thin film morphology and maturation rate were investigated by noncontact atomic force microscopy. Submonolayer coverages of 3,4,9,10-perylenetetracarboxylic diimide deposited on alkali-halides form films of needle-shaped islands, and undergo a dewetting transition when deposited on NaCl. The resulting island surface distribution, size, shape, and rate of dewetting may be varied by changing growth conditions such as temperature and by templating the substrate with single atomic layer deep pits or depositing gold nanoclusters to modify island nucleation. This characterization is an important step in controlling the structure of thin organic films for devices that are sensitive to nanoscale film structure.

Formation, surface characterization, and electrocatalytic application of self-assembled monolayer films of tetra-substituted manganese, iron, and cobalt benzylthio phthalocyanine complexes

Molecular thin films of manganese (SAM-2), iron (SAM-3), and cobalt (SAM-4) phthalocyanine complexes, non-peripherally tetra-substituted with benzylmercapto, were formed on polycrystalline gold disc electrode by self-assembly technique. Surface characteristics of the films were interrogated by cyclic voltammetry. Significant passivation of voltammetry processes associated with bare gold surface (gold oxidation and underpotential deposition of copper) confirmed formation of the films. Electrocatalytic property of the films was evidenced from better voltammetry responses (less positive oxidation potential and better current signal) of the insecticide, carbofuran, on these films, relative to that on bare gold electrode. In terms of less positive oxidation potential, the FePc derivative (3) gave the best response, while the best current signal was observed on SAM-2-modified gold electrode. The average heterogeneous rate constant, k, for the oxidation of carbofuran was 3.6 × 10−2 cm s−1 on the SAM film with the best current signal (SAM-2).

Single crystalline Pr2−xYxO3 (x=–2) dielectrics on Si with tailored electronic and crystallographic structure

Crystalline oxides on Si with tailored electronic and crystallographic properties are of importance for the integration of functional oxides or alternative semiconductors to enable novel device concepts in Si microelectronics. We present an electronic band gap study of single crystalline Pr2−xYxO3 (0≤x≤2) heterostructures on Si(111). The perfect solubility of the isomorphic bixbyites Pr2O3 and Y2O3 during molecular beam epitaxy thin film growth on Si enables a linear band gap tuning. Special focus is devoted to the determination of the electronic band offsets across the dielectric/Si interface. In addition, the composition x allows to control the crystallographic lattice parameter where, for example, Pr0.8Y1.2O3 enables the growth of fully lattice matched oxide heterostructures on Si.