Conducting AFM and 2D GIXD studies on pentacene thin films

Among all organic semiconductors, pentacene has been shown to have the highest thin film mobility reported to date. The crystalline structure of the first few pentacene layers deposited on a dielectric substrate is strongly dependent on the dielectric surface properties, directly affecting the charge mobility of pentacene thin film OTFTs. Herein, we report that there is a direct correlation between the crystalline structure of the initial submonolayer of a pentacene film and the mobility of the corresponding 60-nm-thick films showing terrace-like structure, as confirmed by 2D grazing-incidence X-ray diffraction and atomic force microscopy. Specifically, multilayered pentacene films, grown from single crystal-like faceted islands on HMDS-treated surface, have shown much higher charge mobility (mu = 3.4 +/- 0.5 cm2/Vs) than those with polycrystalline dendritic islands (mu = 0.5 +/- 0.15 cm2/Vs) on OTS-treated ones.     

XRD and AFM studies of ZnS thin films produced by electrodeposition method

The structure and morphology of ZnS thin films were investigated. ZnS thin films have been grown on an indium tin oxide glass substrate by electrodeposition method using zinc chloride and sodium thiosulfate solutions at room temperature. The X-ray diffraction patterns confirm the presence of ZnS thin films. From the AFM images, grain size decreases as the cathodic potential becomes more negative (from ?1.1 to ?1.3 V) at various deposition periods. Comparison between all the samples reveals that the intensity of the peaks increased, indicating better crystalline phase for the films deposited at ?1.1 V. These films show homogeneous and uniform distribution according to AFM images. On the other hand, XRD analysis shows that the number of ZnS peaks increased as deposition time was increased from 15 to 30 min at ?1.1 V. The AFM images show thicker films to be formed at ?1.1 V indicating more favourable condition for the formation of ZnS thin films.     

Using wavelets to analyze AFM images of thin films: surface micelles and supported lipid bilayers

This paper presents micro- and nanoanalysis of thin films based on images obtained by atomic force microscopy (AFM). The analysis exploits the discrete wavelet transform and the resulting wavelet spectrum to study surface features. It is demonstrated that the wavelet technique can characterize micro- and nanosurface features and distinguish between similar surface structures. The use of a feature extraction method is shown. The method involves the separation of certain frequency content from the original AFM images and analyzing the data independently to gain quantitative information about the images. By using the feature extraction method, soft surfaces in water are analyzed and nanofeatures are measured. The packing of surface micelles of sodium dodecyl sulfate on a self-assembled monolayer is analyzed. The characteristics of pore formation, due to penetration of the antibacterial peptide protegrin, into a solid-supported lipid bilayer are quantified. The sizes of the pores are obtained, and it is observed that the line tension of the pores reduces the fluctuations of the lipid bilayer.     

Nanotribological behavior of thermal treatment of zinc titanate thin films

We present a study of the nanotribological behavior of ZnTiO₃ films; the surface morphology, stoichiometry, and friction (μ) were analyzed using atomic force microscopy, X‐ray photoelectron spectroscopy, and nanoscratch system. It is confirmed that the measured values of H and μ of the ZnTiO₃ films were in the range from 8.5 ± 0.4 to 5.6 ± 0.4 GPa and from 0.164 to 0.226, respectively. It is suggested that the hexagonal ZnTiO₃ decomposes into cubic Zn₂TiO₄ and rutile TiO₂ based on the thermal treatment; the H, μ, and R_MS were changed owing to the grain growth and recovery that results in a relax crystallinity of ZnTiO₃ films. From X‐ray photoelectron spectroscopy measured, core levels of O 1 can attribute the weaker bonds as well as lower resistance after thermal treatment. The XRD patterns showed that as‐deposited films are mainly amorphous; however, the hexagonal ZnTiO₃ phase was observed with the ZnTiO₃ (104), (110), (116), and (214) peaks from 620 to 820 °C, indicating that there is highly (104)‐oriented ZnTiO₃ on the silicon substrate. Copyright © 2012 John Wiley & Sons, Ltd.     

In situ optical studies of thermal stability of iodine-doped polyazomethine thin films

Chemical vapour deposition (CVD) method, via polycondensation reaction, is used to obtain poly(1,4-phenylene-methylidynitrilo-1,4-phenylenenitrilomethylidyne) (PPI) thin films. The iodine (I₂) doping process of these films is observed in situ, by means of changes of the absorbance spectra. A distinct reduction of the energy gap, being a result of p-type doping, is connected with extracting electrons from π-conjugated system and formation of the polaron states inside the gap. On the basis of UV–Vis–NIR(T) experiments, the heat treatment effect on the absorption coefficient spectra of iodine doped PPI thin films is reported, as a new method, to estimate thermal stability and to obtain the border temperature of doped polymer films. Moreover it is demonstrated that the analysis of absorption bands (shape, level and position) and absorption edge parameters, such as the Urbach energy (E_U) and the energy gap (E_G) can be used to evaluate the changes of polymer chain conformation and conjugation.     

An AFM, XPS and wettability study of the surface heterogeneity of PS/PMMA-r-PMAA demixed thin films

A series of homopolymer/random copolymer blends was used to produce heterogeneous surfaces by demixing in thin films. The chosen homopolymer is polystyrene (PS) and the random copolymer is poly(methyl methacrylate)-r-poly(methacrylic acid) (PMMA-r-PMAA), whose acidic functions could be used as reactive sites in view of further surface functionalization. The proportion of each polymer at the interface was deduced from X-ray photoelectron spectroscopy (XPS) data using, on the one hand, the O/C ratio, and on the other hand, decomposition of the carbon peak of the blends in two components corresponding to the carbon peaks of PS and PMMA-r-PMAA. Combining the information from XPS with atomic force microscopy (AFM) images, water contact angle measurements and PS selective dissolution, it appears that the surfaces obtained from blends with a high PS content (90/10 to 70/30) display pits with a bottom made of PMMA-r-PMAA, randomly distributed in a PS matrix. On the other hand, the surfaces obtained from blends with a low PS content (30/70 to 10/90) display randomly distributed PS islands surrounded by a PMMA-r-PMAA matrix. The characteristics of the heterogeneous films are thought to be governed by the higher affinity of PMMA-r-PMAA for the solvent (dioxane), which leads to the elevation of the PS phase compared to the PMMA-r-PMAA phase, and to surface enrichment in PMMA-r-PMAA.     

AFM analysis of organic silane thin films for bioMEMS applications

Designing surfaces that elicit the desirable response is essential for bioMEMS (biological microelectromechanical systems) applications. To this end, we have developed two different types of silane film—hydrophobic and hydrophilic—using vinyltrichlorosilane and poly(ethylene glycol) silane, respectively. As the surface topography plays a very important role in governing protein or cell interactions, these films were characterized extensively using atomic force microscopy. All the films developed were found to have a very low root‐mean‐square roughness value (<1.3 nm). Furthermore, the topography of protein‐adsorbed silane‐modified surfaces was investigated because cell adhesion is mediated primarily by proteins. Three‐dimensional and section plots were able to differentiate the way in which protein interacts with hydrophobic and hydrophilic surfaces. Copyright © 2003 John Wiley & Sons, Ltd.     

Rupture of thin stagnant films on a solid surface due to random thermal and mechanical perturbations

A generalized formalism for the rupture of a nondraining thin film on a solid support due to imposed random thermal and mechanical perturbations, modeled as a Gaussian white noise, is presented. The evolution of amplitude of perturbation is described by a stochastic differential equation. The average film rupture time is the average time for the amplitude of perturbation to equal to the film thickness and is calculated by employing a first passage time analysis for different amplitudes of imposed perturbations, wavenumbers, film thickness, van der Waals and electrostatic interactions and surface tensions. The results indicate the existence of an optimum wavenumber at which the rupture time is minimum. A critical film thickness is identified based on the sign of the disjoining pressure gradient, below which the film is unstable in that the rupture time is very small. The calculated values of rupture time as well as the optimum wavenumber in the present analysis agree well with the results of linear stability analysis for immobile as well as completely mobile gas-liquid film interfaces. For stable films, the rupture time is found to increase dramatically with film thickness near the critical film thickness. As expected, the average rupture time was found to be higher for smaller amplitudes of imposed perturbations, larger surface potentials, larger surface tensions and smaller Hamaker constants.     

Different surface corrugations occurring during drainage of axisymmetric thin liquid films

This paper deals with the different surface corrugations observable during the thinning of axisymmetric thin and large aqueous films, stabilized by saponin. The films are observed using a thin film balance under a constant driving pressure. This device allows measurement of the thicknesses of the film surface shapes arising all along the drainage, as well as the following-up of their evolution before equilibrium is attained. Depending on the electrolyte (NaCl) concentration, three different sorts of corrugation were originally observed in such suspended thin liquid films. At the lowest NaCl concentrations, corresponding to repulsive potential between film walls, only the hydrodynamic corrugations deformed the film surfaces. Regarding the higher NaCl concentrations, when van der Waals forces become predominant, and following the thickness of the first-established thin film, the experiments disclose either that the thinner films are broken up by spinodal decomposition, or that the thicker ones are broken by nucleation and growth of black film. In addition, an original aspect of these works appears in the fact that these observations of the spontaneous decomposition of suspended thin films are relatively similar to those usually described for dewetting experiments on solid substrates, and are well fitted by the existing theoretical models.     

Probing structural changes of spin-coated polystyrene film after swelling and precipitation by synchrotron GIUSAXS and AFM

Polystyrene film of about 50 nm in thickness on silicon wafer was obtained by spin-coating in tetrahydrofuran solution. The film exhibits a rough surface as shown by atomic force microscopy images and ellipsometry data. Furthermore, such surface roughness produced a characteristic lateral correlation peak in an “out-ofplane” scan in the synchrotron grazing incidence ultra-small angle X-ray scattering pattern. The film was treated with liquids of solvent and non-solvent sequentially, resulting in a process of swelling and precipitation of the polystyrene film. Such a solvent/non-solvent treatment completely changed the original surface structure of the film. Aggregates of polystyrene of different sizes were observed both in atomic force microscopy and synchrotron grazing incidence ultra-small angle X-ray scattering measurements. The results demonstrate that synchrotron grazing incidence ultra-small angle X-ray scattering is a unique means to investigate large area micro-structural features of thin films supported on smooth surfaces.     

Some aspects of AFM nanomechanical probing of surface polymer films

We analyzed how the approach developed for the microindentation of non-uniform elastic solids can be adapted to analyze the atomic force microscopy (AFM) probing of ultrathin (1-100 nm thick) polymer films on a solid substrate, as well as polymer films with a multilayered structure. We suggested that recent Johnson's modification of the contact mechanics model that included a viscoelastic contribution could also be utilized to analyze rate-dependent loading data for polymer surfaces. The graded model proposed for microindentation experiments was modified allowing to account not only for variable elastic moduli within different layers but also for the gradient of properties between layers within a transition zone. Two examples of a recent application of this model for molecularly thick hyperbranched polymer monolayers (<3 nm thick) and tri-layered polymer films (20-40 nm thick) tethered to a solid substrate were presented and discussed. In both cases, complex shapes of both loading curves and elastic modulus depth profiles obtained from experimental AFM data were successfully fitted by the graded model with realistic structural parameters.     

Nanostructural modifications of V2O5 thin films during Li intercalation studied in situ by AFM

EC-AFM was used to study in situ the surface nanostructure of V₂O₅ thin films grown on vanadium metal and its changes during lithium electrochemical intercalation corresponding to the reversible α-to-δ phase transition. The results evidence the lateral extension and contraction of the oxide nanograins and the flattening of the oxide film surface. The increase and decrease of the lateral dimensions of the grains show that the surface reflects the volume expansion and contraction resulting from the dimensional changes of the oxide structure when lithium is inserted and de-inserted. An increase of ∼10% and ∼15% with respect to pristine oxide film was observed after the formation of the ε and δ phases, respectively. The lateral extension of the grains (∼7%) and surface flattening subsist after de-intercalation showing the non-fully reversible change of the oxide nanostructure at the interface with the electrolyte. Repeated cycling causes aging characterized by the amplification of the lateral extension of the grains (∼17%) and flattening of the oxide surface with respect to the pristine oxide film. It also modifies the surface of the oxide grains by creating new planes indicative of surface reconstruction and/or the emergence of slip planes.     

An atomic force microscope (AFM) and tapping mode AFM study of the solvent-induced crystallization of polycarbonate thin films

Thin films of bisphenol-A polycarbonate (PC) were progressively crystallized in a controllable manner under the action of an appropriate plasticizing agent. No conducting layer was applied to the polymer surface, so that imaging with the atomic force microscope (AFM) left the sample exposed for additional plasticizer treatment. The PC sample was observed many times throughout the crystallization process using the AFM in both the contact and tapping modes. Data regarding the growth process for spherulites in addition to high-resolution morphology studies were achieved. © 1996 John Wiley & Sons, Inc.     

Characterization of very low thermal conductivity thin films

Characterization of thermal transport in nanoscale thin films with very low thermal conductivity (<1 W m^?1 K^?1) is challenging due to the difficulties in accurately measuring spatial variations in temperature field as well as the heat losses. In this paper, we present a new experimental technique involving freestanding nanofabricated specimens that are anchored at the ends, while the entire chip is heated by a macroscopic heater. The unique aspect of this technique is to remove uncertainty in measurement of convective heat transfer, which can be of the same magnitude as through the specimen in a low conductivity material. Spatial mapping of temperature field as well as the natural convective heat transfer coefficient allows us to calculate the thermal conductivity of the specimen using an energy balance modeling approach. The technique is demonstrated on thermally grown silicon oxide and low dielectric constant carbon-doped oxide films. The thermal conductivity of 400 nm silicon dioxide films was found to be 1.2 W m^?1 K^?1, and is in good agreement with the literature. Experimental results for 200 nm thin low dielectric constant oxide films demonstrate that the model is also capable of accurately determining the thermal conductivity for materials with values <1 W m^?1 K^?1.     

XPS studies of thin polycyanurate films on silicon wafers

Thin films of a diandicyanato bisphenol A (DCBA) prepolymer on silicon substrates have been investigated. Angle dependent X-ray photoelectron spectroscopy reveals some thickness-dependent features, which lead to an adsorption model for the DCBA prepolymer molecules. The adsorption of the first layer is governed by the interaction of the triazine rings with the substrate surface.     

Thermoanalytical and morphological studies of cross-linked latex films by advanced techniques

In the present report, thermal analysis (TMDSC, DMA, TG, stress-strain analysis), nano-indentation and AFM morphological characterization of cross-linkable latexes, prepared with either a pre-coalescence cross-linker (1,3-butylene glycol dimethacrylate) or post-coalescence cross-linker (adipic dihydrazide) at various levels of cross-linking, were done. The study assesses the effect of type and level of cross-linking on the film formation process through the evolution of mechanical properties and latex morphology. In addition, the final fundamental thermal and mechanical properties, specific end-use properties and latex morphology resulting from the film formation process are reported.     

XPS studies of thin polycyanurate films on silicon wafers

Thin films of a diandicyanato bisphenol A (DCBA) prepolymer on silicon substrates have been investigated. Angle dependent X-ray photoelectron spectroscopy reveals some thickness-dependent features, which lead to an adsorption model for the DCBA prepolymer molecules. The adsorption of the first layer is governed by the interaction of the triazine rings with the substrate surface.     

Ultra thin films of nanocrystalline Ge studied by AFM and interference enhanced Raman scattering

Initial growth stages of the ultra thin films of germanium (Ge) prepared by ion beam sputter deposition have been studied using atomic force microscope (AFM) and interference enhanced Raman scattering. The growth of the films follows Volmer-Weber growth mechanism. Analysis of the AFM images shows that Ostwald ripening of the grains occurs as the thickness of the film increases. Raman spectra of the Ge films reveal phonon confinement along the growth direction and show that the misfit strain is relieved for film thickness greater than 4 nm. Dedicated to Professor C N R Rao on his 70th birthday