Pentacene Islands Research Articles

In situ Auger Electron Spectroscopy Study of the Pentacene Film Grown on the SrTiO3(001) Surface

We have investigated the growth and photodecomposition of the pentacene ultrathin film grown on the TiO2-terminated SrTiO3(001) single crystal substrate by in situ Auger electron spectroscopy (AES) and ex situ atomic force microscopy (AFM). The careful analyses of AES and AFM data have revealed that the AES intensities of C(KLL) in the growth of the pentacene were to some extent larger than those in its photodecomposition, for the coverage of pentacene islands estimated by AFM. This result supports the existence of adsorbed-like pentacene precursors before the photodecomposition as was proposed in our previous report.

Imaging and Detection of Single Molecule Recognition Events on Organic Semiconductor Surfaces

The combination of organic thin film transistors and biological molecules could open new approaches for the detection and measurement of properties of biological entities. To generate specific addressable binding sites on such substrates, it is necessary to determine how single biological molecules, capable of serving as such binding sites behave upon attachment to semiconductor surfaces. Here, we use a combination of high-resolution atomic force microscopy topographical imaging and single molecule force spectroscopy (TREC), to study the functionality of antibiotin antibodies upon adsorption on pentacene islands, using biotin-functionalized, magnetically coated AFM tips. The antibodies could be stably adsorbed on the pentacene, preserving their functionality of recognizing biotin over the whole observation time of more than one hour. We have resolved individual antigen binding sites on single antibodies for the first time. This highlights the resolution capacity of the technique.

Determining the optimum pentacene channel thickness on hydrophobic and hydrophilic dielectric surface

We report that the optimum pentacene channel thickness is dependent on the surface energy state of its dielectric substrate. Pentacene thin-film transistor (TFT) with hydrophobic substrate displays a peak linear mobility at an optimum channel thickness of 50nm, below or above which the linear mobility decreases. In contrast, the linear mobility of the TFT with hydrophilic substrate monotonically increases until the channel thickness decreases to 15nm. According to atomic force microscopy of 15-nm-thin pentacene grown on the SiO2 and poly-4-vinyphenol (PVP) dielectrics, the pentacene islands on PVP are not perfectly interconnected unlike the case on SiO2.

Variable Temperature Scanning Tunneling Microscopy of Pentacene Monolayer and Bilayer Phases on Ag(111)

Pentacene deposited onto a Ag(111) surface at 300 K is studied using scanning tunneling microscopy at temperatures of 300 and 50 K, providing structural insight into its unusual growth habit. At room temperature, an unexpectedly high pentacene coverage is needed to nucleate ordered pentacene islands, which appear surrounded by a disordered pentacene phase. These room temperature pentacene nuclei are revealed as bilayer structures from their coverage-dependent size evolution and molecularly resolved images of domain boundaries, recorded at 50 K. At this reduced temperature, two different monolayer phases with long-range order and commensurate with the Ag(111) surface lattice further emerge. These two monolayer phases exhibit comparable (0.7 vs 0.8 molecule/nm2) packing densities, but distinct intermolecular registration and alignment with respect to the silver sublattice.

Effect of impurities on pentacene island nucleation

Pentacenequinone (PnQ) impurities have been introduced into a pentacene source material in a controlled manner to quantify the relative effects of the impurity content on the grain boundary structure and thin film nucleation. Atomic force microscopy has been employed to directly characterize by weight films grown by using 0.0%--7.5% PnQ in the source material. Analysis of the distribution of capture zone areas of submonolayer islands as a function of impurity content shows that for a large PnQ content, the critical nucleus size for forming a Pn island is smaller than for a low PnQ content. This result indicates a favorable energy for the formation of Pn-PnQ complexes, which, in turn, suggests that the primary effect of PnQ on Pn mobility may arise from the homogeneous distribution of PnQ defects.

A defect-free thin film pentacene diode: Interplay between transport and scanning tunneling microscope tip tunneling injection

A diodelike device has been fabricated by organic molecular beam deposition of pentacene on a surface-modified, single-crystalline Au substrate. Using the substrate as the first and the tip of a scanning tunneling microscope (STM) as the second electrode, transport characteristics of this organic semiconductor (OSC) device were investigated. The probed pentacene islands are single crystalline and defect-free and consist of few molecular layers only. The current-voltage characteristics of this device reveal a pronounced asymmetry. For negative polarity, the current characteristics is almost independent of the layer thickness. For positive polarity, the current onset is shifted significantly to larger voltages with increasing layer thickness. Numerical simulations for a two-dimensional model system allow us to identify the injection properties of the STM tip as reasons for this pronounced asymmetry. For negative substrate bias the creation of holes in the valence band occurs by tunneling of electrons to the tip whereas in the opposite case holes have to be transported through the OSC layer from the substrate. Thus, for low positive voltage the hole current limits the device current. Once the resulting voltage drop between layer and tip becomes larger than the barrier for electron injection, direct tunneling of electrons into the pentacene conduction band becomes possible and n conduction begins to dominate.

Photodecomposition of Pentacene Films on Atomically Controlled SrTiO3(001) Surfaces

Photodecomposition of pentacene films grown on the atomically controlled SrTiO3(001) (STO) surfaces was investigated by atomic force microscopy. The photodecomposition was found to occur only when the coverage of the pentacene film was less than 1 ML on the TiO2-terminated STO(001). When the oxygen adsorption is a rate-limiting step, the decomposition rate is nearly proportional to the surface area uncovered with pentacene island, while at the last stage of the decomposition it is in proportion not to the coverage of the pentacene islands but to the perimeter of the pentacene islands. The SrO-terminated STO(001) surface exhibits very little activity in the photodecomposition of the pentacene. The exponential decay of the rate constant with the SrO coverage suggests that it is attributed to some chemical effect such as the formation of Sr hydroxide at the SrO-terminated surface.

Channel formation in single-monolayer pentacene thin film transistors

The geometrical arrangement of single-molecule-high islands and the contact between them have large roles in determining the electrical properties of field effect transistors (FETs) based on monolayer-scale pentacene thin films. As the pentacene coverage increases through the submonolayer regime there is a percolation transition where islands come into contact and a simultaneous rapid onset of current. At coverages just above the percolation threshold, the electrical properties vary with geometrical changes in the contacts between the pentacene islands. At higher coverages, the FET mobility is much lower than the mobility measured by the van der Pauw method because of high contact resistances in monolayer-scale pentacene film devices. An increase in the van der Pauw mobility of holes as a function of pentacene coverage shows that second layer islands take part in charge transport.

Tunnelling charge injection into a pentacene layer using dynamic-mode scanning forcemicroscopy

Conductive atomic force microscopy has been utilized to study the charge injectionphenomenon on various materials. We proposed a gentle method for local charge injectionand observation based on self-excitation dynamic-mode atomic force microscopy, whichinvolves switching between amplitude modulation mode for tunnelling charge injection andfrequency modulation modes for surface potential measurement. We demonstrated that themethod was effective in investigating charge injection and dissipation processes withoutserious degradation of the tip and samples. To examine the method we measured goldnanoparticles on an insulator and an isolated pentacene island on a substrate.

Enhanced Electrical Percolation Due to Interconnection of Three-Dimensional Pentacene Islands in Thin Films on Low Surface Energy Polyimide Gate Dielectrics

The role of lateral interconnections between three-dimensional pentacene islands on low surface energy polyimide gate dielectrics was investigated by the measurement of the surface coverage dependence of the charge mobility and the use of conducting-probe atomic force microscopy (CP-AFM). From the correlation between the electrical characteristics and the morphological evolution of the three-dimensionally grown pentacene films-based field-effect transistors, we found that during film growth, the formation of interconnections between the three-dimensional pentacene islands that are isolated at the early stage contributes significantly to the enhancement process of charge mobility. The CP-AFM current mapping images of the pentacene films also indicate that the lateral interconnections play an important role in the formation of good electrical percolation pathways between the three-dimensional pentacene islands.

Photoinduced Interfacial Charging and “Explosion” of Monolayer Pentacene Islands

Electrostatic force microscopy shows that the electric field gradients above pentacene monolayer islands on 2-nm SiO2/Si substrates, in a dark, dry nitrogen environment, display a wide distribution of signs and magnitude that is dependent on sample history. Under 12 mW/cm2 green (532 nm) illumination, pentacene islands accumulate positive charge because of photoexcited electron transfer across the oxide to the Si substrate. At a strong illumination of 60 mW/cm2, pentacene islands reform into small spherical particles, apparently because the positive charge Coulomb repulsion energy becomes comparable to the cohesive energy of the pentacene monolayer.

Electrostatic Field and Partial Fermi Level Pinning at the Pentacene−SiO2Interface

Monolayer islands of pentacene deposited on silicon substrates with thermally grown oxides were studied by electric force microscopy (EFM) and scanning Kelvin probe microscopy (SKPM) in ultrahigh vacuum (UHV) after prior 10 min exposure to atmospheric ambient. On 25-nm-thick oxides, the pentacene islands are 0.5 V higher in electrostatic potential than the silicon dioxide background because of intrinsic contact potential differences. On 2-nm-thin oxides, tunneling across the oxides allows Fermi level equilibration with pentacene associated states. The surface potential difference depends on the doping of the underlying Si substrates. The Fermi level movement at the pentacene SiO(2) interface was restricted and estimated to lie between 0.3 and 0.6 eV above the pentacene valence band maximum. It is proposed that hole traps in the pentacene or at the pentacene-oxide interface are responsible for the observations.

Nucleation and growth of ultrathin pentacene films on silicon dioxide: effect of deposition rate and substrate temperature

We report on a study of pentacene thin-films grown by high vacuum deposition on silicon dioxide, using atomic force microscopy (AFM) and transmission electron microscopy (TEM). The nucleation density of pentacene islands on SiO 2 is found to rise as the deposition rate is increased but decreases at higher substrate temperature. This observation is consistent with the predictions of the rate-equation formalism for homogeneous nucleation. Hence, by controlling the two deposition parameters we are able to tune the mean size of the pentacene islands, as well as the grain boundary density in the first pentacene layer. The deposition rate and the substrate temperature also affect the subsequent growth of the pentacene islands, which can be satisfactorily described by a capture zone model under given conditions of deposition. From a structural point of view, the use of selected area electron diffraction (SAED) reveals the crystalline nature of the pentacene islands. For pentacene films grown at the substrate temperature of 353 K (80 °C), the observed SAED pattern matches well the calculated patterns obtained for the bulk triclinic structure.

Correlated growth in ultrathin pentacene films on silicon oxide: Effect of deposition rate

Understanding the growth mechanism in molecular organic thin films is fundamental to their applications in organic electronics. We present an extensive study of the growth mechanism of pentacene thin films on silicon dioxide $({\mathrm{SiO}}_{2})$ using atomic force microscopy. For a fixed substrate temperature ${T}_{s},$ the deposition rate \ensuremath{\kappa} is found to be a key parameter in controlling the nucleation density in the submonolayer regime and hence transport properties in the first layer of the organic field effect transistors. At a fixed ${T}_{s}=338\mathrm{K}$ the maximum number of pentacene islands per unit area $N$ follows the scaling law $N\ensuremath{\propto}{\ensuremath{\kappa}}^{\ensuremath{\delta}}$ with $\ensuremath{\delta}=1.16\ifmmode\pm\else\textpm\fi{}0.10.$ A mechanism of homogeneous nucleation followed by diffusive growth accounts for this behavior and allows us to estimate the critical nucleus size of the pentacene islands. The results obtained from a statistical analysis of the island size distribution are fully consistent with a phenomenological capture zone model. The validity of this model depends on the extent of reevaporation of pentacene admolecules during deposition, which is moderated by the deposition rate. We demonstrate that the rate dependence of island nucleation has important implications for the density of grain boundaries, which may play an important role in the transport mechanism.

Dynamic scaling, island size distribution, and morphology in the aggregation regime of submonolayer pentacene films.

Scaling behavior of the island size distribution through a universal scaling function f(u) is demonstrated for submonolayer pentacene islands in the aggregation regime (0.1<theta<0.5) grown on oxidized silicon surfaces. The distribution of f(u) suggests that four molecules constitute the smallest stable island. The structure factor S(k) of the submonolayer films calculated from AFM micrographs compares well with diffuse x-ray intensities from in situ experiments. The structure factor was decomposed into the contribution from the average island shape and the interisland distribution confirming that a unique characteristic length scale regulates the growth dynamics.