Single Pentacene Research Articles

Chemically Cross-Linked Thin Poly(vinylidene fluoride-co-trifluoroethylene)Films for Nonvolatile Ferroelectric Polymer Memory

Both chemically and electrically robust ferroelectric poly(vinylidene fluoride-co-trifluoro ethylene) (PVDF-TrFE) films were developed by spin-coating and subsequent thermal annealing with the thermal cross-linking agent 2,4,4-trimethyl-1,6-hexanediamine (THDA). Well-defined ferroelectric β crystalline domains were developed with THDA up to approximately 50 wt %, with respect to polymer concentration, resulting in characteristic ferroelectric hysteresis polarization-voltage loops in metal/cross-linked ferroelectric layer/metal capacitors with remnant polarization of approximately 4 μC/cm(2). Our chemically networked film allowed for facile stacking of a solution-processable organic semiconductor on top of the film, leading to a bottom-gate ferroelectric field effect transistor (FeFET). A low-voltage operating FeFET was realized with a networked PVDF-TrFE film, which had significantly reduced gate leakage current between the drain and gate electrodes. A solution-processed single crystalline tri-isopropylsilylethynyl pentacene FeFET with a chemically cross-linked PVDF-TrFE film showed reliable I-V hysteresis with source-drain ON/OFF current bistablility of 1 × 10(3) at a sweeping gate voltage of ±20 V. Furthermore, both thermal micro/nanoimprinting and transfer printing techniques were conveniently combined for micro/nanopatterning of chemically resistant cross-linked PVDF-TrFE films.

ChemInform Abstract: Detection and Spectroscopy of Single Pentacene Molecules in a p- Terphenyl Crystal by Means of Fluorescence Excitation.

Recent advances in fluorescence excitation spectroscopy with high efficiency have produced greatly improved optical spectra for the first electronic transition of individual single molecules of pentacene in p‐terphenyl crystals at low temperatures (1.5 to 10 K). Two classes of single molecule behavior are observed: class I molecules have time‐independent resonance frequencies, and class II molecules show a diffusive motion among several resonant frequencies with time which we term ‘‘spectral diffusion’’ by analogy with a similar effect which is common in amorphous materials. The temperature dependence of the linewidth and the power dependence of the fluorescence emission rate and of the linewidth are reported and analyzed. Various forms of the surprising class II behavior are described, including jumping among several discrete frequencies, creeping toward the center of the inhomogeneous line in many small steps, and a wandering among many possible resonance frequencies. The occurrence of class II behavior is restricted to the wings of the inhomogeneous line suggesting that the effect is correlated with some form of local disorder. The spectral diffusion rate increases with increasing temperature, suggesting that the effect may be due to phonon‐assisted transitions of local degrees of freedom around the pentacene defect whose source remains to be identified conclusively.

Formation and electrical transport properties of pentacene nanorod crystal

The monophasic formation of an uncharted pentacene crystal, the pentacene nanorod, hasbeen investigated. The restricted formation of the pentacene nanorod on a bare micasurface reveals a peculiar surface catalytic crystal growth mode of the pentacene. Wedemonstrated the charge transport measurements through a single pentacene nanorod andanalyzed the data using a periodic hopping conduction model. The results revealed that thepentacene nanorod has a periodic conductive node within their one-dimensional crystal.

Imaging Molecular Orbitals by Scanning Tunneling Microscopy on a Passivated Semiconductor

Decoupling the electronic properties of a molecule from a substrate is of crucial importance for the development of single-molecule electronics. This is achieved here by adsorbing pentacene molecules at low temperature on a hydrogenated Si(100) surface (12 K). The low temperature (5 K) scanning tunneling microscope (STM) topography of the single pentacene molecule at the energy of the highest occupied molecular orbital (HOMO) tunnel resonance clearly resembles the native HOMO of the free molecule. The negligible electronic coupling between the molecule and the substrate is confirmed by theoretical STM topography and diffusion barrier energy calculations.

A scanning Kelvin probe study of charge trapping in zone-cast pentacene thin filmtransistors

We have used scanning Kelvin probe microscopy (SKPM) as a local probe tostudy charge trapping in zone-cast pentacene field effect transistors on bothSiO2 and benzocyclobutene (BCB) substrates. Annealing at130 °C was found to reduce the threshold voltage, susceptibility to negative gate bias stress andtrapping of positive charges within single pentacene grains. We conclude that oxygen isable to penetrate and disassociatively incorporate into crystalline pentacene, chemicallycreating electrically active defect states. Screening of a positive gate bias caused by electroninjection from Au into pentacene was directly observed with SKPM. The rate of screeningwas found to change significantly after annealing of the film and depended on the choice ofgate dielectric.

Effects of anisotropy and the proximity to the surface on the spontaneous emission rate of pentacene in p-terphenyl crystal

Due to the anisotropy of a birefringent host crystal, the spontaneous emission rate of a single-impurity molecule depends on the orientation of the transition dipole moment relative to the host crystal. In p -terphenyl crystal, the transition dipole moment of a single pentacene impurity can have four different orientations. Here the relative spontaneous emission rate of the electric quadrupole transition is calculated depending on the unknown component of the transition dipole moment normal to the a ′/ b ′-plane. Maximum difference in the spontaneous emission rate of the electric quadrupole transition is up to 9%. For comparison, the effects caused by the proximity to the plane surface of the crystal are calculated.

Anisotropic field effect mobility in single crystal pentacene

Fan-shaped electrodes were designed on Si∕SiO2 substrate to measure the anisotropic field effect mobility in freestanding single crystal pentacene. Field effect transistor was fabricated by placing single crystal pentacene on the prepatterned electrodes. The contact between the electrodes and single crystal pentacene was enhanced by applying pressure. Angle dependence of field effect mobility in single crystal pentacene showed remarkably anisotropic behavior. The highest mobility value was estimated to be ∼2.3cm2∕Vs at room temperature.

Difference values of spontaneous emission rate of a single-impurity molecule of pentacene in biaxial p-terphenyl crystal

The spontaneous emission rate of a single-impurity molecule in a birefringent host crystal depends on the angles between the transition dipole moment and the principal axes of the dielectric (permittivity) tensor as well as on the main dielectric constants of a host crystal. The transition dipole moment of a single pentacene impurity in p-terphenyl crystal may have four different orientations depending on the spectroscopic sites O 1 to O 4. Here the orientation of the transition dipole moment is defined by the angle Ω between the projection of the transition dipole moment to the a ′ / b ′ -plane of the crystal and the b′-axis. The relative spontaneous emission rate is calculated for these four spectroscopic sites in dependence on unknown component of the transition dipole moment, which is normal to the a ′ / b ′ -plane. Possible differences in the spontaneous emission rate of the electric dipole transition of pentacene in p-terphenyl crystal are up to 24%.

Electrical characterization of single grain and single grain boundary of pentacene thin film by nano-scale electrode array

Abstract Organic thin film transistors (O-TFTs) are attracting attentions recently for their possible application to flexible field effect transistor (FET) devices and even for candidate devices in post-silicon information systems. However, the organic thin films usually grow in a polycrystalline state, and the electrical properties are very difficult to set at an appropriate value because orientation and size of the polycrystalline grains are almost uncontrollable. Thus, it should be necessary to clarify the conduction characteristics within the single grain and between the grains through grain boundaries in order to precisely control the electrical properties of the organic thin films. This paper reports on a nano-scale electrode array, which provides a method to characterize single organic semiconductor grain, as well as to determine the single grain boundary effect on the electrical conduction. The fabrication method of the nano-scale electrode array and evaluation results are described. Single grain and single grain boundary in MBE grown pentacene thin film were detected and were measured for the first time using FET structures. The resistance of single grain was estimated to be around 10 M Ohms while that of single grain boundary to be about 100 M Ohms. This is the first experimental verification of the resistance of single grain pentacene thin film and single pentacene grain boundary.

Orientation Control of Standing Epitaxial Pentacene Monolayers Using Surface Steps and In-plane Band Dispersion Analysis by Angle Resolved Photoelectron Spectroscopy

ABSTRACTWe prepared quasi single crystalline pentacene monolayer films on Bi-terminated Si(111) by using bunched steps on vicinally-cut surfaces as an orientation template. Band dispersion in the conduction plane of pentacene was clearly observed by angle-resolved photoelectron spectroscopy.

Demonstration of logic AND device using a split-gate pentacene field effect transistor

We report on the fabrication and performance of pentacene-based split-gate field effect transistors (FETs) on doped Si/SiO 2 substrates. Several transistors with split gate structures were fabricated and demonstrated AND logic functionality. The transistor’s functionality was controlled by applying either 0 or − 10 V to each of the gate electrodes. When − 10 V was simultaneously applied to both gates, the transistor was conductive (ON), while any other combination of gate voltages rendered the transistor highly resistive (OFF). A significant advantage of this device is that AND logic devices with multiple inputs can be built using a single pentacene channel with multiple gates. The p-type carrier mobility of charge within the pentacene active layer of these transistors was about 10 − 5 cm 2/V-s. We attribute the low value of mobility primarily to the sharp contours of the pentacene film between the drain and the source contacts and to defects in the pentacene film. The average charge density was 1.4 × 10 12 holes/cm 2. Despite low mobility, the devices operated at lower drain-source ( V DS) and gate-source ( V GS) voltages as compared with previously reported pentacene based FETs.

Complementary inverter based on interface doped pentacene

An organic complementary metal–oxide–semiconductor (O-CMOS) inverter is presented, which is based on a single pentacene layer acting both as n- and p-type organic semiconductors. The circuit consists of two spatially separated transistors realized by pairs of calcium and gold source and drain electrodes, respectively. The p transistor is obtained by utilizing the conventional pentacene∕SiO2 channel interface whereas the n transistor is realized by doping the SiO2 interface with traces of calcium prior to pentacene deposition. Both, n and p transistors work exclusively in unipolar mode within the range of the supply voltage of 60 V. The O-CMOS inverter works reliably with a gain in between 17 and 24, and the respective electron and hole mobilities were found around 0.1cm2V−1s−1. The circuit shows hysteresis, which can be explained by a gate voltage-dependent electron trapping in the n channel. Electron accumulation can also be realized by the application of a polymethylmethacrylate interlayer between SiO2 and pentacene.

Interaction of single pentacene molecules with monatomic Cu∕Cu(111) quantum wires

An ultrahigh vacuum low-temperature scanning tunneling microscope operated at 7 K is used to assemble Cu adatom chains on a Cu(111) surface by atom manipulation. Cu atoms within the close-packed chain reside on nearest-neighbor fcc hollow sites (Cu–Cu spacing 2.55 Å) along the ⟨110⟩ in-plane directions. Spectroscopic measurements of the differential tunneling conductance dI∕dV reveal that the monatomic Cu chain exhibits unoccupied one-dimensional (1D) quantum well states trapped in the pseudogap of the ⟨111⟩-projected Cu bulk bands. These chain-localized states are described by a 1D energy band centered 3.2 eV above the Fermi level (total band width 3.6 eV) and derive from spz hybrid atomic orbitals associated with the single Cu∕Cu(111) adatom. Pentacene molecules (C22H14) deposited on Cu(111) by thermal evaporation adopt a planar adsorption geometry with their long molecular axis aligned with the ⟨110⟩ in-plane directions. The organic molecule can be laterally manipulated along different high-symmetry directions of the substrate via attractive tip/molecule interactions. Lateral manipulation is also capable to attach single pentacene molecules to the ends of assembled Cu chains with atomic-level precision. We find (i) an enhanced adsorptive binding of the attached molecule characterized by spatial overlap between its carbon framework and the outermost chain atoms, (ii) persistence of the chain-localized states for the molecule-chain hybrid structure, and (iii) a clear correspondence between the number of Cu chain atoms involved in the spatial overlap and the observed energetic upward shift of the chain-localized quantum levels.

Ab initio study of pentacene on Au(0 0 1) surface

We report results of our first-principles density-functional studies on the nature of intermolecular interactions between pentacene (C 22H 14) molecules and adsorption properties of the molecule on Au(0 0 1) surface. The energetics and electronic structure of two pentacene molecules in various configurations show an anisotropic nature of intermolecular interactions between the molecules. From total energy calculations, it is predicted that a single pentacene molecule adsorbed on Au(0 0 1) surface prefer to be centered at the bridge site, aligning its long-axis to the [1 1 0] direction of the Au(0 0 1) substrate. The structural distortions of the adsorbed molecule are closely related to the orbital characteristics of the HOMO and LUMO states and indicate a possible back-donation from the metal substrate to the molecule.

Organic field-effect transistors with single and double pentacene layers

We report the characterization of field-effect transistors fabricated within individual grains of single and double pentacene layers grown on silicon oxide. Field-effect mobilities are found to increase with increasing gate voltage and exhibit a thermally activated form for the temperature dependence. These characteristics can be explained by the multiple trapping and release model. The mobilities of double-layer devices are one order of magnitude higher than those of single-layer devices. Possible origins of the traps are discussed. The geometry of these ultrathin devices makes them suitable for sensing applications.

Manipulation and adsorption-site mapping of single pentacene molecules on Cu(111)

Low-temperature scanning tunneling microscopy at $7\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ is used to study the adsorption and manipulation of single pentacene molecules on Cu(111). Controlled lateral translations of the molecule are performed along different high-symmetry directions of the substrate via attractive tip-molecule interactions. By means of a detailed site-mapping technique combining lateral manipulations of the molecule and of native substrate adatoms we determine the adsorptive configuration of the aromatic molecule. We find a planar adsorption geometry with the long molecular axis aligned with the close-packed Cu atom rows and the benzene units centered over hexagonal close-packed hollow sites of the substrate.

Ambipolar Charge Injection and Transport in a Single Pentacene Monolayer Island

Electrons and holes are locally injected in a single pentacene monolayer island. The two-dimensional distribution and concentration of the injected carriers are measured by electrical force microscopy. In crystalline monolayer islands, both carriers are delocalized over the whole island. On disordered monolayer, carriers stay localized at their injection point. These results provide insight into the electronic properties, at the nanometer scale, of organic monolayers governing performances of organic transistors and molecular devices.

An ab-initio study of pentacene on aluminum (100) surface

In this work we report the energetics and the electronic properties at the interface formed by a single pentacene molecule deposited onto aluminum (1 0 0) surface, studied by means of ab-initio calculations within the density-functional theory. For the envisaged planar adsorption configuration, the charge density profiles and binding energies seem to be consistent with physisorption of the pentacene molecule onto the Al substrate. We estimate the Schottky barrier heights both in the local density and generalized gradient approximations: the calculated n-type Schottky barrier height shows a rectifying behavior, in good agreement with previous experiments.

Effect of impurities on the mobility of single crystal pentacene

We have obtained a hole mobility for the organic conductor pentacene of μ=35 cm2/V s at room temperature increasing to μ=58 cm2/V s at 225 K. These high mobilities result from a purification process in which 6,13-pentacenequinone was removed by vacuum sublimation. The number of traps is reduced by two orders of magnitude compared with conventional methods. The temperature dependence of the mobility is consistent with the band model for electronic transport.

Structural and electronic properties of pentacene molecule and molecular pentacene solid

The structural and electronic properties of a single pentacene molecule and a pentacene molecular crystal, an organic semiconductor, are examined by a first-principles method based on the generalized gradient approximation of density functional theory. Calculations on the crystal were carried out for a triclinic unit cell containing two pentacene molecules. The bandwidths of the valence and conduction bands which determine the charge migration mechanism are found to depend strongly on the crystallographic direction. Along the triclinic reciprocal lattice vectors a ∗ and b ∗ which are orientated approximately perpendicular to the molecular axes the maximal valence (conduction) band width amounts to only 75 (59) meV, even smaller values are obtained for the c ∗ direction parallel to molecular axes. Along the stacking directions a ∗+b ∗ and a ∗−b ∗ , however, the maximal valence (conduction) band width is found to reach 145 (260) meV. The value for the conduction band width is larger than estimates for the polaron binding energy but significantly smaller than recent results obtained by semiempirical methods. The single molecule has a HOMO–LUMO gap of about 1.1 eV as deduced from the Kohn–Sham eigenvalue differences. When using the self-consistent field method, which is expected to yield more reliable results, a value of 4.73 eV is obtained. The theoretical value for the band gap in the molecular solid amounts to 1.0 eV at the Γ-point.