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Pentacene Films (pentacene + film)

Distribution by Scientific Domains

Polymers and Materials Science	100%

Selected Abstracts

Observation of Unusual Homoepitaxy in Ultrathin Pentacene Films and Correlation with Surface Electrostatic Potential

ADVANCED MATERIALS, Issue 30 2009
Vivek Kalihari
Pentacene second layer grows with a twist (,76,°) on the first layer, while the third and subsequent layers show commensurism with their respective underlayers. The preferred twisting of the second layer on the first layer gives rise to epitaxial and non-epitaxial domains which also reflect in the surface electrostatic potential measurements. [source]

Thickness-Driven Orthorhombic to Triclinic Phase Transformation in Pentacene Thin Films,

ADVANCED MATERIALS, Issue 7 2005
F. Drummy
Pentacene films are thermally evaporated onto amorphous carbon-coated mica substrates held at room temperature. The crystal structure and morphology of the films are analyzed using electron microscopy and diffraction, and a new orthorhombic structure is characterized for films below a critical thickness (see Figure). Evidence that the orthorhombic structure is thermodynamically stable at low film thickness due to its low (001) surface energy is obtained. [source]

Cover Picture: Microscopic Evidence for Spatially Inhomogeneous Charge Trapping in Pentacene (Adv. Mater.

ADVANCED MATERIALS, Issue 11 2005
11/2005)
Abstract Trapped charge has been observed in polycrystalline pentacene films by electric force microscopy in work reported by Muller and Marohn on p.,1410. The cover shows an optical fiber watching the motion of a metal-coated cantilever hovering over a polycrystalline pentacene film. The film occupies the 6.5,,,m gap in a working field-effect transistor. Trapped charge (dark blue and black areas in the pentacene) appears as large patches randomly distributed throughout the device, implying that long-lived traps in polycrystalline pentacene are not correlated with grain boundaries. [source]

Influence of Electric Field on Microstructures of Pentacene Thin-Films in Field-Effect Transistors,

ADVANCED FUNCTIONAL MATERIALS, Issue 2 2008
L. Cheng
Abstract We report on electric-field-induced irreversible structural modifications in pentacene thin films after long-term operation of organic field-effect transistor (OFET) devices. Micro-Raman spectroscopy allows for the analysis of the microstructural modifications of pentacene in the small active channel of OFET during device operation. The results suggest that the herringbone packing of pentacene molecules in a solid film is affected by an external electric field, particularly the source-to-drain field that parallels the a,b lattice plane. The analysis of vibrational frequency and Davydov splitting in the Raman spectra reveals a singular behavior suggesting a reduced separation distance between pentacene molecules after long-term operations and, thus, large intermolecular interactions. These results provide evidence for improved OFET performance after long-term operation, related to the microstructures of organic semiconductors. It is known that the application of large electric fields alters the semiconductor properties of the material owing to the generation of defects and the trapping of charges. However, we first suggest that large electric fields may alter the molecular geometry and further induce structural phase transitions in the pentacene films. These results provide a basis for understanding the improved electronic properties in test devices after long-term operations, including enhanced field-effect mobility, improved on/off current ratio, sharp sub-threshold swing, and a slower decay rate in the output drain current. In addition, the effects of source-to-drain electric field, gate electric field, current and charge carriers, and thermal annealing on the pentacene films during OFET operations are discussed. [source]

Thickness-Dependent Structural Evolutions and Growth Models in Relation to Carrier Transport Properties in Polycrystalline Pentacene Thin Films,

ADVANCED FUNCTIONAL MATERIALS, Issue 17 2007
H.-L. Cheng
Abstract Thickness-dependent crystal structure, surface morphology, surface energy, and molecular structure and microstructure of a series of polycrystalline pentacene films with different film thickness ranging from several monolayers to the several hundred nanometers have been investigated using X-ray diffraction (XRD), atomic force microscopy (AFM), contact angle meter, and Raman spectroscopy. XRD studies indicate that thin film polymorphs transformation behaviours are from the orthorhombic phase to the thin-film phase and then to the triclinic bulk phase as measured by the increased tilt angle (,tilt) of the pentacene molecule from the c- axis toward the a- axis. We propose a growth model that rationalizes the ,tilt increased along with increasing film thickness in terms of grain size and surface energy varying with film growth using AFM combined with contact angle measurements. The vibrational characterizations of pentacene molecules in different thickness films were investigated by Raman spectroscopy compared to density functional theory calculations of an isolated molecule. In combination with XRD and AFM the method enables us to distinguish the molecular microstructures in different thin film polymorphs. We proposed a methodology to probe the microscopic parameters determining the carrier transport properties based on Davydov splitting and the characteristics of aromatic C,C stretching modes in Raman spectra. When compared to the triclinic bulk phase at a high thickness, we suggest that the first few monolayer structures located at the dielectric surface could have inferior carrier transport properties due to weak intermolecular interactions, large molecular relaxation energy, and more grain boundaries. [source]

Enhancement of Interconnectivity in the Channels of Pentacene Thin-Film Transistors and Its Effect on Field-Effect Mobility,

ADVANCED FUNCTIONAL MATERIALS, Issue 14 2006
S. Lee
Abstract With the aim of improving the field-effect mobility of transistors by promoting the interconnectivity of the grains in pentacene thin films, deposition conditions of the pentacene molecules using one-step (total thickness of layer 50,nm: 0.1,Å,s,1) and two-step (first layer 10,nm: 0.1,Å,s,1, second layer 40,nm: 4.0,Å,s,1) depositions are controlled. Significantly, it is found that the continuities of the pentacene thin films vary with the deposition conditions of the pentacene molecules. Specifically, a smaller number of voids is observed at the interface for the two-step deposition, which results in field-effect mobilities as high as 1.2,cm2,V,1,s,1; these are higher by more than a factor of two than those of the pentacene films deposited in one step. This remarkable increase in field-effect mobility is due in particular to the interconnectivity of the pentacene grains near the insulator substrate. [source]