Growth Of Pentacene Films Research Articles

Surfactant-like Additives Assisted the Lateral Growth of Pentacene Films.

Lateral growth of thin films is crucial for organic electronic devices, such as field-effect transistors. Here, we report a strategy to improve the lateral growth of pentacene films using rubrene as a surfactant-like additive. Atomic force microscopy (AFM) images confirm the enhanced lateral growth with the presence of rubrene, resulting in smooth and enlarged molecule domains in the films in comparison to those without rubrene. Molecular dynamics simulations are conducted to explore the interlayer diffusion of pentacene molecules during the growth. With the rubrene molecules as surfactant-like additives, mean square displacement (MSD) analysis shows that the pentacene molecules have a descending diffusion coefficient of 2.0 × 10-5 cm2 s-1, which is greater than the ascending diffusion coefficient of 1.6 × 10-5 cm2 s-1. The more descending molecules lead to an enhanced lateral growth of pentacene films, which is in good agreement with the experiments. As a result, the pentacene films grown with rubrene exhibit a rapid increase in carrier mobility over thickness due to the well-connected domains resulting from enhanced lateral growth. This finding will provide a new strategy to modulate the morphology of organic films for high-performance devices.

Unravelling molecular disorder at SAM-functionalized charge injection interfaces in organic field-effect transistors

Charge injection in Organic Field Effect Transistors is investigated by monitoring in real-time the electrical conduction, while in situ growing pentacene films. The source and drain electrodes are made of gold functionalized with alkanethiol Self-Assembled Monolayers (SAMs), whose number of carbon atoms ranges from 5 to 10. Measuring the current flowing into the channel allows us to establish a correlation between the charge injection and the pentacene film growth. In particular, the thickness dependence of both the charge mobility and the charge carrier accumulation can be explained in terms of the molecular order at the interface between the pentacene films and the SAMs. Specifically, the threshold voltage, contact resistance and temperature-dependent constant exhibit parabolic trends for increasing alkanethiol chain length. Conversely, both the band mobility and the charge mobility are characterized by oscillations that follow an odd–even trend with a linear decay for increasing number of the carbon atoms. These results corroborate the theoretical interpretation, previously proposed by Stoliar et al., that the odd–even effect is ascribable to the order degree and orientation of the pentacene molecules interacting with the SAMs, while the parabolic trend depends on the chain length of the alkanethiol SAMs. • Charge injection is monitored in real-time for in situ growing pentacene films • Charge injection versus alkanethiol SAMs is investigated • SAMs dis(order) determines the parabolic trend of the charge injection • The arrangement of molecules at the SAM interface determines the odd-even effect • A new empirical equation rationalizes the drain-source current during the film growth

Influence of the terrace width of pentacene underlayers on the crystallinity of C60 overlayers

Pentacene and fullerene (C60) form an efficient stacked heterostructure in organic solar cells, and C60 films exhibit high crystallinity on pentacene films. However, the mechanism of the crystallinity improvement in C60 overlayers remains to be fully elucidated. In this work, the surface (step and terrace structure) of pentacene films is investigated in terms of the terrace width using atomic force microscopy (AFM) to clarify the mechanism. We found that the terrace width becomes wider as the substrate temperature for pentacene film growth increases, and the terrace width becomes narrower as the thickness of pentacene films increases. In addition, we investigate the influence of the terrace width of pentacene underlayers on C60 overlayers using AFM and X-ray diffraction. We found that pentacene underlayers with wide terraces increase the grain size and improve the crystallinity of C60 overlayers. These findings indicate that growing pentacene underlayers with wider terraces allows for further enhancement in the crystallinity of C60 overlayers, leading to higher quality pentacene/C60 stacked heterostructures.

Surface morphology of vacuum-evaporated pentacene film on Si substrate studied by in situ grazing-incidence small-angle X-ray scattering: I. The initial stage of formation of pentacene film

The progress of the surface morphology of a growing sub-monolayered pentacene film on a Si substrate was studied by in situ grazing-incidence small angle X-ray scattering (GISAXS). The observed GISAXS profiles did not show sizes of pentacene islands but mainly protuberances on the boundaries around pentacene film. Scattering of X-ray by residual pits in the pentacene film was also detected in the GISAXS profiles of an almost fully covered film. The average radius of pentacene protuberances increased from 13 to 24 nm as the coverage increased to 0.83 monolayer, and the most frequent radius was almost constant at approximately 9 nm. This result suggests that the population of larger protuberances increase with increasing lengths of boundaries of the pentacene film. It can also be considered that the detected protuberances were crystallites of pentacene, since the average size of protuberances was nearly equal to crystallite sizes of pentacene films. The almost constant characteristic distance of 610 nm and amplitudes of pair correlation functions at low coverages suggest that the growth of pentacene films obeyed the diffusion-limited aggregation (DLA) model, as previously reported. It is also considered that the sites of islands show a triangular distribution for small variations of estimated correlation distances.

A low temperature solution-processed ormosil film for low-voltage organic field-effect transistors

A high performance, low operating voltage organic field effect transistor (OFET) has been fabricated by utilizing organically modified silicate (ormosil) based film as dielectric. The ormosil film is fabricated by a sol-gel method at low temperature (180 °C) and exhibits a high dielectric (k = 33), a smooth surface (Rq = 0.29 nm) and a low leakage current density (5 × 10−9 A cm−2 at −2 V). The ormosil film contains hydrophobic methyl (CH3) functional groups derived from methyltriethoxysilane (MTES) and these groups produce a surface with hydrophobic character and low surface energy for pentacene film growth. The OFET with the ormosil dielectric exhibits excellent performs with high mobility (0.80 cm2 V−1 s−1), low operating voltage (−1.5 V), low threshold voltage (−0.25 V) and low sub-threshold swing (192 mV dec−1). The result demonstrates that the ormosil film can be used as a high performance dielectric for OFETs, and provides a promising way for low power and low cost organic electronics.

Study of copper-phthalocyanine and pentacene film growth on transferred graphene: The influence of polymer residues

Wet transferring of chemical vapor deposition (CVD) graphene with the assistance of poly(methyl methacrylate) (PMMA) usually leaves polymer contaminations onto graphene surface. Knowledge about the influence of PMMA residues on organic film growth properties is crucial for graphene-related organic electronic devices. In this study, we investigate growth properties of coper-phthalocyanine (CuPc) and pentacene on PMMA contaminated graphene substrate. Grain size and morphologies of CuPc and pentacene films were found to be governed by the density of PMMA residues on graphene. Nevertheless, the molecular packing structures of CuPc and pentacene respond differently with the density with PMMA residues. CuPc molecules tend to nucleated into grains with molecular π-π stacking parallel to graphene surface, which is irrespective of the PMMA residues. Whereas the packing behavior of pentacene on PMMA-transferred CVD graphene is sensitive to the density of PMMA residues. Molecular π-π stacking perpendicular to graphene is observed when pentacene was deposited onto as-transferred graphene. Parallel mode was observed only on the substrates without PMMA residues.

Structure of clusters of pentacene molecules and their polarizabilities

We calculate and compare the ground state structures of one, two and three pentacene molecule clusters, using B3LYP and B97-D density functionals, and polarizabilities in the ground state structure to examine the extent of anisotropy for applications in electronic device fabrication. B3LYP and B97-D functionals perform sufficiently well for a single pentacene molecule, but for two and three molecule clusters, the inclusion of dispersive interactions through B97-D becomes necessary. For two and three molecule clusters the energy landscape near ground state is flat in molecular coordinates, implying multiple related structures may be accessible at room temperature. Further, for the ground state structures, the polarizability along the long axis of the molecule is larger than the other two directions. This may make it possible to explain the beneficial effect of applying an electric field during processing of pentacene films in electronic devices, as observed experimentally; specifically, in this context, the structural and polarizability data presented could allow for development of models for nucleation and growth of pentacene films.

Charge transport and morphology of pentacene films confined in nano-patterned region

The transport and morphology of macroscopic organic transistors have been thoroughly explained. However, the relationship between charge transport and the morphology of organic semiconductors in nano-confined regions is not well understood, which is required to design high-performance nano-electronics. Therefore, in this study, the electrical performance of pentacene thin-film transistors (TFTs) fabricated on nano-scale confined geometries was compared with that of other TFT systems fabricated on micro-scale confined or flat dielectrics. The results showed that the photoresist (PR) reliefs patterned onto silicon dioxide (SiO2) dielectrics could control the growth mode of pentacene in the confined regions. As the line spacing between the PR patterns decreased to below the average size of pentacene normally grown on SiO2 dielectrics (∼1.7 μm), field-effect mobility (μFET) of pentacene TFTs improved considerably. Specifically, the μFET values for the 250 nm patterned dielectric system were as high as 0.33 cm2 V−1 s−1, which was greater than those of the wider-patterned PR or flat dielectric systems (⩽ 0.15 cm2 V−1 s−1). These findings were related to the enhancement of charge-carrier transport owing to the anisotropic π-conjugated crystals nucleated from the nano-confined edges, providing important information that can be used in the design of high-performance nano-scale organic electronics. We have analyzed the effect of the morphology of the pentacene films confined in the micro- and nano-scale relief patterns on the charge transport. As the patterned space region for confining the growth of organic semiconductors decreased to below the average grain size (∼ 1.7 μm), the field-effect mobility increased. Because the molecular ordering is influenced by the pattern’s confining effect on pentacene film growth. Our results will provide an approach that can be used to further increase the charge transport mobility of polycrystalline organic semiconductors by increasing the molecular ordering in the direction of charge transport. While the effect of morphology on an organic semiconductor's charge transport is generally well understood, less is known about the effect when the material is confined, despite this being a highly relevant situation for nanoelectronic devices. A South Korea-based research team led by Tae-Woo Lee has now determined how the performance of a thin-film transistor is affected when its pentacene semiconductor is confined in a nanoscale pattern rather than a microscale pattern or a flat dielectric system. The researchers observed higher field-effect mobility in the nanopattern-based device compared to the other configurations. The enhancement arises as a result of the nanopattern's effect on the arrangement of the pentacene molecules in the thin film — favouring their ordering in the direction of the charge transport, which in turn improves the device's performance. This understanding of the effect of confinement on charge mobility may contribute to the engineering of more efficient organic semiconductors.

Helium diffraction study of pentacene films on Au(1 1 1)

Here we present a helium atom diffraction study of pentacene films on Au(111) surface prepared by supersonic molecular beam deposition. Though investigated parameter space was limited no significant difference between the films prepared by different deposition energies was observed. Completion of monolayer coverage was confirmed by simultaneous helium scattering and quartz crystal resonance frequency shift measurements during pentacene film growth on the gold electrode of a quartz resonator. Monolayer films were found to adopt a (6×3) unit cell which was also observed for pentacene monolayers on Ag(111). However no ordered multilayer film structure could be observed which is in contrast with the previous Ag(111) studies.

Nucleation of Organic Molecules via a Hot Precursor State: Pentacene on Amorphous Mica

Organic thin films have attracted considerable interest due to their applicability in organic electronics. The classical scenario for thin film nucleation is the diffusion-limited aggregation (DLA). Recently, it has been shown that organic thin film growth is better described by attachment-limited aggregation (ALA). However, in both cases, an unusual relationship between the island density and the substrate temperature was observed. Here, we present an aggregation model that goes beyond the classical DLA or ALA models to explain this behavior. We propose that the (hot) molecules impinging on the surface cannot immediately equilibrate to the substrate temperature but remain in a hot precursor state. In this state, the molecules can migrate considerable distances before attaching to a stable or unstable island. This results in a significantly smaller island density than expected by assuming fast equilibration and random diffusion. We have applied our model to pentacene film growth on amorphous Muscovite mica.

Behavior of pentacene molecules deposited onto roughness-controlled polymer dielectrics films and its effect on FET performance

We describe the effects of the polymer dielectric surface roughness on the nucleation and initial island growth of pentacene films in the submonolayer regime using poly(methylmethacrylate) and polystyrene films with highly controlled surface roughness value. The pentacene film structures were found to significantly affect the performances of pentacene field-effect transistors (FETs). As the dielectric surface roughness increased, the activation energies for pentacene island nucleation and molecule reevaporation from the surface were dramatically reduced. A high surface roughness greater than 0.9nm was found to limit the degree to which the FET performances could be improved. These results suggest a means for optimizing the nanostructural surfaces of polymer dielectrics by engineering OFET interfaces in an effort to improve device performance.

Effect of Multi-Dimensional Ultraviolet Light Exposure on the Growth of Pentacene Film and Application to Organic Field-Effect Transistors

We report on the multi-dimensional alignment of pentacene molecules on a poly(methyl methacrylate)-based photosensitive polymer (PMMA-polymer) and its effect on the electrical performance of the pentacene-based field-effect transistor (FET). Pentacene molecules are shown to be preferentially aligned on the linearly polarized ultraviolet (LPUV)-exposed PMMA-polymer layer, which is contrast to an isotropic alignment on the bare PMMA-polymer layer. Multi-dimensional alignment of pentacene molecules in the film could be achieved by adjusting the direction of LPUV exposed to the PMMA-polymer. The control of pentacene molecular alignment is found to be promising for the field-effect mobility enhancement in the pentacene FET.

Organic Film Growth on In Plane Heterogeneous Substrates

This paper introduces two examples of organic thin film growth on in plane heterogeneous substrates. One is growth of pentacene films on SiO2 with metal electrodes, in which nucleation and initial growth process was observed real time by photoemission electron microscopy. Another is growth of sexithiophene (6T) films on SiO2 with periodic grooves (graphoepitaxy). Elongated 6T grains could be obtained along the groove under an optimal growth condition. The mechanism of these growth processes could be basically understood according to a conventional thin film growth theory.

Formation of intra-island grain boundaries in pentacene monolayers

To assess the formation of intra-island grain boundaries during the early stages of pentacene film growth, we studied sub-monolayers of pentacene on pristine silicon oxide and silicon oxide with high pinning centre density (induced by UV/O(3) treatment). We investigated the influence of the kinetic energy of the impinging molecules on the sub-monolayer growth by comparing organic molecular beam deposition (OMBD) and supersonic molecular beam deposition (SuMBD). For pentacene films fabricated by OMBD, higher pentacene island-density and higher polycrystalline island density were observed on UV/O(3)-treated silicon oxide as compared to pristine silicon oxide. Pentacene films deposited by SuMBD exhibited about one order of magnitude lower island- and polycrystalline island densities compared to OMBD, on both types of substrates. Our results suggest that polycrystalline growth of single islands on amorphous silicon oxide is facilitated by structural/chemical surface pinning centres, which act as nucleation centres for multiple grain formation in a single island. Furthermore, the overall lower intra-island grain boundary density in pentacene films fabricated by SuMBD reduces the number of charge carrier trapping sites specific to grain boundaries and should thus help achieving higher charge carrier mobilities, which are advantageous for their use in organic thin-film transistors.

Behavior of Pentacene Initial Nucleation on Various Dielectrics and Its Effect on Carrier Transport in Organic Field-Effect Transistor

The influence of dielectric surface energy on the initial nucleation and the growth of pentacene films as well as the electrical properties of the pentacene-based field-effect transistors are investigated. We have examined a range of organic and inorganic dielectrics with different surface energies, such as polycarbonate/SiO2, polystyrene/SiO2, and PMMA/SiO2 bi-layered dielectrics and also the bare SiO2 dielectric. Atomic force microscopy measurements of sub-monolayer and thick pentacene films indicated that the growth of pentacene film was in Stranski-Kranstanow growth mode on all the dielectrics. However, the initial nucleation density and the size of the first-layered pentacene islands deposited on different dielectrics are drastically influenced by the dielectric surface energy. With the increasing of the surface energy, the nucleation density increased and thus the average size of pentacene islands for the first mono-layer deposition decreased. The performance of fabricated pentacene-based thin film transistors was found to be highly related to nucleation density and the island size of deposited Pentacene film, and it had no relationship to the final particle size of the thick pentacene film. The field effect mobility of the thin film transistor could be achieved as high as 1.38 cm2Ns with on/off ratio over 3 x 10(7) on the PS/SiO2 where the lowest surface energy existed among all the dielectrics. For comparison, the values of mobility and on/off ratio were 0.42 cm2Ns and 1 x 10(6) for thin film transistor deposited directly on bare SiO2 having the highest surface energy.

Polymorphic transformation induced by nanoimprinted technology in pentacene-film early-stage growth

Polymorphism control of organic semiconductor films greatly affects optoelectronic device performance. The thin-film phase is always the dominant phase in the early growth of pentacene films; however, the performance of the bulk phase is superior to that of the thin-film phase. In the present study, a nanoimprinted polyimide layer is used to induce the growth of the bulk phase in the early stage. The bulk phase appears in the initial few layers and its dominance of the polymorphisms in the pentacene film increases with increasing thickness. The mechanism of the phase transformation is explained.

Energetics and stability of pentacene thin films on amorphous and crystalline octadecylsilane modified surfaces

Charge transport in organic thin film transistors (OTFTs) is directly related to the morphology and growth of the organic semiconductor at the dielectric interface. The most commonly used dielectric interface in OTFT research is alkylsilane-modified silicon oxide (SiO2). In this report, the nucleation, energetics, and stability of pentacene thin films on methyl-terminated surfaces are discussed. The density of the terminal methyl group was found to be an important parameter for controlling the growth of organic semiconductors. Pentacene growth is two-dimensional (2D) on SiO2 dielectrics modified with a crystalline, densely packed octadecylsilane (OTS) monolayer. However, it is primarily three-dimensional (3D) on SiO2 dielectrics modified with an amorphous OTS layer. Beyond a critical OTS density, the interaction between the OTS and pentacene exceeds the pentacene interlayer interaction energy engendering 2D growth which is preferential for high charge carrier mobility. The nucleation density is also much higher on the crystalline OTS compared to the amorphous OTS. The sub-monolayer thin films of pentacene were found to be much more stable on the ordered OTS compared to disordered OTS. Atomic force microscopy (AFM) and Monte Carlo simulations were used to develop a thorough analysis of pentacene film growth and energetics on OTS surfaces.

Real-time observation and control of pentacene film growth on an artificially structured substrate.

Suppression of nucleation around a gold electrode during pentacene growth on a SiO2 channel is found by photoemission electron microscopy. Mass flow is driven by the difference between the molecular orientations on SiO2 and gold. The poor connectivity at the channel/electrode boundary causes degradation in the performance of a field-effect transistor, which is found to be improved by self-assembled monolayer treatment on the electrode (see figure; thickness in monolayers (ML)).

Enhancement of Electrical Conductance for Pentacene Thin Film Transistor by Controlling an Initial Layer-by-Layer Growth Mode Directly on SiO2 Insulator

Initial nucleation and growth of pentacene films on various pre-cleaning treated SiO2 gate insulators were systematically examined by atomic force microscope. The performance of fabricated pentacene thin film transistor devices was found to be highly related to the initial film growth modes. In contrast to the film in the three-dimensional island-like growth mode on SiO2 under an organic cleaning process, a layer-by-layer initial growth mode occurred on the SiO2 insulator cleaned with ammonia solution which has shown much improved electrical properties of the thin film transistors. Field effect mobility of the thin film transistor devices could be achieved as high as 1.0 cm2 V-1 s-1 on the bared SiO2/Si substrate and the on/off ratio was over 106. The enhanced electrical conductance was further confirmed by an electrostatic force microscopic observation of quantized electrical potentials via charge-injection to the submonolayer pentacene islands with layer-by-layer growth mode.

Low-voltage organic transistors with titanium oxide/polystyrene bilayer dielectrics

Pentacene-based low-voltage organic transistors were realized with titanium oxide/polystyrene (TiO2/PS) bilayer dielectrics. Significantly, the TiO2 bottom layer was fabricated by a layer-by-layer deposition procedure and had an ultrathin thickness (<5 nm). The nonpolar PS top layer not only shields out the high polarity TiO2 layer, but also reduces the roughness of the dielectric, which results in low gate leakage of dielectric and the more ordered growth of pentacene film. This organic transistor with a TiO2/PS bilayer dielectric was found to exhibit high performance with a low operating voltage (≤3 V), and a high on/off ratio (>107).