Crystalline Organic Thin Films Research Articles

Data storage with 0.7 nm recording marks on a crystalline organic thin film by a scanning tunneling microscope

Ultrahigh density data storage on a novel organic thin film by scanning tunneling microscope (STM) under ambient conditions is demonstrated. The material, N-(3-nitrobenzylidene)p-phenylenediamine (NBPDA), is used for preparing thin film by vacuum evaporation method. Crystalline NBPDA films with electrical bistability are obtained by this method. Recording experiment on the films is made by applying voltage pulses between the STM tip and substrate. The recorded marks are 0.7 nm in size, corresponding to a storage density of 1014 bit/cm2. Current–voltage characteristic measurement shows that the resistance of the unrecorded region of the NBPDA films is much higher than that of the recorded region. The mechanism of recording is discussed.

STUDY OF ULTRAHIGH DENSITY DATA STORAGE ON ORGANIC-COMPLEX THIN FILMS

Crystalline organic complex thin films have been performed by vacuum evaporation method on HOPG substrates.Using Scanning Tunneling Microscope storage,array has been made on the film by applying voltage pulses between the tip and the substrate.The recording marks are 1.3nm in diameter.Voltage-current characterization shows that the recorded region has a conductor behavior while the unrecorded regions shows an insulator behavior.It is confirmed from experiments that the distance between two recording marks can be as short as 2nm,which corresponds to the storage density of 2.5×1013bit/cm2.The mechanism about the recording is briefly discussed.

Three-Color, Tunable, Organic Light-Emitting Devices

An independently controlled, three-color, organic light-emitting device was constructed with a vertically stacked pixel architecture that allows for independent tuning of color, gray scale, and intensity. The 12-layer device structure consists of sequentially stacked layers of metal oxide, amorphous organic, crystalline organic, and metal thin films deposited by a combination of thermal evaporation and radio-frequency sputtering. Each of the three addressable colors is sufficiently bright for flat panel video display applications. A novel inverted structure is used for the middle device in the stack to lower the maximum drive voltage of the compound pixel.

Layer-by-layer quasi-epitaxial growth of a crystalline organic thin film

We use reflection high energy electron diffraction and grazing incidence X-ray diffraction to study films of the archetype crystalline organic material: 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA) grown on Au(111) by organic molecular beam deposition. Although the PTCDA lattice is significantly mismatched to the Au(111) substrate, the films are orientationally aligned and have a well defined film thickness, providing evidence for layer-by-layer “quasi-epitaxial” growth. Furthermore, we have performed the first characterization of the molecular level disorder in these films, and have determined the presence of a significant (3.8%) strain in the 2D lattice parameters.

Excitons and exciton confinement in crystalline organic thin films grown by organic molecular-beam deposition.

We study excitons in thin films of varying thickness of two organic molecular crystals grown by the ultrahigh-vacuum process of organic molecular-beam deposition; 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA), and 3,4,9,10-perylenetetracarboxylic bis-benzimidazole (PTCBI). By using electroabsorption spectroscopy, low-temperature fluorescence, and absorption techniques, we find that nearly spatially symmetric charge-transfer/Wannier-like excitons are present in crystalline PTCDA films, and that these excitons can be confined in multilayer stacks of ultrathin layers of thicknesses ranging from 10 to 100 \AA{}. In PTCBI films, however, the exciton has a smaller radius, and thus much weaker spectral and electric-field dependences on layer thickness are observed. In addition, we present theoretical and experimental evidence showing that exciton-lattice interactions may be modified by the growth of ultrathin layers and the subsequent confinement of the charge-transfer excitons.

Photonic Devices Based on Crystalline Organic Semiconductors for Optoelectronic Integrated Circuits

Crystalline organic thin films are deposited under ultrahigh vacuum conditions at substrate temperatures from 77 K to 300 K. Due to the weak van der Waals bonding forces between the organic molecules, lattice matching is not required for layer growth. Structures can be prepared both on dielectric or amorphous substrates and on top of completely processed III-V or Si wafers. In this paper, we discuss the fabrication and performance of organic semiconductor electroluminescent devices, waveguides, and photodetectors.

Finite size effects observed in the fluorescence of ultrathin crystalline organic films grown by organic molecular beam deposition

We study the low-temperature fluorescence spectra of stacks of alternating layers of crystalline organic thin films grown by organic molecular beam deposition. We find that there is an increase in molecular vibrational frequency and a redistribution of fluorescence intensity to higher energy as the layer thickness decreases from 500 to 10 Å. These observations indicate that the width of the molecular potential energy well of the ground state and the displacement of the excited and ground states are functions of layer thickness. We explain these results using a model based on electronic wavefunction confinement and a systematic change in the exciton—phonon coupling in ultrathin molecular films.

Optoelectronic and Structural Properties of Vacuum-Deposited Crystalline Organic Thin Films

ABSTRACTRecently, it has been discovered that crystalline organic thin films can be deposited in nearly single crystalline form on a variety of substrates such as glass, polymers, etc. Since then, this discovery has led to the growth of crystalline organic quantum wells, waveguides, coupler/switches, and organic/ inorganic heterojunction devices such as field effect transistors and avalanche photodiodes. Organic light emitting diodes (LEDs) which luminesce in the red, green and blue have also been demonstrated. In this paper, we will report on several recent advances in the growth of organic thin films deposited by organic molecular beam deposition. We report on modeling of organic monolayer growth based on the atom-atom potential Method. The Model provides insight into the factors which control “quasi-epitaxial growth” i.e. the ordered growth of one layer of an organic film which is incommensurate with the substrate lattice. We also observe large optical nonlinearities which are a feature of both single and Multi-layer crystalline organic films. The growth of organic, nonlinear optically active crystalline organic compounds are also discussed.

Observation and modeling of quasiepitaxial growth of a crystalline organic thin film

We directly observe, using the scanning tunneling microscope, a two-dimensional crystal of the organic compound 3, 4, 9, 10-perylenetetracarboxylic dianhydride (PTCDA). The surface unit cell dimension is found to be 21.6±2.2 Å by 15.2±1.6 Å, or approximately 20% larger than the bulk unit cell. Furthermore, the organic lattice is oriented with respect to the graphite substrate even though the two lattices are incommensurate. These observations are consistent with reflection high energy electron diffraction measurements, and energy minimization calculations, assuming that the van der Waals bond is the predominant intermolecular force which determines the equilibrium crystal structure. The combination of measurement and theory provides the first step in developing tools for predicting the conditions which lead to quasiepitaxial growth of these technologically important van der Waals solids.

Evolution of quasi-epitaxial growth of a crystalline organic semiconductor on graphite

We have studied the evolution of the growth of a crystalline organic semiconductor thin film using reflection high energy electron diffraction (RHEED). Our results indicate that highly ordered crystalline films of an organic compound; namely 3, 4, 9, 10-perylenetetracarboxylic dianhydride (PTCDA), can be grown on graphite by the ultrahigh vacuum process of organic molecular beam deposition even though the crystal structures of the two materials are highly mismatched. The RHEED patterns show the evolution of planar crystal growth from 2 monolayer (∼6 Å) coverage until at least 50 Å as the films are deposited onto substrates cooled to 100 K. At larger film thicknesses, somewhat nonplanar but still crystalline growth occurs. Furthermore, crystalline thin films were obtained independent of growth rate, which was as high as 3.0 Å/s. To our knowledge, this is the first direct experimental evidence of the evolution of growth from monolayer to bulk film coverage into an ordered, quasi-epitaxial structure.