Organic Molecular Thin Films Research Articles

Orientation-controlled growth of molecular organic thin films

The control of the structure and preferential orientation of molecular organic thin films might be achieved by the selection of suitable substrates that, in general, have to be designed. Langmuir–Blodgett (LB) and self-assembled monolayers (SAMs) films are good candidates as substrates due to the chemical versatility of their surfaces. We illustrate an example of the selection of either of the two energetically most probable preferential orientations of polycrystalline neutral tetramethyltetraselenafulvalene (TMTSF) thin films by the choice of different substrates. The films (thickness ∼1 µm), obtained by sublimation of TMTSF microcrystals in high vacuum (∼10−6 mbar), are dominated by (0 −2 1)- and (100)-oriented microcrystals when using KBr (100) and methyl-terminated films, respectively.

Molecular Organic Thin Films of p-Nitrophenyl Nitronyl Nitroxide: Surface Morphology and Polymorphism

Highly oriented thin films (thickness ≈︂1 to 2 μm) of the molecular organic radical p-nitrophenyl nitronyl nitroxide (p-NPNN) have been grown by thermal evaporation in high vacuum on three different substrates: NaCl (001), mica, and highly oriented pyrolytic graphite (HOPG). The surfaces associated to three polymorphs (α, β and δ) reveal clearly differentiated nanometer-scale morphologies as evidenced by Tapping Mode Atomic Force Microscopy (TMAFM) measurements.

Tuning the color emission of thin film molecular organic light emitting devices by the solid state solvation effect

We demonstrate a general method for tuning the luminescent emission spectrum of dipolar molecules by adjusting the strength of intermolecular dipole–dipole interactions using a doped guest–host molecular organic thin film system. Spectral shifting is achieved by introducing dopants that change the local electric field in the film. This `solid state solvation effect' is used to continuously tune, by up to 75 nm, the peak emission wavelength of organic light emitting devices consisting of triarylamine host materials doped with polar luminescent dyes DCM2, aluminum tris(8-hydroxyquinoline), or both. Red, orange, yellow, green, blue, and white emission is demonstrated with the same set of organic molecules.

Optical properties of molecular organic semiconductor thin films under intense electrical excitation

Using electro-optical pump and probe measurements, we studied optical properties of a thin film of tris-(8-hydroxyquinoline) aluminum (Alq3) doped with DCM2 laser dye under pulsed injection currents as high as 300 A/cm2. Pump-induced transparency and absorption were observed in the spectral ranges of 570–610 nm and 620–705 nm, respectively. We attribute these optical effects to molecular polarons formed under electrical excitation. Polaron absorption dominates the optical properties of electrically pumped Alq3:DCM2 films in the spectral range where optical gain is anticipated (i.e., from 630 to 700 nm). Polaronic effects may present a significant obstacle for realization of organic diode lasers based on this material system.

Dislocation hollow cores observed on surfaces of molecular organic thin films: p-nitrophenyl nitroxyl nitroxide radical

We have studied by means of tapping mode atomic force microscopy ex situ grown surfaces of highly-oriented α-phase thin films of the molecular organic radical p-nitrophenyl nitroxyl nitroxide. The films are grown by thermal evaporation of the precursor radical in high vacuum on glass and NaCl(001) substrates. The surfaces reveal a complex nanometer-scale morphology formed by a random distribution of dislocations identified as spirals (screw dislocations) of opposite sign interacting in pairs. Each spiral emerges from a hollow core with a diameter of ca 15–20 nm. The dislocation density is rather high due to the way in which the films grow under the given experimental conditions. The out-of-plane growth of the films is governed by the Frank–Read source mechanism.

Ultrahigh Si+ implant activation efficiency in GaN using a high-temperature rapid thermal process system

Si + implant activation efficiencies above 90%, even at doses of 5×1015 cm−2, have been achieved in GaN by rapid thermal processing at 1400–1500 °C for 10 s. The annealing system utilizes molybdenum intermetallic heating elements capable of operation up to 1900 °C, producing high heating and cooling rates (up to 100 °C s−1). Unencapsulated GaN shows severe surface pitting at 1300 °C and complete loss of the film by evaporation at 1400 °C. Dissociation of nitrogen from the surface is found to occur with an approximate activation energy of 3.8 eV for GaN (compared to 4.4 eV for AlN and 3.4 eV for InN). Encapsulation with either rf magnetron reactively sputtered or metal organic molecular beam epitaxy-grown AlN thin films provides protection against GaN surface degradation up to 1400 °C, where peak electron concentrations of ∼5×1020 cm−3 can be achieved in Si-implanted GaN. Secondary ion mass spectrometry profiling showed little measurable redistribution of Si, suggesting DSi⩽10−13 cm2 s−1 at 1400 °C. The implant activation efficiency decreases at higher temperatures, which may result from SiGa to SiN site switching and resultant self-compensation.

Thin film organic light emitting devices and lasers

Abstract We describe recent advances in luminescent devices based on Forster energy transfer in vacuum deposited, small molecular weight organic thin films. As examples, we concentrate our discussion on a novel, 12 layer stacked organic light emitting device useful for full color, high resolution flat panel display applications, and optically pumped semiconducting organic thin film lasers. Both devices demonstrate the unusual properties characteristic of these materials which open the door to a wide range of new device possibilities.

Quantum size effects of charge-transfer excitonsin nonpolar molecular organic thin films

We use a quantum-mechanical model to analyze charge-transfer (CT) excitons in closely packed, nonpolar organic molecular crystal thin films grown by the ultrahigh-vacuum process of organic molecular beam deposition. The exciton Hamiltonian includes both polarization effects and the periodic pseudopotential of the crystal. This model takes into account the very large anisotropy characteristic of many organic materials such as the archetype molecular crystal, 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA) and PTCDA-based multilayers. Using a single-exciton Hamiltonian, we quantitatively model experimental electroabsorption data and the absorption spectral shifts observed in ultrathin organic multilayers or ``multiple quantum wells.'' The data analyzed from several such experiments give independent and consistent estimations of the anisotropic effective mass tensor and exciton radii for PTCDA along different crystal axes. This treatment is general, and is found to extend to other CT and Wannier exciton systems found in many interesting, nonpolar organic molecular and inorganic semiconductor crystals such GaAs, suggesting similar physical origins for Wannier and CT excitons in a wide range of materials.

Chemistry and electronic properties of metal contacts on an organic molecular semiconductor

This paper gives a brief review of the electronic properties of the surface of an organic molecular semiconductor, i.e. PTCDA, and of the chemical and electrical properties of its contacts with to metals, i.e. Ti and Au. The combination of direct and inverse photoemission spectroscopies demonstrates that correlation effects are small in PTCDA. Synchrotron radiation photoemission spectroscopy shows that the Ti PTCDA interface is reacted and that chemistry-induced electronic states in the PTCDA band gap lead to an Ohmic contact. The Au PTCDA interface, on the other hand, is unreacted and abrupt and the Au PTCDA contact is rectifying. This duality, which applies also to other metal/PTCDA interfaces, suggests the overwhelming importance of chemistry in defining electrical properties of contacts to organic molecular thin films.

The nonlinear Schrödinger system: collapse, nonlinear damping, noise, impurities and nonlocal dispersion

The paper reports on the effects of adding noise, damping and impurities to the 2-dimensional nonlinear Schrödinger equation. The effects of including long-range dispersion in the 1-dimensional nonlinear Schrödinger equation are also examined. The framework of the reported results is the application of nonlinear Schrödinger systems in the study of molecular systems and organic thin films. The paper includes numerical as well as analytical results.

Characteristic Energy Losses by Slow Electrons in Organic Molecular Thin Films at 77°K

Characteristic energy losses by slow electron impact have been measured for ⪖102 Å films of several aromatic compounds as well as alkanes, alkenes, alcohols, acetone and ice at 77°K. Resonances are clearly indicated by the electron current I transmitted by the dielectric as a function of the modulated thermionic emitter voltage V, displayed as dI/dV vs V. Energy losses are measured directly by retarding potential analysis of backscattered electrons. Measurements at several incident electron energies provide the V-dependence of the cross section. The method is particularly useful for measuring small losses (∼1 eV). There is no potential barrier for injecting electrons into the dielectrics examined, and the evidence indicates that they have positive bulk electron affinities. All systems show resonances at 0.5–1 eV which are attributed to transient negative ions. Acetone, n-hexane and ice were examined in some detail for 0–4 eV losses by retarding potential analysis. The results indicate that electrons are also inelastically scattered by higher transient negative ion states. All compounds examined show resonances in the 3–4 eV range, which may be due to low lying triplet states. The alkanes also have considerable oscillator strength in the 4–7 eV range, approximating that in the 7–10 eV range.