Organic Molecular Beam Deposition Research Articles

Effects of temperature on structural and morphological features of CoPc and CoPcF 16 thin films

Herein investigation of the effects of substrate temperature on the structural and morphological features of both cobalt(II) phthalocyanine (CoPc) and cobalt(II) hexadecafluorophthalocyanine (CoPcF 16) thin films is presented. For these purposes thin films of CoPc and CoPcF 16 prepared by organic molecular beam deposition were investigated by means of optical absorption spectroscopy, X-ray diffraction, Raman spectroscopy, and atomic force microscopy. Concerning the degree of crystallinity, the morphology, the phase composition and the preferential molecular orientation of both CoPc and CoPcF 16 thin films, we found out that the increase of substrate temperature during growth influences these properties of the above-mentioned thin film systems (CoPc vs. CoPcF 16) in a different way.

Highly Efficient p‐i‐n and Tandem Organic Light‐Emitting Devices Using an Air‐Stable and Low‐Temperature‐Evaporable Metal Azide as an n‐Dopant

AbstractCesium azide (CsN3) is employed as a novel n‐dopant because of its air stability and low deposition temperature. CsN3 is easily co‐deposited with the electron transporting materials in an organic molecular beam deposition chamber so that it works well as an n‐dopant in the electron transport layer because its evaporation temperature is similar to that of common organic materials. The driving voltage of the p‐i‐n device with the CsN3‐doped n‐type layer and a MoO3‐doped p‐type layer is greatly reduced, and this device exhibits a very high power efficiency (57 lm W−1). Additionally, an n‐doping mechanism study reveals that CsN3 was decomposed into Cs and N2 during the evaporation. The charge injection mechanism was investigated using transient electroluminescence and capacitance–voltage measurements. A very highly efficient tandem organic light‐emitting diodes (OLED; 84 cd A−1) is also created using an n–p junction that is composed of the CsN3‐doped n‐type organic layer/MoO3 p‐type inorganic layer as the interconnecting unit. This work demonstrates that an air‐stable and low‐temperature‐evaporable inorganic n‐dopant can very effectively enhance the device performance in p‐i‐n and tandem OLEDs, as well as simplify the material handling for the vacuum deposition process.

Development of organic light‐emitting diodes for electro‐optical integrated devices

AbstractOrganic light‐emitting diodes (OLEDs) are discussed for electro‐optical integrated devices that are used for optical signal transmission. Organic optical devices including polymeric optical fibers are used for optical communication applications to realize polymeric electro‐optical integrated devices. The OLEDs were fabricated by vacuum process, i.e. the organic molecular beam deposition (OMBD) technique or a solution process on a polymeric or a glass substrate, for comparison. Optical signals faster than 100 MHz have been created by applying pulsed voltage directly to the OLED utilizing rubrene doped in 8‐hydoxyquinolinum aluminum (Alq3), as an emissive layer. OLEDs fabricated by solution process utilizing rubrene doped in carrier‐transporting materials have also discussed. OLEDs utilizing polymeric materials by solution process are also fabricated and discussed. Moving‐picture signals are transmitted utilizing both vacuum‐ and solution‐processed OLEDs, respectively.

Spectroscopic ellipsometry and reflection anisotropy spectroscopy of lutetium diphthalocyanine films on silicon

AbstractThe main aim of this work is to investigate organic thin films by the use of Reflection Anisotropy Spectroscopy (RAS) and Spectroscopic Ellipsometry (SE) in situ during growth of the thin films on silicon substrates. The investigated material is the rare‐earth diphthalocyanine LuPc2. The LuPc2 films were deposited on hydrogen passtivated Si(111) and on oxidised Si substrates by Organic Molecular Beam Deposition (OMBD). The films on H‐Si(111) show a high anisotropic behaviour. Model calculations for in situ SE measurements revealed angles between θ = 60° and 90° for LuPc2 on H‐Si(111). Furthermore, the energy transitions found match to those determined by Density Functional Theory calculations (DFT). (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Optical investigation of CuPc thin films on vicinal Si(111)

AbstractCuPc thin films were grown on the hydrogenated vicinal Si(111) surfaces by organic molecular beam deposition (OMBD). Spectroscopic ellipsometry was employed to determine the refractive index (n) and extinction coefficient (k). Furthermore, the ratio of the out‐of‐plane to the in‐plane absorption intensity provides the average tilting angle of molecules in the film. Reflectance anisotropy spectroscopy (RAS) shows the optical anisotropy of CuPc thin films induced by the vicinal Si(111) surfaces, which indicates the in‐plane orientation of molecules. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

In‐plane optical anisotropy of copper‐phthalocyanine films: RAS studies and modelling

AbstractCopper‐phthalocyanine (CuPc) films prepared by organic molecular beam deposition on H‐passivated silicon and Si covered with native oxide were investigated by means of ellipsometry and reflection anisotropy spectroscopy (RAS). In addition to the out‐of‐plane anisotropy found from the evaluation of the ellipsometry data (Aspnes and Studna, Phys. Rev. Lett. 54, 1954 (1985) [1]) an in‐plane anisotropy in the order of 10‐2 was detected by RAS. In a first step model calculations assuming homogeneous in‐plane anisotropy over the whole layer thickness were performed in order to extract the anisotropy of the optical constants n and k.In a second step, more advanced models taking into account a change in the anisotropy with film thickness were constructed in order to describe the in‐plane anisotropy spectral evolution with film thickness. For the films on Si/SiO2 the in‐plane anisotropy is found to increase with increasing thickness. On H‐Si this effect is not observed and the behaviour of CuPc is more complicated to model on this substrate. Thus, the interaction of the molecules with the differently treated substrate surfaces plays a crucial role for the molecular alignment. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Morphology and Structure Transitions of Copper Hexadecafluorophthalocyanine (F16CuPc) Thin Films

The thickness-dependent morphology and structure transitions of pristine and thermally annealed F16CuPc thin films deposited from the vapor phase by organic molecular beam deposition have been studied with use of atomic force microscopy, scanning electron microscopy, and X-ray diffraction. Pristine films show clear morphology transitions with increasing thickness, changing from spherical-like crystals to flexible-fiber-like crystals via standing-up needle-like crystals. Two different roughening processes are identified: kinetic roughening for films <30 nm thick (scaling exponent, β ≈ 0.123) and rapid roughening for films ≥30 nm thick (β ≈ 3.089). Thicker pristine films are composed of two crystal layers: a spherical-crystal layer and a flexible-fiber-crystal layer. Controlled thermal annealing leads to further reorganization of the packing geometry and large crystals dominate the resulting film morphology. Mechanisms are proposed for the structure and morphology transitions.

Electric Field Assisted Effects on Molecular Orientation and Surface Morphology of Thin Titanyl(IV)phthalocyanine Films

Herein we present electric field assisted effects on the molecular orientation, the polymorphism, and the surface morphology of thin titanyl(IV)phthalocyanine (TiOPc) films. The ability of electric fields to affect the thin film structure of polar molecules is demonstrated using titanyl(IV)phthalocyanine as a model compound exhibiting both a permanent and an induced electric dipole moment. Thin films of TiOPc prepared by organic molecular beam deposition (OMBD) in the absence and in the presence of an electric field during the thin film growth are characterized using polarization dependent Raman spectroscopy and atomic force microscopy (AFM). The distinctive pattern of vibrational modes of the phthalocyanine skeleton indicates different molecular orientations in these thin films: Raman spectra of regions where an electric field is present during thin film growth reveal a preferential molecular orientation with an inclination angle of the molecular plane (pseudoplanar macrocycle) with respect to the substrate plane of nearly 90 degrees. Contrary to that, in regions where the electric field was absent, the molecules adopt predominantly a configuration with a smaller tilt angle (approximately 60 degrees). In addition, an electric field assisted change is apparent in AFM images: A large amount of well-formed steplike crystallites lying parallel to the substrate is observed when no electric field was present, whereas in the case when an electric field was applied during thin film growth the crystallites exhibit a tilt with respect to the substrate plane.

Very high temperature chemical vapor deposition of new carbon thin films using organic semiconductor molecular beam sources

We carried out the preparation and characterization of new carbon films deposited using an organic molecular beam deposition apparatus with very high substrate temperature (from room temperature to 2670 K), which we newly developed. When we irradiated molecular beam of organic semiconductor perylene tetracarboxylic acid dianhydride (PTCDA) on Y 0.07Zr 0.93O 2 (111) at 2170 K, a new carbon material was formed via decomposition and fusing of the molecules. The films were characterized with an atomic force microscope (AFM), Raman spectroscopy, X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). Zirconium carbide (ZrC) films were identified beneath the topmost carbon layer by XRD and XPS analyses, which results from chemical reactions of the substrate and the molecules. Partially graphitized aromatic rings of PTCDA were observed from Raman spectroscopy. The present technique – very high temperature chemical vapor deposition using organic semiconductor sources – will be useful to study a vast unexplored field of covalent carbon solids.

Raman polarization studies of highly oriented organic thin films

AbstractIn this work we report on the capability of polarized Raman spectroscopy to investigate the structure of thin organic films. Diindenoperylene (DIP) thin films on (1 × 1)‐rutile(110) were prepared via organic molecular beam deposition (OMBD). Raman spectra of DIP thin films showed several strong Raman modes in the wavenumber region from 1200 to 1650 cm−1. The Ag mode at 1284 cm−1 shows two contributions, thereby indicating the coexistence of at least two DIP film structures. Polarized Raman spectroscopy was applied to characterize the molecular orientation and the dominance of the σ‐configuration (i.e. upright standing DIP molecules) was found. Copyright © 2009 John Wiley &amp; Sons, Ltd.

Electric‐Field‐Induced Gap States in Pentacene

Adv. Mater. 2009, 21, 1–5 2009 WILEY-VCH Verlag Gmb IC A T IO N Electronics based on organic materials have experienced unprecedented progress in recent years. The prospect of flexible, unbreakable, extremely low-weight electronics at relatively low cost has stimulated a lot of research and development on flexible display media, organic memories, and organic radio frequency identification (RFID) tags. Progress in this field has been sustained by the synthesis of new materials, the improvement of electronic devices, and the development of novel and improved processing techniques, such as inkjet printing, and microcontact printing. Even though significant advances have been achieved in recent years, fundamental issues surrounding the origin of gap states in small molecule materials are not resolved, and relatively little is known about the influence of these states on the electronic properties and the electrical stability of organic devices. It is important to address these questions because stable device operation is a major requirement in realizing organic displays and RFID tags. Furthermore, gap states affect the lifetime and the reliability of organic transistors and integrated circuits. Defect states might be caused by environmental conditions such as exposure to oxygen andmoisture. Therefore, it is essential to gain a fundamental understanding of such effects and their underlying microscopic processes. The formation of gap states and the influence of oxygen on the formation process were studied in polycrystalline pentacene (Pn) thin-film transistors (TFTs) using in situ electrical measurements. The experimental results are complemented by pseudopotential density functional calculations based on first principles for different types of oxygen-related defects in Pn. The results of the density functional calculations were used as input parameters in the simulation of the current/voltage characteristics of Pn TFTs. The transistor characteristics weremodeled by a density-of-states transport model. Simulations of the device characteristics will be presented and compared to experimental results. The investigation of gap states or doping effects in organic films is very often hindered by the unintentional incorporation of impurities during the purification, fabrication, and characterization of the materials and devices. Furthermore, the structural properties of the organic films have a distinct influence on the electronic properties of the materials. For example, several studies suggest that oxygen acts as a p-type dopant in a variety of semiconducting polymers and small molecules, such as Pn. Other studies show rather stable device operation under ambient conditions for weeks and months. Therefore, a systematic investigation of doping and trapping effects is essential to clarify the picture. In particular, the threshold voltage of the transistor has to be stable to realize complex organic circuitry, which is required for applications, such as organic RFID tags. The cross-section of a staggered-Pn TFTwith bottom drain and source contacts is shown in Figure 1b. The Pnmolecules (Fig. 1a) were deposited using organic molecular beam deposition (OMBD) onto a silicon oxide gate dielectric. The transfer characteristics of a polycrystalline TFT are shown in Figure 2a. The transistor had a channel length of 20mm and a width-tolength (W/L) ratio of 4000. Measurements were carried out under vacuum at room temperature. The transistor exhibited a mobility of 0.6 cmV 1 s , an on/off ratio larger than seven orders of magnitude, and a threshold voltage of 1V. Furthermore, the transistor exhibited a subthreshold slope of< 100mV/decade. The onset voltage of the drain current, defined as the gate voltage for which the drain current started to increase, was 0 V. In another experiment, the sample was exposed to dry oxygen for more than 30min. The oxygen concentration was controlled by the pressure in the high vacuum chamber (10 1 Pa). During oxygen exposure, no electrical bias was applied to the electrodes of the transistor. After exposing the device to oxygen, the transistor was pumped back to high vacuum (<10 6 Pa) and characterized. In this case, the transfer characteristics were not affected by the oxygen exposure (Fig. 2a). The experiment was repeated several times at different pressures and exposure times (up to 12 h). However, the characteristics did not change. The device behaved

Organic nanofibers from thiophene oligomers

We report on the growth and morphology of α-quaterthiophene and α-sexithiophene clusters, needles (or “fibers”), and flat islands on muscovite mica (001). Samples are prepared by organic molecular beam deposition, and characterized ex situ by polarized fluorescence microscopy and by atomic force microscopy. The growth mechanism of such thiophene fibers is compared to that of para-phenylenes. Distinct needle orientations are present on the surface, their formation being explained by a combination of epitaxy and electric field assisted alignment.

Comparative study on structural and optical properties of ZnO films grown by metalorganic molecular beam deposition and metalorganic chemical vapor deposition

Polycrystalline zinc oxide films were grown on glass substrates by low pressure metal organic chemical vapor deposition (LP-MOCVD) and metal organic molecular beam deposition (MOMBD) using diethlyzinc and water as precursors. With increasing substrate temperature (TS), preferential growth direction of the LP-MOCVD films changed from c-axis direction to a-axis direction, while that of the MOMBD films changed from random direction to c-axis direction. Photoluminescence (PL) spectra of the LP-MOCVD films exhibited an orange band emission related to interstitial oxygen atoms, while those of the MOMBD films exhibited a green band emission related to oxygen vacancies. This PL result suggests that the LP-MOCVD films are in oxygen excess condition, but the MOMBD films are in oxygen deficient condition. Photoacoustic measurements revealed that the energy level of the nonradiative center formed in the LP-MOCVD films is deeper than that in the MOMBD films.

Spectroscopic evidence for aggregate formation in N, N′-bis(phenylethyl)-perylene-3,4,9,10-bis(dicarboximide) (PPEI) films grown at room-temperature

We investigate the photophysical properties of as-deposited films of the perylene derivative N, N′-bis(phenylethyl)-perylene-3,4,9,10-bis(dicarboximide) (PPEI), grown by organic molecular beam deposition. The film morphology, consisting of coexisting phases, was studied using optical and atomic force microscopy and was found to be independent of film thickness. However, a reduction in PPEI film thickness results in an increase in the relative contribution from aggregated species to the photophysical properties of the films, which can be attributed to a room-temperature annealing process.

Electronic structure of epitaxial thin films of bay-substituted perylene bisimide dyes

We have investigated epitaxial thin films of three air-stable organic n-type semiconducting perylene tetracarboxylic acid bisimides (PBIs) with fluoroalkyl groups in the imide position with a combination of X-ray, UV, and inverse photoelectron spectroscopy. We explore the applicability of film preparation by organic molecular beam deposition (OMBD) under ultra-high vacuum conditions to these compounds. By means of fluorine and chlorine substituents in the bay area a systematic torsion of the π-conjugated core of these three PBIs is achieved. Accordingly, these molecules offer a model system to analyze the interplay between the molecular conformation, the film structure, and the electronic structure. Our results show that the PBIs can be deposited intact and contamination free by OMBD and relatively smooth films with layer-to-layer growth can be established. Moreover, the valence spectra reveal the effect of the electro-negative bay substituents as well as of the twisting of the perylene core, which leads to energy shifts of the occupied and unoccupied frontier orbitals.

Key role of molecular kinetic energy in early stages of pentacene island growth

Organic molecular beam deposition is studied systematically at thermal and hyperthermal regimes aiming at investigating the role of molecular kinetic energy on the growth mechanism of pentacene submonolayers on SiOx/Si. We show that the kinetic energy of the impinging molecule (Ek) plays a crucial role in determining island structure and shape, distribution of island sizes, the crystalline quality of the first monolayer, and even the growth mode of subsequent layers. With increasing Ek, the island structure changes from fractal to nonfractal, the shape becomes more anisotropic and the island size more uniform, pointing to correlated island growth. Moreover, while 3D island growth is observed for thermal organic molecular beam deposition, supersonic molecular beam deposition gives rise to layer-by-layer growth, at least for the first two layers. When Ek≥5.0 eV, the first monolayer is composed of large single crystalline domains which can extend over up to 10 μm, inferred from comparing atomic force micrographs of height and net transverse shear force. In these growth conditions both the high surface diffusivity and energy redistribution play a major role. We propose a mechanism where the energy dissipation occurring during the molecule–surface collision leads to the reorientation of whole islands during island coalescence, resulting in the elimination of grain boundaries.

Thermally activated dewetting of organic thin films: the case of pentacene on SiO2 and gold

The morphology of pentacene organic thin films deposited on SiO2 and Au(111) surfaces using organic molecular beam deposition (OMBD) has been characterized by a multi-technique approach. Among the techniques applied were X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), scanning electron microscopy (SEM) and thermal desorption spectroscopy (TDS). Our rather detailed studies reveal that on both substrates the growth is strongly influenced by dewetting and islanding phenomena, yielding very rough surfaces. Surprisingly, substantial changes in the morphology were observed also after deposition on room-temperature samples on a time scale of several hours. The rather extensive set of in situ XPS data was analyzed in the framework of a simple model, which allows us to derive rather detailed information on the roughness parameters.

Photoemission electron microscopy of diindenoperylene thin films

We present the results of photoemission electron microscopy investigations of diindenoperylene thin films deposited by organic molecular-beam deposition on polycrystalline gold at room temperature. Our findings revealed the existence of a different molecular orientation, besides the already known $\ensuremath{\lambda}$ and $\ensuremath{\sigma}$ phases, characterized by an intermediate tilt angle. In thicker film regions, the energy of the films is minimized by a molecular arrangement that has an asymptotic tendency to the upright standing position. We have interpreted our results by using basic concepts in terms of relative strength of the substrate-molecule and molecule-molecule interactions.

Vacuum ultraviolet ellipsometry investigation of ultrathin organic films and their heterostructures

Thin layers and multilayer structures of the DNA bases guanine and cytosine on ZnO substrates were prepared by organic molecular beam deposition under ultra-high vacuum conditions and measured in situ by means of vacuum ultraviolet spectroscopic ellipsometry at the synchrotron source BESSY. Using the dielectric function of the individual layers the optical response of the G/C and C/G heterostructures was modeled. Deviations between simulated and experimental data were mainly attributed to the ordering of cytosine over guanine in the G/C and C/G structures and to the influence of the substrate.

Spectral characterization of thin films of vanadyl hexadecafluorophthalocyanine VOPcF 16

In this work, structural features and optical properties of hexadecafluorosubstituted vanadyl phthalocyanine (VOPcF 16) films deposited by Organic Molecular Beam Deposition (OMBD) technique have been investigated. Thin films of VOPcF 16 were characterised by ellipsometry, optical absorption and vibrational (IR and Raman) spectroscopy. VOPcF 16 films deposited onto silicon and quartz substrates held at room temperature are well organized and characterised by a predominantly co-facial parallel arrangement of molecules vertical to the surface. Thermal treatment of these films at 220 °C leads to the transition from one phase to another and formation of the polycrystalline VOPcF 16 films with randomly distributed crystallites on the substrate surface.