Organic Molecular Beam Deposition Research Articles

The dibenzoterrylene-C60 composite structure as a possible new single-photon source - growth and optical analysis

Abstract A sample fabrication approach based on organic molecular beam deposition creating a new molecule-based emitter structure and its optical analysis is presented. The polycyclic aromatic hydrocarbon dibenzoterrylene (DBT, C38H20) was deposited between layers of C60-fullerenes, which serve as a protective matrix. It was found that this structure maintains the DBT’s optical properties [1, 2], but a potentially quenching impact cannot be excluded. Raman spectra of the composite were collected proving the molecules’ preserved integrity after being evaporated at high temperatures. Finally, the results of confocal laser scanning microscopy show fluorescent molecular clusters.

Dual functions of light-emission and light-harvesting using organic donor and acceptor co-deposition

The device working principles of light emission and light harvesting using p-n (or donor-acceptor) heterostructures are reciprocal relations in terms of thermodynamics and charge transfer mechanisms. The dual-functional optoelectronic systems of organic light-emitting diodes (OLEDs) and organic photovoltaic cells (OPVCs) including organic photodiodes (OPDs) were obtained by fabricating the devices with bi-layer (BL) and triple-layer (TL) structures through the co-deposition of p-type rubrene and n-type NDI-C6 molecules using organic molecular beam deposition. The concentration ratio of rubrene:NDI-C6 for the co-deposition layers (CDLs) was controlled by deposition rates of 3:7, 5:5, and 7:3, resulting in the distinctive performance of the devices. The OLEDs with rubrene/CDL 3:7/NDI-C6 TL structures show considerably broad and enhanced electroluminescence emission by approximately 40 times including a drastic increase in the exciplex EL peak at 695 nm, compared to those of the OLEDs with rubrene/NDI-C6 BLs or other TL structures. As the concentration of n-type NDI-C6 in the CDL increased, the driving voltage decreased, and the current increased for the OLED with the TL structure. The fill factor and power conversion efficiency of the OPVCs and photodetectivity using the same devices increased with increasing concentration of n-type NDI-C6. The dual performance of the OLED and OPVC, including the OPD of the elaborated co-deposition of organic molecules, was clearly observed, even though the efficiencies of the devices were relatively low. In our optoelectronic devices, the optimal concentration ratio of p-type rubrene and n-type NDI-C6 of the CDL in the TL devices was 3:7 for the dual function of light emission and light harvesting.

Octafluoro-Substituted Phthalocyanines of Zinc, Cobalt, and Vanadyl: Single Crystal Structure, Spectral Study and Oriented Thin Films.

In this work, octafluoro-substituted phthalocyanines of zinc, vanadyl, and cobalt (MPcF8, M = Zn(II), Co(II), VO) were synthesized and studied. The structures of single crystals of the obtained phthalocyanines were determined. To visualize and compare intermolecular contacts in MPcF8, an analysis of Hirshfeld surfaces (HS) was performed. MPcF8 nanoscale thickness films were deposited by organic molecular beam deposition technique and their structure and orientation were studied using X-ray diffraction. Comparison of X-ray diffraction patterns of thin films with the calculated diffractograms showed that all three films consisted of a single crystal phase, which corresponded to a phase of single crystals. Only one strong diffraction peak corresponding to the plane (001) was observed on the diffraction pattern of each film, which indicated a strong preferred orientation with the vast majority of crystallites oriented with a (001) crystallographic plane parallel to the substrate surface. The effect of the central metals on the electronic absorption and vibrational spectra of the studied phthalocyanines as well as on the electrical conductivity of their films is also discussed.

Effect of the deposition method and ageing in atmosphere on the optical properties of tetraphenylporphyrins (TPPs) films

In this work thin films of 5,10,15,20-tetraphenylporphyrin (H2TPP) and 5,10,15,20-tetrakis(4-methoxyphenyl)porpyhrin (H2TMPP)) were prepared by organic molecular beam deposition (OMBD). In addition, spin coating was applied as an alternative preparation technique for thin films of H2TPP to investigate the influence of the deposition method on the optical properties.The preservation of the molecular structure was proven by comparative Raman spectroscopy studies between films and powder. Modelling of the spectroscopic ellipsometry data yields a uniaxial anisotropy of the optical constants for the films of both molecules deposited by OMBD, which was used to determine the relative orientation of the molecules with respect to the substrate. The effect of the storage in air on the optical properties of the OMBD films was investigated using the example of H2TMPP. The molecular order in the films was found to decrease while the film roughness increased compared to samples stored in nitrogen.

Index matching in multilayered organic waveguides

Index matching of guided modes in birefringent multilayered organic waveguides opens new prospects for the design of mode coupling and mode switching devices. We demonstrate index matching of guided modes in two multilayered structures, in (a) a PTCDA-Alq3-PTCDA three-layer and (b) a PTCDA-Alq3 effective medium multilayer waveguide. The optical waveguides were grown on a Pyrex substrate by organic molecular beam deposition. The occurrence of index matching was investigated both experimentally by measuring the effective refractive index dispersion of transverse electric and magnetic modes using the m-line technique and theoretically by modelling the index dispersion with a transfer matrix algorithm.

Properties and degradation of manganese(III) porphyrin thin films formed by high vacuum sublimation

Manganese porphyrins are of interest due to the optical, electronic and magnetic properties of the central metal ion, coupled to the low bandgap of the polyaromatic ring. These attractive characteristics are harnessed in solutions or in ultra-thin films, such as, for example, self-assembled monolayers. However, for devices, thicker films deposited using a controlled and reproducible method are required. Here we present the morphological, structural, chemical and optical properties of manganese(III) tetraphenylporphyrin chloride (MnTPPCl) thin films deposited using organic molecular beam deposition, typically employed to process analogue molecules for applications such as organic photovoltaics. We find, using a combination of UV-vis and X-ray photoelectron spectroscopies, that the sublimation process leads to the scission of the Mn–Cl bond. The resultant film is a Mn(II)TPP:Mn(III)TPPCl blend where approximately half the molecules have been reduced. Following growth, exposure to air oxidizes the Mn(II)TPP molecule. Through quantitative analysis of the time-dependent optical properties, the oxygen diffusion coefficient (D) [Formula: see text] is obtained, corresponding to a slow bulk oxidation following fast oxidation of a 8-nm-thick surface layer. The bulk diffusion D is lower than for analogous polycrystalline films, suggestion that grain boundaries, rather than molecular packing, are the rate-limiting steps in oxidation of molecular films. Our results highlight that the stability of the axial ligands should be considered when depositing metal porphyrins from the vapor phase, and offer a solvent-free route to obtain reproducible and smooth thin films of complex materials for engineering film functionalities.

Self-organization of complete organic monolayers via sequential post-deposition annealing

Organic molecular beam deposition has been used to surpass the chemical approach commonly adopted for coating SiOx surfaces, obtaining a smooth and uniform monomolecular layer of sexithiophene that fully covers the SiOx surface on the centimetre length scale. This result has been achieved by submitting sexithiophene sub-monolayer films grown at different substrate temperatures to a post-deposition annealing process. Through Scanning Probe Microscopy techniques, morphological, tribological and mechanical measurements have highlighted the existence of face-on molecular aggregates on the SiOx surface and their re-organization by means of a post-annealing process. Atomistic molecular dynamics simulations complement experimental observations, shedding light on the microscopic aspects of molecular diffusion and aggregates reorganization. Exploiting the molecular reorganization upon post-annealing, almost perfect 6 T monolayers were grown through a sequence of deposition and annealing steps. This preparation technique represents a new route for changing surface properties by using high controlled monomolecular layers.

Fluorosubstituted lead phthalocyanines: Crystal structure, spectral and sensing properties

In this work, we synthesized and investigated tetra- (PbPcF4) and hexadecafluorinated (PbPcF16) lead phthalocyanines. Their single crystal structures were refined for the first time. PbPcF4 crystallizes in stacks with all molecules facing in one direction identically to the monoclinic PbPc polymorph in the tetragonal space group I4, while PbPcF16 crystallizes in the triclinic P-1 space group. Moreover, the effect of fluorosubstitution on the structural features of thin films grown by organic molecular beam deposition (OMBD), vibrational and UV–visible optical absorption spectra is discussed. Apart from this, the chemiresistive sensor response of PbPcFx (x = 0, 4, 16) films to ammonia (1–5 ppm) in dependence on fluorination degree is also studied. It was shown that the sensor response to ammonia decreases in the order of PbPcF4 > PbPcF16 > PbPc. The nature of interaction between PbPcFx and ammonia molecules was studied using DFT calculations.

Thin-film growth behavior of non-planar vanadium oxide phthalocyanine**Project supported by the National Natural Science Foundation of China (Grant No. 51673214) and the National Key Research and Development Program of China (Grant No. 2017YFA0206600).

The thin film properties of organic semiconductors are very important to the device performance. Herein, non-planar vanadyl phthalocyanine (VOPc) thin films grown on rigid substrates of indium tin oxide, silicon dioxide, and flexible substrate of kapton by organic molecular beam deposition under vacuum conditions are systematically studied via atomic force microscopy and x-ray diffraction. The results clearly reveal that the morphology and grain size are strongly dependent on the substrate temperature during the process of film deposition. Meanwhile, the VOPc films with the structure of phase I or phase II can be modulated via in situ annealing and post-annealing temperature. Furthermore, the crystalline structure and molecular orientation of vapor-deposited VOPc can be controlled using molecular template layer 3, 4, 9, 10-perylene-tetracarboxylic dianhydride (PTCDA), the VOPc film of which exhibits the phase I structure. The deep understanding of growth mechanism of non-planar VOPc film provides valuable information for controlling structure-property relationship and accelerates the application in electronic and optoelectronic devices.

Template-Free Orientation Selection of Rod-Like Molecular Semiconductors in Polycrystalline Films.

Many organic semiconductors (OSCs) feature strong optical anisotropy that can be exploited to increase the efficiency of optoelectronic devices. We demonstrate that for the technologically relevant, rod-like OSCs diindenoperylene (DIP), pentacene (PEN), and α-sexithiophene (6T) deposited on silicon oxide surfaces it is possible to prepare polycrystalline thin films in which the long molecular axis is oriented parallel to the substrate plane in a template-free fashion. In films grown by organic molecular beam deposition at room temperature or higher, the molecules are oriented upright standing (σ-orientation). Instead, the here-presented growth at low temperatures followed by slow annealing up to a temperature near molecular desorption has the effect of "freezing" the molecules with their long axis oriented parallel to the substrate plane (λ-orientation) while conferring them crystalline long-range order. We discuss the huge impact on the optical anisotropy of the films observed as a consequence of the orientation transition. Finally, we propose a mechanism for explaining the achieved λ-orientation, which is stable under environmental conditions.

Ultrahigh-vacuum organic molecular-beam deposition system for in situ growth and characterization.

A compact ultrahigh-vacuum molecular-beam deposition system has been developed for the in situ synthesis of organic thin films and multilayers. The system incorporates all the features (heater, thickness monitor, evaporators) necessary for controlled organic thin-film growth. It can be used independently, or it can be docked to the in situ growth system and transferred to other instruments of the PGM beamline, thus allowing extensive film preparation and characterization. A manipulator dedicated to specimen preparation and organic-film deposition with temperature control between 200 K and ∼800 K has been developed. The design and performance of the system are reported with emphasis on a novel solution of masks developed to achieve position-dependent film deposition. To demonstrate the enhanced capabilities of the PGM beamline in the growth and in the characterization of electronic-structure studies of organic molecular films and their heterostructures through synchrotron-based spectroscopies, this paper presents some preliminary results of a study of Fe-phthalocyanine growth on Si substrates and on in situ prepared La0.67Sr0.33MnO3 buffer layers on SrTiO3 single crystal.

Structural transition of pentacene monolayer on Cd(0001)

Pentacene is a typical organic semiconductor molecule, which has important application value in organic electronics. In this paper, the two-dimensional self-assembled structures of pentacene on the Cd(0001) surface were investigated by means of organic molecular beam deposition (OMBD) and low-temperature scanning tunneling microscopy (LT-STM). Three structural transitions induced by temperature were found. (1) Pentacene molecules form a disordered molecular monolayer under the room-temperature deposition conditions. (2) For the disorder monolayer, annealing the sample at 320 K leads to the transition from disordered phase to a well ordered monolayer, in which two kinds of molecular chains with different molecular orientations are alternately arranged, constituting the herringbone structure. (3) When the annealing temperature is increased to 350 K, the herringbone packing transforms into a unique chiral clover structure that has never been reported: three same pentacene dimers arrange into a chiral pinwheel structure. (4) Further increasing the annealing temperature to 400 K, the chiral clover structure transforms into a common and ordered brick-wall structure. With the combination of density functional theory (DFT) calculation, it is found that the change of charge density at both ends of individual pentacene molecule is opposite, which indicates that the each pentacene molecule has the net dipole moment. It is particularly interesting that the organizational chirality of chiral clover structure can be switched from one enantiomer to another driven by the electric filed from a STM tip.

Fluorinated Metal Phthalocyanines: Interplay between Fluorination Degree, Films Orientation, and Ammonia Sensing Properties.

In this work, the sensor response of MPcFx (M = Cu, Co, Zn; x = 0, 4, 16) films toward gaseous NH3 (10–50 ppm) was studied by a chemiresistive method and compared to that of unsubstituted MPc films to reveal the effects of central metals and F-substituents on the sensing properties. A combination of atomic force microscopy and X-ray diffraction techniques have been used to elucidate the structural features of thin MPcFx films deposited by organic molecular beam deposition. It has been shown that the sensor response of MPcF4 films to ammonia is noticeably higher than that of MPc films, which is in good correlation with the values of binding energy between the metal phthalocyanine and NH3 molecules, as calculated by the density functional theory (DFT) method. At the same time, in contrast to the DFT calculations, MPcF16 demonstrated the lesser sensor response compared with MPcF4, which appeared to be connected with the different structure and morphology of their films. The ZnPcF4 films were shown to exhibit a sensitivity to ammonia up to concentrations as low as 0.1 ppm, and can be used for the selective detection of ammonia in the presence of some reducing gases and volatile organic compounds. Moreover, the ZnPcF4 films can be used for the detection of NH3 in the gas mixture simulating exhaled air (N2 76%, O2 16%, H2O 5%, and CO2 3%).

Polydimethylsiloxane films engineered for smart nanostructures

Herein, we present organic molecular beam deposition to fabricate (sub-)micrometer-thin polydimethylsiloxane (PDMS) substrate-fixed films with a thickness homogeneity better than 2% on square centimeters. Their surface roughness and wrinkle morphology is controlled by the evaporation temperature, the growth rate, and the ultraviolet-light irradiation. The tailoring of the elastic modulus for the selected vinyl-functionalized PDMS nano-membranes is demonstrated. Both the surface morphology and mechanics are key parameters, which crucially determine the tissue-to-implant interactions for applications in bioelectronics. The cross-linked PDMS nano-membranes with elastic moduli of only a few hundred kPa are realized – a compliance similar to human soft tissues. The in situ characterization of their mechanical properties is presented based on their temperature sensitivity by spectroscopic ellipsometry and correlated to subsequently performed nano-indentation using a sophisticated atomic force microscopy instrument. Such soft sub-micrometer-thin elastomer membranes will become an essential component of dielectric elastomer transducers with strains comparable to human muscles, operated at the conventional battery voltages for future artificial muscles or skin implants.

Emission dynamics of hybrid plasmonic gold/organic GaN nanorods

We studied the emission of bare and aluminum quinoline (Alq3)/gold coated wurtzite GaN nanorods by temperature- and intensity-dependent time-integrated and time-resolved photoluminescence (PL). The GaN nanorods of ∼1.5 μm length and ∼250 nm diameter were grown by plasma-assisted molecular beam epitaxy. Gold/Alq3 coated GaN nanorods were synthesized by organic molecular beam deposition. The near band-edge and donor-acceptor pair luminescence was investigated in bare GaN nanorods and compared with multilevel model calculations providing the dynamical parameters for electron–hole pairs, excitons, impurity bound excitons, donors and acceptors. Subsequently, the influence of a 10 nm gold coating without and with an Alq3 spacer layer was studied and the experimental results were analyzed with the multilevel model. Without a spacer layer, a significant PL quenching and lifetime reduction of the near band-edge emission is found. The behavior is attributed to surface band-bending and Förster energy transfer from excitons to surface plasmons in the gold layer. Inserting a 5 nm Alq3 spacer layer reduces the PL quenching and lifetime reduction which is consistent with a reduced band-bending and Förster energy transfer. Increasing the spacer layer to 30 nm results in lifetimes which are similar to uncoated structures, showing a significantly decreased influence of the gold coating on the excitonic dynamics.

Effect of fluorosubstitution on the structure of single crystals, thin films and spectral properties of palladium phthalocyanines

In this work, the crystalline structure of single crystals grown by vacuum sublimation of unsubstituted palladium phthalocyanines (PdPc), its tetrafluorinated (PdPcF4) and hexadecafluorinated (PdPcF16) derivatives have been investigated using X-ray diffraction measurements. Two crystalline phases have been identified for PdPc; the molecules in both phases crystallize in stacks with herringbone arrangement in the monoclinic space groups (C2/c for α-PdPc; P21/n for γ-PdPc). Both PdPcF4 and PdPcF16 crystallize in the triclinic P-1 space group, forming stacks of molecules in columnar arrangement with molecules in adjacent columns aligned parallel to one another. X-ray diffraction measurements have also been used to elucidate the structural features and molecular orientation of thin films of PdPc, PdPcF4 and PdPcF16, grown by organic molecular beam deposition at different substrate temperatures. The effect of fluorosubstitution on UV-visible optical absorption and vibrational spectra of palladium phthalocyanine derivatives is also discussed.

High-Vacuum Deposition of Biferrocene Thin Films on Room-Temperature Substrates

Metallocenes are a promising candidate for future spintronic devices due to their versatile and tunable magnetic properties. However, single metallocenes, e.g., ferrocene, sublimate below room temperature, and therefore the implementation for future applications is challenging. Here, a method to prepare biferrocene thin films using organic molecular beam deposition (OMBD) is presented, and the effect of substrate and deposition rate on the film structure and morphology as well as its chemical and magnetic properties is investigated. On Kapton and Si substrates, biferrocene interacts only weakly with the substrate, and distinct grains scattered over the surface are observed. By incorporating a 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA) seeding layer and depositing biferrocene at high deposition rates of 1.0 Å s–1, it is possible to achieve a well-ordered densely packed film. With spintronic applications in mind, the magnetic properties of the thin films are characterized using superconducting quantum interference device (SQUID) magnetometry. Whereas initial SQUID measurements show weak ferromagnetic behavior up to room temperature due to oxidized molecule fragments, measurements of biferrocene on PTCDA capped with LiF show the diamagnetic behavior expected of biferrocene. Through the successful deposition of biferrocene thin films and the ability to control the spin state, these results demonstrate a first step toward metallocene-based spintronics.

Growth, Structure, and Anisotropic Optical Properties of Difluoro-anthradithiophene Thin Films

Anthradithiophene (ADT) and its functionalized derivatives have proven to be attractive for high-performance electronic devices based on small-molecule organic semiconductors. In this manuscript we investigate the structural and optical properties of thin films of difluoro-anthradithiophene (diF-ADT), an ADT derivative, grown by organic molecular beam deposition (OMBD). By grazing incidence X-ray diffraction and reciprocal space maps, we show that diF-ADT crystallizes in a thin film structure similar to the single crystal unit cell. In addition, we investigate the growth characteristics with atomic force microscopy (AFM) and show an increase of surface mound sizes with elevated substrate temperature. Optical absorption measurements reveal a clear vibronic progression in both solution and thin film spectra along with a distinct optical anisotropy related to the molecular orientation in thin films.

Formation of ferromagnetic molecular thin films from blends by annealing.

We report on a new approach for the fabrication of ferromagnetic molecular thin films. Co-evaporated films of manganese phthalocyanine (MnPc) and tetracyanoquinodimethane (TCNQ) have been produced by organic molecular beam deposition (OMBD) on rigid (glass, silicon) and flexible (Kapton) substrates kept at room temperature. The MnPc:TCNQ films are found to be entirely amorphous due to the size mismatch of the molecules. However, by annealing while covering the samples highly crystalline MnPc films in the β-polymorph can be obtained at 60 °C lower than when starting with pure MnPc films. The resulting films exhibit substantial coercivity (13 mT) at 2 K and a Curie temperature of 11.5 K.

Synthesis, spectroscopic characterization and thermogravimetric analysis of two series of substituted (metallo)tetraphenylporphyrins.

Subsequent treatment of H2TPP(CO2H)4 (tetra(p-carboxylic acid phenyl)porphyrin, 1) with an excess of oxalyl chloride and HNR2 afforded H2TPP(C(O)NR2)4 (R = Me, 2; iPr, 3) with yields exceeding 80%. The porphyrins 2 and 3 could be converted to the corresponding metalloporphyrins MTPP(C(O)NR2)4 (R = Me/iPr for M = Zn (2a, 3a); Cu (2b, 3b); Ni (2c, 3c); Co (2d, 3d)) by the addition of 3 equiv of anhydrous MCl2 (M = Zn, Cu, Ni, Co) to dimethylformamide solutions of 2 and 3 at elevated temperatures. Metalloporphyrins 2a–d and 3a–d were obtained in yields exceeding 60% and have been, as well as 2 and 3, characterized by elemental analysis, electrospray ionization mass spectrometry (ESIMS) and IR and UV–vis spectroscopy. Porphyrins 2, 2a–d and 3, 3a–d are not suitable for organic molecular beam deposition (OMBD), which is attributed to their comparatively low thermal stability as determined by thermogravimetric analysis (TG) of selected representatives.