Optical properties of pentacene and perfluoropentacene thin films

The optical properties of pentacene (PEN) and perfluoropentacene (PFP) thin films on various SiO(2) substrates were studied using variable angle spectroscopic ellipsometry. Structural characterization was performed using x-ray reflectivity and atomic force microscopy. A uniaxial model with the optic axis normal to the sample surface was used to analyze the ellipsometry data. A strong optical anisotropy was observed, and enabled the direction of the transition dipole of the absorption bands to be determined. Furthermore, comparison of the optical constants of PEN and PFP thin films with the absorption spectra of the monomers in solution shows significant changes due to the crystalline environment. Relative to the monomer spectrum, the highest occupied molecular orbital to lowest unoccupied molecular orbital transition observed in PEN (PFP) thin film is reduced by 210 meV (280 meV). A second absorption band in the PFP thin film shows a slight blueshift (40 meV) compared to the spectrum of the monomer with its transition dipole perpendicular to that of the first absorption band.     

Epitaxial growth of pentacene films on Cu110

The molecular structure of thin pentacene (C(22)H(14)) films grown on a Cu(110) surface has been studied by means of He atom scattering, low energy electron diffraction, thermal desorption spectroscopy, x-ray photoelectron spectroscopy, and x-ray absorption spectroscopy. Depending on the actual film thickness three different crystalline phases have been identified which reveal a characteristic reorientation of the molecular plane relative to the substrate surface. In the monolayer regime the molecules form a highly ordered commensurate (6.5x2) structure with a planar adsorption geometry. For thin multilayers (thickness <2 nm) a second phase is observed which is characterized by a lateral ((-0.65 5.69) ( 1.90 1.37)) structure and a tilting of the molecular plane of about 28 degrees around their long axis which remains parallel to the surface. Finally, when exceeding a thickness of about 2 nm subsequent growth proceeds with an upright molecular orientation and leads to the formation of crystalline films which are epitaxially oriented with respect to the substrate. The present study thus demonstrates that also on metal substrates highly ordered pentacene films with an upright orientation of the molecular planes can be grown. Photoelectron spectroscopy data indicate further that thick films do not grow in a layer-by-layer mode but reveal a significant degree of roughness.     

Photoluminescence spectroscopy of pure pentacene, perfluoropentacene, and mixed thin films

We report detailed temperature dependent photoluminescence (PL) spectra of pentacene (PEN), perfluoropentacene (PFP), and PEN:PFP mixed thin films grown on SiO(2). PEN and PFP are particularly suitable for this study, since they are structurally compatible for good intermixing and form a model donor/acceptor system. The PL spectra of PEN are discussed in the context of existing literature and compared to the new findings for PFP. We analyze the optical transitions observed in the spectra of PEN and PFP using time-dependent density functional theory calculations. Importantly, for the mixed PEN:PFP film we observe an optical transition in PL at 1.4 eV providing evidence for coupling effects in the blend. We discuss a possible charge-transfer (CT) and provide a tentative scheme of the optical transitions in the blended films.     

Structural order in perfluoropentacene thin films and heterostructures with pentacene

Synchrotron x-ray diffraction reciprocal space mapping was performed on perfluoropentacene (PFP) thin films on SiO2 in order to determine the crystal structure of a novel, substrate-induced thin film phase to be monoclinic with unit cell parameters of a = 15.76 +/- 0.02 A, b = 4.51 +/- 0.02 A, c = 11.48 +/- 0.02 A, and beta = 90.4 +/- 0.1 degrees . Moreover, layered and co-deposited heterostructures of PFP and pentacene (P) were investigated by specular and grazing-incidence x-ray diffraction, atomic force microscopy, and Fourier-transform infrared spectroscopy. For a ca. three-monolayers-thick PFP film grown on a P underlayer, slightly increased lattice spacing was found. In contrast, co-deposited P/PFP films form a new mixed-crystal structure with no detectable degree of phase separation. These results highlight the structural complexity of these technically relevant molecular heterojunctions for use in organic electronics.     

Adsorption-induced intramolecular dipole: correlating molecular conformation and interface electronic structure

The interfaces formed between pentacene (PEN) and perfluoropentacene (PFP) molecules and Cu(111) were studied using photoelectron spectroscopy, X-ray standing wave (XSW), and scanning tunneling microscopy measurements, in conjunction with theoretical modeling. The average carbon bonding distances for PEN and PFP differ strongly, that is, 2.34 A for PEN versus 2.98 A for PFP. An adsorption-induced nonplanar conformation of PFP is suggested by XSW (F atoms 0.1 A above the carbon plane), which causes an intramolecular dipole of approximately 0.5 D. These observations explain why the hole injection barriers at both molecule/metal interfaces are comparable (1.10 eV for PEN and 1.35 eV for PFP) whereas the molecular ionization energies differ significantly (5.00 eV for PEN and 5.85 eV for PFP). Our results show that the hypothesis of charge injection barrier tuning at organic/metal interfaces by adjusting the ionization energy of molecules is not always readily applicable.     

Engineering of TMDC–OSC hybrid interfaces: the thermodynamics of unitary and mixed acene monolayers on MoS2

Hybrid systems of two-dimensional (2D) materials such as transition metal dichalcogenides (TMDCs) and organic semiconductors (OSCs) have become subject of great interest for future device architectures. Although OSC–TMDC hybrid systems have been used in first device demonstrations, the precise preparation of ultra-thin OSC films on TMDCs has not been addressed. Due to the weak van der Waals interaction between TMDCs and OSCs, this requires precise knowledge of the thermodynamics at hand. Here, we use temperature-programmed desorption (TPD) and Monte Carlo (MC) simulations of TPD traces to characterize the desorption kinetics of pentacene (PEN) and perfluoropentacene (PFP) on MoS₂ as a model system for OSCs on TMDCs. We show that the monolayers of PEN and PFP are thermally stabilized compared to their multilayers, which allows preparation of nominal monolayers by selective desorption of multilayers. This stabilization is, however, caused by entropy due to a high molecular mobility rather than an enhanced molecule–substrate bond. Consequently, the nominal monolayers are not densely packed films. Molecular mobility can be suppressed in mixed monolayers of PEN and PFP that, due to intermolecular attraction, form highly ordered films as shown by scanning tunneling microscopy. Although this reduces the entropic stabilization, the intermolecular attraction further stabilizes mixed films.

Entropic stabilization enables the fabrication of (perfluoro-)pentacene monolayers on MoS₂ by selective multilayer desorption but reduces order and packing density. Intermolecular attraction allows to create ordered close-packed molecular monolayers.     

Crystallographic and morphological characterization of thin pentacene films on polycrystalline copper surfaces

The degree of crystallinity, the structure and orientation of crystallites, and the morphology of thin pentacene films grown by vapor deposition in an ultrahigh vacuum environment on polycrystalline copper substrates have been investigated by x-ray diffraction and tapping-mode scanning force microscopy (TM-SFM). Depending on the substrate temperature during deposition, very different results are obtained: While at 77 K a long-range order is missing, the films become crystalline at elevated temperatures. From a high-resolution x-ray-diffraction profile analysis, the volume-weighted size of the crystallites perpendicular to the film surface could be determined. This size of the crystallites increases strongly upon changing temperature between room temperature and 333 K, at which point the size of individual crystallites typically exceeds 100 nm. In this temperature region, three different polymorphs are identified. The vast majority of crystallites have a fiber texture with the (001) net planes parallel to the substrate. In this geometry, the molecules are oriented standing up on the substrate (end-on arrangement). This alignment is remarkably different from that on single-crystalline metal surfaces, indicating that the growth is not epitaxial. Additionally, TM-SFM images show needlelike structures which suggest the presence of at least one additional orientation of crystallites (flat-on or edge-on). These results indicate that properties of thin crystalline pentacene films prepared on technologically relevant polycrystalline metal substrates for fast electronic applications may be compromised by the simultaneous presence of different local molecular aggregation states at all temperatures.     

Chain-length dependent growth dynamics of n-alkanes on silica investigated by energy-dispersive x-ray reflectivity in situ and in real-time

We compare the growth dynamics of the three n-alkanes C(36)H(74), C(40)H(82), and C(44)H(90) on SiO(2) using real-time and in situ energy-dispersive x-ray reflectivity. All molecules investigated align in an upright-standing orientation on the substrate and exhibit a transition from layer-by-layer growth to island growth after about 4 monolayers under the conditions employed. Simultaneous fits of the reflected intensity at five distinct points in reciprocal space show that films formed by longer n-alkanes roughen faster during growth. This behavior can be explained by a chain-length dependent height of the Ehrlich-Schwoebel barrier. Further x-ray diffraction measurements after growth indicate that films consisting of longer n-alkanes also incorporate more lying-down molecules in the top region. While the results reveal behavior typical for chain-like molecules, the findings can also be useful for the optimization of organic field effect transistors where smooth interlayers of n-alkanes without coexistence of two or more molecular orientations are required.     

Origins for epitaxial order of sexiphenyl crystals on muscovite(001)

The epitaxial order of sexiphenyl crystals on muscovite(001) is investigated by x‐ray diffraction, lattice misfit calculations and atomic force microscopy. Depending on the substrate temperature during the thin film growth process, different epitaxial orientations are formed. Sexiphenyl thin films prepared at 370 K preferentially form crystals with the crystallographic (11‐1) planes parallel to the substrate surface while at 434 K a strong fraction of crystals with (11‐2) orientations is grown. The epitaxial orders of sexiphenyl crystals are compared with lattice misfit calculations. The in‐plane order of the {11‐1} crystals can be explained by a point‐on‐line coincidence I, which reveals that the interface is formed by undisturbed crystal surfaces. The epitaxial order of the {11‐2} oriented crystals is characterised by the experimental observation that low indexed crystal directions in the sexiphenyl(11‐2) plane and the muscovite(001) surface coincide with each other, forming a near‐coincidence case. Corrugations of the substrate surface are responsible for this second type of epitaxial order. Characteristic features in the thin film morphology could be correlated to the two observed epitaxial orientations of the sexiphenyl crystals. Copyright © 2009 John Wiley & Sons, Ltd.     

A high-resolution near-edge x-ray absorption fine structure investigation of the molecular orientation in the pentacene/poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) pentacene/system

We present x-ray photoemission spectroscopy and highly resolved near-edge x-ray absorption fine structure spectroscopy measurements taken on pentacene thin films of different thicknesses deposited on a spin coated poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) substrate. Thin films of pentacene were prepared by using organic molecular beam deposition in situ using strictly controlled evaporation conditions. Our investigations show that pentacene thin films on PEDOT:PSS are characterized by upright standing molecules. Due to the strong dichroic behavior, the calculated values of the molecular orientation give a clear indication not only of the real molecular arrangement in the films but also of a high orientational order. This high degree of molecular orientation order is a characteristic already of the first layer. The films show the tendency to grow on the PEDOT:PSS substrate following an island-fashion mode, with a relatively narrow intermixing zone at the interface between the pentacene and the polymer blend. The peculiarity of the growth of pentacene on PEDOT:PSS is due to the fact that the substrate does not offer any template for the nucleated films and thus exerts a lateral order toward the crystal structure arrangement. Under these conditions, the upright orientation of the molecules in the films minimizes the energy required for the system stability.     

In-plane orientation of polyimide

As a nondestructive method of evaluation, we have used integrated optics to investigate thin films (<8 μm) of polyamic acid and polyimide as a function of molecular weight, initial imidization temperature, method of imidization, and annealing treatment. Polyamic acid films were found to exhibit a large optical anisotropy, indicating preferential alignment of the long axis of the molecule in the plane of the film. Imidization increased the birefringence of the film by a factor of 2.5 and reduced the film thickness. The only parameter that was found to effect the anisotropy of the films was the method of imidization. Chemical imidization was found to increase the birefringence by a factor of 3, indicative of a higher degree of molecular orientation parallel to the film surface. This effect was not observed in thicker (>25-μm) films using x-ray diffraction where the orientation function was found to be independent of the method of imidization.     

Crystal growth of para-sexiphenyl on clean and oxygen reconstructed Cu(110) surfaces

The formation of crystalline para-sexiphenyl (6P) films on Cu(110) and Cu(110)-(2 × 1)O (Cu-O) has been studied by low energy electron diffraction, X-ray absorption spectroscopy and both in situ and ex situ X-ray diffraction methods to elucidate the transition from the initial monolayers to crystalline thin films. It is found that, for Cu-O, a single and, for Cu(110), a double wetting layer is formed which then acts as a template for the subsequent 3D crystal growth. For both substrates the orientation of the long molecular axes of the 6P molecules in the first layers is conserved for the molecules in the bulk crystals growing on them. The main difference between both systems is that on Cu-O the first monolayer assembles in a form close to that of a 6P bulk plane which can be easily continued by crystallites grown upon them, while on the Cu(110) surface the 6P mono- and bi-layers differ substantially from the bulk structure. The bi-layer forms a complex periodically striped phase. Thin 6P films grow with the 6P(203) crystal plane parallel to the Cu-O substrate surface. For this orientation, the 6P molecules are stacked in layers and the molecules demonstrate only one tilt of the mean molecular plane with respect to the sample surface. On clean Cu(110), a more complex 6P(629) plane is parallel to the substrate surface and this orientation is likely a consequence of the super-molecular long-range periodicity of the second molecular layer striped phase.     

Induced alignment of a solution-cast discotic hexabenzocoronene derivative for electronic devices investigated by surface X-ray diffraction

A surface X-ray diffraction study is presented showing that highly ordered and uniaxially aligned hexa(3,7-dimethyl-octanyl)hexa-peri-hexabenzocoronene (HBC-C8,2) films can be fabricated by crystallization from solution onto friction-transferred poly(tetrafluoroethylene) (PTFE) layers. Three crystalline HBC-C8,2 majority phases result. In all three phases, the HBC-C8,2 molecules self-organize into columns which are uniaxially aligned along the direction defined by the PTFE macromolecules of the substrate. The three phases are quite similar, the major difference being their orientation with respect to the substrate. A quasi-2D epitaxial growth mechanism with a grapho-epitaxial component for one of the three phases explains the formation of the three rotational HBC-C8,2 variants. A method to obtain a thin film with only one phase is proposed. The results show that standard THETAV;-2THETAV; X-ray diffraction and transmission electron diffraction can be very misleading tools to estimate the crystalline quality in a thin film of complex structure.     

Buried interfacial layer of highly oriented molecules in copper phthalocyanine thin films on polycrystalline gold

The growth of copper phthalocyanine thin films evaporated on polycrystalline gold is examined in detail using near edge x-ray absorption fine structure spectroscopy and surface sensitive x-ray photoemission spectroscopy. The combination of both methods allows distinguishing between the uppermost layers and buried interface layers in films up to approximately 3 nm thickness. An interfacial layer of approximately 3 ML of molecules with an orientation parallel to the substrate surface was found, whereas the subsequent molecules are perpendicular to the metal surface. It was shown that even if the preferred molecular orientation in thin films is perpendicular, the buried interfacial layer can be oriented differently.     

衬底温度对Fe3N薄膜生长和磁性的影响

采用直流磁控溅射方法, 以Si(100) 单晶片为衬底, 在衬底温度为150～450 ℃的范围内得到了ε-Fe3N薄膜样品. 利用 XRD, SEM和VSM等表征手段, 研究了衬底温度对ε-Fe3N薄膜的生长和磁性的影响. 实验结果表明, 随着衬底温度的升高, 薄膜的生长速率、晶粒尺寸和单位质量磁化强度均增大, 而矫顽力呈现先增加后减小的变化趋势, 当衬底温度为350 ℃时, 矫顽力达到最大值18.72 kA/m, 可以认为此时薄膜样品的晶粒尺寸接近于交换作用长度.     

Highly ordered thin films of 5,5'-bis(3'-fluoro-biphenyl-4-yl)-2,2' : 5',2'-terthiophene with two meso-phases

The growth process and phase state of 5,5'-bis(3'-fluoro-biphenyl-4-yl)-2,2'?:?5',2'- terthiophene (m-F2BP3T) thin films were investigated by atomic force microscopy (AFM), in-plane and out-of-plane X-ray diffraction (XRD), and selected area electron diffraction (SAED). Two meso-phases (thin film phases) of m-F2BP3T films on SiO(2) surface were obtained in the early stages. The m-F2BP3T films initially exhibited two-dimensional (2D) layers (≤4 ML) followed by three-dimensional (3D) island growth. The film structure evolved two thin film phases in the first four layers and the bulk phase was formed from the fifth layer, which occurred concomitantly with the change of the growth mode. Moreover, the variation of weak epitaxy growth behavior of ZnPc from 2D to 3D growth further reflects that the phase state of the first three layers is different from that of the fourth layer, in spite of ZnPc crystals showing just one orientation corresponding to commensurate epitaxy. The novel phase behavior is closely related to the synergistic effects of the outstanding soft matter properties, limited elasticity of organic molecules, and strain originating from the SiO(2) substrate. This study investigates novel phase behavior in organic thin films and provides significant insight into the mechanism of the phase transition.     

Substrate-induced anisotropy of c-axis textured NaxCoO2 thin films

Na_xCoO₂ [x = 0.51, 0.54, and 0.59] thin films have been grown on SrTiO₃ (100)-oriented single crystals with a 5° vicinal cut towards [010] by pulsed laser deposition. We analysed the films by X-ray diffractometry, atomic force microscopy (AFM), and dc-transport measurements. X-ray diffraction patterns of the films show single phase and c-axis textured growth with the film plane closely aligned to the [001]-direction of 5° miscut SrTiO₃ (001) substrates. In addition to the structural analysis of these films we performed transport measurements along and perpendicular to the substrate tilt direction and determined the resistivity anisotropy as a function of temperature. The results enable the development of a strategy for the fabrication of Na_xCoO₂ based thermoelectric thin film devices.     

The effect of different substrate temperatures on the structure and luminescence properties of Y2O3:Bi3+ thin films

Y₂O₃:Bi³⁺ phosphor thin films were prepared by pulsed laser deposition in the presence of oxygen (O₂) gas. The microstructure and photoluminescence (PL) of these films were found to be highly dependent on the substrate temperature. X-ray diffraction analysis showed that the Y₂O₃:Bi³⁺ films transformed from amorphous to cubic and monoclinic phases when the substrate temperature was increased up to 600 °C. At the higher substrate temperature of 600 °C, the cubic phase became dominant. The crystallinity of the thin films, therefore, increased with increasing substrate temperatures. Surface morphology results obtained by atomic force microscopy showed a decrease in the surface roughness with an increase in substrate temperature. The increase in the PL intensities was attributed to the crystallinity improvement and surface roughness decrease. The main PL emission peak position of the thin films prepared at substrate temperatures of 450 °C and 600 °C showed a shift to shorter wavelengths of 460 and 480 nm respectively, if compared to the main PL peak position of the powder at 495 nm. The shift was attributed to a different Bi³⁺ ion environment in the monoclinic and cubic phases.     

Substrate selected polymorphism of epitaxially aligned tetraphenyl-porphyrin thin films

Porphyrin molecules, of interest as versatile materials for organic electronics, are highly prone to formation of significantly different polymorphic phases. To elucidate the determinants for the specific polymorphic phase formed in thin films as well as for the arrangement of the molecules on a given substrate two different anisotropic substrate surfaces have been selected: KCl(100) and the oxygen reconstructed Cu(110) surface. We observe that the crystal structure of the thin films depends on the substrate, whereas the relative molecular orientations in both cases are similar. X-Ray and transmission electron diffraction of 30 nm thick tetraphenyl-porphyrin (H(2)TPP) and platinum tetraphenyl-porphyrin (PtTPP) thin films deposited on KCl(100) surfaces reveals that both kinds of molecules crystallize in a tetragonal polymorph with the (001) lattice planes, i.e. with their macrocycles, parallel to the substrate. Films deposited on the oxygen reconstructed Cu(110)-(2 × 1)O surface exhibit in contrast the triclinic polymorph even though molecules again align nearly parallel to the substrate surface as observed by LEED and X-ray diffraction. On both substrates we identify two driving forces for the epitaxial alignment of porphyrins: (i) molecules aligning with their macrocycles (nearly) parallel to the substrate surface and (ii) the porphyrin molecules forming a commensurate unit cell with the respective substrate. The polymorphic phase meeting both requirements is the most favorable to be formed on a given substrate and due to this structural flexibility in both cases well-ordered, epitaxially aligned porphyrin thin films are achieved.     

电荷转移配合物薄膜制备方法和结构表征的研究进展

回顾了与Langmuir-Blodgett(LB)技术有关的电荷转移配合物薄膜的各类制备方法、结构表征结果,并比较了制备方法对薄膜结构的影响.例如,将LB膜C18H37TCNQ(电子受体)插入到电子给体3,3’,5,5’-tetramethylbenzidine(四甲基联苯胺, TMB)的石油醚溶液中进行掺杂,制备了TMB•C18H37TCNQ电荷转移配合物薄膜.在这种薄膜中,给体和受体以面对面的方式堆积,两者的环平面与基片平面接近垂直.而采用硬脂酸和C18H37TCNQ的混合LB膜通过类似的掺杂路线制备的TMB•C18H37TCNQ薄膜的结构发生了一些变化,例如其长的烃链C18H37更加垂直于基片平面.通过比较以前的各种实验结果可以得出以下结论:电荷转移配合物的结构可以通过制备方法得到控制.