Morphology and electronic properties of the pentacene on cobalt interface

Abstract

In this paper, we report the structural and electronic properties of pentacene thin films grown on a polycrystalline Co film using atomic force microscopy and ultraviolet photoemission spectroscopy ͑UPS͒, respectively. Investigation of this type of interface is of importance for the engineering of hybrid organometallic spintronic devices for which the use of spin polarized electrodes is a prerequisite. Uniform single crystalline areas of pentacene as large as several micrometers, with molecules arranging almost perpendicular to the substrate, were obtained. For the electronic properties at this interface, we have found an energy barrier for the hole injection of about 1 eV, in spite of the fact that the ionization potential of pentacene reported previously equals the work function of Co. A shift of the vacuum level of the same magnitude has also been observed. A comparison of the UPS spectra of the pentacene films with the gas phase spectrum directly indicates that hybridization effects are present at this interface. © 2006 American Institute of Physics. ͓DOI: 10.1063/1.2363707͔ INTRODUCTIONMolecular organic semiconductors have become a very active area of research during the past decade. The structural flexibility due to weak intermolecular bonds and the endless possibilities for chemical synthesis of new molecules give molecular systems a unique advantage over inorganic semiconductors for relatively simple fabrication of ͑opto-͒ electronic devices such as light emitting diodes for flexible or even transparent displays, transistors, solar cells, sensors, and solid state lasers. Applications into electronic devices in which the spin degree of freedom of the electron is exploited ͑spintronics͒ have recently started to be considered, motivated by the notion that organic semiconductors are materials with large spin relaxation times. [1][2][3][4][5] Spin valves with a significant magnetoresistance have been produced with Alq 3 thin films sandwiched between two ferromagnetic electrodes. 4,5 In response to the technological success of the organic semiconductors, fundamental research, with the goal of understanding the surface and interface properties of these systems, has followed. Such studies have proven to be valuable for device engineering, as well as from the fundamental viewpoint, electronic properties of interfaces between organic semiconductors and metals being still puzzling for scientists. While the electronic and structural properties of large organic molecules on noble metals and semiconducting or semimetallic substrates have been extensively studied, only a limited number of studies on more reactive substrates, such as the ferromagnetic transition metals, have been reported. 6 Such investigations are of crucial relevance to understand and control efficient spin injection from ferromagnetic electrodes into organic thin films.In the ferromagnetic transition metals, both the magnetic behavior and the reactivity originate from the high density of the d states near the Fermi level. For the structural propertie...

Pentacene thin films on ferromagnetic oxide: Growth mechanism and spintronic devices

“…40 C, in the latter because desorption started to play a significant role. From the Arrhenius plot in Figure 4(a), we extracted E N ¼ 0.65 6 0.05 eV for 40 T 60 C. This energy scale is confirmed by ultra-violet photoemission spectroscopy measurements of a 0.4 eV energy dipole between pentacene and LSMO, 27 which is low if compared to the 1 eV found for pentacene deposited on Co. 33,34 The E N value for pentacene on LSMO can be compared with the nucleation energy found for pentacene grown on SiO 2 over the same temperature range: E N ¼ 0.78 6 0.05 eV was found for a growth rate of 0.007 nm/s, 35 which is similar to the one used in the present experiment. At higher growth rates (0.03 nm/s) P i à m¼2 E m þ i à E D ¼ 1.13 6 0.04 eV and i* ¼ 3 were reported, 36 hence E N ¼ 0.23 6 0.01 eV.…”

“…Considering that E N varies with the growth rate, 31 the value we extracted for pentacene on LSMO is compatible with the one for SiO 2 substrates (Figure 4(b)), assuming that at lower rates E N levels off rather than diverging. 35,37 Pentacene nucleation and growth on LSMO are much closer to those on SiO 2 than those on Co. 34 The diffusion mechanism was analysed in order to disentangle the contributions of diffusion and nucleation in E N . The surface diffusion length is defined as x s ¼ ffiffiffiffiffiffiffi ffi D s s p , where D s is the diffusion coefficient and s is the residence time, i.e., the time the molecule moves on the substrate before attaching or desorbing.…”

Engineering spin propagation across a hybrid organic/inorganic interface using a polar layer

“…Strong electrical dipoles are present at many OSC/metal interfaces and these interfacial dipoles can significantly alter the non-interacting equilibrium energy levels 11,13,14,[22][23][24][25][26][27][28] . Thus far, little is experimentally known about the role of these vacuum level shifts on spin injection and extraction.…”

Charge Trapping in Intergrain Regions of Pentacene Thin Film Transistors

“…There have been several reports of pentacene phases showing a higher ionization potential than the bulk crystal. Tiba et al [27] have reported a 0.3 V shift of the HOMO level in a pentacene monolayer grown on cobalt, while Fukagawa et al [28] observed a 0.7 eV difference when the molecules form a monolayer in the face-on configuration, compared to molecules forming a crystal. This difference is even higher when the pentacene molecules are isolated, similarly to the gas phase, where the shift is 1.6 eV.…”