Electronic structure and molecular orientation of pentacene thin films on ferromagnetic ${\text{La}}_{0.7}{\text{Sr}}_{0.3}{\text{MnO}}_3$

Pentacene thin films deposited on a ferromagnetic electrode, ${\text{La}}_{0.7}{\text{Sr}}_{0.3}{\text{MnO}}_3$ (LSMO), have been studied using near-edge x-ray absorption fine structure (NEXAFS), ultraviolet photoemission spectroscopy (UPS), and atomic force microscopy (AFM). Here we present electronic structure and molecular orientation of pentacene thin film on LSMO. No evidence related to covalent bonding or significant charge transfer between pentacene and LSMO has been found in the NEXAFS or UPS results. UPS measurements suggest that the vertical ionization potential of pentacene on LSMO is 4.9 eV. Our results extracted from NEXAFS indicate that molecular long axis of pentacene stands on the LSMO substrate surface with a tilt angle of about $22°\pm 2°$ between the main molecular axis and the substrate surface normal. AFM images show the terracelike crystalline grain formed by stacking pentacene crystalline layers and a rough crystal-layer spacing of $14–15\text{Å}$. Findings deduced from UPS, NEXAFS, and AFM consistently demonstrate that pentacene stands on LSMO with a tilt angle.

Figure 1

(Color online) X-ray absorption spectra of $\text{Mn}L$ edge of LSMO exposed to ambient atmosphere for three weeks and ultrasonically cleaned in acetone and isopropanol (bottom), LSMO treated by TL1 (middle), and 5 nm pentacene vacuum deposited on LSMO treated by TL1 (top). All spectra were collected by means of both total yield mode (black line) and partial yield mode (300 V suppression, red dashed line and 500 V suppression, blue dashed line, which has a strongest intensity at 639.7 eV in the bottom spectra and a weakest intensity at 640.6 eV in the middle spectra).

Figure 2

(Color online) (a) UPS spectra of LSMO treated by TL1 and pentacene thin films with thickness from 0.8 nm (red line) to 10 nm (blue line) on LSMO. Inset shows valence-band spectra blown up from $-0.5$ to $2.5\text{eV}$. (b) Energy-level alignment diagram derived from (a) for a bulk pentacene thin film on LSMO treated by TL1.

Figure 3

(Color online) (a) $\text{C}K$-edge NEXAFS spectra collected at different angles ($90°$, black line; $70°$, red line; $50°$, green line; $30°$, blue line; and $10°$, cyan line) of the beam relative to the substrate plane for a 5 nm pentacene thin film deposited on LSMO treated by TL1. (b) ${\pi }^{\ast }$ intensity normalized versus the squared sine of incident angle.

Figure 4

(Color online) AFM images of (a) bare LSMO, (b) 1 nm pentacene, and (C) 20 nm pentacene thin films on LSMO. (b) shows different steps for diffusion-mediated growth of a submonolayer (1nm) of pentacene on the LSMO substrate and step height of the terrace equal to 1.4–1.5 nm obtained from the bottom inset of (b1). (c) shows AFM images of a 20 nm pentancene thin film deposited on LSMO with the sizes of $5\times 5\mu {\text{m}}^2$ (c1) and local enlargement by the area of $1.5\times 1.5\mu {\text{m}}^2$ (c2) which displays an obvious stepwise structure in each grain and where height line gives a rough step height of stacking pentacene crystalline layers (c3).