Effect of molecular ordering on spin and charge injection in rubrene

Studies have shown that interfaces play a crucial role for efficient spin injection and transport. Here, we address the complex role of interface in spin and charge injection into organic materials by various interface-sensitive characterization tools. Inelastic tunneling spectroscopy and polarized neutron reflectometry were mainly adopted to explore the interfaces of high mobility organic semiconductor rubrene sandwiched by two ferromagnetic electrodes. The dramatic difference in the reported magnetotransport properties and charge injection characteristics in trilayer magnetic junctions has been attributed to the different growth morphology of rubrene molecules at the interface dictated by the presence or absence of a 0.6 nm alumina seed layer. The magnetic contribution of the top ferromagnetic electrode is also influenced by the morphology of the rubrene layer underneath, directly affecting the spin injection efficiency. This work highlights the importance of interface engineering in the development of organic-based spintronics devices.

Figure 1

(Color online) (a) IETS plots of RJ [Al(8)/Rub(10)/Al(10)] (circle) and HJ $\left[\text{Al}\left(8\right)/{\text{Al}}_2{\text{O}}_3\left(0.6\right)/\text{Rub}\left(6\right)/\text{Al}\left(10\right)\right]$ (square) with IETS plot of control junction (CJ) $\left[\text{Al}\left(8\right)/{\text{Al}}_2{\text{O}}_3\left(1\right)/\text{Al}\left(8\right)\right]$ (triangles), and computed Raman (solid bar) and IR (hollow bar) peaks for single-crystal rubrene included for reference (all thicknesses are in nm). The IETS plots are shifted vertically for clarity. The arrows on the $x$-axis correspond to the position of Al phonon mode ($\sim 33$ mV) and Al-O stretching mode ($\sim 107$ mV), (b) Active molecular vibrational modes observed in hybrid junctions at (i) $1201{\text{cm}}^{-1}$ and (ii) $650.1{\text{cm}}^{-1}$ (arrows represents vibrations in plane along the tetracene axis) and in rubrene junctions at (iii) $432{\text{cm}}^{-1}$ and (iv) $616.4{\text{cm}}^{-1}$ (arrows represent vibrations out of plane perpendicular to the tetracene axis). Schematics of the molecular vibration shown here is taken from Ref. 19.

Figure 2

(Color online) (a) Shows the rubrene film thickness measured using a profilometer for $\text{Al}\left(6\right)/\text{Rub}\left(x\right)/\text{Al}\left(3\right)$ and $\text{Al}\left(6\right)/{\text{Al}}_2{\text{O}}_3\left(0.7\right)/\text{Rub}\left(x\right)/\text{Al}\left(3\right)$ with two different values of the nominal thickness: $x=15$ and 20 nm. (b) Schematics: (Left) azimuthal growh of rubrene molecules on the alumina seed layer in HJ leading to stronger inter-molecular electronic coupling; (Right) disordered nonazimuthal growth of rubrene on metal electrode in RJ leading to a strong hopping dominant transport.

Figure 3

(Color online) Normalized conductance (log scale) vs inverse temperature plotted in (a) HJ [$\text{Co}\left(8\right)/{\text{Al}}_2{\text{O}}_3\left(0.6\right)/\text{Rub}\left(40\right)/\text{Fe}\left(8\right)$] and (b) RJ [Co(8)/Rub(40)/Fe(8)]. The fit of experimental curves to charge injection and bulk transport models with the extracted parameters are shown.

Figure 4

(Color online) (a) Measured NSF reflectivity data ($R++$ and $R--$) and the corresponding fits for both the hybrid $\text{Si}/\text{SiO}\left(1\right)/\text{Co}\left(8\right)/{\text{Al}}_2{\text{O}}_3\left(0.6\right)/\text{Rub}\left(15\right)/\text{Fe}\left(8\right)/\text{CoO}\left(1.5\right)$ and rubrene: $\text{Si}/\text{SiO}\left(1\right)/\text{Co}\left(8\right)/\text{Rub}\left(20\right)/\text{Fe}\left(8\right)/\text{CoO}\left(1.5\right)$ samples at $H_{\text{a}}=0.7\text{T}$. (b) Depth profile of the nuclear SLD (top) and the layer magnetic moment (bottom) obtained from the PNR fits for the hybrid and rubrene samples.

Figure 5

(Color online) MR measurement (minor loop) performed on HJ: $\text{CoO}\left(1\right)/\text{Co}\left(8\right)/{\text{Al}}_2{\text{O}}_3\left(0.6\right)/\text{Rub}\left(25\right)/\text{Fe}\left(12\right)$ at 77 and 4.2 K. The bottom Co electrode is pinned to the CoO layer by cooling the sample in negative field. Insets: top shows the increase in junction resistance with cool down and the bottom shows the gradual decrease in MR with applied bias at 77 K. The shift in the curve (toward left) is due to a residual current from the power supply, flowing through the magnet.