Spatial Extent of Wave Functions of Gate-Induced Hole Carriers in Pentacene Field-Effect Devices as Investigated by Electron Spin Resonance

An electron spin resonance (ESR) method is applied to a pentacene field-effect device to investigate gate-induced hole carriers in such devices. Clear field-induced ESR signals are observed, demonstrating that all of the field-injected carriers have $S=1/2$ spins. Anisotropic ESR signals due to unpaired $\pi$ electrons show the molecular orientation at the interface in the devices. The spatial extent of the spin density distribution (wave function) of the carriers is evaluated from the ESR linewidth, accounting for the hyperfine structure, to be of the order of 10 molecules.

Figure 1

(a) Chemical structure of pentacene. The principal axes of the proton hyperfine coupling of the $\pi$ electrons are shown. (b) Schematic of the FET structure used in this study. The channel length ($L$) and width ($W$) of the Au source and drain electrodes are $100\mu \mathrm{m}$ and 2.17 cm, respectively.

Figure 2

Gate voltage $V_G$ dependence of capacitance $C$ of pentacene FET structure at a modulation frequency of 120 Hz at 290 K. Inset: Drain current ($I_D$) versus drain voltage ($V_D$) at different $V_G$ for FET at 290 K.

Figure 3

Field-induced ESR signal of pentacene at 290 K. The external magnetic field is parallel to the substrate. Inset: $V_G$ dependence of the number of field-induced spins $N_{\mathrm{spin}}$ (solid circles) and the number of field-induced charges $N_{\mathrm{charge}}$ (open squares) at 290 K.

Figure 4

(a) Schematic diagram of molecular orientation of pentacene in an FET. The $z$ axis is parallel to the $p\pi$ orbital, and the $y$ axis is perpendicular to the $p\pi$ orbital and the C-H bond [see Fig. 1a]. (b) Anisotropy of the $g$ values and the linewidth of the field-induced ESR signals for an external magnetic field parallel ($H_{\parallel }$) and perpendicular ($H_{\perp }$) to the substrate at 290 K.