Resonant inelastic x-ray scattering studies of the organic semiconductor copper phthalocyanine

We report resonant inelastic x-ray scattering (RIXS) measurements on polycrystalline and single crystal samples of the organic semiconductor $\beta$-copper phthalocyanine (CuPc) as well as time dependent density functional theory calculations of the electronic properties of the CuPc molecule. Resonant and nonresonant excitations were measured along the three crystal axes with $120\mathrm{meV}$ resolution. We observe molecular excitations as well as charge-transfer excitons along certain crystal directions and compare our data with the calculations. Our results demonstrate that RIXS is a powerful tool for studying excitons and other electronic excitations in organic semiconductors.

Figure 1

(Color online) (a) CuPc molecule. (b) The $\beta \text{−}\mathrm{Cu}\mathrm{Pc}$ unit cell. Molecules on the corners are parallel to each other, and molecules in the bases are parallel to each other.

Figure 2

(Color online) X-ray absorption spectra of f, $25\mu \mathrm{m}$ copper foil; p, CuPc powder; c, along the $c^{*}$ direction; b, along the $b^{*}$ direction; and a, along the $a^{*}$ direction of a single crystal of CuPc.

Figure 3

(Color online) Left: IXS measurements of the HOMO-LUMO gap of a stacked crystal sample of $\beta \text{−}\mathrm{Cu}\mathrm{Pc}$ ($300\mathrm{meV}$ resolution). Right: Orbitals calculated using the B3LYP method. Different spins have different colors, and the surfaces shown have equal energy surfaces. (a) HOMO-1 (0), (b) HOMO (2), (c) LUMO (4), and (d) $\mathrm{LUMO}+1$ (0); number of degenerate states are given in parentheses.

Figure 4

(Color online) (a) Calculated density of states of CuPc using the B3LYP method. Red shows the VOs and blue shows the OOs. (b) Spectra of CuPc at constant incident energy. $b^{*}$ is parallel to the momentum transfer $\left(\mathit{Q}\right)$.

Figure 5

(Color online) (a) Resonance scans with $b^{*}$ parallel to the momentum transfer. The left oval in the plot shows the $1.3\mathrm{eV}$ excitation, and the right oval shows the resonant excitation at $2.6\mathrm{eV}$. The dashed line shows that the exciton at $1.7\mathrm{eV}$ is nonresonant. The data are shifted vertically for clarity. (b) Possible states involved in $2.6\mathrm{eV}$ resonant excitations.