Hyperthermal Molecular Beam Deposition of Highly Ordered Organic Thin Films

We use a seeded supersonic molecular beam to control the kinetic energy of pentacene (${\mathrm{C}}_{22}{\mathrm{H}}_{14}$) during deposition and growth on Ag(111). Highly ordered thin films are grown at low substrate temperatures ($\sim 200\mathrm{K}$) at kinetic energies of a few electron volts, as shown by low energy He diffraction and x-ray reflectivity spectra. In contrast, deposition of thermal molecules yields only amorphous films. Growth at room or higher temperature substrates yields films of poorer quality irrespective of the depositing beam energy. We find that after the first wetting layer is completed, a new ordered phase is formed, whose in-plane lattice spacings match one of the bulk crystal planes. The high quality of the films can be interpreted as the result of local annealing induced by the impact of the impinging high-energy molecules.

Figure 1

Surface specular reflectivity as a function of deposition time, relative to the clean Ag(111) specular reflectivity $I_0$. Inset: expanded view of the initial decay.

Figure 2

(a) He diffraction spectrum along the Ag(111) $⟨11\overline{2}⟩$ direction (defined here as $\varphi =0°$) for 1 ML pentacene deposited with $E_{\mathrm{t}\mathrm{r}\mathrm{a}\mathrm{s}\mathrm{l}}=5\mathrm{e}\mathrm{V}$. Expected peak positions relative to the ($6\times$) periodicity (see text) are indicated by full lines. (b) Diffraction spectra from $\sim 50\mathrm{M}\mathrm{L}$ pentacene/Ag(111) deposited with $E_{\mathrm{t}\mathrm{r}\mathrm{a}\mathrm{s}\mathrm{l}}=5\mathrm{e}\mathrm{V}$. Full (dashed) lines correspond to a periodicity of $6.1\pm 0.1\AA$ ($15.3\pm 0.6\AA$) along Ag(111) $⟨11\overline{2}⟩$ ($⟨1\overline{1}0⟩$) direction. Inset: ex situ x-ray reflectivity spectrum from the same film. Structural model of the film: (c) top view; (d) side view.

Figure 3

LEAD data taken along the surface $⟨11\overline{2}⟩$ direction for films grown at different temperatures and kinetic energies.