Stable and metastable phases of PTCDA on epitaxial NaCl films on Ag(100)

Thin films of the model molecule perylene-3,4,9,10-tetracarboxylic acid dianhydride (PTCDA) were vacuum deposited on epitaxial NaCl films on Ag(100) and investigated by spot-profile-analysis low-energy electron diffraction (SPA-LEED). At growth temperatures between 193 K and room temperature (RT), the first layer (monolayer) exhibits a commensurate $\left(3\sqrt{2}\times 3\sqrt{2}\right)R45°$ superstructure with a quadratic unit cell, which is identical to that found for PTCDA growth on bulk NaCl crystals [Burke et al., Phys. Rev. Lett. 100, 186104 (2008)]. At RT, dewetting occurs and complicates the formation of multilayers. However, at low surface temperatures (223 K), closed PTCDA multilayers grow. The corresponding unit cell $\left[19.69\left(4\right)\text{Å}\times 12.23\left(3\right)\text{Å}\right]$ is rectangular and similar to the herringbone structure of the (102) bulk plane of PTCDA. This growth form is metastable and the surface dewets upon annealing at 300 K. Submonolayers deposited at low temperatures (20–100 K) are disordered; upon annealing, they transform into the $\left(3\sqrt{2}\times 3\sqrt{2}\right)R45°$ structure via a third metastable phase with a herringbone-type rectangular unit cell.

Figure 1

(a) SPA-LEED pattern of a PTCDA film with a nominal thickness of about 0.7 ML, grown on a 10-ML-thick NaCl film on Ag(100) at 223 K. The electron energy was 34 eV. (b) Schematic presentation of the LEED pattern according to the structure model illustrated in Fig. 2a. The reciprocal unit vectors ${\mathbf{b}}_{\mathbf{1}}^{\ast }$ and ${\mathbf{b}}_{\mathbf{2}}^{\ast }$ of the commensurate quadratic PTCDA structure are given, as well as the corresponding unit cell (solid line) and that of the NaCl(100) plane (dotted line).

Figure 2

(Color online) (a) Real-space structure model for the commensurate growth of the first PTCDA monolayer on NaCl(100). Large red spheres and small green spheres represent chlorine and sodium atoms, respectively. The main directions of the NaCl(100) plane are referred to as [010] and [001] with respect to the axes of the bulk fcc crystal. The centered unit cell (dashed black line) and the primitive unit cell (solid black line) of NaCl(100) are indicated with their dimensions at RT (Refs. 45, 46). The unit cell of the commensurate quadratic PTCDA structure is marked (black line) with the dimensions defined by its relationship of commensurability with NaCl(100). (b) Real-space structure model of the incommensurate herringbone PTCDA structure in multilayered PTCDA films on NaCl(100). Its experimentally measured dimensions are indicated $\left(19.69\text{Å}\times 12.23\text{Å}\right)$. Its unit cell (solid black line) is compared to those of the (102) plane of the PTCDA bulk structure in the $\alpha$ (dashed-dotted blue line) and $\beta$ phase (dashed red line).

Figure 3

SPA-LEED patterns of a multilayer PTCDA film with about 2.7 ML in nominal thickness, grown on a 10-ML-thick NaCl film on Ag(100) at 223 K. (a) just after growth and (b) after annealing at 300 K for 72 h. In the superimposed simulations of LEED pattern, the full circles and bold arrows represent the diffraction spots and the reciprocal unit vectors (${\mathbf{b}}_{\mathbf{1}}^{\ast }$ and ${\mathbf{b}}_{\mathbf{2}}^{\ast }$) of the commensurate PTCDA structure (Q phase). The empty circles and thin arrows represent the diffraction spots and the reciprocal unit vectors (${\mathbf{c}}_{\mathbf{1}}^{\ast }$ and ${\mathbf{c}}_{\mathbf{2}}^{\ast }$) of the incommensurate PTCDA structure (HB phase). The crosses indicate the $\left(n,0\right)$ and $\left(0,k\right)$ spots of the incommensurate PTCDA structure where $n$ and $k$ are odd numbers, which are expected to be absent in the presence of two glide planes. A dashed circle on the left side of the LEED patterns indicates the position of the NaCl $\left(\overline{1},0\right)$ spot. [10] and $\left[1\overline{1}\right]$ are the substrate directions along which the spot profiles were measured.

Figure 4

(Color online) Radial SPA-LEED profiles, measured along the $\left[1\overline{1}\right]$ direction of Fig. 3a (black dots) and Fig. 3b (blue triangles) under the same conditions of observation. Spots from both the commensurate (Q) and incommensurate (HB) PTCDA structures are observed along this direction. The exact positions of the diffraction spots were determined by fitting the profiles with a set of Lorentzian peaks (green curves and red envelope).

Figure 5

Radial SPA-LEED profiles of PTCDA films ranging from 0.14 to 1.85 ML in nominal thickness, grown on a 10-ML-thick NaCl film on Ag(100) at 223 K, measured along the [10] direction under identical experimental conditions at a sample temperature of 130 K.

Figure 6

(Color online) Radial SPA-LEED profiles of a thin PTCDA film of about 0.7 ML nominal thickness, grown on a 10-ML-thick NaCl film on Ag(100) at 100 K. Profiles (a)–(e) and (f)–(k) were measured along the [10] and $\left[1\overline{1}\right]$ direction, respectively. Profiles (a) and (f) were measured directly after growth. The other profiles demonstrate the evolution after consecutive annealing. In detail: [(b) and (g)] after annealing at 200 K for 10 min, [(c) and (h)] after further annealing at 270 K for 15 min, [(d) and (i)] after further annealing at 270 K for 10 min and at 285 K for 10 min, [(e) and (k)] after further annealing at 285 K for 30 min and at 300 K for 20 min. The profiles are vertically shifted for clarity. The gray curves show individual peaks fitted to the data, the black curves are the sums of these. In order to avoid overloading, not all fitted components are shown. Note that spots from both the Q and the ${\text{HB}}^{\ast }$ structures are observed along both directions. For further details see text.