Structure and growth of tetracene on Ag(111)

The structure of the tetracene/Ag(111) interface in the coverage range θ  = 0 to 2.4 ML is studied with scanning tunneling microscopy (STM) at 8 K and with low energy electron diffraction (LEED) at T = 300 … 100 K. For θ $\lesssim$ 0.01 ML, one-dimensional (1D) diffusion of single molecules along $\langle 01\overline{1}\rangle$-directions is observed even at 8 K. For 0.1 ML < θ < 0.5 ML molecules are homogeneously distributed over the surface forming a disordered phase (static at T = 8 K, dynamic at T = 25 K), indicating a repulsive intermolecular interaction (δ-phase). For θ $\gtrsim$ 0.5 ML, local ordering in the commensurate γ-phase is observed. Further increase of the coverage yields a compressed monolayer (ML) phase (θ ≡ 1 ML) with point-on-line registry (α-phase). The interaction between molecules has been calculated with the force-field approach to rationalize the molecular packing motifs in the various phases. Under most circumstances molecule-molecule interactions are repulsive, in agreement with experimental findings. A simulation of the adsorption up to θ  = 1 ML according to the random sequential adsorption (RSA) algorithm shows that the disorder-to-order transition from the δ- to γ-phase occurs close to random close packing (RCP), θ  = 0.5–0.6 ML. Since tetracene molecules are a two-dimensional (2D) representation of Onsager's hard rod model, this suggests that this phase transition is driven both energetically and entropically. For θ ≈ 2.23 ML a metastable bilayer phase with point-on-line coincidence is observed (β-phase). The basic structural unit of this phase is a triplet of molecules that are tilted along the long molecular axis against each other; at least one of these molecules is tilted out of the surface plane. Within the β-phase a superstructure of alternating rotation domains is observed. This superstructure has a period of 7.4 nm. The molecular packing in the β-phase resembles the packing in the bulk crystal structure of tetracene, its formation can therefore be interpreted as incipient pseudomorphic growth of tetracene on Ag(111). However, pseudomorphic growth cannot be continued beyond the β-phase.

Figure 1

Phase diagrams of Tc/Ag(111) after deposition at (a) room temperature and (b) low temperature (T ≤ 230 K). Vertical bars denote experimentally observed α, β, and γ phases and the estimated coverage of the RCP layer. Hatched areas indicate phase coexistence. High temperature boarders for all phases are approximated. Points for which STM data are shown in this article are marked with ✦, , , , and . Inset in (b): chemical structure of Tc.

Figure 2

(a) Surface mobility of Tc molecules on Ag(111) at 8 K. The STM image was recorded with 0.4 V bias voltage and95 pA tunneling current (A shadow filter was applied for improvedcontrast[68]). The stripes represent the traces of moving Tc molecules. Single immobilized Tc molecules can be recognized at the step edges of the substrate. The symbol ✦ indicates the point in the phase diagram of Fig. 1 that corresponds to the STM image. (b) Time spectrum of the tunneling current recorded above a diffusion trace. (c)–(e) Subsequent STM images of the same sample area, recorded with 0.08 V bias voltage and 95 pA tunneling current. Dashed lines mark diffusion traces of molecules which leave the image frame (dash line) or appear (dash-dot line).

Figure 3

(a), (b) STM images of Tc on Ag(111) at different conditions corresponding to points and in the phase diagram of Fig. 1. Tc surface densities (a) 4.4 Å × 10${}^{13}$ and (b) 5.6 Å × 10${}^{13}$ cm${}^{-2}$. Inset in (b): Small domain of γ-phase. (c) Typical arrangement of Tc molecules at coverages less than RCP. (d) Unit cell of the γ-phase.

Figure 4

Simulation of the random close-packed (RCP) Tc layer in the random sequential adsorption (RSA) approximation. (a) Number of single adsorption attempts needed for a successful event as function of the total number of Tc molecules $N_{\mathrm{Tc}}$ already placed on the surface (bottom axis) and the corresponding surface density of the layer $n_{\mathrm{Tc}}$ (top axis). Three cases are simulated—full registry with the Ag(111) substrate, i.e., commensurate structure (red dots); point-on-line registry with Ag(111), i.e., molecules placed on top of Ag atomic rows (black dots); and incommensurate adsorption (blue dots). The results of three independent simulations are accumulated for each case in the plot. The surface density at which the number of required attempts diverges was taken as the density of the corresponding RCP phase (red, black, and blue vertical lines). Black dashed lines and symbols , represent the surface densities in the STM images of Figs. 3a and 3b. (b)–(d) Distribution of Tc molecules from the RCP simulations in cases of incommensurate registry (b), commensurate registry (c), and point-on-line registry (d).

Figure 5

(a) LEED pattern (beam energy 12 eV) and (b), (c) STM images (tunneling current 0.1 nA, bias voltage 1.5 V) of the Tc/Ag(111) α-phase. The unit cell is marked in (b) and (c). (d) dI/dV spectra of a Tc molecule in the α-phase, recorded at its center [blue curve, approximate position marked in blue in (c)] and its edge [red curve, approximate position marked in red in (c)]. The green curve is the spectrum of the clean Ag(111) surface. The black horizontal arrow marks the shift of the Ag(111) surface state upon Tc adsorption. (e) Unit cell of the α-phase (angle and unit cell vectors are taken from SPA-LEED data of Langner et al.[1]: ${\mathbf{a}}_1$ = 12.9 Å, ${\mathbf{a}}_2$ = 7.8 Å, α = 83.2°).

Figure 6

Unit cells of the (a) γ-phase and (b) α-phase of Tc/Ag(111). The γ-phase is plotted as a commensurate superstructure. The α-phase is plotted according to SPA-LEED data from Ref. 1. (c) The γ-phase can be transformed into the α-phase by a rigid displacement of Tc chains as indicated with the yellow arrows.

Figure 7

(a) Intermolecular interaction energy Φ as a function of relative position of two co-planar Tc molecules that are aligned parallel. One molecule (shown) is located at the center of the graph, the other (not shown) is displaced by ΔX, ΔY relative to it. (b) Intermolecular interaction energy Φ as a function of relative position of two co-planar, parallel chains of Tc molecules (nine molecules in each chain). One chain (shown) is located at the center of the graph, the other (not shown) is rigidly displaced by ΔX, ΔY relative to it. The black region in the (ΔX, ΔY) map is excluded because of steric hindrance. The vectors${\mathbf{g}}_1$′, ${\mathbf{g}}_2$′, −${\mathbf{g}}_1$′, −${\mathbf{g}}_2$′, and Σg′ point to the local potential minima which agree with unit cell vectors of the $\mathit{\gamma }$-phase. Vectors${\mathbf{a}}_1$ and ${\mathbf{a}}_2$ are the unit cell vectors of $\mathit{\alpha }$-phase. Inset in (a) shows the potential profile for a Tc molecule diffusing along a silver atomic row (purple arrow) and approaching another Tc molecule fixed in the center of the map.

Figure 8

(a) STM image (bias voltage 2 V, tunneling current 0.7 nA) of Tc/Ag(111) showing α- and β-phases of Tc coexisting on the same terrace of the substrate. The symbol indicates the point in the phase diagram of Fig. 1 that corresponds to the STM image. The unit cells of the α- and β-phases are marked in black and shaded in yellow or red, respectively. For comparison, the Ag(111) unit cell is shown by small black rhombus. (b) STM height profile recorded along the dashed line in (a).

Figure 9

(a)–(c) STM images of the Tc/Ag(111) β-phase acquired at different bias voltages and tunneling currents: (a) 2.0 V, 47 pA; (b) 1.0 V, 47 pA; (c) 0.6 V, 47 pA. Molecules in different sublattices are marked with colored ovals and letters A, B (B′), C, D, E; The center positions of molecules in the A, B, and C sublattices are marked with green, light blue, and dark blue circles in (a), (b), and (c), respectively. The positions of domain walls are marked with black dotted lines that are labeled DW1 or DW2. Red dotted lines show the unit cell of the β-phase. Black circular arrows mark the positions of two-fold rotation axes R${}_2$ (see main text). (d) Structure model for the Tc/Ag(111) β-phase: top view (top) and side view (bottom) of the unit cell (red dashed lines) with 22 molecules, color-coded as in panels (a)–(c). Between the top and side views the various sublattices are shown separately. Positions of DW1 and DW2 are indicated. For comparison with the β-phase, the molecules in the blue boxes show the structure motif of bulk Tc. Inset: Unit cell of bulk Tc.

Figure 10

(a)–(b) LEED patterns (beam energy 12 eV) of the Tc/Ag(111) interface. Tc deposition at T = 230 K; Initial Tc coverage approximately 2.23 ML. (a) Before annealing. The pattern corresponds to the β-phase. (b) After annealing at RT and subsequent cooling to 100 K. The pattern corresponds to the α-phase. (c) STM image (bias voltage 0.4 V, tunneling current 0.4 nA) of the molecular layer recorded after annealing, i.e. corresponding to the LEED pattern shown in (b). Areas with molecular arrangement corresponding to the α-phase and γ-phase are shaded in orange and green, respectively. Disordered areas are marked in blue. (d)–(e) Two consecutive STM scans demonstrate a change ofthe local structure in the layer—the border between two neighboring translational domains of α-phase (white arrows) moves to the left; this is effected by the rigid translation of a molecular row (as shown in Fig. 7b) to the right. Note that the molecular arrangement at the border agrees well with the structure of the γ-phase. Corresponding unit cells are marked in red and blue.

Figure 11

Surface registry of the Tc/Ag(111) β-phase. Grey lines: lattice lines of Ag(111). Tc structure is shown as in Fig. 9d. The angles Θ${}_{\mathrm{A}}={71}^{\circ }$, Θ${}_{\mathrm{B}}={76}^{\circ }$, and Θ${}_{\mathrm{C}}={76}^{\circ }$ are the orientations of molecular rows in the respective sublattices relative to the $\langle 01\overline{1}\rangle$-direction of silver.