Metastable phase of lead phthalocyanine films on graphite: Correlation between geometrical and electronic structures

The geometrical and electronic structures of a metastable phase of lead phthalocyanine (PbPc) films on graphite have been studied by combined use of low energy electron diffraction (LEED) and two-photon photoemission (2PPE) spectroscopy. In submonolayer (sub-ML) PbPc films on graphite, islands in a metastable phase are formed just after deposition, as we reported previously by use of photoelectron emission microscopy (PEEM) [I. Yamamoto, N. Matsuura, M. Mikamori, R. Yamamoto, T. Yamada, K. Miyakubo, N. Ueno, and T. Munakata, Surf. Sci. 602, 2232 (2008)]. On single crystalline graphite substrates, the metastable islands produce clearly discernible LEED spots. By comparing the unit cell with that of annealed 1 ML films, molecules in the metastable islands are standing upright with a molecular density 1.8 times higher than that in the well-ordered 1 ML films. The LEED spots for the sub-ML films disappear after annealing. The islands in the metastable phase are surrounded by areas of a two-dimensional (2D) gaslike phase composed of flat-lying molecules. The metastable islands melt into the 2D gas phase, consistent with the PEEM results. In 2PPE spectroscopy, the lowest unoccupied molecular orbital (LUMO) derived level of the metastable phase is clearly distinguishable from that of flat-lying molecules. By tracking the thermal annealing process of the films by 2PPE spectroscopy, we clarify the decay of the LUMO derived peak intensity, the work function shift, and the energy shifts of molecular states associated with the transition from the metastable phase to the 2D gas phase. With this, we demonstrate the complementary capabilities of LEED and 2PPE spectroscopy to probe phase transitions of organic films in a nondestructive manner.

Figure 1

PEEM images of (a) as-deposited and (b) annealed (373 K, 1 h) 0.3 ML PbPc films on an HOPG substrate measured at a photon energy of 5.70 eV, which is sufficient to ionize the HOMO derived band (reprinted with permission from Ref. [17]; copyright 2016, Elsevier). The field of view is 36 μm. The brightness is displayed by the gray scale shown on the right-hand side. The bright islands labeled by a circle in (a) disappear in (b), and the “half-tone region” labeled by a square in (a) becomes brighter in (b).

Figure 2

Contrast-enhanced LEED images of (a) an annealed 1 ML film, (b) an as-deposited 0.7 ML film, and (c) an annealed 0.7 ML film on a single crystalline graphite substrate. The (0, 0) spot in (c) is at the center of the disk pattern. The reciprocal unit cells and some of the simulated spots are highlighted. (d) LEED intensity profiles (as extracted from the raw data) for films of different coverages given in the legend. The profiles are measured along lines equivalent to the red line in (c). The primary energy of the electron beam is shown at the bottom of each panel.

Figure 3

(a) STM image of a 0.5 ML annealed PbPc film observed at room temperature, imaged at a sample bias $\left(V_{\mathrm{s}}\right)$ of 3.00 V and a tunneling current $\left(I_{\mathrm{t}}\right)$ of 0.30 nA. The scan size is $55\times 55\mathrm{n}{\mathrm{m}}^2$. Only scratchlike patterns were observed. (b) STM image of (a) observed at 78 K. $V_{\mathrm{s}}=-1.10\mathrm{V},I_{\mathrm{t}}=0.10\mathrm{nA}$. The size is $55\times 55\mathrm{n}{\mathrm{m}}^2$. The inset shows the magnified image of the island, imaged at $V_{\mathrm{s}}=0.82\mathrm{V},I_{\mathrm{t}}=0.08\mathrm{nA};10\times 10\mathrm{n}{\mathrm{m}}^2$. PbPc island and substrate areas are clearly separated. The nearly square unit cells are highlighted in the images. All scale bars in the images correspond to 5 nm.

Figure 4

2PPE spectra of as-deposited (blue) and annealed (magenta) 0.3 ML PbPc films on HOPG measured at a photon energy of 4.33 eV. Peak X was not known previously and is identified to arise from the $\mathrm{LUMO}/\mathrm{LUMO}+1$ level of the metastable phase.

Figure 5

Photon energy–dependent 2PPE spectra for the as-deposited 0.3 ML PbPc film on HOPG. The photon energies are shown on the right-hand side. All spectra were normalized to the intensity of the $\mathrm{LUMO}/\mathrm{LUMO}+1$ peak. The energy positions of the peaks are marked by vertical bars.

Figure 6

(a) A series of 2PPE spectra measured after the deposition of 0.3 ML PbPc on single crystalline graphite. Spectra are shown on the measured raw intensity scale without artificial normalization. The time after deposition is shown on the right-hand side. 2PPE spectra of clean graphite and the annealed film are given at the bottom and top, respectively, for comparison. The traces in the right panel are expanded by a factor of 3 relative to those in the left panel. The energies of the $\mathrm{LUMO}/\mathrm{LUMO}+1$ peak, the X peak, and HOMO peak are shown by bars. Thin vertical lines at 3.5 and 3.6 eV show the energies of IPS2 and IPS1, respectively. (b) Filled circles show the intensity of peak X relative to that of the $\mathrm{LUMO}/\mathrm{LUMO}+1$ peak. (c) Time evolutions of the work function (square), the $\mathrm{LUMO}/\mathrm{LUMO}+1$ peak energy (circle), and the HOMO peak energy (triangle).