Formation and regulation of unoccupied hybridized band with image potential states at perylene/graphite interface

Abstract

Occupied and unoccupied electronic structures of submonolayer perylene (C20H12) on a graphite surface have been investigated using two-photon photoemission (2PPE) spectroscopy for two phases at room and low temperatures. Low energy electron diffraction measurements indicated that the molecules are disordered at room temperature and form a well-ordered superstructure below 180 K. In 2PPE, a specific unoccupied peak (Lx) was observed at around room temperature (>180 K) but not at low temperature (<180 K). The temperature-dependence of the excitation probability was attributed to a contribution of a diffuse unoccupied state, which is characterized by the molecular orbital extending outside the perylene molecular framework. At around room temperature, perylene adopts a flat-lying molecular orientation so that the diffuse state can hybridize with a free-electron-like unoccupied surface state, image potential states (IPS). As a result, the hybridized Lx state can be excited from the occupied bulk band through the IPS-mediated process. In contrast, hybridization is not efficient in the low-temperature phase due to the standing molecular orientation, which decouples the molecule away from the image plane of the substrate. The size of molecular islands also affects hybridization between the diffuse states and IPS because the two states encounter each other at the edge part of molecular aggregates. The temperature-dependent 2PPE results indicate that the molecular orientation and island size of perylene are directly linked to the formation of hybridized states, and thus, the excitation probability at the interface can be regulated by the morphology on the surface.

Structural Characterization and Photoluminescence Properties of Monolayer Perylene on a Graphite Surface

“…Furthermore, the unit cell length as short as 5.9 Å for the b -axis of the β-crystal (see Figure c) was not observed in our STM and LEED measurements. It should be noted that the same unit cell parameters listed in Figure d were also obtained for sub-ML films at low temperatures . At the perylene/graphite interface, only one type of well-ordered superstructure shown in Figure d was observed by STM and LEED.…”

“…Irrespective of high exposure, the LEED patterns observed at low temperatures remained sharp, as typically shown in Figure c, and no second layer growth was confirmed by STM. In addition, the work functions and spectral features of two-photon photoemission (2PPE) were unchanged after completion of 1 ML, , which is different from the cases of multilayer growth of perylene on metal substrates. , When the monolayer film was left in UHV at room temperature, a part of the monolayer film desorbed by the next day following the experiments. Therefore, it is estimated that the desorption temperature of perylene on the graphite surface is close to room temperature.…”

“…To highlight the image intensity, the image is processed with a rainbow color code. Schematic drawings with the van der Waals isosurfaces , of standing and flat-lying perylene molecules are superimposed. The standing perylene molecule is modeled with its short molecular edge parallel to the surface.…”

Accessing the Conduction Band Dispersion in CH3NH3PbI3 Single Crystals

“…The experiments was carried out in a home-built apparatus with an angle-resolved hemispherical energy analyzer (R3000, VG-Scienta, the emission acceptance angle in angle-resolved and angleintegrated mode is ±10° and 30°, respectively) and the overall energy resolution including the bandwidth of the laser light was typically less than 20 meV. [34] All 2PPE measurements were acquired with a sample bias of -3V to collect low-energy photoelectrons effectively. The influence of adding -3 V bias during AR-2PPE measurements has also been carefully considered, results in the underestimation of m* with a value of ~25% at most, especially when the value is evaluated far from the gamma point (see Fig.…”

“…The power of the laser light was set to ∼0.0125 nJ/pulse at a repetition rate of 80 MHz, which was one order of magnitude less than that used for conventional 2PPE measurements . The experiments were carried out in a home-built apparatus with an angle-resolved hemispherical energy analyzer (R3000, VG-Scienta), and the overall energy resolution was typically less than 20 meV . All 2PPE measurements were acquired with a sample bias of −3 V to collect low-energy photoelectrons effectively.…”