Pentacene on Au(1 1 1), Ag(1 1 1) and Cu(1 1 1): From physisorption to chemisorption

Abstract

We measured the electronic and the molecular surface structure of pentacene deposited on the (1 1 1)-surfaces of coinage metals by means of ultraviolet photoelectron spectroscopy (UPS) and low-energy electron diffraction (LEED). Pentacene is almost flat-lying in monolayers on all three substrates and highly ordered on Au(1 1 1) and on Cu(1 1 1). On Ag(1 1 1), however, weak chemisorption leads to almost disordered monolayers, both, at room temperature and at 78 K. On Cu(1 1 1) pentacene is strongly chemisorbed and the lowest unoccupied molecular orbital becomes observable in UPS by a charge transfer from the substrate. On Ag(1 1 1) and Cu(1 1 1) multilayers adopt a tilted orientation and a high degree of crystallinity. On Au(1 1 1), most likely, also in multilayers the molecular short and long axes are parallel to the substrate, leading to a distinctively different electronic structure than on Ag(1 1 1) and Cu(1 1 1). Overall, it could be demonstrated that the substrate not only determines the geometric and electronic characteristics of molecular monolayer films but also plays a crucial role for multilayer film growth.

The role of the van der Waals interactions in the adsorption of anthracene and pentacene on the Ag(111) surface

“…Both PBE and PBE+vdW surf , however, predict flat conformation for pentacene/Ag(111), as can be seen in Fig. 6, in agreement with experimental observations 10,45,46 . Our results are also in good agreement with previous theoretical studies: from PBE calculations we found d = 3.938Å and E ads = −0.119 eV, which compare well with previous GGA results (d = 3.7-4.12Å and E ads within the range of −0.078 to −0.108 eV) 47,48 ; using the PBE+vdW surf method we found d = 2.910Å and E ads = −2.396 eV, which is in good agreement with previous results obtained by Toyoda et al 47 using the pairwise DFT-D method (d = 2.9Å and E ads = −2.28 eV), but differ from the values obtained by the same authors using the nonlocal vdW-DF method (d = 3.7Å and E ads = −1.62 eV) 47 .…”

Investigation of MoOx/n‐Si strong inversion layer interfaces via dopant‐free heterocontact

“…A monochromatized Al Kα source (1486.7 eV) and a monochromatized HeI (21.22 eV) were used as XPS and UPS excitation source, respectively. Detailed sketch of the measurement geometry was shown in our previous work . Kinetic energy scale in the plot of the secondary electron region of UPS was corrected by a −3 V bias voltage and the analyzer work function.…”

Contact Resistance in Organic Field‐Effect Transistors: Conquering the Barrier

“…In addition, the conformation and orientation of the organic semiconductor molecules on the metal and amount of chemical bonding can vary based on the surface energetics and chemistry, a process that also results in shifts in φ B . When molecules are physisorbed, they couple weakly to the metal surface and the alignment of energy levels can approach the Schottky–Mott limit, where φ B is controlled by the metal work function and the semiconductor HOMO/LUMO levels . For strong molecule–metal interactions, such as the case of chemisorbed molecules, charge transfer from the metal to the semiconductor molecules leads to the formation of large surface dipoles, making the band diagram of the isolated material surfaces irrelevant .…”

“…When molecules are physisorbed, they couple weakly to the metal surface and the alignment of energy levels can approach the Schottky–Mott limit, where φ B is controlled by the metal work function and the semiconductor HOMO/LUMO levels . For strong molecule–metal interactions, such as the case of chemisorbed molecules, charge transfer from the metal to the semiconductor molecules leads to the formation of large surface dipoles, making the band diagram of the isolated material surfaces irrelevant . The strength of the coupling between the organic semiconductor and the electrode can be tuned, to a certain extent, by processing.…”

“…For example, Zimmerling and Batlogg showed that exposure of the metal interface to air before making the metal‐semiconductor contact results in reduced contact resistance in single crystal rubrene OFETs with gold contacts, likely due to a contamination layer that reduces the interface interaction and charge transfer leading to a more physisorbed‐type contact . The energetics at the interface and the processing steps taken to form the interface can modify the ordering of molecules into the bulk, introducing an additional barrier related to charge movement within the first few layers of organic material. Surface modifications can also be used to intentionally shift the work‐function, tune the degree of charge transfer, and modify semiconductor conformation by applying structures like self‐assembled monolayers (SAMs), dopant layers, and oxide interlayers, as we will discuss in the Section .…”