Portrait of the potential barrier at metal–organic nanocontacts

Abstract

Electron transport through metal-molecule contacts greatly affects the operation and performance of electronic devices based on organic semiconductors [1][2][3][4] and is at the heart of molecular electronics exploiting single-molecule junctions [5][6][7][8] . Much of our understanding of the charge injection and extraction processes in these systems relies on our knowledge of the potential barrier at the contact. Despite significant experimental and theoretical advances a clear rationale of the contact barrier at the single-molecule level is still missing. Here, we use scanning tunnelling microscopy to probe directly the nanocontact between a single molecule and a metal electrode in unprecedented detail. Our experiments show a significant variation on the submolecular scale. The local barrier modulation across an isolated 4-[trans-2-(pyrid-4-yl-vinyl)] benzoic acid molecule bound to a copper(111) electrode exceeds 1 eV. The giant modulation reflects the interaction between specific molecular groups and the metal and illustrates the critical processes determining the interface potential. Guided by our results, we introduce a new scheme to locally manipulate the potential barrier of the molecular nanocontacts with atomic precision.The electronic structure at the interface between a bulk metal and an organic semiconductor thin film has been extensively studied [9][10][11] and is commonly described in the framework of a band alignment model at the interface. In the single-molecule case, however, this model faces its limits. Chemical bonding between an organic molecule and a metal surface can result in significant charge transfer and rearrangement, which depend critically on the local atomic geometry 5,12 . In this situation of a strongly hybridized electronic system, a good indicator of the physical and chemical processes determining the molecule-metal contact is the work function, which for metal substrates is defined as the energy difference between the vacuum level far above the surface and the Fermi level (see also Fig. 1a; ref. 13).The formation of induced dipoles at the interface owing to the bonding of molecules can substantially modify the work function, which thereby provides valuable information on the degree of charge reorganization at the interface [13][14][15] . Photoemission experiments, often used to determine the averaged, coverage-dependent work function of a surface 11,16,17 , have generated considerable progress in understanding the formation of the interface built-in potential. However, these experiments cannot provide any information at the molecular length scale. Lateral resolution can be achieved by photoemission of adsorbed Xe, scanning tunnelling or Kelvin probe measurements 13,[18][19][20] . revealed local modifications of the work function but could not probe a single-molecule contact with submolecular resolution. A quantity closely related to the work function is the local potential-energy barrier experienced by tunnelling electrons during scanning tunnelling microscopy (STM) measurement...

Self-assembly of strongly dipolar molecules on metal surfaces

“…[45][46][47]50 Such a redistribution of electronic charge around adsorbates on coinage metal surfaces has been shown experimentally in several studies, 19,28,48 and is actually visible in the STM images in Figures 4(c) and 4(d). This mechanism is often found to be more pronounced on Cu(111) than on Ag(111) or Au(111).…”

Epitaxial hexagonal boron nitride on Ir(111): A work function template

“…4(c); it clearly can be seen that on both the pore and wire, the current decays much more slowly than on the clean iridium, indicating a lower apparent tunneling barrier. Approximating the barrier height b as the average work function of the tipsample system, i.e., b = ( t + s )/2 [66], we can deduce from an exponential fit and via s = 2( Ir − hBN ) [66] an overall work function reduction with respect to the iridium surface of ∼1.6 eV for the wire and ∼1.2 eV for the pore (note that Ir and hBN refer to the potential barriers as determined from the exponential fit). Combining the results of the FER and I (z) measurements, we find a modulation of the work function within the moiré of roughly 0.5 eV.…”

Electronic Mapping of Molecular Orbitals at the Molecule-Metal Interface

“…DOI: 10.1103/PhysRevLett.105.066801 PACS numbers: 73.63.Àb, 31.15.AÀ, 68.37.Ef, 82.37.Gk In the past decade, we have witnessed significant progress in integrating organic molecules in functional molecular devices like single molecule diodes [1,2] or in organic field-effect transistors [3,4]. To improve the functionality of such molecular electronic devices, an essential prerequisite is to gain a substantial understanding of the electronic structure of molecule-surface interfaces near the Fermi level that ultimately controls the performance of such a device [5][6][7].In this context, the precise energetic alignment of the molecular orbitals with respect to the Fermi level of the substrate, in particular, that of the highest occupied molecular orbitals and the lowest unoccupied molecular orbitals, is a key component of the electronic structure of the molecule-surface system under consideration. Therefore, in this Letter we present a detailed electronic mapping of molecular orbitals (MOs) by distance-dependent currentvoltage (I-V) spectroscopy.…”

“…DOI: 10.1103/PhysRevLett.105.066801 PACS numbers: 73.63.Àb, 31.15.AÀ, 68.37.Ef, 82.37.Gk In the past decade, we have witnessed significant progress in integrating organic molecules in functional molecular devices like single molecule diodes [1,2] or in organic field-effect transistors [3,4]. To improve the functionality of such molecular electronic devices, an essential prerequisite is to gain a substantial understanding of the electronic structure of molecule-surface interfaces near the Fermi level that ultimately controls the performance of such a device [5][6][7].…”