Substrate-interaction, long-range order, and epitaxy of large organic adsorbates (original) (raw)
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The Journal of Physical Chemistry C, 2014
The electronic structure of a hexa-cata-hexabenzocoronene (HBC)/ Cu(111) interface is investigated by two-photon photoemission over a range of coverage from 0 to 2 ML monolayers. It is found that increasing the HBC coverage shifts the vacuum level of the Cu substrate until this shift saturates at a coverage of ∼2 ML. Over this same range of coverage, the Shockley and the bare-surface Cu(111) image-potential states are shown to be quenched, while new unoccupied states appear and grow in strength with coverage. The use of momentum-and polarization-resolved photoemission spectra reveals that the new states are modified image states.
Self-assembled monolayers on organic semiconductors
Self-assembled monolayers (SAMs) are widely used in a variety of emerging applications for surface modification of metals and oxides. Here, we demonstrate a new type of molecular self-assembly: the growth of organosilane SAMs at the surface of organic semiconductors. Remarkably, SAM growth results in a pronounced increase of the surface conductivity of organic materials, which can be very large for SAMs with a strong electron-withdrawing ability. For example, the conductivity induced by perfluorinated alkyl silanes in organic molecular crystals approaches 10 −5 S per square, two orders of magnitude greater than the maximum conductivity typically achieved in organic field-effect transistors. The observed large electronic effect opens new opportunities for nanoscale surface functionalization of organic semiconductors with molecular self-assembly. In particular, SAM-induced conductivity shows sensitivity to different molecular species present in the environment, which makes this system very attractive for chemical sensing applications.
In-situ study of the interface formation in organic multilayers
Superlattices and Microstructures, 2008
Organic molecular beam epitaxy (OMBE) is the growth technique assuring high quality films and properly designed multilayers of organic molecular materials in view of both their fundamental studies and applications. To monitor in-situ the OMBE growth, we have applied reflectance anisotropy spectroscopy (RAS), which is particularly effective and sensitive, being non destructive for organic materials. In this work, RAS is used to monitor, in-situ, the growth of organic heterostructures. The experiment is carried out using organic single crystals as substrates and oligothiophenes as molecular materials to build a stack of layers. A layer-by-layer growth of the films is demonstrated, together with the formation of interfaces with peculiar optical behavior and structure.
Surface Science, 2008
The structural order of 1,4,9,10-naphthalene-tetracarboxylicacid-dianhydride (NTCDA) monolayers on Ag(1 1 1) has been investigated by spot profile analysis low energy electron diffraction (SPA-LEED). For increasing coverage, we find a sequence of three highly ordered structures: a commensurate structure (a), a uniaxially incommensurate structure (a 2 ), and an incommensurate structure (b) with coverages of 0.9 ML, 0.95 ML, and 1 (saturated) monolayer (ML), respectively. In the high coverage regime, the structures coexist and a coverage increase causes a change of their relative fractions. The a and b structures were known before [U. Stahl, D. Gador, A. Soukopp, R. Fink, E. Umbach, Surf. Sci. 414 (1998) 423], but the b structure was proposed as commensurate, since its very small misfit with respect to a commensurate structure could not be resolved. This misfit leads to a periodic modulation, causing additional Moiré satellites in the diffraction pattern. This finding demonstrates the importance of high resolution methods for the geometry determination of large organic adsorbates.
Fundamental studies of the interactions of adsorbates on organic surfaces
Proceedings of the …, 1987
We have undertaken fundamental studies that characterize the important interactions of adsorbates on organic solids. Model substrates were prepared by the molecular self-assembly of organosulfur compounds of the general structure [S(CH2).-X]2 (a = 15-19, X = CH3, CONH2, CO2H,
Electronic functionalization of the surface of organic semiconductors with self-assembled monolayers
Nature Materials, 2008
Molecular self-assembly has been extensively used for surface modification of metals and oxides for a variety of applications, including molecular 1,2,3,4 and organic electronics 5,6,7,8 . One of the goals of this research is to learn how the electronic properties of these surfaces can be modified by self-assembled monolayers (SAM). Here, we demonstrate a new type of molecular self-assembly: the growth of organosilane SAMs at the surface of organic semiconductors, which results in a dramatic increase of the surface conductivity of organic materials. For organosilane molecules with a large dipole moment, SAM-induced surface conductivity of organic molecular crystals approaches 10 -5 S per square, which is comparable to the highest conductivity realized in organic field-effect transistors (OFETs) at ultra-high densities of charge carriers 9,10,11 . SAM-functionalized organic surfaces are fully accessible to the environment which makes them very attractive for sensing applications. We have observed that the interaction of vapors of polar molecules with SAM-functionalized organic semiconductors results in a fast and reversible change of the conductivity, proportional to the pressure of an analyte vapor.
Substrate induced ordered structures in monomolecular adlayers
Supramolecular Science, 1994
The monolayer structure of 1,4-didodecylbenzene (DDB) at the interface between organic solutions and the basal planes of single crystalline MoSe2 and MoS2 has been investigated in situ by scanning tunnelling microscopy (STM), and compared to the corresponding monolayer structures on graphite, as well as to the monolayer structure of a long-chain alkane, dotriacontane. Common to the structures on all substrates is the fact that they are (1) close packed, (2) oriented relative to the substrate lattices, and (3) not simply commensurate to the substrates. However, details of the structures, including the unit cell symmetries, do depend on the particular substrate. It is concluded that for flexible and non-covalently bound chain molecules the atomic flatness of the substrate generally favours a high degree of order in the molecular adsorbate. The specific molecular structures depend on the particular surface properties of the substrate, i.e. lattice constants and corrugation of the adsorption potential across the surface. The less symmetric unit cells of DDB on the molybdenum dichalcogenides, when compared to the unit cell on graphite, is attributed to the larger corrugation in the adsorption potentials, which in turn is due to the larger lattice constants of the dichalcogenides.
Langmuir, 1987
The considerable activity in the area of organic thin f i i , involving very thin polymeric f i i and molecular monolayers and multilayers, led to the formation of a panel, sponsored by the Materials Sciences Division of the Department of Energy, to review this field. Its purpose was to better understand the relevant scientific topics and to suggest suitable areas of research. In particular, a number of potential applications were identified, which require further scientific advances for them to see fruition. These include nonlinear and active optical devices, chemical, biochemical, and physical sensors, protective layers (e.g., for passivation), patternable materials both for resists and for mass information storage, surface modification (e.g., wetting and electrochemical electrode properties), and synthetic biomacromolecules. Studies of these films have the added advantage that they could lead to a better scientific understanding of such subjects as the relationships between the microstructure of ordered molecular arrays and their collective properties, the tailoring of interfaces and surfaces, especially when used to model multibody interactions, and the physical and chemical reactions of films involving phase transitions and intra-and interfilm transport. The areas that appear to require the most attention include the application of new characterization techniques, such m the scanning tunneling microscope, the improvement of mechanical and thermal stability, the identification and characterization of physical and chemical defects, and the effects of internal ordering on macroscopic properties. It is further recommended that strong interdisciplinary efforts be mounted to address and solve these problems.
Growth-Mode-Induced Narrowing of Optical Spectra of an Organic Adlayer
Advanced Materials, 2008
Devices based on organic semiconductors allow novel applications, stimulating intensive research on the physicochemical characteristics of organic thin films. With true molecular electronics at the horizon, understanding of the behavior of molecules on surfaces is crucial. The interactions occurring in the thin film phase and with the substrate often lead to new electronic and optical properties. Due to the strong and wavelength-selective absorption of light by organic molecules, optical absorption spectroscopy of the organic adsorbate-substrate interface is a valuable tool to study these new properties. As measurements of light absorption by molecular (sub-) monolayers on inorganic surfaces became recently feasible, the substrate influence can now be analyzed in detail. Here we report an in situ growth study of an organic layer on a salt substrate, displaying surprisingly sharp optical transitions already at room temperature. We conclude that commensurate growth minimizes the inhomogeneous broadening in those optical spectra. Both Coulomb interactions and van der Waals-bonding between inorganic substrate and molecules lead to an unexpected formation of a quadratic structure for the first monolayer, which could be directly visualized by noncontact atomic force microscopy (AFM). Through either continued deposition or annealing, the molecules rearrange and form islands that exhibit the structure of the bulk single crystal. Advanced potential energy calculations explain this transition well. Our results clearly demonstrate the significant impact of the physical structure on the resulting physicochemical properties of molecular layers. These findings can be a starting point for further theoretical and experimental studies concerning the structure-properties-relation in molecular layers on surfaces.