183,196 research outputs found
Spin dependent transport in organic light-emitting diodes
Electrically Detected Magnetic Resonance (EDMR) was used to study a series of
multilayer organic devices based on aluminum (III) 8-hydroxyquinoline. These
devices were designed to identify the micoscopic origin of different spin
dependent process, i.e. hopping and exciton formation. EDMR is demonstrated to
probe molecular orbitals of charge, and thus indirectly explore interfaces,
exciton formation, charge accumalation and electric fields in operating organic
based devices
Doping Effects on Organic Interfaces
Organic electronics are an integral area of research because organic semiconductors can be lightweight, flexible, and biodegradable with low-cost production methods such as ink-jet or roll-to-roll printing. These inexpensive mass production techniques require solution deposition of the organic material. While single crystal organic semiconductors have been shown to exhibit carrier mobilities comparable to the silicon currently used in photovoltaics, during solution deposition of common organic semiconducting materials the resultant thin-film is often polycrystalline. Device performance and electrical properties of organic thin-film transistors are highly dependent on crystal structure and molecular packing. In polycrystalline thin-films, boundary regions between crystal grains can affect the overall performance of devices, as crystal structure and packing may differ from that of the surrounding crystal regions. These boundary regions may also serve as defect sites, allowing environmental factors, such as oxygen content and humidity, to alter local charge transport through devices. We utilize Kelvin Probe Force Microscopy (KPFM) to characterize how grain boundaries alter local conductivity and device performance as a function of doping in 2,8-difluoro-5,11-triethysilylethynyl anthradithiophene (diF TESADT) thin-film transistor surfaces. Device voltage drops at grain boundaries are characterized as a function of both atmospheric dopants and transition time between dopants
Inelastic Decay of Electrons in the Shockley-type Metal-Organic Interface States
We present a theoretical study of lifetimes of interface states (IS) on
metal-organic interfaces PTCDA/Ag(111), NTCDA/Ag(111), PFP/Ag(111), and
PTCDA/Ag(100), describing and explaining the recent experimental data. By means
of unfolding the band structure of one of the interfaces under study onto the
Ag(111) Brillouin zone we demonstrate, that the Brillouin zone folding upon
organic monolayer deposition plays a minor role in the phase space for electron
decay, and hence weakly affects the resulting lifetimes. The presence of the
unoccupied molecular states below the IS gives a small contribution to the IS
decay rate mostly determined by the change of the phase space of bulk states
upon the energy shift of the IS. The calculated lifetimes follow the
experimentally observed trends. In particular, we explain the trend of the
unusual increase of the IS lifetimes with rising temperature.Comment: 8 pages, 5 figure
Surface melting of methane and methane film on magnesium oxide
Experiments on surface melting of several organic materials have shown
contradictory results. We study the Van der Waals interactions between
interfaces in surface melting of the bulk CH_4 and interfacial melting of the
CH_4 film on the MgO substrate. This analysis is based on the theory of
Dzyaloshinskii, Lifshitz, and Pitaevskii for dispersion forces in materials
characterized by the frequency dependent dielectric functions. These functions
for magnesium oxide and methane are obtained from optical data using an
oscillator model of the dielectric response. The results show that a repulsive
interaction between the solid-liquid and liquid-vapor interfaces exists for the
bulk methane. We also found that the van der Waals forces between two
solid-liquid interfaces are attractive for the CH_4 film on the MgO substrate.
This implies that the van der Waals forces induce the presence of complete
surface melting for the bulk methane and the absence of interfacial melting for
CH_4 on the MgO substrate.Comment: 11 pages, 4 ps figure
Single-Crystal Organic Charge-Transfer Interfaces probed using Schottky-Gated Heterostructures
Organic semiconductors based on small conjugated molecules generally behave
as insulators when undoped, but the hetero-interfaces of two such materials can
show electrical conductivity as large as in a metal. Although charge transfer
is commonly invoked to explain the phenomenon, the details of the process and
the nature of the interfacial charge carriers remain largely unexplored. Here
we use Schottky-gated heterostructures to probe the conducting layer at the
interface between rubrene and PDIF-CN2 single crystals. Gate-modulated
conductivity measurements demonstrate that interfacial transport is due to
electrons, whose mobility exhibits band-like behavior from room temperature to
~ 150 K, and remains as high as ~ 1 cm2V-1s-1 at 30 K for the best devices. The
electron density decreases linearly with decreasing temperature, an observation
that can be explained quantitatively based on the heterostructure band diagram.
These results elucidate the electronic structure of rubrene-PDIF-CN2 interfaces
and show the potential of Schottky-gated organic heterostructures for the
investigation of transport in molecular semiconductors.Comment: 37 pages, 9 Figures (including supplementary information
Interpretation of X-ray Absorption Spectroscopy in the Presence of Surface Hybridization
X-ray absorption spectroscopy yields direct access to the electronic and
geometric structure of hybrid inorganic-organic interfaces formed upon
adsorption of complex molecules at metal surfaces. The unambiguous
interpretation of corresponding spectra is challenged by the intrinsic
geometric flexibility of the adsorbates and the chemical interactions with the
interface. Density-functional theory (DFT) calculations of the extended
adsorbate-substrate system are an established tool to guide peak assignment in
X-ray photoelectron spectroscopy (XPS) of complex interfaces. We extend this to
the simulation and interpretation of X-ray absorption spectroscopy (XAS) data
in the context of functional organic molecules on metal surfaces using
dispersion-corrected DFT calculations within the transition potential approach.
On the example of X-ray absorption signatures for the prototypical case of
2H-porphine adsorbed on Ag(111) and Cu(111) substrates, we follow the two main
effects of the molecule/surface interaction on XAS: (1) the substrate-induced
chemical shift of the 1s core levels that dominates in physisorbed systems and
(2) the hybridization-induced broadening and loss of distinct resonances that
dominates in more chemisorbed systems.Comment: 13 pages, 4 figure
Soil Infrastructure, Interfaces & Translocation Processes in Inner Space (“Soil-it-is”): Towards a Road Map for the Constraints and Crossroads of Soil Architecture and Biophysical Processes
Soil functions and their impact on health, economy, and the environment are evident at the macro scale but determined at the micro scale, based on interactions between soil micro-architecture and the transport and transformation processes occurring in the soil infrastructure comprising pore and particle networks and at their interfaces. Soil structure formation and its resilience to disturbance are highly dynamic features affected by management (energy input), moisture (matric potential), and solids composition and complexation (organic matter and clay interactions). In this paper we review and put into perspective preliminary results of the newly started research program “Soil-it-is” on functional soil architecture. To identify and quantify biophysical constraints on soil structure changes and resilience, we claim that new approaches are needed to better interpret processes and parameters measured at the bulk soil scale and their links to the seemingly chaotic soil inner space behavior at the micro scale. As a first step, we revisit the soil matrix (solids phase) and pore system (water and air phases), constituting the complementary and interactive networks of soil infrastructure. For a field-pair with contrasting soil management, we suggest new ways of data analysis on measured soil-gas transport parameters at different moisture conditions to evaluate controls of soil matrix and pore network formation. Results imply that some soils form sponge-like pore networks (mostly healthy soils in terms of agricultural and environmental functions), while other soils form pipe-like structures (agriculturally poorly functioning soils), with the difference related to both complexation of organic matter and degradation of soil structure. The recently presented Dexter et al. (2008) threshold (ratio of clay to organic carbon of 10 kg kg-1) is found to be a promising constraint for a soil’s ability to maintain or regenerate functional structure. Next, we show the Dexter et al. (2008) threshold may also apply to hydrological and physical-chemical interface phenomena including soil-water repellency and sorption of volatile organic vapors (gas-water-solids interfaces) as well as polycyclic aromatic hydrocarbons (water-solids interfaces). However, data for differently-managed soils imply that energy input, soil-moisture status, and vegetation (quality of eluded organic matter) may be equally important constraints together with the complexation and degradation of organic carbon in deciding functional soil architecture and interface processes. Finally, we envision a road map to soil inner space where we search for the main controls of particle and pore network changes and structure build-up and resilience at each crossroad of biophysical parameters, where, for example, complexation between organic matter and clay, and moisture-induced changes from hydrophilic to hydrophobic surface conditions can play a role. We hypothesize that each crossroad (e.g. between organic carbon/clay ratio and matric potential) may control how soil self-organization will manifest itself at a given time as affected by gradients in energy and moisture from soil use and climate. The road map may serve as inspiration for renewed and multi-disciplinary focus on functional soil architecture
Strongly Enhanced Thermal Stability of Crystalline Organic Thin Films Induced by Aluminum Oxide Capping Layers
We show that the thermal stability of thin films of the organic semiconductor
diindenoperylene (DIP) can be strongly enhanced by aluminum oxide capping
layers. By thermal desorption spectroscopy and in-situ X-ray diffraction we
demonstrate that organic films do not only stay on the substrate, but even
remain crystalline up to 460C, i.e. 270 deg. above their desorption point for
uncapped films (190C). We argue that this strong enhancement of the thermal
stability compared to uncapped and also metal-capped organic layers is related
to the very weak diffusion of aluminum oxide and the structurally well-defined
as-grown interfaces. We discuss possible mechanisms for the eventual breakdown
at high temperatures.Comment: 5 pages, 4 figures, submitted to Adv. Mat., for further information
see http://www.physchem.ox.ac.uk/~f
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