Session Z16: Focus Session: Organic Electronics and Photonics: Fundamentals

Near-infrared photoresponse in single walled carbon nanotube/polymer composite films

We present a near-infrared photoresponse study of single-walled carbon nanotube/poly(3-hexylthiophene)-block-polystyrene polymer (SWCNT/P3HT-b-PS) composite films for different loading ratios of SWCNT in the polymer matrix. Compared to the pure SWCNT film, the photoresponse [(light current -- dark current)/dark current] is much larger in the SWCNT/polymer composite films. The photoresponse is up to 157{\%} when SWCNTs are embedded in P3HT-b-PS while for a pure SWCNT film it is only 40{\%}. We also show that the photocurrent strongly depends on the position of the laser spot with maximum photocurrent occurring at the metal--film interface. We explain the photoresponse due to exciton dissociations and charge carrier separation caused by a Schottky barrier at the metallic electrode - SWCNT interface   

Single molecule photoluminescence excitation spectroscopy of polyfluorene

Polyfluorene is a remarkable conjugated polymer with a uniquely rich polymorphism [1]. Because of this characteristic it can be considered as a model playground to understand structure-property relationships in conjugated polymers. Here, by low temperature single polymer chain photoluminescence excitation spectroscopy [2], we look at the spectral characteristics of the absorbing and emitting chromophores on a chain. These experiments are performed on both the $\beta $-phase and the glassy disordered structure, elucidating the role of chain polymorphism on conformational relaxation and energy transfer. Moreover, we compare results on multichromophoric polymers with short oligomers, where a single chromophore is responsible for the optical response. These experiments illuminate directly the emergence of chromophores in conjugated polymers through delocalization: how a pi-electron system evolves from a localized molecular (oligomeric) unit into a delocalized species. \\[4pt] [1] Da Como Nano Lett. \\[0pt] [2] Walter PRL 2009   

Quadratic Electro-Optic Effect in Doped Nonconjugated Conductive Polymer 1,4-Trans-Polyisoprene, an Organic Quantum Dot System

Thin, optically uniform films of 1,4-trans-polyisoprene have been prepared on quartz substrate from a toluene solution. These films have been characterized using FTIR and optical absorption spectroscopy before and after doping with iodine. The optical absorption spectrum at low doping shows two peaks: one at $\sim $ 4.2 eV due to radical cation and the other at $\sim $ 3.2 eV due to charge-transfer. Doping leads to a reduction of the intensity of =C-H bending vibration-band due to formation of radical cations upon charge-transfer. Quadratic electro-optic measurements have been made using field-induced birefringence method at 633 nm. A modulation depth of $\sim $ 0.13{\%} has been observed for an applied field of 1.1 V/$\mu $m for a 0.37 $\mu $m thick film. The modulation depth had a quadratic dependence on applied field. The Kerr coefficient as measured is exceptionally large and has been attributed to the subnanometer size metallic domains (quantum dots) formed upon doping and charge-transfer.   

Spin-orbit coupling and spin relaxation rate in singly charged pi-conjugated polymer chains

In inorganic semiconductor spintronics the spin-diffusion length is usually limited by spin-orbit coupling. Here we examine the effect of spin-orbit coupling in organic spintronics. We consider singly charged pi-conjugated polymer chains. We show that the diagonal matrix elements for spin-orbit coupling are zero. Even the off-diagonal matrix elements are zero or negligibly small unless a twisted, non-planar polymer chain is considered. We calculate these matrix elements as a function of twist-angle using tight-binding wavefunctions. We show that time reversal symmetry prevents spin-orbit induced spin-precession and propose a phonon-assisted spin-flip process.   

Polarizability, susceptibility, and dielectric constant of nano-scale molecular films: a microscopic view

We explore theoretically the size-dependence of the polarizability, susceptibility, and dielectric constant of nano-scale molecular layers. This is achieved by comparing first principles calculations based on density functional theory to phenomenological modeling based on polarizable dipolar arrays, for a model system of organized monolayers comprised of oligophenyl chains. We show that molecular packing density is the single most important factor controlling the bulk limit of all three quantities as well as the rate at which they are approached. Finally, we show that the polarization does not reach its ``bulk'' limit, as determined from the Clausius-Mossotti (CM) model, but the susceptibility and dielectric constant do converge to the correct bulk limit. However, whereas the CM model describes the dielectric constant well at low lateral densities, finite size effects of the monomer units cause it to be increasingly inaccurate at high lateral densities.   

Tuning vibrations in single-molecule junctions: inelastic electron tunneling spectroscopy of an alkanedithiol

We study pentanedithiol molecular junctions formed by means of the break-junction technique with a scanning tunneling microscope at low temperatures. Using inelastic electron tunneling spectroscopy and first-principles calculations, the response of the junction to elastic deformation is examined. We show that this procedure makes a detailed characterization of the molecular junction possible. In particular, our results show unequivocally that tunneling takes place through just a single molecule.   

Towards molecular electronics with scalable nanopore junctions

We have fabricated and measured nanoscale molecular junctions. Each device consists of a shallow pore in an oxide layer, with a self-assembled monolayer (SAM) on a gold surface at the bottom. The use of a conductive polymer as a top-contact avoids previously noted issues of metal diffusion into contacted SAMs. Larger pores are more likely to contain monolayer defects and dislocations, thus nanometer-scale control over the pore size allows us to investigate transport through the SAM as a function of defect density. The planar geometry and use of robust materials in the device allows for additional molecular synthesis after monolayer formation. For example, we use ``click'' chemistry to alter the functionality of SAMs of azide-terminated alkanethiols. The use of mixed monolayers to substantially dilute the number of conducting molecules in a 50-nanometer diameter pore allows us to observe few to single-molecule transport behavior.   

Photophysics of MEH-PPV film under High Hydrostatic Pressure: The Role of Charge-Transfer Excitons

We report theoretical calculations for interacting pairs of PPV oligomers that explain the pressure-dependent experimental observations on MEH-PPV films by E.~Olejnik {\it et al}. We use the Pariser-Parr-Pople (PPP) model Hamiltonian for single chains. We assume that pressure decreases the intermolecular distance in the ordered phase of this two-phase material, and simulate pressure effects by assuming distance-dependent interchain Coulomb interactions and electron hopping. Our calculations show that the photophysics of the ordered phase is dominated by a charge-transfer exciton, which is a quantum-mechanical superposition of the covalent delocalized exciton state and the ionic polaron-pair state. We are able to explain the pressure-induced (i) quenching of the photoluminescence and its redshift, (ii) the redshift of the cw triplet photoinduced absorption, (iii) the appearance of ps PA bands at $\sim$ 0.35 eV and 0.9 eV and their strong blueshifts. Detailed comparisons between experiments and theory are made.   

Optical Probes of MEH-PPV films at High Hydrostatic Pressure

We investigate the primary and long-lived photoexcitations in $\pi$-conjugated polymer films with increased interchain coupling by studying the photophysics of substituted PPV derivative thin films, namely 2-methoxy-5-(2'-ethylhexyloxy) [MEH-PPV] at high hydrostatic pressure, P up to 120 kbar in a diamond anvil cell, using both ultrafast transient mid- and near-IR spectroscopies with 0.1 ps resolution, and cw optical techniques (photo induced absorption (PA) and photoluminescence (PL) in a broad spectral range from 0.2 to 2.2 eV). With increasing P the cw PL band weakens, broadens, and red-shifts by $\sim$ 2 meV/kbar; whereas the triplet PA red shifts to a lesser extent. The ultrafast PA band of the singlet exciton at $\sim$ 0.95 eV at ambient splits, blue shifts and acquires a much longer decay component. A second, weak PA band at $\sim$ 0.33 eV at ambient, dramatically blue-shifts ($\sim$ 3 meV/kbar) and substantially intensifies with P. These pressure-induced effects are discussed considering the interplay of two phases in the MEH-PPV film: a disordered phase with large PL efficiency, and PA that does not change much with P; and a less emissive ordered phase that increases with P, where the interchain coupling substantially increases with P.   

Self Assembled Dipole Monolayers on CNTs: Effect on Transport and Charge Collection

We propose a method of quickly and dramatically increasing the conductivity of carbon nanotubes via growth of a self assembled monolayer (SAM) of fluoroalkyl trichlorosilane dipoles following the method demonstrated with organic semiconductors in [1,2]. Growth of a SAM on carbon nanotubes results in a strong p-type doping which improves the conductivity by a factor of two or more. Additionally, this doping is nonvolatile and persists in high vacuum and inert atmospheres. Improvements to conductivity are most dramatic in the case of predominantly semi-conducting, single walled carbon nanotubes (SWCNT) due to the remarkable introduction of about 1.2e14 holes/sq. cm, but this method is also an effective means to improve metallic, multi-walled carbon nanotubes (MWCNT). We will demonstrate improvement of transport and charge collection properties of both SWCNTs and MWCNTs by these SAM coatings in FETs and also in organic photovoltaic solar cells and in OLEDs. [1] M. F. Calhoun et al. Nature Materials 7, 84 - 89 (2008). [2] C. Y. Kao et al. Adv. Func. Mater. 19, 1 (2009).   

Efficient electron and hole injection in organic transistors with carbon nanotube electrodes

Single Wall Carbon Nanotubes (SWCNTs) are of great interest as electrode materials in Organic Field Effect Transistors (OFETs) since they are easy to process and stable in ambient contitions. Thanks to their field emission properties, SWCNTs electrodes, in principle, are able to inject both electrons and holes into organics with low injection barriers, promoting tunneling injection. We well present recent result on the electrical properties of p-type and n-type OFETs using hairy SWCNTs electrodes, where the CNTs are attached on the substrate by means of metallic Ti contact pads. Devices with SWCNTs electrodes show improved injection characteristics compared with those using conventional metallic electrodes both for and p-type (pentacene) and n-type (fullerene) materials.   

Octadecanethiol Island Formation on Single Crystal Zinc Oxide Surfaces

Organic photovoltaic devices, containing ZnO nanorod electron acceptor arrays intercalated with organic polymers, could lead to low-cost solar cells. Surface modifications of ZnO with octadecanethiol (ODT) monolayers have been shown to improve charge transfer in such devices. The present work is an effort to understand these monolayers through studies of ODT on single crystals of ZnO with well-defined oxygen-terminated or zinc-terminated surfaces. Both bare and ODT- functionalized surfaces were characterized with atomic force microscopy, Fourier transform infrared spectroscopy, x-ray photoemission spectroscopy, and water contact angle measurements. ODT seemed to form islands of multilayers on zinc-terminated surfaces and islands of monolayers on oxygen- terminated surfaces. While ODT was expected to preferentially bond along defects and terraces on oxygen-terminated surfaces, this was not observed. ODT was also expected to more effectively bond to the zinc-terminated surface, which was observed. This work was supported by the National Science Foundation Division of Materials Research DMR-0606054, DMR-0907409, and the Renewable Energy Materials Research Science and Engineering Center at the Colorado School of Mines.   

Mechanical control of morphology in fluorene oligomers: First principles calculations

Mechanically induced strain can have a significant impact on the optical properties of conjugated polymers. We use \textit{ab-initio} computational methods to investigate tensile stretching and compression of polyfluorene oligomers. We show that strain can result in changes in backbone morphology which relate to shifts in transition energies in a non-trivial manner. In particular, compression of oligomers results in two distinct morphologies which shift the signal in opposite directions, despite an equal distance between terminal atoms. We also consider the application of strain through adsorption on a silicon substrate. Extension or compression can be induced through mismatch between the lattice of the substrate and the size of the repeat unit of the molecule.   

Large Single Grain Thin Film by Hollow Capillary Method

Using 6,13-bis(triisopropyl-silylethynyl) pentacene (TIPS-Pentacene), we have made solution processed thin film transistor by hollow capillary method. The grain size is 1mm wide and more than 10mm long along with the writing direction routinely, which cover the channels of entire device. Thickness of the highly uniform and continuous film can be varied from 10nm to 100nm by tuning the concentration and speed of substrate. We also show the Evidence of two preferred growth orientation along with the writing direction. The field effect mobility shows highly orientation-dependence.   

Exploring photomechanical switching capability and self-assembly of individual molecules on semiconductor surfaces

Surface-bound photoactive organic molecules reveal substantially different photomechanical switching properties compared to when they are in solution-based environments. Metal surfaces, for example, often reduce photomechanical activity due to molecule-substrate interactions. Semiconductor surfaces are expected to induce different molecular switching behavior due to the presence of a band gap, potentially resulting in longer excited-state lifetimes and enhanced control of photomechanical properties. Here we report our exploration of single-molecule-resolved self-assembly and photomechanical switching capability of azobenzene derivatives on semiconducting GaAs(110) using variable temperature scanning tunneling microscopy.