Marsha A. Loth

Wake Forest University, Winston-Salem, NC, United States

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Publications (34)124.75 Total impact

  • [Show abstract] [Hide abstract]
    ABSTRACT: Triethylsilylethynyl anthradithiophene (TESADT) holds considerable promise for organic transistor applications due to the high electrical mobilities attained by post-deposition crystallization using solvent vapour annealing. We have studied thermal annealing as an alternative route to post-deposition crystallization of TESADT films. Thermal annealing initially appears promising, producing mm-sized crystal domains, but poor electrical performance is obtained, which we attribute to a combination of crack formation and potentially also structural transition during the anneal process. We also find that illumination has a significant positive effect on crystallization, possibly due to an optically-induced enhancement in molecular mobility during annealing. This suggests further studies of how solvent exposure, heat, substrate surface properties and particularly light exposure influence the ordering kinetics of TESADT are warranted.
    03/2014;
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    ABSTRACT: Despite the great interest organic spintronics has recently attracted, there is only a partial understanding of the fundamental physics behind electron spin relaxation in organic semiconductors. Mechanisms based on hyperfine interaction have been demonstrated, but the role of the spin-orbit interaction remains elusive. Here, we report muon spin spectroscopy and time-resolved photoluminescence measurements on two series of molecular semiconductors in which the strength of the spin-orbit interaction has been systematically modified with a targeted chemical substitution of different atoms at a particular molecular site. We find that the spin-orbit interaction is a significant source of electron spin relaxation in these materials.
    Physical Review Letters 05/2013; 110(21):216602. · 7.73 Impact Factor
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    ABSTRACT: The rich phase behavior of 5,11-bis(triethylsilylethynyl) anthradithiophene (TES ADT) – one of the most promising, solution-processable small-molecular organic semiconductors – is analyzed, revealing the highest performing polymorph among four solid-state phases, opening pathways toward the reliable fabrication of high-performance bottom-gate/bottom-contact transistors.
    Chemistry of Materials. 04/2013; 25(9):1823–1828.
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    ABSTRACT: Trialkylgermyl functionalization allows development of high-performance soluble small-molecule organic semiconductors with mobilities greater than 5 cm(2) V(-1) s(-1) . Spray-deposited organic thin-film transistors show a record mobility of 2.2 cm(2) V(-1) s(-1) and demonstrate the potential for incorporation in large-area, low-cost electronic applications.
    Advanced Materials 04/2013; · 14.83 Impact Factor
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    ABSTRACT: We utilize Atomic Force Microscopy (AFM) and Kelvin Probe Force Microscopy (KPFM) to characterize the dynamics of electronic transport across 2,8-difluoro-5,11-triethylsilylethynyl anthradithiophene (diF TESADT) grain boundaries. We show that the morphology of grain boundaries and the adsorption of atmospheric dopants at these local boundaries have a direct impact on the electrical behavior of diF TESADT in thin film transistor (TFT) devices. Device voltage drops at grain boundaries are characterized as a function of both atmospheric dopants and transition time between dopants. The morphology, including crystallization and packing motifs, of diF TESADT grown on thermally grown SiO2 will be discussed and related to other semiconducting small organic molecules. This work will be put in the context of other, recent advances in small molecule organics.
    03/2013;
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    ABSTRACT: Organic thin film transistors (OTFT) partially composed of solution processed 2,8-difluoro-5,11-bis(triethylsilylethynyl)-anthradithiophene (diF-TES-ADT) have shown high performance with hole mobilities up to 1 cm^2/(V s). Pretreatment of the gold electrodes results in growth of large diF-TES-ADT crystals extending well out into the channel of the OTFT. Without electrode pretreatment, the crystal sizes are small and possess a non-preferred molecular orientation. We have chosen to investigate the reasons for the reduced crystal size of these films on untreated gold electrodes by studying a model system generated by vapor deposition of multilayers of diF-TES-ADT on Au(111). The initial wetting layer forms a highly ordered film such that the anthradithiophene backbone is oriented parallel to the substrate and the unit cell is 1.49 nm x 1.25 nm with an included angle of 56.8^o. The second layer is poorly ordered with only weak evidence of crystallinity in small regions. Growth beyond the second layer appears essentially bulk-like and crystalline with domain sizes that are potentially limited by the disordered bilayer growth.
    03/2013;
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    ABSTRACT: We utilize scanning tunneling microscopy (STM) to characterize the initial growth and crystallization of the high-performance, small organic molecule 2,8-difluoro-5,11-triethylsilylethynyl (diF TESADT) on Au(111). Two ordered structures are observed with diF TESADT backbone planes parallel to the substrate. Submolecular resolution imaging of the first monolayer ordered film regions realizes structures with close approach of fluorine-sulfur and fluorine-fluorine atoms of alternating molecules. These measurements provide evidence for the importance of non-covalent F-S and F-F interactions in driving 2D self-assembly. Scanning Tunneling Spectroscopy indicates a 2.4 eV transport gap which is insensitive to the local domain. Structures and growth are put in context of bulk measurements and device performance measurements.
    03/2013;
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    ABSTRACT: The nanoscale boundaries formed when neighboring spherulites impinge in polycrystalline, solution-processed organic semiconductor thin films act as bottlenecks to charge transport, significantly reducing organic thin-film transistor mobility in devices comprising spherulitic thin films as the active layers. These interspherulite boundaries (ISBs) are structurally complex, with varying angles of molecular orientation mismatch along their lengths. We have successfully engineered exclusively low- and exclusively high-angle ISBs to elucidate how the angle of molecular orientation mismatch at ISBs affects their resistivities in triethylsilylethynyl anthradithiophene thin films. Conductive AFM and four-probe measurements reveal that current flow is unaffected by the presence of low-angle ISBs, whereas current flow is significantly disrupted across high-angle ISBs. In the latter case, we estimate the resistivity to be 22 MΩμm(2)/width of the ISB, only less than a quarter of the resistivity measured across low-angle grain boundaries in thermally evaporated sexithiophene thin films. This discrepancy in resistivities across ISBs in solution-processed organic semiconductor thin films and grain boundaries in thermally evaporated organic semiconductor thin films likely arises from inherent differences in the nature of film formation in the respective systems.
    ACS Nano 09/2012; · 12.03 Impact Factor
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    ABSTRACT: Scanning tunneling microscopy studies of the first monolayer of 2,8-difluoro-5,11-(bis)triethylsilylethynyl anthradithiophene on Au(111) reveal two ordered structures with anthradithiophene planes parallel to the substrate. Submolecular resolution STM images demonstrate structures with a close approach of fluorine–sulfur and fluorine–fluorine atoms in the ordered structures. This provides evidence for the importance of noncovalent F–S and F–F in driving 2D self-assembly in the monolayer. Spectroscopic studies indicate a transport gap of 2.4 eV that is insensitive to the local domain structures, as expected for weak intermolecular interactions.
    The Journal of Physical Chemistry C. 09/2012; 116(40):21465–21471.
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    ABSTRACT: Patterning organic thin-film transistors (OTFTs) is critical in achieving high electronic performance and low power consumption. We report on a high-yield, low-complexity patterning method based on exploiting the strong tendency of halogen-substituted organic semiconductors to crystallize along chemically tailored interfaces. We demonstrate that the organic semiconductor molecules self-align on the contacts, when the halogen–halogen interaction is allowed by the chemical structures and conformations of the self-assembled monolayer and organic semiconductor. The ordered films exhibit high mobilities and constrain the current paths. The regions surrounding the devices, where the interaction is inhibited, consist of randomly oriented molecules, exhibiting high-resistivity and electrically insulating neighboring devices. To identify the role of F–F interactions in the development of crystalline order, we investigate OTFTs fabricated on mono-fluorinated benzene thiol treated contacts, which allows us to isolate the interactions between the F originating from the organic semiconductor and the F in each position on the benzene ring of the thiol, and to selectively study the role of each interaction. Combining the results obtained from quantitative grazing incidence X-ray diffraction and Kelvin probe measurements, we show that the surface treatments induce structural changes in the films, but also alter the injection picture as a result of work function shifts that they introduce. We show that both effects yield variations in the field-effect transistor characteristics, and we are able to tune the field-effect mobility more than two orders of magnitude in the same material.
    Journal of Materials Chemistry 08/2012; 22(36):19047-19053. · 5.97 Impact Factor
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    ABSTRACT: Heterogeneous nucleation is often the precursor to crystallization in solution-processed organic semiconductor thin films. Here, we study the efficacy of a series of nine small-molecule organic semiconductor additives in seeding the crystallization of solution-processable triethylsilylethynyl anthradithiophene (TES ADT). By systematically varying the concentrations of the additives in TES ADT thin films, we found the tendency of the additives to crystallize, their solubility in the casting solvent, and their similarity in chemical structure to TES ADT, to determine the nucleation and resulting density of nuclei. Tracking the crystallization process further yields information about the mechanism of nucleation. While pure TES ADT nucleates instantaneously at the onset of crystallization, nucleation transitions to a distributed process occurring throughout crystallization with the incorporation of increasing amounts of additives.
    Chemistry of Materials. 07/2012; 24(15):2920–2928.
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    ABSTRACT: Herein, time-dependent scanning Kelvin probe microscopy of solution processed organic thin film transistors (OTFTs) reveals a correlation between film microstructure and OTFT device performance with the location of trapped charge within the device channel. The accumulation of the observed trapped charge is concurrent with the decrease in ISD during operation (VG = −40 V, VSD = −10 V). We discuss the charge trapping and dissipation dynamics as they relate to the film structure and show that application of light quickly dissipates the observed trapped charge.
    Applied Physics Letters 06/2012; 5(6). · 3.79 Impact Factor
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    ABSTRACT: The growth rate of spherulites in solution-processed, triethylsilylethynyl anthra-dithiophene thin films can vary by a factor of three depending on the surface energy of the underlying substrate. By selectively patterning the underlying substrate to have regions of different surface energies, Y.-L. Loo and co-workers show on page 2692 how they are able to guide crystallization along pre-specified, nonlinear patterns over large areas in the plane of the film.
    Advanced Materials 05/2012; 24(20):2691. · 14.83 Impact Factor
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    ABSTRACT: Control over the molecular orientation in organic thin films is demonstrated with precise in-plane spatial resolution over large areas. By exploiting the differential crystallization rates on substrates with different surface energies, the radial symmetry of spherulitic growth can be disrupted by preferentially selecting the molecular orientations that promote growth along the paths of the underlying patterns.
    Advanced Materials 04/2012; 24(20):2692-8. · 14.83 Impact Factor
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    ABSTRACT: Due to the rapidity of morphological development during deposition, solution-processed organic semiconductor thin films exist in semicrystalline or polycrystalline states, incorporating a high degree of local variations in molecular orientation compared to their single-crystal counterparts. Spherulites, a common crystalline superstructure found in these systems, for example, incorporate a large distribution of molecular orientations about the radial axis to maintain their space-filling growth habit. Here, we aim to determine how this distribution of molecular orientations influences charge transport by fabricating arrays of devices on single spherulites. Given that the orientation distribution that is present about the radial axis mandates the presence of low-angle grain boundaries within single spherulites, we find intraspherulitic charge transport to be independent of the general direction of π-stacking; organic field-effect transistors exhibit comparable mobilities regardless of how their channels are oriented with respect to the general π-stacking direction.
    Journal of the American Chemical Society 02/2012; 134(12):5436-9. · 10.68 Impact Factor
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    ABSTRACT: We utilize conductive atomic force microscopy (C-AFM) and tunneling atomic force microscopy (TUNA) to characterize dynamics of electronic transport across fluorinated triethylsilylethynyl anthradithiophene (diF-TES ADT) grain boundaries. The crystallization of diF-TES ADT grown on SiO2 will be discussed and related to comparable molecules. The resulting voltage drop between individual crystals as a function of dopants will be discussed in terms of charge transport models and compared to current device work.
    02/2012;
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    ABSTRACT: Over the past two decades organic molecules have shown increasing promise as active layers in electrical devices such as field effect transistors, organic light emitting diodes, and organic photovoltaic devices. The suitability of organic molecules for use in these devices is governed by several properties, chief among them being the ability to self-organize into a film showing high carrier mobility. Organic thin film transistors (OTFT) partially composed of solution processed 2,8-difluoro-5,11-bis(triethylsilylethynyl)-anthradithiophene have shown high performance. To date these OTFTs have been constructed solely by solution processing. As such, we have chosen to investigate the possibility of vapor deposition as an alternative. Our investigation of the viability of vapor deposition of this promising molecule begins with deposition on Ag(111) as a model system. Preliminary STM results will be presented and discussed.
    02/2012;
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    ABSTRACT: We report on organic thin-film transistors fabricated on a novel soluble small molecule organic semiconductor difluoro bis(triethylgermyl) anthradithiophene. Fabrication techniques are all applicable at room temperature and ambient pressure, and include drop-casting, spin-coating, and spray deposition. Devices exhibit remarkable electronic properties, including charge carrier mobilities as high as 3.5 cm^2/Vs, on/off current ratios of 10^5, and good environmental and operational stability. Chemical treatment of the contact surface with self-assembled monolayers allows us great control of the crystalline order within the organic semiconductor layer. Because thin-film microstructure defects such as grain boundaries reduce the charge transport capabilities of the active layer, high quality single crystals are grown by physical vapor transport for comparison. By correlating the electrical properties with the structural data obtained from X-ray diffraction, we find that a good π-π overlap is responsible for this superior electronic behavior.
    02/2012;
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    ABSTRACT: The preparation of uniform large-area highly crystalline organic semiconductor thin films that show outstanding carrier mobilities remains a challenge in the field of organic electronics, including organic field-effect transistors. Quantitative control over the drying speed during dip-coating permits optimization of the organic semiconductor film formation, although the kinetics of crystallization at the air–solution–substrate contact line are still not well understood. Here, we report the facile one-step growth of self-aligning, highly crystalline soluble acene crystal arrays that exhibit excellent field-effect mobilities (up to 1.5 cm V^−1s^−1) via an optimized dip-coating process. We discover that optimized acene crystals grew at a particular substrate lifting-rate in the presence of low boiling point solvents, such as dichloromethane (b.p. of 40.0 °C) or chloroform (b.p. of 60.4 °C). Variable-temperature dip-coating experiments using various solvents and lift rates are performed to elucidate the crystallization behavior. This bottom-up study of soluble acene crystal growth during dip-coating provides conditions under which one may obtain uniform organic semiconductor crystal arrays with high crystallinity and mobilities over large substrate areas, regardless of the substrate geometry (wafer substrates or cylinder-shaped substrates).
    Advanced Functional Materials 01/2012; 22:1005. · 10.44 Impact Factor
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    ABSTRACT: We report on physical mechanisms behind the temperature-dependent optical absorption, photoluminescence (PL), and photoconductivity in spin-coated films of a functionalized anthradithiophene (ADT) derivative, ADT-triethylsilylethynyl (TES)-F, and its composites with C60 and another ADT derivative, ADT-TIPS-CN. Measurements of absorption and PL spectra, PL lifetimes, and transient photocurrent were performed at temperatures between 98 and 300 K as a function of applied electric field. In pristine ADT-TES-F films, absorptive and emissive species were identified to be disordered H aggregates whose properties are affected by static and dynamic disorder. The exciton bandwidths were ≤0.06 and ∼0.115 eV for absorptive and emissive aggregates, respectively, indicative of higher disorder in the emissive species. The exciton in the latter was found to be delocalized over approximately four to five molecules. The PL properties were significantly modified upon adding a guest molecule to the ADT-TES-F host. In ADT-TES-F/C60 composites, the PL was considerably quenched due to photoinduced electron transfer from ADT-TES-F to C60, while in ADT-TES-F/ADT-TIPS-CN blends, the PL was dominated by emission from an exciplex formed between ADT-TES-F and ADT-TIPS-CN molecules. In all materials, the PL quantum yield dramatically decreased as the temperature increased due to thermally activated nonradiative recombination. Considerable electric-field-induced PL quenching was observed at low temperatures at electric fields above ∼105 V/cm due to tunneling into dark states. No significant contribution of ADT-TES-F emissive exciton dissociation to transient photocurrent was observed. In all materials, charge carriers were photogenerated at sub-500-ps time scales, limited by the laser pulse width, with temperature- and electric-field-independent photogeneration efficiency. In ADT-TES-F/C60 (2%) composites, the photogeneration efficiency was a factor of 2–3 higher than that in pristine ADT-TES-F films. In ADT-TES-F/ADT-TIPS-CN (2%) blends, an additional charge carrier photogeneration component was observed at room temperature at time scales of ∼20 ns due to exciplex dissociation. At ∼0.5–5 ns after photoexcitation, the carriers propagated via thermally and electric-field-activated hopping with an activation energy of ∼0.025 eV. At time scales longer than ∼5 ns, charge transport of carriers that are not frozen in traps proceeded through tunneling via isoenergetic sites.
    Physical review. B, Condensed matter 12/2011; 84(23). · 3.77 Impact Factor

Publication Stats

32 Citations
124.75 Total Impact Points

Institutions

  • 2013
    • Wake Forest University
      • Department of Physics
      Winston-Salem, NC, United States
  • 2010–2013
    • University of Kentucky
      • Department of Chemistry
      Lexington, Kentucky, United States
  • 2012
    • Princeton University
      • Department of Chemical and Biological Engineering
      Princeton, NJ, United States
  • 2010–2011
    • Oregon State University
      • Department of Physics
      Corvallis, OR, United States