Publications by authors named "Alejandro L Briseño"

71 Publications

Orienting and shaping organic semiconductor single crystals through selective nanoconfinement.

Soft Matter 2021 Jan 8. Epub 2021 Jan 8.

Department of Chemical Engineering and Materials Science, Stevens Institute of Technology, Hoboken, NJ 07030, USA.

For organic semiconductor crystals exhibiting anisotropic charge transport along different crystallographic directions, nanoconfinement is a powerful strategy to control crystal orientation by aligning the fast crystallographic growth direction(s) with the unconfined axis(es) of nanoconfining scaffolds. Here, design rules are presented to relate crystal morphology, scaffold geometry, and orientation control in solution-processed small-molecule crystals. Specifically, organic semiconductor triisopropylsilylethynyl pyranthrene needle-like crystals with a dimensionality of n = 1 and perylene platelike crystals with n = 2 were grown from solution within nanoconfining scaffolds comprising cylindrical nanopores with a dimensionality of m = 1, representing one unconfined dimension along the cylinder axis, and those comprising nanopillar arrays with a dimensionality of m = 2. For m = n systems, native crystal growth habits were preserved while the crystal orientation in n = m direction(s) was dictated by the geometry of the scaffold. For n ≠ m systems, on the other hand, orientation control was restricted within a single plane, either parallel or perpendicular to the substrate surface. Intriguingly, control over crystal shape was also observed for perylene crystals grown in cylindrical nanopores (n > m). Within the nanopores, crystal growth was restricted along a single direction to form a needle-like morphology. Once growth proceeded above the scaffold surface, the crystals adopted their native growth habit to form asymmetric T-shaped single crystals with concave corners. These findings suggest that nanoporous scaffolds with spatially-varying dimensionalities can be used to grow single crystals of complex shapes.
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http://dx.doi.org/10.1039/d0sm01928cDOI Listing
January 2021

Vibrational Excitation Mechanism in Tunneling Spectroscopy beyond the Franck-Condon Model.

Phys Rev Lett 2020 Mar;124(11):116804

Fachbereich Physik, Freie Universität Berlin, Arnimallee 14, 14195 Berlin, Germany.

Vibronic spectra of molecules are typically described within the Franck-Condon model. Here, we show that highly resolved vibronic spectra of large organic molecules on a single layer of MoS_{2} on Au(111) show spatial variations in their intensities, which cannot be captured within this picture. We explain that vibrationally mediated perturbations of the molecular wave functions need to be included into the Franck-Condon model. Our simple model calculations reproduce the experimental spectra at arbitrary position of the scanning tunneling microscope's tip over the molecule in great detail.
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http://dx.doi.org/10.1103/PhysRevLett.124.116804DOI Listing
March 2020

A Large Anisotropic Enhancement of the Charge Carrier Mobility of Flexible Organic Transistors with Strain: A Hall Effect and Raman Study.

Adv Sci (Weinh) 2020 Jan 13;7(1):1901824. Epub 2019 Nov 13.

Department of Physics Rutgers University Piscataway NJ 08854 USA.

Utilizing the intrinsic mobility-strain relationship in semiconductors is critical for enabling strain engineering applications in high-performance flexible electronics. Here, measurements of Hall effect and Raman spectra of an organic semiconductor as a function of uniaxial mechanical strain are reported. This study reveals a very strong, anisotropic, and reversible modulation of the intrinsic (trap-free) charge carrier mobility of single-crystal rubrene transistors with strain, showing that the effective mobility of organic circuits can be enhanced by up to 100% with only 1% of compressive strain. Consistently, Raman spectroscopy reveals a systematic shift of the low-frequency Raman modes of rubrene to higher (lower) frequencies with compressive (tensile) strain, which is indicative of a reduction (enhancement) of thermal molecular disorder in the crystal with strain. This study lays the foundation of the strain engineering in organic electronics and advances the knowledge of the relationship between the carrier mobility, low-frequency vibrational modes, strain, and molecular disorder in organic semiconductors.
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http://dx.doi.org/10.1002/advs.201901824DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6947506PMC
January 2020

Short Excited-State Lifetimes Enable Photo-Oxidatively Stable Rubrene Derivatives.

J Phys Chem A 2019 Sep 26;123(35):7558-7566. Epub 2019 Aug 26.

Department of Polymer Science and Engineering, University of Massachusetts, 120 Governors Drive, Amherst, Massachusetts 01003, United States.

A series of rubrene derivatives were synthesized and the influence of the side group in enhancing photo-oxidative stability was evaluated. Photo-oxidation half-lives were determined via UV-vis absorption spectroscopy, which revealed thiophene containing derivatives to be the most stable species. The electron affinity of the compounds did not correlate with stability as previously reported in literature. Our work shows that shorter excited-state lifetimes result in increased photo-oxidative stability in these rubrene derivatives. These results confirm that faster relaxation kinetics out-compete the formation of reactive oxygen species that ultimately degrade linear oligoacenes. This report highlights the importance of using molecular design to tune excited-state lifetimes in order to generate more stable oligoacenes.
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http://dx.doi.org/10.1021/acs.jpca.9b04203DOI Listing
September 2019

Three-Dimensional CeO Woodpile Nanostructures To Enhance Performance of Enzymatic Glucose Biosensors.

ACS Appl Mater Interfaces 2019 Jan 4;11(2):1821-1828. Epub 2019 Jan 4.

Department of Polymer Science and Engineering , University of Massachusetts Amherst , Amherst , Massachusetts 01003 , United States.

Fabrication of detection elements with ultrahigh surface area is essential for improving the sensitivity of analyte detection. Here, we report a direct patterning technique to fabricate three-dimensional CeO nanoelectrode arrays for biosensor application over relatively large areas. The fabrication approach, which employs nanoimprint lithography and a CeO nanoparticle-based ink, enables the direct, high-throughput patterning of nanostructures and is scalable, integrable, and of low cost. With the convenience of sequential imprinting, multilayered woodpile nanostructures with prescribed numbers of layers were achieved in a "stacked-up" architecture and were successfully fabricated over large areas. To demonstrate application as a biosensor, an enzymatic glucose sensor was developed. The sensitivity of glucose sensors can be enhanced simply by increasing the number of layers, which multiplies surface area while maintaining a constant footprint. The four-layer woodpile nanostructure of CeO glucose sensor exhibited enhanced sensitivity (42.8 μA mM cm) and good selectivity. This direct imprinting strategy for three-dimensional sensing architectures is potentially extendable to other electroactive materials and other sensing applications.
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http://dx.doi.org/10.1021/acsami.8b16985DOI Listing
January 2019

High-Resolution Vibronic Spectra of Molecules on Molybdenum Disulfide Allow for Rotamer Identification.

ACS Nano 2018 Nov 5;12(11):11698-11703. Epub 2018 Nov 5.

Fachbereich Physik , Freie Universität Berlin , Arnimallee 14 , 14195 Berlin , Germany.

Tunneling spectroscopy is an important tool for the chemical identification of single molecules on surfaces. Here, we show that oligothiophene-based large organic molecules which only differ by single bond orientations can be distinguished by their vibronic fingerprint. These molecules were deposited on a monolayer of the transition metal dichalcogenide molybdenum disulfide (MoS) on top of a Au(111) substrate. MoS features an electronic band gap for efficient decoupling of the molecular states. Furthermore, it exhibits a small electron-phonon coupling strength. Both of these material properties allow for the resolution of vibronic states in the range of the limit set by temperature broadening in our scanning tunneling microscope at 4.6 K. Using DFT calculations of the molecule in gas phase provides all details for an accurate simulation of the vibronic spectra of both rotamers.
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http://dx.doi.org/10.1021/acsnano.8b07414DOI Listing
November 2018

Efficient Electron Mobility in an All-Acceptor Napthalenediimide-Bithiazole Polymer Semiconductor with Large Backbone Torsion.

ACS Appl Mater Interfaces 2018 Nov 12;10(46):40070-40077. Epub 2018 Nov 12.

Department of Polymer Science and Engineering , University of Massachusetts , 120 Governors Drive , Amherst , Massachusetts 01003 , United States.

An all-acceptor napthalenediimide-bithiazole-based co-polymer, P(NDI2OD-BiTz), was synthesized and characterized for application in thin-film transistors. Density functional theory calculations point to an optimal perpendicular dihedral angle of 90° between acceptor units along isolated polymer chains; yet optimized transistors yield electron mobility of 0.11 cm/(V s) with the use of a zwitterionic naphthalene diimide interlayer. Grazing incidence X-ray diffraction measurements of annealed films reveal that P(NDI2OD-BiTz) adopts a highly ordered edge-on orientation, exactly opposite to similar bithiophene analogs. This report highlights an NDI and thiazole all-acceptor polymer and demonstrates high electron mobility despite its nonplanar backbone conformation.
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http://dx.doi.org/10.1021/acsami.8b11234DOI Listing
November 2018

Enhanced Device Efficiency and Long-Term Stability via Boronic Acid-Based Self-Assembled Monolayer Modification of Indium Tin Oxide in a Planar Perovskite Solar Cell.

ACS Appl Mater Interfaces 2018 Sep 22;10(35):30000-30007. Epub 2018 Aug 22.

Polymer Science and Engineering , University of Massachusetts Amherst , Amherst , Massachusetts 01003 , United States.

Interfacial engineering is essential for the development of highly efficient and stable solar cells through minimizing energetic losses at interfaces. Self-assembled monolayers (SAMs) have been shown as a handle to tune the work function (WF) of indium tin oxide (ITO), improving photovoltaic cell performance and device stability. In this study, we utilize a new class of boronic acid-based fluorine-terminated SAMs to modify ITO surfaces in planar perovskite solar cells. The SAM treatment demonstrates an increase of the WF of ITO, an enhancement of the short-circuit current, and a passivation of trap states at the ITO/[poly(3,4ethylenedioxylenethiophene):poly(styrenesulfonic acid)] interface. Device stability improves upon SAM modification, with efficiency decreasing only 20% after one month. Our work highlights a simple treatment route to achieve hysteresis-free, reproducible, stable, and highly efficient (16%) planar perovskite solar cells.
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http://dx.doi.org/10.1021/acsami.8b10445DOI Listing
September 2018

Gecko-Inspired Biocidal Organic Nanocrystals Initiated from a Pencil-Drawn Graphite Template.

Sci Rep 2018 08 2;8(1):11618. Epub 2018 Aug 2.

Department of Chemical Engineering, University of Massachusetts Amherst, Amherst, Massachusetts, 01003-9303, USA.

The biocidal properties of gecko skin and cicada wings have inspired the synthesis of synthetic surfaces decorated with high aspect ratio nanostructures that inactivate microorganisms. Here, we investigate the bactericidal activity of oriented zinc phthalocyanine (ZnPc) nanopillars grown using a simple pencil-drawn graphite templating technique. By varying the evaporation time, nanopillars initiated from graphite that was scribbled using a pencil onto silicon substrates were optimized to yield a high inactivation of the Gram-negative bacteria, Escherichia coli. We next adapted the procedure so that analogous nanopillars could be grown from pencil-drawn graphite scribbled onto stainless steel, flexible polyimide foil, and glass substrates. Time-dependent bacterial cytotoxicity studies indicate that the oriented nanopillars grown on all four substrates inactivated up to 97% of the E. coli quickly, in 15 min or less. These results suggest that organic nanostructures, which can be easily grown on a broad range of substrates hold potential as a new class of biocidal surfaces that kill microbes quickly and potentially, without spreading antibiotic-resistance genes.
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http://dx.doi.org/10.1038/s41598-018-29994-3DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6072760PMC
August 2018

Correlating Crystal Thickness, Surface Morphology, and Charge Transport in Pristine and Doped Rubrene Single Crystals.

ACS Appl Mater Interfaces 2018 Aug 30;10(31):26745-26751. Epub 2018 Jul 30.

Polymer Science and Engineering , University of Massachusetts Amherst , Amherst , Massachusetts 01003 , United States.

The relationship between charge transport and surface morphology is investigated by utilizing rubrene single crystals of varying thicknesses. In the case of pristine crystals, the surface conductivities decrease exponentially as the crystal thickness increases until ∼4 μm, beyond which the surface conductivity saturates. Investigation of the surface morphology using optical and atomic force microscopy reveals that thicker crystals have a higher number of molecular steps, increasing the overall surface roughness compared with thin crystals. The density of molecular steps as a surface trap is further quantified with the subthreshold slope of rubrene air-gap transistors. This thickness-dependent surface conductivity is rationalized by a shift from in-plane to out-of-plane transport governed by surface roughness. The surface transport is disrupted by roughening of the crystal surface and becomes limited by the slower vertical crystallographic axis on molecular step edges. Separately, we investigate surface-doping of rubrene crystals by using fluoroalkyltrichrolosilane and observe a different mechanism for charge transport which is independent of surface roughness. This work demonstrates that the correlation between crystal thickness, surface morphology, and charge transport must be taken into account when measuring organic single crystals. Considering the fact that these molecular steps are universally observed on organic/inorganic and single/polycrystals, we believe that our findings can be widely applied to improve charge transport understanding.
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http://dx.doi.org/10.1021/acsami.8b04451DOI Listing
August 2018

Phase Transition of Graphene-Templated Vertical Zinc Phthalocyanine Nanopillars.

J Am Chem Soc 2018 07 22;140(26):8185-8191. Epub 2018 Jun 22.

Department of Polymer Science and Engineering , University of Massachusetts Amherst , Amherst , Massachusetts 01003 , United States.

We report on the graphene-assisted growth, crystallization, and phase transition of zinc phthalocyanine (ZnPc) vertically oriented single crystal nanopillars. Postcrystallization thermal annealing of the nanostructures results in a molecular packing change while maintaining the vertical orientation of the single crystals orthogonal to the underlying substrate. Grazing incidence X-ray diffraction and high-resolution TEM studies characterized this phase transition from a metastable crystal phase to the more stable β-phase commonly observed in bulk crystals. These vertical arrays of crystalline nanopillars exhibit a high-surface-to-volume ratio, which is advantageous for applications such as gas sensors. We fabricated chemiresistor sensors with ZnPc nanopillars grown on graphene and demonstrated its selectivity for ammonia vapors, and improvement in sensitivity in the β-phase crystal packing pillars due to their molecular orientation increasing the exposure of the Zn ion to the ammonia analyte. This work highlights the first morphology-retentive phase transition in organic single crystal nanopillars through simple postprocessing thermal annealing. This study opens up the possibility of molecular packing control without large variations in morphology, a necessity for high-performance devices and establishing structure-property relations.
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http://dx.doi.org/10.1021/jacs.8b03078DOI Listing
July 2018

Direct Printing of Graphene Electrodes for High-Performance Organic Inverters.

ACS Appl Mater Interfaces 2018 May 26;10(18):15988-15995. Epub 2018 Apr 26.

Department of Polymer Science and Engineering , University of Massachusetts , Amherst , Massachusetts 01003 , United States.

Scalable fabrication of high-resolution electrodes and interconnects is necessary to enable advanced, high-performance, printed, and flexible electronics. Here, we demonstrate the direct printing of graphene patterns with feature widths from 300 μm to ∼310 nm by liquid-bridge-mediated nanotransfer molding. This solution-based technique enables residue-free printing of graphene patterns on a variety of substrates with surface energies between ∼43 and 73 mN m. Using printed graphene source and drain electrodes, high-performance organic field-effect transistors (OFETs) are fabricated with single-crystal rubrene (p-type) and fluorocarbon-substituted dicyanoperylene-3,4:9,10-bis(dicarboximide) (PDIF-CN) (n-type) semiconductors. Measured mobilities range from 2.1 to 0.2 cm V s for rubrene and from 0.6 to 0.1 cm V s for PDIF-CN. Complementary inverter circuits are fabricated from these single-crystal OFETs with gains as high as ∼50. Finally, these high-resolution graphene patterns are compatible with scalable processing, offering compelling opportunities for inexpensive printed electronics with increased performance and integration density.
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http://dx.doi.org/10.1021/acsami.8b01302DOI Listing
May 2018

Polarization-Dependent Photoinduced Bias-Stress Effect in Single-Crystal Organic Field-Effect Transistors.

ACS Appl Mater Interfaces 2017 Oct 21;9(39):34153-34161. Epub 2017 Sep 21.

National University of Science and Technology MISiS , Moscow 119049, Russia.

Photoinduced charge transfer between semiconductors and gate dielectrics can occur in organic field-effect transistors (OFETs) operating under illumination, leading to a pronounced bias-stress effect in devices that are normally stable while operating in the dark. Here, we report an observation of a polarization-dependent photoinduced bias-stress effect in two prototypical single-crystal OFETs, based on rubrene and tetraphenylbis(indolo{1,2-a})quinolin. We find that the decay rate of the source-drain current in these OFETs under illumination is a periodic function of the polarization angle of incident photoexcitation with respect to the crystal axes, with a periodicity of π. The angular positions of maxima and minima of the bias-stress rate match those of the optical absorption coefficient of the corresponding crystals. The analysis of the effect shows that it stems from a charge transfer of "hot" holes, photogenerated in the crystal within a very short thermalization length (≪μm) from the semiconductor-dielectric interface. The observed phenomenon is a type of intrinsic structure-property relationship, revealing how molecular packing affects parameter drift in organic transistors under illumination. We also demonstrate that a photoinduced charge transfer in OFETs can be used for recording rewritable accumulation channels with an optically defined geometry and resolution, which can be used in a number of potential applications.
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http://dx.doi.org/10.1021/acsami.7b11134DOI Listing
October 2017

Tunable fluorescence quenching near the graphene-aqueous interface.

J Colloid Interface Sci 2017 Nov 6;506:76-82. Epub 2017 Jul 6.

Department of Polymer Science and Engineering, University of Massachusetts, Amherst, MA 01003, United States. Electronic address:

With increased interest in graphene-based sensors for biomolecules and other targets, we investigated the impact of ionic strength on the steady-state emissions from fluorescein labels on proteins adsorbing on pristine CVD (chemical vapor deposited)-graphene on a silica support. Using the model system of fluorescein-tagged fibrinogen we demonstrated that, for fluorescein tags on adsorbed fibrinogen, emission intensity was very sensitive to the salt concentration. This behavior was not seen for fluorescein-tagged fibrinogen in solution. We demonstrated that fluorescein "quenching" in this system was a result of fluorescein's pH sensitivity: with changes in salt concentration, near-surface fluorescein experiences either the neutral bulk pH or, with a negatively charged surface, an acidic environment. The findings carry the important implication that the aqueous environment near silica-supported graphene is substantially acidic as a result of near-surface negative charge. This further implies, because of the purity of the graphene in this study and its lack of oxidation, that negative charge arises from ion adsorption and/or from the underlying silica support, which may be hydrated and present dissociated surface silanols. That is, the electrostatic potential from silica beneath the graphene may pass through the graphene, much as van der Waals interactions have been proven to do. Results were semi-quantitatively consistent with calculations that employed a Guoy Chapman model of the interface and the established pKa of the fluorescein. While these findings were obtained with adsorbed proteins, similar fluorescence quenching would be expected for any fluorescein-tagged species in the vicinity of silica-supported graphene. Thus, because of the negative charge at the aqueous graphene interface, ionic strength can be exploited as means of creating a molecular ruler of fluorescein emissions, and the emissions can be assessed within different distances, corresponding to the Debye length, from the graphene interface.
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http://dx.doi.org/10.1016/j.jcis.2017.07.019DOI Listing
November 2017

Dinaphthotetrathiafulvalene Bisimides: A New Member of the Family of π-Extended TTF Stable p-Type Semiconductors.

Chemistry 2017 Oct 27;23(60):15002-15007. Epub 2017 Jul 27.

Graduate School of Materials Science, Nara Institute of Science and Technology (NAIST), 8916-5 Takayama-cho, Ikoma, Nara, 630-0192, Japan.

Air-stable organic semiconductors based on tetrathiafuluvalene (TTF) were developed by synthesising a series of dinaphthotetrathiafulvalene bisimides (DNTTF-Im) using electron-donating TTF, π-extended naphthalene, and electron-withdrawing imide. Electron-spin-resonance spectroscopy and X-ray single-crystal structure analysis of aryl-substituted DNTTF-Im radical cations confirmed that localisation of the spin resides on the electron-donating TTF moiety. The organic field-effect transistor properties derived from the use of highly crystalline n-butyl (C4) and n-hexyl(C6)-substituted DNTTF-Im were assessed. The hole carrier mobility of C6-DNTTF-Im was improved from 3.7×10  cm  V  s to 0.30 cm  V  s in ambient conditions. This is attributed to the raise of the substrate temperature from 25 °C to 200 °C during sublimation. The XRD and microscopy analysis suggested that increasing the substrate temperature accelerates the end-on packing resulting in larger grains suitable for hole charge transport parallel to the substrate.
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http://dx.doi.org/10.1002/chem.201702657DOI Listing
October 2017

Poly[2,5-bis(3-dodecylthiophen-2-yl)thieno[3,2-b]thiophene] Oligomer Single-Crystal Nanowires from Supercritical Solution and Their Anisotropic Exciton Dynamics.

J Phys Chem Lett 2017 Jul 16;8(13):2984-2989. Epub 2017 Jun 16.

Department of Physics, University of Massachusetts Amherst , Amherst, Massachusetts 01003, United States.

Supercritical fluids, exhibiting a combination of liquid-like solvation power and gas-like diffusivity, are a relatively unexplored medium for processing and crystallization of oligomer and polymeric semiconductors whose optoelectronic properties critically depend on the microstructure. Here we report oligomer crystallization from the polymer organic semiconductor, poly[2,5-bis(3-dodecylthiophen-2-yl)thieno[3,2-b]thiophene] (PBTTT) in supercritical hexane, yielding needle-like single crystals up to several microns in length. We characterize the crystals' photophysical properties by time- and polarization-resolved photoluminescence (TPRPL) spectroscopy. These techniques reveal two-dimensional interchromophore coupling facilitated by the high degree of π-stacking order within the crystal. Furthermore, the crystals obtained from supercritical fluid were found to be similar photophysically as the crystallites found in solution-cast thin films and distinct from solution-grown crystals that exhibited spectroscopic signatures indicative of different packing geometries.
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http://dx.doi.org/10.1021/acs.jpclett.7b01128DOI Listing
July 2017

Homoepitaxy of Crystalline Rubrene Thin Films.

Nano Lett 2017 05 12;17(5):3040-3046. Epub 2017 Apr 12.

Department of Electrical Engineering, Princeton University , Princeton, New Jersey 08544 United States.

The smooth surface of crystalline rubrene films formed through an abrupt heating process provides a valuable platform to study organic homoepitaxy. By varying growth rate and substrate temperature, we are able to manipulate the onset of a transition from layer-by-layer to island growth modes, while the crystalline thin films maintain a remarkably smooth surface (less than 2.3 nm root-mean-square roughness) even with thick (80 nm) adlayers. We also uncover evidence of point and line defect formation in these films, indicating that homoepitaxy under our conditions is not at equilibrium or strain-free. Point defects that are resolved as screw dislocations can be eliminated under closer-to-equilibrium conditions, whereas we are not able to eliminate the formation of line defects within our experimental constraints at adlayer thicknesses above ∼25 nm. We are, however, able to eliminate these line defects by growing on a bulk single crystal of rubrene, indicating that the line defects are a result of strain built into the thin film template. We utilize electron backscatter diffraction, which is a first for organics, to investigate the origin of these line defects and find that they preferentially occur parallel to the (002) plane, which is in agreement with expectations based on calculated surface energies of various rubrene crystal facets. By combining the benefits of crystallinity, low surface roughness, and thickness-tunability, this system provides an important study of attributes valuable to high-performance organic electronic devices.
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http://dx.doi.org/10.1021/acs.nanolett.7b00380DOI Listing
May 2017

The Structural Origin of Electron Injection Enhancements with Fulleropyrrolidine Interlayers.

Adv Mater Interfaces 2016 May 25;3(10). Epub 2016 Feb 25.

Department of Polymer Science and Engineering, University of Massachusetts, 120 Governors Drive, Amherst, Massachusetts 01003, United States of America.

The orientation of the substituent groups in a new class of work function modification layers, based on functionalized fulleropyrrolidines, is measured and found to directly account for the sign of the work function change.
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http://dx.doi.org/10.1002/admi.201500852DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5259752PMC
May 2016

Impact of morphology on polaron delocalization in a semicrystalline conjugated polymer.

Phys Chem Chem Phys 2017 Feb;19(5):3627-3639

Freie Universität Berlin, Berlin Joint EPR Lab, Institut für Experimentalphysik, Berlin, Germany.

We investigate the delocalization of holes in the semicrystalline conjugated polymer poly(2,5-bis(3-alkylthiophene-2-yl)thieno[3,2-b]thiophene) (PBTTT) by directly measuring the hyperfine coupling between photogenerated polarons and bound nuclear spins using electron nuclear double resonance spectroscopy. An extrapolation of the corresponding oligomer spectra reveals that charges tend to delocalize over 4.0-4.8 nm with delocalization strongly dependent on molecular order and crystallinity of the PBTTT polymer thin films. Density functional theory calculations of hyperfine couplings confirm that long-range corrected functionals appropriately describe the change in coupling strength with increasing oligomer size and agree well with the experimentally measured polymer limit. Our discussion presents general guidelines illustrating the various pitfalls and opportunities when deducing polaron localization lengths from hyperfine coupling spectra of conjugated polymers.
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http://dx.doi.org/10.1039/c6cp07485eDOI Listing
February 2017

Evidence for negative charge near large area supported graphene in water: A study of silica microsphere interactions.

J Colloid Interface Sci 2017 04 2;492:15-24. Epub 2016 Dec 2.

Department of Polymer Science and Engineering, UMass Amherst, 120 Governors Drive, Amherst, MA 01003, United States. Electronic address:

This study addresses the electrostatic and van der Waals interactions at the aqueous interface of large area CVD graphene, 1-3 layers thick on a silica support and assessed by Raman spectroscopy to have exclusive sp2 character. Ionic strength was found to substantially alter the interactions of silica microspheres with silica-supported graphene. Particles were nonadhesive at large Debye lengths but became irreversibly adherent at reduced Debye lengths about 2nm or less. This was demonstrated to be qualitatively parallel to the influence of ionic strength on silica-silica interactions. The observed ionic strength effects are best explained by negative charges in the vicinity, within a few nanometers, of the supported graphene. DLVO-based modeling of the silica-water-supported graphene interaction suggests that van der Waals interactions drive particle capture and that the surface potential at the supported graphene surface is at least -10 to -15mV (corresponding to a charge density of 0.02-0.06/nm). This charge could result from ion adsorption or from charges on silica beneath the graphene. The conclusions are not substantially affected by inclusion of nanometer-scale interfacial roughness in the modeling.
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http://dx.doi.org/10.1016/j.jcis.2016.12.002DOI Listing
April 2017

Graphene Ink as a Conductive Templating Interlayer for Enhanced Charge Transport of C-Based Devices.

ACS Appl Mater Interfaces 2016 Nov 20;8(43):29594-29599. Epub 2016 Oct 20.

Department of Polymer Science and Engineering, University of Massachusetts , Amherst, Massachusetts 01003, United States.

We demonstrate conductive templating interlayers of graphene ink, integrating the electronic and chemical properties of graphene in a solution-based process relevant for scalable manufacturing. Thin films of graphene ink are coated onto ITO, following thermal annealing, to form a percolating network used as interlayer. We employ a benchmark n-type semiconductor, C, to study the interface of the active layer/interlayer. On bare ITO, C molecules form films of homogeneously distributed grains; with a graphene interlayer, a preferential orientation of C molecules is observed in the individual graphene plates. This leads to crystal growth favoring enhanced charge transport. We fabricate devices to characterize the electron injection and the effect of graphene on the device performance. We observe a significant increase in the current density with the interlayer. Current densities as high as ∼1 mA/cm and ∼70 mA/cm are realized for C deposited with the substrate at 25 °C and 150 °C, respectively.
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http://dx.doi.org/10.1021/acsami.6b05536DOI Listing
November 2016

Surface Grafting of Functionalized Poly(thiophene)s Using Thiol-Ene Click Chemistry for Thin Film Stabilization.

ACS Appl Mater Interfaces 2016 Nov 31;8(44):30543-30551. Epub 2016 Oct 31.

Conte Polymer Center for Polymer Research, University of Massachusetts-Amherst , 120 Governors Drive, Amherst, Massachusetts 01003, United States.

Regioregular poly[(3-hexylthiophene)-ran-(3-undecenylthiophene)] (pP3HT) and vinyl terminated poly(3-hexylthiophene) (xP3HT) were synthesized by the McCullough method and surface grafted to thiol modified silicon dioxide wafers using thiol-ene click chemistry. Utilizing this method, semiconducting, solvent impervious films were easily generated. Thiol-ene click chemistry is convenient for film stabilization in electronics because it does not produce side products that could be inimical to charge transport in the active layer. It was found through grazing incidence wide-angle X-ray scattering (GIWAXS) that there is no change in microstructure between as-spun films and thiol-ene grafted films, while there was a change after the thiol-ene grafted film was exposed to solvent. Organic field-effect transistors (oFETs) were fabricated from grafted films that had been swelled with chloroform, and these devices had mobilities on the order of 10 cm V s, which are consistent with poly(thiophene) monolayer devices.
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http://dx.doi.org/10.1021/acsami.6b08667DOI Listing
November 2016

Controlling Chain Conformations of High-k Fluoropolymer Dielectrics to Enhance Charge Mobilities in Rubrene Single-Crystal Field-Effect Transistors.

Adv Mater 2016 Dec 26;28(45):10095-10102. Epub 2016 Sep 26.

Department of Chemical Engineering and Materials Research Institute, 106 Fenske Laboratory, The Pennsylvania State University, University Park, PA, 16802, USA.

A novel photopatternable high-k fluoropolymer, poly(vinylidene fluoride-bromotrifluoroethylene) P(VDF-BTFE), with a dielectric constant (k) between 8 and 11 is demonstrated in thin-film transistors. Crosslinking P(VDF-BTFE) reduces energetic disorder at the dielectric-semiconductor interface by controlling the chain conformations of P(VDF-BTFE), thereby leading to approximately a threefold enhancement in the charge mobility of rubrene single-crystal field-effect transistors.
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http://dx.doi.org/10.1002/adma.201602873DOI Listing
December 2016

Orthogonal Ambipolar Semiconductor Nanostructures for Complementary Logic Gates.

ACS Nano 2016 09 22;10(9):8610-9. Epub 2016 Aug 22.

Department of Polymer Science and Engineering, University of Massachusetts , Amherst, Massachusetts 01003, United States.

We report orthogonal ambipolar semiconductors that exhibit hole and electron transport in perpendicular directions based on aligned films of nanocrystalline "shish-kebabs" containing poly(3-hexylthiophene) (P3HT) and N,N'-di-n-octyl-3,4,9,10-perylenetetracarboxylic diimide (PDI) as p- and n-type components, respectively. Polarized optical microscopy, scanning electron microscopy, and X-ray diffraction measurements reveal a high degree of in-plane alignment. Relying on the orientation of interdigitated electrodes to enable efficient charge transport from either the respective p- or n-channel materials, we demonstrate semiconductor films with high anisotropy in the sign of charge carriers. Films of these aligned crystalline semiconductors were used to fabricate complementary inverter devices, which exhibited good switching behavior and a high noise margin of 80% of 1/2 Vdd. Moreover, complementary "NAND" and "NOR" logic gates were fabricated and found to exhibit excellent voltage transfer characteristics and low static power consumption. The ability to optimize the performance of these devices, simply by adjusting the solution concentrations of P3HT and PDI, makes this a simple and versatile method for preparing ambipolar organic semiconductor devices and high-performance logic gates. Further, we demonstrate that this method can also be applied to mixtures of PDI with another conjugated polymer, poly[2,5-bis(3-tetradecylthiophen-2-yl)thieno[3,2-b]thiophene]) (PBTTT), with better hole transport characteristics than P3HT, opening the door to orthogonal ambipolar semiconductors with higher performance.
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http://dx.doi.org/10.1021/acsnano.6b03942DOI Listing
September 2016

Real-space visualization of conformation-independent oligothiophene electronic structure.

J Chem Phys 2016 May;144(19):194703

Department of Chemistry and Biochemistry, Materials Science Institute, Oregon Center for Optical, Molecular and Quantum Science, University of Oregon, 1253 University of Oregon, Eugene, Oregon 97403, USA.

We present scanning tunneling microscopy and spectroscopy (STM/STS) investigations of the electronic structures of different alkyl-substituted oligothiophenes on the Au(111) surface. STM imaging showed that on Au(111), oligothiophenes adopted distinct straight and bent conformations. By combining STS maps with STM images, we visualize, in real space, particle-in-a-box-like oligothiophene molecular orbitals. We demonstrate that different planar conformers with significant geometrical distortions of oligothiophene backbones surprisingly exhibit very similar electronic structures, indicating a low degree of conformation-induced electronic disorder. The agreement of these results with gas-phase density functional theory calculations implies that the oligothiophene interaction with the Au(111) surface is generally insensitive to molecular conformation.
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http://dx.doi.org/10.1063/1.4949765DOI Listing
May 2016

TiO2/BiVO4 Nanowire Heterostructure Photoanodes Based on Type II Band Alignment.

ACS Cent Sci 2016 Feb 3;2(2):80-8. Epub 2016 Feb 3.

Department of Chemical Engineering and Department of Chemistry, University of California, Berkeley, California 94720, United States; Materials Sciences Division and Kavli Energy NanoSciences Institute, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States; Materials Sciences Division and Kavli Energy NanoSciences Institute, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States.

Metal oxides that absorb visible light are attractive for use as photoanodes in photoelectrosynthetic cells. However, their performance is often limited by poor charge carrier transport. We show that this problem can be addressed by using separate materials for light absorption and carrier transport. Here, we report a Ta:TiO2|BiVO4 nanowire photoanode, in which BiVO4 acts as a visible light-absorber and Ta:TiO2 acts as a high surface area electron conductor. Electrochemical and spectroscopic measurements provide experimental evidence for the type II band alignment necessary for favorable electron transfer from BiVO4 to TiO2. The host-guest nanowire architecture presented here allows for simultaneously high light absorption and carrier collection efficiency, with an onset of anodic photocurrent near 0.2 V vs RHE, and a photocurrent density of 2.1 mA/cm(2) at 1.23 V vs RHE.
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http://dx.doi.org/10.1021/acscentsci.5b00402DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4827543PMC
February 2016

Controlled Directional Crystallization of Oligothiophenes Using Zone Annealing of Preseeded Thin Films.

ACS Appl Mater Interfaces 2015 Oct 6;7(41):23008-14. Epub 2015 Oct 6.

Department of Polymer Engineering, University of Akron , Akron, Ohio 44325, United States.

We demonstrate a simple route to directionally grow crystals of oligothiophenes, based on 2,5-bis(3-alkylthiophen-2-yl)thieno[3,2-b]thiophene with degrees of polymerization of 2 (BTTT-2) and 4 (BTTT-4) via zone annealing (ZA) of preseeded films. ZA of spun-cast films of BTTT-2 does not yield highly aligned crystals. However, if the film is oven-annealed briefly prior to ZA, highly aligned crystals that are millimeters in length can be grown, whose length depends on the velocity of the ZA front. The precrystallized region provides existing nuclei that promote crystal growth and limit nucleation of new crystals in the melted region. Aligned crystals of BTTT-2 can be obtained even when the moving velocity for ZA is an order of magnitude greater than the crystal growth rate. The relative nucleation rate to the crystallization rate for BTTT-4 is greater than that for BTTT-2, which decreases the length over which BTTT-4 can be aligned to ∼500 μm for the conditions examined. The temperature gradient and moving velocity of ZA enable control of the length of the aligned crystalline structure at the macroscale.
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http://dx.doi.org/10.1021/acsami.5b06344DOI Listing
October 2015

Water Processable Polythiophene Nanowires by Photo-Cross-Linking and Click-Functionalization.

Nano Lett 2015 Sep 20;15(9):5689-95. Epub 2015 Aug 20.

Department of Polymer Science and Engineering, University of Massachusetts , Amherst, Massachusetts 01003, United States.

Replacing or minimizing the use of halogenated organic solvents in the processing and manufacturing of conjugated polymer-based organic electronics has emerged as an important issue due to concerns regarding toxicity, environmental impact, and high cost. To date, however, the processing of well-ordered conjugated polymer nanostructures has been difficult to achieve using environmentally benign solvents. In this work, we report the development of water and alcohol processable nanowires (NWs) with well-defined crystalline nanostructure based on the solution assembly of azide functionalized poly(3-hexylthiophene) (P3HT-azide) and subsequent photo-cross-linking and functionalization of these NWs. The solution-assembled P3HT-azide NWs were successfully cross-linked by exposure to UV light, yielding good thermal and chemical stability. Residual azide units on the photo-cross-linked NWs were then functionalized with alkyne terminated polyethylene glycol (PEG-alkyne) using copper catalyzed azide-alkyne cycloaddition chemistry. PEG functionalization of the cross-linked P3HT-azide NWs allowed for stable dispersion in alcohols and water, while maintaining well-ordered NW structures with electronic properties suitable for the fabrication of organic field effect transistors (OFETs).
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http://dx.doi.org/10.1021/acs.nanolett.5b01185DOI Listing
September 2015