@article{nevola_hoffman_bataller_ade_gundogdu_dougherty_2019, title={Rigid valence band shift due to molecular surface counter-doping of MoS2}, volume={679}, ISSN={["1879-2758"]}, DOI={10.1016/j.susc.2018.09.016}, abstractNote={Adsorption of the acceptor material tetracyanoquinodimethane can control optoelectronic properties of MoS2 by accepting defect generated excess negative charge from the surface that would otherwise interfere with radiative decay processes. Angle Resolved Photoelectron Spectroscopy measurements show that the MoS2 band structure near the Γ point shifts rigidly upward by ∼0.2 eV for a complete surface coverage of acceptor species as expected for an upward Fermi level shift due to charge transfer to the TCNQ. The molecular adsorbate orbitals visible in photoemission are indicative of an anionic species, consistent with interfacial charge transfer but without evidence for hybrid states arising from covalent adsorbate-surface interactions. Thus, our interface studies support the notion that molecular adsorbates are a useful tool for controlling optoelectronic functionality in 2D materials without fundamentally modifying their favorable band structures.}, journal={SURFACE SCIENCE}, author={Nevola, D. and Hoffman, B. C. and Bataller, A. and Ade, H. and Gundogdu, K. and Dougherty, D. B.}, year={2019}, month={Jan}, pages={254–258} }
 @article{hoffman_pazoki_apperson_dougherty_2019, title={Spatially Uniform Shallow Trap Distribution in an Ultrathin Organic Transistor}, volume={13}, ISSN={["1862-6270"]}, DOI={10.1002/pssr.201800486}, abstractNote={In organic electronic materials, charge carrier transport is often limited by disorder‐induced trap states very close in energy to the ideal band transport states. We directly view the location and impact of these “shallow” traps on an ultrathin transistor active layer using Kelvin Probe Force Microscopy. As the transistor turns on, dramatic fluctuations in the surface potential of the active channel suddenly arise due to charge trapping and release processes. Importantly, the spatial distribution of rapid fluctuations in surface potential is uniform throughout the active channel. These facts strongly constrain the microscopic origin of shallow charge traps, and associated efforts to optimize the mobility and noise performance baseline in device applications.}, number={5}, journal={PHYSICA STATUS SOLIDI-RAPID RESEARCH LETTERS}, author={Hoffman, Benjamin C. and Pazoki, Sara and Apperson, Aubrey and Dougherty, Daniel B.}, year={2019}, month={May} }
 @article{hoffman_mcafee_pazoki_apperson_brendan t. o'connor_dougherty_2017, title={Temperature controlled interlayer disorder in ultrathin films of alpha-sexithiophene}, volume={642}, ISSN={["0040-6090"]}, DOI={10.1016/j.tsf.2017.09.011}, abstractNote={The surface potential difference between the first and second layer of α-sexithiophene (6T) films on silane passivated SiO2 is observed using Kelvin Probe Force Microscopy. The relative surface potential between the first two layers changes sign depending on whether the films are grown at 70 °C or 120 °C substrate temperature. Complementary wide angle X-ray scattering observations show that this difference can be interpreted as the result of a higher concentration of interlayer hole traps due to poor out-of-plane ordering in films deposited at lower temperature. Molecular sliding and shifts in tilt angle are proposed as the microscopic origin of out-of-plane disorder leading to trapped charges.}, journal={THIN SOLID FILMS}, author={Hoffman, Benjamin C. and Mcafee, Terry and Pazoki, Sara and Apperson, Aubrey and Brendan T. O'Connor and Dougherty, Daniel B.}, year={2017}, month={Nov}, pages={182–187} }
 @article{hoffman_mcafee_conrad_loth_anthony_ade_dougherty_2016, title={Intrinsic Charge Trapping Observed as Surface Potential Variations in diF-TES-ADT Films}, volume={8}, ISSN={["1944-8244"]}, DOI={10.1021/acsami.6b03886}, abstractNote={Spatial variations in surface potential are measured with Kelvin probe force microscopy for thin films of 2,8-difluoro-5,11-bis(triethylsilylethynyl)anthradithiophenes (diF-TES-ADT) grown on SiO2 and silane-treated SiO2 substrates by organic molecular beam deposition. The variations are observed both between and within grains of the polycrystalline organic film and are quantitatively different than electrostatic variations on the substrate surfaces. The skewness of surface potential distributions is larger on SiO2 than on HMDS-treated substrates. This observation is attributed to the impact of substrate functionalization on minimizing intrinsic crystallographic defects in the organic film that can trap charge.}, number={33}, journal={ACS APPLIED MATERIALS & INTERFACES}, author={Hoffman, Benjamin C. and McAfee, Terry and Conrad, Brad R. and Loth, Marsha A. and Anthony, John E. and Ade, Harald W. and Dougherty, Daniel B.}, year={2016}, month={Aug}, pages={21490–21496} }
 @article{bondarev_popescu_younts_hoffman_mcafee_dougherty_gundogdu_ade_2016, title={Lowest energy Frenkel and charge transfer exciton intermixing in one-dimensional copper phthalocyanine molecular lattice}, volume={109}, ISSN={["1077-3118"]}, DOI={10.1063/1.4968821}, abstractNote={We report the results of the combined experimental and theoretical studies of the low-lying exciton states in crystalline copper phthalocyanine. We derive the eigen energy spectrum for the two lowest intramolecular Frenkel excitons coupled to the intermolecular charge transfer exciton state and compare it with temperature dependent optical absorption spectra measured experimentally, to obtain the parameters of the Frenkel-charge-transfer exciton intermixing. The two Frenkel exciton states are spaced apart by 0.26 eV, and the charge transfer exciton state is 50 meV above the lowest Frenkel exciton. Both Frenkel excitons are strongly mixed with the charge transfer exciton, showing the coupling constant 0.17 eV which agrees with earlier experimental measurements. These results can be used for the proper interpretation of the physical properties of crystalline phthalocyanines.}, number={21}, journal={APPLIED PHYSICS LETTERS}, author={Bondarev, I. V. and Popescu, A. and Younts, R. A. and Hoffman, B. and McAfee, T. and Dougherty, D. B. and Gundogdu, K. and Ade, H. W.}, year={2016}, month={Nov} }
 @article{mcafee_hoffman_you_atkin_ade_dougherty_2016, title={Morphological, Optical, and Electronic Consequences of Coexisting Crystal Orientations in beta-Copper Phthalocyanine Thin Films}, volume={120}, ISSN={["1932-7455"]}, DOI={10.1021/acs.jpcc.6b05043}, abstractNote={The crystal structure of the β phase of copper phthalocyanine (CuPc) is a monoclinic herringbone, which is commonly created in thin films by either thermal annealing of α-CuPc at ∼300 °C or deposition on heated substrates. Of the several known CuPc crystal polymorphs, the β phase is of particular interest due to its thermodynamic stability. We observe three coexisting crystal orientations for thin films of β-CuPc to be (101), (105), and (502) using grazing incidence wide-angle X-ray scattering. Each of the three crystal orientations have distinct surface roughness, as measured by atomic force microscopy, but are electrostatically similar by Kelvin probe force microscopy. However, local optical properties, as measured by micro-UV–vis spectroscopy, are very different in the different domains.}, number={33}, journal={JOURNAL OF PHYSICAL CHEMISTRY C}, author={McAfee, Terry and Hoffman, Benjamin C. and You, Xiao and Atkin, Joanna M. and Ade, Harald and Dougherty, Daniel B.}, year={2016}, month={Aug}, pages={18616–18621} }
 @article{scott_xue_wang_kline_hoffman_dougherty_zhou_bazan_brendan t. o'connor_2016, title={Significantly Increasing the Ductility of High Performance Polymer Semiconductors through Polymer Blending}, volume={8}, ISSN={["1944-8244"]}, DOI={10.1021/acsami.6b01852}, abstractNote={Polymer semiconductors based on donor-acceptor monomers have recently resulted in significant gains in field effect mobility in organic thin film transistors (OTFTs). These polymers incorporate fused aromatic rings and have been designed to have stiff planar backbones, resulting in strong intermolecular interactions, which subsequently result in stiff and brittle films. The complex synthesis typically required for these materials may also result in increased production costs. Thus, the development of methods to improve mechanical plasticity while lowering material consumption during fabrication will significantly improve opportunities for adoption in flexible and stretchable electronics. To achieve these goals, we consider blending a brittle donor-acceptor polymer, poly[4-(4,4-dihexadecyl-4H-cyclopenta[1,2-b:5,4-b']dithiophen-2-yl)-alt-[1,2,5]thiadiazolo[3,4-c]pyridine] (PCDTPT), with ductile poly(3-hexylthiophene). We found that the ductility of the blend films is significantly improved compared to that of neat PCDTPT films, and when the blend film is employed in an OTFT, the performance is largely maintained. The ability to maintain charge transport character is due to vertical segregation within the blend, while the improved ductility is due to intermixing of the polymers throughout the film thickness. Importantly, the application of large strains to the ductile films is shown to orient both polymers, which further increases charge carrier mobility. These results highlight a processing approach to achieve high performance polymer OTFTs that are electrically and mechanically optimized.}, number={22}, journal={ACS APPLIED MATERIALS & INTERFACES}, author={Scott, Joshua I. and Xue, Xiao and Wang, Ming and Kline, R. Joseph and Hoffman, Benjamin C. and Dougherty, Daniel and Zhou, Chuanzhen and Bazan, Guillermo and Brendan T. O'Connor}, year={2016}, month={Jun}, pages={14037–14045} }