@article{frick_sridhar_khansari_comstock_norman_o'donnell_maggard_sun_dougherty_2023, title={Spreading resistance effects in tunneling spectroscopy of α-RuCl3 and Ir0.5Ru0.5Cl3}, volume={108}, ISSN={["2469-9969"]}, url={https://doi.org/10.1103/PhysRevB.108.245410}, DOI={10.1103/PhysRevB.108.245410}, abstractNote={The Mott insulating state is the progenitor of many interesting quantum phases of matter including the famous high-temperature superconductors and quantum spin liquids. A recent candidate for novel spin liquid phenomena is $\ensuremath{\alpha}\text{\ensuremath{-}}{\mathrm{RuCl}}_{3}$, a layered honeycomb Mott insulator whose electronic structure has been a source of mystery. In particular, scanning tunneling spectroscopy has indicated a Mott gap in $\ensuremath{\alpha}\text{\ensuremath{-}}{\mathrm{RuCl}}_{3}$ that is much lower than the 2-eV value observed in photoemission measurements. Here, we show that the origin of this discrepancy is a spreading resistance artifact associated with tunneling into highly resistive materials by comparing with prior experiments and numerical modeling. A similar phenomenon is also observed in a substitutional alloy, ${\mathrm{Ir}}_{0.5}{\mathrm{Ru}}_{0.5}{\mathrm{Cl}}_{3}$, that has a higher resistivity than the parent compound. While the tunneling measurements cannot be used to accurately measure the sample density of states for these materials, we can take advantage of the spreading resistance sensitivity to quantify the anisotropic resistivity of these layered materials and connect to previous macroscopic transport observations.}, number={24}, journal={PHYSICAL REVIEW B}, author={Frick, Jordan R. and Sridhar, Samanvitha and Khansari, Ario and Comstock, Andrew H. and Norman, Elizabeth and O'Donnell, Shaun and Maggard, Paul A. and Sun, Dali and Dougherty, Daniel B.}, year={2023}, month={Dec} }
 @article{worku_ben-akacha_sridhar_frick_yin_he_robb_chaaban_liu_winfred_et al._2021, title={Band Edge Control of Quasi-2D Metal Halide Perovskites for Blue Light-Emitting Diodes with Enhanced Performance}, volume={8}, ISSN={["1616-3028"]}, DOI={10.1002/adfm.202103299}, abstractNote={Perovskite light‐emitting diodes (PeLEDs) have received great attention for their potential as next‐generation display technology. While remarkable progress has been achieved in green, red, and near‐infrared PeLEDs with external quantum efficiencies (EQEs) exceeding 20%, obtaining high performance blue PeLEDs remains a challenge. Poor charge balance due to large charge injection barriers in blue PeLEDs has been identified as one of the major roadblocks to achieve high efficiency. Here band edge control of perovskite emitting layers for blue PeLEDs with enhanced charge balance and device performance is reported. By using organic spacer cations with different dipole moments, that is, phenethyl ammonium (PEA), methoxy phenethyl ammonium (MePEA), and 4‐fluoro phenethyl ammonium (4FPEA), the band edges of quasi‐2D perovskites are tuned without affecting their band gaps. Detailed characterization and computational studies have confirmed the effect of dipole moment modification to be mostly electrostatic, resulting in changes in the ionization energies of ≈0.45 eV for MePEA and ≈ −0.65 eV for 4FPEA based thin films relative to PEA‐based thin films. With improved charge balance, blue PeLEDs based on MePEA quasi‐2D perovskites show twofold increase of the EQE as compared to the control PEA based devices.}, journal={ADVANCED FUNCTIONAL MATERIALS}, author={Worku, Michael and Ben-Akacha, Azza and Sridhar, Samanvitha and Frick, Jordan R. and Yin, Shichen and He, Qingquan and Robb, Alex J. and Chaaban, Maya and Liu, He and Winfred, J. S. Raaj Vellore and et al.}, year={2021}, month={Aug} }
 @article{pazoki_frick_dougherty_2021, title={Dynamics of domain boundaries at metal-organic interfaces}, volume={154}, ISSN={["1089-7690"]}, DOI={10.1063/5.0029313}, abstractNote={Domain boundaries are a determining factor in the performance of organic electronic devices since they can trap mobile charge carriers. We point out the possibility of time-dependent motion of these boundaries and suggest that their thermal fluctuations can be a source of dynamic disorder in organic films. In particular, we study the C8-BTBT monolayer films with several different domain boundaries. After characterizing the crystallography and diversity of structures in the first layer of C8-BTBT on Au(111), we focus on quantifying the domain boundary fluctuations in the saturated monolayer. We find that the mean squared displacement of the boundary position grows linearly with time at early times but tends to saturate after about 7 s. This behavior is ascribed to confined diffusion of the interface position based on fits and numerical integration of a Langevin equation for the interface motion.}, number={12}, journal={JOURNAL OF CHEMICAL PHYSICS}, author={Pazoki, Sara and Frick, Jordan and Dougherty, Daniel B.}, year={2021}, month={Mar} }
 @article{nevola_bataller_kumar_sridhar_frick_o'donnell_ade_maggard_kemper_gundogdu_et al._2021, title={Timescales of excited state relaxation in alpha-RuCl3 observed by time-resolved two-photon photoemission spectroscopy}, volume={103}, ISSN={["2469-9969"]}, url={https://doi.org/10.1103/PhysRevB.103.245105}, DOI={10.1103/PhysRevB.103.245105}, abstractNote={The nonequilibrium properties of strongly correlated materials present a target in the search for new phases of matter. It is important to observe the types of excitations that exist in these materials and their associated relaxation dynamics. We have studied the photoexcitations in a spin-orbit assisted Mott insulator $\ensuremath{\alpha}\text{\ensuremath{-}}\mathrm{Ru}{\mathrm{Cl}}_{3}$ using time-resolved two-photon photoemission spectroscopy and transient reflection spectroscopy. We find that photoexcited carriers (doublons) in the upper Hubbard band rapidly relax to Mott-Hubbard excitons on a timescale of less than 200 fs. Subsequently, further relaxation of these lower-energy quasiparticles occurs with an energy-dependent time constant of that ranges from 370 to 600 fs due to exciton cooling. The population of Mott-Hubbard excitons persists for timescales up to several microseconds.}, number={24}, journal={PHYSICAL REVIEW B}, author={Nevola, Dan and Bataller, Alexander and Kumar, Ankit and Sridhar, Samanvitha and Frick, Jordan and O'Donnell, Shaun and Ade, Harald and Maggard, Paul A. and Kemper, Alexander F. and Gundogdu, Kenan and et al.}, year={2021}, month={Jun} }
 @article{frick_sridhar_o'donnell_maggard_dougherty_2020, title={An interface-controlled Mott memristor in α-RuCl3}, volume={116}, ISSN={0003-6951 1077-3118}, url={http://dx.doi.org/10.1063/5.0009670}, DOI={10.1063/5.0009670}, abstractNote={Memristor devices have history-dependent charge transport properties that are ideal for neuromorphic computing applications. We reveal a memristor material and mechanism in the layered Mott insulator α-RuCl3. The pinched hysteresis loops and S-shaped negative differential resistance in bulk crystals verify memristor behavior and are attributed to a nonlinear coupling between charge injection over a Schottky barrier at the electrical contacts and concurrent Joule heating. Direct simulations of this coupling can reproduce the device characteristics.}, number={18}, journal={Applied Physics Letters}, publisher={AIP Publishing}, author={Frick, Jordan R. and Sridhar, Samanvitha and O'Donnell, Shaun and Maggard, Paul A. and Dougherty, Daniel B.}, year={2020}, month={May}, pages={183501} }
 @article{wang_wang_dougherty_2020, title={Direct molecular quantification of electronic disorder in N,N '-Di-[(1-naphthyl)-N,N '-diphenyl]-1,1 '-biphenyl)-4,4 '-diamine on Au(111)}, volume={38}, ISSN={["2166-2754"]}, DOI={10.1116/6.0000401}, abstractNote={Organic light-emitting diodes are important in display applications, but thin films used in these devices often exhibit complex and highly disordered structures. We have studied the adsorption of a typical hole transport material used in such devices, N,N′-Di-[(1-naphthyl)-N,N′-diphenyl]-1,1′-biphenyl)-4,4′-diamine (α-NPD), on the Au(111) surface. Scanning tunneling microscopy images reveal the appearance of different conformations in the first monolayer with submolecular resolution. Scanning tunneling spectra identify the highest occupied molecular orbital on several different adsorption structures. We directly compare the statistical distribution of this orbital energy between an ordered monolayer structure and a disordered bilayer structure of α-NPD on Au(111). The disordered structure exhibits a very broad distribution that is consistent with inferences from prior organic device studies and that we propose arises from minor conformational variations.}, number={5}, journal={JOURNAL OF VACUUM SCIENCE & TECHNOLOGY B}, author={Wang, Jiuyang and Wang, Jingying and Dougherty, Daniel B.}, year={2020}, month={Sep} }
 @article{al-tawhid_frick_dougherty_kumah_2019, title={Growth-temperature dependence of conductivity at the LaCrO3/SrTiO3 (001) interface}, volume={37}, ISSN={0734-2101 1520-8559}, url={http://dx.doi.org/10.1116/1.5085334}, DOI={10.1116/1.5085334}, abstractNote={The effect of growth conditions and postgrowth treatment on the structural and electronic properties of the polar/nonpolar LaCrO3/SrTiO3 (LCO/STO) interface has been investigated. Under low oxygen partial pressure, oxygen vacancies are formed in the STO substrate resulting in metallicity with a measured sheet carrier concentration of 1016cm−−2. Annealing postgrowth in flowing oxygen causes the sheet carrier concentration to saturate to 1013cm−−2, which is consistent with the reported values of a two-dimensional gas at other polar/STO interfaces. However, growth under nonreducing growth conditions leads to insulating behavior. High-resolution synchrotron x-ray-based structural determination of the atomic-scale structures of both metallic and insulating LCO/STO interfaces shows chemical intermixing and an interfacial lattice expansion.}, number={2}, journal={Journal of Vacuum Science & Technology A}, publisher={American Vacuum Society}, author={Al-Tawhid, Athby and Frick, Jordan R. and Dougherty, Daniel B. and Kumah, Divine P.}, year={2019}, month={Mar}, pages={021102} }
 @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{pazoki_dougherty_2019, title={Suppression of dynamic disorder in fullerenes at metal-organic interfaces}, volume={151}, ISSN={["1089-7690"]}, DOI={10.1063/1.5123739}, abstractNote={Organic semiconductors are prone to strong disorder effects that often exhibit significant dynamic characteristics. In this study, static disorder and dynamic disorder of fullerene molecules at the interface of Au(111) are directly distinguished using Scanning Tunneling Microscopy (STM) and Scanning Tunneling Spectroscopy. We consider an STM image consisting of hundreds of different molecules, as an ensemble which exhibits conductance fluctuations due to both dynamic disorder and static disorder. On the other hand, local conductance measurement of single molecules over time reflects only dynamic disorder. We demonstrate that dynamic disorder is always smaller than static disorder at the C60/Au(111) interface due to structural constraints of molecules at the interface. Dynamic disorder in our experiment is due to small librations of individual molecules that are restricted by the metal surface, while static disorder is related to different bonding orientations of the C60 cage that are frozen-in at room temperature. Our experimental results can be modeled with direct simulations of differential tunneling conductance.}, number={21}, journal={JOURNAL OF CHEMICAL PHYSICS}, author={Pazoki, Sara and Dougherty, Daniel B.}, year={2019}, month={Dec} }
 @article{abdurazakov_nevola_rustagi_freericks_dougherty_kemper_2018, title={Nonequilibrium electron dynamics in pump-probe spectroscopy: Role of excited phonon populations}, volume={98}, ISSN={["2469-9969"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85058282950&partnerID=MN8TOARS}, DOI={10.1103/PhysRevB.98.245110}, abstractNote={We study the role of excited phonon populations in the relaxation rates of nonequilibrium electrons using a nonequilibrium Green's function formalism. The transient modifications in the phononic properties are accounted for by self-consistently solving the Dyson equation for the electron and phonon Green's functions. The pump-induced changes manifest in both the electronic and phononic spectral functions. We find that the excited phonon populations suppress the decay rates of nonequilibrium electrons due to enhanced phonon absorption. The increased phonon occupation also sets the nonequilibrium decay rates and the equilibrium scattering rates apart. The decay rates are found to be time dependent, and this is illustrated in the experimentally observed population decay of photoexcited ${\mathrm{Bi}}_{1.5}{\mathrm{Sb}}_{0.5}{\mathrm{Te}}_{1.7}{\mathrm{Se}}_{1.3}$.}, number={24}, journal={PHYSICAL REVIEW B}, publisher={American Physical Society (APS)}, author={Abdurazakov, O. and Nevola, D. and Rustagi, A. and Freericks, J. K. and Dougherty, D. B. and Kemper, A. F.}, year={2018}, month={Dec} }
 @article{hou_powel_dougherty_sommer_maggard_2018, title={Tunable Optical and Photocatalytic Properties of Low-Dimensional Copper(I)-Iodide Hybrids Using Coordinating Organic Ligands}, volume={18}, ISSN={["1528-7505"]}, DOI={10.1021/acs.cgd.8b00788}, abstractNote={A family of copper(I)-iodide/organic hybrid compounds was investigated for the impact of coordinating organic ligands on their structures, as well as on their optical and photocatalytic properties. This included the synthesis of two new crystalline compounds, [(CuI)2(bpmd)] and [(CuI)2(bpp)] (bpmd = 2,2′-bipyrimidine, bpp = 2,3-bis(2-pyridyl)pyrazine), both of which consist of chain structures formed by (CuI)2 rhombus-shaped dimers that are further coordinated to the N-groups of the bridging organic ligands. To more broadly investigate structure–property relationships within this system, nine related copper(I)-iodide/organic hybrid compounds, that is, [(CuI)2Ln] (n = 1 or 2; L = 1,2-bis(4-pyridyl) ethylene (bpe); 2,2′-bipyrimidine (bpmd); 2,3-bis(2-pyridyl) pyrazine (bpp); 4,4′-bipyridine (44bpy); pyridazine (pdz); pyrimidine (pmd); pyrazine (pz); pyrazinamide (pza); quinoxaline (quin)) were also prepared in high purity containing extended (CuI)∞ chains or sheets coordinated to bridging or terminating org...}, number={9}, journal={CRYSTAL GROWTH & DESIGN}, author={Hou, Feier and Powel, Matthew and Dougherty, Daniel B. and Sommer, Roger D. and Maggard, Paul A.}, year={2018}, month={Sep}, pages={5406–5416} }
 @article{deloach_conrad_einstein_dougherty_2017, title={Coverage dependent molecular assembly of anthraquinone on Au(111)}, volume={147}, ISSN={["1089-7690"]}, DOI={10.1063/1.4999623}, abstractNote={A scanning tunneling microscopy study of anthraquinone (AQ) on the Au(111) surface shows that the molecules self-assemble into several structures depending on the local surface coverage. At high coverages, a close-packed saturated monolayer is observed, while at low coverages, mobile surface molecules coexist with stable chiral hexamer clusters. At intermediate coverages, a disordered 2D porous network interlinking close-packed islands is observed in contrast to the giant honeycomb networks observed for the same molecule on Cu(111). This difference verifies the predicted extreme sensitivity [J. Wyrick et al., Nano Lett. 11, 2944 (2011)] of the pore network to small changes in the surface electronic structure. Quantitative analysis of the 2D pore network reveals that the areas of the vacancy islands are distributed log-normally. Log-normal distributions are typically associated with the product of random variables (multiplicative noise), and we propose that the distribution of pore sizes for AQ on Au(111) originates from random linear rate constants for molecules to either desorb from the surface or detach from the region of a nucleated pore.}, number={18}, journal={JOURNAL OF CHEMICAL PHYSICS}, author={DeLoach, Andrew S. and Conrad, Brad R. and Einstein, T. L. and Dougherty, Daniel B.}, year={2017}, month={Nov} }
 @article{popescu_younts_hoffman_mcafee_dougherty_ade_gundogdu_bondarev_2017, title={Monitoring Charge Separation Processes in Quasi-One-Dimensional Organic Crystalline Structures}, volume={17}, ISSN={1530-6984 1530-6992}, url={http://dx.doi.org/10.1021/ACS.NANOLETT.7B02471}, DOI={10.1021/ACS.NANOLETT.7B02471}, abstractNote={We perform the transient absorption spectroscopy experiments to investigate the dynamics of the low-energy collective electron-hole excitations in α-copper phthalocyanine thin films. The results are interpreted in terms of the third-order nonlinear polarization response function. It is found that, initially excited in the molecular plane, the intramolecular Frenkel exciton polarization reorients with time to align along the molecular chain direction to form coupled Frenkel-charge-transfer exciton states, the eigenstates of the one-dimensional periodic molecular lattice. The process pinpoints the direction of the charge separation in α-copper phthalocyanine and similar organic molecular structures. Being able to observe and monitor such processes is important both for understanding the physical principles of organic thin film solar energy conversion device operation and for the development of organic optoelectronics in general.}, number={10}, journal={Nano Letters}, publisher={American Chemical Society (ACS)}, author={Popescu, Adrian and Younts, Robert A. and Hoffman, Benjamin and McAfee, Terry and Dougherty, Daniel B. and Ade, Harald W. and Gundogdu, Kenan and Bondarev, Igor V.}, year={2017}, month={Sep}, pages={6056–6061} }
 @article{hewitt_boltersdorf_maggard_dougherty_2017, title={Recovery of of the bulk-like electronic structure of manganese phthalocyanine beyond the first monolayer on Bi2Te3}, volume={662}, ISSN={["1879-2758"]}, DOI={10.1016/j.susc.2017.03.014}, abstractNote={The evolution of electronic structure of manganese phthalocyanine on Bi2Te3 shows a transition to a bulk-like aspect abruptly after completion of the first layer. This allows the inference that, in the first layer, there is charge transfer and electronic hybridization involving the occupied Mn-derived d orbitals of the molecule into the conduction band of the substrate. The charge transfer coupling is seen using angle-resolved ultraviolet photoelectron spectroscopy by monitoring the evolution of work function and band structure with increasing molecular film thickness. The electronic structure in the second layer is more bulk-like as indicated by the reappearance of well-known low energy d orbitals that were depopulated in the first layer. Scanning tunneling microscopy shows that the transition to bulk like behavior is also reflected in film structure as a transition from a unique disordered monolayer to a locally ordered and dense second layer. These observations are relevant to ongoing efforts to control topological insulator interfaces especially for spintronics applications.}, journal={SURFACE SCIENCE}, author={Hewitt, A. S. and Boltersdorf, J. and Maggard, P. A. and Dougherty, D. B.}, year={2017}, month={Aug}, pages={87–92} }
 @article{balar_xiong_ye_li_nevola_dougherty_hou_ade_o’connor_2017, title={Role of Polymer Segregation on the Mechanical Behavior of All-Polymer Solar Cell Active Layers}, volume={9}, ISSN={1944-8244 1944-8252}, url={http://dx.doi.org/10.1021/ACSAMI.7B13719}, DOI={10.1021/ACSAMI.7B13719}, abstractNote={An all-polymer bulk heterojunction (BHJ) active layer that removes the use of commonly used small molecule electron acceptors is a promising approach to improve the thermomechanical behavior of organic solar cells. However, there has been limited research on their mechanical properties. Here, we report on the mechanical behavior of high-performance blade-coated all-polymer BHJ films cast using eco-friendly solvents. The mechanical properties considered include the elastic modulus, crack onset strain, and cohesive fracture energy. We show that the mechanical behavior of the blend is largely unaffected by significant changes in the segregation characteristics of the polymers, which was varied systematically through solvent formulation. In comparison to a polymer:fullerene BHJ counterpart, the all-polymer films were found to have lower stiffness and increased ductility. Yet, the fracture energy of the all-polymer films is not significantly improved compared to that of the polymer:fullerene films. This study highlights that improved mechanical behavior of all-polymer systems cannot be assumed, and that details of the molecular structure, molecular weight, and film morphology play an important role in both the optoelectronic and mechanical properties. Furthermore, we show that simple composite modeling provides a predictive tool for the mechanical properties of the polymer blend films, providing a framework to guide future optimization of the mechanical behavior.}, number={50}, journal={ACS Applied Materials & Interfaces}, publisher={American Chemical Society (ACS)}, author={Balar, Nrup and Xiong, Yuan and Ye, Long and Li, Sunsun and Nevola, Daniel and Dougherty, Daniel B. and Hou, Jianhui and Ade, Harald and O’Connor, Brendan T.}, year={2017}, month={Dec}, pages={43886–43892} }
 @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{wang_deloach_jiang_papa_myahkostupov_castellano_liu_dougherty_2017, title={Tuning interfacial spin filters from metallic to resistive within a single organic semiconductor family}, volume={95}, ISSN={2469-9950 2469-9969}, url={http://dx.doi.org/10.1103/PhysRevB.95.241410}, DOI={10.1103/physrevb.95.241410}, abstractNote={Jingying Wang,1 Andrew Deloach,1 Wei Jiang,2 Christopher M. Papa,3 Mykhaylo Myahkostupov,3 Felix N. Castellano,3 Feng Liu,2 and Daniel B. Dougherty1,* 1Department of Physics, North Carolina State University, Raleigh, North Carolina 27695-8202, USA 2Department of Material Science and Engineering, University of Utah, Salt Lake City, Utah 84112, USA 3Department of Chemistry, North Carolina State University, Raleigh, North Carolina 27695-8204, USA (Received 6 January 2017; revised manuscript received 26 May 2017; published 30 June 2017)}, number={24}, journal={Physical Review B}, publisher={American Physical Society (APS)}, author={Wang, Jingying and Deloach, Andrew and Jiang, Wei and Papa, Christopher M. and Myahkostupov, Mykhaylo and Castellano, Felix N. and Liu, Feng and Dougherty, Daniel B.}, year={2017}, month={Jun} }
 @article{stuart_gray_nevola_su_sachet_ulrich_dougherty_2016, title={Back Cover: Magnetoelectric oxide films for spin manipulation in graphene (Phys. Status Solidi RRL 3/2016)}, volume={10}, ISSN={1862-6254}, url={http://dx.doi.org/10.1002/PSSR.201670718}, DOI={10.1002/PSSR.201670718}, abstractNote={No abstract is available for this article.}, number={3}, journal={physica status solidi (RRL) - Rapid Research Letters}, publisher={Wiley}, author={Stuart, S. C. and Gray, B. and Nevola, D. and Su, L. and Sachet, E. and Ulrich, M. and Dougherty, D. B.}, year={2016}, month={Mar} }
 @article{zoellner_stuart_chung_dougherty_jones_maggard_2016, title={CuNb1−xTaxO3 (x ≤ 0.25) solid solutions: impact of Ta(v) substitution and Cu(i) deficiency on their structure, photocatalytic, and photoelectrochemical properties}, volume={4}, ISSN={2050-7488 2050-7496}, url={http://dx.doi.org/10.1039/c5ta06609c}, DOI={10.1039/c5ta06609c}, abstractNote={Investigation of CuNb1−xTaxO3has led to new insights into the visible-light photocurrents and photocatalytic activities of solid solutions.}, number={8}, journal={Journal of Materials Chemistry A}, publisher={Royal Society of Chemistry (RSC)}, author={Zoellner, Brandon and Stuart, Sean and Chung, Ching-Chang and Dougherty, Daniel B. and Jones, Jacob L. and Maggard, Paul A.}, year={2016}, pages={3115–3126} }
 @article{mcafee_apperson_ade_dougherty_2016, title={Growth of thermally stable crystalline C60 films on flat-lying copper phthalocyanine}, volume={4}, ISSN={2050-7488 2050-7496}, url={http://dx.doi.org/10.1039/C5TA06820G}, DOI={10.1039/c5ta06820g}, abstractNote={We observe thermally stable growth of fcc(111) films of fullerene-C60 on top of crystalline, flat-lying, CuPc film structures on graphite using combined grazing incidence wide-angle X-ray scattering and atomic force microscopy.}, number={3}, journal={Journal of Materials Chemistry A}, publisher={Royal Society of Chemistry (RSC)}, author={McAfee, Terry and Apperson, Aubrey and Ade, Harald and Dougherty, Daniel B.}, year={2016}, pages={1028–1032} }
 @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{stuart_gray_nevola_su_sachet_ulrich_dougherty_2016, title={Magnetoelectric oxide films for spin manipulation in graphene}, volume={10}, ISSN={["1862-6270"]}, DOI={10.1002/pssr.201510433}, abstractNote={The challenge of creating a graphene spin field effect transistor (spin‐FET) demands a magnetic gate dielectric material whose magnetization can be switched electrically. We have grown films of Cr2O3 on top of graphite and graphene by pulsed laser deposition that shows this crucial functionality. We demonstrate that the Cr2O3 films are magnetoelectric by poling them in combined electric and magnetic fields and then using magnetic force microscopy to observe spontaneous surface domain structure as a function of poling field. In addition, we show that the electric field created by a conducting AFM tip can be used to write magnetic patterns in the film that demonstrate the kind of continuous magnetoelectric control needed for a prototype spin‐FET. (© 2016 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)}, number={3}, journal={PHYSICA STATUS SOLIDI-RAPID RESEARCH LETTERS}, author={Stuart, S. C. and Gray, B. and Nevola, D. and Su, L. and Sachet, E. and Ulrich, M. and Dougherty, D. B.}, year={2016}, month={Mar}, pages={242–247} }
 @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} }
 @article{lee_sorescu_lee_dougherty_2016, title={Supramolecular clusters and chains of 2,6-dimethylpyridine on Cu(110): Observation of dynamic configuration change with real-space surface science techniques and DFT calculations}, volume={652}, ISSN={["1879-2758"]}, DOI={10.1016/j.susc.2016.01.020}, abstractNote={The adsorption of 2,6-dimethylpyridine (2,6-DMP) on Cu(110) has been studied using low temperature scanning tunneling microscopy (LT-STM), time-of-flight electron stimulated desorption ion angular distribution (TOF-ESDIAD), and density functional theory (DFT) calculations. At low temperatures (T < ~ 150 K), the 2,6-DMP adsorbs in a flat configuration on Cu(110) producing clusters and extended domains via weak hydrogen bonding (C—H···N) with the molecular symmetry axis aligned along the < 001 > surface direction. At near-saturation coverage, a c(6 × 2) long-range ordered structure was observed. Upon annealing to T = 200 K, the 2,6-DMP molecules adopt an upright configuration with their pyridine ring plane oriented parallel to the <11̅0> azimuth. These upright 2,6-DMP molecules produce extended molecular chains where the repulsive interactions between the molecular chains give rise to coverage-dependent interchain distances. At near-saturation coverage, a 1260 surface structure is observed for the upright configuration. The DFT calculations suggest that the Cu adatom plays an important role in the adsorption configuration change of the 2,6-DMP molecule.}, journal={SURFACE SCIENCE}, author={Lee, Junseok and Sorescu, Dan C. and Lee, Jae-Gook and Dougherty, Dan}, year={2016}, month={Oct}, pages={82–90} }
 @article{jin_liu_dougherty_cullen_reutt-robey_weeks_robey_2015, title={C-60 chain phases on ZnPc/Ag(111) surfaces: Supramolecular organization driven by competing interactions}, volume={142}, ISSN={["1089-7690"]}, DOI={10.1063/1.4906044}, abstractNote={Serpentine chain C60 phases were observed in scanning tunneling microscopy (STM) images of C60 layers on zinc phthalocyanine (ZnPc) or pentacene covered Ag(111) and Au(111) surfaces. This low-density, quasi-one-dimensional organization contrasts starkly with the close-packed hexagonal phases observed for C60 layers on bare metal substrates. STM was employed to perform a detailed investigation of these chain structures for C60/ZnPc/Ag(111) heterolayers. Motivated by the similarity of these chain phases, and the chain and stripe organization occurring in dipole-fluid systems, we investigated a model based on competing van der Waals attractions and electrostatic repulsions between C60 molecules as an explanation for the driving force behind these monolayer phases. Density functional theory (DFT) calculations revealed significant charge transfer to C60 from the Ag(111) substrate, through the intervening ZnPc layer, inducing electrostatic interactions between C60 molecules. Molecular dynamics simulations performed with attractive van der Waals interactions plus repulsive dipole-dipole interactions reproduced the C60 chain phases with dipole magnitudes consistent with DFT calculations.}, number={10}, journal={JOURNAL OF CHEMICAL PHYSICS}, author={Jin, W. and Liu, Q. and Dougherty, D. B. and Cullen, W. G. and Reutt-Robey, J. E. and Weeks, J. and Robey, S. W.}, year={2015}, month={Mar} }
 @article{huston_wang_mcafee_loth_anthony_ade_conrad_dougherty_2015, title={Disruption of Molecular Ordering over Several Layers near the Au/2,8-Difluoro-5,11-bis(triethylsilylethynyl) Anthradithiophene Interface}, volume={15}, ISSN={["1528-7505"]}, DOI={10.1021/cg501621k}, abstractNote={The transition from an electrode-dominated ordered monolayer structure to a bulk-like thin film crystal structure of 2,8-difluoro-5,11-bis(triethylsilylethynyl) anthradithiophene (diF-TES-ADT) is observed to occur over at least the first four molecular layers near the electrode surface. Scanning tunneling microscopy studies of the growth of diF-TES-ADT on Au(111) show that the first two molecular layers assemble with aromatic planes parallel to the substrate surface. The monolayer structures are highly stable and well-ordered, while the bilayer structures are more loosely packed and poorly ordered. Subsequent diF-TES-ADT growth results in a more bulk-like layer containing standing up molecular configurations approaching the (001) crystal face as observed by grazing incidence wide angle X-ray scattering measurements. However, the third and fourth monolayers also show poor long-range ordering and an apparent height modulation that indicate significant strain effects from the substrate still persist.}, number={2}, journal={CRYSTAL GROWTH & DESIGN}, author={Huston, Shawn M. and Wang, Jiuyang and McAfee, Terry and Loth, Marsha and Anthony, John E. and Ade, Harald W. and Conrad, Brad R. and Dougherty, Daniel B.}, year={2015}, month={Feb}, pages={822–828} }
 @article{wang_dougherty_2015, title={Indirect coupling of an organic semiconductor to a d-orbital surface state}, volume={92}, ISSN={["1550-235X"]}, DOI={10.1103/physrevb.92.161401}, abstractNote={Adsorption of the organic semiconductor perylene tetracarboxylic dianhydride onto $\mathrm{Cr}(001)$ decreases the metal $d$-derived surface-state lifetime without causing a shift in its energy. This suggests an indirect electronic interaction that contrasts sharply with expectations of p-d electronic coupling based on direct chemisorption. Lifetime changes are measured with scanning tunneling spectroscopy as a function of temperature and quantified as arising from a molecule-induced increase in electron-electron scattering rate into bulk bands within Fermi-liquid theory. Adsorbate-induced effects extend far beyond the adsorption site of the molecule, decaying exponentially away with a characteristic length scale of $\ensuremath{\sim}2.4\phantom{\rule{0.16em}{0ex}}\mathrm{nm}$, similar to the carrier mean free path in Cr.}, number={16}, journal={PHYSICAL REVIEW B}, author={Wang, Jingying and Dougherty, Daniel B.}, year={2015}, month={Oct} }
 @article{hewitt_wang_boltersdorf_maggard_dougherty_2014, title={Coexisting Bi and Se surface terminations of cleaved Bi2Se3 single crystals}, volume={32}, number={4}, journal={Journal of Vacuum Science & Technology. B, Microelectronics and Nanometer Structures}, author={Hewitt, A. S. and Wang, J. Y. and Boltersdorf, J. and Maggard, P. A. and Dougherty, D. B.}, year={2014} }
 @article{king_sahoo_fuoco_stuart_dougherty_liu_maggard_2014, title={Copper Deficiency in the p-Type Semiconductor Cu1–xNb3O8}, volume={26}, ISSN={0897-4756 1520-5002}, url={http://dx.doi.org/10.1021/CM404147J}, DOI={10.1021/CM404147J}, abstractNote={The p-type semiconductor CuNb3O8 has been synthesized by solid-state and flux reactions and investigated for the effects of copper extrusion from its structure at 250–750 °C in air. High purity CuNb3O8 could be prepared by solid-state reactions at 750 °C at reaction times of 15 min and 48 h, and within a CuCl flux (10:1 molar ratio) at 750 °C at reaction times of 15 min and 12 h. The CuNb3O8 phase grows rapidly into well-faceted micrometer-sized crystals under these conditions, even with the use of Cu2O and Nb2O5 nanoparticle reactants. Heating CuNb3O8 in air to 450 °C for 3 h yields Cu-deficient Cu0.79(2)Nb3O8 that was characterized by powder X-ray Rietveld refinements (Sp. Grp. P21/a, Z = 4, a = 15.322(2) A, b = 5.0476(6) A, c = 7.4930(6) A, β = 107.07(1)o, and V = 554.0(1) A3). The parent structure of CuNb3O8 is maintained with ∼21% copper vacancies but with notably shorter Cu–O distances (by 0.16–0.27 A) within the Cu–O–Nb1 zigzag chains down its b-axis. Copper is extruded at high temperatures in air ...}, number={6}, journal={Chemistry of Materials}, publisher={American Chemical Society (ACS)}, author={King, Nacole and Sahoo, Prangya Parimita and Fuoco, Lindsay and Stuart, Sean and Dougherty, Daniel and Liu, Yi and Maggard, Paul A.}, year={2014}, month={Mar}, pages={2095–2104} }
 @article{dougherty_sandin_vescovo_rowe_2014, title={Coverage-dependent surface magnetism of iron phthalocyanine on an O-Fe(110) surface}, volume={90}, ISSN={["2469-9969"]}, DOI={10.1103/physrevb.90.045406}, abstractNote={Iron phthalocyanine adsorbed on an oxygen covered Fe(110) surface shows a complex coverage-dependent spin polarization during growth of a molecular monolayer. Spin polarization is modified at low submonolayer coverages, absent at intermediate submonolayer coverages, and reappears in modified form for a complete monolayer. This is attributed to coverage-dependent adsorption configurations from a random adsorption system to a packed monolayer with a well-defined interfacial spin polarization. In addition, we report on the observation of a rotation of the spin direction of photoelectrons in the presence of molecules which is attributed to molecular modifications of surface magnetic anisotropy.}, number={4}, journal={PHYSICAL REVIEW B}, author={Dougherty, Daniel B. and Sandin, Andreas and Vescovo, Elio and Rowe, J. E.}, year={2014}, month={Jul} }
 @article{sullivan_sahoo_fuoco_hewitt_stuart_dougherty_maggard_2014, title={Cu-Deficiency in the p-Type Semiconductor Cu5–xTa11O30: Impact on Its Crystalline Structure, Surfaces, and Photoelectrochemical Properties}, volume={26}, ISSN={0897-4756 1520-5002}, url={http://dx.doi.org/10.1021/CM502891T}, DOI={10.1021/CM502891T}, abstractNote={The p-type semiconductor Cu5Ta11O30 has been investigated for the effect of Cu extrusion on its crystalline structure, surface chemistry, and photoelectrochemical properties. The Cu5Ta11O30 phase was prepared in high purity using a CuCl-mediated flux synthesis route, followed by heating the products in air from 250 to 750 °C in order to investigate the effects of its reported film preparation conditions as a p-type photoelectrode. At 650 °C and higher temperatures, Cu5Ta11O30 is found to decompose into CuTa2O6 and Ta2O5. At lower temperatures of 250 to 550 °C, nanosized CuIIO surface islands and a Cu-deficient Cu5–xTa11O30 crystalline structure (i.e., x ∼ 1.8(1) after 450 °C for 3 h in air) is found by electron microscopy and Rietveld structural refinement results, respectively. Its crystalline structure exhibits a decrease in the unit cell volume with increasing reaction temperature and time, owing to the increasing removal of Cu(I) ions from its structure. The parent structure of Cu5Ta11O30 is conserved...}, number={23}, journal={Chemistry of Materials}, publisher={American Chemical Society (ACS)}, author={Sullivan, Ian and Sahoo, Prangya P. and Fuoco, Lindsay and Hewitt, Andrew S. and Stuart, Sean and Dougherty, Daniel and Maggard, Paul A.}, year={2014}, month={Nov}, pages={6711–6721} }
 @article{wang_dougherty_2014, title={Extrinsic origins of electronic disorder in 2D organic crystals}, volume={32}, ISSN={["2166-2746"]}, DOI={10.1116/1.4869196}, abstractNote={The dominant impact of extrinsic electronic disorder was observed in α-sexithiophene (α-6T) films grown on C60/Au(111) substrates by scanning tunneling microscopy and spectroscopy measurements. Sexithiophene molecules adopt an upright orientation with a structure that reflects both bulklike α-6T packing and C60 substrate templating effects. Apparent height variations in these films are assigned as molecular sliding defects but are observed to have negligible impact on the energy of the highest occupied molecular orbital. This orbital energy varies more significantly in response to random changes in probe tip, implying that the dominant origin of electronic disorder in two-dimensional organic crystals can be extrinsic to the film.}, number={3}, journal={JOURNAL OF VACUUM SCIENCE & TECHNOLOGY B}, author={Wang, Jiuyang and Dougherty, Daniel B.}, year={2014}, month={May} }
 @article{xu_hewitt_wang_guan_boltersdorf_maggard_dougherty_gundogdu_2014, title={Intrinsic and extrinsic effects on the electrostatic field at the surface of Bi2Se3}, volume={116}, ISSN={0021-8979 1089-7550}, url={http://dx.doi.org/10.1063/1.4891719}, DOI={10.1063/1.4891719}, abstractNote={The time evolution of electrostatic fields near a Bi2Se3 surface after a mechanical cleave was observed using Second Harmonic Generation. By comparing samples with different bulk doping levels and samples cleaved in different gas environments, these observations indicate multiple contributions to electric field evolution. These include the intrinsic process of Se vacancy diffusion as well as extrinsic processes due to both reactive and nonreactive surface adsorbates.}, number={4}, journal={Journal of Applied Physics}, publisher={AIP Publishing}, author={Xu, Chao and Hewitt, Andy and Wang, Jingying and Guan, Tianshuai and Boltersdorf, Jonathan and Maggard, Paul A. and Dougherty, Daniel B. and Gundogdu, Kenan}, year={2014}, month={Jul}, pages={043519} }
 @article{wang_hewitt_kumar_boltersdorf_guan_hunte_maggard_brom_redwing_dougherty_2014, title={Molecular Doping Control at a Topological Insulator Surface: F-4-TCNQ on Bi2Se3}, volume={118}, ISSN={["1932-7447"]}, DOI={10.1021/jp412690h}, abstractNote={Recent electrical measurements have accessed transport in the topological surface state band of thin exfoliated samples of Bi2Se3 by removing the bulk n-type doping by contact with thin films of the molecular acceptor F4-TCNQ. Here we report on the film growth and interfacial electronic characterization of F4-TCNQ grown on Bi2Se3. Atomic force microscopy shows wetting layer formation followed by 3D island growth. X-ray photoelectron spectroscopy is consistent with this picture and also shows that charge transferred to the molecular layer is localized on nitrogen atoms. Ultraviolet photoelectron spectroscopy shows a work function increase and an upward shift of the valence band edge that suggest significant reduction in carrier density at the Bi2Se3 surface.}, number={27}, journal={JOURNAL OF PHYSICAL CHEMISTRY C}, author={Wang, J. and Hewitt, A. S. and Kumar, R. and Boltersdorf, J. and Guan, T. and Hunte, F. and Maggard, P. A. and Brom, J. E. and Redwing, J. M. and Dougherty, D. B.}, year={2014}, month={Jul}, pages={14860–14865} }
 @article{zhao_feng_dougherty_sun_petek_2014, title={Molecular Electronic Level Alignment at Weakly Coupled Organic Film/Metal Interfaces}, volume={8}, ISSN={["1936-086X"]}, DOI={10.1021/nn5049969}, abstractNote={Electronic level alignment at interfaces of molecular materials with inorganic semiconductors and metals controls many interfacial phenomena. How the intrinsic properties of the interacting systems define the electronic structure of their interface remains one of the most important problems in molecular electronics and nanotechnology that can be solved through a combination of surface science experimental techniques and theoretical modeling. In this article, we address this fundamental problem through experimental and computational studies of molecular electronic level alignment of thin films of C(6)F(6) on noble metal surfaces. The unoccupied electronic structure of C(6)F(6) is characterized with single molecule resolution using low-temperature scanning tunneling microscopy-based constant-current distance-voltage spectroscopy. The experiments are performed on several noble metal surfaces with different work functions and distinct surface-normal projected band structures. In parallel, the electronic structures of the quantum wells (QWs) formed by the lowest unoccupied molecular orbital state of the C(6)F(6) monolayer and multilayer films and their alignment with respect to the vacuum level of the metallic substrates are calculated by solving the Schrödinger equation for a semiempirical one-dimensional (1D) potential of the combined system using input from density functional theory. Our analysis shows that the level alignment for C(6)F(6) molecules bound through weak van der Waals interactions to noble metal surfaces is primarily defined by the image potential of metal, the electron affinity of the molecule, and the molecule surface distance. We expect the same factors to determine the interfacial electronic structure for a broad range of molecule/metal interfaces.}, number={10}, journal={ACS NANO}, author={Zhao, Jin and Feng, Min and Dougherty, Daniel Barker and Sun, Hao and Petek, Hrvoje}, year={2014}, month={Oct}, pages={10988–10997} }
 @article{an_rowe_dougherty_lee_diebold_2014, title={Optical second-harmonic generation induced by electric current in graphene on Si and SiC substrates}, volume={89}, ISSN={["2469-9969"]}, DOI={10.1103/physrevb.89.115310}, abstractNote={We find that the flow of direct electric current (dc) through graphene on substrate enhances surface optical second-harmonic generation (SHG) from the graphene/substrate system. The current can enhance surface SHG by about 300% for a chemical-vapor-deposition (CVD) graphene monolayer on a SiO${}_{2}/\mathrm{Si}(001)$ substrate, and by about 25% for an epitaxial four-layer-graphene film on a 3.5\ifmmode^\circ\else\textdegree\fi{}-miscut vicinal SiC(0001) substrate. The enhancement in both the CVD and epitaxial graphene samples is due to electric field-induced SHG, which is produced by the current-associated vertical electric field at the SiO${}_{2}/$Si interface or at the graphene/SiC interface. Measurements of rotational-anisotropy SHG (RA-SH) from both samples revealed that the current-induced SHG varies strongly with the measurement location along the current flow direction. By measuring RA-SH from the vicinal SiC(0001) substrate, we determined all three second-order susceptibility tensor elements (${d}_{33}$ = \ensuremath{-}52.0 pm/V, ${d}_{15}$ = 20.0 pm/V, and ${d}_{31}$ = 18.7 pm/V) that characterize the SHG response of hexagonal SiC at the fundamental wavelength of 740 nm. We further determined the three effective susceptibility tensor elements (${d}_{33}$ = \ensuremath{-}135.8 pm/V, ${d}_{15}$ = 18.5 pm/V, and ${d}_{31}$ = 14.6 pm/V) that characterize the surface SHG from the graphene/vicinal-SiC(0001) sample and finally showed that the current-dependent tensor element ${d}_{33}$ can be enhanced to a large value of ${d}_{33}$ = \ensuremath{-}199.0 pm/V by electric current in epitaxial graphene.}, number={11}, journal={PHYSICAL REVIEW B}, author={An, Yong Q. and Rowe, J. E. and Dougherty, Daniel B. and Lee, Ji Ung and Diebold, Alain C.}, year={2014}, month={Mar} }
 @article{mcafee_gann_guan_stuart_rowe_dougherty_ade_2014, title={Toward Single-Crystal Hybrid-Carbon Electronics: Impact of Graphene Substrate Defect Density on Copper Phthalocyanine Film Growth}, volume={14}, ISSN={["1528-7505"]}, DOI={10.1021/cg500504u}, abstractNote={Graphene has long been recognized as a potential replacement for indium tin oxide as a transparent conducting substrate that may not only be cheaper to manufacture but also may provide mechanical flexibility and templating for preferential organic film growth. Here, we report the discovery that the thin film growth mode and crystal structure of copper phthalocyanine (CuPc), a prototype organic semiconductor, is extremely sensitive to even atomic-scale defects (e.g., steps) on the graphene surface and that high quality films can be grown with a well-defined crystal orientation that should be favorable for optimized solar cell applications. The initial growth involves flat-lying copper phthalocyanine molecules in a triclinic brickstone crystal with (012) orientation. Thicker films on pristine graphite, as well as thin films on lower quality graphene, show an orientational transition to the flat-lying (112)-oriented brickstone, which nucleates near film defects and grows in more compact 3D islands. The thi...}, number={9}, journal={CRYSTAL GROWTH & DESIGN}, author={McAfee, Terry and Gann, Eliot and Guan, Tianshuai and Stuart, Sean C. and Rowe, Jack and Dougherty, Daniel B. and Ade, Harald}, year={2014}, month={Sep}, pages={4394–4401} }
 @article{sandin_rowe_dougherty_2013, title={Improved graphene growth in UHV: Pit-free surfaces by selective Si etching of SiC(0001)-Si with atomic hydrogen}, volume={611}, ISSN={["1879-2758"]}, DOI={10.1016/j.susc.2013.01.010}, abstractNote={We present a novel technique of growing UHV graphene using atomic hydrogen etching of SiC(0001)–Si surfaces. Hydrogen atoms generated from a hot tungsten filament selectively etch silicon surface atoms thereby facilitating the Si-sublimation process at temperatures around 1000 °C according to Auger Electron Spectroscopy. This allows for separate, non-thermal control of the rate of formation of the interfacial buffer layer formation to yield reduced pit formation observed by scanning tunneling microscopy during subsequent UHV graphene growth.}, journal={SURFACE SCIENCE}, author={Sandin, Andreas and Rowe, J. E. and Dougherty, Daniel B.}, year={2013}, month={May}, pages={25–31} }
 @article{pronschinske_bruce_lewis_chen_calzolari_buongiorno-nardelli_shultz_you_dougherty_2013, title={Iron(ii) spin crossover films on Au(111): scanning probe microscopy and photoelectron spectroscopy}, volume={49}, ISSN={1359-7345 1364-548X}, url={http://dx.doi.org/10.1039/C3CC44904A}, DOI={10.1039/c3cc44904a}, abstractNote={The growth of films of [H2B(pz)2]Fe(ii)(bpy) on Au(111) is characterized from the bilayer film to multilayer film regime. Scanning tunneling microscopy shows a transition from a well-ordered, uniform bilayer film to a poorly-ordered film at larger thicknesses. Previous local tunneling spectroscopy and conductance mapping in bilayer films permit the identification of coexisting molecular spin-states at all temperatures. New ultraviolet photoelectron spectroscopy is consistent with this picture and in agreement with the density of states calculated by density functional theory. In thicker films with a polycrystalline morphology, evidence for a more bulk-like change in spin composition as a function of temperature is obtained by observing the reduction in intensity of Fe 2p core level satellites in X-ray photoelectron spectra.}, number={89}, journal={Chemical Communications}, publisher={Royal Society of Chemistry (RSC)}, author={Pronschinske, Alex and Bruce, Robert C. and Lewis, Geoff and Chen, Yifeng and Calzolari, Arrigo and Buongiorno-Nardelli, Marco and Shultz, David A. and You, Wei and Dougherty, Daniel B.}, year={2013}, pages={10446} }
 @article{pronschinske_chen_lewis_shultz_calzolari_buongiorno nardelli_dougherty_2013, title={Modification of Molecular Spin Crossover in Ultrathin Films}, volume={13}, ISSN={1530-6984 1530-6992}, url={http://dx.doi.org/10.1021/nl304304e}, DOI={10.1021/nl304304e}, abstractNote={Scanning tunneling microscopy and local conductance mapping show spin-state coexistence in bilayer films of Fe[(H2Bpz2)2bpy] on Au(111) that is independent of temperature between 131 and 300 K. This modification of bulk behavior is attributed in part to the unique packing constraints of the bilayer film that promote deviations from bulk behavior. The local density of states measured for different spin states shows that high-spin molecules have a smaller transport gap than low-spin molecules and are in agreement with density functional theory calculations.}, number={4}, journal={Nano Letters}, publisher={American Chemical Society (ACS)}, author={Pronschinske, Alex and Chen, Yifeng and Lewis, Geoffrey F. and Shultz, David A. and Calzolari, Arrigo and Buongiorno Nardelli, Marco and Dougherty, Daniel B.}, year={2013}, month={Mar}, pages={1429–1434} }
 @article{stuart_satchet_sandin_maria_rowe_dougherty_ulrich_2013, title={Smooth MgO films grown on graphite and graphene by pulsed laser deposition}, volume={31}, ISSN={["2166-2746"]}, DOI={10.1116/1.4818511}, abstractNote={Pulsed laser deposition was used to grow thin (1–100 nm) magnesium oxide films directly on graphite and epitaxial graphene on SiC(0001). The authors observe very smooth (typical rms roughness of ∼0.4 nm) film morphologies that are nearly independent of film thickness and conformal to the substrate for films grown on room temperature substrates. Surface roughness is less than 1 nm for thicknesses up to 100 nm and is independent of oxygen background pressure during growth. X-ray diffraction shows no evidence of crystallinity for films grown on room temperature substrates but shows ⟨100⟩ texture for films grown on heated substrates that also have very rough surface morphologies. X-ray photoelectron spectroscopy shows hydroxylation of films due to air exposure that can only be partially removed by annealing, indicating the presence of atomic defects in the films.}, number={5}, journal={JOURNAL OF VACUUM SCIENCE & TECHNOLOGY B}, author={Stuart, Sean C. and Satchet, Edward and Sandin, Andreas and Maria, Jon-Paul and Rowe, John E. and Dougherty, Daniel B. and Ulrich, Marc}, year={2013}, month={Sep} }
 @article{mcafee_gann_ade_dougherty_2013, title={Thermally Induced Dewetting in Ultrathin C-60 Films on Copper Phthalocyanine}, volume={117}, ISSN={["1932-7447"]}, DOI={10.1021/jp4067372}, abstractNote={The evolution of thermally annealed ultrathin fullerene–C60 layers on copper phthalocyanine is examined by atomic force microscopy and near-edge X-ray absorption fine structure spectroscopy. Annealing causes C60 films to dewet the copper phthalocyanine substrate surface via lateral surface mass transport. Coarsening of C60 clusters is observed that creates mounds that exceed the nominal C60 thickness by more than an order of magnitude and is consistent with surface diffusion-mediated mass transport. Implications for thermal morphology control in organic solar cells, such as the destabilization of multilayered C60:CuPc with individual layers only ∼5–10 nm thick, are discussed.}, number={49}, journal={JOURNAL OF PHYSICAL CHEMISTRY C}, author={McAfee, T. and Gann, E. and Ade, H. and Dougherty, D. B.}, year={2013}, month={Dec}, pages={26007–26012} }
 @article{calzolari_chen_lewis_dougherty_shultz_buongiorno nardelli_2012, title={Complex Materials for Molecular Spintronics Applications: Cobalt Bis(dioxolene) Valence Tautomers, from Molecules to Polymers}, volume={116}, ISSN={1520-6106 1520-5207}, url={http://dx.doi.org/10.1021/jp3099895}, DOI={10.1021/jp3099895}, abstractNote={Using first principles calculations, we predict a complex multifunctional behavior in cobalt bis(dioxolene) valence tautomeric compounds. Molecular spin-state switching is shown to dramatically alter electronic properties and corresponding transport properties. This spin state dependence has been demonstrated for technologically relevant coordination polymers of valence tautomers as well as for novel conjugated polymers with valence tautomeric functionalization. As a result, these materials are proposed as promising candidates for spintronic devices that can couple magnetic bistability with novel electrical and spin conduction properties. Our findings pave the way to the fundamental understanding and future design of active multifunctional organic materials for spintronics applications.}, number={43}, journal={The Journal of Physical Chemistry B}, publisher={American Chemical Society (ACS)}, author={Calzolari, Arrigo and Chen, Yifeng and Lewis, Geoffrey F. and Dougherty, Daniel B. and Shultz, David and Buongiorno Nardelli, Marco}, year={2012}, month={Oct}, pages={13141–13148} }
 @article{gokce_dougherty_gundogdu_2012, title={Effect of p-type doping on the oxidation of H-Si(111) studied by second-harmonic generation}, volume={30}, number={4}, journal={Journal of Vacuum Science & Technology. A, Vacuum, Surfaces, and Films}, author={Gokce, B. and Dougherty, D. B. and Gundogdu, K.}, year={2012} }
 @article{sandin_jayasekera_rowe_kim_buongiorno nardelli_dougherty_2012, title={Multiple coexisting intercalation structures of sodium in epitaxial graphene-SiC interfaces}, volume={85}, ISSN={1098-0121 1550-235X}, url={http://dx.doi.org/10.1103/PhysRevB.85.125410}, DOI={10.1103/physrevb.85.125410}, abstractNote={This article discusses multiple coexisting intercalation structures of sodium in peitaxial graphene-SiC interfaces.}, number={12}, journal={Physical Review B}, publisher={American Physical Society (APS)}, author={Sandin, Andreas and Jayasekera, Thushari and Rowe, J. E. and Kim, Ki Wook and Buongiorno Nardelli, M. and Dougherty, Daniel B.}, year={2012}, month={Mar} }
 @article{nelson_sandin_dougherty_aspnes_rowe_diebold_2012, title={Optical and structural characterization of epitaxial graphene on vicinal 6H-SiC(0001)-Si by spectroscopic ellipsometry, Auger spectroscopy, and STM}, volume={30}, ISSN={["2166-2754"]}, DOI={10.1116/1.4726199}, abstractNote={The authors report results of spectroscopic ellipsometry (SE) measurements in the near-IR, visible, and near-UV spectral ranges using a Woollam dual rotating-compensator ellipsometer, analyzing data in terms of both epitaxial graphene and interface contributions. The SiC samples were cleaned by standard methods of CMP and HF etching prior to mounting in UHV and growing epitaxial graphene by thermal annealing at ∼1400 °C. Most samples were vicinally cut 3.5° off (0001) toward [11−20]. STM measurements show that the initial regular step edges were replaced by somewhat irregular edges after graphene growth. From growth-temperature and Auger data the authors estimate that the graphene is ∼3–4 ML thick. The authors find significant differences among the spectral features of the interface “buffer” layer and those of graphene. Specifically, the hyperbolic-exciton peak reported previously at ∼4.5 eV in graphene shifts to a similarly shaped peak at ∼4 eV in the interface buffer layer. The authors attribute this shift to a significant component of sp3 bonded carbon in the buffer, which occurs in addition to the sp2 bonded carbon that is present in the graphene layer. SE data in the terahertz range obtained by Hoffman et al. [Thin Solid Films 519, 2593 (2011)] show that the mobility values of graphene grown on the carbon face of SiC vary with proximity to the substrate. This leads to the question as to whether an interface layer at the Si face has properties (i.e., dielectric function/complex refractive index) that are different from and/or affect those of the graphene layers.}, number={4}, journal={JOURNAL OF VACUUM SCIENCE & TECHNOLOGY B}, author={Nelson, Florence and Sandin, Andreas and Dougherty, Daniel B. and Aspnes, David E. and Rowe, Jack E. and Diebold, Alain C.}, year={2012}, month={Jul} }
 @article{huston_wang_loth_anthony_conrad_dougherty_2012, title={Role of Fluorine Interactions in the Self-Assembly of a Functionalized Anthradithiophene Monolayer on Au(111)}, volume={116}, ISSN={["1932-7455"]}, DOI={10.1021/jp307539q}, abstractNote={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.}, number={40}, journal={JOURNAL OF PHYSICAL CHEMISTRY C}, author={Huston, Shawn M. and Wang, Jiuyang and Loth, Marsha A. and Anthony, John E. and Conrad, Brad R. and Dougherty, Daniel B.}, year={2012}, month={Oct}, pages={21465–21471} }
 @article{pronschinske_mardit_dougherty_2011, title={Modeling the constant-current distance-voltage mode of scanning tunneling spectroscopy}, volume={84}, ISSN={["2469-9969"]}, DOI={10.1103/physrevb.84.205427}, abstractNote={We demonstrate the relationship between sample density of electronic states and constant-current distance-voltage spectra starting from the usual expressions for tunneling current in scanning tunneling microscopy experiments. First-order differential equations are derived for the tip position as a function of voltage drop across the tunnel junction for both square and trapezoidal barrier transmission functions. Numerical solutions of the square barrier equation are carried out for different sample density of states and compared with self-consistent integration of the tunneling integral equation. It is shown that normalization of the distance vs voltage spectra by taking logarithmic derivatives reproduces the peak positions in the sample density of states usually to within 0.1 eV. The use of differential equations is proposed as an accurate method for analyzing experimental data and applied to the example case of the \ensuremath{\pi}${}^{*}$ orbital of the $c$(8 \ifmmode\times\else\texttimes\fi{} 2) phase of benzoate on Cu(110).}, number={20}, journal={PHYSICAL REVIEW B}, author={Pronschinske, Alex and Mardit, Daniel J. and Dougherty, Daniel B.}, year={2011}, month={Nov} }
 @article{wang_pronschinske_dougherty_2011, title={Scanning tunneling microscopy of a disordered Alq3-metal interface}, volume={12}, ISSN={["1878-5530"]}, DOI={10.1016/j.orgel.2011.07.020}, abstractNote={Scanning tunneling microscopy is used to observe the structure of the first monolayer of Alq3 on Cu(1 1 0). Individual molecules can be resolved at very low coverage where there is also evidence of significant surface mobility. As Alq3 surface coverage increases, molecules aggregate by forming pairs and chain-like structures without ever establishing long range order. Interfacial disorder is maintained even after annealing to 590 K. The tendency to form chain-like aggregates is attributed to anisotropic intermolecular interactions. Anisotropies in these interactions are implicated in chain formation by illustrative density functional theory calculations and the role of direct dipolar interactions, anisotropic van der Waals interactions, and substrate-mediated interactions is discussed.}, number={11}, journal={ORGANIC ELECTRONICS}, author={Wang, Zhengang and Pronschinske, Alex and Dougherty, Daniel B.}, year={2011}, month={Nov}, pages={1920–1926} }
 @article{dutton_dougherty_jin_reutt-robey_robey_2011, title={Superatom orbitals of C-60 on Ag(111): Two-photon photoemission and scanning tunneling spectroscopy}, volume={84}, ISSN={["1098-0121"]}, DOI={10.1103/physrevb.84.195435}, abstractNote={The unoccupied electronic structure of C${}_{60}$ layers on Ag(111) was investigated with two-photon photoemission (2PPE) and scanning tunneling spectroscopy (STS). The focus, in addition to providing unoccupied level assignments for C${}_{60}$ monolayers, is on identifying signatures of C${}_{60}$ superatom molecular orbitals (SAMOs), diffuse unoccupied levels weakly bound by the spherical potential of the fullerene shell, in the two complementary techniques. The s-SAMO is identified 3.3 eV above the Fermi level in 2PPE and at a bias of 3.5 eV in STS. Possible contributions from $p$-SAMO levels are also discussed for both measurement techniques. The results are compared with recent low-temperature scanning tunneling microscopy (LT-STM) work that investigated C${}_{60}$ SAMOs on Cu(111) and oxygen-covered Cu(110) surfaces and with previous 2PPE measurements for C${}_{60}$ monolayers on Au(111) and Cu(111).}, number={19}, journal={PHYSICAL REVIEW B}, author={Dutton, G. J. and Dougherty, D. B. and Jin, W. and Reutt-Robey, J. E. and Robey, S. W.}, year={2011}, month={Nov} }
 @misc{maksymovych_voznyy_dougherty_sorescu_yates_2010, title={Gold adatom as a key structural component in self-assembled monolayers of organosulfur molecules on Au(111)}, volume={85}, ISSN={["0079-6816"]}, DOI={10.1016/j.progsurf.2010.05.001}, abstractNote={Chemisorption of organosulfur molecules, such as alkanethiols, arenethiols and disulfide compounds on gold surfaces and their subsequent self-organization is the archetypal process for molecular self-assembly on surfaces. Owing to their ease of preparation and high versatility, alkanethiol self-assembled monolayers (SAMs) have been widely studied for potential applications including surface functionalization, molecular motors, molecular electronics, and immobilization of biological molecules. Despite fundamental advances, the dissociative chemistry of the sulfur headgroup on gold leading to the formation of the sulfur–gold anchor bond has remained controversial. This review summarizes the recent progress in the understanding of the geometrical and electronic structure of the anchor bond. Particular attention is drawn to the involvement of gold adatoms at all stages of alkanethiol self-assembly, including the dissociation of the disulfide (S–S) and hydrogen-sulfide (S–H) bonds and subsequent formation of the self-assembled structure. Gold adatom chemistry is proposed here to be a unifying theme that explains various aspects of the alkanethiol self-assembly and reconciles experimental evidence provided by scanning probe microscopy and spectroscopic methods of surface science. While several features of alkanethiol self-assembly have yet to be revisited in light of the new adatom-based models, the successes of alkanethiol SAMs suggest that adatom-mediated surface chemistry may be a viable future approach for the construction of self-assembled monolayers involving molecules which do not contain sulfur.}, number={5-8}, journal={PROGRESS IN SURFACE SCIENCE}, author={Maksymovych, Peter and Voznyy, Oleksandr and Dougherty, Daniel B. and Sorescu, Dan C. and Yates, John T., Jr.}, year={2010}, pages={206–240} }
 @article{pronschinske_dougherty_2010, title={Impact of Local Molecular Environment on the Decay of Image Potential States}, volume={1}, ISSN={["1948-7185"]}, DOI={10.1021/jz100854g}, abstractNote={Image potential-derived states of upright chemisorbed benzoate molecules on a Cu(110) surface have been measured with scanning tunneling spectroscopy. The widths of image-derived features for these adsorbates are sensitive to the presence of coexisting flat-lying benzoate molecules. This dependence is attributed to the different couplings between image potential state wave functions and bulk metal states that result from differences in substrate-mediated intermolecular interactions. The comparative trends are semiquantitatively modeled using a dielectric continuum approach.}, number={17}, journal={JOURNAL OF PHYSICAL CHEMISTRY LETTERS}, author={Pronschinske, Alex and Dougherty, Daniel B.}, year={2010}, month={Sep}, pages={2613–2617} }
 @article{sandin_pronschinske_rowe_dougherty_2010, title={Incomplete screening by epitaxial graphene on the Si face of 6H-SiC(0001)}, volume={97}, ISSN={["1077-3118"]}, DOI={10.1063/1.3484966}, abstractNote={A biased scanning tunneling microscope (STM) tip is used to study the ability of carriers in graphene to screen external electrostatic fields by monitoring the effect of tunneling-junction width on the position of image potential-derived surface states. These states are unusually sensitive to local electric fields due to the STM tip in both single layer and bilayer epitaxial graphene. This is attributed to the incomplete screening of applied fields in epitaxial graphene on SiC(0001). Our observations imply that charged impurity scattering is likely to be a dominant factor in the transport properties of epitaxial graphene on SiC.}, number={11}, journal={APPLIED PHYSICS LETTERS}, author={Sandin, Andreas and Pronschinske, Alex and Rowe, J. E. and Dougherty, Daniel B.}, year={2010}, month={Sep} }
 @article{jin_dougherty_cullen_robey_reutt-robey_2009, title={C-60-Pentacene Network Formation by 2-D Co-Crystallization}, volume={25}, ISSN={["0743-7463"]}, DOI={10.1021/la900968d}, abstractNote={We report experiments highlighting the mechanistic role of mobile pentacene precursors in the formation of a network C(60)-pentacene co-crystalline structure on Ag(111). This co-crystalline arrangement was first observed by low temperature scanning tunneling microscopy (STM) by Zhang et al. (Zhang, H. L.; Chen, W.; Huang, H.; Chen, L.; Wee, A. T. S. J. Am. Chem. Soc. 2008, 130, 2720-2721). We now show that this structure forms readily at room temperature from a two-dimensional (2-D) mixture. Pentacene, evaporated onto Ag(111) to coverages of 0.4-1.0 ML, produces a two-dimensional (2-D) gas. Subsequently deposited C(60) molecules combine with the pentacene 2-D gas to generate a network structure, consisting of chains of close-packed C(60) molecules, spaced by individual C(60) linkers and 1 nm x 2.5 nm pores containing individual pentacene molecules. Spontaneous formation of this stoichiometric (C(60))(4)-pentacene network from a range of excess pentacene surface coverage (0.4 to 1.0 ML) indicates a self-limiting assembly process. We refine the structure model for this phase and discuss the generality of this co-crystallization mechanism.}, number={17}, journal={LANGMUIR}, author={Jin, Wei and Dougherty, Daniel B. and Cullen, William G. and Robey, Steven and Reutt-Robey, Janice E.}, year={2009}, month={Sep}, pages={9857–9862} }
 @article{conrad_tosado_dutton_dougherty_jin_bonnen_schuldenfrei_cullen_williams_reutt-robey_et al._2009, title={C60 cluster formation at interfaces with pentacene thin-film phases}, volume={95}, ISSN={0003-6951 1077-3118}, url={http://dx.doi.org/10.1063/1.3266857}, DOI={10.1063/1.3266857}, abstractNote={The C60-thin film pentacene interface was investigated using scanning tunneling microscopy, atomic force microscopy, and ultraviolet photoemission spectroscopy. C60 deposition on a multilayer pentacene film (standing) yields an interface dominated by C60 clusters, regardless of the underlying substrate. Three-dimensional cluster growth dominates due to weak interactions with the underlying Pn. C60 cluster size and density on sequential Pn layers suggest an Ehrlich–Schwoebel-type barrier at Pn layer boundaries. Cluster formation reduces the C60 lowest unoccupied molecular orbital–Pn highest occupied molecular orbital (HOMO) separation, while increasing the respective HOMO-HOMO offset. Heterostructure fabrication protocols can alter interface morphology and induce band shifts on the order of 0.3 eV.}, number={21}, journal={Applied Physics Letters}, publisher={AIP Publishing}, author={Conrad, B. R. and Tosado, J. and Dutton, G. and Dougherty, D. B. and Jin, W. and Bonnen, T. and Schuldenfrei, A. and Cullen, W. G. and Williams, E. D. and Reutt-Robey, J. E. and et al.}, year={2009}, month={Nov}, pages={213302} }
 @article{dougherty_jin_cullen_reutt-robey_robey_2009, title={Striped domains at the pentacene: C60 interface}, volume={94}, ISSN={0003-6951 1077-3118}, url={http://dx.doi.org/10.1063/1.3046721}, DOI={10.1063/1.3046721}, abstractNote={Scanning tunneling microscopy observations of the initial growth stages of the first layer of pentacene on a monolayer film of C60 on Ag(111) are presented. Pentacene films nucleate and grow with molecules standing up at the pentacene:C60 interface similar to thin film phases observed on weakly interacting substrates such as SiO2. Unlike reported thin film pentacene phases, those on 1 monolayer C60/Ag(111) exhibit a striated morphology with domains of 4 nm nominal width. This long range pattern of periodic pentacene displacements relative to the substrate is the response to stress induced in the pentacene film by its interaction with the rigid C60 support.}, number={2}, journal={Applied Physics Letters}, publisher={AIP Publishing}, author={Dougherty, Daniel B. and Jin, Wei and Cullen, William G. and Reutt-Robey, Janice E. and Robey, Steven W.}, year={2009}, month={Jan}, pages={023103} }
 @article{maksymovych_dougherty_2008, title={Molecular self-assembly guided by surface reconstruction: CH3SH monolayer on the Au(111) surface}, volume={602}, ISSN={0039-6028}, url={http://dx.doi.org/10.1016/j.susc.2008.03.047}, DOI={10.1016/j.susc.2008.03.047}, abstractNote={Self-assembly of methanethiol (CH3SH) on Au(1 1 1) was studied using scanning tunneling microscopy at T < 150 K when the S–H bond is intact. The CH3SH monolayer assumes a commensurate structure with a 6202 unit-cell. Only one of three possible azimuthal domains of the monolayer was observed with domain walls pinned at the rotational boundaries of the herringbone reconstruction. From a real-time observation of monolayer formation at T = 60 K we propose a phenomenological model for the growth of preferential domains, which is based on the interplay between molecular detachment from domain boundaries and anisotropic mass transport on the Au(1 1 1) surface due to its herringbone reconstruction.}, number={12}, journal={Surface Science}, publisher={Elsevier BV}, author={Maksymovych, Peter and Dougherty, Daniel B.}, year={2008}, month={Jun}, pages={2017–2024} }
 @article{dougherty_jin_cullen_reutt-robey_robey_2008, title={Variable Temperature Scanning Tunneling Microscopy of Pentacene Monolayer and Bilayer Phases on Ag(111)}, volume={112}, ISSN={1932-7447 1932-7455}, url={http://dx.doi.org/10.1021/jp804682v}, DOI={10.1021/jp804682v}, abstractNote={Pentacene deposited onto a Ag(111) surface at 300 K is studied using scanning tunneling microscopy at temperatures of 300 and 50 K, providing structural insight into its unusual growth habit. At room temperature, an unexpectedly high pentacene coverage is needed to nucleate ordered pentacene islands, which appear surrounded by a disordered pentacene phase. These room temperature pentacene nuclei are revealed as bilayer structures from their coverage-dependent size evolution and molecularly resolved images of domain boundaries, recorded at 50 K. At this reduced temperature, two different monolayer phases with long-range order and commensurate with the Ag(111) surface lattice further emerge. These two monolayer phases exhibit comparable (0.7 vs 0.8 molecule/nm2) packing densities, but distinct intermolecular registration and alignment with respect to the silver sublattice.}, number={51}, journal={The Journal of Physical Chemistry C}, publisher={American Chemical Society (ACS)}, author={Dougherty, D. B. and Jin, W. and Cullen, W. G. and Reutt-Robey, J. E. and Robey, S. W.}, year={2008}, month={Nov}, pages={20334–20339} }
 @article{lee_dougherty_yates_2007, title={Self-assembly of 2,6-dimethylpyridine on Cu(110) directed by weak hydrogen bonding}, volume={601}, ISSN={0039-6028}, url={http://dx.doi.org/10.1016/j.susc.2007.04.248}, DOI={10.1016/j.susc.2007.04.248}, abstractNote={Sequential stages of formation of a self-assembled monolayer of flat-lying 2,6-dimethylpyridine molecules on a single crystal Cu(1 1 0) surface have been observed by low-temperature scanning tunneling microscopy (LT-STM). At an adsorption temperature of 10 K, all of the molecules are randomly distributed at low coverage upon adsorption. The isolated molecules align their molecular axes parallel to the 〈0 0 1〉 azimuth of the Cu lattice. The nitrogen atom in the molecule is located at the four-fold hollow site. Upon annealing to 100 K, the molecules associate to form head-to-head dimers. The dimer units involve a pair of weak hydrogen bonds between methyl group-hydrogen atoms and N moieties on adjacent molecules, forming a core structure for further growth. In a later stage of self-assembly, single head-to-tail weak hydrogen bonds between ring C–H bonds and N moieties form in chains on the periphery of the central cores, leading to larger domains with a c(6 × 2) overlayer structure.}, number={16}, journal={Surface Science}, publisher={Elsevier BV}, author={Lee, Junseok and Dougherty, Daniel B. and Yates, John T., Jr.}, year={2007}, month={Aug}, pages={L91–L94} }
 @article{dougherty_lee_yates_2006, title={Assembly of Linear Clusters of Iodobenzene Dimers on Cu(110)}, volume={110}, ISSN={1520-6106 1520-5207}, url={http://dx.doi.org/10.1021/jp0645420}, DOI={10.1021/jp0645420}, abstractNote={Low-temperature STM observations of the low-coverage chemisorption behavior of iodobenzene on Cu(110) are presented at two annealing temperatures. When a random distribution of isolated iodobenzene molecules deposited at T < 100 K is annealed to approximately 170 K, the undissociated molecules assemble into chainlike clusters composed of identical subunits. An STM tip-induced dissociation reaction is used to determine that the individual units in the chains are composed of pairs of iodobenzene molecules. A model is proposed in which iodine atoms on each member of the pair are directed toward one another. The driving force for the formation of such clusters is suggested to be the dispersion interactions between the polarizable iodobenzene molecules.}, number={41}, journal={The Journal of Physical Chemistry B}, publisher={American Chemical Society (ACS)}, author={Dougherty, Daniel B. and Lee, Junseok and Yates, John T.}, year={2006}, month={Oct}, pages={20077–20080} }
 @article{lee_dougherty_yates_2006, title={Chemisorbed Benzoate-to-Benzene Conversion via Phenyl Radicals on Cu(110):  Kinetic Observation of Conformational Effects}, volume={128}, ISSN={0002-7863 1520-5126}, url={http://dx.doi.org/10.1021/ja060533t}, DOI={10.1021/ja060533t}, abstractNote={The adsorption and decomposition of benzoic acid on the Cu(110) surface has been investigated using temperature-programmed reaction (TPR) spectroscopy and scanning tunneling microscopy (STM). The benzoate species is found to exist in two conformations: a phase containing upright species at monolayer saturation and a phase containing many tilted species at lower coverages. Thermal decomposition begins to occur near 500 K, yielding benzene and CO2. It is found that phenyl radicals, generated preferentially from the tilted benzoate species, efficiently abstract H atoms from undecomposed benzoate species to produce benzene in a rate-controlling process with an activation energy of about 29 kcal/mol. Using deuterium atom substitution at the 4-C position on the benzoate ring, it is found that the hydrogen abstraction reaction is selective for 2-,3- and 5-,6-C-H bonds. This observation indicates that the mobile phenyl radical is surface bound and preferentially attacks C-H bonds which are nearest the Cu surface binding the benzoate species, either as an upright species or as a tilted species.}, number={18}, journal={Journal of the American Chemical Society}, publisher={American Chemical Society (ACS)}, author={Lee, Junseok and Dougherty, Daniel B. and Yates, John T.}, year={2006}, month={May}, pages={6008–6009} }
 @article{dougherty_maksymovych_yates_2006, title={Direct STM evidence for Cu-benzoate surface complexes on Cu(110)}, volume={600}, ISSN={0039-6028}, url={http://dx.doi.org/10.1016/j.susc.2006.04.048}, DOI={10.1016/j.susc.2006.04.048}, abstractNote={Abstract The chemisorption of benzoate on a Cu(1 1 0) crystal at room temperature was studied using low temperature scanning tunneling microscopy. STM images, obtained at 5 K for low benzoate coverage, show isolated surface species that consist of a single Cu adatom stabilizing two benzoate molecules in a flat orientation. These species are discussed in relation to other known metal-organic surface compounds. At higher coverage the 4 - 3 1 5 overlayer, called the α-phase, was also observed at 5 K and found to contain features attributable to two Cu adatoms associated with two pairs of non-equivalent benzoate species. The observed topographic features are used to suggest refinements of the structural model of the ordered α-phase overlayer.}, number={19}, journal={Surface Science}, publisher={Elsevier BV}, author={Dougherty, D.B. and Maksymovych, P. and Yates, J.T., Jr.}, year={2006}, month={Oct}, pages={4484–4491} }
 @article{lee_dougherty_yates_2006, title={Edge-On Bonding of Benzene Molecules in the Second Adsorbed Layer on Cu(110)}, volume={110}, ISSN={1520-6106 1520-5207}, url={http://dx.doi.org/10.1021/jp0625661}, DOI={10.1021/jp0625661}, abstractNote={The bilayer of benzene on Cu(110) was studied with temperature-programmed desorption (TPD), time-of-flight electron stimulated desorption ion angular distribution (TOF-ESDIAD), and scanning tunneling microscopy (STM). TPD spectra show that three well-defined adsorption states exist. The alpha layer corresponds to the first layer containing flat-lying benzene molecules. As coverage increases, the beta layer forms on top of the alpha layer, and eventually, a multilayer, gamma, forms. TPD measurements show that the number of benzene molecules in the beta layer is equal to the number of benzene molecules in the alpha layer. ESDIAD measurements establish that the orientation of the benzene molecules in the beta layer is edge-on, with two C-H bonds directed toward the surface. STM images of the beta layer reveal closely spaced edge-on benzene molecules arranged in repeating hexagons, as well as loosely spaced benzene molecules with greater apparent height, which are also edge-on species. Correlation between the different measurements suggests a structural model for the benzene bilayer.}, number={32}, journal={The Journal of Physical Chemistry B}, publisher={American Chemical Society (ACS)}, author={Lee, Junseok and Dougherty, Daniel B. and Yates, John T.}, year={2006}, month={Aug}, pages={15645–15649} }
 @article{lee_dougherty_yates_2006, title={Phenyl Species Formation and Preferential Hydrogen Abstraction in the Decomposition of Chemisorbed Benzoate on Cu(110)}, volume={110}, ISSN={1520-6106 1520-5207}, url={http://dx.doi.org/10.1021/jp0606863}, DOI={10.1021/jp0606863}, abstractNote={The adsorption and decomposition of benzoic acid on the Cu(110) surface has been investigated using temperature-programmed reaction (TPR) spectroscopy and scanning tunneling microscopy (STM). The benzoate species is found to exist in two conformations--a phase containing upright species at monolayer saturation and a phase containing many lying-down species at lower coverages. Thermal decomposition begins to occur near 500 K, yielding benzene and CO(2). It is found that phenyl species, generated preferentially from the lying-down benzoate species, efficiently abstract H atoms from undecomposed benzoate species to produce benzene in a rate-controlling process with an activation energy of about 29 kcal/mol. Using deuterium-atom substitution at the 4-C position on the benzoate ring it is found that the hydrogen-abstraction reaction is selective for 2,3 and 5,6 C-H bonds. This observation indicates that the mobile phenyl species is surface bound and preferentially attacks C-H bonds which are nearest the Cu surface and bind the benzoate species as either an upright species or a tilted species.}, number={20}, journal={The Journal of Physical Chemistry B}, publisher={American Chemical Society (ACS)}, author={Lee, Junseok and Dougherty, Daniel B. and Yates, John T.}, year={2006}, month={May}, pages={9939–9946} }
 @article{dougherty_lee_yates_2006, title={Role of Conformation in the Electronic Properties of Chemisorbed Pyridine on Cu(110):  An STM/STS Study}, volume={110}, ISSN={1520-6106 1520-5207}, url={http://dx.doi.org/10.1021/jp060733r}, DOI={10.1021/jp060733r}, abstractNote={Pyridine was chemisorbed on Cu(110) at 10 K and observed using STM at 5 K as dosed and after annealing to temperatures between 20 and 300 K. At very low coverage, two molecular species with different apparent heights are observed to coexist. The higher species is assigned to a pyridine molecule bonded with its symmetry axis perpendicular to the surface plane, while the lower species is assigned to a pyridine molecule that is tilted down toward the surface plane. At low coverage, the tilted pyridine species predominates on the surface, but as the total surface coverage of pyridine increases, the molecules stand up until the upright geometry becomes favored. Measurements of the STS of the two species show different molecular resonances derived from the lowest unoccupied pyridine pi* orbitals. The tilted pyridine species has a peak in the unoccupied local density of states at 2.6 +/- 0.1 eV, whereas the upright pyridine species has a peak at 2.3 +/- 0.1 eV.}, number={24}, journal={The Journal of Physical Chemistry B}, publisher={American Chemical Society (ACS)}, author={Dougherty, D. B. and Lee, J. and Yates, J. T.}, year={2006}, month={Jun}, pages={11991–11996} }
 @article{dougherty_maksymovych_lee_yates_2006, title={Stark-shifted image potential states of benzene bilayers on Cu(110) and Cu(111)}, volume={431}, ISSN={0009-2614}, url={http://dx.doi.org/10.1016/j.cplett.2006.09.104}, DOI={10.1016/j.cplett.2006.09.104}, abstractNote={Benzene bilayer islands were observed by STM to be structurally similar on both Cu(1 1 1) and Cu(1 1 0) surfaces. The local electronic structure of the bilayers was studied using tunneling spectroscopy of Stark-shifted image potential states. On both Cu substrates, the n = 1 bilayer state was locally shifted away from the Fermi level when compared to the n = 1 monolayer state. The Stark-shift due to the electric field of the STM tip was observed to be greater for the n = 1 bilayer state than the n = 1 monolayer state on Cu(1 1 1).}, number={4-6}, journal={Chemical Physics Letters}, publisher={Elsevier BV}, author={Dougherty, D.B. and Maksymovych, P. and Lee, J. and Yates, J.T., Jr.}, year={2006}, month={Nov}, pages={303–307} }
 @article{maksymovych_sorescu_dougherty_yates_2005, title={Surface Bonding and Dynamical Behavior of the CH3SH Molecule on Au(111)}, volume={109}, ISSN={1520-6106 1520-5207}, url={http://dx.doi.org/10.1021/jp058154u}, DOI={10.1021/jp058154u}, abstractNote={The chemisorption of the undissociated CH3SH molecule on the Au(111) surface has been studied at 5 K using scanning tunneling microscopy (STM) and density functional theory (DFT) calculations. The molecule was found to adsorb on atop Au sites on the defect-free surface. CH3SH undergoes hindered rotation about the Au-S bond on the defect-free surface which is seen in STM as a time-averaged 6-fold pattern. The pattern suggests that the potential minima directions occur for the rotating molecule at the six hollow sites surrounding the atop adsorption site. The barrier for rotation, obtained by DFT calculations, is approximately 0.1 kcal.mol(-1). At low coverages, preferential adsorption occurs at defect sites in the surface, namely, the herringbone "elbows" and random atomic step sites. Molecules adsorbed on these sites do not exhibit rotational freedom.}, number={47}, journal={The Journal of Physical Chemistry B}, publisher={American Chemical Society (ACS)}, author={Maksymovych, Peter and Sorescu, Dan C. and Dougherty, Dan and Yates, John T.}, year={2005}, month={Dec}, pages={22463–22468} }
 @article{dougherty_thürmer_degawa_cullen_reutt-robey_williams_2004, title={Triggered fast relaxation of metastable Pb crystallites}, volume={554}, ISSN={0039-6028}, url={http://dx.doi.org/10.1016/j.susc.2004.01.035}, DOI={10.1016/j.susc.2004.01.035}, abstractNote={A small fraction of Ru-supported Pb crystallites, prepared by quenching from just below the melting point (∼600 K), is found in a nearly spherical metastable configuration that relaxes after an STM tip interaction. The metastable structure could not be reproduced by exposure to hydrogen, water, carbon monoxide or oxygen. However, AES measurements demonstrate the segregation of oxygen from the Pb/Ru interface to the surface of the continuous Pb film upon post-deposition annealing, suggesting surface oxygen as a possible origin of the metastability. The rapid relaxation of the metastable crystallites has been measured using STM, providing access to the early stages of relaxation where crystalline layers are removed on the time scale of seconds, in comparison with the time scales of minutes to hours for quenched structures. The shape evolution (facet radius vs. time) for different crystallites is consistent with predictions of shape-preserving relaxation from an initial Pokrovsky–Talapov shape for T<400 K. Evaluation of the overall kinetics of single layer relaxation shows that the concentration gradients of the diffusing species across the top facet of the crystallite are on the order of 1% of the equilibrium concentration.}, number={2-3}, journal={Surface Science}, publisher={Elsevier BV}, author={Dougherty, D.B. and Thürmer, K. and Degawa, M. and Cullen, W.G. and Reutt-Robey, J.E. and Williams, E.D.}, year={2004}, month={Apr}, pages={233–244} }