Geoffrey Rojas
University of Nebraska–Lincoln
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Publication
Featured researches published by Geoffrey Rojas.
Advanced Materials | 2015
Woo Jin Hyun; Ethan B. Secor; Geoffrey Rojas; Mark C. Hersam; Lorraine F. Francis; C. Daniel Frisbie
All-printed, foldable organic thin-film transistors are demonstrated on glassine paper with a combination of advanced materials and processing techniques. Glassine paper provides a suitable surface for high-performance printing methods, while graphene electrodes and an ion-gel gate dielectric enable robust stability over 100 folding cycles. Altogether, this study features a practical platform for low-cost, large-area, and foldable electronics.
Physical Chemistry Chemical Physics | 2012
Geoffrey Rojas; Scott Simpson; Xumin Chen; Donna A. Kunkel; Justin Nitz; Jie Xiao; Peter A. Dowben; Eva Zurek; Axel Enders
Engineering the electronic structure of organics through interface manipulation, particularly the interface dipole and the barriers to charge carrier injection, is of essential importance to improve organic devices. This requires the meticulous fabrication of desired organic structures by precisely controlling the interactions between molecules. The well-known principles of organic coordination chemistry cannot be applied without proper consideration of extra molecular hybridization, charge transfer and dipole formation at the interfaces. Here we identify the interplay between energy level alignment, charge transfer, surface dipole and charge pillow effect and show how these effects collectively determine the net force between adsorbed porphyrin 2H-TPP on Cu(111). We show that the forces between supported porphyrins can be altered by controlling the amount of charge transferred across the interface accurately through the relative alignment of molecular electronic levels with respect to the Shockley surface state of the metal substrate, and hence govern the self-assembly of the molecules.
Nature Communications | 2016
Yanfei Wu; Annabel R. Chew; Geoffrey Rojas; Gjergji Sini; Greg Haugstad; Alexei Belianinov; Sergei V. Kalinin; Hong Li; Chad Risko; Jean-Luc Brédas; Alberto Salleo; C. Daniel Frisbie
Establishing fundamental relationships between strain and work function (WF) in organic semiconductors is important not only for understanding electrical properties of organic thin films, which are subject to both intrinsic and extrinsic strains, but also for developing flexible electronic devices. Here we investigate tensile and compressive strain effects on the WF of rubrene single crystals. Mechanical strain induced by thermal expansion mismatch between the substrate and rubrene is quantified by X-ray diffraction. The corresponding WF change is measured by scanning Kelvin probe microscopy. The WF of rubrene increases (decreases) significantly with in-plane tensile (compressive) strain, which agrees qualitatively with density functional theory calculations. An elastic-to-plastic transition, characterized by a steep rise of the WF, occurs at ∼0.05% tensile strain along the rubrene π-stacking direction. The results provide the first concrete link between mechanical strain and WF of an organic semiconductor and have important implications for understanding the connection between structural and electronic disorder in soft organic electronic materials.
Chemical Communications | 2012
Donna A. Kunkel; Scott Simpson; Justin Nitz; Geoffrey Rojas; Eva Zurek; Lucie Routaboul; Bernard Doudin; Pierre Braunstein; Peter A. Dowben; Axel Enders
The permanent dipole of quinonoid zwitterions changes significantly when the molecules adsorb on Ag(111) and Cu(111) surfaces. STM reveals that sub-monolayers of adsorbed molecules can exhibit parallel dipole alignment on Ag(111), in strong contrast with the antiparallel ordering prevailing in the crystalline state and retrieved on Cu(111) surfaces, which minimizes the dipoles electrostatic interaction energy. DFT shows that the rearrangement of electron density upon adsorption is a result of donation from the molecular HOMO to the surface, and back donation to the LUMO with a concomitant charge transfer that effectively reduces the overall charge dipole.
Langmuir | 2011
Geoffrey Rojas; Xumin Chen; Donna A. Kunkel; M. Bode; Axel Enders
The nucleation and growth of 2D layers of tetraphenyl porphyrin molecules on Ag(111) are studied with variable-temperature scanning tunneling microscopy. The organic/metal heteroepitaxy occurs by strict analogy to established principles for metal heteroepitaxy. A hierarchy of energy barriers for diffusion on terraces and along edges and around corners of adislands is established. The temperature is key to activating these barriers selectively, thus determining the shape of the organic aggregates, from a fractal shape at lower temperatures to a compact shape at higher temperatures. The energy barriers for the terrace diffusion of porpyrins and the molecule-molecule binding energy were determined to be 30 meV < E(terrace) < 60 and 130 meV < E(diss) < 160 meV, respectively, from measurements of island sizes as a function of temperature. This study provides an experimental verification of the validity of current models of epitaxy for the heteroepitaxy of organics and is thus expected to help establish design principles for complex metal-organic hybrid structures.
ACS Applied Materials & Interfaces | 2016
Geoffrey Rojas; Yanfei Wu; Greg Haugstad; C. Daniel Frisbie
Scanning Kelvin probe microscopy was used to measure band-bending at the model donor/acceptor heterojunction poly(3-hexylthiophene) (P3HT)/fullerene (C60). Specifically, we measured the variation in the surface potential of C60 films with increasing thicknesses grown on P3HT to produce a surface potential profile normal to the substrate both in the dark and under illumination. The results confirm a space-charge carrier region with a thickness of 10 nm, consistent with previous observations. We discuss the possibility that the domain size in bulk heterojunction organic solar cells, which is comparable to the space-charge layer thickness, is actually partly responsible for less than expected electron/hole recombination rates.
New Journal of Physics | 2011
Xumin Chen; Seolun Yang; Ji Hyun Kim; Hyung Do Kim; Jae-Sung Kim; Geoffrey Rojas; Ralph Skomski; Haidong Lu; Anand Bhattacharya; T. S. Santos; Nathan P. Guisinger; M. Bode; Alexei Gruverman; Axel Enders
The structural, electronic and dielectric properties of high-quality ultrathin BaTiO3 films were investigated. The films, which were grown by ozone-assisted molecular beam epitaxy on Nb-doped SrTiO3(001) substrates and have thicknesses as low as 8 unit cells (u.c.) (3.2nm), are unreconstructed and atomically smooth with large crystalline terraces. A strain-driven transition to three-dimensional (3D) island formation is observed forfilms of 13u.c. thickness (5.2nm). The high structural quality of the surfaces, together with dielectric properties similar to bulk BaTiO3 and dominantly TiO2 surface termination, makes these films suitable templates for the synthesis of high-quality metal- oxide multiferroic heterostructures for the fundamental study and exploitation of magneto-electric effects, such as a recently proposed interface effect in Fe/BaTiO3 heterostructures based on Fe-Ti interface bonds.
Journal of Physics: Condensed Matter | 2013
Pavel Lukashev; Ji-Hyun Kim; Seolun Yang; Jae-Sung Kim; Xumin Chen; Geoffrey Rojas; Jan Honolka; Ralph Skomski; Axel Enders; Renat Sabirianov
First-principle calculations are employed to show that the magnetic structure of small atomic clusters of Co, formed on a crystalline W(110) surface and containing 3-12 atoms, strongly deviates from the usual stable ferromagnetism of Co in other systems. The clusters are ferri-, ferro- or non-magnetic, depending on cluster size and geometry. We determine the atomic Co moments and their relative alignment, and show that antiferromagnetic spin alignment in the Co clusters is caused by hybridization with the tungsten substrate and band filling. This is in contrast with the typical strong ferromagnetism of bulk Co alloys, and ferromagnetic coupling in Fe/W(110) clusters.
Journal of Physical Chemistry C | 2010
Geoffrey Rojas; Xumin Chen; Cameron Bravo; Ji-Hyun Kim; Jae-Sung Kim; Jie Xiao; Peter A. Dowben; Yi Gao; Xiao Cheng Zeng; Wonyoung Choe; Axel Enders
Physical Review B | 2013
Donna A. Kunkel; James Hooper; Scott Simpson; Geoffrey Rojas; Stephen Ducharme; Timothy Usher; Eva Zurek; Axel Enders