Jaewu Choi

Photoemission study of direct photomicromachining in poly(vinylidene fluoride)

Direct pattern transfer onto poly(vinylidene fluoride) was achieved by using x-ray photons from a synchrotron radiation source. Quadrupole mass spectrometry and ultraviolet photoemission spectroscopy, combined with ab initio molecular orbital calculations, were employed to investigate the mechanism of direct photomicromachining. The mass spectrometry identified H2, F, and HF as the etched products, with no carbon containing species being detected. The changes in photoemission spectra due to photodegradation were analyzed by comparison with ab initio molecular orbital calculations. This analysis indicated that a high degree of conjugation is generated in the degraded polymer due to the loss of fluorine atoms. It is concluded that the mechanism of direct photomicromachining is ascribable to the shrinking of the irradiated polymer region due to defluorination and the generation of conjugation.

Lattice and electronic band structure changes across the surface ferroelectric transition

Abstract The origin of the surface ferroelectric phase transition in crystalline copolymer films of vinylidene fluoride (70%) with trifluoroethylene (30%) is explored. We report a uniaxial doubling of the surface Brillouin zone in the conduction band dispersion across the surface ferroelectric phase transition. The temperature dependent changes in the electronic structure occur primarily in the conduction band and are accompanied by a surface dipole reorientation.

Surface electronic phase transition in colossal magnetoresistive manganese perovskites: La0.65Sr0.35MnO3

We have observed a distinct surface phase transition for an important class of giant magnetoresistance materials [La1−xSrxMnO3(x=0.35)]. The surface phase transition occurs at 240 K compared to 370 K for the bulk and is fundamentally different. In the bulk, a ferromagnetic metal to paramagnetic bad-metal transition occurs, while the lower-temperature surface transition is from an insulator to a semimetal. The surface of this manganese perovskite is electronically and compositionally quite different from the bulk with important implications for the behavior of artificially grown layered transition-metal oxides and for the use of surface sensitive techniques to probe the bulk.

Band filling and depletion through the doping of polyaniline thin films

The position of the molecular orbitals of polyaniline, relative to the Fermi level, shifts with the doping of polyaniline by both charge donor and charge acceptor species. Sodium as an electron donor, when added to the polyaniline system, results in an increase in electron population in the polyaniline bands but a decrease in density of states near EF. This is evident from the consistent shifts toward the greater binding energy of the occupied as well as unoccupied molecular orbitals, the valence bands and conduction band edges and characteristic core levels. In the case of iodine doping, an electron acceptor, all the states were shifted toward smaller binding energy. These results provide compelling evidence that the molecular orbitals of vapor deposited polyaniline thin films are delocalized.

Influence of dynamical scattering in crystalline poly(vinylidene fluoride-trifluoroethylene) copolymers

The effective Debye temperature of poly(vinylidene fluoride-trifluoroethylene) copolymers was measured using photoemission and neutron diffraction techniques. An effective Debye temperature of 53±11 K is obtained from the photoemission data and 69±3.5 K from neutron diffraction measurements. This effective Debye temperature is a consequence of the temperature-dependent dynamic motions perpendicular to the surface of these crystalline polymer films.

Band structure and orientation of molecular adsorbates on surfaces by angle-resolved electron spectroscopies

Publisher Summary Band structure is the energy dependence of electron states as a function of wave vector. This is the result of the combination of atomic orbitals in a periodic lattice. Similarly, molecular orbitals can also combine to form a band structure. Though molecular orbitals are highly localized in molecular overlayers and molecular crystals, there is sufficient interaction among adjacent molecules to result in dispersion, as in many examples of crystalline molecular overlayers and thin films. The band structure of the occupied and unoccupied molecular orbitals can be measured using angle-resolved photoemission spectroscopy (ARPES or ARUPS) and angle-resolved inverse photoemission spectroscopy (ARIPES). It is important that the molecular overlayer or crystal be ordered if electronic band structure is to be clearly and accurately determined. Such ordering includes well-defined molecular orientation(s). The dipole and symmetry selection rules in photoemission and the principles for mapping the experimental band structure have long been applied to the study of small molecule overlayers.

The nonmetal to metal transition with alkali doping of films of molecular icosahedra

Abstract Using photoemission, inverse photoemission, and electron energy loss spectroscopy, we have probed the changes in the electronic structure of molecularly adsorbed closo -1,2-dicarbadodecaborane ( closo -C 2 B 10 H 12 or orthocarborane) films as a function of Na doping. The insulator gap of this film of molecular icosahedra closes with Na doping and both new occupied and unoccupied states are introduced. The Na-doped system resembles a Mott-Hubbard insulator, even at high doping levels, and demonstrates the defining role of correlation.

Thin crystalline functional group copolymer poly(vinylidene fluoride–trifluoroethylene) film patterning using synchrotron radiation

The photodegradation mechanism due to synchrotron radiation exposure of crystalline poly[vinylidene fluoride–trifluoroetylene, P(VDF–TrFE)] copolymer thin films has been studied with ultraviolet photoemission spectroscopy (UPS) and mass spectroscopy. Upon increasing exposure to x-ray white light (hν⩽1000 eV), UPS measurements reveal that substantial chemical modifications occur in P(VDF–TrFE) 5 monolayer films, including the emergence of new valence band features near the Fermi level, indicating a semimetallic photodegradeted product. The photodetached fragments of the copolymer consist mainly of H2, HF, CHF, CH2. This x-ray exposure study demonstrates that P(VDF–TrFE) films, possessing unique technologically important properties, can be directly patterned by x-ray lithographic processes.

Can photoemission accurately probe the bulk electronic structure of the complex oxides

The surface of the complex oxides, in particular the manganese perovskites, is dominated by a different free enthalpy at the surface. This can be demonstrated by surface segregation after annealing the samples. By comparing relative x-ray photoelectron spectroscopy intensities of the manganese perovskite La1−xCaxMnO3 (x=0.1, 0.35) for different emission angles, the profile of elemental concentration was determined. Analysis shows evidence for pronounced surface segregation of calcium. Our results suggest that there are strong differences between surface and bulk enthalpies without segregation or surface relaxation. This is consistent with the previously observed surface resonance in the electronic band structure.

Comparison of aluminum and sodium doped poly(vinylidene fluoride-trifluoroethylene) copolymers by x-ray photoemission spectroscopy

The chemical interaction between the simple metals, aluminum and sodium, and crystalline copolymer thin films of vinylidene fluoride (70%) with trifluoroethylene (30%), has been studied using x-ray photoemission spectroscopy. Aluminum and sodium metalize the polymer differently and different binding sites for the two metals can be inferred from the corresponding core level shifts. Aluminum leads to enhanced screening of final photoemission states associated with the polymer, while sodium doping strongly influences the fluorine, but perturbs the carbon backbone only slightly.