[Show abstract][Hide abstract] ABSTRACT: Electron irradiation can alter electronic charge transport through Si-CH2(CH2)(12)-CH3//Hg molecular junctions. Applying UPS, XPS, Auger, NEXAFS, and electrical transport measurements, we show that irradiation induces defects, most likely CC bonds and C-C cross-links, which introduce new electronic states into the HOMO-LUMO gap of the alkyl chains, and, hence, effectively dope these layers. We demonstrate a 1-2 order of magnitude enhancement in current, clearly distinguishable from that of defects in as-prepared layers.
Journal of the American Chemical Society 07/2007; 129(24):7494-5. DOI:10.1021/ja071960p · 11.44 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The electronic transport through alkyl chains of a molecular level was compared and analyzed through C-Si bonds, and contacted by charge transfer between the molecule and the electrode as a result of this bond formation. A semiconductor/saturated molecule-metal junction, two transport barriers can exist simultaneously, a Schottky barrier inside the semiconductor. The tunnel barrier is affected by the molecular length or molecular layer width, and by the molecular levels closest to the Fermi level and band edges of each electrode. The existence of two distinct barriers in a system allows to extract complementary information about the molecular junction. Changing the doping type of the electrode can effect both the Schottky barrier and the tunnel barrier. For p-Si molecular junctions in both reverse-and forward-bias directions, the current is dominated by majority carriers, that is, holes, which flow from the Si to the Hg in forward bias and from the Hg to the Si in reverse bias.
[Show abstract][Hide abstract] ABSTRACT: A series of p- and n-GaAs-S-C(n)H(2n+1) || Hg junctions are prepared, and the electronic transport through them is measured. From current-voltage measurements, we find that, for n-GaAs, transport occurs by both thermionic emission and tunneling, with the former dominating at low forward bias and the latter dominating at higher forward bias. For p-GaAs, tunneling dominates at all bias voltages. By combining the analysis of the transport data with results from direct and inverse photoemission spectroscopy, we deduce an energy band diagram of the system, including the tunnel barrier and, with this barrier and within the Simmons tunneling model, extract an effective mass value of 1.5-1.6m(e) for the electronic carriers that cross the junctions. We find that transport is well-described by lowest unoccupied and highest occupied states at 1.3-1.4 eV above and 2.0-2.2 eV below the Fermi level. At the same time, the photoemission data indicate that there are continua of states from the conduction band minimum and the valence band maximum, the density of which varies with energy. On the basis of our results, it appears likely that, for both types of junctions, electrons are the main carrier type, although holes may contribute significantly to the transport in the p-GaAs system.
The Journal of Physical Chemistry B 08/2006; 110(29):14363-71. DOI:10.1021/jp062181i · 3.30 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The electroluminescence efficiency of Ir-based green emitter devices is very sensitive to the nature of the hole transport layer used. We show that by inserting a 1 nm layer of bis[4-(N,N-diethylamino)-2-methylphenyl](4-methylphenyl)methane (MPMP) in a 4,4′-bis-(carbazol-9-yl)biphenyl (CBP) hole transport layer, a device that combines the positive attributes of both MPMP (high efficiency) and CBP (low injection voltage) is obtained. These results can be understood based on a combined ultraviolet photoemission spectroscopy/inverse photoemission spectroscopy study, which reveals the very low electron affinity and superior electron blocking capability of MPMP.
[Show abstract][Hide abstract] ABSTRACT: We present a comprehensive experimental and theoretical characterization of the electronic structure of four 1,1-diaryl-2,3,4,5-tetraphenylsiloles (aryl = phenyl, 2-(9,9-dimethylfluorenyl), 2-thienyl, pentafluorophenyl). Solid-state electron affinities and ionization potentials of these siloles were measured using inverse-photoelectron spectroscopy (IPES) and photoelectron spectroscopy (PES), respectively; the density of electronic states obtained from calculations performed at the density functional theory (DFT) level corresponds very well to the PES and IPES data. The direct IPES measurements of electron affinity were then used to assess alternative estimates based on electrochemical and/or optical data. We also used DFT to calculate the reorganization energies for the electron-transfer reactions between these siloles and their radical anions. Additionally, optical data and ionization potential and electron affinity data were utilized to estimate the binding energies of excitons in these siloles.
Journal of the American Chemical Society 07/2005; 127(25):9021-9. DOI:10.1021/ja051139i · 11.44 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The possibility of nonequilibrium conditions in doped organic molecular thin films is investigated using a combination of ultraviolet photoemission spectroscopy (UPS) and contact potential difference measurements. Surface or interface photovoltage is of particular concern in materials with large band gap and appreciable band (or energy level) bending at interfaces. We investigate here zinc phthalocyanine (ZnPc) and N,N'-diphenyl-N,N'-bis(1-naphthyl)-1,1'biphenyl-4,4'' diamine (alpha-NPD) p-doped with the acceptor molecule, tetrafluorotetracyanoquinodimethane (F4-TCNQ). In both cases, we observe an upward movement of the vacuum level away from the metal interface with respect to the Fermi level, consistent with the formation of a depletion region. We show that photovoltage is not a significant factor in these doped films, under ultraviolet illumination during UPS. We suggest that the carrier recombination rate in organic films is sufficiently fast to exclude any photovoltage effects at room temperature. .
Journal of Vacuum Science & Technology A Vacuum Surfaces and Films 07/2004; 22(4):1488-1492. DOI:10.1116/1.1688363 · 2.14 Impact Factor