Buildup Of Surface Potential Research Articles

N2O Decomposition on Singly and Doubly (K and Li)-Doped Co3O4 Nanocubes─Establishing Key Factors Governing Redox Behavior of Catalysts.

The intimate mechanism of N2O decomposition on bare and redox-tuned Co3O4 nanocubes (achieved by single (Li or K) and double (Li and K) doping) was elucidated. The catalysts synthesized by the hydrothermal method were characterized by X-ray electron absorption fine structure measurements, X-ray diffraction, Raman spectroscopy, scanning electron microscopy, transmission electron microscopy, and Kelvin Probe techniques. TPSR and steady-state isothermal catalytic tests reveal that the N2O turnover frequencies are critically sensitive to the work function of the catalysts, adjusted purposely by doping. For the catalysts obtained by one-pot hydrothermal synthesis, lithiation of the Co3O4 nanocubes leads to the formation of {Li'8a, Co·16d} species, decreasing steadily the work function and the activity, while for the catalysts prepared by postsynthesis impregnation, formation of {Li'8a, Co'16d, Co··16c} species leads to a volcano-type dependence of the catalytic activity and the work function in parallel. The beneficial effect of potassium was discussed in terms of mitigation of surface potential buildup due to the accumulation of ionosorbed oxygen intermediates (surface electrostatics), which hinders the interfacial electron transfer. Analysis of the catalytic activity response to the redox tuning of Co3O4, substantiated by DFT calculations, allowed for a straightforward conceptualization of the redox nature of the N2O decomposition in terms of the lineup of frontier orbitals of the N2O/N2O- and O2-/O2 reactants with the surface DOS structure and the resultant molecular orbital interactions. The positions of the virtual bonding 3πg0(N2O)-α-3dz2 and the occupied 2πg1(O2-)-α-3dz2 states relative to the Fermi energy level play a crucial role in the regulation of the forward and backward interfacial electron transfer events, which drive the redox process.

Monolayer Phases of a Dipolar Perylene Derivative on Au(111) and Surface Potential Build-Up in Multilayers.

9-(Bis-p-tert-octylphenyl)-amino-perylene-3,4-dicarboxy anhydride (BOPA-PDCA) is a strongly dipolar molecule representing a group of asymmetrically substituted perylenes that are employed in dye-sensitized solar cells and hold great promise for discotic liquid crystal applications. Thin BOPA-PDCA films with orientated dipole moments can potentially be used to tune the energy-level alignment in electronic devices and store information. To help assessing these prospects, we here elucidate the molecular self-assembly and electronic structure of BOPA-PCDA employing room temperature scanning tunneling microscopy and spectroscopy in combination with ultraviolet and X-ray photoelectron spectroscopies. BOPA-PCDA monolayers on Au(111) exclusively form in-plane antiferroelectric phases. The molecular arrangements, the increase of the average number of molecules per unit cell via ripening, and the rearrangement upon manipulation with the STM tip indicate an influence of the dipole moment on the molecular assembly and the rearrangement. A slightly preferred out-of-plane orientation of the molecules in the multilayer induces a surface potential of 1.2 eV. This resembles the giant surface potential effect that was reported for vacuum-deposited tris(8-hydroxyquinoline)aluminum and deemed applicable for data storage. Notably, the surface potential in the case of BOPA-PDCA can in part be reversibly removed by visible light irradiation.

Methods for measurement of electron emission yield under low energy electron-irradiation by collector method and Kelvin probe method

Secondary electron emission yield of gold under electron impact at normal incidence below 50 eV was investigated by the classical collector method and by the Kelvin probe method. The authors show that biasing a collector to ensure secondary electron collection while keeping the target grounded can lead to primary electron beam perturbations. Thus reliable secondary electron emission yield at low primary electron energy cannot be obtained with a biased collector. The authors present two collector-free methods based on current measurement and on electron pulse surface potential buildup (Kelvin probe method). These methods are consistent, but at very low energy, measurements become sensitive to the earth magnetic field (below 10 eV). For gold, the authors can extrapolate total emission yield at 0 eV to 0.5, while a total electron emission yield of 1 is obtained at 40±1 eV.

Spontaneous buildup of surface potential with a thin film of a zwitterionic molecule giving noncentrosymmetric crystal structure

Surface potentials were examined using the Kelvin method for thin films of zwitterionic molecules, pyridinium 1,3-dihydro-1,3-dioxo-2H-inden-2-ylide (PI or IPB) and two nitrogen-substituted derivatives. Spontaneous buildup of the surface potential on the film (5.5 V at a film thickness of 300 nm) was only observed for the compound with unidirectional orientation of the molecular dipole moments in the single crystal. The relationship between the alignment of the molecular dipole moments in the film and the measured surface potentials was investigated using grazing incidence x-ray diffraction, pole-figure measurements, atomic force microscopy, and Kelvin probe force microscopy.

Modified electron-beam-induced deposition of metal nanostructure arrays using a parallel electron beam

A modified electron-beam-induced deposition method using a parallel beam of electrons is developed. The method relies on the buildup of surface potential on an insulating surface exposed to an electron beam. Presence of sharp edges on the insulating surface implies presence of large electric fields that lead to site-specific nucleation of metal vapor on those regions. Feature sizes as small as 20nm can be deposited without the need to use fine probes, and thus the limitation of probe size imposed on the resolution is overcome. The use of pure metal vapor also renders the process inherently clean.

Enhancement of Surface Potential Buildup and Decay of Tris(8-quinolinato)aluminum Film Deposited on Ultraviolet/Ozone-Treated Gold Electrodes

The enhancement of the surface potential of tris(8-quinolinato)aluminum (Alq3) film deposited on ultraviolet (UV)/ozone-treated gold electrodes was confirmed by surface potential measurement, and the ordering of molecular dipoles was suggested. The enhanced surface potential of the order of 10 V decayed rapidly by photoirradiation, due to molecular dipole disordering, but a surface potential of 100–400 mV remained. Interestingly, this remaining surface potential of Alq3 on the UV/ozone-treated gold electrode was always smaller than that of Alq3 film on an untreated gold electrode. It was concluded that the UV/ozone treatment is effective in decreasing the density of surface states at the Alq3-metal interface as well as for the enhancement of molecular dipole ordering.

Spontaneous buildup of giant surface potential by vacuum deposition of Alq3 and its removal by visible light irradiation

We observed high and persistent spontaneous buildup of the surface potential (SP) upon vacuum deposition of tris(8-hydroxyquinolinato) aluminum(III) (Alq3) on an Au substrate under dark conditions. SP determined by the Kelvin probe method reached 28 V at a thickness of 560 nm and the surface of the Alq3 film was positively charged. We propose a model in which preferential orientation of the dipole moments of Alq3 molecules is the origin of this buildup of the SP. The intensity of second-harmonic generation was also dramatically increased by the deposition of Alq3 under dark conditions, which supports the notion of a buildup of dipole layers. This giant surface potential was almost completely removed by irradiation of Alq3 molecules with visible light, and irradiation during deposition also prevented the buildup of SP.

Electric-field-induced phase changes in polyvinylidene fluoride: Effects from corona polarity and moisture

The constant-current corona triode has been used to charge and monitor the surface potential buildup of polyvinylidene fluoride samples in α form. Under dry air and positive polarity it is possible to pole the samples to sufficiently high electric fields so as to cause a phase change from the nonpolar to the polar δ form. This phase transition is confirmed by x-ray diffractograms. Surface potential measurements on δ samples display a ferroelectric behavior in which a plateau appears in the surface potential buildup curve due to the dipole reorientation. The values of the remanent polarization and the coercive field were estimated to be ∼50 mC/m2 and ∼65 MV/m, respectively.

The trapping of electrons in polystyrene

The trapping of electrons in localized states in polystyrene has been studied by means of an electron beam technique. A 2.2-kV beam is used to inject a short pulse of charge into the free surface of a thin film of the polymer, and a second electron beam monitors the surface potential of the film. The surface potential is related to trapped charge density and to the depth of charge penetration. By defining a trapping parameter alpha =1/ mu tau E, one can relate the incremental buildup of surface potential to the injected charge density and carrier range. The release of electrons from traps is analyzed in terms of a time and temperature dependent demarcation energy E/sub m/. This energy is related to elapsed time since charge injection via the equation E/sub m/=kT1n(vt). Time dependence of charge decay is thus related to the energy distribution of traps in the polymer.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>