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Bi-based catalysts have attracted great attention for efficient electrocatalytic carbon dioxide (CO2) reduction to formic acid (HCOOH). However, the effect of the growth kinetics of Bi nanostructures on morphology and their catalytic performance has not been studied. Here, we varied the Bi3+ precursor concentration in the electrolyte to control the electrochemical growth rate of Bi nanostructures. It was found that the growth rate determines not only the geometric structure but also the microstructure of Bi nanostructures. The slow growth with a low precursor concentration (1 mM) produced Bi nano-sheet (NS) with high crystallinity in (012) preferred orientation. But, the polycrystalline Bi nano-branch (NB) with a larger surface area was formed by a faster growth condition (precursor concentration = 30 mM). As a result, Bi NB achieved a higher FEHCOOH of 97.1% than Bi NS (FEHCOOH = 81.5%) at −1.0 VRHE. This work reveals that the growth condition of the Bi nanostructures plays a significant role in designing the catalysts for the efficient CO2 reduction reaction.

for use as a Transparent Electrode Kyoung Soon Choi, Yensil Park, Ki-Chang Kwon, Jooheon Kim, Chang Keun Kim, Soo Young Kim, Kihyon Hong and Jong-Lam Lee b,z School of Chemical Engineering and Materials Science, Chung-Ang University, Dongjak-gu, Seoul, 156-756, Korea Division of Advanced Materials Science and Department of Materials Science and Engineering, Pohang University of Science and Technology, Pohang, Gyeongbuk, 790-784, Korea

We report the correlation between charge injection and charge balance using lithium fluoride/aluminum (LiF/Al) cathode and iridium oxide (IrO x )-coated indium tin oxide (ITO) anode in organic light emitting diodes (OLEDs). The turn-on voltage decreased from 6.7 to 1.0 V and luminous efficiency at 80 mA/cm 2 increased from 1.24 to 4.60 cd/A due to the presence of LiF and the high work function of IrO x . When IrC x was used in the anode region, the luminous efficiency increment rate decreased from 40% in Al cathode OLED to 7% in LiF/Al cathode OLED. The value of the carrier density suggested that the majority carrier changed from electrons in ITO-LiF/Al OLEDs to holes in ITO/IrC x -LiF/Al OLEDs, reducing the rate of increase. Therefore, it has been considered that charge injection should be increased and balanced in order to maximize the luminous efficiency.

We report the change in the crystallinity of pentacene film on O 2 -plasma-treated molybdenum (Mo) and the electrical properties of pentacene thin-film transistors using Mo as source-drain electrodes. The surface energy of the Mo film was increased with O 2 -plasma treatment. Synchrotron radiation photoemission spectroscopy results revealed that the valence band split from the metallic Fermi level as the surface of the Mo was treated with O 2 plasma. This provides evidence that the surface of the Mo film changed to Mo oxide (MoO x ). Grazing-angle incidence X-ray diffraction results showed that pentacene molecules on O 2 -plasma-treated Mo has well-aligned features in the vertical direction as well as in the lateral direction, leading to a high crystalline film growth. The growth mode of pentacene changed from a tilted bulklike structure to a thin-film phase type by the treatment. This change in the growth mode is attributed to the reduction in interaction energy between pentacene and Mo via the formation of oxide at the Mo/pentacene interface. Secondary electron emission spectra showed that the work function increased by 0.55 eV after O 2 -plasma treatment on the Mo surface. Thus, the Mo oxides lowered the potential barrier for the hole injection from Mo to pentacene, increasing the drain current of bottom-contact organic thin-film transistors. © 2009 The Electrochemical Society. [DOI: 10.1149/1.3153121] All rights reserved.

We report the enhancement of the electroluminescent property of organic light-emitting diodes (OLEDs) using MoO x as a hole injection layer between indium tin oxide (ITO) anodes and 4,4'-bis[N-(1-naphthyl)-N-phenyl-amino]biphenyl (α-NPD). The operation voltage for 100 mA/cm 2 of OLEDs with a 10 A thick MoO x layer decreased from 24.5 to 14.7 V, and the luminescence value at 190 mA/cm 2 increased from 1280 to 9750 cd/m 2 . α-NPD was deposited in situ on O 2 -plasma-treated ITO (O 2 -ITO) anodes and O 2 -ITO anodes coated with an O 2 -plasma-treated MoO, layer (O 2 -MoO x ,), and their interface dipole energies were quantitatively determined using synchrotron radiation photoemission spectroscopy. Secondary electron emission spectra after the deposition of α-NPD on anodes revealed that the work function of O 2 -MoO x , is 0.55 eV higher than that of O 2 -ITO. The corresponding interface dipole energies were -0.20 eV for O 2 -ITO and -0.50 eV for O 2 -MoO x . The hole injection barrier at the interface was reduced from 1.15 to 0.85 eV by the deposition of MoO x , resulting in an enhancement of the electroluminescence property of OLEDs.

Low resistance and high reflectance ohmic contacts on p-type GaN were achieved using an Ag-based metallization scheme. Oxidation annealing was the key to achieve ohmic behavior of Ag-based contacts on p-type GaN. A low contact resistivity of ∼5 X 10 -5 Ω cm 2 could be achieved from Me (=Ni, Ir, Pt, or Ru)/Ag (50/1200 A) contacts after annealing at 500°C for 1 min in O 2 ambient. Oxidation annealing promoted the out-diffusion of Ga atoms from the GaN layer, and Ga atoms dissolved in the in-diffused Ag layer with the formation of Ag-Ga solid solution, resulting in ohmic contact formation. Using Ru/Ni/Au (500/200/500 A) overlayers on the Me/Ag contacts, the excessive incorporation of oxygen molecules into the contact interfacial region, and the out-diffusion and agglomeration of Ag, were effectively prevented during oxidation annealing. As a result, a high reflectance of 87.2% at the 460 nm wavelength and a smooth surface morphology could be obtained simultaneously.

The electroluminescence performance of devices as enhanced by using ruthenium oxide (RUO x ) as a hole injection layer (HIL) between indium-tin-oxide (ITO) anodes and 4,4'-bis[N-(1-naphthyl)-N-phenyl-amino]biphenyl (a-NPD) and using Mg-Al alloy as a cathode. Both the HIL and the Mg-Al alloy were applied to organic light emitting diodes (OLEDs) on both glass and flexible substrates. The operation voltage at a current density of 100 mA/cm 2 decreased from 14.7 to 9.9 V when the RUO x interlayer and Mg-Al cathode were applied to OLEDs on glass substrate. Also, the operation voltage reduced from 17.1 to 10.8 V for OLED on polyethylene terephthalate (PET) substrate. The maximum luminance value increased from 1510 to 5400 cd/m 2 in OLEDs on glass substrate using the RuO x interlayer and Mg-Al cathode. Synchrotron radiation photoelectron spectroscopy results showed that core level of the C Is spectra shifted 0.25 eV toward higher binding energy when α-NPD was deposited on RuO x -coated ITO/PET substrate. Secondary cutoff spectra revealed that the work function increased by 0.45 eV when the RuO x interlayer was deposited on ITO/PET substrate. Thus, the RuO x interlayer lowered the potential barrier for hole injection, reducing the turn-on voltage of OLEDs and increasing the quantum efficiency.

The effects of implanted nitrogen on microstructural, magnetic, and optical properties of Mn-implanted GaN were studied. N and Mn ions were coimplanted into p-GaN and subsequently annealed at 973-1173 K. Compared with Mn-implanted samples, the Curie temperature and magnetic moment significantly increased. From high-resolution transmission electron microscopy, Mn nitrides such as Mn 6 N 2 . 5 8 and Mn 3 N 2 drastically decreased, and the concentration of N vacancies was reduced by the N implantation. This led to the increase of Mn concentration occupying Ga lattice sites, evident by the shift of Raman mode (E 2 ) at 567 cm - 1 to higher energy by about 2.5 cm - 1 . The photoluminescence peak at 2.92 eV shifted to 2.86 eV and became strong with N implantation, indicating an increase in effective hole concentration increased due to an enhanced activation of Mn impurities in p-GaN. Consequently, the enhancement of ferromagnetic property by coimplantation of Mn and N ions originated from the increase of hole concentration via the increase of Mn concentration in GaN, due to the suppression of production of Mn-N compounds.

Ruthenium oxide and iridium oxide were used as the Schottky barrier materials of GaN metal-semiconductor-metal (MSM) ultraviolet photodetector. After annealing Ru and Ir contacts at 500°C under O 2 ambient, the reverse leakage current density at -5 V drastically decreased by four orders of magnitude. Furthermore, the Schottky barrier height and optical transmittance at 360 nm simultaneously increased to 1.43 eV and 71.5% for the Ru, and to 1.48 eV and 74.8% for the Ir contact. The dramatic improvements originated from the formation of thermally stable oxide phases, RuO 2 and IrO 2 , by the annealing, resulting in the increase in the responsivity of the GaN MSM photodetector by one order of magnitude in comparison with the photodetector with Pt Schottky contact.

The chemistry of oxygen atoms at the surface of an AlGaN layer for Al 0.35 Ga 0.65 N/GaN heterostructures was investigated by scanning photoemission microscopy (SPEM) using synchrotron radiation. SPEM imaging and space-resolved photoemission spectroscopy showed that the oxygen atoms were preferentially incorporated into AlGaN rather than GaN due to the high reactivity of Al with oxygen. In situ annealing at 1000°C could lead to the outdiffusion of oxygen impurities from the bulk AlGaN, resulting in a significant increase in the intensity of Al-O bonds at the AlGaN surface. Therefore, it is suggested that the unintentional doping of oxygen impurities in AlGaN could yield a heavily doped n-type AlGaN layer, resulting in a drastic reduction in effective Schottky barrier heights of metal contacts on AlGaN/GaN heterostructures.

The microstructural reaction of Pd/Ni contact to p-type GaN has been investigated using transmission electron microscopy, and the results were used to interpret the electrical properties of the ohmic contact. When the contact was annealed at 500°C, the contact resistivity decreased to and the layer structure changed to The NiO layer produced at the surface acted as a diffusion barrier for the outdiffusion of Pd atoms to the surface. This promoted the formation of such Pd gallides due to the reaction of Pd layer with GaN substrate. Subsequently, Ga vacancies, acting as effective acceptors for electrons, were produced below the contact. The NiO layer could also suppress the outdiffusion of N atoms released from the decomposed GaN, leading to the interfacial region to be a N-rich condition. Therefore, the good thermal stability as well as low contact resistivity could be simultaneously realized with the Pd/Ni contact. © 2003 The Electrochemical Society. All rights reserved.

The effect of surface treatment in SiC on the change of Schottky characteristics was interpreted using X-ray photoelectron spectroscopy. The binding energies of both C-Si and Si-C bonds were simultaneously increased about 0.5 eV at the inductively coupled plasma (ICP)-treated surface, meaning that the Fermi level, E F , shifted toward the conduction bandedge. The increase in the Si/C atomic ratio with the ICP treatment provides evidence that a number of carbon vacancies, V C , were produced at the treated surface. This supports that E F pins at the energy level of V C near the conduction bandedge, leading to the reduction of Schottky harrier height for the transport of electrons as well as independence of Schottky barrier height on the metal work function.

The effect of surface treatment using Cl 2 inductively coupled plasma on ohmic contacts to both n-type and p-type GaN was investigated via synchrotron photoemission spectroscopy. Compared to HCI treatment, the Cl 2 plasma treatment led to a shift of the surface Fermi level by 1.0 eV toward the conduction band maximum for n-type GaN and by 0.8 eV toward the valence band minimum for p-type GaN. N vacancies were produced near the surface by the plasma treatment, playing a role in reducing the surface band bending of n-type GaN, and thereby causing a dramatic decrease in contact resistivity. A significant degradation in the ohmic contact to p-type GaN could be attributed to an n-type layer formed near the surface region of the p-type GaN after the plasma treatment. The n-p junction formed below the surface led to an increased Schottky barrier for the transport of holes.

The chemical bonding state and atomic composition at the surface of p-type GaN were studied by synchrotron radiation photoemission spectroscopy. Ga-related oxides existed even after surface treatment using HCl solution, causing pinning of the surface Fermi level at 0.62 eV above valence band maximum Meanwhile, aqua regia treatment is effective in removing the surface oxide, resulting in the movement of the Fermi level toward This resulted from the existence of Ga vacancies underneath the surface oxide. As a result, the slope in the plot of Schottky barrier height with the metal work function was increased. © 2003 The Electrochemical Society. All rights reserved.

The effect of a Au overlayer on the thermal stability of Pt transparent ohmic contacts on p-type GaN was investigated. The contact resistivity for the Pt contact (100 A) rapidly increased from 3.7 × 10 -4 to 8.8 Ω cm 2 after annealing for 16 h at 550°C. However. no degradation in the contact resistivity was found by incorporating the Au overlayer (50 A) on the Pt contact (50 A). The degradation of the Pt contact originated from the out-diffusion of N atoms producing N vacancies below the contact, resulting in a decrease in the net concentration of holes. For the Pt/Au contact, the Au layer acts as a sink for Ga atoms, causing the production of Ga vacancies below the contact. This suppresses the decrease of net concentration of holes, leading to a thermally stable ohmic contact.

Electrical properties of Ni contacts on lightly doped n-type 4H-SiC (4.2 × 10 15 cm -3 ) were interpreted in terms of changes in microstructure and atomic composition at the Ni/SiC interface. The effective Schottky barrier height was increased from 1.55 to 1.81 eV after annealing at 600°C, but decreased to 1.25 eV at 800°C. When the Ni contact was annealed at 900°C, ohmic contact was formed. Two kinds of Ni silicides, Ni 31 Si 12 and Ni 2 Si, were observed in the annealed contact and the work function of silicide increased by 0.36 eV. The atomic composition of Si in nickel silicides forming at the interface increased with the elevation of annealing temperature. This implies that silicidation process proceeded through the outdiffusion of Si, leaving Si vacancies below the contact via increase in Schottky barrier height. The temperature (900°C) for ohmic contact formation was higher than the silicidation temperature (600°C) in forming Ni 31 Si 12 and Ni 2 Si phases. An abundance of C atoms were outdiffused to the surface of contact layer, leaving C vacancies below the contact. C vacancies act as donors for electrons, which caused the increase in net electron concentration, leading to the formation of the ohmic contact on n-type SiC through the reduction of depletion layer width.

The surface of p-type GaN was treated with HCl, H 3 PO 4 , KOH, and aqua regia solutions before the deposition of Pd metal, and the ohmic contact formation mechanism was studied by observing the change of ohmic characteristics with the pretreatment of the surface. Contact resistivity decreased in sequence with treatment using HCI, H 3 PO 4 , KOH, and aqua regia solutions. The contact resistivity on p-type GaN treated with boiling aqua regia was decreased by two orders of magnitude in comparison with the HCI treated one. The amounts of oxygen and carbon atoms on p-type GaN were reduced with the same sequence of surface treatment and the aqua regia solution is most effective in selectively removing surface oxides without etching the p-type GaN. This provides evidence that the reduction of contact resistivity originates from the selective removal of surface oxides, acting as an impeding harrier for hole injection from metal to p-type GaN. Contact resistivities did not change when the hole concentration was varied from 2.8 X 10 16 to 2.5 X 10 17 cm -3 in both the HCI treated and aqua regia treated samples. This suggests that the formation of ohmic contact on p-type GaN can be explained hy the thermionic emission of holes at the interface of Pd with p-type GaN. The Schottky barrier height deduced from both electron affinity of GaN and work function of Pd is much differ from the value of about 0.35 to 0.45 eV experimentally determined using contact resistivities. This proposes that the Fermi level pinning at acceptor-type point defects on the surface of p-type GaN lead to the ohmic contact on p-type GaN.

Microstructural changes in Ni/Au contacts to p-type GaN as a function of annealing temperature were investigated using X-ray di ffraction, field emission scanning electron microscopy, and cross-sectional transmission electron microscopy combined with energy dispersive spectroscopy. The results obtained were used to interpret the electrical properties of Ni/Au ohmic contacts to p- type GaN. The contact resistivity decreased from 1.4 3 10 22 to 6.1 3 10 24 V cm 2 after annealing at 6008C. The reduction in contact resistivity resulted from the dissolution of Ga atoms into the Au-Ni solid solution produced during annealing. Au atoms diffuse d to the GaN substrate through grain boundaries and reacted with Ni atoms at the grain boundaries of Ni, producing an Au-Ni solid solution. This resulted in the evolution of microstructure with island-shaped Ni grains surrounded by the Au-Ni solid solution. At this stage, Ga atoms outdiffused from the GaN substrate and dissolved into the Au-Ni solid solution, leading to the generation of Ga vacancies below the contact. Thus, the net concentration of holes below the contact increased, and so the contact resistivity was reduced. When the sample was annealed at 8008C, the Au layer completely reacted with the Ni layer, uniformly producing an AuNi solid solution containing Au-Ga and Ni-Ga compounds. Simultaneously, N atoms reacted with Ni, and produced cubic Ni 4N in the vicinity of the metal/GaN interface. Consequently, N vacancies, acting as donors in GaN, were generated below the contact, leading to an increase of contact resistivity to 4.0 3 10 22 V cm 2 .