Haeri Kim

Silver Schottky contacts to a-plane bulk ZnO

The temperature dependent electrical properties of Ag Schottky contacts to a-plane bulk ZnO single crystal were investigated in the temperature range of 100–300 K. The variation in the barrier heights was described by a double Gaussian distribution with two different regions in the temperature range of 200–300 and 100–180 K. The modified Richardson plot in the temperature range of 200–300 K produced the Richardson constant of 29 A cm−2 K−2 which is similar to the theoretical value of 32 A cm−2 K−2 for n-type ZnO, indicating that the inhomogeneous barrier height with the thermionic emission model can explain the current transport well in this region. Below 200 K, the bulk carriers start to freeze out and the induced oxygen vacancies in the interface region, probably due to the formation of silver oxide at the Ag–ZnO interface, will influence significantly the current transport by creating very thin interfacial layer that is susceptible to electron tunneling.

Ambient effects on electric-field-induced local charge modification of TiO2

We investigated the surface potential of TiO2 single crystals using scanning probe microscopy (SPM) under different gas environment. The SPM tip-induced electrical stress resulted in reversal in the surface potential, Vsurf, polarity only in H2/Ar (ΔVsurf = 0.30 eV) and not in Ar and O2. Quantitative measurement of the influence of ambient gas on the surface potential led us to develop a model where the adsorbed oxygen molecules and oxygen vacancies interact to change their relative concentration leading to different surface potential in TiO2. These results will give us insights into ambient-dependent physical phenomena in oxide thin film nanostructures.

Nanoscale Resistive Switching Schottky Contacts on Self-Assembled Pt Nanodots on SrTiO3

A nanoscale Schottky diode using Pt nanodisks on a Nb-doped SrTiO3 (Nb:STO) single crystal was fabricated, and resistive switching (RS) was demonstrated with conductive atomic force microscopy at ultrahigh vacuum. Pt disks with diameters on the order of 10 nm were formed using colloidal self-assembled patterns of silica nanospheres, followed by evaporation of the Pt layers on the Nb:STO single crystal. Here we show that the reproducible bipolar RS behavior of the nanoscale Pt/Nb:STO Schottky junction was achieved by utilizing local current-voltage spectroscopy. The conductance images, obtained simultaneously with topographic images, show the homogeneous current distribution of selected triangular-shaped Pt nanodisks during repetitive resistive switching between the high-resistance state (HRS) and low-resistance state (LRS). The endurance characteristics of the Pt/Nb:STO junction exhibit reliable switching behavior. These results suggest that the rectifying and resistive Pt/Nb:STO junction can be scaled down to the 10 nm range and their position can be controlled.

Enhanced Surface-and-Interface Coupling in Pd-Nanoparticle-coated LaAlO3/SrTiO3 Heterostructures: Strong Gas- and Photo-Induced Conductance Modulation

Pd nanoparticle (NP) coated LaAlO3/SrTiO3 (LAO/STO) heterointerface exhibits more notable conductance (G) change while varying the ambient gas (N2, H2/N2, and O2) and illuminating with UV light (wavelength: 365 nm) than a sample without the NPs. Simultaneous Kelvin probe force microscopy and transport measurements reveal close relationships between the surface work function (W) and G of the samples. Quantitative analyses suggest that a surface adsorption/desorption-mediated reaction and redox, resulting in a band-alignment modification and charge-transfer, could explain the gas- and photo-induced conductance modulation at the LAO/STO interface. Such surface-and-interface coupling enhanced by catalytic Pd NPs is a unique feature, quite distinct from conventional semiconductor hetero-junctions, which enables the significant conductance tunability at ultrathin oxide heterointerfaces by external stimuli.

Evolution of local work function in epitaxial VO2 thin films spanning the metal-insulator transition

Transport and Kelvin probe force microscopy measurements were simultaneously conducted on epitaxial VO2 thin films. The samples work function abruptly dropped from 4.88 eV to 4.70 eV during heating from 333 K to 353 K, suggesting a significant change in its electronic band structure spanning the metal insulator transition. The work function showed nearly no statistical deviation across the films surface during the transition, likely due to band bending at the boundaries of the small domains. Resistance profiles confirmed that the local work function corresponded closely to the resistance of the corresponding area.

Enhanced organic solar cells efficiency through electronic and electro-optic effects resulting from charge transfers in polymer hole transport blends

We demonstrate that blending fluorinated molecules in PEDOT:PSS hole transport layers (HTL) induces charge transfers which impact on both charge extraction and photogeneration within organic photovoltaic (OPV) devices. OPVs fabricated with modified HTL and two photoactive polymer blends led systematically to power conversion efficiencies (PCE) increases, with PTB7:PC70BM blend exhibiting PCE of ∼8.3%, i.e. ∼15% increase compared to pristine HTL devices. A reduced device-to-device characteristics variations was also noticed when fluorinated additives were used to modify the PEDOT:PSS. Shading lights onto the effect of HTL fluorination, we show that the morphology of the polymer:PCBM blends remains surprisingly unaffected by the fluorinated HTL surface energy but that, instead, the OPVs are impacted not only by the HTL electronic properties (work function, dipole layer, open circuit voltage, charge transfer dynamic) but also by alteration of the complex refractive indices (photogeneration, short circuit current density, external quantum efficiencies, electro-optic modelling). Both mechanisms find their origin in fluorination induced charge transfers. This work points towards fluorination as a promising strategy toward combining both external quantum efficiency modulation and power conversion efficiency enhancement in OPVs. Charge transfers could also be used more broadly to tune the optical constants and electric field distribution, as well as to reduce interfacial charge recombinations within OPVs.

YBa2Cu3O7−δ/NdBa2Cu3O7−δ/YBa2Cu3O7−δ edge junctions and SQUIDS

YBa2Cu3O7−δ (YBCO) Josephson junctions in a ramp edge geometry with NdBa2Cu3O7−δ (NBCO) barriers were fabricated by pulsed laser deposition on (100) SrTiO3 substrates. The barrier layer thicknesses were d=100, 200, and 300 A . The I–V characteristics of YBCO/NBCO/YBCO junctions changed from the resistively shunted junction type to flux‐creeplike behavior as temperature decreased. Shapiro steps due to the ac Josephson effect clearly developed under microwave irradiation. The interface between the superconducting (YBCO) and the normal layer (NBCO) turned out to be fairly clean with small interface resistance. The mean value of the measured IcRn product for junctions with 100‐A‐thick barriers was 71±34 μV at 77 K. The normal coherence length of the NBCO barrier material was ∼40 A. The SQUIDs, made of superconductor–normal metal–superconductor junctions, showed voltage modulation at 77 K.

A self-generated and degradation-resistive cratered stainless steel electrocatalyst for efficient water oxidation in a neutral electrolyte

An electron-mediated CO2-to-chemical conversion system is regarded as one of the effective solutions for the depletion of fossil fuels and the accumulation of atmospheric CO2. In this process, the protons and electrons generated from the water-oxidation reaction at an anode are used during the reduction of CO2 at a cathode, in order to produce high-value hydrocarbon chemicals. Therefore, water oxidation is also a key reaction for the overall electron-mediated CO2-to-chemical conversion. In this work, a facile preparation method is developed for a highly efficient water oxidation electrocatalyst which stably operates in a neutral bicarbonate electrolyte optimized for CO2-reduction conditions. Ni-rich cratered structures were spontaneously formed on the stainless steel surface by harsh electro-oxidation, and the chemical composition changes of Fe and Ni on the catalyst surface dramatically enhance water-oxidation activity showing an overpotential value of 504 mV at 10 mA cm−2 in a CO2-saturated bicarbonate electrolyte. In contrast to a severe degradation in the phosphate electrolyte, the cratered stainless-steel (CSS) catalyst is very stable for an 18 h reaction in the bicarbonate electrolyte. Surface spectroscopic analyses of CSS consistently revealed that the active-surface structure of the NiOOH and adsorbed water molecules is remarkably stable throughout water-oxidation in the neutral bicarbonate electrolyte, while the destruction of Ni structures by the phosphate electrolyte is proposed to cause concomitant activity loss for water oxidation.

Influence of Gas Ambient on Charge Writing at the LaAlO3/SrTiO3 Heterointerface

We investigated the influences of charge writing on the surface work function (W) and sheet resistance (R) of the LaAlO3/SrTiO3 (LAO/STO) heterointerface in several gas environments: H2(2%)/N2(98%), air, N2, and O2. The decrease in W and R due to charge writing was much larger in air (ΔW = -0.45 eV and ΔR = -40 kΩ/S) than in O2 (ΔW = -0.21 eV and ΔR = -19 kΩ/S). The reduced R could be maintained more than 100 h in H2/N2. Such distinct behaviors were quantitatively discussed, based on the proposed charge-writing mechanisms. Such analyses showed how several processes, such as carrier transfer via surface adsorbates, surface redox, electronic state modification, and electrochemical surface reactions, contributed to charge writing in each gas.

Influence of perfluorinated ionomer in PEDOT:PSS on the rectification and degradation of organic photovoltaic cells

Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) is widely used to build optoelectronic devices. However, as a hygroscopic water-based acidic material, it brings major concerns for stability and degradation, resulting in an intense effort to replace it in organic photovoltaic (OPV) devices. In this work, we focus on the perfluorinated ionomer (PFI) polymeric additive to PEDOT:PSS. We demonstrate that it can reduce the relative amplitude of OPV device burn-in, and find two distinct regimes of influence. At low concentrations there is a subtle effect on wetting and work function, for instance, with a detrimental impact on the device characteristics, and above a threshold it changes the electronic and device properties. The abrupt threshold in the conducting polymer occurs for PFI concentrations greater than or equal to the PSS concentration and was revealed by monitoring variations in transmission, topography, work-function, wettability and OPV device characteristics. Below this PFI concentration threshold, the power conversion efficiency (PCE) of OPVs based on poly(3-hexylthiophene-2,5-diyl):[6,6]-phenyl-C61-butyric acid methyl ester (P3HT:PCBM) are impaired largely by low fill-factors due to poor charge extraction. Above the PFI concentration threshold, we recover the PCE before it is improved beyond the pristine PEDOT:PSS layer based OPV devices. Supplementary to the performance enhancement, PFI improves OPV device stability and lifetime. Our degradation study leads to the conclusion that PFI prevents water from diffusing to and from the hygroscopic PEDOT:PSS layer, which slows down the deterioration of the PEDOT:PSS layer and the aluminum electrode. These findings reveal mechanisms and opportunities that should be taken into consideration when developing components to inhibit OPV degradation.