Huiseong Jeong

Fabrication of 0.1 μm gate aperture Mo-tip field-emitter arrays using interferometric lithography

A fabrication process has been developed which enables us to make matrix-addressable Mo-tip field-emitter arrays (FEAs) with 0.1 μm gate aperture and 0.2 μm tip-to-tip distance. An interferometric lithography combined with a trilevel resist process which uses an imaging resist layer, a silicon oxide interlayer, and antireflective coating has been implemented to fabricate the periodic structure of the gated FEAs in an addressable matrix. The matrix-addressable FEAs have shown a turn-on voltage as low as 13 V and an emission current density of 17 mA/cm2 at a gate voltage of 30 V.

Inverted planar perovskite solar cells with dopant free hole transporting material: Lewis base-assisted passivation and reduced charge recombination

Three novel triarylamine derivatives (TPACs: TPAC0M, TPAC2M and TPAC3M), sharing the same conjugation scaffold but possessing different numbers of methoxy units, were designed for high performance hole transport materials (HTMs) of p–i–n planar perovskite solar cells (PSCs). Cyclic voltammetry and absorption results showed that their energy levels would be beneficial to work as HTMs of PSCs, and time-resolved photoluminescence and transient photo-voltage studies proved that TPAC-based PSCs had better charge extraction and more suppressed non-radiative recombination properties than PEDOT:PSS-based PSCs, leading to superior power conversion efficiency (PCE). We confirmed that the methoxy units introduced to the triarylamine derivatives did not cause noticeable changes in their optical properties, such as absorption and bandgap, electrical conductance, measured by conductive atomic force microscopy, and mobility, but the charge extraction and recombination behaviors of TPAC-based PSCs could be improved by increasing the number of methoxy units in the arylamine moiety. X-ray photoelectron spectroscopy revealed that methoxy units could act as a Lewis base passivating the defect sites at the interface between HTM and perovskite, consequently resulting in an increase of their PCE to 17.54% without any dopants.

Charge Transport in Light Emitting Devices Based on Colloidal Quantum Dots and a Solution-Processed Nickel Oxide Layer

We fabricated hybrid light emitting devices based on colloidal CdSe/ZnS core/shell quantum dots and a solution-processed NiO layer. The use of a sol-gel NiO layer as a hole injection layer (HIL) resulted in overall improvement in device operation compared to a control device with a more conventional poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) HIL. In particular, luminous efficiency increased substantially because of the suppression of excessive currents and became as large as 2.45 cd/A. To manifest the origin of current reduction, temperature- and electric field-dependent variations of currents with respect to bias voltages were investigated. In a low bias voltage range below the threshold for luminance turn-on, the Poole-Frenkel (PF) emission mechanism was responsible for the current-density variation. However, the space-charge-limited current modified with PF-type mobility ruled the current-density variation in high bias voltage range above the threshold.

Resistance switching in epitaxial SrCoOx thin films

We observed bipolar switching behavior from an epitaxial strontium cobaltite film grown on a SrTiO3 (001) substrate. The crystal structure of strontium cobaltite has been known to undergo topotactic phase transformation between two distinct phases: insulating brownmillerite (SrCoO2.5) and conducting perovskite (SrCoO3−δ) depending on the oxygen content. The current–voltage characteristics of the strontium cobaltite film showed that it could have a reversible insulator-to-metal transition triggered by electrical bias voltage. We propose that the resistance switching in the SrCoOx thin film could be related to the topotactic phase transformation and the peculiar structure of SrCoO2.5.

Uncovering operational mechanisms of a single-walled carbon nanotube network device using local probe electrical characterizations.

The apparent field-effect-transistor (FET)-like operations of a device based on a network of single-walled carbon nanotubes (SWCNTs) are elucidated with the help of local probe electrical characterization methods using an atomic force microscope. The apparent switching behavior of the device with an on-off ratio>10(4) is found to be due to just two localized areas in the network of SWCNTs based on the measurements by electrostatic force microscopy and scanning gate microscopy. The result demonstrates that the conductance of a network of SWCNTs can be dominated by localized perturbations.

Quantitative non-contact voltage profiling of quasi one-dimensional nanoelectronic devices

Local electrical characterization tools, such as Electrostatic force microscopy (EFM), can provide local electrical information of nanoelectronic devices, albeit mostly qualitative. For example, EFM images are convolution of local surface potential, capacitance, and contact potential variations in the device. In this study, we demonstrate a calibration procedure to obtain quantitative local voltage distributions of quasi one-dimensional nanoelectronic devices based on carbon nanotubes and ZnO nanowires. By comparing the results with IV measurements of the same devices, we can obtain local electrical properties of devices such as contact resistance, intrinsic resistivity of the nanomaterial, and resistance of a defect.

Non-Contact Local Conductance Mapping of Individual Graphene Oxide Sheets during the Reduction Process

We used electrostatic force microscopy (EFM) to investigate local conducting states of atomically thin individual graphene oxide (GO) sheets and monitor the spatial evolution of their conducting properties during the reduction process. Because of the thinness of the GO sheets and finite carrier density, the electric field is partially screened in the reduced GO, which is manifested in the EFM phase signals. We found inhomogeneous oxidation states in as-prepared GO sheets and followed the evolution of reduction process in the individual GO sheets during both thermal and chemical reduction. We also compared the EFM measurement results with simultaneous IV characteristics to assess correlations between two measurements.