Jong Yeog Son

Direct observation of conducting filaments on resistive switching of NiO thin films

The Hg∕NiO∕Pt capacitor with a Hg top electrode diameter of about 35μm showed the typical bistable resistive switching characteristic. After the removal of the Hg top electrode, we directly observed the formation and removal of filaments for a high resistive state (Roff) and a low resistive state (Ron) by conducting atomic force microscope (CAFM). CAFM images for Roff and Ron states directly exhibit evidence of the formation and removal of filaments on the surface, which supports well the filament model as a switching mechanism of resistive random access memory.

NiO Resistive Random Access Memory Nanocapacitor Array on Graphene

In this study, a NiO RRAM nanocapacitor array was fabricated on a graphene sheet, which was on a Nb-doped SrTiO(3) substrate containing terraces with a regular interval of about 100 nm and an atomically smooth surface. For the formation of the NiO RRAM nanocapacitor (Pt/NiO/graphene capacitor) array, an anodic aluminum oxide (AAO) nanotemplate with a pore diameter of about 30 nm and an interpore distance of about 100 nm was used. NiO and Pt were subsequently deposited on the graphene sheet. The NiO RRAM nanocapacitor had a diameter of about 30 +/- 2 nm and a thickness of about 33 +/- 3 nm. Typical unipolar switching characteristics of the NiO RRAM nanocapacitor array were confirmed. The NiO RRAM nanocapacitor array on graphene exhibited lower SET and RESET voltages than that on a bare surface of Nb-doped SrTiO(3).

Size-tunable mesoporous spherical TiO2 as a scattering overlayer in high-performance dye-sensitized solar cells

Size-tunable mesoporous spherical TiO2 (MS TiO2) with a surface area of ∼110 m2 g−1 have been prepared through combination of “dilute mixing”-driven hydrolysis of titanium(iv) tetraethoxide and solvothermal treatment. The hierarchically structured MS TiO2 are characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), and nitrogen sorption analysis. Using three different MS TiO2 (587, 757, and 1554 nm in diameter) as a scattering overlayer on a transparent nanocrystalline TiO2 film, bi-layered dye-sensitized solar cells (DSCs) have been fabricated. Since the MS TiO2 particles are comprised of ∼10 nm nanocrystallites that cluster together to form large secondary spheres, they can function as light scatterers without sacrificing the surface area for dye-uptake. As a result, the present MS TiO2-based cells perform a noticeable improvement in the overall efficiency: maximum 9.37% versus 6.80% for the reference cell made of a TiO2 nanocrystalline film. This extraordinary result is attributed to the dual effects of enhanced dye loading and light scattering.

Writing polarization bits on the multiferroic BiMnO3 thin film using Kelvin probe force microscope

We report the multiferroic properties of epitaxially (100) oriented BiMnO3 thin film on (100) LaAlO3 substrate and preferentially (111) oriented BiMnO3 thin film on (111) Pt/TiO2/SiO2/Si substrate. Nano-size bits of ferroelectric polarization on the BiMnO3 thin film on (111) Pt/TiO2/SiO2/Si substrate can be easily written and read by Kelvin force microscope (KFM). We found that, for the preferentially (111) oriented BiMnO3 thin film, only ferroelectric polarization has been induced at the low writing biases, which makes the writing and reading process simple. This suggests that the preferentially oriented BiMnO3 thin film is a potential candidate for the high-density data storage device based on KFM.

Heteroepitaxial ferroelectric ZnSnO3 thin film.

We investigated the ferroelectric characteristics of an epitaxial perovskite ZnSnO(3) thin film on a (111) SrRuO(3)/(111) SrTiO(3) substrate fabricated by pulsed laser deposition. We confirmed that the ZnSnO(3) thin film was epitaxially grown on the substrate, forming large terraces on the surface of the ZnSnO(3) thin film. The ZnSnO(3) thin film exhibited a high ferroelectric polarization of approximately 47 microC/cm(2), which was further supported by first-principles calculations.

Synthesis of horizontally aligned ZnO nanowires localized at terrace edges and application for high sensitivity gas sensor

We developed step edge decoration method for the fabrication of semiconductor ZnO nanodots and nanowires using pulsed laser deposition. We synthesized high quality ZnO nanowires with the small diameter of about 20nm and the uniform interval of about 80nm between each nanowire, which has a simple structure for the formation of contact electrodes. The ZnO nanowire-based sensor was prepared only with the simple process of a gold electrode formation. The ZnO nanowire-based sensor exhibited the high surface-to-volume ratio of 58.6μm−1 and the significantly high sensitivity of about 10 even for the low ethanol concentration of 0.2ppm.

A Nonvolatile Memory Device Made of a Ferroelectric Polymer Gate Nanodot and a Single-Walled Carbon Nanotube

We demonstrate a field-effect nonvolatile memory device made of a ferroelectric copolymer gate nanodot and a single-walled carbon nanotube (SW-CNT). A position-controlled dip-pen nanolithography was performed to deposit a poly(vinylidene fluoride-ran-trifluoroethylene) (PVDF-TrFE) nanodot onto the SW-CNT channel with both a source and drain for field-effect transistor (FET) function. PVDF-TrFE was chosen as a gate dielectric nanodot in order to efficiently exploit its bipolar chemical nature. A piezoelectric force microscopy study confirmed the canonical ferroelectric responses of the PVDF-TrFE nanodot fabricated at the center of the SW-CNT channel. The two distinct ferroelectric polarization states with the stable current retention and fatigue-resistant characteristics make the present PVDF-TrFE-based FET suitable for nonvolatile memory applications.

High-mobility graphene nanoribbons prepared using polystyrene dip-pen nanolithography.

Graphene nanoribbons (GNRs) are fabricated by dip-pen nanolithography and polystyrene etching techniques on a SrTiO(3)/Nb-doped SrTiO(3) substrate. A GNR field-effect transistor (FET) shows bipolar FET behavior with a high mobility and low operation voltage at room temperature because of the atomically flat surface and the large dielectric constant of the insulating SrTiO(3) layer, respectively.

Dip-Pen Lithography of Ferroelectric PbTiO3 Nanodots

Dip-pen nanolithography of ferroelectric PTO nanodots is described. This position-controlled dip-pen nanolithography using a silicon nitride cantilever produced an array of ferroelectric nanodots with a minimum lateral dimension of approximately 37 nm on a Nb-doped SrTiO(3) substrate. This minimum-sized PTO dot is characterized by single-domain epitaxial growth with an enhanced tetragonality (c/a ratio) of 1.08.

Self-formed exchange bias of switchable conducting filaments in NiO resistive random access memory capacitors.

We report on the ferromagnetism of conducting filaments formed in a NiO thin film, which exhibited a typical bistable resistive switching characteristic. The NiO thin film showed an antiferromagnetic hysteresis loop for a high resistive state (R(OFF)). However, for a low resistive state (R(ON)), the conducting filaments exhibited a ferromagnetic hysteresis loop for the field cooling. The ferromagnetic hysteresis behavior of the R(ON) state reveals switchable exchange coupling between the ferromagnetic Ni conducting filaments and the antiferromagnetic NiO layer.