Atomic Imaging of Interface Defects in an Insulating Film on Diamond.

The insulator/semiconductor interface structure is the key to electric device performance, and much interest has been focused on understanding the origin of interfacial defects. However, with conventional techniques, it is difficult to analyze the interfacial atomic structure buried in the insulating film. Here, we reveal the atomic structure at the interface between an amorphous aluminum oxide and diamond using a developed electron energy analyzer for photoelectron holography. We find that the three-dimensional atomic structure of a C-O-Al-O-C bridge between two dimer rows of the hydrogen-terminated diamond surface. Our results demonstrate that photoelectron holography can be used to reveal the three-dimensional atomic structure of the interface between a crystal and an amorphous film. We also find that the photoelectron intensity originating from the C-O bonds is strongly related to the interfacial defect density. We anticipate significant progress in the study of amorphous/crystalline interfaces based on their three-dimensional atomic structures analysis.

Floating gate memory with charge storage dots array formed by Dps protein modified with site-specific binding peptides.

We report a nanodot (ND) floating gate memory (NFGM) with a high-density ND array formed by a biological nano process. We utilized two kinds of cage-shaped proteins displaying SiO2 binding peptide (minTBP-1) on their outer surfaces: ferritin and Dps, which accommodate cobalt oxide NDs in their cavities. The diameters of the cobalt NDs were regulated by the cavity sizes of the proteins. Because minTBP-1 is strongly adsorbed on the SiO2 surface, high-density cobalt oxide ND arrays were obtained by a simple spin coating process. The densities of cobalt oxide ND arrays based on ferritin and Dps were 6.8 × 10(11) dots cm(-2) and 1.2 × 10(12) dots cm(-2), respectively. After selective protein elimination and embedding in a metal-oxide-semiconductor (MOS) capacitor, the charge capacities of both ND arrays were evaluated by measuring their C-V characteristics. The MOS capacitor embedded with the Dps ND array showed a wider memory window than the device embedded with the ferritin ND array. Finally, we fabricated an NFGM with a high-density ND array based on Dps, and confirmed its competent writing/erasing characteristics and long retention time.

Thermo-stable carbon nanotube-TiO2 nanocompsite as electron highways in dye-sensitized solar cell produced by bio-nano-process.

We produced a thermostable TiO2-(anatase)-coated multi-walled-carbon-nanotube (MWNT) nanocomposite for use in dye-sensitized solar cells (DSSCs) using biological supuramolecules as catalysts. We synthesized two different sizes of iron oxide nanoparticles (NPs) and arrayed the NPs on a silicon substrate utilizing two kinds of genetically modified cage-shaped proteins with silicon-binding peptide aptamers on their outer surfaces. Chemical vapor deposition (CVD) with the vapor-liquid-solid phase (VLS) method was applied to the substrate, and thermostable MWNTs with a diameter of 6 ± 1 nm were produced. Using a genetically modified cage-shaped protein with carbon-nanomaterials binding and Ti-mineralizing peptides as a catalyst, we were able to mineralize a titanium compound around the surface of the MWNT. The products were sintered, and thin TiO2-layer-coated MWNTs nanocomoposites were successfully produced. Addition of a 0.2 wt% TiO2-coated MWNT nanocomposite to a DSSC photoelectrode improved current density by 11% and decreased electric resistance by 20% compared to MWNT-free reference DSSCs. These results indicate that a nanoscale TiO2-layer-coated thermostable MWNT structure produced by our mutant proteins works as a superior electron transfer highway within TiO2 photoelectrodes.

Nonvolatile flash memory based on biologically integrated hierarchical nanostructures.

The first six peptides of multifunctional titanium binding peptide-1 bestowed recombinant L-ferritin, minT1-LF, was genetically engineered and used to fabricate multilayered nanoparticle architecture. The multifunctionality of minT1-LF enables specific binding of nanoparticle-accommodated minT1-LF to the silicon substrate surface and wet biochemical fabrication of gate oxide layer by its biomineralization activity. Three-dimensional (3D) nanoparticle architecture with multilayered structure was fabricated by the biological layer-by-layer method and embedded in a metal oxide-semiconductor device structure as a charge storage node of a flash memory device. The 3D-integrated multilayered nanoparticle architecture successfully worked as a charge storage node in flash memory devices that exhibited improved charge storage capacity compared with that of a conventional monolayer structure device.

Gold nanoparticle-induced formation of artificial protein capsids.

Gold nanoparticles are generally considered to be biologically inactive. However, in this study we show that the addition of 1.4 nm diameter gold nanoparticle induces the remodeling of the ring-shaped protein TRAP into a hollow, capsid-like configuration. This structural remodeling is dependent upon the presence of cysteine residues on the TRAP surface as well as the specific type of gold nanoparticle. The results reveal an apparent novel catalytic role of gold nanoparticles.

Interface State Density Prediction between an Insulator and a Semiconductor by Gaussian Process Regression Models for a Modified Process.

During data-driven process condition optimization on a laboratory scale, only a small-size data set is accessible and should be effectively utilized. On the other hand, during process development, new operations are frequently inserted or current operations are modified. These accessible data sets are somewhat related but not exactly the same type. In this study, we focus on the prediction of the quality of the interface between an insulator and GaN as a semiconductor for the potential application of GaN power semiconductor devices. The quality of the interface was represented as the interface state density, Dit, and the inserted operation to the process was the ultraviolet (UV)/O3-gas treatment. Our retrospective evaluation of model-building approaches for Dit prediction from a process condition revealed that for the UV/O3-treated interfaces, data of interfaces without the treatment contributed to performance improvement. Such performance improvement was not observed when using a data set of Si as the semiconductor. As a modeling method, the automatic relevance vector-based Gaussian process regression with the prior distribution of the length-scale parameters exhibited a relatively high predictive performance and represented a reasonable uncertainty of prediction as reflected by the distance to the training data set. This feature is a prerequisite for a potential application of Bayesian optimization. Furthermore, hyperparameters in the prior distribution of the length-scales could be optimized by leave-one-out cross-validation.

High-k Solution-Processed Barium Titanate/Polysiloxane Nanocomposite for Low-Temperature Ferroelectric Thin-Film Transistors.

Ferroelectric nanoparticles have attracted much attention for numerous electronic applications owing to their nanoscale structure and size-dependent behavior. Barium titanate (BTO) nanoparticles with two different sizes (20 and 100 nm) were synthesized and mixed with a polysiloxane (PSX) polymer forming a nanocomposite solution for high-k nanodielectric films. Transition from the ferroelectric to paraelectric phase of BTO with different nanoparticle dimensions was evaluated through variable-temperature X-ray diffraction measurement accompanied by electrical analysis using capacitor structures. A symmetric single 200 peak was constantly detected at different measurement temperatures for the 20 nm BTO sample, marking a stable cubic crystal structure. 100 nm BTO on the other hand shows splitting of 200/002 peaks correlating to a tetragonal crystal form which further merged, thus forming a single 200 peak at higher temperatures. Smaller BTO dimension exhibits clockwise hysteresis in capacitance-voltage measurement and correlates to a cubic crystal structure which possesses paraelectric properties. Bigger BTO dimension in contrast, demonstrates counterclockwise hysteresis owing to their tetragonal crystal form. Through further Rietveld refinement analysis, we found that the tetragonality (c/a) of 100 nm BTO decreases at a higher temperature which narrows the hysteresis window. A wider hysteresis window was observed when utilizing 100 nm BTO compared to 20 nm BTO even at a lower loading ratio. The present findings imply different hysteresis mechanisms for BTO nanoparticles with varying dimensions which is crucial in understanding the role of how the BTO size tunes the crystal structures for integration in thin-film transistor devices.

Chiral meta-molecules consisting of gold nanoparticles and genetically engineered tobacco mosaic virus.

We demonstrate a chiral meta-molecule in the ultraviolet (UV) and visible (VIS) regions using a complex of Au nanoparticles (NPs) and rod-shaped tobacco mosaic virus (TMV). Au NPs five nm in diameter are uniformly formed on peptide-modified TMV. The peptide-modified TMV with uniform-sized Au NPs has improved dispersion in solution. A negative circular dichroism (CD) peak is produced around 540 nm, at plasmonic resonance wavelength of Au NPs. Additionally, modification of a CD peak in the UV region is observed. Attaching NPs to a virus causes the enhancement and modification of CD peaks in both the UV and VIS regions. Our results open a new avenue for the preparation of three dimensional chiral metamaterials at optical frequencies.

Epitaxial growth of SiGe films by annealing Al-Ge alloyed pastes on Si substrate.

A simple, low-cost, and non-vacuum epitaxial growth method to realize large-area semiconductors on crystalline silicon will become the game-changer for various applications. For example, we can expect the disruptive effect on the cost of large-scale III-V multi-junction solar cells if we could replace the high-cost germanium substrate with silicon-germanium (SiGe) on Si. For SiGe epitaxial growth, we attempted to develop a process using original Al-Ge pastes for screen printing and subsequent annealing. We compare two pastes including Al-Ge alloyed pastes with compositional uniformity in each particle and Al-Ge mixed pastes. We revealed that Al-Ge alloyed paste could form flatter SiGe film with much less residual pastes, supported by in-situ observations. The uniform and sufficient dissolution of the alloyed paste is responsible for these and led to higher average Ge-composition by annealing at 500 °C. The composition in SiGe was vertically graded up to ~ 90% at the topmost surface. These results show that printing and firing of Al-Ge alloyed paste on Si is the desirable, simple, and high-speed process for epitaxial growth of SiGe, which could be potentially used as the lattice-matched virtual substrate with III-V semiconductors.

Resistive random access memory utilizing ferritin protein with Pt nanoparticles.

This study reports controlled single conductive paths found in resistive random access memory (ReRAM) formed by embedding Pt nanoparticles (Pt NPs) in NiO film. Homogeneous Pt NPs produced and placed by ferritin protein produce electric field convergence which leads to controlled conductive path formation. The ReRAM with Pt NPs shows stable switching behavior. A Pt NP density decrease results in an increase of OFF state resistance and decrease of forming voltage, whereas ON resistance was independent of the Pt NP density, which indicates that a single metal NP in a memory cell will achieve low power and stable operation.

Enhanced Thermoelectric Transport and Stability in Atomic Layer Deposited-HfO2/ZnO and TiO2/ZnO-Sandwiched Multilayer Thin Films.

Herein, enhancements in thermoelectric (TE) performance, both the power factor (PF) and thermal stability, are exhibited by sandwiching HfO2 and TiO2 layers onto atomic layer deposited-ZnO thin films. High-temperature TE measurements from 300 to 450 K revealed an almost two-fold improvement in electrical conductivity for TiO2/ZnO (TZO) samples, primarily owing to an increase in carrier concentration by Ti doping. On the other hand, HfO2/ZnO (HZO) achieved the highest PF values owing to maintaining Seebeck coefficients comparable to pure ZnO. HZO also exhibited excellent stability after multiple thermal cycles, which has not been previously observed for pure or doped ZnO thin films. Such improvement in both TE properties and thermal stability of HZO can be attributed to a shift in crystalline orientation from the a axis to c axis, as well as the high bond dissociation energy of Hf-O, stabilizing the ZnO structure. These unique properties exhibited by HZO and TZO thin films synthesized by atomic layer deposition pave the way for next-generation transparent TE devices.

The characterization of a single discrete bionanodot for memory device applications.

We investigated electronic properties of a biochemically synthesized cobalt oxide bionanodot (Co-BND) by means of scanning tunneling microscopy/spectroscopy (STM/STS) and Kelvin-probe force microscopy (KFM). Experimentally obtained I-V characteristics and numerically obtained dI/dV and (dI/dV)/(I/V) from I-V revealed the band gap energy, band position of valence and conduction band of the Co-BND. KFM observation shows that bias polarity dependent surface potential change after charge injection. The observed surface potential change indicates that the Co-BND has a charge storage capability. We demonstrated the application of Co-BNDs for electronic devices by choosing flash memory as the example device. The fabricated Co-BND embedded MOS memory showed clear memory operation due to the charge confinement in the embedded Co-BNDs.

Segregation-free bromine-doped perovskite solar cells for IoT applications.

Perovskite solar cells have attracted much attention as next-generation solar cells because of their high efficiency and low fabrication costs. Moreover, perovskite solar cells are a promising candidate for indoor energy harvesting. We investigated the effect of bandgap tuning on the characteristics of triple cation-based perovskite solar cells under fluorescent lamp illumination. According to the current density-voltage curves, perovskite solar cells with a wider bandgap than the conventional one exhibited improved open-circuit voltage without sacrificing short-circuit current density under fluorescent lamp illumination. Moreover, the wider bandgap perovskite films including a large amount of bromine in the composition did not show phase segregation, which can degrade the photovoltaic performance of perovskite solar cells, after fluorescent lamp illumination. Our results demonstrate the facile strategy to improve the performance of perovskite solar cells under ambient lighting and great potential of perovskite solar cells for indoor applications such as power sources for the internet of things.

Instantaneous Semiconductor-to-Conductor Transformation of a Transparent Oxide Semiconductor a-InGaZnO at 45 °C.

The emphasis on ubiquitous technology means that future technological applications will depend heavily on transparent conducting materials. To facilitate truly ubiquitous applications, transparent conductors should be fabricated at low temperatures (<50 °C). Here, we demonstrate an instantaneous (<100 ns) and low-temperature (<45 °C at the substrate) method, excimer laser irradiation, for the transformation of an a-InGaZnO semiconductor into a transparent highly conductive oxide with performance rivaling traditional and emerging transparent conductors. Our analysis shows that the instantaneous and substantial conductivity enhancement is due to the generation of a large amount of oxygen vacancies in a-InGaZnO after irradiation. The method's combination of low temperature, extremely rapid process, and applicability to other materials will create a new class of transparent conductors for the high-throughput roll-to-roll fabrication of future flexible devices.

Easy and green preparation of a graphene-TiO2 nanohybrid using a supramolecular biomaterial consisting of artificially bifunctionalized proteins and its application for a perovskite solar cell.

We report a novel preparation method for a graphene/TiO2 nanohybrid using a supramolecular biomaterial (CDT1). CDT1 can offer an increase in the dispersibility of graphene in water and subsequent complexation of graphene and TiO2. This nanohybrid was applied to a perovskite solar cell and success was achieved in improving its photoelectric conversion efficiency.

Effect of Gold Nanoparticle Distribution in TiO2 on the Optical and Electrical Characteristics of Dye-Sensitized Solar Cells.

Photoanodes comprising Au nanoparticles (GNPs) and thin TiO2 layers with a stacked structure were fabricated by repeating the application of TiO2 paste and GNP solutions on conductive glass to vary the distribution of GNPs in the TiO2 layer. The plasmon-enhanced characteristics of dye-sensitized solar cells (DSSCs) with such photoanodes were investigated. Both the absorption of the TiO2 layer and the performance of the DSSC are found to be most increased by plasmonic enhancement when GNPs are concentrated near the position in the TiO2 layer, which is the penetration depth of the incident light of wavelength corresponding to the maximum absorption of the N719 dye (~ 520 nm). When a GNP layer with a relatively high density of 1.3 µg/cm2 density was formed at its position, and two GNP layers with a relatively low density of 0.65 µg/cm2 were formed near the front side of the incident light, the short-circuit current density (Jsc) and energy conversion efficiency (η) of the DSSC were found to be 10.8 mA/cm2 and 5.0%, increases of 15 and 11%, respectively, compared with those of the DSSC without GNPs. Our work suggests that optimization of the distribution of GNPs in the TiO2 layer is very important for improving the performance of DSSCs fabricated by utilizing GNPs.

Biotemplated Synthesis of TiO2-Coated Gold Nanowire for Perovskite Solar Cells.

Fibrous nanomaterials have been widely employed toward the improvement of photovoltaic devices. Their light-trapping capabilities, owing to their unique structure, provide a direct pathway for carrier transport. This paper reports the improvement of perovskite solar cell (PSC) performance by a well-dispersed TiO2-coated gold nanowire (GNW) in a TiO2 cell layer. We used an artificially designed cage-shaped protein to synthesize a TiO2-coated GNW in aqueous solution under atmospheric pressure. The artificially cage-shaped protein with gold-binding peptides and titanium-compound-biomineralizing peptides can bind GNWs and selectively deposit a thin TiO2 layer on the gold surface. The TiO2-coated GNW incorporated in the photoelectrodes of PSCs increased the external quantum efficiency within the range of 350-750 nm and decreased the internal resistance by 12%. The efficient collection of photogenerated electrons by the nanowires boosted the power conversion efficiency by 33% compared to a typical mesoporous-TiO2-nanoparticle-only electrode.

Fabrication of Nanoshell-Based 3D Periodic Structures by Templating Process using Solution-derived ZnO.

Fabrication methods for a 3D periodic nanostructure with excellent and unique properties for various applications, such as photonic and phononic crystals, have attracted considerable interest. Templating processes using colloidal crystals have been proposed to create nanoshell-based 3D structures over a large area with ease. However, there are technical limitations in structural design, resulting in difficulties for structural flexibility. Here, we demonstrate a combination of proximity field nanopatterning and infiltration processes using solution-derived ZnO for a nanoshell-based 3D periodic structure with high structural flexibility and controllability. A unique process of infiltration of a solution-derived material into a polymeric template prepared by a proximity field nanopatterning process achieves the fabrication of a pre-formed layer that works as a protective layer for the template and framework for the inverse structure. Subsequently, this process shows the controllability of nanoshell thickness and significant improvement in the structure height shrinkage factor (16%) compared to those of a previous non-vacuum infiltration method (34%). The proposed method offers high controllability and flexibility in the design of structural sizes, leading to further development toward nanoshell-based 3D structures for various applications including energy devices and sensors.

Creating Reversible p-n Junction on Graphene through Ferritin Adsorption.

An alternative way to construct a stable p-n junction on graphene-based field effect transistor (G-FET) through physical adsorption of ferritin (spherical protein shell) is presented. The produced p-n junction on G-FET could also operate through water-gate. Native ferritins are known to be negatively charged in wet condition; however, we found that native negatively charged ferritins became positively charged after performing electron beam (EB)-irradiation. We utilized this property to construct p-n junction on G-FET. We found also that EB-irradiation could remove the effect of charged impurity adsorbed on graphene layer, thus the Dirac point was adjusted to gate voltage Vg = 0.

Selection of a novel peptide aptamer with high affinity for TiO2-nanoparticle through a direct electroporation with TiO2-binding phage complexes.

We have developed an easy and rapid screening method of peptide aptamers with high affinity for a target material TiO2 using M13 phage-display and panning procedure. In a selection step, the phage-substrate complexes and Escherichia coli cells were directly applied by electric pulse for electroporation, without separating the objective phages from the TiO2 nanoparticles. Using this simple and rapid method, we obtained a novel peptide aptamer (named ST-1 with the sequence AYPQKFNNNFMS) with highly strong binding activity for TiO2. A cage-shaped protein fused with both ST-1 and an available carbon nanotube-affinity peptide was designed and produced in E. coli. The multi-functional supraprotein could efficiently mineralize a titanium-compound around the surface of single-wall carbon nanotubes (SWNTs), indicating that the ST-1 is valuable in the fabrication of nano-composite materials with titanium-compounds. The structural analysis of ST-1 variants indicated the importance of the N-terminal region (as a motif of AXPQKX6S) of the aptamer in the TiO2-binding activity.