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1.
ACS Omega ; 9(24): 25968-25975, 2024 Jun 18.
Article in English | MEDLINE | ID: mdl-38911734

ABSTRACT

In order to obtain thermally stable thin-film materials with high dielectric constant, A2B2O7 thin films (Sr2Ta2O7, Sr2Nb2O7, La2Zr2O7, and La2Ti2O7) containing Sr or La and their solid solutions were grown on Pt/Ti/SiO2/Si substrates by RF sputtering and their crystal structures and dielectric properties were investigated. The Sr2Ta2O7 and La2Ti2O7 films exhibit highly oriented crystal structures. By contrast, the Sr2Nb2O7 and La2Zr2O7 films exhibit polycrystalline structures. The leakage properties of the Sr-containing films are lower and more stable in the high-temperature region (up to 300 °C) than those of the La-containing films. Among the investigated films, the Sr2Ta2O7 film grown at 500 °C and annealed at 900 °C shows the most stable dielectric constant with respect to temperature in the temperature range from room temperature to 300 °C. In addition, the xSr2Ta2O7-(1-x)La2Ti2O7 solid solutions exhibit enhanced dielectric properties at x = 0.35. The dielectric constant is greater than 100, and its variation with temperature is less than 10%. The Sr-containing A2B2O7 ferroelectric thin films have potential applications as high-temperature film capacitors that can operate at temperatures as high as 300 °C.

2.
Chem Sci ; 15(24): 9147-9154, 2024 Jun 19.
Article in English | MEDLINE | ID: mdl-38903239

ABSTRACT

Lattice strain effects on the piezoelectric properties of crystalline ferroelectrics have been extensively studied for decades; however, the strain dependence of the piezoelectric properties at nano-level has yet to be investigated. Herein, a new overview of the super-strain of nanoporous polycrystalline ferroelectrics is reported for the first time using a nanoengineered barium calcium zirconium titanate composition (Ba0.85Ca0.15)(Ti0.9Zr0.1)O3 (BCZT). Atomic-level investigations show that the controlled pore wall thickness contributes to highly strained lattice structures that also retain the crystal size at the optimal value (<30 nm), which is the primary contributor to high piezoelectricity. The strain field derived from geometric phase analysis at the atomic level and aberration-corrected high-resolution scanning transmission electron microscopy (STEM) yields of over 30% clearly show theoretical agreement with high piezoelectric properties. The uniqueness of this work is the simplicity of the synthesis; moreover the piezoresponse d 33 becomes giant, at around 7500 pm V-1. This response is an order of magnitude greater than that of lead zirconate titanate (PZT), which is known to be the most successful ferroelectric over the past 50 years. This concept utilizing nanoporous BCZT will be highly useful for a promising high-density electrolyte-free dielectric capacitor and generator for energy harvesting in the future.

3.
ACS Omega ; 7(23): 19380-19387, 2022 Jun 14.
Article in English | MEDLINE | ID: mdl-35721998

ABSTRACT

The ability to control the polarity of an all-sputtered epitaxial GaN/AlN/Al film on a Si(111) substrate via intermediate oxidization was investigated. A stable surface of GaN on a Si substrate is a N-terminated surface (-c surface); hence, for electric device applications, the Ga-terminated surface (+c surface) is preferable. The GaN/AlN/Al film on Si(111) showed a -c surface, as confirmed by time-of-flight low-energy atom scattering spectroscopy (TOFLAS) and X-ray photoelectron spectroscopy (XPS). The AlN layer was intentionally oxidized via air exposure during film growth. The GaN surface subjected to the oxidization process had the +c surface. Secondary-ion mass spectrometry measurements indicated a high oxygen concentration after the intentional oxidization. However, the intentional oxidization degraded the crystallinity of the GaN/AlN layer. By changing the oxidization point and repeating the GaN/AlN growth, the crystallinity of GaN was recovered. Such polarity control of GaN on Si grown by sputtering shows strong potential for the fabrication of large-diameter +c-GaN template substrates at low cost.

4.
Nat Comput Sci ; 1(7): 470-478, 2021 Jul.
Article in English | MEDLINE | ID: mdl-38217117

ABSTRACT

Existing data-driven approaches for exploring high-entropy alloys (HEAs) face three challenges: numerous element-combination candidates, designing appropriate descriptors, and limited and biased existing data. To overcome these issues, here we show the development of an evidence-based material recommender system (ERS) that adopts Dempster-Shafer theory, a general framework for reasoning with uncertainty. Herein, without using material descriptors, we model, collect and combine pieces of evidence from data about the HEA phase existence of alloys. To evaluate the ERS, we compared its HEA-recommendation capability with those of matrix-factorization- and supervised-learning-based recommender systems on four widely known datasets of up-to-five-component alloys. The k-fold cross-validation on the datasets suggests that the ERS outperforms all competitors. Furthermore, the ERS shows good extrapolation capabilities in recommending quaternary and quinary HEAs. We experimentally validated the most strongly recommended Fe-Co-based magnetic HEA (namely, FeCoMnNi) and confirmed that its thin film shows a body-centered cubic structure.

5.
ACS Comb Sci ; 22(9): 433-439, 2020 09 14.
Article in English | MEDLINE | ID: mdl-32659073

ABSTRACT

The correlation between the crystal structure and valence band structure of a (GaxIn1-x)2O3 solid solution system was investigated by using combinatorial synthesis. At a low Ga content of (GaxIn1-x)2O3 with a single-phase cubic In2O3 crystal structure, a surface electron accumulation layer (SEAL), which is an important electrical phenomenon in In2O3, was confirmed. When the Ga content increased to approximately x = 0.4, mixed crystal structures of Ga2O3 and In2O3 were produced. Above x = 0.5, the dominant valence band structure was attributed to Ga2O3, the SEAL disappeared, and the sheet resistance increased greatly by 5 orders of magnitude or more. The in-gap state and valence band structure of the (GaxIn1-x)2O3 solid solution system were strongly affected by Ga2O3; however, the valence band maximum position shifted to a higher binding energy.


Subject(s)
Combinatorial Chemistry Techniques , Germanium/chemistry , Indium/chemistry , Oxygen/chemistry , Solutions
6.
ChemSusChem ; 13(12): 3269-3276, 2020 Jun 19.
Article in English | MEDLINE | ID: mdl-32133787

ABSTRACT

A new heterostructured material is synthesized with lamellar arrangements in nanoscale precision through an innovative synthetic approach. The self-assembled Ni-based cyano-bridged coordination polymer flakes (Ni-CP) and graphene oxide (GO) nanosheets with a layered morphology (Ni-CP/GO) are used as precursors for the synthesis of multicomponent hybrid materials. Annealing of Ni-CP/GO in nitrogen at 450 °C allows the formation of Ni3 C/rGO nanocomposites. Grinding Ni-CP/GO and thiourea and annealing under the same conditions produces N,S-codoped reduced GO-wrapped NiS2 flakes (NiS2 /NS-rGO). Interestingly, further heating up to 550 °C allows the phase transformation of NiS2 into NiS accompanied by the formation of a face-centered cubic (FCC-Ni) metal phase between NS-rGO layers (FCC-Ni-NiS/NS-rGO). Among all the materials, the resulting FCC-Ni-NiS/NS-rGO exhibits good electrocatalytic activity and stability toward the oxygen evolution reaction (OER) owing to the synergistic effect of multiphases, the well-designed alternating layered structures on the nanoscale with abundant active sites.

7.
ACS Omega ; 4(12): 14680-14687, 2019 Sep 17.
Article in English | MEDLINE | ID: mdl-31552307

ABSTRACT

Mesoporous hafnium dioxide (HfO2) thin films (around 20 nm thick) were fabricated by a sol-gel-based spin-coating process, followed by an annealing process at 600 °C to realize the ion-conducting media for the ionics (e.g., Na+ and K+ for rechargeable ion batteries). Another film of aluminum metal (10 nm thick) was deposited by direct current sputtering to soak into the mesopores. A monitored thermal treatment process at 500 °C in the air yields mesostructured HfO2/Al2O3 composite thin films. However, aluminum dioxide (Al2O3) is formed during annealing as an insulating film to reduce the leakage current while retaining the ionic conductivity. The obtained mesostructured HfO2/Al2O3 films show a leakage current at 3.2 × 10-9 A cm-2, which is significantly smaller than that of the mesoporous HfO2 film (1.37 × 10-5 A cm-2) or HfO2/Al film (0.037 A cm-2) at a bias voltage of 1.0 V, which is enough for ion conduction. In the meantime, among all the thin films, the mesostructured HfO2/Al2O3 composite thin films display the smallest Nyquist arc diameter in 1.0 M KOH electrolyte, implying a lower impedance at the electrode/electrolyte interface and reflecting a better ion diffusion and movement.

8.
ACS Comb Sci ; 21(4): 310-315, 2019 04 08.
Article in English | MEDLINE | ID: mdl-30790519

ABSTRACT

High-throughput and combinatorial materials science methods were used to investigate the dependence of the work function in the Ni-Si system on the B content (0-30 at. %). Alloying of NiSi is used to adapt its properties to suit the needs as a gate electrode material. Thin-film materials libraries were fabricated and investigated with respect to their structural and electrical properties. Further the work function values of selected samples in the region of interest were analyzed. The results show that the work function can be adjusted between 4.86 eV (B = 4.2 at. %) and 5.16 eV (B = 29.2 at. %) for (NiSi)B x.


Subject(s)
Boron/chemistry , Electric Conductivity , Nickel/chemistry , Silicon/chemistry , Combinatorial Chemistry Techniques/methods , Molecular Structure , Small Molecule Libraries/chemistry , Structure-Activity Relationship , Surface Properties
9.
ChemistryOpen ; 7(8): 599-603, 2018 Aug.
Article in English | MEDLINE | ID: mdl-30094126

ABSTRACT

Herein, we report a novel method for the formation of hollow Prussian blue analogue (CoFe-PBA) nanocubes, using spherical silica particles as sacrificial templates. In the first step, silica cores are coated by a CoFe-PBA shell and then removed by etching with hydrofluoric acid (HF). The cubic shape of CoFe-PBA is well-retained even after the removal of the silica cores, resulting in the formation of hollow CoFe-PBA cubes. The specific capacity of the hollow CoFe-PBA nanocubes electrodes is about two times higher than that of solid CoFe-PBA nanocubes as storage materials for sodium ions. Such an improvement in the electrochemical properties can be attributed to their hollow internal nanostructure. The hollow architecture can offer a larger interfacial area between the electrolyte and the electrode, leading to an improvement in the electrochemical activity. This strategy can be applied to develop PBAs with hollow interiors for a wide range of applications.

10.
Nano Lett ; 18(7): 4355-4359, 2018 07 11.
Article in English | MEDLINE | ID: mdl-29961329

ABSTRACT

Multivalued logic circuits, which can handle more information than conventional binary logic circuits, have attracted much attention as a promising way to improve the data-processing capabilities of integrated circuits. In this study, we developed a ternary inverter based on organic field-effect transistors (OFET) as a potential component of high-performance and flexible integrated circuits. Key elements are anti-ambipolar and n-type OFETs connected in series. First, we demonstrate an organic ternary inverter that exhibits three distinct logic states. Second, the operating voltage was greatly reduced by taking advantage of an Al2O3 gate dielectric. Finally, the operating voltage was finely tuned by the designing of the device geometry. These results are achievable owing to the flexible controllability of the device configuration, suggesting that the organic ternary inverter plays an important role with regard to high-performance organic integrated circuits.

11.
Inorg Chem ; 57(5): 2773-2781, 2018 Mar 05.
Article in English | MEDLINE | ID: mdl-29431431

ABSTRACT

Orthorhombic rare-earth trivalent manganites RMnO3 (R = Er-Lu) were self-doped with Mn to form (R0.667Mn0.333)MnO3 compositions, which were synthesized by a high-pressure, high-temperature method at 6 GPa and about 1670 K from R2O3 and Mn2O3. The average oxidation state of Mn is 3+ in (R0.667Mn0.333)MnO3. However, Mn enters the A site in the oxidation state of 2+, creating the average oxidation state of 3.333+ at the B site. The presence of Mn2+ was confirmed by hard X-ray photoelectron spectroscopy measurements. Crystal structures were studied by synchrotron powder X-ray diffraction. (R0.667Mn0.333)MnO3 crystallizes in space group Pnma with a = 5.50348(2) Å, b = 7.37564(1) Å, and c = 5.18686(1) Å for (Lu0.667Mn0.333)MnO3 at 293 K, and they are isostructural with the parent RMnO3 manganites. Compared with RMnO3, (R0.667Mn0.333)MnO3 exhibits enhanced Néel temperatures of about TN1 = 106-110 K and ferrimagnetic or canted antiferromagnetic properties. Compounds with R = Er and Tm show additional magnetic transitions at about TN2 = 9-16 K. (Tm0.667Mn0.333)MnO3 exhibits a magnetization reversal or negative magnetization effect with a compensation temperature of about 16 K.

12.
ACS Appl Mater Interfaces ; 10(3): 2762-2767, 2018 Jan 24.
Article in English | MEDLINE | ID: mdl-29277988

ABSTRACT

The main purpose of this study is to establish a guideline for controlling the device properties of organic antiambipolar transistors. Our key strategy is to use interface engineering to promote carrier injection at channel/electrode interfaces and carrier accumulation at a channel/dielectric interface. The effective use of carrier injection interlayers and an insulator layer with a high dielectric constant (high-k) enabled the fine tuning of device parameters and, in particular, the onset (Von) and offset (Voff) voltages. A well-matched combination of the interlayers and a high-k dielectric layer achieved a low peak voltage (0.25 V) and a narrow on-state bias range (2.2 V), indicating that organic antiambipolar transistors have high potential as negative differential resistance devices for multivalued logic circuits.

13.
Nanoscale ; 9(31): 11297-11302, 2017 Aug 10.
Article in English | MEDLINE | ID: mdl-28761943

ABSTRACT

Quantum molecular devices have a potential for the construction of new data processing architectures that cannot be achieved using current complementary metal-oxide-semiconductor (CMOS) technology. The relevant basic quantum transport properties have been examined by specific methods such as scanning probe and break-junction techniques. However, these methodologies are not compatible with current CMOS applications, and the development of practical molecular devices remains a persistent challenge. Here, we demonstrate a new vertical resonant tunneling transistor for large-scale integration. The transistor channel is comprised of a MOS structure with C60 molecules as quantum dots, and the structure behaves like a double tunnel junction. Notably, the transistors enabled the observation of stepwise drain currents, which originated from resonant tunneling via the discrete molecular orbitals. Applying side-gate voltages produced depletion layers in Si substrates, to achieve effective modulation of the drain currents and obvious peak shifts in the differential conductance curves. Our device configuration thus provides a promising means of integrating molecular functions into future CMOS applications.

14.
ACS Appl Mater Interfaces ; 9(4): 3581-3589, 2017 Feb 01.
Article in English | MEDLINE | ID: mdl-28058843

ABSTRACT

A NaF/KF postdeposition treatment (PDT) has recently been employed to achieve new record efficiencies of Cu(In,Ga)Se2 (CIGSe) thin film solar cells. We have used a combination of depth-dependent soft and hard X-ray photoelectron spectroscopy as well as soft X-ray absorption and emission spectroscopy to gain detailed insight into the chemical structure of the CIGSe surface and how it is changed by different PDTs. Alkali-free CIGSe, NaF-PDT CIGSe, and NaF/KF-PDT CIGSe absorbers grown by low-temperature coevaporation have been interrogated. We find that the alkali-free and NaF-PDT CIGSe surfaces both display the well-known Cu-poor CIGSe chemical surface structure. The NaF/KF-PDT, however, leads to the formation of bilayer structure in which a K-In-Se species covers the CIGSe compound that in composition is identical to the chalcopyrite structure of the alkali-free and NaF-PDT absorber.

15.
Dalton Trans ; 44(3): 997-1008, 2015 Jan 21.
Article in English | MEDLINE | ID: mdl-25407768

ABSTRACT

This work investigates the spectroscopic properties of (Co1-xZnx)Al2O4 with a range of x of 0 ≤ x ≤ 1. Spectroscopic and crystallographic evaluations using XRD, Raman, FT-IR and UV-Vis spectroscopy reveal that Zn(2+) substitution systematically changes the lattice constant, which mainly depends on the Co-O bonds, and the related optical characteristics of this material. The x dependence of these properties shows two trends, and the mutation point seems to be at x ≈ 0.5. This implies that the electronic structure of (Co1-xZnx)Al2O4 is not changed monotonically by Zn(2+) substitution. Interestingly, some of the optical phenomena observed in this study become prominent for samples with x ≥ 0.5. That is, we observed sideband peaks near the main peaks in the Raman spectra, and their relative intensities systematically and significantly increased with increasing Zn(2+) substitution. The rates of increase are not constant, and are fast for samples with x ≥ 0.5. The sideband peaks are considered to reflect the unique changes in the local electronic structure of (Co1-xZnx)Al2O4, and they are useful for evaluating the substitution level without the influence of the site change phenomenon. Thus, clarifying them is expected to be important for understanding and controlling the electronic structure of the spinel oxide. On the other hand, investigation of the visible light absorption due to the d-d transition of Co(2+) reveals that the efficiency is also high for samples with high Zn(2+) substitution (x ≥ 0.5). This is also considered to be valuable information for investigation of the optical properties and/or the catalytic function of the spinel oxide. Moreover, the fluorescence of the (Co1-xZnx)Al2O4 samples is also identified as a novel functional property of this material. The intensity of the fluorescence peak also dramatically increases for samples with x ≥ 0.7. The effect of Zn(2+) substitution on the local electronic structure of (Co1-xZnx)Al2O4 has not been clarified yet. However, some of the interesting characteristics reviewed in this study are worth investigating from the viewpoint of materials science and applications.

16.
ACS Appl Mater Interfaces ; 6(13): 10415-20, 2014 Jul 09.
Article in English | MEDLINE | ID: mdl-24911949

ABSTRACT

We produced an optically controllable dual-gate organic field-effect transistor by a simple one-step spin-coating of a mixed solution of photochromic spiropyran (SP) and poly(3-hexylthiophene) (P3HT). Postannealing enhanced polymer chain ordering of P3HT to induce phase separation into an SP-rich lower layer and an SP-free upper layer. These layers worked independently as transistor channels with distinct optical responsivity. The top channel was optically inactive, but the bottom channel was optically active, because of the photoisomerization of SP. These results demonstrate the potential of our technique to produce a multifunctional photoactive organic transistor by a simple process.

17.
ACS Appl Mater Interfaces ; 6(12): 9201-6, 2014 Jun 25.
Article in English | MEDLINE | ID: mdl-24869419

ABSTRACT

This paper describes the interface reactions and diffusion between silver/polypyrrole (Ag/PPy) composite and silicon substrate. This composite material can be used as a novel technique for 3D-LSI (large-scale integration) by the fast infilling of through-silicon vias (TSV). By immersion of the silicon wafer with via holes into the dispersed solution of Ag/PPy composite, the holes are filled with the composite. It is important to develop a layer between the composite and the Si substrate with good diffusion barrier and adhesion characteristics. In this paper, SiOx and two types of SiOxNy barrier layers with various thicknesses were investigated. The interface structure between the Si substrate, the barrier, and the Ag/PPy composite was characterized by transmission electron microscopy. The adhesion and diffusion properties of the layers were established for Ag/PPy composite. Increasing thickness of SiOx proved to permit less Ag to transport into the Si substrate. SiOxNy barrier layers showed very good diffusion barrier characteristics; however, their adhesion depended strongly on their composition. A barrier layer composition with good adhesion and Ag barrier properties has been identified in this paper. These results are useful for filling conductive metal/polymer composites into TSV.

18.
ACS Appl Mater Interfaces ; 5(21): 11371-6, 2013 Nov 13.
Article in English | MEDLINE | ID: mdl-24099440

ABSTRACT

We demonstrated optical manipulation of single-electron tunneling (SET) by photoisomerization of diarylethene molecules in a metal-insulator-semiconductor (MIS) structure. Stress is placed on the fact that device operation is realized in the practical device configuration of MIS structure and that it is not achieved in structures based on nanogap electrodes and scanning probe techniques. Namely, this is a basic memory device configuration that has the potential for large-scale integration. In our device, the threshold voltage of SET was clearly modulated as a reversible change in the molecular orbital induced by photoisomerization, indicating that diarylethene molecules worked as optically controllable quantum dots. These findings will allow the integration of photonic functionality into current Si-based memory devices, which is a unique feature of organic molecules that is unobtainable with inorganic materials. Our proposed device therefore has enormous potential for providing a breakthrough in Si technology.

19.
ACS Appl Mater Interfaces ; 5(19): 9726-31, 2013 Oct 09.
Article in English | MEDLINE | ID: mdl-24040885

ABSTRACT

We demonstrate a new device that combines a light-field effect and an electrical-gate effect to control the drain current in a dual-gate transistor. We used two organic layers, photochromic spiropyran (SP)-doped poly(triarylamine) (PTAA) and pristine PTAA, as top and bottom channels, respectively, connected to common source and drain electrodes. The application of voltage to the top and bottom gates modulated the drain current through each layer independently. UV irradiation suppressed the drain current through the top channel. The suppressed current was then maintained even after the UV light was turned off because of an optical memory effect induced by photoisomerization of SP. In contrast, UV irradiation did not change the drain current in the bottom channel. Our dual-gate transistor thus has two organic channels with distinct photosensitivities: an optically active SP-PTAA film and an optically inactive PTAA film. This device configuration allows multi-level switching via top- and bottom-gate electrical fields with an optical-memory effect.

20.
ACS Comb Sci ; 15(8): 435-8, 2013 Aug 12.
Article in English | MEDLINE | ID: mdl-23883301

ABSTRACT

Resistive random access memory (ReRAM) has been proposed as a new application for oxide materials. We have proposed a Cu electrode/dielectric oxide/bottom electrode stack structure as a potential ReRAM material that is compatible with the LSI process. Control of the switching voltage and the initial conductive filament formation process is beneficial for actual applications. To control the switching property by controlling the valence state of metals, we investigated the Ta-Nb binary oxide ((TaxNb1-x)2O5) system as a dielectric oxide layer using a combinatorial method. A combinatorial pulsed laser deposition method was used to fabricate the (TaxNb1-x)2O5 system systematically on a Pt/Si substrate. X-ray photoelectron spectroscopy revealed defect formation relating to Ta and the compensation of oxygen vacancies caused by a change in the valence number of Nb. As the Ta content decreased, there were a decrease in the threshold voltage of the low resistive state and an enhancement of the leakage current, meaning that the switching properties can be controlled by controlling the (TaxNb1-x)2O5 system.


Subject(s)
Combinatorial Chemistry Techniques , Copper/chemistry , Nanotechnology , X-Ray Diffraction
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