Rational Assembly of Two-Dimensional Perovskite Nanosheets as Building Blocks for New Ferroelectrics.

Artificial materials in the form of superlattices have been studied actively in quest of new engineering methods or design rules for the development of desired functionalities, in particular high-k ferroelectricity, ferromagnetism, and high mobility electron gas. This work presents a controlled assembly strategy for fabricating atomically precise interfaces of two-dimensional (2D) homologous perovskite nanosheets (Ca2Nam-3NbmO3m+1-; m = 3-6) to construct artificial superlattices. The distinctive thickness of each 2D homologous perovskite nanosheets attributed to the presence of different number of NbO6 octahedra provides an exquisite control to engineer interfacial properties for tailored design of superior high-k properties and emergence of ferroelectricity. The higher dielectric constant (εr = 427) and development of ferroelectricity for (Ca2Nb3O10-/Ca2Na2Nb5O16-)6 superlattice indicate that superlattice films with both odd number of NbO6 octahedra possess extended polarization due to the potential effect of heterointerface and ferroelectric instabilities. Furthermore, the increased discontinuities/offsets in Ca2Nb3O10- and Ca2Na3Nb6O19- nanosheets band alignment results in superior insulating properties (∼1 × 10-11 A cm-2 at 1 V) for (Ca2Nb3O10-/Ca2Na3Nb6O19-)6 superlattice. These findings exhibit new research opportunities for the development of novel artificial high-k dielectric/ferroelectric via precise control of interfaces at the atomic level and can be extended to the large family of 2D perovskite compounds.

Scalable Design of Two-Dimensional Oxide Nanosheets for Construction of Ultrathin Multilayer Nanocapacitor.

Large size of capacitors is the main hurdle in miniaturization of current electronic devices. Herein, a scalable solution-based layer-by-layer engineering of metallic and high-κ dielectric nanosheets into multilayer nanosheet capacitors (MNCs) with overall thickness of ≈20 nm is presented. The MNCs are built through neat tiling of 2D metallic Ru0.95 O2 0.2- and high-κ dielectric Ca2 NaNb4 O13 - nanosheets via the Langmuir-Blodgett (LB) approach at room temperature which is verified by cross-sectional high-resolution transmission electron microscopy (HRTEM). The resultant MNCs demonstrate a high capacitance of 40-52 µF cm-2 and low leakage currents down to 10-5 -10-6 A cm-2 . Such MNCs also possess complimentary in situ robust dielectric properties under high-temperature measurements up to 250 °C. Based on capacitance normalized by the thickness, the developed MNC outperforms state-of-the-art multilayer ceramic capacitors (MLCC, ≈22 µF cm-2 /5 × 104  nm) present in the market. The strategy is effective due to the advantages of facile, economical, and ambient temperature solution assembly.

Atomic Layer Engineering of High-κ Ferroelectricity in 2D Perovskites.

Complex perovskite oxides offer tremendous potential for controlling their rich variety of electronic properties, including high-TC superconductivity, high-κ ferroelectricity, and quantum magnetism. Atomic-scale control of these intriguing properties in ultrathin perovskites is an important challenge for exploring new physics and device functionality at atomic dimensions. Here, we demonstrate atomic-scale engineering of dielectric responses using two-dimensional (2D) homologous perovskite nanosheets (Ca2Nam-3NbmO3m+1; m = 3-6). In this homologous 2D material, the thickness of the perovskite layers can be incrementally controlled by changing m, and such atomic layer engineering enhances the high-κ dielectric response and local ferroelectric instability. The end member (m = 6) attains a high dielectric constant of ∼470, which is the highest among all known dielectrics in the ultrathin region (<10 nm). These results provide a new strategy for achieving high-κ ferroelectrics for use in ultrascaled high-density capacitors and post-graphene technology.

Biomass-Derived Thermally Annealed Interconnected Sulfur-Doped Graphene as a Shield against Electromagnetic Interference.

Electrically conductive thin carbon materials have attracted remarkable interest as a shielding material to mitigate the electromagnetic interference (EMI) produced by many telecommunication devices. Herein, we developed a sulfur-doped reduced graphene oxide (SrGO) with high electrical conductivity through using a novel biomass, mushroom-based sulfur compound (lenthionine) via a two-step thermal treatment. The resultant SrGO product exhibited excellent electrical conductivity of 311 S cm(-1), which is 52% larger than 205 S cm(-1) for undoped rGO. SrGO also exhibited an excellent EMI shielding effectiveness of 38.6 dB, which is 61% larger than 24.4 dB measured for undoped rGO. Analytical examinations indicate that a sulfur content of 1.95 atom % acts as n-type dopant, increasing electrical conductivity and, therefore, EMI shielding of doped graphene.

Artificial design for new ferroelectrics using nanosheet-architectonics concept.

Control over the emergence of ferroelectric order remains a fundamental challenge for the rational design of artificial materials with novel properties. Here we report a new strategy for artificial design of layered perovskite ferroelectrics by using oxide nanosheets (high-k dielectric Ca2Nb3O10 and insulating Ti0.87O2) as a building block. We approached the preparation of superlattice films by a layer-by-layer assembly involving Langmuir-Blodgett deposition. The artificially fabricated (Ti0.87O2/Ca2Nb3O10)2(Ti0.87O2) superlattices are structurally unique, which is not feasible to create in the bulk form. By such an artificial structuring, we found that (Ti0.87O2/Ca2Nb3O10)2(Ti0.87O2) superlattices possess a new form of interface coupling, which gives rise to ferroelectricity with a good fatigue-free characteristic. Considering the flexibility of self-assembled nanosheet interfaces, this technique provides a route to synthesize a new kind of layered ferroelectric oxides.

2D perovskite nanosheets with thermally-stable high-κ response: a new platform for high-temperature capacitors.

We investigated high-temperature dielectric responses of high-κ perovskite nanosheet (Ca2Nb3O10), an important material platform for postgraphene technology and ultrascale electronic devices. Through in situ characterizations using conducting atomic force microscopy, we found a robust high-temperature property of Ca2Nb3O10 nanosheet even in a monolayer form (∼2 nm). Furthermore, layer-by-layer assembled nanocapacitors retained both size-free high-εr characteristic (∼200) and high insulation resistance (∼1×10(-7) A/cm2) at high temperatures up to 250 °C. The simultaneous improvement of εr and thermal stability in high-κ nanodielectrics is of critical technological importance, and perovskite nanosheet has great potential for a rational design and construction of high-temperature capacitors.

A self-reducible and alcohol-soluble copper-based metal-organic decomposition ink for printed electronics.

We report a novel method for the synthesis of a self-reducible (thermally reducible without a reducing atmosphere) and alcohol-soluble copper-based metal-organic decomposition (MOD) ink for printed electronics. Alcohol-solvent-based conductive inks are necessary for commercial printing processes such as reverse offset printing. We selected copper(II) formate as a precursor and alkanolamine (2-amino-2-methyl-1-propanol) as a ligand to make an alcohol-solvent-based conductive ink and to assist in the reduction reaction of copper(II) formate. In addition, a co-complexing agent (octylamine) and a sintering helper (hexanoic acid) were introduced to improve the metallic copper film. The specific resistivity of copper-based MOD ink (Cuf-AMP-OH ink) after heat treatment at 350 °C is 9.46 µΩ·cm, which is 5.5 times higher than the specific resistivity of bulk copper. A simple stamping transfer was conducted to demonstrate the potential of our ink for commercial printing processes.

Endovascular stenting as a first choice for the palliation of superior vena cava syndrome.

To assess the effectiveness of endovascular stenting for the palliation of superior vena cava (SVC) syndrome, endovascular stent insertion was attempted in 10 patients with symptomatic occlusion of the SVC. All the patients had known malignant disease of the thorax. Eight patients had been treated previously with chemotherapy and radiotherapy (n=5), chemotherapy alone (n=2), or pneumonectomy and radiotherapy (n=1). After developing SVC syndrome, all the patients were stented before receiving any other treatment. After single or multiple endovascular stents were inserted, five of eight patients were treated with chemotherapy and radiotherapy (n=2) or chemotherapy alone (n=3). Resolution of symptoms was achieved in nine patients within 72 hr (90%). In one patient, the symptoms did not disappear until a second intervention. At follow up, symptoms had recurred in two of ten patients (20%) after intervals of 15 and 60 days. Five patients have died from their cancers, although they remained free of symptoms of SVC occlusion until death. In conclusion, endovascular stent insertion is an effective treatment for palliation of SVC syndrome. Endovascular stent insertion can be considered the first choice of treatment, due to the immediate relief of symptoms and excellent sustained symptomatic relief.