Advanced Materials

Materials nanoarchitectonics is a new paradigm for constructing novel functional materials in the same way architecture is used in building con-struction. This Special Issue features the research at the World Premier International (WPI) Research Center for Materials Nanoarchitectonics (MANA). A new building is being constructed for MANA, where research on atomic- and molecular-level nanoarchitectonics is the focus. The cover image represents both macroscopic architecture and nanoarchitecture.

Alan Turing, famous for developing his algorithmic approach to computation, also explored the possibility of computing in complex, unorganized assemblies of logic gates. On page 286, a hardware-based, neuromorphic electronic device built by Adam Stieg and co-workers exhibits features similar to the electrical activity of biological brains and neuron assemblies using a complex network of self-assembled atomic switches.

Bend them, shake them, any which way you like them. But can mechanical forces be used to control materials at the nano- or molecular scales? A new generation of smart materials whose properties can be controlled by external stimuli is emerging and is discussed in the Progress Report by Katsuhiko Ariga and co-workers on page 158.

This Progress Report summarizes examples on mechanical control of nanomaterials and nanosystems and mainly focuses on recent research in this field. Approaches to create macroscopic mechanical outputs upon accumulation of molecular-level phenomena and controls of molecular systems by macroscopic mechanical stimuli are described. As mechanical forces are much reliable and widely applicable, we believe these efforts have significant contributions for improving our life.

Diverse experiments on one- and two-dimensional inorganic nanostructures employing atomic force microscope and scanning tunneling microscope units integrated with a high-resolution transmission electron microscope are presented.

Recent developments in electrolyte, anode, and cathode materials for protonic SOFCs are here reported, addressing the issue of chemical stability, processability, and good power performance below 600 °C. Recent findings show significant improvements in the power density output of cells based on doped barium zirconate electrolytes, pointing out towards the feasibility of the next generation of protonic SOFCs.

2D inorganic nanosheets can be used as a dielectric analogue of graphene. A layer-by-layer engineering approach to inorganic nanosheets promises unique possibilities in the design of thin-film device architectures, such as capacitors, gate dielectrics, transistors, and artificial ferroelectrics. Graphene is only the tip of the iceberg, and we are now starting to discover new possibilities afforded by 2D materials. Minoru Osada and Takayoshi Sasaki review the recent developments on page 210.

2D inorganic nanosheets are interesting dielectric building blocks that can be used as a dielectric analogue of graphene. Layer-by-layer engineering approach of these nanosheets promises unique possibilities to design thin-film device architectures such as capacitors, gate dielectrics, transistors, and artificial ferroelectrics. Trends and recent progress in this class of materials are highlighted and provide a perspective for future nanoelectronics.

The state-of-the-art research activities in the field of photocatalysis is reviewed to reveal the key issues, challenges, and opportunities facing present and future research on photocatalytic materials. Energy band engineering, nanotechnology, modern materials characterization methods and advanced theoretical studies promise to sustain rapid development of photocatalytic materials to contribute an encouraging prospect in the realization of a sustainable society.

An atomic switch is a nanoionic-device that controls the diffusion of metal ions/atoms and their reduction/oxidation processes in the switching operation to form/annihilate a conductive path. We review various atomic switches, such as the gap-type and the gapless-type two-terminal atomic switches and three-terminal atomic switches, as well as the novel functions of these devices, such as selective volatile/nonvolatile, synaptic, memristive, and photo-assisted operations.

Two examples of confined molecular catalysts are presented. PtCl₄²⁻ complexes are attached to a thiol-terminated monolayer by ligand exchange of Cl⁻ with a thiolate group and incorporated in a multilayer of viologen moieties by ion exchange. All Cl⁻ ligands are replaced by OH⁻ or H₂O before HER takes place. Ex situ and in situ XAFS measurements confirm that the Pt complexes accelerate HER without being converted into Pt particles.

Shape-memory surfaces with on-demand, tunable nanopatterns are developed to observe time dependent changes in cell alignment using temperature-responsive poly(ϵ-caprolactone) (PCL) films. Temporary grooved nanopatterns are easily programmed on the films and triggered to transition quickly to permanent surface patterns by the application of body heat. A time-dependent cytoskeleton remodeling is also observed under biologically relevant conditions.

Utilizing the piezoelectric effect of ZnO nanowires, nanogenerators have been fabricated for producing an output of 10–20 V. On page 280, Zhong Lin Wang demonstrates a self-powered system by integrating a nanogenerator with sensors, energy storage units, a data processor, and a wireless transmitter, which can operate without a battery by harvesting mechanical energy from the environment. Such systems have application in biomedical science, environmental monitoring, structural monitoring, and personal electronics.

Sensor networks are a key technological and economic driver for global industries in the near future, with applications in health care, environmental monitoring, infrastructure monitoring, national security, and more. This paper introduces a technology that is capable of providing sustainable self-sufficient micro/nano-power sources for future sensor networks.

A complex, self-assembled network of highly interconnected atomic switches similar in structure to Turing's “B-Type unorganized machine” exhibits emergent criticality similar in nature to the electrical activity of biological brains and neuron assemblies. Power law scaling of rapid fluctuations in electrical conductance indicates a potential utility in hardware-based implementations of real-time, multi-input processing through reservoir computation.

Half-metals are a class of materials that behave as a metal in one spin direction and an insulator in the opposite spin direction. Half-metallic antiferromagnets as a subclass of half-metals are characterized further by totally compensated spin moments in a unit cell. Being able to generate fully spin-polarized current while exhibiting zero macroscopic magnetization, half-metallic antiferromagnets are expected as one of the most ideal materials for spintronic applications.

Bottom-up fabrication methods based on self assembly of molecules are developed for solution-processed production of organic semiconductor devices. Our methods enable area selective crystallization of molecules and direct formation of organic single crystals from solution. Since these methods can be processed under ambient condition at room temperature, they are fully compatible with printable electronics technology.

In superconductor–semiconductor–superconductor junctions with superlattice structure, differential resistance as a function of voltage shows oscillatory behavior under the irradiation of radio-frequency waves as shown in the figure. We interpret quantitatively this newly discovered phenomenon in terms of the formation of novel composite particles, Andreev polarons, which is caused by the strong coupling of superconducting quasiparticles with long-wavelength acoustic phonons.

Intramolecular single-molecule logic gate designs: the semi-classical, the quantum Hamiltonian and the qubit circuits are demonstrated to be different versions of intramolecular quantum control. They differ from the way the classical input data are encoded on the molecule and from the quantum-to-classical conversion output reading procedure.