Advanced Materials

With the advent of nanotechnology, metals will have to be processed at a much smaller scale and new engineering concepts are required. On page 91, Khattiya Chalapat and co-workers demonstrate the assembly of metallic nano-3D-structures by plastic deformation that is induced either on the surface by reactive ion etching or within the metal matrix by focused ion beam. The front cover image illustrates a metallic nanocubic structure being unfolded by an ion beam.

Jennifer A. Lewis and co-workers report on page 96 the design and fabrication of microvascular multinozzle arrays composed of multi-generation, bifurcating microchannels embedded in plastic. Both single and dual multinozzle printheads are produced, which enable rapid printing of functional materials in multilayered structures over large areas (1 m²).

A highly efficient nanorod photoanode for solar-driven photoelectrochemical (PEC) water splitting is reported by Kazunari Domen and co-workers on page 125. The cover image illustrates vertically aligned semiconductor nanorods grown in situ on a metal substrate. The single crystalline 1D nanostructure with low contact barrier makes them ideal building blocks of PEC cells for efficient solar energy conversion.

Solvation of graphene and its derivatives engenders a range of graphene-based functional soft materials from colloids, amphiphiles, liquid crystals, and gels to membranes. Centered on various colloidal interactions activated via solvation, this report illustrates solvated graphenes as an emerging class of functional soft materials.

Many animal species, including fish, reptiles, and mammals, have armor that offers both protection against predators without sacrificing flexibility. This flexible armor has appeared independently through convergent evolution. We discuss the underlying principles and review the principal contributions. The concept of flexible armor presents a bright potential for synthetic bioinspired systems.

The paper reviews the experimental and theoretical studies on the growth of ultrathin silica films onto metal single crystal substrates. Monolayer and bilayer silica films are discussed in more detail. The results show the structural complexity and diversity of silica overlayers on metals, providing further information towards our understanding of the atomic structure and properties of silica.

Linear arrays of vertically aligned carbon nanotubes (VACNTs) can be produced by directly exposing thin linear features written on a Si wafer to non-uniform plasmas, as reported by Kostya (Ken) Ostrikov and co-workers on page 69. This approach can be used to produce virtually any pattern of linear arrays of VACNTs with variable and graded morphology and structure, without any catalyst or complex nanofabrication. The graded nanotube arrays show competitive performance in electron field emission and biosensing nanodevices.

Low-temperature plasmas in direct contact with arbitrary, written linear features on a Si wafer enable catalyst-free integration of carbon nanotubes into a Si-based nanodevice platform and in situ resolution of individual nucleation events. The graded nanotube arrays show reliable, reproducible, and competitive performance in electron field emission and biosensing nanodevices.

A facile method to fabricate high-surface area functional carbons via convenient “salt templating” is presented. Exemplarily, nitrogen- as well as nitrogen-/boron-co-doped carbons were synthesized using ionic liquids as precursors and eutectics as porogen. The porogen is easily removable with water and the porosities can be adjusted from micro- to mesoporous depending on the salt nature and amount.

Metallic membranes of about 2.5 nm thick but with macroscopic lateral dimensions have been successfully fabricated from ultrathin gold nanowires. Such metallic nanomembranes are transparent, conductive and mechanically strong, with an optical transmittance of 90–97%, an electrical resistance of ∼1142 kΩ sq⁻¹, and a breaking strength of ∼14 N m⁻¹ with a typical atomic force microscope probe.

New insights into the atomistic and electronic structure of the oxygen vacancy in SrTiO₃ are presented through first-principles calculations. The oxygen vacancy induces a local anti-ferrodistortive-like oxygen-octahedron rotation, even in the cubic phase. This feature leads to localized electronic states in the bandgap, giving an excellent explanation to the thermal ionization and optical transition observed experimentally.

Ion processing of the reactive surface of a free-standing polycrystalline metal film induces a flow of atoms into grain boundaries, resulting in plastic deformation. A thorough experimental and theoretical analysis of this process is presented, along with the demonstration of novel engineering concepts for precisely controlled 3D assembly at micro- and nanoscopic scales.

Microvascular multinozzle arrays are designed and fabricated for high-throughput printing of functional materials. Ink-flow uniformity within these multigeneration, bifurcating microchannel arrays is characterized by computer modeling and microscopic particle image velocimetry (micro-PIV) measurements. Both single and dual multinozzle printheads are produced to enable rapid printing of multilayered periodic structures over large areas (≈1 m²).

An organic ultralow voltage field effect transistor for DNA hybridization detection is presented. The transduction mechanism is based on a field-effect modulation due to the electrical charge of the oligonucleotides, so label-free detection can be performed. The device shows a sub-nanometer detection limit and unprecedented selectivity with respect to single nucleotide polymorphism.

Extremely high carrier mobilities are achieved in layered molybdenum oxide by Sivacarendran Balendhran, Kourosh Kalantar-zadeh, and co-workers on page 109. This is achieved by intercalation of the octahedral molybdenum oxide lattice with hydrogen, which tunes the bandgap of the material. Carrier mobilities of 1100 cm² V⁻¹ s⁻¹ and greater are attained, highlighting the potential of ultrathin molybdenum oxide for high speed nanoelectronics.

We demonstrate that the energy bandgap of layered, high-dielectric α-MoO₃ can be reduced to values viable for the fabrication of 2D electronic devices. This is achieved through embedding Coulomb charges within the high dielectric media, advantageously limiting charge scattering. As a result, devices with α-MoO₃ of ∼11 nm thickness and carrier mobilities larger than 1100 cm² V⁻¹ s⁻¹ are obtained.

Compact metallo-dielectric hybrid clusters with subwavelength dimensions are fabricated by template guided self-assembly. Elastic and inelastic scattering spectroscopy and electromagnetic simulations reveal that hybrid clusters comprising TiO₂ nanoparticles on top of a cluster of strongly coupled gold nanoparticles harness synergistic electromagnetic interactions between the building blocks. This results in a boost of the peak electric field intensity and a redistribution of the field in the ambient medium. The complex phase landscape in the clusters features optical vortices that enhance the magnetic field.

Surface-immobilized monolayers of fluorescent molecular sensors consisting of a short conjugated oligo(p-phenylene ethynylene) core end-capped with an acceptor fluorophore (analyte receptor) display significant signal amplification due to enhanced intermolecular energy transfer within the monolayer. This general phenomenon offers a superior platform for designing ratiometric fluorescent sensors. An example of how this can be used to convert a narrow-range threshold fluorescent pH indicator (fluorescein) to a broad-range ratiometric fluorescent chemosensor is described.

A vertically aligned Ta₃N₅ nanorod photoelectrode is fabricated by through-mask anodization and nitridation for water splitting. The Ta₃N₅ nanorods, working as photoanodes of a photoelectrochemical cell, yield a high photocurrent density of 3.8 mA cm⁻² at 1.23 V versus a reversible hydrogen electrode under AM 1.5G simulated sunlight and an incident photon-to-current conversion efficiency of 41.3% at 440 nm, one of the highest activities reported for photoanodes so far.

Dumbbell-like Pt₄₈Pd₅₂-Fe₃O₄ nanoparticles are synthesized and functionalized with oleylamine-polyethyleneglycol to serve as an efficient catalyst for H₂O₂ reduction and tetramethylbenzidine (TMB) oxidation in biological solutions. The Pt₄₈Pd₅₂-Fe₃O₄/TMB kit is even more active than the natural enzyme for H₂O₂ detection with a detection limit reaching 2 μM, and is successfully used to quantitatively monitor the extracellular H₂O₂ generated by neutrophils.

HgTe colloidal quantum dots (CQD) in an inorganic As₂S₃ matrix allow 100-fold higher mobility with optimized transport properties compared to HgTe-organic CQD film while remaining intrinsic. The material's electronic properties are measured by field effect transistors as a function of temperature and the responsivity and detectivity of the mid-IR photoconductors are discussed.

The Stöber method is a well-known and versatile process for synthesizing silica spheres with controllable particle size, narrow size distribution, smooth surface, and porosity. This contribution highlights recent advances on the extension of the Stöber method to construct mesoporous SiO₂ and TiO₂ shells for uniform multifunctional core–shell nanostructures.