Dielectrics with ultralow permittivity within 2 times that of air, excellent mechanical performance, and high thermal stability are highly attractive to many applications. However, since the finding of silica aerogels in the 1930s, no alternative ultralight porous dielectric with density below 10 mg/cm(3) has been developed. Here we present three-dimensional hierarchical boron nitride foam with permittivity of 1.03 times that of air, density of 1.6 mg/cm(3), and thermal stability up to 1200 °C obtained by chemical vapor deposition on a nickel foam template. This BN foam exhibits complete recovery after cyclic compression exceeding 70% with permittivity within 1.12 times that of air. Gathering all these exceptional characters, the BN foam should create a breakthrough development of flexible ultralow-permittivity dielectrics and ultralight materials.

This tutorial review provides a fundamental understanding of different mechanisms, material selection, device optimization and applications of hydrovoltaic technology and provides a systematic collection of recent advances.

Coating is the most widely applied technology to improve surface properties of substrates, and nanotechnology has been playing an important role in enhancing the coating performance. However, the tunability of surface properties by a single atomic layer remains poorly understood. Here we demonstrate that a chemical vapor deposited hexagonal boron nitride (h-BN) monolayer of large area and high quality can serve as a perfect coating to significantly improve friction, oxidation and electric resistance of the substrates. The exceptional low friction and insulation of h-BN monolayer coating facilitate the characterization of the h-BN film vividly by atomic force microscopy, showing the h-BN monolayer consists of domains with size within a few micrometers. This excellent coating performance together with the exceptional high thermal and chemical stability make the h-BN monolayer a promising coating material.

Hexagonal boron nitride monolayers with domain sizes up to 700 μm2 and geometry from triangle to hexagon are fabricated through a refined control over the precursor and morphology of the copper substrate. Hydrogen etching is shown to tailor the h‐BN monolayers precisely along the grain boundaries, providing their morphology over micro­meter scale and a new avenue toward fabricating nanoribbons.

Continuous and ultrathin platinum (Pt) films were deposited on tungsten (W) adhesion layers using atomic layer deposition (ALD) techniques. Pt ALD films were deposited at 120 °C using MeCpPtMe3 and H2 plasma as the reactants. X-ray reflectivity studies observed the rapid nucleation of the Pt film. X-ray photoelectron results were consistent with layer-by-layer growth suggesting a continuous Pt film at thicknesses ≥1.5 nm. The high surface energy of the W ALD adhesion layer enables the growth of continuous and ultrathin films of lower surface energy metals and should facilitate the use of precious metals for a variety of applications.