Atomic layer deposition (ALD) is a thin film technology that in the past two decades rapidly developed from a niche technology to an established method. It proved to be a key technology for the surface modification and the fabrication of complex nanostructured materials. In this Progress Report, after a short introduction to ALD and its chemistry, the versatility of the technique for the fabrication of novel functional materials will be discussed. Selected examples, focused on its use for the engineering of nanostructures targeting applications in energy conversion and storage, and on environmental issues, will be discussed. Finally, the challenges that ALD is now facing in terms of materials fabrication and processing will be also tackled.

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A new atomic layer deposition (ALD) process for nanocrystalline tin dioxide films is developed and applied for the coating of nanostructured materials. This approach, which is adapted from non‐hydrolytic sol‐gel chemistry, permits the deposition of SnO2 at temperatures as low as 75 °C. It allows the coating of the inner and outer surface of multiwalled carbon nanotubes with a highly conformal film of controllable thickness. The ALD‐coated tubes are investigated as active components in gas‐sensor devices. Due to the formation of a p‐n heterojunction between the highly conductive support and the SnO2 thin film an enhancement of the gas sensing response is observed.

Photonic crystals are metamaterials designed to display a periodic modulation of the refractive index. [ 1,2 ] For light wavelengths that match the Bragg condition, such materials display a photonic gap. For suffi cient refractive index contrast, a complete band gap emerges, the density of states in the gap is zero, transmission vanishes and incident light is specularly refl ected. The concept was initially proposed by Yablonovitch and John 25 years ago. Research in the fi eld initially developed rapidly but has matured over the last decade. Although materials with complete band gaps have been reported from the infrared to the visible, there still exist many challenges in fabrication and for possible applications. [ 3,4 ] For example only recently threedimensional guiding of photons in photonic crystals has been demonstrated. [ 5,6 ] Interestingly disordered photonic structures are also candidates for complete band gap materials. Numerical data show that peculiar hyperuniform disordered materials can display complete bandgaps in two dimensions [ 7 ] that allows the design of cavities and optical waveguides. [ 8 ] Recent experimental data for two-dimensional hyperuniform structures in the microwave regime provide support for these claims. [ 9,10 ] Independent numerical calculations suggest that these concepts can also be applied to three-dimensional hyperuniform structures where a band gap is predicted to open for refractive indices n ≥ 3 in air. [ 11 ] Here we present such hyperuniform structures made from silicon with a broad and pronounced gap in the shortwave infrared for the fi rst time.One of the intrinsic shortcomings of photonic crystals is the highly selective refl ection from Bragg planes due to crystalline symmetries. For many practical applications this feature is detrimental. For example dye-free refl ective color displays, colored packing materials or cosmetics are preferentially noniridescent and thus non-crystalline. Moreover the design of optical integrated devices is based on the realization of waveguides, switches and optical cavities that suffer from the anisotropic optical response of crystalline solids. [ 8 ] While initially largely ignored, the design of amorphous photonic materials has gained increasing attention over the last decade. [12][13][14][15][16] Disordered dielectric structures with short-range order display wideangle refl ection and broad spectral features. Early experiments demonstrated that the transmission and refl ection properties are governed by an interplay between Mie scattering and local order via the modulation of the single scattering cross section. [ 13,14 ] To some extent both properties can be tuned independently which in turn allows to tailor solid and liquid materials with a specifi c optical response, fi nding use in random lasers [ 15 ] or for materials where angle-independent structural colors are desired. [ 16 ] While engineering disordered photonic materials is just at its beginning, many examples can already be found in nature such as in non-iridescent colo...

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