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.
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...
Following the graphene isolation, strong interest in two dimensional (2D) materials has been driven by their outstanding properties. Their typical intrinsic structure, including strong in-plane covalent bonding and weak out-of-plane Van der Waals interaction, makes them highly promising in diverse areas such as electronics, catalysis, and environment. Growth of 2D materials requires a synthesis approach able to control the deposition onto a support at the atomic scale. Thanks to their simplicity, versatility and ability to control thickness at the angstrom level, Atomic Layer Deposition (ALD) and its variant Atomic Layer Etching (ALET) appear as ones of the most suited techniques to synthesize 2D materials. The development of ALD technique for fabricating 2D materials in the last ten years justifies reviewing its most recent groundbreaking discoveries and progresses. Particular attention will be paid to stable 2D materials especially graphene, h-BN, Mo and W dichalcogenides and few monolayered metal oxides. Specificities and outputs of ALD for 2D material as well as emerging directions and remaining technical challenges will be highlighted. layers. This kind of materials exhibits interesting properties related to their size restriction: electronic confinement, modifying both optical and electronic properties, and high surface to volume ratio that affects the mechanical and chemical properties. 4,5 Therefore, 2D materials are highly attractive due to their potential in cutting edge domains such as micro-, and opto-electronics 4,6 as well as renewable energy, 4,7-9 catalysis, 10,11 gas sensing 12,13 and environment. 4,7,14,15 They are also exciting because of their possibility to be stacked into VdW heterostructures combining and/or tuning their chemical and physical properties. 6,[16][17][18] However, since 2D materials are atomically thin, a suitable manufacturing process capable of controlling their fabrication in terms of structure, composition, thickness, defects, purity and crystallinity without degradation of their original properties is necessary. Two approaches can be considered: the top-down and bottom-up synthetic routes. 19 Historically, top-down approach has first been developed using mechanical and chemical exfoliation techniques, 1,2,20,21 the first example being the well-known graphene exfoliation using scotch tape.Later on, etching processes have been adapted. Recently, Atomic Layer Etching (ALET), 22 the top down variant of Atomic Layer Deposition (ALD), has been introduced for fabricating 2D materials. On the other hand, bottom-up techniques have largely been developed using conventional thin film deposition. These techniques include sputtering, evaporation and Chemical Vapor Deposition (CVD). 3,23,24 However, they are mainly based on either high temperature processes, substrate restrictive deposition or low thickness control. 21,25,26 Amongst all the fabrication processes, ALD appears to be one of the most suited techniques to synthesize 2D materials, because of its simplicity, versatility and c...
The chemical inertness of graphite and, in the case of tubes, of rolled up few layer graphene sheets, requires some degree of “defect engineering” for the fabrication of carbon based heterostructured materials. It is shown that atomic layer deposition provides a means to specifically label anchoring sites and can be used to characterize the surface functionality of differently treated carbon nanotubes. Direct observation of deposited titania by analytical transmission electron microscopy reveals the location and density of anchoring sites as well as structure related concentrations of functional groups on the surface of the tubes. Controlled functionalization of the tubes therefore allows us to tailor the distribution of deposited material and, hence, fabricate complex heterostructures
Carbon materials such as carbon nanotubes (CNTs), graphene, and reduced graphene oxide (RGO) exhibit unique electrical properties, which are also influenced by the surrounding atmosphere. They are therefore promising sensing materials. Despite the existence of studies reporting the gas-sensing properties of metal oxide (MOx) coated nanostructured carbon, an incomplete understanding of their sensing mechanism remains. Here we report a systematic study on the preparation, characterization, and sensing properties of CNT and RGO composites with SnO2 coating. Atomic layer deposition (ALD) was applied to the conformal coating of the inner and outer walls of CNTs with thin films of SnO2 of various thicknesses, while nonaqueous sol-gel chemistry assisted by microwave heating was used to deposit tin dioxide onto RGO in one step. The sensing properties of SnO2/CNTs and SnO 2/RGO heterostructures toward NO2 target gas were investigated as a function of the morphology and density of the metal oxide coating. The general sensing mechanism of carbon-based heterostructures and the role of the various junctions involved are established. ABSTRACT: Carbon materials such as carbon nanotubes (CNTs), graphene, and reduced graphene oxide (RGO) exhibit unique electrical properties, which are also influenced by the surrounding atmosphere. They are therefore promising sensing materials. Despite the existence of studies reporting the gas-sensing properties of metal oxide (MO x ) coated nanostructured carbon, an incomplete understanding of their sensing mechanism remains. Here we report a systematic study on the preparation, characterization, and sensing properties of CNT and RGO composites with SnO 2 coating. Atomic layer deposition (ALD) was applied to the conformal coating of the inner and outer walls of CNTs with thin films of SnO 2 of various thicknesses, while nonaqueous sol−gel chemistry assisted by microwave heating was used to deposit tin dioxide onto RGO in one step. The sensing properties of SnO 2 / CNTs and SnO 2 /RGO heterostructures toward NO 2 target gas were investigated as a function of the morphology and density of the metal oxide coating. The general sensing mechanism of carbon-based heterostructures and the role of the various junctions involved are established.
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