Surface modification and fabrication of 3D nanostructures by atomic layer deposition
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Introduction Atomic layer deposition (ALD) in combination with the strategy of templating, which utilizes the existing materials as scaffolds for ALD coating, is a powerful tool for fabricating nanomaterials and nanostructures. It allows control over the composition and physical properties in an exceptionally accurate manner. Basically, the ALD process involves self-limiting surface reactions from the gas phase with atomic monolayer precision. This permits perfect conformal coatings for thin films, even on surfaces with complex shapes. With rapid developments in nanoscience and nanotechnology research, different classes of nanomaterials such as nanoparticles, nanowires (NWs), nanotubes (NTs), and nanoporous membranes have become available in large quantities and varieties. Simply by templating or coating, ALD allows the creation of a second generation of nanomaterials and devices that produces novel, unexplored prototypes such as core-shell and multilayered material systems in three dimensions. Insightful reviews on ALD in general from a historical survey to emerging applications can be found elsewhere1–4 and in the other articles in this issue of MRS Bulletin. In this article, we focus on the fabrication of three-dimensional (3D) nanostructures by ALD on the basis of surface modification
of nanoporous materials, as described in the next section. By the connectivity of pores in space, briefly, we choose anodized alumina membranes and colloidal crystals because both have similar porosity but different scattering paths in terms of gasphase diffusion. In accordance with ballistic-transport-based and random-walk-based ALD results, therefore, we first classify the ALD-modified nanoporous materials into NTs and inverted opals. We also point out the unusual crystallization behaviors due to the high curvature of surfaces from nanopores or NWs. The implication for applications is, in particular, governed by surface-limited reactions and charge-collection efficiency. Examples of conformal coating of the surfaces of semiconductor NWs and NTs and of site-selective ALD on patterned surfaces are summarized as well. These structures allow for the functional and direct fabrication of nanoscale devices in many emerging applications. Finally, we address recent device development using 3D structures by ALD and provide future research directions.
Surface modification of nanoporous materials The capability of ALD has revealed improved material properties through surface modification and/or the replication of nanoporous materials and coating the insides of the pores with
Changdeuck Bae, Institute of Applied Physics, University of Hamburg, Germany; [email protected] Hyunjung Shin, Kookmin University, Seoul, Korea; [email protected] Kornelius Nielsch, University of Hamburg, Germany; [email protected] DOI: 10.1557/mrs.2011.264
© 2011 Materials Research Society
MRS BULLETIN • VOLUME 36 • NOVEMBER 2011 • www.mrs.org/bulletin
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SURFACE MODIFICATION AND FABRICATION OF 3D NANOSTRUCTURES BY ATOMIC LAYER DEPOSITIO
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