“Top-Down” and “Bottom-Up” Approaches

Abstract

There are plenty of methods to synthesis nano materials. These methods are grouped into two categories namely Top-Down and Bottom-Up techniques. The techniques are classified based on the phase of the starting material. In the Top-down class of techniques, the starting material is in solid state, whereas in Bottom-Up techniques the starting material is either in gaseous state or in liquid state. Top-down approach refers to a subtractive process in which a bulk starting material is divided into smaller ones of nanosize. Bottom-up approach refers to an additive process that starts with precursor atoms (or) molecules which combine to form nanosized structure. In bottom-up approach, nanostructures are built atom-by-atom or molecule-by-molecule. Depending on the requirement we have to select an appropriate method for prepration of nano materials. Few methods are listed here Top-Down techniques Bottom-Up techniques 1) Mechanical grinding(Ball Milling) 2) Lithography 3) Etching 4) Erosion 1) Sol gel method 2) Chemical Vapour Deposition 3) Physical Vapour Deposition 4) Electrochemical deposition 5) Hydrothermal method 6) Atomic layer deposition TOP-DOWN TECHNIQUES Ball milling Ball milling also known as mechanical grinding is a simple and popular method. Here the materials are grinded into extremely fine powders. It is one of the most important processes used in industries to synthesize nanomaterials. This method has