Methods of making nanomaterials by assembling atoms and molecules through nucleation and growth, such as liquid-phase reduction and thermal decomposition, the sol-gel method and CVD. The counterpart of top-down methods, which break down larger materials.
Japanese name: ボトムアップ法
Bottom-up synthesis appears in 1 practice question across 1 topic, including Nanomaterials.
Field: Nanomaterials
Bottom-up synthesis is the general term for methods that build nanomaterials by gathering atoms, ions and molecules. They include reducing metal ions in solution, thermal decomposition of organometallic precursors, the sol-gel method that hydrolyzes and condenses metal alkoxides, condensation of atoms from the vapor, CVD that decomposes source gases on a substrate, and ALD that deposits one atomic layer at a time. Compared with top-down methods, which cut or crush large materials, they produce less disorder in the arrangement of atoms, and their strength is that particles of uniform size and shape can be designed through chemical conditions.
The key to uniform particle size is to separate nucleation and growth in time. When the monomer concentration in solution exceeds the critical supersaturation, nuclei form all at once; as the concentration falls, nucleation stops and only the existing nuclei grow (the LaMer model). The hot-injection method reported in 1993 creates this situation by rapidly injecting precursors into a hot solvent and spread as a way of obtaining semiconductor nanocrystals of uniform size. In the growth stage, the final size is adjusted through temperature, time and precursor concentration, and the reaction is stopped by cooling at the right moment.
In liquid-phase synthesis, molecules with long chains such as oleic acid and oleylamine, as well as citric acid and polymers, adsorb on particle surfaces. These ligands (capping agents) prevent particles from agglomerating, and because they adsorb strongly on particular crystal faces and slow the growth of those faces, they allow cubes, rods, plates and other shapes to be made selectively. The synthesis of gold nanorods, in which small seed crystals are grown in one direction with the help of a surfactant, is an example. On the other hand, when the particles are used as catalysts or electronic materials, the surface ligands hinder reactions or the flow of current, so a step is needed to remove them by heat treatment or ligand exchange.
The challenges of bottom-up methods are mass production and control of placement. Liquid-phase synthesis yields only small amounts per reaction and can require large volumes of organic solvent and purification steps. Placing the resulting particles at defined positions on a substrate is also difficult. Top-down methods such as lithography can define position and shape precisely, but making dimensions of a few nanometers or surfaces smooth at the atomic level is hard. Methods combining the two are therefore also widely used, such as self-assembling molecules along patterns drawn by lithography, or printing nanoparticle inks to form wiring.
Answer: C. Reducing metal salts or decomposing precursors in solution so that nanoparticles form by nucleation and growth
Reduction or thermal decomposition in solution (chemical liquid-phase synthesis) is a bottom-up method in which atoms and molecules gather to form nuclei that grow into particles. The sol-gel method, condensation from the vapor phase and CVD growth of carbon nanotubes are also bottom-up methods. Ball milling is a top-down method that mechanically crushes large particles into smaller ones; the equipment is simple and suited to mass production, but the size distribution tends to be broad, and contamination from the milling media and introduction of strain occur. Lithography and etching are also top-down methods, removing unwanted parts from a larger film, with the advantage of precise control over shape and position. Bottom-up methods make it easy to control size and shape at the atomic level but face challenges in mass production and positional control.
Bottom-up methods build materials from atoms and molecules by nucleation and growth, while top-down methods make large materials smaller by grinding, etching or machining. The former easily produce particles of uniform size and shape with few defects, while the latter are strong in mass production and control of position.
Yes. It hydrolyzes and condenses metal alkoxides or similar compounds to make a dispersion of particles (a sol) from molecules and then a connected network (a gel), so it is a typical bottom-up method. In the method that makes silica spheres with ammonia as the catalyst, particles of very uniform size can be obtained under suitable conditions.
The basic approach is to concentrate nucleation in a short time and then let only growth proceed under conditions where no new nuclei form. Methods include rapidly injecting precursors into a hot solvent and making seed crystals first and then growing them. After growth, the distribution can be narrowed further by size-selective precipitation, in which a poor solvent is added to precipitate particles by size.
Reduction or thermal decomposition in solution (chemical liquid-phase synthesis) is a bottom-up method in which atoms and molecules gather to form nuclei that grow into particles.
Methods for making nanomaterials are divided into top-down …Carbon nanotube BET method Graphene Melting point depression of nanoparticles Quantum dot Specific surface area Zeta potential