Химия - Пар-жидкость-кристалл - Введение

01 марта 2011


Оглавление:
1. Пар-жидкость-кристалл
2. Введение
3. Механизм роста
4. Схожие методы выращивания

В механизме ПЖК обычно выделяются следующие стадии:

• Подготовка капель жидкого сплава, под которыми будет происходить рост.
• Введение осаждаемого вещества в газообразном виде, его адсорбция поверхностью капли и диффузия вглубь
• Пересыщение капли приводит к кристаллизации вещества на границе раздела жидкость/кристалл.

Экспериментальная технология

Процесс ПЖК состоит из следующих этапов:

1. Тонкая пленка Au осаждается на кремниевую подложку методом распыления или термовакуумным испарением.
2. The wafer is annealed at temperatures higher than the Au-Si eutectic point, creating Au-Si alloy droplets on the wafer surface. Mixing Au with Si greatly reduces the melting temperature of the alloy as compared to the alloy constituents. The melting temperature of the Au:Si alloy reaches a minimum when the ratio of its constituents is 4:1 Au:Si, also known as the Au:Si eutectic point.
3. Lithography techniques can also be used to controllably manipulate the diameter and position of the droplets.
4. One-dimensional crystalline nanowires are then grown by a liquid metal-alloy droplet-catalyzed chemical or physical vapor deposition process, which takes place in a vacuum deposition system. Au-Si droplets on the surface of the substrate act to lower the activation energy of normal vapor-solid growth. For example, Si can be deposited by means of a SiCl₄:H₂ gaseous mixture reaction, only at temperatures above 800 °C, in normal vapor-solid growth. Moreover, below this temperature almost no Si is deposited on the growth surface. However, Au particles can form Au-Si eutectic droplets at temperatures above 363 °C and adsorb Si from the vapor state until reaching a supersaturated state of Si in Au. Furthermore, nanosized Au-Si droplets have much lower melting points due to the fact that the surface area-to-volume ratio is increasing, becoming energetically unfavorable, and nanometer-sized particles act to minimize their surface energy by forming droplets.
5. Si has a much higher melting point than that of the eutectic alloy, therefore Si atoms precipitate out of the supersaturated liquid-alloy droplet at the liquid-alloy/solid-Si interface, and the droplet rises from the surface. This process is illustrated in figure 1.

Характерные черты ПЖК

• Greatly lowered reaction energy compared to normal vapor-solid growth.
• Wires grow only in the areas activated by the metal catalysts and the size and position of the wires are determined by that of the metal catalysts.
• This growth mechanism can also produce highly anisotropic nanowire arrays from a variety of materials.

Требования к свойствам катализатора

The requirements for catalyst are:

• It must form a liquid solution with the crystalline material to be grown at the nanowire growth temperature.
• The solid solubility of the catalyzing agent is low in the solid and liquid phases of the substrate material.
• The equilibrium vapor pressure of the catalyst over the liquid alloy be small so that the droplet does not vaporize, shrink in volume, and decrease the radius of the growing wire until, ultimately, growth is terminated.
• The catalyst must be inert to the reaction products.
• The vapor-solid, vapor-liquid, and liquid-solid interfacial energies play a key role in the shape of the droplets and therefore must be examined before choosing a suitable catalyst; small contact angle between the droplet and solid are more suitable for large area growth, while large contacts angles result in smaller whisker formations.
• The solid-liquid interface must be well-defined crystallographically in order to produce highly directional growth of nanowires. It is also important to point out that the solid-liquid interface cannot, however, be completely smooth. Furthermore, if the solid liquid interface was atomically smooth, atoms near the interface trying to attach to the solid would have no place to attach to until a new island nucleates, leading to an extremely long growth rate. Therefore, “rough” solid surfaces, or surfaces containing a large number of surface atomic steps are needed for depositing atoms to attach and nanowire growth to proceed.

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