„VLS-Mechanismus“ – Versionsunterschied
| [ungesichtete Version] | [ungesichtete Version] |
imported>Smile a While add a ref |
expand |
||
| Zeile 4: | Zeile 4: | ||
<!-- End of AfD message, feel free to edit beyond this point --> |
<!-- End of AfD message, feel free to edit beyond this point --> |
||
{{context}} |
{{context}} |
||
The '''vapor-liquid-solid process''' ('''VLS''') is a mechanism for the growth of one-dimensional structures, such as [[nanowire]]s, from [[chemical vapor deposition]]. Growth of a crystal through direct [[adsorption]] of a gas phase on to a solid surface is generally very slow. The VLS mechanism circumvents this by introducing a [[catalytic]] liquid alloy phase which can rapidly adsorb a vapor to [[supersaturation]] levels, and from which [[crystal growth]] can subsequently occur from [[nucleation|nucleated]] seeds at the liquid-solid interface. The physical characteristics of nanowires grown in this manner depend, in a controllable way, upon the size and physical properties of the liquid alloy. |
|||
| ⚫ | |||
== VLS mechanism == |
|||
Generally speaking, it is a method to get a nano-single-line. The line is one dimensional. V means vapor; another kind of vapor such as Ar or others carries the resource of the material which you want to get a single-crystal-line. L is liquid; it always means the catalyst. The catalyst stays in the liquid during the reaction. S is solid; the surface where you can see the result.<ref>[http://books.google.co.uk/books?id=tSu--K8OUykC&pg=PA3191&lpg=PA3191&dq=%22Vapor-Liquid-Solid%22+VLS&source=web&ots=Z8ZJaO4MJg&sig=tXSkTrMyoDEB89m37oi2TMK1gr8&hl=en&sa=X&oi=book_result&resnum=9&ct=result "Vapor-Liquid-Solid Synthesis of Nanowires"], Encyclopedia of Chemical Processing</ref><ref>[http://www.hbci.com/~wenonah/new/crystals.htm "Growing Crystals in the Lab"], John V. Milewski</ref> |
|||
The VLS mechanism was proposed in 1964 as an explanation for [[silicon]] whisker growth from the gas phase in the presence of a liquid [[gold]] droplet placed upon a silicon substrate.<ref>{{cite journal|last=Wagner |first=R. S.|coauthors=Ellis, W. C.|date=1964|title=Vapor-liquid-solid mechanism of single crystal growth|journal=Appl. Phys. Lett.|volume=4|issue=5|pages=89|doi=10.1063/1.1753975}}</ref> The explanation was motivated by the absence of axial [[screw dislocation]]s in the whiskers (which in themselves are a growth mechanism), the requirement of the gold droplet for growth, and the presence of the droplet at the tip of the whisker during the entire growth process. |
|||
| ⚫ | |||
The VLS mechanism is typically described in three stages:<ref>{{cite book|last=Lu|first=Yicheng|coauthors=Zhong, Jian|title=Semiconductor Nanostructures for Optoelectronic Applications|editor=Todd Steiner|publisher=Artech House, Inc.|location=Norwood, MA|date=2004|pages=p.191-192|isbn=978-1-580-53751-3}}</ref> |
|||
*Preparation of a liquid [[alloy]] droplet upon the substrate from which a wire is to be grown |
|||
*Introduction of the substance to be grown as a vapor, which [[adsorb]]s on to the liquid surface, and diffuses in to the droplet |
|||
*[[Supersaturation]] and [[nucleation]] at the liquid/solid interface leading to axial crystal growth |
|||
The diameter of the nanowire which is grown depends upon the properties of the alloy droplet. The growth of nano-sized wires requires nano-size droplets to be prepared on the substrate. In an equilibrium situation this is not possible as the minimum radius of a metal droplet is given by<ref>{{cite journal|last=Huang|first=M. H.|coauthors=Wu, Y; Feick, H; Tran, N.; Weber, E.; Yang, P.|date=2001|title=Catalytic Growth of Zinc Oxide Nanowires by Vapor Transport|journal=Adv. Mater.|volume=13|issue=2|pages=113 - 116|doi=10.1002/1521-4095}}</ref> |
|||
:<math>R_{\mathrm{min}} = \frac{2V_{l}}{RT}\sigma_{lv}\ln(s),</math> |
|||
where ''V<sub>l</sub>'' is the [[molar volume]] of the droplet, ''σ<sub>lv</sub>'' the liquid-vapor [[surface energy]], and ''s'' is the degree of supersaturation<ref>{{cite book|last=Wang|first=Ji-Tao|title=Nonequilibrium Nondissipative Thermodynamics: With Application to Low-pressure Diamond Synthesis|publisher=Springer Verlag|location=Berlin|date=2002|pages=p. 65|isbn=978-3-540-42802-2}}</ref> of the vapor. This equations restricts the minimum diameter of the droplet, and of any crystals which can be grown from it, under typically conditions to well above the nanometer level. Several techniques to generate smaller droplets have been developed, including the use of [[monodisperse nanoparticles]] spread in low dilution on the substrate, and the laser ablation of a substrate-catalyst mixture so to form a plasma which allows well-separated nanoclusters of the catalyst to form as the systems cools.<ref>{{cite book|last=Bhushan|first=Bharat|title=Springer Handbook of Nanotechnology|publisher=Spinger-Verlag|location=Berlin|pages=p.105|isbn=3-540-01218-4}}</ref> |
|||
| ⚫ | |||
{{reflist}} |
{{reflist}} |
||
== External links == |
|||
*[http://www.hbci.com/~wenonah/new/crystals.htm Growing Crystals in the Lab] |
|||
[[Category:Nanomaterials]] |
[[Category:Nanomaterials]] |
||
[[Category:Chemical processes]] |
[[Category:Chemical processes]] |
||
| ⚫ | |||
<ref>[http://books.google.co.uk/books?id=tSu--K8OUykC&pg=PA3191&lpg=PA3191&dq=%22Vapor-Liquid-Solid%22+VLS&source=web&ots=Z8ZJaO4MJg&sig=tXSkTrMyoDEB89m37oi2TMK1gr8&hl=en&sa=X&oi=book_result&resnum=9&ct=result "Vapor-Liquid-Solid Synthesis of Nanowires"], Encyclopedia of Chemical Processing</ref>--> |
|||
Version vom 28. Oktober 2008, 15:55 Uhr
Vorlage:AfDM Vorlage:Context The vapor-liquid-solid process (VLS) is a mechanism for the growth of one-dimensional structures, such as nanowires, from chemical vapor deposition. Growth of a crystal through direct adsorption of a gas phase on to a solid surface is generally very slow. The VLS mechanism circumvents this by introducing a catalytic liquid alloy phase which can rapidly adsorb a vapor to supersaturation levels, and from which crystal growth can subsequently occur from nucleated seeds at the liquid-solid interface. The physical characteristics of nanowires grown in this manner depend, in a controllable way, upon the size and physical properties of the liquid alloy.
VLS mechanism
The VLS mechanism was proposed in 1964 as an explanation for silicon whisker growth from the gas phase in the presence of a liquid gold droplet placed upon a silicon substrate.[1] The explanation was motivated by the absence of axial screw dislocations in the whiskers (which in themselves are a growth mechanism), the requirement of the gold droplet for growth, and the presence of the droplet at the tip of the whisker during the entire growth process.
The VLS mechanism is typically described in three stages:[2]
- Preparation of a liquid alloy droplet upon the substrate from which a wire is to be grown
- Introduction of the substance to be grown as a vapor, which adsorbs on to the liquid surface, and diffuses in to the droplet
- Supersaturation and nucleation at the liquid/solid interface leading to axial crystal growth
The diameter of the nanowire which is grown depends upon the properties of the alloy droplet. The growth of nano-sized wires requires nano-size droplets to be prepared on the substrate. In an equilibrium situation this is not possible as the minimum radius of a metal droplet is given by[3]
where Vl is the molar volume of the droplet, σlv the liquid-vapor surface energy, and s is the degree of supersaturation[4] of the vapor. This equations restricts the minimum diameter of the droplet, and of any crystals which can be grown from it, under typically conditions to well above the nanometer level. Several techniques to generate smaller droplets have been developed, including the use of monodisperse nanoparticles spread in low dilution on the substrate, and the laser ablation of a substrate-catalyst mixture so to form a plasma which allows well-separated nanoclusters of the catalyst to form as the systems cools.[5]
References
External links
- ↑ R. S. Wagner, Ellis, W. C.: Vapor-liquid-solid mechanism of single crystal growth. In: Appl. Phys. Lett. 4. Jahrgang, Nr. 5, 1964, S. 89, doi:10.1063/1.1753975.
- ↑ Yicheng Lu, Zhong, Jian: Semiconductor Nanostructures for Optoelectronic Applications. Hrsg.: Todd Steiner. Artech House, Inc., Norwood, MA 2004, ISBN 978-1-58053-751-3, S. 191–192.
- ↑ M. H. Huang, Wu, Y; Feick, H; Tran, N.; Weber, E.; Yang, P.: Catalytic Growth of Zinc Oxide Nanowires by Vapor Transport. In: Adv. Mater. 13. Jahrgang, Nr. 2, 2001, S. 113 - 116, doi:10.1002/1521-4095.
- ↑ Ji-Tao Wang: Nonequilibrium Nondissipative Thermodynamics: With Application to Low-pressure Diamond Synthesis. Springer Verlag, Berlin 2002, ISBN 978-3-540-42802-2, S. 65.
- ↑ Bharat Bhushan: Springer Handbook of Nanotechnology. Spinger-Verlag, Berlin, ISBN 3-540-01218-4, S. 105.