The vapor–liquid–solid method ( VLS ) is a mechanism for the growth of one-dimensional structures, such as nanowires , from chemical vapor deposition . The 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.
29-457: VLS may refer to: Vapor-Liquid-Solid method , a method of growing nanocrystals Vermont Law School Vertical Launching System for firing missiles Von Luschan's chromatic scale of skin colour West Flemish , a dialect in Belgium, ISO 639-3 code Vertical Lift System, a style of Scissor doors VideoLAN Server Valk Last Slot ,
58-404: A given temperature and pressure, the molar volume is the same for all ideal gases and is based on the gas constant : R = 8.314 462 618 153 24 m ⋅Pa⋅K ⋅mol , or about 8.205 736 608 095 96 × 10 m ⋅atm⋅K ⋅mol . The molar volume of an ideal gas at 100 kPa (1 bar ) is The molar volume of an ideal gas at 1 atmosphere of pressure is For crystalline solids ,
87-428: 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: The VLS process takes place as follows: The requirements for catalysts are: The materials system used, as well as the cleanliness of the vacuum system and therefore the amount of contamination and/or
116-424: A relatively simple setup composed of a dual-zone vacuum furnace. The hot end of the furnace contains the evaporating source material, while the evaporated particles are carrier downstream, (by way of a carrier gas) to the colder end of the furnace where they can absorb, nucleate, and grow on a desired substrate. Molecular beam epitaxy (MBE) has been used since 2000 to create high-quality semiconductor wires based on
145-625: A set of speedcubing algorithms See also [ edit ] VLS-1 , the Brazilian Space Agency satellite launcher Topics referred to by the same term [REDACTED] This disambiguation page lists articles associated with the title VLS . If an internal link led you here, you may wish to change the link to point directly to the intended article. Retrieved from " https://en.wikipedia.org/w/index.php?title=VLS&oldid=1002322091 " Category : Disambiguation pages Hidden categories: Short description
174-415: A substance is the ratio of the volume ( V ) occupied by a substance to the amount of substance ( n ), usually at a given temperature and pressure . It is also equal to the molar mass ( M ) divided by the mass density ( ρ ): V m = V n = M ρ {\displaystyle V_{\text{m}}={\frac {V}{n}}={\frac {M}{\rho }}} The molar volume has
203-408: Is dependent on the surface (σ s ) and liquid–solid interface (σ ls ) tensions, as well as an additional line tension (τ) which comes into effect when the initial radius of the droplet is small (nanosized). As a nanowire begins to grow, its height increases by an amount dh and the radius of the contact area decreases by an amount dr (see Figure 4). As the growth continues, the inclination angle at
232-710: Is dependent on the whisker diameter: the larger the whisker diameter, the faster the nanowire grows axially. This is because the supersaturation of the metal-alloy catalyst ( Δ μ {\displaystyle \Delta \mu } ) is the main driving force for nanowhisker growth and decreases with decreasing whisker diameter (also known as the Gibbs-Thomson effect): Δ μ = Δ μ o − 4 α Ω d {\displaystyle \Delta \mu =\Delta \mu _{\mathrm {o} }-{\frac {4\alpha \Omega }{d}}} . Again, Δμ
261-434: Is different from Wikidata All article disambiguation pages All disambiguation pages Vapor-Liquid-Solid method 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. The explanation was motivated by the absence of axial screw dislocations in the whiskers (which in themselves are
290-515: Is on the order of meters. Therefore, evaporated source atoms (from, say, an effusion cell) act as a beam of particles directed towards the substrate. The growth rate of the process is very slow, the deposition conditions are very clean, and as a result four superior capabilities arise, when compared to other deposition methods: Molar volume In chemistry and related fields, the molar volume , symbol V m , or V ~ {\displaystyle {\tilde {V}}} of
319-528: Is represented by the quantity excess volume of the mixture, an example of excess property . Molar volume is related to specific volume by the product with molar mass . This follows from above where the specific volume is the reciprocal of the density of a substance: V m , i = M i ρ i 0 = M i v i {\displaystyle V_{\rm {m,i}}={M_{i} \over \rho _{i}^{0}}=M_{i}v_{i}} For ideal gases ,
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#1732783584160348-537: Is the Avogadro constant and Z is the number of formula units in the unit cell. The result is normally reported as the "crystallographic density". Ultra-pure silicon is routinely made for the electronics industry , and the measurement of the molar volume of silicon, both by X-ray crystallography and by the ratio of molar mass to mass density, has attracted much attention since the pioneering work at NIST in 1974. The interest stems from that accurate measurements of
377-463: Is the atomic volume of Si and α {\displaystyle \alpha } the specific free energy of the wire surface. Examination of the above equation, indeed reveals that small diameters ( < {\displaystyle <} 100 nm) exhibit small driving forces for whisker growth while large wire diameters exhibit large driving forces. Involves the removal of material from metal-containing solid targets by irradiating
406-475: Is the main driving force for nanowhisker growth (the supersaturation of the metal droplet). More specifically, Δμ 0 is the difference between the chemical potential of the depositing species (Si in the above example) in the vapor and solid whisker phase. Δμ 0 is the initial difference proceeding whisker growth (when d → ∞ {\displaystyle d\rightarrow \infty } ), while Ω {\displaystyle \Omega }
435-494: Is the radius of the contact area and β 0 is defined by a modified Young’s equation: σ 1 cos ( β o ) = σ s − σ l s − τ r o {\displaystyle \sigma _{\mathrm {1} }\cos(\beta _{\mathrm {o} })=\sigma _{\mathrm {s} }-\sigma _{\mathrm {ls} }-{\frac {\tau }{r_{\mathrm {o} }}}} , It
464-598: The SI unit of cubic metres per mole (m /mol), although it is more typical to use the units cubic decimetres per mole (dm /mol) for gases , and cubic centimetres per mole (cm /mol) for liquids and solids . The molar volume of a substance i is defined as its molar mass divided by its density ρ i : V m , i = M i ρ i 0 {\displaystyle V_{\rm {m,i}}={M_{i} \over \rho _{i}^{0}}} For an ideal mixture containing N components,
493-468: The VLS growth mechanism. However, in metal-catalyzed MBE the metal particles do not catalyze a reaction between precursors but rather adsorb vapor phase particles. This is because the chemical potential of the vapor can be drastically lowered by entering the liquid phase. MBE is carried out under ultra-high vacuum (UHV) conditions where the mean-free-path (distance between collisions) of source atoms or molecules
522-597: The base of the nanowires (α, set as zero before whisker growth) increases, as does β 0 : σ 1 cos ( β o ) = σ s cos ( α ) − σ l s − τ r o {\displaystyle \sigma _{\mathrm {1} }\cos(\beta _{\mathrm {o} })=\sigma _{\mathrm {s} }\cos(\alpha )-\sigma _{\mathrm {ls} }-{\frac {\tau }{r_{\mathrm {o} }}}} . The line tension therefore greatly influences
551-407: The catalyst contact area. The most import result from this conclusion is that different line tensions will result in different growth modes. If the line tensions are too large, nanohillock growth will result and thus stop the 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
580-539: The material using extremely high temperature resistive or electron bombardment heating. Furthermore, targets can simply be made from a mixture of materials or even a liquid. Finally, the plasma formed during the laser absorption process allows for the deposition of charged particles as well as a catalytic means to lower the activation barrier of reactions between target constituents. Some very interesting nanowires microstructures can be obtained by simply thermally evaporating solid materials. This technique can be carried out in
609-561: The molar volume can be measured by X-ray crystallography . The unit cell volume ( V cell ) may be calculated from the unit cell parameters, whose determination is the first step in an X-ray crystallography experiment (the calculation is performed automatically by the structure determination software). This is related to the molar volume by V m = N A V c e l l Z {\displaystyle V_{\rm {m}}={{N_{\rm {A}}V_{\rm {cell}}} \over {Z}}} where N A
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#1732783584160638-415: The molar volume is given by the ideal gas equation ; this is a good approximation for many common gases at standard temperature and pressure . The ideal gas equation can be rearranged to give an expression for the molar volume of an ideal gas: V m = V n = R T P {\displaystyle V_{\rm {m}}={\frac {V}{n}}={\frac {RT}{P}}} Hence, for
667-661: The molar volume of the mixture is the weighted sum of the molar volumes of its individual components. For a real mixture the molar volume cannot be calculated without knowing the density: V m = ∑ i = 1 N x i M i ρ m i x t u r e {\displaystyle V_{\rm {m}}={\frac {\displaystyle \sum _{i=1}^{N}x_{i}M_{i}}{\rho _{\mathrm {mixture} }}}} There are many liquid–liquid mixtures, for instance mixing pure ethanol and pure water , which may experience contraction or expansion upon mixing. This effect
696-403: The nanometer level. Several techniques to generate smaller droplets have been developed, including the use of monodispersed 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. During VLS whisker growth, the rate at which whiskers grow
725-454: The presence of oxide layers at the droplet and wafer surface during the experiment, both greatly influence the absolute magnitude of the forces present at the droplet/surface interface and, in turn, determine the shape of the droplets. The shape of the droplet, i.e. the contact angle (β 0 , see Figure 4) can, be modeled mathematically, however, the actual forces present during growth are extremely difficult to measure experimentally. Nevertheless,
754-458: The shape of a catalyst particle at the surface of a crystalline substrate is determined by a balance of the forces of surface tension and the liquid–solid interface tension. The radius of the droplet varies with the contact angle as: R = r o sin ( β o ) , {\displaystyle R={\frac {r_{\mathrm {o} }}{\sin(\beta _{\mathrm {o} })}},} where r 0
783-439: The substrate where they can nucleate and grow into nanowires . The laser-assisted growth technique is particularly useful for growing nanowires with high melting temperatures , multicomponent or doped nanowires, as well as nanowires with extremely high crystalline quality. The high intensity of the laser pulse incident at the target allows the deposition of high melting point materials, without having to try to evaporate
812-424: The substrate. In an equilibrium situation this is not possible as the minimum radius of a metal droplet is given by where V l is the molar volume of the droplet, σ lv the liquid-vapor surface energy , and s is the degree of supersaturation 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
841-441: The surface with high-powered (~100 mJ/pulse) short (10 Hz) laser pulses, usually with wavelengths in the ultraviolet (UV) region of the light spectrum. When such a laser pulse is adsorbed by a solid target, material from the surface region of the target absorbs the laser energy and either (a) evaporates or sublimates from the surface or is (b) converted into a plasma (see laser ablation ). These particles are easily transferred to
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