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A chemical vapor deposition route in which catalyst particles form or travel in the gas stream rather than remaining on a macroscopic support. For carbon nanotubes, particle nucleation, tube growth and aggregation can occur in one flowing reactor. Product withdrawal becomes possible when the growing network is sufficiently connected.

A compatible, reaction-limited chemical vapor deposition process can give nearly conformal coverage of tops, sidewalls and floor. Gas-phase precursors reach recessed surfaces and react there; the coating is not restricted to the line of sight of a solid source. The result is a continuous film over the resist pattern, which is unfavorable for ordinary lift-off process.

If each wall receives the same $0.20\mu \mathrm{m}$ coating as a horizontal surface, the nominal $0.50\mu \mathrm{m}$ gap narrows toThat numerical result is the conformal idealization. Precursor depletion or rapid surface reaction can instead give poorer bottom coverage or premature closure near the mouth. Actual silver chemistry and deposition temperature must also be compatible with the resist; a process that decomposes the resist does not preserve the illustrated mask.

Use substrate-supported chemical vapor deposition. Prepare a clean quartz support with a suitable supported transition-metal catalyst, often nanoscale Fe-containing particles. Catalyst loading, support chemistry and pretreatment control the active particle size and density. A heated carbon-containing gas supplies carbon to those particles, from which carbon nanotubes grow.

At sufficiently high nucleation density, neighboring tubes constrain one another's lateral motion, producing a roughly aligned carbon nanotube forest normal to the support. This is collective crowding, not a requirement that quartz epitaxially orient every tube. Gas delivery, catalyst lifetime and growth time set length and uniformity. Depending on catalyst–support adhesion, catalyst particles can remain near the support or move with growing tips.

The catalyst is supported and the carpet remains attached to a substrate. Check particle distribution and film uniformity separately from tube diameter and alignment; a dense black coating alone does not establish a nanotube forest. The original growth sketch contrasts this supported route with a floating-catalyst reactor.

In physical vapor deposition, material is physically released from a source and transported to the substrate, where it condenses. Three examples are thermal evaporative deposition, electron-beam evaporation and sputtering. Their angular distributions, source heating and energetic bombardment differ, even though all belong to the physical-deposition family.

In chemical vapor deposition, volatile reactants undergo chemical reactions that produce a solid film at or near the substrate, with gaseous products removed. Examples are atmospheric-pressure thermal CVD, low-pressure CVD and plasma-enhanced chemical vapor deposition. These are three established process variants, distinguished by pressure and activation method. Surface chemistry, precursor delivery and temperature determine growth, rather than condensation alone.

Atomic layer deposition is a related cyclic surface-reaction method, but need not be counted as a separate conventional CVD example to satisfy the requested three. Reactive sputtering can involve chemistry, so the boundary is best explained by the dominant source and transport mechanism rather than by claiming that a PVD chamber can contain no chemical reactions.

A simple chemical vapor deposition model separates transport through a gas boundary layer, with coefficient $h_m$, from surface reaction, with coefficient $k_s$:For $k_s\gg h_m$, growth is mass-transfer-limited deposition. A typical cold-wall, flowing reactor heats the substrate strongly while keeping most chamber walls cooler. A shaped inlet, controlled flow and tilted or rotating susceptor help control boundary-layer thickness and precursor depletion. The essential design problem is delivering a similar flux everywhere; heating a surface still further gives little benefit once delivery limits growth.

For $k_s\ll h_m$, growth is reaction-limited deposition. A hot-wall, low-pressure batch tube can hold many wafers in a nearly uniform-temperature zone. Faster gas transport makes surface concentrations more uniform, while slow chemistry reduces consumption before precursors reach downstream surfaces. Temperature uniformity is particularly important because surface rates can depend strongly on temperature.

These are illustrative architectures, not a rule that wall type alone determines the regime. Temperature, pressure, flow and precursor chemistry jointly set $k_s/h_m$.

In catalytic chemical vapor deposition, a metal nanoparticle activates a carbon-containing feedstock and supplies mobile carbon to the growing rim. Carbon can diffuse over the particle surface and, depending on the catalyst state, through its bulk. Once a curved carbon cap nucleates, continued incorporation extends the tube. Strong catalyst-support adhesion favors base growth; weaker adhesion can allow the particle to lift with the tube tip. A liquid catalyst is not universally required.

Particle size constrains tube diameter, but semiconducting character requires a particular wrapping family: $n-m\not\equiv 0(\mathrm{mod}3)$. Merely making particles smaller does not select that congruence class. Routes toward predominantly semiconducting carbon nanotubes include controlling catalyst composition, exposed facets and size distribution to bias cap/rim nucleation and growth, or adjusting carbon chemical potential and growth/etching conditions so that metallic tubes are less likely to survive.

These are mechanistic strategies rather than a universal recipe. Experiments with catalyst conditioning found that high semiconducting selectivity depended sensitively on oxidizing/reducing gas conditions and came with a strong yield penalty, motivating a catalyst-deactivation model.^[ref]

The useful distinction is between selecting a structure at nucleation, selecting its growth rate, and selectively terminating or removing unwanted tubes. Each can enrich the semiconducting fraction, but they do not establish the same growth mechanism.