Nucleation and Growth
Summary
TLDRThis video explores the evolution of TV displays, focusing on the use of quantum dots in modern screens. It delves into the process of nanoparticle synthesis, particularly the thermodynamic approach, which involves supersaturation, nucleation, and growth. The video explains how nanoparticles form through the reduction of free energy and how nucleation varies between homogeneous and heterogeneous mechanisms. It also contrasts diffusion-controlled growth with surface-controlled growth, showing how these processes impact nanoparticle size distribution. The discussion highlights key factors influencing nanoparticle formation, including temperature and growth mechanisms.
Takeaways
- đș TVs have evolved from bulky boxes to slim, flat screens with quantum dot technology.
- đŹ Quantum dots are nanoparticles ranging from 2 to 10 nanometers in diameter, used in modern displays.
- âïž Nanoparticle synthesis involves two main mechanisms: kinetic and thermodynamic approaches.
- đĄïž The thermodynamic approach includes supersaturation, nucleation, and growth to form nanoparticles.
- đ§Ș Supersaturation happens when the solution exceeds equilibrium solubility, driving nucleation and solid-phase formation.
- đ Free energy change drives nucleation, with the energy balance involving volume free energy and surface energy.
- đ Critical nucleation concepts include 'R star' (critical radius) and 'Delta G star' (energy barrier for nucleation).
- âïž Lower temperatures promote supercooling, leading to smaller nanoparticle sizes, while higher temperatures yield larger particles.
- âïž Growth of nanoparticles can be diffusion-controlled or surface-process-controlled, influencing size distribution.
- 𧱠Polynuclear growth results in faster particle growth, while mononuclear growth is slower but more uniform.
Q & A
What are quantum dots and how are they related to modern displays?
-Quantum dots are nanosized particles, ranging from 2 to 10 nanometers in diameter, used in modern displays like QLED. They enhance color and brightness in screens by emitting specific light frequencies.
What are the two main mechanisms for synthesizing nanoparticles?
-The two main mechanisms for synthesizing nanoparticles are the kinetic approach, which limits precursor or space, and the thermodynamic approach, which involves supersaturation, nucleation, and growth.
What is supersaturation, and how is it achieved?
-Supersaturation occurs when a solution exceeds its equilibrium solubility, leading to the formation of a new phase. It can be achieved by reducing temperature or through in-situ chemical reactions.
What is the role of free energy in the synthesis of nanoparticles?
-Free energy drives nanoparticle synthesis, where the reduction in free energy from a supersaturated state encourages the formation of a solid phase. This is expressed as the free volume energy difference between the solid and liquid phases.
What is the critical radius (R*) and how does it relate to nanoparticle growth?
-The critical radius (R*) represents the size at which nuclei become stable. Nuclei larger than R* grow, while those smaller dissolve back into the solution. This concept is key to controlling nanoparticle formation.
How does temperature affect nanoparticle nucleation and growth?
-Higher temperatures reduce the energy barrier for nucleation, leading to larger particles, while lower temperatures (supercooling) result in smaller particles due to faster nucleation.
What is the difference between homogeneous and heterogeneous nucleation?
-Homogeneous nucleation occurs in a pure solution without external surfaces, while heterogeneous nucleation happens on pre-existing surfaces, lowering the energy barrier for nuclei formation.
What is diffusion-controlled growth in nanoparticle synthesis?
-Diffusion-controlled growth occurs when nucleation stops due to a reduction in the concentration of growth species, but the growth of existing particles continues as growth species diffuse from the bulk solution to the particle surface.
What is surface-controlled growth and how does it differ from diffusion-controlled growth?
-Surface-controlled growth happens when the concentration of growth species on the particle surface is high, causing growth to be limited by surface processes rather than diffusion. This leads to size disparities compared to diffusion-controlled growth.
What is the impact of mononuclear versus polynuclear growth on nanoparticle size uniformity?
-Mononuclear growth, which occurs layer by layer, promotes more uniform particle sizes, while polynuclear growth, where multiple layers form simultaneously, can lead to non-uniform particle sizes.
Outlines
đș Evolution of TVs and Quantum Dots
The script discusses the evolution of televisions from bulky boxes to flat monitors, highlighting the use of quantum dots in modern displays. Quantum dots are nano-sized particles that enhance display quality. The synthesis of these nanoparticles is divided into kinetic and thermodynamic approaches. The thermodynamic approach, which is the focus of this paragraph, involves processes such as super saturation, nucleation, and growth. Super saturation is achieved by reducing temperature or chemical reaction, leading to nucleation where growth species are saturated. The process is driven by the reduction of free energy, which is the difference between the solid and liquid phases, represented by ÎGv. This energy change is dependent on solute concentration, temperature, and super saturation.
đŹ Nucleation and Growth of Nanoparticles
This section delves into the critical terms and processes involved in nucleation, including the formation of nuclei and the energy barrier (ÎG*) that must be overcome. It introduces the concept of the critical radius (R*), beyond which nuclei are stable and grow. The discussion also covers how temperature affects these critical values, with lower temperatures favoring nucleation. The phenomenon of super cooling is mentioned, which results in smaller particle sizes. Heterogeneous nucleation is also explored, where pre-existing surfaces reduce the energy barrier for nucleation. The size distribution of nanoparticles is dependent on the growth process, which includes steps like generation of growth species, diffusion, absorption, and surface growth. The paragraph distinguishes between diffusion-controlled and growth-controlled processes, affecting the size distribution of nanoparticles.
đ Diffusion and Surface-Controlled Growth
The final paragraph discusses two mechanisms of growth: diffusion-controlled and surface-controlled. In diffusion-controlled growth, the rate of particle growth is determined by the diffusion of growth species to the particle surface. This process leads to the formation of uniformly sized particles. The mathematical relationship between the radius of a spherical nucleus, diffusion coefficient, and concentration gradients is provided. In contrast, surface-controlled growth occurs when the diffusion of growth species is rapid, and the growth rate is determined by surface processes. Two types of surface processes are identified: mononuclear and polynuclear growth. Mononuclear growth occurs layer by layer, while polynuclear growth happens when the surface concentration is high, leading to a constant growth rate independent of particle size or time. The implications of these growth mechanisms on the synthesis of monosized particles are also discussed.
Mindmap
Keywords
đĄQuantum Dots
đĄNanoparticles
đĄSynthesis of Nanoparticles
đĄSupersaturation
đĄNucleation
đĄFree Energy
đĄDiffusion
đĄHomogeneous Nucleation
đĄHeterogeneous Nucleation
đĄSurface Growth
Highlights
TV evolution from bulky boxes to slim, flat monitors with quantum dot technology.
Quantum dots are nanoparticles ranging from 2 to 10 nanometers in diameter.
Two primary mechanisms for nanoparticle synthesis: kinetic and thermodynamic approaches.
Thermodynamic approach involves super saturation, nucleation, and growth processes.
Super saturation is achieved through temperature reduction or chemical reactions.
Nucleation occurs once a system becomes super saturated, leading to the formation of a new phase.
Free energy reduction drives the process of nanoparticle formation, balancing surface and volume energies.
Nuclei larger than the critical radius grow, while smaller ones dissolve back into the solution.
Temperature impacts nucleation and particle size: lower temperatures lead to smaller particles.
Heterogeneous nucleation lowers the energy barrier due to pre-existing surfaces.
Growth of nanoparticles involves generation, diffusion, and incorporation of growth species.
Diffusion-controlled growth results in uniformly sized particles.
Mononuclear growth involves layer-by-layer addition, favoring surface area and size-dependent growth.
Polynuclear growth allows faster growth before the previous layer is complete, leading to linear growth.
Diffusion versus growth processes lead to different size distributions of nanoparticles.
Transcripts
[Music]
have you ever wondered how TVs evolve
from fat and bulky boxes to slim and
flat monitors with revolutionary lead
displays that they are built with most
of the known and Big Time Brands now use
the so-called
Qs this type of displays makes use of
quantum dots which are nanop particles
that range between 2 to 10 nanometers in
diameter now the question is how do we
make nanosized particles like Quantum
dots in this video we reveal the major
mechanisms involved in the synthesis of
nanop
particles the synthesis of nanop
particles is generally divided into two
mechanisms kinetic approach and
thermodynamic approach the kinetic
approach can be done by introducing a
limiting amount of precursor for growth
or limiting the space for the process
like in myel
synthesis on the other hand the
thermodynamic approach which is the
focus of this video can be accomplished
through these subprocesses super
saturation
nucleation and
growth in order to undergo nucleation
the growth species must be
saturated this is the state of the
solution where adding more solute to the
solvent will not increase the
concentration of the
system super Satur rating can be
attained through a reduction in
temperature or in C2 chemical
reaction once super saturation is
achieved nucleation commences for the
succeeding discussion solution synthesis
will be looked upon to tackle the
fundamentals of the process particularly
concerning the formation of nanop
particles this type of synthesis is
defined by having a system with a high
solute concentration which exceeds the
equilibrium solubility of the chemical
to form a new phase a super saturated
solution has a high free energy the
formation of a solid phase reduces the
free energy at the same time conserves
the overall equilibrium concentration of
the system the reduction of this free
energy is the driving force of the
process this indicated energy is also a
free energy difference between the solid
phase and the liquid phase or is also
referred to as the free volume energy
denoted by Delta
GV the free volume energy is dependent
on the solute concentration C
temperature T and the super saturation
Sigma as shown in the equation here in
we can see that the absence of super
saturation consequently leads to no
nucleation additionally spontaneous
nucleation occurs for systems with large
larger solute concentrations compared to
the equilibrium concentration c not
because of a negative free energy value
assuming that the nuclei form is
spherically shaped the volume free
energy can be expressed
[Music]
in we can see here the trend of the
overall free energy is a function of the
atomic radius R the increase in the
atomic radius would lead to a more
negative free energy change a more
favored formation of the solid phase
specifically third folds of the radius
value however the energy reduction which
is caused by this factor is
counterbalanced by the formation of an
interface between the solid and liquid
phases hence the formation of nuclei in
the system is accompanied by an increase
in the change surface energy Delta Mu s
described through the following
equation in the interface exists gamma
the surface free energy the contribution
of the surface energy term is coupled
with the surface area of the nucleus 4
pi r 2 reflected in the equation the
increase in surface energy is factored
by twice the radius value the free
energy versus atomic radius graph would
then result to
this overall the resultant of these
factors is the total chemical potential
change for nucleus formation shown in
dashes this shows the behavior of the
atoms in the super saturated solution
initially atoms in the liquid phase
gather together to form the solid phase
thus having a positive free energy
change from a higher contribution of the
increase in surface energy than from the
free volume energy Factor
a maximum is observed in the graph
wherein we introduce some critical terms
R star and Delta G Star the decrease in
the surface energy will then be observed
after attaining this maximum value where
the process is there there after
accompanied by the growth of the nucleus
in such case nuclei with Redi larger
than R star or stable whereas those
which are smaller dissolves back to the
solution effectively decreasing the
overall free energy the corresponding
change in free energy to achieve our
star is the value Delta G Star
physically we can also treat Delta G
Star as the energy barrier needed to be
surpassed in order to form a
[Music]
clear these values can be obtained by
getting the slope of the graph at the
maximum point which is T Delta G over Dr
equal Z the resulting values
are from this
equation it is shown that the free
volume energy change is a function of
temperature which consequently means
that R star and Delta G Star are also
functions of
[Music]
temperature therefore as shown in the
graph the increase in temperature
results in the decrease of these
critical values as a result a processed
temperature that is lower than the
equilibrium solidification temperature
readily produces nucleating sites after
this temperature is attained a
considerable nucleation rate is achieved
the phenomenon Associated to this
process is called super cooling hence at
Super cooling smaller particle sizes are
achieved whereas for higher temperatures
larger particles are obtained
[Music]
the foregoing nucleation mechanism
revolved on a perfect system consisting
of the solution components only however
in some systems the activation energy
for creating nuclei Delta G star is
reduced brought by the presence of
pre-existing surfaces which effectively
decrease the surface energy
contribution this mechanism is termed
heterogeneous
nucleation the critical values are then
equated
to it is important to note here that the
critical radius is similar for both
homogeneous and heterogeneous cases and
only the energy barrier decrease in
value differentiating these values is
the factor s Theta which is
deterministic of the shape of the
nucleus wherein its values lie between
zero and one hence for this type of
nucleation the energy barrier for the
heterogeneous process is factored with s
Theta with that of the homogeneous
process the size distribution of nanop
particles is dependent on the subsequent
growth process of the
nuclei the growth process of the nuclei
involves multi-steps and the major steps
are generation of growth species
diffusion of the growth species from bul
to the growth
surface absorption of the growth species
onto the growth surface and surface
growth through irreversible
incorporation of growth species onto the
solid
surface these steps can be further
grouped into two processes supplying the
growth species to the growth surface is
termed as diffusion which includes the
generation diffusion and absorption of
growth species onto the growth surface
whereas incorporation of growth species
absorbed on the growth surface into
solid structure is denoted as growth a
diffusion limited growth would result in
a differen size distribution of nanop
particles as compared with that by
growth limited
processes let us first talk about
defusion controlled growth when the
concentration of growth species reduces
below the minimum concentration for
nucleation nucleation stops whereas the
growth
continues if the growth process is
controlled by the diffusion of growth
species from the bulk solution to the
particle surface the growth rate is
given by where R is the radius of
spherical
nucleus D is the diffusion coefficient
of the growth
species C is a bul
concentration CS is the concentration on
the surface of solid
particles and VM is the m volume of the
nuclei as Illustrated in figure
[Music]
3.6 by solving this differential
equation and assuming the initial size
of nucleus R not prime is a change of B
concentration negligible we
have for two particles with initial
radius difference the radius difference
decreases as time increases or particles
grow bigger according
to combining with equation three we
have both equations four and five
indicate that the radius difference
decreases with increase of nuclear
radius and prolonged growth
time the diffusion controlled growth
promotes the formation of uniformly
sized
[Music]
particles now let's talk about growth
controlled by surface
process when the diffusion of growth
species from the bulk to the growth
surface is sufficiently rapid that is
the concentration on the surface is the
same as that in the bulk as illustrated
by a dash line also in the figure the
growth rate is controlled by the surface
process there are two mechanisms for the
surface processes monuclear growth and
polynuclear growth for the monuclear
growth the growth proceeds layer by
layer the growth species are
incorporated into one layer and proceeds
to another layer only after the growth
of the previous layer is
complete there is sufficient time for
the growth species to diffuse on the
surface the growth rate is thus
proportional to the surface
area where km is a proportionality
constant dependent on the concentration
of growth species the growth rate is
given by solving the
equation the radius difference increases
with an increasing radius of the nuclei
substituting equation 7 into 8
yields this boundary condition is
derived from equation 7 and it means
that the radius is not infinitely large
that is R is less than M equation 9
shows that the radius difference
increases with a prolonged Pro
time obviously this growth mechanism
does not favor the synthesis of
monosized
[Music]
particles during polynuclear growth
which occurs when the surface
concentration is very high surface
process is so fast that second layer
growth proceeds before the first layer
growth grow is complete the growth rate
of particles is independent of particle
size or time that is the growth rate is
constant where KP is a constant only
dependent on temperature hence the
particles grow linearly with
[Music]
time
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