Nucleation and Growth

Summary

This 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

• 📺 TVs have evolved from bulky boxes to slim, flat screens with quantum dot technology.
• 🔬 Quantum dots are nanoparticles ranging from 2 to 10 nanometers in diameter, used in modern displays.
• ⚛️ Nanoparticle synthesis involves two main mechanisms: kinetic and thermodynamic approaches.
• 🌡️ The thermodynamic approach includes supersaturation, nucleation, and growth to form nanoparticles.
• 🧪 Supersaturation happens when the solution exceeds equilibrium solubility, driving nucleation and solid-phase formation.
• 📈 Free energy change drives nucleation, with the energy balance involving volume free energy and surface energy.
• 🌟 Critical nucleation concepts include 'R star' (critical radius) and 'Delta G star' (energy barrier for nucleation).
• ❄️ Lower temperatures promote supercooling, leading to smaller nanoparticle sizes, while higher temperatures yield larger particles.
• ⚙️ Growth of nanoparticles can be diffusion-controlled or surface-process-controlled, influencing size distribution.
• 🧱 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

00:00

📺 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 ΔGv. This energy change is dependent on solute concentration, temperature, and super saturation.

05:00

🔬 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 (ΔG*) 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.

10:02

🌐 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.

Keywords

💡Quantum Dots

Quantum dots are nanoscale semiconductor particles that have unique electronic properties due to their small size. In the video, they are mentioned as a key component in modern display technology, like those used in QLED TVs. Their size, which ranges from 2 to 10 nanometers, enables precise control over light emissions, resulting in better color accuracy and brightness.

💡Nanoparticles

Nanoparticles refer to particles with sizes ranging from 1 to 100 nanometers. The video discusses the synthesis of nanoparticles like quantum dots, focusing on how their nanoscale size is crucial for their optical and electronic properties. Their small size allows them to exhibit unique behaviors, which are important in various technological applications, including electronics and materials science.

💡Synthesis of Nanoparticles

This term refers to the processes involved in creating nanoparticles. In the video, it is explained that nanoparticles can be synthesized through two main approaches: the kinetic approach, which limits the growth of particles, and the thermodynamic approach, which relies on processes like supersaturation, nucleation, and growth to form nanoparticles. These methods are crucial for producing nanoparticles with specific sizes and properties.

💡Supersaturation

Supersaturation occurs when a solution contains more solute than it can hold at equilibrium, leading to the formation of new solid phases, such as nanoparticles. The video highlights that supersaturation is a critical step in the thermodynamic synthesis of nanoparticles, as it creates the conditions necessary for nucleation and subsequent particle growth.

💡Nucleation

Nucleation is the initial process where solute particles cluster together to form a new phase, such as a solid, from a supersaturated solution. In the video, nucleation is described as the first stage in the formation of nanoparticles. Once nucleation occurs, further growth of particles can happen, leading to the development of larger nanoparticles.

💡Free Energy

Free energy is the thermodynamic quantity that drives the transformation of a system. The video discusses how the reduction in free energy, particularly the free volume energy, facilitates the formation of solid phases during the nucleation and growth of nanoparticles. Lowering the free energy stabilizes the newly formed particles, allowing them to grow further.

💡Diffusion

Diffusion is the process by which molecules move from areas of high concentration to low concentration. In the context of the video, diffusion is essential in the growth of nanoparticles, as growth species need to diffuse from the bulk solution to the surface of growing particles. The diffusion rate impacts the size distribution of nanoparticles, with slower diffusion leading to more uniform particle sizes.

💡Homogeneous Nucleation

Homogeneous nucleation occurs when nuclei form uniformly throughout a solution without any foreign surfaces or particles acting as catalysts. The video explains that this form of nucleation happens in ideal systems and requires overcoming a significant energy barrier. It contrasts with heterogeneous nucleation, which is catalyzed by surfaces already present in the system.

💡Heterogeneous Nucleation

Heterogeneous nucleation refers to nucleation that occurs on pre-existing surfaces or particles, which lower the energy barrier required for particle formation. In the video, it is mentioned that the presence of such surfaces can facilitate the nucleation process, making it more efficient than homogeneous nucleation. This is important for controlling the size and distribution of nanoparticles.

💡Surface Growth

Surface growth is the process where atoms or molecules are added to the surface of a growing particle. The video describes surface growth as a key step in the overall growth process of nanoparticles. Once growth species are adsorbed onto the particle surface, they are irreversibly incorporated into the solid structure, allowing the particle to increase in size.

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