Assignment 2 Process

FAKULTI KEJURUTERAAN KIMIA PROCESS CHEMISTRY

(CHE434)

NAME :MUHAMMAD AIMAN BIN HASSAN, MUHAMMAD KIFLAIN BIN ZULKFILI, NURSUHAILI ALIAH BINTI ABDULLAH.

STUDENT NO.

FACULTY

PROGRAM

:2014822384,2014293198,2014442196

: FACULTY OF CHEMICAL ENGINEERING

: BACHELOR OF CHEMICAL ENGINEERING (PURE)

GROUP

CODE AND COURSE

TOPIC

LECTURER’S NAME

DATE OF SUBMISSION

:EH2201B

: CHE434 PROCESS CHEMISTRY

: ASSIGNMENT 2

: DR. NURUL FADHILAH BINTI KAMALUL ARIPIN

: 19th DISEMBER 2014

UNIVERSITI TEKNOLOGI MARA

ABSTRACT

This research is conducted to find out about the synthesis of nanomaterial

specifically quantum dots[1] , the characteristics of nanomaterials and also it’s

applications. Nanoparticles are not new and their history can be traced back to the

Roman period[2] However, there still many undiscovered knowledge about

nanomaterials. This research is important because the nanotechnology can be used for

many purposes to create a better future in cosmetics, protection, androids[3] and so

much more. This research is conducted to understand more about ‘Quantum Dots’.

Nanocrystal[4] made of semiconductor[5] materials that are small enough to exhibit

quantum mechanical properties. This research was conducted and achieved by various

research on the internet and from the help of an expert. At the end of the research,

results on the synthesis of quantum dots, it’s characteristics and applications became

more clear.

REASEARCH BACKGROUND

INTRODUCTION

A nanocrystal is a material particle having at least one dimension smaller than 100

nanometers and composed of atoms in either a single or poly crystalline arrangement.

The size of nanocrystals distinguishes them from larger crystals. When the

nanomaterials embedded in solids nanocrystals may exhibit much more complex

melting behavior than conventional solids and may form the basis of a special class of

solids. They can behave as single-domain systems (a volume within the system having

the same atomic or molecular arrangement throughout) that can help explain the

behavior of macroscopic samples of a similar material without the complicating

presence of grain boundaries and other defects.

Semiconductor nanocrystals having dimensions smaller than 10nm are also described

as quantum dots (QD). A QD is a nanocrystal made of semiconductor materials that are

small enough to exhibit quantum mechanical properties. Specifically, its

cutting are confined in all three spatial dimensions. The electronic properties of these

materials are intermediate between those of bulk semiconductors and of

discrete molecules. QD were discovered in a glass matrix by Alexey Ekimov in

1981 and in colloidal solutions by Louis E. Brus in 1985. The term "quantum dot" was

coined by Mark Reed.

Electronic characteristics of a QD are closely related to its size and shape.

Consequently, the color of emitted light shifts from red to blue when the size of the QD

is made smaller. This allows the excitation and emission of quantum dots to be highly

tunable. Since the size of a quantum dot may be set when it is made, its conductive

properties may be carefully controlled. Quantum dot assemblies consisting of many

different sizes, such as gradient multi-layer nanofilms, can be made to exhibit a range of

desirable emission properties.

SYNTHESIS

1. Colloidal Synthesis

Colloidal semiconductor nanocrystals are synthesized from precursor compounds

dissolved in solutions, like traditional chemical processes. The synthesis of quantum

dots is done by using precursors, organic surfactants and solvents. Firstly, the solution

is done by heating the solution at high temperature, this will decompose the precursors

and it will form monomers which then nucleate and generate nanocrystals. During the

process, the temperature is a critical factor to determine optimal conditions for the

nanocrystal growth. It has to be high enough to allow rearrangement and annealing of

atoms during the synthesis process. The concentration of monomers is another critical

factor that has to be stringently controlled during nanocrystal growth. The growth

process of nanocrystals can occur in two different regimes, "focusing" and "defocusing".

At high monomer concentrations, the critical size is relatively small, resulting in growth

of nearly all particles. In this regime, smaller particles grow faster than large ones

resulting in "focusing" of the size distribution to yield nearly mono disperse particles.

The size focusing is optimal when the monomer concentration is kept such that the

average nanocrystal size present is always slightly larger than the critical size. Over

time, the monomer concentration diminishes, the critical size becomes larger than the

average size present, and the distribution "defocuses".

2. Lithography

Quantum wells are covered with a polymer mask and exposed to an electron or ion

beam. The surface is covered with a thin layer of metal, then cleaned and only the

exposed areas keep the metal layer. Pillars are attached into the entire surface. The

multiple layers are applied this way to build up the properties and size wanted. This

method has its disadvantages which is slow, contamination, low density and defect

formation.

Figure 1.1 : Shows a lithography process.

3. Epitaxy : Patterned Growth

Semiconductor compounds with a smaller band gap (GaAs) are grown on the

surface of a compound with a larger band gap (AlGaAs). Growth is restricted by

coating it with a masking compound (SiO2) and etching that mask with the shape of

the required crystal cell wall shape. Disadvantage: density of quantum dots limited

by mask pattern.

Figure 1.2 : Shows the shape and surface of the compound

4. Epitaxy: Self-Organized growth

Uses a large difference in the lattice[6] constant of the substrate and the crystalling

material. When the crystallized layer is thicker than the critical thickness , there is a

strong strain on the layeers. The breakdown results in randomly distributed islets of

regular shape and size. Disadvantages: size and flunctuaion.

Figure 1.3 : Shows the thickness of the crystallized layer

CHARACTERISTIC

A wide variety of imaging methods scanning tunneling microscopy (STM), atomic

force microscopy (AFM), scanning transmission electron microscopy (STEM), energy

filtered electron microscopy (EFTEM) are used to investigate the growth, the self-

assembling, and the physical properties of quantum dots. The peaks in the diffraction

pattern are less intense and are broad structural studies. Therefore based on high

resolution transmission electron microscopy (HRTEM), extended X-ray absorption fine

structure (EXAFS), scanning tunneling microscopy (STM) and atomic force microscopy

(AFM).

Figure 2: Shows a high resolution TEM image showing the icosahedral shape

and

five fold symmetry axis of a Ag nanoparticle.

HRTEM with its ability to image atomic distributions in real space, is a popular and

powerful method. The icosahedral structure of nanocrystals is directly observed by

HRTEM and evidence for twinning (required to transform a crystalline arrangement to

an icosahedron [7] ) is also obtained by this means. The images are often compared

with the simulated ones. High resolution imaging provides compelling evidence for the

presence of multiply twinned [8] crystallites specialy in the case of Au and Ag

nanoparticles. Characterization by electron microscopy also has certain problems. For

example, the ligands [9] are stripped from the clusters under the electron beam; the

beam could also induce phase transitions and other dynamic events like quasi [10] -

melting and lattice reconstruction. The fact that ligands desorb from clusters has made it

impossible to follow the influence of the ligand shell on cluster packing.

STM, with its ability to resolve atoms, provides exciting opportunities to study the size

and morphology of individual nanoparticles. In the case of ligated nanocrystals, the

diameters obtained by STM include the thickness of the ligand shell. Ultra high vacuum

STM facilitates in situ studies of clusters deposited on a substrate. Furthermore, it is

possible to manipulate individual nanoscale particles using STM. However, it is not

possible to probe the internal structure of a nanocrystal, especially if it is covered with a

ligand shell. AFM supplements STM and provides softer ways of imaging nanocrystals.

EXAFS has advantages over the other techniques in providing an ensemble average,

and is complimentary to HRTEM.

Applications of Quantum Dots

TRANSISTOR/ QUANTUM COMPUTATION

Key active component in practically all modern electronics.

Mass-produced using a highly automated process (semiconductor device

fabrication) that achieves astonishingly low per-transistor costs.

Produced in integrated circuits (often shortened to IC, microchips or

simply chips), along with diodes, resistors, capacitors and other electronic

components, to produce complete electronic circuits.

SOLAR/PHOTOVOLTAIC CELLS

A mesoporous layer of titanium dioxide nanoparticles forms the backbone of the

cell, much like in a DSSC.

 TiO2 layer can then be made photoactive by coating with semiconductor

quantum dots using chemical bath deposition, electrophoretic deposition or

successive ionic layer adsorption and reaction.

The electrical circuit is then completed through the use of a liquid or solid redox

couple. The efficiency of QDSCs has increased to over 5% shown for both liquid-

junction and solid state cells.

LED

Their emission colour can be tuned from the visible throughout the infrared

spectrum.

Allows quantum dot LEDs to create almost any colour on the CIE diagram.

One uses photo excitation with a primary light source LED (typically blue or

UV LEDs are used).

Quantum dots (QD) are also being considered for use in white light-emitting

diodes in liquid crystal display (LCD) televisions.

The structure of QD-LEDs used for the electrical-excitation scheme is similar

to basic design of OLED.

An applied electric field causes electrons and holes to move into the quantum

dot layer and recombine forming an exciton that excites a QD.

MEDICAL IMAGING

The technique, process and art of creating visual representations of the interior of

a body for clinical analysis and medical intervention.

Reveal internal structures hidden by the skin and bones, as well as to diagnose

and treat disease.

Imaging modalities example Radiography, Magnetic Resonance Imaging (MRI),

Nuclear medicine and Ultrasound.

References

1. Quantum dots(QD); a semiconductor crystal or nanometre dimensions with

distinctive conductive properties determined by it’s size.

2. Roman period ;the roman empire was the post-republican period of the ancient

roman civilization.

3. Android ; a mobile operating system(OS) based on the Linux kernel and currently

developed by google.

4. Nanocrystal ; it is a material particle having at least one dimension smaller than

100 nanometres and composed of atoms in either a single- or polycrystalline

arrangement.

5. Semiconductor ; a solid substance that has a conductivity between that of an

insulator and that of most metals, either due to the addition of an impurity or

because of temperature effects.

6. Lattice ; An arrangement of the particles in a regular periodic pattern in 2/3

dimension.

7. Icosahedron ; any polygon having twenty plane faces.

8. Twinned ; being same or having similar design, colour as another.

9. Ligand ; a substance that forms a complex around a central atom.

10.Quasi ; partly or some degree (semi-).