Assignment 2 Process
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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-).