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NANOSTRUCTURED MATERIALS (MM-407)

Synthesis and Characterization of Semi-Conducting Nanoparticles

Group Members:• Aqib Raza (MM-001)• Muhammad Ali Khan (MM-017)• Taha Ahmed Siddiqui (MM-028)• Abdul Rehman Anis (MM-037)

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Nanomaterials (Size)

Semiconductors (Property)

Those materials that are in the range of 1 to 100 nm size and one can reduce the dimension in one, two or all three directions to obtain thin films, wires or dots, respectively

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.

INTRODUCTION

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Why SEMICONDUCTING Nanoparticles?

Semiconductor have been useful in making devicesFocus towards miniaturization of electronic

components integration to accommodate huge number in small

volumeTo enable compact digital watches, calculators,

computers, laptops etc.Lead to interesting devices like:

Single electron transistors Tunnel junctions Magnetic spin valves

Miniaturization: the process of making something very small using modern technology

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Quantum Refinement Effect

In bulk particle behaves as if it was free Because the confining dimension is large

compared to the wavelength of the particle the bandgap remains at its original energy due to

a continuous energy stateAs the dimension decreases and reaches nanoscale

the energy spectrum becomes discrete the bandgap becomes size-dependent This ultimately results in a blue shift in light

emission as the size of the particles decreases

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Quantum Refinement Effect

Electrons and holes being squeezed into a dimension that approaches a critical quantum measurement, called the exciton Bohr radius a quantum dot such as a

small sphere confines in three dimensions

a quantum wire confines in two dimensions

a quantum well confines only in one dimension

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Semi-conducting NanomaterialsClassification

2-D Nanostructures Quantum Well

1-D Nanostructures Quantum Wire

0-D Nanostructures Quantum Dots

Quantum: minimum amount of any physical entity (in this case, electrons or holes) involved in an interaction

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Core–Shell Semiconductor Nanocrystal

Properties intermediate between small, individual molecules and bulk, crystalline semiconductors

Composed of a quantum dot semiconducting core material and a shell of a distinct semiconducting material

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Typically composed configurations such as CdS/ZnS, CdSe/ZnS, CdSe/CdS, and InAs/CdSe(typical notation is: core/shell)

Classification: (based on conduction and valence band alignment of the core and the shell) Type I Reverse Type I Type II

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SYNTHESIS OF SEMICONDCUTING NANOPARTICLESTop down ApproachControlled particle size is hard to achieveColloidal synthesis of nanoparticles in a suitable solvent medium not achievable

Bottom down approach Desired particle sizes over the largest possible rangeNarrow size distributionsGood crystallinityControllable surface functionalization

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SYNTHESIS OF SEMICONDCUTING NANOPARTICLESThe two well established route for synthesis are

1. Aqueous synthesis of II-VI semiconductor nanoparticles

In the presence of various short-chain thiols as stabilizing agents

2. Organometallic synthesis of either II-VI (CdSe, CdTe) or III-V (InP, InAs) nanocrystals

Based on the high-temperature thermolysis of precursors

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Synthesis in Colloidal Form

The colloidal synthesis of nanoparticles generally involves several consecutive stages: 1. Nucleation from initially homogeneous

solution2. Growth of the pre-formed nuclei3. Isolation of particles reaching the desired

size from the reaction mixture4. Post-preparative size fractionation

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Synthesis in Aqueous Media

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Semiconductor Nanoparticles Synthesized in Aqueous MediaSTAGE 11. Hydrogen chalcogenide produced by reaction

between Al2Te3 and H2SO4 at room temperature 2. It is then introduced in vessel containing metal

salt and stabilizer3. Water-soluble precursors of nanoparticles are

formed in the form of some metal-chalcogen-thiol complexes.

STAGE 21. Applying heat in the second stage promotes the

chemical reaction between metal and chalcogenide leading to the formation of semiconductor nuclei followed by their growth.

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Parameters Of Aqueous Media

The use of different thiols and their mixtures as stabilizing agents

The concentrations of the reactantsthe pH value of the solution, and the duration

of the heat treatmentAs a result, the size range of 1.2-6.0 nm is

generally accessible

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Using Organometallic Precursors

High-temperature (200-360 °C) thermolysis of organometallic precursors

In the presence of stabilizing agents. Organometallic precursors are rapidly injected

into a very hot (300-320°C) stabilizing solvent resulting in explosive nucleation

Followed by a temperature drop to ~200°C which terminates the nucleation.

The time of subsequent growth at a desired temperature and the proper choice of stabilizing agents provide control over the particle size

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CdSe Nanoparticles

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Commonly Prepared Semi-conducting Nano Particles

CdSZnSe PbSe InAs Nanocrystals InP Nanocrystals GaP and GaAs Nanocrystals

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CHARACTERIZATION

Why?

MorphologyChemical CompositionElectronic propertiesOptical properties

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1. XRD

Measure the average spacing's between layers or rows of atoms

Determine the orientation of a single crystal or grain

Find the crystal structure of an unknown material

Measure the size, shape and internal stress of small crystalline regions

Works on the principle of Bragg’s Law: n ƛ =2d sin θ

Scehrrer Formula:d = 0.94 ƛ/β cos θ

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Electron Microscopy

1. SEM3D imagesUpto 20,000 times magnifiesRaster scanning over the sampleBulk and small samples can be analyzedConductive sampleParticle size confirmation upto 10nm

2. TEM3. AFM4. SPM

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Semiconducting nanoparticlesposses higher band gap then their bulk.

Due to Quantum Confinement, theband gap is discrete.

Samples Band Gap (Bulk ) eV

Band Gap (Nano) eV

CdO 2.3 3.4

CdS 2.42 3.25

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UV-Vis Absorption Spectroscopy

1. Measurement of the attenuation of a beam of light after it passes through a sample or after reflection from a sample surface

2. Determination of Absorption Constant, then by formula:

άhυ= A(hυ-Eg)^n

RADIANT SOURCE

WAVELENGTHSELECTOR

SOLVENTPHOTO-DETECTO

RREADOUT

SAMPLE

25ApplicationsNano-electronicsNano-photonicsEnergy conversionNon-linear opticsMiniaturized Sensors and Imaging devicesSolar cellsCatalysisDetectorsPhotography Biomedicine

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Nanocrystals in Light-Emitting Devices

Luminescent semiconductor nanocrystals were successfully integrated into the thin film polymer-based LEDs as emitting materials

Advantages of using polymer-nanocrystal composites are the processing of both polymer and nanocrystals from solution and the superior luminescent properties of the nanocrystals.

Additionally, there is a possibility to tune the emission color via control of the nanocrystal size.

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Nanocrystals in Solar Cells

The optical absorption and band edge positions of the nanoparticles can be easily designed both by their elemental composition and by the nanocrystal size via the quantum confinement effect.

Efficient charge transfer from. the nanocrystals to the conduction band of wide-band gap semiconductors (TiO2, ZnO. Ta2O5) makes them attractive

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CONCLUSION