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ZnO QUANTUM DOTS Sample Essay & Outline

Objective

The main objective of this experiment was to synthesize Nano scale particles for Zinc Oxide (ZnO) to study their growth over time and describe its three dimensional quantum confinement.

Introduction

Quantum dots are zero-dimensional nanoparticles of semiconducting material whose excitons are confined in three spatial dimensions. As a result of their small size, they exhibit optical and electronic properties that lie between those of bulk solids and those of discrete molecules. Nanocrystals are capable of absorbing light and remitting it in a different colour. The absorption wavelength by the nanocrystals is usually a function of the size of particles at the Angstrom scale. When an electron in a bulk semiconductor is excited from the valence to the conduction band, an electron “hole” of opposite electric charge is left behind.

Due to the Coulomb force between the excited electron and the “hole”, the excited electron is attracted to the “hole” to form an electron-hole excitons or pair. In a bulk semiconductor, this electron-hole excitons are typically bound within the Bohr characteristic exciton radius. By constraining the electron-hole pair further causes changes in the properties of the semiconductor. This effect is a form of quantum confinement that is a key feature in a range of emerging electronic structures.

The effect specifically describes the phenomenon results electron holes and electrons being constrained to a critical quantum measurement. Under confined conditions, the quantity of energy that can be possessed by the electrons is quantized to discrete amounts, rather than the continuous bands in the bulk solid. When the hole and the electron recombine energy is released in the form of a photon. The photon is emitted with discrete energy values and wavelengths.

In addition, the difference in the electron energy levels depends on the degree of quantum confinement. A greater degree of quantum confinement produces larger gaps between the energy levels, which result to emission of photons with smaller wavelengths. For the case of semi-conduction nano crystals, quantum confinement can be explained using a 3-dimentional “particle-in-a-box” model (Bhattacharya, Fornari and Kamimura).

Quantum dots are significant for optical applications, particularly because of their high quantum yield. They have proven to function like a single-electron transistor, and have also shown the Coulomb blockade effect, thus, making them to find applications in electronics. In addition to these, quantum dots have been suggested as implementations of qubits for processing of quantum information (Rodriguez and Fernández-García).

In this lab experiment, nanocrystals of ZnO were prepared by reacting zinc acetate dehydrate, Zn(C2H3O2)2.2H2O, with tetramethyl ammonium hydroxide pentahydrate, N(CH4)4OH.5H2O. The growth of ZnO nanoparticles was then monitored by measurement of the UV-vis absorption spectrum at wavelength 230 to 390 nm as a function of time. The first sample was not placed in the ice bath, the second one was not. The absorption spectra was then acquired at intervals of 6 minutes after initiation of the formation of ZnO nanoparticles over a duration of 90 minutes. The solutions were then sealed and held for 2 days, and the final absorption spectra was measured.

Experimental Procedure

Please refer to the lab manual “ZnO Quantum Dots”, page 3.

Analysis of Results

Absorption Spectra

The size of the ZnO nanoparticles is calculated using the empirical formula shown below:

-----------------Eq. 1
Where:
D – particle diameter in angstroms (Å)
- is the wavelength at which the rising edge of the bsorption band edge is half of the bend maximum, and,
a, b & c are empirical constants ( a = 3.301, b = -294.0, c = -1.09)

Analysis of sample 1 (no ice)
At time = 0 minutes:

From the equation of the linear regression line, x intercept = 263.174 nm
Therefore, = 263.174 nm
Using equation 1 above:

Solving this quadratic equation;
D = 14.055 Angstroms
Band-gap energy:
Ebg =
Where:
h = 6.626x10-34 J s
c = 2.998x108 m/s
Ebg = = 7.548 eV

At time = 6 minutes:
= 265.105 nm
Therefore,

Solving for D;
D = 14.224 Angstroms
Ebg = = 7.493 eV

The above calculations were repeated for every time interval and the results obtained were tabulated in the table below:

Table 1: Particle size and band-gap energy of ZnO as a function of time for sample 1 (no ice)

Time (minutes) Particle diameter (Angstroms) Particle Radius (R-2) Band-gap energy (Ebg)
Ebg = (eV)
0 263.174 14.055 0.0202 7.548
6 265.105 14.224 0.0198 7.493
12 265.243 14.2366 0.0197 7.489
18 264.211 14.1457 0.0200 7.518
24 265.600 14.2684 0.0196 7.479
30 265.989 14.3036 0.0196 7.468
36 266.012 14.3056 0.0195 7.468
42 267.234 14.4158 0.0192 7.433
48 266.957 14.3908 0.0193 7.441
54 267.235 14.4158 0.0192 7.433
60 268.245 14.5115 0.0190 7.405
66 267.132 14.4064 0.0193 7.436
72 267.454 14.4373 0.0192 7.427
78 268.346 14.5174 0.0190 7.402
84 266.090 14.3123 0.0195 7.465
90 268.000 14.4860 0.0191 7.412

One unit of angstrom is equivalent to 0.1 nm. This means that the size of our ZnO nanoparticles range from 1.4055nm to 1.44860nm. The ideal particle size usually ranges from 2.0 to 7.8 nm. This means that the size particles in this experiment are slightly smaller than the ideal size. The size of particles are controlled by a number of factors, mainly the temperature of synthesis. Temperature control is very significant in controlling the size of particles produced.

The graph in figure 5 below shows how particle size of ZnO varies with time. From this graph, it can be observed that the particles grow slightly larger as time moves. This trend is evident within the first 60 minutes. Beyond this time, the particle size tends to remain constant. However, the variation in particle size is very narrow, with all particles being within a range of 14.055 to 14.5115 angstroms.

From the results in figure 6, it shows that the band-gap energy increases as the particle radius (R-2) increases. In other words, as the particle radius increases, the band-gap energy begins to fall.

Analysis of sample 2 (with ice):
At time = 0 minutes:

From the equation of the linear regression line, x intercept = 261.00 nm
Therefore, = 261.00 nm
Using equation 1:

Solving this quadratic equation;
D = 13.868 Angstroms
Band-gap energy:
Ebg =
Where:
h = 6.626x10-34 J s
c = 2.998x108 m/s
Ebg = = 7.611 eV

At time = 6 minutes:
= 261.55 nm
Therefore,

Solving for D;
D = 13.915 Angstroms
Ebg = = 7.595 eV

These rest of the calculations were done same as above and the results recorded in the table 2 below.

Time (minutes) Particle diameter (Angstroms) Particle Radius (R-2) Band-gap energy (Ebg)
Ebg = (eV)
0 261.00 13.868 0.0208 7.611
6 261.55 13.915 0.0207 7.595
12 261.59 13.920 0.0206 7.594
18 262.002 13.953 0.0205 7.
24 262.324 13.982 0.0205 7.573
30 261.238 13.887 0.0207 7.604
36 262.578 14.005 0.0204 7.565
42 262.850 14.030 0.0203 7.557
48 263.150 14.054 0.0203 7.549
54 262.456 13.991 0.0204 7.569
60 263.284 14.063 0.0202 7.545
66 263.760 14.107 0.0201 7.531
72 264.345 14.157 0.0200 7.515
78 263.868 14.117 0.0201 7.528
84 264.880 14.206 0.0198 7.500
90 264.884 14.206 0.0198 7.500

As observed in the results obtained for sample 1 (no ice), the results for sample 2 presented in table 2 above show that the particle size grows with time. An interesting observation in this result is that the particles are smaller in size compared to the size of particles in sample 1. This means that temperature plays a significant role in the formation of ZnO nanoparticles.

The Effect of Temperature on the Growth of the Particles.

Synthesis of three dimensional ZnO nanostructures through the hydrothermal process by use of ammonia and zinc acetate at a temperature of 65oC yields successful results. The temperature during the synthesis process has an effect on the particle size, band gap, sensitivity and structural defects. Increase in temperature beyond the required temperature results in increased size of particles. Higher temperatures cause increased rates of growth dynamics, and the later introduce defects in the nano structure, hence, reducing the crystal quality (Tartakovskii).

Conclusions

The temperature plays a very significant role in the formation and growth of ZnO nanoparticles. At a lower temperature, the particles formed are slightly smaller in size compared to a higher temperature. The particles increase in size as time increases after they are formed. As the particle size increases, band-gap energy increases.

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References

Bhattacharya, Pallab, Roberto Fornari and Hiroshi Kamimura. Comprehensive Semiconductor Science and Technology. New South Wales: Newnes, 2011.
Rodriguez, José A. and Marcos Fernández-García. Synthesis, Properties, and Applications of Oxide Nanomaterials. London: John Wiley & Sons, 2007.
Sami Almoqayyad, Jamil Salem, Talat Hammad. ZnO Nanoparticles: Synthesis,Growth. UK: Lap Lambert Academic Publishing GmbH KG, 2013.
Synthesis and Characterization of Zno Nanoparticles and Nanorods. UK, 2011.
Tartakovskii, Alexander. Quantum Dots: Optics, Electron Transport and Future Applications. U.K: Cambridge University Press, 2012.