CHAPTER ONE

1.0                                           INTRODUCTION

1.1 BACKGROUND OF THE RESEARCH

Nanocrystalline semiconductors have been the subject of intense research ¹ due to their unique physical and chemical properties^2,3. Evidence shows that reducing the particle (grain) size of semiconductors (chalcogenide) to several tens of nanometers or below produce significant changes in their properties⁴. For instance, Gold which is chemically inert at normal scales, can serve as a potent chemical catalyst at nanoscale. This is practically possible because of quantum size effects⁵.

Therefore, the study of the synthesis and characterization of semiconductor materials in the nanoscale size range is of great importance.

As an important binary iv-vi group semiconductor material, PbS is a direct narrow band gap semiconductor with optical energy gap of 0.41 eV at room temperature⁶ and relatively large excitation Bohr radius 18 nm ⁷. The band gap can be changed by changing the grain size of the nanocrystalline structure with lattice constant of 5.936 Å⁸. Quantum-sized PbS has been widely used in many fields, particularly in lead ion-selective sensors⁹, IR detector,¹⁰ photography¹¹, electroluminescent devices such as light-emitting diodes and high-speed switching ¹². It can also be used as a photoconductive and photovoltaic detector, making them among the earliest that have been used as high performance photoconductive detectors ¹³. Also as in solar absorbers, photon switch, optical switch ¹⁴, photo resistance, humidity and temperature sensors, decorative coating, solar control coating ¹⁵ and the biological fluorescence probe ¹⁶ among other uses. For these reasons, many research group have an increasing interest in the development and study of PbS nanostructures with different morphologies, such as nanowires, nanotubes¹⁷,   nanorods, nanoballs¹⁸, nanopieces ^19-20, nanocubes²¹, nanostars^22-23, and hierarchical nanostructures^24-25 .

These nanostructures can be obtained by several methods such as electrodeposition²⁶, spray pyrolysis²⁷, photoaccelerated, chemical deposition ²⁸, microwave heating ²⁹ and chemical bath deposition ³⁰. Up-to-date, chemical bath deposition method has been successfully used to deposit many different semiconductor thin films including SnS, CdSe, ZnSe, CdS, CuS, CuInS₂ and CuBiS.

Chemical bath deposition technique is chosen in this work owing to its many advantages like low cost³¹, low working temperatures, easy coating of large surface, simplicity in instrumental operation and also homogeneity with high quality thin films,¹³. This technique also offers the advantage of being able to deposit thin films on different kinds, shapes, and sizes of substrate ³³ .

1.2 STATEMENT OF THE PROBLEM

Scientists in all ramifications have been striving to produce devices that are smaller, lighter and faster with improved efficiency. The recent discovery of nanotechnology made this possible. The small size of nanoparticles endowed them with unusual structural and optical properties unlike their bulk counterparts. It is the interest in this direction that led to the synthesis of PbS nanostructures in this work.

1.3 RESEARCH OBJECTIVE

The goal of this research was to synthesize and characterize PbS nanostructures using chemical bath deposition method. The research objectives were to:

• Develop low cost method to synthesize PbS. This was achieved by Chemical Bath Deposition method.
• Synthesize high yield PbS thin films with variable morphologies
• Study the influence of precursors concentration on the structural, optical and surface morphological properties of PbS nanostructures annealed at a temperature of 150 ^oC
• Characterized the PbS nanostructures produced using scanning electron microscope (SEM), X-ray diffractometer (XRD), Ultra-violet visible (UV) spectrophotometer.
• Investigate the effect of annealing on the optical property of the deposited PbS nanostructures.
• Determine the crystal size of the PbS nanostructures synthesized

• JUSTIFICATION OF THE STUDY

Despite the increasing interest on the synthesis of PbS nanostructures, there is a dearth of information on PbS nanostructures grown using different molarities. This study is to fill the knowledge gap and enable material scientist to synthesize PbS nanostructures with well-defined morphologies suitable for various applications in electronics and opto-electronics.

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