SYNTHESIS AND CHARACTERIZATION OF ZnONANO-PARTICLES AS PHOTODETECTOR

Submitted by

Bhuginath Sharma

In partial fulfillment for the award of the Degree of

MASTER OF SCIENCE

IN

PHYSICS

UNDER THE ESTEEMED GUIDANCE OF

Dr. Dr. D. Mohanta

DEPARTMENT OF PHYSICS SCHOOL OF SCIENCE AND TECHNOLOGY TEZPUR UNIVERSITY NAPAAM , SONITPUR ,ASSM:784028

SCHOOL OF SCIENCE AND TECHNOLOGY

TEZPUR

CERTIFICATE

This is to certify that the thesis entitled “Synthesis and Characterization of ZnO nanoparticles” is submitted by Mr. Bhuginath sharma (Roll NO-PHY10030) To this Institute in partial fulfillment of the Requirement for the award of the degree of Master of Science Department Physics, is a bonafied record of the work carried out under my supervision and guidance. It is further certified that no part of this thesis is submitted for the award of any degree.

Tezpur

Dr. Dr. D. Mohanta Date- Supervisor

Department of Physics

ACKNOWLEDGEMENT

I express my sincere thanks to my supervisor, Dr. D MOHATA Department of Physics, TEZPUR UNIVERSITY for his esteemed supervision, incessant support, inspiration and constructive criticism throughout my project work.

I accord my thanks to Mr Shayan bayan, Department of Physics Tezpur university, for providing me with all necessary characterization facilities during the project work. I would like to take the opportunity to thank Dr. U.K Mohanty, Department of MME, for allowing me to do SEM in his department. Also I thank Dr. S. Paria, Department of Chemical engineering, NIT Rourkela for permit me to use Particle size analyzer for my project work. I would also thank to H.O.D and all my faculty members, office staffs and Technical staffs of Department of Physics , NIT Rourkela, for their co-operation. I wish to extend my sincere thanks to Mr. Alok all for his help and moral support, without whom it couldn’t be possible. I convey my thanks to all my class mates, Miss. Annapurna Patra, Mr. Prakash, Mr. Naresh (PhD scholars) for their valuable suggestion and help. Finally, I would also express my deep sense of gratitude to my parents and family members for their encouragement and support throughout, which always inspired me.

CONTENTS

1. INTRODUCTION ………………………………………………………..1

2. LITERATURE REVIEW…………………………………………………6

3. SYNTHESIS

I. SYNTHESIS OF ZnO-2 ………………………………………………..8

II. SYNTHESIS OF ZnO-1………………………………………………..94. CHARACTERIZATION

I. XRD ……………………………………10

1. INTRODUCTION

Nanomaterials

Nanomaterials  is a field that takes a materials science-based approach to nanotechnology. It

studies materials with morphological features on thenanoscale, and especially those that have

special properties stemming from their nanoscale dimensions. Nanoscale is usually defined as

smaller than a one tenth of a micrometer in at least one dimension, though this term is sometimes

also used for materials smaller than one micrometer.

On 18 October 2011, the European Commission adopted the following definition of a nonmaterial:[2]

A natural, incidental or manufactured material containing particles, in an unbound state or as an

aggregate or as an agglomerate and where, for 50% or more of the particles in the number size

distribution, one or more external dimensions is in the size range 1 nm – 100 nm. In specific cases

and where warranted by concerns for the environment, health, safety or competitiveness the

number size distribution threshold of 50% may be replaced by a threshold between 1 and 50%.

An important aspect of nanotechnology is the vastly increased ratio of surface area to volume

present in many nanoscale materials, which makes possible newquantum mechanical effects. One

example is the “quantum size effect” where the electronic properties of solids are altered with great

reductions in particle size. This effect does not come into play by going from macro to micro

dimensions. However, it becomes pronounced when the nanometer size range is reached. A

certain number of physical properties also alter with the change from macroscopic systems. Novel

mechanical properties of nanomaterals is a subject ofnanomechanics research. Catalytic activities

also reveal new behaviour in the interaction with biomaterials.

Nanoparticles or nanocrystals made of metals, semiconductors, or oxides

are of particular interest for their mechanical, electrical, magnetic, optical, chemical and other

properties. Nanoparticles have been used as quantum dots and as chemical catalysts such

as nanomaterial-based catalysts.

Nanoparticles are of great scientific interest as they are effectively a bridge between bulk materials

and atomic ormolecular structures. A bulk material should have constant physical properties

regardless of its size, but at the nano-scale this is often not the case. Size-dependent properties are

observed such as quantum confinement insemiconductor particles, surface plasmon resonance in

some metal particles and superparamagnetism in magneticmaterials

Nanoparticles exhibit a number of special properties relative to bulk material. For example, the

bending of bulk copper(wire, ribbon, etc.) occurs with movement of copper atoms/clusters at about

the 50 nm scale. Copper nanoparticles smaller than 50 nm are considered super hard materials

that do not exhibit the same malleability and ductility as bulk copper. The change in properties is

not always desirable. Ferroelectric materials smaller than 10 nm can switch their magnetisation

direction using room temperature thermal energy, thus making them useless for memory

storage.Suspensions of nanoparticles are possible because the interaction of the particle surface

with the solvent is strong enough to overcome differences in density, which usually result in a

material either sinking or floating in a liquid. Nanoparticles often have unexpected visual properties

because they are small enough to confine their electrons and produce quantum effects. For

example gold nanoparticles appear deep red to black in solution.

The often very high surface area to volume ratio of nanoparticles provides a tremendous driving

force for diffusion, especially at elevated temperatures. Sintering is possible at lower temperatures

and over shorter durations than for larger particles. This theoretically does not affect the density of

the final product, though flow difficulties and the tendency of nanoparticles to agglomerate do

complicate matters. The surface effects of nanoparticles also reduces the incipientmelting

temperature.

Properties of Zno…….sssssssssZinc oxide is an inorganic compound with the formula ZnO. It usually appears as a white powder, nearly insoluble in water. The powder is widely used as an additive into numerous materials and products including plastics, ceramics, glass, cement, rubber (e.g. car tyres), lubricants, paints, ointments, adhesives, sealants, pigments, foods (source of Zn nutrient), batteries, ferrites, fire retardants, etc. ZnO is present in the Earth crust as a mineral zincite; however, most ZnO used commercially is produced synthetically.

In materials science, ZnO is often called a II-VI semiconductor because zinc and oxygen belong to the 2nd and 6th groups of the periodic table, respectively. This semiconductor has several favorable properties: good transparency, high electron mobility, wide bandgap, strong room temperature luminescence, etc. Those properties are already used in emerging applications for transparent electrodes in liquid crystal displays and in energy-saving or heat-protecting windows, and electronic applications of ZnO as thin-film transistor and light-emitting diode are forthcoming as of 2009.

CHEMICAL PROPERTIES

ZnO occurs as white powder commonly known as zinc white or as the mineral zincite. Themineral usually contains a certain amount of manganese and other elements and is of yellow to red color. Crystalline zinc oxide is thermo-chromic, changing from white to yellow when heated and in air reverting to white on cooling. This is caused by a very small loss of oxygen at high temperatures to form the non-stoichiometric Zn1+xO, where at 800 °C, x= 0.00007.

Zinc oxide is an amphoteric oxide. It is nearly insoluble in water and alcohol, but it is soluble in (degraded by) most acids, such as hydrochloric acid:

ZnO + 2 HCl → ZnCl2 + H2O

Bases also degrade the solid to give soluble zincates:

ZnO + 2NaOH + H2O → Na2(Zn(OH)4)

ZnO reacts slowly with fatty acids in oils to produce the corresponding carboxylates, such as oleate or stearate. ZnO forms cement-like products when mixed with a strong aqueous solution of zinc chloride and

these are best described as zinc hydroxy chlorides. This cement was used in dentistry.

ZnO also forms cement-like products when reacted with phosphoric acid, and this forms the basis of zinc phosphate cements used in dentistry. A major component of zinc phosphate cement produced by this reaction is hopeite, Zn3(PO4)2·4H2O.

ZnO decomposes into zinc vapor and oxygen only at around 1975 °C, reflecting its considerable stability. Heating with carbon converts the oxide into zinc vapor:

ZnO + C → Zn + CO Zinc oxide reacts violently with aluminum and magnesium powders, with chlorinated rubber and linseed oil on heating causing fire and explosion hazard. It reacts with hydrogen sulfide to give the sulfide: this reaction is used commercially in removing H2S using ZnO powder (e.g., as deodorant).

ZnO + H2S → ZnS + H2O When ointments containing ZnO and water are melted and exposed to ultraviolet light, hydrogen peroxide is produced.

CRYSTAL STRUCTURES

Zinc oxide crystallizes in three forms: hexagonal wurtzite, cubic zincblende, and the rarelyobserved cubic rocksalt. The wurtzite structure is most stable and thus most common at ambient conditions. The zincblende form can be stabilized by growing ZnO on substrates with cubic lattice structure. In both cases, the zinc and oxide are tetrahedral. The rocksalt NaCl-type structure is only observed at relatively high pressures - ~10 GPa.

The hexagonal and zincblende ZnO lattices have no inversion symmetry (reflection of a crystal relatively any given point does not transform it into itself). This and other lattice symmetry properties result in piezoelectricity of the hexagonal and zinc blende ZnO, and in pyro-electricity of hexagonal ZnO.

The hexagonal structure has a point group 6 mm (Hermann-Mauguin notation) or C6v Schoenflies notation), and the space group is P63mc or C6v. The lattice constants are a = 3.25 Å and c = 5.2 Å; their ratio c/a ~ 1.60 is close to the ideal value for hexagonal cell c/a = 1.633. As in most II-VI materials, the bonding in ZnO is largely ionic, which explains its strong piezoelectricity. Due to this ionicity, zinc and oxygen planes bear

electric charge (positive and negative, respectively). Therefore, to maintain electrical neutrality, those planes reconstruct at atomic level in most relative materials, but not in ZnO - its surfaces are atomically flat, stable and exhibit no reconstruction. This anomaly of ZnO is not fully explained yet.

1. Wurtzite structure 2. Zinc blende structure Fig. 1 Crystal structures of ZnO

ELECTRONIC PROPERTIES

ZnO has a relatively large direct band gap of ~3.3 eV at room temperature; therefore, pure ZnO is colorless and transparent. Advantages associated with a large band gap include higher breakdown voltages, ability to sustain large electric fields, lower electronic noise, and high temperature and high-power operation. The bandgap of ZnO can further be tuned from ~3–4 eV by its alloying with magnesium oxide or cadmium oxide.

Most ZnO has n-type character, even in the absence of intentional doping. Native defects such as oxygen vacancies or zinc interstitials are often assumed to be the origin of this, but the subject remains controversial. An alternative explanation has been proposed, based on theoretical calculations, that unintentional substitutional hydrogen impurities are responsible. Controllable n-type doping is easily achieved by substituting Zn with group-III elements Al, Ga, In or by substituting oxygen with group-VII elements chlorine or iodine. Reliable p-type doping of ZnO remains difficult. This problem originates from low solubility of p-type dopants and their compensation by abundant n-type impurities, and it is pertinent not only to ZnO, but also to similar compounds GaN and ZnSe. Measurement of p-type in "intrinsically" n-type material is also not easy because inhomogeneity results in spurious signals.

Current absence of p-type ZnO does limit its electronic and optoelectronic applications which usually require junctions of n-type and p-type material. Known p-type dopants include group-I elements Li, Na,

K; group-V elements N, P and As; as well as copper and silver. However, many of these form deep acceptors and do not produce significant p-type conduction at room temperature. Electron mobility of ZnO strongly varies with temperature and has a maximum of ~2000 cm2/(V·s) at ~80 Kelvin.[21] Data on hole mobility are scarce with values in the range 5-30 cm2/(V·s).

Photodetector…………

Photosensors or photodetectors are sensors of light or other electromagnetic energy. There are several varieties:

(1) Active pixel sensors are image sensors consisting of an integrated circuit that contains an array of pixel sensors, each pixel containing a both a light sensor and an active amplifier. There are many types of active pixel sensors including the CMOS APS commonly used in cell phone cameras, web cameras, and some DSLRs. An image sensor produced by a CMOS process is also known as a CMOS sensor, and has emerged as an alternative to Charge (CCD) sensors.

(2) Charge-coupled devices (CCD), which are used to record images in astronomy, digital photography, and digital cinematography. Although before the 1990s photographic plates were the most common in astronomy. Glass-backed plates were used rather than film, because they do not shrink or deform in going between wet and dry condition, or under other disturbances. Unfortunately, Kodak discontinued producing several kinds of plates between 1980 and 2000, terminating the production of important sky surveys. The next generation of astronomical instruments, such as the Astro-E2, include cryogenic detectors. In experimental particle physics, a detectors a device used to track and identify elementary particles

(3) Chemical detectors, such as photographic plates, in which a silver halide molecule is split into an atom of metallic silver and a halogen atom. The photographic developer causes adjacent molecules to split similarly.

(4)Cryogenic detectors are sufficiently sensitive to measure the energy of single x-ray, visible and infrared photons.

(5)LEDs reverse-biased to act as photodiodes. See LEDs as Photodiode Light Sensors.

(6)Optical detectors, which are mostly quantum devices in which an individual photon produces a

discrete effect.

(7)Optical detectors that are effectively thermometers, responding purely to the heating effect of the

incoming radiation, such as pyroelectric detectors, Golaycells, thermocouples and thermistors, but

the latter two are much less sensitive.

(8)Photoresistors or Light Dependent Resistors (LDR) which change resistance according to light

intensity

(9)Photovoltaic cells or solar cells which produce a voltage and supply an electric current when

illuminated

(10)Photodiodes which can operate in photovoltaic mode or photoconductive mode

(11)Photomultiplier tubes containing a photocathode which emits electrons when illuminated, the

electrons are then amplified by a chain of dynodes.

(12)Phototubes containing a photocathode which emits electrons when illuminated, such that the

tube conducts a current proportional to the light intensity.

(13)Phototransistors, which act like amplifying photodiodes.

(14)Quantum dot photoconductors or photodiodes, which can handle wavelengths in the visible and

infrared spectral regions.

Ion implantation……………

Ion implantation is a materials engineering process by which ions of a material are accelerated in an electrical field and impacted into a solid. This process is used to change the physical, chemical, or electrical properties of the solid. Ion implantation is used insemiconductor device fabrication and in metal finishing, as well as various applications in materials science research. The ions alter the elemental composition of the target, if the ions differ in composition from the target, stop in the target and stay there. They also cause much chemical and physical change in the target by transferring their energy and momentum to the electrons and atomic nuclei of the target material. This causes a structural change, in that the crystal structure of the target can be damaged or even destroyed by the energetic collision cascades. Because the ions have masses comparable to those of the target atoms, they knock the target atoms out of place more than electron beams do. If the ion energy is sufficiently high (usually tens of MeV) to overcome the coulomb barrier, there can even be a small amount of nuclear transmutation

GENERAL PRINCIPLE.

Ion implantation setup with mass separator

Ion implantation equipment typically consists of an ion source, where ions of the desired element are produced, an accelerator, where the ions are electrostatically accelerated to a high energy, and a target chamber, where the ions impinge on a target, which is the material to be implanted. Thus ion implantation is a special case of particle radiation. Each ion is typically a single atom or molecule, and thus the actual amount of material implanted in the target is the integral over time of the ion current. This amount is called the dose. The currents supplied by implanters are typically small (microamperes), and thus the dose which can be implanted in a reasonable amount of time is small. Therefore, ion implantation finds application in cases where the amount of chemical change required is small.

Typical ion energies are in the range of 10 to 500 keV (1,600 to 80,000 aJ). Energies in the range 1

to 10 keV (160 to 1,600 aJ) can be used, but result in a penetration of only a few nanometers or

less. Energies lower than this result in very little damage to the target, and fall under the

designation ion beam deposition. Higher energies can also be used: accelerators capable of 5 MeV

(800,000 aJ) are common. However, there is often great structural damage to the target, and

because the depth distribution is broad (Bragg peak), the net composition change at any point in

the target will be small.

The energy of the ions, as well as the ion species and the composition of the target determine the

depth of penetration of the ions in the solid: A monoenergetic ion beam will generally have a broad

depth distribution. The average penetration depth is called the range of the ions. Under typical

circumstances ion ranges will be between 10 nanometers and 1 micrometer. Thus, ion implantation

is especially useful in cases where the chemical or structural change is desired to be near the

surface of the target. Ions gradually lose their energy as they travel through the solid, both from

occasional collisions with target atoms (which cause abrupt energy transfers) and from a mild drag

from overlap of electron orbitals, which is a continuous process. The loss of ion energy in the target

is called stopping and can be simulated with thebinary collision approximation method.