Essay: Nanochemistry

Sample Preparation: General description
Over a last decade there has been increased interest in nanochemistry. In order to explore novel physical properties and phenomena and realize potential applications of nanostructures and nanomaterials, the ability to fabricate and process nanomaterials and nanostructures is the first corner stone in nanotechnology. Variety of nanoparticles have been synthesized and proposed as potential building blocks of various optical and electronic devices. Fundamental importance to understanding and development of nanoscale materials is the development of synthetic method, which allows scientist to control over such parameter as particle size, shape, size distributions and compositions.
The property of mixed oxide systems greatly depend on their nanostructure is much sensitive to the selection of appropriate method for the preparation, provides control over their optical. electrical, magnetic and other physical properties.
Numerous synthesis techniques have been reported as a favorable for synthesis of Nanomaterials with uniform size distribution, homogeneous, controllable shape and good stability. We have adopted several techniques for thin film fabrication, both by chemical approach and evaporation in ultra high vacuum systems to produce nanoparticles with narrow size distribution. Although, our main focus is on rare-earth based perovskite systems with a generic formula ‘ ABO3 thin films , we shall discuss generalized synthesis procedure for any mixed oxide compounds.
2.1.1 Solid state reaction method for bulk materials
The solid-state method can be used to prepare a whole range of materials including mixed metal oxides, sulfides, nitrides, aluminosilicates, etc.
Non-volatile solids are used as starting materials. Solids do not usually react together at room temperature over normal time scale so it is necessary to heat them at much higher temperature for long time duration for proper reaction to occur at an appreciable rate. The major advantage of SSR method is, the final product in solid form is structurally pure with the desired properties depending on the final sintering temperatures. Reactions between or within solid reactants to yield a solid product are solvent free reactions hence no waste disposal issues associated with the solvent need be considered. This method is environment friendly and no toxic or unwanted waste is produced after the solid state reaction route is complete.
Solid state reaction route for bulk material involves mixing, pulverizing, calcinations, compacting and sintering of component shown in fig 2.1
2.1.2 Wet chemistry routes for synthesis for bulk
To obtain metal oxide nanoparticle with well define shape, crystallinity , size distribution, and surface functionality, is not only energy extensive but also extremely difficult to achieve using traditional solid state reaction route. This makes it a challenging task to use the solid state route to synthesize high purity conductors with high density and reproducible nanostructure. On the other hand, wet chemistry route provides attractive alternative. It is emerging as an inexpensive route to synthesize a single crystalline nanoparticle .This process, need judicious choice of molecular precursor, reaction medium and good kinetic control of the reaction parameter to form Nanomaterials with high purity and good homogeneity.
There exist a number of methods to synthesize the thin films which are categorized broadly as:
2.1.3 Various techniques for synthesizing thin films
Thin film materials are the key element of continued technological advance made in the field of electronic, photonic and magnetic device. Thin film deposition of metallic, insulating, conductive and dielectric materials plays an important role in a large number of applications compared to the bulk compounds. It is well known that the physical and chemical properties of thin films of metal oxides on a foreign substrate are different from those of the bulk sample.[1,2] The processing of materials into thin films allows easy integration into various types of device. Based on the nature of deposition process the methods employed for thin oxide film deposition can be divided into two group i.e. physical and chemical methods.
Chemical deposition:
1. Chemical Solution Deposition or Solgel spin coating & Dip coating
2. Chemical Vapor deposition (CVD)
3. Plating
4. Spray-pyrolysis technique (SPT)
5. Atomic layer deposition (ALD)
Physical deposition:
1. RF/Magnetron sputtering
2. Thermal evaporation
3. e-beam evaporation
4. Pulse Laser ablation (PLD)
5. MBE (Molecular Beam Epitaxy)
6. Electrospray deposition
7. Cathode arc deposition
Among the various techniques given above, spin coating and thermal evaporation techniques is employed in present investigation.
2.1.3.1 Chemical Solution Deposition (Spin coating) using Sol-gel chemistry route:
Spin coating is currently the predominant technique employed to produce uniform thin films with thickness of the order of micrometers and nanometers. The pioneering analysis of spin coating was performed more than fifty years ago by Emil et al [3] .The sol-gel process is a versatile solution process for making advanced materials. In general, the sol-gel process involves the transition of a solution system from a liquid “sol” (mostly colloidal) into a solid “gel” phase. In addition to the sol-gel method we have used precursor based solution deposition route (spin coating) for the nanostructure films. By chemical solution deposition (CSD), most any composition of metal oxide can be deposited as films, by applying the precursor solutions available. It is a low temperature processing route for synthesis of nanoparticles which provide good control over grain size and also on the phase formation.
The sol-gel process is a synthesis route consisting in the preparation of a sol and successive gelation and solvent removal. At the first stage of the sol-gel process is preparing homogeneous solution of easily purified precursor. Colloidal solution (i.e ‘sol’) is prepared by the hydrolysis reaction. Removal of the liquid from the sol yields the gel, and the sol/gel transition controls the particle size and shape. In the spin coating process, the substrate spins around an axis that should be perpendicular to the coating area. The process is carried out in four stages shown in fig 2.2. During the deposition stage, solution for instance a photosensitive resist, is allowed to fall on rotating substrates from microsyrings and the substrate is accelerated to the desired speed. Spreading of the solution takes place due to centrifugal force. The second stage is when the substrate is accelerated up to its final, desired, rotation speed called spin up stage. This stage is usually characterized by aggressive fluid expulsion from the wafer surface by the rotational motion. The third stage, spin off, is when the substrate is spinning at a constant rate and fluid viscous forces dominate fluid thinning behavior. In this stage gradual fluid thinning takes place. Subquent acceleration as well as the rotation speed and the time allotted to the individual steps ensure that a homogeneous layer thickness remains after excess resist is spin off. The process is ends with drying process where solvent are loss from the surface and these results in formation of thin film.
Fig 2.2: Schematic diagram of spin coating process : deposition of sol, spin up , spin off and gelation by solvent evopration.
Final film thickness and’other properties will depend on the nature of the solution (viscosity, drying rate, percent solids, surface tension, etc.) and the parameters chosen for the spin coating process. Factors such as rotational speed, acceleration, and fume exhaust contribute to how the properties of spin coated films are defined.
Fig. 2.3: Spin Coating Unit (Model SCU 2005)
Fig.2.3 shows the photograph of the thin film spin coating unit (Model No. SCU 2005, M/S. Apex Instruments, Kolkota). This unit was used to fabricate nanostructured thin films of various perovskite manganite thin films with different thickness.
2.1.3.2 Thermal Evaporation Method
The thermal evaporation technique comprises evaporating source materials in a vacuum chamber below 10-6 torr and condensing the evaporated particles on a substrate. In this technique, thermal energy is supplied to sources from which atoms are evaporated which are deposited on the substrate. Thermal Evaporation was done using Vacuum Coating unit, Model 12CU4 (MS Vacuum Techniques Pvt. Ltd., Bangalore)shown in fig 2.4. The vacuum system consists of Rotary Vacuum Pump, Diffusion Pump with Liquid Nitrogen trap and various valves like Roughing, Backing, Needle valve, Air admittance and High Vacuum Valves for operating system and Pirani and Penning gauges for measuring vacuum. After the coating unit has been vacuumised Ion cleaning for the cleaning of specimen surface through glow discharge is done.
Fig. 2.4: Photograph of Vacuum Coating Unit, Model 12CU4
Thermal Evaporation process consists of heating the metal to be deposited in a boat until its evaporation. The metal vapour condenses as thin film on the substrate surface. The process takes place inside vacuum enabling free movement of molecules. The process is advantageous where purity of metal deposition is required.
2.2 Characterization Methods
2.2.1 General Description
The need to fine-tune different exotic nanoparticle properties to make them suitable for specific applications has sparked a large number of worldwide research efforts aimed at their tailoring. Full use of these structures in these applications requires more detailed information and a feedback of data coming from reliable characterization techniques [4-6]. Accurate characterization of any nanomaterials requires a precise understanding and measurement of the surface and interfacial phenomena along with the other factors such as phase analysis, compositional characterization, structural elucidation, micro-structural analysis and surface characterization, which have strong bearing on the properties of materials. This has led to the emergence of variety of advanced techniques in the field of materials science. Simplicity we may think of this information as being divided into:
‘ Surface Morphology (the microstructural or nanostructural architecture );
‘ Elemental composition (the elements and the possibly molecular groupings present);
‘ Crystal structure (the detail atomic arrangement in the chemical phases contained within the microstructure);
Film property Characterization Technique
‘ Surface Morphology : 1. SEM (Scanning Electron Microscopy)
2. ATEM (Analytical Transmission Electron
Microscopy)
3. STM (Scanning Tunneling Microscopy)
4. AFM (Atomic Force Microscopy)
‘ Elemental Composition : 1. XRF (X-ray Fluorescence)
2. XRD (X-ray Diffraction)
3. EPMA (Electron Probe Microanalysis)
4. RBS (Rutherford Backscattering Spectroscopy)
5. SIMS (Secondary Ion Mass Spectroscopy)
6. AES (Auger Electron Spectroscopy)
7. XPS (X-ray Photoelectron Spectroscopy)
8. EDAX (Energy Dispersive X-ray analysis)
‘ Crystal Structure : 1. XRD (X-ray Diffraction)
2. STEM (Scanning Transmission Electron
Microscopy)
3.LEED (Low Energy Electron Diffraction) & RHEED (Reflection High Energy Electron Diffraction)
In this section some of the above mentioned analytical instrumental techniques used to characterize our thin films are described with relevant principles of their operation and working in detail.
2.2.2 X-ray diffraction (XRD) technique:
X-ray techniques hold a leading role as a tool for material characterization. The physical properties of thin films must relate to their structure and X-ray diffraction (XRD) is a powerful tool of probing materials structures in atomic scale. X-ray diffractograms provide a wealth of information – related to the crystal structure of solids, including lattice constant and geometry , particle size , the lattice mismatches and strain between the thin films and substrates ,identification of different crystallographic phases , including the impurities and unreacted components ,orientation of single crystals and preferred orientation of polycrystals, defects, etc. Such information provides a very useful guideline towards modification of synthesis procedures.
X-ray diffraction is a rapid analytical technique primarily used in the powder diffraction mode. In conventional X-ray diffraction and fluorescence with large incident angles, an incident x-ray beam penetrates upto few to several hundred micrometers deep into a material. So that when it comes to thin film analysis the beam penetration depth may be much greater than the sample thickness, X-rays get penetrated through the thin films, without undergoing significant scattering. To limit the incident X-ray beam to the surface, grazing-incidence X-rays are needed.[7] At an incident angle near or below the critical angle for total reflection, the incident beam is evanescent and penetrates only the top 100 A?? or less into the surface [8-9]. Grazing incidence XRD(GIXRD) refers to a method where the incident X-ray beam makes a small (typically about 1 degree) angle to the sample surface , hence the X-ray beam sufficiently passes through the film material, causing sizable scattering before being transmitted. The GIXRD is very popular mode of XRD as it does not require dedicated instrumentation in the standard XRD set up. In the present work, the GIXRD method is applied as a innovative tool for the identification of phase, particle size and lattice parameters in multilayer deposits. By means of this technique, valuable information can be obtained [10]. Some of them are discussed.
2.2.2.1. Unit cell volume
X-ray diffraction provides us the lattice cell parameters of crystal unit cell and therefore its volume.
Cubic: V = a 3(a=b=c)
Tetragonal: V = a 2 c
Hexagonal: V = a2c sin(60??)
Trigonal: V = a2c sin(60??)
Rhombohedral: V = a 3 ‘ 1 ‘ 3 cos2 ?? + 2 cos3 ??
Orthorhombic: V = abc
Monoclinic: V = abc (sin ??)
Triclinic: V = abc (1- cos2 ?? – cos2 ?? – cos2 ??) + 2(cos(??) cos(??) cos(??))??
Where a, b, and c are the unit cell axes dimensions and ??, ??, and ?? are the inclination angles of the axes in the unit cell.
2.2.2.2. X-ray density
X-ray density = Weight of the atoms in unit cell
Volume of unit cell
?? = ?? A
V N
where ?? = density (gm cm-3)
??A = atomic weight of all atoms in the unit cell
N = Avogadro number
V = Vol. of unit cell (cm3)
2.2.2.3. Porosity
By comparing X-ray density to that of macroscopic density of a pressed and sintered pellet one can calculate the percent porosity in the compact.
2.2.2.4 Particle size
When the particle size of the individual crystal is less than 1000 ??, the term particle size is used. Crystals of this size range cause broadening of the Bragg peak in the XRD spectrograph, the extent of the broadening is given by –
B = 0.9 ??
t cos??
where, B = broadening of the diffraction line (Bragg peak) measured at full width at half
its maximum intensity (FWHM) (in radian)
?? = wavelength of target
t = diameter of the crystal particle
2.2.2.5 Strain
If film is nit free from micro-stain then broadening of the peak is due to crystalline size and strain. Strain in the film can be calculated from,
Strain (??)= ??s/tan??
Where , ??s= broadening of peak due to strain
By plotting graph between ??cos?? vs. sin?? slope gives value of strain
in the film.
2.2.2.6 Lattice Mismatch
The lattice mismatch between the substrate and the film leads to the lattice strain which can be calculated using the formula, Positive or negative value of ??, respectively, indicates a tensile strain or compressive strain in the film. The strain can be very effective in manipulating the electrical transport and magnetotransport properties of the thin films.
??% = [(dsubstrate – dfilms) / dsubstrate] ?? 100
2.2.3 AFM (Atomic Force Microscopy)
Scanning probe microscope (SPM) defines a broad group of instruments used to image and measure the properties of the material .Two primary form of SPM are; Scanning Tunneling Microscopy (STM) and Atomic Force Microscopy (AFM). Atomic force microscopy was developed about five years after STM (Scanning Tunneling Microscopy) by Binnig [11] and has rapidly developed into a powerful and invaluable surface analysis technique. STM has fundamental disadvantage, one can investigate only the conductive or conductive layers coated samples. While AFM images flat surfaces with atomic resolution in direct Space, regardless of their electrical conductivity.
An atomic force microscope includes a sharp tip of, about 2??m long and down to a minimum of 20 nm is diameter mounted on a micromachined cantilever. The cantilever is typically silicon or silicon nitride which is 100 to 200 ??m long. As the tip scans a surface to be investigated, interatomic forces between the tip and the sample surface induce displacement of the tip which causes the cantilever bend or deflect. A laser beam is transmitted to and reflected from the cantilever for measuring the cantilever orientation. In a preferred embodiment the laser beam has an elliptical shape. The reflected laser beam is detected with a position-sensitive detector, preferably a bicell. A schematic of working principle of AFM is as shown in fig.2.5. The output of the bicell is provided to a computer for processing of the data for providing a topographical image of the surface with atomic resolution. Any bending of the cantilever results in a shift in position of the focused laser spot on the detector, and the PSD can measure such displacements to an accuracy of < 1nm [12]. The AFM can be operated in a number of modes, depending on the application. In general, possible imaging modes are divided into static (also called contact) modes and a variety of dynamic (or non-contact and tapping mode) modes where the cantilever is vibrated. AFM's can operate under UHV, ambient air conditions or even with a solid sample and tip submerged in a liquid cell, which is highly useful in biological systems Fig. 2.5: Schematic of working principle of atomic force microscope (AFM) Generally AFM is used to provide information about the surface structures of a sample, the particle size, RMS roughness and sometimes the film thickness. However, AFM operates usually in three modes according to the nature of the tip motion. ' Contact mode : The contact mode where the tip scans the sample in close contact with the surface is the common mode of operation used in the force microscope useful for obtaining 3D topographical information on nanostructures and surfaces. It is also known as repulsive mode, an AFM tip makes soft 'physical contact' with the surface. As tip scanned, cantilever flex and produced a measurable displacement due to Van der Waal forces . ' Non-Contact mode: A new era in imaging was opened when a system with the non-contact mode is used in situations where tip contact might alter the sample in subtle ways. When lifting the tip by at least 50 - 150 A?? above the sample surface .Attractive Van der Waals forces acting between the tip and the sample are detected and topographic images are constructed by scanning the tip above the surface. ' Tapping mode : Tapping mode is a key advance in AFM. Unlike the operation of contact mode, where the tip is in constant contact with the surface, in tapping mode the tip makes intermittent contact with the surface. Contact time is less so lateral forces are reduced dramatically. Tapping mode is usually preferred to image samples with structures that are weakly bound to the surface or samples that are soft (polymers, thin films). In the present study, a large number of thin films, prepared by various routes, are characterized by the atomic force microscopy (AFM). Fig. 2.6 shows the photograph of the basic AFM machine (Nanosurf AG, Switzerland) installed and being used at the Department of Physics, Gujarat University. Fig .2.6:AFM at Department of Physics, Gujarat University