Chapter 2: Experimental Techniques
2.1. Synthesis Techniques of GQDs and Its Heterostructures
2.2.1. Microscopy Techniques
2.2.1.1. Field Emission Scanning Electron Microscopy (FESEM)
FESEM is an electron microscopy technique used for imaging of the materials with a resolution micro-nano meter range. The use of electrons in microscope has two main advantages as compared to the conventional microscopy (optical). Much larger magnifications are possible here, since electron wavelengths are much smaller than photon wavelengths and the depth of field is much higher. The electron wavelength depends on the electron velocity or the accelerating voltage expressed as,
=
√ =
√ nm …….………….. (2.1)
In the FESEM, electrons are emitted from a field emission source under extreme vacuum.
The vacuum allows the electrons movement along a column without scattering, which helps to prevent discharges inside the instrument. The field emission source is a tungsten filament (cathode) with sharp tip, which is placed in a huge electrical potential gradient.
The significance of extremely thin and sharp tip (diameter 10-100 nm) is that an electric field can be concentrated to an extreme level so that the work function of the material is lowered and electrons can leave from the cathode. After emission, the electrons are accelerated by the two anodes. An accelerating voltage (0.5-30 kV) between the cathode and anode is commonly used. This voltage combined with the beam diameter determines the resolution of the image. As the voltage increases, better point-to-point resolution can be achieved. Because of the smaller beam size of the electron source, the beam produced by this emitter is about 1000 times smaller in diameter than that in a standard SEM, which markedly improves the image resolution. The beam is collimated by electromagnetic condenser lenses, focused by an objective lens, and scanned across the surface of the sample by electromagnetic deflection coils. The primary imaging method is by collecting emitted secondary electrons that are released by the sample. A secondary electron detector is placed near to the specimen. By correlating the sample scan position with the resulting signal, an image is formed on the screen that is strikingly similar to what would be seen through an optical microscope. The FESEM is equipped with a
special objective or focusing lens that projects the magnetic field below the lens. Very high resolution is obtained by shortening the specimen–lens distance and using a specially designed in-lens. The distance is shortened by placing the specimen in the lens magnetic field. In this case, secondary electron detector is placed above the objective in-lens (called as in-lens detector), which makes difference in the image compared to the conventional image of the secondary electron detector. Very high resolution and contrast can be obtained by using in-lens detector. Interaction of the primary electron beam with atoms in the material results in the shell transitions. This process leads to the emission of X-ray.
The emitted X-ray has an energy characteristic of the parent element. Detection and measurement of the energy permits elemental analysis by means of Energy Dispersive X- ray Spectroscopy (EDS). EDS provides rapid qualitative, or with adequate standards, elemental composition with a sampling depth of 1-2 microns. FESEM (Sigma Zeiss, Germany) is used in the present study. The resolution of the FESEM instruments is 1nm.
FESEM characterization of Au NPs decorated GQD-GCN sample was done for imaging as well as the compositional analysis.
2.2.1.2. High Resolution Transmission Electron Microscopy (HRTEM)
TEM is one of the advanced imaging techniques for nanoscale materials characterization.
HRTEM is well known for studying the structural information, lattice imaging (defects or dislocations) and the morphology of the nanostructured samples. In case of a crystalline material, electron diffraction will occur only at specific angles, which are the characteristic features for the crystal structures present. In the present thesis, TEM and HRTEM are extensively used to study the morphology and crystallinity of GQD and GQDs based heterostructures. TEM works on the principle of optical projection; when an object is placed in front of a light source, its image is enlarged and a shadow is created on the screen placed far distance behind this object. Electrons emitted from an electron gun are accelerated to high voltages (typically 100 to 400 kV) and focused on the sample by a number of condenser lenses. Lanthanum hexaboride (LaB6) crystal is used for generation of thermionic electron emission. The emitted electrons pass through a series of lenses to be focused and scanned across the sample. The static beam has a diameter of a few microns. The sample must be sufficiently thin (a few tens to a few hundred nm) to be transparent to electrons. The transmitted and forward scattered electrons form a diffraction pattern in the back focal plane and a magnified image in the image plane. With
additional lenses, either the image or the diffraction pattern is projected onto a fluorescent screen for viewing or photographic recording.
In our present study, a TEM (JEM2100, JEOL, Japan) operating at 200 kV with a high resolution CCD camera (Gatan, USA) is used for the TEM and HRTEM imaging. A photograph of the TEM instrument is shown in Fig. 2.5. Some components of the TEM instrument are labelled in the figure. For TEM imaging, the powder samples were dispersed in methanol solvent and sonicated for a few minutes. The GQD or GQD heterostructure in solvent sample is drop casted on a copper grid having 400 mesh size (Pacific Grid, USA). After normal drying for a prolonged time, this grid is used for the TEM imaging. Improved resolution of the lattice image is obtained after processing of inverse fast Fourier transformation (IFFT) image using a 'Digital Micrograph' (Gatan, USA) image analysis software. TEM imaging is a powerful tool for analysing the shape of the nanostructures. In the present thesis, TEM analysis is used for studying the size of GQDs and understanding the formation of GQDs at different stages of reaction using two different precursor materials.
Figure 2.5: photograph of the TEM (JEM 2100, JEOL, Japan) used for the present study. Some of the components are marked in image.