Physics Class Notes
  1. What is Sol-gel method for the synthesis of nanostructure materials?
     

Ans: In a sol-gel system, two types of materials or components, are involved, such as ‘sol’ and ‘gel’. Here, sols refer to solid particles in a liquid. Hence, sols can be considered as a subclass of colloids. On the other side, gels are the continuous network of particles with pores filled with liquid (or polymers containing liquid).

A sol-gel process involves the formation of ‘sols’ in a liquid and then connecting the sol particles (or some subunits capable of forming a porous network) to build a network (‘gel’). By evaporating the liquid, it is possible to obtain powders, thin films or even monolithic solid. The sol-gel method is particularly useful to synthesize ceramics or metal oxides although sulphides, borides and nitrides also are possible. The schematics are given below;

 

 

 

 

 

 

 

 (a) Sol, (b) gel, and (c) sol-gel monolithic solid

There are several synthesis routes to prepare the sol-gel materials, such as hydrothermal synthesis, sonochemical synthesis, microwave synthesis, synthesis using micro-reactor etc.

There are several advantages of the sol-gel process over the other material preparation process. Some of the key benefits are mentioned below;

  • It is a low-temperature process, which is very useful for future generation flexible electronics applications. Due to low-temperature energy consumption becomes low and less pollution.

  • High purity and well-controlled material structure of the sol-gel process leads it comparable to conventional CVDs or other deposition systems.

  • In most of the cases, particularly for the oxide material formation process, the overall sol-gel process can be less expensive as compared to other deposition processes.

  • Some of the benefits, like getting unique materials such as aerogels, zeolites, and ordered porous solids by organic-inorganic hybridization are unique to the sol-gel process.

 

2. What is Quantum Confinement?

Ans: The word confinement means to confine the motion of randomly moving electron (and/or holes) to restrict its movement in specific energy levels (discreteness), and quantum reflects the atomic realm of particles. The quantum confinement effect is observed when the size of the particle is too small (typically 10 nanometers or less) to be comparable to the wavelength of the electron. So as the size of a particle decrease till we reach a nanoscale, the reduction in confining dimension makes the energy levels discrete and this increases or widens up the bandgap and ultimately the bandgap energy also increases. Specifically, the phenomenon results from electrons and holes being squeezed into a dimension that approaches a critical quantum measurement called the exciton Bohr radius. QDs are the class of materials in which quantum confinement effects can be evidenced. They are tiny semiconductor crystals on the order of nanometer size, containing merely a hundred to a thousand atoms. As a result, they tightly confine electrons or electron-hole pairs called “excitons” in all three dimensions. QDs are a subgroup in the family of nanomaterials, which comprises metals, insulators, semiconductors, and organic materials. Specifically, the term “quantum dot” refers only to semiconductor nanocrystals, whereas any other inorganic material in the nano regime is referred to as a “nanocrystal.”

 

3. Write down the difference between SEM and TEM. Write down the use of STM.

Ans: The scanning electron microscope (SEM) and transmission electron microscope (TEM) both are electron microscopes. Their versatility and exceptionally high spatial resolution render them a valuable tool for many applications.

The main difference between SEM and TEM is that SEM creates an image by detecting reflected or knocked-off electrons, while TEM uses transmitted electrons (electrons that are passing through the sample) to create an image. As a result, TEM offers valuable information on the inner structure of the sample, such as crystal structure, morphology and stress state information, while SEM provides information on the sample’s surface and its composition. Moreover, one of the most pronounced differences between the two methods is the optimal spatial resolution that they can achieve. SEM resolution is limited to ~0.5 nm, while with the recent development in aberration-corrected TEMs, images with a spatial resolution of even less than 50 pm have been reported. In terms of analysis point of view, SEMs provide a 3D image of the surface of the sample, whereas TEM images are 2D projections of the sample, which in some cases interprets the results more difficult for the operator. SEMs usually use acceleration voltages up to 30 kV, while TEM users can set it in the range of 60–300 kV. The magnifications that TEMs offer are also much higher compared to SEMs. TEM users can magnify their samples by more than 50 million times, while for the SEM, this is limited to 1–2 million times. The samples preparation technique for the SEM require little or no effort for sample preparation and can be directly imaged by mounting them on an aluminium stub. On the other hand, TEM sample preparation is a quite complicated and tedious procedure that only trained and experienced users can follow successfully. The samples need to be very thin, as flat as possible, and the preparation technique should not introduce any artifacts (such as precipitates or amorphization) to the sample. Many methods have been developed, including electropolishing, mechanical polishing, and focused ion beam milling. Dedicated grids and holders are used to mount the TEM samples.

 

Scanning Tunneling Microscopy (STM) is a powerful type of electron microscopy for imaging the surface of a sample at the atomic scale. STM is known as a surface-sensitive technique and, unlike TEM, it needs an absolutely clean surface; thus, its success rate is low. However, in recent time, STM produces a current profile of oxide surface is being used to analysis the memristive behaviours.

 

5. Mention one useful application of carbon nanotube (CNT). What is the full form of NEMS?

Ans: Carbon nanotubes have been of great interest because of their simplicity and ease of synthesis. Research on carbon nanotubes has shown the application in the field of energy storage, hydrogen storage, electrochemical supercapacitor, field-emitting devices, transistors, nanoprobes and sensors, composite material, templates, etc. Among these applications, CNTs in sensors applications are promising. The conductivity of nanotube systems is highly sensitive to gaseous ambient, which affects the sign and amount of injected charge. The dimensions of a nanotube sensing element are such that very low quantities of analyte species will produce a measurable response. Nanotube gas sensors certainly have prospects to challenge conventional gas sensors for specific uses. In addition to transduction, bio-sensors require a bioreceptor (e.g., enzyme or cell) immobilisation matrix. Carbon-nanotube-based bio-sensors meet both requirements and have been found to promote homogeneous electron-transfer reactions.

NEMS, or nanoelectromechanical systems, are devices in which an electronic circuit or vice versa controls the physical motion of a nanometre-scale structure.

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