This set of Nanotechnology Multiple Choice Questions & Answers (MCQs) focuses on “Far Field and Near Field Optical Microscopy”.
1. Choose the incorrect statement from the following.
a) NSOM employs a detector that is rastered across sample
b) NSOM makes use of non-propagating fields
c) NSOM has excitation laser light focused through an aperture
d) NSOM is a form of electron microscopy
View Answer
Explanation: NSOM is a scanning probe microscope. It makes use of non-propagating or evanescent fields for carrying high frequency spatial information. This evanescent field is created when excitation laser light is focused through an aperture having diameter smaller than the excitation wavelength. The microscope uses a detector that rasters across the sample using a piezoelectric stage.
2. Which of the following is a broadband nanoscale spectroscopy?
a) Nano-FTIR
b) TERS
c) SERS
d) Direct local Raman NSOM
View Answer
Explanation: The Nano-FTIR (nanoscale Fourier transform infrared) spectroscopy is a broadband nanoscale spectroscopy. It is a combination of FTIR spectroscopy and s-SNOM microscopy. This method uses the FTIR detection combined with broadband illumination to obtain a complete infrared spectrum at every spatial location.
3. What is Abbe’s theory of image formation?
a) Objects in focal plane of microscope are illuminated using convergent light from condenser
b) Inside a medium, particles of light are equally surrounded by particles of the medium
c) All points of a wavefront of light in vacuum is a new source of wavelet expanding in every direction
d) Bright objects sent out beams that would rebound off objects and enter the eye
View Answer
Explanation: Abbe’s theory of image formation was proposed in the year 1873. It states that objects in the focal plane of microscope are illuminated by the convergent light from a condenser. The light from the source can be considered as waves propagating in a particular direction and superimposed to form the incident illumination.
4. What is the purpose of a polarizer in an NSOM set-up?
a) Shear-force arrangement
b) Removal of stray light
c) Mounting sample on pieze stage
d) Surface analysis
View Answer
Explanation: There are various components of an NSOM set-up. These include source, beam splitter, feedback mechanism, detector, scanning probe, piezoelectric sample stage, polarizer, coupler etc. The polarizer along with the beam splitter serves the purpose of removing stray light from the returning reflected light.
5. A contrast enhancing technique cannot be provided by which of the following methods?
a) Change in the refractive index
b) Constant local stress
c) Alteration in the polarization of light
d) Modification of the magnetic properties
View Answer
Explanation: It is possible to make use of contrast enhancing techniques by changing the polarization of light or the intensity of the light as a function of the incident wavelength. This is also possible by changing the refractive index, reflectivity, magnetic properties and local stress.
6. Who was the first scientist to develop the idea of an imaging instrument in near field?
a) M.J.Fleming
b) Edwin Ostertag
c) Edward Hutchinson Synge
d) Anton Smirnov
View Answer
Explanation: It was Edward Hutchinson Synge who created and developed the idea of an imaging instrument that was capable of imaging in the near field. His original idea was based on the application of intense, nearly planar light from an arc placed behind an orifice that is kept near the surface of sample. This would facilitate the collection of information via point-by-point scattering.
7. Find out a near field spectroscopy method that uses merocyanine based dyes embedded in resin.
a) Nano-FTIR
b) Aperture Raman NSOM
c) Near field dielectric microscopy
d) Fluorescence NSOM
View Answer
Explanation: Fluorescence NSOM is a highly sensitive technique making use of fluorescence for near field imaging. This method of imaging uses merocyanine based dyes that remain embedded in appropriate resins. It is often used in the field of bioscience since it can achieve resolutions as low as 10nm.
8. Which of the following is not a drawback of NSOM?
a) Low working distance
b) High resolution
c) Limited to surface studies
d) Shallow depth of field
View Answer
Explanation: Near field optical microscopes have certain limitations such as very low working distance, shallow depth of field, longer scan times for larger samples and limited surface analysis of samples. Further, it is non-conducive for analyzing soft materials in shear force mode and other contrast operations.
9. How can we express Abbe’s equation?
a) d = (0.621λ0)/n sin a
b) d = λ0/n cos a
c) d = 0.621/(λ0n sin a)
d) d = (n sin a)/(0.621λ0)
View Answer
Explanation: Abbe’s equation can be given by,
d = (0.621λ0)/NA or d = (0.621λ0)/n sin a
where d = resolution; λ0 = wavelength of imaging radiation in vacuum; NA is the numerical aperture for
the optical component. It is also given by n sin a; n is the refractive index of medium between lens and source and a is half the aperture angle.
10. Identify the apertured mode of operation in NSOM.
a) PSTM with sharp transparent tip
b) Scanning inferometric microscopy with double modulation
c) Illumination collection
d) PSTM with opaque tip
View Answer
Explanation: NSOMs can be operated in both apertured and aperture less modes. The apertured modes of operation are: i) Illumination, ii) Reflection, iii) Collection, iv) Illumination collection and v) Reflection collection, while the aperturless modes of operation involve:- i) Scanning inferometric apertureless microscopy with double modulation.
11. Which of the following is an offshoot of surface enhanced Raman Spectroscopy (SERS)?
a) NFS
b) Fluorescence SNOM
c) TERS
d) Nano-FTIR
View Answer
Explanation: TERS, the acronym for Tip-enhanced Raman spectroscopy is an offshoot of the (SERS). This technique is used in the apertureless shear-force NSOM setup or in AFM having gold or silver coated tip. Using AFM tips can significantly enhance the Raman Signal.
12. In which year was the modern theory of image formation in the microscope founded?
a) 1932
b) 1873
c) 1947
d) 1825
View Answer
Explanation: The modern theory of image formation in a microscope was founded in the year 1873 by the German physicist Ernst Abbe. In 1932, neutron was discovered by James Chadwick. The year 1825 marked the invention of electromagnet. In 1947, transistor was invented.
13. NSOMs are vulnerable to artifacts.
a) True
b) False
View Answer
Explanation: NSOMs can be vulnerable towards artifacts that are not created from contrast mode. In apertureless NSOM, also called as scattering type SNOM, many of the artifacts are eliminated by the usage of proper techniques.
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