[Music] hello everyone welcome to this nptel online certification course on material characterization the title of the course is scanning electron ion prop microscopy in material characterization myself deva prata pradhan associate professor at material science center iit kharagpur today i will mention three major microscopic techniques that will be covered in this course and the brief history on microscope development magnification the resolution of microscope and the lens separation that play spoil sort to deteriorate the resolution these are the topics we will discuss today let us go to three major microscopic technique that will be covered in this course first will be scanning electron microscopy then scanning ion microscopy in particular helium ion microscope then scanning prop microscopy so in each case i will be covering the basic principle different parts and their functions how signals are obtained using this technique and they are used for constructing images how different parameters are varied to get the desired information and how samples are prepared to be examined under these microscopes and all these microscopes provides us three dimensional images let understand each word of this technique first is microscope microscope what comes from the two greek word one is microage that means small and another is scorpion to look at so an instrument or system that is used to view objects that are too small that we cannot see by naked eye is called microscope and the science of investigating such small objects using the microscope is called microscopy we visualize all the objects through our eye just mere presence of i is enough to see an object just mere presence of i may not be enough to see an object for example you are in a room painted with a black color paint in the wall and you switch up the light window is closed door is closed so there is complete darkness let us say in the middle of the night can you see an object even though your eyes are present you cannot see any object present in the room because of the darkness so light has to be there to get reflected from the object and pass through this space and reach your eyes once it reach your eyes then the signal is transferred to the brain and it allows us to detect the presence of an object's its appearance its color and its shape and size if we do not have light in the room then we cannot see the objects so we can say that without light there is no sight in addition to the presence of light we must have a eye that is for detector here therefore we are able to see any object present in the room however as you know light has a wavelength in the range of 400 nanometer to 700 nanometer so if a object is of size smaller than that then this visible light will not enough to see an object of size less than the wavelength of the light and there comes the electrons to rescue us electrons being a charged particle of very light mass can be pass through through the space if we made it to vacuum and we can change the wavelength of the electron to the desired value by changing its velocity if we apply potential difference between two electrodes then the velocity of the electrons will change therefore wavelength will change and we we can have wavelength much smaller than the wavelength of the light and therefore using the electron we could able to see the objects of a much smaller size though the light microscopy is used extensively in pathological laboratory to see objects which are little bigger or in the range of the wavelength of the light we extensively used electron microscope to see the objects of much smaller size that we cannot see by the light microscope so the middle or what is electrons here if we use electron beam to see an object then that is called electron microscopy we can also use ion ion electrons are all same but ions can be different and ions can penetrate less into a specimen as compared to the electron because electron is much smaller in mass smaller in size that that can penetrate much deeper into the sample as compared to the ions so the ions can provide us much superior quality images as compared to the electron microscopy so that is where ion microsoft copy come into the picture and then we have probe microscopy we were talking about that we are in the dark room and there was no presence of light and we discussed that we cannot see an object if there is no light but actually still you can know the presence of object in the room in the absence of light how you how do you know the presence of light in the absence of light how do you know the presence of an object in the absence of light if you crawl around with your hand and legs probably you can locate where a table is placed and if you carefully touch by your hand you can find if a sheet of paper is placed on the table if there is a pen on the table or if a tennis ball is there or cricket poll is there or a cone is there on the table you can locate it using your hand and fingers even by touching the pen you can tell whether the pen body is metallic or plastic so here the fingers in our hand acts as a prop to detect the presence of an object inside the dark room so it is always not necessary that light to be present in the absence of light also we can know the presence or absence of an object at a particular place this is the middle one coming that probe so if we use a physical prop like a blind man dodge he uses white cane to locate an object or something is close to him or her similarly we can use physical prop to measure the size shape of an object so here you see the middle one middleward electron ion prop coming into picture of microscopy then the first word is our scanning scanning means running down or systematically mobbing a prop it can be focused ion beam or focused electron beam on the surface of a surface of an object to produce an image so therefore we say scanning so in this case what we do is that we use either electron or ion or prop to visualize an object or create an image to know its shape size morphology etcetera so all these three techniques will gives us three dimensional image all these three techniques gives us three dimensional image and in the nanometer scale less than one nanometer then lets go to the history on microscope development so microscope is developed quite long back first in 1590 hans johnson and his son zakaria johnson developed the first microscope then in the 17th century early 17th century galileo first used the compound microscope then in the same 17th century leon hook used the microscope to study the bacteria and other microscopic creature and then robot hook used the compound microscope to visualize microorganisms and he has published his book in micrographia with the photographs hand sketched of the microorganisms and in the 18th century more developments occur to the optical microscope particularly on the lens and how to increase the magnification and so far up to 18th century scientists were particularly focusing on the optical microscope to see smaller size objects then the breakthrough on understanding the maximum resolution except came in 19th century and how small object can be viewed by using the microscope was postulated using in the 19th century then in the 20th century a major alternative to the light microscope camp all these previous years we have optical microscope or light microscope then in the 20th century the use of electron beam instead of light is used to create the image and this use of electron beam could provide us much superior resolution in the nanometer scale now coming to the 20th and 21st century this present century we have iron microscopes that has been commercialized in 2007 approximately last within last 10 to 15 years and at present there are about hundreds of such ion microscope whether those ion microscope could replace the electron microscope remain as a question because iron microscope could certainly provide much superior resolution as compared to the electron microscope we will see in this century in next 30 40 years how much of the electron microscope will be replaced by the ion microscope indeed expert believes that both will be exist for next several couple of degrees or next several regards and they are unique in their own sense in this slides what you have seen there is two important words magnification and resolution so let try to understand what is magnification and what is resolution so magnification is nothing but the ratio of image size to the object size theoretically one can have an image of any size and thus magnification can be of any value but by increasing the magnification to any extent would produce the blurry image if the resolution is not good enough so just by increasing the magnification if the resolution is not good will not be enough then what is this resolution it is defined as the closest spacing between two points that can be seen through a microscope as separate and deities like if we have two spots here and i could see this as a separate entities then i can say this is my resolution let us say i have another two point which are much more closer than this then my resolution is even better even better if these two spots are far away then resolution is worse so resolution this value the spacing between two points or objects should be as small as possible the the value if it is small that means we have better resolution or higher resolution so first let us try to understand what is the what is the resolving power of human eye how small object our eye can see or detect so resolving power of a human eye so h per relay criteria theta minimum which is the angle of resolution or minimum angle of resolution is one point two two lambda by d so here theta minimum is the angular resolution and lambda is the wavelength of light and d is the diameter of light gathering element diameter of optical element let us say objective lens this angular resolution theta minimum can be converted to spatial resolution by multiplying with focal length f so we can write that del l is equal to theta minimum into f where f is the distance between object and objective length or our i and which is equal then we can write which is equal to 1.22 f lambda by d this is our angular this is our spatial resolution so now if lets say if our i is placed 1 meter away from object let us say for f is 1 meter let us say f is 1 meter and say lambda is let us say green light 500 nanometer and d is the detector that is our eye the let us say the diameter of our pupil in the eye is 4 millimeter let us say 4 millimeter then we will have del l is equal to 1.22 then 1 meter we can write thousand millimeter into 500 nanometer then this is 4 millimeter and that would give us around ah zero point one five millimeter remember this is the diameter of pure pill in the eye so our eye can detect objects placed as close as point one five millimeter so this is the resulting power of a human eye or resolution of a human eye this is the resolution of a humanoid point one five millimeter so this again ah if let us say ah we can write again del l is nothing but d minimum that is the minimum distance between two object is equal to one point two two f lambda divided by d let us say we have a lens we have a lens and let us say its focal point is here focal point is here and then then this is the angle of aperture alpha and this is the optical lens its let us say diameter d and here we can write here we can write 10 alpha is equal to d d by 2 into this will be equal to d by 2 f so now i can put that here which is equal to 1.22 f lambda divided by ah 2 f into 10 alpha 2 f into 10 alpha so which is nothing but 0 points 6 1 lambda divided by 10 alpha so here alpha is angle of aperture or we can say half angle of the cone of light from one of the object so then ah we can write ah this alpha is very small actually this alpha is very small which is equal to sine alpha which is equal to tan alpha and we also have called refractive index of the medium between the lens objective lens and the object so therefore this t minimum d minimum is written a can be written as point six one lambda divided by mu sine sine alpha so this is what or we can write it it which is equal to 0.61 lambda divided by n a this is called average formula on resolution this is called a this is reported in derived by ah ernst karl abe in eighteen seventies and this underlines the importance of lambda that is wavelength of the light and numerical aperture numerical aperture is mu sine alpha here mu is the refractive index where mu is the refractive index of the medium medium between the object and objective lens so as you see this d minimum that mean that means minimum distance between two objects is directly proportional to the wavelength of the light or wavelength of the near red or electromagnetic radiation so having smaller the wavelength means we have d d minimum smaller that means good resolution otherwise in order to decrease the value of ah d minimum we should increase the value of refractive index and angle of aperture so so that is what we can do so now using a ah light ah visible light let us say we can use minimum 400 nanometer and if you if we use ah uv light then we can go ahead to use 200 nanometer ah of wavelength and using that ah and if you if we use ah oil immersion lens which has a refractive little higher refractive index at the same time if we use the larger angle angle of aperture or aperture angle then ah we can have this numerical aperture using the oil immersion lens within a larger aperture this numerical aperture value can go as high as ah 1.4 to 1.7 and with that value we can achieve a resolution of around ah 150 nanometer so let me clear this we can have like we had got formula d minimum is equal to point six one lambda divided by mu sine alpha using ah let us say ultraviolet light let us say violet light of 400 nanometer and oil immersion lens and with a large aperture with a large aperture we can achieve d minimum is approximately 150 nanometer which is good enough to see bacterial cells this is can be done using a optical microscope however we cannot see any object of smaller than c this size using the visible light visible light so for that we need the electromagnetic radiation or anything any particles which travels it is a much faster rate with a smaller wavelength lets talk little more about the ah resolution resolving power ok this go to next slide ok let us talk about resolution even though lenses used in the microscope are perfect resolution would be limited by the diffraction diffraction occurs when ah a weapon wave front is ah blocked or impeded by an object as you see here ah this almost parallel wave wavefront emerge as spherical wavefront after it passing through an aperture this is due to the diffraction and interference so in any microscope light has to pass through a series of aperture and lenses themselves and therefore they will undergo the production so the beam of light which supposed to be seen as a perfect spot will no more be appearing as a perfect spot rather than series of the cons with diminishing intensity from the center that you see here ah we have ah in the center that's that is called ah that is that is called aerytics here when light passed through a small aperture instead of giving us a perfect spot it is gives a dicks and then around that there is a cones of rings which these rings are called airy rings area ring with dim diminishing intensity from the center so this is due to the diffraction now now if we and this diameter of the dicks the diameter of this sticks is inversional to the aperture diameter so this diameter of the disk lets the diameter of the dicks is d this diameter is inversely proportional to the aperture diameter a pressure diameter now if there is two spots or two aperture or maybe two objects from her diffraction occurs if they are too close to each other then they will not be resolved they are almost overlapping on each other and if these two spots are little away from each other and this is what the result condition at which they are not overlapping each other and as per the lord relay when the maximum intensity of the reduce coincide with the first minimum of this second then we can say these two points are just resolved this is what happens the best condition of our resolving two objects when the intensity maximum intensity of an aridix coincides with the first minimum of the second then we can say these two points are just resolved is that only the fraction ah that spoils the resolution no we have several other aberrations too and we will be discussing what are those aberration in our next lecture thank you