 So, in this lecture, we are going to discuss about the scanning probe microscopy. This microscopic techniques have been developed late 1980s and early 1990s and there are lot of development taken place even after 2000. So, we are going to discuss only three important methods in this connection. Scanning microscopic techniques atom force microscopic technique and near field scanning optical microscopic technique. So, as you understand scanning probe means there is a probe which is going to scan a surface of a material and these techniques are widely used for study of surfaces both external and internal surfaces. The first two techniques that is scanning trailing microscopy and the atom force microscopy both are used for the surface imaging of the material. On the other hand near field scanning optical microscopic technique is used for both surface and also internal surfaces of the material. What I mean to say is that the external internal surfaces both can be image using the scanning near field scanning optical microscopic. So, most important technique which is used for near field scanning optical microscopic is the laser confocal microscopy and this is getting more importance over the time scale as it can give us information regarding the structure of the internal surface. Initially it was used for biological specimens nowadays people are using it for material science application also. So, in this lecture when we discuss one by one these three techniques like there is testium AFM and NSOM and part of the NSOM is basically LCM that is laser confocal microscopy. So, let us first discuss about the scanning trailing microscopy as you understand it is a scanning technique. So, therefore ligand scanning electron microscope you probably have used it there is a raster which makes the tip of this microscope or the probe of the microscope to scan of the surface and tunneling is basically coming from the tunneling of currents. So, let us first see history of it. It was discovered quite sometime back about 30 years back by two scientists Henry H. Ruehrer and Binning at the IBM labs anyways and for the discover immediately after few years they got Nobel Prize in 1986 for this discovery along with another stalwart in the microscopic technique that is Ernst Ruska. So, Ruska actually finally at the discovery of Tasmus Electromicroscopy. So, he along with these two scientists Ruehrer and Binning received the Nobel Prize in 1986 for the discovery the central figure shows this machine which these two scientists made at the IBM labs. So, before discussing about the scanning trailing microscopy let me just talk about something about tunneling. The concept of trailing came after the advent of quantum mechanics. As you know the if we take a small tip very fine tip of the diameter of suppose few nanometers 1 to 2 nanometers and bring close to a material surface and then if you apply a small bias voltage V to the tip because of this small bias voltage there will be electrical field generated and this electrical field will lead to tunneling of electrons from the tip to the sample surface and this tunneling of electron can result into tunneling current. This is well known in the literature thus this is the reason impact this current depends on the height of the barrier and it has been found that this height of the barrier this is the tip here you can see and this is sample. So, therefore, once the tip is bought close to the sample and a small voltage basically bias voltage applied between the sample and the tip and then there will be tunneling of current like this electrons like this and it has been found that the height barrier that is the distance between the tip and the sample surface this is a strong function of average work function of the tip and the sample. So therefore, obviously tip material has to be have as to a very low work function material normally tungsten is used for that very fine tips tungsten tip which is oriented along 110 is used for such a tunneling. In fact, this same tungsten tip is used for the fake electron guns which we discussed for the electron microscopes. So, therefore, this concept of tunneling can be used for imaging the surfaces of a material. Let us see how it is done. So, here I am showing atomic scale view on the left side of this picture is the STM tip consisting of atomic atoms of the material which is used to make this STM tip normally tungsten and then there is a surface. So, therefore, when the tip is bought very close to the surface as I have already told there and there is a small bias voltage applied that is this tunneling of electron from tip to the sample surface and this height H which is known as a barrier for tunneling is directly proportional to the average work function of the STM tip and the surface. Now, so, therefore, if we do some kind of arrangement to scan this tip on a sample surface and depending on this height tunneling current will vary. So, therefore, we can use this tunneling current variation to image the surface of the material the other ways of doing that also which I will discuss subsequently. Now, this is on the right side of the picture that is shown in a schematic video which is made by making many number of images. So, as you see there is a small tip here at the top consisting of several atoms several atoms and several hundreds of atoms and the surface and this is scanning either here actually sample is shown to be scanning or moving by CNC plate or CNC device at a very controlled velocity and the distance between the tip and the sample surface is kept constant. Depending on the atoms the height atom plane or atomic positions the height of this barrier will change as you see the green atoms or the blue atoms at a lower height than the green atoms. So, therefore, the tunneling current will vary and once we obtain this kind of information and store it we can plot it as a function of spatial variable and we can get the image. So, this is the basically the principle of scanning tunneling microscope. This was well known even after the discovery of the quantum mechanics and other theories came up in 1910's and even 1920, 1930's but the actual device making took a long time only in 1980's the actual device could be made because of the preparation of fine tip at the same time controlling this movement of the tip because tip needs to be brought to the very close to sample surface for tunneling to happen. So ideally tending the steam tip is very much pointed in fact it should be as pointed as that it will contain 1 to 2 atoms at the end and relatively very low arc functions. So, what is used is basically H tungsten crystals they are ideal and in fact these H tungsten crystals which are oriented along 110 direction this is 110 direction 110 direction of the tungsten crystal this is also used as a field emitter as I said in a field emission gun and so therefore by there are lot of what is called complex mechanism by the steam works which I will not going to discuss but I will discuss about the basic things of the principles of the scanning transmission electron microscopy and scanning tunneling microscopy and also some applications. So, as you know this is actually the device the way it looks like schematically the sample surface there is a tungsten tip and then there are attachment here and which will make this device to move on XY plane and this the tunneling voltage applied across the tip and the sample and you measure the current. So, and atomistically this can be shown like this again I am showing several times this thing so that it can be clear to you there is a tip here which is a positively biased as compared to sample surface and because of that current electron will flow from the sample surface to the tip and current will flow opposite direction a tunneling current is basically actually exponentially varies as proportional to the distance and thus a feedback loop thus a high feedback loop the tip can be maintained at a constant distance when a sample surface or it can be even being close or to see the variation of the tunneling current. So, there are obviously there are two ways of operating it if the tunneling current is kept constant suppose between the tip and the sample surface the jet position of the tip must be moved up and down depends on the sample surface as you see sample surface is very rough here it varies. So, therefore the as the tip current tunneling current is constant to maintain this constant current in the tunneling current in the tip one is to bring the tip closer or the longer distance depends on the sample atom positions on the sample surface. So, this movement is normally recorded the movement of the tip can be recorded as is moving on the sample surface and then it can be plotted and on a sample on a basically raster mode it can be plotted and you will give us a topographic image this is a first mechanism by which so in the first case we kept the tunneling current tunneling current constant and move the tip is moved up and down to keep it constant and the position of the tip is then used to plot to obtain a topographic image this is a first or the a we can say mechanism of scanning tunneling microscope another way we can do is that we can keep the jet position of the tip constant that the distance of the tip from this on the sample surface can be kept constant and tunneling current obviously will change depending on the surface configuration. So, whenever the atoms are closer to the tip there will be more tunneling current atoms are close avoided to the tip there will be less tunneling current. So, therefore if this changes in the tunneling current is recorded and then if you plot the tunneling current variation as a function of spatial variable x and y or z then basically we can get a topographic image. So, this is called b so in this case z is kept constant and that is distance between the tip and the sample surface at a particular position and then tunneling current tunneling current is measured it will change as a function of sample position and then this is plotted I t c is plotted as a function of spatial variables to obtain a topographic image. So, this is very simple this is all well known even long back but discovery always takes time because of the implication because of the you know problem of integrating the whole instrument. So, and obviously as you understand the probe needs to be scanned or suppose needs to be scanned actually on the sample surface the scanning is done by raster is similar to like a c m and each coordinate is can be recorded each coordinate of the sample tip can be recorded on a on a computer just like that. So, whether you use a or b it does not matter all the positions are recorded and then values of the current or values of the distance of the of the movement of the tip can be recorded at whichever you want you can plot on the on the on a computer and get the image. So, this is basically the way these are the ways different ways a scanning tunneling microscope can be used and this array used only. So, remember this requires the tunneling of electron therefore, the whole thing has to be kept under a very high vacuum system this is another problem second problem is that the sample must be conducting otherwise there will be no tunneling of current. So, these are the basically routine problems one phase the sample is not conducting much there will be not much tunneling happening of the electron from the sample surface to the tip and because of that current flow will not happen and this this microscope will not occur. Second important thing is that the the the whole system has to be kept under a ultra high vacuum and that makes the the process quite costly because you need a vacuum system attached to it. On a broad scale actually if you remove the if you increase the scale so this will look like this. So, this tunneling tip is controlled by PGO's electric scanner and nowadays very precisely controlled PGO's scanner available and this tip is basically can be bought close or wider from the sample surface as you understand the sample surface will be very rough there will be atoms arranged in a differently order manner. In fact, there will be absorb atoms sitting on the surface. So, all those things makes the sample surface very rough. So, and then there is a feedback loop to control the if you want to keep this tip at a constant distance from the sample surface there is a feedback loop or and you need a voltage amplifier also to amplify the whatever voltage is coming recorded. So, ability to precisely position the probe of an STM is possible only because of X, Y, Z PGO's scanner and this X, Y, Z PGO's electric canals came long it took long time for it to be operational and to make it be to made be used for this kind of microscope purpose. So, this kind of PGO's electric scanner is always coupled to feedback regulator to keep the track of a tunneling current also precisely positioning the tip. So, therefore, we need several electronics for such a kind of device to run and this all become possible only when the technology had advanced sufficiently. So, give you some examples and I am just taking example from the literature because we have personally I must I have not worked on the STM and some of our colleagues has but the images from the books are much better let us look at that this is actually taken from a solid state physics book by Charles Kittel which is a standard book you can see this image available in backer white contrast this is actually platinum atoms on a 111 plane what you see is that hexagonal array of atoms 1, 2, 3, 4, 5, 6 this is an hexagonal so 1 on 1 plane a surface will be containing this kind of attenuation this is central atom and this goes on repeating. So, what you can see here this just goes on there is another one here like this. So, it gives us very good not even that these are actually images of atoms looks like but they are not as you can understand they are not actually images of the atoms they this is basically plot of a tunneling current as a function of the space. So, this side suppose this is at a higher height so this will give us very small high tunneling current that is why it looks like a dark on the other hand these surfaces which are a deeper side which are looking like bright and what again the groups here looking like also darker, darker means very black color so they lead to very small amount of tunneling current. So, that is how we can actually generate images to show you another one this is for the nickel surface see there is a distinct difference between nickel surface 1 1 1 surface nickel 1 1 1 and the platinum 1 1 1 in a platinum 1 1 you could see big atoms filling the whole space here you can see large gap between the rows of the atoms this is these are the rows of the atoms and this is again obtained from the same group meaning meaning and the steward which they have done on when they are working in IBM labs. So, these are the tips of the these are the positions which are close to tip of the scanning tunneling microscope that is why they are looking like very bright and blue on the other hand this surface is looking darker because they are at an angle and this is again looking the brighter depends on the obviously how the surface is aligned with respect to tip the surface are inclined they are not flat really at even at any scale surface will not be perfectly flat because the atoms atom reconstructions takes place on the surface always to give you another example which is from very recent work this is one of my co-leavertors professor A. P. S. I. from toque university from quasicrystal surfaces. So, we know that quasicrystals are very you know new and very exotic materials people think about it also that even normal price for the discovery of quasicrystal was awarded this year in last year actually 2011 to professor Dan Sedman for discovery of that. So, this has become a big aspect of research nowadays this is a scanning trans tending microscope image of the icosaddle L, P, D, M and quasicrystals quasicrystals all are multiple element alloys. So, what you see here is that five atom clusters here 1 2 3 4 5 or ten atom cluster like this you can see on the surface there are many such clusters here one there one many many such clusters which are deemed to be present on this quasicrystal material this is the atomic azimuth we really cannot say which atom is what but we can really say that on the surface of the quasicrystal also the same atomic azimuth is observed as in the bulk. So, one can actually see it by doing the STM remember to do this STM analysis we need to clean the surface. So, any aluminum alloy will have very thin layer of aluminoxide presence. So, these samples are taken in the alta high vacuum and then they are spattered using the argon gas to remove the whatever oxygen atom presence and this then the virgin surfaces created which was then probed by the STM and it was taken. So, therefore, STM is obviously very costly equipment because you not only need a vacuum system and other things but also you need sputtering devices also cause to study different metallic samples and as I told you mostly the conduct sample can be studying. So, metallic samples looks to be ideal for these conducting samples that is what they are done. Now, let me go to the next the surface probe technique that is called atomic force microscopy. As I told you the scanning tiling microscopic technique has its own problem like it cannot be used for non conducting samples. It cannot be used for liquids or it cannot be used in normal atmosphere because you need to have tiling of currents that requires tiling of electron that requires vacuum system to be very good and whole system to be kept under vacuum. So, therefore, these limitations lead to the discovery of another microscopic surface microscopic technique called atomic force microscopy and this is again based on the principle that if you have a very fine tip attached with a cantilever beam and whenever this tip is bought close to the sample surface. So, there will be atomic force acting on the tip because of the atoms which are present in the sample surface and depending on the force they are actually there can be repulsive or attractive forces which I will tell you depending on the distance between tip and the and the sample surface. So, depending on these forces the tip can be going down or going up. So, if you measure the tip position by using a laser beam that is if I have a laser beam falling on the tip surface and reflected back on a 4 quadrant photo detector then I can precisely determine the tip height as it scans on the sample surface and then plot it just like its STM and we can get a better image. Remember this does not require any vacuum system this does not require you know the sample to be conducting because it depends on the atomic forces and this kind of atomic forces between the tip atoms of the tip that is this red color atoms and the sample atoms they depend on the what is called distance between the tip and the atom it does not depend on whether sample is conducting non conducting insulating nothing. So, this is the basically the idea now I will discuss you in detail how does it work. Well AFM brings a probe probe very close to the close possible surface that is the tip and this force it then is detected by deflecting spring actually it is not a deflecting it is a cantilever you can see this is a big cantilever beam which is attached to the microscope and this force is detected now forces between the probe tip that is this tip and the sample are sensed to control the distance between the tip and the sample. So, as you know if you have two atoms suppose the force buses distance between the atom can be plotted like this and this is how the force buses distance curve vary. So, whenever the atoms are very close to each other there will be repulsive forces whenever atoms are far apart from each other there will be attractive forces all you know that. So, therefore depending on the closeness of the tip to sample surfaces the forces between the atoms of the tip and the sample will vary whether repulsive type or may be attractive type and this can be used to operate the this kind of microscope AFM in different modes the one of the mode is called contact mode other one is called non-contact mode. So, we will discuss one by one this contact mode and non-contact mode but I hope you have understood what is the basic mechanism basic mechanism is the forces between the atoms of the surface and the tip atom this tips. So, thus those are the probes now before that let me just tell you how it is it is again discovered by Binnie Gower and the quarter at Stanford you remember after discovery of STM Binnie moved to the Stanford University and it requires a cantilever beam a tip sample surface laser beam and obviously you need a photo detector all of this I have shown you at the first slide and the tips actually can be different type this is a normal tip as you see it is just like a tetrahedron or you can have a super tip okay one tip and another one is attached to that or you can have a ultra-laver tip which is three micontall and these are all obtained from J Paul lever from Caltech and all this you means available in this website you can see that. So, tip manufacturing is a big thing it is normally done using a very complex process like dipping electrode and looking at in the microscope subsequently every time you dip it some all amount of the material is getting corroded this kind of from this kind of tips and that is how we can find it up to the atomic level remember the tip is very fine of the order of few maybe one nanometer or less so that there are very few number of atoms present the tip well as I said there are two modes of operation one is called contact other is called non-contact contact mode is caused by the repulsive and non-contact mode called as attractive force is here we clear from this plot force process distance whenever two atoms are bought close to each other and as we know that the force process distance if the atoms are even impinging on each other will be very high and then this course comes down and then there is optimum distance where force is the minimum and then if we move the atom far apart from each other again force increases and it becomes constant after some time and remains there and obviously up to certain distance there will be no force of attraction repulsion whatever between the two atoms that is how the force is vary and this basically called as this can be obtained from any physics or chemistry text book you can see that and so therefore whenever the probe or the tip is very close to sample surface the distance are very short the forces will be repulsive in nature as you can see here repulsive in nature or whenever the distance will be large the force will be attractive in after nature and K team cantilever can be used to both measure the both attractive force or the repulsive force in different modes so if we measure the repulsive force is called a contact mode we measure the repulsive force is called non-contact mode this is also known as tapping mode I will discuss in detail. So in a contact mode where the short range interaction of between the inter dynamic forces are very important tip is normally 5 to 20 nanometer diameter radius and 10 to 15, 25 minimum micron heart okay and the cantilever is approximately 50 to 400 micron long and cantilevers are normally very low stiffness because they needs to be you know going up and down deform actually but it cannot deform the sample surface because it fits the sample surface in contact mode sample surface will get deform and then you do not actually detect exactly what is that sample surface tip some can scan the surface either the tip of the specimen can be moved by a piezoelectric poisoning system and detector system can measure the reflection of the tip as its contacts make a contact of the sample surface because you are bringing the tip to the very close sample surface so there will be contact and then it will be deflected and the deflected can be measured by a leather beam. So as I said here also there are two ways of doing so one is called constant force that means if you suppose one to have the constant force between the sample surface and the tip to be maintained then the there will be feedback loop which will keep this constant force between the sample and the tip but to keep this constant force the sample tip has to deflect more when the distance is higher between the tip and the sample surface but distance is smaller that attractive force will be sorry whenever distance are the attack forces the pulse force will be a little less and when the distance is close the pulse force will be high so depending on that you can detect the tip reflection and in the jet directions and then plot it later on to get an image or you can actually have a constant height so no feedback system is really used when the sample roughness is small, higher scan rate is possible here also and in this case the tip is kept as constant height from sample surface and force actually varies and that force can lead to the reflection of the tip and that can be measured to plot. Well in attractive mode that is called non-conduct mode is called also tapping mode is used for the interact normally attractive forces to interact a sample with the tip it operates within the Van der Waal ready of the atoms you know there is a Van der Waal force of attraction and this attraction happens when the ready of the atom is within the Van der Waal force of attraction limits and in this case actually the cantilever resonance, cantilever actually oscillates near the resonance frequency normally 200 kilowatts to improve this resonance basically if I have a tip like this I just keep a tap so that is resonance or it is do like this and once it does it it measure the long range forces that is why it is called a tapping mode it has many advantages in the contact mode no lateral forces will act on this on the tip because whenever tip is very close to sample surface there will be lateral forces which can act also other than the this vertical forces here there will be no lateral force but distance is higher this is non-conduct so non-destructive so it does not lead to any contamination of the sample it does not lead to deformation of the sample also so this is basically suitable for all kinds of soft material and the other mode which is contact mode is basically suitable for the hard materials because even if the tip hits the sample surface there will be no deformation activities on the sample surface which will not change the material surface or the chemical surface because of contamination or the change on these atoms on the sample surface. So the attractive force is basically as I just now said is basically depends on the van der Waal forces that is so distance should be maintained such that the van der Waal forces ready of the atoms actually becomes comparable with the tip distance. Now how is the force actually vary let me just tell you in a simple slide like this so how do we actually measure as you know in AFM actually one can measure the force was a distance plot also so this is the sample position this is a force can deliver force and these are the different position A B C D E and I am showing you the position of the tip as you see tip is close to the coming close to the sample surface at position A but not at course so therefore the force is very constant and whenever tip is touching the sample surface or very close sample surface tip gets point so the force started dropping or rather it will be start is taking on this curve which is the rising curve and a sample at position C force is very high and whenever the this is what is happen is the engagement whenever tip comes down and basically hits the sample surface if you want retraction that is disengagement from the sample surface then again it will be going C D E say E D C B A so this is the position C which is that is exactly at the contact point D is basically it has again gone down much so you gone up so the force has decreased E as it has decreased A means it has got up and this is the set point here so therefore depending on the kind of disengagement the engagement the forces on the cantilever beam varies how the forces are measured well this obviously is very important aspect suppose this is the X Y Z scanner and this is the sample that means sample scans on the tip doesn't move a sample scans and as the tips basically goes down the sample surface as this is tip trajectory the forces acting on the tip will be varying so therefore that deflection on the tip will also vary and if you have a laser beam following the sample tip surface and then depending on the position of the tip that it is deflected more or less because of attraction or whatever force attractive repulsive force on is with the sample surface this the laser beam will precisely detect the sample deflection on the of the cantilever tip and this will go back to the sensor actually consists to a four quadrant system is just like this A B C D so if the laser beam hits A B C D any positions it can be detected very precisely so cantilever beam is designed with a very low spring constant as I said so that is easy to bend and very sensitive with the force and laser is focused to reflect this of the cantilever beam there is focus on the cantilever surface and so that it can be reflected back the sensor and position of the beam the sensor measure the reflection of the cantilever beam and this it turn can give us the value of the force obviously this has to be doing the rastering just like ACM or STM and these are the images taken from nano device compression which which are making this this instruments another one is taken from stephanie rose from biophysics department of Boston University in Germany and as you see here the as this is a tip cantilever and there is a tip which I told you that I will show you and here you have seen the same thing this is a tip this is a cantilever is a tip and this is attached to the microscope and laser beam falls on the tip and as the tip deflects more it it can sense the positions of the tip by using that and this is how it is done and tip rub make a rastering here just like this which is shown here as this raster on the sample surface and every time it position can be detected by using a CNC device or physical device rather and the tip position can also be detected by this if you store all the data in a computer that around you can plot the position of the deflection of the tip as a function of the distance or the function of the special coordinates and get the image this is used many purposes like digital image for the topography surface or this can be used to determine the roughness of a surface sample or to measure the thickness of a crystal growth layer in fact this is mostly done to measure the topography sample surface this can be done to the level of atomic resolution that is Armstrong level. So all the actually people are able to get resolution of the order of one Armstrong so you can see the atoms of the sample surface which I have shown you which I am going to show you also in this and I have shown you in case of STM. Now this can be also used for non-connected surface like proteins and DNA as I showed you this can also be used to study dynamical behavior of living and free shells remember this is not exhaustive applications are keep on coming as the technique is getting used more and more so therefore if you want to really know about the real application one is to look at the literature available in the different journals topography is obviously done this is again right side of this topography image 2.5 to 2.5 nanometer cross this is 2.5 nanometer 2.5 nanometer distance of pyrolytic graphene graphite in fact you know the graphine is basically graphine is basically prepared for pyrolytic graphite graphite this is graphite and bumps are represent topographic atomic configuration these are the ones while coloring reflects the lateral forces of the tip okay there is a coloring scheme these are the force varies and scan direction was right to left this is the way this is taken again from the Nabaleses laboratories of US they have obtained this images. So this can be used for highly imaging for the contact mode contact mode actually lead to very good resolution image but it can damage and also can measure basically friction forces also a non contact mode mode you have a lower resolutions but no resistant to the sample tapping mode as I said the tip can be tapped the better resolution but minimal damage to the sample surface again to show you some of the images which we have taken recently for initial crystals which were basically ball mill to see the defect structure on the sample surface these are not to the atomic resolution images but will really tell us the images on the sample surface okay. So as you see here this is the initial initial sample with the clear steps which can begin seen on a topographic plot if you deform it by ball milling 4 hours and 8 hours you can 4 hours you can see the steps created on the sample surface damages are done in 8 hours even the surface steps are much finer and you can even come you can see this this kind of small and dilution cater sample surfaces important one is a laser confocal microscopy but let me tell you the basic principle first all of we know that the optical microscope resolution is basically governed by the wavelength wavelength of the light and as you know the that is resolution is normally obtained to about 300 nanometers and it is very difficult to go down because the optical microscope uses light normal light but in near field scanning optical microscope we can beat the resolution limit and not only that we can even scan the surfaces we can look at the internal surfaces of the many things in fact in the medical or in the biological research one can image the inside structure of artery or vein by using these microscopes and for material science we are learning it how to use it I will show you some example how it can be used or there are lot of you know potential for this technique to be used for the materials and applications this technique are actually is called NOSOM near field scanning optical microscope is we use a sub wavelength aperture okay normally we as I told you in a transmissive electron microscope or even the other microscopy techniques that scanning transmissive you are scanning transmissive electron microscope also use apertures and aperture is nothing but a plate in a small hole is present and it can be used to select a particular beam in a transmissive electron microscope in this case we use a sub wavelength aperture and this is about 200 to 20 nanometers is the diameter and this can be placed very close proximity to the sample surface that is the actually challenge actually it can be placed very close means of the order of 10 some nanometer like 10 20 nanometers and then if you allow the light to pass through this aperture light passing through the aperture will remain collimated because the very small size for the distance of the order of one aperture diameter aperture diameter is of the order of 20 to 200 nanometers and aperture diameter is maintained in near field position and then it is scanned on sample surface so image can be reconstructed point by point with a special resolution limited by the aperture not by the wavelength of the light because aperture is smaller than the wavelength of the light here aperture normally then in the light of microscope optical microscopy wavelength of the light is a couple of 100 nanometers that is 300 to 400 nanometers but here aperture is less than the size of that so if you keep the aperture in the near field region that is very near to the close to the sample surface the actually the resolution of these microscopes will depend on the aperture size not on the wavelength that is how we beat the resolution determined by the wavelength of the light so this has increased this will lead to the rapid change of the you know study of the different kind of material by using the optical microscopy just by using a near field aperture I hope this is clear from this picture so what is here actually you have a taper aperture as you see here which is kept close to the sample surface and then this is a poly crystalline sample obviously and you pass a light beam to that and then this aperture can be actually scanned over the sample surface and if you scan the aperture light beam also be scanning the sample surface and then this is the constructed image which is diffraction limited diffraction limited resolution image basically and this can be obtained in a far field lens and then plotted now these are all done digitally obviously because one can use cause reconstruct the image using computers this has been possible because of one major discovery so as you see here in the earlier picture this tip at the tapered of the tip of the NOSM the upper tip of the NOSM is basically what is dictates resolution so this was done by developed by tapered optical fiber probe was developed by Betis and Trotman in 1991 and tapered optical fibers are fabricated from single mode optical fiber using a commercially available micro pipette puller with a focused carbon dioxide laser as a heat source this aperture is formed by coating the tapered fiber with a high reflectivity material that is aluminum or silver via standard thermal operation so you can see there actually this is the AG coated on a tapered optical fiber and AG actually increase reflectivity reflectivity should be very high because using optical light and then this is actually high numerical aperture optical microscope objective so this is detect whatever is reconstructed after this light beam passes through the sample surface and that is what is done in a near field optical microscope so instead of a light one can use a laser beam as you can understand there are optical lasers available like laser beams which are common in optical web lens that can be used that is why it is called laser confocal microscopy and because the aperture is kept the near field that is why it is called confocal and this is how it is done really there is a laser beam which comes with fiber optics and then this is the real aperture which is tapered one sample and there is a collection optics and there is a detection optics as with STM probe is this is also raster using a piezoelectric device this is means this probe is nothing but tapered optical fiber that is raster using a piezoelectric device because that movement needs to be controlled very precisely to give you some idea how it can be used this are all taken from different literature this is again from professor Dr. Hans Jugen but from the MPIP this is Max Planck Institute NIMS joint international lab this is basically ironing liquid micro drop stained with fluorescent dye red color staying on a soft PDMS surface which is a green color you can clearly see the clarity of the image in when one can see the tapered nature here of this of the liquid drop that is actually increases our capability to image even small liquid drops present on the surfaces of any material. Another example of this is corrosion surface which is a very new this is published just few months back by Sanchez Stover et al in corrosion science and as you see is that this is actually copper surface at 75 degrees after the certain kind of corrosion test called ZRA and you can see in fact that the corroded regions in sample surface the black color regions are the sample surface remember this is an optical microscopic image using confocal microscope it is not a scanning electron microscopic image so the clarity and the this is actually the micron distances 40 56 and there is a large scale and the resolution is much better than the normal optical microscopic image so one can actually probe this kind of effects and as I said in the lecture that the new new applications are coming into picture as people started learning this techniques more and more so one can use it more and more to compare this three different techniques FM STM and the NSOM I like to tell you three things in all these three techniques a resolution is largely dependent on the probe size okay that is the tip size and in case of FM STM the tip size in case of NSOM this is the aperture size STM requires a conductive specimen FM NSOM does not require do not require and both of this can be used in air vacuum or liquids but STM needs to be used only in vacuum FM physically contact the specimen by contact mode but STM and NSOM does not so AFM in the contact mode can damage the specimen surface contaminate but STM and NSOM will not well there are many other techniques in this world as gamma pulse so I will just tell you several ones but I will not discuss each of these techniques first one is lateral force microscopy is called LFM in this case fictional forces can be measured by twisting or sideways forces on the cantilever again it is basically a type of AFM you can also a magnetic force microscopy MFM and this is again popular for the magnetic material if ever tip is magnetic it detects the magnetic fields or measure the magnetic properties of the sample or you can also have a electric force microscopy is called EFM in which electrically charged PT tip the platinum tip detects the electric fields or measure the dielectric and the electro static properties of the sample in fact people can do chemical force microscopy also the chemical chemically functionalized tip in AFM can interact to the molecular sample surface giving information about the bond stems and I just discussed about the near field optical microscopy I discussed about one of the techniques like laser confocal there are many others available so in this technique also probe technique in which small apertures are scanned molecular sample and probe is a quartz fiber it is made the discovery made in 1991 pulled to a sharp point and coated with aluminum or silver to give a soluble length aperture and there are many such interesting application for each of techniques available which one can actually get from literature.