[Music] hello my name is ashitosh and i am here to talk about a very popular technique in atomic force microscope which is called as electrostatic force microscope or efn and this talk will provide you an overview on how to get information about electric properties of your sample surfaces using efm so in efm the electrostatic force between the tip and the sample is measured and this electrostatic force on the tip is because of the tipped sample capacitance generally a voltage is applied between the tip and the sample and the tip hovers few nanometers above the sample
surface to give you a map of the electric properties of the sample surface this right hand side image depicts the interaction forces on the tip and generally we find that these interaction forces are both long range as well as short range forces since we are talking about electrostatic force we find that this electrostatic force come come under this long range interaction force and uh it comes under this non-con technology of this interaction force versus the tip sample distance graph there are basically two main approaches you know using which we can perform an efm experiment the
first approach exploits the tip distance dependence of electrostatic and van der waals force by employing something called as the dual pass lift mode technique so generally uh what we find is that whenever the tip is very close to the sample surface you know the van der waals forces are dominant however uh you know when the tip is lifted to a certain height then the van der waals force is naturally decreased and the electrostatic force uh in this case uh become become the dominant force so what we generally do in this dual pass lift mode technique
is that um we perform two passes so it's the first pass scan and the second pass scan in the first pass the tip scans the sample surface to reveal the topography where the van der waals forces are dominant the tip is then lifted let's say you know 50 nanometers or so uh away from the sample surface all right where the van der waals forces are negligible now and a second pass now is performed along the same topography lines to detect the electric properties of the sample surface at every point along the sample so if you
take a look at this figure then this black line reveals the topography of the sample and this is generally done in park's true non-contact mode then the tip is lifted to a certain height and then this second pass is performed along this topographical lines to reveal the electric properties of the sample surface where the electrostatic force are dominant then there is a second approach or a second technique which is called as a single pass technique which uses a different frequency for detection of the electrostatic force um with respect to your you know topographical imaging in
this case the van der waals forces and the electrostatic forces are decoupled by using an additional lock-in amplifier you know for the efm signal [Music] so what happens in this single pass method is that the tip scans the surface of the sample right by you know at a resonant frequency which is omega not and then this is nothing but it's the cantilever resonant frequency omega naught and this surface to cool off is uh generally uh mapped you know using this first lock-in amplifier okay which uh locks in the surface topographic signals at this cantilever regional
frequency omega naught in park's true non-contact mode at the same time an ac bias okay at the same time an ac bias with a frequency omega tip okay is applied to the cantilever or the tip along with the dc bias so what happens here is that you know this second lock-in amplifier now comes into picture and the second of local amplifier then decouples the motion of the tip and separates the electrostatic signal from the topographical signal and this is done at the um you know frequency which is given by omega tip or it is the
frequency at which you apply your ac sinusoidal voltage so if you take a look at you know this uh this schematic diagram then uh in this figure what we find is that you know there is a single pass that is performed and in this single pass both the topography which is locked in at this cantilever resonant frequency omega naught plus the efm signal which is locked in at a you know frequency which is given by omega tip okay is uh you know both these signals are then captured at the same time this single pass method
is superior to your lift pass is superior to your lift mode technique because you know this method gives you high spatial resolution and it also requires less time for scanning this is a schematic diagram of the entire setup or or the experimental setup for the dual pass frequency efm so here we can clearly see that you know there is uh this first lock-in amplifier which locks in at this cantilever resonant frequency omega naught all right and and this feedback loop you know then provides you or reveals the topography of your sample surface you know then
we apply this ac voltage and this dc bounce right and what happens here is that now there is a second lock-in amplifier which locks in at a frequency omega tip which is the frequency of your ac signature voltage and then this reveals the or it you know basically decouples the electrostatic force or electrostatic signal from your topographical signal and it reveals the map of your electric properties of the sample surface now generally what we find is that mathematically the electrostatic force is given by this equation number one and whenever we talk about um you know
electrostatic force uh we need to really uh you know see which voltages are involved and generally we find that uh you know you have the surface potential voltage which is given by vs uh plus you apply a dc voltage plus you apply a sinusoidal ac voltage to the to the tip okay now if you take a look at this equation number one what we can do is we can expand this equation and when we expand this equation these three terms right pop up as we expand this equation number one now all these three terms have
got their own importance and what we can do is you know very briefly we can see each of this term in efm what we can do is we can you know let's say just apply a dc voltage to the sample or to the tip and then the electrostatic force that is measured you know is is given by this equation number two which is nothing but the first term that uh popped up you know when you expand equation number one now uh since you know we apply only dc voltage to the tip what we find is
that you know this particular electrostatic force that we try to map uh okay it it it is only measured you know with respect to the static bending of the cantilever and generally you know this kind of measurement derives a qualitative map of the uh you know sample potential or the surface potential of your sample surface one example that is that is shown here is a pt coated nano wires so on the left hand side you have the height image and on the right hand side you have the efm amplitude image if you happen to take
a look at the height image then what we find is that you know these uh nanowires they are not much distinguishable uh you know in in your height image so if you happen to take a look at this region right here then you actually cannot see any nano wire but if you apply a you know a dc bias to such a sample then what we can really find is that now these nano wires or or these pt coated nanowires they have got higher surface charging okay and they are clearly visible in your efm amplitude image
okay um you know as compared to your height image of that particular sample so here you find that you know these these nanowires are now clearly visible which were not visible in your height image uh there is this another example uh which is given by this standard sample so this blue electrodes are nothing but you know these gold golden electrodes or these golden patterns which are which are you know which are on the on your uh silicon oxide surface or substrate um so these blue uh patterns are your grounded lines right and the opposite electrodes
are your biasing lines now if you happen to take the height image of this particular you know of these particular gold golden electrodes right then the height image is given by the top figure right here where you have these alternate ground and bias lines the same can be revealed in your line profile where this ground lines and bias lines are clearly visible and clearly distinguishing however what we can do here is that we can apply a sample bias of let's say around 0.5 volts in this case um right here and [Music] then we find that
you know the efm signals between the grounded and the biased electors are clearly distinguishable okay when you take a look at the efm amplitude image and the efm phase image so this middle image right here is your efm amplitude image and the bottom image is your ef and phase image where you can clearly find out uh you know the bias lines okay as well as the grounded lines in case of you know this particular experiment which was performed in the efm there is another very important mode which is called as kelvin probe microscopy and this
kermit microscopy is governed by this equation number three so whenever you know you you take a look at or expand that equation number one then the second term which pops out you know is given by this equation number three in this particular case which we which we use it for kelvin pro microscopy what we really find here is that the electrostatic force in this case is mapped at a frequency which is given by omega tip and this frequency omega tip is nothing but it is the frequency of your ac sinusoidal voltage uh which was applied
to your tip and what we do is you know there is a second lock-in amplifier which locks in at this particular frequency omega tip to to reveal the electrostatic force now what is done additionally in this case is that you know if you happen to take a look at equation number three then we have this term which is b dc minus vs now in this case if this bias voltage vdc becomes equal to your surface potential then the electrostatic force you know goes down to zero okay so whenever we perform this kelvin probe kelvin probe
microscopy experiment then what we do is at every point or at you know every pixel of your sample surface or the image what we try to do is we try to use a dc bias feedback right so in this case what we do is we sweep this dc bias in such a way that you know this dc bias becomes equal to your surface potential and your electrostatic force at the frequency omega tip goes down to zero and this is what we want so you know whenever the force you know goes down to 0 we map
that dc bias at every point or at every pixel of your image and then you know you you get a complete image where you have the dc bias that had been applied where the electrostatic force went down to zero and that entire map of this vdc is then nothing but it is going to be a map of your actual surface potential of your sample surface so you know just by you know sweeping this dc bias we are in a position to map the or map quantitatively what is the surface potential of your sample kelvin probe
microscope you know it it not only measures you know quantitatively what is the surface potential but you know it also can give you uh the absolute values for the work function of your sample surface as well there is a nice example where we we use a hopg surface and the the top image is the height image of your hopg and the bottom image is your surface potential image of that particular sample so what we do in this particular case is that you know while uh while we are scanning the hopg surface what we do is
we apply step wise sample bias of 1 volt 2 volt 3 volt 0 volt and minus 2 volt now by applying these bias it doesn't really uh you know display any change to the height image but as you sweep or you know change this sample bias this particular change of the sample buttons or or on the surface potential you know gets reflected in your kpm channel itself and it then reveals uh you know how uh you know how the sample bias was changed and you know what is the uh sample potential uh at every point
or at every pixel of that particular image now in this case you know since you know initially we applied one volt to the sample surface or the sample bias then you know this kp m signal or kpm channel actually gave you that one volt which is revealed in this line profile so if you take the line profile then it actually removes that one volt then the moment you change it to two volt the kpm channel you know revealed that two volt as well as your surface potential and then you uh then as we changed uh
it to three volts uh the kpm channel you know reflects that three volts uh in its image so on and so forth so kpm you know not only uh you know gives you uh how the surface potential looks like but it actually quantitatively gives you what is the surface potential of your sample this is the schematic diagram for the kpfm um experiment that we do so in this case um you know the first lock-in amplifier locks at a frequency of cantilever resonance frequency omega naught and the feedback loop then reveals the topography of the sample
uh now what we do is we apply ac bias with the frequency omega tip along with the dc voltage and then what we can do is we can lock in at a frequency of omega tip using the second document amplifier to reveal what is the surface potential map quantitatively now what we can also do in the experiment is that now using the second locking amplifier we can actually lock in at a frequency of two omega and uh you know the moment you're locking at a frequency of two omega uh you are in a position to
actually map the uh dc by dz variation of your sample surface and this comes into picture you know when you take a look at the third term that that you know that that popped out when you expanded that equation number one now in this case what we can do is uh we can lock in at a frequency of two omega and we find that you know uh in this case this force f2 omega the dc dependence is actually removed and the measurement yields the value of tc by dz map which is independent of the surface
potential given by equation number four from this dc by dz map what we can do is we can actually try to derive what is the dialectic constant of the sample itself here is one example where you know on the left hand side you you find that you know there is this e coli bacteria and this is the cross sectional height profile of this e coli bacteria and then on the right hand side what we can see is we can see the derived dynamic constant map of that same e coli bacteria this you can you know
achieve by simulations uh and by using simple analytical modems you know where from the dc by digital map you are in a position to derive the dielectric constant map of your sample surface so in summary uh i would like to conclude that electrostatic force microscopy is a very popular technique and it operates in electrical mode in afm to give you the maps of variations of electric properties of the sample particularly the surface potential and the surface charge distribution of the sample surface thank you very much [Music]
