 So in the last class we have started discussing about the advanced surface characterization techniques and I have introduced you the subject given different examples from our own studies showing the importance of the surface characterization techniques from the perspectives of material science engineering. So we need to now go into details of each of these techniques the first one which will discuss is X-ray photoelectron spectroscopy or what is known as XPS in the literature. Many of these slides which I am going to show you can be actually seen in this first book which is by Terry Burr published in 1994 but in the encyclopedia also you can get lot of information first one is what is XPS XPS as I said it is known as X-ray photoelectron spectroscopy that means you have X-ray source and photoelectron as the output that is very clear is it not. So what is actually done as you know photoelectric effect was discovered long back almost like 90 years back in by Salbert Einstein and he received a Nobel Prize because of the discovery in the photoelectric effect normally if a photon of certain energy suppose H new falls on a photoelectrically sensitive material the photo electricity is generated because of the production of electrons that is what is shown in the first slide you see there is a light source with energy of the photon quanta which is coming as energy of H new and this can actually eject out electrons from the photoactive elements and that can produce electricity fine so that same thing can be used to make photoelectron spectroscopy per source which can be used in the photoelectron spectroscopy is not the photon but an X-ray. So this was done by K Sackman and he received Nobel Prize after 60 years of Einstein's discovery both of these two scientists are from German and Sackman received Nobel Prize in 1981 for the discovery of this so what is done here you have X-ray photon coming from X-ray source it can be any actual X-ray source like copper or nickel or chromium or iron and this X-ray photon falls on a certain suppose molecule here is an oxygen or certain atom actually oxygen atom and then because of its high energy its ejects one electron from the inner cell that is one a cell of the oxygen atom and this electron comes out this electron is basically oxygen one in electron photo is photoelectrically ejected electron and we can use this electron to study different kinds of you know behavior different kinds of aspects of these spectroscopy that is what is the basic principle of the technique in a nutshell what I can say you. So and as I said in the last class also this is known as ESCA electron spectroscopy for chemical analysis but it has different acronyms like XPS except photoelectron spectroscopy or UPS if you use ultraviolet as a source of energy ultraviolet photoelectron spectroscopy or you can use a photo light like photo so which can lead to emission of electrons then it is called a photo emission spectroscopy but we are going to discuss these two mainly so we are not going to discuss what will happen if you use ultraviolet or photon light actually and then produce spectroscopy well what is actually the process what is actually the analytical methods let us show it in terms of energy diagrams what I am showing here is basically energy levels of certain atom you could see 1s 2s 2p 3s like that so 1s has two electrons 2s has two electrons and then 2p has six electrons in the shells so then you have 3s which is unfilled and then your balance band after that you have a Fermi level so and below Fermi level actually all the specified levels of the energy levels are occupied by atoms electron sorry and then you have ev at a higher level then basically if by difference of 5 5 is known as work function as you know that from the Einstein's theory so this is what is called kinetic energy this is what is called binding energy now if a photoelectrons or x3 actually comes at false on this metric atom and it rejects suppose all the electrons in 2p shell and this electrons then has sufficient kinetic energy to travel to this balance band and may even pass the Fermi level and come out this is known as photoelectron so we can calculate the kinetic energy photoelectron as h nu which is the incident energy of the photon minus eb eb is basically the binding energy plus 5 5 is the work function so this is work function you know and eb is basically binding energy that is how we denote so that means the binding energy is the energy with which this electron is bound to the nucleus so that means excess XPS spectrum basically can always plot the energy or the intensity of the photoelectrons I versus either the binding energy that is eb or the kinetic energy they are related as you can see 5 is that is work function for a material is well known h nu is the energy of the incident x-ray that is also well known so therefore the two things which can vary is the ke or eb depending on which atom which electron you are rejecting out so that is why the XPS plots basically will be intensity versus either eb that is the binding energy or kinetic energy that is how the plots are made now what all we can do with this this is a basically the principle which I told you that is how things are actually happening inside a material and as you know why it is called surface characterization tool as you know this electron photoelectron which is coming out for the sample will have energies I know something kind of energy not very high so therefore if it is ejected deep inside the sample then this electron may not be able to come out to a sample surface and we cannot analyze that is why we use surface actually characterization this state is used for surface characterization tool now what all we can do we can do elemental identification obviously because we know what is the exact energy of the electron which is coming out so we can categorically say which element is present we can actually even tell chemical state of the element that is whether it is a oxidized reduced what kind of stated is why it is a neutral we can actually measure the electric composition the constant in the surface region and we can obtain the valence band structure in fact nowadays we can actually do mapping of different elements present on the surface in the surface so therefore lot of things can be done in the XPS that is why XPS is considered as a versatile tool before I get inside into the other things of XPS let me just explain a little bit about what this is RGR because RG is also used in the purpose let us see what can be done in RGR so this is what is this the first picture is basically correspond to the XPS as you see K electron case level L1 L2 3 M then the family level then the vacuum level that is EV and these two are differentiated by work function okay so now it makes a false and eject electron comes out this whole thing is known as XPS right now in this case of the last picture the last basically schematic diagram what is happening if you see if this becomes vacant by ejecting the electrons out by the x-rays and higher level electron higher level means suppose at L1 level electron can fall back into the K level and thus creating releasing certain energy when this comes to this some energy released and this energy then can be used to knock out an electron at a higher much higher energy level that is L2 3 this electron will have sufficient little bit of kinetic energy not as high as the XPS electron X-ray photo ejected electron but it will have certain energy this electron is known as Auger electron so therefore in a nutshell I can say Auger electron production is totally different from the photo except photo electrons spectroscopy here basically jump of the electron from a higher level to lower level leads to production of certain other energy h nu 1 which can basically eject and the electron from L2 3 cell and that electron comes out from the sample surface is known as Auger electrons so obviously the kinetic energy of this Auger electron is much lower than the XPS that is why Auger spectroscopy is always done from this region near to sample surface you cannot actually probe little inside the sample surface where the XPS can be used to prove the sample surface which is deeper compared to Auger so that is why XPS and Auger these are complementary tool very you know widely use remember Auger was discovered by Auger so that is why this process is named after him when we discuss this Auger spectroscopy in detail so let us not talk about this right now but for the sake of your understanding I am just going to tell you what is Auger spectroscopy now let us me go back to the XPS again in XPS binding energy reference is like done like this let us do that so this is sample they have a core electrons then your binding energy binding energy means this is what the electrons are there then this is as a formidable as I said EF and there is a work function then your vacuum and angel level okay so once they actually falls it basically crosses the binding energy then familiar well then this work function comes out that is how actually you get kind of energy of the XPS as this h nu – BF – 5 of the sample so and that is basically then you know if in a case a spectrometer what happens there is a vacuum level and there is a spectrometer work function so this needs to be you know subtracted so basically is depends the kind of energy is basically when this comes to the electron pulse on spectrometer kinetic energy becomes like this h nu – BF – 5 spec because Q sample 5 sample 5 sample basically cancelled out so finally we get binding energy is basically equal to binding energy of the electron h nu is the energy of the incident photon the x-ray photon – the kinetic energy – the 5 spectrometer now if you know this is fixed well known in a very particular spectrometer this is the fixed also because we know what is the incident energy of the x-ray so basically what is varying is this and this are these two are dependent so now we can actually calculate if I know this the kinetic energy or if I know the kinetic energy you can calculate binding energy basically what you measure is the kinetic energy of the electron and that can be used to calculate the binding energy which is presented the electron so that is how actually we can characterize the electron which is ejected out how the instrumentation is done we are going to discuss in detail later on but to show you that so what you have is basically you have a lot of different stops are there here this is the basically heart of the system you have a sample you have x-ray gun and you can have electron gun basically x-ray has always electron gun and then you have iron gun to basically you know clean the sample surface many times you can use argon iron because this is done inside a vacuum chamber so once I electron ejected this passes through this electrostatic transfer lens this is what is shown you here and then it is basically goes through a hemispherical you know portion and then falls on the electron channel electron channel electron multiplier which is nothing but basically a gate and then it is getting multiplied amplified the signal through when it passes through this multiplier and then it is recorded you know after it passes to a red meter it recorded and you get a plot between intensity versus kinetic energy or intensity versus my energy that is what is shown you here well that is actually simplest possible thing but it is not so then you have a control unit and there are many other stops related to it but this is what is done remember it is done in a very high vacuum system because electrons needs to travel through this that is why we need a very high vacuum system vacuum level should be written as minus 10 and we need also any kind of surface contamination or the surface reactions so that is why you know x-rays energies which are allowed to fall on the samples to be teller so that there will be no surface reaction or contamination form because of the x-ray falling on the sample surface well let me show you a typical x-ray x-ray spectrum I am going to show you two spectrums one from our own other ones from taken from literature as I said in the last class we have been working lots on the hydroxyapatite and hydroxyapatite metal that is titanium composites so from the surface the spectrum is taken both from hydroxyapatite and hydroxyapatite titanium composites is basically intensity versus binary energy plot intensity is in arbitrary units that is called the basically the energy the number of counts CPS sometimes we call CPS counts per second and binary energy is always written in units of electron volts so as you see we can see different peaks as usual any spectroscopy have a background and a peak okay and the most intense peak coming from both the sample is oxygen 1s for which I have already discussed with you then you have a peak coming from carbon 1s which is basically used at a standard in a in XPS because you know all samples of some kind of carbon so you are going to see these peaks so many times the spectrometers actually are basically calibrated using this peak position then because it is a hydroxyapatite hydroxyapatite formula of CA 10 PO4 O2 OH O2 okay so as you see there are phosphorus there is calcium the hydrogen hydrogen normally not detected and you have you see oxygen peaks calcium peaks is there calcium 2p 3 by 2 calcium 2p half we are going to discuss what is the meaning of that this are present both in hydroxyapatite titanium then you have phosphorus peaks P2P and P2S coming in this in this positions and all together you have basically little bit of titanium peaks here you can see carbon 1 is a carbon 2s will be present always this titanium peak is not visible here but little bit small peak is visible here so that is how we can say titanium is present but basically titanium is not present as a metallic state its presence as an oxide of state or basically oxidized state so that is actually how these XPS spectrum looks like and we can analyze different things from this you can calculate the element present the chemical state of the element present and we can calculate the percentage of the particular element present by calcule the area under the peaks we can calculate in fact the chemical shift because of the binding of the elements with any other things lot of lot of things can be done which is the part of the topics of this lecture typical XPS spectrum for indium phosphide 110 surface as you see here you see only indium and phosphorus peaks you see indium 3s indium 3 1 by 2 this is 3 P 1 by 2 this is 3 P 3 by 5 3 D 3 by 2 this is 3 D 5 by 2 okay this is 3 by 2 this is phosphorus phosphorus will college have a high binding energy is when the indium as you know phosphorus 2s phosphorus 2P phosphorus so indium 4P indium 4D so indium as 3S 3P 3D 4P and 4D phosphorus as 2S 2P peaks which is clearly detected and then your background in this that is how actually the peaks are the spectrum looks like I hope things are clear in your mind how XPS works and how the spectrum looks like because we want to use the spectrum now to give you more idea let us look at a very high atomic number elements like urinium and talk about binding energies and the adhesion constructions this is binding as it was and the constructions as you see here different kind of levels are there you have 4 P 2 spins 1 1 by 2 5 D 3 by 2 okay 5 D 3 by 2 is small also there and this is different then 5P half 5S and 5D on the other hand F levels are there also say F 5 by 2 F 7 by 2 you can see the other than constructions are very high another construction means how much I know and that electrons are created after you ionize with this X-rays very high for F electrons followed by 5 4D then 5D and 4P are there and these are very low for these electrons like 5D 5P 5P and 5D and 5S where they are very close to family level so that means the electron yield photoelectron yield by X-rays X spectroscopy will depend on this ionization cross section that means this will be higher a for these energy levels for case of I urinium then others so that means we can do such a common as this for all elements and then figure out whether this actually happens in a particular system or not well now let us get into the nitty-gitty of that as you see for PD and F peaks there are always two pictures of job here you see for gold 3 4D 3 by 2 and 4D 5 by 2 please I mean these peaks are actually comes as a doublet suppression between these two peaks are named as a spin orbital splitting the values of the spin orbital pitting of a core level of an electron in different compounds are nearly same you must remember this is very important that means values of the spin orbital splitting this is the spin orbital pitting value this is for a core level electron in different compounds are nearly same peak ratios peak array ratio that means this is the peak area for these two electrons peak array ratios of a core level of an electron element in different compounds are also nearly same these two are very important now I have not discussed what is a spin orbital splitting I am going to do that but this is what we observe and we have seen some of this you know pattern the spin out is pitting in the peak array ratios assist in any mental identification okay now let us look at what is it what is spin orbital pitting this is first to tell you how actually done you know electron has a half spin or down spin it is half minus half plus half so what we do here basically use something known as ALS coupling we do not use only S as you know the principal quantum number NLS all these things you are very aware of it so L and S S actually talks about the spin of the electrons so here you are using L and S coupling so this number J J is what is this number 3 by 2 1 by 2 is basically L plus minus S as you know S can be either plus half or minus half depending on the spin so if you have this kind of situations one up and down so one is up is called plus up down is called minus half so in this case when is up is basically J equal to L plus half if it is down with the J is equal to L minus half okay so first one the way picks and notices first is the principal quantum number 2p okay as you know 1 1s 2s 2p 3s 4p that is how the point of principal quantum number actually goes so first one first digit in a peak notation is corresponds to N at the principal quantum N second one cost point to L the magnetic quantum number that is PSDF there are four sets as you know and so thus that means S is equal to what is basically S is equal to 0p equal to 1d equal to 3f equal to 3 sorry d equal to 2f equal to 3 so that means P is equal to 1 so it is to correspond to N principal quantum of Np corresponds to what 1 then if it is 1 plus half there is an electron is up spin that is positive so it is become 1 plus half equal to 3 by 2 correct so that is what is only here when L equal to 1 that is for P cells either it is P 1 by half or P 3 by half if it is P 1 by half that means P 1 minus half one is basically L and minus half is basically down spin so that is what is that and then here this P 1 plus 1 half that is 3 by 2 so one is corresponding to the Lp equal to 1 that is this magnetic one number azimuthal quantum or non-magnetic quantum sorry azimuthal quantum number and half is basically coming from of spin so it has been found that area ratios that is what I have been discussed of these two peaks will be one is to two that means three by two will be double area having double area than the two now D I will D correspond to L equal to two right so the L equal to so that means D it will be two minus half that is D 3 by 2 where minus half is corresponding to down spin right and either wise it will be two plus half then half plus upper cost amount of spin this is how actually we are designating area ratios of these two split spin orbital splitting is 2 is to 3 and then f correspond to L equal to 3 so L will the 1 for P L 2 for L P D and L 3 for f so as you see 3 minus half will be 5 and half so that becomes down spin electron and 3 plus half will be 7 by 2 that is will have spin electron area ratios of these two speaks after splitting will be 3 is to 4 so in a nutshell this slide is that is why it is very important for you to understand in a nutshell we can actually obtain spin orbital splitting information in XPS and this is how things are designated this is how things are actually you know pics are marked in the XPS spectrum now let us look at you know how we can use this for gold gold has you know is as 4S 4P 4D 5S 4F 5F and 5P electrons so what are they you know the binding as your four election is 763 this is what is there 763 4P half that is down spin is basically 643 this is the one 4P 3 by 2 half spin it is 547 you see the area ratios are 1 is to this is 2 is to 1 then 4D you have a 3 by 2 that is down spin this D is correspond to 2 L equal to so therefore L 2 minus half is 3 by 2 this is what is that this is coming around 353 electron volts binding energy and 5 by 2 will come 335 they are very close by area ratios is about how much is about 2 is to 3 now if you look at 4F sorry 4F so 4F has 88 and 84 as a binding energies this is the one this is the one very close by 8884 so 5 by 2 is 3 minus half down spin and 7 by 2 is 3 plus half spin erase is 3 is to 4 then you have 5P others many times are not observed 5F and 5P is very 5P is observed 5 wave is very close so it is not observed but not seen now we can actually use this you know area ratios for quantitative analysis that is what has done in this case you have used aluminum as a excess also so in case of Aussie earth if you look at it the binding at this fall up of say I will not discussing here but I will discuss later is N 4 O is 1116 electron volts N5 N6 N67 where we need to discuss let us not discuss about this part right now once you come back to us I will discuss so first let us see how we can identify pics just like except the fraction here we can identify pics so what is the general method first is take pics position and relative intensities of two or more pics both in case of XPS and RGL lines of an elements takes the spin orbital splitting and area ratios of PDF first thing is the peak position relative peak in cities or two or more pics that is what you should do second thing is the spin orbital pit splitting of our area ratios so let us see the following things here it is basically the plot is showing for aluminum and silicon you see this is coming for aluminum 2S so we can actually know the peak position for TA2S what is the binding energy so binding energy chemical that aluminum 2P also silicon 2S silicon 2P but it doesn't tell about what is the spin orbital splitting okay so to do that whether is 2P 1 or 2P 3 by 2 we have one has to look into the area ratios so as you can see this is aluminum 2P so therefore this will be 2P 1 and this will be 2P 3 by 2 this will be 2P 3 by 2 this will be small one 2P 3 by half silicon is also like that one can detect the and if you scan the whole thing there are many many peaks first thing is this you can find oxygen carbon chlorine silicon nitrogen all aluminum sodium so many pics are observed so that is what the done by just taking the peak positions second thing which you must know before even going proceed further is what is sampling depth how much information is coming from what to the till what kind of you know depth of the sample so normally this is again we use the same formula like x-rays because it is a except penetration only but electrons coming out after generation from the sample surface so if we know the lambda I I means for the particular element in last this mean free path of any element in a solid we can do that so for a electric electron intensity of I0 which is created inside the sample as a depth D below the surface the intensity is attenuated according to the BS Lumber law we have already discussed BS Lumber law and you will be visual spectroscopy so intensity is which is coming on the surface is basically exponentially varies with dy lambda so with a path length of one lambda okay that is you know 63% of the electrons are scattered this is very clear when D equal to lambda is basically when D equal to lambda is basically is equal to I0 divided by E so that means 1 by E of I0 that is equal to 63% of I0 that is what is said here so therefore you must remember this formula it is I0 exponential D by lambda not lambda by D okay it is D by lambda lambda is the in a sticks mean free path of an electron in a solid that you must have knowledge fast we must have time this are actually in a state scattered electron not elastic is categorically on because these electrons are losing energies so therefore one is to know what is the mean free path of that this is all available sampling this is identified as a depth from which 95% of all photoelectrons are scattered by the time they read the surface correct so it is the depth from which 95% of all the photoelectron does scatter by the time there is the surface so more lambdas are in range of 1 2 1 3.5 nanometer for aluminum k radiation so sampling depth at 3 lambda for XPS under this condition is about 3 to 10 nanometers the spread is small about 100 Armstrong depth you cannot get more than that so if you have monolayer of 30 Armstrong okay this is how actually get this is a depth in nanometers electron energy as you see here depth is very high this is 100 this is 1000 as you say this is depth is high when you have binding at this less as obvious there will be more kind of a electron as the binding as it reduces kind of as it will be more so it will be coming out as the binding as increases it also decreases but for a very high bending at this again there is the increase that is because of the you know effect which is different from the nil assist scattering this is basically universal curve so lambda actually depends on the kinetic energy of photoelectrons and the specific elements this is that so as specific element means what type of element it is that is basically the details the binding energy so now after knowing all these things because you know sampling depth now you know how the pigs to be identified let us see how quantity analysis can be done now in XPS instrument measurements are accurate to the level of plus minus 10% it is not very accurate error is approximately 10% the intensity which is coming out of sample surface is a function of ni sigma i lambda i k what is ni ni is the average atomic concentration of element i in the surface under analysis that is what actually want to measure I the intensity of electrons peak can be measured very easily ni is the average atomic concentration that is our variable or that is what is actually want to measure so it will sigma is what photoelectron cross section basically is called scope scofill factor expressed by any peak position P lambda I have already discussed is the analysis mean prepath of photoelectron and case all the other factors dumpting like for quantity assessment you know eds how many other factors dumpting in edx actually there are factors like you know atomic number difference the flow sense all these factors are dumpting in a particular parameter same way k has all the other factors so basically frankly speaking intensity depends on the concentration of the element photoelectron cross-section of the element and the elastic mean prepath of the photoelectron these two factors k is normally constant for any experiment so as if you know if this is measurable if you know these two parameters we can calculate ni that is what is the quantification time now how to measure this I that is what actually errors comes in and that is actually propagates into the analysis so if you look at this intensity by secantic energy part peaks shape is like this this is not a typical x-ray diffraction peaks little bit distorted so one has to remove the background to calculate peak height if you fit the background like this like this one this is wrong if you fit the background like this this is also not best the best one is to fit the background like this that means background fitting is very important so the standard measure on this some instrument or full expression about the accuracy can be done if you have a standard but reproducibility is always very high here for 2% so vast cases in this measurement is this best one is this these are the middle that is why actually the human error comes into picture because depends it depends on how the measurement of the area depends on how you perceive the peak second important thing which you want is to consider for measurements of this quantum element is called transmission function which is basically to the detector efficiency so this is nothing but detection efficiency of an electron energy analyzer which is a function of electron energy transmission function depends on parameters of the electron energy analyzer such as pass energy to give you an idea if I pass energy of 8 electron 80 electron volts and this is the you know the energies of the beam current and this is done for gold after sputtering as you see the peak peak height here is this much but when you have pass energy of 20 electron volts peak height is this much so that means the peak height or the area on the peaks get affected by the pass energy pass energy basically the energy which is used in the detector to energize to get to the electrons can be detected very easily so this is what is basically dependent on the machine so what you must have a knowledge about that normally solve very you know fixed quantity for a particular setup but one must know what is pass energy how it can be controlled to get a good peak position peak of that now what is school field cross section factor if this can be calculated for each element from a scattering theory basically basically for you know basically for aluminum and magnesium cal for adhesion and similarly lambda i is varies with the kinetic energy of a photo electron it can be also estimated for universal curve which I have told you universal curve is this one this is the one this is lambda i this can be used to calculate all the lambda is of different elements okay so for a multi-element system supposedly which is normally the case this is we can write down the constants of element ni divided by ni plus nj plus k over j ij k are three elements so this is how it is done i i divided by sigma i by lambda i divided by this is the whole thing the whole divided by this one this is for i this is for j this is for k as you say the factor which comes with capital k nullified because of the ratio taking so that is how actually done so I can also calculate nj now nj is nj divided by ni plus nj plus nk similarly nk can be calculated let us see how this are done some examples for oxide surface that is oxygen metal atomic ratios determined for cost many intensities mg o pilotized one o s o 2 s by metal metal is basically mg mg here 2p ratio is this much plus minus 10% okay now l 2 o 3 is also like that si u 2 calculated c u 2 o these are the different and oxygen atoms oxygen energy levels divided by metal energy levels ratios so these are the measurements actually tells us how quantitatively intensities can be correlated.