 This flagship was originated by the discovery of the demonstration of graphene in 2004 in the Nobel Prize in 2010. The goal of this project was to bring two-dimensional materials to practical applications. I'll tell you something about graphene and then graphene is just a prototype of all kaj je atomične tine, kaj je zelo reduzirati do nekaj atomičnih ljahov. Zato, da je menej expertični in ultrafast spektroskopi, po začeljenju vse materijali imamo vse materijali, da imamo vse materijali o ultrafastu optika, kaj je materijali, kaj je vse materijali, kaj je vse materijali vse materijali vse materijali. Tako, kaj je vse materijali, And then I'll first give a very broad introduction to graphene. Then I'm going to focus on the optical properties of graphene. And then I'm going to move on to two dimensional semiconductors, so other materials that can be exfoliated to atomicity, atomically thin materials, but they are semiconductors differently from graphene, which is a seme metal. And then I'm going to show you how one now can start frosta from the atomicity into dimensional materials and stack them on top of each other and one can form heterostructures of two-dimensional materials so now you can fabricate new materials just by stacking atomic layer by atomic layer different materials and this gives a very great freedom of engineering new material properties, so let's start with a broad introduction to graphine. So, what is graphine? Grafin je, kaj je zelo, halotropi karbon. Zelo halotropi karbon je izgleda z grekimi halosentropi. Zelo je jedno z vseh formu komikovih elementov. Zelo komikovih elementov karbon je izgleda v vseh formu komikovih halotropi. In tudi je dva izgleda z halotropi karbon, kaj je diamon in grafite. Zelo je zelo izgleda v diamon, karboni halosentropi karbon je izgleda z vseh z SP3. Zelo je zelo izgleda z halotropi karbon, kaj je zelo izgleda z grafite. Zelo je halosentropi karbon, kaj je zelo izgleda z grafite. Zelo se zelo izgleda z vseh formu komikovih halosentropi karbon. Zelo je grafite, akim v vseh formu komikovih halosentropi. Zelo se kaj doživajo, kaj je zelo izgleda z grafite sem solim odlično. Zelo se zelo izgleda z grafite sem solim odlično. Zelo se zelo izgleda z grafite. Zelo grafite sem zelo izgleda za 1 atom tih grafite. V ovom različenju se vidimo, da karboni atomi je zelo vsečen v honeycom, čekan, vajer, lettis. To je prototyp v materijali, kaj je tudi tudi dimensijali materijali, ker je atomični. Tudi nekaj nekaj nekaj nekaj atomični. Zdaj, kako se pravite grafini? Prostavno, zelo, da vidimo, taj materijal, zelo se, da karbon karbon vzelo v grafini, je 0,14 nanometer in interplanar spacing v grafajti taj vzelo vzelo v 0,335 nanometeri. Tako, je taj za 3 milijon vzelo v 1 milimeter v grafajti. Tako, nekaj tega predbiti signa znača? Govoriš, da vzelo vkuljega. Tega je skupan karbon vzelo vzelo vzelo v grafini, vzel malo v 0,1,3 gdaj. Tako, kaj je v 3 gdaj, muzala v grafini, vzelo v grafini, vzelo v grafini. da je pričočiti grafite. If you take a piece of graphene and you wrap it around itself in a 0 dimensional, you obtain a ball of atoms of carbon, which is a fuller. Or you can wrap the graphene in 1 dimensional, then you obtain a tube, which is known as a carbon nanotube. So these are the different forms of carbon that you can obtain. Now if you look at fulleries, Mojtukje are molecules made of 60 carbon atoms arranged in a sphere. Nenam jih se prišli v honor of the architect Richard McMaster Fuller, who designed the geodesic domes that recall these fuller in molecules. These were first synthesized in 1985 by Aral Crot, Robert Kal, Richard Molly, who got for these the Nobel Prize in 1996. In karbonanotubi je vsega trajba grafinja in vsega vsega. Zelo je tudi vsega vsega vzgivost, ali nekaj 1 nanometer. In to je počkaj nekaj, da je počkaj nekaj, da je počkaj nekaj, da je počkaj nekaj. And they also have been discovered well before graphene, and they also have been investigated for the unique mechanical thermal and optical properties. Now if you go to graphene, how can you obtain graphene? So the first method of production for graphene was simply by micromechanical cleavage. So you start from this graphite which is strongly layered, and then you slice it down, you clean it down til you get a single atomic plane. And this was performed by Gaim and Novozelo, actually by Kostya Novozelo. And this is really, you see the signature of Andre Gaim, this is really the equipment that was used. You take a piece of graphite and you take some scotch tape. And by progressively trying to exfoliate thinner and thinner amounts of graphite and depositing it on a substrate, you get down to a single atomic layer. So this was at the time, but there still are, I think, in Manchester. Novozelo was very young, probably already was PhD student or postdoc when these experiments were done. And so this was one of the so-called Friday afternoon experiments. When you are on a Friday afternoon, you have worked the whole week, you try something a bit unusual. And so they said, okay, why don't we try to exfoliate graphite and see if we can get to a single atomic layer. And that's indeed how they succeeded in producing the first graphene. So this is a way also telling us that sometimes you don't really need, as was mentioned in the previous lecture, what is important are ideas more than funding, because the funding required to make the first graphene was really very limited. So anybody could have done this. And so these are Andre Geim and Costi Novozelo, who received the Nobel Prize in 2010 in physics. Costi Novozelo was really young, was 36, for groundbreaking experiments regarding the two-dimensional material graphene. Now, this is the first image of a single layer graphene flake that was exfoliated and deposited on a silicon dioxide substrate. And you can see optical contrast, which is different for a monolayer and a bilayer, that allows you to recognize when you have a monolayer. And by exfoliation, you typically get flakes of graphene that are on the range of a few tens of microns. So very small flakes, but which are truly two-dimensional, and fully of atomic thickness. And this is an AFM image where you can really see the carbon atoms arranged in this kind of chicken wire arrangement. So this was, of course, graphene was predicted, was known since many decades, but it was thought that it would not be stable. It was thought that once you make graphene, graphene would kind of change either wrap into a nanotube or into a fuller. It would not remain stable two-dimensional. So this was the pioneering result. Already only about 15 years ago was the first demonstration of a two-dimensional material. And now, since then, technology has evolved very rapidly. Of course, with micromechanical cleavage, you can make a very limited surface, very limited size of this flake. Maybe if you are really good, you can make a millimeter squared, but not more. And these are still the flakes that have the best physical properties, the ones that are mechanically cleaved. But this is really more an art to be able to cleave and get really a big flake, which is a single layer. Since then there have been other methods. You can have a carbon segregation on metal or silicon carbide. In particular, chemical vapor deposition on a metal substrate allows you to make very large, to cover very large surfaces with a single layer graphene. It is not as high quality as the micromechanically cleaved, but you can really cover, as I will show you in a moment, very large surfaces. Another method worth mentioning is liquid phase exfoliation. So you start from some graphite by using liquid with combination with ultrasounds, you exfoliate it until you get a dispersion of flakes in liquid. Sometimes it's called a graphene ink. And this is maybe not necessarily a single layer. It can be maybe a mixture of several multilayers with different thickness. But with these you can generate easily large amounts of material. But the CVD process on copper allows you really to create very large surfaces. And there has been a dramatic progress, especially in the Asian world. People have really realized this potential. And this is a procedure for making graphene roll-to-roll production. So you grow the graphene on a copper foil. Then you use a polymer support. So you transfer the graphene to the polymer. And then now you are able then to transfer graphene to a single layer graphene to a very large substrate. Here you can see a 30-inch single layer graphene. So this of course allows this material to be used in many practical applications exploiting its unique properties. Now what is so special about graphene? Of course in this lecture I can cover the amount of properties because there are really many properties in many different applications. So first of all it is atomically thin sheet of carbon. It is a flexible, light and abundant material. As I will show you, it is a very good electrical conductor. It has electrical mobility much higher than the best semiconductors such as silicon. So it has potential for high speed electronics. It is a conductor but it is transparent. So the combination of these two properties is not common because typically transparent materials such as glasses are insulators, electrical insulators while good electrical conductors such as metals don't transmit light. Now there is the most important transparent conductor that is used in all displays is indium tin oxide, ITO which combines two properties. Now graphene can compete with ITO in combining these two properties. So it can be used as an electrode that also transmits light. So this is very important for this place. And then it has very remarkable mechanical properties that I will not cover in this lecture. So it is one of the strongest materials known so it can be used or incorporated in lightweight composites. So there is really a very broad range of multidisciplinary applications of graphene that range from composites, transparent conductors, photovoltaics transistors, electronics components, even membranes, gas barriers for example is another important application and all these applications rely on its peculiar physical properties. So in this lecture I am going to focus in particular on the unique electronic and optical properties of graphene. So let's discuss now the optical properties of graphene. So first of all let us consider the electron, in order to understand the optical properties we need to understand the electronic structure of graphene. And now you can quite simply understand that if you consider this graphene structure each carbon atom has four electrons in the outer shell so three of these electrons are hybridized sp2 so you have this very strong covalent bond but then the fourth electron is free to move and is delocalized along the plane. So graphene is a very high electrical conductivity is a very good electrical conductor because of these three electrons. So graphene you see that it is very peculiar so in a typical semiconductor you would expect a parabolic band structure of energy with respect to momentum. So in graphene you get these two cones in the valence and the conduction bands these two cones they touch at this point that is known as a direct point. Now if you consider intrinsic and open graphene you have that this lower band the lower cone is the valence band is completely full and the conduction band is completely open. So graphene behaves as a semi-metal so you have field valence band and empty conduction band. But now if you look at the dispersion relationship meaning energy with respect to wave number you get this expression where you see that there is a linear as you would expect for a cone there is a linear relationship between energy and wave number k and now this proportionality is the so-called Fermi velocity which is 8 to 10 to 5 meter per second so particles behave as so-called massless Dirac fermions. Now this very peculiar electronic structure has some important consequence on the optical properties. So you know an optical transition basically since a photon has no momentum or negligible momentum let's say an optical transition is typically vertical so it connects state with the same wave vector. So here you can see that basically if you have an intrinsic graphene you have a electrons in the valence band and you have a completely empty valence band so you can make this transition at any energy of the photon at any frequency so while in a semiconductor when you have a band gap you cannot absorb below band gap in graphene you can absorb light at any wavelength so you can make these vertical transitions connecting lower and upper Dirac cones at any wavelength so graphene it is the only material that can couple light to light and the mid infrared to the ultraviolet so what if you consider undoped graphene of course if you doped the graphene then things change because then you will occupy either the conduction you will create either electrons or conduction holes in the valence band now you would think if you consider any material that you would tin it down to a single atomic layer you would think that the absorption is negligible how much can a single layer of atoms absorb but you find that the absorption of graphene is constant and it is relatively high it is 2.3 percent so this is of course not a very high absorption but if you consider it comes from a single atomic layer it still demonstrates a very strong light matter interaction so graphene in say from the mid IR to the ultraviolet range in general absorbs as a constant absorption of 2.3 percent per layer now this of course is a quite unique property so if you take any light detector based on a semiconductor this will have a band gap so it will work only in a limited range of frequencies now graphene due to this universal absorption it can absorb light at any wavelength so it can absorb any color of light from the mid IR to the visible to the ultraviolet and it has a potential for an ultra broadband detector so a detector that can respond to light at any color from the mid IR to the visible and of course each of these regions of the electromagnetic spectrum is important so having a detector that can address all of this simultaneously has very big potential of course you have a problem that you absorb in a single layer only to a low amount but you can try to enhance this absorption by different techniques then also these detectors have the potential to operate at much higher speeds with respect to conventional photo detectors now another property of graphene as I was mentioning is that it is a transparent conductor so graphene combines high electrical conductivity with high optical transparency so if you see the other way you consider a single layer of graphene transmits more than 97% of the light so it's basically transparent but still it's a conductor so you can use it to replace indium tin oxide in screens such as liquid crystal displays flat panel displays plasma displays and so on touch panels so you can really use it and this is one of the first real practical applications to use graphene as a transparent conductor also in photovoltaics of course because you need it every time that you need an electrode that can also couple light out of the device now another really remarkable property of graphene is electrical tunability so typically a material has some absorption which cannot be easily tuned electrically you can shift it a bit maybe by stock shift but with graphene you can really have dramatic effects so as I was saying graphene couple strongly to light because a mono layer displays this constant absorption of 2.3% across visible and infrared ranges but now here I'm showing you intrinsic graphene so meaning that the valence band is completely filled with electrons the conduction band is empty so I can make transitions I can promote electrons in these vertical transitions basically with photons of arbitrary energy so I can absorb light at any wavelength but now if I tune my graphene if I put a gate and I shift my Fermi energy now for example here I am depleting the valence band of electrons so graphene now cannot do these transitions anymore because they are no more electrons but they are transparent and I can very easily shift it so I can make graphene either absorptive or transparent electrically and this is not so easy to do with a material to control its optical properties electrically and also in a very fast way so this is ideal for applications to optoelectronics when you want to modulate the transmission so you can make a very efficient graphene modulators as it work here you have an optical waveguide that transmits light this is typically say 1.5 micro which is the wavelength used in optical communications so this is a silicon waveguide light is transmitted to this silicon waveguide and on top of this waveguide you have your graphene layer so you see that you will have this is the mod of the waveguide there will be an evanescent part of the electric field that sees the graphene so part of the light transmitted this waveguide sees the graphene on top and now of course the graphene will absorb even if it absorbs a little but if you put it on top of this waveguide it can result in a very large absorption but now you can change by changing the voltage to the graphene you can now shift the Fermi level graphene completely transparent so electrically you can very quickly modulate and this you can do at any frequency from the infrared to the visible you can modulate the transmission the optical transmission of the material because by shifting the Fermi energy you can you can change the you can bring it from absorbing to transparent essentially now as I told you the main specialty is looking at ultrafast optical response of materials so what graphene is also a material that is an extremely fast optical response so what happens if you come, if you promote with light with some photons you promote some electrons from the valence to the conduction band so here I'm showing you let's say if you do it with a short light path suddenly you bring these electrons up from the valence to the conduction band so you bring you have electrons in the conduction band you have holes left down in the valence band now these are of course this is not an equilibrium this is not like a Fermi Dirac distribution then the first thing that will happen is that these electrons will very quickly thermalize in a time less than 50 femtoseconds and they establish this Fermi Dirac distribution so the electrons equilibrate with themselves but now the electrons are hot, they can reach very high temperatures of thousands of degrees but the lattice of the graphene is still cold so the next step will be electron phonon scattering so this takes place on a very fast time scale on the order of a picosecond now we can so graphene then recovers its original state within a few picoseconds so it has an extremely fast optical response so it is an optical switch that can be open and closed very fast and this has also applications so how can you detect these as I will show you also later on in different cases you can study this ultra fast carrier relaxation by so called pump probe experiments so you take a first light pulse so called pump that promotes the electrons in the conduction band and then with a given delay you come with a second pulse the probe that measures how the transmission changes so what you typically see in graphene is that you are promoting electrons in the conduction band so now you are putting electrons in the conduction band now if I come with a delayed pulse now these states in the conduction band that I could have reached without the pump now they are occupied and by the Pauli exclusion principle you cannot put other electrons there so you have what is called Pauli blocking and so you cannot absorb anymore or you reduce your absorption at this specific wavelength so by exciting graphene its transmission increases so you have in this sense a switch because from absorptive it becomes transmissive so it increases its transmission but then this recovers very quickly so first of all this is a measurement of how the transmission of graphene you see first it increases very quickly and then it recovers there is a first very fast decay due to the coupling with a so called strongly coupled optical phonos and then you have a slower decay due to equilibration with acoustic phonos but after a few picoseconds basically the material has returned to its original condition so it's an ultra fast switch and this also has applications for example you can use graphene as a so called saturable absorber so you can put graphene in a laser and now if you this is like a switch that opens when you have high intensity but then closes immediately so it forces the laser to emit light in form of a train of pulses and you can put this graphene inside the laser and automatically the laser starts to go into this so called modlocked regime and because it finds more convenient to emit instead of a continuous wave a train of pulses because this train of pulses having high peak power are transmitted more have higher transmission coefficients from the graphene so this is another quite unique property of graphene now graphene as I said was just the beginning of the old field of two dimensional semiconductors in fact if you consider now all the good properties of graphene it's the thinest material however it is transparent but still electrically conductive it has an absorption quasi independent of optical wavelength it is strong but flexible it has high thermal conductivity high electron mobility it is made of carbon so very abundant material however it is not a semiconductor it is a semi metal so this makes it difficult to use graphene in electronics because electronics is based on semiconductors so soon after the discovery of the demonstration of graphene in 2004 it was realized that many materials can be exfoliated just in the same way of graphene to being atomically thin two dimensional materials and now the field is exploding and there are really many materials that can be exfoliated and that have different properties some of them are semiconductors some of them are insulators like boron nitride some of them are superconductors so there is really wide variety of materials that can be exfoliated and they become atomically thin so here I am going to focus in particular on the semiconducting two dimensional materials and on this class known as a transition metal decalcogenized so if you look at the periodic table you are combining some transition metals M with some calcogen atoms like S, S, E or T and you have the chemical form S2 where M is a transition metal and X is a calcogen so you can have okay here I wrote 40 but it is probably much more it is probably hundreds nowadays of different compounds that are all layered and can be reduced to atomically thin two dimensional materials so the first one to be exfoliated is a molybdenum disulfide MOS2 so I am going to focus mainly on this material but as I said there are many more now how is this how is this done it is not a really single atomic plane here you see you have three layers three planes so you have the transition metal, the molybdenum that is strongly covalent, strong covalent rebound the sulfur in this case so you have a very strong intra layer bond between the metal and the calcogen so you form this structure and then you can have several of these layers that are held together by very weak bonds essentially van der Waals bonds so you can easily exfoliate it so when you go to a single layer in this case layer thickness is on the order of 6 to 7 so 0.6 to 0.7 nanometer here you can see an atomic force microscope of a single layer you can really see here when you go you see the thickness of 6.5 and so again you can produce this material by mechanical exfoliation just like you did with graphene but again mechanical exfoliation of course gives very small materials here they are even smaller than graphene so maybe you get flakes of a few micro squared that are truly single layer but again now there is a very intense activity in developing different technologies that are not mechanical scotch tape method for example chemical vapor deposition in chemical surfaces with a very broad coverage although you typically tend to create this kind of triangles but you can kind of homogeniously cover with these triangles of single layer the surface or you can have again this liquid phase exfoliation so you can make inks of these materials so again the progress is being very rapid although these materials are very old they discover that they can be exfoliated to a single layer that is back to the maybe 2010 2011 and since then there is really been an explosion of interest in these materials, why? because they share the properties of graphene of being two dimensional materials and now they are semiconductors and in addition they have some really remarkable physical properties that I will now discuss in the remaining of my talk so first of all taking MOS 2 and this is true for all these materials you know semiconductor can have a direct or indirect gap so indirect gap means that the minimum conduction band the maximum of the valence band are different wave vectors and this is not good for emitting light because light emission requires that the wave vector is conserved on the other end a direct band gap as the maximum of the valence band, the minimum of conduction band with the same wave vector and this has a very strong light emission now the peculiar properties of these materials is that they are in direct band gap when they are multilayer but then when they are when they go to a single layer due to quantum confinement these levels go up in energy and down here and then the lowest energy transition remains remains at the direct point so they become suddenly when going from two layers to one layer they go from indirect to direct band gap and this is a dramatic effect you can see the lumines since indirect band conductors do not emit light you see the luminescence is nearly zero for a two layer then you go from one layer the luminescence increases by two orders of magnitude so suddenly it becomes emissive and only when it is a single layer another very interesting properties is that there are very strongly excitons with very high binding energy so what are excitons so in a semiconductor you typically think you promote an electron in the conduction band you leave a hole in the valence band but then you can think that this electron and hole form like a nitrogen like system so there is an attraction between the two so they form like a small hydrogen atom which has a certain binding energy now if you take a standard semiconductor this binding energy is extremely low is a few milliretron volts so in a semiconductor at room temperature this binding thermal energy is much higher so these excitons are destroyed so in a semiconductor you think about free electrons in the conduction band and free holes in the valence band but here this increases enormously for two reasons first of all because now you are getting a two dimensional quantum confinement so now you have a semiconductor that is only one atomic layer thick and then the second reason is that if you it's quite trivial problem of physics if you consider the hydrogen atom the binding energy of the exciton depends on the dielectric constant of your medium now if you go from a bulk material you will see that the lines of force of this field they essentially see the dielectric constant of the semiconductor but now if you have a material that is only one layer thick then you see that the field lines of the electric field they go outside where there is no the dielectric constant is basically one there is no screening there is no screening from the medium so this increases the exciton binding energy by two orders of magnitude so in a normal semiconductor you have a binding energy of a few milletron volt here you have hundreds of milletron volts up to one electron volt so you really see the excitons at room temperature by naked eye very strongly so you get something like this you really see these very beautiful peaks and indeed there is also spectra of the most popular transition metalical coogenites MOS2, MOS2, WS2, WS2 you see they all have these very strong peaks at room temperature that are due to these excitonic transitions now ok ok maybe I can skip this this is just a comparison here you see in gallium are bark semiconductor you have a binding energy of a few MEV here you have a binding energy of 0.3 EV in this case you can nicely see the so-called readberg series of the exciton for different levels so you can really measure it experimentally so these excitons are stable even at room temperature now I told you it's a direct band gap semiconductor at the K point and it has these two excitons, A and B excitons which are due to a strong spin orbit interaction in the valence band that splits these states in the valence band so here you can see the typical absorption spectrum with these two peaks that are so-called A and B excitons and with the this is the so-called C exciton now there is another very interesting property of these materials which is so-called valedependent optical selection rules so you have these two now you see that you have here these excitons now here since you have different elements you will have a broken symmetry so the result is that you have two valleys the K valley and the K prime valley which are not degenerate which are degenerate in energy but they can be selectively populated by circularly polarized light so if you excite a light with if you excite this semiconductor when it is one dimensional with circularly polarized light with for example one elicity one is either right or left circular polarization you will get you will access one of these two valleys in circular polarization you will access the other valley and now this can be is remarkable because in typical in semiconductor these two valleys cannot be selectively excited so this means that when you excite with circularly polarized light you can you can address one valley and so the emission will have the same circular polarization so you basically get emission which is again with a very high degree of circular polarization and this is a unique property of this material because of the so called spin valley locking that you have due to this two dimensional character now this can give rise to a field that is called a balletronics so you know that typical electronics works in charge of of electrons as a degree of freedom you can think of spin tronics where you use the spin as an additional degree of freedom here you can now use the valley as an additional way of storing information so in principle you can address one or the other valley and this can be taught as possible ways of writing information for example in one valley or the other valley now let me see how much time I have not much maybe I'm just gonna give you one example of how we use ultrafast optical spectroscopy to study non-equilibrium properties so let me give you a very brief introduction to ultrafast pan-prop spectroscopy so this is a technique that is very general you one of these two-dimensional semiconductors but it can be applied to any material and so what you do is you excite the material with a light pulse so called pump pulse and then you come with a time delay the probe pulse and you measure the transmission of this probe and then you repeat the same experiment you still measure the transmission of the probe without the pump and then you calculate what is called the transient transmission which is the difference between these two values and of course this difference will depend on the time, on the delay between these two pulses and now essentially how do I do it I have a sequence of pulses of pump and probe and then typically with a chopper that can be a mechanical or other kind of chopper I can switch on and off the pump pulse and so I can measure the transmission of a probe with pump one without and then measure this transmission now in the end I move my delay with a mechanical delay line and then I will get a map like this where I get here the time delay which I control with a mechanical delay line and here I get how the transmission spectrum of my probe is as a function of frequency and as a function of delay and then I can take a cut of this map at the given delay I get a spectrum I can take a cut at the given time and I get a dynamics so this is a very powerful technique that gives me very rich information on how the excitation evolve in these materials so maybe this is just one example of what you can learn from these materials so for example I can see that in some cases my transmission increases or in other cases with the pump my transmission decreases so I will get my differential signal that can be either positive or negative and so I can now consider the different possible sources of this signal I create for example with my pump electrons in the conduction band and now what I can have is increased transmission simply because I am bleaching this is called photo bleaching or poly blocking because now I am occupying some states so if I come with my probe I will see an increased transmission or I can have a stimulated emission now I am creating I am putting some electrons in my valence band with my conduction band with my probe I can stimulate the emission down to the valence band or I can have a photo induced absorption from the conduction band to some higher lying states so let me maybe I am going to skip a few slides see if I can maybe because I think in the last few minutes I want to show you the last topic and I am going to show you some examples of spectroscopy on this which are now heterostructures of 2D materials now having established that you can make graphene you can make these two dimensional semiconductors now you can add another very big degree of freedom by stacking these materials on top of each other and so essentially you can mechanically assemble you see one layer of graphene one layer of a transition methodical cogenites, another layer of graphene so you can really assemble these materials on top of each other and the good thing is that they are held together by the same van der Waals interactions that hold together them in the layer material so there is a very big you preserve essentially the individual layer character because you have this very strong covalent bonds but if you want to make an heterostructure of semiconductors like you do for example in optoelectronics so if you want to put together two different semiconductors and make two different layers you have to account that the lektis have different they have different lektis periods so there is a lektis mismatch that limits the capabilities but here since you have this van der Waals interaction you don't have any limitations so if you are able of course to assemble these heterostructures you can really put together and you have an almost unlimited freedom so you can make many different devices which consist of heterostructures of these two dimensional semiconductors so I just want to show you two examples one example is a heterostructure of graphene with one of these transition metardical cogenites for example WS2 here you have graphene which is semimetal so has no gap and you combine it with this WS2 which is a semiconductor so in this case you will you have also some very interesting optoelectronic properties the other case is an heterostructure of two semiconductors in this case I am taking for example WSE2 and MOSE2 I am taking two single layers and I am stacking them on top of each other so what happens now here you get two semiconductors you see you get two different electronic gaps in this case WSE2 as the higher band with respect to MOSE2 but now you get this so called type 2 band alignment you see that now you have the minima of the conduction band the maximum of the valence band so what happens now if you excite for example the MOSE2 which is the lowest energy semiconductor so you create an electron in the conduction band you leave a hole in the valence band you don't excite this semiconductor because you are below the band but now you know that also electrons tend to go down holes tend to float up so here this hole finds this state which is a higher energy so there will be a very fast hole transfer so now the hole goes to the other material that has not been excite by light but now I am separating especially the electron and the hole now I get what is called I still get a bound electron hole pair but now the electron and the hole are sitting on different layers I get what is called an interlayer exciton and this has completely different properties compared to the electron and hole sitting on the same material so you see that I get a huge new degree of freedom and just to show you this example this is an experiment that we did in collaborations with the University of Texas in IKFO so here you see there are extremely small flakes on the order of a few microns squared but you can stack them on top of each other in this case and you get here a region where you have your heterostructure so you have MOAC2 and WAC2 put on top of each other and you can see here the absorption spectrum of one, the absorption spectrum of the other and when you combine them you see that you can more or less recognize, there is some shifts but you can recognize the peaks of the two materials now we do a two color pump probe spectroscopy we excite the exiton, the bound electron hole pair in the MOAC2 and then we look at what happens in the other material so essentially this is the result so when I excite the MOAC2 I'm exciting at this material and I'm looking at this other material so when I excite only the MOAC2 I see a weak negative signal which is a photo induced absorption now, if I excite only the WAC2 because on this structure I have all the three materials separate then I don't see any signal because I am exciting with a too small energy but now if I excite my heterostructure I see a signal that is growing on the WAC2 and it's becoming much stronger and it's growing not instantaneously but with a given time so what this signal is due to I'm exciting now this electron hole and the hole is hopping from one material to the other and I can time resolve very clearly that it takes 230 seconds for the hole to jump from these to these so this is the interlayer hole transfer time for example now if I look at also what happens once we have formed this interlayer then it decays on a very long time scale because now the electron and the hole are separated and it takes much longer to decay and so ok there is a temperature depends maybe we can skip this now the last example I want to show you and then I finish is an heterostructure of one of these two dimensional semiconductors with graphene again now I am putting together a material with a band gap and a material with no band gap that absorbs light at any wavelength so I am taking WS2 on top of a single layer graphene now what happens when I excited this structure above the band gap above the band gap of the semiconductor I see a very characteristic signal of the A and the B exciton of the semiconductor that decays in some time exponential decay in a picosecond time scale so I am essentially exciting my semiconductor above band gap but now if I go below band gap the semiconductor doesn't absorb anymore but now the graphene will absorb but still I see pretty much the same signal so essentially I am exciting even if I excite the graphene I am transferring some excitation to the semiconductor now this goes to some higher line gap because here I am exciting well below the band gap of the semiconductor so I can do some influence dependence but basically the result is that if I excite my heterostructure I see the signal but if I excite with low energy just the semiconductor I see nothing so it is really something coming from graphene and going to the semiconductor so ok let me maybe skip these and come directly to the result so what we are seeing is hot electron transfer so essentially we are creating with graphene some electrons in the conduction band as I was telling you before these electrons very quickly thermalize and so you will have a familiar distribution so you will have this tail that have quite high energies because they will collide and so some will acquire very high energies now I am putting my semiconductor and the electrons can very quickly tunnel from the while they are still hot they can very quickly tunnel from graphene to the semiconductor and in fact this is very consistent with previous measurements our power low dependence of the signal but essentially we are exciting the graphene below bandgap and we are resolving these very hot electron tunneling to the semiconductor and we can do this with very high time resolution and we see that this is essentially a time constant on the order of 20 femtosecond so we see that if we excite the graphene very rapidly the electrons will tunnel from the graphene to the semiconductor so you can see that really by putting together this two dimensional material you get completely new optical properties and you get a lot of physical new properties that you can exploit for many applications and with this I think I come to an end of my presentation and thank you for your attention so thank you and I think now it's time for some questions can you show the last slide where you have this exactly the efficiency of the process depends on this thermalized electrons how many are above the conduction band of the semiconductor yes exactly so it could also be that there is no effect at all depending on the position of the conduction band well essentially you will generate with what you do is you generate this hot electron distribution which of course will depend on how many electrons you put in the conduction band so on the electronic temperature and then of course depending on this band also you will have a certain tunneling probability for doing this but the efficiency of the process how much is it well it's compared to the number of excited electrons on graphene how many are ending up in the conduction band it's not a very high efficiency probably on the order of percent or lower because you are getting a tail of these electrons but the nice thing is that you are exciting well below the band of the semiconductor and still you are getting an excitation how much well below do you serve a threshold we go essentially down to let's say 0.8 so this semiconductor is a band of 2EV and we still get up to 0.8EV so more than two times lower we get we see the signal on the semiconductor and you can think that you can sometimes you can do what is called multi photon excitation so you can absorb more than two photons or two or more photons but then this would give a non-linear dependence which we don't see in our data so we have dependence of the excitation power also yes it is clear that it is not a multi photon excitation also because if you go to the semiconductor alone you don't see anything because you are well below threshold then I go there what about the redshift that you see in the you mean in which redshift this one in the stack this depends essentially because now when you do an heterostructure you have a different refractive index and the refractive index shifts the exitons so if you have an exit on essentially you have a substrate you can have one layer and then air or you can have another layer on top and these influences induces some redshift of these bands well, you can see by this redshift that you have two semiconductors on top of each other but then you don't know whether they interact, whether they exchange to do this you need to do one of these kind of time resolved experiments just general question do you think that this calcogenite can be used as, how long is the lifetime of the emission of this materials more or less quite debated question actually in principle it can be as long as a few nanoseconds it depends, there are what are called OJ processes so when you put you create electron holes, you create exitons these exitons can annihilate so they can meet each other one goes relaxes back to the and the other is excited to higher energy and this destroys from two you get one so if you go at lower intensities you can prevent this and the other point is that these materials are quite new so there is not yet a very systematic study on the effect of the quality of the material so some of the materials are exfoliated so for example they have defects some have more or less defects more or less doping so I think if you have a really pure material without any defects then you can have very high lifetime in very high photo luminescence quantum yield there is a future for this material in photo induced electron transfer or something like that in order to use the excited state of this material to do something well for example with these heterostructures you can get, it's a very nice way of separating the charges and then they get really long lived charges so for sure then they can be extracted they can be used in photo induced electron transfer it works also in solution well yes let's say these experiments that I showed are on flakes put on a substrate on a transparent substrate but you can also work in solution you can make what are called the inks of these materials so you can exfoliate them and then you can in solution essentially other question? in which sense well they can go in both ways in the sense that typically well typically ok, if you have a single layer of graphene you go perpendicularly so you see the absorption but for example in this modulator that I showed you then you have some evanescent wave that sees the graphene and then can go also transversely you mean the pump power when you do this pump probe experiment well, we try to stay in a so called linear regime where we would like to use typically as low power as we can we need the problem is we need to detect the signal so if you have a very stable instrument you can very gently excite this material but typically it's you're talking of fluences in the order of micro juice per square centimeter which are say you typically the densities are not very high of charges that you put well here ok, this experiment is a non-linear experiment so the transient absorption is a third order linear experiment these materials also have a strong have non-linearities for example graphene if you think of these materials so the first time I started people told me you should do experiments on these materials said ok, I can do it but I will see nothing because what can I expect to see from an anatomically thin material but surprisingly these materials have an extremely strong non-linear response so for example graphene has a third order non-linear response coefficient it's called chi-3 which is 6 orders of magnitude higher than most of standard materials like transpiled materials on the other end you have to consider that these materials are extremely thin so if you make you have an extremely strong non-linear response in these materials but the overall effect is still very small because the thickness is just one atomic layer compared to let's say millimeters from graphene well graphene you can use it for storage one hypothesis that is for example people in the IIT in Geneva are working a lot on these is to use graphene for storing hydrogen so you can kind of think that hydrogen can attach to the graphene and then you can do this in this case for storage of fuel other applications you can use graphene in super capacitors so if you want to make very large capacitors you can also exploit it so these are several applications in energy well they are quite stable these materials actually of course what you can do if you want to make them even more stable and what people are doing you can encapsulate them so you have one of these two-dimensional materials which is boron nitride boron nitride is a large gap semiconductor insulator let's say and you can also make it one layer or few layers so if you encapsulate these materials between these boron nitride then it becomes even more say the properties increase a lot because this kind of suppresses interaction with the environment but in general they are quite stable I would say yes yes ok so first of all to your first questions I think it is true that if you put one of these materials on top of each other the specter shift but on the other end here you see you have A and B excitons now if you put them together it's true that they shift a bit because of this dielectric environment but still the individual layers maintain essentially their properties they don't form a new compound with different properties so you really have one semiconductor and the other now they can be separated now the question is how are they available this is something that maybe one should investigate but for sure you are separating in principle now what people are starting to do is they can put of course at the moment these are really something that came out in the last three or four years it's really new you can now start to put three or four of these materials so you can make a cascade you can really make a gradient and you can separate them even more in principle so they hopefully can become even more available for doing photochemistry so thank you again professor Cerullo and I think for today it's all we will see here tomorrow at 9 9 have a nice day