Bruno Uchoa, Ling Yang, S.-W. Tsai, N. M. R. Peres, and A. H. Castro Neto
We examine theoretically the signatures of magnetic adatoms in graphene probed by scanning tunneling spectroscopy (STS). When the adatom hybridizes equally with the two graphene sublattices, the broadening of the local adatom level is anomalous and can scale with the cube of the energy. In contrast to ordinary metal surfaces, the adatom local moment can be suppressed by the proximity of the probing scanning tip. We propose that the dependence of the tunneling conductance on the distance between the tip and the adatom can provide a clear signature for the presence of local magnetic moments. We also show that tunneling conductance can distinguish whether the adatom is located on top of a carbon atom or in the center of a honeycomb hexagon.
Analysis
1. This paper is published in Physics Review Letters in November, 2009 and presents theoretical study of Scanning Tunneling Spectroscopy of magnetic adatoms in graphene.
2. Authors propose that the dependence of tunneling conductance on the distance between the tip and the adatom can provide clear signature for the local magnetic moments.
3. Authors show that tunneling conductance can distinguished the position of adatom i.e. whether the magnetic adatom is on the top of carbon atom or in the middle of hexagon.
4. Authors propose that an adatom localized level can hybridize strongly with STM tip unlike ordinary metals that have low density of states and by analyzing the dependence of differential conductance (DC) with the distance between nonmagnetic tip and adatom one can get experimental signature of local magnetic moment.
5. Electrons have different sub-lattice quantum number in graphene so the destructive interference between different tunneling paths causes substantial change to the form of Fano Factor and shape of differential conductance (DC) when the magnetic adatom sits at top of carbon atom as opposed to when it sits in the middle of a hexagon.
6. Observation in (5) gives STM a capability to identify adatoms and defects in graphene.
7. It is know that adatom can occupy different sites in graphene. In this study authors have considered two cases (i) when adatom is sitting at the top of a carbon atom where adatom breaks local sublattice symmetry, (ii) when adatom is sitting in the center of hexagon without breaking any symmetry.
8. This study indicates that when the distance between the tip and the adatom become progressively becomes very small differential conductance DC peaks can shift strongly. If this is the case the peak at the right of the Fermi energy will have a red shift and it will eventually cross the experimentally accessible bias window providing an experimental signature for the presence of local magnetic moment using a nonmagnetic tip even in the presence of Kondo peak.
Conclusion
9. To conclude, authors have derived the fingerprint of Fano resonance of a magnetic adatom in graphene.
10. It also concludes that presence of adatom adsorbed on graphene can be identified using shift in differential conductance (DC) with the changing tip-adatom distance.
11. This study also gives information about the possibility of local magnetic moment away from Kondo regime in graphene.
Sunday, November 29, 2009
Friday, November 27, 2009
Atomic Hydrogen Adsorbate Structures on Graphene
Richard Balog, Bjarke Jørgensen, Justin Wells, Erik Lægsgaard, Philip Hofmann, Flemming Besenbacher, and Liv Hornekær
Analysis
1. This paper is published in |J|A|C|S| Communications in May, 2009 and presents Scanning Tunneling Microscopy study of hydrogen adsorption on graphene grown by thermal annealing of silicon carbide (SiC).
2. Chemical reaction of hydrogen atom with graphene is a potentially viable method to introduce bad gap in graphene (semi metal) to use it for electronic devices.
3. Authors present Scanning Tunneling Microscopy (STM) study of adsorbed hydrogen atoms at the basel plane of graphene grown on SiC.
4. At low coverage, hydrogen dimer structure and at high coverage hydrogen clusters are observed by STM.
5. Hydrogen dimer formation occur at the high tunneling probability area of graphene that is altered by 6×6 reconstruction of SiC(0001) (1×1) surface.
6. Observed hydrogenation of graphene on SiC was reversible by thermal annealing.
7. For hydrogenation graphene was exposed to 1600 K D-atom beam for 5 s at a flux of 1012-1013 atoms/cm2 s.
8. This article talks about the papers that have demonstrated that ortho and para positions are most favorable for hydrogen attachment i.e. energetically ortho and para dimmers are most stable configuration at the basel plane of graphene and graphite (1-3).
9. Authors did not observe significant difference in dimer formation on graphite or graphene. In their study, hydrogen monomers were also observed on graphene surface as opposed to fact that there is no reported study of hydrogen monomer on graphite.
10. The observation of monomer suggests that atomic hydrogen is more strongly bind to graphene than graphite.
11. Authors have fond that at low coverage the majority of hydrogen adsorbate follows 6x6 modulation of the SiC surface.
12. Both theory and experiment have suggested that deformation plays a key role in the reactivity of graphene.
13. Barrier for sticking decreases for higher curvature surface and binding energy increases. The decrease in barrier for sticking can be explained by hydrogen atom (H) chemisorptions causes change of sp2 hybridization to sp3, thus relaxation of carbon atoms towards hydrogen adsorbate.
14. During STM study and hydrogen deposition the sample was kept at room temperature. In STM adsorbed hydrogen should be visible as bright protrusion.
15. As the coverage is increased hydrogen tends to make large clusters similar to hydrogen on graphite.
16. It has been predicted theoretically that one sided hydrogenation is thermodynamically unstable (4,5).
17. Authors have observed that tip induced tunneling can desorb the hydrogen from the surface that implies weak binding of hydrogen with the graphene surface. They also observe that graphene can be recovered after annealing the hydrogen adsorbed surface at 800 0C.
Conclusion
18. Authors have studied graphene grown by thermal annealing on SiC (0001) substrate and found presence of different dimer structure at low hydrogen coverage and hydrogen clusters at high hydrogen coverage (high coverage of hydrogen is similar to the hydrogen on graphite)
19. Authors have observed tip induced desorption of hydrogen from graphene surface and also find the graphene can be recovered after annealing the hydrogenated graphene at 800 0C.
References
(1) Casolo, S.; Lovvik, O. M.; Martinazzo, R.; Tantardini, G. F. J. Chem. Phys.2009, 130, 054704.
(2) Hornekaer, L.; Sljivancanin, Z.; Xu, W.; Otero, R.; Rauls, E.; Stensgaard, I.; Laegsgaard, E.; Hammer, B.; Besenbacher, F. Phys. ReV. Lett. 2006, 96, 156104.
(3) Ferro, Y.; Teillet-Billy, D.; Rougeau, N.; Sidis, V.; Morisset, S.; Allouche, A. Phys. ReV. B 2008, 78, 8.
(4) Sofo, J. O.; Chaudhari, A. S.; Barber, G. D. Phys. ReV. B 2007, 75, p153401.
(5) Boukhvalov, D. W.; Katsnelson, M. I.; Lichtenstein, A. I. Phys. ReV. B 2008, 77, 035427
Analysis
1. This paper is published in |J|A|C|S| Communications in May, 2009 and presents Scanning Tunneling Microscopy study of hydrogen adsorption on graphene grown by thermal annealing of silicon carbide (SiC).
2. Chemical reaction of hydrogen atom with graphene is a potentially viable method to introduce bad gap in graphene (semi metal) to use it for electronic devices.
3. Authors present Scanning Tunneling Microscopy (STM) study of adsorbed hydrogen atoms at the basel plane of graphene grown on SiC.
4. At low coverage, hydrogen dimer structure and at high coverage hydrogen clusters are observed by STM.
5. Hydrogen dimer formation occur at the high tunneling probability area of graphene that is altered by 6×6 reconstruction of SiC(0001) (1×1) surface.
6. Observed hydrogenation of graphene on SiC was reversible by thermal annealing.
7. For hydrogenation graphene was exposed to 1600 K D-atom beam for 5 s at a flux of 1012-1013 atoms/cm2 s.
8. This article talks about the papers that have demonstrated that ortho and para positions are most favorable for hydrogen attachment i.e. energetically ortho and para dimmers are most stable configuration at the basel plane of graphene and graphite (1-3).
9. Authors did not observe significant difference in dimer formation on graphite or graphene. In their study, hydrogen monomers were also observed on graphene surface as opposed to fact that there is no reported study of hydrogen monomer on graphite.
10. The observation of monomer suggests that atomic hydrogen is more strongly bind to graphene than graphite.
11. Authors have fond that at low coverage the majority of hydrogen adsorbate follows 6x6 modulation of the SiC surface.
12. Both theory and experiment have suggested that deformation plays a key role in the reactivity of graphene.
13. Barrier for sticking decreases for higher curvature surface and binding energy increases. The decrease in barrier for sticking can be explained by hydrogen atom (H) chemisorptions causes change of sp2 hybridization to sp3, thus relaxation of carbon atoms towards hydrogen adsorbate.
14. During STM study and hydrogen deposition the sample was kept at room temperature. In STM adsorbed hydrogen should be visible as bright protrusion.
15. As the coverage is increased hydrogen tends to make large clusters similar to hydrogen on graphite.
16. It has been predicted theoretically that one sided hydrogenation is thermodynamically unstable (4,5).
17. Authors have observed that tip induced tunneling can desorb the hydrogen from the surface that implies weak binding of hydrogen with the graphene surface. They also observe that graphene can be recovered after annealing the hydrogen adsorbed surface at 800 0C.
Conclusion
18. Authors have studied graphene grown by thermal annealing on SiC (0001) substrate and found presence of different dimer structure at low hydrogen coverage and hydrogen clusters at high hydrogen coverage (high coverage of hydrogen is similar to the hydrogen on graphite)
19. Authors have observed tip induced desorption of hydrogen from graphene surface and also find the graphene can be recovered after annealing the hydrogenated graphene at 800 0C.
References
(1) Casolo, S.; Lovvik, O. M.; Martinazzo, R.; Tantardini, G. F. J. Chem. Phys.2009, 130, 054704.
(2) Hornekaer, L.; Sljivancanin, Z.; Xu, W.; Otero, R.; Rauls, E.; Stensgaard, I.; Laegsgaard, E.; Hammer, B.; Besenbacher, F. Phys. ReV. Lett. 2006, 96, 156104.
(3) Ferro, Y.; Teillet-Billy, D.; Rougeau, N.; Sidis, V.; Morisset, S.; Allouche, A. Phys. ReV. B 2008, 78, 8.
(4) Sofo, J. O.; Chaudhari, A. S.; Barber, G. D. Phys. ReV. B 2007, 75, p153401.
(5) Boukhvalov, D. W.; Katsnelson, M. I.; Lichtenstein, A. I. Phys. ReV. B 2008, 77, 035427
Ferromagnetism in Semihydrogenated Graphene Sheet
J. Zhou, Q. Wang, Q. Sun, X. S. Chen, Y. Kawazoe, and P. Jena
Single layer of graphite (graphene) was predicted and later experimentally confirmed to undergo metal-semiconductor transition when fully hydrogenated (graphane). Using density functional theory we show that when half of the hydrogen in this graphane sheet is removed, the resulting semihydrogenated graphene (which we refer to as graphone) becomes a ferromagnetic semiconductor with a small indirect gap. Half-hydrogenation breaks the delocalized π bonding network of graphene, leaving the electrons in the unhydrogenated carbon atoms localized and unpaired. The magnetic moments at these sites couple ferromagnetically with an estimated Curie temperature between 278 and 417 K, giving rise to an infinite magnetic sheet with structural integrity and magnetic homogeneity. This is very different from the widely studied finite graphene nanostrucures such as one-dimensional nanoribbons and two-dimensional nanoholes, where zigzag edges are necessary for magnetism. From graphene to graphane and to graphone, the system evolves from metallic to semiconducting and from nonmagnetic to magnetic. Hydrogenation provides a novel way to tune the properties with unprecedented potentials for applications.
Analysis
1. Published in June 2009 this is a DFT based theoretical study that talks about the effect of hydrogenation of graphene film.
2. Fully hydrogenated graphene, also known as “graphane” was experimentally prepared by K. S. Novoselov et al. and the results were published in Science (30 January 2009, Vol. 323). It was demonstrate that hydrogenation of graphene is reversible.
3. This paper discusses how the properties of graphene film will change if half hydrogenation can be achieved.
4. It has been demonstrated that hydrogen has potential to alter the properties of non-magnetic as well as magnetic materials.
5. Both graphene and graphane are non-magnetic in nature.
6. Using DFT calculation, the author of this paper shows that half hydrogenated graphene (authors call it graphone) is ferromagnetic in nature.
7. Partial saturation of carbon atoms in graphene breaks the pi (π) bonds and the p-electrons associated with unhydrogenated bond are localized and unpaired.
8. Along with magnetic properties, electronic properties of graphene also changes due to partial (half) hydrogenation of graphene. For example graphone becomes an indirect band gap semiconductor with a very small band gap as opposed to graphene (zero band gap conductors) or graphane (very large band gap insulator).
9. Magnetism also arises in 0D graphene nanodots, 1D graphene nanoribbons and 2D graphen nanoholes due to the presence of zigzag edges. However, Okada showed that arm chair nanoribbons are more favorable than zigzag ones. Thus the challenge is to produce the carbon material with zigzag edges still remains.
10. This theoretical study shows that magnetic graphene material can be achieved by selective hydrogenation of graphene.
11. When half the carbon atoms are hydrogenated, strong σ bonds are formed between C-H atoms and pi (π) bonds are broken, thus leaving electrons in unhydrogenated C atoms unpaired and localized.
12. Authors have found that unhyddrogenated C atoms have magnetic moment of 1 µB and are main cause of magnetization due to unpaired 2p electrons.
13. Authors have found that controlling the hydrogenation, i.e. after removing the hydrogen from the hydrogenated graphene (graphone) revert the magnetism and thus makes graphene non magnetic. The reason lies in that fact that two unsaturated carbon atoms form nearest neighbor where pz orbital forms pi (π) bonds, which quenches the magnetization.
14. Based on the above claims, authors infer that controlling the amount of hydrogenation can control the magnetic properties of graphene. Thus controlling the hydrogenation and the geometry is key to achieve predicted ferromagnetism.
Conclusion
15. In conclusion, it is shown theoretically that it is possible introduce ferromagnetism into graphene by surface modification with hydrogenation.
16. This method has the following advantages over the existing ones:
*.It is not necessary to substitutionally dope C atoms by foreign atoms such as transition metal atoms or B and N.
*. It is not necessary to cut the two-dimensional graphene into finite systems with zigzag edges like one-dimensional nanoribbon or zero-dimensional quantum dots.
*. It is not necessary to introduce carbon vacancies like nanoholes in graphene, where magnetism appears at the edge of the vacancy.
17. The disavantages of the above processes are the following:
*. Integrity of graphene structure is destroyed by vacancies or by substitution with foreign atoms or cutting into nanosize, where magnetism is inhomogeneously distributed.
*. In practical applications, nanoribbons need to be assembled, and the magnetic moments in the zigzag edges can be easily quenched in the assembly.
*. Magnetism can also be introduced by transition metal adsorption, but due to the strong d-d interactions, it is easy for transition metal atoms to form clusters.
18. All these methods are difficult to control and they are not reversible. Therefore, reversibility, controllability, integrity of structure, and homogeneity of magnetism make the graphone sheet very appealing for further experimental study.
Single layer of graphite (graphene) was predicted and later experimentally confirmed to undergo metal-semiconductor transition when fully hydrogenated (graphane). Using density functional theory we show that when half of the hydrogen in this graphane sheet is removed, the resulting semihydrogenated graphene (which we refer to as graphone) becomes a ferromagnetic semiconductor with a small indirect gap. Half-hydrogenation breaks the delocalized π bonding network of graphene, leaving the electrons in the unhydrogenated carbon atoms localized and unpaired. The magnetic moments at these sites couple ferromagnetically with an estimated Curie temperature between 278 and 417 K, giving rise to an infinite magnetic sheet with structural integrity and magnetic homogeneity. This is very different from the widely studied finite graphene nanostrucures such as one-dimensional nanoribbons and two-dimensional nanoholes, where zigzag edges are necessary for magnetism. From graphene to graphane and to graphone, the system evolves from metallic to semiconducting and from nonmagnetic to magnetic. Hydrogenation provides a novel way to tune the properties with unprecedented potentials for applications.
Analysis
1. Published in June 2009 this is a DFT based theoretical study that talks about the effect of hydrogenation of graphene film.
2. Fully hydrogenated graphene, also known as “graphane” was experimentally prepared by K. S. Novoselov et al. and the results were published in Science (30 January 2009, Vol. 323). It was demonstrate that hydrogenation of graphene is reversible.
3. This paper discusses how the properties of graphene film will change if half hydrogenation can be achieved.
4. It has been demonstrated that hydrogen has potential to alter the properties of non-magnetic as well as magnetic materials.
5. Both graphene and graphane are non-magnetic in nature.
6. Using DFT calculation, the author of this paper shows that half hydrogenated graphene (authors call it graphone) is ferromagnetic in nature.
7. Partial saturation of carbon atoms in graphene breaks the pi (π) bonds and the p-electrons associated with unhydrogenated bond are localized and unpaired.
8. Along with magnetic properties, electronic properties of graphene also changes due to partial (half) hydrogenation of graphene. For example graphone becomes an indirect band gap semiconductor with a very small band gap as opposed to graphene (zero band gap conductors) or graphane (very large band gap insulator).
9. Magnetism also arises in 0D graphene nanodots, 1D graphene nanoribbons and 2D graphen nanoholes due to the presence of zigzag edges. However, Okada showed that arm chair nanoribbons are more favorable than zigzag ones. Thus the challenge is to produce the carbon material with zigzag edges still remains.
10. This theoretical study shows that magnetic graphene material can be achieved by selective hydrogenation of graphene.
11. When half the carbon atoms are hydrogenated, strong σ bonds are formed between C-H atoms and pi (π) bonds are broken, thus leaving electrons in unhydrogenated C atoms unpaired and localized.
12. Authors have found that unhyddrogenated C atoms have magnetic moment of 1 µB and are main cause of magnetization due to unpaired 2p electrons.
13. Authors have found that controlling the hydrogenation, i.e. after removing the hydrogen from the hydrogenated graphene (graphone) revert the magnetism and thus makes graphene non magnetic. The reason lies in that fact that two unsaturated carbon atoms form nearest neighbor where pz orbital forms pi (π) bonds, which quenches the magnetization.
14. Based on the above claims, authors infer that controlling the amount of hydrogenation can control the magnetic properties of graphene. Thus controlling the hydrogenation and the geometry is key to achieve predicted ferromagnetism.
Conclusion
15. In conclusion, it is shown theoretically that it is possible introduce ferromagnetism into graphene by surface modification with hydrogenation.
16. This method has the following advantages over the existing ones:
*.It is not necessary to substitutionally dope C atoms by foreign atoms such as transition metal atoms or B and N.
*. It is not necessary to cut the two-dimensional graphene into finite systems with zigzag edges like one-dimensional nanoribbon or zero-dimensional quantum dots.
*. It is not necessary to introduce carbon vacancies like nanoholes in graphene, where magnetism appears at the edge of the vacancy.
17. The disavantages of the above processes are the following:
*. Integrity of graphene structure is destroyed by vacancies or by substitution with foreign atoms or cutting into nanosize, where magnetism is inhomogeneously distributed.
*. In practical applications, nanoribbons need to be assembled, and the magnetic moments in the zigzag edges can be easily quenched in the assembly.
*. Magnetism can also be introduced by transition metal adsorption, but due to the strong d-d interactions, it is easy for transition metal atoms to form clusters.
18. All these methods are difficult to control and they are not reversible. Therefore, reversibility, controllability, integrity of structure, and homogeneity of magnetism make the graphone sheet very appealing for further experimental study.
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