Graphene: The First Studies of its

Transcription

1 Graphene: The First Studies of its Formation, Structure and Energetics Jack Blakely, MSE, Cornell Single crystal surfaces, mainly Ni but also Pt,Pd,Co Formation of surface carbon layers by near-equilibrium segregation Characterization of monolayer and multilayer graphite by electron spectroscopy and low energy electron diffraction Monolayer graphite formation as a phase transition Range of stability of the graphene monolayer before multilayer formation Binding energies of carbon in the different surface states Interplay between surface morphology (steps,facets) and adsorbed carbon Step-free substrates for graphene support SiO 2, Al 2 O 3

2 Some references to early Cornell work on Carbon layer formation on Metals "Electron Energy Losses in Thin Graphite Layers", H.R. Patil and J.M.Blakely, J.C Shelton, J. Appl. Phys. 9, 3806 (1974). "Equilibrium Segregation of Carbon to a Nickel (111) Surface: A Surface Phase Transition", J.C. Shelton, H.R. Patil and J.M. Blakely, Surface Science 43, 25 (1974). "Equilibrium Adsorption and Segregation", J.M. Blakely and J.C. Shelton, review article published in "Surface Physics of Materials", edited by J.M. Blakely, Academic Press (1975). "Binding Energies of Carbon to Ni (100) from Equilibrium Segregation Studies", L.C. Isett and J.M. Blakely, Surface Science 47, 645 (1975). "Binding of Carbon Atoms at a Stepped Nickel Surface", L.C. Isett and J.M. Blakely, J. Vac. Sci. Tech. 12, 237 (1975). "Segregation Isosteres for Carbon at the (100) Surface of Nickel", L.C. Isett and J.M. Blakely, Surface Science 58, 397 (1976). "Structure of the Gas-Solid Interface", J.M. Blakely, in proceedings of the Darken Conference: Physical Chemistry in Metallurgy (1976). "Reconstruction of Stepped Nickel Surfaces", H.V. Thapliyal and J.M. Blakely, J. Vac. Sci. Tech. 15, 600 (1978). "Carbon Layer Formation on the Pt(111) Surface as a Function of Temperature", J.C. Hamilton and J.M. Blakely, J. Vac. Sci. Tech. 15, 559 (1978). "Segregation to Surfaces: Dilute Alloys of the Transition Metals, J.M. Blakely, CRC Critical Reviews in Solid State Sciences and Materials Sciences, Nov. (1978), p "Structure and Phase Transitions of Segregated Surface Layers", J.M. Blakely and H.V. Thapliyal, in Interfacial Segregation, A.S.M. (1978). "Carbon Monolayer Phase Condensation on Ni(111)", M. Eizenberg and J.M. Blakely, Surface Sci. 82, 228 (1979). "Surface Carbon Segregation in Dilute Alloys of Pt, Pd and Co", J.C. Hamilton and J.M. Blakely, Surface Science 91, 199 (1980). "Carbon Interaction with Ni Surfaces: Monolayer Formation and Structural Stability", M. Eizenberg and J.M.Blakely, J. Chem. Phys. 71, 3467 (1979). "Morphology and Composition of Crystal Surfaces", Chapter 1, Vol. 1 of Chemical Physics of Solid Surfaces and Heterogeneous Catalysis, edited by D.P. Woodruff and D.A. King (1981). "Estimating the Density of Carbon Atoms on a Ni Catalyst Surface in Equilibrium with a Carbonaceous Gas", R. Ramanathan and J.M. Blakely, Appl. Surf. Sci., 29, 427 (1987).

3 Various graphite-related forms of carbon. Figure from Geim & Novoselov 2010 Carbon-carbon bond energy is ~2.5eV, spacing~0.12nm and not very sensitive to small displacements normal to basal plane Distance between (0001) planes in graphite is ~0.35nm

4 Some of the methods to make and characterize graphene Cleavage of 3D graphite Reduction of graphitic oxide C 8 O 2 (OH) 2 (intercalated graphite) Decomposition of SiC Challenge to identify graphene or thin graphite layers Optical reflection from graphene on SiO 2 Raman microscopy, graphene and multilayer graphite have different characteristic spectra Segregation from solid or liquid metals containing dissolved carbon Secondary electron spectroscopy Auger, py ARUPS Decomposition of hydocarbon etc or other carbon containing gas Electron and X-ray diffraction

5 Relationship between equilibrium adsorption and equilibrium segregation from solid/liquid solution

6 On Ni(100) the low carbon coverage region shows Langmuir type of behavior Data on Ni surfaces shown here is for low carbon coverages Data on Fe(100) from Grabke et al Ni(100)6 o [013] is a stepped surface vicinal to (100) Surfaces near Ni(111) do not show this smooth variation of carbon concentration with temperature "Segregation Isosteres for Carbon at the (100) Surface of Nickel", L.C. Isett and J.M. Blakely, Surface Science 58, 397 (1976).

7 Segregation Isosteres for low carbon coverage region on Ni(100) The fractional coverage, θ, is normalized to 1/4 monolayer. At carbon coverages on Ni(100) below ~1/4 monolayers the variation of coverage with temperature is gradual By using a range of bulk doping levels to vary the carbon chemical potential isosteres can be constructed and give a heat of segregation to (100) "Segregation Isosteres for Carbon at the (100) Surface of Nickel", L.C. Isett and J.M. Blakely, Surface Science 58, 397 (1976).

8 Variation of carbon concentration as a function of temperature on Ni(111) due to segregation The surface monolayer phase, graphene, has a range of stability of ~ 100K around ~1100K "Equilibrium Segregation of Carbon to a Nickel (111) Surface: A Surface Phase Transition", J.C. Shelton, H.R. Patil and J.M. Blakely, Surface Science (1974)

9 Model of graphene as an epitaxial carbon layer on the Ni(111) surface. From LEED patterns and quantitative Auger spectroscopy the overlayer is identified as p(1x1)c 1)C 2 The lattice mismatch between Ni(111) and graphite (0001) is < 1% Without extensive analysis of the variation of diffracted intensities versus electron energy the exact lateral and vertical positions of the carbons is not well defined.

10 Carbon Binding Energies in and on Ni crystals Note the small fractional change for different sites. p(1x1)c 2 on Ni(111) is graphene "Binding Energies of Carbon to Ni (100) from Equilibrium Segregation Studies", L.C. Isett and J.M. Blakely, Surface Science 47, 645 (1975). "Structure and Phase Transitions of Segregated Surface Layers", JM J.M. Blakely and H.V. Thapliyal, in Interfacial Segregation, A.S.M. (1978).

11 Dependence of the temperature of formation and range of stability of the graphene phase on the doping level of the Ni(111) crystal "Carbon Monolayer Phase Condensation on Ni(111)", M. Eizenberg and J.M. Blakely, Surface Sci. 82, 228 (1979).

12 Dependence of the graphene formation temperature and range of stability on the orientation or step structure of the Ni substrate The formation of graphene is accompanied by facetting to expose {111} planes Data for Ni surfaces along the [110] zone "Carbon Interaction with Ni Surfaces: Monolayer Formation and Structural Stability", M. Eizenberg and J.M.Blakely, J. Chem. Phys. 71, 3467 (1979).

13 Carbon Segregation to Ni surfaces of various orientations Formation of the graphite monolayer phase seems to be insensitive to the orientation of the Ni substrate. "Carbon Interaction with Ni Surfaces: Monolayer Formation and Structural Stability", M. Eizenberg and J.M.Blakely, J. Chem. Phys. 71, 3467 (1979).

14 Electron diffraction from stepped surfaces shows splitting of reflections corresponding to the average inter-step spacing. Segregation of carbon at low coverages causes a pairing or clustering of the steps. "Reconstruction of Stepped Nickel Surfaces", H.V. Thapliyal and J.M. Blakely, J. Vac. Sci. Tech. 15, 600 (1978).

15 Segregation of Carbon to surfaces of different materials Pt, Pd, Co Surfaces studied included Pt(100), Pd(100), Pd(111)and Co(0001) Pd(100), Pd(111) and Co(0001) all exhibited the formation of a monolayer graphite (graphene) phase; similar il to Ni(111) Pt(100) showed no signification carbon accumulation before the onset of multilayer graphite formation. These results are consistent with the Ni data and again suggest that t the exact orientation of the substrate has little effect on the quasi-equilibrium formation of the graphene phase. "Surface Carbon Segregation in Dilute Alloys of Pt, Pd and Co", J.C. Hamilton and J.M. Blakely, Surface Science 91, 199 (1980).

16 Occurrence of the graphene phase of carbon on Ni surfaces is insensitive iti to the exact Ni orientation ti The morphology and transport properties of graphene formed on Ni surfaces that are atomically rough or have atomic steps or facets may be different from that formed on flat (111)

17 Ranges of stability of different carbon states on Ni(111) surfaces in equilibrium with dissolved C from our data. The corresponding ranges for coexistence with CO/CO 2 and CH 4 /H 2 gas mixtures are calculated using the data.

18 Typical Atomic Step Array on a Surface Vicinal to Si(001) 2 x 1 1x2 Terrace 2 x 1 Step "STM Studies of Phase Separation on Si(100) Surfaces with Periodic Step Arrays", C.C. Umbach, M.E. Keeffe, and J.M. Blakely,J. Amer Vac. Soc.,B9,721,(1991).

19 Making Step-free substrates 1 Growth of Si on Mesa Structures using Energetic Beam of Si 2 H 6 In this work the trench width and depth were 2μm and 0.5μm respectively, with the mesa edge dimensions being 2, 4, 8,12 and 24μm. "A Growth Method for Creating Arrays of Atomically Flat Mesas on Silicon", Doohan Lee, Jack Blakely, Todd W. Schroeder and J. R. Engstrom, Applied Physics Letters, 78,1349, (2001).

20 Formation of arrays of step-free mesas on Si(111) due to Si deposition in the step-flow growth regime Doohan Lee, Jack Blakely, Todd Schroeder and Jim Engstrom, Appl. Phys. Lett.,78,1349, (2001)

21 Patterned Si Surface used as starting structure for producing large Atomically Flat Areas by the Sublimation Method L 1 = 2-50 microns L 2 =1micron Ridges ~ 1micron high "Atomic Step Distributions on Annealed Periodic Si(001) Gratings, So Tanaka, C.C. Umbach, Jack Blakely, Ruud Tromp and Marion Mankos, J. Vac. Sci. Technol. A, 15, p (1997). "Formation and Stability of Large Step-free Areas on Si(001) and Si(111), Doohan Lee and Jack Blakely, Surface Science,445,32,(2000).

22 Removal of Steps by Flow into the ridges step bunch single step evaporated atoms Steps flow into ridges and become pinned No new steps nucleated Allows surfaces to be prepared with up to 50μm x 50μm step free areas

23 Capacitor Structures formed both on Step-Free Areas between the Ridges and on normal stepped wafer surface Valerian Ignatescu and Jack.M. Blakely, ""Leakage currents through thin silicon oxide grown on atomically flat silicon surfaces"", MRS Proceedings, 849, KK7.11.1(2005).

24 Comparison of Stepped Surface Morphology Before/After Oxidation Upper: before oxidation Lower: after oxidation ~7nm oxide thickness 5μm 5μm Steps did not move significantly during oxidation! "Surface and Interfacial Morphology of Oxides on Si(111) with Ultra-Low Atomic Step Density", Antonio Oliver and Jack Blakely, JVST B, 18, 2862, (2000)

25 Formation of step-free mesas on Al 2 O 3 (0001) by high temperature annealing only The dynamics of the step motion controlled by a balance of surface The dynamics of the step motion controlled by a balance of surface diffusion and evaporation rates Kee-Chul Chang, Doohan Lee, Christopher Umbach and Jack Blakely,. "Scanning Probe Microscopy of Atomically Engineered Silicon and Sapphire Surfaces" in Microscopy of Semiconducting Materials, Inst. Physics Conferences #180, UK, , (2004).

26 Occurrence of the graphene phase of carbon on Ni surfaces is insensitive iti to the exact Ni orientation ti The morphology and transport properties of graphene formed on Ni surfaces that are atomically rough or have atomic steps or facets may be different from that formed on flat (111)