Dirac Fermions in Graphene and Graphiteâa view from angle ...

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Figure 5.1. Dispersions through the K point taken from the as-grown single layer graphene (a) and from the sample with the highest doping of NO 2 (c). Panels (b) and (d) show the angle integrated spectra for (a) and (c) respectively. In panels (c) and (d) note the appearance of the NO 2 states at ≈ 5 and 11 eV below E F . 5.2 Valence band of graphene with adsorption of nitrogen dioxide Before discussing the effect of molecular doping on the electronic structure of bilayer and single layer graphene, we shall prove that NO 2 is present on the surface of our samples. Figure 5.1 shows ARPES data of the valence band of a single layer graphene sample before (panel a) and after (panel b) exposure to NO 2 . The exposure to NO 2 leads to the appearance of two broad peaks centered at 5 eV and 11 eV below E F . Similar structures were reported previously in the photoemission studies of NO 2 adsorbed on the surfaces of W(110) 90 and Si 91 . In the case of W(110), the spectra of NO 2 adsorbed on the surface are similar to that of the gaseous N 2 O 4 , suggesting dimerization of NO 2 on the surface 90 . Previous study of electron energy loss spectroscopy (EELS) of NO 2 adsorbed on graphite at 90 K has found vibrational modes which were consistent with the formation of NO 2 dimers 92 , suggesting that a similar dimerization might also take place in the present study. An exact estimation of the amount of NO 2 adsorbed on the surface of our samples is difficult due to the significant photo-stimulated desorption. In fact, to minimize the impact of the photo desorption on our data we had to intentionally reduce the photon flux and accumulate the spectra for only 30 seconds. This explains a relatively poor statistics of the data taken from graphene with adsorbed NO 2 . 37
Figure 5.2. (a) Dispersions through the K point taken in as-grown bilayer graphene. Data were taken along the red line through the K point shown in the inset. (b) Dispersions taken after 0.6 L (1 Langmuir=10 6 torr · s) NO 2 adsorption. (c) data taken 6 minutes after panel b. (d) EDCs taken at k 1 , k 2 , k 3 and k 4 as labeled on the top of panels a and b. The black and red arrows point to the midpoint of the leading edge, which is shifted to higher binding energy after NO 2 doping. (e) Zoom in of data shown in panel b. The white dotted line is a guide for the eye for the dispersions. (f) Momentum distribution curve (MDC) at the energy labeled by the dotted black line in panel d. The dots are the raw data and the solid line is the fit using three Lorentzian peaks simulating the cross-section of the hat-like dispersion in panel e. 5.3 Gas adsorption and metal-insulator transition in bilayer graphene Fig. 5.2 shows ARPES data taken on bilayer graphene before and after NO 2 adsorption at 20K. Panel a shows dispersions through the K point in the as-grown graphene. The Dirac point energy is readily identified at the binding energy of 0.3 eV. Strictly speaking, the Dirac point, which separates the upward dispersing conduction band from downward dispersing valence band, is located within the gap. Existence of this gap between the conduction and valence bands in bilayer graphene has already been shown in previous ARPES studies 78,63 . Panel b shows data taken right after dosing NO 2 gas at 1×10 −8 torr for 60 seconds (0.6 Langmuir of NO 2 gas). Clearly, adsorption of NO 2 leads to the shift of the entire band structure toward E F , which indicates hole doping of the bilayer graphene. Although the electron doping of epitaxial graphene has been studied 78 , this is the first demonstration of hole doping of epitaxial graphene. Close inspection of the region near E F shows that it lies within the gap separating the conduction and valence bands with the latter band located above E F . To illustrate this, in Fig. 5.2(d) we plot the energy distribution curves (EDCs) at the momentum values where the bands are closest to E F for data shown in panel a (black curves) and panel b (red curves). It is obvious that after NO 2 adsorption, the EDCs move toward higher binding energy, with the leading edge shifted by ≈ 30 meV. The shift of the leading edge is characteristic of a gap, showing unambiguously that the sample in panel b is semiconducting. This process is reversible: the sample becomes a metal again if we remove most of the NO 2 from the surface by exposure to high photon flux, or by annealing the sample. Fig. 5.2(c) displays the data taken after desorption of NO 2 by photon flux. The bands of graphene are shifted back to their original positions with the Dirac point at 0.3 eV below E F . It is clear however that a small fraction of NO 2 is still present, since there is still considerable broadening by up to 200% in the photoemission signal in Fig.5.2(c), which might be associated with the scattering of electrons by the residual NO 2 molecules. By thermal annealing at ≈ 400C the residual molecules can be completely removed, as confirmed by the sharpening of the peaks and the complete recovery of the line width of pure graphene. In addition to demonstrating molecular hole doping of graphene, the data in Fig.5.2(b) also allow an unobscured view of the top of the valence band which exhibits a characteristic hat-like shape (see Fig.5.2(e)). Such shape was calculated 77,93 but to the best of our knowledge it was not observed in the previous ARPES studies with the present degree of clarity, mainly because of the contribution from the conduction band (see 38
Page 1 and 2: Dirac Fermions in Graphene and Grap
Page 3 and 4: Dirac Fermions in Graphene and Grap
Page 5 and 6: Contents Abstract 1 Contents Acknow
Page 7 and 8: 7.6 Conclusions and implication of
Page 9 and 10: Curriculum Vitæ Shuyun Zhou Educat
Page 11 and 12: • Selected as Science Highlight f
Page 13 and 14: The unit cell of graphene contains
Page 15 and 16: Projecting this equation into φ(r
Page 17 and 18: Figure 1.5. Schematic drawing of th
Page 19 and 20: Chapter 2 Experimental techniques 2
Page 21 and 22: dispersion E(k z ). Thus k z values
Page 23 and 24: where ɛ is infinitely small. Accor
Page 25 and 26: Langmuirs (1L=10 −6 torr·s) to c
Page 27 and 28: mean free path, E kin is the kineti
Page 29 and 30: a b c d e f graphite SiC f graphite
Page 31 and 32: Figure 3.3. XPS results showing dat
Page 33 and 34: Figure 3.5. Intensity map at -1 eV
Page 35 and 36: Chapter 4 Gap opening in epitaxial
Page 37 and 38: Figure 4.2. Observation of the gap
Page 39 and 40: the EDCs as well as the region of e
Page 41 and 42: Figure 4.7. Dispersions measured in
Page 43 and 44: Figure 4.9. (a-c) LEEM images taken
Page 45 and 46: Figure 4.10. Proposed mechanism for
Page 47: Chapter 5 Metal-insulator transitio
Page 51 and 52: Figure 5.3. (a-f) Data taken throug
Page 53 and 54: Chapter 6 Dirac fermions and effect
Page 55 and 56: Figure 6.2. Linear Λ-shaped disper
Page 57 and 58: Figure 6.4. Detailed low energy dis
Page 59 and 60: Figure 6.6. Detailed dispersion nea
Page 61 and 62: Chapter 7 ARPES study of partially
Page 63 and 64: Figure 7.2. (a) Fermi energy intens
Page 65 and 66: direction, i.e. perpendicular to gr
Page 67 and 68: Figure 7.5. ARPES intensity map mea
Page 69 and 70: Figure 7.7. (a) ARPES intensity map
Page 71 and 72: Figure 7.9. (a) ARPES intensity map
Page 73 and 74: 19. Hüfner, S. Photoelectron Spect
Page 75 and 76: 59. Jiang, Y. J., Yao, D.-Y., Carls
Page 77 and 78: 99. Luk’yanchuk, I. A. & Kopelevi
Page 79 and 80: List of Figures 1.1 (a) Sp 2 bondin
Page 81 and 82: 4.8 Thickness dependence of E D and
Page 83 and 84: 7.2 (a) Fermi energy intensity map.

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