Universita’ Degli Studi di Milano-Bicocca (UMB), Italy
Understanding the adsorption mechanisms of organic molecules on graphene and their subsequent influence on the electronic and magnetic properties of this interface is essential in designing graphene based devices. In this thesis we perform first principles calculations based on density functional theory (DFT) in an effort to understand these phenomena. Most organic electronic devices are composed of interfaces formed by the organic overlayer and a metallic electrode. Understanding the charge transfer dynamics at the interface would help engineer efficient organic devices.
With this in mind, the first part of research we present is the adsorption of core-excited organic molecules on graphene. We predict the induction or suppression of magnetism in the valence shell of physisorbed and chemisorbed organic molecules on graphene occurring on the femtosecond time scale as a result of core level excitations. We consider three organic molecules: Pyridine – whose interaction with graphene is mainly facilitated by van der Waals forces, Picoline radical – an intermediate case where there is a strong van der Waals interaction of the pyridine π ring with graphene but a covalent bonding of the molecule and pyridine radical – where the interaction is mainly through covalent bonding, and study the ground state and N 1s core excited state electronic properties for these systems. For physisorbed molecules, where the interaction with graphene is dominated by van der Waals forces and the system is non-magnetic in the ground state, numerical simulations based on density functional theory show that the valence electrons relax towards a spin polarized configuration upon excitation of a core-level electron. The magnetism depends on efficient electron transfer from graphene on the femtosecond time scale. On the other hand, when graphene is covalently functionalized, the system is magnetic in the ground state showing two spin dependent mid gap states localized around the adsorption site. At variance with the physisorbed case upon core-level excitation, the LUMO of the molecule and the mid gap states of graphene hybridize and the relaxed valence shell is not magnetic anymore.
Next we discuss the interplay between the charge transfer lifetime of core excited organic molecules adsorbed on graphene and the modification of its electronic structure by a variable coupling with a metal substrate. The nitrogen 1s core electron of 1,10 – bipyridine (C5H4N)2 is photo-excited and adsorbed on bilayer graphene/nickel(111) (BP/BLG/Ni) and epitaxially grown graphene/Ni(111) (BP/ EG/ Ni). We predict from first principle calculations that the charge transfer time of core excited molecules depend strongly on the coupling of graphene to the underlying Ni substrate. In the ground state, the LUMO of the molecule is quite strongly coupled with the substrate in both the cases (BP/BLG/Ni and BP/EG/Ni). In the case of BP/BLG/Ni, the layer of graphene in contact with nickel substrate strongly hybridizes but the upper layer of graphene remains fairly decoupled. The excited molecular LUMO* finds very few states of graphene close to the Dirac point at the Fermi level to hybridize with. This leads to a decoupled molecular LUMO* and the lifetime increases significantly (∼ 116 fs). But in the case of BP/EG/Ni, the strong hybridization of graphene with the underlying nickel substrate significantly distorts the electronic structure of graphene generating states close to the Fermi level. The LUMO* of the molecule strongly couples with these states resulting in a substantially smaller lifetime (∼ 33 fs). We also find experimental evidence to confirm this trend by performing core-hole-clock spectroscopy. The resonant charge transfer lifetime we find is ∼ 30 fs±5 fs for the BP/BLG/Ni and ∼ 4 fs±1 fs for the BP/EG/Ni, thus clearly demonstrating the effect of substrate on the charge transfer dynamics of organic molecules on graphene.
This thesis is structured as follows: In the second chapter we review graphene and recent advances in graphene technology. The physical properties of graphene and the study of its electronic structure from Tight-Binding approach, DFT and experiments are discussed. This is followed by a summary of its chemical, electronic, magnetic and mechanical properties. Several experimental techniques to synthesize graphene such as exfoliation, epitaxial routes and growth on metal substrates are detailed followed by a discussion on the electronic applications of graphene.
In the third chapter, we briefly review the electronic structure theory for many electron systems such as Hartree-Fock approach, Thomas-Fermi model, HohenbergKohn and Kohn-Sham theorems which setup the backbone for density functional theory. Approximations within DFT such as the local density approximation, local spin density approximation and generalized gradient approximation are discussed. We also summarize the non-equilibrium Green’s function approach, an alternate electronic structure approach which is used to simulate bulk continuum by coupling the system to self-energy operators.
These methods are mainly used to calculate the ground state electronic structure of systems. In the fourth chapter we discuss the excited state methods which can successfully simulate spectroscopic experiments such as x-ray photo-emission spectroscopy (XPS) (evaluated by initial state approximation and ∆ SCF approximation), near-edge x-ray absorption fine structure (NEXAFS) (calculated by the transition potential approach) and core-hole-clock (CHC) technique (which can be calculated by the ∆ SCF approach and the charge transfer lifetime extracted from the intrinsic Lorentzian linewidths of the molecular coupling with the substrate).
Chapter 5 details the phenomenon of femtomagnetism which is induced/ suppressed in graphene by the adsorption of core level excited organic molecules. We find that the adsorption mechanism of the organic molecules play a crucial role in effecting the system magnetism. We study three cases: pyridine, picoline radical and pyridine radical, each with a different adsorption mechanism when interacting with graphene, and study the electronic and magnetic properties of these system upon molecular core-level excitation.
Chapter 6 discusses the dynamic charge transfer lifetime of a molecular overlayer with respect to the variable coupling of graphene with a metal substrate. Core excited bipyridine is adsorbed on epitaxially grown graphene/Ni(111) and bilayer graphene/Ni(111) and study the interaction of the molecule in these two cases. The charge transfer dynamics and extraction of the charge transfer lifetime from these interfaces are discussed and these results are validated using corehole-clock measurements. In chapter 7 we conclude by summarizing our results followed by the references used.