Plasmonic Properties of Nanopatterned Graphene

Graphene is an atomically thin two-dimensional material with carbon atoms arranged in a honeycomb lattice. It has been successfully mechanically exfoliated from graphite and characterized in 2004 by Andre Geim and Konstantin Novoselov at the University of Manchester, UK. While suspended graphene holds the world record in mobility exceeding 100,000 cm2/Vs, which means that graphene could increase the operating frequency of electronic devices up to the THz regime. For example, imaging computer chips running at a few THz frequency, which would be about 1000 times faster than current chips. However, there are two main obstacles for using graphene in electronics and optoelectronics. Firstly, graphene has no band gap, which means that the current through graphene cannot be turned off by a gate voltage, hampering the creation of a graphene transistor. Secondly, graphene\u27s absorbance is only about πα =2.3%, where α = e2/hc = 1/137 is the fine structure constant. In my thesis I focus on a unique method to increase the absorbance of single-layer graphene (SLG) and multi-layer graphene (MLG) to nearly 100%. This method relies on the creation of localized surface plasmons (LSPs) around holes patterned in SLG and MLG. These LSPs are able to absorb almost 100% of incident infrared (IR) light over a wide range of wavelengths from λ =1.3 µm down to λ = 12 µm and beyond, covering the near-infrared (NIR), short-wavelength infrared (SWIR), mid-wavelength infrared (MWIR), and long-wavelength infrared (LWIR) regimes. Taking advantage of the high absorbance in nanopatterned graphene we developed the proof-of-concepts of IR photodetectors based on the photothermoelectric effect in nanopatterned graphene (NPG) and nanopatterned multilayer graphene intercalated with FeCl3 (NPMLG-FeCl3), an IR photodetector based on the combined effect of phase transition from insulator to metal in VO2 and NPG in a heterostructure consisting of NPG on top of VO2, and a thermal IR emission source based on NPG. During my thesis I have studied several physical phenomena for developing these devices. Due to the 2D nature of graphene and its Dirac fermions, it is possible to tune the LSP absorption and emission resonance wavelength by means of a gate voltage. This means all the proposed IR photodetectors and thermal IR sources can be tuned electrostatically. In the case of the IR photodetectors based on NPG and NPMLG-FeCl3 the detection of incident IR light relies on the plasmonically enhanced photothermoelectric effect due to asymmetric nanopatterning of NPG or NPMLG. For the thermal IR emission from NPG we generalized Planck\u27s theory to any grey body and derived the completely general nonlocal fluctuation-dissipation theorem with nonlocal response of surface plasmons in the random phase approximation (RPA). The nonlocality of the fluctuation-dissipation theorem allows for the description of the coherence in the excited LSPs and the thermally emitted photons. For the description of the devices we developed a multiscale modeling platform using various software packages for performing finite-difference time domain (FDTD) simulations, COMSOL solvers for the drift-diffusion and heat equations coupled by thermoelectric effects, density functional theory (DFT) calculations, and analytical formulas