Quantum transport and ab initio calculations

This large work packet has two main thrusts: a) ab initio calculations electronic excitations, and b) nonequilibrium quantum transport properties, and we discuss them separately, even though there are many cross-links and shared tasks.

Electronic excitations from first principles
The basic electronic and optical properties of a material close to equilibrium are governed by the excited electronic states. Numerical calculations of the single-particle and collective excitations of specific graphene-based structures from first principles, i.e. based only on the laws of quantum mechanics and the atomic species and positions, enables the quantitative prediction of the material’s properties from its geometrical and chemical structure. Furthermore such calculations provide a link between model calculations (see e.g. WP2) and experimental data (WP4,WP5), as well as provide support to the plasmonic applications, WP3. Using the electronic structure code GPAW developed at DTU Physics, we shall perform first-principles calculations with the aim of tailoring the optical and plasmonic properties of GALs and related structures by tuning the size of the anti-dots and/or the chemical termination of the carbon edges. Since real devices will inevitably involve the deposition of graphene on a substrate (metallic or semiconducting) we shall also study how the electronic excitations are affected by the coupling to a substrate.

The first-principles calculation of excited states is a non-trivial task that requires the use of many-body perturbation theory or time-dependent DFT for extended inhomogeneous systems. We have recently implemented such a method in GPAW and demonstrated excellent agreement with experiments for a number of properties like the dielectric constants of various semiconductors and plasmon energies of metal surfaces.  Moreover, we have shown that chemical modification of the edges at a graphene nanoribbon can have a large influence on the single-particle bandstructures, and that the plasmonic excitations in graphene can be significantly altered when adsorbed on a surface.  Here we shall study the combined effect of chemical functionalization of the anti-dot edges and the interaction with different substrates, with the aim of modeling, and ultimately rationally designing, the optical and electronic properties of GALs structure with quantitative precision. We shall include the coupling to the electromagnetic field by using the microscopic dielectric function obtained from the first-principles GPAW calculations as input to the Maxwell equations. This will allow us to investigate the detailed distribution of optical near field close to the GAL structure. The results are directly relevant for graphene-based nanoplasmonics and are directly comparable to the experiments performed under WP3.  Finally, shall also study the interaction between the GAL hole edges and DNA nucleotides to estimate the key parameters for structures employed in WP6.

Quantum transport
We will calculate the vibrational properties, the electron-phonon couplings in graphene  nanostructures, and corresponding Raman signals, using the TranSiesta/Inelastica methodology developed at DTU Nanotech. The lattice super-structures we wish to consider include (i) holes, (ii) protrusion-patterns (cf. WP5), (iii) modulated adsorbed structures (through the block co-polymer masks) such as molecules or metallic particles.  In combination with electronic transport calculations we will evaluate the thermo-electric properties of graphene structures. This will be done for medium-sized structures (< 10.000 atoms) based on first principles DFT calculations in combination with empirical potential MD simulations for thermal transport. We will further use the DFT calculations to obtain parameters which can be used in the large-scale simulations (WP2). We will investigate the current-induced reconstruction of the atomic structure and local nano-scale Joule heating by comparing calculations with HR-TEM and Raman experiments.  These can be compared to experimental information in microcopy or Raman signals (Stokes/Anti-stokes). In combination with WP3 we will assess the use of GAL-based structures as molecular sensors based on electronic conductance as well as Raman signals (surface and chemical enhancement).

In a parallel effort, we shall investigate certain basic science issues pertaining to nanostructured graphene.  These include the role of electron-electron interactions, possibilities for magnetic ordering (certain edge structures imply antidots with a magnetic moment), magnetotransport (i.e., Hofstaedter butterflies), and strain engineering of structurally relaxed GAL structures (using the concept of pseudomagnetic fields).  The original qubit concept of we proposed in 2008 will be further developed, i.e., the coupling between nearby qubits will be calculated.  This is clearly “blue sky” research, hopefully leading to suggestions for new experiments.  Finally, we shall develop a theory for an emerging nanostructure characterization tool, nano four-point probe spectroscopy). 

 

graphical_abstract fra mads wp1 (b385).jpg

Figure: We have made theoretical predictions for the formation and electron transport properties of adsorbate-induced kink-structures in graphene: The electronic behaviour can be made ribbon or 1D-like in the graphene confined between two consecutive kinks. The structures might be formed using the enhanced reactivity of bend graphene.

 

Antti-Pekka Jauho

Antti-Pekka Jauho (leader)
Professor
DTU Nanotech

Quantum transport

 

http://www.cng.dtu.dk/research-1-/quantum-transport-and-ab-initio-calculations
21 NOVEMBER 2019