Optoelettronica in eterostrutture verticali basate sul grafene: come le proprietà di assorbimento di luce nel THz e nell'infrarosso dipendono dall'allineamento reticolare.

Responsabili di progetto

Andrea Tomadin, Jeil Jung

Accordo

COREA DEL SUD - NRF - National Research Foundation of Korea

Bando

CNR-NRF 2016-2017

Dipartimento

Scienze fisiche e tecnologie della materia

Area tematica

Scienze fisiche e tecnologie della materia

Stato del progetto

Nuovo

Proposta di ricerca

Vertical heterostructures comprising graphene (GR) and other two-dimensional (2D) crystals have emerged in the last few years as a new paradigm of "complex materials on demand" [1], enabled by rapid progress in material science. These materials offer novel features related with: in-plane electron transport; vertical electron tunneling; light-matter hybridization; and electron-electron interactions. These features can find applications in new technologies enhancing current device designs, thus there is a need for a broad study of their optoelectronic properties. Among the available 2D crystals, hexagonal boron nitride (hBN) has emerged as an insulating substrate of choice. GR on top of a hBN substrate, or "encapsulated" between two hBN layers, features an enhanced electron mobility [2] and plasmon lifetime [3]. Moreover, it has been found that hBN substantially modifies the optoelectronic properties of GR.

The first way in which hBN modifies the optoelectronic properties of GR arises from the fact that electrons roaming on a GR sample are subjected to the local potential of the hBN atoms. The effective electronic potential has a periodicity of several microns and follows the moiré pattern of the GR and hBN slightly mismatched lattices. The effective moiré potential is responsible for a reconstruction of the electronic spectrum and the emergence of minibands in the moiré superlattice Brillouin zone. Experiments have addressed the topographic properties of the moiré pattern [4] and its effects on the electronic spectrum [5] and on the transport characteristic in a magnetic field [6].

In our recent works [7,8] we addressed the problem of the effects of the moiré pattern on its plasmonic spectrum and THz conductivity. We found that both properties depend sensibly on the chemical potential of the electrons in GR, which is gate-tunable. Our calculations are fully numerical and start from a moiré-band Hamiltonian for GR pi-band electrons, in which the local periodic substrate interaction term is added to the k.p continuum Dirac model of an isolated graphen sheet. The calculation of the plasmon spectrum, however, uses a generic but effective interaction model.

In this project, we first propose to improve the calculation of the plasmon spectrum by applying the ab-initio electron structure calculations [9,10] which have been successfully employed in the calculation of the THz conductivity. We will combine our multi-band method for the calculation of the density-density response function with the derivation of an explicit effective Hamiltonian that has the periodicity of the moiré pattern. Accurate determination of plasmonic spectrum is valuable because, by tuning the chemical potential in GR/hBN systems, it is possible to achieve low-energy plasmon resonances (THz) localized around the satellite Dirac points [7]. A plasmonic device which operates in this regime, away from the charge neutrality point, is much less hampered by local disorder and electron-hole puddles.

Then we propose to study the plasmonic spectrum when a very good alignment between the GR and hBN lattices forces a local matching of the lattice constants and generates periodic stacking defects in the system. In such regime, it has been observed that the electronic bands display topological properties [12]. We aim to understand how the topology of the electron spectrum influences the plasmon spectrum. Such result would be valuable to engineer plasmonic materials at the nanoscale, taking advantage of the well-known robustness of topological features.

A thick hBN substrate modifies the optoelectronic properties of GR in a second important way, i.e. bulk hBN supports the propagation of phonon-polariton (PP) modes within so-called "reststrahlen" frequency bands. Highly directional, these modes exhibit deep subdiffraction confinement of light and have been shown to propagate with low losses [12], holding promise for transformation optics applications.

In our recent work [13], we have shown that PP modes in hBN hybridize with plasmons in GR, and give origin to a rich spectrum of plasmon-phonon polariton modes. We have shown that coupling of these polaritons to electrons in GR enables angle-resolved photoemission spectroscopy (ARPES) experiments to measure signatures of the collective modes in hBN which, being an insulator, could not be probed by ARPES on its own.

In this project, we propose to extend these results to include the effects of the misalignment between GR and hBN on the spectral properties of plasmons and electrons in GR. We propose to calculate the spectrum of the hybridized PP modes and GR plasmons at the satellite Dirac points. The calculation of the density-density response function must now take into account the minibands of the reconstructed electronic spectrum. We anticipate that the interplay between the tunability of the chemical potential in the GR sample and the width of the hBN layer yields a rich spectrum, with several sweet spots for the operation of a plasmonic device based on this setup.

[1] K.S. Novoselov et al., Physica Scripta T146, 014006 (2012).
[2] C.R.Dean et al., Nature Nanotechnol. 5, 722 (2010).
[3] A. Woessner et al., Nature Mater. 14, 421 (2015).
[4] J. Xue et al., Nature Mater. 10, 282 (2011); R. Decker et al., Nano Lett. 11, 2291 (2011).
[5] M. Yankowitz et al., Nature Phys. 8, 382 (2012); L.A. Ponomarenko et al., Nature 497, 594 (2013).
[6] C.R. Dean et al., Nature 497, 598 (2013).
[7] A. Tomadin et al., Phys. Rev. B 90, 161406(R) (2014).
[8] A.M. DaSilva, J. Jung et al., arXiv:1507.01834 (2015).
[9] J. Jung et al., Phys. Rev. B 89, 205414 (2014).
[10] J. Jung et al., Nature Commun. 6, 6308 (2015).
[11] J.C.W. Song et al., PNAS 112, 10879 (2015).
[12] S. Dai et al., Science 343, 1125 (2014); J.D. Caldwell et al., Nature Commun. 5, 5221 (2014).
[13] A. Tomadin et al., Phys. Rev. Lett. 115, 087401 (2015).

Obiettivi della ricerca

(1a) Extension of our numerical technique for the calculation of the plasmonic spectrum of graphene carriers on a hBN substrate. The extended code will interface to our methods for ab-initio electron structure calculations in the presence of a moiré potential. We think that this calculation builds on our respective core competencies and takes advantage of a merging of techniques which we have already mastered in separate projects.

(1b) We will use our technique to calculate the plasmonic spectrum of graphene carriers on a hBN substrate when electronic bands display topological properties. Our expertise in many-body theory and our long acquaintance with calculations of density-density response functions makes our collaboration an ideal venue to tackle this difficult project.

(2) Calculation of the hybrid phonon-plasmon polariton spectrum of graphene on top of a thick hBN layer, taking into account the effects of the misalignment between graphene and hBN.

Since these goals involve the merging of two complementary numerical techniques which we have indipendently developed, it will be important to collaborate closely on a technical level to succeed. Travelling and meetings, allowed by the funding of this grant, will greatly facilitate our collaboration.

Ultimo aggiornamento: 19/05/2024