Campus of International Excellence

Abstract:  

Continuous media and metamaterials with a near-zero refractive index (NZI media) provide alternative pathways for the control and manipulation of light-matter interactions.  The exotic behavior of NZI media is rooted in the fact that the wavelength gets effectively stretched as the refractive index vanishes. This allows for pathological solutions to the wave equation, including   spatially static fields distributions which nevertheless dynamically oscillate in time. This paradoxical behavior gives access to a regime of qualitatively different wave dynamics, where the importance of the geometry is lessened, and certain observables are invariant with respect to geometrical deformations, even including changes in the topology of the system.

In this talk, I’ll review and discuss some of the geometry-invariant phenomena related to near-zero-index media. Examples will include: (i) transmission (tunneling) of waves through deformed waveguides. (ii) Unconventional resonators supporting modes whose eigenfrequency is independent of the geometry of their external boundary. (iii) Violation of effective medium theory geometrical restrictions, enabling, for example, single unit-cell metamaterials. (iv) Existence of bound states in open 3D compact resonators with arbitrarily shaped boundaries.

Different technological applications and implementations of these concepts will be discussed.

Más información: http://www.ifimac.uam.es/category/seminars/

Abstract:  

In this talk, an overview of surface-science investigations on the chemical reactivity of epitaxial graphene (Gr) and materials “beyond graphene” (van der Waals semiconductors, topological insulators, Dirac semimetals, Weyl semimetals) will be provided.

By means of time-resolved X-ray photoemission spectroscopy and high-resolution electron energy loss spectroscopy, different surface chemical reactions in epitaxial Gr have been followed in real time (Gr growth by cracking of hydrocarbons, oxidation, intercalation of chemical species).

In particular, we have observed that, due to the similar work functions, Gr grows as an undoped sheet on the Pt-skin of Pt3Ni(111) but in an oxygen environment Ni segregation toward the surface occurs with selective oxidation of Ni. Subsequently, the Pt skin underneath Gr is replaced by a nickel-oxide skin. In the oxidation process, Gr acquires a p-type doping of 0.3 eV [1].

We have also used surface-science tools for investigating Gr employment in the fields of energy and catalysis. By means of a combination of surface-science spectroscopies and density functional theory [2], we have unveiled the mechanisms ruling the catalytic role of epitaxial Gr grown on transition-metal substrates for the production of hydrogen from water. Water decomposition at the Gr/metal interface at room temperature provides a hydrogenated Gr sheet, which is buckled and decoupled from the metal substrate. Molecular hydrogen is released upon heating above T=400 K.

Moreover, the analysis of the chemical reactivity of surface defects of two-dimensional materials provides important information for the nanofabrication process of electronic devices with active channels of ultrathin flakes of black phosphorus (few-layer phosphorene) [3] or InSe [4], which require the use of capping layers in order to avoid surface degradation in ambient conditions. In particular, we find high reactivity of phosphorene toward water, oxygen and CO [5], while water decomposition at room temperature occurs at Se vacancies of InSe [4].

Furthermore, the chemical inertness of high-quality single crystals of topological insulators toward ambient gases [6] will be discussed. The subsequent ambient stability of uncapped topological insulator-based nanodevices [7] paves the way for the technological exploitation of topological insulators in the fields of plasmonics [8] and Terahertz photodetection [7].

Finally, surface-science investigations on the chemical reactivity of Weyl semimetals and Dirac semimetals, also highlighting their potential applications in catalysis, will be presented.

References

[1]         A. Politano and G. Chiarello, 2D Mater. 4 (2017) 035003.

[2]          A. Politano et al., ACS Nano 10 (2016) 4543.

[3]          L. Viti et al., Adv. Mater. 27 (2015) 5567; L. Viti et al., Adv. Mater. 28 (2016) 7390.

[4]          A. Politano et al. Nanoscale 8 (2016) 8474.

[5           A. Politano et al., Nano Res. 9 (2016) 2598.

[6]          A. Politano et al., J. Phys. Chem. C 118 (2014) 21517.

[7]           L. Viti et al., Nano Lett. 16 (2016) 80; A. Politano et al. APL Mater. 5 (2017) 035504.

[8]          A. Politano et al., Phys. Rev. Lett. 115 (2015) 216802.

Más información: http://www.ifimac.uam.es/category/seminars/

Abstract:  

Continuous media and metamaterials with a near-zero refractive index (NZI media) provide alternative pathways for the control and manipulation of light-matter interactions.  The exotic behavior of NZI media is rooted in the fact that the wavelength gets effectively stretched as the refractive index vanishes. This allows for pathological solutions to the wave equation, including   spatially static fields distributions which nevertheless dynamically oscillate in time. This paradoxical behavior gives access to a regime of qualitatively different wave dynamics, where the importance of the geometry is lessened, and certain observables are invariant with respect to geometrical deformations, even including changes in the topology of the system.

In this talk, I’ll review and discuss some of the geometry-invariant phenomena related to near-zero-index media. Examples will include: (i) transmission (tunneling) of waves through deformed waveguides. (ii) Unconventional resonators supporting modes whose eigenfrequency is independent of the geometry of their external boundary. (iii) Violation of effective medium theory geometrical restrictions, enabling, for example, single unit-cell metamaterials. (iv) Existence of bound states in open 3D compact resonators with arbitrarily shaped boundaries.

Different technological applications and implementations of these concepts will be discussed.

Más información: http://www.ifimac.uam.es/category/seminars/

Abstract:  

In this talk, an overview of surface-science investigations on the chemical reactivity of epitaxial graphene (Gr) and materials “beyond graphene” (van der Waals semiconductors, topological insulators, Dirac semimetals, Weyl semimetals) will be provided.

By means of time-resolved X-ray photoemission spectroscopy and high-resolution electron energy loss spectroscopy, different surface chemical reactions in epitaxial Gr have been followed in real time (Gr growth by cracking of hydrocarbons, oxidation, intercalation of chemical species).

In particular, we have observed that, due to the similar work functions, Gr grows as an undoped sheet on the Pt-skin of Pt3Ni(111) but in an oxygen environment Ni segregation toward the surface occurs with selective oxidation of Ni. Subsequently, the Pt skin underneath Gr is replaced by a nickel-oxide skin. In the oxidation process, Gr acquires a p-type doping of 0.3 eV [1].

We have also used surface-science tools for investigating Gr employment in the fields of energy and catalysis. By means of a combination of surface-science spectroscopies and density functional theory [2], we have unveiled the mechanisms ruling the catalytic role of epitaxial Gr grown on transition-metal substrates for the production of hydrogen from water. Water decomposition at the Gr/metal interface at room temperature provides a hydrogenated Gr sheet, which is buckled and decoupled from the metal substrate. Molecular hydrogen is released upon heating above T=400 K.

Moreover, the analysis of the chemical reactivity of surface defects of two-dimensional materials provides important information for the nanofabrication process of electronic devices with active channels of ultrathin flakes of black phosphorus (few-layer phosphorene) [3] or InSe [4], which require the use of capping layers in order to avoid surface degradation in ambient conditions. In particular, we find high reactivity of phosphorene toward water, oxygen and CO [5], while water decomposition at room temperature occurs at Se vacancies of InSe [4].

Furthermore, the chemical inertness of high-quality single crystals of topological insulators toward ambient gases [6] will be discussed. The subsequent ambient stability of uncapped topological insulator-based nanodevices [7] paves the way for the technological exploitation of topological insulators in the fields of plasmonics [8] and Terahertz photodetection [7].

Finally, surface-science investigations on the chemical reactivity of Weyl semimetals and Dirac semimetals, also highlighting their potential applications in catalysis, will be presented.

References

[1]         A. Politano and G. Chiarello, 2D Mater. 4 (2017) 035003.

[2]          A. Politano et al., ACS Nano 10 (2016) 4543.

[3]          L. Viti et al., Adv. Mater. 27 (2015) 5567; L. Viti et al., Adv. Mater. 28 (2016) 7390.

[4]          A. Politano et al. Nanoscale 8 (2016) 8474.

[5           A. Politano et al., Nano Res. 9 (2016) 2598.

[6]          A. Politano et al., J. Phys. Chem. C 118 (2014) 21517.

[7]           L. Viti et al., Nano Lett. 16 (2016) 80; A. Politano et al. APL Mater. 5 (2017) 035504.

[8]          A. Politano et al., Phys. Rev. Lett. 115 (2015) 216802.

Más información: http://www.ifimac.uam.es/category/seminars/