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Controlling the plasmonic properties of ultrathin TiN films at the atomic level Deesha Shah, Alessandra Catellani, Harsha Reddy, Nathaniel Kinsey, Vladimir M. Shalaev, Alexandra Boltasseva, and Arrigo Calzolari ACS Photonics, Just Accepted Manuscript • DOI: 10.1021/acsphotonics.7b01553 • Publication Date (Web): 11 May 2018 Downloaded from http://pubs.acs.org on May 14, 2018
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ACS Photonics
Controlling the plasmonic properties of ultrathin TiN films at the atomic level Deesha Shah1, Alessandra Catellani2, Harsha Reddy1, Nathaniel Kinsey3, Vladimir Shalaev1, Alexandra Boltasseva1, Arrigo Calzolari2* 1
School of Electrical and Computer Engineering and Birck Nanotechnology Center, Purdue University, West Lafayette, Indiana 47907, USA 2 3
CNR-NANO Istituto Nanoscienze, Centro S3, I-41125 Modena, IT
School of Electrical and Computer Engineering, Virginia Commonwealth University, Richmond, Virginia 23220, USA
*
[email protected] RECEIVED DATE (to be automatically inserted after your manuscript is accepted if required according to the journal that you are submitting your paper to)
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ABSTRACT
By combining first principles theoretical calculations and experimental optical and structural characterization such as spectroscopic ellipsometry, X-ray spectroscopy, and electron microscopy, we study the dielectric permittivity and plasmonic properties of ultrathin TiN films at an atomistic level. Our theoretical results indicate a remarkably persistent metallic character of ultrathin TiN films and a progressive red shift of the plasmon energy as the thickness of the film is reduced, which is consistent with previous experimental studies. The microscopic origin of this trend is interpreted in terms of the characteristic two-band electronic structure of the system. Surface oxidation and substrate strain are also investigated to explain the deviation of the optical properties from the ideal case. This paves the way to the realization of ultrathin TiN films with tailorable and tunable plasmonic properties in the visible range for applications in ultrathin metasurfaces and nonlinear optics.
KEYWORDS plasmonics, optical properties, DFT, ellipsometry, titanium nitride, ultrathin films
Recent developments in nanofabrication techniques have facilitated a surge of nanoplasmonic devices whose properties can be engineered by careful structural control of their metallic building blocks1. The usage of plasmonic materials – mostly metals – that support the lightcoupled subwavelength oscillations of free electron clouds has led to advances in various applications, such as sensing2–4, photovoltaics5–7, and optical circuitry8,9, unachievable with conventional dielectric photonic materials. Along with the control of the lateral dimensions of plasmonic structures, it is also possible to tailor the properties by adjusting the thickness of plasmonic materials, all the way down to a few monolayers10,11. As the dimensions shrink down
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ACS Photonics
to nanometer range thicknesses, quantum phenomena emerge as a result of the strong electron confinement, making ultra-thin plasmonic materials ideal platforms to study light – matter interactions at the nanoscale12,13. For example, the strong confinement leads to an enhanced nonlinear optical response in ultra-thin films in comparison to their bulk counterparts14,15. Most importantly, ultrathin plasmonic films are predicted to have a greatly increased sensitivity to the local dielectric environment, strain, and external perturbations10. Consequently, unlike conventional metals with properties that are challenging to tailor, atomically thin plasmonic materials exhibit optical responses that can be engineered by precise control of their thickness, composition, and the electronic and structural properties of the substrate and superstrate10,16,17. This unique tailorability, unachievable with bulk or relatively thick metallic layers, establishes ultrathin plasmonic films as an attractive material platform for the design of tunable and dynamically switchable metasurfaces18. In order to realize these applications, a deeper insight into plasmonic material properties at the nanoscale is required. To investigate the sought after atomically-thin plasmonic regime, there have been several efforts to grow ultra-thin ( 100 nm) have been largely studied24,25,26,27,28 using several
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experimental and theoretical techniques, including spectroscopic ellipsometry29,30, X-ray, UV photoemission and Auger spectroscopy31,32, electron energy loss33,34, Hall electron transport35, scanning electron and tunneling microscopies36,37, tight-binding and ab initio simulations24,38,39, 40 On the contrary, even though a few reports on growth41,42 and electron transport43,44 appeared in the last years, the optical properties of ultrathin TiN films (
