Probing Morphological Alteration of Carrier Interactions with Confined Field Modes on Two Dimensional Materials

Donald Roper Engineering Education and Centers, National Science Foundation, Arlington, Virginia, USA Greg Forcherio Microelectronics-Photonics Graduate Program, University of Arkansas, Fayetteville, Arkansas, USA Drew DeJarnette Mechanical Engineering, University of Tulsa, Tulsa, Oklahoma, USA

Properties of collective charge oscillations constrained to surface support emerging materials and devices. Compact field confinement of plasmons offers enhanced nonlinear susceptibility for transition metal dichalcogenides (TMD). Gate tunability of plasmons in graphene could improve optoelectronic coupling. The dispersion of electromagnetic local density of states associated to surface plasmons is modulated by carrier density and distribution in associated nanostructures. Understanding the influence of nanostructure morphologies that support plasmon-electron interactions is therefore critical.

This work probed carrier interactions with plasmonic modes using scanning transmission electron microscopy (STEM) for energy electron loss spectroscopy (EELS). This technique permitted high spatial resolution while avoiding artifacts (e.g., direct electron-hole pair generation) from optical approaches. The discrete dipole approximation (DDA) to Maxwell’s equations was used to generate EELS spectra and topological surface plasmon maps to compare with EELS measurements. This comparison distinguished effects of nanostructure morphology on emergence of discrete and hybrid modes and carrier injection.

Plasmonic structures arising from self-assembly and redox devolution in native environments are of increasing significance. Therefore, annular and irregular morphologies were explored for the first time in this work, after validation with symmetric shapes.[1] Fig. 1 illustrates resonances from 1.0 to 2.08 eV supported by nanoellipses impacted at center, half-major/minor, and edge points. These energies were correlated with bright, dark, and hybrid modes in EELS maps in Fig. 2. Irregular nanoellipses convoluted and blue-shifted resonance energies, as in Fig 3. A proximal graphene layer further shifted dark and edge modes. Quantification of bright mode losses distinguished hot carrier injection to graphene from plasmons for the first time.[2]

Plasmonic lattice resonances in ordered nanostructures offer enhanced nonlinear susceptibility in TMD. [3] Morphological features that optimize this benefit are identified here across a broad range of parameter values.

[1] G. Forcherio, D. DeJarnette, M. Benamara, and D.K. Roper, in preparation. [2] D. DeJarnette and D.K. Roper, J. Appl. Phys. 116, 054313 (2014). [3] G. Forcherio, P. Blake, D. DeJarnette, and D.K. Roper, Opt. Expr. 22(15) 17791(2014).

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