Structure-Function Relations for Carrier Interactions with Confined Field Modes on Two-Dimensional Materials

            Nanoarchitectures that support confined fields can affect carrier dynamics of emerging materials and devices. Field confinement offers enhanced nonlinear susceptibility for transition metal dichalcogenides (TMD).  Gate tunability of confined fields in graphene could improve optoelectronic coupling. The dispersion of electromagnetic local density of states associated to confined fields of surface plasmons is modulated by carrier density and distribution in associated nanostructures.  Examining structure-function relations of nanoarchitectures that support plasmon-electron interactions is fundamental to advances in the field.    

     This work evaluated carrier interactions with plasmonic modes using scanning transmission electron microscopy (STEM) for energy electron loss spectroscopy (EELS). High spatial resolution was achieved, while artifacts (e.g., direct electron-hole pair generation) from alternate optical methods were avoided.  Using the discrete dipole approximation (DDA) to Maxwell's equations, EELS spectra and topological surface plasmon maps were simulated to compare with EELS measurements. Comparing simulated and experimental spectra distinguished effects of nanoarchitecture variations on emergence of discrete and hybrid modes and carrier injection.

      Plasmonic structures resulting from self-assembly and redox devolution in native environments are of increasing significance.  Therefore, annular architectures and irregularities 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] Nanoarchitectures that optimize these interactions 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).