(158c) Structure-Function Relations for Carrier Interactions with Confined Field Modes on Two-Dimensional Materials

Fig 1. DDA computed EELS spectra for 270x115 nm ellipse.
Fig. 2 EELS maps distinguish light, dark, and hybrid modes.
Fig 3. Carrier density affects electron energy loss on graphene.[2]
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       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).