an excellent environment for exploration of fundamental physical and chemical pheno mena due to their extreme degrees of tunability. [5][6][7] As a result, they have been applied in a variety of fields ranging from nanophotonics, quantum optics, catalysis, medicine, mechanical or optical coatings, to surface enhanced Raman spectroscopy. [8][9][10][11][12][13] Nanoparticles on their own or in fewparticle clusters exhibit properties not found in the bulk due to a large ratio of surface area to volume; however, controlled ordering of nanoparticles assemblies allows development of new functionalities, leading to concepts such as single molecule sensing, [14] artificial molecules, [15] and molecular imaging. [16] This in turn leads to the dual challenge of preparation of nanoparticle assemblies with desired geometries and exploration of their functionalities of interest. An enormous variety of methods exist for nanoparticle synthesis including chemical synthesis and physical vapor deposition. [17][18][19][20][21][22] Electron beam lithography (EBL), focused ion beam (FIB) milling, and laser-induced dewetting methods can be used for control over material positioning and growth to some degree, but resulting particle sizes are currently too large for applications requiring particles with less than about 20 nm as the critical dimension. Colloidal self-assembly of nanocrystals is a chemical growth alternative that allows for high packing density and high-quality nanoparticle ordering for particle sizes below 10 nm and has been thoroughly reviewed, [23] but in general, the particle chemistry, stoichiometry, morphology, and relative position largely cannot be locally modified after the assembly process, which hinders opportunities to explore phenomena that depend on these traits.Perhaps the best way to extend the correlations between structure and properties is through a combination of these methods via controllable modification of synthesized plasmonic material. Intentionally modifying the plasmonic structures dynamically after synthesis has largely not been explored. Direct-write techniques such as EBL and FIB have high degrees of precision, and can be used for patterning down to the ≈1 nm level with an aberration corrected beam in specific geometries. [24] An even higher-precision alternative is the electron beam of the scanning transmission electron microscope Spatial confinement of matter in functional nanostructures has propelled these systems to the forefront of nanoscience, both as a playground for exotic physics and quantum phenomena and in multiple applications including plasmonics, optoelectronics, and sensing. In parallel, the emergence of monochromated electron energy loss spectroscopy (EELS) has enabled exploration of local nanoplasmonic functionalities within single nanoparticles and the collective response of nanoparticle assemblies, providing deep insight into associated mechanisms. However, modern synthesis processes for plasmonic nanostructures are often limited in the types of accessible geometry, and mat...

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