By mechanically distorting a crystal lattice it is possible to engineer the electronic and optical properties of a material. In graphene, one of the major effects of such a distortion is an energy shift of the Dirac point, often described as a scalar potential. We demonstrate how such a scalar potential can be generated systematically over an entire electronic device and how the resulting changes in the graphene work function can be detected in transport experiments. Combined with Raman spectroscopy, we obtain a characteristic scalar potential consistent with recent theoretical estimates. This direct evidence for a scalar potential on a macroscopic scale due to deterministically generated strain in graphene paves the way for engineering the optical and electronic properties of graphene and similar materials by using external strain.
Assembling plasmonic nanoparticles on metal films is an elegant method for the design of SERS platforms with dense hot spots and amplified electric fields. Interaction between the localized surface plasmons of metal nanoparticles within assemblies induces collective plasmonic modes which can further couple with the propagating surface plasmons prevailing on the metal films. Herein, we report on the electric field effects as well as Raman signal enhancements arising due to the sandwiching of Au and Ag core−shell nanoparticle assemblies on Au films with varying thicknesses of the underlying metal film. The sandwich plasmonic platforms are prepared by linking Ag@SiO 2 as well as Au@SiO 2 nanoparticles on Au films using (3-mercaptopropyl)trimethoxysilane (3-MPTS). The interaction between the SiO 2 shell on the Ag/Au nanoparticles and free silanol groups on 3-MPTS provides a monolayer of core−shell systems on the Au films, as corroborated by SEM images. Finite-difference time-domain simulations with heptamer models of Ag@SiO 2 and Au@SiO 2 particles on Au films confirm an enhancement in the electric field upon sandwiching the nanoparticle aggregates on the Au films. The Raman signal enhancement factors for the dye Rhodamine 6G are estimated, and the enhancement in the Raman signal intensities on Ag@SiO 2 over Au@SiO 2 assembled on a 20 nm Au film is attributed to the higher Q-factor of Ag. The largest measured Raman signal intensity on Ag@SiO 2 on a 60 nm thick Au film, ∼10 7 , is reasoned based on the variation of the electric field intensity of the Au film as a function of its thickness.
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