| Autor: |
Kim JY; Department of Materials Science and Engineering, University of Michigan, Ann Arbor, MI 48109, USA., Han MG; Condensed Matter Physics and Materials Science Department, Brookhaven National Laboratory, Upton, NY 11973, USA., Lien MB; Department of Electrical Engineering, University of Michigan, Ann Arbor, MI 48109, USA., Magonov S; SPM Labs LLC, Tempe, AZ 85283, USA., Zhu Y; Condensed Matter Physics and Materials Science Department, Brookhaven National Laboratory, Upton, NY 11973, USA., George H; Department of Electrical Engineering, University of Michigan, Ann Arbor, MI 48109, USA., Norris TB; Department of Electrical Engineering, University of Michigan, Ann Arbor, MI 48109, USA., Kotov NA; Department of Materials Science and Engineering, University of Michigan, Ann Arbor, MI 48109, USA.; Department of Chemical Engineering, University of Michigan, Ann Arbor, MI 48109, USA.; Department of Biomedical Engineering, University of Michigan, Ann Arbor, MI 48109, USA.; Biointerfaces Institute, University of Michigan, Ann Arbor, MI 48109, USA. |
| Abstrakt: |
The symmetry of metallic nanocolloids, typically envisaged as simple geometrical shapes, is rarely questioned. However, the symmetry considerations are so essential for understanding their electronic structure, optical properties, and biological effects that it is important to reexamine these foundational assumptions for nanocolloids. Gold nanorods (AuNRs) are generally presumed to have nearly perfect geometry of a cylinder and therefore are centrosymmetric. We show that AuNRs, in fact, have a built-in electrostatic potential gradient on their surface and behave as noncentrosymmetric particles. The electrostatic potential gradient of 0.11 to 0.07 V/nm along the long axes of nanorods is observed by off-axis electron holography. Kelvin probe microscopy, secondary electron imaging, energy-filtered transmission electron microscopy, and plasmon mapping reveal that the axial asymmetry is associated with a consistently unequal number of cetyltrimethylammonium bromide moieties capping the two ends of the AuNRs. Electrostatic field maps simulated for the AuNR surface reproduce the holography images. The dipole-like surface potential gradient explains previously puzzling discrepancies in nonlinear optical effects originating from the noncentrosymmetric nature of AuNRs. Similar considerations of symmetry breaking are applicable to other nanoscale structures for which the property-governing symmetry of the organic shell may differ from the apparent symmetry of inorganic core observed in standard electron microscopy images. |
