Warefta Hasan

Nanofabrication of plasmonic structures.

This review focuses on nanofabrication tools, based on soft lithography, which can generate a wide range of noble-metal structures with exceptional optical properties. These techniques offer a scalable and practical approach for producing arrays of complementary plasmonic structures (nanoholes and nanoparticles) and, in addition, expand the possible architectures of plasmonic materials because the metal building blocks can be organized over multiple length scales. We describe the preparation and characterization of five different systems: subwavelength nanohole arrays, finite arrays of nanoholes, microscale arrays of nanoholes, multiscale arrays of nanoparticles, and pyramidal particles. We also discuss how the surface plasmon resonances of these structures can be tuned across visible and near-infrared wavelengths by varying different parameters. Applications and future prospects of these nanostructured metals are addressed.

Tailoring the structure of nanopyramids for optimal heat generation.

This paper investigates how structural features of noble metal nanoparticles affect their photothermal properties. Using PEEL, we fabricated a range of Au nanopyramid-like particles with surface plasmon resonances tunable from visible to near-infrared wavelengths. By systematically varying geometric parameters including size, shell thickness, and presence or absence of tips, we determined which factors were most important in heat generation. For solutions with the same Au content, we discovered that pyramidal particles with thin shells and having sharp tips showed the largest photothermal response.

Pyramids: A platform for designing multifunctional plasmonic particles

This Account explores nanofabricated pyramids, a new class of nanoparticles with tunable optical properties at visible and near-infrared wavelengths. This system is ideally suited for designing multifunctional plasmonic materials for use in diagnostics, imaging, sensing, and therapeutics. The nanofabrication scheme that we developed (called PEEL) for these asymmetric metal particles is extremely versatile and offers several advantages over synthetic methodologies. The PEEL approach yields pyramids with variable sizes, thicknesses, and multimetal compositions, as well as blunt or ultrasharp tips or no tips. In addition, we have prepared pyramids with site-specific chemical and biological functionality on different portions of the pyramids. This is an important design feature for biological applications, as suggested by the generation of amphiphilic gold pyramids functionalized with alkanethiols on the hydrophobic portions and DNA on the hydrophilic portions. The optical characteristics of these pyramids depend on particle orientation, wavevector direction, and polarization direction and can be tuned. Using the multipolar surface plasmon resonances of large (>250 nm) pyramids, imaging and spectral identification of pyramid orientation in condensed media was possible. We were also able to direct pyramids to assemble into one- and two-dimensional arrays with interesting optical properties. Furthermore, modification of the PEEL fabrication scheme allowed the production of multimaterial pyramidal structures with complex attributes, highlighting the power of this platform for exacting nanometer-scale control over particle structure and composition.

Optical properties of tipless gold nanopyramids.

Controlling the size and shape of plasmonic nanoparticles (NPs) is a convenient approach to tune their optical properties for a broad range of applications. For example, metal NPs can function as discrete refractive index elements in optical sensing devices[1] as well as contrast agents in biological imaging.[2] An emerging area in plasmonics that has received less attention, however, is the tailoring of specific structural features of a NP (e.g., sharp asperities) to manipulate their optical properties. Systems of interest that could display signatures of such nanoscale geometrical changes are large (>100 nm) anisotropic NPs because they can support tunable dipole, quadrupole, and higher-order plasmon modes.[3] Although synthetic methods can access different shapes by changing the morphology of the seed (hard) or molecular (soft) templates[4] or by introducing metal ions and other molecules into the growth solution,[5] the overall sizes are usually too small to access higher-order plasmons. One important shape is Au nanorods, which can be grown to various lengths by keeping the width constant and increasing the length[6] or to various widths by preferential overgrowth on their sides.[7] In general, fabrication methods offer a more systematic approach to control different aspects of large particle shapes.[8] Also, the shapes of fabricated NPs tend to have a higher degree of monodispersity because their structures are governed by hard templates (e.g., anodized alumina membranes).[3e, 9]

Optical Properties of Anisotropic Core-Shell Pyramidal Particles

This paper describes an approach to fabricate anisotropic core-shell particles by assembling dielectric beads within fabricated noble metal pyramidal structures. Particles with gold (Au) shells and different dielectric cores were generated, and their optical properties were characterized by single particle spectroscopy. Because of their unique geometry, these particles exhibit multiple plasmon resonances from visible to near-IR wavelengths.