Shuzhou Li

Methods for Describing the Electromagnetic Properties of Silver and Gold Nanoparticles

This Account provides an overview of the methods that are currently being used to study the electromagnetics of silver and gold nanoparticles, with an emphasis on the determination of extinction and surface-enhanced Raman scattering (SERS) spectra. These methods have proven to be immensely useful in recent years for interpreting a wide range of nanoscience experiments and providing the capability to describe optical properties of particles up to several hundred nanometers in dimension, including arbitrary particle structures and complex dielectric environments (adsorbed layers of molecules, nearby metal films, and other particles). While some of the methods date back to Mies celebrated work a century ago, others are still at the forefront of algorithm development in computational electromagnetics. This Account gives a qualitative description of the physical and mathematical basis behind the most commonly used methods, including both analytical and numerical methods, as well as representative results of applications that are relevant to current experiments. The analytical methods that we discuss are either derived from Mie theory for spheres or from the quasistatic (Gans) model as applied to spheres and spheroids. In this discussion, we describe the use of Mie theory to determine electromagnetic contributions to SERS enhancements that include for retarded dipole emission effects, and the use of the quasistatic approximation for spheroidal particles interacting with dye adsorbate layers. The numerical methods include the discrete dipole approximation (DDA), the finite difference time domain (FDTD) method, and the finite element method (FEM) based on Whitney forms. We discuss applications such as using DDA to describe the interaction of two gold disks to define electromagnetic hot spots, FDTD for light interacting with metal wires that go from particle-like plasmonic response to the film-like transmission as wire dimension is varied, and FEM studies of electromagnetic fields near cubic particles.

Synthesis of hexagonal close-packed gold nanostructures

Solid gold is usually most stable as a face-centred cubic (fcc) structure. To date, no one has synthesized a colloidal form of Au that is exclusively hexagonal close-packed (hcp) and stable under ambient conditions. Here we report the first in situ synthesis of dispersible hcp Au square sheets on graphene oxide sheets, which exhibit an edge length of 200-500 nm and a thickness of ~ 2.4 nm (~ 16 Au atomic layers). Interestingly, the Au square sheet transforms from hcp to a fcc structure on exposure to an electron beam during transmission electron microscopy analysis. In addition, as the square sheet grows thicker (from ~ 2.4 to 6 nm), fcc segments begin to appear. A detailed experimental analysis of these structures shows that for structures with ultrasmall dimensions (for example, <~ 6 nm thickness for the square sheets), the previously unobserved pure hcp structure becomes stable and isolable.

Surface Plasmon Resonance Enhanced Light Absorption and Photothermal Therapy in the Second Near-Infrared Window

Enhanced near-field at noble metal nanoparticle surfaces due to localized surface plasmon resonance (LSPR) has been researched in fields ranging from biomedical to photoelectrical applications. However, it is rarely explored on nonmetallic nanomaterials discovered in recent years, which can also support LSPR by doping-induced free charge carriers, let alone the investigation of an intricate system involving both. Here we construct a dual plasmonic hybrid nanosystem Au-Cu9S5 with well controlled interfaces to study the coupling effect of LSPR originating from the collective electron and hole oscillations. Cu9S5 LSPR is enhanced by 50% in the presence of Au, and the simulation results confirm the coupling effect and the enhanced local field as well as the optical power absorption on Cu9S5 surface. This enhanced optical absorption cross section, high photothermal transduction efficiency (37%), large light penetration depth at 1064 nm, excellent X-ray attenuation ability, and low cytotoxicity enable Au-Cu9S5 hybrids for robust photothermal therapy in the second near-infrared (NIR) window with low nanomaterial dose and laser flux, making them potential theranostic nanomaterials with X-ray CT imaging capability. This study will benefit future design and optimization of photoabsorbers and photothermal nanoheaters utilizing surface plasmon resonance enhancement phenomena for a broad range of applications.

Lead-free germanium iodide perovskite materials for photovoltaic applications

Computational screening based on density-functional-theory calculations reveals Ge as a candidate element for replacing Pb in halide perovskite compounds suitable for light harvesting. Experimentally, three AGeI3 (A = Cs, CH3NH3 or HC(NH2)2) halide perovskite materials have been synthesized. These compounds are stable up to 150 °C, and have bandgaps correlated with the A-site cation size. CsGeI3-based solar cells display higher photocurrents, of about 6 mA cm−2, but are limited by poor film forming abilities and oxidising tendencies. The present results demonstrate the utility of combining computational screening and experimental efforts to develop lead-free halide perovskite compounds for photovoltaic applications.

Ordered array of gold semishells on TiO2 spheres: an ultrasensitive and recyclable SERS substrate.

Ordered array of Au semishells on TiO(2) spheres with controlled size are prepared by combining the nanosphere self-assembly and atomic layer deposition (ALD). This ordered 2-D structure with designed array of metal nanogaps can be used as an ultrasensitive surface-enhanced Raman scattering (SERS) substrate with high reproducibility and stability. More importantly, the SERS substrates are recyclable, as enabled by their self-cleaning function due to the TiO(2) photocatalytic degradation of the target molecules. The high SERS sensitivity and recyclability are demonstrated by the detection of Rhodamine 6G (R6G) and brilliant cresyl blue (BCB) molecules. As both the nanosphere lithography and ALD are scalable processes, such 2-D ordered substrates may find applications in chemical sensing.

Plasmon-mediated synthesis of silver triangular bipyramids.

Synthetic methods have been developed for producing a wide variety of nanostructures that differ in size, shape, and composition. Indeed, methods now exist for preparing nonspherical, monodisperse samples of Au and Ag triangular prisms,[1–5] cubes,[6, 7] wires,[8] bars,[9] tetrahedra,[7, 10] octahedra,[11] decahedra,[12, 13] bipyramids,[14] and disks.[15] Synthetic control over particle shape allows one to chemically tailor the optical, electric, magnetic, and catalytic properties of such structures, making them useful for many applications, including biological diagnostics,[16] therapeutics,[17, 18] catalysis,[19, 20] and optics.[21, 22]

Correlated optical measurements and plasmon mapping of silver nanorods.

Plasmonics is a rapidly growing field, yet imaging of the plasmonic modes in complex nanoscale architectures is extremely challenging. Here we obtain spatial maps of the localized surface plasmon modes of high-aspect-ratio silver nanorods using electron energy loss spectroscopy (EELS) and correlate to optical data and classical electrodynamics calculations from the exact same particles. EELS mapping is thus demonstrated to be an invaluable technique for elucidating complex and overlapping plasmon modes.

Programmable photo-electrochemical hydrogen evolution based on multi-segmented CdS-Au nanorod arrays.

Programmable photocatalysts for hydrogen evolution have been fabricated based on multi-segmented CdS-Au nanorod arrays, which exhibited high-efficiency and programmability in hydrogen evolution as the photoanodes in the photoelectrochemical cell. Multiple different components each possess unique physical and chemical properties that provide these cascade nanostructures with multiformity, programmability, and adaptability. These advantages allow these nanostructures as promising candidates for high efficient harvesting and conversion of solar energy.

Gap structure effects on surface-enhanced raman scattering intensities for gold gapped rods

Gapped rods provide a unique platform for elucidating structure/function relationships, both for single-molecule electrochemical techniques and for surface-enhanced Raman scattering (SERS). This paper attempts to elucidate the dependence of SERS intensities on gap topography and gap distance for gold gapped rods with segment lengths varying over a wide range (40-2000 nm). Significantly, we have determined that rough gaps lead to a smaller SERS enhancement than smooth gaps for these structures even though the rough gaps have a larger total surface area. Both theory and experiment show periodic variation of SERS intensity with segment length as determined by odd-symmetry plasmon multipoles. Excitation of even-symmetry modes is dipole forbidden (for polarization along the rod axis), but this selection rule can be relaxed by roughness or, for smooth gaps, by near-field coupling between the rod segments.

Chemical Fabrication of Heterometallic Nanogaps for Molecular Transport Junctions

We report a simple and reproducible method for fabricating heterometallic nanogaps, which are made of two different metal nanorods separated by a nanometer-sized gap. The method is based upon on-wire lithography, which is a chemically enabled technique used to synthesize a wide variety of nanowire-based structures (e.g., nanogaps and disk arrays). This method can be used to fabricate pairs of metallic electrodes, which exhibit distinct work functions and are separated by gaps as small as 2 nm. Furthermore, we demonstrate that a symmetric thiol-terminated molecule can be assembled into such heterometallic nanogaps to form molecular transport junctions (MTJs) that exhibit molecular diode behavior. Theoretical calculations demonstrate that the coupling strength between gold and sulfur (Au-S) is 2.5 times stronger than that of Pt-S. In addition, the structures form Raman hot spots in the gap, allowing the spectroscopic characterization of the molecules that make up the MTJs.