Symposium EL06—Molecular and Colloidal Plasmonics—Synthesis and Applications

Over the years, tremendous advances have been made in the design and fabrication of nano-size objects in a variety of geometries. Nanosphere Lithography is a large-area nanostructurization technique introduced in the early 80s which utilizes dielectric nanospheres for the preparation of nanoapertures or nanoparticles formed in the gaps between the nanospheres. The process is cost-effective and easily scalable, therefore it is often chosen in favor of expensive and time-consuming electron lithography or low-resolution photolithography.

In this report, we describe a simple method of preparing versatile plasmonic nanomasks based on electrostatic-driven assembly of silica nanoparticles (SNPs) of diameters ranging from 60 to 300 nm. This technique makes it possible to produce homogenous masks of uniform apertures on bare or PMMA-coated glass substrates covering an area of a few cm². Fabricating different nanomasks starting from single holes, through dimers or trimers, up to long, nanowire-like apertures is possible due to a high degree of control of the sizes and concentrations of the SNPs, as well as of the temperature during the process. In combination with physical vapor deposition of metals, such as silver or gold, the masks can be used further as platforms for obtaining many different plasmonic nanostructures like discs, cones, dimers, or even more sophisticated shapes by simply changing the evaporation angle. Such plasmonic resonators are very suitable for SERS, photovoltaic and biosensing applications because of their high tunability stemming from a number of degrees of freedom within the described method of fabrication.

The application of metal and metal oxides in the form of nanostructures, to enhance physicochemical properties of materials has increasingly attracted the interest of materials scientists in different fields. Among other features, these nanomaterials show a broad antimicrobial activity and can be advantageous to design bioactive coatings and/or surfaces, with controlled metal ion release, exerting significant biological action and associated low toxicity for humans. In recent years, we have developed and deeply characterized many different nanoantimicrobial systems [1-3] as a powerful alternative route to fight bacterial resistance towards conventional antibiotics and disinfecting agents. In this study, bioactive Cu- and Ag- nanoparticles were produced as ultra-stable [4] nanocolloids by means of laser ablation synthesis in solution (LASiS) for hypothetical food packaging application. We exploited the key features of LASiS, which is a green and versatile route to obtain nanoparticles without any toxic reductant or stabilizer. The resulting nanocolloids were used as additives for the controlled modification of different biodegradable polymeric matrices. Antibacterial ion release kinetics from modified surfaces was monitored, showing a tuneable and long-term release of bioactive species over time. The risk of entire nanoparticle release was ruled out by electron microscopy investigations of the contact solutions. Finally, the coatings’ surface chemical composition (assessed by photoelectron and vibrational spectroscopies) was correlated with the ion release and bioactivity properties. They were examined in different cases of study, in order to evaluate their employment in active food packaging and bacteriostatic coatings for the car industry.

MCS acknowledges the project “Extension of the shelf-life of agri-food products through nanoantimicrobial and antibiofilm packaging with low environmental impact”, Research for Innovation – European Social Fund network, n° 435A866B.

[1] M. C. Sportelli et al., Scientific Reports, 7 (2017), 11870.
[2] M.C. Sportelli et al., Trends in Analytical Chemistry, 84 (2016), 131-138.
[3] M.C. Sportelli et al., Nanomaterials, 7 (2016), 6.
[4] M. C. Sportelli et al., Colloids and Surfaces A, 559 (2018), 148–158.

Liquid metals are an interesting class of materials with fascinating properties deriving from their simultaneous metallic and liquid nature.¹ Besides their use as self-healing contacts in stretchable electronics, shrinking the size of the particles down to the nanoscale adds an extra dimension of complexity which generates new physicochemical properties and opens up new applications. Liquid metal micron- and nano-sized particles are being explored for biomedical applications, chemical sensors, imaging, and batteries. Nevertheless, the knowledge of their chemistry is still very limited compared to other classes of materials. Ga-based nanoparticles (NPs) are the most studied systems so far. Interestingly, Ga NPs have been demonstrated to possess plasmonic properties that are influenced by their size-dependent solid-liquid transition.^2,3 Along with Ga, Cu is another non-noble metal presenting attracting catalytic and plasmonic behavior.⁴ Yet, the combination of these two metals at the nanoscale has not previously been reported.
In this work, we explore the reactivity of colloidal liquid Ga nanoparticles (NPs) toward a copper molecular precursor to synthesize bimetallic Cu-Ga NPs.⁵ Anisotropic mushroom-shaped Cu-Ga nanodimers, where the two segregated domains of the constituent metals share an interface, form as the reaction product. We combine transmission electron microscopy techniques, ICP elemental analysis, cyclic voltammetry and X-Ray absorption spectroscopy to investigate the formation mechanism of these anisotropic bimetallic nanostructures. We demonstrate that a galvanic replacement reaction (GRR) between the Ga seeds and a copper-amine complex takes place. GRRs are spontaneous electrochemical processes wherein one metallic domain (the sacrificial template) is oxidized by the cations of another metal that possesses a more positive reduction potential, usually resulting in alloyed hollowed structures in crystalline noble-metal systems. We, therefore, attribute the unusual final morphology of the bimetallic NPs to both the presence of the native oxide shell around the Ga NPs and their liquid nature, and we explain the reaction mechanism involving these phenomena. The same reaction scheme was then extended to the synthesis of Ag-Ga and Cu-In NPs, and their final morphologies further supported the above-stated hypothesis.
Based on this understanding, we also demonstrate that sequential GRRs to include more metal domains are possible and trimers including Ag-Cu-Ga were also obtained.
Overall, this first study on colloidal liquid metal NPs as sacrificial templates in GRRs showcases the very intriguing and peculiar reactivity of this class of materials at the nanoscale which has certainly been underexplored to date. Furthermore, it opens the way toward achieving much more sophisticated and complex structures of Ga-based nanomaterials, which possess promising properties for top-scientific challenges including CO₂ electroreduction, sensing, plasmonics, and self-healing electronics.

1. Daeneke, T. et al. Chem. Soc. Rev. 2018, 47, 4073–4111.
2. Yarema, M. et al. J. Am. Chem. Soc. 2014, 136, 12422–12430.
3. Knight, M. W. et al. ACS Nano 2015, 9, 2049–2060.
4. Chan, G. H. et al. Nano Lett. 2007, 7, 1947-1952
5. Castilla-Amorós, L. et al. J. Am. Chem. Soc. 2020, Accepted (10.1021/jacs.0c09458)

Halide perovskite semiconductors can merge the highly efficient operational principles of conventional inorganic semiconductors with the low temperature solution processability of emerging organic and hybrid materials, offering a promising route towards cheaply generating electricity as well as light. Following a surge of interest in this class of materials, research on the corresponding halide perovskite nanocrystals (NCs) as well has gathered momentum in the last years.1 Another class of materials that has been recently investigate by us is that of metal chalcohalides. These materials offer a broad solid-state chemistry and they been investigated for applications in solar energy conversion, thermoelectrics, hard radiation detection, and superconductivity. Among them, lead chalcohalides have been rarely studied in the past. We have recently reported the synthesis of a series of lead chalcohalides, by means of colloidal approaches, delivering phases and compositions that had not been previously identified in the bulk.2 These materials present indirect band gaps, they emit in the NIR region of the spectrum at cryogenic temperatures and have unique crystal structures. We will also show our recent results on the synthesis and advanced characterization of heterostructured nanocrystals in which one domain is a lead chalcohalide and the other domain is a cesium lead halide perovskite.3 The two domains are separated by a flat, atomically defined, epitaxial interface. In these materials, the photogenerated carries are separated at their interface, and as such they might be promising in applications ranging from catalysis to photovoltaics.
References
1. Q. Akkerman et al. “Genesis, challenges and opportunities for colloidal lead halide perovskite nanocrystals”, Nat. Mater. 2018, 17, 394–405.
2. S. Toso et al. “Nanocrystals of Lead Chalcohalides: A Series of Kinetically Trapped Metastable Nanostructures”, J. Am. Chem. Soc. 2020, 142, 22, 10198.
3. S. Toso et al. “Halide Perovskite-Lead Chalcohalide Nanocrystal Heterostructures”, under review.

Silicon nanowires (Si-NWs) represent useful building blocks for nanoelectronic devices in many fields such as photovoltaics, photocatalysis, sensing or photodetectors. This is because they show interesting optical properties, including plasmon resonance (PR), the collective oscillation of free electrons, induced by an electromagnetic field at the proper frequency. PR is a versatile phenomenon because it is tunable depending on the intended application by modulating the nanosystem geometry, the medium covering or surrounding it and its shape. It allows, for example, to use Si-NWs to collect and amplify, by several orders of magnitude, radiant energy by generating a locally amplified electric field, a beneficial mechanism in many applications. Till now, however, there are no direct observations neither deep understanding in the literature on PR in Si-NWs. In this talk, the surface plasmon resonances triggered in isolated Si-NWs with diameters below than 100 nm are visualized at high spatial resolution. We characterize the systems through transmission electron microscopy (TEM) coupled to electron energy loss spectroscopy (EELS) with a subnanometer electron probe. The plasmon behavior of the SiNWs is then modeled through theoretical calculations, and the results are in good agreement with the experimental data. The electrical field spatial distribution generated by the PR is mapped and rationalized. As an extension of our study, we show experimental and modeling data on SiNWs coated with different materials and structures, and we compare the plasmonic behavior to the one of pristine SiNWs.

Noble metal nanoparticles (NPs) are known for their ability to confine visible and near-infrared light at the nanoscale through plasmonic resonances. The plasmonic resonance frequency can be tuned by changing the nanostructure shape, material, or adjacent environment. To extend the features of plasmonic resonances, NPs can be arranged into periodic structures, also called superlattices. Such lattice structures potentially foster inter-particle and inter-cluster interactions through photonic coupling, giving rise to so-called lattice plasmons, which have interesting properties, the potential for high Q factors, strong near-field enhancement, and the potential to be tuned by period, incident angle and polarization. Moreover, the inner structure of a unit cell (cluster) of such periodic arrangement and its homogeneity has a strong influence on the plasmonic response.
Here, we present a self-assembly process that allows us to assemble a controlled number of nanospheres, nanorods or nano triangles into superlattices on large-scale. Capillary forces drive the NP self-assembly, via a nanostructured mold that confines the NPs in periodically arranged wells, yielding NP clusters in a superlattice. The chemical composition of the NP dispersion controls the formation of homogeneous NP superlattices. We studied the impact of homogeneity concerning extinction properties, near-field enhancement and irregularities of superlattices. Further, we demonstrate the generalization of our process regarding different shapes of NPs. The fabrication of superlattices with a defined number of NPs in each cluster allows us to compare their extinction properties, moreover, to study the formation of hybrid plasmonic modes originating from the individual clusters and the lattice.
We support our experimental studies by numerical modelling, thereby calculating the superlattice-induced extinction. To minimize the gap between electromagnetic simulations and experimental characterization, we combine statistical image analysis (based on SEM images of self-assembled clusters), finite-element modelling, and material and structure optimization. To identify the superlattice plasmonic shift for a varying lattice parameter, we analyze the cluster’s near-field electromagnetic response, for weighed superpositions of statistically relevant particle arrangements identified by image analysis. Based on statistical analysis, we introduce and superimpose various geometric irregularities in our model, for matching the fabricated superlattice extinction curve, alongside optimization of particle radius, potential shell thickness and refractive index, and particle materials. By Mie theory and colloidal particle measurements, we calibrate our material model and incorporate the results regarding the gap between self-assembled NPs. We demonstrate polarization effects and extinction spectra with respect to variations of aforementioned parameters. As a special case, we additionally show how sensitive the system responds when irregularities are introduced.
Our results, corroborated by electromagnetic simulations and complemented by surface-enhanced Raman scattering (SERS) measurements, provide insight into the near-field enhancement of nanospheres, nanorods and nanotriangles, arranged in sub-wavelength superlattices of macroscopic dimensions, and give rise to new ideas for plasmon-coupled sensing.

We report the use of colloidal Au NCs as building blocks in the design of optical metamaterials. Chemical exchange of the long ligands used in NC synthesis with more compact ligand chemistries brings neighboring NCs into proximity and increases interparticle coupling.^1,2 This ligand-controlled coupling allows us to tune through a dielectric-to-metal phase transition seen by a 10¹⁰ range in DC conductivity and a dielectric permittivity ranging from everywhere positive to everywhere negative across the whole range of optical frequencies.¹ For example, by partially exchanging the NC assemblies, we create strong, ultrathin film optical absorbers with a 6x increase in extinction in the infrared compared to that of bulk Au thin films.² For more complete ligand exchange and with thermal annealing, we realize strong optical scatterers useful in the design of optical metamaterials.³ Ligand exchange and annealing of NC films also triggers a large volume shrinkage. By juxtaposing plasmonic NCs and bulk materials, we exploit their different chemical and mechanical properties to transform lithographically-defined two-dimensional structures, upon ligand exchange, into three-dimensional structures.⁴ We use the three-dimensional structures to demonstrate large-area metamaterials with chiroptical responses of ~40% transmission difference between left-hand and right-hand circularly polarized light and that are suitable broadband circular polarizers.⁵
(1) Fafarman, A. T.; Hong, S.-H.; Caglayan, H.; Ye, X.; Diroll, B. T.; Paik, T.; Engheta, N.; Murray, C. B.; Kagan, C. R. Chemically Tailored Dielectric-to-Metal Transition for the Design of Metamaterials from Nanoimprinted Colloidal Nanocrystals. Nano Lett. 2013, 13 (2), 350–357. https://doi.org/10.1021/nl303161d.
(2) Chen, W.; Guo, J.; Zhao, Q.; Gopalan, P.; Fafarman, A. T.; Keller, A.; Zhang, M.; Wu, Y.; Murray, C. B.; Kagan, C. R. Designing Strong Optical Absorbers via Continuous Tuning of Interparticle Interaction in Colloidal Gold Nanocrystal Assemblies. ACS Nano 2019, acsnano.9b02818. https://doi.org/10.1021/acsnano.9b02818.
(3) Chen, W.; Tymchenko, M.; Gopalan, P.; Ye, X.; Wu, Y.; Zhang, M.; Murray, C. B. C. B.; Alu, A.; Kagan, C. R. C. R. Large-Area Nanoimprinted Colloidal Au Nanocrystal-Based Nanoantennas for Ultrathin Polarizing Plasmonic Metasurfaces. Nano Lett. 2015, 15 (8), 5254–5260. https://doi.org/10.1021/acs.nanolett.5b02647.
(4) Zhang, M.; Guo, J.; Yu, Y.; Wu, Y.; Yun, H.; Jishkariani, D.; Chen, W.; Greybush, N. J.; Kübel, C.; Stein, A.; Murray, C. B.; Kagan, C. R.; Kubel, C.; Stein, A.; Murray, C. B.; Kagan, C. R. 3D Nanofabrication via Chemo-Mechanical Transformation of Nanocrystal/Bulk Heterostructures. Adv. Mat. 2018, 30 (22), 1800233. https://doi.org/10.1002/adma.201800233.
(5) Guo, J.; Kim, J.-Y.; Zhang, M.; Wang, H.; Stein, A.; Murray, C. B.; Kotov, N. A.; Kagan, C. R. Chemo- and Thermomechanically Configurable 3D Optical Metamaterials Constructed from Colloidal Nanocrystal Assemblies. ACS Nano 2019, acsnano.9b08452. https://doi.org/10.1021/acsnano.9b08452.

Gold nanorods, first made in the mid-late 1990’s by several groups, are the quintessential examples of shape-controlled properties at the nanoscale: the aspect ratio of the rods governs the positions of their transverse and longitudinal plasmon bands. The “standard” synthesis, a seed-mediated growth approach in aqueous solution, yields gold nanorods with diameters of 10-15 nm and tunable lengths from 20-80 nm. These standard gold nanorods exhibit corresponding extinction spectra that show a transverse plasmon band at ~520 nm and longitudinal plasmon bands at ~600-850 nm. Recently our laboratory has expanded its capability to create “mini,” “maxi” and “mega” rods, where the range of aspect ratios are similar, and in some cases beyond, the standard rods. The fundamental absorption and scattering of light by this library of nanomaterials have been measured; and while the general expectation that larger particles scatter more light, and smaller particles absorb more light, is generally held, the details offer some surprises. The smallest nanorods enable surface chemical characterization by NMR; and recent STEM/EELS experiments show that the density of ligands at the ends and sides of the nanorods depends on ligand type and absolute rod dimension.

Gold and silver nanostructures are common for applications in plasmonics. However, additional metals can be added to bring new function and enhanced properties. Here, the synthesis of branched gold-palladium nanostructures by seed-mediated co-reduction will be discussed. These nanocrystals have localized surface plasmon resonances (LSPRs) that can be tuned throughout the visible and near-infrared through control of particle size, composition, and finer features such as tip sharpness. Significantly, the LSPRs of the branched gold-palladium nanostructures display higher sensitivity to changes in refractive index compared to all-gold analogues. This feature opens up the potential of such materials in LSPR sensor applications. The branched gold-palladium nanostructures also display high structural stability in photothermal applications. Electromagnetic simulations reveal that the Pd content, and specifically its distribution, has a significant impact on optical properties and is an essential criterion for efficient heating. This insight provides new synthetic targets and highlights the importance of developing synthetic methods where the distribution of different metals in multimetallic structures can be precisely controlled.

The plasmonic photothermal effect involves nonradiative conversion of light to heat by plasmonic nanostructures. It has attracted significant attention due to the widespread potential applications in developing energy conversion devices, therapeutic agents, and sensors and actuators. Here we report our recent progress on the design and preparation of plasmonic nanostructures for photothermal conversion. We first introduce the general principle of plasmonic photothermal conversion and then discuss the strategies for improving efficiency, which has been the focus of this field. We then discuss a number of typical application types, such as solar energy harvesting, steam generation, photothermal actuation, and color printing, to elucidate how to tailor the nanomaterials to meet the requirements of these specific applications. In addition to the photothermal effect, other unique physical and chemical properties can be coupled to further explore the application scenarios of plasmonic photothermal materials.

We report a mechanistic study of the Ag-catalyzed redox reaction between surface-bound nitroaromatic and isocyanide molecules for the production of an aromatic azo compound and isocyanate. We elucidate the mechanistic details by tracking the vibrational bands of all chemical species involved in the reaction through in situ surface-enhanced Raman spectroscopy (SERS). In a typical study, we functionalized the surface of Ag nanocubes with 1,4-phenylene diisocyanide (1,4-PDI) and demonstrated their adsorption by analyzing the Ag-bound N-C stretching band (ν_NC(Ag)). When 4-nitrothiophenol (4-NTP) molecules were introduced onto the Ag surface covered with 1,4-PDI, we observed the decrease of the n_NC(Ag) band and the appearance of the –NCO stretching band (ν_NCO) of isocyanate. Concurrently, we detected the formation of trans-4,4’-dimercaptoazobezene (trans-DMAB) at the expense of 4-NTP by characterizing the vibrational bands of these two species. Because the binding of isocyanide to Ag contributed to the formation of an electron-rich surface through s donation, the redox reaction could occur when the oxygen atoms of the electron deficient nitro-groups of 4-NTP were extracted by the electron-rich Ag surface and subsequently used to oxidize the isocyanide to isocyanate. The coverage densities of both 1,4-PDI and 4-NTP on the Ag surface had a strong impact on the production of trans-DMAB. The redox reaction still took place when 1,4-PDI was replaced with 4,4’-biphenyldiisocyanide, confirming the pivotal role played by the isocyanide group.

Replacing one atom or linkage in an organic molecule or polymer can dramatically affect its structure and properties. Chemists have long leveraged the power of synthesis to adjust and fine tune the properties of molecules and materials. Nanoparticles are a class of fundamental structural and functional building blocks for the construction of new materials. The properties of these materials depend on the size, shape, and composition of the constituent nanoparticles as well as their precise organization within the material. In order to fine tune the properties of the material, we must be able to carefully adjust the organization of its component nanoparticles. We are interested in using the power of synthetic chemistry to program and carefully adjust the structure and properties of hierarchical nanoparticle-based materials. This talk deals with peptide-based methods for controlling the synthesis and assembly of nanoparticles into well-defined chiral helical architectures. It will be demonstrated that the atomic make-up of the peptide constructs can be carefully adjusted and that these subtle yet purposeful modifications lead to significant structural changes to the chiral nanoparticle superstructure assembly and properties.

Bimetallic nanomaterials composed of a less reactive core metal with a dilute surface coverage of a second, catalytically active metal can exhibit improved catalytic performance resulting from a fine balance of activity and selectivity. Similarly, hybrid nanoparticles with a plasmonic core can be excited by visible light to influence the reactivity or selectivity at atoms or satellites of a more catalytically active metal localized at their surface. In both cases, the differing catalytic reactivity of the two component materials in the solid state is also reflected in differences in the reactivity of their metal salt precursors during nanomaterials synthesis. Our research group has developed a set of versatile synthetic tools for differentially controlling the rates of reduction of metal precursors to facilitate the growth of nanostructures with desired morphologies and surface compositions. Recently, we demonstrated that visible light excitation of a silver core nanostructure can be used to drive the plasmon-assisted reduction of platinum ions—a material that is not plasmonic in the visible region. This plasmon-mediated synthesis relies on the oxidation of a weak reducing agent, sodium citrate, by plasmonic hot holes to generate thermalized electrons that reduce platinum ions at the surface of the silver nanoparticle core. The approach overcomes key challenges in bimetallic nanomaterials synthesis, including accelerating the kinetically slow reduction of one metal precursor while also preventing competing galvanic exchange processes. Tuning of reduction kinetics controls the localization and amount of platinum deposited on the silver cores and enables the selective generation of silver-platinum core-shell and core-satellite architectures, the latter of which are not accessible using standard thermal synthesis approaches.

An overview of the Solid State and Materials Chemistry (SSMC) program will be discussed in the context of the 2019 workshop “Frontiers in hybrid and interfacial materials chemistry research” (workshop report by B. S. Guiton et al., MRS Bulletin, 45(11), 951-964. doi:10.1557/mrs.2020.271). The workshop participants identified interdisciplinary challenges and opportunities as well as current areas of progress in subdisciplines including hybrid synthesis, functional surfaces, and functional interfaces. The scope of the workshop included several research topics beyond the scope of the SSMC program. To address this, the presentation will also briefly describe the other Topical Materials Research Programs (TMRPs) in DMR in addition to showcasing highlight outcomes of some recent research projects supported by SSMC.

Nanoparticle assemblies show interesting optical properties and show distinct advances in sensing and therapeutics. Short, thiolated DNA strands are common linkers in gold nanoparticle (NP) assemblies; however, the precise placement of different DNA sequences on single NPs is challenging, which limits possible architectures. Moreover, because of the similar physical and chemical proprieties of different DNA sequences, the uniform gold NP core material makes regiospecific functionalization challenging. This talk will describe a gold NP platform and functionalization process that allows for the spatially-controlled placement of multiple DNA sequences on single NPs. We will discuss how gold nanostars fully conjugated with one DNA sequence can selectively release the DNA from the tips under excitation of fs-pulses at the plasmon wavelength of the branch. Then, we will show how these now bare Au regions can be functionalized with a different thiolated DNA sequence to result in constructs with different DNA strands at specific structural NP features. This selective functionalization can be further exploited to create unique nanoparticle assemblies by hybridization to other DNA-functionalized gold colloids.

Colloidal synthesis of nanoparticles is a complex process, which is subject to both thermodynamic and kinetic restrictions. Understanding and controlling these thermodynamic and kinetic factors are critical for the reproducible and reliable synthesis of nanoparticles. A common thermodynamic factor is the reduction potential of the reactants, whose role in nano-synthesis is not yet well understood. Here we took the gold bipyramids (GBPs) synthesis as an example to explore the quantitative relationship between reduction potential and synthetic products. For the convenience of comparison, phenol derivatives with similar structure (4-R-phenol) but different standard reduction potential were chosen as reductants, where R is chlorine, hydrogen, methyl, and methyl oxide, respectively. When they were used to reduce gold precursor, their actual reduction potential can be regulated easily through the pH value of the growth solution to study the effect of reduction potential on the products. For each phenol derivative we tested, it was found that there is an appropriate pH range, in which GBPs of high quality were synthesized and their longitudinal LSPR peak red-shifted with the increase of the initial pH value. This pH range is related to the substituent on phenol, those with electron-withdrawing groups are higher while those with electron-donating groups are lower. Although these phenol derivatives work in different pH ranges, the plots of the longitudinal LSPR peak of their resultant products versus the pH are quite similar. When pH was substituted by the corresponding reduction potential calculated from the Nernst equation, these plots coincide together, meaning that the influence of reduction potential on the formation of GBPs is independent of the specific phenol derivatives (reductants), so the reduction potential is a fundamental factor in the synthesis of GBPs. This role of reduction potential is expected to be general in the synthesis of other types of nanoparticles, which is important not only for understanding the mechanism of nano-synthesis but for improving the reproducibility and reliability of nano-synthesis.

Many industrial oxidation processes based on peroxide, have a difficulty of over oxidation in homogeneous solution principally due to faster oxidation rates of already oxidized species and high statistical probability of secondary oxidation near the completion of reaction. Ideal for stepwise oxidation is to limit the residence time of target molecule in a reaction zone to allow for single (or quantized) reaction events. This can be achieved in a membrane geometry where catalyst along a pore length and set flow velocity can precisely control residence time for oxidation. It is also known that Au nanoparticles can catalytically activate peroxide under light irradiation due to the formation of concentrated surface plasmon electromagnetic fields and thus hypothesized to be present in nanoporous planes. A plasmonic membrane was synthesized by evaporation of 25nm thick Au films onto pore entrances of anodized aluminum oxide membranes (AAO) with pore diameters of 20-200nm. This allowed solutions of peroxide and sec-phenethyl alcohol to flow through membrane and interact with Au surface plasmon upon exit of the membrane. Under light illumination of 10-100 mW/cm2, quantum efficiencies (photon/peroxide radical) above 100% were seen (as high as around 250%), indicating a combination of both field induced activation of peroxide as well as a hot electron injection mechanism. By flowing sec-phenethyl alcohol and peroxide through the optimized Au@AAO system, the controlled single oxidation product of acetophenone was observed demonstrating the promise of the plasmonic flow membrane reactor design.

Hot carriers refer to energetic electrons/holes with energy distributions deviated from equilibrium Fermi-Dirac distributions. In metallic nanostructures, hot carriers can be generated from the decay of surface plasmons (SPs), which holds great promise for plasmon-enhanced photocatalysis, solar-energy harvesting devices, etc. One key knowledge needed for developing these applications is the hot-carrier energy distributions (HCEDs). Recently, an effective way to determine HCED experimentally utilizing a scanning tunneling microscope (STM)1 was reported. With carefully chosen molecules that possess appropriate transmission characteristics between the plasmonic gold film and the gold tip of a STM, single-molecule junctions are created for current-voltage measurements at various voltage bias. The difference in the measured currents for the cases with and without plasmonic excitation reflects the so-called hot-carrier current which enables the direct quantification of HCED.
The knowledge of HCED is of fundamental and practical importance. Hot electrons/holes generated in metallic nanostructures can be transferred/injected to adjacent semiconductors with appropriate band structures for plasmon-enhanced photocatalytic or photovoltaic processes. Gold- and silver-semiconductor heterostructures are most commonly studied for such applications based on hot electron transfer. In the last few years, titanium nitride (TiN) has been studied as an alternative plasmonic material with optical properties comparable to gold. Its chemical inertness plasmon resonance in the bio-transparent window (750 nm- 900 nm) makes it promising for biophotonic applications. We synthesized TiN@TiO2 core-shell nanoparticles (NPs) and demonstrate that with the illumination of a 700 nm laser, such NPs efficiently convert free oxygen into singlet oxygen molecules (1O2) which are key molecules used to kill cancer cells in photodynamic therapy (PDT).2 We also show that hot electrons injected from TiN to TiO2 play a major role in this photocatalytic process and provide an analytical model for evaluating the hot electron injection efficiency. Our study confirms the promise of TiN in hot electron-mediated applications especially in areas of biomedical and therapeutic interest.
Yet another direction that will be covered is a machine learning assisted topology optimization of thermal emitters for far-field thermophotovoltaic (TPV) applications. We have developed a unique optimization framework that allows to significantly speed-up meta-structure design optimization, and allows to significantly increase thermal emission reshaping efficiency of TiN based thermal emitters.3 Specifically, we have demonstrated that ML assisted optimization framework ensures more than 4900-time speed up of thermal emitters design development in comparison with conventionally used topology optimization. The developed thermal emitter design ensures more than 97% thermal emission efficiency.
1. H. Reddy, K. Wang, Z. Kudyshev, L. Zhu, S. Yan, A. Vezzoli, S. J. Higgins, V. Gavini, A. Boltasseva, P. Reddy, V. M. Shalaev, E. Meyhofer, Science 369, 6502 (2020).
2. X. Xu, A. Dutta, J. Khurgin, A. Wei, V. M. Shalaev, A. Boltasseva, Laser Photonics Rev. 14, 5 (2020).
3. Z. A. Kudyshev, A. V. Kildishev, V. M. Shalaev, A. Boltasseva. Appl. Phys. Rev. 7, 2 (2020).

This talk will discuss the plasmon-induced resonance energy transfer process as the mechanism of energy transfer from plasmonic metals to semiconductors. It will show the effort to develop solar energy conversion devices, biosensors, and therapeutic agents based on this mechanism. This talk will demonstrate some application examples across several relevant fields.

Metal nanoparticles exhibit unique size and composition dependent optical properties, known as localized surface plasmon resonance (LSPR). When they assemble into small clusters, the optical coupling between the nanoparticles lead to new plasmonic properties. Specifically, when Ag or Au nanoparticles assemble into dimers and trimers, they exhibit polarization dependent optical spectra due to short range coupling. When they formed randomly arrays, the long-range coupling could result in changes in the LSPR band width and peak position. In addition, when semiconductor quantum dots (QDs) were placed closed to the metal nanoparticle assemblies, the plasmonic feature is altered due to the coupling between the exciton in the QDs and the plasmons. The fluorescence of the QDs changes as well due to this interaction. Under specific conditions, we observed peak splitting in both the dark-field scattering and fluorescence spectra of Au-QD-Au hybrid nanostructures.

A significant investigation has been done on understanding and implementing the well-known effect of surface plasmon resonance (SPR) in nanomaterials, and it has opened extensive research possibilities. Our lab has grown different nanomaterials such as Si nanoparticles and hybrid Au-ZnO nanowires to explore their opportunity in solar cells, water splitting, and chemical/biosensing applications. The aim is to control these nanostructures' size and shape and study their influence on different properties. I will present the embedding of Si nanoparticles in various matrices such as SiOx, SiNx, SiOxCy, and their unique properties for downshifting applications in the third generation of solar cells. Further, I will report the growth of hybrid Au-ZnO nanowires and the associated opportunities in hydrogen production and chemical/biosensing applications.

Widespread misuse and overuse of antimicrobial drugs had led to emergence of multidrug-resistant (MDR) bacteria, placing tremendous pressure and even posing threat on health care worldwide. According to the recently released data by Infectious Diseases Society of America (IDSA), MDR infections are becoming the third leading cause of deaths (~160,000 deaths annually) in the United States. If left unchecked, MDR bacteria caused diseases could kill more people than cancer. Therefore, it is of crucial importance to develop effective antimicrobial approaches to combat bacterial infections, rather than merely relying on identification and development of new classes of antibiotics or antimicrobial agents. Recently, light-activated disinfection (LAD) has emerged as an efficient and efficacious way to eradicate biofilm-mediated microbial infections, advancing conventional concepts and bolstering current prevention efforts of bacterial disinfection. LAD approaches have shown excellent ability to instantaneously photo-inactivate a wide range of microbes and can work synergistically with medical antimicrobial agents. Ultraviolet (UV) germicidal irradiation is among one of most reliable and well-studied LAD-based antimicrobial technologies. It provides rapid and effective inactivation of drug-resistant microorganisms by destroying their nucleic acids and disrupting their DNA, rendering them incapable of reproducing and infecting. However, prolonged UV exposure can pose significant health risks to humans, causing skin cancer and eye damage, thus can only be applied at discrete moments in time (i.e. episodically). Especially for implementing the tool for continuous environmental cleaning activities within the clinical environment, this disinfection method is not suitable for prevention and control of healthcare-associated infections (HAI), which are a significant burden globally with millions of patients affected annually.

Violet-blue 405-nm light is emerging as a safe and cost-effective alternative to UV-based LAD technologies for continuous environmental decontamination since it can be used automatically without any downtime or side effects. Using sophisticated configured light emitting diode (LED) ballistic photons that are non-harmful to humans, this bactericidal visible light possesses outstanding antimicrobial properties against a wide range of bacterial and fungal pathogens. Nevertheless, its photo-inactivation efficiency is still several orders of magnitude lower than that of UV light. Here, we introduce a plasmonic approach enabling rapid (minutes) and extremely efficient (> 99.995%) inactivation of MDR Vibrio Cholerae biofilms with 405-nm light. We demonstrate nearly three orders of magnitude improvement in photo-inactivation efficiencies using 405-nm light activated radiatively coupled aluminum plasmonic nanoantenna arrays. We achieve highly effective light-induced local hyperthermia and toxic reactive oxidative species to irreversibly disrupt MDR biofilms. Our technique opens the door to light-activated smart antimicrobial coatings for continuous decontamination in occupied areas, thereby can help infection control practices by preventing mature biofilm development using low-intensity incoherent light.

Chiral structure controlled at nanoscale provides a new route to achieve intriguing optical properties such as polarization control and negative refractive index. However, asymmetric structure control with nanometer precision is difficult to accomplish due to limited resolution and complex processes of conventional methods. In this regards, utilizing chirality transfer occurring at organic-inorganic materials offers viable route to overcome these limitations. Previously we developed a unique synthesis strategy that characteristic of molecule is transferred to gold nanoparticle morphology. Based on the system, here, we demonstrated novel chiral gold nanostructures exploiting chirality transfer between peptide and high-Miller-index gold surfaces. Enantioselective adsorption of peptides results in unequal development of nanoparticle surface and this asymmetric evolution leads to highly twisted chiral element in single nanoparticle making unprecedented 432 helicoid morphology. The synthesized helicoid nanoparticle showed strong optical activity (dissymmetry factor of 0.2 at 622 nm) which was substantiated by distinct transmittance color change of helicoid solution under polarized light. Modulation of peptide recognition and crystal growth enabled diverse morphological evolution and the structural alterations provided tailored optical response, such as optical activity, handedness, and resonance wavelength. We believe that our peptide directed synthesis strategy offers a truly new paradigm in chiral metamaterial fabrication and will be beneficial in the rational design of chiral nanostructures for use in novel applications.

Plasmonic nanoparticles (e.g., gold, silver particles) show unique optical properties, such as localized surface plasmon resonance. Organization of plasmonic nanoparticles into discrete nanostructures with precise control allows for fine-tuning their interparticle plasmon coupling and collective properties. The assemblies of plasmonic nanoparticles have shown diverse applications in photonics, sensing, catalysis, and energy harvesting. In this talk, I will present our efforts to the fabrication of discrete plasmonic nanostructures with molecular-like and polymer-like configurations through directional bonding of nanoparticles and the exploitation of the plasmonic properties of the hierarchically assembled nanoparticles.

Stabilization of nanoparticles is usually achieved by either charging the nanoparticle surface (electrostatic stabilization) or capping the nanoparticle surface with ligands (steric stabilization). Because organic capping ligands can also be used to tune the size, morphology, and chemical reactivity of nanoparticles during the nanoparticle formation process, attention has largely focused on how organic molecules influence the growth of nanoparticles.^1-2 However, the roles of ionic precursors and the influence of their presence in the reaction mixture on their properties have been mostly overlooked. Gold nanoparticles are excellent species for investigating the impact of the ion residue in the reaction solution on the synthesis and properties of gold nanoparticles owing to their good stability and characteristic photophysical properties.³ We used polyethyleneimine (PEI) as the reducing and stabilizing agent for gold nanoparticles and tuned the ratio between PEI and Au(III) to adjust the abundance of gold ions in the nanoparticles. We found that a low level of the elemental gold and the stabilization of gold nanoparticle by an excess of gold ions contributed to the production of ultra-small nearly neutral gold nanoparticles.⁴ The cross-linking between gold ions/PEI/nanoparticles further led to the assembly of these small gold nanoparticles into suprananoparticles that were stable in water. The hygroscopic Au(III) residues in the suprananoparticles absorbed moisture to form a micro-water pool and subsequently the nanoparticles in the new aqueous solution reshuffled to form larger nanoparticles, resulting in significant changes in their optical properties. This phenomenon was used to formulate a material for a fast, sensitive and straightforward detection of water content in organic solvents. Our results indicate that ionic precursors may play an important role in the formation and stabilization of nanoparticles. Moreover, the ionic residues, regardless of whether they are a part of the nanostructure or impurities, may introduce further characteristics and significantly change the properties of the nanostructure.

References.
1. Personick, M. L., et al., J. Am. Chem. Soc. 2013, 135, 18238.
2. Rycenga, M., et al., Chem. Rev. 2011, 111, 3669.
3. Daniel, M. C., et al., Chem. Rev. 2004, 104, 293.
4. Choi, S., et al., Nanoscale Advances 2019, 1, 1331.

Nature and humans are inextricably linked to colors, and they adopt nanostructures—not a dye or chemical pigment—to generate nonfading colors that have unique optical properties. The nanostructures are dominantly embodied in two optical phenomena: photonic crystals and plasmonics. Photonic crystals give rise to the selective reflection of certain wavelengths as the periodic nanostructures create photonic bandgap inside the structures. Plasmonic colors happen at the metal surface of which morphology is defined in nanoscale; surface plasmon polariton (SPP) and localized surface plasmon resonance (LSPR) are largely known for the origins of plasmonic resonance. The two colors that have structural origins have great potentials when they are realized in a granular format. The structurally-colored granules serve as aesthetic pigments, microsensors that monitors microenvironment, optical barcodes, and nanoscale blocks constructing macroscopic structures. The granular combination of photonic and plasmonic colors are expected to couple two unique advantages into a single template.
Here, we utilize emulsion drops to create a plasmonically resonant surface on a microsphere with a photonic bandgap inside. The dual-mode Janus microspheres that exhibit plasmonic and photonic colors on the opposite side as a consequence of directional metal deposition. We microfluidically fabricate single emulsions with highly saturated colloids inside and on the interface at the same time, where the 3D crystallized colloids inside the emulsions develop a photonic bandgap, and the 2D crystallized colloids at the surface lead to the plasmonic color. We found the optimal range of suitable nanoparticle diameter for plasmonic colors from aluminum, from gold, and photonic colors to be 300-400, 200-300, and 180-250 nm in this system, respectively. It indicates that there is a limit to independently control the two colors so we adopt double emulsions of which the core and shell have crystallized nanoparticles with different diameters.
For the fabrication of Janus photonic/plasmonic microspheres, we disperse silica nanoparticles in an acrylate polymer, where silica nanoparticles are spontaneously crystallized by repulsive interactions from disjoining pressure. Silica nanoparticles are anchored at the emulsion interface and slowly exposed to the water phase as the emulsions are incubated in water. Also, the distance between nanoparticles becomes shortened with time because the polymer is slowly dissolved into water. The colloidal aging determines the surface nanostructure of the microparticles afterward, which determines plasmonic colors whereas the crystallization inside determines the photonic colors. Silica particles are removed and metal is deposited unidirectionally, which results in a continuous film with periodic pores on the top and metal bowls inside the pores. SPP occurs at the former and LSPR occurs at the latter, two of which are coupled to make plasmonic resonance by the incoming light. When the resonance occurs in the visible range, the surface generates plasmonic colors and longer incubation time blue-shifts the resonant wavelength so that the colors change from blue to purple to red. An incubation time of emulsions thus changes the nanostructure and it is first to sophisticatedly control plasmonic colors in a granular format including all the rainbow colors. The colors are also adjusted by the metal type, metal thickness, and the relative location on the microspheres with respect to the metal-deposited apex. Finally, the anisotropic location of metal renders the microspheres react to an electric field so that the group of them makes a reflective display. The Janus microspheres are great candidates for microsensors with photonic barcodes, aesthetic colorants for secondary macrostructures, and active pixels for a reflective display.

Raman spectroscopy is one of the riveting methods for molecular sensing as it provides noninvasive and label-free molecular identification. However, the intensity of Raman scattering is remarkably low, which makes it challenging to detect the infinitesimal amount of molecules. Metal nanostructures or nanoparticles can localize electromagnetic field on their surface for specific wavelengths of incident light through surface plasmon resonance, which can enhance Raman signal extraordinarily for the molecules near the surface; the phenomenon is referred to as surface-enhanced Raman scattering (SERS). SERS is a key to overcoming the weakness of Raman spectroscopy for molecular detection. However, metals are prone to contamination by adhesives such as protein, resulting in a reduction in Raman signal. Therefore, the pretreatment of samples is unavoidable for Raman analysis because many analytes are complex mixtures of small molecules, proteins, and/or cells.
To prevent contamination, hydrogel networks have been employed as a protection layer. The hydrogels allow the infusion of smaller molecules than the mesh size while excluding larger molecules. Therefore, the hydrogel layer with a proper mesh size obviates the need for a time consuming and costly pretreatment, allowing the on-site analysis. One of the simple and practical structures as pretreatment-free SERS-based molecular sensors is metal nanoparticles encapsulated in hydrogel microparticles or microgels. For high Raman intensity and signal reproducibility, metal nanoparticles are required to be homogeneously embedded in the hydrogel matrix at a high concentration without local aggregates. However, it is very challenging to produce such a structure through post-encapsulation of metal nanoparticles due to the low dispersion stability of metal nanoparticles at a high concentration.
Here, we design SERS-active microgels containing a high density of gold nanoparticles through simultaneous photoreduction of gold precursor and photocross-linking of hydrogel precursor. To produce the microgels, the mixture of gold precursor, polyethylene glycol diacrylate (PEGDA), photoinitiator, sodium citrate, and distilled water is emulsified into monodisperse water-in-oil (W/O) emulsion droplets using a microfluidic device. The droplets are irradiated by ultraviolet (UV) for 2 min and incubated at 50°C for 2 days. During the UV irradiation, the photoinitiator is decomposed to form radicals, which cause photoreduction of gold precursors to form gold nanoparticles and photocross-linking of PEGDA to form a hydrogel network. During the thermal treatment, the unreacted gold precursor is reduced in the presence of sodium citrate, forming new gold nanoparticles or promoting the growth of preformed gold nanoparticles. The size and number density of gold nanoparticles formed in the hydrogel matrix strongly depend on the concentration of sodium citrate. The concentration of sodium citrate is optimized according to the intensity of Raman signals of the rhodamine 6G molecule. The microgels made using optimized sodium citrate concentration exhibit high Raman intensity as well as high signal uniformity, indicating high dispersion stability of gold nanoparticles at a high concentration. With the SERS-active microgels, we can detect pyocyanin dissolved in saliva without any sample pretreatment; the pyocyanin is one of the biomarkers for sepsis which is caused by infection of Pseudomonas aeruginosa. The limit of detection (LOD) is found to be 100 nM, which is two orders of magnitude lower than the typical concentration in clinical saliva samples of sepsis patients. It is noteworthy that the SERS-active microgels can detect the small molecules in the presence of large adhesives and cells in saliva. We believe that our microgels can be used as a general sensing platform for the on-site detection of small toxic molecules and biomarkers in various samples, including foods, drugs, biological fluids, and many others.

Conformation and dynamics of small molecules, peptides, and proteins on the surface of metal nanostructures strongly affect the properties and applicability of these materials. However, despite the urgent need in understanding of molecule-surface and surface-induced intermolecular interactions, because of a great complexity of the associated phenomena and limitations of the available analytical methods, in many cases these details still remain elusive. Infrared spectroscopy, and especially its nonlinear two-dimensional variant conducted with sequences of femtosecond laser pulses, 2DIR, is a powerful method to elucidate molecular structure. Recently, the application of 2DIR spectroscopy was extended to studies of molecules on nanostructures. I will present results of our 2DIR experiments with molecules on two different classes of nanostructures: sub-wavelength colloidal silver nanoparticles with sizes of few to few tens of nanometers and half-wavelength gold infrared antennas of micrometer size.
Frequently, molecular organization within the nanoparticle capping layer has highly disordered character. Surprisingly, our 2DIR studies of a small tripeptide glutathione on silver nanoparticles revealed that under favorable conditions the interaction with silver surface leads to formation of highly-ordered intermolecular aggregates, with structure that apparently resembles tightly-stacked β-sheet-like layers wrapping the nanoparticle. This finding may have important implications in situations where ordered structure of the capping layer can impart unique chemical properties to the nanoparticles or can serve as a basis for design of multilayer macromolecular architectures.
In order to boost the sensitivity in 2DIR studies of surface molecules, we use resonant plasmonic infrared nano-antennas. The optical properties of these antennas can be controlled by the design of their size and shape to optimally suit the needs of the advanced nonlinear experiments. Signal enhancements factors of up to 10⁵ have been observed, whereas molecular quantum dynamics probed by 2DIR spectroscopy have not been affected by the interaction with plasmonic antennas. Among the direct consequences of signal amplification are dispersive line shapes of the vibrational transitions, competition between the near-field coupling and the radiation damping enhancement mechanisms, and lack of the selectivity of molecular excitation to the laser light polarization, imposed by the surface boundary conditions. Interestingly, we observed that molecules subjected to the enhanced near-field of the plasmonic antennas are excited within the highly non-perturbative excitation regime, even though weak laser pulses are used for the excitation. This result can pave a way towards magnetic resonance-style spectroscopic experiments in the infrared.

Working with plasmonic materials involves many scientific steps, including, aside from the laboratory-level experiments, the numerical creation, their comparison, and the device fabrication. Besides these challenging steps, the design of new plasmonic materials with unique physical and chemical characteristics, and outstanding optical properties, which are traditional realms of gold and silver, merits an important place. Optimizing the material properties to improve their functionality and performance in plasmonic applications is a subsequent challenge to be tackled, also through iterative feedback from the experiments.

This presentation will demonstrate an overview of recent advances in the computational design of potential future plasmonic materials, such as translational metals, transparent conducting oxides, or plasmonically active semiconductor allotropes, and their application in plasmonic structures, concepts, and devices. The extraction of complex dispersion characteristics from density functional theory (DFT) calculations allows the integration into subsequent electromagnetic modeling steps. This talk will illustrate the panorama of applying the so-developed materials into plasmonic particle investigations, ranging from isolated particles of various shapes and materials to self-assembled regular structures, exhibiting collective plasmonic crystal responses. The applications range from plasmonic sensing, metamaterials, and self-assembled particle clusters for surface-enhanced Raman scattering and catalysis.

The Computational Materials Group at SDU investigates computational pathways, from the first-principles calculation-based molecular design to the three-dimensional multi-physical modeling of plasmonic nanostructures and plasmonically enabled devices for sensing, lighting, and catalysis applications. Our group recently developed the "Photonic Materials Cloud," a cloud-based platform to support streamline the experimental, numerical, research, and education-based work on plasmonic materials. It allows for creating and comparing various material data via various methods and applying them to standard photonic applications, such as nanoparticle scattering and layered thin-film responses. The export of publication-ready graphics and column-based data facilitates its easy integration into a photonic materials science research line.

The rapid heating induced by ultrafast excitation of metal nanostructures coherently excites the breathing vibrational modes of the structure. Detailed information about these modes can be obtained by studying single nanoparticles with transient absorption microscopy. However, in single particle experiments the breathing modes are usually heavily damped by radiation losses into the substrate. Recently we discovered that using low density substrates drastically reduces radiation losses and creates very high vibrational quality factors for gold nanoplates. This allows several new phenomena to be explored, including strong vibrational coupling between stacked nanoplates, and new vibrational modes that correspond to motion of the nanoplates relative to the substrate and, for the stacked nanoplates, relative to each other. The results from these experiments will be described in this talk, along with studies of mass loading effects for the nanoplates. In the mass loading experiments a localized change in vibrational frequency is observed when a particle is adsorbed onto the surface of a nanoplate. This is an unusual result, as the breathing modes of the nanoplates are normal modes that are spread out over the entire plate. This implies that the adsorbed mass hybridizes the normal modes. Insight into this effect is obtained from finite element simulations that model the vibrational modes of the coupled system.

Over the years, tremendous advances have been made in the design and fabrication of nano-size objects in a variety of geometries. Nanosphere Lithography is a large-area nanostructurization technique introduced in the early 80s which utilizes dielectric nanospheres for the preparation of nanoapertures or nanoparticles formed in the gaps between the nanospheres. The process is cost-effective and easily scalable, therefore it is often chosen in favor of expensive and time-consuming electron lithography or low-resolution photolithography.

In this report, we describe a simple method of preparing versatile plasmonic nanomasks based on electrostatic-driven assembly of silica nanoparticles (SNPs) of diameters ranging from 60 to 300 nm. This technique makes it possible to produce homogenous masks of uniform apertures on bare or PMMA-coated glass substrates covering an area of a few cm². Fabricating different nanomasks starting from single holes, through dimers or trimers, up to long, nanowire-like apertures is possible due to a high degree of control of the sizes and concentrations of the SNPs, as well as of the temperature during the process. In combination with physical vapor deposition of metals, such as silver or gold, the masks can be used further as platforms for obtaining many different plasmonic nanostructures like discs, cones, dimers, or even more sophisticated shapes by simply changing the evaporation angle. Such plasmonic resonators are very suitable for SERS, photovoltaic and biosensing applications because of their high tunability stemming from a number of degrees of freedom within the described method of fabrication.

High light field enhancement and its nanoscale confinement in plasmonic nanostructures facilitate stronger light-matter interaction, including spontaneous emission rate, nonlinear response, light absorption, and so on. Plasmonic metasurfaces are two-dimensional nanoantennas arrays enabling subwavelength field localization. Because of the nanoscale light confinement, the metasurfaces provide an exceptional ability to manipulate light accompanied by unique spectral features, including high quality factor resonances.

We design the plasmonic metasurface that consists of multi-segment silver-silicon nanopillars, and we analyze the multiple mode excitations in this nanopillar array. The effect is similar to the excitations of multiple plasmonic modes at the complex interfaces of multilayer metal and dielectric structure. We numerically study the spectral response of a single and paired nanopillars in the unit cell of the periodic array [1]. Because of the hybrid silver-silicon multilayer design, we can realize Fano resonances and Rabi splitting in the metasurface's spectral response.

The study's emphasis has been placed on the analysis of bright and dark modes excited in the nanopillar array. Mode interplay and coupling give rise to asymmetric spectral profiles in often referred to as Fano resonances. Strong coupling between plasmonic states causes the energy levels to repel by splitting, and because of the mode coupling facilitated by the lattice, we observe their Rabi splitting.
The effects of multiple mode excitations and their Rabi splitting facilitated by the lattice can also be observed in another spectral range providing all parameters of the structure are scaled proportionally. However, it also imposes requirements on material properties in the spectral range of interest. In this case, a common condition for the observation of plasmonic resonances needs to be satisfied, and in particular, the real part of permittivity of the metal-like material should be comparable in magnitude to the semiconductor permittivity.

Support from the Navy HBCU-MI program under the grant N00014-18-1-2739 and the Office of Naval Research under the grant N00014-19-1-2117 is gratefully acknowledged.

[1] D. Bosomtwi, M. Osin'ski, and V. E. Babicheva, "Mode Coupling and Rabi Splitting in Transdimensional Photonic Lattices," 2020 IEEE 20th International Conference on Nanotechnology (IEEE-NANO), pp. 107-110 (2020).

Even in the 21^st century, bacterial infections are still treated empirically. Antibiotics are prescribed preemptively, because a proper diagnosis relies on culture growth which takes a day or more and can delay patient treatment, increasing morbidity. Yet the unnecessary administration of powerful, broad-spectrum antibiotics leads to the proliferation of antibiotic resistance. A bioinspired sensing platform, composed of 2-dimensional physically activated chemically (2PAC) assembled surface enhanced Raman scattering (SERS) sensors coupled with machine learning (ML) algorithms, has been developed for rapid phenotypic antibiotic susceptibility tests (AST). This platform provides new diagnostic tools for antibiotic stewardship, complementing existing genomic tests that only provide feedback on known antimicrobial resistance mechanisms. Just as one can smell the difference between coffee and chocolate amongst multiple odors, SERS+ML rapidly measures and classifies spectral features of bacterial metabolite signatures in response to antibiotics, which are correlated with antibiotic lethality mechanisms. As individual odor receptors are incapable of identifying odorant molecules, so are individual SERS spectra unable to differentiate in a complex background.
SERS + ML is an exciting nascent area where some recent work using vector machines and artificial neural networks has greatly improved concentration predictions from SERS data. However, this is a longstanding challenge to produce large, high quality and reproducible data sets are needed for any ML analysis with SERS. Since the nanogap distance between gold nanospheres determines performance in SERS devices, small variations on the order of angstroms lead to large signal variations. 2PAC is low-cost, scalable, and reproducible chemical assembly method able to control of sub-nm nanogaps and is capable of reproducibly probing individual molecules over mm² areas. An implemented CNN regression model trained on SERS data of Rhodamine 800 resulted in limits of detection (LOD) and quantification (LOQ) of 10 fM (~10^-5 ng/mL) with prediction accuracy (r² value) of 0.96 over a dynamic range of 6 orders of magnitude.
In order to reduce the time required for phenotypic AST, we present a metabolomics approach rather than direct measurement of cell growth or viability to detect phenotypic susceptibility or resistance to antibiotics Yet there is an important price to pay to reduce AST time in this way: metabolomics approaches introduce an enormous parameter space. Consider that the E. coli metabolome contains over 2600 different metabolites. Nevertheless, SERS + ML allows for a fingerprinting approach to handle the complex data. Analysis of SERS data using a generative model, the variational autoencoder (VAE), is able to identify spectral features associated with molecular fingerprints the in the cell lysate data associated with antibiotic efficacy. The high interpretability of the VAE generated SERS spectra allows us to identify useful vibrational information which guides additional targeted data collection to improve classification accuracy. Culture-free and easily acquired datasets of bacterial metabolites in aqueous solution can be leveraged to improve predictive models of complex metabolite response of bacterial communities and is one of the most significant advantages of using a generative model. Greater than 99% accuracy is achieved with unsupervised Bayesian Gaussian Mixture analysis when using data informed transfer learning with only a few training examples. Our results show differentiation of bacterial populations of ESKAPE pathogens based on antibiotic susceptibility in 10 min when using SERS + ML. This enormously reduces the amount of time needed to validate phenotypic AST with conventional growth assays and outlines a promising approach towards practical SERS AST.

The assembly of plasmonic metal nanostructures can efficiently merge the reactivity and energy harvesting abilities with enhanced plasmonic performance for visible light photocatalysis. Here we explore the influence of the presence of electromagnetic hotspots in the ability of plasmonic colloidal structures to induce efficient energy transfer for plasmonic catalysis. We report a novel synthetic strategy for the fabrication of colloidally assembled nanoparticles (NPs) in aqueous solution through fine controlled galvanic replacement between Ag nanoprisms and Au precursors.^1-5 Colloidally assembled nanostructures, e.g. Au particle-in-a frame and bimetallic assembled Au@M (M=Pd, Pt) nanostructures with catalytically active metals, exhibited superior performance over their constituent nanostructure counterparts in plasmonic sensing, surface-enhanced Raman scattering (SERS), and plasmonic catalysis.^1-4 Recently, we successfully synthesize colloidal Au and Au@M (M=Pd, Pt) NP trimers with remarkable structural stability in various solutions.¹ These model systems allow us to study the synergy effect of hot spots and bimetallic composition in plasmonic catalysis. Our computational and experimental results highlight the synergy effect of geometry and composition of plasmonic catalysts in plasmon-driven chemical reactions. The plasmonic properties of NP trimers show that core-shell bimetallic NPs with hot spots can induce efficient light-to-chemical energy conversion as an increment of non-radiative plasmon decay to the catalyst surface. We monitored the plasmon-mediated reduction of 4-nitrobenzenethiol (4-NBT) using SERS for studying hot electron-induced chemical conversion. Core-shell bimetallic NP trimers show distinguishable photo-induced reduction of 4-NBT compared to their monomer or monometallic counterparts due to their efficient energy transfer. We expect that the present study can provide a new direction for the development of efficient photocatalysts.

References
1. S. Lee, H. Hwang, W. Lee, D. Scherbarchov, Y. Wy, J. Grand, B. Auguié, D. H. Wi, E. Cortés,* S. W, Han,* "Core-shell bimetallic nanoparticle trimers for efficient light to chemical energy conversion" (Just accepted in ACS Energy Letters).
2. S. Lee, J. Kim, H. Yang, E. Cortés, S. Kang, S. W. Han,* "Particle-in-a-Frame nanostructures with interior nanogaps" (Angew. Chem. Int. Ed. 2019, 58(44), 15890-15894).
3. Y. Wy, S. Lee, D. H. Wi, S. W. Han,* "Colloidal clusters of bimetallic core-shell nanoparticles for enhanced sensing of hydrogen in aqueous solution" (Part. Part. Syst. Charact. 2018, 35(5), 1700380).
4. S. Lee, Y. Wy, Y. W. Lee, K. Ham, S. W. Han,* "Core-shell nanoparticle clusters enable synergistic integration of plasmonic and catalytic functions in a single platform" (Small, 2017, 13(43), 1701633).
5. S. Lee, J. W. Hong, S.-U. Lee, Y. W, Lee, S. W. Han,* "The controlled synthesis of plasmonic nanoparticle clusters for efficient surface-enhanced Raman scattering platforms" (Chem. Commun. 2015, 51(42), 8793-8796).

Laser diodes (LDs) are regarded as the next generation of ultra-efficient light sources, being able to produce more photons at high power densities than conventional light-emitting diodes.^[1] Even though most state-of-the-art technologies are based on a blue LD pumping a white-light-emitting phosphor, an all-laser wavelength mixing approach, e.g. a combination of three (RGB) or even four (RGBY) laser wavelengths would outperform the efficiency of any other known white-light source.^[2] However, in illumination applications, laser-based lighting systems still suffer from their monochromatic, low-divergent, and coherent nature, which demands a new generation of extremely efficient and versatile optical diffusers based on disordered nanostructured materials.^[2] In the here presented study, we demonstrate a macroscopically expanded, three-dimensional (3D) laser light diffuser based on a highly porous (>99.99%) nanoarchitecture, composed of interconnected hollow hexagonal boron nitride (h-BN) microtubes, with a wall thickness below 25 nm. The 3D hollow h-BN microtubular framework structure is synthesized by a novel template approach, which is based on a highly porous ceramic network consisting of tetrapodal-shaped microparticles.^[3] The synthesis results in a disordered and non-absorbing photonic network with thinly spread Rayleigh-type scattering centers, based on a combination of feature sizes greater than, equal to, and well below the magnitude of the impinging wavelength.^[4] With densities close to that of air, these aero-materials basically resemble an artificial solid fog, but with a defined hierarchical internal structure. This enables an isotropic 3D light distribution from energetic, highly directional, as well as coherent laser light, with speckle contrasts well below the human sensitivity limit. In combination with the excellent heat management of the 3D h-BN exceptionally high laser damage thresholds can be reached, enabling the usage of these aero-materials for high brightness illumination applications. Functionalization of these structures with photoactive materials will additionally open new fundamental research prospects in the field of disordered photonics, including 3D plasmonic systems, random lasing, up/down conversion, and other non-linear optical effects.

References:
[1] Laser & Photonics Reviews 7, 2013, 963–993
[2] Optics Express 19, 2011, A982-90
[3] Nature Communications 8, 2017, 1215
[4] Nature Communications 11, 2020, 1437

*Corresponding Author: [email protected]

Nanoparticles of gold and silver have been widely used in biosensing due to their fascinating plasmonic properties. Most of previously reported gold and silver nanoparticles are solid particles. Our recent studies showed that gold-silver alloyed nanoparticles with hollow interiors possess unique plasmonic properties that make them particularly suitable for biosensing applications. In the first part of this talk, I will introduce our new strategy to craft hollow gold-silver nanoparticles with unique physical and chemical parameters. Rational design and chemical synthesis of the nanostructures will be specified. In the second part, I will discuss the application of these hollow plasmonic nanostructures in biosensing. Examples of in vitro diagnostics of disease biomarkers will be highlighted.

The incompatibility of solar energy and the light absorption band of a chemical bond prevents the use of light to activate the chemical bond for interesting chemical reactions directly. This presentation will focus on a strategy that enables the efficient coupling of photon energy into chemical bonds to selectively promote the desired chemical reactions. The strategy relies on the excitation of hot electrons in quantum-sized metal nanoparticles (QSMNPs, with size in the range of 2 ~10 nm) upon photo-illumination and the following efficient injection of the hot electrons into specific chemical bonds. The redistribution of hot electrons in the chemical bonds dissipates the kinetic energy of hot electrons to the chemical bonds, activating the chemical bonds to promote the target chemical reactions. These sequential processes occur in a confined space, representing a series of quantum transitions, i) optical-to-electronic transition in quantum-sized metal nanoparticles (i.e., hot electron generation), ii) electronic-to-electrical transition at the nanoparticle/adsorbate interface (i.e., hot electron injection), and iii) electrical-to-electronic transition in adsorbate molecules (i.e., chemical bond activation). Selective oxidation of alcohols to aldehydes rather than ketones/acids, a class of important chemical reactions for many industrial processes (e.g., esterification), will be used as an example to highlight the use of QSMNPs for photo-driven selective chemical transformation on platinum group metal (PMG) nanoparticle catalysts, which do not exhibit strong optical absorption.

Plasmonic nanomaterials interact strongly with light, and as a consequence there are of great interest in a broad variety of fields, such as photocatalysis, photochemistry, biophotonics, sensing, and wave-guiding. We present a novel technique for the synthesis of plasmonic zirconium nitride (ZrN) nanoparticles using a scalable non-thermal plasma process.¹ Cost, production concerns and most importantly thermal and chemical stability motivate the search for alternative plasmonic materials to gold and silver², like Group IV transition metal-nitrides³ such as TiN⁴ and the relatively unexplored ZrN. Our ZrN nanoparticles display a plasmonic peak around 620 nm and from XRD and TEM we infer the crystallinity of the particles to be a cubic rock salt structure and a tunable size distribution below 10 nm. An attractive application of these plasmonic particles is the reduction of metals like platinum and chromium (VI) species in water which are extremely toxic.⁵ Here we have provided evidence of plasmon-driven photocatalytic activity within visible wavelengths to reduce platinum ions in solution.⁶ An aqueous solution of ZrN, methanol, and chloroplatinic acid (H₂PtCl₆) was illuminated using a monochromator to spectrally select wavelengths in the visible regime. Energy dispersive X-ray spectroscopy (EDS) was then used to determine the ratio of reduced platinum to zirconium at a given wavelength. A similar method was used to reduce Chromium (VI), a carcinogen commonly found in water, except we use the diphenyl carbazide method to determine the amount of Chromium (VI) in solution before and after exposing it to light. We then spectrally select visible wavelengths to compare the quantum yields of Chromium (VI) reduction between our synthesized ZrN and commonly used TiO₂ nanoparticles. We are able to achieve a quantum yield of up to 1.50% reducing Chromium (VI) to Chromium (III) using visible wavelengths, providing convincing evidence of photocatalytic response in this class of alternative plasmonic materials.

[1] Stephen Exarhos, Alejandro Alvarez-Barragan, Ece Aytan, Alexander A. Balandin, and Lorenzo Mangolini, Plasmonic Core−Shell Zirconium Nitride− Silicon Oxynitride Nanoparticles, ACS Energy Letters, 3 (10), 2349-2356, 2018.
[2] Naik, G. V., Shalaev, V. M., & Boltasseva, A. Alternative plasmonic materials: Beyond gold and silver. Advanced Materials, 25(24), 3264–3294, 2013.
[3] Guler, U., Shalaev, V. M., & Boltasseva, A. Nanoparticle plasmonics : going practical with transition metal nitrides. Biochemical Pharmacology, 18(4), 227–237, 2015.
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