Symposium NM02 : Nanometal---Synthesis, Properties and Applications

Nanoparticles play a central role in the rapidly growing nanoscience and nanotechnology fields. Atomic level tailoring of nanoparticles is of great importance in order to map out the structure-property relationships. Recent success in the synthesis of atomically well-defined nanoparticles has offered exciting opportunities to pursue many fundamental issues that were difficult to tackle with polydisperse nanoparticles. This talk will present several cases of single-atom, single-electron level manipulations of metal nanoparticles, such as doping a single Ag or Cu atom into a gold nanoparticle and site-specific “surgery” of surface motifs of a nanoparticle with controlled charge state. Such atomic-level manipulations provide unique opportunities for investigating how the structure and composition precisely impact the particle’s properties and functionality at the single-atom, single-electron level. New strategies for achieving such goals have been devised. Overall, the pursuit of single-atom level tailoring opens up new opportunities for controlling nanoparticles on an atom-by-atom basis.

Total synthesis, where desired organic- and/or bio-molecules could be produced from simple precursors at atomic precision and with known step-by-step reactions, has prompted centuries-lasting bloom of organic chemistry since its conceptualization in 1828 (Wöhler synthesis of urea). Such expressive science is also highly desirable in nanoscience, since it represents a decisive step towards atom-by-atom customization of nanomaterials for basic and applied research. Although total synthesis chemistry is less established in nanoscience, recent years have witnessed seminal advances and increasing research efforts devoted into this field. In this talk, I will discuss our recent work on introducing and developing total synthesis routes and mechanisms for atomically precise metal nanoclusters (NCs). Due to their molecular like formula and properties (e.g., HOMO-LUMO transition, strong luminescence and stereochemical activity), atomically precise metal NCs could be regarded as “molecular metals” (or metallic molecules / molecular-like metals), holding potential applications in various practical sectors such as biomedicine, energy, catalysis and many others. More importantly, the molecular-like properties of metal NCs are sensitively dictated by their size and composition, suggesting total synthesis of them as an indispensable basis for reliably realizing their practical applications.

In recent years, it has become possible to synthesize gold clusters, silver clusters, and alloy clusters with atomic precision using thiolate or phosphine (PR₃) as a ligand. The electronic/geometric structures and size-specific physical/chemical properties of these metal clusters have also been investigated extensively. Similar to these metal clusters, platinum (Pt) clusters have also attracted much interest. An attractive feature of Pt clusters is their high catalytic activity in a variety of reactions. In the precise synthesis of these Pt clusters, carbon monoxide (CO) or PR₃ is used as the main ligand. However, little information has been obtained on the electronic structure and physical/chemical properties of Pt_n(CO)_m(PR₃)_l clusters to date. In this research, the final objective is to obtain experimental information about the largely unknown electronic structure of Pt_n(CO)_m(PR₃)_l clusters. To this end, we precisely synthesized a Pt₁₇ cluster ([Pt₁₇(CO)₁₂(PPh₃)₈]ⁿ⁺; n = 1, 2) protected by CO and triphenylphosphine (PPh₃) by a simple method and studied its geometric and electronic structure. Mass spectrometry, elemental analysis, and single-crystal X-ray structural analysis of the product revealed that the obtained Pt₁₇(CO)₁₂(PPh₃)₈ comprises positively charged [Pt₁₇(CO)₁₂(PPh₃)₈]⁺ and [Pt₁₇(CO)₁₂(PPh₃)₈]²⁺, having a geometric structure similar to that³⁶ of neutral Pt₁₇(CO)₁₂(PEt₃)₈. The optical absorption spectroscopy and electrochemical measurements of [Pt₁₇(CO)₁₂(PPh₃)₈][(SbF₆)_n] (n = 1, 2) demonstrated that [Pt₁₇(CO)₁₂(PPh₃)₈][(SbF₆)_n] (n = 1, 2) has a discrete electronic structure. Furthermore, the emission spectroscopy revealed that [Pt₁₇(CO)₁₂(PPh₃)₈][(SbF₆)_n] (n = 1, 2) exhibits photoluminescence in the near-infrared region. In this presentation, I also talk about our recent results on the precise synthesis and one-dimensional structures of alloy clusters including Pt element.

Ligand-protected gold clusters with defined compositions and structures have attracted special attention because of diverse geometric structures and unique optical/electronic properties associated with their molecule-like features. During our recent studies on phosphine-ligated gold clusters in the subnanometer regime (nuclearity ~10), some examples of peculiar interactions at the metal-ligand interface, such as weak attractive forces involving heteroatoms and π-functionalities in the ligand moieties, have been found [1]. Herein we report unusual Au…H-C hydrogen bonds, which were characterized by X-ray crystallography and solution NMR [2].
The cluster we used here is core+exo type Au₆ cluster carrying m-phenylene-bridged diphosphines. The crystal structure showed close contacts of the gold framework to the bridging m-phenylene units. The hydrogen atoms at the 2-position of the bridges (H-2) are located in proximity to the tetrahedral core with Au-H distances of 2.60 – 2.65 Å, which are shorter than the sum of the van der Waals radii (2.86 Å). Accordingly, the distances to the C-2 atoms (3.641 – 3.699 Å) were shorter than that estimated with the assumption that C, H and Au atoms are aligned with van der Waals Au-H contact (3.95 Å). These observations imply the presence of hydrogen-bond-type interactions between the H-2 atoms and the Au cluster unit. It should be noted that the Au…H-C interactions were also observed in solution NMR, which showed ¹H and ¹³C NMR signals due to the C-H units at considerably downfield regions (δ_H = 11.6, δ_C = 147.3); The downfield shifts from normal aromatic protons were more than 4 and 10 ppm for ¹H and¹³C, respectively. We also show similar metal-H hydrogen bonds in related cluster systems. These results not only demonstrate the unique capability of small gold cluster to interact with unfunctionalized C-H groups but also shed light on the elucidation of recently emerging gold cluster catalysis.
[1] K. Konishi, et al. J. Phys. Chem. Lett. 2016, 7, 4267 [2] Md. A. Bakar, K. Konishi et al, Nature Commun. 2017, 8, 576).

Ligand-protected gold nanoclusters have been attracting great interest, because they display aesthetical structural diversity and are promising materials for both fundamental research and applications in catalysis, biosensing, luminescence and molecular electronics, etc. Their precise compositions and well-defined sizes are very helpful for understanding the structure-property relationships of metal clusters and nanoparticles.
Beyond the conventional thiolate and phosphine ligands, alkynyl groups have emerged as a promising ligand in protecting gold nanoclusters. As the ligand space is further broadened, new cluster compositions, structures and interfaces can be discovered. We will provide an atomic-level view of alkynyl-protected gold nanoclusters. The syntheses of a series of gold nanoclusters with “direct reduction” method will be reported. The total structure determination of these clusters will be described. The stability and optical properties of these nanoclusters will be discussed based on electronic structural analysis. In addition, ligand effects on the catalytic performance of gold nanoclusters will be presented. We believe the success in these systems will stimulate more effort in discovering new aspects this class of gold nanoclusters with alkynyl groups in the protective layer.

Ligands play an important role in the stabilization and functionalization of Au nanostructures and in self-assembled monolayers. Despite the increasing number of organic groups capable of stabilizing gold in addition to the popular thiolate ligands and the emerging alkynyl groups, there has been no systematic comparison of organic ligands regarding their binding strength to gold. To facilitate the future experimental design of promising ligands for gold surfaces, nanostructures, and nanoclusters, we provide a comprehensive view of the ligand-gold interface for six types and 27 ligands from first principles dispersion-corrected density functional theory. We find a surprising contrast between simple and bulky N-heterocyclic carbenes (NHCs). The bulk NHCs benefit from greater van der Waals contributions and additional Au---H-R hydrogen bonds. In fact, we find that alkynyl groups and bulky NHCs demonstrate the strongest binding with the gold surfaces. We further explore the computational design and show the viability of NHC-protected gold nanoclusters of magic stability. The overall trend from the present work not only confirms the emerging role of alkynyl ligands but also predicts the very promising direction of using bulky NHCs to achieve stable gold nanoclusters and interfaces.

Quaternary ammonium is a cationic group which is always show positive change at any pH conditions. Many reports have been published on thiolate-gold nanoclusters but almost all of them were covered by non-ionic or anionic thiol compounds. Only a few reports of amino-thiol-stabilized gold nanoclusters can be found but no report of quaternary ammonium thiolate-stabilized ones. For the first time, we prepared quaternary ammonium-stabilized fluorescent gold nanoclusters by sputtering processes. Furthermore, we have successfully prepared Au₂₅ clusters stabilized by quaternary ammonium thiols. In this case, counter anion plays an important key. Bromides cannot well stabilize Au₂₅ clusters and they decomposed rapidly. But after changing this counter anion to PF₆^- or other soft anions, very stable fully cationic Au₂₅ nanoclusters could be obtained.

In this work, we demonstrate three different doping modes when an atomically-precise nanocluster: [Au₂₃(SR)₁₆]− is doped with different metals (Cd, Cu, Ag), including (i) simple substitution, (ii) surface reconstruction and (iii) total structure transformation. Both experimental and theoretical results demonstrate that the dopant concentration is critical: the original structure of [Au₂₃(SR)₁₆]⁻ is retained under slight doping, but it starts to change under high concentrations of Cd and Ag dopants. Our results suggest that doping in nanoclusters is not just a simple substitution of original atoms or filling a vacant site; instead, it can be explored as a useful method to tailor the structure of nanoclusters partially (e.g., Cd doping of Au₂₃) or totally (e.g., Ag doping of Au₂₃). Overall, this work greatly expands doping chemistry for tailoring the structures of nanoclusters and is expected to open new avenues for designing nanoclusters with novel structures using different dopants.

The first atomic structure of a gold nanoparticle (AuNP) was determined by X-ray crystallography (1). The atomic structure that identified the Au₁₀₂NP as a molecule led to the idea of the gold cluster as a “super-atom”, and revealed a layer of alternating gold and ligand molecules at the interface (2). Subsequent X-ray structures of smaller organo-soluble AuNPs have supported the super-atom idea, and have shown a similar gold-thiol surface layer (3). Nevertheless, using X-ray crystallography as a general method for solving AuNP structures has been limited by the capability of forming well-ordered crystals. The structure determination of AuNPs at atomic resolution by aberration corrected electron microscopy (EM) was demonstrated (4,5). The successful structural determination was due to the implementation of a low dose strategy combined with a homogeneous sample that could be subjected to single particle reconstruction. Last but not least, the new structures do not respond to the super-atom model. Yet, they are well-defined particles of remarkable stability and reactivity. Such reactivity makes possible to form bio-conjugates that can be used to monitor traffic of small proteins inside human cells by cryo-electron tomography.

(1)Jadzinsky et al. Science (2007).

(2) Walter, et al. Proc. Natl. Acad. Sci. (2008)

(3)Häkkinen. Nature Chemistry (2012).

(4) Azubel et al. Science (2014).

(5) Azubel et al. ACSNano (2017)

Ultrasmall nanoparticles with a diameter below 2 nm are promising as specialized carriers for targeted drug delivery. Functionalized with specific binding motifs, they open up innovative opportunities in a wide range of applications, e.g. nanomedicine. A possible application is the specific targeting of protein-epitopes.
A versatile approach to the functionalization with specific epitope-binding motifs, i.e. peptides and proteins, is covalent binding of the gold-surface with sulfur-containing molecules. NMR spectroscopy gives valuable insights into the characteristics of the binding situation on the surface.
L-cysteine as a typical sulfur-containing biomolecule was chosen to elucidate the binding of ligands to ultrasmall gold nanoparticles (d < 2 nm). Cysteine is the only thiol-containing amino acid and therefore an ideal model compound for the binding to gold nanoparticles.
The nanoparticle preparation was carried out by reduction of HAuCl₄ with NaBH₄. L-cysteine was directly attached to the gold nanoparticles via gold-sulfur binding. The purification of the cysteine-capped nanoparticles was performed by multiple centrifugation and washing steps.
To investigate the binding of L-cysteine to the gold nanoparticle surface, isotope-labeled L-cysteine (¹³C and ¹⁵N) was used. 2D ¹H-DOSY,¹³C-DOSY and 3D ¹H-¹³C-HSQC-iDOSY NMR spectroscopy of the gold nanoparticles enabled the determination ofthe hydrodynamic particle diameter in excellent agreement with the metallic core diameter by high-resolution transmission electron microscopy.
The binding of L-cysteine to the gold nanoparticles via the thiol group was confirmed by ¹H and ¹³C NMR spectroscopy and 2D-NMR spectroscopy (¹H, ¹H-COSY, ¹H,¹³C-HSQC and ¹³C,¹³C-INADEQUATE). To exclude the binding of L-cysteine to the gold nanoparticles via the amino group, ¹⁵N NMR spectroscopy was carried out.
Quantitative¹³C NMR spectroscopy and atomic absorption spectroscopy enabled the calculation of a loading of approximately 100 L-cysteine molecules on each gold nanoparticle. By X-ray photoelectron spectroscopy, 95% elemental gold was identified whereas about 5% was oxidized. This confirms the existence of metallic gold nanoparticles, functionalized with L-cysteine as model compound.

We show that NMR spectroscopy is especially well suited to analyze ultrasmall gold nanoparticles due to their small particle size, leading to excellent spectra of dispersed nanoparticles that elucidate the binding situation of L-cysteine on the gold surface.

Polydopamine (PDA) is a nature inspired biopolymer that can be synthesized through self-assembly under mild controlled environment. Due to the catechol functional groups, PDA can form coordination bonding with various metallic ions. In our work, we utilized the strong affinity between PDA and various metal cations, including Cu²⁺, Ni²⁺, and Co²⁺, to obtain metal ion doped PDA (mPDA). The mPDA powders were irradiated under electron beam using a transmission electron microscope. As the electron beam intensity increased, nucleation and growth of nanoscale precipitates were observed. Electron diffraction and EDX analysis indicated these precipitations were metal nanocrystals. The morphology of the metal nanoparticles varied depending on the metal specie. This discovery indicates electron and perhaps other irradiation sources can be used to induce nanoparticle in metal-doped PDA, which may be useful to modify the properties of PDA-based thin film materials. On the other hand, our study implies PDA can be used as a platform for growth of nanoparticles, which may have potentials in various engineering applications.

In recent years, small metal nanoclusters (with sizes below 2 nm) have emerged as quite interesting materials because their optical properties differ from those of larger clusters or plasmonic nanoparticles. One of the new properties exhibited by these systems is their photoluminescence, probably due to the discretization of the electron density of states. Light emission from metal nanoclusters can be relevant in technological applications as primary nanosource of light or as a nanoantenna to enhance light-matter interaction. Herein, we study the photoluminescence (PL) emission from Pt metal nanoclusters embedded in silica and sapphire plates, by using the ion implantation technique. The PL signal ranges from 400 nm to 600 nm, with a peak at 550 nm for all samples irrespective of the matrix type. We have observed a gradual change in the behavior of the PL intensity as increasing the power density of picosecond pulse excitation in the UV range. PL intensity increases linearly at lower pump excitation and reaches a saturation behavior for intermediate power density. However, as the excitation power increases above ~7 kW/m², the PL response become more intense, and it can be considered as superlinear with respect the excitation laser intensity. This optical behavior is in clear contrast compared with the usual observations in semiconductor quantum dots, where the PL intensity saturates at higher fluence excitation. The observed superlinear PL from Pt nanoclusters is analyzed in terms of multi-photon excitations.

The emergent properties of colloidal nanomaterials are dependent on their shape and exposed facets, so mechanistic understanding of atomic formation and removal is critical. Non-equilibrium synthetic methods are powerful tools for making energetically unfavorable shapes and facets, but studying these processes is challenging. Liquid cell Transmission Electron Microscopy (TEM) enables single nanoparticle dynamics to be monitored in their native liquid environment with the necessary spatial and temporal resolution to observe shape and facet evolution. Studying non-equilibrium etching of gold nanocrystals has provided insight into fundamental formation mechanisms of nanocrystals with high-energy facets.
Graphene liquid cell TEM encapsulates small pockets of liquid between graphene sheets for imaging using a transmission electron microscope. Oxidative etching of gold nanocrystals in the graphene liquid cell was induced through a combination of pre-loaded iron chloride and oxidative species generated by electron beam induced radiolysis. Control of the chemistry in the graphene liquid cell pockets allowed the nanocrystal dynamics and mechanisms to be related back traditional synthetic techniques. The electron beam dose rate controlled the rate of atom removal, and the initial concentration of iron chloride established the potential of the oxidative etching.
Pre-synthesized gold nanocubes and nano-rhombic dodecahedra (RDD), with {100} and {110} surface facets respectively, were oxidatively etched while monitoring the effect of chemical potential on the facet trajectories. Both the cubes and RDD transformed to intermediate tetrahexahedra (THH) shapes with {hk0} surface facets. When etching the cubes in this non-equilibrium regime, lower initial concentrations of iron chloride led to intermediate {hk0} facets with lower h/k values. However, etching the RDD at differing initial iron chloride concentrations led to same intermediate THH with {hk0} facets of h/k = 2.5. Monte Carlo simulations corroborated the role of chemical potential in controlling the facets for the cubes but not the RDD. Zero temperature kinetic models show that removing a 6-coordinated edge atom on the nanocrystals reveals 7-coordinate inner atoms for cubes but 6-coordinate inner atoms for RDD. Therefore, chemical potential controls the facets for cubes by modulating the probability ratio of removing inner versus edge atoms. This fundamental understanding of kinetically-driven shape transformations will aid efforts to make nanocrystals with high-energy facets.
Through these in situ TEM studies, the formation of non-equilibrium nanocrystal structures were watched at the single particle level in solution, and the mechanisms of etching were elucidated. This mechanistic understanding of nanocrystal etching will hopefully inform future synthetic efforts to control facets and structures for energetically unfavorable shapes.

Gold clusters protected by ligands or stabilized by polymers are viewed as superatoms from the analogy of the electronic structures with those of the conventional atoms. It has been demonstrated that H atom mimics Au atoms in bare Au clusters: the electronic and geometric structures of Au clusters are retained after replacement of the Au atom with H atom [1]. A recent theoretical study proposed that an H atom behaves as an Au atom in the Au cluster Au₂₅(SR)₁₈ (SR = thiolate) and contributes its 1s electron to the superatomic electron count [2]. Small nonplasmonic Au clusters stabilized by polymer exhibit localized surface plasmon resonance in the presence of NaBH₄^– due to electron donation from the adsorbed H atoms [3,4]. These results suggest that structures and properties of gold superatoms can be tuned through doping H atoms. However, a molecular-level understanding has not been attained about the interaction between H and Au superatoms.
The present work focuses on the interaction of H with phosphine-protected Au-based clusters [Au₉(PPh₃)₈]³⁺ and [PdAu₈(PPh₃)₈]²⁺ which can be viewed as oblate gold superatoms with 6 electrons. We observed by mass spectrometry and NMR spectroscopy that a hydride (H^–) was doped into these gold-based oblate superatoms upon the reaction with NaBH₄^–. Density functional theory calculations of the products [Au₉H(PPh₃)₈]²⁺ and [PdHAu₈(PPh₃)₈]²⁺ demonstrated that hydride is bonded to a coordinatively unsaturated site at the center and that the (Au₉H)²⁺ and (PdHAu₈)⁺ core can be viewed as nearly spherical superatom with closed electronic shells. The hydride-doped superatoms (Au₉H)²⁺ was successfully converted to the well-known Au₁₁³⁺ superatoms by the reaction with Au(I) complexes, while the hydride in the (PdHAu₈)⁺ remained during the growth to (PdHAu₁₀)⁺. These hydride-mediated growth processes will provide a new atomically precise synthesis of Au clusters via bottom-up approach.

[1] Buckart, S.; Ganteför, G.; Kim, Y. D.; Jena, P. J. Am. Chem. Soc. 2003, 125, 14205.
[2] Hu, G.; Tang, Q.; Lee, D.; Wu, Z.; Jiang, D.-e. Chem. Mater. 2017, 29, 4840.
[3] Ishida, R.; Yamazoe, S.; Koyasu, K.; Tsukuda, T. Nanoscale 2016, 8, 2544.
[4]Ishida, R.; Hayashi, S.; Yamazoe, S.; Kato, K.; Tsukuda, T. J. Phys. Chem. Lett. 2017, 8, 2368.

Atomically precise metal nanoclusters have recently emerged as a novel class of catalysts for the hydrogen evolution reaction. From first-principles density functional theory, we show that the eight coordinatively unsaturated (cus) Au atoms in the Au₂₂(L⁸)₆ cluster [L⁸ = 1,8-bis(diphenylphosphino) octane] can adsorb H stronger than Pt, thereby being a potentially promising catalyst for the hydrogen evolution reaction (HER). We find that up to six H atoms can adsorb onto the Au₂₂(L⁸)₆cluster and they have close-to-zero Gibbs free adsorption energies (ΔG_H). From the HOMO–LUMO gaps, frontier orbitals, and Bader charge analysis, we conclude that H behaves as a hydride or electron-withdrawing ligand in the Au₂₂(L⁸)₆clusters, in contrast to the metallic H in thiolate-protected Au nanoclusters. Our study demonstrates that ligand-protected Au clusters with cus Au sites will be the most promising candidates for realizing Au–H nanoclusters and can act as excellent electrocatalysts for the HER.

Monolayer-protected clusters (MPCs) represent a class of nanomaterials that can be synthesized and isolated with structural (both compositional and geometric) specificity. Hence, MPCs provide model systems for understanding the nanoscale structure-function interplay. We have recently shown that femtosecond time-resolved two-dimensional electronic spectroscopy (2DES) can be used to isolate carrier dynamics of specific MPC electronic states.Here, 2DES studies of a family of MPCs in the 1-2 nm size range will be presented. These results show that the optical, electronic, and spin-state properties of MPCs are extremely sensitive to the electronic configuration of nanometal orbitals. For example, the magnetic properties of Au₂₅(SR)₁₈, where SR represents an alkanethiol, can be switched reversibly by oxidative opening of the eight-electron Superatom P orbital. Electronic interactions between assembled MPCs also exhibit spin-dependent magnetic phenomena not present in the isolated building blocks. Ultrafast spectroscopy on dimerized 20-atom MPCs reveal inter-particle spin-dependent dynamics not observed for the monomer. In contrast to the discrete carrier dynamics typical for MPCs, larger nanoparticles exhibit collective electronic behavior. I will provide a comprehensive description of how 2DES can be employed to describe the dynamics of metals spanning the non-metallic cluster and metallic particle domains.

It has been known that silver ions released by silver nanoparticles have bactericidal effect against a wide spectrum of bacteria. Silver ions can induce redox reactions on the membrane transport proteins of bacteria thereby deactivating them. Silver ions are also capable of impairing the energy transfer mechanism of bacteria during respiration process. However, the nanoparticles are unstable and easily form aggregates, which decreases their antibacterial activity.
To improve the dispersion stability of silver nanoparticles in aqueous media, and to increase their effectiveness as antibacterial agents, we coated triangular plate-like silver nanoparticles (silver nanoplates, Ag NPLs) with one or two layers of gold atoms (Ag@Au1L NPLs and Ag@Au2L NPLs, respectively). These gold coatings improved the dispersion stability in aqueous media with high salt concentrations. Ag@Au1L NPLs showed stronger antibacterial activity on pathogenic bacteria than Ag NPLs and Ag@Au2L NPLs. Furthermore, the Ag@Au1L NPLs decreased the number of bacteria living in RAW 264.7 cells. The Ag@Au1L NPLs displayed no cytotoxicity towards RAW 264.7 cells, and the Ag@Au1L NPLs could be used as an antibacterial agent for intracellular bacterial infections.
Next, we prepared hollow-shaped alloy nanoparticles made of silver and gold atoms (Ag/Au NPs) by treating silver nanoparticles with gold ions, not core-shell type gold-coated silver nanoparticles as described in the previous section. The antibacterial activity of the hollowed Ag/Au alloy nanoparticles was stronger than the original silver nanoparticles. Additionally, the release of silver ions from the hollowed Ag/Au nanoparticles was higher than the original silver nanoparticles.
The gold atoms on the surface of silver nanoparticles or in the alloy nanoparticles made of silver and gold atoms affected the oxidation of the silver atoms in the nanoparticles. Since chloride ions can affect the migration of gold atoms on silver, in turn, the migration of gold atoms might affect the exposure of silver metals to the culture medium and the subsequent release of silver ions. These gold treatments are effective methods to improve the antimicrobial activity of silver nanoparticles.

Morphology dependent properties of nanoparticles have been utilized in various applications and especially nanoparticles composed of high-index facets have attracted significant attention in catalysis with its high density of atomic steps serving as active sites for reactions. However, despite vigorous efforts for delicate control of nanoparticle morphology, controlled fabrication of specific surface structures within a few nanometer scale still remains as a big challenge due to instability of high-index facets. Previously, we demonstrated aqueous based seed-mediated two step growth method to synthesize high-indexed gold nanoparticles with morphological variations by control of reaction parameter and introduction of organo-thiol shape controlling additives. Strong interactions between the organo-thiol based additives and the metal surface induces directional growth depending on the shape-modifier structures.
Herein, based on this established system, we demonstrate precise surface structure control with organo-thiol additive during palladium nanoparticle synthesis. Resulting nanoparticles show multi-stepped surface structures which varies from cubic based planar step patterns to spiral step patterns composed of multiple high-index facets. We demonstrate superior catalytic activity of “multi-stepped” nanoparticles. Morphology variation is originated from collaborative interactions between kinetic modifiers and shape modifying additives during the growth step. Reduction kinetic was controlled by growth solution pH, where various types of acids were introduced to the growth solution. Higher proton concentration in the synthesis environment induced slower electron withdrawing of reducing agent to affect geometrical shape and uniformity. Additionally, dramatic change in nanoparticle morphology was observed respect to the acid type when shape modifying organo-thiol group was introduced to the reaction solution. These anion dependent morphology change is related to specific interactions between organo-thiol molecule and anions to modify growth directionality. Our study provides crucial design constraints for fine morphology tuning during nanoparticle synthesis which allows systematical control of nanoparticle surface structures for potential catalysis application.

In this work, Ni-B amorphous alloyed catalysts used for a gas-liquid-solid three phase reaction were synthesized on Ni foam by electroless plating method, which was evaluated by the reference reaction of nitrobenzene hydrogenation. Such design offered several advantages. First of all, the low-cost Ni-B amorphous alloyed catalysts showed superior catalytic activity and selectivity towards the hydrogenation reaction. Besides, the use of Ni foam as the support materials could increase the surface area for loading the Ni-B amorphous alloyed catalysts. Moreover, its interconnected macropore structure was beneficial for the transport of the gas/liquid reactants to the active sites. The morphologies and size distributions and the chemical composition of the prepared Ni-B amorphous alloyed catalysts were characterized by SEM, XRD and XPS, respectively. The actual loading of the Ni-B amorphous alloyed catalysts was analyzed using ICP-MS technology. The hydrogen chemisorption on the Ni-B amorphous alloyed catalysts was characterized by temperature programmed desorption of H₂ (H₂-TPD) analyses. Finally, the catalytic performance was evaluated by nitrobenzene hydrogenation in a continuous flow monolithic microreactor under various parameters. The experimental results indicated that the developed Ni-B amorphous alloyed catalysts deposited on the Ni foam was able to improve the catalytic performance.

Metallic thin films are unstable in as deposited state. These films tend to break up and agglomerates when annealed. This is known as dewetting and driven by surface energy minimization of the film – substrate interface. Dewetting occurs in both solid and liquid state and while it can be detrimental, where high temperature performance is required, it can also be used to fabricate nanoparticles arrays on substrate for different applications [1].
Enhancing the thermal stability of metallic thin films is of importance, especially for high temperature applications. Recently, we reported that addition of silver nanoparticles on copper thin films suppresses the solid state dewetting of copper thin film [2]. In this work, we explore the effect of added nanoparticles on the liquid state dewetting behaviour of bismuth thin films. We study three system, copper, silver and gold nanoparticles deposited on bismuth thin films on amorphous carbon TEM grids. Transmission electron microscopy results shows that gold and silver nanoparticles accelerate bismuth dewetting, while copper nanoparticles suppress the process. A model is presented to explain these differences.
References
[1] K. Kumar and P. Swaminathan, “Fabrication of silica nanopillars by templated etching using bimetallic nanoparticles for anti-reflection applications,” Applied Surface Science, under review (2018).
[2] K. Kumar and P. Swaminathan, “Role of silver nanoparticles in the dewetting behavior of copper thin films,” Thin Solid Films, vol. 642, pp. 364–369, 2017.

The large specific surface area of nano-porous materials realizes both high catalytic activity and resource savings. Thin film materials have homogeneity in large areas and can be applied to sensor devices or electrode materials for batteries. The traditional and general method of fabricating a nano-porous noble metallic thin film is the dealloying method, i.e., the selective corrosion of an alloy of a noble metal and a base metal by a strong acid, such as hydrochloric acid or sulfuric acid. In this study, we fabricated a palladium (Pd) thin film with three-dimensional (3D) nano-network structure (3DPdNNF) by the dealloying method without discharging any material with possibly high environmental impact, such as waste solution containing heavy metals or strong acids. The dealloying was conducted in aqueous solutions of organic chelating agents of citric acid and ethylene-diamine-tetraacetic acid (EDTA) and sodium carbonate. All agents are used as a food additive and have environmentally friendly characteristics. Furthermore, the base metal used was aluminum (Al). However, Pd has a hydrogen-storage property with hydrogen gas selective permeation, and it is expected to be useful for developing sensor devices to detect only hydrogen gas. However, pure Pd metal has low repetitive durability for hydrogen gas exposure due to irreversible deformation and destruction caused by its hydrogen-storage property. In this study, we aimed to fabricate a 3DPdNNF by the above method and utilize its stress relaxing action due to the nano-network structure to apply it to a hydrogen gas sensor with repetitive durability.
The thin film preparation was performed using the RF-sputtering method, and Al–Pd alloy (82at% Al–18 at% Pd) films were deposited on substrate (glass, Si wafer, and elastic carbon film) to a thickness of 70 nm. The dealloying process was conducted in mixed aqueous solution of organic chelating acid (100 μmol/L) and sodium carbonate for pH adjustment. The optimized pH value and temperature were 10.0 and 368 K, respectively.
The Al–Pd film was reformed to uniform 3DPdNNF with high Pd purity (> 99 at% Pd). The pore size of the network could be controlled to within the range of 2.90–12.5 nm by three parameters of the gas conditions during the sputtering deposition and the pH values and chelating agents in the dealloying processes. Furthermore, we evaluated the hydrogen gas sensing performance as a 3DPdNNF utilization of a high Pd purity and large specific surface area film. The fabricated 3DPdNNF showed a response to hydrogen gas by changing the electrical resistance in the kΩcm range with repetitive durability for hydrogen gas exposure.

Platinum (Pt) and Pt-based (e.g. Pt/Cu, Pt/Au, Pt/Ag) alloy nanoparticles (NPs) have been demonstrated as high performance catalysts. Usually Pt and Pt-based alloy NPs have been synthesized via chemical reduction methods. However, impurity and incomplete removal of byproducts and toxic reductants can hinder catalytic properties. On the other hand, due to the difference in reduction potential of metal salts and/or difference in decomposition temperature of metal complexes, core-shell structure or phase segregation were observed in the formed bimetallic NPs. This is often seen in bimetallic NPs of immiscible elemental components in the bulk state such as Pt/Au. In our research, we have proposed to prepare highly uniform Pt and Pt alloy NPs, such as Pt/Cu and Pt/Au alloy NPs, by sputtering at room temperature onto a low vapor pressure liquid, polyethylene glycol (PEG, Mw 600). The method combines the advantages of sputtering to produce atoms and clusters for any metals from the bulk counterparts and the suppression and control of particle growth by the liquid medium in vacuum sputtering chamber. Thus varying the sputtering parameters allows for particle size control and tunable alloy NPs’ composition.
Sputtering was performed onto PEG and TEM grid. Sputtering current applied to each magnetron was separately controlled and varied in order to tune the composition in the resulting bimetallic NPs. Various characterization methods such as UV-Vis, XRD, XPS, TEM, HRTEM and STEM-HAADF and STEM-EDX mapping have been used to analyze the obtained NPs. TEM, HRTEM and STEM showed that Pt NPs with tunable sizes from 0.9 nm to 1.4 nm and narrow size distribution were produced. In addition, we found that negligible particle aggregation happened in PEG and Pt NPs were stable even after keeping in the dark at room temperature for several months. The slight growth of Pt NPs in PEG during storage was found accompanied with the consumption of free Pt atoms in PEG. The method was applied for a Pt/Cu alloy target and the effect of sputtering parameters such as sputtering time, the rotation speed of PEG on particle size has been studied. Pt₂₉Cu₇₁ alloy NPs have been synthesized by sputter deposition for the first time. Furthermore, we showed that we were able to produce Pt/Au alloy NPs using a double-target sputtering. The target design allowed us to finely control Pt/Au alloy NPs’ composition via simultaneous sputter deposition onto PEG. The results showed that particle size and composition are strongly correlated and can be tailored by varying the sputtering current. Increasing Pt content resulted in smaller particle size and particles with same composition had similar sizes. Moreover, the agglomeration of NPs is dependent on the Pt content. Our findings in the relation among particle size, particle composition, and aggregation state of the formed NPs with respect to its composition can shed light into the formation mechanism of Pt-based alloy NPs.

Fire through dielectric (FTD) contact is the dominant technology for contacting a commercial silicon solar cell because of its low-cost and high throughput attributes. During the FTD process, the glass frits in the Ag paste melt and etch the dielectric (anti-reflecting layer) first, to have Ag metal contact directly to the bulk Si. The redox reaction between the glass frits melting and dielectric etching leads to the formation of a recrystallized glass layer, which distributes within the Ag gridlines and bulk Si emitter surface, and silver nano-particles, which are mainly silver alloys (Ag₂Te and PbTe) encapsulated in the glass layer. When without the Ag₂Te and PbTe nano-particles/colloids, due to the existence of glass layer, silver gridlines rarely directly contact with the bulk silicon and electrons have to tunnel through the glass layer. However, in this work, Ag₂Te and PbTe are semimetals which have very narrow bandgaps. The existence of Ag₂Te and PbTe nano-particles/colloids in the glass layer can change the electrical property of the glass layer. Thus, the specific contact resistance of silver gridlines cannot be calculated based on simplified metal-semiconductor contacts. The possible electron transport on the silver/silicon interface includes: 1. through silver gridline contact directly with bulk silicon; 2. tunneling through an ultrathin glass layer; 3. through Ag₂Te and PbTe nano-colloids assisted tunneling; 4. through multistep tunneling. This suggests that the contact resistivity in the presence of Ag₂Te and PbTe nano-particles/colloids is independent of the inverse square root of emitter peak surface concentration. Lower contact resistances were measured in the presence of Ag₂Te and PbTe nano-particles on the silicon emitter with relatively low peak surface concentration.

Metal-doped titanium oxide (TiO₂) nanoparticles were developed as enhanced photocatalysts with disinfection and purification potential. However, they can be hazardous to both the environment and human health. In this study, the toxicity of nickel (Ni) and platinum (Pt)-doped TiO₂ nanoparticles (Ni-TiO₂ and Pt-TiO₂ nanoparticles) to skin and eye cells and a mouse skin model was evaluated. Physicochemical properties of nanoparticles were characterized using field emission-scanning electron microscopy, field emission-transmission electron microscopy, and X-ray photoelectron spectroscopy. Cytotoxicity was also evaluated at concentrations of 0.0001 to 10 mg/mL in HaCaT and ARPE-19. To investigate in vivo acute toxicity, 1 to 123 mg nanoparticles/1.5 cm² were applied to hair-removed dorsal skin in a mouse model, and skin tissues and body condition were monitored. Metal-doped TiO₂ nanoparticles had a spherical crystalline shape with nanoscale sizes less than 100 nm. In nanoparticles, 1.0% Ni2p and 0.26% Pt4f were detected at 871.2 eV and 74.4 eV for Ni-TiO₂ and Pt-TiO₂ nanoparticles, respectively. Cell viability in HaCaT was higher than that in ARPE-19 at a range of 0.1 - 10 mg/mL nanoparticles. In ARPE-19, superior cell viability was observed at 1.0 mg/mL and more than 1.0 mg/mL Ni-TiO₂ nanoparticles compared with Pt-TiO₂ nanoparticles. In vivo, body weight and AST/ALT levels were not significantly altered by Ni-TiO₂ nanoparticle exposure. Histology, skin thickness, and inflammation grade were also comparable to control mice. Thus, metal-doped TiO₂ nanoparticles show minimal toxicity in skin cells and a mouse skin model, indicating their potential for various applications.

A mixed metal inorganic complex which resembles redox sensitivity will be presented. Molecular switches experience a reversible physical change due to various external stimuli such as: high energy radiation, electrical current, heat, magnetic fields, and ultraviolet or visible light. We have synthesized a series of Group IVB and VB complexed to VIIIB metals that undergo intermolecular electron transfer upon high energy exposure resulting in a state change. In addition, we have found that substituting a Group IVB with selected Lanthanide metals results in fluorescently active compounds. We will present how the structure dictates the function of these complexes as sensors by analysis using nuclear magnetic resonance spectroscopy (NMR), Fourier-transform infrared spectroscopy (FT-IR), Raman spectroscopy, ultraviolet and visible spectroscopy (UV-Vis), fluorescence spectroscopy, energy dispersive x-ray spectroscopy, and cyclic voltammetry.

A high-speed electrodeposition regime that enables nanocrystalline nickel (Ni), cobalt (Co) and alloy (Ni-Co) film depositions with excellent adhesion characteristics on untreated titanium (Ti) surfaces was recently reported by our group [1]. This was the first report of strong adhesion to an untreated Ti surface without any pretreatment, displacement reaction, or intermediate layer pre-deposition to mitigate the thin tenacious oxide that renders adhesion to Ti a challenge. Under conditions that combined the high flow rate electrodeposition with a higher current density than previously investigated, a new and additional mechanism was enabled that resulted in the growth of highly crystalline Ni, Co and Ni-Co nanowires in addition to the nanocrystalline film. Excellent quality nanowires for high melting point metals are difficult to grow using the standard electrodeposition into nanoporous anodized aluminium templates method [2,3]. The new template-less growth conditions reported here indicate that an alternative method to produce highly crystalline nanowires for high melting point metals has been identified. Novel roles for oxygen, carbon and chlorine chemistries in the nanocrystallite and nanowire growth processes are identified. We identify specific growth models for both nanocrystal and nanowire from quantitative TEM, SEM, SAED, EDS and AFM results.

[1] Hussain, MS. Direct Ni-Co alloy plating of titanium alloy surfaces by high speed electrodeposition. Trans Inst of Metal Finishing 90 (2012) 15. DOI: 10.1179/174591911X13188464808876

[2] H. Pan, B. Liu, J. Yi, C. Poh, S. Lim, J. Ding, Y. Feng, C.H.A. Huan, J. Lin. Growth of Single-Crystalline Ni and Co Nanowires via Electrochemical Deposition and Their Magnetic Properties. J. Phys. Chem. B 109 (2005) 3094-3098. DOI: 10.1021/jp0451997

[3] L.A. Meier, A.E. Alvarez, S.G. García, M.C. del Barrio. Formation of Cu and Ni Nanowires by Electrodeposition. Procedia Materials Science 8 (2015) 617-622. DOI: 10.1016/j.mspro.2015.04.116

A solution-processed transparent conducting electrode was fabricated via layer-by-layer (LBL) deposition of graphene oxide (GO) and silver nanowires (Ag NWs). First, graphite was oxidized with a modified Hummer’s method to obtain negatively-charged GO sheets, and Ag NWs were functionalized with cysteamine hydrochloride to acquire positively-charged silver nanowires. Oppositely-charged GO and Ag NWs were then sequentially coated on a 3-aminopropyltriethoxysilane modified glass substrate via LBL deposition, which provided highly controllable thin films in terms of optical transmittance and sheet resistance. Next, the reduction of GO sheets was performed to improve the electrical conductivity of the multilayer films. The resulting GO/Ag NWs multilayer was characterized by a UV-Vis spectrometer, field emission scanning electron microscope, optical microscope and sheet resistance using a four-point probe method. The best result was achieved with a 2-bilayer film, resulting in a sheet resistance of 6.5 Ω sq^-1 with an optical transmittance of 78.5% at 550 nm,which values are comparable to those of commercial ITO electrodes. The device based on a 2-bilayer hybrid film exhibited the highest device efficiency of 1.30% among the devices with different number of graphene/Ag NW LBL depositions.
Ag NWs films exhibited low sheet resistance and high optical transmittance, which are comparable to those of commercial ITO electrode. We fabricated an organic photovoltaic device based on Ag NWs as the anode/V2O5 as the HTL/P3HT:PCBM/Al, and optimized device with Ag NWs exhibited power conversion efficiency of 1.30% and 1.72%.

We introduce a strategy to design a highly sensitive temperature sensors using Silver (Ag)
nanocrystal (NC) thin films. Resistive temperature sensors were fabricated on using
poly(dimethylsiloxane) (PDMS) substrate through ligand exchange process [1] and interfacial
engineering process. We take advantages of high thermal expansion coefficient of PDMS to
control the interparticle distances of Ag NCs, and eventually to improve the sensitivity. We
achieved a very sensitivity with temperature coefficient of resistance of 0.1/K. We discussed
the origin of the high sensitivity by investigating structural, chemical, electrical, and
electromechanical properties as well as the charge transport mechanisms. We also examine
the stability, reliability, and sensitivity of our sensors. This work offers and effective way to
design low-cost and high-performance sensors.

Through all solution-processed methods, wearable strain sensors with high gauge factor are demonstrated using thin film of Ag nanocrystals (Ag NCs). Controlling of interval between Ag NCs enables to control a flow of electrons through hopping mechanism and/or tunneling effect, which can enhance gauge factor. In this study, halides ligand treatment is proposed to design interval between nanoparticles to obtain high performances at solid state. Higher sensitivity of ligand treatment with I^- is demonstrated by comparisons of experimental results of halides; Br^-, Cl^-, I^-. The origin of the higher performance is discussed with various characterization methods. Careful studies to further enhance the sensitivity is conducted by controlling various conditions in ligand exchange process. These high sensitivity thin film Ag NCs strain gauge sensors allow to measure a delicate movement or signal from various objects. This work provides a low cost and simple method to design high sensitivity wearable sensors for various field such as robotics, or healthcare systems.

Recently, early diagnosis of Alzheimer’s disease (AD) has attracted considerable attention in the field of nanomedicine and biosensing. In particular, detection of plasma biomarkers specific for AD, including amyloid beta (Aβ40 and Aβ42), is considered a promising, cost-effective, and non-invasive method for diagnosis of AD. Herein, we present a facile approach for designing a nanoprobe based on surface-enhanced Raman scattering (SERS) for the sensitive and selective detection of Aβ40 and Aβ42 in blood. To this end, Ag nanoshells (AgNSs) bearing Raman labels were synthesized in a single step under very mild conditions (25 ^oC and 60 min). As-prepared AgNSs had a uniform shell thickness and nanogaps of 2 nm, resulting in significant electromagnetic field enhancement. In addition, each AgNS exhibited a unique and intense SERS signal without overlapping, allowing the multiplex and sensitive detection of Aβ40 and Aβ42. The AgNSs conjugated with a specific antibody for Aβ40 or Aβ42 was able to detect each target protein at concentration as low as 100 fg/ml in a fast and simple manner.

When a sufficient energy is applied at a local point in multilayered nano-films composed of alternating layers with a thickness of 10 nm order, an exothermic reaction occurs and a compound is generated at the local region. The generated heat then spreads into an adjacent unreacted region. If the temperature in the neighboring region becomes high sufficient to cause additional mixing, a self-sustained propagating reaction (SPR) occurs. SPR is induced by not only by electrostatic discharge, electrical heating, and laser irradiation, but also by mechanical impact. SPR by mechanical impact can be divided into two phenomena: i.e., (i) initial reaction by mechanical loading and (ii) ignition and propagation of reaction wave. Many studies have been conducted on (ii) SPR properties such as the ignition threshold and the speed of propagation. On the other hand, for (i), the detailed mechanisms of the initial chemical reaction due to mechanical loading has been unclear. This chemical reaction occurs at well below melting points and eutectic temperature, implying that the chemical reaction can occur by solid-state reaction. The atomic mixing in the solid-state can be brought about by diffusion or plastic deformation. Since the reaction occurs in very short time, we focus the role of plastic deformation.
The purpose of the study is to clarify the mechanisms of initial chemical reaction of Ti/Si multilayered nano-films by mechanical loading. Since the multilayered nano-films have structural anisotropy, the deformation mechanism depends on the loading mode with respect to the stacking direction. In this study, we focus the deformation and chemical reaction under compression loading in the stacking direction as a basic mode. We fabricated truncated cone-shaped specimens by focused ion beam from a polycrystalline-Ti/amorphous-Si multilayered nano-films (bilayer thickness: ~30 nm) deposited by electron beam evaporation, and then conducted compression experiments under in situ observation by field emission scanning electron microscopy. We evaluated the chemical reaction or change in crystal structure by transmission electron microscopy (TEM) observation and selected-area electron diffraction to the deformed specimens.
The specimens yielded at a true stress of ~3 GPa under compression, and showed gradual hardening behavior. TEM observation confirmed that each layer was plastically deformed or thinned, and then the mixing of Ti and Si occurred. The selected-area electron diffraction revealed that a new crystal structure, estimated to be Ti₅Si₄ or TiSi, was generated on the Ti/Si interface and in the Ti layer. These results indicated that the mixing of each layer was induced by plastic deformation due to compressive stress and the compound of Ti and Si was generated. The exothermic reaction can be controlled or generated at a desired site by the local mechanical loading, and so it can be used for various applications such as local heating in large-scale devices.

Metal nanostructures have been studied quite extensively in the research area of heterogeneous catalysis long before the introduction of the concept of nanoscience and nanotechnology. The significant progress achieved in the past twenty years in chemical synthesis has enabled precise control over not only the size but also the shape of the metal nanostructures, and therefore attracted intense interest not only in catalysis but also optoelectronics due to the well-known effect of localized surface plasmon resonance. In this presentation, I will introduce our recent progress in the synthesis of colloidal metal nanostructures in confined spaces using various templating methods, and further manipulation of their secondary structures. By combining the confinement of templates with the seed-mediated growth strategy, we demonstrate the significant advantages of this general method over the conventional ones in creating a large variety of nanomaterials with novel plasmonic properties.

During non-classical growth of nanostructures via assembly of primary nuclei, nucleation and assembly are assumed to be distinct processes: nanoparticles nucleate randomly and aggregate to form extended structures through Brownian motion in the presence of long-range attractive interactions. Here we investigate the relationship between these two processes by using in situ AFM, in situ, ex situ and cryo TEM and UV-Vis spectroscopy to observe growth of colloidal gold and simulations to develop a mechanistic model of the process. Our results reveal an inexorable link between nucleation and assembly with nuclei forming almost exclusively within a ~ 1 nm interfacial region of existing particles. The new particles immediately close the gap either through a diffusive jump or via growth of a neck between the seed and new particle, generating aggregates exhibiting features commonly attributed to oriented attachment of independently nucleated particles. In addition, pH was identified as a control parameter for manipulating crystallization pathway through shifting the balance of chemical potentials of solution species. The results demonstrate that creation of initial particle interfaces leads to local environments that redirect growth towards non-classical processes.

Gold nanoparticles of controlled size and shape display brilliant optical properties throughout the visible and near-infrared portions of the electromagnetic spectrum. Gold nanorods, usually 15-20 nm in diameter with tunable lengths of 20, 30, 40, 50, 60 nm, show two plasmon bands corresponding to tranverse and longitudinal excitations. The length/width ratio (aspect ratio) of gold nanorods is well-known to dictate the relative positions of the two plasmon bands. Recently in my laboratory we have developed a synthesis of "mini gold nanorods" in which the particle diameters are 5-8 nm, and lengths are 10, 20, 30 etc. nm. These absolutely smaller nanorods display similar plasmon bands to their larger counterparts, but the relative proportion of light scattering compared to light absorption differs. This talk will describe the synthetic procedures to create mini-rods as well as comparisons of properties between mini-rods and "regular" rods.

Symmetry-breaking is the essential step required for an isotropic seed particle to grow into an anisotropic shape. Using specially designed electron microscopy methods and strategically-chosen synthetic routes, we investigate the mechanisms behind symmetry breaking and shape control in gold nanorod growth, from conception to old age [1-4]. We observe the symmetry-breaking event that triggers the formation of the embryonic nanorod; an asynchronous formation of new surface structures in the cuboctahedral gold seed particles [1,3]. We show that the size at which the seed particle breaks symmetry depends sensitively on the Au/Ag ion ratio and describe a mechanism as to how this, in turn, controls the initial nanorod aspect ratio [2,3]. After observing the initial symmetry breaking event, we investigate the evolution of the nanorod morphology over an extended timeframe, for periods up to 3 orders of magnitude longer than the conventional 1 - 2 hours [4]. Following initial rapid anisotropic growth, the nanorods grow isotropically, and over longer times (weeks), tend ultimately towards the same reduced aspect ratio, irrespective of the AgNO₃ concentrations. Furthermore, we measure the orientation and stability of different facets [5,6] and show how the nanorod transitions from faceted to curved when allowed to grow for extended periods [4]. Collectively, these observations suggest that the ultimate final aspect ratio is dictated by the surface energetics of the cylindrical sides compared to the hemispherical tips, with little dependence on the initial AgNO₃ concentration. In other words, the AgNO₃ concentration mainly asserts control over aspect ratio at the symmetry breaking point, rather than during later growth. Altogether, these observations provide a rational framework for controlling width, aspect ratio and facet orientation in the growth of single crystal gold nanorods.

[1] M. J. Walsh, S. J. Barrow, W. Tong, A. M. Funston and J. Etheridge. Symmetry Breaking and Silver in Gold Nanorod Growth, ACS Nano 9, 715 (2015)
[2] W. Tong, M. J. Walsh, P. Mulvaney, J. Etheridge and A. M. Funston. Control of symmetry breaking size and aspect ratio in gold nanorods: underlying role of silver nitrate J Phys Chem C 121 3549 (2017).
[3] M.J. Walsh, W. Tong, H.Katz-Boon, P. Mulvaney, J. Etheridge, AM Funston. A Mechanism for Symmetry Breaking and Shape Control in Single-Crystal Gold Nanorods Accounts in Chemical Research 50, 2925 (2017)
[4] W. Tong, H. Katz-Boon, M. J. Walsh, M. Weyland, J. Etheridge and A. M. Funston The Evolution of Size, Shape, and Surface Morphology of Gold Nanorods Chem Comm, 54, 3022 (2018)
[5] H. Katz-Boon, C. J. Rossouw, M. Weyland, A. M. Funston, P. Mulvaney and J. Etheridge. Three-Dimensional Morphology and Crystallography of Gold Nanorods, Nano Letters 11, 273 (2011)
[6] H. Katz-Boon, M. J. Walsh, C. Dwyer, P. Mulvaney, A. M. Funston and J. Etheridge. Stability of Crystal Facets in Gold Nanorods, Nano Letters 15, 1635 (2015)

Soft-hard interfaces at the surface of nanoparticles (NPs) determine interaction potentials, including the mechanisms of growth, spatial reactivity, colloidal stability, and nanoparticle functionality [1]. For example, soft molecular ligands are thought to guide growth and symmetry breaking in anisotropic NPs. These ligands can also act as soft templates for site-selective deposition of functional coatings [2, 3]. Thus, quantitative details of the local attachment, distribution, and structure of soft-hard interfaces would enable the development of methods for high-yield, monodisperse NP synthesis.

Conventional techniques to characterize soft-hard NP interfaces—such as nuclear magnetic resonance, small angle x-ray scattering, and other bulk methods—lack the spatial resolution necessary to probe key details, including how the surface structure and chemistry varies within and between individual particles [1]. Yet in NPs, polydispersity is a defining characteristic, and surface energies and interactions vary widely based on local facet, curvature, and composition. While electron microscopy can address this challenge, an ideal approach requires a combination of: low-background substrates, the ability to quantify elemental distributions of small molecules, and the efficiency necessary to probe multiple NPs. Here we report a particle-by-particle analysis of soft-hard interfaces on gold nanorods (AuNRs) using aberration-corrected scanning transmission electron microscopy and electron energy loss spectroscopy (EELS) spectrum imaging on graphene substrates.

In order to demonstrate the ability of electron microscopy methods to probe soft-hard interfaces, we investigate anisotropic mesoporous silica functionalization of AuNRs. Mesoporous silica can be deposited with site selectivity to either the ends or the sides of AuNRs. Such growth is thought to be templated by the anisotropic distribution of capping ligand cetyl trimethylammonium bromide (CTAB) [2, 3]. We deposit AuNRs onto suspended graphene substrates and use EELS spectrum imaging to map the presence of carbon, silicon, and oxygen. We directly observe a mesoporous silica frame with carbon containing pores and an increased carbon signal on the surface of the rods, indicating the presence of a residual CTAB shell surrounding the particle. Using graphene as a reference, we quantify the CTAB present before and after silica deposition. These results indicate that before deposition, AuNRs are coated with a few-nm thick CTAB layer corresponding to ~5,000 ligands/particle. This density is small when compared to the ~60,000 ligands/particle on silica-coated AuNRs. Our methods thus represent the first direct, quantitative chemical analysis of soft-hard interfaces of metal nanoparticles.

References:
[1] A Smith, et al., Analyst 142 (2017), p. 11.
[2] J Hinman, et al., J. Am. Chem. Soc. 139 (2017), p. 9851.
[3] F Wang et al., Angew. Chemie - Int. Ed. 52 (2013), p. 10344.

A key way to impart specific function to metallic nanoparticles is to functionalize their surface with ligands, allowing a range of applications including chemical and biological sensing; theranostics; and catalysis. For gold nanoparticles, this typically involves incubating nanoparticles in solutions containing thiol-terminated ligands and allowing the ligands to self-assemble on the surface of the gold through the formation of gold-thiol bonds. However, the structure and organization of the resulting ligand shell at the metal-organic interface is difficult to visualize. This talk will describe how single molecule fluorescence and super-resolution microscopy provide insight into surface organization of DNA-functionalized gold nanorods as a function of different preparation strategies. In cases when the DNA binds in a collapsed, disordered configuration, plasmon heating facilitates dynamic surface reorganization to a more ordered, upright geometry. By understanding how changes in surface preparation protocols impact the resulting ligand shell, we will be able to synthesize more reproducible functionalized nanostructures.

Understanding the behavior of chiral molecules in macroscopic systems requires amplification. Here we investigate the amplification of chirality by biocompatible chiral ligand-functionalized gold nanorods (GNRs) in a nematic liquid crystal. We synthesized two aspect ratios of cholesterol-capped and 1,1’-binaphthyl-capped GNRs. Both types of GNRs, featuring either centrally or axially chiral ligands, were fully characterized by NMR, transmission electron microscopy, TEM, UV-vis-NIR spectroscopy, TGA, and CD spectropolarimetry. Characterization of mixtures of these GNRs in 5CB with induced CD spectropolarimetry and polarized light optical microscopy (free surface, homeotropic boundary conditions, and Cano wedge cells) show that the GNRs not only induce chirality in the N-LC phase but also that the chirality in both systems is remarkably amplified in comparison to quasi-spherical gold nanoparticles (NPs) functionalized with the same ligands.^1,2In each case the helical pitch (p)of the induced N*-LC phase was measured and the molar helical twisting power (β_M