Chanaka Kumara

X-ray Crystal Structure and Theoretical Analysis of Au25-xAgx(SCH2CH2Ph)18(-) Alloy.

The atomic arrangement of Au and Ag atoms in Au25-xAgx(SR)18 was determined by X-ray crystallography. Ag atoms were selectively incorporated in the 12 vertices of the icosahedral core. The central atom and the metal atoms in the six [-SR-Au-SR-Au-SR-] units were exclusively gold, with 100% Au occupancy. The composition of the crystals determined by X-ray crystallography was Au18.3Ag6.7(SCH2CH2Ph)18. This composition is in reasonable agreement with the composition Au18.8Ag6.2(SCH2CH2Ph)18 measured by electrospray mass spectrometry. The structure can be described in terms of shells as Au1@Au5.3Ag6.7@6×[-SR-Au-SR-Au-SR-]. Density functional theory calculations show that the electronic structure and optical absorption spectra are sensitive to the silver atom arrangement within the nanocluster.

Experimental and Theoretical Determination of the Optical Gap of the Au144(SC2H4Ph)60 Cluster and the (Au/Ag)144(SC2H4Ph)60 Nanoalloys

Au144PET60 and Au144-xAgxPET60 (PET = SC2H4Ph, phenylethylthiolate, and 30 ≤ x ≤ 53) clusters were studied by optical spectroscopy and linear response time-dependent density functional theory. Spectra of thin dry films were measured in order to reveal the onset for electronic absorption. The optical gap of the Au144PET60 cluster was determined at 0.19 ± 0.01 eV, which agrees well with the computed energy for the first optical transition at 0.32 eV for a model cluster Au144(SH)60 when the line width of individual transitions is taken into account. The optical gaps for the Au144-xAgxPET60 alloy clusters were observed in a range of 0.12-0.26 eV, in good agreement with the calculations giving 0.16-0.36 eV for the lowest-energy optical transitions for corresponding Au144-xAgx(SH)60 models. This indicates that the gap is only moderately affected by doping Au with Ag. This work constitutes the first accurate determination of the fundamental spectroscopic gap of these compounds.

X-ray Crystal Structure of Au38–xAgx(SCH2CH2Ph)24 Alloy Nanomolecules

Herein, we report the X-ray crystallographic structure of a 38-metal atom Au-Ag alloy nanomolecule. The structure of monometallic Au38(SR)24 consists of 2 central Au atoms and 21 Au atoms forming a bi-icosahedral core protected by 6 dimeric and 3 monomeric units. In Au38-xAgx(SR)24,where x ranges from 1 to 5, the silver atoms are selectively incorporated into the Au21 bi-icosahedral core. Within the Au21 core, the silver atoms preferentially occupy nine selected locations: (a) the two vertex edges, three atoms on each edge and six atoms total, and (b) the middle face-shared three-atom ring, adding to a total of nine locations. X-ray crystallography yielded a composition of Au34.04Ag3.96(SCH2CH2Ph)24. The crystal structure of the alloy nanomolecule can be described in terms of shells as Au2@Au17.04Ag3.96@ 6×[-SR-Au-SR-Au-SR] 3×[-SR-Au-SR-].

Super-Stable, Highly Monodisperse Plasmonic Faradaurate-500 Nanocrystals with 500 Gold Atoms: Au∼500(SR)∼120

Determining the composition of plasmonic nanoparticles is challenging due to a lack of tools to accurately quantify the number of atoms within the particle. Mass spectrometry plays a significant role in determining the nanoparticle composition at the atomic level. Significant progress has been made in understanding ultrasmall gold nanoparticles such as Au25(SR)18 and Au38(SR)24, which have Au core diameters of 0.97 and 1.3 nm, respectively. However, progress in 2-5 nm-diameter small plasmonic nanoparticles is currently impeded, partially because of the challenges in synthesizing monodisperse nanoparticles. Here, we report a plasmonic nanocrystal that is highly monodisperse, with unprecedentedly small size variability. The composition of the superstable plasmonic nanocrystals at 115 kDa was determined as Au(500±10)SR(120±3). The Au(~500) system, named Faradaurate-500, is the largest system to be characterized using high resolution electrospray (ESI) mass spectrometry. Atomic pair distribution function (PDF) data indicate that the local atomic structure is consistent with a face-centered cubic (fcc) or Marks decahedral arrangement. High resolution scanning transmission electron microscopy (STEM) images show that the diameter is 2.4 ± 0.1 nm. The size and the shape of the molecular envelope measured by small-angle X-ray scattering (SAXS) confirms the STEM and PDF analysis.

Faradaurate-940: Synthesis, Mass Spectrometry, Electron Microscopy, High-Energy X-ray Diffraction, and X-ray Scattering Study of Au∼940±20(SR)∼160±4 Nanocrystals

Obtaining monodisperse nanocrystals and determining their composition to the atomic level and their atomic structure is highly desirable but is generally lacking. Here, we report the discovery and comprehensive characterization of a 2.9 nm plasmonic nanocrystal with a composition of Au940±20(SCH2CH2Ph)160±4, which is the largest mass spectrometrically characterized gold thiolate nanoparticle produced to date. The compositional assignment has been made using electrospray ionization and matrix-assisted laser desorption ionization mass spectrometry (MS). The MS results show an unprecedented size monodispersity, where the number of Au atoms varies by only 40 atoms (940 ± 20). The mass spectrometrically determined composition and size are supported by aberration-corrected scanning transmission electron microscopy (STEM) and synchrotron-based methods such as atomic pair distribution function (PDF) and small-angle X-ray scattering (SAXS). Lower-resolution STEM images show an ensemble of particles-1000s per frame-visually demonstrating monodispersity. Modeling of SAXS data on statistically significant nanoparticle population-approximately 10(12) individual nanoparticles-shows that the diameter is 3.0 ± 0.2 nm, supporting mass spectrometry and electron microscopy results on monodispersity. Atomic PDF based on high-energy X-ray diffraction experiments shows decent match with either a Marks decahedral or truncated octahedral structure. Atomic resolution STEM images of single particles and their fast Fourier transform suggest face-centered cubic arrangement. UV-visible spectroscopy data show that Faradaurate-940 supports a surface plasmon resonance peak at ̃505 nm. These monodisperse plasmonic nanoparticles minimize averaging effects and have potential application in solar cells, nano-optical devices, catalysis, and drug delivery.

Au329(SR)84 Nanomolecules: Compositional Assignment of the 76.3 kDa Plasmonic Faradaurates

The purpose of this work is to determine the chemical composition of the previously reported faradaurates, which is a large 76.3 kDa thiolated gold nanomolecule. Electrospray ionization quadrupole-time-of-flight (ESI Q-TOF) mass spectrometry of the title compound using three different thiols yield the 329:84 gold to thiol compositional assignment. The purity of the title compound was checked by matrix-assisted laser desorption ionization-time-of-flight (MALDI-TOF) mass spectrometry. Positive and negative mode ESI-MS spectra show identical peaks denoting that there are no counterions, further reinforcing the accuracy of the assigned composition. We intentionally added Cs(+) ions to show that the Au329(SR)84 is the base molecular ion, with several Cs(+) adducts. A comprehensive investigation including analysis of the title compound with three ligands, in positive and negative mode and Cs(+) adduction, leads to a conclusive composition of Au329(SR)84. This formula determination will facilitate the fundamental understanding of emergence of surface plasmon resonance in Au329(SR)84 with 245 free electrons.

Transformation of Au144(SCH2CH2Ph)60 to Au133(SPh-tBu)52 Nanomolecules: Theoretical and Experimental Study

Ultrastable gold nanomolecule Au144(SCH2CH2Ph)60 upon etching with excess tert-butylbenzenethiol undergoes a core-size conversion and compositional change to form an entirely new core of Au133(SPh-tBu)52. This conversion was studied using high-resolution electrospray mass spectrometry which shows that the core size conversion is initiated after 22 ligand exchanges, suggesting a relatively high stability of the Au144(SCH2CH2Ph)38(SPh-tBu)22 intermediate. The Au144 → Au133 core size conversion is surprisingly different from the Au144 → Au99 core conversion reported in the case of thiophenol, -SPh. Theoretical analysis and ab initio molecular dynamics simulations show that rigid p-tBu groups play a crucial role by reducing the cluster structural freedom, and protecting the cluster from adsorption of exogenous and reactive species, thus rationalizing the kinetic factors that stabilize the Au133 core size. This 144-atom to 133-atom nanomolecules compositional change is reflected in optical spectroscopy and electrochemistry.

Au(144-x)Pd(x)(SR)60 nanomolecules.

Au144-xPdx(SR)60 alloy nanomolecules were synthesized and characterized by ESI mass spectrometry to atomic precision. The number of Pd atoms can be varied by changing the incoming metal ratio and plateaus at 7 Pd atoms. Based on the proposed 3-shell structure of Au144(SR)60, we hypothesize that the Pd atoms are selectively incorporated into the central Au12 icosahedral core.

Gold Thiolate Nanomolecules: Synthesis, Mass Spectrometry, and Characterization

This chapter summarizes the synthetic routes used for the following HS-CH2-CH2-Ph protected gold nanoclusters: Au25(SR)18, Au38(SR)24, Au40(SR)24, Au67(SR)35, Au103–105(SR)45–46, Au130(SR)50, and Au144(SR)60. The synthetic routes are based on either (a) direct synthetic route or (b) a core-size conversion route. The synthetic routes leading to the most stable clusters are discussed and the characterizational techniques used to study the products are described.

Understanding Au∼98Ag∼46(SR)60 nanoclusters through investigation of their electronic and local structure by X-ray absorption fine structure

Here we report the electronic and local atomic structure of thiol-stabilized Au∼98Ag∼46(SR)60 nanoclusters investigated by synchrotron radiation-based X-ray absorption fine structure (XAFS). Au L3-edge X-ray absorption near edge fine structure (XANES) was used to examine the d band character of Au, which is highly related to the electronic, magnetic and catalytic activities of Au. It was observed that the d band hole population of Au in Au∼98Ag∼46(SR)60 was higher than that of bulk Au. The formation of the AuAg alloy was confirmed by extended X-ray absorption fine structure (EXAFS). The EXAFS results also suggested that Au atoms in Au∼98Ag∼46(SR)60 nanoclusters preferred to occupy the metal core sites, while the Ag atoms were mainly on the surface.