X-ray studies of the structure and electronic behavior of alkanethiolate-capped gold nanoparticles: the interplay of size and surface effects

Epitaxial deposition of silver ultra-fine nano-clusters on defect-free surfaces of HOPG-derived few-layer graphene in a UHV multi-chamber by in situ STM, ex situ XPS, and ab initio calculations

The growth of three-dimensional ultra-fine spherical nano-particles of silver on few layers of graphene derived from highly oriented pyrolytic graphite in ultra-high vacuum were characterized using in situ scanning tunneling microscopy (STM) in conjunction with X-ray photoelectron spectroscopy. The energetics of the Ag clusters was determined by DFT simulations. The Ag clusters appeared spherical with size distribution averaging approximately 2 nm in diameter. STM revealed the preferred site for the position of the Ag atom in the C-benzene ring of graphene. Of the three sites, the C-C bridge, the C-hexagon hollow, and the direct top of the C atom, Ag prefers to stay on top of the C atom, contrary to expectation of the hexagon-close packing. Ab initio calculations confirm the lowest potential energy between Ag and the graphene structure to be at the exact site determined from STM imaging.

Tuning the properties of ZnO, hematite, and Ag nanoparticles by adjusting the surface charge

A poly (acryl acid) (PAA) post-treatment method is performed to modify the surface charge of ZnO nanospheres, hematite nanocubes, and Ag nanoprisms from highly positive to very negative by adjusting the PAA concentration, to and greatly modify their photoluminescence, cytotoxicity, magnetism, and surface plasmon resonance. This method provides a general way to tune the nanoparticle properties for broad physicochemical and biological applications.

Critical role of water and the structure of inverse micelles in the Brust-Schiffrin synthesis of metal nanoparticles

Although Brust-Schiffrin two-phase synthesis is a popular method for synthesizing ligand-protected metal nanoparticles with an average size of less than 5 nm, the details on how the reactions can be controlled from a mechanistic point of view are still unclear, therefore hindering efforts to synthesize monodisperse metal nanoparticles. It was recently discovered that this method is basically an inverse-micelle-based synthesis (Li, Y.; Zaluzhna, O.; Xu, B.; Gao, Y.; Modest, J. M.; Tong, Y. Y. J. J. Am. Chem. Soc.2011, 133, 2092). In this letter, the critical role of water and the structure of inverse micelles in typical synthesis of gold nanoparticles were further investigated. We found that (1) water encapsulated in the inverse micelles of [TOA](+) that also hosted metal ions formed a hydrophilic microenvironment that acted as a reaction-enabling proton-accepting medium for the thiol protons (RS-H) and (2) not only the presence but also the amount of water in the reaction medium has a profound effect on the Au(I) precursor species (a pure [TOA][AuX(2)] complex or a mixture of a [TOA][AuX(2)] complex and polymeric [Au(I)SR](n) species), the reduction of Au(III) by thiols, and the formation of uniform small metal nanoparticles. A quantitative analysis of the (1)H NMR of the encapsulated water enabled an estimation of the size and composition of the involved inverse micelles.

High yield synthesis of pure alkanethiolate-capped silver nanoparticles

One-phase, one-pot synthesis of Ag(0) nanoparticles capped with alkanethiolate molecules has been optimized to easily achieve a pure product in quantitative yield. We report the synthesis of dodecanethiolate-capped silver particles and the chemophysical, structural, and morphologic characterization performed by way of UV-vis, (1)H NMR, and X-ray photoelectron (XPS) spectroscopies, X-ray powder diffraction (XRD) and X-ray absorption fine structure analysis (XFAS), electron diffraction and high-resolution transmission electron microscopy (HR-TEM), and scanning and transmission electron microscopy (SEM and TEM). Depending on the molar ratio of the reagents (dodecylthiosulphate/Ag(+)), the mean Ag(0) particle size D(XRD) is tuned from 4 to 3 nm with a narrow size distribution. The particles are highly soluble, very stable in organic solvents (hexane, toluene, dichloromethane, etc.), and resistant to oxidation; the hexane solution after one year at room temperature does not show any precipitation or formation of oxidation byproducts.

Insights into initial kinetic nucleation of gold nanocrystals

Understanding the initial nucleation mechanism of monodisperse nanocrystals (NCs) during synthesis process is an important prerequisite to control the desired sizes and to manipulate the properties of nanoscale materials. The acquisition of information for the small nanocluster nucleation process, however, still remains challenging. Here, using a continuous-flow in situ X-ray absorption fine structure (XAFS) spectroscopy for time-resolved studies, we have clarified the initial kinetic nucleation of Au clusters under the grain size of 1 nm for the classical Au NCs synthesis via the reduction of AuCl(4)(-) in aqueous solution. The in situ XAFS results present the experimental revelation of the formation of intermediate Cl(3)(-)Au-AuCl(3)(-) dimer and the subsequent higher complexes 'Au(n)Cl(n+x)' in the initial nucleation stage. We propose a kinetic three-step mechanism involving the initial nucleation, slow growth, and eventual coalescence for the Au NCs formation, which may be helpful for the synthesis of metallic nanomaterials.

Behind the Quantum and Size Effects: Broken-Bond-Induced Local Strain and Skin-Depth Densified Quantum Trapping of Charge and Energy

Shrinking the size of a solid down to nanometer scale is indeed fascinating, which makes all the otherwise constant physical quantities to be tunable such as the Young’s modulus, dielectric constant, melting point, etc. The variation of size also generates novel properties that can hardly be seen in the bulk such as the conductor-insulator and nonmagnetic-magnetic transition of noble metals at the nanoscale. Although the physics of materials at the nanoscale has been extensively investigated, the laws governing the energetic and dynamic behavior of electrons at such a scale and their consequences on the tunable physical properties of nanostructures have not been well understood [C. Q. Sun, Prog Solid State Chem 35, 1-159 (2007); Prog Mater Sci 54, 179-307 (2009)]. The objective of the contribution is to update the recent progress in dealing with the coordination-resolved energetic and dynamic behavior of bonds in the low-dimensional systems with consideration of the joint effect of temperature and pressure. It is shown that the broken-bond-induced local strain and the associated charge and energy quantum trapping at the defect sites perturbs the atomic cohesive energy, electroaffinity, the Hamiltonian and the associated properties of entities ranging from point defects, surfaces, nanocavities and nanostructures. Application of the theories to observations has led to consistent understanding of the behavior of nanometer-sized materials and the interdependence of these entities as well as the means of determining the bond energy through the temperature-dependent measurements.

Synthesis and characterization of gold at gold(i)-thiomalate core at shell nanoparticles

In this paper, the synthesis of gold at gold(I)-thiolate core at shell nanoparticles is described for the first time. The chemical nature and structure of these nanoparticles were characterized by a multi-technique approach. The prepared particles consist of gold metallic cores, about 1 nm in size, surrounded by stable gold(I)-thiomalate shells (Au at Au(I)-TM). These nanoparticles could be useful in medicine due to the interesting properties that gold(I)-thiomalate has against rheumatoid arthritis. Furthermore, the described results give new insights in the synthesis and characterization of metallic and core at shell nanoparticles.

Self-assembled monolayers of thiols and dithiols on gold: new challenges for a well-known system

Self-assembled monolayers (SAMs) of alkanethiols and dialkanethiols on gold are key elements for building many systems and devices with applications in the wide field of nanotechnology. Despite the progress made in the knowledge of these fascinating two-dimensional molecular systems, there are still several "hot topics" that deserve special attention in order to understand and to control their physical and chemistry properties at the molecular level. This critical review focuses on some of these topics, including the nature of the molecule-gold interface, whose chemistry and structure remain elusive, the self-assembly process on planar and irregular surfaces, and on nanometre-sized objects, and the chemical reactivity and thermal stability of these systems in ambient and aqueous solutions, an issue which seriously limits their technological applications (375 references).

Thiol-capped ZnO nanowire/nanotube arrays with tunable magnetic properties at room temperature

The present study reports room-temperature ferromagnetic behaviors in three-dimensional (3D)-aligned thiol-capped single-crystalline ZnO nanowire (NW) and nanotube (NT) arrays as well as polycrystalline ZnO NT arrays. Besides the observation of height-dependent saturation magnetization, a much higher M(s) of 166 microemu cm(-2) has been found in NTs compared to NWs (36 microemu cm(-2)) due to larger surface area in ZnO NTs, indicating morphology-dependent magnetic properties in ZnO NW/NT systems. Density functional calculations have revealed that the origin of ferromagnetism is mainly attributed to spin-polarized 3p electrons in S sites and, therefore, has a strong correlation with Zn-S bond anisotropy. The preferential magnetization direction of both single-crystalline NTs and NWs lies perpendicular to the tube/wire axis due to the aligned high anisotropy orientation of the Zn-S bonds on the lateral (100) face of ZnO NWs and NTs. Polycrystalline ZnO NTs, however, exhibit a preferential magnetization direction parallel to the tube axis which is ascribed to shape anisotropy dominating the magnetic response. Our results demonstrate the interplay of morphology, dimensions, and crystallinity on spin alignment and magnetic anisotropy in a 3D semiconductor nanosystem with interfacial magnetism.

Ligand-stabilized metal nanoparticles in organic solvent

This critical review reports the fundamental behavior of metal nanoparticles in different organic solvents, i.e., metal organosol. An overview on metal organosol and then their smart synthetic approaches, characterization, and potential applications in the fields of catalysis and spectroscopy with special emphasis on SERS are embodied. Aspects of organosol fabrication, stabilization, morphology control, growth mechanisms, and physical properties as mono- and bimetallic nanoparticles are discussed. The article inspires the repetitive usage of metal nanoparticles as stable deliverable organic and molecular compounds.

Synthesis and characterization of accessible metal surfaces in calixarene-bound gold nanoparticles

Use of organic ligands to partially passivate nanoparticles against sintering yet retain a degree of small molecule accessibility to the metal surface has been a lofty goal in functional materials synthesis, which in principle also enables the design of preferred electronic and steric environments on a nanoparticle surface. Catalysis using gold in particular requires donor ligands that facilitate an electron-rich metal surface and generalizable strategies for dealing with deactivation due to sintering. Here, synthesis and characterization of gold nanoparticles postsynthetically modified with the chelating ligand cone-5,11,17,23,29,35-hexa(tert-butyl)-37,39,41-tris(diphenylphosphinomethoxy)-38,40,42-trimethoxycalix[6]arene (1) is reported. In solution as well as when supported on the surface of TiO2, nanoparticles modified with tripodal calix[6]arene phosphine ligand 1 demonstrate enhanced protection against sintering relative to unmodified, tetraoctylammonium bromide-surfactant-stabilized gold nanoparticles. In between adsorbed calixarene ligands, there is accessible gold surface area in these nanoparticles, and this is measured quantitatively for the first time for a calixarene-modified nanoparticle, using a newly developed fluorescence methodology involving 2-naphthalenethiol as a relevant chemisorption probe molecule. Ligand steric bulk critically influences amount of accessible surface on the metal nanoparticle since the use of a smaller calix[4]arene ligand (MBC) results in a 7-fold lower accessible surface area relative to using 1 under otherwise similar conditions. In addition, surface coverage of 1 controls accessible surface area in an unintuitive fashion: a 4-fold increase in accessible metal surface area is observed upon increasing the surface coverage of 1 to be 1.5-fold higher than the minimum required for surface saturation. This is presumably the result of a more open ligand packing of 1 at higher surface coverages, which allows greater accessibility to 2-napthalenethiol.

Interfacial electronic structure of gold nanoparticles on Si(100): alloying versus quantum size effects

Gold nanoparticles (Au NPs) were prepared on a native-oxide-covered Si(100) substrate by sputter-deposition followed by thermal annealing. The size of Au NPs could be controlled in the range of 8-48 nm by varying the sputter-deposition time and post-annealing temperature. The interparticle separation was found to be directly related to the size of Au NPs, with smaller separations for particles of smaller size. The surface morphology, crystal structure, and interfacial composition of the chemical states of these supported Au NPs were studied as a function of their average size by using scanning electron microscopy, glancing-incidence X-ray diffraction, and depth-profiling X-ray photoelectron spectroscopy (XPS), respectively. The new Au 4f7/2 peak found at 1.1-1.2 eV higher in binding energy than that for the metallic Au feature (at 84.0 eV) can be attributed to the formation of Au silicide at the interface between Au NPs and the Si substrate. Depth-profiling XPS experiments revealed no discernible change in the binding energies of the Au silicide and metallic Au 4f features with increasing Ar+ sputtering time, indicating that the Au-to-silicide interface is abrupt. Furthermore, the shift in the Au 5d5/2 valence band to a higher binding energy and the reduction of the Au 5d spin-orbit splitting with increasing Ar+ sputtering time also support the formation of Au silicide. No clear evidence for the quantum size effect was observed for the supported NPs. The finite density of state at the Fermi level and the fixed Au 4f7/2 peak position clearly indicate the metallic nature of the Au silicide at the Au-Si interface.

Chemical synthesis and structural studies of thiol-capped gold nanoparticles

We report the chemical synthesis and structural studies of thiol-capped Au nanoparticles (NPs) using extended X-ray absorption fine structures (EXAFS) and high-resolution transmission electron microscopy (HRTEM). Synthesis of Au NPs was conducted in one case in a toluene/water two-phase system using alkanethiols with varied hydrocarbon chain length (C6, C12, and C18), resulting in NPs of sizes ranging from 1.6 nm to 5.4 nm. Au L3-edge EXAFS reveals a systematical trend of the local structure of Au in the NPs when the Au/S ratio and chain-length of thiols are varied. In another synthesis, thiol-capped Au NPs were prepared on the surface of silicon nanowires, which act as both substrates and reducing agents. HRTEM reveals that not only spherical particles but also very small quasi-1D nanostructures of Au were formed. The formation and structure of these Au NPs was discussed in terms of ligand and template effect associated with the silicon nanowire substrates.Key words: thiol-capped Au nanoparticles, EXAFS, silicon nanowires, electroless deposition, quasi-1D Au nanostructures.

Monolayer of amphiphilic functionalized gold nanoparticles at an air-water interface

Langmuir films at the air-water interface exhibit a variety of surface phases which arise primarily due to the molecular interaction governed by intermolecular separation. We have studied the thermodynamical aspects of Langmuir monolayers of amphiphilic functionalized gold nanoparticles (AGNs) at the air-water (A-W) interface. Interestingly, the AGN monolayer exhibits phases like gas, a low-ordered liquid (L1) , a high-ordered liquid (L2) , and a collapsed state. We find that the first-order phase transition between L1 and L2 vanishes above a critical temperature of 28.4 degrees C . Surprisingly, for a range of higher temperatures ( > or = 29.4 degrees C and < or = 36.3 degrees C ), the L1 phase undergoes a transition to a bilayer of the L2 phase before entering into the collapsed state.

Alkanetelluroxide-protected gold nanoparticles

The synthesis and characterization of the first air-stable tellurium-containing ligand-protected gold nanoparticles (NPs) are reported. Although the synthesis largely followed the well-known Brust two-phase approach, the starting ligand was dioctyl ditelluride rather than alkanetellurol, which is an analogue of the widely used alkanethiol. Dioctyl ditelluride was used because alkanetellurol is unstable. The 1H and 13C NMR spectra, as well as infrared spectra (IR) of the formed Au NPs, indicated that the Te-Te bond in the starting ligand was broken but the octyl group was intact. This was further corroborated by the solid-state 125Te NMR spectrum that displayed a very broad and significantly downfield-shifted peak, indicating that tellurium was directly bound to the Au core. Furthermore, the O 1s and Te 3d XPS spectra of the Au NPs indicated that the capping ligands were octanetelluroxide. An average particle size of 2.7 nm diameter as measured by transmission electron microscopy (TEM) corresponded to an Au607 core. A two-step weight loss of approximately 22.2% in total was observed in the thermogravimetric analysis, which indicated about 53% ligand monolayer coverage (i.e., Au607(Te(=O)C8H17)133). Additionally, dioctyl ditelluride demonstrated an intriguing reductive power that led to a more sophisticated chemistry of forming the air-stable octanetelluroxide-protected gold NPs. It has been found that (1) when the ratio of Au to Te was about 1.5 a colorless intermediate state similar to Au(I)-SR (the intermediate state widely accepted in the synthesis of thiolate-protected Au NPs) could be obtained and (2) this kind of intermediate state played a key role in the formation of stable Au NPs.

Surface structural characteristics and tunable electronic properties of wet-chemically prepared Pd nanoparticles

The ligand substitution reaction, Pd L(3,2,1)-edge and S K-edge x-ray absorption fine structure (XAFS), XAFS simulations, and valence-band and core-level x-ray photoelectron spectroscopy (XPS) have been used to systematically study the surface chemical and electronic properties of wet-chemically prepared Pd nanoparticles of varied size, molecular capping, and metal composition. It was found that the replacement of weakly interacting capping molecules (amine and tetra-alkylphosphonium bromide) with strongly binding thiols caused a considerable change in the surface bonding of Pd nanoparticles. However, the Pd d-electron counts (number of d electrons) remained almost unchanged before and after ligand substitution, which is unexpected since Pd atoms normally lose electrons to the more electronegative S atoms. XAFS results and simulations provided useful insights into the surface structural characteristics of Pd nanoparticles and satisfactorily accounted for the unexpected d-electron behavior involved in the ligand substitution process. XPS valence and core-level spectra further revealed a size-dependent d-band narrowing and presented complementary information to XAFS about the surface electronic properties of Pd atoms. The small weakly bound Pd nanoparticles seem inevitably to have a net d-electron depletion due to the influence of the surface effect (chemical adsorption by oxygen), which is more significant than the d-electron enriching nanosize effect. However, it was demonstrated that by forming Pd-Ag alloy nanoparticles, a net increase of the Pd d-electron counts can be realized. Therefore, it is illustrated that by manipulating the surface, size, and alloying effects, the electronic properties of Pd nanoparticles can be possibly tuned.

Coordination-dependent surface atomic contraction in nanocrystals revealed by coherent diffraction

Surface atoms have fewer interatomic bonds than those in the bulk that they often relax and reconstruct on extended two-dimensional surfaces. Far less is known about the surface structures of nanocrystals. Here, we show that coherent diffraction patterns recorded from individual nanocrystals are very sensitive to the atomic structure of nanocrystal surfaces. Nanocrystals of Au of 3-5 nm in diameter were studied by examining diffraction intensity oscillations around the Bragg peaks. Both results obtained from modelling the experimental data and molecular dynamics simulations strongly suggest inhomogeneous relaxations, involving large out-of-plane bond length contractions for the edge atoms (approximately 0.2 A); a significant contraction (approximately 0.13 A) for {100} surface atoms; and a much smaller contraction (approximately 0.05 A) for atoms in the middle of the {111} facets. These results denote a coordination/facet dependence that markedly differentiates the structural dynamics of nanocrystals from bulk crystalline surfaces.

Extended x-ray absorption fine structure study of local structure and atomic correlations of tellurium nanoparticles

Extended x-ray absorption fine structure measurements were performed on trigonal tellurium (t-Te) and tellurium nanoparticles in very thin films. Twofold coordinated chains, much like those in t-Te, exist even in the nanoparticles, but covalent bonds or intrachain interactions are stronger than those of t-Te as shown by a shorter bond length and higher Einstein temperature. The interchain coordination number of the nanoparticles is about half that of t-Te, suggesting reduction of interchain interactions.

Electronic structures of CdSe nanocrystals — An X-ray absorption near-edge structure (XANES) investigation

CdSe nanocrystals (NCs), capped with trioctylphosphine oxide and 1-octadecanamine, have been synthesized with colloidal methods. Both UV–vis absorption and photoluminescence spectra show that the prepared nanostructures exhibit a quantum confinement effect. X-ray absorption near–edge structure (XANES) at Se K-edge recorded in fluorescence yield mode have been carried out on these NCs, with different size and capping ligands to gain insight into how their electronic structures are affected by the NC size and chemical properties of the ligands. The results indicate that XANES are sensitive to both the NC size and the nature of the capping ligands. The whiteline intensity in XANES spectra can be related to the photoluminescence properties of these NCs.Key words: CdSe nanocrystals, capping ligands, X-ray absorption near-edge structure, UV–vis spectroscopy, photoluminescence.

Magnetic properties of ZnO nanoparticles

We experimentally show that it is possible to induce room-temperature ferromagnetic-like behavior in ZnO nanoparticles without doping with magnetic impurities but simply inducing an alteration of their electronic configuration. Capping ZnO nanoparticles ( approximately 10 nm size) with different organic molecules produces an alteration of their electronic configuration that depends on the particular molecule, as evidenced by photoluminescence and X-ray absorption spectroscopies and altering their magnetic properties that varies from diamagnetic to ferromagnetic-like behavior.

Metal Core Bonding Motifs of Monodisperse Icosahedral Au13 and Larger Au Monolayer-Protected Clusters As Revealed by X-ray Absorption Spectroscopy and Transmission Electron Microscopy

The atomic metal core structures of the subnanometer clusters Au13[PPh3]4[S(CH2)11CH3]2Cl2 (1) and Au13[PPh3]4[S(CH2)11CH3]4 (2) were characterized using advanced methods of electron microscopy and X-ray absorption spectroscopy. The number of gold atoms in the cores of these two clusters was determined quantitatively using high-angle annular dark field scanning transmission electron microscopy. Multiple-scattering-path analyses of extended X-ray absorption fine structure (EXAFS) spectra suggest that the Au metal cores of each of these complexes adopt an icosahedral structure with a relaxation of the icosahedral strain. Data from microscopy and spectroscopy studies extended to larger thiolate-protected gold clusters showing a broader distribution in nanoparticle core sizes (183 +/- 116 Au atoms) reveal a bulklike fcc structure. These results further support a model for the monolayer-protected clusters (MPCs) in which the thiolate ligands bond preferentially at 3-fold atomic sites on the nanoparticle surface, establishing an average composition for the MPC of Au180[S(CH2)11CH3]40. Results from EXAFS measurements of a gold(I) dodecanethiolate polymer are presented that offer an alternative explanation for observations in previous reports that were interpreted as indicating Au MPC structures consisting of a Au core, Au2S shell, and thiolate monolayer.

Chiroptical Activity of BINAP-Stabilized Undecagold Clusters

Undecagold cluster compounds [Au11(BINAP)4X2]+ (X = Cl and Br) were synthesized by chemical reduction of the corresponding precursor complexes, Au2X2(BINAP), where BINAP represents the bidentate phosphine ligand 2,2'-bis(diphenylphosphino)-1,1'-binaphthyl. The circular dichroism spectra of Au11 stabilized by the enantiomers [Au11(R-BINAP)4X2]+ and [Au11(S-BINAP)4X2]+ exhibited intense and mirror-image Cotton effect, whereas those of Au11(3+) clusters stabilized by achiral monodentate phosphine ligands did not. The origin of the chiroptical activity of [Au11(BINAP)4X2]+ is discussed in the context of the structural deformation of the Au11(3+) core.