Probing Single Nanometer-scale Particles with Scanning Tunneling Microscopy and Spectroscopies

Scanning tunneling microscopy can be used to isolate single particles on surfaces for further study. Local optical and electronic properties coupled with topographic information collected by the scanning tunneling microscope (STM) give insight into the intrinsic properties of the species under study. Since each spectroscopic measurement is done on a single particle, each sample is monodisperse, regardless of the degree of heterogeneity of the original preparation. We illustrate this with three example systems – a metal cluster of known atomic structure, metal nanoparticles dispersed from colloid suspensions, and metallocarbohedrenes (Met-Cars) deposited with other reaction products. Au and Ag nanoparticles were imaged using a photon emission STM. The threshold voltage, the lowest bias voltage at which photons are produced, was determined for Au nanoparticles. Electronic spectra of small clusters of Ni atoms on MoS₂ were recorded. Preliminary images of Zr-based Met-Car-containing soot were obtained on Au and MoS₂ substrates and partial electronic spectra were recorded of these possible Met-Car particles.     

Tunable Aminooxy‐Functionalized Monolayer‐Protected Gold Clusters for Nonpolar and Aqueous Oximation Reactions

Aminooxy (–ONH₂) groups are well known for their chemoselective reactions with carbonyl compounds, specifically aldehydes and ketones. The versatility of aminooxy chemistry has proven to be an attractive feature that continues to stimulate new applications. This work describes application of aminooxy click chemistry on the surface of gold nanoparticles. A trifunctional amine‐containing aminooxy alkane thiol ligand for use in the functionalization of gold monolayer‐protected clusters (Au MPCs) is presented. Diethanolamine is readily transformed into an organic‐soluble aminooxy thiol ( AOT ) ligand using a short synthetic path. The synthesized AOT ligand is coated on ≤2‐nm‐diameter hexanethiolate‐(C₆S)‐capped Au MPCs using a ligand‐exchange protocol to afford organic‐soluble AOT /C₆S (1:1 ratio) Au mixed monolayer‐protected clusters (MMPCs). The synthesis of these Au(C₆S)( AOT ) MMPCs and representative oximation reactions with various types of aldehyde‐containing molecules is described, highlighting the ease and versatility of the chemistry and how amine protonation can be used to switch solubility characteristics.     

Novel fabrication and catalytic application of poly(ethylenimine)-stabilized gold–silver alloy nanoparticles

Novel synthesis of amine-stabilized Au–Ag alloy nanoparticles with controlled composition has been devised using poly(ethylenimine) (PEI) as a reducing and a stabilizing agent simultaneously. The composition of Au–Ag alloy nanoparticles was readily controlled by varying the initial relative amount of HAuCl₄ and AgNO₃. Due to the presence of abundant amine functional groups in PEI, which could act as the dissolving ligand for AgCl, the precipitation problem of Ag⁺ in the presence of Cl⁻ from the gold salt was avoided. On this basis, the relatively high concentrations of HAuCl₄ and AgNO₃ salts were used for the fabrication of Au–Ag alloy nanoparticles. The PEI thus plays triple roles in this study that include the co-reducing agents for HAuCl₄ and AgNO₃, the stabilizing agents for Au–Ag alloy nanoparticles, and even the dissolving agents for AgCl. As a novel material for use in catalysis, the Au–Ag alloy nanoparticles including pure Au and Ag samples were exploited as catalysts for the reduction of 4-nitrophenol in the presence of NaBH₄. As the Au content was increased in the Au–Ag alloy nanoparticles, the rate constant of the reduction was exponentially increased from pure Ag to pure Au.     

Thick-target X-ray bremsstrahlung spectra produced in 6.5 keV and 7.5 keVe--Hf collisions

The absolute doubly differential cross-sections (DDCS) for production of the thick-target X-ray bremsstrahlung spectra in collisions of 6.5 keV and 7.5 keV electrons with thick Hf target are measured. The X-ray photons are counted by a Si(Li) detector placed at 90° to the electron beam direction. The bremsstrahlung spectra are corrected for various ‘solid-state effects’ namely, electron energy-loss, electron back-scattering, and photon-attenuation in the target, in addition to the correction for detector’s efficiency. The DDCS values after correction, are compared with the predictions of a most accurate thin-target bremsstrahlung theory [H K Tseng and R H Pratt,Phys. Rev. A3, 100 (1971); Kisselet al, Atomic Data Nucl. Data Tables 28, 381 (1983)]. Also, a dependence of the absolute DDCS on atomic numberZ of the targets (₄₇Ag,₇₉Au and₇₂Hf) at 7.0 keV and 7.5 keV electron energies has been studied. The agreement between experiment and theory is found to be satisfactory within 27% systematic error of measurements. However, an apparent systematic difference between experiment and theory in the region of low-energy photons has been explained qualitatively by considering the fact that the hexagonal atomic structure of Hf offers possibly a greater magnitude of ‘solid-state effects’ in respect of blocking the low-energy bremsstrahlung photons from coming out of the target surface than does the cubic-face centered structure of Ag and Au target in similar conditions of the experiment.     

1-Hexadecylamine as both reducing agent and stabilizer to synthesize Au and Ag nanoparticles and their SERS application

1-Hexadecylamine (HDA)-capped Au and Ag nanoparticles (NPs) have been successfully prepared by a one-pot solution growth method. The HDA is used as both reducing agent and stabilizer in the synthetic process is favorable for investigating the capping mechanism of Au and Ag NPs’ surface. The growth process and characterization of Au and Ag NPs are determined by Ultraviolet–visible (UV–vis) spectroscopy, transmission electron microscopy (TEM), and X-ray diffraction (XRD). Experimental results demonstrate that the HDA-capped Au and Ag NPs are highly crystalline and have good optical properties. Furthermore, surface-enhanced Raman scattering (SERS) spectra of 2-thionaphthol are obtained on the Au and Ag NPs modified glass surface, respectively, indicating that the as-synthesized noble metal NPs have potentially high sensitive optical detection application.     

Laser ablation of silver and gold in liquid ammonia

Laser ablation of a silver (Ag) and/or gold (Au) target was performed in liquid ammonia (l-NH₃) at 233 K using nanosecond laser pulses of 1064, 532 and 355 nm wavelengths. An “in situ” monitoring of the ablation process by UV/vis/NIR spectroscopy has shown the evolution of the surface plasmon extinction band of silver or gold nanoparticles and thus confirmed their formation. While sols of Au nanoparticles in l-NH₃ are quite stable in air, those of Ag nanoparticles undergo oxidation to Ag(I) complexes with NH₃ ligands. On the other hand, formation of solvated electrons, namely of the (e⁻)NH₃ solvates, has not been unequivocally confirmed under the conditions of our laser ablation/nanoparticle fragmentation experiment, since only very weak vis/NIR spectral features of these solvates were observed with a low reproducibility. Reference experiments have shown that the well-known chemical production of these solvates is hindered by the presence of Ag and Au plates. Ag and Au targets can thus possibly act as electron scavengers in our ablation experiments.     

Metal nanoparticle-doped coloured films on glass and polycarbonate substrates

In a program on the development of metal (e.g. Au, Ag, Cu and their alloy) nanoparticles in sol-gel derived films, attempts were made to synthesize different coloured coatings on glasses and plastics. The absorption position of surface plasmon resonance (SPR) band arising from the embedded metal nanoparticles was tailored by controlling the refractive index of the matrix for the development of different colours. Thus different coloured (pink to blue) coatings on ordinary sheet glasses were prepared by generating Au nanoparticles in mixed SiO₂-TiO₂ matrices having refractive index values ranging from about 1.41 to 1.93. In another development,in situ generation of Ag nanoparticles in the inorganic-organic hybrid host leads to the formation of different abrasion resistant coloured coatings (yellow to pink) on polycarbonate substrates after curing. As expected, the SPR peak of Ag or Au is gradually red-shifted due to the increase of refractive index of the coating matrices causing a systematic change of colour     

Design of Enhanced Catalysts by Coupling of Noble Metals (Au,Ag) with Semiconductor SnO2 for Catalytic Reduction of 4‐Nitrophenol

The reduction of 4‐nitrophenol (Nip) into 4‐aminophenol (Amp) by NaBH₄, which is catalyzed by both binary and ternary yolk–shell noble‐metal/SnO₂ heterostructures, is reported. The binary heterostructures contain individual Au or Ag nanoparticles (NPs) and the ternary heterostructures contain both Au and Ag NPs. The Au@SnO₂ yolk–shell NPs are synthesized via a silica seeds‐mediated hydrothermal method. Subsequently, the Au@SnO₂@Ag and Au@SnO₂@Au yolk–shell–shell (YSS) NPs are synthesized, whereby SnO₂ is located between the Au and Ag NPs. The morphology, composition, and optical properties of the as‐prepared samples are analyzed. For the binary heterostructures, the rate of the reduction reaction increases with decreasing particle size. The catalytic results demonstrate the synergistic effect of Au and Ag in the ternary metal–semiconductor heterostructures, which is beneficial to the catalytic reduction of Nip into Amp. Both the binary and ternary heterostructures exhibit significantly better catalytic performances than the corresponding bare Au and Ag NPs. It is envisaged that the current synthesized strategy will promote further interest in the field of bimetal NP‐based catalysis.     

银/二氧化钛核壳纳米颗粒对碲化镉纳米晶的荧光增强研究

利用新合成的复合纳米结构银/二氧化钛核壳纳米颗粒,研究了金属银纳米颗粒对碲化镉纳米晶层荧光的增强情况.结果表明,这种新型复合金属纳米结构能极大地增强发光纳米晶层的荧光强度.银/二氧化钛核壳纳米颗粒是以水合肼、硝酸银和四异丙氧基钛为原材料,利用胶体化学法在水溶液中合成.透射电子显微镜图片表明这种新合成的银/二氧化钛纳米材料基本上呈球形,有较为明显的核壳结构,中间黑色的核是银纳米颗粒,外层颜色较浅部分是二氧化钛壳层.另外,包裹二氧化钛壳层后,银纳米颗粒的表面等离子吸收带从409 nm红移至430 nm,也证实了这种新型核壳纳米材料的形成.将此合成方法得到的银/二氧化钛纳米颗粒和碲化镉纳米晶用旋转涂覆方法进行直接组合后,得到了银纳米颗粒对碲化镉纳米晶荧光的明显增强,并对其增强的物理过程进行了讨论.这种能够增强荧光团发光的新型复合银纳米结构将在发光器件、荧光成像、生物探测等方面具有一定的应用价值.     

Structure and properties of TiO<Subscript>2</Subscript> surfaces: a brief review

Titanium oxides are used in a wide variety of technological applications where surface properties play a role. TiO₂ surfaces, especially the (110) face of rutile, have become prototypical model systems in the surface science of metal oxides. Reduced TiO₂ single crystals are easy to work with experimentally, and their surfaces have been characterized with virtually all surface-science techniques. Recently, TiO₂ has also been used to refine computational ab initio approaches and to calculate properties of adsorption systems. Scanning tunneling microscopy (STM) studies have shown that the surface structure of TiO₂(110) is more complex than originally anticipated. The reduction state of the sample, i.e. the number and type of bulk defects, as well as the surface treatment (annealing in vacuum vs. annealing in oxygen), can give rise to different structures, such as two different (1×2) reconstructions, a ‘rosette’ overlayer, and crystallographic shear planes. Single point defects can be identified with STM and influence the surface chemistry in a variety of ways; the adsorption of water is discussed as one example. The growth of a large number of different metal overlayers has been studied on TiO₂(110). Some of these studies have been instrumental in furthering the understanding of the ‘strong metal support interaction’ between group-VIII metals and TiO₂, as well as low-temperature oxidation reactions on TiO₂-supported nanoscopic gold clusters. The growth morphology, interfacial oxidation/reduction reaction, thermal stability, and geometric structure of ultra-thin metal overlayers follow general trends where the most critical parameter is the reactivity of the overlayer metal towards oxygen. It has been shown recently that the technologically more relevant TiO₂ anatase phase can also be made accessible to surface investigations. Received: 4 March 2002 / Accepted: 20 October 2002 / Published online: 5 February 2003 RID="*" ID="*"Corresponding author. Fax: +1-504/862-8279, E-mail: diebold@tulane.edu     

Silver,gold and the corresponding core shell nanoparticles: synthesis and characterization

Simple strategies for producing silver and gold nanoparticles (AgNP and AuNP) along with the corresponding core shell nanoparticles (Au–Ag and Ag–Au) by reduction of the metal salts AgBF₄ and HAuCl₄ by NaBH₄ in water will be presented. The morphologies of the obtained nanoparticles are determined by the order of addition of reactants. The obtained NPs, with sizes in the range 3–40 nm, are characterized by transmission electronic microscopy (TEM) and UV–Vis absorption spectroscopy, so as to evaluate their qualities. Moreover, a direct electrochemical detection protocol based on a cyclic voltammetry in water solution that involves the use of glassy carbon electrode is also applied to characterize the prepared NPs. The developed NPs and the related electroanalytical method seem to be with interest for future sensing and biosensing applications including DNA sensors and immunosensors.     

Metal-on-oxide nanoparticles produced using laser ablation of microparticle aerosols

A continuous aerosol process has been studied for producing nanoparticles of oxides that were decorated with smaller metallic nanoparticles and are free of organic stabilizers. To produce the oxide carrier nanoparticles, an aerosol of 3–6 μm oxide particles was ablated using a pulsed excimer laser. The resulting oxide nanoparticle aerosol was then mixed with 1.5–2.0 μm metallic particles and this mixed aerosol was exposed to the laser for a second time. The metallic micron-sized particles were ablated during this second exposure, and the resulting nanoparticles deposited on the surface of the oxide nanoparticles producing an aerosol of 10–60 nm oxide nanoparticles that were decorated with smaller 1–5 nm metallic nanoparticles. The metal and oxide nanoparticle sizes were varied by changing the laser fluence and gas type in the aerosol. The flexibility of this approach was demonstrated by producing metal-decorated oxide nanoparticles using two oxides, SiO₂ and TiO₂, and two metals, Au and Ag.     

Synchrotron-Radiation Photoemission Study of Growth and Stability of Au Clusters on Rutile TiO2(110)-1×1

利用同步辐射高分辨光电子能谱研究了金团簇在部分还原TiO₂-(1×1)表面的生长和稳定性．价带谱实验结果观察到非常少量金团簇的沉积导致了Ti3+的3d峰完全消失,表明金团簇成核在TiO₂-(1×1)表面的氧缺陷位．Au4f芯电子光电子能谱实验结果证明了TiO₂-(1×1)表面氧缺陷位向金团簇转移电荷．还对比研究了化学剂量比和部分还原的TiO₂-(1×1)表面上金团簇的热稳定性．当金团簇尺寸相近时部分还原的TiO₂-(1×1)表面上金团簇要比化学剂量比的TiO₂-(1×1)面上金团簇稳定;在相同的表面上尺寸大的金团簇要比尺寸小的金团簇稳定．     

Peak effect at microwave frequencies in swift heavy ion irradiated YBa2Cu3O7-δ thin films

The vortex dynamics at microwave frequencies in YBa₂Cu₃O_7-δ (YBCO) films have been studied. We observe a peak in the microwave (4.88 and 9.55 GHz) surface resistance in some films in magnetic fields up to 0.8 T. This is associated with the ‘peak-effect’ phenomenon and reflects the order-disorder transformation of the flux line lattice near the transition temperature. Introduction of artificial pinning centers like columnar defects created as a result of irradiation with 200 MeV Ag ion (at a fluence of 4×10¹⁰ ions/cm²) leads to the suppression of the peak in films previously exhibiting ‘peak effect’.     

Negative chemical pressure effects induced by Y substitution for Ca on the ‘exotic’ magnetic behavior of the spin-chain compound, Ca3Co2O6

The magnetic behavior of a solid solution, Ca_{3 x} Y_x CO₂ O₆, based on the ‘exotic’ spin-chain compound, Ca₃Co₂O₆, crystallizing in K₄CdCl₆-derived rhombohedral structure is investigated. Among the compositions investigated(x = 0.0, 0.3, 0.5, 0.75 and 1.0), single-phase formation persists up tox = 0.75, with the elongation of the c-axis. The present investigations reveal that the temperature at which the ‘so-called’ ‘partially disordered antiferromagnetic structure’ sets in (which occurs at 24 K for the parent compound,x = 0.0) undergoes gradual reduction with the substitution of Y for Ca, attaining the value of about 2.2 K for the nominalx = 1.0. The trend observed in this characteristic temperature is opposite to that reported under external pressure, thereby establishing that Y substitution exerts negative chemical pressure. Anomalous steps observed in the isothermal magnetization at very low temperatures (around 2 K) forx = 0.0, which have been proposed to arise from ‘quantum tunneling effects’ are found to vanish by a small substitution (x = 0.3) of Y for Ca. Systematics in AC and DC magnetic susceptibility behavior with Y substitution for Ca have also been probed. We believe that the present results involving the expansion of chain length without disrupting the magnetic chain may be useful to the overall understanding of the novel magnetism of the parent compound.     

Sers measurement of oxidation-reduction reactions of adsorbates on gold microstructures

The use of surface enhanced Raman scattering (SERS) to study oxidation-reduction and complexation chemical reactions on Au surfaces is illustrated by: (1) the reaction of Au(CN)₂^? adsorbed on a Au colloid (2140 cm^?) to form Au(CN)₃^2? (2131 cm^?1) on the surface in excess CN^?; (2) the oxidation of Au(CN)₂^? by HNO₃, Cl₂, or Br₂ solutions to form Au(CN)₄^? (2190 cm^?) on a Au colloid; and (3) the dissolution of Au in excess CN^? with O₂. Unlike with Ag surfaces, no SERS is observed when Au powder is exposed to NO, NO₂, SO₂, CO, or CO₂ gases. The surface chemistry of Au is discussed in the light of these reactions.     

Solution templating of Au and Ag nanoparticles by linear poly[2-(diethylamino)ethyl methacrylate]

Abstract  

Linear poly[2-(diethylamino)ethyl methacrylate], poly(DEAMA), is an uncommon example of a homopolymer that can reduce salts of Au and Ag in solution to yield stable dispersions of nanoparticles (5–25 nm typical size). Poly(DEAMA)-stabilized Au and Ag nanoparticles were prepared in a mixture of water and 2-butoxyethanol, an amphiphilic organic solvent. The “loading ratio” (mole ratio of metal atoms to amines), a key parameter influencing particle size and clustering, was systematically varied. The size distribution and clustering of the nanoparticles were characterized by transmission electron microscopy and small-angle X–ray scattering. The maximum loading ratio achievable without inducing precipitation was approximately 0.3 for Au, but the maximum loading ratio for Ag was only about 0.04. The preparation of both Au and Ag nanoparticles in solution with a linear polymeric template illustrates that dendritic or hyperbranched architecture of the polymer is not a prerequisite for obtaining stable, non-aggregated dispersions. From a practical standpoint, poly(DEAMA) is an inexpensive template material that is readily immobilized on silica, which could facilitate development of novel, nanoparticle-based heterogeneous catalysts.     

A facile strategy to synthesize bimetallic Au/Ag nanocomposite film by layer-by-layer assembly technique

A facile strategy has been developed for the preparation of bimetallic gold–silver (Au–Ag) nanocomposite films by alternating absorption of poly-(ethyleneimine)–silver ions and Au onto substrates and subsequent reduction of the silver ions. The composition, micro-structure and properties of the {PEI–Ag/Au}_n nanocomposite films were characterized by ultraviolet visible spectroscopy (UV–vis), transmisson electron microscopy (TEM), field emission scanning electron microscopy (FESEM), X-ray photoelectron spectroscopy (XPS), surface enhanced Raman scattering (SERS) and cyclic voltammetry (CV). The UV–vis characteristic absorbances of {PEI–Ag/Au}_n nanocomposite thin film increase almost linear with the number of bilayers, which indicates a process of uniform assembling. Appearance of a double plasmon bands in the visible region and the lack of apparent core–shell structures in the TEM images confirm the formation of bimetallic Au–Ag nanoparticles. The result of XPS also demonstrates the existence of Ag and Au nanoparticles in the nanocomposite films. TEM and FESEM images show that these Ag and Au nanoparticles in the films possess sphere structure with the size of 20–25 nm. The resulting {PEI–Ag/Au}_n films inherit the properties from both the metal Ag and Au, which exhibits a unique performance in SERS and electrocatalytic activities to the oxidation of dopamine. As a result, the {PEI–Ag/Au}_n films are more attractive compared to {PEI–Ag/PSS}_n and {PEI/Au}_n films.     

Metal clusters in metal/C thin film nanosystems

Structure of metal clusters and of the C₆₀ matrix in Au/C₆₀ and Cu/C₆₀ nanosystems was investigated by X-ray diffraction. Results support a charge-transfer-type interaction at the metal-C₆₀ interface, which affects the size distribution of metal clusters, favouring interstitial location of metal ions in the fullerite lattice. Received 5 February 1999 and Received in final form 7 July 1999     

Synthesis of nanocrystalline material by sputtering and laser ablation at low temperatures

Physical vapor deposition techniques such as sputtering and laser ablation – which are very commonly used in thin film technology – appear to hold much promise for the synthesis of nanocrystalline thin films as well as loosely aggregated nanoparticles. We present a systematic study of the process parameters that facilitate the growth of nanocrystalline metals and oxides. The systems studied include TiO₂, ZnO, γ-Al₂O₃, Cu₂O, Ag and Cu. The mean particle size and crystallographic orientation are influenced mainly by the sputtering power, the substrate temperature and the nature, pressure and flow rate of the sputtering gas. In general, nanocrystalline thin films were formed at or close to 300 K, while loosely adhering nanoparticles were deposited at lower temperatures. Received: 31 October 2000 / Accepted: 9 January 2001 / Published online: 26 April 2001