Shishan Zhang

Preparation, characterization, and chemical stability of gold nanoparticles coated with mono-, bis-, and tris-chelating alkanethiols.

A systematically varying series of monolayer-protected clusters (MPCs) was prepared by exposing small gold nanoparticles ( approximately 2 nm in diameter) to the following four adsorbates: n-octadecanethiol ( n - C18), 2-hexadecylpropane-1,3-dithiol ( C18C2), 2-hexadecyl-2-methylpropane-1,3-dithiol ( C18C3), and 1,1,1-tris(mercaptomethyl)heptadecane ( t - C18). The resultant MPCs were characterized by solubility studies, UV-vis spectroscopy, transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and Fourier-transform infrared spectroscopy (FT-IR). All of the MPCs were soluble in common organic solvents; moreover, analysis by TEM showed that the core dimensions were unaffected by exposure to any of the adsorbates. Separate studies by XPS revealed that the sulfur atoms in all MPCs were predominantly bound to the surface of gold (i.e., approximately 85% or better). Analysis by FT-IR showed that MPCs functionalized with n - C18 possessed alkyl chains having the greatest conformational order in both the solid-state and dispersed in solution; in contrast, those generated from the other three adsorbates were more liquid-like with reduced conformational order (or crystallinity). The rate of nanoparticle decomposition induced by cyanide ions was monitored by UV-vis spectroscopy. While MPCs functionalized with n - C18 showed the fastest rate of decomposition, those functionalized with C18C3 were the most resistant to decomposition. Overall, the following trend in chemical stability was observed, C18C3 >> C18C2 > t - C18 >> n - C18.

Monolayer-Protected Gold Nanoparticles Prepared Using Long-Chain Alkanethioacetates†

This letter describes the preparation of monolayer-protected nanoparticle clusters (MPCs) from the adsorption of n-tetradecanethioacetate onto colloidal gold nanoparticles using the Brust-Schiffrin two-phase synthesis method. The MPCs were characterized by transmission electron microscopy (TEM), ultraviolet-visible (UV-vis) spectroscopy, (1)H nuclear magnetic resonance (NMR) spectroscopy, X-ray photoelectron spectroscopy (XPS), and Fourier transform infrared (FTIR) spectroscopy. These studies found that the monolayer coatings on the gold nanoparticles were nearly indistinct with regard to chemical composition, monolayer structure, and Au-S ligation when compared to those prepared from the analogous adsorption of n-tetradecanethiol (i.e., the thioacetate headgroup adsorbs to gold as a thiolate, with concurrent loss of the acetyl group). Under equivalent conditions of formation, however, the size of the gold nanoparticles formed was larger when using the alkanethioacetate adsorbate (e.g., 4.9 +/- 1.2 nm) compared to the alkanethiol adsorbate (e.g., 1.6 +/- 0.3 nm). The observed difference in size is rationalized on the basis of the stronger ligating ability of the thiol compared to that of the thioacetate during gold nanoparticle nucleation and/or growth. The use of alkanethioacetates affords significant control of particle size and allows the formation of MPCs with thiol-sensitive omega-functional groups.

Correlating linactant efficiency and self-assembly: structural basis of line activity in molecular monolayers.

Surfactants exhibit characteristic phenomena, including the reduction of interfacial free energy, self-assembly into aggregates, and even the formation of lyotropic liquid crystalline phases at high concentrations. Our research has shown that a semifluorinated phosphonic acid can act as the two-dimensional analogue of a surfactant-a linactant-by reducing the line tension between hydrocarbon-rich and fluorocarbon-rich phases in a Langmuir monolayer. This linactant can also self-assemble into nanoscale clusters in a monolayer. Here, we explore the dependence of linactant behavior on molecular structure. Members of a homologous series of partially fluorinated phosphonic acids were synthesized and tested as linactants: CF(3)(CF(2))(n-1)(CH(2))(m)PO(3)H (abbreviated as FnHmPO(3)). The tests revealed that linactants with longer hydrophobic chains were most efficient in lowering line tension. For linactants with the same overall chain length, the length of the fluorocarbon block was correlated with efficiency. Thus, the linactant efficiency was ranked in the order F8H8PO(3) < F10H6PO(3) < F8H11PO(3) < F10H9PO(3). In all cases, linactant-containing Langmuir-Blodgett monolayers exhibited nanoscale molecular clusters with characteristic dimensions of 20-30 nm; enhanced linactant efficiency was systematically correlated with larger clusters.

Self-Assembled Monolayers Derived from a Double-Chained Monothiol Having Chemically Dissimilar Chains

The structure and conformation of self-assembled monolayers (SAMs) derived from the adsorption of a specifically designed double-chained partially fluorinated thiol having the formula 12,12,13,13,14,14,15,15,16,16,17,17,18,18,19,19,19-heptadecafluoro-2-tetradecylnona-decane-1-thiol (2) onto the surface of evaporated gold were examined by ellipsometry, contact angle goniometry, polarization modulation infrared reflection-absorption spectroscopy (PM-IRRAS), and X-ray photoelectron spectroscopy (XPS). The results were compared to those of SAMs generated from normal hexadecanethiol (1) and a structurally related single-chained partially fluorinated thiol having the formula 12,12,13,13,14,14,15,15,16,16,17,17,18,18,19,19,19-heptadecafluorononadecane-1-thiol ( 3). Collectively, the studies demonstrate that the double-chained adsorbate 2 forms SAMs on gold in which the alkyl chains are less densely packed and less conformationally ordered than those in the SAMs derived from each of the single-chained adsorbates. Furthermore, the fluorocarbon moieties in the SAMs derived from 2 are more tilted from the surface normal than those in the SAMs derived from 3. The low values of contact angle hysteresis suggest, however, that the double-chained adsorbate 2 generates homogeneous monolayer films on the surface of gold.

Light-Induced Covalent Immobilization of Monolayers of Magnetic Nanoparticles on Hydrogen-Terminated Silicon

Specifically tailored ω-alkenyl-1-carboxylic acids were synthesized for use as surfactants in the single-step preparation of manganese ferrite (MnFe2O4) nanoparticles (NPs). Monodisperse manganese ferrite NPs terminated with ω-alkenyl moieties were prepared via a one-pot reaction at high temperature without the need of ligand exchange. Using this approach, simple adjustment of the rate of heating allowed precise tuning of the size of the nanoparticles, which were characterized in bulk form by transmission electron microscopy (TEM), Fourier-transform infrared (FT-IR) spectroscopy, and X-ray diffraction (XRD). These surfactant-coated magnetic nanoparticles were then deposited onto hydrogen-terminated silicon(111) wafers and covalently anchored to the surface by UV-initiated covalent bonding. Analysis by scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS) confirmed that the UV treatment led to covalent immobilization of the NPs on the silicon surface with a consistent packing density across the surface. The magnetic properties of the stable, surface-bound nanoparticle arrays were characterized using a superconducting quantum interference device (SQUID) magnetometer. The materials and methods described here are being developed for use in bit-patterned ultrahigh density magnetic recording media and nanoscale biomagnetic sensing.

Magnetization reversal and magnetic anisotropy in patterned Co/Pd multilayer thin films

(Co/Pd)N multilayers exhibit high vertical magnetic anisotropy and have been extensively explored as recording medium candidates for high density magnetic recording applications. In this work, (Co/Pd)N multilayers are deposited by magnetron sputtering and patterned into large periodic arrays of 200 nm islands to enable controlled domain wall injection for quantitative comparison of magnetic anisotropy energies. Magnetic properties are correlated with x-ray photoelectron spectroscopy data, an approach commonly used to probe the binding energies and valence band positions. Confirming theoretical predictions, it is demonstrated that the degree of d-shell hybridization at Co/Pd interfaces directly correlated with the magnitude of magnetic anisotropy.

Semi-fluorinated phosphonic acids form stable nanoscale clusters in Langmuir–Blodgett and self-assembled monolayers

We compared molecular monolayers of semifluorinated phosphonic acids (F8H11PO3, F10H6PO3, F8H8PO3, and F6H10PO3) on mica substrates prepared by two different methods: Langmuir–Blodgett (LB) transfer of pre-formed monolayers from the air–water interface, and self-assembled monolayers (SAMs) formed spontaneously at the interface between mica and solution (with tetrahydrofuran as the solvent). The films were investigated with atomic force microscopy (AFM) and contact angle measurements. Nanometer-scale two-dimensional (2D) clusters (20–30 nm in size) were observed in both the LB films and the SAMs. Time-dependent AFM images suggested that for SAMs derived from F8H11PO3, F8H8PO3, and F6H10PO3, small clusters nucleated, but stopped growing once a stable size was reached. In situAFM images suggested that the clusters were dome-shaped. The LB monolayer structures were consistent with those of the SAMs, but with greater long-range order of the clusters in some cases. For SAMs derived from F10H6PO3, however, long immersion times led to continued growth and coalescence of the 2D clusters and formation of a flat, untextured monolayer. Similarly, LB films generated from F10H6PO3 exhibited cluster coalescence. The similarity of the observed structures in LB films and SAMs suggests that these nanostructured films represent an equilibrium state based on intrinsic self-organization of the semifluorinated molecules, even in SAMs, where mobility is significantly restricted at the solid–liquid interface. Based on these observations, we hypothesize that the packing incommensurability between the hydrocarbon and fluorocarbon blocks leads to splay of neighboring chains and spontaneous curvature. However, when intermolecular interactions are dominated by a long fluorocarbon block, there is a transition to a flat structure, where the packing mismatch is compensated for by increased disorder within the hydrocarbon block.

Fibrillar Self-Organization of a Line-Active Partially Fluorinated Thiol within Binary Self-Assembled Monolayers

Self-assembled monolayers (SAMs) were prepared from a novel two-tailed partially fluorinated thiol (F8C11/C16), possessing one hydrocarbon chain and one chain with an extended fluorinated segment, and from mixtures of F8C11/C16 and hexadecanethiol (C16) on gold, with the expectation that the internal chemical dissimilarity and wedge-like shape of F8C11/C16 would lead to unique self-organizational motifs. The SAMs were systematically characterized using ellipsometry, atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), contact angle goniometry, and polarization modulation infrared reflection-absorption spectroscopy (PM-IRRAS). Based on this characterization, the one-component F8C11/C16 SAMs exhibited relatively poor molecular organization compared to traditional alkanethiols, forming low coverage monolayers with significant molecular disorder. However, the series of mixed SAMs formed from F8C11 and F8C11/C16 were anomalously well ordered as indicated by film thickness, surface coverage, and the frequencies of characteristic vibrational modes. AFM images of these mixed SAMs exhibited nanoscale fibrillar structures in a birds-nest morphology, suggesting that in the presence of a C16 matrix, the F8C11/C16 component organized into the two-dimensional analogue of discrete bilayers. Control experiments involving mixed SAMs comprised of F8C11/C16 and a single-tailed partially fluorinated thiol (F8C11) or C16 and F8C11 exhibited no appreciable indication of interesting self-organization beyond an evenly dispersed mixing of the thiolates or phase separation, respectively.

Low temperature vacuum annealing study of (Co∕Pd)n magnetic multilayers

The correlation between the magnetic properties and the microstructural and chemical composition modifications of Co∕Pd magnetic multilayers upon annealing in ultrahigh vacuum at 250°C is presented. Magnetic characterization using magnetic sample magnetometer shows the vertical magnetic anisotropy increase and the switching field distribution decrease in the annealed samples. The larger values of magnetic anisotropy in the annealed samples are further shown using the magnetic force microscopy of the ac demagnetized states in the Co∕Pd multilayer films. X-ray diffraction rocking curves show an improvement in the texture and the initial magnetization curve slopes indicate the decreases in defect densities. Overall, vacuum annealing under optimal conditions improves the magnetic properties of Co∕Pd multilayers for applications in ultrahigh density magnetic recording.

Swelling of a cluster phase in Langmuir monolayers containing semi-fluorinated phosphonic acids

Langmuir monolayers of semi-fluorinated nonadecylphosphonic acid (F8H11PO), hexadecylphosphonic acid (H16PO), and their mixtures were investigated by Brewster angle microscopy (BAM), atomic force microscopy (AFM) and surface-pressure measurements. Nanometre-scale two-dimensional clusters were observed by AFM in a spread monolayer of pure F8H11PO transferred to mica. Two different organized arrangements of clusters were observed. AFM and BAM observations showed that the mixture exhibits a solid phase over a large range of mole fraction and surface pressure, sometimes in coexistence with clusters. With increasing mole fraction of H16PO, the lateral shape of these clusters remains the same while their organization and their height change.