https://is.muni.cz/el/1431/podzim2006/C7780/um/Read/2711172/SAM_str_grwth_ProgSurfSci00_151.pdf

Structure and growth of self-assembling monolayers

Gold nanoparticles, ca. 30 A in diameter, have been derivatized with specifically deuterated (position 1 and positions 10 to 13) and perdeuterated (positions 2 to 18) octadecanethiols (C18SH). The phase behavior of the octadecanethiolate monolayers chemisorbed onto the colloidal gold surface was characterized by differential scanning calorimetry (DSC). The DSC thermograms show that the C18SH-derivatized Au nanoparticles undergo distinct phase transitions which can be associated with the reversible disordering of the alkyl chains. Despite the highly curved geometry of these Au particles, there is a remarkable degree of conformational order in the alkanethiolate chains and the thermotropic behavior of the thiol-modified gold nanoparticles is very similar to that of conventional, planar self-assembled monolayers. Both the peak maximum temperature and the enthalpy associated with the DSC transition strongly parallel those of the gel-to-liquid crystalline transition of n-diacylphosphatidylcholine lipid bilayer...

Structure and Dynamics in Alkanethiolate Monolayers Self-Assembled on Gold Nanoparticles: A DSC, FT-IR, and Deuterium NMR Study

Self-assembled monolayers (SAMs) of n-alkanethiolates on gold, silver, and copper have been intensively studied both as model organic surfaces and as modulators of metal surface properties. Sensitivity restrictions imposed by monolayer coverage and the low surface area of planar metal substrates, however, limit the characterization of these films in molecular terms to surface enhancement techniques. As a result, key aspects such as film dynamics and alkyl chain ordering remain ill-defined. The characterization of the thermal behaviour of SAMs is important not only for the design of stable, well-ordered organic superlattices, but also for the fundamental understanding of the factors that drive molecular interactions in two dimensions. Phase properties in SAMs have been addressed here through the synthesis of gold nanoparticles of 20–30 A in diameter and fully covered with alkylthiol chains. These thiolmodified gold nanoparticles with large surface areas have enabled the monolayer film structure to be uniquely characterized by transmission FT-IR spectroscopy, NMR spectroscopy, and differential scanning calorimetry. Our studies reveal that for long-chain thiols (≥ C16), the alkyl chains exist predominantly in an extended, all-trans ordered conformation at 25°C. Furthermore, calorimetry, variable temperature transmission FT-IR spectroscopy, and solid-state 13C NMR studies have established that a cooperative chain melting process occurs in these alkylated metal colloids. How this arises is not immediately evident, given the relation between the extended chain conformation and the geometry of the spherical nanoparticles. Transmission electron microscopy (TEM) reveals that adjacent gold particles are separated by approximately one chain length; this suggests that chain ordering arises from an interdigitation of chains on neighboring particles. The thermotropic behavior is sensitive to the alkyl chain length and chain packing density. The alkylated nanoparticles can thus serve as a highly dispersed analogue to the much-studied planar SAMs.

Self-Assembled Monolayers on Gold Nanoparticles

A revised view of Brust−Schiffrin metal nanoparticle syntheses is presented here. Precursor species of these reactions are identified and quantified for Au, Ag, and Cu systems. Contrary to the assumptions of previous reports, tetraalkylammonium metal complexes are shown to be precursors of the two-phase reactions, whereas M(I) thiolates are shown to be precursors of the one-phase reactions. A new scheme is outlined for the two-phase synthesis, and the implications of this scheme are discussed. A new synthetic strategy employing well-defined precursors is also introduced. Finally, M(I) thiolate formation, and its impact on nanoparticle synthesis, is discussed. It is expected that the results presented here will lead to modifications in the manner in which these important syntheses are conducted.

New insights into Brust-Schiffrin metal nanoparticle synthesis.

In this Account, we discuss the dynamic features of self-assembled monolayers (SAMs) on both planar and nonplanar substrates. Our focus is on organothiol and organic acid-based monolayers. Results from variable-temperature electrochemical, calorimetric, vibrational, and solid-state NMR spectroscopic measurements lead to a convergent description of SAM dynamics.

A dynamic view of self-assembled monolayers.

In the present work, we have used reflection−absorption infrared spectroscopy (RAIRS) to investigate the temperature behavior of n-alkanethiols adsorbed on gold substrates A careful examination of the spectra (frequencies, integrated intensities, and line widths) reveals at least two different types of thermally induced disorder as the temperature is increased These observations are discussed in light of other recent studies using complementary techniques The overall picture that emerges is that an initial stage is characterized by a gradual untilting of the alkyl chain and the appearance of gauche conformational defects near the surface, and both of these effects are largely reversible The subsequent nonreversible disordering of the monolayer is characterized by a large number of defects located below the surface Infrared spectroscopy is particularly well suited for detecting these changes

Thermal Treatment of n-Alkanethiolate Monolayers on Gold, As Observed by Infrared Spectroscopy

Reflection‐absorption infrared spectroscopy at near grazing incidence is used to investigate the thermal stability of self‐assembled monolayers of alkyl thiols, HS–(CH2)n−1‐R, adsorbed on polycrystalline metal surfaces. The spectral features of the C–H stretching region have been carefully analyzed in the temperature range from 150 to 450 K. These features are highly sensitive to chain orientation and chain conformation. On gold, the chains are found to gradually untilt but remain largely all‐trans up to about 350 K. Above this temperature, a broad, irreversible transition to a liquidlike phase is observed, and which is characterized by a large number of gauche conformational defects. This overall behavior did not depend upon the chain length or terminal group for the systems studied. In contrast, significant differences were observed on a silver substrate.

Probing the different phases of self‐assembled monolayers on metal surfaces: Temperature dependence of the C–H stretching modes

The thermal behavior of a self-assembled silver alkanethiolate layered material (AgSCn, n = 12) has been explored using two spectroscopic techniques:  13C CPMAS NMR (13C cross-polarization magic an...

Thermal Behavior of a Self-Assembled Silver n-Dodecanethiolate Layered Material Monitored by DSC, FTIR, and 13C NMR Spectroscopy†

The dynamics of the alkyl chains below and above the chain-melting transition temperature of long-chain silver n-alkanethiolate layered compounds were studied using variable-temperature 2H NMR and IR spectroscopies. Two specifically deuterated compounds, silver-d35-octadecanethiolate [AgSCH2(CD2)16CD3] and silver-1,1-d2-octadecanethiolate [AgSCD2(CH2)16CH3], were used to identify the behavior of different regions of the chains. The 2H NMR studies provide details about the alkyl chain motion, whereas IR spectroscopy provides details of the alkyl chain conformation. A key structural component of these materials is a polymeric −Ag−S− backbone from which the alkyl chains extend outward from both sides. The result is a two-dimensional, ribbonlike structure. The alkyl chains are highly ordered and adopt an extended all-trans conformation, which is maintained up to a few degrees below the phase transition temperature. The occurrence of a significant gauche population is observed above the phase transition temper...

T. H. Ellis

Papers

Thermal Treatment of n-Alkanethiolate Monolayers on Gold, As Observed by Infrared Spectroscopy

Probing the different phases of self‐assembled monolayers on metal surfaces: Temperature dependence of the C–H stretching modes

Thermal Behavior of a Self-Assembled Silver n-Dodecanethiolate Layered Material Monitored by DSC, FTIR, and 13C NMR Spectroscopy†

Structure and Dynamics of Selectively Deuterated Self-Assembled Silver n-Octadecanethiolate Layered Materials†