Going beyond the self-assembled monolayer: metal intercalated dithiol multilayers and their conductance (original) (raw)
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Surface Science - SURFACE SCI, 1999
We describe a structural study of the S/Au interface for decanethiol monolayers (C10) on a Au(111) surface using the technique of X-ray standing waves (XSWs). The XSW results for full-coverage monolayers are inconsistent with any model incorporating a single sulfur adsorption site, such as the widely assumed threefold hollow site on the Au(lll) surface. Instead, the XSW results reveal two distinct sulfur head group sites, each with a distinct lateral and vertical location with respect to the underlying gold lattice. We discuss structural models that are consistent with these results. We have also studied the evolution of the structure versus coverage with XSW and X-ray photoelectron spectroscopy (XPS) and have determined that the local S/Au interface structure of the lying down striped phase at low coverages (0.27 ML, I ML=4.62 x 1014 molecules cm-2) is indistinguishable from that of the standing up c(4 x 2) phase at saturation (I ML). Some important implications concerning the stru...
Transport studies of isolated molecular wires in self-assembled monolayer devices
Journal of Applied Physics, 2005
We have fabricated a variety of isolated molecule diodes based on self-assembled monolayers ͑SAMs͒ of solid-state mixture ͑SSM͒ of molecular wires ͓1,4-methane benzene dithiol ͑Me-BDT͔͒, and molecular insulator spacers ͓penthane 1-thiol ͑PT͔͒ with different concentration ratios r of wires/spacers, which were sandwiched between two gold ͑Au͒ electrodes. We introduce two specialized methods borrowed from surface science to ͑i͒ confirm the connectivity between the Me-BDT molecules with the upper Au electrode, and ͑ii͒ count the number of isolated molecular wires in the devices. The electrical transport properties of the SSM SAM diodes were studied at different temperatures via the conductance and differential conductance spectra. We found that a potential barrier caused by the spatial connectivity gap between the PT molecules and the upper Au electrode dominates the transport properties of the pure PT SAM diode ͑r =0͒. The transport properties of SSM diodes with r values in the range 10 −8 Ͻ r Ͻ 10 −4 are dominated by the conductance of the isolated Me-BDT molecules in the device. We found that the temperature dependence of the SSM diodes is much weaker than that of the pure PT device, indicating the importance of the Me-BDT simultaneous bonding to the two Au electrodes that facilitates electrical transport. From the differential conductance spectra we also found that the energy difference between the Au electrode Fermi level and the Me-BDT highest occupied molecular-orbital ͑or lowest unoccupied molecular-orbital͒ level is ϳ1.5 eV; where it is ϳ2.5 eV for the PT molecule. The weak temperature-dependent transport that we obtained for the SSM diodes reflects the weak temperature dependence of ⌬. In addition, our measurements reveal that the conductance of SSM diodes scales linearly with r, showing that the charge transport in these devices is dominated by the sum of the isolated Me-BDT molecular conductance in the device. Based on this finding, and the measured number of the Me-BDT molecules in the device we obtained the "single molecule resistance," R M . We measured R M =6ϫ 10 9 ⍀ for isolated Me-BDT molecules, which is consistent with previous measurements using other transport measuring techniques. A simple model for calculating R M , where the transport is governed by electron tunneling through the Me-BDT molecule using the WKB approximation, is in good agreement with the experimental data, thus validating the procedures used for our measurements.
We investigated the effects of tunneling current on scanning tunneling microscopy (STM) images of 1-octanethiol (OT) and 1-decanethiol (DT) self-assembled monolayers (SAMs). At a low tunneling current, the domain boundaries and ordered alkanethiol molecules were clearly resolved. As the tunneling current was increased at a constant bias voltage, however, the STM images showed disordered structures of the OT and DT SAMs. As the tunneling current was reduced back to low values, the ordered structures of the alkanethiol molecules reappeared. The reversibility of the process suggests that the sulfur head groups did not rearrange under any of the tunneling current conditions. On the basis of our observations, which are inconsistent with the standard model for STM imaging of molecules on metal surfaces, we consider the STM imaging mechanism in terms of a two-region tunneling junction model.
Bulletin of the Korean Chemical Society, 2011
We investigated the effects of tunneling current on scanning tunneling microscopy (STM) images of 1-octanethiol (OT) and 1-decanethiol (DT) self-assembled monolayers (SAMs). At a low tunneling current, the domain boundaries and ordered alkanethiol molecules were clearly resolved. As the tunneling current was increased at a constant bias voltage, however, the STM images showed disordered structures of the OT and DT SAMs. As the tunneling current was reduced back to low values, the ordered structures of the alkanethiol molecules reappeared. The reversibility of the process suggests that the sulfur head groups did not rearrange under any of the tunneling current conditions. On the basis of our observations, which are inconsistent with the standard model for STM imaging of molecules on metal surfaces, we consider the STM imaging mechanism in terms of a two-region tunneling junction model.
The Bulletin of the Korean Chemical Society , 2011
We investigated the effects of tunneling current on scanning tunneling microscopy (STM) images of 1-octanethiol (OT) and 1-decanethiol (DT) self-assembled monolayers (SAMs). At a low tunneling current, the domain boundaries and ordered alkanethiol molecules were clearly resolved. As the tunneling current was increased at a constant bias voltage, however, the STM images showed disordered structures of the OT and DT SAMs. As the tunneling current was reduced back to low values, the ordered structures of the alkanethiol molecules reappeared. The eversibility of the process suggests that the sulfur head groups did not rearrange under any of the tunneling current conditions. On the basis of our observations, which are inconsistent with the standard model for STM imaging of molecules on metal surfaces, we consider the STM imaging mechanism in terms of a two-region tunneling junction model.
Despite their ubiquity, self-assembled monolayers (SAMs) of thiols on coinage metals are difficult to study and are still not completely understood, particularly with respect to the nature of thiol−metal bonding. Recent advances in molecular electronics have highlighted this deficiency due to the sensitivity of tunneling charge-transport to the subtle differences in the overall composition of SAMs and the chemistry of their attachment to surfaces. These advances have also challenged assumptions about the spontaneous formation of covalent thiol−metal bonds. This paper describes a series of experiments that correlate changes in the physical properties of SAMs to photoelectron spectroscopy to unambiguously assign binding energies of noncovalent interactions to physisorbed disulfides. These disulfides can be converted to covalent metal−thiolate bonds by exposure to free thiols, leading to the remarkable observation of the total loss and recovery of length-dependent tunneling charge-transport. The identification and assignment of physisorbed disulfides solve a long-standing mystery and reveal new, dynamic properties in SAMs of thiols.
Journal of Vacuum Science & Technology A: Vacuum, Surfaces, and Films, 2005
X-ray photoelectron spectroscopy ͑XPS͒ is widely applied for the chemical characterization of surfaces and multilayers of thin films. In order to obtain quantitative results, XPS peak areas generally are divided by sensitivity factors and normalized to 100 at. % to obtain so-called raw concentrations. For homogeneous materials, materials with randomly distributed atoms within the analyzed surface layer, these concentrations may be a useful quantity. Yet, for a material consisting of a substrate on top of which a number of chemically different layers are present, the raw concentrations depend on measuring details like the takeoff angle during the XPS analyses and clearly are not a satisfactory way to describe the sample. The main purpose of this article is to present a calculation method that converts raw concentrations into more meaningful quantities. The method is applicable to a restricted but technologically relevant class of samples: substrates on top of which one or more homogeneous layers are present. Examples are: gate dielectrics on Si or GaAs, self-assembling monolayers on a metallic substrate, thin oxide films on metals with an organic contamination on top. The method is based upon standard exponential attenuation of the photoelectron intensity as a function of traveled distance. For each element or chemical state in the system it has to be known to which layer͑s͒ it belongs. Sensitivity factors are corrected for matrix effects and for intrinsic excitations. Starting from the raw concentrations, the method calculates in a self-consistent way the composition of all layers in the system and the thickness of each layer. Only one measurement at one measuring angle is required to obtain these results. To obtain insight into the accuracy of the calculation method, XPS results obtained on ultrathin SiO 2 layers on Si that were slightly contaminated with hydrocarbons have been analyzed with the method. The obtained thicknesses were in good agreement with values for the thickness of the SiO 2 layer and the organic surface contamination as obtained by other methods. Consistent values were also obtained for the concentration ratio O / Si in the SiO 2 layers. The calculation method has also been verified for three types of self-assembled monolayers ͑SAM layers͒ on gold. Layers of C18 ͑octadecane-thiol͒ and of EG4 ͑a mercaptoalkyloligo-ethyleneglycol͒ deposited from solutions with different concentrations were examined. Also, SAM layers deposited from mixtures with molecules with different chain lengths, mercapto-undecanol ͑MUO͒, and a biotinylated oligo-ethyleneglycol-alkyl thiol ͑BAT͒, were investigated. The model analysis provided the thickness of the organic layers, the concentrations of the components in the layers, and the coverage of the gold with sulphur ͑in atoms/ cm 2 ͒. Rutherford backscattering spectrometry ͑RBS͒ was applied to determine ͑in an independent way͒ the amount of sulphur at the gold surface. The RBS results correlated well with the XPS data. The obtained values for the concentration ratios of the SAM layers were in agreement with the theoretically expected values. It is shown in the article that it is essential to model the mixtures of MUO and BAT as a three-layer system ͑gold substrate, aliphatic interlayer, and top layer containing the ethylene oxide groups͒ in order to obtain agreement.