Effects of molecular structure and interfacial ligation on the precision of Cu-bound α,ω-mercaptoalkanoic acid "molecular ruler" stacks

Langmuir

Volumn 23, Issue 2, 2007, Pages 638-648

  Daniel, Thomas A ^a   Uppili, Sundarajan ^a   McCarty, Gregory ^a,b   Allara, David L ^a  

^a PENNSYLVANIA STATE UNIVERSITY   (United States)

^b NORTH CAROLINA STATE UNIVERSITY   (United States)

Author keywords

[No Author keywords available]

Indexed keywords

ADLAYERS; MOLECULAR LAYER STRUCTURES; NANOSCIENCE; THIOL GROUPS;

COPPER; LITHOGRAPHY; MOLECULAR STRUCTURE; NANOTECHNOLOGY; SURFACE ROUGHNESS; THIN FILMS;

ORGANIC ACIDS;

EID: 33846885688     PISSN: 07437463     EISSN: None     Source Type: Journal    
DOI: 10.1021/la0621719     Document Type: Article

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59. An average of 5% of the original intensity often remainsm and this reduced intensity does not significantly decrease with longer copper solution deposition times. The remaining signal is attributed to a small fraction of the reactive carboxylic acid sites at the interface being protected by hexadecanethiol molecules physically blocking the path to the acid group.
60. With deposition times above 2 h, the presence of free thiol at 164 eV can sometimes be seen in the sulfur XPS spectra, an indication of significant amounts of physisorbed hexadecanethiol on the surface.
61. We also note that with longer solution deposition times (up to 24 h) impurity signatures increasingly begin to appear in the IR spectra which can be associated with adsorption and trapping of organic materials such as solvent and ambient contaminants.
62. 33 is 21.3 Å; for 16-mercaptohexadecanoic acid it is 22.3 Å.
63. It is possible that some of the adlayer did not react with the copper, thus forming an incomplete adlayer; however, longer deposition times did not reduce these peaks significantly.
64. Without knowing the surface density of these adsorbates and the problems with the overlapping CH wagging modes, it is difficult to determine an accurate orientation.
65. 10 Our experimental methods do not allow any definitive conclusion regarding further addition, but if it did occur, as expected, it would contribute to "roughening" of the multilayer structure by formation of islands of chains at the terminal layer of the multilayer stack with deterioration of the "molecular ruler" characteristics.
66. We were unable to determine the amount of Cu(II) and S in the MHA|MHA|MH film relative to the MHA|MHA bilayer due to the complexity of the trilayer structure in terms of modeling the attenuation of the photoelectron fluxes. Also, X-ray beam induced Cu(II) autoreduction made the Cu analysis somewhat problematic.
67. Unfortunately, with the densified trilayer sample point the overlayer thickness becomes too large to allow useful interpretations of the XPS Au-S S 2p core level spectra. This can be seen in Figure 17, spectrum J, where the Cu-S species dominate with only a shoulder of the Au-S observable.