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The application of reflection infrared and surface enhanced Raman spectroscopy to the characterization of chemisorbed organic disulfides on Au
Journal of Electron Spectroscopy and Related Phenomena, 30 (1983) 11 Elsevier Scientific Publishing Company, Amsterdam -Printed in The Netherlands
11
THE APPLICATION OF REFLECTION INFRARED AND SURFACE RAMAN SPECTROSCOPY TO THE CHARACTERIZATION ORGANIC DISULFIDES ON Au
ENHANCED OF CHEMISORBED
D. L. Allara and R. G. Nuzzo Bell Laboratories,
Murray Hill, NJ 07974
ABSTRACT We have found that a variety of organic molecules containing the S-S linkage strongly adsorb on freshly evaporated or electrochemically cleaned gold surfaces. Adsorption stops at monolayer coverage and the packing density of the molecules can be close to the limit set by the size of the molecules. Reflection infrared spectra show that the films are strongly oriented and in the case of long alkyl-chain-substituted disulfides the films have structures similar to those of Langmuir-Blodgett type films. Most of the films were prepared by freshly evaporating gold onto polished single crystal silicon substrates. Immediately after the deposition the films were removed from the vacuum system (under N2) and placed in solutions of the disulfides. After rinsing the fiis were examined by ellipsometry at 6328A and 442OA and in general, the results are consistent with monolayer structures with close packing similar to bulk crystallites. Infrared spectra were obtained using a Digilab 15B Fourier transform infrared spectrometer with a high f/number optical system and small beam spot size for glancing reflection. Spectra were obtained between 800 and 32OOcm-1 using a mercury cadmium telluride detector. The spectrum of a monolayer of the di-p-nitrobenzoate ester of trans-o-Dithiane-4,5-diol is shown in Figure 1. The structure of the diol is given below. The spectrum of the monolayer of the di-ester of the Cl6 acid (n-C15H3tC02H) is given in Figure 2. Notice the alkyl chain mode band series in the monolayer spectrum which indicates crystalline packing of the chains. The infrared spectra are consistent with oriented structures which maximize packing densities. No direct evidence is yet available for defining a Au-S bond. However, a sample in which the monolayer of the p-nitrobenzoate derivative has been covered with 4OOA average thickness of CaF2 and 2OOA of smooth silver has yielded an enhanced Raman spectrum (h(excite)==4880& -5OmW power) in which the normally strong S-S stretching mode observed at 504cm-1 in bulk samples is absent in the monolayer. These studies demonstrate an application of surface vibrational spectroscopy to defining Work is presently underway to quantitatively calculate molecular structures at surfaces. molecular orientation from the infrared spectra and to more carefully determine the extent of any ordering in appropriate monolayers.
0 1983 Elsevier Scientific Publishing Company 036%2048/83/0000-0000/$03.00
11
THE APPLICATION OF REFLECTION INFRARED AND SURFACE RAMAN SPECTROSCOPY TO THE CHARACTERIZATION ORGANIC DISULFIDES ON Au
ENHANCED OF CHEMISORBED
D. L. Allara and R. G. Nuzzo Bell Laboratories,
Murray Hill, NJ 07974
ABSTRACT We have found that a variety of organic molecules containing the S-S linkage strongly adsorb on freshly evaporated or electrochemically cleaned gold surfaces. Adsorption stops at monolayer coverage and the packing density of the molecules can be close to the limit set by the size of the molecules. Reflection infrared spectra show that the films are strongly oriented and in the case of long alkyl-chain-substituted disulfides the films have structures similar to those of Langmuir-Blodgett type films. Most of the films were prepared by freshly evaporating gold onto polished single crystal silicon substrates. Immediately after the deposition the films were removed from the vacuum system (under N2) and placed in solutions of the disulfides. After rinsing the fiis were examined by ellipsometry at 6328A and 442OA and in general, the results are consistent with monolayer structures with close packing similar to bulk crystallites. Infrared spectra were obtained using a Digilab 15B Fourier transform infrared spectrometer with a high f/number optical system and small beam spot size for glancing reflection. Spectra were obtained between 800 and 32OOcm-1 using a mercury cadmium telluride detector. The spectrum of a monolayer of the di-p-nitrobenzoate ester of trans-o-Dithiane-4,5-diol is shown in Figure 1. The structure of the diol is given below. The spectrum of the monolayer of the di-ester of the Cl6 acid (n-C15H3tC02H) is given in Figure 2. Notice the alkyl chain mode band series in the monolayer spectrum which indicates crystalline packing of the chains. The infrared spectra are consistent with oriented structures which maximize packing densities. No direct evidence is yet available for defining a Au-S bond. However, a sample in which the monolayer of the p-nitrobenzoate derivative has been covered with 4OOA average thickness of CaF2 and 2OOA of smooth silver has yielded an enhanced Raman spectrum (h(excite)==4880& -5OmW power) in which the normally strong S-S stretching mode observed at 504cm-1 in bulk samples is absent in the monolayer. These studies demonstrate an application of surface vibrational spectroscopy to defining Work is presently underway to quantitatively calculate molecular structures at surfaces. molecular orientation from the infrared spectra and to more carefully determine the extent of any ordering in appropriate monolayers.
0 1983 Elsevier Scientific Publishing Company 036%2048/83/0000-0000/$03.00