TY - JOUR
T1 - Self-assembly of alkanethiolates directs sulfur bonding with GaAs(100)
AU - Mancheno-Posso, Pablo
AU - Muscat, Anthony J.
N1 - Funding Information:
The authors are grateful to Intel Corporation for funding and to Sanaz Gardner, Scott B. Clendenning, and Mansour Moinpour for valuable discussions. The authors thank Yissel Contreras for acquiring and analyzing the AFM images.
Publisher Copyright:
© 2016 Elsevier B.V.
PY - 2017/3/1
Y1 - 2017/3/1
N2 - Molecules that contain linear alkane chains self-assemble on a variety of surfaces changing the degree of wetting, lubricity, and reactivity. We report on the reoxidation of GaAs(100) in air after adsorbing five alkanethiols (C n H 2n+1 -SH where n = 3, 6, 12, 18, 20) and one alkanedithiol (HS-(CH 2 ) 8 -SH) deposited from the liquid phase. The alignment of the alkane chains forms a self-assembled layer, however, air diffuses readily through the carbon layer and reaches the surface. The impact of alignment is to improve the bonding of sulfur with the surface atoms which reduces the oxidation rate based on fitting the data to a reaction-diffusion model. The layer thickness and molecular density scale linearly with the number of carbon atoms in the alkane chain. The thickness of the alkanethiolate (RS − ) layer grows by 0.87 ± 0.06 Å for each C atom in the chain and the surface density by 0.13 ± 0.03 molecule per nm 2 per C atom up to a coverage of 5.0 molecules/nm 2 for n = 20 or 0.8 monolayer. The surface coverage increases with length because interactions between methylene (CH 2 ) groups in neighboring chains reduce the tilt angle of the molecules with the surface normal. The tight packing yields areas per alkanethiolate as low as 20 Å 2 for n = 20. The amount of C in the layer divided by the chain length is approximately constant up to n = 12 but increases sharply by a factor of 2–4× for n = 18 and 20 based on the C 1s X-ray photoelectron spectroscopy (XPS) peak. Fourier transform infrared (FTIR) spectroscopy shows that the asymmetric methylene stretch shifts continuously to lower wavenumber and the relative peak area increases sharply with the length of the alkane chain. Fitting the data to a reaction-diffusion model shows that for times less than 30 min the surface oxide coverage does not depend on the thickness of the self-assembled layer nor the diffusivity of oxygen through the layer. Instead increasing the layer thickness makes more S available for bonding with the predominately As termination and reduces the rate coefficient for reaction of oxygen with the GaAs surface.
AB - Molecules that contain linear alkane chains self-assemble on a variety of surfaces changing the degree of wetting, lubricity, and reactivity. We report on the reoxidation of GaAs(100) in air after adsorbing five alkanethiols (C n H 2n+1 -SH where n = 3, 6, 12, 18, 20) and one alkanedithiol (HS-(CH 2 ) 8 -SH) deposited from the liquid phase. The alignment of the alkane chains forms a self-assembled layer, however, air diffuses readily through the carbon layer and reaches the surface. The impact of alignment is to improve the bonding of sulfur with the surface atoms which reduces the oxidation rate based on fitting the data to a reaction-diffusion model. The layer thickness and molecular density scale linearly with the number of carbon atoms in the alkane chain. The thickness of the alkanethiolate (RS − ) layer grows by 0.87 ± 0.06 Å for each C atom in the chain and the surface density by 0.13 ± 0.03 molecule per nm 2 per C atom up to a coverage of 5.0 molecules/nm 2 for n = 20 or 0.8 monolayer. The surface coverage increases with length because interactions between methylene (CH 2 ) groups in neighboring chains reduce the tilt angle of the molecules with the surface normal. The tight packing yields areas per alkanethiolate as low as 20 Å 2 for n = 20. The amount of C in the layer divided by the chain length is approximately constant up to n = 12 but increases sharply by a factor of 2–4× for n = 18 and 20 based on the C 1s X-ray photoelectron spectroscopy (XPS) peak. Fourier transform infrared (FTIR) spectroscopy shows that the asymmetric methylene stretch shifts continuously to lower wavenumber and the relative peak area increases sharply with the length of the alkane chain. Fitting the data to a reaction-diffusion model shows that for times less than 30 min the surface oxide coverage does not depend on the thickness of the self-assembled layer nor the diffusivity of oxygen through the layer. Instead increasing the layer thickness makes more S available for bonding with the predominately As termination and reduces the rate coefficient for reaction of oxygen with the GaAs surface.
KW - Alkanethiol
KW - GaAs(100)
KW - Liquid phase
KW - Passivation
KW - Self-assembled monolayer
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U2 - 10.1016/j.apsusc.2016.11.016
DO - 10.1016/j.apsusc.2016.11.016
M3 - Article
AN - SCOPUS:84998865157
SN - 0169-4332
VL - 397
SP - 1
EP - 12
JO - Applied Surface Science
JF - Applied Surface Science
ER -