Self-assembly of alkanethiolates directs sulfur bonding with GaAs(100)

Pablo Mancheno-Posso, Anthony J Muscat

Research output: Contribution to journalArticle

7 Citations (Scopus)

Abstract

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 (CnH2n+1-SH where n = 3, 6, 12, 18, 20) and one alkanedithiol (HS-(CH2)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 nm2 per C atom up to a coverage of 5.0 molecules/nm2 for n = 20 or 0.8 monolayer. The surface coverage increases with length because interactions between methylene (CH2) 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 Å2 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.

Original languageEnglish (US)
Pages (from-to)1-12
Number of pages12
JournalApplied Surface Science
Volume397
DOIs
StatePublished - Mar 1 2017

Fingerprint

Sulfur
Self assembly
Alkanes
Paraffins
Atoms
Carbon
Oxygen
Molecules
gallium arsenide
Air
Chain length
Oxides
Fourier transform infrared spectroscopy
Wetting
Monolayers
X ray photoelectron spectroscopy
Oxidation
Liquids

Keywords

  • Alkanethiol
  • GaAs(100)
  • Liquid phase
  • Passivation
  • Self-assembled monolayer

ASJC Scopus subject areas

  • Surfaces, Coatings and Films

Cite this

Self-assembly of alkanethiolates directs sulfur bonding with GaAs(100). / Mancheno-Posso, Pablo; Muscat, Anthony J.

In: Applied Surface Science, Vol. 397, 01.03.2017, p. 1-12.

Research output: Contribution to journalArticle

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abstract = "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 (CnH2n+1-SH where n = 3, 6, 12, 18, 20) and one alkanedithiol (HS-(CH2)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 {\AA} for each C atom in the chain and the surface density by 0.13 ± 0.03 molecule per nm2 per C atom up to a coverage of 5.0 molecules/nm2 for n = 20 or 0.8 monolayer. The surface coverage increases with length because interactions between methylene (CH2) 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 {\AA}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.",
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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 (CnH2n+1-SH where n = 3, 6, 12, 18, 20) and one alkanedithiol (HS-(CH2)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 nm2 per C atom up to a coverage of 5.0 molecules/nm2 for n = 20 or 0.8 monolayer. The surface coverage increases with length because interactions between methylene (CH2) 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 Å2 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.

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KW - Alkanethiol

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KW - Passivation

KW - Self-assembled monolayer

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