Aldehydes react with scribed silicon to form alkyl monolayers: Characterization by ToF-SIMS suggests an even–odd effect

doi:10.1016/j.apsusc.2004.03.078

Applied Surface Science

Volumes 231–232, 15 June 2004, Pages 323-327

https://doi.org/10.1016/j.apsusc.2004.03.078 Get rights and content

Abstract

Alkyl monolayers are formed when silicon is chemomechanically scribed in the presence of aldehydes (from butanal to nonanal). X-ray photoelectron spectroscopy (XPS), wetting, and time-of-flight secondary ion mass spectrometry (ToF-SIMS) suggest increasingly thick and hydrophobic monolayers with increasing aldehyde chain length. Superimposed on the general trend of stronger ToF-SIMS signals for hydrocarbon fragments from longer aldehyde precursors is an even–odd effect. This effect is most pronounced for smaller (one- and two-carbon) hydrocarbon fragments and for monolayers prepared with shorter aldehyde precursors. This is the first time than an even–odd effect has been demonstrated for monolayers on scribed silicon.

Introduction

Chemomechanical surface modification has recently been developed as an effective method for simultaneously functionalizing and patterning silicon. This method consists of wetting a silicon shard with a reactive compound and then mechanically scribing it. Scribing activates silicon so that it reacts with 1-alkenes [1], [2], 1-alkynes [1], 1-haloalkanes [3], alcohols [2], and epoxides [4]. Chemomechanical surface modification takes place conveniently in the air with compounds that have not been degassed. Twenty micrometer wide and 2 nm deep functionalized features can be made by scribing with a miniature tungsten carbide ball [5], and 30 nm wide features can be made with an AFM tip [6].

Here we show that aldehydes react chemomechanically with scribed silicon to produce monolayers. Aldehydes are known to react dissociatively with Si(1 1 1)-7×7 [7], [8] and they have been predicted to add to the silicon dimers on Si(1 0 0)-2×1 by cycloaddition to produce four-membered rings [9] (see Fig. 1). Si(1 0 0)-2×1, Si(1 1 1)-7×7 and scribed silicon appear to react similarly, so the former surfaces are considered to be good models for the latter. Aldehydes also form monolayers on hydrogen-terminated porous and planar silicon [10], [11]. X-ray photoelectron spectroscopy (XPS) and wetting provide evidence for monolayer formation from aldehydes on scribed silicon. Time-of-flight secondary ion mass spectrometry (ToF-SIMS), which has played an important role in characterizing other chemomechanically functionalized silicon surfaces [2], [3], [4], [5], [6], supports and helps clarify XPS and wetting results, and further provides evidence for an even–odd effect in the structure of these monolayers. The significance of an even–off effect in that it implies a degree of homogeneity/ordering in these monolayers. This report contains the first observation of an even–odd effect in monolayers on scribed silicon. We are only aware of one other SIMS study of monolayers (thiols on gold) where evidence for an even–odd effect has been observed [12].

Section snippets

Materials

Butanal (95%), hexanal (98%), heptanal (95%), octanal (99%), and nonanal (95%) were obtained from Aldrich. Pentanal (97%) was purchased from Fluka. Acetone was reagent grade, and water was from a Millipore Milli-Q water system. Sodium dodecyl sulfate (NF grade) was obtained from Columbus Chemical Industries. All chemicals were used as received. Silicon (1 0 0) wafers (p-boron, 0–100 Ω cm, test grade) were obtained from TTI Silicon (Sunnyvale, CA).

Sample preparation

Silicon shards were cleaned as described previously

Results and discussion

Fig. 2 shows XPS and wetting results for silicon that was scribed under the homologous series of aldehydes from butanal to nonanal. For each reactive compound XPS shows monolayer quantities of chemisorbed carbon, which steadily increase as the alkyl chain length of the aldehyde increases. Interestingly, the C 1s narrow scans (not shown) do not appear to contain chemically shifted carbon, which is indicative of CO or CO bonds, i.e., each carbonoxygen bond to carbon typically shifts the C 1s

Acknowledgements

Sincere thanks to the Petroleum Research Fund and to the Research Corporation for support of this research.

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Aldehydes react with scribed silicon to form alkyl monolayers: Characterization by ToF-SIMS suggests an even–odd effect

Abstract

Introduction

Section snippets

Materials

Sample preparation

Results and discussion

Acknowledgements

Langmuir

Angew. Chem., Int. Ed. Engl.

Chem. Mater.

Angew. Chem., Int. Ed. Engl.

Langmuir

Appl. Phys. Lett.

J. Phys. Chem. B

J. Phys. Chem.