Simultaneous Control of Surface Chemistry and Nanoscale Topography on the Si(100) Surface

Madeleine Beasley and K. T. Queeney

Department of Chemistry, Smith College, Northampton MA 01063

Introduction

Acknowledgments

Conclusions

Surface Hydride Species

Hydrosilylation

Multifunctionalization

Henry Dreyfus Teacher Scholar Award from the Camille and Henry Dreyfus Foundation, Nancy Kershaw Tomlinson Memorial Fund

{110}{110

} {110}{110

}

(100) (100)

2071

2082

2088

2109

2115

2134

2000

1 x 10-4

Frequency (cm-1)

Abs

orba

nce

Hydride Species

Frequency (cm-1)

Face Si-H Stretch Vibration

Mono

Mono

Mono

Di

Di

Di

2071

2082

2088

2109

2115

2134

{110}

{110}

{110}

(100)

(100)

(100)

antisymmetric

strained

symmetricUnstrained,

antisymmetricstrained

strained

10471166 2088 2250

3 hr.

2 hr.

1 hr.

4 x 10-4 2 x 10-4

Frequency (cm-1)

Abs

orba

nce

900

2070 2080 2090 2100 2110

2088

5 x 10-4

Abs

orba

nce 40:20 Min.

60:40 Min.

60:20 Min.

Frequency (cm-1)

Figure 1. Confocal laser scanning microsope images showing micro-organism behavior on a variety of surface chemistries and topographies.1

Figure 2. a) Molecular model of a {110}-faceted hillock. Unstrained {110} monohydrides are shown in pink, strained {110} monohydrides are shown in orange,

and (100) dihydrides are shown in green.2

b) AFM image (tapping mode) of an H-terminatedSi(100) surface after etching in deoxygenated water to

prduce the surface modeled above.

1 x 10-3

2800 2900 3000

Abs

orba

nce

Figure 3. FTIR spectrum showing all hydride species present on initial H-terminated surface. Details for each are illustrated in the table above.2

Figure 4. FTIR difference spectra showing the oxidation preference for the {110} unstrained monohydride with a corresponding frequency at 2088 cm-1.

1. J. Zhang, J. Huang, C. Say, M. Beasley, R. Gerdes, Rob Dorit and K.T. Queeney (In Preparation).2. Aldinger, B.S.; Hines, M.A. J. Phys. Chem. C. 2012, 116, 21499-21507. 3. Faggin, M.F.; Green, S.K.; Clark, I.T.; Queeney, K.T.; Hines, M.A.J. Am. Chem. Soc. 2006, 128, 11455-11462.4. Linford, M.R.; Chidsey, C.E.D. J. Am. Chem. Soc. 1993, 115, 12631-12632.

Figure 5. FTIR showing the replacement of SiH bonds with SiO bonds through an O3SiH intermediate.

Figure 9. Evidence for multifunctionalization of a surface with oxidation and hydrosilylation. Figure 8. Comparison of CHx stretch region for the multifunctionalized

surface to reference hydrosilylated surfaces.

Initial H-terminated Surface

Site-Selective Oxidation

Figure 6. CHx stretch region of a completely hydrosilylated flat surface. Figure 7. Comparison of hydrosilylation on a rough (water-etched), H-terminated surface withthe same reaction on a fully oxidized (SiO2) surface.

Previous work3 has demonstrated that anH-terminated Si(100) surface covered withregular, nanoscale (~50-100 nm) hillocks can be generated by etching flatH-terminated surfaces in deoxygenated water. While these features can be seen directly with scanning probe techniques, the best insight into theprecise surface termination of the hillocks is provided by surface infraredspectroscopy, which is exquisitely sensitive both to different silicon hydridespecies and their local chemical environments.

2800 2850 2900 2950 3000

H Reference

Alkylated

CH3stretch

asymmetricCH2 stretch

symmetricCH2 stretch

ν(CH)

Frequency (cm-1)

5 x 10-4

Abs

orba

nce

asymmetricCH2 stretch

symmetricCH3 stretch

symmetricCH2 stretch

5 x 10-4

Abs

orba

nce

Alkylated

H Reference

ν(CHx)

Frequency (cm-1)

2800 2850 2900 2950 3000

5 x 10-4

Abs

orba

nce

2750 2800 2850 2900 2950 3000 3050

ν(CH)

asymmetricCH2 stretch

Frequency (cm-1)

Rough

SC-2

5 x 10-4

asymmetricCH2 stretch

Abs

orba

nce

ν(CHx)

Frequency (cm-1)

Rough

Fully oxidized

ν(SiOx)

5 x 10-4

2800 2850 2900 2950 3000

Frequency (cm-1)

5 x 10-4

Abs

orba

nce

ν(CH)

Frequency (cm-1)

Abs

orba

nce

5 x 10-4

ν(CHx)

2800

2800 2900 3000 900 1000 1100 1200 1300

Rough

Fully oxidized

Fully oxidized,no alkylation

Frequency (cm-1)

900 1000 130012001100Frequency (cm-1)

5 x 10-4

ν(CHx) ν(SiOx)

partially oxidized

Multi-functionalized

Alkylated

a)

b)

H H H H H

Initial Surface

H H HH H H H H H H

HH HHHH HH

Rough, H-Terminated Surface24-hr Etch in

Ar-purged H2O

An extensive body of workhas demonstrated thatcells and micro-organismsbehave differently on surfaceswith nanoscale (<100 nm)topography than they do on flat surfaces or on substrateswith conventional-scalefeatures. Recent work in our lab has demonstratedthe combined effects ofsurface chemistry andnanoscale topography on nucleation of biofilms of Pseudomonas aeruginosa.

The goal of the current work is to expand our ability to control thecombined effects of surface topography and chemistry by developinga method to pattern both chemistry and topography simultaneouslyon the nanoscale. To do this we will exploit naturally-occuring chemistrythat creates nanoscale topography on Si(100) and then explore whether the resulting surface allows site-specific chemistry that will promotenanoscale chemical patterning without the need for lithography.

Transmission FTIR reveals the characteristicSi-H stretching modes of the hillock-coveredsurface. Notably, this surface is reproduciblycreated by a room-temperature etchingprocess. Work by Hines and coworkers2

has correlated the distinct Si-H peakfrequencies with surface species that areattributed to the crystallographic planes thatform the sides and tops of the hillock featuresthat dominate this surface after long (~24-hr)etch times.

Our goal was to determine whether we could selectively react (in this case, oxidize)either the sides or tops of the hillocks preferentially, leaving the remaining face(s) H-terminated for susbequent hydrosilylation. As shown in Figure 4, early stages ofcontrolled oxidation lead to preferential removal of monohydride species associatedwith the sides of the hillocks, suggesting that the bulk of surface oxidation shown inFigure 5 is confined to hillock sides as we had hoped.

Thermally-induced hydrosilylationby reaction with 1-octadecene4 has been widely used to form close-packedalkyl layers on flat, hydrogen-terminated Si surfaces. Figure 6shows the resulting IR spectrumof such a monolayer on flatH:Si(100) in our lab. As shown in Figure 8,this same hydrosilylation reaction has been used successfully in our lab to generate close-packed alkyl layers on the surfaces with nanoscale hillocks.

For our goal of nanoscale chemical patterning to be achieved, this hydrosilylation reaction needs to be much less favorable on the pre-oxidizedhillock sides than on the unoxidized SiHx sites on the hillock tops. Figure 8 shows that, in fact, hydrosilylation will occur on fully oxidized sites, presumablyby reaction with surface silanols. However, this reaction is less extensive than on the unoxidized surface and results in a less well-packed monolayer, as indicated by the asymmetric methylene stretch frequency. Analysis of the Si-O stretching region shows that hydrosilylation of oxidized sites ischaracterized by growth of a ν(Si-O-C) band around 1100 cm-1.

Multifunctionalized surfaces were attempted by partial (1 hour) oxidation to preferentially oxidize sites on the hillock sides, followed by short (15 minutes) thermal hydrosilylation to minimize the less-favorable reaction with oxidized sites. Analysis of the CH stretching region of IR spectra of the resulting surfacesreveals that the alkyl layers thus formed are close packed, which suggests they occur predominantly on regions of extended H termination and not onoxidized areas. The Si-O region of the same IR spectra does not show evidence of extensive Si-O-C bond formation, which is also consistent with a lackof hydrosilylation on oxidized regions.

We have strong evidence that we have successfully carried out spatially differentiated chemistry on the nanoscale on a surface with nanoscale topography.Additional experiments (contact angle goniometry, atomic force microscopy) will be used to probe this reaction in more detail. We are also interested inexploring the possibility of reacting the oxidized sites (e.g. via silanization) to introduce greater variety of chemical functionalization and perhaps to moreeffectively hinder subsequent reaction at oxidized sites.

The ability to pattern surfaces with both topography and chemistry on length scales of 50-100 nm opens up new possibilities for surfaces that can betailored to enhance (or inhibit) cell and/or micro-organism attachment and growth.

Alkylated

Multifunctionalized

Multi-functionalized

Fully oxidized,hydrosilylated

Fully hydrosilylated

2050 2100 2150 2200

1000 1100 1200 1300

2850 2900 2950 3000

Rough, H-Terminated Surface

H H H H H H

HH HHHH HH

Partially Oxidized Surface Multifunctionalized Surface

2000 2100 2200 2300