Subscriber access provided by UNIV OF CALIFORNIA SAN DIEGO LIBRARIES
Article
Passivation of p-Si(100) by covalent grafting of tethered homopolymer brushes Junhong Zhang, and Ming Li Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.6b00343 • Publication Date (Web): 29 Mar 2016 Downloaded from http://pubs.acs.org on April 1, 2016
Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.
Langmuir is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.
Page 1 of 36
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Langmuir
Covalent grafting of tethered homopolymer film on p-Si(100) Junhong Zhang, Ming Li* State Key Laboratory of Metal Matrix Composites, School of Material Science and Engineering, Shanghai Jiao Tong University, No. 800 Dongchuan Rd., Shanghai 200240, China.
KEYWORDS: PMMA, electrografting, pulse voltage, diazonium salt, Si (100)
ABSTRACT: Covalent grafting of homopolymer film onto p-Si(100) surface was performed by the use of surface electroinitiated emulsion polymerization. This polymerization method was one-step and cost-effective, that worked in the aqueous dispersed media containing both nitrobenzenediazonium (NBD) tetrafluoroborate and methyl methacrylate (MMA) monomer. NBD was reduced in the vicinity of substrate at cut-on voltage of pulsed voltage and successive grafted on Si surface, leading to polynitrophenyl (PNP) layer. The reduction of NBD simultaneously initiated the polymerization of MMA. The resulting PMMA macroradicals in the solution can reacted with the previously grafted PNP at cut-off voltage of pulsed voltage for thickening the polymer film. The prepared polymer layer completely covered the Si surface as investigated by the scanning electron microscopy (SEM) and atomic force microscopy (AFM) technique. Importantly, chemical components and the thickness of the grafted polymer film were well controlled by the applied voltage and the charge 1 ACS Paragon Plus Environment
Langmuir
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Page 2 of 36
flow passing through the Si electrode. X-ray photoelectron spectroscopy (XPS) measurement further confirmed the Si-C bond between polymer layer and Si substrate. The homopolymer film was extremely effective in protecting the underlying Si from atmospheric oxidation.
1. INTRODUCTION
The preparation of organic layer on silicon (Si) surface is of great importance for future applications in sensors, new devices, microelectronics, and biology 1, 2. Organic layers can be deposited on surfaces by different methods, such as plasma deposition 3, 4
, spin coating
electrografitng
5, 6
, self-assembly
7, 8
, atom transfer radical polymerization
9, 10
and
11, 12
. Among these methods, it is a key point for the properties of the
final interfaces to form a strong covalent bond between organic molecules and Si surface. However, the direct covalent bonding organic molecule onto Si surface is not simple as for metals. Firstly, Si is covalent material, so the covalent bond between organic molecule and Si surface atom is rigid and directional. The maximum attainable molecule at the interface thereby is relative to the lattice of Si. Secondly, the Si surface is originally covered by oxide unless one works in the ultra high vacuum (UHV). Therefore, a direct molecular attachment implies the substitution of the oxygen atom, but it is difficult because of the high binding energy of Si-O. In order to easily attach molecule, H-terminated Si crystal surfaces was usually prepared because the binding energy of Si-H is weaker 13, 14. Many studies concerned with achievement of covalently attached organic layer 2 ACS Paragon Plus Environment
Page 3 of 36
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Langmuir
onto H-terminated Si surface. To date, such covalently anchored polymer brush is generally prepared using the “grafting from” 15, 16, and “grafting to” techniques 17. The “grafting from” approach provides polymer brushes with high grafting densities and allows for a common mismatch between the brushes 18, 19. In contrast, the “grafting to” approach only tethers end-functionalized polymer chains to a surface under appropriate conditions 20. In this technology, polymer molecules must diffuse through an existing attached polymer layer to reach the reactive sites. Steric hindrance for surface attachment thus formed increases as the thickness of the polymer film increases. Therefore, the polymer layer prepared by “grafting to” approach is thinner and has low grafting density. In addition, the two approaches generally require linkers between surface and polymer chains. In the “grafting from” situation, an initiator molecule is first assembled directly onto the Si surface, and used to initiate the free radical or living polymerization, such as ATRP 21, 22, ROP or ROMP 23, 24. By contrast, a silane or other functional moiety is usually used as linker in the “grafting to” approach
25, 26
. Both methodologies are effective in producing polymer brush on Si
surface. However, they are anyway two-step processes, and require for higher monomer concentration, longer polymerization time, heating and synthesis of appropriate tethering functionality. That limits their use in industrial applications, and is a variety of reasons favoring single-step in-situ process. Efforts should be explored to graft polymer onto the H-terminated Si surface in a simple process. Surface electroinitiated emulsion polymerization is a new and one-pot method, which works in aqueous media
27
. It can chemically bond the copolymer, 3
ACS Paragon Plus Environment
Langmuir
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Page 4 of 36
composed of purely polynitrophenlene (PNP) in the inner part of the layer and purely vinylic in the outer part, to any conducting and semiconducting substrate. The first PNP layer is used as a linker of the substrate and vinylic polymer. Compared with conventionally grafting process, in this method, the precursor and tethered polymer brush were simultaneously grafted in the one solution, which via the “grafting from” and “grafting to” mechanism, respectively. It has been used to functionalize gold stainless steel
29
and multi-walled carbon nanotubes
28
,
30
, but has not been used to
immobilize polymer film on Si surface. In this paper, we reported that homopolymer film was successfully grafted on p-Si(100) via surface electroinitiated emulsion polymerization method. The polymer layer was uniform, smooth and thickness-controllable. It was composed of PNP and poly(methyl methacrylate) (PMMA) and its chemical composition was homogeneous throughout the entire film, which was different with the copolymer film observed in the previously literature
29
. Importantly, the chemical components of homopolymer
slightly changed with the thickening of the film, which characterized by attenuated total reflection-infrared radiation (ATR-IR) and X-ray photoelectron spectroscopy (XPS). Furthermore, we also observed that PMMA components of copolymer were strongly dependent on the applied pulsed voltage.
2. EXPERIMENTAL SECTION
2.1 Substrate treatment Si substrate was cut into 1×2 cm2 pieces from 0.1 Ω·cm p-type Si (100) wafers. 4 ACS Paragon Plus Environment
Page 5 of 36
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Langmuir
The substrate was then ultrasonically cleaned for 5 min in succession with acetone, ethanol, and water. After that, the cleaned substrate was etched with 3% hydrofluoric acid solution, yielding a fully hydride-terminated Si surface. 2.2 Emulsions preparation Methyl
methacrylate
(MMA)
and
4-nitrobenzene
diazonium
(NBD)
tetrafluoroborate were used as received. A master acidic solution at 0.61 M (PH
