Structure and Order of Phosphonic Acid-Based Self-Assembled Monolayers on Si(100)

Abstract

Organophosphonic acid self-assembled monolayers (SAMs) on oxide surfaces have recently seen increased use in electrical and biological sensor applications. The reliability and reproducibility of these sensors require good molecular organization in these SAMs. In this regard, packing, order and alignment in the SAMs is important, as it influences the electron transport measurements. In this study, we examine the order of hydroxyl- and methyl- terminated phosphonate films deposited onto silicon oxide surfaces by the tethering by aggregation and growth method using complementary, state-of-art surface characterization tools. Near edge x-ray absorption fine structure (NEXAFS) spectroscopy and in situ sum frequency generation (SFG) spectroscopy are used to study the order of the phosphonate SAMs in vacuum and under aqueous conditions, respectively. X-ray photoelectron spectroscopy and time of flight secondary ion mass spectrometry results show that these samples form chemically intact monolayer phosphonate films. NEXAFS and SFG spectroscopy showed that molecular order exists in the octadecylphosphonic acid and 11-hydroxyundecylphosphonic acid SAMs. The chain tilt angles in these SAMs were approximately 37° and 45°, respectively.

An Organophosphonate Strategy for Functionalizing Silicon Photonic Biosensors

“…As compared to previous organophosphonate SAM studies, the relative percentage of phosphorous observed is lower than the reported value (0.8 atom %). 12 However, this result was expected, as we adopted an expedient one-cycle SAM deposition approach, instead of three cycles as previously reported. Using the “T-BAG” method, the organophosphonic acid-adsorbed silicon substrates must be heated in order to covalently deposit onto silicon oxide; 37 however, the repeated thermal cycling may cause functional loss of silicon waveguide-based biosensors.…”

“…9,11 Furthermore, formation of multi-layer silane networks attenuates the sensitivity and reduces the stability of functional surfaces for biosensing. 12 Therefore, more robust surface functionalization strategies could result in stable and reliable silicon-based biosensors. Recently, organophosphonate self-assembled monolayers (SAMs) have been employed successfully to modify various inorganic oxide surfaces, such as Al 2 O 3 13 , TiO 2 14 and SiO 2 15 .…”

Alternative Methods of Silicon Porosification and Properties of These PSi Layers

“…These organophosphonates have superior physicochemical properties. Relative to silanes, phosphonate layers can form densely packed monolayers with higher surface coverage (Cattani-Scholz et al 2009) and are much more stable in both acidic and alkaline solutions (Silverman et al 2005;Hoque et al 2006;Dubey et al 2010) fabrication of complementary circuits and transistors (Klauk et al 2007;Zschieschang et al 2008), modification of DNA biosensors (Cattani-Scholz et al 2009), and preparation of cell adhesion substrates (Midwood et al 2004;Adden et al 2006;Luo et al 2008). However, to the best of our knowledge there is no example in the literature of such PSi functionalization, despite the huge potential of such functionalization.…”

Spin‐Cast and Patterned Organophosphonate Self‐Assembled Monolayer Dielectrics on Metal‐Oxide‐Activated Si

“…[5, 10] In order to form stable P-O-Si phosphonates on SiO 2 , 48 hours of solid-state thermal annealing of PA films at 140 °C has been employed. [9, 16] This “long-term” processing may present a bottleneck for the integration of phosphonate SAMs into practical applications using SiO 2 substrates.…”

“…In addition, bound molecules are clearly present in spectra of SAMs from 2 as indicated by ion peaks of [ 2 -H] − calcd for C 18 H 38 PO 3 − , 333.26; found, 333.25, [ 2 +AlO 2 ] − calcd for AlC 18 H 39 PO 5 − , 393.24; found, 393.23, in good agreement with previous studies. [12,16] Similarly, for SAMs of 1 , ion peaks of bound alkyl chain fragments of [ 1 -(C 6 H 5 OC 11 H 22 )+AlH] − calcd for AlC 8 H 19 PO 4 − , 237.08; found, 237.09, and phenyl ring fragments of C 6 H 6 OC − , 106.04; found, 106.04, C 6 H 6 OCH − , 107.05; found, 107.05, C 6 H 6 H + , 79.05; found, 79.06, and C 6 H 6 OH + , 95.05; found, 95.05. FTIR spectra in the ν P-O region show that SAMs form a predominantly bidentate binding conformation to AlO x (Fig.…”