Characterization of self-assembled organic films using differential charging in X-ray photoelectron spectroscopy (original) (raw)
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Surface Science, 2006
Combining functional organic self-assembled monolayers (SAMs) with conventional semiconductor materials is a key step in the development of integrated electronics-based devices. T-BAG (Tethering by Aggregation and Growth) has been shown to be a simple and reliable method to grow SAMs of alkylphosphonates on oxide surfaces. However, distinguishing SAMs from ultra-thin multilayers is a challenge for most conventional surface characterization techniques. Self-assembled films of octadecylphosphonic acid (ODPA) were deposited on oxide-covered silicon coupons, converted to the corresponding phosphonates, and characterized by high resolution angularly resolved X-ray photoelectron spectroscopy (XPS). It was our goal to distinguish among different bonding configurations for phosphorous in the phosphonate head groups, namely, mono-dentate, bi-dentate or tri-dentate interactions with the oxide surface, as well as to assess quantitatively the near-surface layer composition. We also present an innovative method that allows us to distinguish between monolayer and multilayer films of ODPA on silicon oxide surfaces. This method is based on differential surface charging effects in XPS. It was found that variation in the ODPA film thickness causes differential responses of various spectral characteristics to an electrical bias applied to the sample during XPS measurements. Both positive and negative applied biases were found to affect the carbon core-level (C1s) line-shape and intensity in the case of the multilayer ODPA film, whereas line-shapes and intensities of all XPS lines measured for the monolayer film were unaffected by the application of a dc bias in the ±30 V range.
Structure and Order of Phosphonic Acid-Based Self-Assembled Monolayers on Si(100)
Langmuir, 2010
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.
Response of Polyelectrolyte Layers to the SiO2 Substrate Charging As Probed by XPS
Langmuir, 2009
A single layer of the cationic polyelectrolyte poly(allyamine) hydrochloride (PAH) deposited, using the layerby-layer technique, on a silicon substrate containing 5 nm oxide layer is investigated by XPS while applying an external potential bias to the sample to control and manipulate the charge built-up on the oxide layer. Under application of a-10 V bias, the oxide layer is positively charged due to photoemission process, evidenced by the measured Si2p binding energy of 104.4 eV. Application of a +10 V bias attracts the low energy neutralizing electrons, stemming from a hot filament, and leads to a negatively charged oxide layer, also evidenced by the measured Si2p binding energy of 102.9 eV. The single polyelectrolyte overlayer also responds to this polarity change of the oxide layer underneath by displaying a somewhat larger shifts both in the C1s and N1s peaks. In addition to the shifts in the positions, the N1s peaks undergo a significant intensity depletion, mostly on the positively charged-N + component. We interpret this intensity depletion to be the result of reorientation of some of the dangling positively charged groups by moving toward the negatively charged oxide underlayer. To our knowledge this is the first time that a chemically specific response to an electrical stimuli is reported using XPS. A bilayer LbL film consisting of PAH and PSS, exhibits even a larger charging shift, but this time no intensity alteration is observed, most probably due to locking of theN + groups by the-SO 3 + counterions of the second layer.
ECS Transactions, 2007
The presented works prepares the ground for galvanic deposition of metallic overlayers on metal oxide surfaces functionalized by phosphonic acid monolayers. In view of this goal, it is necessary to control the most important factors of the electrodeposition process (e.g. flatness and uniformity of distribution). Because of these requirements, in this work, we study the morphology and the interaction between PO(OH) 2 -functionalities and an oxide surface. The analysis is carried out using complementary surface analysis techniques, like Atomic Force Microscopy (AFM) and X-ray Photoelectron Spectroscopy (XPS). We have succeeded in creating a blank surface with a RMS roughness of less than 1 nm and a hydroxyl fraction of 28%. The organic deposition was followed in time and it was observed that Volmer Weber multilayer growth was present. Also the influence of carbon contamination was characterized and a methodology is presented to correct the experimental data for this ambient contamination.
Surface and Interface Analysis, 2024
Strategies to deal with sample charging effects on X-ray photoelectron spectroscopy(XPS) spectra are presented. These strategies combine charge compensation (or lackof) via a flow of electrons and an electrical connection (or lack of) of samples to theground. Practical examples involving samples with a range of different electrical prop-erties, sample structure/composition and sensitivity to X-rays, illustrate the correla-tion between sample properties, measurement strategies, and the resulting XPS data.The most appropriate measurement strategy for a particular sample is also recom-mended. We highlight the crucial importance of appropriate XPS data acquisition toobtain a correct data interpretation
Self-assembled monolayers of phosphonates (SAMPs) of 11-hydroxyundecylphosphonic acid, 2,6-diphosphonoanthracene, 9,10-diphenyl-2,6-diphosphonoanthracene, and 10,10′-diphosphono-9,9′-bianthracene and a novel self-assembled organophosphonate duplex ensemble were synthesized on nanometer-thick SiO 2 -coated, highly doped silicon electrodes. The duplex ensemble was synthesized by first treating the SAMP prepared from an aromatic diphosphonic acid to form a titanium complex-terminated one; this was followed by addition of a second equivalent of the aromatic diphosphonic acid. SAMP homogeneity, roughness, and thickness were evaluated by AFM; SAMP film thickness and the structural contributions of each unit in the duplex were measured by X-ray reflection (XRR). The duplex was compared with the aliphatic and aromatic monolayer SAMPs to determine the effect of stacking on electrochemical properties; these were measured by impedance spectroscopy using aqueous electrolytes in the frequency range 20 Hz to 100 kHz, and data were analyzed using resistance−capacitance network based equivalent circuits. For the 11-hydroxyundecylphosphonate SAMP, C SAMP = 2.6 ± 0.2 μF/cm 2 , consistent with its measured layer thickness (ca. 1.1 nm). For the anthracene-based SAMPs, C SAMP = 6−10 μF/cm 2 , which is attributed primarily to a higher effective dielectric constant for the aromatic moieties (ε = 5−10) compared to the aliphatic one; impedance spectroscopy measured the additional capacitance of the second aromatic monolayer in the duplex (2ndSAMP) to be C Ti/2ndSAMP = 6.8 ± 0.7 μF/cm 2 , in series with the first.
Bonding Self-Assembled, Compact Organophosphonate Monolayers to the Native Oxide Surface of Silicon
Journal of the American Chemical Society, 2003
A new method is described to prepare strongly bonded, compact monolayer films of alkyl-or arylphosphonates on the native oxide surface of Si (SiO2/Si). This method is illustrated for octadecyl-and R-quarterthiophene-2-phosphonates. For both cases, AFM shows comprehensive coverage of the SiO2/Si surface. The thickness of the continuous film of 4TP/SiO2/Si was measured both by AFM and by X-ray reflectivity to be ca. 18 Å. Direct gravimetric analysis shows surface coverage by R-quarterthiophene-2phosphonate to be about 0.66 nmol/cm 2 , which corresponds to molecular packing in the film close to that of crystalline R-quarterthiophene. Coverage by octadecylphosphonate was ca. 0.90 nmol/cm 2 , corresponding to a cross-sectional area of about 18.5 Å 2 /molecule, consistent with close-packed alkyl chains.
Analytical Chemistry, 2007
An external bias is applied to two samples containing composite surface structures, while recording an XPS spectrum. Altering the polarity of the bias affects the extent of differential charging in domains that are chemically or electronically different to create a charge contrast. By utilizing this charge contrast, we show that two distinct silicon nitride and silicon oxynitride domains are present in one of the composite samples. Similarly, we use this technique to show that titanium oxide and silicon oxide domains exist as separate chemical entities in another composite sample. Figure 4. Ti 2p, Si 2p, and O 1s regions of the XPS spectrum of a TiO2/SiO2 composite sample recorded while applying -10 and +10 V external bias.
Traditional X-ray photoelectron spectroscopy can inappropriately indicate the presence of two or more oxidation states for metal(s) in certain polycrystalline phase pure metal-oxides. We attribute this behavior to differential charging of samples. By employing an external bias, DC or AC, to the sample holder during spectral acquisition, we were able to minimize charging effects and determine properties dependent on grain/grain and substrate/grain interactions of complex composite polycrystalline materials.