X-ray Standing-wave Research Articles

The Charge of Solid–Liquid Interfaces Measured by X‐Ray Standing Waves and Streaming Current

Measurements of ion distributions at a charged solid-liquid interface using X-ray standing waves (XSW) are presented. High energy synchrotron radiation (17.48 keV) is used to produce an XSW pattern inside a thin water film on a silicon wafer. The liquid phase is an aqueous solution containing Br and Rb ions. The surface charge is adjusted by titration. Measurements are performed over a pH range from 2.2-9, using the native Si oxide layer and functional (amine) groups as surface charge. The Debye length, indicating the extension of the diffuse layer, could be measured with values varying between 1-4 nm. For functionalized wafers, the pH dependent change from attraction to repulsion of an ion species could be detected, indicating the isoelectric point. In combination with the measurement of the streaming current, the surface charge of the sample could be quantified.

In Situ Studies on the Switching Behavior of Ultrathin Poly(acrylic acid) Polyelectrolyte Brushes in Different Aqueous Environments

The pH-dependent switching of a poly(acrylic acid) (PAA) polyelectrolyte brush was investigated in situ using infrared spectroscopic ellipsometry (IRSE). The brush was synthesized by a "grafting to" procedure on silicon substrate with a native oxide layer. The overall thickness of the PAA brush in the dry state was approximately 5 nm. Reversible switching of the polymer brush was studied at titration from pH 2 to 10 and back in steps of 1 pH unit. The switching process was observed by monitoring the characteristic vibrational bands of the carboxylic groups of the PAA molecules. Decreasing of the C=O vibrational band amplitude and arising of a COO(-) vibrational band proved the chemical changes in the molecular structure of the brushes due to changes of the pH value in the aqueous solution. Due to the strong absorption of these bands in the IR region, the switching process could be monitored clearly. Switching the brush in several cycles with increasing and decreasing pH value showed a hysteresis-like behavior. For the first time, such hysteresis is observed in titration experiments of polyelectrolyte brushes. This behavior is attributed to the complex mechanisms of the ion's mobility in the brush layer which is explained with a suggested simplifying model describing the influence of ions inside the brush layer. In addition to the IRSE measurements, X-ray standing waves (XSW), in situ visible ellipsometry, and contact angle measurements have been performed and were in good agreement with the results from IRSE. Repetition of the in situ measurement cycles proved the good reversibility of the switching process which is highly important for practical applications of polymer brushes.

X-ray standing waves and scanning electron microscopy — Energy dispersive X-ray emission spectroscopy study of gold nanoparticles

A systematic characterization of mono-disperse nanoparticles with nominal diameters of 25 nm, 46 nm, 73 nm, 100 nm, 115 nm, and 250 nm was performed using X-ray standing waves (XSW). The samples were prepared on Si-wafer pieces and analyzed at DELTA synchrotron facility at beamline BL8 under grazing incidence geometry of the primary radiation. Additionally, SEM-EDX inspections of single particles as well as population-density checks were conducted. Particles with smaller diameters were able be characterized by XSW while the larger ones were not completely covered by the interference field produced by the provided 15 keV monochromatic radiation of BL8. The results of the measurements were compared with those of numerical simulations. The extension of the interference field perpendicular to the Si-wafer reflector was determined to be 83 nm ± 4 nm.

Combined ellipsometry and X-ray related techniques for studies of ultrathin organic nanocomposite films

Ultrathin nanocomposite films of nitrobenzene on silicon were analyzed by Infrared Spectroscopic Ellipsometry (IRSE), X-ray reflectivity (XRR) and X-ray standing waves (XSW) before and after evaporation of gold. Infrared Spectroscopic Ellipsometry measurements were performed for identification of adsorbates and for investigation of the molecular orientation. Results for film thickness were correlated with XRR measurements. Further, XSW measurements of elements incorporated in nitrobenzene (C, N, and O) were performed with soft X-rays. The combination of the different methods allowed to confirm a model for the electrochemically deposited nitrobenzene films before and after gold evaporation. The characterization by XRR and XSW scans using hard X-rays showed that gold had penetrated into the nitrobenzene film and thus changed density and optical properties of this layer significantly. A depth profile correlated to the electron density is deduced from the XRR measurements. This profile allows to localize—in vertical direction—gold islands within the composite film.

Normal-incidence x-ray standing-wave determination of the adsorption geometry of PTCDA on Ag(111): Comparison of the ordered room-temperature and disordered low-temperature phases

Normal incidence x-ray standing wave (NIXSW) experiments have been performed for monolayers of 3,4,9,10-perylene-tetracarboxylic-dianhydride (PTCDA) adsorbed on the Ag(111) surface. Two phases were analyzed: the low-temperature phase (LT phase), which is disordered and obtained for deposition at substrate temperatures below 150 K, and the ordered phase, which is obtained for deposition at room temperature (RT phase). From the NIXSW analysis the vertical bonding distances to the Ag surface were obtained for the averaged carbon atoms and the two types of chemically different oxygen atoms in the terminal anhydride groups. For the LT phase, we find about 2% $(0.05\text{ }\text{\AA{}})$ and 8% $(0.21\text{ }\text{\AA{}})$ smaller averaged bonding distances for the C and O atoms, respectively, compared to the RT phase. In both phases, the planar geometry of the free molecule is distorted; in particular, the carboxylic O atoms are closer to the surface by $0.20\text{ }\text{\AA{}}$ (RT) and $0.31\text{ }\text{\AA{}}$ (LT) with respect to the averaged C distance. The difference between the vertical bonding distances of the carboxylic and anhydride O atoms is found to be 0.32 (RT) and $0.33\text{ }\text{\AA{}}$ (LT). These structural parameters of the two phases are compared to those of PTCDA monolayers adsorbed on Au(111) and Cu(111) surfaces and are discussed in the frame of current bonding models.

Direct method for imaging elemental distribution profiles with long-period x-ray standing waves

A model-independent Fourier-inversion method for imaging elemental profiles from multilayer and total-external reflection x-ray standing wave (XSW) data is developed for the purpose of understanding the assembly of atoms, ions, and molecules at well-defined interfaces in complex environments. The direct-method formalism is derived for the case of a long-period XSW generated by low-angle specular reflection in an attenuating overlayer medium. It is validated through comparison with simulated and experimental data to directly obtain an elemental distribution contained within the overlayer. We demonstrate this formalism by extracting the one-dimensional profile of Ti normal to the surface for a ${\text{TiO}}_{2}/\text{Si}/\text{Mo}$ trilayer deposited on a Si substrate using the $\text{Ti}\text{ }K\ensuremath{\alpha}$ fluorescence yield measured in air and under an aqueous electrolyte. The model-independent results demonstrate reduced coherent fractions for the in situ results associated with an incoherency of the x-ray beam (which are attributed to fluorescence excitation by diffusely or incoherently scattered x-rays). The uniqueness and limitations of the approach are discussed.

Building Conjugated Organic Structures on Si(111) Surfaces via Microwave-Assisted Sonogashira Coupling

A novel step-by-step method employing microwave-assisted Sonogashira coupling is developed to grow fully conjugated organosilicon structures. As the first case study, p-(4-bromophenyl)acetylene is covalently conjugated to a p-(4-iodophenyl)acetylene-derived monolayer on a Si(111) surface. By bridging the two aromatic rings with C[triple bond]C, the pregrown monolayer is structurally extended outward from the Si surface, forming a fully conjugated (p-(4-bromophenylethynyl)phenyl)vinylene film. The film growth process, which reaches 90% yield after 2 h, is characterized thoroughly at each step by using X-ray reflectivity (XRR), X-ray standing waves (XSW), and X-ray fluorescence (XRF). The high yield and short reaction time offered by microwave-assisted surface Sonogashira coupling chemistry make it a promising strategy for functionalizing Si surfaces.

Theoretical and experimental comparison of SnPc, PbPc, and CoPc adsorption on Ag(111)

A combination of normal-incidence x-ray standing-wave (NIXSW) spectroscopy, x-ray photoelectron spectroscopy (XPS), scanning tunneling microscopy (STM), and density-functional theory (DFT) has been used to investigate the interaction of a number of phthalocyanine molecules (specifically, SnPc, PbPc, and CoPc) with the Ag(111) surface. The metal-surface distances predicted by the DFT calculations for SnPc/Ag(111) $(2.48\text{ }\text{\AA{}})$ and CoPc/Ag(111) $(2.88\text{ }\text{\AA{}})$ are in good agreement with our NIXSW experimental results for these systems ($2.31\ifmmode\pm\else\textpm\fi{}0.09$ and $2.90\ifmmode\pm\else\textpm\fi{}0.05\text{ }\text{\AA{}}$, respectively). Good agreement is also found between calculated partial density-of-states plots and STM images of CoPc on Ag(111). Although the DFT and $\text{Pb}\text{ }4f$ NIXSW results for the Pb-Ag(111) distance are similarly in apparently good agreement, the $\text{Pb}\text{ }4f$ core-level data suggest that a chemical reaction between PbPc and Ag(111) occurs due to the annealing procedure used in our experiments and that the similarity of the DFT and $\text{Pb}\text{ }4f$ NIXSW values for the Pb-Ag(111) distance is likely to be fortuitous. We interpret the $\text{Pb}\text{ }4f$ XPS data as indicating that the Pb atom can detach from the PbPc molecule when it is adsorbed in the ``Pb-down'' position, leading to the formation of a Pb-Ag alloy and the concomitant reduction in Pb from a ${\text{Pb}}^{2+}$ state (in bulklike films of PbPc) to ${\text{Pb}}^{0}$. In contrast to SnPc, neither PbPc nor CoPc forms a well-ordered monolayer on Ag(111) via the deposition and annealing procedures we have used. Our DFT calculations show that each of the phthalocyanine molecules donate charge to the silver surface, and that back donation from Ag to the metal atom (Co, Sn, or Pb) is only significant for CoPc.

Influence of intramolecular polar bonds on interface energetics in perfluoro-pentacene on Ag(111)

We investigated the structural and electronic properties of vacuum sublimed perfluoro-pentacene (PFP) thin films on Ag(111) substrates using x-ray standing waves (XSW), x-ray diffraction (XRD) and ultraviolet photoelectron spectroscopy (UPS). XSW results reveal a flat adsorption geometry of the monolayer PFP/Ag(111) with a relatively large bonding distance of $3.16\text{ }\text{\AA{}}$ for both, the carbon and fluorine atoms. Multilayers PFP/Ag(111) adopt a herringbone structure with the molecular long axis parallel to the substrate and a vertical lattice spacing of $3.06\text{ }\text{\AA{}}$ as evidenced by XRD. The strong intramolecular polar bond character of the fluorine-carbon bonds in PFP leads to an orientation dependent ionization energy (IE) that is experimentally observed by UPS for the monolayer-multilayer transition: The inclined molecular plane orientation in the multilayer herringbone arrangement leads to an increase of the PFP IE by $>0.4\text{ }\text{eV}$ compared to the flat lying monolayer.

Characterization of trace embedded impurities in thin multilayer structures using synchrotron X‐ray standing waves

AbstractX‐ray standing wave (XSW) field generated under Bragg reflection condition in a periodic Mo/Si multilayer structure has been used to determine the concentration and location of various trace element contaminants embedded in different layers of that multilayer structure. We have used intense synchrotron X rays for XSW analysis. It is observed that various trace element impurities such as Cr, Fe, Ni and W get embedded unintentionally in the multilayer structure during the deposition process. Consequences of such impurity incorporation on the optical properties of the multilayer structure are discussed in hard and soft X‐ray regions. Present measurements are important in order to optimize the deposition methods on one hand and to better correlate the measured optical properties of a multilayer structure with theoretical models on the other. Copyright © 2010 John Wiley & Sons, Ltd.

Depth profile characterization of ultra shallow junction implants

A need for analysis techniques, complementary to secondary ion mass spectrometry (SIMS), for depth profiling dopants in silicon for ultra shallow junction (USJ) applications in CMOS technologies has recently emerged following the difficulties SIMS is facing there. Grazing incidence X-ray fluorescence (GIXRF) analysis in the soft X-ray range is a high-potential tool for this purpose. It provides excellent conditions for the excitation of the B-K and the As-L(iii,ii) shells. The X-ray standing wave (XSW) field associated with GIXRF on flat samples is used here as a tunable sensor to obtain information about the implantation profile because the in-depth changes of the XSW intensity are dependent on the angle of incidence. This technique is very sensitive to near-surface layers and is therefore well suited for the analysis of USJ distributions. Si wafers implanted with either arsenic or boron at different fluences and implantation energies were used to compare SIMS with synchrotron radiation-induced GIXRF analysis. GIXRF measurements were carried out at the laboratory of the Physikalisch-Technische Bundesanstalt (PTB) at the electron storage ring BESSY II using monochromatized undulator radiation of well-known radiant power and spectral purity. The use of an absolutely calibrated energy-dispersive detector for the acquisition of the B-Kalpha and As-Lalpha fluorescence radiation enabled the absolute determination of the total retained dose. The concentration profile was obtained by ab initio calculation and comparison with the angular measurements of the X-ray fluorescence.

Analysis of the Ion Distribution at a Charged Solid−Liquid Interface Using X-ray Standing Waves

Functionalized solid-liquid interfaces were analyzed by X-ray standing waves (XSW) combined with streaming current measurements to study surface charges, interfacial potential, and ion distributions. Thin films of aqueous solution containing Br(-) anions and Fe(3+) cations at a concentration of 10 mg/L were prepared on functionalized silicon wafers. Functionalization of Si surfaces was accomplished by aminosilane groups shifting the interfacial potential toward positive values. The ion distribution was measured with nanometer resolution, which allows distinguishing between absorbed and mobile ions at the surface and in the diffusive layer, respectively. For Br(-), different degrees of ion attraction were measured for the pH values 5.7 and 2.8. The ion Debye length values of the diffuse layer were 4 and 2 nm, respectively.

Rb+ and Sr2+ Adsorption at the TiO2 (110)−Electrolyte Interface Observed with Resonant Anomalous X-ray Reflectivity

We report the vertical density profiles of Rb(+) and Sr(2+) at the rutile TiO(2)(110)-electrolyte interface for the following bulk electrolyte conditions, [Rb(+)] = 1 mM at pH 11 and [Sr(2+)] = 0.1 mM at pH 10.3, using X-ray reflectivity and resonant anomalous X-ray reflectivity. We find that Rb(+) specifically adsorbs with a coverage of 0.080 +/- 0.003 monolayer (ML) and a height of 3.72 +/- 0.03 A above the surface Ti-O plane. In comparison, Sr(2+) adsorbs with a coverage of 0.40 +/- 0.07 ML and an average height of 3.05 +/- 0.16 A, but with a significant vertical distribution width (0.35 +/- 0.02 A). The Sr(2+) distribution in the presence of a background electrolyte ([Na(+)] = 30 mM) was also investigated, and it is found that, while the ion height and coverage are unchanged within the uncertainties of the measurements, the width of the distribution is apparently increased in the presence of Na(+). Comparison is made with previous results, including XR and X-ray standing waves (XSW) measurements, and molecular dynamics simulations. Our results are in excellent agreement with a recently proposed multisite adsorption mechanism that suggests simultaneous adsorption in two inner-sphere adsorption geometries, the tetradentate and the bidentate sites.

Characterization of Intermetallic Layer with Nanoresolution Using X-Ray Standing Wave Technique

X-ray standing wave technique has been used to measure the kinetics of CoSi intermetallic phase growth in a-Si/Co/a-Si sandwich structure. The a-Si/Co/a-Si arrangement were placed into a waveguide structure formed by two Ta films. X-ray fluorescence and extended X-ray absorption fine structure analysis has been used in a combination with X-ray standing wave technique for depth profiling with sub-nanometer resolution of specimens annealed at 493K for different annealing time. The position and the thickness of the growing CoSi intermetallic phase have been monitored.

Nanoresolution interface studies in thin films by synchrotron x‐ray diffraction and by using x‐ray waveguide structure

AbstractIn the last years we performed several measurements with synchrotron radiation of several facilities to reveal interesting interface phenomena on the nanoscale. We used both x‐ray diffraction1, 2 (XRD) and spectrometry techniques. In this paper, we briefly summarize the results obtained from diffraction measurements, which lead us to our recent grazing incidence x‐ray fluorescence analysis (GIXRF) and extended x‐ray absorption fine structure (EXAFS) experiments.We show how a combination of experimental methods of GIXRF analysis and EXAFS spectroscopy in fluorescence detection with x‐ray standing waves (XSW) technique was applied for the depth profiling of a‐Si/Co/a‐Si layers with nanometer resolution to monitor the growth of CoSi intermetallic phase. The investigated layers were placed into the waveguide structure formed by two Ta films to increase sensitivity and accuracy of the measurements. Copyright © 2009 John Wiley & Sons, Ltd.

Study of swift heavy-ion-induced modification in Ti/Si using x-ray standing waves

Intermixing in a Si/Ti/Si tri-layer induced by 120 MeV Au ions has been studied. X-raystanding wave analysis combined with x-ray reflectivity has been used to get a depth profileof the Ti marker layer with an accuracy of a fraction of a nanometer. Two differentthicknesses of the Ti marker layer have been used to study the possible effect of layerthickness on intermixing. In the case of a 2 nm thick Ti layer intermixing is stronger ascompared to a 6 nm Ti film, which can be understood in terms of a stronger confinementof the dissipated energy in the Ti layer due to increased interface scattering ofδ-electrons in the case of the 2 nm thick Ti layer. In the 6 nm thick Ti layer, intermixing isasymmetric at the two interfaces, which may be due to a possible asymmetry in theinterface structure in the as-deposited film itself.

Atomic-scale X-ray structural analysis of self-assembled monolayers on Silicon

Two related self-assembled monolayers (SAMs), 4-bromostyrene (BrSty) and 4-bromophenylacetylene (BPA), are photochemically grown from solution on to the monohydride-terminated Si(111) surface. The atomic-scale structures of the resulting SAMs are examined by X-ray standing waves (XSW), X-ray reflectivity (XRR), X-ray fluorescence, atomic-force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), and density functional theory (DFT). The coverage is 0.5 ML. The results show that in each case the molecule covalently bonds to a single Si T1 site and stands up-right with a slight molecular tilt of 17○ that leaves the Br terminal end over a neighboring T4 site. The Br height is 8.5 A (BrSty) and 8.6 A (BPA) above the top surface Si atom. The combined XSW and XRR results rule-out two alternative bonding models predicted by DFT that have the root of the molecule bonded to two neighboring top Si surface atoms. Based on the XSW 111 and 333 coherent fractions, the BPA/Si(111) has a reduced vertical Br distribution width in comparison to BrSty. This greater rigidity in the molecular structure is correlated to a C=C bond at the root.

Hydration and Reduction of Molecular Beam Epitaxy Grown VOx/α-Fe2O3(0001): Ambient Pressure Study

Supported vanadium oxides processed under ambient environments have been studied by using X-ray standing wave (XSW) analysis of X-ray fluorescence spectroscopy and X-ray photoelectron spectroscopy (XPS). For the VOx/α-Fe2O3(0001) system, hydration and hydrogen annealing have been carried out under ambient pressure. Vanadium in the hydrated oxide phase occupies two high-symmetry surface adsorption sites with distinct adsorption heights, which resembles the adsorption geometry of fully oxidized vanadium. Reduction by the hydrogen annealing enhanced the V overlayer ordering by relocating a portion of the disordered V to high-symmetry sites. The V atoms located closer to the substrate oxygen layer in the hydrated phase moved toward the substrate after hydrogen reduction, while the V in the higher adsorption site stayed at the same height. The different responses of two adsorption sites to the reduction process are discussed and related to activities of the two sites.

The influence of X-ray coherence length on TXRF and XSW and the characterization of nanoparticles observed under grazing incidence of X-rays

An approach to the chemical characterization and a study of the morphology of very fine fractions of nanoparticles on surfaces can be deduced from experiments using the grazing incidence X-ray Standing Waves (XSW) by means of Total Reflection X-ray Fluorescence (TXRF). Some theoretical aspects not considered until now for TXRF and XSW, e.g. the influence of coherence length of X-rays from different X-ray sources, the particle form, particle size and distribution, are presented and discussed. Results of numerical simulations of XSW-scans considering the modelling of particles are compared with the experimental data obtained from XSW experiments performed at a synchrotron radiation facility. Advantages and limitations of the applied techniques are discussed for the characterization of Au- and CdTe-nanoparticles.

X-ray Optics with Small Vertical Divergence and Horizontal Focusing Foran X-ray Standing-wave Measurement

An x-ray optics was proposed for dynamical diffraction measurements of a non-ideal crystal. It was composed of a pair of channel-cut Si (004) crystals and a one-dimensional focusing lens system. Vertical angular divergence values were measured as FWHM ’s of ca. 1.3 arc secs for an incident photon energy of 12.4 keV during a rocking scan around the analyzer sapphire 0006 reflection when the second channel-cut crystal was fixed at five deviation angles. The experimental angular resolution deconvoluted is about 1.3 arc secs at the deviation angle of ca. 0.7 arc sec. A typical photon flux was around 10 photons /s as a peak value of the rocking scan for an incident slit of 0.2 x 0.1 mm. The horizontal beam size measured at the focal distance of ca. 200 mm was 4.3 μm.