Grafting cavitands on the Si(100) surface

Cavitand molecules having double bond terminated alkyl chains and different bridging groups at the upper rim have been grafted on H-terminated Si(100) surface via photochemical hydrosilylation of the double bonds. Pure and mixed monolayers have been obtained from mesitylene solutions of either pure cavitand or cavitand/1-octene mixtures. Angle resolved high-resolution X-ray photoelectron spectroscopy has been used as the main tool for the monolayer characterization. The cavitand decorated surface consists of Si-C bonded layers with the upper rim at the top of the layer. Grafting of pure cavitands leads to not-well-packed layers, which are not able to efficiently passivate the Si(100) surface. By contrast, monolayers obtained from cavitand/1-octene mixtures consist of well-packed layers since they prevent silicon oxidation after aging. AFM measurements showed that these monolayers have a structured topography, with objects protruding from the Si(100) surface with average heights compatible with the expected ones for cavitand molecules.     

Depth profiles of shallow implanted layers by soft ion sputtering and total-reflection X-ray fluorescence

A new method suitable for depth profiling of shallow layers on different materials is presented. It is based on a soft and planar ion sputtering combined with differential weighing, total-reflection X-ray fluorescence (TXRF) spectrometry and Tolansky interferometry. By means of a stepwise repetition of these techniques it is possible to determine both density/depth and concentration/depth profiles. The respective quantities are expressed in terms inherent only to the sample and traceable to the SI-units or subunits gram, nanometer and mole. It is a unique feature of this method that density/depth profiles can directly be obtained from measurements without any calibration or theoretical approximation. The method is applied to a Si wafer implanted with Co ions of 25 keV energy and a nominal dose of 1×10¹⁶ cm⁻². The depth resolution is shown to be <3 nm while a total depth of some 100 nm can be reached. The concentration/depth profile is compared with RBS measurements, wet-chemical etching plus TXRF and Monte Carlo simulations. In view of the fact that only similar but not exactly the same samples have been examined by these methods, a good correspondence can be noticed.     

Effects of substrate on the melting behavior of Cd arachidate Langmuir–Blodgett films

Nineteen monolayered Cd arachidate films were deposited on float glass substrate coated with Si and Ni over-layers. Two layers have been chosen with very different surface free energies. Melting behavior of films were studied using variable temperature X-ray specular reflectivity and Fourier transform infrared spectroscopy measurements. In conformity with earlier studies, melting of the multilayer precede by a transition from distorted hexagonal to hexaticlike phase. However, the transition temperature to hexaticlike phase as well as the melting temperature depend significantly on the type of layer. Both the transition temperatures are higher for the multilayer deposited on Ni layer as compared to those for the film deposited on Si layer. These results can be understood in terms of different surface free energies of Ni and Si layers. Further, in case of Ni layer, transition to hexaticlike phase is relatively sharper. Even in the molten state there is a significant difference in the structure of the Cd arachidate film on two layers; packing density of molecules in molten state is lower in case of substrate with higher surface free energy. These results suggest that the surface free energy of substrate plays a significant role in melting behavior of Langmuir–Blodgett films.     

Characterization of Laser-Arc deposited multilayer systems by means of AES, Factor Analysis and XPS

In a vacuum chamber at 5 · 10^–4 Pa, multilayer systems (single layer thickness 20 nm) consisting of Ti/C and Al/C, respectively, have been deposited on Si (111) disks by the laser assisted coating (Laser-Arc). Structure and composition have been investigated by means of Scanning Electron Microscopy (SEM), X-ray Photoelectron Spectroscopy (XPS), and Auger Electron Spectroscopy (AES) in conjunction with Factor Analysis. AES depth profile measurements through the outermost part of the layers show for both the Ti/C and the Al/C samples a regular structure of the layer sequence metal/ carbon with a constant distance along the sample normal and sharply formed interfaces. In the metallic layers an oxygen enrichment was found, which is more intensive in the Ti/C deposit than in the Al/C one. By means of Factor Analysis in the evaluation of the differentiated spectra as a function of sputtering time, the formation of carbides at the metal/carbon interfaces has been detected. However, in the present state of the investigations it can not be decided, whether the observed carbide formation is the result of the energy impact due to ion sputtering or the coating fabrication process itself.     

Grazing incidence X-ray photoemission spectroscopy of SiO2 on Si

We have applied grazing incidence X-ray photoemission spectroscopy to the determination of the thickness of SiO₂ layers on Si, as well as surface carbon that is present. The measurements take advantage of the different optical constants of the layers. X-rays incident on the surface at grazing angle undergo total external reflection, where the fields in each layer are subject to highly non-linear changes as a function of incidence angle. X-ray photoemission excited by these fields gives information on atomic species, chemical state, and layer thickness. Simultaneous fits are made to the photoemission spectra in each layer. The method is illustrated for a thermally grown oxide layer and a native oxide on Si.     

Determination of activation energies of the U(Mo)/Si and U(Mo)/Al solid state reaction using in-situ X-ray diffraction and Kissinger analysis

The solid state reaction between U(Mo) and Si, leading to the formation of silicides, has been studied using in-situ X-ray Diffraction. Samples were prepared by sputter depositing Si in thin layers on U(Mo) substrates (8 wt% Mo) and vice versa. In a similar way the reaction between U(Mo) and Al has been studied using U(Mo) substrates covered with a thin layer of Al. The samples were heated to temperatures up to 950 °C in a static purified helium atmosphere. Even though the measurements were hampered by the undesired oxidation of uranium, the formation of various silicides and aluminides could be observed. Kissinger analysis on ramp anneals with ramp rates of 0.2, 0.5, 1 and 3 °C/s have been performed to investigate the kinetics of the formed silicides. Using this method, the apparent activation energy for the different silicide formation reactions was deduced. Using the effective heat of formation rule, a prediction was made on the first phase formed and the subsequent phase sequence. A good agreement was found between the measurements and prediction.     

Formation of aromatic siloxane self-assembled monolayers

We describe reproducible protocols for the chemisorption of self-assembled monolayers (SAMs), useful as imaging layers for nanolithography applications, from p-chloromethylphenyltrichlorosilane (CMPS) and 1-(dimethylchlorosilyl)-2-(p,m-chloromethylphenyl)ethane on native oxide Si wafers. Film chemisorption was monitored and characterized using water contact angle, X-ray photoelectron spectroscopy, and ellipsometry measurements. Atomic force microscopy was used to monitor the onset of multilayer deposition for CMPS films, ultimately allowing film macroscopic properties to be correlated with their surface coverage and nanoscale morphologies. Although our results indicate the deposition of moderate coverage, disordered SAMs under our conditions, their quality is sufficient for the fabrication of sub-100-nm-resolution metal features. The significance of our observations on the design of future imaging layers capable of molecular scale resolution in nanolithography applications is briefly discussed.     

An X-ray diffraction and MAS NMR study of the thermal expansion properties of calcined siliceous ferrierite

Powder and single-crystal X-ray diffraction, combined with MAS NMR measurements, has been used to study the thermal expansion of siliceous zeolite ferrierite as it approaches a second-order displacive phase transition from a low-symmetry (Pnnm) to a high-symmetry (Immm) structure. Below the transition temperature, ferrierite exhibits positive thermal expansivity. However, above the transition temperature a significant change in thermal behavior is seen, and ferrierite becomes a negative thermal expansion material. Accurate variable-temperature single-crystal X-ray diffraction measurements confirm the transition temperature and allow the changes in average atomic position to be followed with temperature. The results from the single-crystal X-ray diffraction study can be correlated with (29)Si MAS NMR chemical shifts for the low-temperature phase. At low temperatures the results show that the positive thermal expansivity is driven by an overall increase in Si[bond]Si distances related to an increase in Si[bond]O[bond]Si bond angles. However, in the high-temperature phase the Si[bond]O[bond]Si angles are approximately invariant with temperature, and the negative thermal expansion in this case is caused by transverse vibrations of the Si[bond]O[bond]Si units.     

Depth profiling by means of the combination of glancing incidence X-ray fluorescence spectrometry with low energy ion beam etching technique

Experiments using a combination of laterally resolved glancing incidence X-ray fluorescence spectrometry in the vicinity of the critical angle of total reflection and ion beam ramp etching were performed to work out a new technique for depth profiling in solid-state thin films with nanometre resolution. The lateral point-to-point resolution of the total-reflection X-ray fluorescence (TXRF) spectrometer used was determined as 200 ± 10 μm by means of standard samples (Cr bars on Si). At an inclination angle in the range of 10⁻⁴ degrees for the ramp, which has been produced by ion beam etching, the geometrically covered depth is in the range of 1 nm. In order to demonstrate the potential of the new technique, preliminary results on Cu/Cr multilayers on Si substrate are presented.     

RE2MAl6Si4 (RE = Gd, Tb, Dy; M = Au, Pt): layered quaternary intermetallics featuring CaAl2Si2-type and YNiAl4Ge2-type slabs grown from aluminum flux

Six new intermetallic aluminum silicides--Gd(2)PtAl(6)Si(4), Gd(2)AuAl(6)Si(4), Tb(2)PtAl(6)Si(4), Tb(2)AuAl(6)Si(4), Dy(2)PtAl(6)Si(4), and Dy(2)AuAl(6)Si(4)--have been obtained from reactions carried out in aluminum flux. The structure of these compounds was determined by single-crystal X-ray diffraction. They form in space group Rthremacr;m with cell constants of a = 4.1623(3) A and c = 51.048(5) A for the Gd(2)PtAl(6)Si(4) compound. The crystal structure is comprised of hexagonal nets of rare earth atoms alternating with two kinds of layers that have been observed in other multinary aluminide intermetallic compounds (CaAl(2)Si(2) and YNiAl(4)Ge(2)). All six RE(2)MAl(6)Si(4) compounds show antiferromagnetic transitions at low temperatures (T(N) < 20 K); magnetization studies of the Dy compounds show metamagnetic behavior with reorientation of spins at 6000 G. Band structure calculations indicate that the AlSi puckered hexagonal sheets in this structure are electronically distinct from the other surrounding structural motifs.     

Nanotransfer of the polythiophene molecular alignment onto the step-bunched vicinal Si(111) substrate

A poly(3-dodecylthiophene-2,5-diyl) film having in-plane anisotropic molecular arrangement was successfully fabricated by transferring its Langmuir-Blodgett film onto a step-bunched Si(111) substrate. Polarized near-edge X-ray absorption fine structure measurements revealed that the polythiophene main chains are preferentially orientated along periodic facet/terrace nanostructures on the step-bunched substrate, whereas less anisotropy was found on a flat substrate. The step-bunched Si substrate has been proved to be effective for controlling the in-plane molecular arrangement in the polymer thin film.     

X-ray nanoscale profiling of layer-by-layer assembled metal/organophosphonate films

The nanoscale structure of multilayer metal/phosphonate thin films prepared via a layer-by-layer assembly process was studied using specular X-ray reflectivity (XRR), X-ray fluorescence (XRF), and long-period X-ray standing wave (XSW) analysis. After the SiO(2) X-ray mirror surfaces were functionalized with a monolayer film terminated with phosphonate groups, the organic multilayer films were assembled by alternating immersions in (a) aqueous solutions containing Zr(4+), Hf(4+), or Y(3+) cations and then (b) organic solvent solutions of PO(3)-R-PO(3), where R was a porphyrin or porphyrin-square spacer molecule. The different heavy metal cations provided X-ray fluorescence marker layers at different heights within the different multilayer assemblies. The XSW measurements used a 22 nm period Si/Mo multilayer mirror. The long-period XSW generated by the zeroth-order (total external reflection) through fourth-order Bragg diffraction conditions made it possible to examine the Fourier transforms of the fluorescent atom distributions over a much larger q(z)() range in reciprocal space than previously achieved.     

Depth-resolved molecular structure and orientation of polymer thin films by synchrotron X-ray diffraction

An exemplary system suitable for optoelectronics applications, i.e. poly(3-hexylthiophene), hereinafter P3HT, deposited by spin casting onto silicon substrates functionalised by three selected molecules and then properly annealed, has been examined. Grazing Incidence X-ray Scattering (GIXS) measurements have been performed with 4-circle diffractometer, allowing for a fine control of sample axes movement.By choosing different grazing incident angles, diffraction patterns from different layers of polymeric thin films have been recorded. Both in-plane and out-of-plane geometries have been combined in order to obtain complementary structural information. In this way structural and orientational differences of the polymer along with the film thickness (?50 nm) have been highlighted. For all P3HT films spun on functionalized Si wafer, macromolecular layers close to the substrate surface give some evidence of higher order and orientation than those outmost the surface, and this behaviour is pronounced to a different extent depending on the functionalized molecules used. Contrariwise P3HT layers deposited onto bare Si wafer display reduced orientation and decreased crystallite size, especially at buried interface.     

Resolving the aluminum ordering in aluminosilicates by a combined experimental/theoretical study of 27Al electric field gradients

The discrimination between atomic species in light-element materials is a challenging question. An archetypal example is the resolution of the Al/Si ordering in aluminosilicates. Only an average long-range order can be deduced from powder X-ray or neutron diffraction, while magic-angle-spinning NMR provides an accurate picture of the short-range order. The long- and short-range orders thus obtained usually differ, hence raising the question of whether the difference between local and extended orders is intrinsic or caused by the difficulty of obtaining an accurate picture of the long-range order from diffraction techniques. In this communication we resolve this question for the monoclinic phases of BaAl2Si2O8 and SrAl2Si2O8 on the basis of 27Al NMR measurements and ab initio simulation of electric field gradient. Although the long- and short-range orders deduced from our XRD and NMR experiments differ, they become similar when the XRD atomic positions are optimized by ab initio electronic structure calculations.     

Microsecond studies of layer motion in a ferroelectric liquid crystal device

Abstract

Small angle X-ray scattering has been employed to study dynamically the layer motion in a ferroelectric liquid crystal device on application of low electric fields. Microsecond time resolution was achieved and the use of an area detector in the experiment allowed the examination of layer motion in two orthogonal planes. The X-ray data show that during switching the chevron structure adopted by the layers distorts, implying a variation in the chevron angle. A rotation of the layers in the plane of the device is also observed, coincident in time with the change in chevron angle. The motion of the layers takes place on a ten microsecond time scale and the angular rotation of the layers is approximately 1°.     

Semicarbazide-functionalized Si(111) surfaces for the site-specific immobilization of peptides

The covalent attachment of semicarbazide-functionalized layers to hydrogen-terminated Si(111) surfaces is reported. The surface modification, based on the photoinduced hydrosilylation of a Si(111) surface with protected semicarbazide-functionalized alkenes, was investigated by means of X-ray photoelectron spectroscopy (XPS), contact angle measurements, and atomic force microscopy (AFM). The removal of the protecting group yielded a semicarbazide-terminated monolayer which was reacted with peptides bearing a glyoxylyl group for site-specific alpha-oxo semicarbazone ligation.     

Exploring the interfacial structure of protein adsorbates and the kinetics of protein adsorption: an in situ high-energy X-ray reflectivity study

The high energy X-ray reflectivity technique has been applied to study the interfacial structure of protein adsorbates and protein adsorption kinetics in situ. For this purpose, the adsorption of lysozyme at the hydrophilic silica-water interface has been chosen as a model system. The structure of adsorbed lysozyme layers was probed for various aqueous solution conditions. The effect of solution pH and lysozyme concentration on the interfacial structure was measured. Monolayer formation was observed for all cases except for the highest concentration. The adsorbed protein layers consist of adsorbed lysozyme molecules with side-on or end-on orientation. By means of time-dependent X-ray reflectivity scans, the time-evolution of adsorbed proteins was monitored as well. The results of this study demonstrate the capabilities of in situ X-ray reflectivity experiments on protein adsorbates. The great advantages of this method are the broad wave vector range available and the high time resolution.     

Optimization of Parameters of Graphene Synthesis on Copper Foil at Low Methan Pressure

Graphene films on copper foils were synthesized using low-pressure (2200-2800 Pa) chemical vapor deposition (CVD) from methane/hydrogen mixtures. The number of graphene layers is shown to be dependent on the composition of gas mixture and synthesis parameters. The annealing procedure of copper foils used as substrates was optimized to obtain high quality graphene. Atomic and electronic structures of graphene on copper and SiO₂/Si substrates were studied by Raman, X-ray photoelectron, and near-edge X-ray absorption fine structure spectroscopy methods.     

Copper L X-ray spectra measured by a high resolution ion-induced X-ray spectrometer

High resolution L X-ray emission spectra of Cu have been measured by 0.75 MeV/u H, He, and F, 0.73 MeV/u Ar, 0.64 MeV/u Si, and 0.073 MeV/u Si ion impacts with a crystal spectrometer. The X-ray transition energies in the Cu target for Lι, Lη, L_1,2, Lβ₁, and Lβ_3,4 diagram lines induced by H ion impact are determined, which are in good agreement with those given in the reference by Bearden (Rev. Mod. Phys. 39, 78, 1967). The X-ray spectra produced by F, Si, and Ar ions have complicated structures due to multiple L and M shell vacancy production. The L_1,2 and Lβ₁ emission spectra for H and He ions are compared with the calculated ones based on the multiconfiguration Dirac-Fock method. The origin of the broadening of the L_1,2 line to the lower energy for H ion impact is attributed to one 2p plus one 3d electron vacancy production.     

Microsecond studies of layer motion in a ferroelectric liquid crystal device

Small angle X-ray scattering has been employed to study dynamically the layer motion in a ferroelectric liquid crystal device on application of low electric fields. Microsecond time resolution was achieved and the use of an area detector in the experiment allowed the examination of layer motion in two orthogonal planes. The X-ray data show that during switching the chevron structure adopted by the layers distorts, implying a variation in the chevron angle. A rotation of the layers in the plane of the device is also observed, coincident in time with the change in chevron angle. The motion of the layers takes place on a ten microsecond time scale and the angular rotation of the layers is approximately 1°.