Spin-echo Resolved Grazing Incidence Scattering Research Articles

Profile retrieval of a buried periodic structure using spin echo grazing incidence neutron scattering

When the neutron scattering technique, Spin Echo Resolved Grazing Incidence Scattering (SERGIS) concept, was originally put forward by Rekveldt [Physica B 1135, 234–236 (1997)] and Felcher et al. [Proc. SPIE 4785, 164 (2002)], they recognized that the specular scattering and the off-specular scattering could be spatially separated due to the tight neutron beam collimation in the scattering plane, a necessity for any reflectometry experiment. In this Letter, we show that it is possible to make large area measurements of periodic grating structures using SERGIS in a number of interesting scenarios. The SERGIS data can be analyzed using a dynamical theory, which makes it possible to effectively retrieve the lateral profile of a commercial periodic diffraction grating. Interestingly, this is still the case even when that grating is buried beneath a highly deuterated poly(methyl methacrylate-D8) polymer layer. We also clearly demonstrate that the maximum sensitivity to lateral structures is achieved when the specular reflection from the grating is excluded from the data analysis, demonstrating a feature of SERGIS that was proposed over two decades ago.

RF neutron spin flippers in time of flight Spin-Echo Resolved Grazing Incidence Scattering (SERGIS)

RF coils have been routinely used for Larmor labelling on the Offspec instrument at ISIS. These coils encode directional information via the neutron polarization using a series of parallelogram-shaped pole shoes which can be tuned to different angles with an RF gradient flipper in the centre of each magnet. We report on measurements of the magnetic field integral through the coils in reflection geometry for a range of scattering angles and different pole shoe angles. Such information is mapped out by measuring the phase of the neutron polarisation and the measurements are discussed in light of data on patterned silicon gratings with a dewetted polymer and the visibility of in-plane structures to SERGIS.

Using Neutron Spin Echo Resolved Grazing Incidence Scattering to Investigate Organic Solar Cell Materials

The spin echo resolved grazing incidence scattering (SERGIS) technique has been used to probe the length-scales associated with irregularly shaped crystallites. Neutrons are passed through two well defined regions of magnetic field; one before and one after the sample. The two magnetic field regions have opposite polarity and are tuned such that neutrons travelling through both regions, without being perturbed, will undergo the same number of precessions in opposing directions. In this case the neutron precession in the second arm is said to "echo" the first, and the original polarization of the beam is preserved. If the neutron interacts with a sample and scatters elastically the path through the second arm is not the same as the first and the original polarization is not recovered. Depolarization of the neutron beam is a highly sensitive probe at very small angles (<50μrad) but still allows a high intensity, divergent beam to be used. The decrease in polarization of the beam reflected from the sample as compared to that from the reference sample can be directly related to structure within the sample. In comparison to scattering observed in neutron reflection measurements the SERGIS signals are often weak and are unlikely to be observed if the in-plane structures within the sample under investigation are dilute, disordered, small in size and polydisperse or the neutron scattering contrast is low. Therefore, good results will most likely be obtained using the SERGIS technique if the sample being measured consist of thin films on a flat substrate and contain scattering features that contains a high density of moderately sized features (30 nm to 5 µm) which scatter neutrons strongly or the features are arranged on a lattice. An advantage of the SERGIS technique is that it can probe structures in the plane of the sample.

A neutron spin echo resolved grazing incidence scattering study of crystallites in organic photovoltaic thin films

Neutron spin echo resolved grazing incidence scattering (SERGIS) was used to probe crystallites of [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) produced by extensive thermal annealing of a poly(3-hexylthiophene-2,5-diyl)(P3HT):PCBM organic photovoltaic layer. After annealing a thin film of P3HT:PCBM, PCBM crystallites appear on the sample surface, and a strong SERGIS signal is observed superimposed on the specular reflection. Features in the data can be readily correlated with length scales of the crystallites determined using atomic force microscopy and indicate that in such cases the SERGIS signal may be interpreted as a form of small angle neutron scattering.

Dynamical theory: Application to spin-echo resolved grazing incidence scattering from periodic structures

Neutron spin-echo resolved grazing incidence scattering (SERGIS) measurements performed on a silicon diffraction grating with a rectangular profile were shown in our previous publications to be well explained by dynamical theory calculations. The theory is based on a Bloch wave expansion of the neutron wavefunction in the periodic layer of the grating, which includes all multiple scattering within that layer. Calculations show that the spin-echo polarization should be very sensitive to the scattering geometry (i.e., incident angle, sample alignment and beam divergence) and the sample specifications (i.e., grating period, groove depth). To test these predictions, SERGIS measurements have been performed on a set of gratings with different specifications in various scattering geometries. In all cases, simulations based on the dynamical theory, with all the parameters set to their known values, are in good agreement with the collected data.

Phase and Microphase Separation of Polymer Thin Films Dewetted from Silicon—A Spin–Echo Resolved Grazing Incidence Neutron Scattering Study

An understanding of the structure of ultrathin polymer films on solid substrates has scientific importance in applications as well as in fundamental studies of polymer diffusion or adsorption. We present studies of the organization of dewetted droplets of polymers on a silicon surface using a new neutron scattering technique, spin-echo resolved grazing incidence scattering (SERGIS), that has the potential to address at the same time the droplet-droplet correlations and the chemical configuration inside each droplet. For the seminal experiments, the polarized neutron reflectometer EVA at the Institut Laue-Langevin, Grenoble, France, was equipped with a spin-echo setup, and measurements were taken on surface structures previously characterized by different techniques. The dewetted polymers used in our studies were pure polystyrene, a mixture of polystyrene and polyparamethylstyrene, and a diblock copolymer of the two homopolymers. Even with a provisional setup SERGIS, we were able to determine the correlation between the droplets, providing results in excellent agreement with those obtained by atomic force microscopy and grazing incidence small-angle X-ray and neutron scattering. In addition, it was confirmed that the correlation function for diblock copolymer droplets is more complex than for polymer mixtures, exhibiting partial ordering of the copolymer within each droplet.

Offspec, the ISIS spin-echo reflectometer

OffSpec has been purposely built as a low background, polarised neutron reflectometer with spin-echo for Larmor labeling. The instrument received first neutrons in December 2008 and five of the intended seven modes of operation have now been tested and operated with peer reviewed user experiments. The technical performance of the instrument will be reviewed in each of the modes and future scientific possibilities discussed. Particular attention will be paid to results from spin-echo SANS (SESANS) and spin-echo resolved grazing incidence scattering (SERGIS), where the largest number of measurements have been performed.

Some recent results using spin echo resolved grazing incidence scattering (SERGIS)

The advantages of neutrons for probing bulk structures are well known: they provide statistically averaged correlation functions over a large range of length scales and they are sensitive to light atoms such as hydrogen. These same qualities are, in principle, useful in the study of surfaces and buried morphologies in thin films, especially when the films are polymeric or biological. However, because of the limited sample volume for such systems, the scattering is weak, especially if the neutron beam has to be severely collimated in order to resolve distances of interest (typically 10 to several 100nm parallel to the surface of the sample). SERGIS is a technique that can potentially overcome these limitations by allowing high resolution measurements of lateral surface structure without requiring tight beam collimation. In this paper we discuss recent progress towards implementing SERGIS both at the Low Energy Neutron Source at Indiana University and on the Asterix reflectometer at the Los Alamos Neutron Science Center (LANSCE). The architecture we use exploits a robust symmetry-related cancellation of Larmor phase aberrations. The spatial resolution it achieves closely mimics that of the ideal magnetic Wollaston prisms. To make progress in understanding SERGIS, we have applied it to the measurement of simple diffraction gratings and developed a dynamical theory that accounts quantitatively and without adjustable parameters for all of the data sets we have measured to date. We argue here that, if SERGIS is to be applied to the study of weakly scattering thin films, it will be necessary to develop the technique of dark-field spin echo scattering angle measurement.

Spin-Echo Resolved Grazing Incidence Scattering (SERGIS) at Pulsed and CW Neutron Sources

We have used SERGIS to probe the surface structure of a silicon diffraction grating of period 140 nm. Experiments were performed at: the Los Alamos Neutron Science Center (LANSCE) pulsed neutron source and the National Institute of Standards and Technology (NIST) continuous wave (CW) reactor neutron source. Although both sets of data show peaks of the spin echo polarization at integer multiples of the grating period, as expected, the results differ in detail. We have developed a dynamical theory, based on a Bloch wave expansion, to describe neutron diffraction from a grating. The theory explains the differences between the two sets of data without any adjustable parameters.

The CG1 instrument development test station at the high flux isotope reactor

The CG1 instrument development station at the High Flux Isotope Reactor (HFIR) at Oak Ridge National Laboratory began commissioning operation in 2009. When completed, the station will have four beams. CG1A is a 4.22Å monochromatic beam intended for spin-echo resolved grazing incidence scattering (SERGIS) prototype development. Initial beam operation and characterization are in progress. CG1B will be a 2.35Å monochromatic beam for a 2-axis utility diffractometer for sample alignment and monochromator development. CG1C will have a double-bounce monochromator system, which will produce a variable wavelength beam from about 1.8–6.4Å, and will be used for imaging and optical development. The CG1D beam is a single chopper time-of-flight system, used for instrument prototype and component testing. The cold neutron spectrum, with an integrated flux of about 2.7×109n/cm2s, has a guide cutoff at 0.8Å and useful wavelengths greater than 20Å.Initial results from CG1 include spectral characterization, imaging tests, detector trials, and polarizer tests. An overview of recent tests will be presented, and upcoming instrument prototype efforts will be described.

Dynamical theory calculations of spin-echo resolved grazing-incidence scattering from a diffraction grating

Neutrons scattered or reflected from a diffraction grating are subject to a periodic potential analogous to the potential experienced by electrons within a crystal. Hence, the wavefunction of the neutrons can be expanded in terms of Bloch waves and a dynamical theory can be applied to interpret the scattering phenomenon. In this paper, a dynamical theory is used to calculate the results of neutron spin-echo resolved grazing-incidence scattering (SERGIS) from a silicon diffraction grating with a rectangular profile. The calculations are compared with SERGIS measurements made on the same grating at two neutron sources: a pulsed source and a continuous wave source. In both cases, the spin-echo polarization, studied as a function of the spin-echo length, peaks at integer multiples of the grating period but there are some differences between the two sets of data. The dynamical theory explains the differences and gives a good account of both sets of results.

A spin-echo resolved grazing incidence scattering setup for the neutron interrogation of buried nanostructures

We present a dedicated experimental spin-echo resolved grazing incidence scattering (SERGIS) setup for the investigation of surfaces and thin films exhibiting large lateral length scales. This technique uses the neutron spin to encode one in-plane component of the wave-vector transfer in a grazing angle scattering experiment. Instead of the scattering angle, the depolarization of the scattered beam is measured. This allows one to achieve a very high in-plane momentum resolution without collimation of the incident neutron beam in the corresponding direction. SERGIS can therefore offer an alternative or complementary method to conventional grazing incidence neutron scattering experiments. We describe the experimental setup installed at the neutron sources ILL (Grenoble) and FRM II (Garching) and present data obtained with this setup on various samples exhibiting characteristic mesoscopic length scales in the range of several hundred nanometers. We also derive general formulas and error margins for the analysis and interpretation of SERGIS data and apply them to the cases of a one-dimensional structure and of an island morphology.

Combining of neutron spin echo and reflectivity: a new technique for probing surface and interface order

The recently proposed spin-echo resolved grazing-incidence scattering (SERGIS) uses the well-known neutron spin echo effect for encoding the momentum transfer in reflectometry. By the application of tilted magnetic-field borders, SERGIS measures the scattering angle in grazing incidence experiments in absence of any geometrical beam-defining tool, such as slits. The main difficulty in such set-ups is the realization of geometrically flat field borders. The possibility of the application of neutron resonance spin echo (NRSE) for such a purpose is discussed, where the field borders are defined by current sheets. Prototype SERGIS experiments performed on holographically made optical gratings at a NRSE triple-axis spectrometer are shown.