Neutron Reflectometer - ROG

The ROG neutron reflectometer measures the way neutrons are reflected by flat surfaces and interfaces with the aim to provide information about the thickness, composition and roughness of thin films and other layered structures. It can be used to study (stacks of) layers with thicknesses of 5 - 150 nm while providing a resolution up to 0.2 nm. Neutron reflectometry can be used to non-destructively study both liquid and solid samples in a variety of different experimental conditions.

The neutron reflectometer is available to scientist with and without experience with neutron reflectometry from all over the world. Interested? Please contact the instrument scientist to learn more about the possibilities.

The ROG neutron reflectometer is currently being upgraded as part of the OYSTER project. In combination with the installation of a cold neutron source, this will increase its performance by up to 100x. We hope to be fully operational by autumn 2020.

Applications

  • The study of surfaces and interfaces of thin films, multilayers and coatings:
    e.g. in-situ determination of hydrogen concentration in metal hydrides, in-operando study of battery materials, composition of thin films of polymers.
  • Solid-liquid interfaces and membranes: e.g. protein diffusion in membranes, membranes for fuel cells, surfactants, polymers, proteins and enzymes at interfaces, drug delivery systems, study of the interface between a battery electrode and a liquid electrolyte.
  • Liquid-air and liquid-liquid interfaces: e.g. proteins, polymers and enzymes in solution,       

Example: Thin film metal hydrides as optical hydrogen sensor

The detection of hydrogen is crucial in many industrial processes and for its adaptation as an energy carrier. Thin film metal hydrides can be used to optically probe the hydrogen pressure without the need for electric currents near the sensing area. These sensors utilize the fact that the optical properties change when the hydrogenation of a thin film metal hydride changes in response to a different partial hydrogen pressure in the environment of the sensor. In order to fundamentally understand the structural response of these thin films to hydrogen, facilitating the rational design of new hydrogen sensors, neutron reflectometry was used to study the temperature and hydrogen pressure dependence of the hydrogen content and thickness of the sensing layer in a variety of thin film metal hydrides (e.g. Hf, Ta, Pd1-yAuy). Neutron reflectometry provided the hydrogen pressure dependence of the hydrogen content in the metal hydride layer as well as the volumetric expansion upon hydrogen sorption and thus enabled a deeper and more fundamental understanding of these materials.

Figure 1.

Artist impression of an optical hydrogen sensor where a sensing layer (in this case hafnium) is deposited on top of an optical fibre. When exposed to hydrogen, the optical properties of the sensing layer changes, which can be detected and used to probe the hydrogen pressure.  A palladium layer is used to dissociate the hydrogen molecule, catalysing the hydrogen sorption by the sensing layer and thus ensuring a fast response of the sensor to a pressure change.

References

Bannenberg et al. ACS Applied Materials & Interfaces 11(17), 15489-15497 (2019): https://doi.org/10.1021/acsami.8b22455

Bannenberg et al. Sensors and Actuators B: Chemical 283, 538-548 (2019): https://doi.org/10.1016/j.snb.2018.12.029

Bannenberg et al. Nature Communications 18, 15718 (2017): 

https://doi.org/10.1038/ncomms15718

How it works

In a neutron reflectometer, a flat sample is illuminated by a collimated neutron beam under a small angle (< 5 degrees) and the fraction of neutrons reflected relative to the total number of neutrons incident on the sample is determined (i.e. the so-called ‘Reflectivity’).

Similar to the reflections of light and X-rays from a surface, the reflectivity of neutrons depends on the wavelength λ and incident angle θ of the neutron. As illustrated in Fig. 1, for incident angles smaller than the critical angle, all neutrons are reflected. For larger angles, the reflectivity of a material decreases rapidly with increasing angle of incidence. Instead of expressing the reflectivity as a function of the incident angle and/or wavelength, it is typically expressed as a function of the momentum transfer Q of the neutron when reflected by the sample: The momentum transfer incorporates the effect of the incident angle and wavelength in a singly quantity and is related to the angle of incidence and wavelength through .

Figure 2.

(a) schematic representation of a neutron reflectometry experiment. In a neutrons reflectometry experiment, a neutron beam illuminates a sample under a small angle of incidence θ. This neutron beam is partly reflected by the sample and the fraction of neutrons reflected is named the reflectivity. An example neutron reflectogram is provided by panel (b), which displays the reflectivity as a function of q. Q is the momentum transfer of the neutron when reflected by the sample and combines the effect of the wavelength and incident angle on the reflectivity in a single parameter. For a material with a different index of refraction/scattering length density (sld), the reflectogram changes. Note that the angles in the schematic are largely exaggerated and the reflectivity is plotted on a logarithmic scale.

Analogous to the reflection of optical light from surfaces, the critical angle and the subsequent decay of reflectivity with increasing angle of incidence depend on the index of refraction of the material studied. However, for neutrons it is often more convenient to use a different quantity named the Scattering Length Density (SLD). The SLD is the product of the scattering length of a material (an isotope-dependent quantity that is well known) and the number of atoms per volume unit.  In contrast to X-rays, the scattering length of neutrons does not monotonously increase with the atomic mass of an element, making neutrons especially powerful for detecting light elements as H and Li and allowing for isotopic substitution (e.g. hydrogen by deuterium).

When a layer of a material is deposited on top of a substrate composed of a material with a different scattering length density, interference occurs between the neutrons reflected from the surface of the sample and neutrons reflected from the interface of the two materials. As illustrated by Figure 2, this interference results in so-called fringes in the reflectivity pattern, whereby the width of the fringes is related to the thickness of the layer and the amplitude to the difference in scattering length density of the two materials.

Figure 3.

Schematic representation of a sample composed of a thin film deposited on a slap of material. Interference occurs between neutrons that are reflected from the surface of the sample and those that are reflected from the interface between the material. This interference gives rise to so-called ‘fringes’ in the reflectogram, of which the width is related to the thickness of the material and the amplitude to the difference in scattering length density.

In a typical reflectometry experiment, the Q-dependence of the reflectivity of a sample (can be composed of multiple layers) is determined. Subsequently, the measured reflectivity is fitted to a model system, from which estimates about the layer thicknesses, scattering length density (and thus composition of the material!) and roughness can be obtained.

Sample requirements

Neutron reflectometry requires large samples with a surface area of at least 400 mm2 (ideally even larger). Most importantly, the samples have to be homogenous in the in-plane direction and extremely flat. Typically, samples for neutron reflectometry are deposited on specially polished (fused) quartz, SrTiO3, MgF substrates or  relatively thick (>3 mm) Si wafers.  Please contact the instrument scientist for more details.

Sample environment

Currently, we have the following sample environment readily available:

  • Gas pressure (0 – 1.5 bar) and temperature (25 – 150 C) controlled cell for in-situ measurements.
  • Langmuir though with anti-vibration table
  • Laminar flow cell (Physica B 357 (2005) 208-212) 
  • Overflowing cylinder (Physica B 283  (2000) 278-281)
  • Electrochemical Cell (under development)

Not (yet) available what you wish? Please contact the instrument scientist to discuss about the possibilities.

Specifications of the instrument

The ROG neutron reflectometer is an unpolarized time-of-flight neutron reflectometer with a horizontal scattering geometry that can be used to study both liquid and solid samples in a variety of different experimental conditions.  Further specifications of the reflectometer can be found in the table below. 

Specifications of the ROG neutron reflectometer 

 

Chopper and resolution

Double-disk chopper with variable interdisk-distance (0.2-0.7 m) that can operate at 10 – 25 Hz. Typical operation with an interdisk distance of 0.3 m results in Δλ/λ ≈ ΔQ/Q ≈ 0.04

Beam size at sample position

40 mm (width)

Sample table

+/- 150 mrad, +/- 5 mm vertical displacement, maximum weight of 150 kg

Wavelength range

0.3 < λ < 1.5 nm (Cold source, expected)

Detector

3He detector

Sample-to-detector distance

2 m

Chopper-to-sample distance

5 m

Incident angle

 

       Solids: -25 < θ <  75 mrad

       Liquids: 0 < θ <  75 mrad

Wavevector transfer

 

       Solids: 0 < Q <  3 nm-1

       Liquids: 0 < Q <  3 nm-1

 

 

Instrument scientist

Lars Bannenberg