Tripling of the scattering vector range of X -ray reflectivity on liquid surfaces using a double crystal deflector.

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Tripling of the scattering vector range of X-ray

reflectivity on liquid surfaces using a double crystal

deflector.

Oleg Konovalov,1* Valentina Belova,1,2 Mehdi Saedi,3 Irene Groot,3 Gilles Renaud,2 Maciej

Jankowski1*

1 The European Synchrotron- ESRF, 71 Avenue des Martyrs, CS 40220, 38043 Grenoble Cedex 9, France

2 Univ. Grenoble Alpes, CEA, IRIG/MEM/NRS 38000 Grenoble, France

3 Leiden Institute of Chemistry, Leiden University, P.O. Box 9502, 2300 RA Leiden, The Netherlands

Correspondence e-mail: oleg.konovalov@esrf.fr, maciej.jankowski@esrf.fr

Abstract:

We achieved a tripling of the maximum range of perpendicular momentum transfer () of X-ray

scattering from liquid surfaces using a double crystal deflector setup to tilt the incident X-ray beam.

This is obtained by using Miller indices of the reflecting crystal atomic planes that are three times

higher than usual. We calculate the deviation from the exact Bragg angle condition induced by a

misalignment between the X-ray beam axis and the main rotation axis of the double crystal deflector

and deduce a fast and straightforward procedure to align them. We show measurements of X-ray

reflectivity up to  on the bare surface of liquid copper.

Introduction:

Investigation of processes occurring at atomic and molecular levels at the surfaces and interfaces of

liquids is of paramount importance for fundamental surface science and practical applications in

physics, chemistry, and biology (Pershan, 2014; Dong et al., 2018; Zuraiqi et al., 2020; He et al., 2021;

Allioux et al., 2022). However, experimental methods that allow insight into these phenomena are

scarce, making synchrotron-based X-ray scattering the prime choice when sub-nanometer accuracy is

needed. The high intensity of synchrotron X-ray beams, their highly compact beam size, and their very

low divergence allow in situ and operando experiments with sub-second time resolution, which is

impossible with standard laboratory X-ray sources. Furthermore, the recent upgrade of the European

Synchrotron Radiation Facility (ESRF) allows for very demanding experiments using the extremely

bright X-ray source (EBS) with unprecedented parameters (Raimondi, 2016).

One of the most widely used X-ray-based techniques for the characterization of liquid surfaces is X-

ray reflectivity (XRR). It relies on measurements of the intensity of the reflected X-ray beam from a

surface at varying incidence angles, the so-called reflectivity curve, which is used to deduce the

surface's out-of-plane electron density profile. Applications of this method are very diverse. They

range from the determination of the roughness of the water surface (Braslau et al., 1985), lipid layers

on the water-air interface (Helm et al., 1987), free liquid metal surfaces (Magnussen et al., 1995;

Regan et al., 1995) displaying layering, polymer assemblies on water (Kago et al., 1998), to protein

layers on liquid surfaces (Gidalevitz et al., 1999). Recent technical developments of advanced sample

environments and methods further allowed the investigations of even more complex systems. Among

these, we may cite Langmuir troughs (Yun & Bloch, 1989) and specialized reactors (Saedi et al., 2020),

studies of electrochemical systems (Duval et al., 2012), layer-by-layer assembly of DNA (Erokhina et

al., 2008), self-assembled layers (Bronstein et al., 2022), or 2D materials formation on liquid metal

catalysts (Jankowski et al., 2021; Konovalov et al., 2022). Thus, the use of XRR, sometimes in

connection with other methods like grazing-incidence small-angle scattering (GISAXS) (Geuchies et al.,

2016) or X-ray absorption spectroscopy (XAS) (Konovalov et al., 2020), offers a powerful tool for the

characterization of a vast family of materials on liquid surfaces.

However, one general difficulty exists in performing XRR on liquid surfaces since neither the liquid

sample nor the synchrotron source can be tilted. The requirement of variation of the X-ray beam

grazing angle () at the sample surface to change the (vertical) scattering vector perpendicular to the

surface,  ( is the X-ray wavelength), brings significant experimental difficulties.

Different technical solutions were implemented to overcome this problem. The synchrotron X-ray

beam can be inclined with respect to the horizontal sample plane using mirrors or single or double

Bragg reflections from crystals (overview in (Pershan & Schlossman, 2012), Chapter 2). The main

drawback of using a mirror is the maximum achievable qz value, usually limited to several critical angles

of the total surface reflection on the mirror material. The single crystal deflector (SCD) extends this

range to  , where  is the Bragg angle of the chosen scattering planes of the crystal (Smilgies

et al., 2005). However, the use of an SCD demands to move the sample to follow the horizontal and

vertical displacement of the beam on it, concomitantly with the change of the  angle. This has the

drawback to agitate the liquid surface. A more recent solution, the double crystal deflector (DCD)

(Honkimäki et al., 2006), relies on a double Bragg reflection from two crystals in a geometry that does

not require any sample movement with a change of the  angle, thus assuring a more stable

measurement. The maximum obtainable incident grazing angle is  , where  and

 are the Bragg angles of the first and second crystals, respectively, and (Murphy et al.,

2014). Practically, in the case of SCDs or DCDs, the maximum achievable perpendicular momentum

transfer 

, does not depend on the X-ray beam energy (see SI Note 1). The most typical choices of

crystal sets used in realized DCDs are Ge(111)/Ge(220), Si(111)/Si(220), and InSb(111)/InSb(220). The

maximum scattering vector reached for these sets is about 2.5 Å-1 (Honkimäki et al., 2006; Arnold et

al., 2012; Murphy et al., 2014), which might not be sufficient for studies of some liquid metals, e.g.,

the surface layering peak and the first structure peak of liquid copper are present at approximately 3

Å-1 (Eder et al., 1980).

The ID10 beamline at ESRF was equipped with an SCD since 1999 (Smilgies et al., 2005). During more

than 1.5 decades of operating this instrument, deep technological knowledge and experience were

acquired, which led to the design and construction, in collaboration with Huber Diffraktionstechnik

GmbH & Co. KG company, of a new generation instrument to study liquid surfaces and interfaces,

using a DCD. The new 6+2 diffractometer, equipped with a DCD, has been operating since 2016. This

diffractometer has the necessary set of rotation and translation stages to precisely align the DCD and

assure its high rigidity and accuracy during operation. In this paper, we present a method of tripling

the 

 value using a DCD by using higher-order Bragg reflections of the two crystals. In practice, we

use the Ge(333)/Ge(660) reflections instead of the now standard set of Ge(111)/Ge(220) reflections.

In addition, we confirm experimentally that even with a three orders of magnitude loss of photon flux

with these reflections, recording X-ray scattering at high  is still feasible thanks to the recently

upgraded ESRF-EBS synchrotron beam (Raimondi, 2016).

Experimental:

XRR measurements using a DCD at the ESRF beamline ID10 were performed using a monochromatic

X-ray beam with an energy of 22 keV, monochromatized by Si(111) channel-cut monochromator

diffracting in the vertical plane. The DCD was aligned according to the below-described procedure.

The beam intensity reaching the sample after scattering by the Ge(333) and Ge(660) reflections was

71010 ph/s at a synchrotron storage ring current of 200 mA. The full width at half maximum of the

beam at the sample position was measured to be 26×10 m2 (H×V) after focusing with 29 Be parabolic

lenses with a radius of 300 m, located before the DCD at 8.9 m from the sample and 36.2 m from the

X-ray source. The X-ray beam reflected from the surface was measured with a CdTe MaxiPix 2D

photon-counting pixel detector (pixel size: 55×55 m2, detector area: 28.4×28.4 mm2, sensor: 1 mm

thick CdTe) and 5 s counting time at each incident angle.

We performed XRR measurements on bare liquid copper and on a graphene layer on liquid copper in

situ at T = 1400 K (above the copper melting temperature) in a specially designed reactor dedicated

to chemical vapor deposition (CVD) growth of thin layers of graphene on a liquid metal catalyst (Saedi

et al., 2020). Single-layer graphene was grown under the same conditions as described in (Jankowski

et al., 2021). The obtained scattering data, which include non-specular components (diffuse scattering

and scattering from the bulk of liquid copper), were processed following the procedure presented in

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摘要：

TriplingofthescatteringvectorrangeofX-rayreflectivityonliquidsurfacesusingadoublecrystaldeflector.OlegKonovalov,1*ValentinaBelova,1,2MehdiSaedi,3IreneGroot,3GillesRenaud,2MaciejJankowski1*1TheEuropeanSynchrotron-ESRF,71AvenuedesMartyrs,CS40220,38043GrenobleCedex9,France2Univ.GrenobleAlpes,CEA,IRIG/M...

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Tripling of the scattering vector range of X -ray reflectivity on liquid surfaces using a double crystal deflector.

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