BL02U2

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Scientific Scope
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　　Surfaces and interfaces of low-dimensional thin films：Due to spatial confinement and nanoscale coupling between different epitaxial layers, complex materials can exhibit a wealth of physical phenomena, including ferroelectricity, magnetism, multiferroics, electronic and ionic conductivity, superconductivity, and electric, magnetic, elastic, and optical coupling. Surface X-ray diffraction is an ideal tool for examining the atomic structure of thin film materials such as low-dimensional oxides. One of the advantages of X-ray technology is that the surface and structure of the film can be obtained without destroying the sample. X-ray surface diffraction can therefore provide information on the evolution of surface and interfacial structures during in situ material growth or processing, which is essential for controlling the structure of materials and understanding the resulting properties.

　　The atomic level structure of a solid surface: There is a growing need for scientists to accurately determine the structure of surfaces in order to understand processes occurring in solid gas or solid vacuum environments. Determining structure is one of the major challenges in surface science. Surface X-ray diffraction has evolved into a powerful technique to achieve this goal. High-brightness X-rays from triple-generation sources are essential for determining the location of weakly scattering atoms (oxygen, nitrogen, and carbon) on the surfaces of complex materials such as topological insulators, two-dimensional materials, and Group III-V semiconductors. In these systems, reconstruction, relaxation, absorption, and reaction information of surfaces can be monitored in situ.

　　Solid-liquid and liquid-liquid interfaces: X-rays are capable of penetrating matter and probing interfaces of interest, even in environments such as liquid-solid and liquid-liquid interfaces, and activities involve the behavior of crystalline monolayers from solutions to solid substrates, amphiphilic molecules at oil-water interfaces, and lipids and proteins at solid-liquid interfaces. Another important area of science includes electrochemical interfaces, confined liquids, polymer interdiffusion, and colloidal interactions.

　　Self-assembly in biofilms and soft matter: Soft matter is a multi-component, multi-phase, non-equilibrium system composed of molecules in which many atoms (mainly carbon, hydrogen, oxygen and nitrogen) are connected by covalent bonds, e.g. biomaterials, polymers and liquid crystals. In these fields, understanding surface and interfacial properties becomes crucial. Surface X-ray diffraction will enable these structures to be studied in their natural aqueous or chemical environments. Soft matter activities involve in situ studies of materials processing, polymer self-assembly, constrained geometry dynamics, biofilm structure dynamics, and interactions between biomaterials and solid surfaces. 

Beamline Layout and Specifications
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A cryogenic permanent magnet oscillator (CPMU) with magnetic period λu=20 mm and total length L=1.5 m is selected as the beam line source for BL02U2 to meet the demand of high energy range. The brightness profile is calculated using XOP simulation software with high brightness and good continuity. The spectral flux into the 80 μrad × 40 μrad center cone is about 3.8×1014 phs/s/0.1%BW at K=1.37 (corresponding toE=10 keV). The source size is about 380×23 μm2, and the divergence of the source is about 60×16 μrad2 at 10 keV. 
　　The beamline optics consist of the following main components: horizontal deflector mirror, double crystal monochromator (DCM), vertical focusing mirror and horizontal focusing mirror. During design, the beamline layout utilizes a minimum number of optical elements with good beam stability. A white beam slit located at 19.3 m from the light source limits the acceptance angle of the beamline to 0.08 mrad × 0.04 mrad. Behind the white light slit, a double-crystal monochromator (DCM) was used 20.86 m downstream of the source, and a double-crystal Si (111) monochromator was used, taking into account the energy resolution and energy range requirements. The total power on the first crystal is about 160 W, with a maximum power density of about 33 W/mm2, cryogenically cooled using liquid nitrogen. In the beamline, a monochromatic X-ray beam is focused on the sample spot through horizontal and vertical focusing mirrors. The vertical focusing mirrors are cylindrical mirrors with meridian bends, placed horizontally; the horizontal focusing mirrors, also with meridian bends, are placed laterally, with focus points at 46 and 51 meters, respectively. The two focusing mirrors also serve to suppress high harmonics. The focusing mirrors are made of silicon with heavy elemental coatings (Pt and Rh). For energies between 4.8 and 10 keV, the uncoated portion of the focusing mirror can be used, while for energies between 10 and 20 keV, the Rh coating is more suitable. The Pt coating is used for energies between 20 and 28 keV.
　　 
　　Optical design schematic and layout of surface diffraction beamline
　　
According to the simulation of the SHADOW tracing program, the focused beam size of the beamline at 10 keV is about 140×72 m2, the photon flux at the sample position is about 1.45×10-4, the photon flux at the sample position is about 8.8×1012 phs/s (10 keV/300 mA), and the beam divergence at the focal point is about 156×58 μrad2。
Endstations
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　　The Surface Diffraction Beamline consists of two stations focusing on different samples.Hutch 1 is designed as a general purpose surface diffraction station that can perform a variety of surface and interface diffraction experiments for thin film and solid-solid interface structural studies, with enough space to install specially designed in-situ equipment. Hutch 2 is a station specifically designed for structural studies of liquid surfaces, solid-liquid, and liquid-liquid interfaces. Hutch 2 is an experimental station for liquid surface, solid-liquid, and liquid-liquid interfaces. Experimental Station.
　　　
　　General arrangement of the endstations

 

　　The Hutch 1 is equipped with a high-precision multipurpose diffractometer. The diffractometer has 4 sample circles + 2 detector circles + 3 analyzing crystal circles, and the high-resolution positioning of the rotational axes permits a range of 2θ up to 170° with a step accuracy of 0.0005. The diffractometer has a double detector arm with a scintillation detector and a small surface detector (Eiger 500k) that can be rotated around its plane. The total load capacity of the detector arms is up to 150 kg. The scintillation detector-to-sample distance is manually adjustable between 700 mm and 1100 mm. The small surface detector system can be adjusted manually by 520 mm and electrically by an additional 400 mm. The diffractometer is also equipped with a high-precision translation stage, a telephoto microscope, a goniometer head, a slit, a cryogenic holder and an adjustable attenuator. At the end of Hutch 1, a Pilatus 2M (1000 μm Si) large surface detector was also installed. The pixel size of the large surface detector is 172 × 172 μm. For maximum flexibility in detector position, the detector is mounted on an xyz displacement stage of the specified design. Typical sample-to-detector distances range from 150 mm to 400 mm for reflectance (including GIXRD) and transmission modes as well as for X-ray diffraction measurements in in-situ experiments.
　　
　　Experimental station layout. Hutch 1 Equipment:(a) Multi-purpose diffractometer system. (1) Sample stage. (2) Eiger 500K. (3) Point detector. (b) Large surface detector system. (1) Pilatus 2M. (2) Detector Positioning Stage - Sample Stage. Hutch 2 Equipment: (c) Liquid Diffraction System (1) Bicrystal Deflector, (2) Diffractometer. (d) Bicrystal deflector schematic.
　　
　　The diffractometer in Hutch 2 is dedicated to liquid-oriented surface X-ray diffraction experiments. The diffractometer has 4 sample circles (2 horizontal and 2 vertical) + 2 detector circles + double crystal deflector. In particular, a bicrystal deflector placed upstream of the diffractometer and a companion Eiger 4M 2D surface detector were designed for the study of liquid surfaces and liquid-liquid interfaces. By adjusting the direction of the incident beam, reflectance and grazing incidence diffraction measurements of liquid surfaces and interfaces can be realized. For example, for a 10 keV incident beam, the horizontally incident beam is deflected by Ge(111) and Ge(220) crystals, and the beam can be bent downward by up to 14.2°，enabling structural characterization under sample horizontal conditions.
　　The BL02U2 beamline also offers a wide range of experimental environments. Nitrogen/Argon, high temperature (up to 1500 K) and cryogenic (down to the liquid nitrogen range) thin film in-situ devices are available for use on the diffractometer of the Hutch 1, and the Hutch 2 offers an LB tank for liquid scattering. The in-situ electric field unit provides a voltage source (0-3000 V) for investigating the structure of thin film samples under an electric field.
Data Acquisition and Analysis
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Time resolution is up to 3000 frames per second with the Eiger 500K detector and up to 25 frames per second with the Pilatus 2M detector. The line station provides Fit 2D software for teaching and on-site processing by the user.
　　The beamline offers the following main test methods: GIXRD, XRR, RSM, PDF, XRD, and liquid XRD. Additional experimental test methods will be determined by the user after contacting the line station to confirm their feasibility. Please note that the beamline and its methodology are designed primarily for the testing of thin film samples.