Experimental Study On The Soft X-ray Spectro-microscopy

Soft X-ray spectroscopy microscopy plays an important role in the research of battery materials,life science,environment and earth science.Scanning transmission X-ray microscopy(STXM)is a kind of transmission microscopy technique,which can provide chemical sensitive images with spatial resolution as low as about 10 nm,combined with the X-ray absorption spectrum(XAS)technique with a wide energy range.In addition,STXM can be used in atmosphere,low temperature,high vacuum and magnetic field.Therefore,STXM has become a powerful scientific tool to study the microstructures of various nanomaterials,polymers,environmental,biological,inorganic and magnetic materials.However,with the deepening of scientific research,the users’ demand for better energy and spatial resolution,experimental efficiency and experimental environment is increasing,such as through ptychography,computed tomography and fluorescence imaging.With the continuous development of imaging methods such as abovementioned,focal stack imaging,STXM needs to be continuously studied and improved to achieve higher experimental efficiency,higher resolution and stronger anti-noise capability,making it more compatible with other advanced methods.Therefore,to improve the data quality,experimental efficiency and the signal-to-noise ratio of STXM,and solve the limitations of the existing software and hardware,we proposed innovative solutions,and carried out a lot of work including instrument design and construction,software development,and test experiments.The specific pieces of work are as follows:1.In order to realize a new high-performance STXM at the Shanghai Synchrotron Radiation Facility(SSRF),improve the imaging quality and efficiency,and solve the backlash errors caused by the turning-back movement of the piezo stage,we built a new type of STXM at the SSRF BL08 U beamline,and developed a bidirectional scanning technique.This new STXM comprises of the zone plate,OSA,sample holder,detector and laser interferometer as its core components.The incident monochromatic X-ray is focused by the zone plate,and after removing the stray light and high-order light through the OSA,the transmitted light intensity through the sample is recorded by the detector.The core of STXM is the development of a control system,which mainly includes the control of the coarse and fine moving stages,the detector,the laser interferometer,and the data acquisition of related detectors.We used Python language to directly control each positioner with high precision and flexibility,and used field programmable gate array(FPGA)to realize the synchronous acquisition and triggering of the X-ray intensity signal recorded by the detector and the sample position information recorded by the laser interferometer.Among them,the bidirectional scanning technique uses FPGA to synchronously collect the X-ray intensity obtained by the detector and the corresponding sample position recorded by the laser interferometer.The X-ray intensity signals were arranged according to the instant sample positions to form the original image.Subsequently,a self-developed post-processing program was used to grid the original image,average the X-ray intensity values of the data points falling into each grid,and finally produce a uniform and regular microscopic image.Furthermore,we have developed a graphical user interface(GUI)of the system with Py Qt5.The interface has the advantages of friendly interface,high automation,strong operability,simple,and intuitive.With this interface,users can conduct one-click operation for complex STXM experiments.Finally,we used a standard test pattern to test the imaging performance of the new STXM.It only took 16.6 minutes to get an image with a size range of 10 μm*10 μm,1000*1000 pixel points with a dwell time of 1 ms,and the spatial resolution reached an accuracy better than 30 nm.Experimental results show that the bidirectional scanning mode of the new STXM can make the piezo stages uninterruptedly and reciprocally scanning at high speed,which increases the resistance to backlash errors and external vibrations,improves the scanning efficiency and image quality,and reduces the radiation dose of the sample in the process of an experiment.Especially in some imaging experiments with a large amount of pixel points and long-time consumption(such as energy stacks with a large energy range and multi-angle tomography),the new STXM can achieve low radiation dose,high signal-to-noise ratio,high spatial resolution and high-speed imaging,and greatly save users’ machine time or beamtime.2.Ptychography is a new imaging method based on STXM.It uses a zone plate to focus X-rays into a small spot,and uses OSA to eliminate high-order light and stray light.Then,the diffraction patterns generated by the beam spot scanning across a sample region are recorded by a CCD or s CMOS detector,and finally a microscopic image is obtained by an iterative reconstruction of the diffraction patterns.Because of its high spatial resolution,high experimental efficiency and high chemical sensitivity,it has received extensive attention all over the world in recent years.However,due to the lack of suitable data reconstruction software,application of this imaging technique is greatly limited.Therefore,we have developed a visual data reconstruction software Py PIE in Python language.The software supports a variety of reconfiguration modes with high automation,simple and friendly interface,and good reconstruction result.Furthermore,based on the Ambient-STXM of Canadian Light Source,we optimized and improved the instrumentation for ptychography,and characterized and studied the high nickel lithium rich cathode particles.First of all,the conventional STXM and STXM ptychography were used to image the K absorption edge of O.The results showed that ptychography has better absorption contrast and spatial resolution,and the experimental efficiency is higher.Furthermore,the spectral ptychography of the electrode particles was performed at the K-edge of O,L-edge of Mn,K-edge of F and L-edge of Ni.The composition and chemical information were added to the imaging results.We found that the F in the electrolyte was embedded into the lattice of the degraded electrode particles,accompanied by the dissolution of Mn in different degrees.This work provides a new tool for the research of battery materials,provides a direction for exploring the degradation mechanisms and preparation processes of batteries,and makes ptychography from a "developing technique" to a "technique for the public".3.Soft X-ray absorption spectroscopy(XAS)is a complementary characterization technique with STXM,and is also the foundation of STXM for element and valence state speciation.In the spectral chamber at the front end of STXM experimental chamber of BL08 U station,a picoammeter is used to collect Auger and photo-electrons current produced by X-ray irradiation on the sample.Because picoammeters are only highly sensitive,they can not only collect the signal current,but also amplify the noise current,resulting in poor signal-to-noise ratio in spectra,and users often find it difficult to get ideal results.In order to solve this practical application problem,we have designed and developed a new X-ray absorption spectrum measurement system.The original picoammeter was replaced by a combination of a preamplifier with filtering functions,a V2 F converter and a multi-channel data acquisition board.The photocurrent signal of the sample is collected by the preamplifier,and the signal is optimized by its special functions(such as filtering,drift compensation and signal gain).The weak photocurrent signal is converted into a large voltage signal,and then the voltage signal is converted into a frequency signal which is more suitable for long-distance transmission by the V2 F converter.Finally,it is collected by the high-speed multi-channel acquisition board.The control system of the setup was realized by the Experimental Physics and Industrial Control System(EPICS)platform and Python language.We further used CS-studio to develop a graphical user interface for the system,which makes it convenient for users to operate and control STXM.Finally,we used the new absorption spectrum measurement system to carry out experiments.Compared with the results of the old system,the results of the new system are smoother in spectroscopy,higher in signal-to-noise ratio,and larger in acquisition efficiency.