| Scanning tunneling microscopy (STM) has a very high spatial resolution can be used to characterize the local electronic density of states of the surface, verify with the theory, has played a unique role in many areas of surface science. However, in some extreme environments, STM still need in-depth research and development.At present, science and technology of strong magnets and strong magnetic field have been rapid development, and because both of electrons and nuclei in material have magnetic moments, when these materials are placed in a strong magnetic field, will inevitably be subject to influenced, showing some new features (and the mechanism of these new features usually do not know). Thus the development of strong magnetic field STM scientific research has broad prospects.STM mainly work under the lower magnetic field presently, and in the quasi-static conditions. Although static STM can probe many important physical properties of material, but the reality, after all, is mainly dynamic, harsh (chemical reactions, material growth, variable temperature variable field and other dynamic phenomena, as well as environment deterioration caused by these dynamic phenomenon). Therefore, the atomic resolution imaging in a dynamic process, and even non-atomic resolution imaging, is very significant. During the PhD under the guidance of my supervisor, I mainly develop the ultra-fast the STM (study dynamic processes, tracking dynamic atomic, and minimizing adverse environmental impact) and strong magnetic field STM.In order to greatly improve the speed of the STM imaging, the development of the fastest high-definition atomic resolution STM, I found out the main issues restricting traditional fast STM imaging speed:(ⅰ) traditional data output and collection methods lead to the effective sampling rate of the card is far below the nominal sampling rate, and thus difficult to output and collect enough data points (pixels) used to control the scanner and imaging;(ⅱ) the traditional scanners are made by polycrystalline piezoelectric ceramic devices, so the mechanical quality factor of the scanners are very low, poor dynamic characteristics;(ⅲ) traditional tunnel junction controllers are driven by high-voltage amplifier, whose strong electronics noise will make the tunnel junction instability increase, affecting the measurement accuracy. Therefore, a careful analysis of these issues is made by me, and proposed solutions:(ⅰ) changing the data output and acquisition mode of the conventional STM "move to the next point after one point finished" to a new model "blind scanning and Blind collecting", then the effective sampling is greatly increased;(ⅱ) quartz tuning fork, made by single crystal material,(the resonant frequency is high, can reach24kHz or more) as fast scanner replacing the traditional polycrystalline piezoelectric ceramics, the quality factor of the scanner greatly improved. And the structure of tuning fork leading to large deformation (amplitude) is used to scan the sample imaging, although the low dielectric constant of quartz;(ⅲ) low-voltage components to control the tunneling junction gap and scanners, the system noise greatly reduced, the image quality is greatly improved."Blind scanning and Blind collecting" means:Driving quartz tuning fork with a separate controller (signal generator) to scan, data acquisition card is only responsible for collecting the scan voltage and tunneling current data, between the controller driving and data acquisition no waiting and switching, which takes full advantage of the acquisition of the data acquisition card. I finally just use an ordinary520-kHz sampling rate data acquisition card and a quartz tuning fork less than an Yuan completing the highest imaging speed beyond:ultra-fast imaging speed of26000lines per second, the image the atoms is clearly visible. Far more than the previous literature records of the imaging speed of10000lines per second, even more than the quartz tuning fork oscillator resonance frequency (24kHz), it will be very conducive to the low operating voltage for a larger scan range (tuning fork amplitude has maximum at the resonance frequency) Moreover, high-performance, low-cost, wide source of quartz tuning fork; and low-voltage operation, low noise, low cost, making our experimental method can not only get a high test performance, but also has a very high application value. The work has been published in the American Institute of Physics Review of Scientific Instruments (Rev. Sci. Instrum.82,053705(2011)), and accompanied by high recognized.Secondly, in order to study the characteristics of the sample in a strong magnetic field environment, I have also successfully developed a STM with good performance under the18tesla strong magnetic field. Magnetic induction intensity aspects is equal to the documented atomic resolution imaging of the highest field, but also has many other aspects of the outstanding performance:(ⅰ)20Femto-Ampere current resolution (so that the imaging of near insulating samples is possible), and (ⅱ) large scale searching ability on sample surface (millimeters),(ⅲ) the whole low-voltage system (with low noise and high speed),(iv) atomic resolution STM under the strongest magnetic field, low temperature, high vacuum. The work has been published in the American Institute of Physics Review of Scientific Instruments (Rev. Sci. Instrum.82,053705(2012)), also highly praised by reviewers.In order to enable superior performance of the STM in the high magnetic field/magnets, I made some breakthrough in the following points:(ⅰ) assisting the supervisor design and custom a liquid nitrogen cold shield free ultra-quiet18/20Tesla superconducting magnet (18Tesla maximum at4.2K;20Tesla maximum at2.2K with liquid helium evaporation is4.8liters per day). The magnet vibration reduced compare with the liquid nitrogen evaporation in the cold shield of the dewar of the common superconducting magnets, clean up the test environment of the STM.(ⅲ) Independent designing the plug-in unit of superconducting magnet with the aperture of52.8mm, and get good test results. Fully taken into account in the design process, materials used in magnetic permeability, thermal conductivity, the degassing rate, processing, hardness, making it meet the compatibility of strong magnetic field, low temperature compatibility, ultra-high vacuum compatibility, process compatibility (and perfect workpiece design will be one waste paper if the processing does not come out), the vacuum seal compatibility.(ⅲ). cooperation with Yubin Hou of our group, the successful optimization of the design of10femto ampere current resolution preamplifier used in a strong magnetic field/magnets in STM, the current resolution of20femto ampere, and clear atomic resolution images collected in18Tesla strong magnetic fields.(ⅳ) cooperate with the group member Wang Qi, developed a "double inner electrodes piezoelectric ceramic" motor can work in strong magnetic field at low temperatures, ultra-high vacuum environment with low voltages, the minimum operating voltage is only9volts, and horizontal search function. The low-voltage operation reduces interference between the strong magnetic field and the signal collected., thus reducing the noise in the signal line, to improve the accuracy of the instrument.(ⅴ). working together with classmate to design and construction of a magnet damping muffler, springs and weight used to vibration isolation; damping rubber pad used to shock-absorbing; box sound reflection; and porous sponge wedge of sound-absorbing for the STM imaging measurements provide a stable external environment.(Vi) The master of the superconducting magnet maintenance, lift the operating magnetic field and low temperature operation, ultra-high vacuum operation, the operation of the STM test every detail in every aspect to ensure the coordination of the STM test system, optimization.For the low-temperature, high magnetic field and ultra-high vacuum compatibility of the tuning fork scanner, ultra-fast imaging under strong magnetic field will become a reality in the near future.Finally, I am responsible for optimizing the strong magnetic field STM system to a three in one combination of an international first microscope system (SMA, the STM-magnetic force microscope (MFM)-atomic force microscope (AFM)). In the SMA body and chamber structure layout, optimizing the design is compatible with the MFM, AFM.Successful completion of18Tesla strong magnetic field STM system is a good base for the platform of the other microscopes:(ⅰ). Good damping anechoic environment. STM tunneling current distance is less than a nanometer, and proximity with the AFM atomic forces interaction, and far less than the MFM long-range magnetic effect range (several tens of nanometers). That the platform meets the AFM test environment requirements, much better than MFM test environmental requirements needed.(ⅱ) low-voltage operating of the piezoelectric motor, but also to the AFM and MFM measurements with low noise, high-precision advantage.(ⅲ) a wealth of experience about the plug-in system in a strong magnetic field/magnets in low temperature, ultrahigh vacuum, strong magnetic field/magnets operating accumulated, the development of MFM and AFM only system used in the zero field debugging.Currently, together with my classmate Yizhi Shi, we successfully in six months time debugging tape magnetic domain images of the highest magnetic field of10Tesla high resolution MFM, observing magnetic domains with magnetic field strength changes. The work has been organized into the article published, is currently being reviewed:AFM under strong magnetic field/magnets are responsible for the commissioning process debugging by Hou Yubin. Entire SMA combo in good condition. SMA combo importance is reflected in three microscope can complement each other, the electronic density of states of the sample surface, the magnetic domain distribution and atomic structure and other information at the same time, with the same environmental measurement, the sample properties will be get good interpreting. Eventually, I published two first author paper classified as the USTC SCI Ⅱ article, and the two works for the high praise of peers and peer reviewers. Because of my ultra-fast STM work, my supervisor was invited to one of the largest international science and technology book publishers, Taylor&Francis’ new book,"Fundamentals of Picoscience":Atomic-Resolution Ultrafast Scanning tunneling Microscopy.The work of high-performance STM development under strong magnetic field work is evaluated by the reviewer:"The instrument is certainly cutting edge in the field of scanning probe microscopy". |
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