| The scanning tunneling microscope (abbreviated as STM) is an important surface science (including surface physics, material science, surface catalysis, nano devices and so on) tool because of its capabilities in atomic resolution, atomic manipulation, etc. It has nevertheless several very severe drawbacks:(1) STM does not provide true atomic resolution (does not give the true structure of atom arrangement on the sample surface) because the tunneling current data that STM uses to construct an image only reflect the electrons’local density of states (LDOS) in the sample, which is not directly related to the atomic positions in the sample. There may be more electrons at the location where atoms are missing and vice versa. One famous example is graphite sample whose lattice structure is hexagonal with no atom at the center. But, the STM image of graphite is a larger hexagonal lattice with a spot at the center, which is a big mismatch with the true atomic arrange (structure). (2) STM can not measure insuting samples because the tunneling current is too weak to detect. (3) STM is not good at the measurement of another important property of electrons: magnetic property (the spin), because what is directly related to the tunnrling current is the charge of electrons, not the spin. For example, the areas of the sample with different spin orientation (i.e. different domains) may produce tunneling currents that are not correlated.These limitations of STM are definitely not favorable for the fundamental understanding of matters because the properties of matters are mainly the results of the interactions among the electrons (with spin and charge) and the structure. As an example, superconducting and colossal magneto-resistance effects are the results of the interactions of the internal electrons and structure. Therefore, using only STM is insufficient, especially in the studies of the strongly electron correlated materials, magnetic phase transision, conductor-to-insulator transition, etc.In the period of my Ph.D. program, my work is main the constructions of a microscope combination system which integrates a STM, a magnetic force microscope (MFM) and an atomic force microscope (AFM) and is called SMA system for convenience. In this SMA system, the AFM is in charge of the measurement of tip-sample interaction force which can give the true atomic arrangement structure because there is a non-vanishing force between the tip and sample if and only if there is a sample atom (actually nucleus) at the measurement spot. Further more, the AFM can also measure insulating samples because the interaction forces bweteen the tip and sample is not related to the sample’s conductivity. The MFM is responsible to the measurement of magnetic domain structure and their behavior. This is the importance of my work.My research work and its creativity as a Ph.D. student in the 3 years from 2007 to 2010 are:(1) the SMA system is the first 3 microscope combo in the world; (2) the SMA system is miniaturized into a small enough setup that can be put in a 52mm bore 20 Tesla strong magnet is pairly challenge because the strongest magnetic field for only one microscope (STM only) has a field strength of only 15 Tesla; (3) making this SMA capable of searching rare nano targets (such as defect and nano devices) across the macroscopic sample (millimeter range). In the construction of the SMA, my responsibilities are:(a) the construction of the MFM; (b) turn the precise lateral junction STM that Yunin Hou built into a millimeter range searchable STM that are appropriate for the 20T-SMA (this work of mine, combined with Yubin Hou and Jihui Wang’s work participated in the 2009 Anhui Province "Challenge Cup" and "the 11th National Challenge Cup" and was granted the "Special Grade Award" and "2nd Place Award", respectively); (c) set up an AFM oscillator circuit (with Quanfeng Li) and obtained a Q factor of greater than 2×105 (world record), and participated in the AFM test with Yubin Hou and Quanfeng Li; (d) successful in the construction of a SMA vacuum chamber system that can fit on the 52mm magnet bore and is detachable at the SMA body without causing any liquid helium leak; (e) the overall assembly and the test of the whole SMA system.Our lab owns the complete intellectual property rights of the mentioned SMA system (totally 19 invention patent applications, in which I am the first author for 4), whose design is very unique. In this SMA, the tranditional tip-sample regulation by the axial displacement of the piezoelectric scanner tube (PST) is changed to the lateral displacement of the PST. This change greatly reduces the diameter of the SMA to only 28mm, which can be easily set into extreme physical conditions such as low temperature, ultra-high vacuum and strong nagnetic field (20T,52mm). The STM and AFM shear the sample tip which is perpendicularly glued to the free end of one prong of a quartz tuning fork. The other prong of the fork is mounted on a PST (called PSTM/AFM). The tunneling current signal of the tip is used to form an STM image and the oscillation-frequency variation signal (caused by the tip-sample force interations) of the fork in an oscillation circuit is exploited to construct an AFM image. The MFM probe is a piezo-resistive cantilever which is glued on another PST (called PMFM). Similar to the AFM, the MFM produces non-contact magnetic images via the frequency modulation. The two PSTs of PSTM/AFM and PMFM, together with the third PST (called P3) are mounted parallel on a base. Their standing points on the base form a equilateral triangle. A sliding piece stretches over PSTM/AFM and PMFM and sits on P3 and P4 by gravity. The sample is vertically glued underneath the sliding piece and faces the STM/AFM tip and the MFM tip. By inertial stepping, P3 can move the sliding piece (i.e. the sample) towards the tips (coarse approach) as well as along the tip alignment direction (to in-situ select which tip is used for imaging). This tip alignment direction stepping motion of the sample can also be used to search the nanometer targets across the entire sample. Multiple levels of vibration isolations from inside to outside are employed to reduce the vibration issue (including the vibration from the liquid helium dewar).Chapter 1 discusses the working principles of STM, MFM and AFM and the current status of these microscopes. It also briefly describes the principles of several kinds of piezoelectric motors used in scanning probe microscopes.Chapter 2 discusses a high precision fully low voltage scanning tunneling microscope. With our new deign of lateral junction regulation technology, we have implemented the function of large area searching nanoscale targets solely under low voltages and obtained extremely clear graphite images using our patented 10 fA current resolution transimpedance amplifier.In chapter 3, a high performance homemade MFM is introduced, which is also equipped with large area searching capability. Some detailed technical issues encountered during the construction process of the MFM are discussed with our own solutions. At last, the MFM is tested and confirmed by the very high quality non-contact magnetic track images of commercial 8 mm video tape samples.In chapter 4, a home-made non-contact frequency-modulated AFM is described. The issues encountered and their solutions are provided. Some nano-particle images produced by this AFM microscope are offered as a test of its performance.Chapter 5 provides a detailed discussion on the integration of the STM, MFM and the AFM. It has the potential to scan the same area of the sample with all these three microscopes without needing to break the vacuum and take out the sample for imaging on a different microscope. The whole SMA is yet very simple, compact and can be inserted into low temperature, UHV and high magnetic field. Chapter 6 is a systematic introduction of how we make the SMA work in extreme condictions. To work in low temperature, we directly insert the SMA pipe chamber into liquid helium dewar. To obtain high vacuum, we design and have MDC vacuum parts company implement a detechable chamber. It also discusses a low vibration liquid helium dewar that we uniquely design for the SMA.And finally, chapter 7 is a comprehensive summary. |
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