A New Low-temperature Scanning Tunneling Microscope And Its Application On Defects In Layered Materials

At low temperature many novel and important electron behaviors, such as superconductivity, quantum hall effect, charge density wave(CDW), colossal magnetoresistance, charge or orbit order, metal-insulator transition, occur in layered materials due to its low dimension. It has long been fundamental and edge-cutting to understand/measure these phenomena in condensed matter physics. The layered materials include graphene-like materials which recently has received lots of attention, high-temperature superconductors and the newly discovered iron-based superconductor, topological insulator and transition metal dichalcogenides(such as1T-TiSe2) which exhibit CDW transition at low temperature. Scanning tunneling microscope because of its atomic resolution in real space is very suitable for its surface physics.To study these subjects above, we have built a new low temperature STM. Low temperature (liquid nitrogen or helium) is an essential dimension in condensed matter physics, which also bring a series of problems such as the performance of piezoelectric materials being bad, vibration caused by low temperature liquid, etc. These problems will directly affect whether the piezoelectric motor can work normally or not, the imaging quality of a STM, which is basis for a STM. So, two previous works have been done (seen in Chapter4). The first, we proposed a new spiral scan mode which can produce a STM image without distortion when scanning. In compared with traditional line-by-line mode, the spiral mode can:(1) reveal the true lattice arrangement on sample surface;(2) give the strength and orientation of drift;(3) realize fast scan. What more, for an unknown sample, it can give its real structure, which the traditional mode cannot replace. Now this work has been published in Rev. Sci. Instrum. And then it also has been published in chapter17of "Fundamentals of Picoscience"(was invited to write by my advisor Prof. Qingyou Lu) which was published by the famous Taylor&Francis. The second, it has been long puzzled by the problem which people need to operate high voltage to drive a STM motor at low temperature (high voltage would result in high electronic noise and8times or higher cost because we need high-voltage controller). To end it, we carefully investigate two type inertial motors including piezoelectric stacks and tube motor. To our surprise, we found the performance (larger step size and lower onset voltage) of both type motor is apparently improved when adding a delay in driving signals. By simulation, we find this phenomenon is related to the creeping effect of the piezoelectric materials, clearing some controversies. And this work has also been published in Rev. Sci. Instrum.Of course, during building the STM, many actual and important problems also need to be solved. For example, we need to amplify the weak tunneling current (～nA). We also need to realize the preparation of sample, exchange of tip/sample in vacuum and the design of anti-vibration, sound-proofing and electromagnetic shielding for the whole setup. Based on the principle of STM, we present our home-made high atomic resolution STM in Chapter2. Many unique advantages and characteristics are included as follows:(1) a unique scan head which can separate from piezoelectric motor, thus provide higher stability;(2) a novel rotate-suspension mechanism, which is very useful for low temperature STM built in high floor;(3) sample can be prepared in-situ;(4) high reliability on replacing tip and sample under our special design;(5) using a pre-amplifier with better than20fA resolution;(6) a special magnetic cooling method and a low cost way to realize continuous measurement at low temperature. By experiments, our homebuilt STM can reach high atomic resolution, operated at low voltage, and can scan the interested area by positioning precisely. The temperature can reach12K, where would be ultra-high vacuum.Because of the special control of our homemade STM, it needs us to develop a controller software. In Chapter3, we introduce a SPM controller software based labview language which is developed by me. The software can be suitable for many type SPMs readily which is different from commercial one. In present, it has been applied on our SPMs friendly, such as STM, atomic force microscppe and magnetic force microscope. And it can locate at the region of interest at nanometer scale, search with high effectiveness and real-time image.Defects at nanometer scale duo to its low dimension and broken symmetry, thus provide a very good view angle to study the electronic, magnetic and mechanical properties. In Chapter5, we present high atomic resolution STM images of1T-TiSe2(the transition metal dichalcogenides) in which exist lots of atom vacancies, which don’t result in huge electron distortions. Unlike1T-TiSe2, we observed rich and novel superstructures near grain boundaries (GBs) of graphite. At the same time, we also observed the standing wave related to the interference between the scattered electrons in the vicinity of graphite boundary. And, the local electronic modulation is presented near a zigzag edge caused by its special geometrical structure.The interlayer interaction in graphite is weak Van der Waals force. However, up to date, its nature physics at atomic level is poorly understood. Some fundamental problems is not clear, for example, why the true honeycomb structure on graphite surface is hardly observed, in place of, the triangular lattice is shown in STM image. We for the first time observed the edge dislocation in the second layer of graphite, which provide direct evidence on how the underlying atoms distribute the surface charge density. To further study, the interactions between the top two layers of graphite were varied by our special thermal treatment and rich and novel electronic structure have been observed. To explain, we propose a collective interference model, which can simulated the function between surface density states and the interlayer interactions quantitatively. Thus, we provide a new method to study interlayer interaction at atomic level. At last, we captured the motion of the dislocation in graphite and its atomically resolved evolution, and we believe this may be caused by the strains in graphite.Based on the studies above, I have recieved4SCI papers. Two of them have been published in Rev. Sei. Instrunm., which is the second order journal accepted by USTC (I am the first author). So I have met the graduation requirements of USTC. I also have submitted a paper in Nanoscale which is under review and another paper is in preparation. Among many PhDs, my report was award "the first prize" at the "2013-Graduate Forum".