The Design And Application Of Low-temperature Scanning Tunneling Microscope

Scanning tunneling microscopy(STM)enables humans to directly observe the individual atoms and their arrangement on the surface of a substance for the first time.People can study the physical and chemical properties of some materials via STM.Therefore,it is widely used in the fields of surface science,materials science,and life sciences.Many materials exhibit novel physical properties such as superconductivity,quantum Hall effect,charge density wave(CDW),and quantum phase transition under low temperature conditions.Scanning tunneling microscope is especially suitable for studying the surface properties of these materials with atomically resolved imaging in real space.So it has become an important project for many research groups around the world to build a scanning tunneling microscope that can work well at low temperatures.Although the technology related to STM has been developed up to now,there are still many problems and challenges in building a low-temperature scanning tunneling microscope system:(1)The scanner is always fixed directly to the stepping motor in conventional STM designs,so the disturbances to the scanner caused by the instability of the stepping motor can not be reduced.(2)A long magnetic rod is always inserted into the Dewar to capture the probe and sample while the STM body is located at the bottom of the Dewar.This not only increases the overall height of the STM setup in vertical direction,but also brings about the risk of damaging the tube scanner.(3)It is a rather complicated operation to exchange the probe and sample under vacuum conditions,which usually requires multiple transfer mechanisms.(4)Customizing a set of cryogenic system specially for the scanning tunneling microscope faces technical difficulties and high costs.Taking into consideration the above issues,during the doctoral period and under my supervisor's careful guidance my work mainly contains the following several parts.First,a new low-temperature scanning tunneling microscope system was developed.The scanner is mechanically decoupled from the stepping motor,which presents the instability of the motor from being transferred into the scanner,thus resulting in a high stability of the tunnel junction.The use of a rotary manipulator to transfer the STM body not only saves the overall height of the STM setup in vertical direction,making it independent of the floor,but also remains relatively simple and reliable compared to the transfer mechanism based on the use of a long rigid rod.With coaxial design,the exchange of tips and samples can be implemented smoothly by a magnetic transfer rod.A standard liquid helium Dewar was used to cool the STM body through a custom-made cryogenic chamber,which reduces the costs.Now the work about the STM design has been published in the journal of Rev.Sci.Instrum.Using this microscope,we observed the novel electronic structure near the grain boundary of graphite.To explain these features,we proposed a collective interference model that can quantitatively simulate the interaction between the lower layer atoms of graphite and the surface electronic states.This work has been Published in the journal of Carbon.Second,in order to further improve the stability of the microscope,we have developed a compact and rigid STM body which features employing an inner-wall polished sapphire guiding tube as a rail for the scanner to form a short tip-sample mechanical loop.The scanner is mounted on a square rod which is housed in the guiding tube and held by a spring strip.The stiff sapphire guiding tube used here allows the STM body to be made in a simple,compact and rigid form.Also the material of sapphire improves the thermal stability of the STM for its good thermal conductivity.The STM body was used to make an insertion low-temperature scanning tunneling microscope system.Also it was directly fixed to the cold finger,which allows the minimum sample temperature to be 4.2 K.The sample can be cleaved at low temperatures by our specially-designed mechanism.This low-temperature scanning tunneling microscope system retains basic measurement functions and is easy to operate.It can be implanted into any liquid helium magnet with a suitable bore size.Third,a set of scanning tunneling microscope control software was built based on the use of Labview programming language.The software can well realize the functions such as motor stepping,scanning images,spectrum measurement and fast data storage.Using this software we have obtained high-quality image data,which proves the reliability of the software.In addition,it can be flexibly used in other scanning probe microscopes for its excellent compatibility.Forth,a new design of piezoelectric positioner for scanning probe microscope was developed to solve the contradiction between the output force and the threshold voltage in the inertial piezoelectric motor.In conventional inertial piezoelectric motors,the maximum output force is the maximum static friction force of the slider of the inertial piezoelectric motor due to the friction force of the slider is often uncontrollable.If the friction force of the slider is too large,the motor can walk only by increasing the threshold voltage.The inertial piezoelectric motor we design has a controllable total friction force,which means that it sticks with a large total friction force and slips with a severely reduced total friction force.This allows the inertial piezoelectric motor to work with a lower threshold voltage while also providing larger output force and stepping size.