Construction Of Scanning Tunneling Microscope Under Extremely Harsh Conditions

Since its invention,the scanning tunneling microscope(STM)has been famous for its ability to directly observe the atomic arrangement of the material surface and obtain the energy band structure of the material through spectroscopic analysis.Its birth marks the era of atomic manipulation.It has been widely used in the basic research of materials science,biological science,condensed matter physics,surface science,and other disciplines and plays a pivotal role.In the development process of scanning tunneling microscope,low temperature scanning tunneling microscope,electrochemical scanning tunneling microscope,high magnetic field scanning tunneling microscope,and ultra-high vacuum scanning tunneling microscope were developed by combining low temperature,solution,high magnetic field,and ultra-high vacuum.Among them,low temperature and high magnetic field environments are the most widely used,necessary conditions for studying the quantum Hall effect,charge density wave,high-temperature superconductivity,giant reluctance effect,and other physical phenomena.The superconducting state pairing mechanism in high-temperature superconductors is the key to realizing high-temperature superconductivity and even room-temperature superconductivity.A high magnetic field is an essential means to study the second type of superconductor.The local Vortex of the second type of superconductor can be directly observed by scanning tunneling microscope.Under the control of magnetic field and temperature,the sample will change from superconducting to Vortex and then to normal.In addition,under a high magnetic field,the scanning tunneling microscope can visualize the local charge ordering and pseudo gap in the superconductor and reveal the spatial correlation between charge,magnetic ordering,and superconductivity.Therefore,developing a scanning tunneling microscope under a high magnetic field is significant.The common international STMs in high magnetic fields are combined with wet superconducting magnets which have low noise and weak vibration during operation.However,the maintenance of wet superconducting magnets requires a large amount of liquid helium which is now very precious.Hence,the maintenance cost of wet superconducting magnets is relatively high.In contrast,the gradually developing cryogen-free magnet does not require an additional supply of liquid helium and is simple to maintain.However,when it runs,the refrigeration will produce severe vibration,which makes the imaging of STM very challenging.Based on the above,we have built a room-temperature solution STM system working in a 10 T cryogen-free superconducting magnet.The designed STM head has two imaging modes:a large piezoelectric scanning tube is fixed to the bottom of the tantalum frame for large area imaging;a small piezoelectric scanning tube is mounted at the free end of the large piezo tube to achieve high accuracy imaging.The high compactness and rigidity of the STM head enable it to work well in the cryogen-free superconducting magnet with harsh vibration environments.Furthermore,we successfully obtained atomic resolution images of graphite in solution at the field strength of 0 T to 10 T.Sub-molecular images of active antibody and plasmid DNA also demonstrate the capability of the device to image biomolecules.We also built an STM system based on a cryogen-free superconducting magnet with variable temperature,which can provide a temperature of 1.6 K～300 K and a magnetic field of up to 12 T.By using the self-made pluggable STM rod,we can conveniently carry out the imaging work of STM.The STM head was made of sapphire material,and the new design of the single slider structure makes it simple,compact,and rigid.We obtained the atomic-resolution images of samples at temperatures of 1.6 K and 300 K and magnetic fields from 0 T to 9 T,respectively,indicating that our device is very suitable for STM imaging studies in low temperatures and high magnetic fields.Based on the above technology,we introduce the room-temperature STM built in the hybrid magnet in Chapter 3.It adopted the multi-stage suspension vibration reduction and sound insulation and a new structure of the STM head in which the coarse stepping motor and the scanning head are separated,so that we can successfully obtain the atomic-resolution images of the graphite surface under the high magnetic field of 27.5 T.We have also designed a new liquid nitrogen dewar suitable for narrow spaces,which can perform the low-temperature STM experiments in a 32 mm aperture of the hybrid magnet,and have obtained some preliminary experimental results.Chapter 5 proposes an STM imaging mechanism that can locate micron-scale samples.We successfully located the STM tip on a 20 μm×50μm bilayer graphene sheet and obtained atomic-resolution images of the graphene surface.We also observed the image of the transition state from the hexagonal lattice structure to the O-type superstructure.In addition,we successfully captured a graphene sheet as small as 1.3 nm through fast and large-area search operations,which is the first time such a small graphene sheet has been observed at atomic resolution.In Chapter 6,we design a high magnetic field STM system with an ultra-high vacuum interconnected with the pulsed laser deposition and molecular beam epitaxial equipment to enable STM measurements in an in-situ vacuum after growing samples.At present,the design and manufacture of the tip and sample changing mechanism of the STM head and the design and processing of the STM vacuum chamber have been completed,and a series of construction and test work will be carried out later.