Developments Of Ultra-fast Scanning Tunneling Microscopes System

Since the invention of the scanning tunneling microscope(STM),humans have been able to observe the arrangement state of individual atoms on the surface of matter and the physical and chemical properties related to the behavior of surface states of electrons in real space.The ultra-high resolution makes STM has great significance and wide application prospects in many fields,such as life science,materials science and microelectronics technology.However,the application range of STM is still limited.For example,when working in extreme environments such as strong magnetic fields,ultra-low temperatures,and ultra-vacuum,it is still necessary to make in-depth studies.This article focuses on achieving ultra-fast scanning tunneling microscopy in air at room temperature.In this process of continuously breaking the scanning frequency limit,we encountered the problems of lower preamp bandwidth,insufficient amplitude of the tuning fork scanner,and insufficient sampling rate of the acquisition card.Ultra-high speed scanning tunneling microscopes must overcome all these problems.Therefore,we have made some innovations and upgrades in many parts of the instrument.In the end,in continuous trials and experimental verification,a set of ultra-fast scanning tunneling microscopes was created and ultra-high scanning frequency of 50 kHz was achieved,which is the fast world record.The major changes we have made are:(1)Ultra-fast scanning tunneling microscopes require higher preamplifier bandwidths,and methods that generally increase integrated circuit bandwidth include reducing feedback capacitance or eliminating parasitic capacitance,reducing feedback resistance,and so on.We first eliminated the parasitic capacitance of the circuit by upgrading the rod-type circuit to a plate-type circuit;then,in the case where the atomic resolution image of the graphite can be clearly measured,the feedback resistance is continuously decreased to increase the preamplifier bandwidth.When the feedback resistance is reduced to 1 M?,self-oscillation occurs in the preamp circuit.At this time,the circuit cannot perform coarse approximation and scan imaging cannot be performed.Through continuous trials,we finally eliminated the self-oscillation by connecting the fixed-value resistor in parallel to the tunnel junction so that the pre-amplifier circuit can perform rough approximation and test the image.The preamplifier bandwidth is increased to 550 kHz which is helpful to achieve ultra-fast STM.(2)According to the sampling theorem,the sampling rate is 2 times higher than the highest frequency of the signal.For the ultra-high speed scanning tunneling microscope,the sampling rate of the existing acquisition card is only 1 MS/s,which is obviously not enough.Therefore,we proposed the use of oscilloscope acquisition method instead of the traditional acquisition card to capture image information,the former sampling rate of up to 1 GS/s is much higher than the latter and easy to use,and the price is also lower than the latter dozens of times.(3)Currently used tuning fork scanners with a resonance frequency of 32.768 kHz,the vibration amplitude at higher frequencies are much lower than the amplitude near the resonance peak.The formant will be further reduced after bonding platinum-rhodium alloy probes on the tuning fork scanner.In order to achieve higher-frequency scanning speed,we chose a tuning fork scanner with a higher resonant frequency of 100 kHz.When the probe was prepared,the length of the probe was controlled to about 1.5 mm to reduce the load of the tuning fork.Increase the vibration amplitude of the tuning fork at high frequencies.(4)In order to increase the stability of the test and obtain a higher resolution of the image,we use a dual-frequency drive piezoelectric motor instead of the traditional single frequency method.Dual-frequency drive piezoelectric motor can guarantee a larger driving force,and the startup voltage is greatly reduced.Not only reducing energy consumption,but also reducing the impact of electromagnetic interference caused by high voltage on the image,and further widening the scope of the STM.