Design and application of a scanning tunneling microscope experimental apparatus with spectroscopic capabilities
[Objective]Scanning tunneling microscopy(STM)is a pivotal method for achieving atomic-resolution imaging of material surfaces and is widely applied in the instructional context of physics experiments.Beyond imaging,STM facilitates spectroscopic analysis.Specifically,examining the first order differential spectrum of the tunneling current directly revealed the density of the electronic states on the sample surface.This detection significantly enhances the understanding of tunneling phenomena,thus bridging theoretical teaching and cutting edge research.However,the high stability required for tunneling spectra measurements limits their use in teaching experiments.[Methods]This paper introduces an innovative scanning tunneling microscope designed specifically for educational purposes that exhibits spectroscopic capabilities.The proposed apparatus combines a mechanical and control system to achieve ideal performance in a desktop system.The scan head,which is the central part of the mechanical system,features a compact,highly rigid design that uses hard materials such as titanium,alumina,and sapphire to minimize the size without sacrificing strength.A spring suspension damping system was used to further minimize the influence of environmental vibrations,while symmetrical piezoelectric stepper motors and tube scanners in the scan head mitigate thermal drift.The small size of the scan head and the use of high hardness materials also contributed to suppressing thermal drift.The scan head is canopied by a metal and glass cover to shield it from electromagnetic noise and sound waves.The control system includes a preamplifier,signal input and output modules,a voltage amplification module,and a microprocessor.Spectroscopic measurements were enabled by an AC signal generator and a lock in amplifier module.During first order differential spectrum measurements,a 10 mV 933 Hz AC signal supplements the DC bias,with lock-in modules extracting and amplifying the 933 Hz signal from the tunneling current.[Results]The STM system's excellent imaging capability is demonstrated by its clear atomic-resolution images across different scan sizes.Apart from the HOPG standard sample,measurements on other samples that were stable in the air were also satisfactory.The monolayer atomic step and atomic defects of the 2D semiconductor CrSBr were demonstrated in the paper.With the stable acquisition of atomic-resolution images,spectroscopic characterization was undertaken.The dI/dV results show good consistency with the slope of the I-V curve,suggesting that the results accurately reflect the intrinsic nature of the sample.Additionally,the features of the dI/dV curves align with the Dirac band structure of graphene,further indicating the spectroscopic capabilities of the system.In conclusion,the compact symmetrical structure and high rigidity of STM offer the advantages of minimal drift and low noise.Combined with an excellent vibration isolation module,this STM system can stably achieve atomic resolution imaging of various materials in a classroom environment.The control system of STM integrates a lock in module and supports measurements of I-Z,I-V,and first order differential tunneling spectra.[Conclusions]This work provides a design solution for an experimental apparatus that facilitates tunneling spectroscopy measurements with tabletop teaching equipment,expanding the teaching content and depth of scanning probe microscopy experiments.
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