Research On Key Techniques Of Tip Enhanced Raman Instrument

Tip-enhanced Raman spectroscopy（TERS）combines the advantages of Scanning Probe Microscopy（SPM）and Raman spectroscopy,it can realize nanoscale resolution imaging which breaks the optical diffraction limit and obtain Raman spectral fingerprint information.However,at present,commercial TERS instruments mainly rely on imports,and there are no relevant instrument products in China.There are many difficulties to realize the independent development of TERS instrument which requires not only highly stable SPM instrument and highly sensitive Raman instrument,but also effective coupling of Raman excitation and collection optical paths in the limited space of SPM.The development of TERS instrument involves the intersection of optics,electronics and mechanical design and other disciplines.Therefore,this paper mainly focuses on the development of highly sensitive Raman instrument and high-performance SPM instrument,exploring and gradually mastering the key technologies of TERS instrument.This work is the base of subsequent integration of TERS instrument.Based on the previous research work of Raman instrument,a highly sensitive micro-Raman instrument was further developed.Tuning fork parameters measurement system was completed and characterized.And parameters of tuning fork mechanical sensor were obtained.The SPM mechanical structure design,control system and piezoelectric tip rough approximation were completed.The whole SPM system is preliminarily completed.The overall research content of this paper is as follows:1)Based on the research group’s previous work in the development of Raman Microscopy instrument,the mechanical structure of CCD was redesigned.And two-stage semiconductor refrigeration and circulating water cooling technology were employed to realize refrigeration at-30℃.And the dark current noise was reduced to2%of noise at 10℃refrigeration.The CCD drive was optimized to make the exposure time could change from 10 ms to 100 s.Sensitivity test,samples test and signal to noise ratio analysis were carried out.The spectral resolution of the self-made Raman instrument system was 0.1nm/pixel.The spectrum range was 0～3200 cm^-1.The focal length of the corresponding spectrometer was 100 mm.The grating line was 1200 g/mm.Low wavenumber performance is at 70 cm^-1.The instrument has extremely high sensitivity.And 520.6 cm^-1（first order peak,signal-to-noise ratio>500）and 980 cm^-1wide peak（second order peak,signal-to-noise ratio>10）of silicon is clearly visible.And SNR of silicon’s third order peak is bigger than 10（test condition:532 nm,20 m W,50 s）.The sensitivity of homemade Raman instrument is comparable to research grade Raman instrument.2)Tuning fork force sensing module was developed which can be used for scanning probe microscope.And tuning fork parameters measurement system was completed and characterized.A pre-amplification module was designed to characterize the tuning fork.And the equivalent series capacitance C,equivalent series inductance L,equivalent series resistance R,equivalent parallel capacitance Cp,resonant frequency f and quality factor Q of tuning fork were calculated according to the frequency-current response characteristics of tuning fork.According to the excitation voltage-amplitude and excitation voltage-current response,the tuning fork force constant was calculated by harmonic oscillator,oscillation coupling and piezoelectric coupling.The tuning fork force constant meets the application need of SPM.Secondly,a tungsten tip electrochemical etching circuit was designed.MOSFET cutting circuit is its core and could terminate the etching within 100 ns.The etched tip was cut to about0.5mm in length and bonded to the end of the tuning fork with epoxy resin to make a tuning fork force sensor.The resonant frequency of the tuning fork mechanical sensor is 31202 Hz,and the quality factor Q is 2080.The self-made SPM works in FM mode.The tuning fork force sensor could realize self-oscillation by the tuning fork self-oscillation circuit.When the tuning fork oscillation is stable,the tuning fork drive signal output by the control loop can vary from 130 m V_pp（minimum）to 1.44 V_pp（maximum）.The output signal of the resonance frequency shift measuring circuit with the phase-locked loop as the core is well linear in the frequency shift range of 0～424 Hz.3)The SPM structure,control software and hardware control system were design.Firstly,according to the piezoelectric stepping principle,the piezoelectric stepping motor and the corresponding driving circuit were designed for SPM tip rough approximation.The output voltage of driving circuit can vary from-80 V（minimum）to80 V（maximum）（bandwidth 90 k Hz）.The stroke of the piezoelectric stepping motor is9.92 mm.It walks a step in 3 ms.The average bidirectional single step length are 55.11nm/step and 45.50 nm/step,respectively.Secondly,the finite element simulation software COMSOL Multiphysics（?）was used to calculate the resonance frequency of the SPM structure.The resonance frequency of the SPM structure was bigger than 1k Hz,which met the application need of SPM.Finally,software PI control and analog PI control modules were designed.Software PI control has been successfully applied to TEC temperature control.The temperature resolution of the temperature control system is±0.02℃.According to the process of SPM tip rough approximation,tip withdrawal and scanning imaging,the SPM control software is written with Lab VIEW and the overall instrument structure of SPM system is preliminarily completed.