Thermal Transport Property Of Nanotip And Its Application In Thermometry

Nanotip-assisted thermal nanomanufacturing breaks through the diffraction limitation of traditional laser processing to achieve ultra-high spatial resolution processing accuracy.Accurate temperature measurement and regulation is a key factor in the application of this technology,while the heat transfer process of the tip-sample system determines its temperature distribution and heat flow direction,mainly including the heat diffusion inside the nanotip and the sample,and the heat transport across the contact interface.Among them,the size effect of phonon scattering and the interfacial heat transfer mechanism at the nanoscale are research challenges.Therefore,in this work,molecular dynamics simulations and experimental methods are used to study the size effect of the nanotip with variable cross-section and point contact heat transfer characteristics at the sliding interface,and to reveal the mechanism of nanoscale temperature hotspots generation within the sample and the phonon ballistic thermal transport phenomena induced by hotspots.The details of the study are as follows:（1）For the effect of size variation on phonon scattering within the nanotip,size effect verification was conducted,and the influence of cross-sectional size on thermal conductivity was quantitatively investigated.By constructing the molecular dynamics model of nanotip and establishing the temperature gradient in it,it is found that the thermal conductivity of different positions of nanotip is different,which verifies the existence of the tip size effect.The quantitative research found that as the cross-sectional radius increases from 10?to 60?,the thermal conductivity will increase from 0.78W/（m·K）to 2.74 W/（m·K）.In addition,the temperature jump at the contact interface indicates that the thermal contact resistance is the biggest obstacle in the tip-sample heat transfer.（2）To understand the mechanism of heat transfer across the tip-substrate interface,the effects of pressure and interfacial sliding on the thermal conductance were investigated by molecular dynamics methods.It was shown that as the pressure increases,the formation of chemical bonds and the increase in contact area strengthen the heat transfer between the tip and the sample,and the interfacial thermal conductance enhanced by 168.7%when it increases from-11 n N to 104 n N.However,the interface phonon behavior shows that under high pressure,the phonon density of states of the nanotip decreases overall and migrates to the high frequency region,and the phonon spectral overlap factor decreases monotonically.Meanwhile,with the tip sliding on the substrate surface,the increase in contact area and the number of chemical bonds indicates that the contact between the two is more sufficient,and the thermal contact conductance increased from 7.48×10^-9 W/K to 1.22×10^-8 W/K.（3）In response to the uncertainty of thermal contact resistance in existing tip-assisted thermal measurement methods,the tip-enhanced Raman thermometry technology was developed to eliminate the influence of contact thermal resistance.Based on the correlation between the tip enhancement effect and the laser incidence angle and wavelength,the nanotip coated with Au film was designed.The strongest enhancement factor of the electromagnetic field is up to 6930 for 20 nm Au film and incidence angle of 10°,and the self-built system can achieve thermal measurements at a spatial resolution of 10 nm level.The Raman intensity is increased by 63.8%,indicating the enhancement effect of Au-coating tip.In addition,the plasmon resonance in the contact region also exhibits strong absorption of laser energy,resulting in the temperature rise of Ga N samples below the Au-coating tip that is approximately twice that of the bare Si tip.（4）The mechanism of ballistic thermal transport at sub-10 nm laser-induced hot spots in Ga N crystal is investigated based on the tip-enhanced Raman thermometry,and the phonon mean free path（MFP）of the Ga N is measured.It is shown that heat carriers in the hot spot region undergo ballistic heat transport,resulting in an anomalous temperature rise in the Ga N sample below the Au-coated tip.According to the correlation between the local thermal conductivity and the phonon MFP and the correspondence between the local thermal conductivity and its temperature distribution,the phonon MFP of Ga N is measured through Raman spectroscopy.The results indicate that the phonon MFP of single crystal Ga N is 400 nm?800 nm and shows a slight decrease with increasing temperature.The numerical solution of the phonon Boltzmann transport equation shows that the phonon MFP of Ga N at 300K is 768.6 nm,which is close to the experimental value,validating the accuracy of Raman spectroscopy for measuring the phonon MFP of materials.In this work,the thermal transport characteristics of the nanotip-sample system were investigated,and the phonon ballistic thermal transport phenomenon and heat transfer cross contact interface were revealed,which is of great significance for the accurate temperature regulation of nanoscale hotspots and the measurement of thermophysical properties.In addition,it provides theoretical and experimental basis for optimizing the heat transfer path of the tip-sample system to achieve higher hotspot temperatures and for the design and use of nanotips.