Microscale Spatially Resolved Thermal Response of Si Nanotip to Laser Irradiation

When an external laser illuminates a nanoscale tip, heating effect arises from absorption of light. This thermal phenomenon happens in near-field scanning optical microscopy (NSOM), 5 apertureless NSOM, tip-enhanced Raman spectroscopy (TERS), laser-assisted scanning tunneling microscope (STM)/ atomic force microscope (AFM)-assisted surface modification and nanofabrication, 11 and high density data storage. 14 In aperture NSOM, during the process of passing light through the aperture, a large portion of the incident laser is trapped in the tip area, causing tip heating. As heat accumulated near the tip apex, the probe could be damaged as a result of thermal stress caused by different thermal expansions of the fiber and aluminum coating of the coaxial tip. 5 For an apertureless tip, a very strong electromagnetic field is created in the vicinity of the tip apex, which may cause nanoscale heating effects in the tip. Mamin and Rugar first used a laser-heated AFM tip and cantilever to create a series of 100 nm depth pits on polymer substrates. Hamann et al. used a laser-heated AFM tipmagnetic recording method, which is beyond traditional limits, to heat a magnetic material and write less than 40 nm pits, corresponding to a data density of 400 Gbit/inch. The heated tip can be used as a sharp, nanoscale knife. 17 Tarun et al. cut CNTs using a heated silicon tip whose temperature is as high as 1400 K as a nanoscale heat source. Heat flow between the contacted tip and substrate was also investigated in the past. In tip-based experiments, the tip heating is important for controlling the results, the depth and width of nanoscale pits in nanosurface modification. Even in TERS, the heated then thermal expanded tip will affect the results. An annealing effect triggered by a local temperature rise of 20 30 K in the substrate would make the Au surface smoother and cause irreversible Raman signal loss. Usually, the heating needs to be controlled to ensure the tip does not adversely affect the sample, especially in probing biological and polymetric materials. In fact, all of the technologies introduced above did not measure the exact temperature. The only control approach is to tune the input laser intensity to control the temperature of the tip in a reasonable range, which in turn determines the thermal expansion for specific materials. Specifically, the temperature models as well as thermal expansion 25 for the thermal tips have been reported extensively. Thermal expansion of tips was usually measured under STM mode, for the tip substrate distance is readily calculated through tunneling current gap relationship. Also, theoretical calculation of the temperature rise or temperature distribution 9,20 provides powerful inspection for laser heated tips. Experimental work for measuring the temperature of heated tips due to laser absorption was mainly done through Raman spectroscopy. The resonance frequency shift method was introduced to investigate the temperature of both AFM tip and cantilever under lateral laser illumination. The latter one,