1349 Research Article Received: 12 February 2009 Accepted: 15 June 2009 Published online in Wiley Interscience: 4 August 2009 (www.interscience.wiley.com) DOI 10.1002/jrs.2388 Tip-induced heating in apertureless near-field optics A. V. Malkovskiy, a V. I. Malkovsky, b A. M. Kisliuk, a C. A. Barrios, a M. D. Foster a and A. P. Sokolov a* Tip-enhanced Raman spectroscopy is considered a promising technique for imaging with nanoscale lateral resolution. However, its developments and use face many problems. In this paper we provide insight into the level of sample heating by the laser light in the presence of a metal-coated atomic force microscope (AFM) tip. The heating is attributed to the presence of an optical field enhanced by the tip. Sample temperatures were estimated using measurements of the ratio of the Stokes and anti-Stokes signals from a thin 50-nm sample on an Al substrate. A correlation between the heating and optical properties of the tips is established. The results demonstrate significant tip-induced heating (100 K and more) even at very low laser power. Copyright c  2009 John Wiley & Sons, Ltd. Keywords: SNOM; TERS; apertureless; optical properties; heating Introduction Scanning near-field optical microscopy (SNOM) is a technique for optical and spectroscopic imaging of different materials with spatial resolution far better than the diffraction limit. [1–4] Two different approaches exist for implementing SNOM: aperture- limited, [5 – 10] which has laid the foundations of the technique, and apertureless [10 – 12] optics. In the second one a sharp tip is used in order to create a local source of light, which greatly increases resolution. [13 – 15] One of the widely used apertureless techniques is the so-called tip-enhanced spectroscopy (TES). It utilizes a sharp metal or metal-coated tip, which provides high enhancements of the optical field localized in the vicinity of the tip. One particular type of SNOM of interest here is tip-enhanced Raman spectroscopy (TERS). [3,4,8 – 12,16,17] In this case, the atoms or molecules of the analyte are excited by an amplified electric field and the emitted Raman signal is also enhanced by the tip. TERS has received significant attention in recent years [18,19] and optical (Raman and fluorescence) imaging with lateral resolution of 10 – 20 nm has been achieved. [20,21] The key element of the apertureless optics is the metalized (or metallic) tip. The development of tips that can provide enhancements of the optical signal of order of a factor ∼10 6 remains a challenge, [22,23] although it is feasible. There are many other complications in TERS technology, including mechanical and thermal damage as well as chemical alteration of the analyte, substrate and even the tip itself. While the mechanical damage can be significantly reduced by the use of a noncontact scanning probe (e.g. tuning fork assembly), thermal damage and chemical alteration are still issues to be reckoned with. It has been demonstrated recently that coating of the apertureless tips by a thin (∼2 nm) layer of Al 2 O 3 significantly enhances the mechanical and chemical stability of the tips without altering their optical properties. [24] However, another important problem, Joule heating, has not been addressed. It is obvious that the enhancement of the optical field, which, in the case of TERS, can be one order of magnitude or even higher, generates additional heating. However, there are very few papers [25 – 27] that discuss temperature factors arising in TERS. The heating arising from the field enhancement has been partially addressed in recent experimental works both for surface- enhanced Raman spectroscopy (SERS) [25] and for TERS, [26] as well as in a theoretical work. [27] The level of heating has been estimated either theoretically [27] or indirectly through analysis of the light-induced damage. [26] In both cases, Joule heating causing sample temperature increases reaching tens of degrees kelvin has been suggested. However, no attempts at direct temperature measurements have been presented so far. This paper presents an analysis of the tip-induced heating of a polymer film using measurements of the ratio of the Stokes and anti-Stokes Raman signals and model calculations. Our analysis demonstrates that the tip-induced heating in optically absorbing media indeed can reach ∼100 K, even at laser power as low as 1 mW. The heating, however, depends on the wavelength of light and optical resonance of the tip. Model calculations of the film heating agree reasonably well with the experimental results. The presented results clearly demonstrate the importance of Joule heating in TERS measurements. Experimental Sample preparation We chose a conducting polymer poly(3,4-ethylenedioxy- thiophene)/poly(styrenesulfonate) (PEDOT/PSS) (Baytron P from H.C. Starck) because of its high Raman signal in red and green ∗ Correspondence to: A. P. Sokolov, University of Akron, Department of Polymer Science, Akron, OH, USA. E-mail: alexei@uakron.edu a Department of Polymer Science, University of Akron, Akron, OH 44304, USA b InstituteofGeologyofOreDeposits,Petrography,MineralogyandGeochemistry (IGEM), Russian Academy of Sciences, Moscow 119017, Russia J. Raman Spectrosc. 2009, 40, 1349–1354 Copyright c  2009 John Wiley & Sons, Ltd.