Heat transfer between a hot AFM tip and a cold sample: impact of the air pressure
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Heat transfer between a hot AFM tip and a cold sample: impact of the air pressure Pierre-Olivier Chapuis1,2, Emmanuel Rousseau1,3, Ali Assy2, Séverine Gomès2, Stéphane Lefèvre2, and Sebastian Volz1 1 Laboratoire EM2C, Ecole Centrale Paris-CNRS, Grande voie des vignes, 92295 ChatenayMalabry, France 2 Centre de Thermique de Lyon (CETHIL), CNRS-INSA de Lyon-UCBL, 9 rue de la Physique, Campus La Doua-LyonTech, 69621 Villeurbanne (Lyon), France 3 Groupe d’Etude des Semiconducteurs - CC074, Université de Montpellier II, Place Eugène Bataillon, 34095 Montpellier, France ABSTRACT We observe the heat flux exchanged by the hot tip of a scanning thermal microscope, which is an instrument based on the atomic force microscope. We first vary the pressure in order to analyze the impact on the hot tip temperature. Then the distance between the tip and a cold sample is varied down to few nanometers, in order to reach the ballistic regime. We observe the cooling of the tip due to the tip-sample heat flux and compare it to the current models in the literature. INTRODUCTION At nanometer scale probing the temperature [1] or measuring thermal properties such as the thermal conductivity [2] is a difficult task that may be tackled with scanning thermal microscopy (SThM) [3-4]. This technique is based on an atomic force microscope (AFM) with a tip sensitive to heat fluxes and/or temperature variations. SThM has been proposed for data storage [5–7], nano-lithography [8–10] or chemical sensing [11]. A key issue in local thermal analysis is to know the thermal sensitivity and the ultimate spatial resolution of the apparatus [12–14]. The resolution depends on the relative intensities of the heat fluxes transferred from the tip to the sample by various channels: conduction at their solid-solid contact; conduction in the liquid meniscus due to ambient humidity and located around the mechanical contact; air conduction around the tip [14]. The spatial resolution is linked to the contact area and improves when conduction though air and the water meniscus are removed. Working in vacuum (pressure lower than 10í2 mbar) removes the contribution from the surrounding air and might affect the water meniscus. It improves spatial resolution [15-16] but decreases the thermal sensitivity. An intermediate vacuum can be a compromise between spatial resolution and thermal properties sensitivity. In this study we focus on the heat flux due to conduction in air. We analyze the tip losses to the ambient environment and observe the cooling of the tip when approaching it close to the surface. EXPERIMENTAL SETUP We now turn to the description of the first experiment. Our setup consists in a SThM embedded in a vacuum chamber. The probe is a 75 ȝm diameter Wollaston wire which is etched at its ends on approximately 200 ȝm. The outer part (silver) has been removed so that the inner
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Pt90/Rh10 part of diameter 5 ȝm is apparent and bent in a V shape at its extremity. This probe has been extensively described in the past [17-18]: An electrical current heats the etched
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