Heat Transfer at Solid鈥揋as Interfaces by Photoacoustics at Brillouin Frequencies

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• 作者：Chang-Ki Min ; Kejia Chen ; Sung Chul Bae ; David G. Cahill ; Steve Granick
• 刊名：The Journal of Physical Chemistry C
• 出版年：2012
• 出版时间：May 24, 2012
• 年：2012
• 卷：116
• 期：20
• 页码：10896-10903
• 全文大小：383K
• 年卷期：v.116,no.20(May 24, 2012)
• ISSN：1932-7455

文摘

We use time-resolved ellipsometry to investigate the rate of heat transfer at solid鈥揼as interfaces through measurements of the amplitude and phase of acoustic waves at Brillouin frequencies, 100鈥?00 MHz, at pressures 2 orders of magnitude higher than earlier comparable studies. An ultrafast optical pulse heats a thin metal film deposited on a sapphire substrate. Heat flow from the substrate into the gas causes expansion of the gas and generates an acoustic wave that is probed by off-null ellipsometry with subpicosecond time resolution. We compare the amplitudes and phases of photoacoustic signals generated in inert gases Ne, Ar, Kr, and Xe to a continuum theoretical model that includes the thermal accommodation coefficient 伪 at the gas鈥搒olid interface. For the surfaces we have studied, bare Au and Au coated by a self-assembled monolayer of 1-octadecanethiol (ODT), this comparison between experiment and theory for the amplitude of the photoacoustic waves suggests that 伪 values for bare and ODT-terminated Au are similar and 伪 > 0.3. This conclusion is tentative, however, because the phases of the photoacoustic waves show systematic differences that are not predicted by the model. For tetrafluoroethane vapor (R-134a refrigerant), the photoacoustic signal generated by a Au surface coated with a hydrophilic (COOH-terminated) self-assembled monolayer is a factor of 2 larger than the photoacoustic signal generated by a hydrophobic (CH₃-terminated) monolayer. We also report measurements of the ultrafast ellipsometry signals generated by the sudden desorption of physisorbed methanol and water on hydrophobic and hydrophilic self-assembled monolayers.