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Under the terms of the Creative Commons Attribution (CC BY) license to their work. ; We present a novel contactless technique for thermal conductivity determination and thermal field mapping based on creating a thermal distribution of phonons using a heating laser, while a second laser probes the local temperature through the spectral position of a Raman active mode. The spatial resolution can be as small as 300 nm, whereas its temperature accuracy is ±2 K. We validate this technique investigating the thermal properties of three free-standing single crystalline Si membranes with thickness of 250, 1000, and 2000 nm. We show that for two-dimensional materials such as free-standing membranes or thin films, and for small temperature gradients, the thermal field decays as T(r) ∝ ln(r) in the diffusive limit. The case of large temperature gradients within the membranes leads to an exponential decay of the thermal field, T ∝ exp[ − A·ln(r)]. The results demonstrate the full potential of this new contactless method for quantitative determination of thermal properties. The range of materials to which this method is applicable reaches far beyond the here demonstrated case of Si, as the only requirement is the presence of a Raman active mode. ; The authors acknowledge the financial support from the FP7 projects MERGING (Grant No. 309150), NANO-RF (Grant No. 318352), NANOTHERM (Grant No. 318117), NANOTEG-ENIAC (Grant No. 270789-2), and the Spanish MICINN projects nanoTHERM (Grant No. CSD2010-0044) and TAPHOR (MAT2012-31392). E.C.A. gratefully acknowledges a Becas Chile 2010 CONICYT fellowship from the Chilean government. ; Peer Reviewed

Content from this work may be used under the terms of the Creative Commons Attribution 3.0 licence. ; We report on stress induced changes in the dispersion relations of acoustic phonons propagating in 27 nm thick single crystalline Si membranes. The static tensile stress (up to 0.3 GPa) acting on the Si membranes was achieved using an additional strain compensating silicon nitride frame. Dispersion relations of thermally activated hypersonic phonons were measured by means of Brillouin light scattering spectroscopy. The theory of Lamb wave propagation is developed for anisotropic materials subjected to an external static stress field. The dispersion relations were calculated using the elastic continuum approximation and taking into account the acousto-elastic effect. We find an excellent agreement between the theoretical and the experimental dispersion relations. ; The authors acknowledge financial support from the FP7 project MERGING (grant no. 309150); the Spanish MICINN projects nanoTHERM (grant no. CSD2010-0044) and TAPHOR (MAT2012-31392). JGB gratefully acknowledges support from the Spanish government through a Juan de la Cierva fellowship. MP and AS acknowledge funding from the Academy of Finland (grant no. 252598). ; Peer Reviewed

Under the terms of the Creative Commons Attribution (CC BY) license to their work. ; We report on the reduction of the thermal conductivity in ultra-thin suspended Si membranes with high crystalline quality. A series of membranes with thicknesses ranging from 9 nm to 1.5 μm was investigated using Raman thermometry, a novel contactless technique for thermal conductivity determination. A systematic decrease in the thermal conductivity was observed as reducing the thickness, which is explained using the Fuchs-Sondheimer model through the influence of phonon boundary scattering at the surfaces. The thermal conductivity of the thinnest membrane with d = 9 nm resulted in (9 ± 2) W/mK, thus approaching the amorphous limit but still maintaining a high crystalline quality. ; The authors acknowledge the financial support from the FP7 project MERGING (Grant No. 309150), the Spanish MICINN projects nanoTHERM (Grant No. CSD2010–0044), TAPHOR (MAT2012–31392), and the Academy of Finland (Grant No. 252598). E.C.-A. gratefully acknowledges a Becas Chile 2010 CONICYT fellowship from Chilean government. ; Peer Reviewed