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Two-dimensional materials, such as transition-metal dichalcogenides (TMDs), are attractive candidates for sensing applications due to their high surface-to-volume ratio, chemically active edges, and good electrical properties. However, their electrical response to humidity is still under debate and experimental reports remain inconclusive. For instance, in different studies, the impedance of MoS2-based sensors has been found to either decrease or increase with increasing humidity, compromising the use of MoS2 for humidity sensing. In this work, we focus on understanding the interaction between water and TMDs. We fabricated and studied humidity sensors based on MoS2 and WS2 coated with copper chloride and silver nitrate. The devices exhibited high chemical stability and excellent humidity sensing performance in relative humidity between 4 and 80%, with response and recovery times of 2 and 40 s, respectively. We have systematically investigated the humidity response of the materials as a function of the type and amount of induced metal salt and observed the reverse action of sensing mechanisms. This phenomenon is explained based on a detailed structural analysis of the samples considering the Grotthuss mechanism in the presence of charge trapping, which was represented by an appropriate lumped-element model. Our findings open up a possibility to tune the electrical response in a facile manner and without compromising the high performance of the sensor. They offer an insight into the time-dependent performance and aging of the TMD-based sensing devices., The Catalan Institute of Nanoscience and Nanotechnology (ICN2) is funded by the CERCA program/Generalitat de Catalunya and is supported by the Severo Ochoa program from Spanish MINECO (Grant No. SEV-2017-0706). We acknowledge support from the EU Project Nanosmart (H2020 ICT-07-2018) and ICN2 members acknowledge the Spanish MINECO project SIP (PGC2018-101743-B-I00). P.X. acknowledges support by a PhD fellowship from the EU Marie Skłodowska-Curie COFUND PREBIST project (Grant Agreement No. 754558).

This article is part of the themed collection: Recent Review Articles., Phonons play a key role in the physical properties of materials, and have long been a topic of study in physics. While the effects of phonons had historically been considered to be a hindrance, modern research has shown that phonons can be exploited due to their ability to couple to other excitations and consequently affect the thermal, dielectric, and electronic properties of solid state systems, greatly motivating the engineering of phononic structures. Advances in nanofabrication have allowed for structuring and phonon confinement even down to the nanoscale, drastically changing material properties. Despite developments in fabricating such nanoscale devices, the proper manipulation and characterization of phonons continues to be challenging. However, a fundamental understanding of these processes could enable the realization of key applications in diverse fields such as topological phononics, information technologies, sensing, and quantum electrodynamics, especially when integrated with existing electronic and photonic devices. Here, we highlight seven of the available methods for the excitation and detection of acoustic phonons and vibrations in solid materials, as well as advantages, disadvantages, and additional considerations related to their application. We then provide perspectives towards open challenges in nanophononics and how the additional understanding granted by these techniques could serve to enable the next generation of phononic technological applications., We acknowledge the support from the project LEIT funded by the European Research Council, H2020 Grant Agreement No. 885689. ICN2 authors thank the Severo Ochoa (Spanish Research Agency AEI, grant no. SEV-2017-0706) and the CERCA Programme / Generalitat de Catalunya. R. C. N. acknowledges funding from the EU-H2020 Research and Innovation Programme under the Marie Sklodowska Curie Individual Fellowship (Grant No. 897148). A. E. S. acknowledges support by the H2020-MSCA-IF project THERMIC-GA No. 101029727. F. C. acknowledges funding from the scholarship BES-2016-077203 funded by MCIN/AEI/10.13038/501100011033 and by “ESF Investing in your future”. O. F. is supported by BIST PhD fellowship Horizon 2020 Research and Innovation Programme under the Marie Sklodowska-Curie Grant Agreement No. 754558. P. X. additionally acknowledges support by a Ph.D. fellowship from the EU Marie Skłodowska-Curie COFUND PREBIST project (Grant No. 754558).

Understanding heat flow in layered transition metal dichalcogenide (TMD) crystals is crucial for applications exploiting these materials. Despite significant efforts, several basic thermal transport properties of TMDs are currently not well understood, in particular how transport is affected by material thickness and the material's environment. This combined experimental-theoretical study establishes a unifying physical picture of the intrinsic lattice thermal conductivity of the representative TMD MoSe

The degree of thermal anisotropy affects critically key device-relevant properties of layered two-dimensional materials. Here, we systematically study the in-plane and cross-plane thermal conductivity of crystalline SnSe

Many layered materials, such as graphene and transition metal dichalcogenides, can be exfoliated down to atomic or molecular monolayers. These materials exhibit exciting material properties that can be exploited for several promising device concepts. Thinner materials lead to an increased surface-to-volume ratio, with mono- and bi-layers being basically pure surfaces. Thin crystals containing more than two layers also often behave as an all-surface material, depending on the physical property of interest. As a result, flakes of layered materials are typically highly sensitive to their environment, which is undesirable for a broad range of studies and potential devices. Material systems based on suspended flakes overcome this issue, yet often require complex fabrication procedures. Here, we demonstrate the relatively straightforward fabrication of exfoliated MoSe flakes down to the monolayer, suspended over unprecedentedly large holes with a diameter of 15 µm. We describe our fabrication methods in detail, present characterization measurements of the fabricated structures, and, finally, exploit these suspended flakes for accurate optical absorption measurements., The authors would like to thank Emerson Coy (CNBM-AMU) for sharing the HR-TEM images. S V and D S R acknowledge the support of the Spanish Ministry of Economy through FPI-SO 2018 and FPI-SO 2019, respectively. ICN2 was supported by the Severo Ochoa program from Spanish MINECO (Grant No. SEV-2017-0706). K J T acknowledges funding from the European Union's Horizon 2020 research and innovation program under Grant Agreement No. 804349 (ERC StG CUHL), RyC Fellowship No. RYC-2017-22330, and IAE Project PID2019-111673GB-I00. A E S, E C A, M S and C M S T acknowledge support of the Spanish MICINN Project SIP (PGC2018-101743-B-I00).

Two-dimensional materials, such as transition-metal dichalcogenides (TMDs), are attractive candidates for sensing applications due to their high surface-to-volume ratio, chemically active edges, and good electrical properties. However, their electrical response to humidity is still under debate and experimental reports remain inconclusive. For instance, in different studies, the impedance of MoS

Precise patterning of 2D materials into micro- and nanostructures presents a considerable challenge and many efforts are dedicated to the development of processes alternative to the standard lithography. In this work we show a fabrication technique based on direct electron beam lithography (EBL) on MoS

Optomechanical crystal cavities (OMC) have rich perspectives for detecting and indirectly analysing biological particles, such as proteins, bacteria and viruses. In this work we demonstrate the working principle of OMCs operating under ambient conditions as a sensor of submicrometer particles by optically monitoring the frequency shift of thermally activated mechanical modes. The resonator has been specifically designed so that the cavity region supports a particular family of low modal-volume mechanical modes, commonly known as -pinch modes-. These involve the oscillation of only a couple of adjacent cavity cells that are relatively insensitive to perturbations in other parts of the resonator. The eigenfrequency of these modes decreases as the deformation is localized closer to the centre of the resonator. Thus, by identifying specific modes that undergo a frequency shift that amply exceeds the mechanical linewidth, it is possible to infer if there are particles deposited on the resonator, how many are there and their approximate position within the cavity region. OMCs have rich perspectives for detecting and indirectly analysing biological particles, such as proteins, viruses and bacteria., D. N. U. gratefully acknowledges funding from the Ministry of Science, Innovation and Universities (PGC2018-094490-B-C22) and Fundació Bosch i Gimpera (F2I-FVal_2019-012). ICN2 is supported by the Severo Ochoa program from the Spanish Research Agency (AEI, grant no. SEV-2017-0706) by the CERCA Programme / Generalitat de Catalunya and by the Ministry of Science, Innovation and Universities (PGC2018-101743-B-I00). G. A and P.X acknowledge the support of a BIST and COFUND PREBIST studentships, respectively. E.K. and G.F. acknowledge the financial support by ERC AdG SmartPhon (Grant No. 694977).

State-of-the-art fabrication and characterisation techniques have been employed to measure the thermal conductivity of suspended, single-crystalline MoS2 and MoS2/hBN heterostructures. Two-laser Raman scattering thermometry was used combined with real time measurements of the absorbed laser power. Measurements on MoS2 layers with thicknesses of 5 and 14 nm exhibit thermal conductivity in the range between 12 Wm-1 K-1 and 24 Wm-1 K-1. Additionally, after determining the thermal conductivity of the latter MoS2 sample, an hBN flake was transferred onto it and the effective thermal conductivity of the heterostructure was subsequently measured. Remarkably, despite that the thickness of the hBN layer was less than a hal of the thickness of the MoS2 layer, the heterostructure showed an almost eight-fold increase in the thermal conductivity, being able to dissipate more than ten times the laser power without any visible sign of damage. These results are consistent with a high thermal interface conductance G between MoS2 and hBN and an efficient in-plane heat spreading driven by hBN. Indeed, we estimate G ∼ 70 MW m-2 K-1 for hBN layer thermal conductivity of 450 Wm-1 K-1 which is significantly higher than previously reported values. Our work therefore demonstrates that the insertion of hBN layers in potential MoS2-based devices holds the promise for efficient thermal management., This work was partially funded by the European Union under the H2020 FET-OPEN NANOPOLY (GA 289061) and Spanish Ministry of Science projects SIP (PGC2018-101743-B-I00), ADAGIO (PGC2018-094490-B-C22), 2DTecBio (FIS2017-85787-R) and 2DENGINE (PID2019-111773RB- I00/AEI/10.13039/501100011033). E D C acknowledges the Spanish Ministry of Science for the Juan de la Cierva Fellowship (JC-2015-25201) and the Ramon y Cajal fellowship (RYC2019-027879-I). D N U and J F S acknowledge the Ramón y Cajal fellowships RYC2014-15392 and RYC2019-028368-I/AEI/10.13039/501100011033. M V C acknowledges project (Reference No. 103739) funded by the Agencia Estatal de Investigación through the PCI 2019 call. The Catalan Institute of Nanoscience and Nanotechnology (ICN2) is funded by the CERCA program/Generalitat de Catalunya, and is supported by the Severo Ochoa program from Spanish MINECO (Grant No. SEV-2017-0706). K W and T T acknowledge support from the Elemental Strategy Initiative conducted by the MEXT, Japan (Grant Number JPMXP0112101001) and JSPS KAKENHI (Grant Numbers 19H05790 and JP20H00354).

Heat conduction in silicon can be effectively engineered by means of sub-micrometre porous thin free-standing membranes. Tunable thermal properties make these structures good candidates for integrated heat management units such as waste heat recovery, rectification or efficient heat dissipation. However, possible applications require detailed thermal characterisation at high temperatures which, up to now, has been an experimental challenge. In this work we use the contactless two-laser Raman thermometry to study heat dissipation in periodic porous membranes at high temperatures via lattice conduction and air-mediated losses. We find the reduction of the thermal conductivity and its temperature dependence closely correlated with the structure feature size. On the basis of two-phonon Raman spectra, we attribute this behaviour to diffuse (incoherent) phonon-boundary scattering. Furthermore, we investigate and quantify the heat dissipation via natural air-mediated cooling, which can be tuned by engineering the porosity., Nanostructuring of silicon allows acoustic phonon engineering, but the mechanism of related thermal transport in these structures is not fully understood. Here, the authors study the heat dissipation in silicon membranes with periodic nanoholes and show the importance of incoherent scattering.

We report on the structural, mechanical, and thermal analysis of 40 nm thick polystyrene-block-poly(ethylene oxide) (PS-b-PEO) block copolymer (BCP) films coated with evaporated chromium layers of different thicknesses (1, 2, and 5 nm). Solvent annealing processes allow the structural control of the BCP films morphology by rearranging the position of the PEO cylinders parallel to the substrate plane. High-vacuum scanning thermal microscopy and ultrasonic force microscopy measurements performed in ambient pressure revealed that coated ultrathin metal layers strongly influence the heat dissipation in the BCP films and the local surface stiffness of the individual BCP domains, respectively. The measured tip-sample effective thermal resistance decreases from 6.1 × 107 to 2.5 × 107 K W-1 with increasing Cr film thickness. In addition, scanning probe microscopy measurements allow the thermal and mechanical mapping of the two segregated polymer domains (PEO-PS) of sub-50 nm characteristic sizes, with sub-10 nm thermal spatial resolution. The results revealed the effect of the surface morphology of the BCP and the incorporation of the metal film on the nanoscale thermal properties and volume self-assembly on the mechanical properties. The findings from this study provide insight into the formation of high aspect ratio BCP-metal structures with the more established applications in lithography. In addition, knowledge of the thermal and mechanical properties at the nanoscale is required in emergent applications, where BCPs, or polymers in general, are part of the structure or device. The performance of such devices is commonly related to the requirement of increased heat dissipation while maintaining mechanical flexibility., The authors acknowledge the financial support from the EU FP7 project QUANTIHEAT (Grant No. 604668) and The Royal Society Paul Instrument Fund, EPSRC Grant EP/K023373/1. ICN2 is supported by Spanish MINECO (Severo Ochoa Centers of Excellence Program under Grant SEV-2017-0706 and PHENTOM project under Grant FIS2015-70862-P) and by the Generalitat de Catalunya (Grant 2017SGR806 and the CERCA Program)

We report a record low thermal conductivity in polycrystalline MoS obtained for ultrathin films with varying grain sizes and orientations. By optimizing the sulfurization parameters of nanometer-thick Mo layers, five MoS films containing a combination of horizontally and vertically oriented grains, with respect to the bulk (001) monocrystal, were grown. From transmission electron microscopy, the average grain size, typically below 10 nm, and proportion of differently oriented grains were extracted. The thermal conductivity of the suspended samples was extracted from a Raman laser-power-dependent study, and the lowest value of thermal conductivity of 0.27 W m K, which reaches a similar value as that of Teflon, is obtained in a polycrystalline sample formed by a combination of horizontally and vertically oriented grains in similar proportion. Analysis by means of molecular dynamics and finite element method simulations confirm that such a grain arrangement leads to lower grain boundary conductance. We discuss the possible use of these thermal insulating films in the context of electronics and thermoelectricity.

The central concept in phononics is the tuning of the phonon dispersion relation, or phonon engineering, which provides a means of controlling related properties such as group velocity or phonon interactions and, therefore, phonon propagation, in a wide range of frequencies depending on the geometries and sizes of the materials. Phononics exploits the present state of the art in nanofabrication to tailor dispersion relations in the range of GHz for the control of elastic waves/phonons propagation with applications toward new information technology concepts with phonons as state variable. Moreover, phonons provide an adaptable approach for supporting a coherent coupling between different state variables, and the development of nanoscale optomechanical systems during the last decade attests this prospect. The most extended approach to manipulate the phonon dispersion relation is introducing an artificial periodic modulation of the elastic properties, which is referred to as phononic crystal (PnC). Herein, the focus is on the recent experimental achievements in the fabrication and application of 2D PnCs enabling the modification of the dispersion relation of surface and membrane modes, and presenting phononic bandgaps, waveguiding, and confinement in the hypersonic regime. Furthermore, these artificial materials offer the potential of modifying and controlling the heat flow to enable new schemes in thermal management., ICN2 is supported by the Spanish MINECO (Severo Ochoa Centers of Excellence Program under Grant SEV‐2017‐0706), and by the Generalitat de Catalunya (Grant 2017SGR806 and the CERCA Program). B.G. acknowledges the support from the Foundation for Polish Science (POIR.04.04.00‐00‐5D1B/18), Polish National Science Centre (UMO‐2018/31/D/ST3/03882), and ERC AdG SmartPhon (Grant No. 694977).

Altres ajuts: ICN2 is supported by the CERCA Programme/Generalitat de Catalunya. In this study, we use transient thermal gratings-a non-contact, laser-based thermal metrology technique with intrinsically high accuracy-to investigate room-temperature phonon-mediated thermal transport in two nanoporous holey silicon membranes with limiting dimensions of 120 nm and 250 nm, respectively. We compare the experimental results with ab initio calculations of phonon-mediated thermal transport according to the phonon Boltzmann transport equation (BTE) using two different computational techniques. We find that the calculations conducted within the Casimir framework, i.e., based on the BTE with the bulk phonon dispersion and diffuse scattering from surfaces, are in quantitative agreement with the experimental data and thus conclude that this framework is adequate for describing phonon-mediated thermal transport in silicon nanostructures with feature sizes of the order of 100 nm.

The possibility of tailoring the critical strain of two-dimensional (2D) materials will be crucial for the fabrication of flexible and stretchable devices. While crystalline MoS2 monolayer shows tensile strength comparable to that of steel, a large concentration of defects and grain boundaries in polycrystalline MoS2 significantly degrades its mechanical properties. In this paper, the fracture in polycrystalline MoS2 films with an average grain size below 10 nm is studied at the micro- and nanoscale using electron microscopy. Two samples with different thicknesses and grain orientations horizontal and vertical to the sample plane are measured. The critical uniaxial strain is determined to be approximately 5% and independent of the sample morphology. However, electron beam irradiation is found to enhance the interaction between MoS2 and polydimethylsiloxane (PDMS) substrates, leading to an increased critical strain that can exceed 10%. This enhancement of strain resistance was used to fabricate a mechanically robust array of MoS2 lines 1 mm in length. Finally, nanoscale crack propagation studied by transmission electron microscopy showed that cracks propagate along the grain boundaries as well as through the grains, preferentially along van der Waals planes. These results provide insight into the fracture of polycrystalline 2D materials and a method to enhance the critical strain., ICN2 is funded by the CERCA program/Generalitat de Catalunya and supported by the Severo Ochoa program from Spanish MINECO (Grant No. SEV-2017-0706). We acknowledge support from the EU Project NANOPOLY (GA 289061) and the Spanish MICINN project SIP (PGC2018-101743-BI00). M.S. thanks D. Torres Navarro for his support in figure preparation. M.P. acknowledges the support from Spanish Ministry of Science, Innovation and Universities within the Ramon y Cajal Program (RYC-2017-23758). P.X. is supported by a M. Sklodowska-Curie Fellowship (COFUND BIST Predoctoral Programme, PREBIST project, Grant No. 754558).

A thermal rectifier/diode is a nonreciprocal element or system that enables preferential heat transport in one direction. In this work we demonstrate a single-material thermal diode operating at high temperatures. The diode is made of nanostructured silicon membranes exhibiting spatially and temperature-dependent thermal conductivity and, therefore, falling into the category of spatially asymmetric, nonlinear nonreciprocal systems. We used an all-optical state-of-the-art experimental technique to prove rectification along rigorous criteria of the phenomenon. Using sub-milliwatt power we achieve rectification of about 14%. In addition, we demonstrate air-triggered thermal switching and passive cooling. Our findings provide a CMOS-compatible platform for heat rectification and applications in energy harvesting, thermal insulation and cooling, as well as sensing and potentially thermal logic., The work was supported by Polish National Science Centre (Sonata UMO-2018/31/D/ST3/03882 and Preludium UMO-2019/33/N/ST5/02902). B.G. acknowledge the support from the Foundation for Polish Science (POIR.04.04.00-00-5D1B/18). The ICN2 is funded by the CERCA programme/Generalitat de Catalunya. The ICN2 is supported by the Severo Ochoa Centres of Excellence programme, funded by the Spanish Research Agency (AEI, grant no. SEV-2017-0706). M.S., F.A. and C.M.S.T. acknowledge support from Spanish MICINN project SIP (PGC2018-101743-B-I00).

Almost 15 years have gone ever since the discovery of graphene as a single atom layer. Numerous papers have been published to demonstrate its high electron mobility, excellent thermal and mechanical as well as optical properties. We have recently seen more and more applications towards using graphene in commercial products. This paper is an attempt to review and summarize the current status of the research of the thermal properties of graphene and other 2D based materials including the manufacturing and characterization techniques and their applications, especially in electronics and power modules. It is obvious from the review that graphene has penetrated the market and gets more and more applications in commercial electronics thermal management context. In the paper, we also made a critical analysis of how mature the manufacturing processes are; what are the accuracies and challenges with the various characterization techniques and what are the remaining questions and issues left before we see further more applications in this exciting and fascinating field., YF, JH, YL, AZ, MK and JL acknowledge the financial support from The Swedish National Science Foundation (VR under the contract No 621-2007-4660), The Swedish Foundation for Strategic Research (SSF) under contract (No SE13-0061), the Swedish Board for innovation under the Siografen program and from the Production Area of Advance at Chalmers University of Technology, Sweden. SC, YZ and JL acknowledge the financial support by the Key R&D Development Program from the Ministry of Science and Technology of China with the contract No: 2017YFB040600 and the National Natural Science Foundation of China (No. 51872182). XX is supported the National Natural Science Foundation of China (No. 11674245 & No. 11890703). MS and CMST acknowledge financial support from the CERCA programme/Generalitat de Catalunya, the Severo Ochoa Centres of Excellence programme, funded by the Spanish Research Agency (AEI, Grant No. SEV-2017-0706).

In this work we study the effects of disorder on the thermal conductivity of porous 100 nm thick silicon membranes, in which the size, shape and position of the pores were varied randomly. Measurements using two-laser Raman thermometry on both non-patterned and porous membranes revealed more than a 10-fold reduction of the thermal conductivity compared to that of bulk silicon and a six-fold reduction compared to non-patterned membranes for the sample with random pore shapes. Using Monte Carlo methods we solved the Boltzmann transport equation for phonons and compared different possibilities of pore organization and its influence on the thermal conductivity of the samples. The simulations confirmed that the strongest reduction of thermal conductivity is achieved for a distribution of pores with arbitrary shapes that partially overlap. Up to a 15% reduction of the thermal conductivity with respect to the purely circular pores was predicted for a porous membrane with 37% filling fraction. The effect of the pore shape and distribution was further studied. Maps of temperature and heat flux distributions clearly showed that for particular pore placement heat transport can be efficiently blocked and hot spots can be found in narrow channels between pores. These findings have an impact on the fabrication of membrane-based thermoelectric devices, where low thermal conductivity is required. This work shows that for porous membranes with a given filling fraction the thermal conductivity can be further modified by introducing disorder in the shape and placement of the pores., The ICN2 is funded by the CERCA program/Generalitat de Catalunya. ICN2 is supported by the Severo Ochoa program from Spanish MINECO (Grant No. SEV-2017-0706). We acknowledge the financial support from the Spanish MINECO project PHENTOM (FIS2015-70862-P). We acknowledge the financial support from the French ANR with the project MESOPHON (ANR-15-CE30-0019). BG acknowledges the support from the Foundation for Polish Science (Homing/ 2016-1/2 and First Team POIR.04.04.00-00-5D1B/18-00) and ERC AdG SmartPhon (Grant No. 694977).

While the dispersion of nanomaterials is known to be effective in enhancing the thermal conductivity and specific heat capacity of fluids, the mechanisms behind this enhancement remain to be elucidated. Herein, we report on highly stable, surfactant-free graphene nanofluids, based on N,N-dimethylacetamide (DMAc) and N,N-dimethylformamide (DMF), with enhanced thermal properties. An increase of up to 48% in thermal conductivity and 18% in specific heat capacity was measured. The blue shift of several Raman bands with increasing graphene concentration in DMF indicates that there is a modification in the vibrational energy of the bonds associated with these modes, affecting all the molecules in the liquid. This result indicates that graphene has the ability to affect solvent molecules at long-range, in terms of vibrational energy. Density functional theory and molecular dynamics simulations were used to gather data on the interaction between graphene and solvent, and to investigate a possible order induced by graphene on the solvent. The simulations showed a parallel orientation of DMF towards graphene, favoring π–π stacking. Furthermore, a local order of DMF molecules around graphene was observed suggesting that both this special kind of interaction and the induced local order may contribute to the enhancement of the fluid's thermal properties., The Catalan Institute of Nanoscience and Nanotechnology (ICN2) acknowledges support from the Severo Ochoa Program (MINECO, Grant SEV-2013-0295) and funding from the CERCA Programme/Generalitat de Catalunya. Funding from the Spanish Ministry (MINECO/FEDER: MAT2015-68394-R NaCarFLOW, FIS2015-70862-P PHENTOM and FIS2015-64886-C5-3-P SIESTA) is also acknowledged. FC, BE, MP and PO acknowledge support from the EU Center of Excellence MaX-Materials Design at the Exascale (Grant No. 676598), Generalitat de Catalunya (Grant No. 2014SGR301) and supercomputing resources from the Red Española de Supercomputación (RES). ACA acknowledges Fundació Cellex de Barcelona for financial support.

We report a record low thermal conductivity in polycrystalline MoS

Heat conduction in silicon can be effectively reduced by means of periodic patterning of free-standing membranes. In this work we show a straightforward method for fabrication of free-standing phononic crystals based on thin silicon membranes. We use the contactless two-laser Raman thermometry method to measure thermal conductivity of the hexagonal phononic crystals. The aim of the study is to understand and control the behaviour of phonons in phononic crystals, with the target of minimizing the thermal conductivity. In particular, we are interested in the influence of the surface-to-volume ratio on the thermal conductivity.

Optical non-linearities, such as thermo-optic effects and free-carrier-dispersion, are often considered as undesired effects in silicon-based resonators and, more specifically, optomechanical (OM) cavities, affecting the relative detuning between an optical resonance and the excitation laser. However, the interplay between such mechanisms could also enable unexpected physical phenomena to be used in new applications. In the present work, we exploit those non-linearities and their intercoupling with the mechanical degrees of freedom of a silicon OM nanobeam to unveil a rich set of fundamentally different complex dynamics. By smoothly changing the parameters of the excitation laser, namely its power and wavelength, we demonstrate accurate control for activating bi-dimensional and tetra-dimensional limit-cycles, a period doubling route and chaos. In addition, by scanning the laser parameters in opposite senses we demonstrate bistability and hysteresis between bi-dimensional and tetra-dimensional limit-cycles, between different coherent mechanical states and between tetra-dimensional limit-cycles and chaos. As a result of implementing the Rosenstein algorithm to the experimental time series we have extracted a Largest Lyapunov Exponent of LLE=1.3x10^5 s-1, which is between one and two orders of magnitude lower than the typical oscillating frequencies of the system. Most of the experimental features can be well reproduced with a model of first-order non-linear differential equations coupled through the number of intracavity photons. Our findings open several routes towards exploiting silicon-based OM photonic crystals for systems with memory and, more specifically, for chaos based applications such as secure information transfer or sensing., Main file:12 pages, 5 figures Supplementary Information: 15 pages, 14 figures

We investigate the feasibility of activating coherent mechanical oscillations in lasing microspheres by modulating the laser emission at a mechanical eigenfrequency. To this aim, 1.5% Nd3+:Barium-Titanium-Silicate microspheres with diameters around 50 {\mu}m were used as high quality factor (Q>10^6) whispering gallery mode lasing cavities. We have implemented a pump-and-probe technique in which the pump laser used to excite the Nd3+ ions is focused on a single microsphere with a microscope objective and a probe laser excites a specific optical mode with the evanescent field of a tapered fibre. The studied microspheres show monomode and multi-mode lasing action, which can be modulated in the best case up to 10 MHz. We have optically transduced thermally-activated mechanical eigenmodes appearing in the 50-70 MHz range, the frequency of which decreases with increasing the size of the microspheres. In a pump-and-probe configuration we observed modulation of the probe signal up to the maximum pump modulation frequency of our experimental setup, i.e., 20 MHz. This modulation decreases with frequency and is unrelated to lasing emission, pump scattering or thermal effects. We associate this effect to free-carrier-dispersion induced by multiphoton pump light absorption. On the other hand, we conclude that, in our current experimental conditions, it was not possible to resonantly excite the mechanical modes. Finally, we discuss on how to overcome these limitations by increasing the modulation frequency of the lasing emission and decreasing the frequency of the mechanical eigenmodes displaying a strong degree of optomechanical coupling., Comment: 17 pages, 5 figures

Free-standing Si films have been and remain an excellent example to study experimentally the effect of the reduction of the characteristic size on the phonon dispersion relation. A step further in geometrical complexity and, therefore, in increasing the control and manipulation of phonons is achieved by introducing periodicity in the medium to form phononic crystals. Here we report on the development of the fabrication process of large-area, solid-air and solid-solid two-dimensional phononic crystals, directly on free-standing, single crystalline silicon membranes. The patterning of the membranes involved electron-beam lithography and reactive ion etching for holes or metal evaporation and lift-off for pillars. The fabrication was possible due to the external strain induced on the membrane in order to reduce the buckling, which is typically found in large area free-standing structures. As a result, we obtained 250 nm thick structured membranes with patterned areas up to 100 × 100 μm, feature size between 100 and 300 nm and periodicity between 300 and 500 nm. The changes in dispersion relations of hypersonic acoustic phonons due to nanopatterning in free-standing silicon membranes were measured by Brillouin light scattering and the results were compared with numerical calculations by finite elements method. Information on phonon dispersion relation combined with a reliable fabrication process for large-scale structures opens a way for phonon engineering in more complex devices., The authors acknowledge the financial support from the FP7 FET Energy Project MERGING (Grant no. 309150); the Spanish MICINN projects nanoTHERM (Grant no. CSD2010-0044); TAPHOR (MAT2012-31392) and the program Severo Ochoa (Grant SEV-2013-0295).

arXiv:1604.04865.-- et al., Heat conduction in 2D materials can be effectively engineered by means of controlling nanoscale grain structure. Afavorable thermal performance makes these structures excellent candidates for integrated heat management units. Here we show combined experimental and theoretical studies for MoS nanosheets in a nanoscale grain-size limit.Wereport thermal conductivity measurements on 5 nm thick polycrystalline MoS by means of 2-laser Raman thermometry. The free-standing, drum-like MoS nanomembranes were fabricated using a novel polymer- and residue-free, wet transfer, in which we took advantage of the difference in the surface energies between MoS and the growth substrate to transfer the CVD-grown nanosheets. The measurements revealed a strong reduction in the in-plane thermal conductivity down to about 0.73 ± 0.25 W m K. The results are discussed theoretically using finite elements method simulations for a polycrystalline film, and a scaling trend of the thermally conductivity with grain size is proposed., The authors acknowledge the financial support from the FP7 projects MERGING (Grant No. 309150) and QUANTIHEAT (No. 604668); the Spanish MICINN project PHENTOM (FIS2015-70862-P); and the program Severo Ochoa (Grant SEV-2013-0295). SR acknowledges funding from the Spanish Ministry of Economy and Competitiveness and the European Regional Development Fund (FIS2015-67767-P (MINECO/FEDER)); and MP acknowledges the program Juan de la Cierva (FPDI-2013-18968).

The design and fabrication of phononic crystals (PnCs) hold the key to control the propagation of heat and sound at the nanoscale. However, there is a lack of experimental studies addressing the impact of order/disorder on the phononic properties of PnCs. Here, we present a comparative investigation of the influence of disorder on the hypersonic and thermal properties of two-dimensional PnCs. PnCs of ordered and disordered lattices are fabricated of circular holes with equal filling fractions in free-standing Si membranes. Ultrafast pump and probe spectroscopy (asynchronous optical sampling) and Raman thermometry based on a novel two-laser approach are used to study the phononic properties in the gigahertz (GHz) and terahertz (THz) regime, respectively. Finite element method simulations of the phonon dispersion relation and three-dimensional displacement fields furthermore enable the unique identification of the different hypersonic vibrations. The increase of surface roughness and the introduction of short-range disorder are shown to modify the phonon dispersion and phonon coherence in the hypersonic (GHz) range without affecting the room-temperature thermal conductivity. On the basis of these findings, we suggest a criteria for predicting phonon coherence as a function of roughness and disorder., The authors acknowledge financial support from the EU FP7 project MERGING (Grant 309150), NANO-RF (Grant 318352), and QUANTIHEAT (Grant 604668); the Spanish MICINN projects nanoTHERM (Grant CSD2010-0044) and PHENTOM (FIS2015-70862-P); and the Severo Ochoa Program (MINECO, Grant SEV-2013-0295). M.R.W. acknowledges the postdoctoral Marie Curie Fellowship (IEF) HeatProNano (Grant 628197).

et al., The effective thermal conductivity of sintered porous pastes of silver is modeled through two theoretical methods and measured by means of three experimental techniques. The first model is based on the differential effective medium theory and provides a simple analytical description considering the air pores like ellipsoidal voids of different sizes, while the second one arises from the analysis of the scanning-electron-microscope images of the paste cross-sections through the finite element method. The predictions of both approaches are consistent with each other and show that the reduction of the thermal conductivity of porous pastes can be minimized with spherical pores and maximized with pancake-shaped ones, which are the most efficient to block the thermal conducting pathways. A thermal conductivity of 151.6 W/m K is numerically determined for a sintered silver sample with 22% of porosity. This thermal conductivity agrees quite well with the one measured by the Lateral Thermal Interface Material Analysis for a suspended sample and matches, within an experimental uncertainty smaller than 16%, with the values obtained by means of Raman thermometry and the 3u technique, for two samples buried in a silicon chip. The consistence between our theoretical and experimental results demonstrates the good predictive performance of our theoretical models to describe the thermal behavior of porous thermal interface materials and to guide their engineering with a desired thermal conductivity., This work was supported by the NANOTHERM project co-funded by the European Commission under the “Information and Communication Technologies”, Seven Framework Program, and the Grant Agreement No 318117.

et al., Studying thermal transport at the nanoscale poses formidable experimental challenges due both to the physics of the measurement process and to the issues of accuracy and reproducibility. The laser-induced transient thermal grating (TTG) technique permits non-contact measurements on nanostructured samples without a need for metal heaters or any other extraneous structures, offering the advantage of inherently high absolute accuracy. We present a review of recent studies of thermal transport in nanoscale silicon membranes using the TTG technique. An overview of the methodology, including an analysis of measurements errors, is followed by a discussion of new findings obtained from measurements on both >solid> and nanopatterned membranes. The most important results have been a direct observation of non-diffusive phonon-mediated transport at room temperature and measurements of thickness-dependent thermal conductivity of suspended membranes across a wide thickness range, showing good agreement with first-principles-based theory assuming diffuse scattering at the boundaries. Measurements on a membrane with a periodic pattern of nanosized holes (135nm) indicated fully diffusive transport and yielded thermal diffusivity values in agreement with Monte Carlo simulations. Based on the results obtained to-date, we conclude that room-temperature thermal transport in membrane-based silicon nanostructures is now reasonably well understood., The work done at MIT was supported as part of the “Solid State Solar-Thermal Energy Conversion Center (S3TEC),” an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Award No. DE-SC0001299/DEFG02-09ER46577. The contribution by A.V.-F. and J. J. A.-G. was partially supported by Project 192 “Fronteras de la ciencia” and Project 251882 “Investigacion Científica Basica.” A. V.-F. also appreciates support from Conacyt through normal and mixed scholarships. MS and CMST acknowledge support from the Spanish program Severo Ochoa (Grant SEV-2013-0295), projects PHENTOM (FIS2015-70862-P) and nanoTHERM (CSD2010-00044), as well as from the EU project MERGING (309150). Z.L. and E.N.W. further acknowledge support and funding from the Air Force Office of Scientific Research (AFOSR), and are grateful to program manager Dr. Ali Sayir.

The analysis of diffracted light from periodic structures is shown to be a versatile metrology technique applicable to inline metrology for periodic nanostructures. We show that 10 nm changes in periodic structures can be traced optically by means of sub-wavelength diffraction. Polymer gratings were fabricated by electron beam lithography. The gratings have a common periodicity of 6 μm, but different line width, ranging from 370 to 550 nm in 10 nm steps. A comparison between the resulting diffraction patterns shows marked differences in intensity which are used to sense nanometre scale deviations in periodic structures.

Mass production of nanostructured surfaces relies on the periodic repetition of micrometre scale patterns. A unit cell with nanometre features in the micrometre size range is repeated thousands of times. The ensemble can used as a diffraction grating for visible light. The relative intensity distribution of the diffraction orders is characteristic for the grating and sensitive to nanometre scale changes. A newly designed subwavelength diffraction setup allows the measurement in real time of the diffraction pattern of an illuminated polymer grating with only one detector image. The setup records diffraction patterns of, for example, polymer gratings with intentionally low scattering contrast and line features ranging from 610 to 80 nm. Thus, sub-100 nm features can be traced. The comparison of the measured diffraction patterns with simulated patterns allows to sense nanometre scale deviations from fabrication goals.

Under the terms of the Creative Commons Attribution License 3.0 (CC-BY)., We investigate experimentally and theoretically the acoustic phonon propagation in two-dimensional phononic crystal membranes. Solid-air and solid-solid phononic crystals were made of square lattices of holes and Au pillars in and on 250 nm thick single crystalline Si membrane, respectively. The hypersonic phonon dispersion was investigated using Brillouin light scattering. Volume reduction (holes) or mass loading (pillars) accompanied with second-order periodicity and local resonances are shown to significantly modify the propagation of thermally activated GHz phonons. We use numerical modeling based on the finite element method to analyze the experimental results and determine polarization, symmetry, or three-dimensional localization of observed modes., The authors acknowledge financial support from the European FP7 project MERGING (Grant No. 309150), the Spanish MINECO projects nanoTHERM (Grant No. CSD2010-0044) and TAPHOR (MAT2012-31392) and the program Severo Ochoa (Grant SEV-2013-0295). M.R.W. acknowledges support of the Marie Curie Fellowship HeatProNano (Grant No. 628197).

ACS AuthorChoice - This is an open access article published under an ACS AuthorChoice License, which permits copying and redistribution of the article or any adaptations for non-commercial purposes., A detailed understanding of the connections of fabrication and processing to structural and thermal properties of low-dimensional nanostructures is essential to design materials and devices for phononics, nanoscale thermal management, and thermoelectric applications. Silicon provides an ideal platform to study the relations between structure and heat transport since its thermal conductivity can be tuned over 2 orders of magnitude by nanostructuring. Combining realistic atomistic modeling and experiments, we unravel the origin of the thermal conductivity reduction in ultrathin suspended silicon membranes, down to a thickness of 4 nm. Heat transport is mostly controlled by surface scattering: rough layers of native oxide at surfaces limit the mean free path of thermal phonons below 100 nm. Removing the oxide layers by chemical processing allows us to tune the thermal conductivity over 1 order of magnitude. Our results guide materials design for future phononic applications, setting the length scale at which nanostructuring affects thermal phonons most effectively., This work is partly funded by the European Commission FP7-ENERGY-FET project MERGING, NMP QUANTIHEAT and ICT NANOTHERM, with Grant Agreement Nrs: 309150, 604668 and 318117, respectively. S.N. and D.D. acknowledge financial support from MPG under the MPRG program. J.S.R., M.R.W., and C.M.S.T. acknowledge support form the Spanish MINECO projects TAPHOR (MAT-2012-31392) and nanoTHERM (CONSOLIDER CSD2010-00044). M.R.W. acknowledges support of the Marie Curie Postdoctoral Fellowship HeatProNano (Grant No. 628197). M.P. and A.S. acknowledge funding from the Academy of Finland (Grant No. 252598).

Present silicon technology provides single crystal films and membranes with thicknesses on the order of 10 nm and below. Along the years, the thermal studies on such structures have shown a reduction of the thermal conductivity consistent with the decrease of the characteristic size. Although the lowering of thermal conductivity is detrimental for heat dissipation processes in nanoelectronic devices, it becomes advantageous to increase the figure of merit (ZT) of Si and turning it as a promising material for thermoelectric applications. It is widely accepted that, for submicrometer thicknesses and down to 20 nm, the reduction of the in-plane thermal conductivity is mainly determined by the shortening of the phonon mean free path due to the diffusive scattering of phonons at the boundaries. In this case an analytical model that adopts the phonon bulk properties and includes the boundary scattering by a conductivity reduction factor is shown to match the experimental trend of systematical reduction of the thermal conductivity as the thickness of the film or membrane decreases. In this talk we will report on the modification of the phonon properties in Si membranes, such as the dispersion relation, and its dependence on the thickness. The thermal conductivity in Si results from the cumulative contribution of the transport of phonons with a broad range of wavevectors and mainly from long-wavevector phonons. Thus, heat transport at room temperature can be influenced by the reduction of the membrane thickness as the dispersion relation of the short-wave-phonons starts to be affected by emerging new vibrational modes arising from the boundary conditions at the membrane surface. Our measurements of the thermal conductivity in ultra-thin sub-20 nm Si membranes have demonstrated this extreme and, in fact, we have been able to disentangle the effects of phonon confinement from those related to surface morphology and chemical composition. Finally, we will discuss the prospects of employing the increased modification of phonon dispersion relation achieved in phononic crystals in controlling heat transport. While some applications require the existence of absolute band gaps, the approach to reduce the thermal conductivity is more linked to the decrease of phonon’s group velocity and phonon-phonon interactions in an extended range of frequencies.