Two-Dimensional Phononic Crystals: Disorder Matters

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.Comment: 19 pages, 4 figures, final published version, Nano Letters, 201

Exciton tuning and strain imaging in WS2supported on PDMS micropillars

Since the raise of 2D materials, significant research has been dedicated to their strain-dependent electronic and mechanical properties. In this work, we studied exciton energies and low-frequency phonon modes in CVD-grown mono- and few-layer WS2 transferred on PDMS micropillars. The modification of the band structure under strain was investigated by photoluminescence (PL) spectroscopy at room temperature. Machine learning (ML) methods were used to analyze the PL spatial maps and facilitate the spectral deconvolution. For monolayer (1L) WS2, red shift in the exciton energy was detected as a function of the position, which was ascribed to the presence of residual strain. For three-layer (3L) strained WS2, a significant increase in the PL intensity corresponding to direct (K-K) band transition together with a change of exciton energy was observed. From the PL spectra, strain distribution maps were extracted for both studied samples, which strongly resembled the ML clustering results. Finally, the low-frequency Raman modes of WS2 were studied on both Si/SiO2 and PDMS substrates and no significant change of their frequency was observed for the 3L-WS2

Thermal transport in nanoporous holey silicon membranes investigated with optically-induced transient thermal gratings

In this study, we use the transient thermal grating optical technique \textemdash a non-contact, laser-based thermal metrology technique with intrinsically high accuracy \textemdash to investigate room-temperature phonon-mediated thermal transport in two nanoporous holey silicon membranes with limiting dimensions of 100 nm and 250 nm respectively. We compare the experimental results to 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 through holey silicon membranes with feature sizes on the order of 100 nm

Thermal transport in nanoporous holey silicon membranes investigated with optically induced transient thermal gratings

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

Revista Praticando Capoeira: materialidade e representações

O presente estudo analisa a revista Praticando Capoeira, entendendo-a como vestígios de uma rede de práticas, capaz de dar a ver a luta de sentidos em que foi produzida e as relações de força que determinaram sua estruturação. O impresso em questão foi produzido pela editora D+T a partir de 1999, vendido principalmente em bancas de revistas até 2009, período em que foram publicadas 44 edições. Por intermédio dos dispositivos editoriais do impresso e pelas representações elaboradas e postas a circular, buscou-se compreender a organização do mundo da capoeira, considerado como um campo social. Uma revista, por sua agilidade de produção e circulação, consegue captar aspectos do cotidiano ao mesmo tempo em que as informações, os embates e as mudanças acontecem. Para trabalharmos com esse objeto, que se converte em fonte de pesquisa, são necessários considerar três elementos distintos, mas articulados, isto é, considerá-lo como suporte de textos, além de atentar para os próprios textos e para o discurso produzido pelo encadeamento de sentidos no ato da leitura. Ancorado na História Cultural e nas reflexões sobre as lutas de representação, a pesquisa entende a coleção de revistas como objetos culturais que guardam as marcas de sua fabricação. Com base na materialidade da revista, trabalha-se com o periódico nos termos de uma arqueologia dos objetos, em busca de pistas, para localizar os sinais das práticas editoriais, capazes de revelar os vestígios das estratégias, utilizadas pelos editores para a organização e regramento da leitura do impresso. A revista Praticando Capoeira é um documento e um monumento sobre o mundo da capoeira, uma testemunha do presente para o futuro, que proporciona a recordação do nosso tempo. As representações estão nos textos e imagens, materializadas no objeto em função de sua fórmula editorial. Identificamos que a revista apresenta a organização do campo com base nos grupos de capoeira e considera a prática da capoeiragem como um dos elementos da identidade brasileira, com potencial esportivo e pedagógico

Enhanced Thermal Conductivity of Free-Standing Double-Walled Carbon Nanotube Networks

Nanomaterials are driving advances in technology due to their oftentimes superior properties over bulk materials. In particular, their thermal properties become increasingly important as efficient heat dissipation is required to realize high-performance electronic devices, reduce energy consumption, and prevent thermal damage. One application where nanomaterials can play a crucial role is extreme ultraviolet (EUV) lithography, where pellicles that protect the photomask from particle contamination have to be transparent to EUV light, mechanically strong, and thermally conductive in order to withstand the heat associated with high-power EUV radiation. Free-standing carbon nanotube (CNT) films have emerged as candidates due to their high EUV transparency and ability to withstand heat. However, the thermal transport properties of these films are not well understood beyond bulk emissivity measurements. Here, we measure the thermal conductivity of free-standing CNT films using all-optical Raman thermometry at temperatures between 300 and 700 K. We find thermal conductivities up to 50 W m-1 K-1 for films composed of double-walled CNTs, which rises to 257 W m-1 K-1 when considering the CNT network alone. These values are remarkably high for randomly oriented CNT networks, roughly seven times that of single-walled CNT films. The enhanced thermal conduction is due to the additional wall, which likely gives rise to additional heat-carrying phonon modes and provides a certain resilience to defects. Our results demonstrate that free-standing double-walled CNT films efficiently dissipate heat, enhancing our understanding of these promising films and how they are suited to applications in EUV lithography.</p

Enhanced Thermal Conductivity of Free-Standing Double-Walled Carbon Nanotube Networks

Nanomaterials are driving advances in technology due to their oftentimes superior properties over bulk materials. In particular, their thermal properties become increasingly important as efficient heat dissipation is required to realize high-performance electronic devices, reduce energy consumption, and prevent thermal damage. One application where nanomaterials can play a crucial role is extreme ultraviolet (EUV) lithography, where pellicles that protect the photomask from particle contamination have to be transparent to EUV light, mechanically strong, and thermally conductive in order to withstand the heat associated with high-power EUV radiation. Free-standing carbon nanotube (CNT) films have emerged as candidates due to their high EUV transparency and ability to withstand heat. However, the thermal transport properties of these films are not well understood beyond bulk emissivity measurements. Here, we measure the thermal conductivity of free-standing CNT films using all-optical Raman thermometry at temperatures between 300 and 700 K. We find thermal conductivities up to 50 W m-1 K-1 for films composed of double-walled CNTs, which rises to 257 W m-1 K-1 when considering the CNT network alone. These values are remarkably high for randomly oriented CNT networks, roughly seven times that of single-walled CNT films. The enhanced thermal conduction is due to the additional wall, which likely gives rise to additional heat-carrying phonon modes and provides a certain resilience to defects. Our results demonstrate that free-standing double-walled CNT films efficiently dissipate heat, enhancing our understanding of these promising films and how they are suited to applications in EUV lithography.</p

Heat dissipation in few-layer MoS2and MoS2/hBN heterostructure

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)

2D Materials Graphene related materials for thermal management Graphene related materials for thermal management

International audienceAlmost 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. TOPICAL REVIE