Structural and Thermal Characterisation of Nanofilms by Time-Resolved X-ray Scattering.

Anton Plech,Tilo Baumbach,Caroline Girmen,Margarita Zakharova,Gernot Buth,Bärbel Krause,Soizic Eon,Hartmut Bracht

doi:10.3390/nano9040501

Abstract

High time resolution in scattering analysis of thin films allows for determination of thermal conductivity by transient pump-probe detection of dissipation of laser-induced heating, TDXTS. We describe an approach that analyses the picosecond-resolved lattice parameter reaction of a gold transducer layer on pulsed laser heating to determine the thermal conductivity of layered structures below the transducer. A detailed modeling of the cooling kinetics by a Laplace-domain approach allows for discerning effects of conductivity and thermal interface resistance as well as basic depth information. The thermal expansion of the clamped gold film can be calibrated to absolute temperature change and effects of plastic deformation are discriminated. The method is demonstrated on two extreme examples of phononic barriers, isotopically modulated silicon multilayers with very small acoustic impedance mismatch and silicon-molybdenum multilayers, which show a high resistivity.

Highlights

Nanoscale analysis of materials profits from the high brilliance of synchrotron-based sources by e.g., real-space or reciprocal space methods
We explore the limits of the method of Time-domain X-ray thermal scattering (TDXTS, name given in analogy to transducer layers on temperature (TDTR)) to investigate the cross-plane thermal conductivity of layered systems
While the simulation of the thermal conductivity is inherently not temperature-dependent, the absolute temperature determination is important for estimation of non-linear contributions of temperature on thermal strain

Summary

Introduction

Nanoscale analysis of materials profits from the high brilliance of synchrotron-based sources by e.g., real-space or reciprocal space methods. Apart from the high resolution in real and reciprocal space the time structure of light emission at synchrotrons adds the ability to investigate dynamic processes, such as lattice motion or dissipation. Optimization of thermoelectric materials involved minimization of the phonon contribution to thermal transport [3,4,5]. This can happen through introduction of interfaces with acoustic impedance mismatch or defects (pointlike or particles) [6,7,8], which increase phonon scattering. The case of layered isotopic substitution drew attention due to the fact that such crystals may electronically behave as bulk, but modulate phonon transport [14,17]

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Conclusion