Sanghamitra Neogi

Tuning thermal transport in ultrathin silicon membranes by surface nanoscale engineering

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.

Native surface oxide turns alloyed silicon membranes into nanophononic metamaterials with ultralow thermal conductivity

Native surface oxide turns alloyed silicon membranes into nanophononic metamaterials with ultra-low thermal conductivity Shiyun Xiong 1,2 , ∗ Daniele Selli 2 , Sanghamitra Neogi 3 , and Davide Donadio 4† arXiv:1705.03143v1 [cond-mat.mtrl-sci] 9 May 2017 1 Functional Nano and Soft Materials Laboratory (FUNSOM) and Collaborative Innovation Center of Suzhou Nano Science and Technology, Soochow University, Suzhou, Jiangsu 215123 , P.R. China 2 Max Planck Institute for Polymer Research, Ackermannweg 10, 55218 Mainz, Germany 3 Department of Aerospace Engineering Sciences, University of Colorado Boulder, Boulder, Colorado 80309, USA 4 Department of Chemistry, University of California Davis, One Shields Ave. Davis, 95616, CA A detailed understanding of the relation between microscopic structure and phonon propagation at the nanoscale is essential to design materials with desired phononic and thermal properties. Here we uncover a new mechanism of phonon interaction in surface oxidized membranes, i.e., native oxide layers interact with phonons in ultra-thin silicon membranes through local resonances. The local resonances reduce the low frequency phonon group velocities and shorten their mean free path. This effect opens up a new strategy for ultralow thermal conductivity design as it complements the scattering mechanism which scatters higher frequency modes effectively. The combination of native oxide layer and alloying with germanium in concentration as small as 5% reduces the thermal conductivity of silicon membranes to 100 time lower than the bulk. In addition, the resonance mechanism produced by native oxide surface layers is particularly effective for thermal condutivity reduction even at very low temperatures, at which only low frequency modes are populated. Controlling terahertz vibrations and heat transport in nanostructures has a broad impact on several ap- plications, such as thermal management in micro- and nano-electronics, renewable energies harvesting, sensing, biomedical imaging and information and communication technologies [1–8]. Significant efforts have been made to understand and engineer heat transport in nanoscale silicon due to its natural abundance and technological relevance [9–12]. In the past decade researchers ex- plored strategies to obtain silicon based materials with low thermal conductivity (TC) and unaltered electronic transport coefficients, so to achieve high thermoelectric figure of merit and enable silicon-based thermoelectric technology[11–18]. From the earlier studies it was recognized that low- dimensional silicon nanostructures, such as nanowires, thin films and nano membranes feature a largely re- duced TC, up to 50 times lower than that of bulk at room temperature. TC reduction becomes more promi- nent with the reduction of the characteristic dimension of the nanostructures [19–22]. Theory and experiments consistently show that surface disorder and the pres- ence of disordered material at surfaces play a major role in determining the TC of nanostructures [12, 23–25]. However, a comprehensive understanding of the physi- cal mechanisms underlying so large TC reduction is lack- ing. The effect of surface roughness and surface disor- der on phonons has been so far interpreted in terms of phonon scattering [26–30], but scattering would not ac- count for mean free path reduction of long-wavelength low-frequency modes. Recent theoretical work demon- strated that surface nanostructures, such as nanopillars at the surface of thin films or nanowires, can efficiently reduce TC through resonances, a mechanism that is in- trinsically different from scattering [31, 32]. Surface res- onances alter directly phonon dispersion relations by hy- bridizing with propagating modes in the nanostructures, thus hampering their group velocity. In this Communication we unravel the effect of native oxide surface layers on thermal transport in ultra-thin silicon membrane models that closely resemble experi- ments, by atomistic molecular dynamics simulations. We show that the observed low TC in these systems, and plausibly in other oxide coated silicon nanostructures, is predominantly due to resonances analogous to those occurring in nanophononic metamaterials. It is worth stressing that surface oxide layers in low dimensional Si materials grow spontaneously at atmospheric conditions, and do not require any specific processing. Our simu- lations ascertain the occurrence of low frequency reso- nant modes that hybridize with the acoustic branches (ω . 4 THz) of the membrane, effectively suppressing their mean free path (MFP). This discovery opens up the possibility to further optimize the TC of ultra-thin silicon membranes by combining resonances with mass scattering, which affects phonons with higher frequency (ω & 4 THz). We show that alloying the crystalline core of ultra-thin membranes with a small percentage of sub- stitutional germanium atoms brings forth ultra-low TC in silicon membranes with technologically viable thick- ness [33]. All TCs are calculated with equilibrium molecular dy- namics (EMD) simulations at 300 K using LAMMPS [34] with interatomic interactions described by the widely used Tersoff potential [35–37]. The equations of mo- tion are integrated with the velocity Verlet algorithm

Optimal thickness of silicon membranes to achieve maximum thermoelectric efficiency: A first principles study

Silicon nanostructures with reduced dimensionality, such as nanowires, membranes, and thin films, are promising thermoelectric materials, as they exhibit considerably reduced thermal conductivity. Here, we utilize density functional theory and Boltzmann transport equation to compute the electronic properties of ultra-thin crystalline silicon membranes with thickness between 1 and 12 nm. We predict that an optimal thickness of ∼7 nm maximizes the thermoelectric figure of merit of membranes with native oxide surface layers. Further thinning of the membranes, although attainable in experiments, reduces the electrical conductivity and worsens the thermoelectric efficiency.