Surface chemistry mediates thermal transport in three-dimensional nanocrystal arrays

Journal name:
Nature Materials
Volume:
12,
Pages:
410–415
Year published:
DOI:
doi:10.1038/nmat3596
Received
Accepted
Published online

Arrays of ligand-stabilized colloidal nanocrystals with size-tunable electronic structure are promising alternatives to single-crystal semiconductors in electronic, optoelectronic and energy-related applications1, 2, 3, 4, 5. Hard/soft interfaces in these nanocrystal arrays (NCAs) create a complex and uncharted vibrational landscape for thermal energy transport that will influence their technological feasibility. Here, we present thermal conductivity measurements of NCAs (CdSe, PbS, PbSe, PbTe, Fe3O4 and Au) and reveal that energy transport is mediated by the density and chemistry of the organic/inorganic interfaces, and the volume fractions of nanocrystal cores and surface ligands. NCA thermal conductivities are controllable within the range 0.1–0.3 W m−1 K−1, and only weakly depend on the thermal conductivity of the inorganic core material. This range is 1,000 times lower than the thermal conductivity of silicon, presenting challenges for heat dissipation in NCA-based electronics and photonics. It is, however, 10 times smaller than that of Bi2Te3, which is advantageous for NCA-based thermoelectric materials.

At a glance

Figures

  1. Structure and heat capacity of NCA films.
    Figure 1: Structure and heat capacity of NCA films.

    a, Schematic of a gold-coated NCA film where an intensity-modulated pump laser periodically heats the sample and a probe laser senses the resultant temperature change by thermoreflectance to measure thermal conductivity (see Methods for FDTR details). The magnified view to the right depicts the NCA thin film. b, SEM cross-sectional image of a 7.5-nm-diameter PbS NCA film. c, Planar TEM image of an 8-nm-diameter Fe3O4 NCA film showing a regular close-packed arrangement. d, Specific heat capacity data as a function of temperature for a diameter series of PbS nanoparticles coated with oleic acid ligands. e, The specific heat capacity of a NCA at 300 K can be estimated as a mass-weighted function of the specific heat capacities of the core material (Ccore) and the ligand (CLigand) such that, CP  =  mcoreCcore+mLigandCLigand, where mcoreis the mass fraction of the core and mLigand is the mass fraction of the ligand. The plot shows the measured and estimated values using bulk values of PbS (Ccore  =  0.19 J g−1 K−1, ref.  24) and oleic acid (CLigand  =  2.043 J g−1 K−1, ref. 30). Error bars represent uncertainty from DSC and TGA measurements.

  2. vDOS of a 2.8-nm-diameter Au nanocrystal and its individual constituents (Au core and a dodecanethiol ligand) predicted from lattice dynamics calculations.
    Figure 2: vDOS of a 2.8-nm-diameter Au nanocrystal and its individual constituents (Au core and a dodecanethiol ligand) predicted from lattice dynamics calculations.

    The blue vertical lines represent the vibrational spectrum of one ligand. The yellow region is the vDOS of the Au core, which is enclosed in the green vDOS of the nanocrystal. The vertical red line represents the thermal activation frequency (kBT/h) at a temperature of 300 K (kBis the Boltzmann constant and h is the Planck constant).

  3. Effects of NCA geometry and temperature on thermal conductivity.
    Figure 3: Effects of NCA geometry and temperature on thermal conductivity.

    a, Diameter series data for various NCAs have increasing thermal conductivity with core diameter regardless of core composition. The trend of the molecular dynamics (MD) simulation results agrees with the experimental data. The inset shows that kNCA does not strongly depend on the thermal conductivity of the bulk core material. The orange dotted line here and in b is the thermal conductivity of Pb oleate at 300 K. b, EMAs (effective medium theory14 (EMT); Hashin LB, ref. 31) cannot explain the PbS NCA data without the use of a finite thermal conductance at the core/ligand interface, which is integral to HJ–ME and Minnich models (see Supplementary Information for details of EMAs). These models clarify that increased kNCA results from decreased interface density (from 4.37 × 108 m2 m−3 to 3.43 × 108 m2 m−3) and increased core volume fraction (from 0.24 to 0.43) over the diameter range 3.3–7.5 nm. c, Temperature series for CdSe (diameter  =  4.1 nm, θD,CdSe  =  182 K; ref.  19), PbS (diameter  =  7.5 nm, θD,PbS  =  225 K; ref.  20), PbSe (diameter  =  7.5 nm, θD,PbSe  =  175 K; ref.  20), PbTe (diameter  =  7.5 nm, θD,PbTe  =  136 K; ref.  20) and Fe3O4 (diameter  =  8.0 nm, θD,Fe3O4 > 350 K; ref. 21) NCAs, and Pb oleate ligands. The points have been slightly offset in temperature so error bars can be resolved. T2 and T3 scalings are included for reference. d, Normalized temperature series for Pb oleate ligands and PbSe and Fe3O4 NCAs (with the respect to their maximum thermal conductivity) plateau above 150 K for Pb oleate, 200 K for PbSe and 300 K for Fe3O4 NCA. Error bars represent the uncertainty in FDTR measurements (ref.  12).

  4. Effects of NCA chemistry on thermal conductivity.
    Figure 4: Effects of NCA chemistry on thermal conductivity.

    aOleate-capped lead chalcogenide NCAs show a decrease in thermal conductivity with larger mass ratio, that is, smaller core Debye temperature θD (θD from ref. 20). The molecular dynamics (MD) simulation data are displaced horizontally for clarity. b, NCA thermal conductivity for different inorganic (In2Se42−, AsS33−, N2H4) and organic ligands (tetradecylphosphonic acid (TDPA) and oleic acid (OA)) on CdSe and PbS nanocrystals. Shorter inorganic ligands increase the thermal conductivity by 50% relative to organic ligands. Error bars represent the uncertainty in FDTR measurements (ref. 12).

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Author information

  1. These authors contributed equally to this work

    • Wee-Liat Ong &
    • Sara M. Rupich

Affiliations

  1. Department of Mechanical Engineering, Carnegie Mellon University, Pittsburgh, Pennsylvania 15213, USA

    • Wee-Liat Ong,
    • Alan J. H. McGaughey &
    • Jonathan A. Malen
  2. Department of Chemistry, University of Chicago, Chicago, Illinois 60637, USA

    • Sara M. Rupich &
    • Dmitri V. Talapin
  3. Department of Materials Science and Engineering, Carnegie Mellon University, Pittsburgh, Pennsylvania 15213, USA

    • Alan J. H. McGaughey &
    • Jonathan A. Malen

Contributions

W-L.O. conducted the FDTR measurements on the NCAs, molecular dynamics simulations and lattice dynamics calculations. S.M.R. synthesized NCAs, conducted TGA, DSC and absorption spectra measurements, and took SEM, TEM and AFM images. W-L.O. and S.M.R. wrote the paper. D.V.T., A.J.H.M. and J.A.M. edited the paper. All authors discussed the data and commented on the manuscript.

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The authors declare no competing financial interests.

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