Session: 09-01: Computational Methods in Micro/ Nanoscale Transport

Paper Number: 131968

131968 - Searching for Coherent Phonons in Colloidal Quantum Dot Films 

Abstract: 

   Colloidal quantum dots (QDs), made of inorganic core and organic ligands, can self-assemble into two-dimensional or three-dimensional thin films. The thermal properties of these films are vital for their applications in photoelectric, electrical, thermoelectric, and phonon-engineered systems [Science 373(2021)640]. A recent investigation on the thermal conductivity of QDs superlattice [Mater. Today Phys. 22(2022)100601] reported that the total thermal conductivity decays with increasing temperatures and attributed this decay to a scattering of the coherent phonons. Here, using a similar film, we instead found that the thermal conductivity shows a slight increase with increasing temperatures. This discrepancy is a result of a possible incorrect calculation of the heat current autocorrelation function (HCACF) employed in the equilibrium molecular dynamics (EMD).

  We used both the EMD and non-equilibrium molecular dynamics (NEMD) methods to predict the thermal conductivity of a series of QD films. In the EMD method, three different approaches were employed to calculate the HCACF. They are, namely, 1) time auto-correlation in LAMMPS (referred to as L-TAC), 2) in-house time auto-correlation (S-TAC), and 3) in-house Fast Fourier transform auto-correlation (FFT-AC). We benchmark our EMD results with the NEMD value and validated that these three EMD and the NEMD approaches produced almost identical thermal conductivities on an LJ argon system. For the QD films, however, only the thermal conductivity values calculated from the S-TAC and FFT-AC are consistent with the NEMD method. The EMD result using the L-TAC approach overestimates the thermal conductivity of the QD films, especially at lower temperatures, producing a thermal conductivity that decreases with increasing temperature.

  We decomposed the L-TAC result into its convective, virial, and cross terms to find that the convective term dominates the thermal conductivity. This breakdown seems inconsistent with the conduction in solids. As the QD film is a heterogeneous system, we corrected for its partial enthalpy [J. APPL. PHYS. 113(2013)084302]. The resulting thermal conductivity decreased, with an accompanying drop in the convective term, but still remained higher than values obtained using other methods. However, the thermal conductivity for the other EMD approaches is unaffected by the partial enthalpy corrections, suggesting that the erroneous thermal conductivity enhancement from the L-TAC is not due to its partial enthalpy. Another notable point is that the cross term from the L-TAC approach changed from negative to positive after enthalpy correction, while the cross terms from the other EMD approaches remained always positive. Therefore, we hypothesize that in complex hybrid systems, cross term that are negative may have little physical significance but can be an artifact of the calculation.

Presenting Author: Yuchen Li ZJU-UIUC Institute, College of Energy Engineering,Zhejiang University

Presenting Author Biography: Li Yuchen is currently pursuing a doctoral degree in Mechanical Engineering at Zhejiang University under the guidance of Professor Wee-liat Ong. Her research focuses on the nanoscale thermal transport of colloidal quantum dot materials.

Authors:

Yuchen Li ZJU-UIUC Institute, College of Energy Engineering,Zhejiang University
Wee-Liat Ong ZJU-UIUC Institute, College of Energy Engineering, Zhejiang University