Surface chemistry mediates thermal transport in three-dimensional nanocrystal arrays

Abstract

Arrays of ligand-stabilized colloidal nanocrystals with sizetunable electronic structure are promising alternatives to single-crystal semiconductors in electronic, optoelectronic and energy-related applications 1-5 . 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, Fe 3 O 4 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 Bi 2 Te 3 , which is advantageous for NCA-based thermoelectric materials.Colloidal nanocrystals self-assemble into NCAs with electronic and optical properties that can be broadly tuned by nanocrystal composition and size 1-4,6-8 . To be considered as viable replacements for traditional semiconductors, NCA-based technologies must also meet thermal management demands as high operating temperatures degrade device performance and lifetime. Thermal conductivity (k) quantifies a material's ability to dissipate heat and relates temperature gradient (∇T ) to heat flux (q) through Fourier's law, q = −k∇T . In metals, most heat is carried by electrons, whereas in semiconductor and insulating crystals, thermal conductivity arises from the transport and scattering of quantized vibrations that are born from the periodic atomic lattice, that is, phonons. Our NCAs are non-metallic and have a vibrational structure that is complicated by compositional heterogeneity and periodicity at two length scales: the atomic lattice within each core and the array of periodic cores separated by ligand monolayers. Studies of planar self-assembled monolayer (SAM) junctions show that surface chemistry can control energy transport at the interface between two solids 9 , but does it also influence the thermal conductivity of a bulk three-dimensional solid with a complex network of internal interfaces? We herein report on the hitherto unknown thermal transport properties of NCAs, using systematic thermal conductivity measurements complemented by heat capacity measurements and atomistic simulations.A series of NCAs was prepared with varying core diameters, core materials and ligand groups (Table 1) through spin-coating concentrated colloidal solutions onto silicon wafers 6 . During film

Coupling of Organic and Inorganic Vibrational States and Their Thermal Transport in Nanocrystal Arrays

“…On the other hand, when the diameter is much bigger than the Kapitza length, α tends to zero, and eq 2 simplifies to an EMA formulation that has no dependence on the interfacial thermal conductance. 36 In our previous study, 24 the experimentally measured thermal conductivities of PbS NCAs with diameters greater than 3.5 nm agree with the predictions of eq 2 when the interfacial thermal conductance between the core and ligands is used as a fitting parameter. We now use this EMA-h model to interpret our simulation predictions.…”

“…The data agree with the geometry-based analytical model of Jimenez et al 56 Figure 2. Predicted thermal conductivities and corresponding experimental measurements 24 of varying the NCA thickness. Figure 3.…”

“…Experiments found that increasing the core Debye temperature, which is inversely proportional to the square root of atomic mass, 65 increased the NCA thermal conductivity. 24 This result was attributed to an increase in interfacial thermal conductance due to a higher number of overlapping vibrational states between the cores and ligands. 24,66 This proposition holds true only in the event of elastic vibrational scattering at the core−ligand interfacesa condition that we validated in the previous section.…”

“…24 This result was attributed to an increase in interfacial thermal conductance due to a higher number of overlapping vibrational states between the cores and ligands. 24,66 This proposition holds true only in the event of elastic vibrational scattering at the core−ligand interfacesa condition that we validated in the previous section. To further probe the underlying transport physics, we now isolate the core mass effect by using core atoms of different atomic mass while using the same potentials for all interactions.…”

Quantum dot light-emitting devices

“…It has been recently shown that thermal conductivity of QD layers can be orders of magnitude lower than that of traditional semiconductors. 48 The optimization of thermal management in QLEDs offers a promising direction toward improved stability, effi ciency, and brightness. One should also look beyond using organic molecules as surface ligands for QDs-inorganic and hybrid ligands for colloidal NCs could provide an interparticle medium that is more thermally conductive and/or transparent for charge carriers.…”