We report a synthesis procedure for dodecanethiol capped wurtzite ZnO nanocrystals with an average diameter of 4 nm that are monodisperse, highly soluble, and shelf-stable for many months. Compared to previous ZnO ink recipes, we demonstrate improved particle solubility and excellent ink stability, resulting in ZnO nanocrystal inks that are optimized for printed electronics applications. The ZnO nanocrystal solution exhibits an absorption peak at 341 nm (3.63 eV), which represents a blue-shift of approximately 0.3 eV from the bulk ZnO bandgap (∼3.3 eV). This blue shift is consistent with previously reported models for an increased bandgap due to quantum confinement. We used variable-angle spectroscopic ellipsometry (VASE) to determine the optical properties of solution-processed thin films of ZnO nanocrystals, which provides valuable insight into the changes in film composition and morphology that occur during thermal annealing treatments ranging from 150–300 °C. The ZnO nanocrystals maintain their quantum confinement when deposited into a thin film, and the degree of quantum confinement is gradually reduced as the thermal annealing temperature increases. Using infrared absorption measurements (FTIR) and X-ray photoelectron spectroscopy (XPS), we show that the dodecanethiol ligands are removed from the ZnO films during annealing, resulting in a high-purity semiconductor film with verymore »
Transient Lattice Response Upon Photoexcitation in CuInSe2 Nanocrystals with Organic or Inorganic Surface Passivation
CuInSe2 nanocrystals offer promise for optoelectronics including thin-film photovoltaics and printed electronics. Additive manufacturing methods such as photonic curing controllably sinter particles into quasi-continuous films and offer improved device performance. To gain understanding of nanocrystal response under such processing conditions, we investigate impacts of photoexcitation on colloidal nanocrystal lattices via time-resolved X-ray diffraction. We probe three sizes of particles and two capping ligands (oleylamine and inorganic S2–) to evaluate resultant crystal lattice temperature, phase stability, and thermal dissipation. Elevated fluences produce heating and loss of crystallinity, the onset of which exhibits particle size dependence. We find size-dependent recrystallization and cooling lifetimes ranging from 90 to 200 ps with additional slower cooling on the nanosecond time scale. Sulfide-capped nanocrystals show faster recrystallization and cooling compared to oleylamine-capped nanocrystals. Using these lifetimes, we find interfacial thermal conductivities from 3 to 28 MW/(m2 K), demonstrating that ligand identity strongly influences thermal dissipation.
- Publication Date:
- NSF-PAR ID:
- 10295084
- Journal Name:
- ACS nano
- Volume:
- 14
- Issue:
- 10
- Page Range or eLocation-ID:
- 13548–13556
- ISSN:
- 1936-0851
- Sponsoring Org:
- National Science Foundation
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Abstract The ongoing interest in colloidal nanocrystal solids for electronic and photonic devices necessitates that their thermal‐transport properties be well understood because heat dissipation frequently limits performance in these devices. Unfortunately, colloidal nanocrystal solids generally possess very low thermal conductivities. This very low thermal conductivity primarily results from the weak van der Waals interaction between the ligands of adjacent nanocrystals. We overcome this thermal‐transport bottleneck by crosslinking the ligands to exchange a weak van der Waals interaction with a strong covalent bond. We obtain thermal conductivities of up to 1.7 Wm−1 K−1that exceed prior reported values by a factor of 4. This improvement is significant because the entire range of prior reported values themselves only span a factor of 4 (i.e., 0.1–0.4 Wm−1 K−1). We complement our thermal‐conductivity measurements with mechanical nanoindentation measurements that demonstrate ligand crosslinking increases Young's modulus and sound velocity. This increase in sound velocity is a key bridge between mechanical and thermal properties because sound velocity and thermal conductivity are linearly proportional according to kinetic theory. Control experiments with non‐crosslinkable ligands, as well as transport modeling, further confirm that ligand crosslinking boosts thermal transport.
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Accepted Manuscript1.0
- Publisher's Version of Record
- https://doi.org/https://doi.org/10.1021/acsnano.0c05553
