Coherent Acoustic Phonons in Colloidal Semiconductor Nanocrystal Superlattices

Abstract

The phonon properties of films fabricated from colloidal semiconductor nanocrystals play a major role in thermal conductance and electron scattering, which govern the principles for building colloidal-based electronics and optics including thermoelectric devices with a high ZT factor. The key point in understanding the phonon properties is to obtain the strength of the elastic bonds formed by organic ligands connecting the individual nanocrystallites. In the case of very weak bonding, the ligands become the bottleneck for phonon transport between infinitively rigid nanocrystals. In the opposite case of strong bonding, the colloids cannot be considered as infinitively rigid beads and the distortion of the superlattice caused by phonons includes the distortion of the colloids themselves. We use the picosecond acoustics technique to study the acoustic coherent phonons in superlattices of nanometer crystalline CdSe colloids. We observe the quantization of phonons with frequencies up to 30 GHz. The frequencies of quantized phonons depend on the thickness of the colloidal films and possess linear phonon dispersion. The measured speed of sound and corresponding wave modulus in the colloidal films point on the strong elastic coupling provided by organic ligands between colloidal nanocrystals.

Direct observation of polymer surface mobility via nanoparticle vibrations

“…Penciu et al 30 and Kuok et al 31 first applied BLS to measure the eigenfrequencies of giant micelles and silica NPs of submicrometer size. For NPs with a diameter <100 nm, Raman scattering and pump probe techniques could be applied, however, fewer modes are resolved 32 – 36 . Recently, particle vibration spectroscopy by means of BLS has been advanced to measure particle mechanical eigenfrequencies to determine the elastic properties of polymer NPs 37 – 39 .…”

“…Based on this approach, we use BLS to measure the T g of PS NPs and to specify the mobile layer driven NP interactions. The experiments allow us to observe the vibrational modes of cluster of polymer NPs whose frequencies changed depending on the interactions among the NPs 35 , 36 , 45 – 47 . The T g of the PS NPs is measured by the temperature dependence of the vibrational modes.…”

Chemically Triggered Formation of Two-Dimensional Epitaxial Quantum Dot Superlattices

“…[10][11][12][13] Whereas this provides ample possibilities to combine different nanocrystals in a single, ordered crystal, only few studies have shown such an approach to result in metamaterials with new or enhanced properties. [14][15][16] For one thing, this is due to the use of nanocrystal building blocks capped by long, organic ligands, which inevitably leads to electrically insulating nanocrystal solids. Therefore, opto-electronic devices such as transistors, [17][18][19] solar cells [20][21][22][23] or photodetectors, [24][25][26] are based on disordered QD solids, where the interparticle distance is usually decreased by exchanging the long organic ligands with shorter organic or inor-ganic moieties.…”

“…[27][28][29][30][31] Although this makes for QD devices with ever increasing performance, carrier mobilities remain well below 10 cm 2 V −1 s −1 and the approach leaves no room for any symmetry-induced collective effects. 15,32,33 Recently, it was shown that PbSe QDs can be organized into ordered two dimensional (2D) superlattices in which all QDs are epitaxially connected to their nearest neighbors. [34][35][36][37][38] As demonstrated by Evers et al, the strong electronic interdot coupling in such epitaxially connected 2D quantum-dot superlattices -epitaxial superlattices in short -results in carrier mobilities of up to 260 cm 2 V −1 s −1 as deduced from THz conductivity measurements.…”

Directional elastic wave propagation in high-aspect-ratio photoresist gratings: liquid infiltration and aging

“…The state-of-the-art methods for nanomechanic measurements are based on deformation, 15,16 or on acoustic wave propagation. 17,18 Both methods pose limitations: the former is destructive, while the latter requires a complex sample preparation. Both assume linear stress-strain relationship, 19 and cannot resolve the direction-dependent mechanical behaviors.…”