Subscribe to the gold package and get unlimited access to Shamra Academy
Register a new userFour-dimensional Vibrational Spectroscopy for Nanoscale Mapping of Phonon Dispersion in BN Nanotubes
98
0
0.0
(
0
)
Ask ChatGPT about the research
No Arabic abstract
Direct measurement of local phonon dispersion in individual nanostructures can greatly advance our understanding of their electrical, thermal, and mechanical properties. However, such experimental measurements require extremely high detection sensitivity and combined spatial, energy and momentum resolutions, thus has been elusive. Here, we develop a four-dimensional electron energy loss spectroscopy (4D-EELS) technique based a monochromated scanning transmission electron microscope (STEM), and present the position-dependent phonon dispersion measurement in individual boron nitride nanotubes (BNNTs). Our measurement shows that the unfolded phonon dispersion of multi-walled BNNTs is close to hexagonal-boron nitride (h-BN) crystals, suggesting that interlayer coupling and curved geometry have no substantial impacts on phonon dispersion. We also find that the acoustic phonons are extremely sensitive to momentum-dependent defect scattering, while optical phonons are much less susceptible. This work not only provides useful insights into vibrational properties of BNNTs, but also demonstrates huge prospects of the developed 4D-EELS technique in nanoscale phonon dispersion measurements.
rate research
Read More
As the length-scales of materials decrease, heterogeneities associated with interfaces approach the importance of the surrounding materials. Emergent electronic and magnetic interface properties in superlattices have been studied extensively by both experiments and theory. $^{1-6}$ However, the presence of interfacial vibrations that impact phonon-mediated responses, like thermal conductivity $^{7,8}$, has only been inferred in experiments indirectly. While it is accepted that intrinsic phonons change near boundaries $^{9,10}$, the physical mechanisms and length-scales through which interfacial effects influence materials remain unclear. Herein, we demonstrate the localized vibrational response associated with the interfaces in SrTiO$_3$-CaTiO$_3$ superlattices by combining advanced scanning transmission electron microscopy imaging and spectroscopy and density-functional-theory calculations. Symmetries atypical of either constituent material are observed within a few atomic planes near the interface. The local symmetries create local phonon modes that determine the global response of the superlattice once the spacing of the interfaces approaches the phonon spatial extent. The results provide direct visualization and quantification, illustrating the progression of the local symmetries and interface vibrations as they come to determine the vibrational response of an entire superlattice; stated differently, the progression from a material with interfaces, to a material dominated by interfaces, to a material of interfaces as the period decreases. Direct observation of such local atomic and vibrational phenomena demonstrates that their spatial extent needs to be quantified to understand macroscopic behavior. Tailoring interfaces, and knowing their local vibrational response, provides a means of pursuing designer solids having emergent infrared and thermal responses.
Being used in optoelectronic devices as ultra-thin conductor-insulator junctions, detailed investigations are needed about how exactly h-BN and graphene hybridize. Here, we present a comprehensive ab initio study of hot carrier dynamics governed by electron-phonon scattering at the h-BN/graphene interface, using graphite (bulk), monolayer and bilayer graphene as benchmark materials. In contrast to monolayer graphene, all multilayer structures possess low-energy optical phonon modes that facilitate carrier thermalization. We find that the h-BN/graphene interface represents an exception with comparatively weak coupling between low-energy optical phonons and electrons. As a consequence, the thermalization bottleneck effect, known from graphene, survives hybridization with h-BN but is substantially reduced in all other bilayer and multilayer cases considered. In addition, we show that the quantum confinement in bilayer graphene does not have a significant influence on the thermalization time compared to graphite and that bilayer graphene can hence serve as a minimal model for the bulk counterpart.
Despite the ubiquity of applications of heat transport across nanoscale interfaces, including integrated circuits, thermoelectrics, and nanotheranostics, an accurate description of phonon transport in these systems remains elusive. Here we present a theoretical and computational framework to describe phonon transport with position, momentum and scattering event resolution. We apply this framework to a single material spherical nanoparticle for which the multidimensional resolution offers insight into the physical origin of phonon thermalization, and length-scale dependent anisotropy of steady-state phonon distributions. We extend the formalism to handle interfaces explicitly and investigate the specific case of semi-coherent materials interfaces by computing the coupling between phonons and interfacial strain resulting from aperiodic array of misfit dislocations. Our framework quantitatively describes the thermal interface resistance within the technologically relevant Si-Ge heterostructures. In future, this formalism could provide new insight into coherent and driven phonon effects in nanoscale materials increasingly accessible via ultrafast, THz and near-field spectroscopies.
The quantum corrections to the frequencies of the $Gamma$ point longitudinal optical (LO) and transverse optical (TO) phonon modes in carbon nanotubes are investigated theoretically. The frequency shift and broadening of the TO phonon mode strongly depend on the curvature effect due to a special electron-phonon coupling in carbon nanotubes, which is shown by the Fermi energy dependence of the frequency shift for different nanotube chiralities. It is also shown that the TO mode near the $Gamma$ point decouples from electrons due to local gauge symmetry and that a phonon mixing between LO and TO modes is absent due to time-reversal symmetry. Some comparison between theory and experiment is presented.
We have studied emission properties of high-density excitons in single-walled carbon nanotubes through nonlinear photoluminescence excitation spectroscopy. As the excitation intensity was increased, all emission peaks arising from different chiralities showed clear saturation in intensity. Each peak exhibited a saturation value that was independent of the excitation wavelength, indicating that there is an upper limit on the exciton density for each nanotube species. We developed a theoretical model based on exciton diffusion and exciton-exciton annihilation that successfully reproduced the saturation behavior, allowing us to estimate exciton densities. These estimated densities were found to be still substantially smaller than the expected Mott density even in the saturation regime, in contrast to conventional semiconductor quantum wires.
Log in to be able to interact and post comments
comments
Fetching comments
Sign in to be able to follow your search criteria
