skip to main content
US FlagAn official website of the United States government
dot gov icon
Official websites use .gov
A .gov website belongs to an official government organization in the United States.
https lock icon
Secure .gov websites use HTTPS
A lock ( lock ) or https:// means you've safely connected to the .gov website. Share sensitive information only on official, secure websites.

Explore Research Products in the PAR
It may take a few hours for recently added research products to appear in PAR search results.


Title: Acoustic and optical phonons in quasi-two-dimensional MPS3 antiferromagnetic semiconductors
We report the results of the investigation of the acoustic and optical phonons in quasi-two-dimensional antiferromagnetic semiconductors of the transition metal phosphorus trisulfide family with Mn, Fe, Co, Ni, and Cd as metal atoms. The Brillouin–Mandelstam and Raman light scattering spectroscopies were conducted at room temperature to measure the acoustic and optical phonon frequencies close to the Brillouin zone center and the Γ−A high symmetry direction. The absorption and index of refraction were measured in the visible and infrared ranges using the reflectometry technique. We found an intriguing large variation, over ∼28%, in the acoustic phonon group velocities in this group of materials with similar crystal structures. Our data indicate that the full-width-at-half-maximum of the acoustic phonon peaks is strongly affected by the optical properties and the electronic bandgap. The acoustic phonon lifetime extracted for some of the materials was correlated with their thermal properties. The results are important for understanding the layered van der Waals semiconductors and assessing their potential for optoelectronic and spintronic device applications.  more » « less
Award ID(s):
2019056
PAR ID:
10546027
Author(s) / Creator(s):
; ; ; ; ; ;
Publisher / Repository:
AIP Publishing
Date Published:
Journal Name:
Applied Physics Letters
Volume:
124
Issue:
16
ISSN:
0003-6951
Format(s):
Medium: X
Sponsoring Org:
National Science Foundation
More Like this
  1. ; ; ; ; ; ; ; ; (, Applied Physics Letters)
    We report the results of the study of the acoustic and optical phonons in Si-doped AlN thin films grown by metal–organic chemical vapor deposition on sapphire substrates. The Brillouin–Mandelstam and Raman light scattering spectroscopies were used to measure the acoustic and optical phonon frequencies close to the Brillouin zone center. The optical phonon frequencies reveal non-monotonic changes, reflective of the variations in the thin film strain and dislocation densities with the addition of Si dopant atoms. The acoustic phonon velocity decreases monotonically with increasing Si dopant concentration, reducing by ∼300 m/s at the doping level of 3 × 1019 cm−3. The knowledge of the acoustic phonon velocities can be used for the optimization of the ultra-wide bandgap semiconductor heterostructures and for minimizing the thermal boundary resistance of high-power devices. 
    more » « less
  2. ; ; (, Advanced Science)
    Abstract Engineering of phonons, that is, collective lattice vibrations in crystals, is essential for manipulating physical properties of materials such as thermal transport, electron‐phonon interaction, confinement of lattice vibration, and optical polarization. Most approaches to phonon‐engineering have been largely limited to the high‐quality heterostructures of III–V compound semiconductors. Yet, artificial engineering of phonons in a variety of materials with functional properties, such as complex oxides, will yield unprecedented applications of coherent tunable phonons in future quantum acoustic devices. In this study, artificial engineering of phonons in the atomic‐scale SrRuO3/SrTiO3superlattices is demonstrated, wherein tunable phonon modes are observed via confocal Raman spectroscopy. In particular, the coherent superlattices led to the backfolding of acoustic phonon dispersion, resulting in zone‐folded acoustic phonons in the THz frequency domain. The frequencies can be largely tuned from 1 to 2 THz via atomic‐scale precision thickness control. In addition, a polar optical phonon originating from the local inversion symmetry breaking in the artificial oxide superlattices is observed, exhibiting emergent functionality. The approach of atomic‐scale heterostructuring of complex oxides will vastly expand material systems for quantum acoustic devices, especially with the viability of functionality integration. 
    more » « less
  3. ; ; ; ; ; (, Journal of Applied Physics)
    Fluorographene, a fluorinated graphene-derivative, is expected to feature both high thermal conductivity and electrical insulation simultaneously, making it an emerging material for thermal management in electronic devices. In this paper, we investigated the lattice thermal conductivity and phonon transport properties of monolayer fluorographene using first-principles calculation. The solution of the fully linearized phonon Boltzmann transport equation gives the lattice thermal conductivity of monolayer fluorographene as 145.2 W m−1 K−1 at 300 K, which is about 20 times smaller than that of monolayer graphene. We systematically compared the phonon transport properties of all phonon modes in graphene and fluorographene in terms of phonon polarization. The significantly reduced thermal conductivity of fluorographene can be attributed to the lowering of both the lifetime of the flexural acoustic phonons and the group velocities of all acoustic phonons. We concluded that the broken in-plane mirror symmetry and the weaker in-plane chemical bonds induced by fluorination led to the suppression of the lattice thermal conductivity of fluorographene. Finally, we investigated the anomalously large contribution of optical phonons to the thermal transport process in fluorographene, where the large group velocities of selected optical phonons were derived from the in-plane acoustic modes of graphene. Our work provides a new approach to studying the influence of chemical functionalization on the phonon structure and exploring graphene-derived thermal management materials. 
    more » « less
  4. ; ; ; ; ; ; ; ; (, Advanced Materials)
    Abstract Twisted van der Waals materials featuring Moiré patterns present new design possibilities and demonstrate unconventional behaviors in electrical, optical, spintronic, and superconducting properties. However, experimental exploration of thermal transport across Moiré patterns has not been as extensive, despite its critical role in nanoelectronics, thermal management, and energy technologies. Here, the first experimental study is conducted on thermal transport across twisted graphene, demonstrating a phonon polarizer concept from the rotational misalignment between stacked layers. The direct thermal and acoustic measurements, structural characterizations, and atomistic modeling, reveal a modulation up to 631% in thermal conductance with various Moiré angles, while maintaining a high acoustic transmission. By comparing experiments with density functional theory and molecular dynamics simulations, mode‐dependent phonon transmissions are quantified based on the angle alignment of graphene band structures and attributed to the coupling among flexural phonon modes. The agreement confirms the dominant tuning mechanisms in adjusting phonon transmission from high‐frequency thermal modes while having negligible effects on low‐frequency acoustic modes near Brillouin zone center. This study offers crucial insights into the fundamental thermal transport in Moiré structures, opening avenues for the invention of quantum thermal devices and new design methodologies based on manipulations of vibrational band structures and phonon spectra. 
    more » « less
  5. ; ; ; ; ; ; ; (, Applied Physics Letters)
    Micro-Raman spectroscopy has become an important tool in probing thermophysical properties in functional materials. Localized heating by the focused Raman excitation laser beam can produce both stress and local nonequilibrium phonons in the material. Here, we investigate the effects of hot optical phonons in the Raman spectra of molybdenum disulfide and distinguish them from those caused by thermally induced compressive stress, which causes a Raman frequency blue shift. We use a thermomechanical analysis to correct for this stress effect in the equivalent lattice temperature extracted from the measured Raman peak shift. When the heating Gaussian laser beam is reduced to 0.71  μm, the corrected peak shift temperature rise is 17% and 8%, respectively, higher than those determined from the measured peak shift and linewidth without the stress correction, and 32% smaller than the optical phonon temperature rise obtained from the anti-Stokes to Stokes intensity ratio. This nonequilibrium between the hot optical phonons and the lattice vanishes as the beam width increases to 1.53 μm. Much less pronounced than those reported in prior micro-Raman measurements of suspended graphene, this observed hot phonon behavior agrees with a first-principles based multitemperature model of overpopulated zone-center optical phonons compared to other optical phonons in the Brillouin zone and acoustic phonons of this prototypical transition metal dichalcogenide. The findings provide detailed insight into the energy relaxation processes in this emerging electronic and optoelectronic material and clarify an important question in micro-Raman measurements of thermal transport in this and other two-dimensional materials. 
    more » « less
  • Citation Formats
  • MLA
  • APA
  • Chicago
  • BibTeX
  • Save / Share this Record
  • Send to Email