Studies of Intrinsic Hot Phonon Dynamics in Suspended Graphene by Transient Absorption Microscopy

Abstract

Correlated transient absorption and atomic force microscopy (AFM) measurements have been performed for monolayer graphene, both free-standing and supported on a glass substrate. The AFM images allow us to locate regions of the suspended graphene. The transient absorption traces show a fast instrument response limited decay, followed by a slower intensity dependent decay. The fast decay is assigned to a combination of coupling between the excited charge carriers and the optical phonon modes of graphene and the substrate, and diffusion of the charge carrier out of the probe region. The slow decay is due to the hot phonon effect and reflects the lifetime of the intrinsic optical phonons of graphene. The time constant for the slow decay is longer for suspended graphene compared to substrate-supported graphene. This is attributed to interactions between the excited charge carriers and the surface optical phonon modes of the substrate, which supplies an additional relaxation channel for supported graphene.

Ultrafast relaxation of hot phonons in graphene-hBN heterostructures

“…When the temperatures of the carriers and optical phonon bath equalize, this cooling channel slows down and this is termed as the Hot Phonon (HP) bottleneck. 5,[9][10][11][12][13] Cooling through direct acoustic phonon emission is not viable because of a vanishingly small phase space for such a scattering process. 14 The hot optical phonons cool down through anharmonic decay to acoustic phonons which are subsequently absorbed into the substrate.…”

“…The range of values of τ op for g-SiO 2 (0.8-1.5 ps) agrees well with previous measurements of the phonon lifetime in SiO 2 supported graphene. 13,21 At higher fluences (i.e., high initial carrier densities), multibody effects like carrier screening, plasmonic modes, and plasmon-phonon interactions might come into play. 31 While these multibody effects are not captured by the simple two temperature model, it still allows us to phenomenologically deduce the optical phonon relaxation lifetime.…”

Highly sensitive transient absorption imaging of graphene and graphene oxide in living cells and circulating blood

“…TA imaging has been developed for visualizing single nanomaterials, such as gold nanoparticles3435, nanowires36, semiconductor and meta nanostructures3738, single-walled carbon nanotubes394041. Recently, TA spectroscopy and imaging have been employed to study the carrier dynamics in graphene4243 and graphene oxide4445 with limited sensitivity.…”

Layer number dependence of carrier lifetime in graphenes observed using time-resolved mid-infrared luminescence

“…In these applications, interaction of electrons with phonons is important, because transport properties and the optical response depend on the speed of cooling of hot carriers. The dynamics of energetic electrons and coupled phonons have been investigated using various experimental methods, such as transient absorption [3,4,5], transient reflectance [6], anti-Stokes Raman scattering [7], photoemission spectroscopy [8,9], and luminescence [10,11,12]. Photoexcited carriers in graphenes exhibit fast (~100 fs) and slow (1.5-5 ps) life-time components [5,6,8].…”