Femtosecond transient absorption spectroscopy and microscopy were employed to study exciton dynamics in suspended and Si 3 N 4 substrate-supported monolayer and few-layer MoS 2 2D crystals. Exciton dynamics for the monolayer and few-layer structures were found to be remarkably different from those of thick crystals when probed at energies near that of the lowest energy direct exciton (A exciton). The intraband relaxation rate was enhanced by more than 40 fold in the monolayer in comparison to that observed in the thick crystals, which we attributed to defect assisted scattering. Faster electronÀhole recombination was found in monolayer and few-layer structures due to quantum confinement effects that lead to an indirectÀdirect band gap crossover. Nonradiative rather than radiative relaxation pathways dominate the dynamics in the monolayer and few-layer MoS 2 . Fast trapping of excitons by surface trap states was observed in monolayer and few-layer structures, pointing to the importance of controlling surface properties in atomically thin crystals such as MoS 2 along with controlling their dimensions.
The lifetimes of the acoustic vibrations of metal nanostructures depend sensitively on the properties of the environment, such as the acoustic impedance and viscosity. In order to accurately study these effects, they have to be separated from the damping processes that are inherent to the nanostructure. Here we show that this can be done experimentally by investigating individual gold nanowires suspended over a trench in air and liquid environments. The experiments were done by ultrafast pump-probe microscopy, recording transient absorption traces at the same point on the nanowire in both environments. These first experiments were performed with water, and the measured vibrational quality factors due to the presence of water were compared to continuum mechanics calculations for a cylinder in a homogeneous environment. Good agreement was found between the experimental quality factors and the calculated values. The continuum mechanics analysis shows that damping is dominated by the acoustic impedance of the solvent rather than by its viscosity for the nanowires in the present experiments. This experimental technique opens up the possibility of studying the effect of viscosity on the high frequency vibrational motions of nanostructures for a variety of liquids.
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.
The memristor is the fourth fundamental circuit element discovered after resistors, capacitors, and inductors. Although this concept has only been proposed in 1971 and confirmed in 2008, many materials with memristive properties have been found since 1962. Halide perovskites have been widely used in solar cells, and recently it has been found that they also possess good memristive properties. Different halide perovskites have been applied to memristors, including 3D organic–inorganic hybrid perovskites, 2D organic–inorganic hybrid perovskites, all‐inorganic cesium/rubidium lead halide perovskites, lead‐less and lead‐free perovskites, and halide perovskite quantum dots. Flexible and fiber‐shaped halide perovskite memristors have been fabricated. Several resistive switching mechanisms of halide perovskite memristors have been proposed, and the relationships between halide perovskite memristors and perovskite solar cells have been discussed. Based on halide perovskite memristors, light‐induced resistive switching and logic gate, high‐density and cross‐bar array data storage unit, and artificial synapse have been designed. Herein, recent advances in halide perovskite memristors are comprehensively and systematically reviewed. Finally, the current challenges and potential future directions in this field are discussed.
Monolayer MoS 2 , processing flexibility and remarkable physical properties derived from its direct bandgap feature, has been endowed to be one of the potential materials for practical applications such as integrated circuits and logic devices. Recently, a facile CVD method using alkali metal compounds as promoters attracted a lot of attention. Here, we systematically investigated the mechanism of alkali metal compoundpromoted growth of monolayer MoS 2 by CVD and proposed a eutectic intermediate model. In the presence of alkali metal compounds, large monolayer MoS 2 was obtained, regardless of the anions. However, nonalkali metal compounds did not promote the growth of monolayer MoS 2 . We proposed that the formation of eutectic intermediates, containing alkali metal molybdates and molybdenum oxides, played a crucial role in promoting the growth of monolayer MoS 2 . It is because the low melting point of eutectic intermediates could facilitate their mobility, favoring less nuclei and lateral growth. The proposal of the eutectic intermediates model could not only contribute to growing ultralarge monolayer MoS 2 and other 2D materials but also inspire new ideas about growing 2D materials based on low melting point and high mobility of eutectic intermediates.
Resonance Raman spectra of individual strained ultralong single-wall carbon nanotubes (SWNTs) are studied. Torsional and uniaxial strains are introduced by atomic force microscopy manipulation. Torsional strain strongly affects the Raman spectra, inducing a large downshift in the E2 symmetry mode in the G+ band, but a slight upshift for the rest of the G modes and also an upshift in the radial breathing mode (RBM). Whereas uniaxial strain has no effect on the frequency of either the E2 symmetry mode in the G+ band or the RBM, it downshifts the rest of the G modes. The Raman intensity change reflects the effect of these strains on the SWNT electronic band structure.
Femtosecond transient absorption microscopy was employed to study the excited-state dynamics of individual semiconducting single-walled carbon nanotubes (SWNTs) with simultaneously high spatial (∼200 nm) and temporal (∼300 fs) resolution. Isolated SWNTs were located using atomic force microscopy, and Raman spectroscopy was employed to determine the chiral index of select nanotubes. This unique experimental approach removes sample heterogeneity in ultrafast measurements of these complex materials. Transient absorption spectra of the individual SWNTs were obtained by recording transient absorption images at different probe wavelengths. These measurements provide new information about the origin of the photoinduced absorption features of SWNTs. Transient absorption traces were also collected for individual SWNTs. The dynamics show a fast, ∼1 ps, decay for all the semiconducting nanotubes studied, which is significantly faster than the previously reported decay times for SWNT suspensions. We attributed this fast relaxation to coupling between the excitons created by the pump laser pulse and the substrate.
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