Very fast hot carrier diffusion in unconstrained MoS2on a glass substrate: discovered by picosecond ET-Raman

Determination of a Raman shift laser power coefficient based on cross correlation

This work presents a novel, to the best of our knowledge, cross correlation technique for determining the laser heating-induced Raman shift laser power coefficient ψ required for energy transport state-resolved Raman (ET-Raman) methods. The cross correlation method determines the measure of similarity between the experimental intensity data and a varying test Gaussian signal. By circumventing the errors inherent in any curve fittings, the cross correlation method quickly and accurately determines the location where the test Gaussian signal peak is most like the Raman peak, thereby revealing the peak location and ultimately the value of ψ. This method improves the reliability of optothermal Raman-based methods for micro/nanoscale thermal measurements and offers a robust approach to data processing through a global treatment of Raman spectra.

Review of Photothermal Technique for Thermal Measurement of Micro-/Nanomaterials

The extremely small size of micro-/nanomaterials limits the application of conventional thermal measurement methods using a contact heating source or probing sensor. Therefore, non-contact thermal measurement methods are preferable in micro-/nanoscale thermal characterization. In this review, one of the non-contact thermal measurement methods, photothermal (PT) technique based on thermal radiation, is introduced. When subjected to laser heating with controllable modulation frequencies, surface thermal radiation carries fruitful information for thermal property determination. As thermal properties are closely related to the internal structure of materials, for micro-/nanomaterials, PT technique can measure not only thermal properties but also features in the micro-/nanostructure. Practical applications of PT technique in the thermal measurement of micro-/nanomaterials are then reviewed, including special wall-structure investigation in multiwall carbon nanotubes, porosity determination in nanomaterial assemblies, and the observation of amorphous/crystalline structure transformation in proteins in heat treatment. Furthermore, the limitations and future application extensions are discussed.

Critical Problems Faced in Raman-based Energy Transport Characterization of Nanomaterials

In the last two decades, tremendous research has been conducted on discovery, design and synthesis, characterization, and applications of two-dimensional (2D) materials. Thermal conductivity and interface thermal conductance/resistance of 2D...

Review on Techniques for Thermal Characterization of Graphene and Related 2D Materials

The discovery of graphene and its analog, such as MoS2, has boosted research. The thermal transport in 2D materials gains much of the interest, especially when graphene has high thermal conductivity. However, the thermal properties of 2D materials obtained from experiments have large discrepancies. For example, the thermal conductivity of single layer suspended graphene obtained by experiments spans over a large range: 1100-5000 W/m·K. Apart from the different graphene quality in experiments, the thermal characterization methods play an important role in the observed large deviation of experimental data. Here we provide a critical review of the widely used thermal characterization techniques: the optothermal Raman technique and the micro-bridge method. The critical issues in the two methods are carefully revised and discussed in great depth. Furthermore, improvements in Raman-based techniques to investigate the energy transport in 2D materials are discussed.

Direct Characterization of Thermal Nonequilibrium between Optical and Acoustic Phonons in Graphene Paper under Photon Excitation

Raman spectroscopy has been widely used to measure thermophysical properties of 2D materials. The local intense photon heating induces strong thermal nonequilibrium between optical and acoustic phonons. Both first principle calculations and recent indirect Raman measurements prove this phenomenon. To date, no direct measurement of the thermal nonequilibrium between optical and acoustic phonons has been reported. Here, this physical phenomenon is directly characterized for the first time through a novel approach combining both electrothermal and optothermal techniques. While the optical phonon temperature is determined from Raman wavenumber, the acoustic phonon temperature is precisely determined using high-precision thermal conductivity and laser power absorption that are measured with negligible nonequilibrium among energy carriers. For graphene paper, the energy coupling factor between in-plane optical and overall acoustic phonons is found at (1.59-3.10) × 1015 W m-3 K-1, agreeing well with the quantum mechanical modeling result of 4.1 × 1015 W m-3 K-1. Under ≈1 µm diameter laser heating, the optical phonon temperature rise is over 80% higher than that of the acoustic phonons. This observation points out the importance of subtracting optical-acoustic phonon thermal nonequilibrium in Raman-based thermal characterization.

Thermal conductance between water and nm-thick WS2: extremely localized probing using nanosecond energy transport state-resolved Raman

Liquid-solid interface energy transport has been a long-term research topic. Past research mostly focused on theoretical studies while there are only a handful of experimental reports because of the extreme challenges faced in measuring such interfaces. Here, by constructing nanosecond energy transport state-resolved Raman spectroscopy (nET-Raman), we characterize thermal conductance across a liquid-solid interface: water-WS₂ nm film. In the studied system, one side of a nm-thick WS₂ film is in contact with water and the other side is isolated. WS₂ samples are irradiated with 532 nm wavelength lasers and their temperature evolution is monitored by tracking the Raman shift variation in the E_2g mode at several laser powers. Steady and transient heating states are created using continuous wave and nanosecond pulsed lasers, respectively. We find that the thermal conductance between water and WS₂ is in the range of 2.5-11.8 MW m^-2 K^-1 for three measured samples (22, 33, and 88 nm thick). This is in agreement with molecular dynamics simulation results and previous experimental work. The slight differences are attributed mostly to the solid-liquid interaction at the boundary and the surface energies of different solid materials. Our detailed analysis confirms that nET-Raman is very robust in characterizing such interface thermal conductance. It completely eliminates the need for laser power absorption and Raman temperature coefficients, and is insensitive to the large uncertainties in 2D material properties input.

Interfacial Thermal Conductance between Monolayer WSe2 and SiO2 under Consideration of Radiative Electron-Hole Recombination

This work reports the interfacial thermal conductance (G) and radiative recombination efficiency (β), also known as photoluminescence quantum yield (PL QY), of monolayer WSe₂ flakes supported by fused silica substrates via energy-transport state-resolved Raman (ET-Raman). This is the first known work to consider the effect of radiative electron-hole recombination on the thermal transport characteristics of single-layer transition-metal dichalcogenides (TMDs). ET-Raman uses a continuous-wave laser for steady-state heating as well as nanosecond and picosecond lasers for transient energy transport to simultaneously heat the monolayer flakes and extract the Raman signal. The three lasers induce distinct heating phenomena that distinguish the interfacial thermal conductance and radiative recombination efficiency, which can then be determined in tandem with three-dimensional (3D) numerical modeling of the temperature rise from respective laser irradiation. For the five samples measured, G is found to range from 2.10 ± 0.14 to 15.9 ± 5.0 MW m^-2 K^-1 and β ranges from 36 ± 6 to 65 ± 7%. These values support the claim that interfacial phenomena such as surface roughness and two-dimensional (2D) material-substrate bonding strength play critical roles in interfacial thermal transport and electron-hole recombination mechanisms in TMD monolayers. It is also determined that low-level defect density enhances the radiative recombination efficiency of single-layer WSe₂.

Rigorous prediction of Raman intensity from multi-layer films

In the Raman probing of multilayer thin film materials, the intensity of the measured Raman scattered light will be impacted by the thickness of the thin film layers. The Raman signal intensity will vary non-monotonically with thickness due to interference from the multiple reflections of both the incident laser light and the Raman scattered light of thin film interfaces. Here, a method for calculating the Raman signal intensity from a multilayer thin film system based on the transfer matrix method with a rigorous treatment of the Raman signal generation (discontinuity) is presented. This calculation methodology is valid for any thin film stack with an arbitrary number of layers with arbitrary thicknesses. This approach is applied to several thin film material systems, including silicon-on-sapphire thin films, graphene on Si with a SiO₂ capping layer, and multilayer MoS₂ with the presence of a gap between layers and substrate. Different applications where this method can be used in the Raman probing of thin film material properties are discussed.

Energy and Charge Transport in 2D Atomic Layer Materials: Raman-Based Characterization

As they hold extraordinary mechanical and physical properties, two-dimensional (2D) atomic layer materials, including graphene, transition metal dichalcogenides, and MXenes, have attracted a great deal of attention. The characterization of energy and charge transport in these materials is particularly crucial for their applications. As noncontact methods, Raman-based techniques are widely used in exploring the energy and charge transport in 2D materials. In this review, we explain the principle of Raman-based thermometry in detail. We critically review different Raman-based techniques, which include steady state Raman, time-domain differential Raman, frequency-resolved Raman, and energy transport state-resolved Raman techniques constructed in the frequency domain, space domain, and time domain. Detailed outlooks are provided about Raman-based energy and charge transport in 2D materials and issues that need special attention.

Distinguishing Optical and Acoustic Phonon Temperatures and Their Energy Coupling Factor under Photon Excitation in nm 2D Materials

Under photon excitation, 2D materials experience cascading energy transfer from electrons to optical phonons (OPs) and acoustic phonons (APs). Despite few modeling works, it remains a long-history open problem to distinguish the OP and AP temperatures, not to mention characterizing their energy coupling factor (G). Here, the temperatures of longitudinal/transverse optical (LO/TO) phonons, flexural optical (ZO) phonons, and APs are distinguished by constructing steady and nanosecond (ns) interphonon branch energy transport states and simultaneously probing them using nanosecond energy transport state-resolved Raman spectroscopy. ΔT OP -AP is measured to take more than 30% of the Raman-probed temperature rise. A breakthrough is made on measuring the intrinsic in-plane thermal conductivity of suspended nm MoS2 and MoSe2 by completely excluding the interphonon cascading energy transfer effect, rewriting the Raman-based thermal conductivity measurement of 2D materials. G OP↔AP for MoS2, MoSe2, and graphene paper (GP) are characterized. For MoS2 and MoSe2, G OP↔AP is in the order of 1015 and 1014 W m-3 K-1 and G ZO↔AP is much smaller than G LO/TO↔AP. Under ns laser excitation, G OP↔AP is significantly increased, probably due to the reduced phonon scattering time by the significantly increased hot carrier population. For GP, G LO/TO↔AP is 0.549 × 1016 W m-3 K-1, agreeing well with the value of 0.41 × 1016 W m-3 K-1 by first-principles modeling.

Effect of temperature on Raman intensity of nm-thick WS2: combined effects of resonance Raman, optical properties, and interface optical interference

Temperature dependent Raman intensity of 2D materials features very rich information about the material's electronic structure, optical properties, and nm-level interface spacing. To date, there still lacks rigorous consideration of the combined effects. This renders the Raman intensity information less valuable in material studies. In this work, the Raman intensity of four supported multilayered WS2 samples are studied from 77 K to 757 K under 532 nm laser excitation. Resonance Raman scattering is observed, and we are able to evaluate the excitonic transition energy of B exciton and its broadening parameters. However, the resonance Raman effects cannot explain the Raman intensity variation in the high temperature range (room temperature to 757 K). The thermal expansion mismatch between WS2 and Si substrate at high temperatures (room temperature to 757 K) make the optical interference effects very strong and enhances the Raman intensity significantly. This interference effect is studied in detail by rigorously calculating and considering the thermal expansion of samples, the interface spacing change, and the optical indices change with temperature. Considering all of the above factors, it is concluded that the temperature dependent Raman intensity of the WS2 samples cannot be solely interpreted by its resonance behavior. The interface optical interference impacts the Raman intensity more significantly than the change of refractive indices with temperature.

Inhomogeneous photocarrier dynamics and transport in monolayer MoS2 by ultrafast microscopy

Monolayer MoS₂ as a member of two-dimensional transition metal dichalcogenides (TMDs) has attracted considerable attention due to its superior optoelectronic properties. Understanding the photocarrier dynamics and transport in these two dimensional systems is beneficial for applications from photovoltaics to sensing. However, various structural defects strongly impact the dynamics and transport of photocarriers. Especially there lacks a precise measuring and understanding of photocarrier transport in TMDs. Here, femtosecond transient absorption spectroscopy and microscopy were employed to study the photocarrier dynamics and transport in monolayer MoS₂. Defect correlated photocarrier dynamics are observed across the monolayer MoS₂ where exciton formation and nonradiative recombination are the two dominant decay processes. To the best of our knowledge, we report two distinct photocarrier transport regimes in MoS₂ for the first time with diffusion coefficients of [Formula: see text] cm² s^-1 and [Formula: see text] cm² s^-1, by taking advantages of ultrafast microscopy with ∼20 nm spatial precision and ∼200 fs temporal resolution. These two regimes are ascribed to fast hot photocarrier diffusion and slow phonon-limited thermal diffusion, respectively. The results indicate that the initial fast photocarrier transport is less dependent on structural defects compared to photocarrier relaxation dynamics which may be useful for hot photocarrier extraction applications.

Nonmonotonic thickness-dependence of in-plane thermal conductivity of few-layered MoS2: 2.4 to 37.8 nm

Recent first-principles modeling reported a decrease of in-plane thermal conductivity (k) with increased thickness for few layered MoS2, which results from the change in phonon dispersion and missing symmetry in the anharmonic atomic force constant. For other 2D materials, it has been well documented that a higher thickness could cause a higher in-plane k due to a lower density of surface disorder. However, the effect of thickness on the k of MoS2 has not been systematically uncovered by experiments. In addition, from either experimental or theoretical approaches, the in-plane k value of tens-of-nm-thick MoS2 is still missing, which makes the physics on the thickness-dependent k remain ambiguous. In this work, we measure the k of few-layered (FL) MoS2 with thickness spanning a large range: 2.4 nm to 37.8 nm. A novel five energy transport state-resolved Raman (ET-Raman) method is developed for the measurement. For the first time, the critical effects of hot carrier diffusion, electron-hole recombination, and energy coupling with phonons are taken into consideration when determining the k of FL MoS2. By eliminating the use of laser energy absorption data and Raman temperature calibration, unprecedented data confidence is achieved. A nonmonotonic thickness-dependent k trend is discovered. k decreases from 60.3 W m-1 K-1 (2.4 nm thick) to 31.0 W m-1 K-1 (9.2 nm thick), and then increases to 76.2 W m-1 K-1 (37.8 nm thick), which is close to the reported k of bulk MoS2. This nonmonotonic behavior is analyzed in detail and attributed to the change of phonon dispersion for very thin MoS2 and a reduced surface scattering effect for thicker samples.