Critical problems faced in Raman-based energy transport characterization of nanomaterials.

In the last two decades, tremendous research has been conducted on the discovery, design and synthesis, characterization, and applications of two-dimensional (2D) materials. Thermal conductivity and interface thermal conductance/resistance of 2D materials are two critical properties in their applications. Raman spectroscopy, which measures the inelastic scattering of photons by optical phonons, can distinct a 2D material's temperature from its surrounding materials', featuring unprecedented spatial resolution (down to the atomic level). Raman-based thermometry has been used tremendously for characterizing the thermal conductivity of 2D materials (suspended or supported) and interface thermal conductance/resistance. Very large data deviations have been observed in literature, partly due to physical phenomena and factors not considered in measurements. Here, we provide a critical review, analysis, and perspectives about a broad spectrum of physical problems faced in Raman-based thermal characterization of 2D materials, namely interface separation, localized stress due to thermal expansion mismatch, optical interference, conjugated phonon, and hot carrier transport, optical-acoustic phonon thermal nonequilibrium, and radiative electron-hole recombination in monolayer 2D materials. Neglect of these problems will lead to a physically unreasonable understanding of phonon transport and interface energy coupling. In-depth discussions are also provided on the energy transport state-resolved Raman (ET-Raman) technique to overcome these problems and on future research challenges and needs.

Imaging Anisotropic Waveguide Exciton Polaritons in Tin Sulfide.

In recent years, novel materials supporting in-plane anisotropic polaritons have attracted a great deal of research interest due to their capability of shaping nanoscale field distributions and controlling nanophotonic energy flows. Here we report a nano-optical imaging study of waveguide exciton polaritons (EPs) in tin sulfide (SnS) in the near-infrared (near-IR) region using scattering-type scanning near-field optical microscopy (s-SNOM). With s-SNOM, we mapped in real space the propagative EPs in SnS, which show sensitive dependence on the excitation energy and sample thickness. Moreover, we found that both the polariton wavelength and propagation length are anisotropic in the sample plane. In particular, in a narrow spectral range from 1.32 to 1.44 eV, the EPs demonstrate quasi-one-dimensional propagation, which is rarely seen in natural polaritonic materials. A further analysis indicates that the observed polariton anisotropy originates from the different optical band gaps and exciton binding energies along the two principal crystal axes of SnS.

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.

The in-plane structure domain size of nm-thick MoSe2 uncovered by low-momentum phonon scattering.

Although 2D materials have been widely studied for more than a decade, very few studies have been reported on the in-plane structure domain (STD) size even though such a physical property is critical in determining the charge carrier and energy carrier transport. Grazing incidence X-ray diffraction (XRD) can be used for studying the in-plane structure of very thin samples, but it becomes more challenging to study few-layer 2D materials. In this work the nanosecond energy transport state-resolved Raman (nET-Raman) technique is applied to resolve this key problem by directly measuring the thermal reffusivity of 2D materials and determining the residual value at the 0 K-limit. Such a residual value is determined by low-momentum phonon scattering and can be directly used to characterize the in-plane STD size of 2D materials. Three suspended MoSe2 (15, 50 and 62 nm thick) samples are measured using nET-Raman from room temperature down to 77 K. Based on low-momentum phonon scattering, the STD size is determined to be 58.7 nm and 84.5 nm for 50 nm and 62 nm thick samples, respectively. For comparison, the in-plane structure of bulk MoSe2 that is used to prepare the measured nm-thick samples is characterized using XRD. It uncovers crystallite sizes of 64.8 nm in the (100) direction and 121 nm in the (010) direction. The STD size determined by our low momentum phonon scattering is close to the crystallite size determined by XRD, but still shows differences. The STD size by low-momentum phonon scattering is more affected by the crystallite sizes in all in-plane directions rather than that by XRD that is for a specific crystallographic orientation. Their close values demonstrate that during nanosheet preparation (peeling and transfer), the in-plane structure experiences very little damage.

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.

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 (ß), 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 ß 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.

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.

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 E2g 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.

Graphene Aerogel Based Bolometer for Ultrasensitive Sensing from Ultraviolet to Far-Infrared.

This work uncovers that free-standing partly reduced graphene aerogel (PRGA) films in vacuum exhibit extraordinarily bolometric responses. This high performance is mainly attributed to four structure characteristics: extremely low thermal conductivity (6.0-0.6 mW·m-1·K-1 from 295 to 10 K), high porosity, ultralow density (4 mg·cm-3), and abundant functional groups (resulting in tunable band gap). Under infrared radiation (peaked at 5.8-9.7 µm), the PRGA film can detect a temperature change of 0.2, 1.0, and 3.0 K of a target at 3, 25, and 54 cm distance. Even through a quartz window (transmissivity of ∼0.98 in the range of 2-4 µm), it can still successfully detect a temperature change of 0.6 and 5.8 K of a target at 3 and 28 cm distance. At room temperature, a laser power as low as 7.5 µW from a 405 nm laser and 5.9 µW from a 1550 nm laser can be detected. The detecting sensitivity to the 1550 nm laser is further increased by 3-fold when the sensor temperature was reduced from 295 K to 12 K. PRGA films are demonstrated to be a promising ultrasensitive bolometric detector, especially at low temperatures.

Measurement of the thermal conductivities of suspended MoS2 and MoSe2 by nanosecond ET-Raman without temperature calibration and laser absorption evaluation.

Steady state Raman spectroscopy is the most widely used opto-thermal technique for measuring a 2D atomic-layer material's thermal conductivity. It requires the calibration of temperature coefficients of Raman properties and measurement/calculation of the absolution laser absorption in 2D materials. Such a requirement is very laborious and introduces very large measurement errors (of the order of 100%) and hinders gaining a precise and deep understanding of phonon-structure interactions in 2D materials. In this work, a novel nanosecond energy transport state resolved Raman (ns ET-Raman) technique is developed to resolve these critical issues and achieve unprecedented measurement precision, accuracy and ease of implementation. In ns ET-Raman, two energy transport states are constructed: steady state and nanosecond thermal transport and Raman probing. The ratio of the temperature rise under the two states eliminates the need for Raman temperature calibration and laser absorption evaluation. Four suspended MoS2 (45-115 nm thick) and four suspended MoSe2 (45-140 nm thick) samples are measured and compared using ns ET-Raman. With the increase of the sample thickness, the measured thermal conductivity increases from 40.0 ± 2.2 to 74.3 ± 3.2 W m-1 K-1 for MoS2, and from 11.1 ± 0.4 to 20.3 ± 0.9 W m-1 K-1 for MoSe2. This is attributed to the decreased significance of surface phonon scattering in thicker samples. The ns ET-Raman features the most advanced capability to measure the thermal conductivity of 2D materials and will find broad applications in studying low-dimensional materials.