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1. Thermal conductance between water and nm-thick WS 2 : extremely localized probing using nanosecond energy transport state-resolved Raman

   https://doi.org/10.1039/d0na00844c  

   Zobeiri, Hamidreza ; Hunter, Nicholas ; Wang, Ridong ; Liu, Xinman ; Tan, Hong ; Xu, Shen ; Wang, Xinwei ( December 2020 , Nanoscale Advances)

   null (Ed.)

   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–WS 2 nm film. In the studied system, one side of a nm-thick WS 2 film is in contact with water and the other side is isolated. WS 2 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 WS 2 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. 

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2. Effect of temperature on Raman intensity of nm-thick WS 2 : combined effects of resonance Raman, optical properties, and interface optical interference

   https://doi.org/10.1039/C9NR10186A  

   Zobeiri, Hamidreza ; Xu, Shen ; Yue, Yanan ; Zhang, Qianying ; Xie, Yangsu ; Wang, Xinwei ( March 2020 , Nanoscale)

   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 WS 2 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 WS 2 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 WS 2 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. 

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3. A thin film efficient pn-junction thermoelectric device fabricated by self-align shadow mask

   https://doi.org/10.1038/s41598-020-57991-y  

   Kogo, Gilbert ; Xiao, Bo ; Danquah, Samuel ; Lee, Harold ; Niyogushima, Julien ; Yarbrough, Kelsea ; Candadai, Aaditya ; Marconnet, Amy ; Pradhan, Sangram K. ; Bahoura, Messaoud ( January 2020 , Scientific Reports)

   Large area highly crystalline MoS₂and WS₂thin films were successfully grown on different substrates using radio-frequency magnetron sputtering technique. Structural, morphological and thermoelectric transport properties of MoS_2,and WS₂thin films have been investigated systematically to fabricate high-efficient thermal energy harvesting devices. X-ray diffraction data revealed that crystallites of MoS₂and WS₂films are highly oriented in 002 plane with uniform grain size distribution confirmed through atomic force microscopy study. Surface roughness increases with substrate temperature and it plays a big role in electron and phonon scattering. Interestingly, MoS₂films also display low thermal conductivity at room temperature and strongly favors achievement of higher thermoelectric figure of merit value of up to 1.98. Raman spectroscopy data shows two distinct MoS₂vibrational modes at 380 cm⁻¹for E¹_2gand 410 cm⁻¹for A_1g. Thermoelectric transport studies further demonstrated that MoS₂films show p-type thermoelectric characteristics, while WS₂is an n-type material. We demonstrated high efficient pn-junction thermoelectric generator device for waste heat recovery and cooling applications.

    

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

   https://doi.org/10.1002/advs.202000097  

   Wang, Ridong ; Zobeiri, Hamidreza ; Xie, Yangsu ; Wang, Xinwei ; Zhang, Xing ; Yue, Yanan ( May 2020 , Advanced Science)

   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 MoS₂and MoSe₂by completely excluding the interphonon cascading energy transfer effect, rewriting the Raman‐based thermal conductivity measurement of 2D materials.G_OP↔APfor MoS₂, MoSe₂, and graphene paper (GP) are characterized. For MoS₂and MoSe₂,G_OP↔APis in the order of 10¹⁵and 10¹⁴W m⁻³K⁻¹andG_ZO↔APis much smaller thanG_LO/TO↔AP. Under ns laser excitation,G_OP↔APis significantly increased, probably due to the reduced phonon scattering time by the significantly increased hot carrier population. For GP,G_LO/TO↔APis 0.549 × 10¹⁶W m⁻³K⁻¹, agreeing well with the value of 0.41 × 10¹⁶W m⁻³K⁻¹by first‐principles modeling.

    

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5. The in-plane structure domain size of nm-thick MoSe 2 uncovered by low-momentum phonon scattering

   https://doi.org/10.1039/d0nr09099a  

   Lin, Huan ; Wang, Ridong ; Zobeiri, Hamidreza ; Wang, Tianyu ; Xu, Shen ; Wang, Xinwei ( April 2021 , Nanoscale)

   null (Ed.)

   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 MoSe 2 (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 MoSe 2 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. 

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