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1. The influence of crystal thickness and interlayer interactions on the properties of heavy ion irradiated MoS 2

   https://doi.org/10.1088/2053-1583/ab817b  

   Isherwood, Liam H. ; Hennighausen, Zachariah ; Son, Seok-Kyun ; Spencer, Ben F. ; Wady, Paul T. ; Shubeita, Samir M. ; Kar, Swastik ; Casiraghi, Cinzia ; Baidak, Aliaksandr ( May 2020 , 2D Materials)

   Ion irradiation is a versatile tool to introduce controlled defects into two-dimensional (2D) MoS₂on account of its unique spatial resolution and plethora of ion types and energies available. In order to fully realise the potential of this technique, a holistic understanding of ion-induced defect production in 2D MoS₂crystals of different thicknesses is mandatory. X-ray photoelectron spectroscopy, electron diffraction and Raman spectroscopy show that thinner MoS₂crystals are more susceptible to radiation damage caused by 225 keV Xe⁺ions. However, the rate of defect production in quadrilayer and bulk crystals is not significantly different under our experimental conditions. The rate at which S atoms are sputtered as a function of radiation exposure is considerably higher for monolayer MoS₂, compared to bulk crystals, leading to MoO₃formation. P-doping of MoS₂is observed and attributed to the acceptor states introduced by vacancies and charge transfer interactions with adsorbed species. Moreover, the out-of-plane vibrational properties of irradiated MoS₂crystals are shown to be strongly thickness-dependent: in mono- and bilayer MoS₂, the confinement of phonons by defects results in a blueshift of the$A_{1g}$mode. Whereas, a redshift is observed in bulk crystals due to attenuation of the effective restoring forces acting on S atoms caused by vacancies in adjacent MoS₂layers. Consequently, the$A_{1g}$frequency of tri- and quadrilayer crystals is statistically invariant on account oft competition between phonon confinement effects and interlayer interactions. The$A_{1g}$linewidth is observed to decrease in bi- and trilayer crystals after low dose irradiation and is attributed to layer decoupling. This work shows that there is a complex interplay between defect production, crystal thickness and interlayer interactions in MoS₂. Our results demonstrate that ion irradiation is an effective tool to modulate the electronic, vibrational and structural properties of MoS₂, which may prove beneficial for practical applications.

    

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2. Glass‐Like Through‐Plane Thermal Conductivity Induced by Oxygen Vacancies in Nanoscale Epitaxial La 0.5 Sr 0.5 CoO 3− δ

   https://doi.org/10.1002/adfm.201704233  

   Wu, Xuewang ; Walter, Jeff ; Feng, Tianli ; Zhu, Jie ; Zheng, Hong ; Mitchell, John F. ; Biškup, Neven ; Varela, Maria ; Ruan, Xiulin ; Leighton, Chris ; et al ( November 2017 , Advanced Functional Materials)

   Ultrafast time‐domain thermoreflectance (TDTR) is utilized to extract the through‐plane thermal conductivity (Λ_LSCO) of epitaxial La_0.5Sr_0.5CoO_3−δ(LSCO) of varying thickness (<20 nm) on LaAlO₃and SrTiO₃substrates. These LSCO films possess ordered oxygen vacancies as the primary means of lattice mismatch accommodation with the substrate, which induces compressive/tensile strain and thus controls the orientation of the oxygen vacancy ordering (OVO). TDTR results demonstrate that the room‐temperatureΛ_LSCOof LSCO on both substrates (1.7 W m⁻¹K⁻¹) are nearly a factor of four lower than that of bulk single‐crystal LSCO (6.2 W m⁻¹K⁻¹). Remarkably, this approaches the lower limit of amorphous oxides (e.g., 1.3 W m⁻¹K⁻¹for glass), with no dependence on the OVO orientation. Through theoretical simulations, origins of the glass‐like thermal conductivity of LSCO are revealed as a combined effect resulting from oxygen vacancies (the dominant factor), Sr substitution, size effects, and the weak electron/phonon coupling within the LSCO film. The absence of OVO dependence in the measuredΛ_LSCOis rationalized by two main effects: (1) the nearly isotropic phononic thermal conductivity resulting from the imperfect OVO planes when δ is small; (2) the missing electronic contribution toΛ_LSCOalong the through‐plane direction for these ultrathin LSCO films on insulating substrates.

    

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3. Stacking‐Order‐Driven Optical Properties and Carrier Dynamics in ReS 2

   https://doi.org/10.1002/adma.201908311  

   Zhou, Yongjian ; Maity, Nikhilesh ; Rai, Amritesh ; Juneja, Rinkle ; Meng, Xianghai ; Roy, Anupam ; Zhang, Yanyao ; Xu, Xiaochuan ; Lin, Jung‐Fu ; Banerjee, Sanjay K. ; et al ( April 2020 , Advanced Materials)

   Two distinct stacking orders in ReS₂are identified without ambiguity and their influence on vibrational, optical properties and carrier dynamics are investigated. With atomic resolution scanning transmission electron microscopy (STEM), two stacking orders are determined as AA stacking with negligible displacement across layers, and AB stacking with about a one‐unit cell displacement along theaaxis. First‐principles calculations confirm that these two stacking orders correspond to two local energy minima. Raman spectra inform a consistent difference of modes I & III, about 13 cm⁻¹for AA stacking, and 20 cm⁻¹for AB stacking, making a simple tool for determining the stacking orders in ReS₂. Polarized photoluminescence (PL) reveals that AB stacking possesses blueshifted PL peak positions, and broader peak widths, compared with AA stacking, indicating stronger interlayer interaction. Transient transmission measured with femtosecond pump–probe spectroscopy suggests exciton dynamics being more anisotropic in AB stacking, where excited state absorption related to Exc. III mode disappears when probe polarization aligns perpendicular tobaxis. The findings underscore the stacking‐order driven optical properties and carrier dynamics of ReS₂, mediate many seemingly contradictory results in the literature, and open up an opportunity to engineer electronic devices with new functionalities by manipulating the stacking order.

    

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4. 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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5. Anomalous thermal transport behavior in graphene-like carbon nitride (C 3 N)

   https://doi.org/10.1039/d2tc02425j  

   Qin, Guangzhao ; Lin, Jianzhou ; Wang, Huimin ; Hu, Jianjun ; Qin, Zhenzhen ; Hu, Ming ( August 2022 , Journal of Materials Chemistry C)

   The success of graphene created a new era in materials science, especially for two-dimensional (2D) materials. 2D single-crystal carbon nitride (C 3 N) is the first and only crystalline, hole-free, single-layer carbon nitride and its controlled large-scale synthesis has recently attracted tremendous interest in thermal transport. Here, we performed a comparative study of thermal transport between monolayer C 3 N and the parent graphene, and focused on the effect of temperature and strain on the thermal conductivity ( κ ) of C 3 N, by solving the phonon Boltzmann transport equation (BTE) based on first-principles calculations. The κ of C 3 N shows an anomalous temperature dependence, and the κ of C 3 N at high temperatures is larger than the expected value following the common trend of κ ∼ 1/ T . Moreover, the κ of C 3 N is found to be increased by applying a bilateral tensile strain, despite its similar planar honeycomb structure to graphene. The underlying mechanism is revealed by providing direct evidence for the interaction between lone-pair N-s electrons and bonding electrons from C atoms in C 3 N based on the analysis of orbital-projected electronic structures and electron localization function (ELF). Our research not only conduct a comprehensive study on the thermal transport in graphene-like C 3 N, but also reveal the physical origin of its anomalous properties, which would have significant implications on the future studies of nanoscale thermal transport. 

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