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1. The interplay of intra- and inter-layer interactions in bending rigidity of ultrathin 2D materials

   https://doi.org/10.1063/5.0146065  

   Jiang, Yingchun; Sridhar, Srividhya; Liu, Zihan; Wang, Dingli; Zhou, Huimin; Deng, Jia; Chew, Huck Beng; Ke, Changhong (April 2023, Applied Physics Letters)

   Continuum mechanics break down in bending stiffness calculations of mono- and few-layered two-dimensional (2D) van der Waals crystal sheets, because their layered atomistic structures are uniquely characterized by strong in-plane bonding coupled with weak interlayer interactions. Here, we elucidate how the bending rigidities of pristine mono- and few-layered molybdenum disulfide (MoS 2 ), graphene, and hexagonal boron nitride (hBN) are governed by their structural geometry and intra- and inter-layer bonding interactions. Atomic force microscopy experiments on the self-folded conformations of these 2D materials on flat substrates show that the bending rigidity of MoS 2 significantly exceeds those of graphene or hBN of comparable layers, despite its much lower tensile modulus. Even on a per-thickness basis, MoS 2 is found to possess similar bending stiffness to hBN and is much stiffer than graphene. Density functional theory calculations suggest that this high bending rigidity of MoS 2 is due to its large interlayer thickness and strong interlayer shear, which prevail over its weak in-plane bonding. 

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2. Phonon Lifetimes in Boron‐Isotope‐Enriched Graphene‐ Hexagonal Boron Nitride Devices

   https://doi.org/10.1002/pssr.202200030  

   Brasington, Alexandra; Liu, Song; Taniguchi, Takashi; Watanabe, Kenji; Edgar, James H.; LeRoy, Brian J.; Sandhu, Arvinder (March 2022, physica status solidi (RRL) – Rapid Research Letters)

   Using hexagonal boron nitride (hBN) as a substrate for graphene has shown faster carrier cooling which makes it ideal for high‐power graphene‐based devices. However, the effect of using boron‐isotope‐enriched hBN has not been explored. Herein, femtosecond pump‐probe spectroscopy is utilized to measure and compare the time dynamics of photo‐excited carriers in graphene‐hBN heterostructures for hBN with the natural distribution of boron isotopes (20%¹⁰B and 80%¹¹B) and hBN enriched to 100%¹⁰B and¹¹B. The carriers cool down faster for systems with isotopically pure hBN substrates by a factor of ≈1.7 times. This difference in relaxation times arises from the interfacial coupling between carriers in graphene and the hBN phonon modes. The results show that the boron isotopic purity of the hBN substrate can help to reduce the hot phonon bottleneck that limits the cooling in graphene devices. 

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3. High-Contrast Plasmonic-Enhanced Shallow Spin Defects in Hexagonal Boron Nitride for Quantum Sensing

   https://doi.org/10.1021/acs.nanolett.1c02495  

   Gao, Xingyu; Jiang, Boyang; Llacsahuanga Allcca, Andres E.; Shen, Kunhong; Sadi, Mohammad A.; Solanki, Abhishek B.; Ju, Peng; Xu, Zhujing; Upadhyaya, Pramey; Chen, Yong P.; et al (September 2021, Nano Letters)

   null (Ed.)

   The recently discovered spin defects in hexagonal boron nitride (hBN), a layered van der Waals material, have great potential in quantum sensing. However, the photoluminescence and the contrast of the optically detected magnetic resonance (ODMR) of hBN spin defects are relatively low so far, which limits their sensitivity. Here we report a record-high ODMR contrast of 46% at room temperature and simultaneous enhancement of the photoluminescence of hBN spin defects by up to 17-fold by the surface plasmon of a gold film microwave waveguide. Our results are obtained with shallow boron vacancy spin defects in hBN nanosheets created by low-energy He+ ion implantation and a gold film microwave waveguide fabricated by photolithography. We also explore the effects of microwave and laser powers on the ODMR and improve the sensitivity of hBN spin defects for magnetic field detection. Our results support the promising potential of hBN spin defects for nanoscale quantum sensing. 

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4. Hysteresis-free high mobility graphene encapsulated in tungsten disulfide

   https://doi.org/10.1063/5.0151273  

   Soundarapandian, Karuppasamy Pandian; De Fazio, Domenico; Bernal-Texca, Francisco; Hoffmann, Rebecca; Ceccanti, Matteo; De Bonis, Sergio L.; Tongay, Sefaattin; Koppens, Frank H. (August 2023, Applied Physics Letters)

   High mobility is a crucial requirement for a large variety of electronic device applications. The state of the art for high-quality graphene devices is based on heterostructures made with graphene encapsulated in >40 nm-thick flakes of hexagonal boron nitride (hBN). Unfortunately, scaling up multilayer hBN while precisely controlling the number of layers remains an outstanding challenge, resulting in a rough material unable to enhance the mobility of graphene. This leads to the pursuit of alternative, scalable materials, which can be used as substrates and encapsulants for graphene. Tungsten disulfide (WS2) is a transition metal dichalcogenide, which was grown in large (∼mm-size) multi-layers by chemical vapor deposition. However, the resistance vs gate voltage characteristics when gating graphene through WS2 exhibit largely hysteretic shifts of the charge neutrality point on the order of Δn∼ 3 × 1011 cm−2, hindering the use of WS2 as a reliable encapsulant. The hysteresis originates due to the charge traps from sulfur vacancies present in WS2. In this work, we report the use of WS2 as a substrate and overcome the hysteresis issues by chemically treating WS2 with a super-acid, which passivates these vacancies and strips the surface from contaminants. The hysteresis is significantly reduced by about two orders of magnitude, down to values as low as Δn∼ 2 × 109 cm−2, while the room-temperature mobility of WS2-encapsulated graphene is as high as ∼62 × 103 cm2 V−1 s−1 at a carrier density of n ∼ 1 ×1012 cm−2. Our results promote WS2 as a valid alternative to hBN as an encapsulant for high-performance graphene devices. 

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5. Rotationally Aligned Hexagonal Boron Nitride on Sapphire by High-Temperature Molecular Beam Epitaxy

   Ryan Page1, Yongjin Cho2 (July 2019, Physical review and Physical review letters index)

   Hexagonal boron nitride (hBN) has been grown on sapphire substrates by ultrahigh-temperature molecular beam epitaxy (MBE). A wide range of substrate temperatures and boron fluxes have been explored, revealing that high crystalline quality hBN layers are grown at high substrate temperatures, >1600℃ , and low boron fluxes, ∼1 × 10%& Torr beam equivalent pressure. In situ reflection high-energy electron diffraction revealed the growth of hBN layers with 60° rotational symmetry and the [112+ 0] axis of hBN parallel to the [11+ 00] axis of the sapphire substrate. Unlike the rough, polycrystalline films previously reported, atomic force microscopy and transmission electron microscopy characterization of these films demonstrate smooth, layered, few-nanometer hBN films on a nitridated sapphire substrate. This demonstration of high-quality hBN growth by MBE is a step toward its integration into existing epitaxial growth platforms, applications, and technologies. 

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