What is it about?

Layered materials with the Van der Waals gap have been extensively studied due to their fascinating properties. However, non-Van der Waals type layered Bi2O2Se exhibiting remarkable properties is challenging to grow due to the weak electrostatic interaction among layers. Herein, we present a chemical vapor deposition (CVD) growth of air-stable ultrathin Bi2O2Se semiconductor with high structural and chemical uniformity. By tuning the growth temperature, we obtained ultra-smooth single crystals of few-layer Bi2O2Se (LBOS) on mica and quartz substrates, as confirmed from X-ray diffraction, micro-Raman, and high-resolution transmission electron microscopy (TEM) analyses. Further, a low-temperature Raman study has been conducted to better realize phonon dispersion in the as-grown LBOS in the temperature range 78-293 K. It is observed that the A1g phonon mode frequency of LBOS varies linearly with the temperature with a first-order temperature coefficient (α) is - 0.01787 ± 0.0011 cm-1K-1. The broadening of the Raman spectral linewidth with temperature has been explained based on the phonon decay, and a phonon lifetime of 2.08 ps is found for LBOS at absolute zero temperature. Finally, the in-plane thermal conductivity of LBOS is estimated by a non-contact measurement technique in a relatively straightforward way. Taking advantage of the excitation power dependency of A1g mode and using the first-order temperature coefficient, the in-plane thermal conductivity of LBOS is estimated to be ~1.6 W/m.K. Our results pave the way for large-area CVD growth of LBOS on arbitrary substrates and developing insights into electron-phonon and phonon- phonon interactions in non-Van der Waals 2D materials.

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Why is it important?

The performance of electronic devices has a fundamental dependency on the thermal conductivity of the materials used. Thermal conductivity measurement is a unique tool to understand the nanosystems better for scheming heat transport in broad areas of technological applications. There are vast applications where we demand remarkably low thermal conductivity, such as thermal wall coverings, thermoelectric energy reaping, and solid-state cooling [20]. Similarly, a material with high thermal conductivity can rapidly dissipate heat, halting device damage[21]. The vibrational properties of BOS are also fundamental to learning the electron−phonon interaction and the transport attributes and their profound influence on the performance of electronic devices [20, 22, 23]. These traits are novel to apprehend anharmonicity in the structure-potential energy. In layered materials like graphene, the in-plane thermal conductivity is significantly higher than cross-plane because the layers in such cases are accumulated with soft van der Waals forces[24]. In contrast to graphene-like materials, in Bi2O2Se, the layers stacked with soft electrostatic forces might lead to lower in-plane thermal conductivity than cross-plane. Although experimental studies of thermal conductivities of Van der Waals materials have been broadly studied, in the case of non-Van der Waals materials, it is very limited. Recently, Yang et al. reported a very low in-plane thermal conductivity ~0.92 W/mK in a few-layered Bi2O2Se because of low phonon group velocity and phonon dispersion in 2D layered Bi2O2Se.[25] Note that the thermal conductivity was determined based on low phonon group velocity, strong anharmonicity, and large surface scattering of acoustic phonons of the Bi2O2Se. However, it would be interesting to measure the thermal conductivity of LBOS in a more straightforward way and compare it with other conventional 2D materials.

Perspectives

Layered materials with the Van der Waals gap have been extensively studied due to their fascinating properties. However, non-Van der Waals type layered Bi2O2Se exhibiting remarkable properties is challenging to grow due to the weak electrostatic interaction among layers. Herein, we present a chemical vapor deposition (CVD) growth of air-stable ultrathin Bi2O2Se semiconductor with high structural and chemical uniformity In contrast to graphene-like materials, in Bi2O2Se, the layers stacked with soft electrostatic forces might lead to lower in-plane thermal conductivity than cross-plane. Although experimental studies of thermal conductivities of Van der Waals materials have been broadly studied, in the case of non-Van der Waals materials, it is very limited. Recently, Yang et al. reported a very low in-plane thermal conductivity ~0.92 W/mK in a few-layered Bi2O2Se because of low phonon group velocity and phonon dispersion in 2D layered Bi2O2Se.[25] Note that the thermal conductivity was determined based on low phonon group velocity, strong anharmonicity, and large surface scattering of acoustic phonons of the Bi2O2Se. However, it would be interesting to measure the thermal conductivity of LBOS in a more straightforward way and compare it with other conventional 2D materials.

Pravat Giri
Indian Institute of Technology Guwahati

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This page is a summary of: Temperature-dependent Raman studies and thermal conductivity of direct CVD grown non-van der Waals layered Bi2O2Se, Journal of Applied Physics, May 2021, American Institute of Physics,
DOI: 10.1063/5.0049368.
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