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1. Anisotropic Optical Properties of 2D Silicon Telluride

   https://doi.org/10.1557/adv.2020.186  

   Bhattarai, Romakanta; Chen, Jiyang; Hoang, Thang B.; Cui, Jingbiao; Shen, Xiao (April 2020, MRS Advances)

   ABSTRACT Silicon telluride (Si2Te3) is a silicon-based 2D chalcogenide with potential applications in optoelectronics. It has a unique crystal structure where Si atoms form Si-Si dimers to occupy the “metal” sites. In this paper, we report an ab initio computational study of its optical dielectric properties using the GW approximation and the Bethe-Salpeter equation (BSE). Strong in-plane optical anisotropy is discovered. The imaginary part of the dielectric constant in the direction parallel to the Si-Si dimers is found to be much lower than that perpendicular to the dimers. The optical measurement of the absorption spectra of 2D Si2Te3 nanoplates shows modulation of the absorption coefficient under 90-degree rotation, confirming the computational results. We show the optical anisotropy originates from the particular compositions of the wavefunctions in the valence and conduction bands. Because it is associated with the Si dimer orientation, the in-plane optical anisotropy can potentially be dynamically controlled by electrical field and strain, which may be useful for new device design. In addition, BSE calculations reduce GW quasiparticle band gap by 0.3 eV in bulk and 0.6 eV in monolayer, indicating a large excitonic effect in Si2Te3. Furthermore, including electron-hole interaction in bulk calculations significantly reduces the imaginary part of the dielectric constant in the out-of-plane direction, suggesting strong interlayer exciton effect in Si2Te3 multilayers. 

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2. Raman study of silicon telluride nanoplates and their degradation

   https://doi.org/10.1088/1361-6528/ac5c13  

   Hathaway, Evan; Chen, Jiyang; Gonzalez-Rodriguez, Roberto; Lin, Yuankun; Cui, Jingbiao (April 2022, Nanotechnology)

   Abstract Silicon telluride (Si 2 Te 3 ) has emerged as one of the many contenders for 2D materials ideal for the fabrication of atomically thin devices. Despite the progress which has been made in the electric and optical properties of silicon telluride, much work is still needed to better understand this material. We report here on the Raman study of Si 2 Te 3 degradation under both annealing and in situ heating with a laser. Both processes caused pristine Si 2 Te 3 to degrade into tellurium and silicon oxide in air in the absence of a protective coating. A previously unreported Raman peak at ∼140 cm −1 was observed from the degraded samples and is found to be associated with pure tellurium. This peak was previously unresolved with the peak at 144 cm −1 for pristine Si 2 Te 3 in the literature and has been erroneously assigned as a signature Raman peak of pure Si 2 Te 3 , which has caused incorrect interpretations of experimental data. Our study has led to a fundamental understanding of the Raman peaks in Si 2 Te 3 , and helps resolve the inconsistent issues in the literature. This study is not only important for fundamental understanding but also vital for material characterization and applications. 

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3. 3D Printing of Solution‐Processable 2D Nanoplates and 1D Nanorods for Flexible Thermoelectrics with Ultrahigh Power Factor at Low‐Medium Temperatures

   https://doi.org/10.1002/advs.201901788  

   Dun, Chaochao; Kuang, Wenzheng; Kempf, Nicholas; Saeidi‐Javash, Mortaza; Singh, David_J; Zhang, Yanliang (October 2019, Advanced Science)

   Abstract Solution‐processable semiconducting 2D nanoplates and 1D nanorods are attractive building blocks for diverse technologies, including thermoelectrics, optoelectronics, and electronics. However, transforming colloidal nanoparticles into high‐performance and flexible devices remains a challenge. For example, flexible films prepared by solution‐processed semiconducting nanocrystals are typically plagued by poor thermoelectric and electrical transport properties. Here, a highly scalable 3D conformal additive printing approach to directly convert solution‐processed 2D nanoplates and 1D nanorods into high‐performing flexible devices is reported. The flexible films printed using Sb₂Te₃nanoplates and subsequently sintered at 400 °C demonstrate exceptional thermoelectric power factor of 1.5 mW m⁻¹K⁻²over a wide temperature range (350–550 K). By synergistically combining Sb₂Te₃2D nanoplates with Te 1D nanorods, the power factor of the flexible film reaches an unprecedented maximum value of 2.2 mW m⁻¹K⁻²at 500 K, which is significantly higher than the best reported values for p‐type flexible thermoelectric films. A fully printed flexible generator device exhibits a competitive electrical power density of 7.65 mW cm⁻²with a reasonably small temperature difference of 60 K. The versatile printing method for directly transforming nanoscale building blocks into functional devices paves the way for developing not only flexible energy harvesters but also a broad range of flexible/wearable electronics and sensors. 

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4. Transport and optical properties of the chiral semiconductor Ag ₃ AuSe ₂

   https://doi.org/10.1002/zaac.202200055  

   Won, Juyeon; Kim, Soyeun; Gutierrez‐Amigo, Martin; Bettler, Simon; Lee, Bumjoo; Son, Jaeseok; Won Noh, Tae; Errea, Ion; Vergniory, Maia G.; Abbamonte, Peter; et al (May 2022, Zeitschrift für anorganische und allgemeine Chemie)

   Abstract Previous band structure calculations predicted Ag₃AuSe₂to be a semiconductor with a band gap of approximately 1 eV. Here, we report single crystal growth of Ag₃AuSe₂and its transport and optical properties. Single crystals of Ag₃AuSe₂were synthesized by slow‐cooling from the melt, and grain sizes were confirmed to be greater than 2 mm using electron backscatter diffraction. Optical and transport measurements reveal that Ag₃AuSe₂is a highly resistive semiconductor with a band gap and activation energy around 0.3 eV. Our first‐principles calculations show that the experimentally determined band gap lies between the predicted band gaps from GGA and hybrid functionals. We predict band inversion to be possible by applying tensile strain. The sensitivity of the gap to Ag/Au ordering, chemical substitution, and heat treatment merit further investigation. 

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5. Comparison of structural and optical properties of blue emitting In0.15Ga0.85N/GaN multi-quantum-well layers grown on sapphire and silicon substrates

   https://doi.org/10.1063/1.5078743  

   Liu, Richard; McCormick, Callan; Bayram, Can (February 2019, AIP Advances)

   Six periods of 2-nm-thick In0.15Ga0.85N/13-nm-thick GaN blue emitting multi-quantum-well (MQW) layers are grown on sapphire (Al2O3) and silicon (Si) substrates. X-ray diffraction, Raman spectroscopy, atomic force microscopy, temperature-dependent photoluminescence (PL), Micro-PL, and time-resolved PL are used to compare the structural and optical properties, and the carrier dynamics of the blue emitting active layers grown on Al2O3 and Si substrates. Indium clustering in the MQW layers is observed to be more pronounced on Al2O3 than those on Si as revealed through investigating band-filling effects of emission centers, S-shaped peak emission energy shifts with increasing temperature, and PL intensity-peak energy spatial nonuniformity correlations. The smaller indium clustering effects in MQW on Si are attributed to the residual tensile strain in the GaN buffer layer, which decreases the compressive strain and thus the piezoelectric polarization field in the InGaN quantum wells. Despite a 30% thinner total epitaxial thickness of 3.3 µm, MQW on Si exhibits a higher IQE than those on Al2O3 in terms of internal quantum efficiency (IQE) at temperatures below 250 K, and a similar IQE at 300 K (30% vs 33%). These results show that growth of blue emitting MQW layers on Si is a promising approach compared to those conventionally grown on Al2O3. 

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