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1. Effect of shape anisotropy in nanocrystals of semiconductors with small spin–orbit splitting

   https://doi.org/10.1063/5.0187728  

   Goupalov, S. V. (March 2024, The Journal of Chemical Physics)

   We derive an effective spin-Hamiltonian accounting for the shape anisotropy of the zinc blende semiconductor nanocrystals within the k · p formalism explicitly taking into account the spin–orbit split-off valence band. It is shown that, for small InP nanocrystals, neglect of the spin–orbit split-off band can lead to significant underestimation of one of the two parameters determining the exciton fine-structure splittings. This parameter is only important for nanocrystals with shape anisotropy. 

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2. Chiral Perovskite Nanoplatelets with Tunable Circularly Polarized Luminescence in the Strong Confinement Regime

   https://doi.org/10.1002/adom.202203125  

   Cao, Qinxuan; Song, Ruyi; Chan, Christopher_C_S; Wang, Zhiyu; Wong, Pui_Ying; Wong, Kam_Sing; Blum, Volker; Lu, Haipeng (March 2023, Advanced Optical Materials)

   Abstract Chiral perovskite nanocrystals have emerged as an interesting chiral excitonic platform that combines both structural flexibility and superior optoelectronic properties. Despite several recent demonstrations of optical activity in various chiral perovskite nanocrystals, efficient circularly polarized luminescence (CPL) with tunable energies remains a challenge. The chirality imprinting mechanism as a function of perovskite nanocrystal dimensionality remains elusive. Here, atomically thin inorganic perovskite nanoplatelets (NPLs) are synthesized with precise control of layer thickness and are functionalized by chiral surface ligands, serving as a unique platform to probe the chirality transfer mechanism at the organic/perovskite interface. It is found that chirality is successfully imprinted into mono‐, bi‐, and tri‐layer inorganic perovskite NPLs, exhibiting tunable circular dichroism (CD) and CPL responses. However, chirality transfer decreases in thicker NPLs, resulting in decreased CD and CPL dissymmetry factors for thicker NPLs. Aided by large‐scale first‐principles calculations, it is proposed that chirality transfer is mainly mediated through a surface distortion rather than a hybridization of electronic states, giving rise to symmetry breaking in the perovskite lattice and spin‐split conduction bands. The findings described here provide an in‐depth understanding of chirality transfer and design principles for distorted‐surface perovskites for chiral photonic applications. 

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3. Circular photogalvanic spectroscopy of Rashba splitting in 2D hybrid organic–inorganic perovskite multiple quantum wells

   https://doi.org/10.1038/s41467-019-14073-6  

   Liu, Xiaojie; Chanana, Ashish; Huynh, Uyen; Xue, Fei; Haney, Paul; Blair, Steve; Jiang, Xiaomei; Vardeny, Z. V. (January 2020, Nature Communications)

   Abstract The two-dimensional (2D) Ruddlesden−Popper organic-inorganic halide perovskites such as (2D)-phenethylammonium lead iodide (2D-PEPI) have layered structure that resembles multiple quantum wells (MQW). The heavy atoms in 2D-PEPI contribute a large spin-orbit coupling that influences the electronic band structure. Upon breaking the inversion symmetry, a spin splitting (‘Rashba splitting’) occurs in the electronic bands. We have studied the spin splitting in 2D-PEPI single crystals using the circular photogalvanic effect (CPGE). We confirm the existence of Rashba splitting at the electronic band extrema of 35±10 meV, and identify the main inversion symmetry breaking direction perpendicular to the MQW planes. The CPGE action spectrum above the bandgap reveals spin-polarized photocurrent generated by ultrafast relaxation of excited photocarriers separated in momentum space. Whereas the helicity dependent photocurrent with below-gap excitation is due to spin-galvanic effect of the ionized spin-polarized excitons, where spin polarization occurs in the spin-split bands due to asymmetric spin-flip. 

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4. Synthesis of TlBr and Tl ₂ AgBr ₃ Nanocrystals

   https://doi.org/10.1002/cnma.202000100  

   Siegler, Timothy_D; Shah, Tushti; Houck, Daniel_W; Suri, Mokshin; Zhang, Yangning; Korgel, Brian_A (March 2020, ChemNanoMat)

   Abstract There are only a few examples of nanocrystal synthesis with thallium (Tl). Here, we report the synthesis of uniform, ligand‐stabilized colloidal nanocrystals of TlBr and Tl₂AgBr₃nanocrystals with average diameter ranging between 10 and 20 nm. TlBr nanocrystals are made by hot injection of trimethylsilyl bromide (TMSBr) into solutions of oleylamine, oleic acid and octadecene with thallium (III) or thallium (I) acetate. Tl₂AgBr₃nanocrystals form when silver (I) acetate is included in the reaction. The TlBr nanocrystals have CsCl crystal structure with a direct band gap of 3.1 eV. The Tl₂AgBr₃nanocrystals have trigonal dolomite crystal structure with an indirect band gap of 3.1 eV. The TlBr nanocrystals made with thallium (III) were sufficiently uniform to assemble into face‐centered cubic (fcc) superlattices. 

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5. Pure spin current injection of single-layer monochalcogenides

   https://doi.org/10.1088/2053-1591/acbf99  

   Mendoza, Bernardo S; Grillo, Simone; Juárez-Reyes, Lucila; Fregoso, Benjamin M (March 2023, Materials Research Express)

   Cao, Yi; Wu, Judy (Ed.)

   Abstract We compute the spectrum of pure spin current injection in ferroelectric single-layer SnS, SnSe, GeS, and GeSe. The formalism takes into account the coherent spin dynamics of optically excited conduction states split in energy by spin–orbit coupling. The velocity of the electron’s spins is calculated as a function of incoming photon energy and angle of linearly polarized light within a full electronic band structure scheme using density functional theory. We find peak speeds of 520, 360, 270 and 370 Km s⁻¹for SnS, SnSe, GeS and GeSe, respectively which are an order of magnitude larger than those found in bulk semiconductors, e.g., GaAs and CdSe. Interestingly, the spin velocity is almost independent of the direction of polarization of light in a range of photon energies. Our results demonstrate that single-layer SnS, SnSe, GeS and GeSe are candidates to produce on demand spin-current in spintronics applications. 

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