arXiv subjects

Faris Horani

Publications and source records attributed to Faris Horani.

7 recordsLinked to original sources

arXiv2026

Synthesis of the Elusive Bulk Iodide Double-Perovskite Semiconductor, Cs2AgBiI6: Microcrystals and Photoconductive Films

Three-dimensional (3D) iodide double-perovskite (elpasolite) semiconductors have attracted interest as potential lead-free metal-halide absorber layers for solar applications. Although studied extensively by computational methods, they have remained largely inaccessible synthetically, consistent with their predicted thermodynamic instability. Here, we report the first synthesis of bulk Cs2AgBiI6, demonstrating both microcrystalline and thin-film forms. Microcrystalline powders of Cs2AgBiI6 were prepared via anion exchange from phase-pure Cs2AgBiBr6 microcrystals. The resulting iodide elpasolite shows broad absorption throughout the visible with a 1.70 +/- 0.05 eV optical bandgap and near-infrared photoluminescence centered at 1.03 eV. We identify the elimination of trace moisture in bulk Cs2AgBiBr6 as the critical factor enabling complete halide exchange and isolation of bulk Cs2AgBiI6 with phase purity. In inert atmosphere, microcrystalline Cs2AgBiI6 shows no decomposition when stored for months at room temperature or heated to ~70 {\deg}C, and it appears equally stable in dry air. Building upon these insights, we then demonstrate the preparation of phase-pure Cs2AgBiI6 films by flash thermal evaporation of Cs2AgBiBr6 followed by anion exchange. Photoconductivity measurements on such Cs2AgBiI6 films demonstrate photocarrier generation and transport, marking the first optoelectronic measurement on this elusive 3D iodide double perovskite.

cond-mat.mtrl-sci↗

arXiv2026

Photon Statistics from Yb3+-Doped CsPbCl3 are Inconsistent with Quantum Cutting

CsPb(Cl1-xBrx)3:Yb3+ has been widely reported as a broadband quantum-cutting material with a photoluminescence quantum yield exceeding 100%, making it a promising candidate for enhancing the blue-green spectral response of silicon photovoltaics. Many groups have reproduced absolute photoluminescence quantum yields over 100%, but others have struggled to obtain such high values. Here, we test the quantum-cutting capabilities of CsPbCl3:Yb3+ nanocrystals and bulk material using photon-correlation analysis. A quantum-cutting material is expected to exhibit photon bunching, but our experiments on CsPbCl3:Yb3+ show no such behavior. In fact, we observe the opposite -- anti-bunching -- under focused-excitation conditions. This observation can be explained with the previously established Auger-quenching pathway in CsPbCl3:Yb3+. Our results thus confirm high-power Auger quenching but question earlier descriptions of quantum cutting in CsPb(Cl1-xBrx)3:Yb3+.

cond-mat.mtrl-sci↗

arXiv2026

Crystal Anisotropy Implications on the Magneto-Optical Properties of van der Waals FePS3

Antiferromagnetic FePS3 has recently gained significant interest in its potential applications in spin-related devices. Here, we show that in-plane structural anisotropy has a major impact in shaping the optical responses of FePS3 single-crystals from the bulk form down to the monolayer limit. X-ray diffraction on a bulk FePS3 crystal confirms a distorted FeS6 octahedron causing inequivalent Fe-Fe distances and consequently resulting in a higher a/b lattice parameter ratio. Micro-photoluminescence observations on bulk and monolayer FePS3 reveal four emissions: one intra-atomic d-d transition (band A, centered at ~1.24 eV) and three p-d charge transfer transitions (bands B, C, and D, centered around ~1.79 eV, ~2.3 eV, and ~2.56 eV, respectively). These bands exhibit different polarization behaviors, which persist down to the monolayer limit. Density functional theory calculations from bulk to monolayer FePS3 reveal the underlying electronic structure, assign the observed emissions, and indicate why these peaks have contrasting linear and circular polarization responses. These results establish a direct structure-optics relation in FePS3, highlighting the strong coupling between lattice anisotropy, electronic transitions, and symmetry-selective optical selection rules.

cond-mat.mtrl-sci↗

arXiv2025

Optical Spin Sensing and Metamagnetic Phase Control in the 2D Van der Waals Magnet Yb3+-Doped CrPS4

The emergence of two-dimensional magnets within the van der Waals toolkit has introduced unprecedented opportunities to develop ultrathin spintronic technologies. Strong coupling between spin and optical properties in such materials can further enable novel spin-photonic capabilities of both fundamental and technological interest. Here, we investigate the optical and spin properties of the air-stable, layered A-type antiferromagnet chromium thiophosphate (CrPS4) when doped with Yb3+. We show that the collective spin properties of CrPS4 are encoded in the sharp f-f luminescence of isolated Yb3+ dopants via strong magnetic superexchange coupling between the two, and that spontaneous magnetic ordering in CrPS4 induces large exchange splittings in the narrow Yb3+ f-f photoluminescence features below TN. Spin reorientation in CrPS4 via a "spin-flop" metamagnetic transition modulates the Yb3+ f-f luminescence energies and exchange splittings. This pronounced link between spin and optical properties enables the demonstration of optically driven spin-flop transitions in CrPS4.

cond-mat.mtrl-sci↗

arXiv2024

Crystal anisotropy implications on the intrinsic magnetic and optical properties in van der Waals FePS3

Antiferromagnetic (AFM) FePS3 has gained significant interest recently for its potential applications in spin-related devices. A single layer is comprised of a honeycomb network, stabilized by long-range spin-exchange interactions, with a zigzag or Neel arrangement of the Fe-atoms. This study exposed, for the first time, a strong impact of lateral crystal distortion on the magnetic arrangement and optical properties of FePS3. This impact was deciphered by correlating photoluminescence (PL) observations with single-crystal XRD which uncovered anisotropy in the a/b crystallographic plane. Thus, induceing a breakage in the inversion symmetry in FePS3 causing changes in it's electronic and optical transitions. The MPL observations exhibited an unexpected band-edge circularly polarized recombination emission, while off-band-edge transitions were linearly polarized. Also, temperature-dependent MPL measurements reflected zigzag-AFM at low temperatures and the coexistence of zigzag or Neel at mid temperatures. Theoretical calculation implementing anisotropy in spin-exchange interactions among Fe atom's nearest neighbors revealed stabilized zigzag arrangement tilted away from the a-axis. Furthermore, DFT calculations of the electronic band-edge predicted split states in degenerate symmetric points (K+/K-) for zigzag structure and non-degenerate for the Neel arrangement. Highlighting the importance of the inclusion of a crystallographic anisotropy parameter for the simulation of the experimental observations.

cond-mat.mtrl-sci↗

arXiv2024

Tuning Magnetic and Optical Properties in MnxZn1-xPS3 Single Crystals by the Alloying Composition

The exploration of two-dimensional (2D) antiferromagnetic (AFM) materials has shown great promise and interest in tuning the magnetic and electronic properties as well as studying magneto-optical effects. The current work investigates the control of magneto-optical interactions in alloyed MnxZn1-xPS3 lamellar semiconductor single crystals, with the Mn/Zn ratio regulating the coupling strength. Magnetic susceptibility results show a retention of AFM order followed by a decrease in Néel temperatures down to ~ 40% Mn concentration, below which a paramagnetic behavior is observed. Absorption measurements reveal an increase in bandgap energy with higher Zn(II) concentration, and the presence of Mn(II) d-d transition below the absorption edge. DFT+U approach qualitatively explained the origin and the position of the experimentally observed mid band-gap states in pure MnPS3, and corresponding peaks visible in the alloyed systems MnxZn1-xPS3. Accordingly, emission at 1.3 eV in all alloyed compounds results from recombination from a 4T1g Mn(II) excited state to a hybrid p-d state at the valence band. Most significant, temperature-dependent photoluminescence (PL) intensity trends demonstrate strong magneto-optical coupling in compositions with x > 0.65. This study underscores the potential of tailored alloy compositions as a means to control magnetic and optical properties in 2D materials, paving the way for advances in spin-based technologies.

cond-mat.mtrl-sci↗

arXiv2024

Lattice-dynamics and in-plane antiferromagnetism in MnxZn1_xPS3 across the entire composition range

Alloyed MnxZn1_xPS3 samples have been grown covering the whole compositional range and studied by means of Raman spectroscopy at temperatures covering from 4K up to 850K. Our results, supported by SQUID magnetic measurements, allowed, from one hand, to complete the magnetic phase diagram of MnxZn1_xPS3 and establish x>0.3 as the composition at which the alloy retains antiferromagnetism and, from the other hand, to identify the Raman signatures indicative of a magnetic transition. The origin of these Raman signatures is discussed in terms of spin-phonon coupling resulting in the appearance of low- and high-frequency zone-folded phonon modes. For the alloy, an assignment of the 1st and 2nd order modes is provided with the aid of first-principle lattice-dynamical calculations. The compositional dependence of all phonon modes is described and the presence of zone-folded modes is shown to take place for both, the alloy and MnPS3. Finally, a comparison of the Raman spectra of ZnPS3 to other compounds of the transition-metal phosphorous trisulfide family allowed shows that low-frequency phonon peaks exhibit an abnormally large broadening. This is consistent with previous claims on the occurrence of a second-order Jahn-Teller effect that takes place for ZnPS3 and Zn-rich MnxZn1_xPS3.

cond-mat.mtrl-sci↗