arXiv subjects

Rijan Karkee

Publications and source records attributed to Rijan Karkee.

6 recordsLinked to original sources

arXiv2024

Panoply of Ni-Doping-Induced Reconstructions, Electronic Phases, and Ferroelectricity in 1T-MoS$_2$

The distorted phases of monolayer 1T-MoS$_2$ have distinct electronic properties, with potential applications in optoelectronics, catalysis, and batteries. We theoretically investigate the use of Ni-doping to generate distorted 1T phases, and find not only the ones usually reported but also two further phases (${3} \times {3}$ and ${4} \times {4}$), depending on the concentration and the substitutional or adatom doping site. Corresponding pristine phases are stable after dopant removal, which might offer a potential route to experimental synthesis. We find large ferroelectric polarizations, most notably in ${3} \times {3}$ which -- compared to the recently measured 1T$''$ -- has 100 times greater ferroelectric polarization, a lower energy, and a larger bandgap. Doped phases include exotic multiferroic semimetals, ferromagnetic polar metals, and improper ferroelectrics with only in-plane polarization switchable. The pristine phases have unusual multiple gaps in the conduction bands, with possible applications for intermediate band solar cells, transparent conductors, and nonlinear optics.

cond-mat.mtrl-sci↗

arXiv2024

Low-energy nine-layer rhombohedral stacking of transition metal dichalcogenides

Transition-metal dichalcogenides (TMDs) show unique physical, optical, and electronic properties. The known phases of TMDs are 2H and 3R in bulk form, 1T and associated reconstructions, and 1H in monolayer form. This paper reports a hypothetical phase, 9R, that may exist in TMDs (Mo, W)(S, Se, Te)$_2$, meeting both dynamical stability and elastic stability criteria. 9R phase has the same space group as 3R, $i.e.$ rhombohedral $R3m$ without inversion symmetry, and has 9 layers in a conventional unit cell. We find that 9R has an energy within 1 meV per formula unit of 3R and can be energetically favored by a particular strain condition. We further calculate the electronic, elastic, piezoelectric, Raman, and second-harmonic generation signatures of 9R TMDs and compare them with the corresponding 2H and 3R phases. 9R has similar properties to 3R but shows distinctive Raman peaks in the low-frequency regime, improved piezoelectric properties, and unique band splitting arising from layer coupling at the conduction band minimum. These distinct properties make 9R an attractive candidate for applications in piezotronics and valleytronics.

cond-mat.mtrl-sci↗

arXiv2022

Surface Effects on Anisotropic Photoluminescence in One-Dimensional Organic Metal Halide Hybrids

One-dimensional (1D) organic metal halide hybrids exhibit strongly anisotropic optical properties, highly efficient light emission, and large Stokes shift, holding promises for novel photodetection and lighting applications. However, the fundamental mechanisms governing their unique optical properties and in particular the impacts of surface effects are not understood. Here, we investigate 1D C4N2H14PbBr4 by polarization-dependent time-averaged and time-resolved photoluminescence (TRPL) spectroscopy, as a function of photoexcitation energy. Surprisingly, we find that the emission under photoexcitation polarized parallel to the 1D metal halide chains can be either stronger or weaker than that under perpendicular polarization, depending on the excitation energy. We attribute the excitation-energy-dependent anisotropic emission to fast surface recombination, supported by first-principles calculations of optical absorption in this material. The fast surface recombination is directly confirmed by TRPL measurements, when the excitation is polarized parallel to the chains. Our comprehensive studies provide a more complete picture for a deeper understanding of the optical anisotropy in 1D organic metal halide hybrids.

cond-mat.mtrl-sci↗

arXiv2022

Bulk Assembly of Organic Metal Halide Nanoribbons

Organic metal halide hybrids with low-dimensional structures at the molecular level have received great attention recently for their exceptional structural tunability and unique photophysical properties. Here we report for the first time the synthesis and characterization of a one-dimensional (1D) organic metal halide hybrid material, which contains metal halide nanoribbons with a width of three octahedral units. It is found that this material with a chemical formula C$_8$H$_{28}$N$_5$Pb$_3$Cl$_{11}$ shows a dual emission with a photoluminescence quantum efficiency (PLQE) of around 25% under ultraviolet (UV) light irradiation. Photophysical studies and density functional theory (DFT) calculations suggest the coexisting of delocalized free excitons and localized self-trapped excitons in metal halide nanoribbons leading to the dual emission. This work shows once again the exceptional tunability of organic metal halide hybrids that bridge between molecular systems with localized states and crystalline ones with electronic bands.

cond-mat.mtrl-sci↗

arXiv2021

Enhanced interlayer interactions in Ni-doped MoS$_2$, and structural and electronic signatures of doping site

The crystal structure of MoS$_2$ with strong covalent bonds in plane and weak Van der Waals interactions out of plane gives rise to interesting properties for applications such as solid lubrication, optoelectronics, and catalysis, which can be enhanced by transition-metal doping. However, the mechanisms for improvement and even the structure of the doped material can be unclear, which we address with theoretical calculations. Building on our previous work on Ni-doping of the bulk 2H phase, now we compare to polytypes (1H monolayer and 3R bulk), to determine favorable sites for Ni and the doping effect on structure, electronic properties, and the layer dissociation energy. The most favorable intercalation/adatom sites are tetrahedral intercalation for 3R (like 2H) and Mo-atop for 1H. The relative energies indicate a possibility of phase change from 2H to 3R with substitution of Mo or S. We find structural and electronic properties that can be used to identify the doping sites, including metallic behavior in Mo-substituted 3R and 2H, and in-gap states for Mo- and S-substituted 1H, which could have interesting optoelectronic applications. We observe a large enhancement in the interlayer interactions of Ni-doped MoS$_2$, opposite to the effect of other transition metals. For lubrication applications, this increased layer dissociation energy could be the mechanism of low wear. Our systematic study shows the effect of doping concentration and we extrapolate to the low-doping limit. This work gives insight into the previously unclear structure of Ni-doped MoS$_2$ and how it can be detected experimentally, the relation of energy and structures of doped monolayers and bulk systems, the electronic properties under doping, and the effect of doping on interlayer interactions.

cond-mat.mtrl-sci↗

arXiv2021

Phase Stability and Raman/IR Signatures of Ni-Doped MoS$_2$ from Density-Functional Theory Studies

Ni-doped MoS$_2$ is a layered material with useful tribological, optoelectronic, and catalytic properties. Experiment and theory on doped MoS$_2$ has focused mostly on monolayers or finite particles: theoretical studies of bulk Ni-doped MoS$_2$ are lacking and the mechanisms by which Ni alters bulk properties are largely unsettled. We use density functional theory calculations to determine the structure, mechanical properties, electronic properties, and formation energies of bulk Ni-doped 2H-MoS$_2$ as a function of doping concentration. We find four meta-stable structures: Mo or S substitution, and tetrahedral (t-) or octahedral (o-) intercalation. We compute phase diagrams as a function of chemical potential to guide experimental synthesis. A convex hull analysis shows that t-intercalation (favored over o-intercalation) is quite stable against phase segregation and in comparison with other compounds containing Ni, Mo, and S; the doping formation energy is around 0.1 meV/atom. Intercalation forms strong interlayer covalent bonds and does not increase the $c$-parameter. Ni-doping creates new states in the electronic density of states in MoS$_2$ and shifts the Fermi level, which are of interest for tuning the electronic and optical properties. We calculate the infrared and Raman spectra and find new peaks and shifts in existing peaks that are unique to each dopant site, and therefore may be used to identify the site experimentally, which has been a challenge to do conclusively.

cond-mat.mtrl-sci↗