Raman Spectroscopy, Photocatalytic Degradation, and Stabilization of Atomically Thin Chromium Tri-iodide (original) (raw)
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the implementation of new materials, apart from silicon, to accomplish the ambitious technological aims. [4-6] A promising approach to supersede Si electronic devices is the implementation of spintronic components, like in ultrathin CrX 3 layers (X = Cl, Br, or I), that use the electron spins as information memory instead of charge carriers. [7-11] Electrically insulating chromium(III) halides CrX 3 (X = Cl, Br, I) exhibit strongly anisotropic magnetic properties, even in a single layer. [12] By reason of the Cr 3+ ions are in 3d 3 electronic configuration with total spin 3/2 that form isotypic honeycomb nets, Huang et al. finally reported on the first experimental proof using CrI 3 (bulk T C = 61 K), which showed that the respective monolayer (T C = 45 K) and trilayer are ferromagnetic, while the bilayer couples antiferromagnetically. [13,14] The CrX 3 anisotropy in atomically thin layers makes them intriguing candidates for sensing applications or future magnetoelectronics. Recently, Zhang et al. reported on similar investigation of monolayer CrBr 3. [15] Moreover, the efficiency of CrCl 3 , used as catalyst material, could be enhanced due to nanoscaling and an enlarged surface-to-volume ratio, similar to a recently shown approach of TiCl 3. [16,17] However, the preparation of highly crystalline CrX 3 nanolayers down to the monolayer limit is still an experimental challenge. The commonly employed state-of-the-art exfoliation technique proceeding from prior synthesized bulk CrX 3 platelets lacks due to absolute nonreproducibility. Ideally, in trigonal CrBr 3 each The experimental observation of intrinsic ferromagnetism in single layered chromium trihalides CrX 3 (X = Cl, Br, I) has gained outstanding attention recently due to their possible implementation in spintronic devices. However, the reproducible preparation of highly crystalline chromium(III) halide nanolayers without stacking faults is still an experimental challenge. As chromium trihalides consist of adjacent layers with weak interlayer coupling, the preparation of ultrathin CrX 3 nanosheets directly on substrates via vapor transport proves as an advantageous synthesis technique. It is demonstrated that vapor growth of ultrathin highly crystalline CrX 3 micro-and nanosheets succeeds directly on yttrium stabilized zirconia substrates in a one-step process via chemical vapor transport (CVT) in temperature gradients of 100 K (600 °C → 500 °C for CrCl 3 and 650 °C → 550 °C for CrBr 3 or CrI 3) without a need for subsequent delamination. Due to simulation results, optimization of synthesis conditions is realized and phase pure CrX 3 nanosheets with thicknesses ≤25 nm are obtained via short term CVT. The nanosheets morphology, crystallinity, and phase purity are analyzed by several techniques, including microscopy, diffraction, and spectroscopy. Furthermore, a potential subsequent delamination technique is demonstrated to give fast access to CrX 3 monolayers using the example of CrCl 3 .
The Journal of Chemical Physics
The first magnetic 2D material discovered, monolayer (ML) CrI 3 , is particularly fascinating due to its ground state ferromagnetism. Yet, because monolayer materials are difficult to probe experimentally, much remains unresolved about ML CrI 3 's structural, electronic, and magnetic properties. Here, we leverage Density Functional Theory (DFT) and high-accuracy Diffusion Monte Carlo (DMC) simulations to predict lattice parameters, magnetic moments, and spin-phonon and spin-lattice coupling of ML CrI 3. We exploit a recently developed surrogate Hessian DMC line search technique to determine CrI 3 's monolayer geometry with DMC accuracy, yielding lattice parameters in good agreement with recently-published STM measurements-an accomplishment given the ∼ 10% variability in previous DFT-derived estimates depending upon the functional. Strikingly, we find previous DFT predictions of ML CrI 3 's magnetic spin moments are correct on average across a unit cell, but miss critical local spatial fluctuations in the spin density revealed by more accurate DMC. DMC predicts magnetic moments in ML CrI 3 are 3.62 µ B per chromium and-0.145 µ B per iodine; both larger than previous DFT predictions. The large disparate moments together with the large spin-orbit coupling of CrI 3 's I-p orbital suggests a ligand superexchange-dominated magnetic anisotropy in ML CrI 3 , corroborating recent observations of magnons in its 2D limit. We also find ML CrI 3 exhibits a substantial spin-phonon coupling of ∼ 3.32 cm −1. Our work thus establishes many of ML CrI 3 's key properties, while also continuing to demonstrate the pivotal role DMC can assume in the study of magnetic and other 2D materials.
Nanoscale, 2017
Half-metallic ferromagnetic materials with planar forms are promising for spintronics applications. A wide range of 2D lattices like graphene, h-BN, transition metal dichalcogenides, etc. are non-magnetic or weakly magnetic. Using first principles calculations, the existence of graphene-like hexagonal chromium nitride (h-CrN) with an almost flat atomically thin structure is predicted. We find that freestanding h-CrN has a 100% spin-polarized half-metallic nature with possible ferromagnetic ordering and a high rate of optical transparency. As a possible method for stabilization and synthesis, deposition of h-CrN on 2D MoSe2 or on MoS2 is proposed. The formation of composites retains the half-metallic properties and leads to the reduction of spin-down band gaps to 1.43 and 1.71 eV for energetically favorable h-CrN/MoSe2 and h-CrN/MoS2 configurations, respectively. Calculation of the dielectric functions of h-CrN, h-CrN/MoSe2 and h-CrN/MoS2 exhibit the high transparency of all three lo...
Journal of Chemical Physics, 2022
The first magnetic 2D material discovered, monolayer (ML) CrI 3 , is particularly fascinating due to its ground state ferromagnetism. Yet, because monolayer materials are difficult to probe experimentally, much remains unresolved about ML CrI 3 's structural, electronic, and magnetic properties. Here, we leverage Density Functional Theory (DFT) and high-accuracy Diffusion Monte Carlo (DMC) simulations to predict lattice parameters, magnetic moments, and spin-phonon and spin-lattice coupling of ML CrI 3. We exploit a recently developed surrogate Hessian DMC line search technique to determine CrI 3 's monolayer geometry with DMC accuracy, yielding lattice parameters in good agreement with recently-published STM measurements-an accomplishment given the ∼ 10% variability in previous DFT-derived estimates depending upon the functional. Strikingly, we find previous DFT predictions of ML CrI 3 's magnetic spin moments are correct on average across a unit cell, but miss critical local spatial fluctuations in the spin density revealed by more accurate DMC. DMC predicts magnetic moments in ML CrI 3 are 3.62 µ B per chromium and-0.145 µ B per iodine; both larger than previous DFT predictions. The large disparate moments together with the large spin-orbit coupling of CrI 3 's I-p orbital suggests a ligand superexchange-dominated magnetic anisotropy in ML CrI 3 , corroborating recent observations of magnons in its 2D limit. We also find ML CrI 3 exhibits a substantial spin-phonon coupling of ∼ 3.32 cm −1. Our work thus establishes many of ML CrI 3 's key properties, while also continuing to demonstrate the pivotal role DMC can assume in the study of magnetic and other 2D materials.
ACS Omega
In past years, the presence of toxic heavy metal ions in water and soil has caused major health problems. The ternary type semiconductor material, reduced graphene oxide (rGO)− Sm 2 MoO 6 −TiO 2 , has been investigated as a photocatalyst for the reduction of soluble chromium(VI) into (III) for the first time. The as-synthesized rGO−Sm 2 MoO 6 −TiO 2 catalyst was analyzed by Xray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy, X-ray photoelectron spectroscopy, FT-Raman, Fourier transform infrared, and optical spectroscopy. The maximum Cr(VI) reduction of 96% was achieved within 70 min under visible light illumination. The powder XRD analysis confirmed the formation of anatase TiO 2. Field-emission SEM images depicted well-dispersed rGO sheets, and TiO 2 and Sm 2 MoO 6 particles are randomly distributed onto rGO. The reduction of Cr(VI) by rGO−Sm 2 MoO 6 −TiO 2 was considerably greater than the reduction by Sm 2 MoO 6 , TiO 2 , Sm 2 MoO 6 −rGO, TiO 2 −rGO, and Sm 2 MoO 6 −TiO 2. Sm 2 MoO 6 acts as an effective cocatalyst to TiO 2 to enhance the separation of photo-generated electron−holes. Even after six consecutive cycles, the photoreduction of Cr(VI) was more than 85%, which reveals that the excellent reusability performance of the catalyst for practical applications. The photogenerated electron plays an important role in the reduction of Cr(VI) into nontoxic Cr(III), and the synergistic effect of rGO greatly improved the separation of h + and e − pairs.
Graphene as an Ideal Buffer Layer for the Growth of High-Quality Ultrathin Cr2O3Layers on Ni(111)
ACS Nano, 2019
Metal-oxide nanostructures play a fundamental role in a large number of technological applications, ranging from chemical sensors to data storage devices. As the size of the devices shrinks down to the nanoscale, it is mandatory to get sharp and good quality interfaces. Here, it is shown that a two-dimensional material, namely graphene, can be exploited as an ideal buffer layer to tailor the properties of the interface between a metallic substrate and an ultrathin oxide. This is proven at the interface between an ultrathin film of the magnetoelectric antiferromagnetic oxide Cr 2 O 3 and a Ni(111) single crystal substrate. The chemical composition of the samples has been studied by means of X-ray photoemission spectroscopy, showing that the insertion of graphene, which remains buried at the interface, is able to prevent the oxidation of the substrate. This protective action leads to an ordered and layer-by-layer growth, as revealed by scanning tunneling microscopy data. The structural 1
Charge recombination a b s t r a c t Cr-doped-TiO 2 thin films, with three different Cr concentrations (2, 5.5, and 9 at.%), have been synthesized by radio-frequency magnetron sputtering in order to sensitize TiO 2 in visible light. UVevisible spectra showed that maximum narrowing (2.1 eV) of the TiO 2 band gap is obtained for the highest Cr concentration. However, negligible photocurrent was measured with Indium Tin Oxide (ITO)/Cr-doped-TiO 2 (9 at.%) single bilayer sample due to the increased recombination rate of the photo-generated charges on the defects associated to the Cr 3þ ions. To lower the charge recombination rate in the Cr-doped-TiO 2 , multilayer films with different numbers of ITO/Cr-doped-TiO 2 (9 at.%) bilayers (namely, 3-, 4-, 5-, 6and 7-bilayers) were deposited by keeping the total thickness of TiO 2 constant in each multilayer film. When the multilayer films were exposed to visible light, we observed that the photocurrent increases as function of the number of bilayers by reaching the maximum with 6-bilayers of ITO/Cr-doped-TiO 2 . The enhanced photocurrent is attributed to: 1) higher absorption of visible light by Cr-doped-TiO 2 , 2) number of space charge layers in form of ITO/TiO 2 interfaces in multilayer films, and 3) generation of photoelectrons just in/or near to the space charge layer by decreasing the Cr-doped-TiO 2 layer thickness. The reduced charge recombination rate in multilayer films was also confirmed by studying the photocurrent kinetic curve. The superior photocatalytic efficiency of the 6-bilayers film implies higher hydrogen production rate through water-splitting: we obtained indeed 24.4 mmol/h of H 2 production rate, a value about two times higher than that of pure TiO 2 (12.5 mmol/h). (R. Dholam).