Two-dimensional Chromium Research Articles

Extendable piezo/ferroelectricity in nonstoichiometric 2D transition metal dichalcogenides

Engineering piezo/ferroelectricity in two-dimensional materials holds significant implications for advancing the manufacture of state-of-the-art multifunctional materials. The inborn nonstoichiometric propensity of two-dimensional transition metal dichalcogenides provides a spiffy ready-available solution for breaking inversion centrosymmetry, thereby conducing to circumvent size effect challenges in conventional perovskite oxide ferroelectrics. Here, we show the extendable and ubiquitous piezo/ferroelectricity within nonstoichiometric two-dimensional transition metal dichalcogenides that are predominantly centrosymmetric during standard stoichiometric cases. The emerged piezo/ferroelectric traits are aroused from the sliding of van der Waals layers and displacement of interlayer metal atoms triggered by the Frankel defects of heterogeneous interlayer native metal atom intercalation. We demonstrate two-dimensional chromium selenides nanogenerator and iron tellurides ferroelectric multilevel memristors as two representative applications. This innovative approach to engineering piezo/ferroelectricity in ultrathin transition metal dichalcogenides may provide a potential avenue to consolidate piezo/ferroelectricity with featured two-dimensional materials to fabricate multifunctional materials and distinguished multiferroic.

Unraveling the dominance of intrinsic catalytic activities over electrical properties in electrocatalytic performance of two-dimensional chromium chalcogenide nanosheets

In this study, we synthesized a new category of materials, 2D Cr2X3 (where X = S, Se, and Te), using an APCVD method. By leveraging the same space group and identical exposed facets of the as-synthesized Cr2X3 nanoflakes, we compared their catalytic performance for HER and electrical properties to unveil the contribution of electrical conductivity in the overall performance. A descending trend in HER activities from Cr2S3 to Cr2Se3, and Cr2Te3 was found, as evidenced by the increasing overpotentials and decreasing current densities at the same overpotential. Interestingly, electrical properties showed an improving trend from Cr2S3 to Cr2Se3, and Cr2Te3, as revealed by IDS-VG and IDS-VDS tests based on FET devices, respectively. The primary factor affecting overall catalytic performance was found to be intrinsic catalytic activities rather than electrical properties, as supported by DFT calculations. These insights emphasize that the focus of harnessing intrinsic catalytic activities when exploring 2D electrocatalysts.

Theoretical Study of Magnon Spin Currents in Chromium Trihalide Hetero-bilayers: Implications for Magnonic and Spintronic Devices

The intriguing magnetic behavior of two-dimensional chromium trihalide materials holds great promises for spintronics and magnonics. Here, we study the stacking-dependent magnetism in (CrBr3/CrI3) hetero-bilayers by using first-principles calculations. We find that different stackings yield ferromagnetic ground states or favor an antiferromagnetic (AFM) interlayer coupling. Starting from ab initio results, we study the magnonic spectra and magnon spin textures in the hetero-bilayer. Based on our findings, we propose an experimental setup for the optical generation and electric detection of pure magnon spin currents in layered AFM (LAFM) (CrBr3/CrI3) hetero-bilayers. Furthermore, we propose a device to simultaneously realize magnon spin currents and topological domain walls in the hetero-bilayers. The advantages of (CrBr3/CrI3) hetero-bilayers over the (CrI3)2 homo-bilayers to realize magnon spin currents are highlighted. The interesting features of magnons in LAFM hetero-bilayers and their potential for spin-charge conversion could pave the way for their implementation in spin-based transistors and logic devices.

Structural, electronic, and magnetic properties of CrTe2

Two-dimensional chromium ditelluride (${\mathrm{CrTe}}_{2}$) is a promising ferromagnetic layered material that exhibits long-range ferromagnetic ordering in the monolayer limit. The formation energies of the different possible structural phases (1T, 1H, 2H) calculated from density functional theory (DFT) show that the 1T phase is the ground state, and the energetic transition barriers between the phases, calculated by the nudged elastic band method, are large, on the order of 0.5 eV. The self-consistent Hubbard $U$ correction parameters are calculated for all the phases of ${\mathrm{CrTe}}_{2}$. The calculated magnetic moment of 1T-${\mathrm{CrTe}}_{2}$ with $\ensuremath{\ge}2$ layers lies in the plane, whereas the magnetic moment of a monolayer is out-of-plane. Band filling and tensile biaxial strain cause the magnetic moment of a monolayer to switch from out-of-plane to in-plane, and compressive biaxial strain in a bilayer causes the magnetic moment to switch from in-plane to out-of-plane. The magnetic anisotropy is shown to originate from the large spin-orbit coupling (SOC) of the Te atoms and the anisotropy of the exchange coupling constants ${J}_{xy}$ and ${J}_{z}$ in an XXZ type Hamiltonian. Renormalized spin wave theory using experimental values for the magnetic anisotropy energy and Curie temperatures provides a range of values for the nearest neighbor exchange coupling.

Magnetic two-dimensional chromium trihalides: structure, properties and modulation

Magnetic two-dimensional chromium trihalides: structure, properties and modulation

Modeling and Simulation of High-Performance CrTe Intrinsic Half Metal-Based Spin Valve and Spin Diode

Most of the pristine two-dimensional materials such as graphene, silicene, germanene etc. are non-magnetic in nature and the creation of magnetism in these materials is subjected to the doping, external field, vacancy, strain etc., but their control in the experimental is very difficult. That has motivated the researchers for the exploration of 2D materials like Transition metal chalcogenides (TMC) with intrinsic magnetism. In the present study, the ferromagnetism of the two-dimensional Chromium telluride (CrTe) is verified by the spin-dependent density of states. Moreover, taking into consideration the similar crystal structure and comparable lattice constants of Vanadium telluride (VTe) to that of CrTe, we have modelled a three-layer spin valve (CrTe-VTe-CrTe) and a two-layer spin diode (CrTe-VTe), and simulated for spin-dependent transport characteristics. The performance parameters like spin injection efficiency, magnetoresistance, rectification ratio has been calculated to carry out the performance evaluation of the modelled devices. The spin valve displays 100% spin injection efficiency and large magnetoresistance of 3.46 × 108%, a comparative study has been carried out to evaluate the performance of the modelled spin valve. Moreover, the spin diode displays high spin filtering efficiency and good rectification ratio, which suggests the potential spintronic applications of the proposed devices. The spin-dependent transport characteristics have been justified by using spin-dependent transmission spectrum and spin-dependent band structure.

Two-dimensional chromium phosphorus monolayer based gas sensors to detect NOx: A first-principles study

The successive realization of two-dimensional (2D) material sensors to detect diverse gas molecules motivates us to take the first-principles calculations to investigate the adsorption of typical gases molecules (such as NO, NO2, NH3, SO2, H2O, CO2 and CO) on a 2D chromium phosphorus (CrP) monolayer. The adsorption energies are calculated after determining the optimum adsorption location and orientation of these molecules on the CrP monolayer surface. The results display that there is appropriate adsorption strength and obvious charge transfer between the 2D CrP monolayer and NOx (x = 1,2) molecules, indicating that the CrP monolayer is preferable adsorption ability towards NOx. The current–voltage (I-V) curves of the NOx-adsorbed CrP monolayer are further investigated by the nonequilibrium Green's function (NEGF) method. Before and after adsorption of NOx on the CrP monolayer, the transport characteristics show apparent response with a significant change of I-V curves, indicating that the CrP monolayer is a candidate material for high selectivity and sensitivity NOx sensor. These results demonstrate that the CrP monolayer has excellent sensing performance, close to or even better than other 2D materials like phosphorene and Ga-doped graphene, which makes it a potential gas sensor for NOx.

A simple approach to synthesis Cr2CTx MXene for efficient hydrogen evolution reaction

Two-dimensional chromium carbide-based MXenes are predicted as an efficient catalyst for the hydrogen evolution reaction (HER). However, the lack of an effective approach to fabricate this promising material remains an impediment to practical applications. Herein, a simple strategy is proposed to synthesis Cr2CTx ultrathin structures from Cr2AlC MAX. The high-resolution analytical scanning transmission electron microscopy (STEM) corroborated with X-ray diffraction and scanning electron microscopy demonstrates that the Al atomic layers have been detached from the pristine MAX structure. Further atomic-resolution STEM confirms that the ultrathin Cr2CTx sheets with fluoride-free surface, nanostructured Cr2CO2, and Al2O3 phases are originated as by-products. The electrochemical test shows that the fabricated Cr2CTx composite with minimal Al2O3 have a good performance for hydrogen evolution reaction and exceed even that of Pt sheet catalyst at high current density. Moreover, high stability of more than 160 h without attenuation was observed to be better than the other MXenes catalytic performer to date. This work paves a gentle route to fabricate Cr2CTx MXene and demonstrates the potential application for HER.

Magnetic Two-Dimensional Chromium Trihalides: A Theoretical Perspective.

The discovery of ferromagnetic order in monolayer two-dimensional (2D) crystals has opened a new venue in the field of 2D materials. Two-dimensional magnets are not only interesting on their own, but their integration in van der Waals heterostructures allows for the observation of new and exotic effects in the ultrathin limit. The family of chromium trihalides, CrI3, CrBr3, and CrCl3, is so far the most studied among magnetic 2D crystals. In this Mini Review, we provide a perspective of the state of the art of the theoretical understanding of magnetic 2D trihalides, most of which will also be relevant for other 2D magnets, such as vanadium trihalides. We discuss both the well-established facts, such as the origin of the magnetic moment and magnetic anisotropy, and address as well open issues such as the nature of the anisotropic spin couplings and the magnitude of the magnon gap. Recent theoretical predictions on Moiré magnets and magnetic skyrmions are also discussed. Finally, we give some prospects about the future interest of these materials and possible device applications.

Effect of bentonite particles’ presence on two-dimensional chromium transmission

The co-transport of pollutants with colloidal particles to lower depths of groundwater and porous environments has been demonstrated in many studies in recent three decades. Despite the numerous researches, all experimental and numerical studies of pollutant transfer in the presence of colloidal particles have been carried out in one dimension, which causes significant errors in this phenomenon. In this study, the two-dimensional transfer experiment of chromium in the presence of bentonite colloidal particles is done in saturated porous media. In order to conduct the experiment in two-dimensional conditions, the sampling was done in central and lateral of the last experiment column section. The results have been demonstrated that the transmission along the longitudinal direction is higher than lateral in the three tests of the transfer of chromium, bentonite, and chromium in the presence of bentonite colloidal particles at the beginning of the experiment, and due to completed mixing in the section, it reached to a constant value as lateral samples. While the presence of bentonite colloidal particles facilitates the transfer of chromium in both longitudinal and lateral directions, increasing the bentonite colloidal particle concentration causes more getting stuck of colloid particles between the sand grains and reduction of the chromium transfer in both longitudinal and lateral directions. So, it can be concluded that transfer in the lateral direction is lower in bentonite colloidal particles compared with chromium, and the reason is the bentonite colloidal particles getting stuck between sand grains, which is exacerbated by increasing the concentration of the bentonite. Also, due to the chromium co-transport with colloid particles in the fraction of chromium total transport, increasing the bentonite concentration causes decreasing the chromium lateral transfer.

Two-dimensional chromium boride MBenes with high HER catalytic activity

Facing the increasing demands of clean energy, searching low-cost and highly active catalysts of hydrogen evolution reactions (HER) has emerged as a major task. Herein on the basis of density functional theory calculations, we reported that a series of Cr-based MBenes of different atomic-layer thickness (referring to 2D transition metal borides), Crn+1B2n (n = 1–3), exhibit superior electrocatalytic performance towards HER. They are highly stable, mechanical anisotropic and intrinsic ferromagnetic conductive systems. Furthermore, we found that they show thickness-dependent mechanical properties and HER catalytic activity. Especially, Cr4B6 exhibits quite high Young’s module of 335 N/m (comparable to 342 N/m of graphene) and 247 N/m along x and y axes, respectively. More importantly, Cr4B6 is identified as a superior HER electrocatalyst with overpotential of only 0.003 V at H coverage of 1 monolayer (ML), which is a promising alternative of Pt catalyst.

Imaging magnetic 2D crystals with quantum sensors.

Odd- and even-layer variations in magnetization occur in two-dimensional chromium triiodide

Theoretical prediction of two-dimensional CrOF sheet as a ferromagnetic semiconductor or a half-metal

Two-dimensional chromium oxide fluoride CrOF sheet was studied based on density functional theory. The investigation indicates that the CrOF sheet is an intrinsic ferromagnetic semiconductor. The corresponding Curie temperature is 150 K. In particular, the Curie temperature increases up to 410 K under hole doping and the CrOF sheet becomes a half-metal. The versatile electronic and magnetic properties indicate that the two-dimensional CrOF sheet can be a promising candidate for next-generation spintronic devices.

Electrical-field-induced magnetic Skyrmion ground state in a two-dimensional chromium tri-iodide ferromagnetic monolayer

Using fully first-principles non-collinear self-consistent field density functional theory (DFT) calculations with relativistic spin-orbital coupling effects, we show that, by applying an out-of-plane electrical field on a free-standing two-dimensional chromium tri-iodide (CrI3) ferromagnetic monolayer, the Néel-type magnetic Skyrmion spin configurations become more energetically-favorable than the ferromagnetic spin configurations. It is revealed that the topologically-protected Skyrmion ground state is caused by the breaking of inversion symmetry, which induces the non-trivial Dzyaloshinskii-Moriya interaction (DMI) and the energetically-favorable spin-canting configuration. Combining the ferromagnetic and the magnetic Skyrmion ground states, it is shown that 4-level data can be stored in a single monolayer-based spintronic device, which is of practical interests to realize the next-generation energy-efficient quaternary logic devices and multilevel memory devices.

Synthesis, crystal structure and magnetic properties of a new cyanide-bridged two-dimensional chromium(I)-cobalt(II) ferromagnet based on pentacyanonitrosylchromate(I)

Synthesis, crystal structure and magnetic properties of a new cyanide-bridged two-dimensional chromium(I)-cobalt(II) ferromagnet based on pentacyanonitrosylchromate(I)

Structural stability and electronic properties of multi-functionalized two-dimensional chromium carbides

The feasibility of designing two-dimensional (2D) materials was investigated using monolayer Cr2C, a member of the MXene family, with F and OH functional groups on each side of the surface. Interestingly, this study shows that the magnetic and electronic properties of Cr2C with F and OH functional groups (i.e., Cr2CF(OH), with one functional group on each side) can be predicted using previously known properties of Cr2C with only one kind of functional group (i.e., either F or OH on both sides). Unlike typical functionalized 2D materials with different functional groups, each functional group of Cr2CF(OH) has a negligible effect on the magnetic and electronic properties of the other side. This unique phenomenon results from the small differences in geometry between the functional groups, such as the coordination number, bond length, and thickness; the properties themselves come from those of Cr2CF2 and Cr2C(OH)2, despite having different functional groups. Due to the preserved magnetic moments of the Cr atoms bound with each functional group, Cr2CF(OH) exhibits ferrimagnetism. Likewise, semiconducting characters are present because the atomic contributions of each energy level from Cr2CF2 and Cr2C(OH)2 are maintained. Thus, the present work demonstrates that a combination of functional groups that shows certain desired properties can be selected when designing Cr2C-based spintronic and electronic devices.

Synthesis and crystal structure of a new two-dimensional chromium(III) network through hydrogen bonds

A new chromium(III) complex, [Cr(ox)(bdmpza)(H2O)], (ox = oxalate dianion and bdmpza = bis(3,5-dimethylpyrazol-1-yl)acetate, respectively), has been synthesized and characterized by X-ray crystallography. This compound crystallizes in the monoclinic space group P21/c, with a = 7.008(3) A, b = 14.229(5) A, c = 16.497(7) A, β = 101.588(7)°, V = 1611.5(11) A3, Z = 4. The chromium atom has a distorted octahedral environment with different coordination atoms. Bis(3,5-dimethylpyrazol-1-yl)acetate anion is a tridentate ligand, and the oxalate group acts as a chelating bidentate ligand. This compound also forms an infinite two-dimensional network through O–H ⋅ ⋅ ⋅ O hydrogen bonds. The lengths of the hydrogen bonds are 2.765(5) and 2.733(5) A, respectively.

Colossal magnetoresistance in the layered chromium sulfideCr2S3−x(x=0.08)

We report structural, magnetic, and magnetotransport properties of the two-dimensional chromium sulfide ${\mathrm{Cr}}_{2}{\mathrm{S}}_{2.92},$ which contains both ${\mathrm{Cr}}^{3+}$ and ${\mathrm{Cr}}^{2+}$ ions. Below its magnetic-ordering temperature ${(T}_{N}=118 \mathrm{K}),$ ${\mathrm{Cr}}_{2}{\mathrm{S}}_{2.92}$ exhibits a weak spontaneous magnetization with a maximum around 90 K and a value of 0.013(4) ${\ensuremath{\mu}}_{B}$ per Cr ion at 5 K. The resistivity and magnetoresistance exhibit pronounced local maxima around ${T}_{N}$ $(\mathrm{MR}=48%$ in a field of 14 T). The magnetotransport properties are discussed in terms of magnetic-polaron formation.

Lithography using nano-lens arrays made of light

We discuss the fabrication of various one and two-dimensional chromium nanostructures. A chromium atomic beam effusing out of a thermal oven is first transversally laser cooled. This well-collimated beam is then structured and focused by the optical dipole force in a periodic light field. By introducing a substrate at the focus of the atomic beam we are able to capture this pattern directly. We discuss a few applications and challenges of this young lithography technique.

Surface structure and reactivity of CrO 3/SiO 2 catalysts

The molecular structure of the two-dimensional chromium oxide overlayer on the silica support at different chromium oxide contents has been investigated by in situ Raman spectroscopy. Under dehydrated conditions, the surface chromium oxide species on silica consists of a highly distorted, tetrahedral monochromate species regardless of chromium oxide content. The catalysts above 2% CrO 3/SiO 2 also contain crystalline α-Cr 2O 3 particles in addition to the surface monochromate species. The methanol oxidation studies reveal that the catalytic activity per Cr atom, the turnover number (TON), decreases as the chromium oxide content increases. The somewhat higher TON of the initial rate as compared to the steady state rate for the methanol oxidation reaction reflects the higher activity of the fully oxidized chromium oxide species relative to the partially reduced chromium oxide species for the methanol oxidation reaction. The major reaction product is HCHO, while HCOOCH 3, CO, and C0 2 are next in abundance. The selectivity for HCHO increases as the chromium oxide content increases and the precalcination temperature is increased. The opposite trend is observed for the selectivity of HCOOCH 3. These selectivity changes are due to dehydroxylization of the silica surface by precalcining at elevated temperatures and increasing the surface chromium oxide species. The current results suggest that HCHO and HCOOCH 3 are produced on two different catalytic sites: HCHO is formed on the Cr site, whereas HCOOCH 3 is produced via hemiacetal intermediates, which are formed by interaction between HCHO adsorbed on the Cr site and CH 3O adsorbed on the silica site. The CO and CO 2 combustion products are produced on both the Cr and silica site.