Room-Temperature Magnetic Phase Transition in an Electrically Tuned van der Waals Ferromagnet.

Finding tunable van der Waals (vdW) ferromagnets that operate at above room temperature is an important research focus in physics and materials science. Most vdW magnets are only intrinsically magnetic far below room temperature and magnetism with square-shaped hysteresis at room temperature has yet to be observed. Here, we report magnetism in a quasi-2D magnet Cr_{1.2}Te_{2} observed at room temperature (290 K). This magnetism was tuned via a protonic gate with an electron doping concentration up to 3.8×10^{21} cm^{-3}. We observed nonmonotonic evolutions in both coercivity and anomalous Hall resistivity. Under increased electron doping, the coercivities and anomalous Hall effects (AHEs) vanished, indicating a doping-induced magnetic phase transition. This occurred up to room temperature. DFT calculations showed the formation of an antiferromagnetic (AFM) phase caused by the intercalation of protons which induced significant electron doping in the Cr_{1.2}Te_{2}. The tunability of the magnetic properties and phase in room temperature magnetic vdW Cr_{1.2}Te_{2} is a significant step towards practical spintronic devices.

Motif based high-throughput structure prediction of superconducting monolayer titanium boride.

Two-dimensional boron structures, due to their diverse properties, have attracted great attention because of their potential applications in nanoelectronic devices. A series of TiBn (2 ≤ n ≤ 13) monolayers are efficiently constructed through our motif based method and theoretically investigated through high-throughput first-principles calculations. The configurations are generated based on the motifs of boron dimeric/triangular/quadrilateral fragments and multi-coordinate titanium-centered boron molecular wheels. Besides previously reported TiB4 and TiB9 which were discovered by the global search method, we predict that high symmetry monolayer TiB7 (Cmmm), which is octa-coordinate titanium boride, is dynamically stable. The TiB7 monolayer is a BCS superconductor with a transition temperature Tc of up to 8.3 K. The motif based approach is proved to be efficient in searching stable structures with prior knowledge so that the potentially stable transition metal monolayers can be quickly constructed by using basic cluster motifs. As an efficient way of discovering materials, the method is easily extended to predict other types of materials which have common characteristic patterns in the structure.

Atom Classification Model for Total Energy Evaluation of Two-Dimensional Multicomponent Materials.

The tunable properties of materials originate from variety of structures; however, it is still a challenge to give an accurate and fast evaluation of stabilities for screening numerous candidates. Herein, we propose an atom classification model to describe the multicomponent materials based on the structural recognition, in which the atoms are classified to estimate the total energies. Taking two-dimensional planar C1-xBx and C1-2x(BN)x as examples, we have found that the test error of total energies is about 3 meV per atom. Notably, the distributions of classified atoms demonstrate the evolution of configurations as a function of temperature, providing a clearer picture of phase transition. In addition, our method is universal, which can be flexibly extended to the bulk structures with more components.

Two-Dimensional Anti-Van't Hoff/Le Bel Array AlB6 with High Stability, Unique Motif, Triple Dirac Cones, and Superconductivity.

We report the discovery of a rule-breaking two-dimensional aluminum boride (AlB6-ptAl-array) nanosheet with a planar tetracoordinate aluminum (ptAl) array in a tetragonal lattice by comprehensive crystal structure search, first-principles calculations, and molecular dynamics simulations. It is a brand new 2D material with a unique motif, high stability, and exotic properties. These anti-van't Hoff/Le Bel ptAl-arrays are arranged in a highly ordered way and connected by two sheets of boron rhomboidal strips above and below the array. The regular alignment and strong bonding between the constituents of this material lead to very strong mechanical strength (in-plane Young's modulus Y x = 379, Y y = 437 N/m, much larger than that of graphene, Y = 340 N/m) and high thermal stability (the framework survived simulated annealing at 2080 K for 10 ps). Additionally, electronic structure calculations indicate that it is a rare new material with triple Dirac cones, Dirac-like fermions, and node-loop features. Remarkably, this material is predicted to be a 2D phonon-mediated superconductor with Tc = 4.7 K, higher than the boiling point of liquid helium (4.2 K). Surprisingly, the Tc can be greatly enhanced up to 30 K by applying tensile strain at 12%. This is much higher than the temperature of liquid hydrogen (20.3 K). These outstanding properties may pave the way for potential applications of an AlB6-ptAl-array in nanoelectronics and nanomechanics. This work opens up a new branch of two-dimensional aluminum boride materials for exploration. The present study also opens a field of two-dimensional arrays of anti-van't Hoff/Le Bel motifs for study.

Theoretical investigations on diamondoids (CnHm, n = 10-41): Nomenclature, structural stabilities, and gap distributions.

Combining the congruence check and the first-principles calculations, we have systematically investigated the structural stabilities and gap distributions of possible diamondoids (CnHm) with the carbon numbers (n) from 10 to 41. A simple method for the nomenclature is proposed, which can be used to distinguish and screen the candidates with high efficiency. Different from previous theoretical studies, the possible diamondoids can be enumerated according to our nomenclature, without any pre-determination from experiments. The structural stabilities and electronic properties have been studied by density functional based tight binding and first-principles methods, where a nearly linear correlation is found between the energy gaps obtained by these two methods. According to the formation energy of structures, we have determined the stable configurations as a function of chemical potential. The maximum and minimum energy gaps are found to be dominated by the shape of diamondoids for clusters with a given number of carbon atoms, while the gap decreases in general as the size increases due to the quantum confinement.

Two-Dimensional Semiconducting Boron Monolayers.

The two-dimensional boron monolayers were reported to be metallic both in previous theoretical predictions and experimental observations. Unexpectedly, we have first found a family of boron monolayers with the novel semiconducting property as confirmed by the first-principles calculations with the quasi-particle G0W0 approach. We demonstrate that the connected network of hexagonal vacancies dominates the gap opening for both the in-plane s+px,y and pz orbitals, with which various semiconducting boron monolayers are designed to realize the band gap engineering for the potential applications in electronic devices. The semiconducting boron monolayers in our predictions are expected to be synthesized on the proper substrates, due to the similar stabilities to the ones observed experimentally.

Phonon-mediated superconductivity in Mg intercalated bilayer borophenes.

Using first-principles calculations, we investigate the structural, electronic and superconducting properties of Mg intercalated bilayer borophenes BxMgBx (x = 2-5). Remarkably, B2MgB2 and B4MgB4 are predicted to exhibit good phonon-mediated superconductivity with a high transition temperature (Tc) of 23.2 K and 13.3 K, respectively, while B4MgB4 is confirmed to be more practical based on the analyses of its stability. The densities of states of in-plane orbitals at the Fermi level are found to be dominant at the superconducting transition temperature in Mg intercalated bilayer borophenes, providing an effective avenue to explore Mg-B systems with high Tcs.

High-coverage stable structures of 3d transition metal intercalated bilayer graphene.

Alkali-metal intercalated graphite and graphene have been intensively studied for decades, where alkali-metal atoms are found to form ordered structures at the hollow sites of hexagonal carbon rings. Using first-principles calculations, we have predicted various stable structures of high-coverage 3d transition metal (TM) intercalated bilayer graphene (BLG) stabilized by the strain. Specifically, with reference to the bulk metal, Sc and Ti can form stable TM-intercalated BLG without strain, while the stabilization of Fe, Co, and Ni intercalated BLG requires the biaxial strain of over 7%. Under the biaxial strain ranging from 0% to 10%, there are four ordered sandwich structures for Sc with the coverage of 0.25, 0.571, 0.684, and 0.75, in which the Sc atoms are all distributed homogenously instead of locating at the hollow sites. According to the phase diagram, a homogenous configuration of C8Ti3C8 with the coverage of 0.75 and another inhomogeneous structure with the coverage of 0.692 were found. The electronic and magnetic properties as a function of strain were also analyzed to indicate that the strain was important for the stabilities of the high-coverage TM-intercalated BLG.

Understanding the stable boron clusters: A bond model and first-principles calculations based on high-throughput screening.

The unique electronic property induced diversified structure of boron (B) cluster has attracted much interest from experimentalists and theorists. B30-40 were reported to be planar fragments of triangular lattice with proper concentrations of vacancies recently. Here, we have performed high-throughput screening for possible B clusters through the first-principles calculations, including various shapes and distributions of vacancies. As a result, we have determined the structures of Bn clusters with n = 30-51 and found a stable planar cluster of B49 with a double-hexagon vacancy. Considering the 8-electron rule and the electron delocalization, a concise model for the distribution of the 2c-2e and 3c-2e bonds has been proposed to explain the stability of B planar clusters, as well as the reported B cages.

Publisher Correction: An extended cluster expansion for ground states of heterofullerenes.

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

Insights into the unusual semiconducting behavior in low-dimensional boron.

Elementary semiconductors are rare and attractive, especially for low-dimensional materials. Unfortunately, most of the boron nanostructures have been found to be metallic, despite their typical semiconducting bulk structure. Herein, we propose a general recipe to realize low-dimensional semiconducting boron. This unusual semiconducting behavior is attributed to charge transfer and electron localization, induced by symmetry breaking that divides boron atoms into cations and anions. In addition, it is feasible to accomplish band gap engineering by rationally designing various structures. Importantly, the low-dimensional semiconducting boron allotropes are predicted to be an excellent solar-cell material with a power conversion efficiency of up to 22%, paving the way for their promising optoelectronic application.

Opening the Band Gap of Graphene via Fluorination for High-Performance Dual-Mode Photodetector Application.

Fluorination is an effective process to open the band gap of graphene (Gr), which is beneficial to the development of optoelectronic devices working in wide wavelength. Herein, we report a dual-mode broadband photodetector (PD) by integrating fluorinated graphene (F-Gr) with silicon (Si). It is found that when working in photoconductive mode, the F-Gr/Si heterojunction exhibited a remarkable photoresponse over a wide spectral region from ultraviolet (UV), visible to near infrared (NIR) light with a high responsivity ( R) of 1.9 × 107 A W-1 and specific detectivity ( D*) of 4.4 × 1012 Jones at 650 nm. Nonetheless, both parameters will be considerably reduced when the F-Gr/Si heterojunction works in the photodiode mode. In this mode, the Ilight/ Idark ratio is as high as 2.0 × 105 and the response speed is accelerated by more than 3 orders of magnitude from about 5 ms to 6.3 µs. Notably, the responsivity of the device in the UV and NIR regions was remarkably enhanced in comparison with that of pristine Gr/Si-heterojunction-based devices. Considering the F-coverage-dependent band gap of the F-Gr revealed by the first-principle calculations, we believe that the enhancement was ascribed to the opening of the band gap in the partially fluorinated Gr, which is stabilized due to the configuration entropy as the temperature increases. The dual-mode PD enabled the simultaneous weak light detection and fast photodetection, which overcome the limitation of the traditional monomode PD.

An electron compensation mechanism for the polymorphism of boron monolayers.

Boron monolayers have been increasingly attractive, while it is still a challenge to understand their structural stabilities, due to electron deficiency and multi-center bonds. In this work, we propose the average electron compensation (AEC) mechanism for boron monolayers based on high-throughput first-principles calculations. It is found that the AEC parameter (λ) tends to be zero for the stable free-standing boron monolayers. In addition, this mechanism can quantitatively describe the stability of boron monolayers on various metal substrates, providing direct suggestions for experimentalists to synthesize various boron monolayers for practical applications.

An extended cluster expansion for ground states of heterofullerenes.

It is challenging to determine the ground states of heterofullerenes due to the numerous isomers. Taking the C60-n B n heterofullerenes (1 ≤ n ≤ 4) as an example, our first-principles calculations with the isomer enumeration present the most stable structure of C57B3, which is energetically favored by 0.73 eV than the reported counterpart. It was difficult to conduct the enumeration for the isomers with n beyond 4 because of the expensive first-principle calculations. Here, we propose a nomenclature to enhance structural recognition and adopt an extended cluster expansion to describe the structural stabilities, in which the energies of the heterofullerenes with various concentrations are predicted by linear combination of the multi-body interactions. Unlike the conventional cluster expansion, the interaction parameters are derived from the enumeration of C60-n B n (n = 1~4), where there are only 4 coefficients to be fitted as a function of composition for the consideration of local bonding. The cross-validation scores are 1~2 meV per atom for both C55B5 and C54B6, ensuring the ground states obtained from our model are in line with the first-principles results. With the help of the structural recognition, the extended cluster expansion could be further applied to other binary systems as an effective complement to the first-principle calculations.