⇦Chinese Physics Letters, 2020, Vol. 37, No. 10, Article code 107506Express Letter Enhanced Ferromagnetism of CrI$_{3}$ Bilayer by Self-Intercalation Yu Guo (郭宇), Nanshu Liu (刘南舒), Yanyan Zhao (赵艳艳), Xue Jiang (蒋雪), Si Zhou (周思)*, and Jijun Zhao (赵纪军) Affiliations MOE Key Laboratory of Materials Modification by Laser, Ion and Electron Beams, Dalian University of Technology, Dalian 116024, China Received 17 July 2020; accepted 10 September 2020; published online 12 September 2020 Supported by the National Natural Science Foundation of China (Grant Nos. 11974068 and 11874097), the China Postdoctoral Science Foundation (Grant Nos. BX20190052 and 2020M670739), and the Fundamental Research Funds for the Central Universities of China (Grant Nos. DUT20LAB110 and DUT19LK12). The authors acknowledge the computer resources provided by the Supercomputing Center of Dalian University of Technology and Shanghai Supercomputer Center.
*Corresponding author. Email: sizhou@dlut.edu.cn Citation Text: Guo Y, Liu N S, Zhao Y Y, Jiang X and Zhou S et al. 2020 Chin. Phys. Lett. 37 107506    Abstract Two-dimensional (2D) ferromagnets with high Curie temperature have long been the pursuit for electronic and spintronic applications. CrI$_{3}$ is a rising star of intrinsic 2D ferromagnets, however, it suffers from weak exchange coupling. Here we propose a general strategy of self-intercalation to achieve enhanced ferromagnetism in bilayer CrI$_{3}$. We show that filling either Cr or I atoms into the van der Waals gap of stacked and twisted CrI$_{3}$ bilayers can induce the double exchange effect and significantly strengthen the interlayer ferromagnetic coupling. According to our first-principles calculations, the intercalated native atoms act as covalent bridge between two CrI$_{3}$ layers and lead to discrepant oxidation states for the Cr atoms. These theoretical results offer a facile route to achieve high-Curie-temperature 2D magnets for device implementation. DOI:10.1088/0256-307X/37/10/107506 PACS:75.50.Dd, 75.30.Et, 75.50.Pp © 2020 Chinese Physics Society Article Text The discoveries of magnetism at the atomic layer limit have opened a new avenue for the research of two-dimensional (2D) materials.[1–3] Recently, monolayer CrI$_{3}$ has been synthesized in the experiment[4] and confirmed to be an intrinsic 2D Ising ferromagnetic (FM) material.[5] It possesses many excellent characters, such as perfect crystalline order, strong anisotropy in magnetic configuration, and a suitable band gap of about 1.2 eV.[6] However, the Curie temperature of monolayer CrI$_{3}$ is only 45 K due to the weak intralayer exchange,[7] while bilayer CrI$_{3}$ with monoclinic stacking even exhibits antiferromagnetic (AFM) spin order between the two layers, which hinders the practical application of this novel 2D material.[7] Therefore, it is highly desirable to seek effective strategies to enhance the ferromagnetism and raise the Curie temperature of 2D CrI$_{3}$. Intercalation has also been demonstrated as a powerful approach to modulate the physical and chemical properties of 2D layered materials. So far, various atoms, ions, or molecules can be intercalated in numerous kinds of 2D materials with well controlled concentration, triggering exotic phenomena for practical applications. For instance, Li intercalation of graphene film remarkably increases optical transmittance and electrical conductivity,[8] while Ca intercalation of bilayer graphene drives superconductivity.[9] 2D SnS$_{2}$ intercalated by different transition metal atoms can exhibit p-type, n-type, and degenerately doped semiconducting behavior.[10] High Curie temperature up to $\sim$$300$ K can be achieved in 2D magnets Fe$_{3- x}$GeTe$_{2}$ and Cr$_{2}$Ge$_{2}$Te$_{6}$ by intercalation of Na and organic ions, respectively.[11–12] TaS$_{2}$ sheets intercalated with Fe atoms using chemical vapor transport method show tunable magnetic order, magnetic anisotropy, and magnetoresistance.[13–15] Cl intercalation of g-C$_{3}$N$_{4}$ monolayer promotes charge carrier migration and yields favorable band gap and band edge positions to improve the photocatalytic performance for water splitting and CO$_{2}$ reduction.[16] Furthermore, intercalation of native atoms into bilayer transition metal dichalcogenides during growth is feasible in laboratory, which allows the generation of a new class of covalently bonded materials with stoichiometry- or composition-dependent properties. For example, by controlling the metal chemical potential, a range of self-intercalated compounds of TaS(Se)$_{2}$ can be obtained, such as Ta$_{9}$S$_{16}$, Ta$_{7}$S$_{12}$, Ta$_{10}$S$_{16}$, Ta$_{8}$Se$_{12}$, and Ta$_{9}$Se$_{12}$, some of which exhibit ferromagnetism or the unique Kagome lattice.[17] Motivated by recent experimental advances in the intercalated 2D materials, herein we propose a feasible strategy to elevate the Curie temperature for stacked and twisted bilayer CrI$_{3}$ by self-intercalation of Cr or I atoms. By first-principles calculations, the geometries, electronic band structures, and magnetic behavior of bilayer CrI$_{3}$ with various intercalation configurations and concentrations have been systematically investigated. The key parameters that govern the exchange energy of intercalated bilayer CrI$_{3}$ have been determined, and the underlying double exchange mechanism was elucidated. First-principles calculations were performed using spin-polarized density functional theory in conjunction with the projector augmented wave potentials, as implemented in the VASP.[18–19] To describe exchange–correlation interactions, we used the generalized gradient approximation with the Perdew-Burke-Ernzerhof functional.[20] The energy cutoff for the planewave basis was chosen as 500 eV. The convergence criteria of total energy and residual force on each atom were set to be 10$^{-7}$ eV and 0.01 eV/Å, respectively. A vacuum region of 20 Å was added to the perpendicular direction to eliminate the interaction between periodic images. Uniform ${\boldsymbol k}$-point meshes with spacing of $\sim$$0.015$ Å$^{-1}$ were adopted to sample the 2D Brillouin zones. We started from the atomic structures of bilayer CrI$_{3}$ with different stacking geometries. Two types of stacked bilayer CrI$_{3}$ were considered, namely low-temperature (LT) and high-temperature (HT) phases, as shown in Figs. 1(a) and 1(b), respectively. They have nearly identical lattice parameter (6.97 Å) and interlayer distance (3.54 Å). In order to explore the preferred magnetic interaction, we defined the exchange energy per formula (f.u.) as $$ \Delta E = (E_{\rm AFM }- E_{\rm FM})/ n,~~ \tag {1} $$ where $E_{\rm AFM}$ and $E_{\rm FM}$ are the energies of CrI$_{3}$ bilayer with AFM and FM spin configurations between the two layers, respectively; $n$ is the number of CrI$_{3}$ formula in the supercell. Positive (negative) value of $\Delta E$ represents FM (AFM) coupling between the two CrI$_{3}$ layers. Larger $\Delta E$ means stronger interlayer FM coupling and generally leads to higher Curie temperature.[21,22] As given in Table 1, $\Delta E$ for pristine bilayer CrI$_{3}$ systems are as small as 2.95 meV/f.u. (LT) and $-$0.14 meV/f.u. (HT), respectively, which are consistent with previous experimental observation of weak ferromagnetism in LT phase and antiferromagnetism in HT phase and close to the reported theoretical values (3.90 meV/f.u. for LT phase and $-0.17$ meV/f.u. for HT phase).[4,7,21,23–25]
cpl-37-10-107506-fig1.png
Fig. 1. Atomic structures of pristine (a) LT and (b) HT phases of bilayer CrI$_{3}$, (c) Cr and (d) I intercalated LT phase of $\sqrt{3}\times \sqrt{3}$ supercell, and (e) Cr and (f) I intercalated twisted 38.42$^{\circ}$-CrI$_{3}$. The Cr and I atoms are shown in purple and orange colors, respectively.
Table 1. Exchange energy $\Delta E$ (in the unit of meV/f.u.) of pristine, Cr and I intercalated CrI$_{3}$ bilayers, including HT and LT phases, twisted bilayers 21.79$^{\circ}$-CrI$_{3}$ and 38.42$^{\circ}$-CrI$_{3}$. The numbers in the brackets are the exchange energies for 21.79$^{\circ}$-CrI$_{3}$ and 38.42$^{\circ}$-CrI$_{3}$ without intercalation.
$\Delta E$ pristine $ \sqrt{3} \times \sqrt{3}$ $2 \times 2$ $\sqrt{7} \times \sqrt{7}$ $3 \times 3$ 21.79$^{\circ}$ 38.42$^{\circ}$
Cr intercalation LT 2.95 38.58 31.48 21.33 6.78 5.38 (0.61) 13.17 (0.99)
HT $-$0.14 29.99 28.50 18.18 15.02
I intercalation LT 2.95 6.64 6.44 3.15 18.64 3.67 6.30
HT $-$0.14 21.05 16.50 6.31 18.23
During the growth process, 2D layered materials with twisted angles are commonly observed in experiments,[26–28] offering extra degree of freedom for modulating their physical properties.[29–33] Therefore, we also considered bilayer CrI$_{3}$ with twisted angles of 21.79$^{\circ}$ and 38.42$^{\circ}$ displayed in Fig. 1 and Fig. S1 in the Supplementary Material (SM), thereafter named as 21.79$^{\circ}$-CrI$_{3}$ and 38.42$^{\circ}$-CrI$_{3}$, respectively. The interlayer binding energies of these twisted CrI$_{3}$ bilayers are nearly identical to that of the LT and HT phases (0.39 meV/atom), indicating favorable formation of both twisted and stacked CrI$_{3}$ sheets in the experiment. The considered twisted systems exhibit interlayer ferromagnetic coupling with smaller exchange energies than that of the LT phase, i.e., $\Delta E = 0.61$ and 0.99 meV/f.u. for 21.79$^{\circ}$-CrI$_{3}$ and 38.42$^{\circ}$-CrI$_{3}$, respectively. Apparently, it is difficult to enhance the ferromagnetism of bilayer CrI$_{3}$ simply by twisting the two layers, and other strategies are desired to increase the exchange energy or even change the exchange mechanism. Inspired by successful intercalation in various layered materials,[11–15,17,34–42] here we explore the possibility of self-intercalation of Cr or I atoms into bilayer CrI$_{3}$, as displayed in Fig. 1 and Fig. S1. Starting from $\sqrt{3}\times \sqrt{3}$, $2 \times 2$, $\sqrt{7}\times \sqrt{7}$, and $3 \times 3$ supercells of the LT and HT phases, a Cr or I atom is intercalated between CrI$_{3}$ layers, resulting in different concentrations (defined as the ratio of one Cr or I atom to the number of CrI$_{3}$ formula in the supercell). For all the systems, the intercalated Cr or I atom is always bonded with the nearby I atoms with bond lengths of around 2.79 and 2.92 Å, respectively, which are close to the intralayer Cr–I bond length of 2.77 Å. Compared with the interlayer spacing of 3.54 Å for the pristine bilayer, the intercalated Cr atom stays on the hollow site and reduces the interlayer distance to about 3.40 Å, while the intercalated I atom locates on the bridge site of the two nearest intralayer I atoms and enlarges the interlayer spacing to up to 4.26 Å. Such difference originates from the distinct Cr–I and I–I bond nature, which is also reflected in the intercalation energies for Cr and I atoms discussed below. To characterize the energetic stability of intercalated atoms, we define the intercalation energy as $$ E_{\rm int} =E_{\rm tot }- E_{\rm bilayer }- E_{\rm Cr/I},~~ \tag {2} $$ where $E_{\rm tot}$ and $E_{\rm bilayer}$ are the energies of bilayer CrI$_{3}$ with and without intercalation, respectively; $E_{\rm Cr/I}$ is the energy of a single Cr or I atom. As listed in Table S1 of SM, intercalation energies of Cr (I) atom for the LT and HT phases are in the range from $-3.97$ to $-4.11$ eV ($-0.13$ to $-0.31$ eV) and from $-4.29$ to $-4.39$ eV ($-0.69$ to $-1.61$ eV), respectively. The twisted systems have $E_{\rm int}$ close to the values of the LT phase (about $-4.00$ and $-0.39$ eV for Cr and I intercalation, respectively). These results suggest that self-intercalation of Cr or I atoms into the vdW gap of stacked or twisted CrI$_{3}$ bilayer is energetically feasible. Compared with I intercalation, the magnitude of $E_{\rm int}$ for the Cr-intercalated systems is several times larger, again indicating the stronger bond formation between the intercalated Cr atom and nearest in-plane I atoms. Interestingly, the magnitudes of these intercalation energies are comparable to the formation energies of single Cr and I vacancy in the pristine CrI$_{3}$ layers (4.00 and 1.15 eV, respectively), implying that the escaped Cr and I atoms from vacancy defects may be self-intercalated into the vdW gap between CrI$_{3}$ layers. Moreover, tearing the intercalated bilayer off bulk CrI$_{3}$ involves an exfoliation energy of 0.20 J/m$^{2}$, almost the same as that for ripping monolayer CrI$_{3}$ from the bulk material (0.21 J/m$^{2}$, see Fig. S4 in the SM for details). Thus, self-intercalation would not affect the exfoliation behavior of layered CrI$_{3}$. The thermal stability of intercalated CrI$_{3}$ bilayers was further assessed by the Born-Oppenheimer molecular dynamics (BOMD) simulation (see Fig. S2 in the SM). The intercalated CrI$_{3}$ bilayers can maintain their structures at 300 K for at least 10 ps simulation with the largest deviation of 0.41 Å, demonstrating their good stability at room temperature. At last, the diffusion behavior of the intercalated Cr atom in the vdW gap of LT and HT phases was examined (Fig. S3 in the SM), which involves moderate barriers of 0.54 and 0.74 eV, respectively, and implies the easy migration of Cr atom to fill in the vdW gap.
cpl-37-10-107506-fig2.png
Fig. 2. Exchange energy ($\Delta E$) and virtual exchange gap ($G_{\rm ex}$) of various Cr-intercalated CrI$_{3}$ bilayers as a function of Cr intercalation concentration.
Most encouragingly, self-intercalating Cr or I atoms into bilayer CrI$_{3}$ can significantly increase the exchange energy ($\Delta E$) and elevate the Curie temperature. All the considered stacking and twisted systems prefer ferromagnetic order for the interlayer spin configuration. As displayed by the spin charge density in Fig. S5 in the SM, the magnetic moments are carried by both intralayer and intercalated Cr atoms. Table 1 presents $\Delta E$ values ranging from $6.78$ to $38.58$ meV/f.u. for Cr intercalation and from $3.15$ to $21.05$ meV/f.u. for I intercalation, which is one or two orders of magnitude larger than the values of pristine CrI$_{3}$ bilayers (2.95 meV/f.u. for LT phase, $-$0.14 meV/f.u. for HT phase, 0.61 meV/f.u. for 21.79$^{\circ}$-CrI$_{3}$, and 0.99 meV/f.u. for 38.42$^{\circ}$-CrI$_{3}$). Especially, the exchange energy increases almost linearly with the intercalated Cr concentration (Fig. 2), suggesting that the magnetism of CrI$_{3}$ bilayer can be effectively modulated by Cr intercalation. According to our previous study, the exchange energy of CrI$_{3}$ bilayer can be increased up to 9.42 meV/f.u. by proximity effect, which elevates $T_{\rm c}$ up to 130 K.[22] The exchange energy is positively correlated with Curie temperature and can reflect the trend of robustness of FM order. The present self-intercalated CrI$_{3}$ bilayers with much larger exchange energies would possess even higher Curie temperature than 130 K. Similar results are also found for trilayer and bulk CrI$_{3}$ by Cr intercalation with exchange energies increased to 19.66 meV/f.u. for the LT phase (3.30 meV/f.u. for pristine systems), while AFM-to-FM transition occurs for the HT phase (see Fig. S8 and Table S3 in the SM). Therefore, self-intercalation is an effective strategy to enhance the ferromagnetism of layered CrI$_{3}$. The enhanced interlayer FM coupling can be understood by the decomposed charge density, electronic band structures, and density of states in Fig. 3, Fig. S5, Fig. S6, and Fig. S7 in the SM. Under octahedral crystal field, the fivefold degenerate $d$ orbitals of Cr atom split into a threefold occupied $t_{2g}$ ($d_{xy}$, $d_{{x}^{2}-{y}^{2}}$, and $d_{{z}^{2}}$) orbital and a twofold unoccupied $e_{g}$ ($d_{xz}$ and $d_{yz}$) orbital. Self-intercalation induces notable electronic states in the gap region that promote the hybridization between $e_{g}$ and $t_{2g}$ states, which in turn is the key for strengthening the FM state of bilayer CrI$_{3}$. The decomposed spin-up charge density of conduction band minimum further reveals the gradually increasing occupation of $e_{g}$ orbital as the concentration of intercalated Cr atom increases. The $e_{g}$–$t_{2g}$ coupling strength can be quantitatively characterized by the virtual exchange gap ($G_{\rm ex}$) between the $e_{g}$ and $t_{2g}$ orbitals,[43] which can be measured from the projected density of states. Taking Cr intercalation as a representative, $G_{\rm ex}$ values for various intercalated bilayers are in the range of 0.37–0.83 eV, remarkably smaller than the values of LT (0.98 eV) and HT (0.92 eV) phases. Generally speaking, $G_{\rm ex}$ decreases with increasing Cr concentration, leading to the gradually increased $\Delta E$ and enhanced ferromagnetism of bilayer CrI$_{3}$ (see Fig. 2).
cpl-37-10-107506-fig3.png
Fig. 3. (a) Decomposed spin-up charge density of conduction band minimum for Cr-intercalated bilayers in $\sqrt{3}\times \sqrt{3}$, $2 \times 2$, $\sqrt{7}\times \sqrt{7}$, and $3 \times 3$ supercell, respectively. The isosurface value is 0.05 e/Å$^{3}$. The charge values at the intercalated Cr atom are presented. (b)–(d) Band structures and project density of states (PDOS) of pristine HT phase of CrI$_{3}$ bilayer, Cr-intercalated HT phase in $\sqrt{3}\times \sqrt{3}$ and $3 \times 3$ supercell, respectively.
To gain further insights into the impact of self-intercalation on the electronic band structure of bilayer CrI$_{3}$, we examined the bond nature, charge transfer and oxidation states. As revealed by the electron density counter plot in Fig. 4(a), the two CrI$_{3}$ layers have interlayer bonding through the intercalated Cr or I atoms. The intercalated Cr atom gains $0.02e$–$0.12e$ from the intralayer Cr atoms nearby according to Bader charge analysis in Table S5 in the SM, thereby modifying the oxidation states of Cr atoms. Taking the HT phase as a representative [see Fig. 4(b)], the intercalated Cr atom has a formal valence state of $+2$, while the two nearest intralayer Cr atoms (sharing an I atom with intercalated Cr) have a formal valence state of $+2.67$, compared with $+3$ for Cr in pristine CrI$_{3}$ bilayer. The presence of intercalated and intralayer Cr atoms with different oxidation states signifies the double exchange mechanism[44,45] that dominates the interlayer ferromagnetism in CrI$_{3}$ bilayer, as illustrated by Fig. 4(c). In the Cr$^{\delta +}-$Cr$^{2+}$ (2 $ < \delta < $ 3) mixed valence complexes, an electron hops between Cr$^{\delta +}$ and Cr$^{3+}$ cations through a bridging I$^{-}$ anion. Within the double exchange picture, electron transfer from one Cr cation to another would be more easily if it does not have to flip spin orientation in order to conform with Hund's rule, which thus facilitates the ferromagnetic coupling between two Cr cations with different valences. Similar double exchange mechanism is also applicable to bilayer CrI$_{3}$ with I intercalation (see Fig. S9). Even though the intercalated I atom is not spin-polarized, it donates certain amounts of electrons (0.06$e$–0.11$e$) to the adjacent I atoms in the two CrI$_{3}$ layers, which in turn leads to different valence states for the nearby intralayer Cr atoms.
cpl-37-10-107506-fig4.png
Fig. 4. (a) Contour plots for the electron densities of pristine, Cr-intercalated and I-intercalated HT phase of CrI$_{3}$ bilayer from left to right panels. (b) One layer of Cr-intercalated HT phase with the formal valence state $\delta_{0}$ ($+$2) for intercalated Cr atom, $\delta_{1}$ ($+$8/3) and $\delta_{2}$ ($+$17/6) for intralayer Cr atoms, and the valence state of other unmarked Cr atoms is $+$3, i.e., $2 = \delta_{0} < \delta_{1} < \delta_{2} < 3$. (c) Schematic diagram of the double exchange mechanism for Cr/I-intercalated CrI$_{3}$ bilayer. The hollow arrow indicates that the electron is not fully occupied for the orbital.
In summary, we significantly enhance the interlayer ferromagnetic order of bilayer CrI$_{3}$ by self-intercalation of Cr or I atoms. The magnetic behavior of intercalated bilayer CrI$_{3}$ of the LT and HT stacking geometries or with a twisted angle has been systematically explored as a function of intercalation concentration. Both Cr and I intercalation lead to robust interlayer ferromagnetic order for all the considered CrI$_{3}$ bilayers. The exchange energy increases with the intercalated Cr concentration, reaching about 40 meV/f.u. compared to the values of pristine CrI$_{3}$ bilayer (2.95 meV/f.u. for LT phase and $-0.14$ meV/f.u. for HT phase). Such strong ferromagnetism is dominated by the double exchange mechanism originated from the interlayer charge transfer via the intercalated atoms, which results in different oxidation states for the Cr atoms in CrI$_{3}$ bilayers and enhanced $e_{g}$–$t_{2g}$ hybridization. These theoretical results provide a universal and experimentally feasible strategy to effectively raise the Curie temperature of 2D ferromagnets via self-intercalation for practical uses. References Van der Waals heterostructuresElectronics and optoelectronics of two-dimensional transition metal dichalcogenidesMobility of Charge Carriers in Semiconducting Layer StructuresLayer-dependent ferromagnetism in a van der Waals crystal down to the monolayer limitCoexistence of Magnetic Orders in Two-Dimensional Magnet CrI 3Doping enhanced ferromagnetism and induced half-metallicity in CrI 3 monolayerElectrical control of 2D magnetism in bilayer CrI3Approaching the limits of transparency and conductivity in graphitic materials through lithium intercalationSuperconducting Calcium-Intercalated Bilayer GrapheneSpatially controlled doping of two-dimensional SnS2 through intercalation for electronicsTransition from Ferromagnetic Semiconductor to Ferromagnetic Metal with Enhanced Curie Temperature in Cr 2 Ge 2 Te 6 via Organic Ion IntercalationDecomposition-Induced Room-Temperature Magnetism of the Na-Intercalated Layered Ferromagnet Fe 3– x GeTe 2Critical behavior of intercalated quasi-van der Waals ferromagnet F e 0.26 Ta S 2 Very large magnetoresistance in Fe 0.28 TaS 2 single crystalsSharp switching of the magnetization in Fe 1 ∕ 4 Ta S 2 Chlorine intercalation in graphitic carbon nitride for efficient photocatalysisEngineering covalently bonded 2D layered materials by self-intercalationEfficient iterative schemes for ab initio total-energy calculations using a plane-wave basis setFrom ultrasoft pseudopotentials to the projector augmented-wave methodGeneralized Gradient Approximation Made SimpleStacking-Dependent Magnetism in Bilayer CrI 3High-Curie-temperature ferromagnetism in bilayer CrI3 on bulk semiconducting substratesProbing magnetism in 2D van der Waals crystalline insulators via electron tunnelingGiant tunneling magnetoresistance in spin-filter van der Waals heterostructuresControl of magnetism in bilayer CrI 3 by an external electric fieldUnconventional superconductivity in magic-angle graphene superlatticesStrange Metal in Magic-Angle Graphene with near Planckian DissipationCorrelated insulator behaviour at half-filling in magic-angle graphene superlatticesStrain and curvature induced evolution of electronic band structures in twisted graphene bilayerExperimental evidence for non-Abelian gauge potentials in twisted graphene bilayersFabrication and the Interlayer Coupling Effect of Twisted Stacked Black Phosphorus for Optical ApplicationsGrowth of carbon nanotubes via twisted graphene nanoribbonsTuning the electronic properties of bilayer black phosphorene with the twist angleDisorder-induced two-step melting of vortex matter in Co-intercalated NbS e 2 single crystalsApproaching Magnetic Ordering in Graphene Materials by FeCl 3 IntercalationElectrochemical Intercalation of PF[sub 6] into GraphiteEnhanced catalytic activity in strained chemically exfoliated WS2 nanosheets for hydrogen evolutionChemical Intercalation of Zerovalent Metals into 2D Layered Bi 2 Se 3 NanoribbonsTwo-Dimensional Layered Chalcogenides: From Rational Synthesis to Property Control via Orbital Occupation and Electron FillingGeneral Strategy for Zero-Valent Intercalation into Two-Dimensional Layered NanomaterialsReversible Intercalation of Hexagonal Boron Nitride with Brønsted AcidsA self-driven approach for local ion intercalation in vdW crystalsToward Intrinsic Room-Temperature Ferromagnetism in Two-Dimensional SemiconductorsTheory of the Role of Covalence in the Perovskite-Type Manganites [ La ,   M ( II ) ] Mn O 3 Interaction Between the d Shells in the Transition Metals
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