Monolayer Vanadium-doped Tungsten Disulfide: A Room-Temperature Dilute Magnetic Semiconductor

Dilute magnetic semiconductors, achieved through substitutional doping of spin-polarized transition metals into semiconducting systems, enable experimental modulation of spin dynamics in ways that hold great promise for novel magneto-electric or magneto-optical devices, especially for two-dimensional systems such as transition metal dichalcogenides that accentuate interactions and activate valley degrees of freedom. Practical applications of 2D magnetism will likely require room-temperature operation, air stability, and (for magnetic semiconductors) the ability to achieve optimal doping levels without dopant aggregation. Here we describe room-temperature ferromagnetic order obtained in semiconducting vanadium-doped tungsten disulfide monolayers produced by a reliable single-step film sulfidation method across an exceptionally wide range of vanadium concentrations, up to 12 at% with minimal dopant aggregation. These monolayers develop p-type transport as a function of vanadium incorporation and rapidly reach ambipolarity. Ferromagnetism peaks at an intermediate vanadium concentration of a few atomic percent and decreases for higher concentrations, which is consistent with quenching due to orbital hybridization at closer vanadium-vanadium spacings, as supported by transmission electron microscopy, magnetometry and first-principles calculations. Room-temperature two-dimensional dilute magnetic semiconductors provide a new component to expand the functional scope of van der Waals heterostructures and bring semiconducting magnetic 2D heterostructures them into the realm of practical application.


functional scope of van der Waals heterostructures and bring semiconducting magnetic 2D
heterostructures them into the realm of practical application.
Intrinsic ferromagnetism has been confirmed in semiconducting monolayer CrI 3 [1,[4][5] and insulating few-layer Cr 2 Ge 2 Te 6 [2] at cryogenic temperatures. Moreover, a transition from paramagnetic to ferromagnetic in vanadium diselenide has been recorded when this metallic material was isolated in monolayer form [3] . Air sensitive monolayer VSe 2 has displayed ferromagnetic order even at and above room temperatures [18] . Monolayer samples provide compelling advantages in the characterization of atomic structure and integration into van der Waals heterostructures [10] . Monolayer tungsten disulfide is furthermore a direct-gap semiconductor with high photoluminescence yield that can achieve a reasonably high on/off current ratio (>10 5 ) in field-effect transistor geometries [19] . Reliable substitutional cation doping of WS 2 and its sister material MoS 2 can induce degenerate n-type (rhenium [20] ) and p-type (carbon [21] and niobium [22] ) conduction. Beyond simply introducing charge carriers, a judicious choice of dopant may also introduce spin polarization. Scalable and controllable synthesis of single-phase monolayer DMS's with ferromagnetic ordering at room temperature could thus provide a new component for van der Waals heterostructures that express novel modes of magneto-electric and magneto-optical response. [23], [24] We report the single-step and atmospheric pressure deposition (via film sulfidation) of high-quality V-doped WS 2 monolayers exhibiting room-temperature ferromagnetism.
Aberration-corrected high-resolution scanning transmission electron microscopy (AC-HRSTEM) and X-ray photoelectron spectroscopy (XPS) reveal substitutional vanadium concentrations up to 12 atomic percent (at%) without substantial structural deformation or degradation. Interestingly, vanadium doping (or alloying) reduces the optical bandgap and induces p-branch transport that reaches ambipolarity. What appears to be intrinsic ferromagnetic order is achieved at room temperature, with a maximum coercivity (H c =130 Oe) and saturation magnetization at an intermediate vanadium concentration (~2 at%). First-principles calculations suggest that magnetism can be further strengthened by optimizing the distribution of dopant-dopant neighbor separations, and also reveal how spin polarized impurity levels breaks the valley degeneracy.
These results now establish a promising route to room-temperature 2D spintronic devices.
Pristine and V-doped WS 2 monolayers were synthesized by chemical vapor deposition (CVD) [25] (schematics at upper left in Figure 1 Figures 2d-f). An oxygen peak is unavoidable in the EELS spectra, due to oxidation of the sample surface and film support.
Keeping the overall volume of the precursor solution constant, the vanadium precursor concentration was varied from zero to 1×10 -5 , 1×10 -4 , 1×10 -3 and 5×10 -3 mol/L; (higher vanadium concentrations triggered precipitation upon mixing W and V precursor solutions, leading to degradation of the WS 2 monolayer's crystallinity, edge regularity, and flatness). XPS analysis measured overall doping levels of approximately ~1.5 at% and ~10 at% for the 1×10 -4 and 1×10 -3 mol/L solutions, while the vanadium concentration in the 1×10 -5 sample was below the XPS detection limit. Direct enumeration of vanadium dopants by atomic resolution TEM value is used to denote each sample, and this is the region from which photoluminescence (PL) Raman spectroscopy and electrical transport measurements are generally taken.
Representative Raman spectra of pristine and V-doped WS 2 monolayers were obtained using excitation wavelengths of 532 nm ( Figure 1) and 488 nm (Supplementary Figure 5).
Pristine WS 2 monolayers exhibit the representative E′(Γ) and A 1 ′(Γ) first-order phonon modes at 355 and 417 cm -1 , respectively. [21] Both E′(Γ) and A 1 ′(Γ) blueshift as a function of vanadium concentration, which is consistent with previously reported spectra of V-doped MoS 2 . [26] In Vdoped WS 2 samples, the defect-activated longitudinal acoustic mode (LA(M)) gradually emerges as the vanadium concentration increases, indicating lattice disorder induced by V dopants. [27] A high-intensity 2LA(M) second-order double resonance peak involving two longitudinal acoustic phonons [28] characteristic for pristine WS 2 monolayers was progressively suppressed upon increasing vanadium concentration, indicating substantial changes in the WS 2 electronic structure which drive the system out of resonance. [21], [28] Pristine monolayers of WS 2 show an intense PL peak at 1.97 eV corresponding to the A exciton [29] . The optical gap decreases under increasing vanadium doping, while the PL peak broadens (likely due to lattice disorder from dopants possibly accompanied by vacancies), and drops in intensity. This evolution of the PL response is consistent with the Raman results discussed above. vacancies are more likely to be coupled to V atoms (written V W +S vac ), which is consistent with prior work on TMD alloys [11], [31] and first-principles calculations described below.
The magnetic properties of pristine and V-doped WS 2 monolayer samples were measured by a vibrating sample magnetometer. To exclude unwanted effects on the magnetization versus magnetic field (M-H) loops that can arise from subtracting diamagnetic and paramagnetic backgrounds [32], [33] , Figure 2 presents as-measured M-H loops at 300K and deduces the saturation magnetization and coercive field (H C ) directly from these loops. The pristine WS 2 sample exhibits a very weak ferromagnetic signal, which we tentatively ascribe to undercoordinated sulfur atoms at crystallographic defects (e.g. edges) [34] , on a diamagnetic background. Vanadium doping of 0.4 at% greatly increases the ferromagnetic signal (M S and H C ) with further strengthening at 2 at% doping and then a much weaker ferromagnetic response at 8 at% vanadium. The ferromagnetism observed in V-doped samples is too strong to originate from edge effects, and its dependence on vanadium concentration suggests an origin in local moments associated with unpaired electrons in vanadium d orbitals. [11], [16], [32] The 2 at% WS 2 sample shows large, clear hysteresis loops at all temperatures from which M S and H C are extracted and plotted as a function of temperature (the raw loops are close to square when rotated to account for the diamagnetic background). The saturation magnetization and coercivity both increase with decreasing temperature, with an interesting non-monotonicity of both around 150-200K.
Density functional theory calculations show local moments on substitutional vanadium atoms whose spin polarization and coupling are sensitive to the relative placement of dopants, a behavior similar to that seen in other computational investigations of doped TMDs [15], [16], [35] . A single vanadium dopant in a 7×7 supercell hosts a local moment of 0.67μ B with vanadium 2 character that is associated with a partially occupied spin-split defect level sitting ~74 meV below the valence band maximum, as shown in Figure 3. Table 1  This study successfully develops a universal, scalable and controllable synthesis route for V-doped WS 2 atomic layers as a dilute magnetic semiconductor, with intrinsic ferromagnetic ordering at room temperature. As the vanadium concentration increases, V-doped WS 2 monolayers exhibit a reduction of the optical bandgap and the emergence of p-type transport, reaching ambipolarity. The vanadium doping induces inherent ferromagnetic ordering at room temperature, with the strongest ferromagnetic signal for the moderately doped (2 at%.) sample.  [37] , the case of vanadium dopant coupled with sulfur monovacancy is set to be three symmetric neighboring sulfur monovacancies in the crystal structure to simplify the simulation process. The applied parameters, acceleration voltage, convergence angle and inner/outer angle for the HAADF detector and spherical aberration (C 3 and C 5 ), were all adjusted according to the experimental conditions.
DFT calculations: Spin-orbit-coupled DFT calculations were implemented in the Vienna Abinitio Simulation Package(VASP). [38], [39], [40] A 7×7 supercell of WS 2 was tested with different V doping levels. The z-axis cell dimension was 15 Å to isolate a layer from its in periodic images.
The energy cutoff in all calculation was 700 eV and the k-point sampling was set as 4×4×1 centered at Γ. The WS 2 unit cell lattice constant calculated as 3.188 Å matches with previous work [43] and was fixed for doped WS 2 since the doping level is not high enough to significantly change the lattice constant. The residual force after relaxation was smaller than 0.01 eV/ Å for all atoms. All visualizations were done with VESTA [44] and pymatgen [45] .
As the experimental distribution of vanadium dopants is irregular and covers a wide range of pairwise separations, we did not model the system with a regular array of dopants at uniform mutual separations but instead examined a dopant pair hosted within a large 7 ×7 supercell across a range of separations, so that we can elucidate their interactions on a pairwise basis. The two dopants within this supercell are closer to each other than to any periodic replicas.
We considered both ferromagnetic (parallel) and antiferromagnetic (anti-parallel) initial spin configurations for two vanadium dopants in a supercell, with the system also being able to converge self-consistently into an unpolarized state in the case of moment quenching.
Electronic Device fabrication: Pristine and V-doped WS 2 triangles were transferred from the growth substrate (Si/SiO 2 ) to a 50 nm thick and atomic layer deposition grown Al 2 O 3 substrate with Pt/TiN/p ++ Si as the back-gate electrode. All FETs were fabricated with a channel length of 1 µm with 40 nm Ni/30 nm Au as the source/drain contact electrodes defined using standard electron-beam lithography process.
Thermal transport measurements: To examine the thermal boundary conductances (h K ) of devices contingent on the use of doped WS 2 , we deposited a nominally 80 nm Al film via electron beam evaporation. We measured the total conductance of the Al/doped WS 2 /SiO 2 interface via time-domain thermoreflectance (TDTR). The specific analyses can be found elsewhere. [46] In our implementation, the 808.5 nm output of a Ti:Sapphire oscillator is spectrally separated into high-energy pump and low-energy probe paths. The pump is electro-optically    Table 1. Table 1| Net moments and energies for vanadium dopant pairs. Dopant pairs are labeled by their separation in lattice coordinates and colored in reference to the supercell in Figure 3. Systems were initialized with either parallel or anti-parallel local moments around the two dopants. Moments after selfconsistent iterations are perpendicular to the plane except for the (0,2) separation, which is 76° away from this axis. For the closest and next-closest dopant separations (★), the lowest energy state examined has no spatially resolvable spin texture. "--" means that both parallel and anti-parallel initial spin textures converge to the same self-consistent state. Dopant

Table of Contents
Room-temperature ferromagnetic order was obtained in semiconducting vanadium-doped tungsten disulfide monolayers. A reproducible and atmospheric pressure film sulfidation growth method led to doping concentration tunability in samples that preserve their properties while been stored in air. These monolayers developed p-type transport as a function of vanadium incorporation and rapidly reach ambipolarity. The ferromagnetic behavior in this dilute semiconductor was modeled and understood through first-principles calculations.

Supplementary Materials
1. Discussion of several V doping precursors for V-doped WS 2 monolayers synthesis. We have studied several vanadium precursors for doping atoms into WS 2 monolayers, 1. For vanadocene (II) (V(C 5 H 5 ) 2 ), we realize doping by powder vaporization growth; 1 it could be doped into the lattice with high concentration, however there are carbon contaminations or even a defective graphene layers underlying as-grown TMDs, as reported for other metalorganic precursors. 2 The morphology is degraded, meaning it fails to form large-area monolayer triangles. Atomic-resolution HAADF-STEM images of highly doped materials show extensive stripes of V dopants in the lattice ( Supplementary Fig. 1a), which substantially affects the physical properties.
2. For vanadium (III) chloride (VCl 3 ), we realize doping by powder vaporization growth; 1 as this precursor is very air-sensitive, it is handled inside a glove box. Doping could also be realized at high concentration, but the final morphology shows as few-layer WS 2 filled with etched holes probably related to the chloride (Supplementary Fig. 1b).

Vanadium (IV) oxide sulfate (VO[SO 4 ]) is our primary current focus, with different concentrations
of the precursor we can dope into TMDs as high as 12% of V with good morphology control. 4. Vanadium (V) oxide (V 2 O 5 ) does not work due high stability and extremely low vapor pressure which impedes sulfurization, at least for sulfur vaporization.
5. Ammonium metavanadate (V) (NH 4 VO 3 ) can be dissolved in DI water for spin coating; we have not tried this precursor, but it has been reported by a recent study, 3 which claimed highconcentration doping is possible, yet high-quality large-area monolayer triangles were not found for higher doping concentration.
The distribution of the transition metal dopants in the host TMD lattice is also highly dependent on the synthesis process, as the kinetically driven CVD process may result in segregation and stripe formation of V dopants. Vanadium segregation and striping were consistently detected when using vanadocene (V(C 5 H 5 ) 2 ) and vanadium (III) chloride (VCl 3 ) as precursors, which would significantly affect the magnetic properties of the materials as revealed by DFT calculations.       9. Estimation of the optimal doping level to obtain largest saturation magnetization Suppose the doping level is p, i.e. the likelihood of a given metal site being V is and (1 − ) for W. Starting from a given V dopant site, we assign half of the net magnetic moment given in Table 1 according to nearest dopant neighbor, assuming a random alloy. Specifically, the possibility of a V having its nearest dopant neighbor sitting at the 3 rd nearest metal site is (1 − ) 12 [1 − (1 − ) 6 ], and we assign 0.46 μ B to it. The possibility of its nearest dopant neighbor sitting at larger distance is (1 − ) 18 and we assign a rough averaged moment in this region, 0.64 μ B to it. Therefore, the expectation value of the moment contributed by a single V is roughly 0.46(1 − ) 12 18 }, whose maximum occurs at ≈ 7% with a corresponding magnetization of 2 × 10 −3 μ B /Å 2 . This simple estimate assumes an ideal random-alloy distribution of vanadium and a magnetic moment contribution based purely on dopant interactions with its nearest neighbor, but it gives a rough upper limit for optimal doping level, since the interaction with more neighbors will likely make the defect states more dispersive and reduce the net moments.
As an illustrative comparison, we present below the magnetic properties calculated without spin-orbit coupling. Table 1 Energy and net magnetic moment for two vanadium atoms in a supercell (no spin-orbit coupling) (★) shows the result is not spin-polarized. The antiferromagnetic state has a lower energy when neglecting spin-orbit coupling, and the spin-split dopant levels separate from the valence band sufficiently to attain integral occupancy. The general effect of quenching of the moment at close dopant separations is preserved even in the absence of spin-orbit interaction.  Figure 9 Bandstructures without spin-orbit coupling and the partial density of states without spin polarization. The red color of the bands represents a projection onto vanadium 2 orbital to characterize the defect states. The vacuum level has been set to 0.
Without spin-orbit coupling, the vanadium is a shallow dopant. From the one vanadium case, the spin splitting (0.138 eV) of the defect state brings one spin state down to the valence band to become fully occupied while the other spin state is fully unoccupied, resulting in an integral Bohr magneton polarization. For the case of two vanadium atoms sitting at nearest-neighbor sites, the system shows no spin polarization. Anti-bonding state is fully unoccupied and the bonding state hybridizes with WS 2 valence bands. For 2 vanadium dopants sitting farthest from each other with parallel spin directions, the two spin-down defect states are fully unoccupied, resulting in 2 Bohr magnetons of net magnetization. If they have anti-parallel spins, the two spin branches are nearly degenerate and the up and down spin states are localized at different dopant sites. 37 The quenching of the magnetic moments in the cases without spin-orbit coupling could be understood by comparing the bonding/anti-bonding splitting and the spin splitting of the defect state. If the bonding/anti-bonding splitting is larger, the moments are quenched. We choose 2 to track the defect state, since the single vanadium defect state projects only onto the V 2 orbital (and Mo and S orbitals). The bonding/anti-bonding energy splitting is 0.31, 0.20, and 0.15 eV for the closest three separations, beyond which the splitting is not resolved (<0.1 eV) due to the 0.05 eV Gaussian smearing.
For comparison, the spin splitting is 0.138 eV. The pair of bonding/antibonding levels is most apparent at 5.52 and 6.38 Å separations (for 3.19 Å, the lower defect state splits sufficiently that it overlaps with other valence band states).