MoS2 monolayers doped across the periodic table show p-type, n-type, metallic, and magnetic behavior, providing building blocks for all-MoS2 device integration.
(Nanowerk Spotlight) Among the many techniques that make modern processors possible, doping is one of the most fundamental. By introducing trace impurity atoms into a semiconductor, engineers control whether it conducts through electrons or holes, a distinction that underpins complementary circuit design across the silicon industry. As chipmakers approach the physical limits of silicon scaling, atomically thin alternatives have gained serious traction.
Molybdenum disulfide (MoS₂), a crystal just three atoms thick built from a layer of molybdenum between two layers of sulfur, already appears on the International Roadmap for Devices and Systems as a future transistor channel material. Yet the comprehensive doping knowledge that matured over decades for silicon has no equivalent for MoS₂.
The obstacle is partly chemical. Conventional growth techniques such as chemical vapor deposition and molecular beam epitaxy rely on gaseous precursors, restricting the menu of dopant elements to a small handful. P-type doping, which creates hole carriers essential for complementary circuits, has proven especially stubborn. Sulfur vacancies that form naturally during MoS₂ growth introduce electron-donating defect states that overpower most acceptor dopants, pinning the material in n-type territory. The range of doping effects in single-layer semiconductors that researchers have been able to access has therefore remained narrow.
Without both carrier types and the electronic and magnetic tunability that diverse dopants can provide, MoS₂ remains limited as a platform for integrated devices.
A study published in Advanced Materials (“Experimentally Mapping the Elemental Doping of MoS₂ Monolayer”) fills much of that gap. Using a growth technique called liquid phase edge epitaxy (LPEE), the research team doped MoS₂ monolayers with more than 40 elements spanning the periodic table. The collection includes all 3d, 4d, and 5d transition metals (except radioactive technetium), nearly all lanthanide rare earths (except radioactive promethium), and several main group elements: gallium, indium, tin, antimony, lead, and bismuth.
Illustration of the preparation of doped MoS₂ monolayers by liquid phase edge epitaxy (LPEE) method. (a) Schematics of the LPEE method. (b) Overview of the elements doped into MoS₂ monolayers in this work. (Image: Reproduced with permission from Wiley-VCH Verlag) (click on image to enlarge)
The LPEE method dissolves dopant precursors, supplied as sulfides or chlorides, together with MoS₂ powder in a molten cesium chloride flux at 800–900 °C. Because the liquid phase accommodates a far wider range of precursor chemistries than the gas phase, it sidesteps the precursor limitations that have constrained earlier efforts. The monolayers grew on sapphire substrates and were confirmed to be single layers by optical microscopy and atomic force microscopy.
Crystalline quality was verified at the atomic scale. Scanning transmission electron microscopy revealed a pristine honeycomb lattice in titanium-doped MoS₂, with individual dopant atoms clearly visible at molybdenum positions. Atom-by-atom spectroscopic mapping pinpointed each titanium atom and confirmed it had replaced a molybdenum atom rather than sitting elsewhere in the lattice. X-ray photoelectron spectroscopy verified the identity and concentration of each dopant across the full library, with doping levels ranging from 0.1% to 5.5%.
Electrical measurements sorted this library into three categories. The largest group showed n-type transport, consistent with the intrinsic electron-rich character of MoS₂. Among these, chromium, iron, palladium, cerium, neodymium, dysprosium, and holmium doped monolayers maintained high on/off ratios of 10⁴ or above, meaning the transistor could be switched cleanly between conducting and insulating states.
A second group, including copper, gallium, zirconium, niobium, indium, tin, hafnium, tantalum, lead, bismuth, and the rare earths europium, gadolinium, thulium, and lutetium, behaved more like metals, with on/off ratios below 10 and little response to an applied gate voltage.
The third category was p-type transport, achieved with titanium, zinc, and gold. Because sulfur vacancies strongly favor n-type conduction in MoS₂, obtaining reliable hole transport has been a persistent challenge across the field. That these three dopants can overcome that native tendency is one of the study’s key results.
Density functional theory (DFT) calculations helped explain the observed behaviors. For titanium, the calculations showed that when it replaces a molybdenum atom, it acts as an electron acceptor, pulling the Fermi level, the energy boundary between occupied and empty electronic states, toward the valence band and producing the p-type character seen in experiments.
The calculations also mapped where different dopants prefer to sit within the MoS₂ lattice. Early transition metals from groups 3 through 8 favored replacing molybdenum. Group 9 elements showed no strong preference, with several configurations nearly equal in energy. Group 10 elements could occupy either sulfur lattice sites or sulfur-adsorption positions. Groups 11 and 12 transition metals, along with gallium, indium, tin, and lead, preferred to adsorb on top of sulfur atoms rather than substitute into the lattice.
Raman spectroscopy corroborated these predictions. A phonon mode at roughly 227 cm⁻¹, normally forbidden in perfect MoS₂, can be activated by point defects that disrupt the crystal’s regular repeating pattern. This mode appeared in monolayers doped with elements that replace molybdenum, such as vanadium, niobium, tantalum, and tungsten. It was absent in monolayers doped with elements that sit on sulfur-adsorption sites, such as zinc, cadmium, and gold. The presence or absence of this Raman peak serves as a spectroscopic fingerprint for identifying where a dopant resides.
Photoluminescence measurements revealed element-specific optical changes. Emission intensity decreased after doping across the board, but the peak position shifted in ways that tracked with doping configuration. Titanium, manganese, hafnium, tantalum, and rhenium produced a blue shift, while chromium and lanthanum series elements shifted the emission toward longer wavelengths. These tunable shifts suggest that doped MoS₂ monolayers could serve in light-emitting and photodetection devices.
Beyond electronics, the study uncovered magnetic functionality. Monolayers doped with the 2D rare earths cerium, neodymium, dysprosium, and holmium exhibited soft ferromagnetic behavior while retaining semiconducting transport. The researchers attribute this to an indirect exchange mechanism in which localized magnetic moments on the rare earth atoms couple through surrounding conduction electrons, producing collective magnetic order at low temperatures.
Having p-type, n-type, and metallic MoS₂ variants within a single material platform is a prerequisite for complementary circuit design. Rather than combining different two-dimensional materials for each carrier type, which complicates fabrication, all necessary building blocks could in principle come from differently doped versions of the same host crystal.
The doping level itself can be adjusted by changing the ratio of dopant precursor to MoS₂ in the growth mixture. Higher concentrations were shown to lower on/off ratios in titanium and zinc doped samples, while vanadium doped MoS₂ switched from n-type to p-type at elevated dopant levels.
By pairing experimental transport and spectroscopic data with first-principles calculations for each element, this work establishes a periodic-table-wide reference that connects dopant identity, preferred lattice site, and resulting device behavior. Combined with the magnetic functionality observed in rare earth doped variants, the library provides the raw ingredients for all-MoS₂ electronics and, potentially, for spintronic devices that harness both charge and spin in an atomically thin platform.
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Liming Xie (National Center for Nanoscience and Technology)
, 0000-0001-8190-8325 corresponding author
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