172 | Nature | Vol 581 | 14 May 2020
Article
or M 2 X 3 layered 2D compounds (M, metal; X, chalcogen), producing
covalently bonded MxXy compounds. We term this class of materials
ic-2D. Taking TaS 2 as an example, the intercalated Ta atoms occupy the
octahedral vacancies in the vdW gap to form distinct topographical pat-
terns, as verified by atomic resolution scanning transmission electron
microscopy–annular dark field (STEM–ADF) imaging. By varying the
ratio of intercalating atoms to octahedral vacancies in the vdW gap, we
grew TaxSy or TaxSey films and quantified the extent of Ta-intercalation
using σ, the percentage of initial total vacancy sites that are occupied by
intercalated atoms. Our results indicate that self-intercalation is com-
mon to a broad class of vdW crystals, and it offers a powerful approach
through which to transform layered 2D materials into ultrathin, cova-
lently bonded ic-2D crystals with ferromagnetic properties.
We first describe the self-intercalation of native atoms—that is, Ta—
into a TaS 2 bilayer during MBE deposition on a silicon wafer, as a means
to demonstrate the formation of an ic-2D film via octahedral vacancy
filling of a 2D bilayer material. Wafer-scale Ta-intercalated TaS 2 bilayer
films were grown on 2-inch, 285-nm SiO 2 /Si wafers in a dedicated MBE
system^14. Ultra-pure Ta and S molecular beams were evaporated from
an e-beam evaporator and a sulfur cracker cell equipped with a valve,
respectively (Fig. 1a, b). We could routinely grow 2H-phase TaS 2 bilayer
films using a high S chemical potential—that is, a Ta-to-S flux ratio of
around 1:10 (Fig. 1a, Supplementary Fig. 1)—for 3 h and a substrate
temperature of 600 °C. When the Ta:S flux ratio was increased to 1:6
(Fig. 1b, c), the film became non-stoichiometric with respect to TaS 2
owing to the excess of Ta atoms. A fingerprint of the Ta-rich environ-
ment is the presence of Ta adatoms (Fig. 1d) occupying the centre of
the honeycombs (Fig. 1e) or situated on top of the Ta sites (Fig. 1f) in
the monolayer TaS 2 film, as observed by STEM when the growth was
interrupted partway through (Supplementary Fig. 2). When Ta and
S are continually supplied in the appropriate ratio, the Ta adatoms
become embedded in the TaS 2 structure, occupying the octahedral
vacancies between two S layers (Fig. 1g). The ic-2D crystals therefore
have a sequential, TaS 2 -Ta -Ta S 2 -Ta layer-by-layer growth mechanism;
as such, multilayer or bulk-phase ic-2D crystals can be readily accessed
simply by increasing the growth time. The thermodynamic stability
of such intercalated phases was assessed using energy-composition
phase diagrams generated through density functional theory (DFT)
calculations (Fig. 1h). It was found that stoichiometric H-phase TaS 2 is
formed only under S-rich conditions (when the chemical potential of
sulfur, μS, exceeds −5.3 eV), whereas at higher Ta:S flux ratios (low μS),
various Ta-intercalated TaxSy configurations—ranging from Ta 9 S 16 (25%
Ta intercalation) to Ta 8 S 12 (66.7% Ta intercalation)—entered a thermo-
dynamically stable state.
Notably, a Ta:S flux ratio of approximately 1:6 produced a 3×aa 3
superlattice of Ta atoms (Fig. 2a) sandwiched between two TaS 2 mon-
olayers. The extent of intercalation (σ) was 33.3%, and the overall stoi-
chiometry of the crystal became Ta 7 S 12 , as corroborated by both the
real-space STEM image (Fig. 2b) and the corresponding fast Fourier
transform (FFT) pattern (Fig. 2c). Image simulation and sequential
STEM images capturing the diffusion of intercalated atoms showed
that the periodically arranged bright spots in the STEM image were
induced by the intercalation of Ta (Fig. 2d, Supplementary Information
section 1, Supplementary Videos 1, 2). We also collected STEM
cross-section images (Fig. 2e, f) to verify the existence of an intercalated
Ta atomic layer in the vdW gap of ic-2D films grown by CVD.
The homogeneous Ta 7 S 12 phase was grown directly on a 2-inch silicon
wafer (Supplementary Fig. 3). The Ta 7 S 12 film was formed by the coa-
lescence of nano-domain crystals (around 50 nm) separated by mirror
twin boundaries or tilted grain boundaries (Supplementary Informa-
tion section 2). The amorphous islands and gaps seen in the STEM
images were attributed to the poor stability of TaxSy and to sample
damage incurred during transfer. Energy dispersive X-ray spectroscopy
(EDS) and electron energy loss spectroscopy (Supplementary Fig. 4)
verified that the film was composed solely of Ta and S, with no foreign
elements, and X-ray photoelectron spectroscopy (Supplementary
Fig. 5) confirmed that the chemical stoichiometry agreed very well
with Ta 7 S 12. The Raman spectra of the film exhibited two prominent Eg^3
and A1g^3 peaks at 300 cm−1 and 400 cm−1, respectively, matching those
of H-phase TaS 2 films. The fingerprint of the intercalation was a series
of minor peaks in the 100 cm−1 to 170 cm−1 range (Supplementary Fig. 6),
which were absent in pure H-phase TaS 233 and are attributed to the
covalent bonds between the intercalated Ta atoms and their octahe-
drally coordinated S atoms (Supplementary Fig. 7).
25% Ta-intercalated TaS 2 has a stoichiometry of Ta 9 S 16 and was pro-
duced at a slightly lower Ta chemical potential than Ta 7 S 12 , correspond-
ing to a Ta:S ratio of around 1:8. The intercalated Ta atoms occupy the
octahedral vacancies in every 2 aa×3 unit length, and this phase was
distinguished by the square symmetry of the intercalated atomic lattice
(Fig. 2g, k, Supplementary Fig. 8). When the Ta:S flux ratio was further
increased to 1:5, a Ta 10 S 16 phase (σ = 50%) was successfully grown
(Fig. 2h). The intercalation concentration—the percentage of total
vacancy sites that were occupied—was determined to be exactly 50%
via atom counting (Supplementary Fig. 9). Notably, this phase is char-
acterized by atomic chains that are interconnected over a short range,
forming an overall glassy phase. Clear diffusive rings were observed
in the proximity of the first-order FFT spots (Fig. 2l, Supplementary
Fig. 10), confirming this short-range ordered structure^34. When the
Ta:S flux ratio was further increased, the glassy phase was retained,
but the short atomic chains became denser before fully evolving into
a complete atomic plane when σ reached approximately 100% (Sup-
plementary Fig. 11). The use of growth conditions intermediate between
those that give rise to high-symmetry phases resulted in phase separa-
tions, and atomically sharp domain boundaries separating two
high-symmetry phases were apparent (Supplementary Information
section 3).
To verify that ic-2D films could be produced by methods other than
MBE, we used CVD to grow self-intercalated TaxSey crystals using excess
Ta precursors. The crystal domains of these films were in the micro-
metre range—considerably larger than the nanosized domains grown
by MBE (Supplementary Fig. 12). A typical Ta 8 Se 12 crystal (σ = 66.7%) is
depicted in Fig. 2i. Notably, it possesses a Kagome lattice belonging to
the P 6 wallpaper symmetry group. A well-defined 3×aa 3 periodic
lattice can be unambiguously identified in the atomic-resolution STEM
image (Fig. 2m; for the simulated image, see Supplementary Fig. 13).
At even higher Ta chemical potential we successfully synthesized Ta 9 Se 12
crystals (σ = 100%), in which the trigonal prismatic vacant sites in
AA-stacked Ta 9 Se 12 were fully occupied (Fig. 2j)—as seen from the top
view (Fig. 2n) and side view (Fig. 2e, Supplementary Fig. 14) STEM
images. By precisely controlling the metal:chalcogen ratio during
growth, we can prepare a full range of Ta-intercalated TaxSey or TaxSy
compounds with intercalation levels ranging from σ = 25% to over 100%,
as verified by EDS (Supplementary Fig. 15, Supplementary Table 1).
In ic-2D films, the intercalated Ta atoms are octahedrally coordi-
nated to the S 6 cage, as opposed to the trigonal-prismatic coordination
that is adopted in pristine TaS 2. Charge transfer from the intercalated
Ta atoms to the TaS 2 host layers creates new electron ordering and
modifies the Ta d-band splitting. Because the amount of charge trans-
fer is dependent on the concentration of the intercalant, the system
can be tuned. To investigate whether ferromagnetic order is present
in the intercalated samples, magneto-transport measurements were
carried out on MBE-grown Ta 7 S 12 (σ = 33.3%) with a predominantly 2Ha
stacking registry (Fig. 3a, Supplementary Fig. 16) and bilayer thickness
(Supplementary Fig. 17). Figure 3c shows the temperature-dependent
resistivity, in which a non-saturating upturn is observed below 30 K
owing to the disorder-induced metal–insulator transition in the poly-
crystalline sample^35. Linear magnetoresistance up to 9 T is observed
at low temperatures in Ta 7 S 12 (Fig. 3d), owing to density and mobil-
ity fluctuations^36. The anomalous Hall effect (AHE) arises from the
interplay of spin–orbit interactions and ferromagnetic order, and is