Realization of fractional-layer transition metal dichalcogenides

Abstract

Layered van der Waals transition metal dichalcogenides (TMDCs), generally composed of three atomic X-M-X planes in each layer (M = transition metal, X = chalcogen), provide versatile platforms for exploring diverse quantum phenomena. In each MX₂ layer, the M-X bonds are predominantly covalent in nature and, as a result, the cleavage of TMDC crystals normally occurs between the layers. Here we report the controllable realization of fractional-layer WTe₂ via an in-situ scanning tunneling microscopy (STM) tip manipulation technique. By applying STM tip pulses, hundreds of the topmost Te atoms are removed to form a nanoscale monolayer Te pit in the 1 T'-WTe₂, thus realizing a 2/3-layer WTe₂ film. Such a configuration undergoes a spontaneous atomic reconstruction, yielding a unidirectional charge density redistribution with the wavevector and geometry quite distinct from that of pristine 1 T'-WTe₂. Our results expand the conventional understanding of the TMDCs and are expected to stimulate further research on the structure and properties of fractional-layer TMDCs.

Fig. 1. Nanoscale fabrication of 2/3-layer WTe₂.

a Schematic of fabricating 2/3-layer WTe₂. Upper panel: Experimental set-up. 1 T′-WTe₂ crystal is firstly placed onto Si substrates, and then covered by graphene monolayer. The 2/3-layer WTe₂ can be realized by applying a scanning tunneling microscopy (STM) tip pulse. Bottom panel: The 2/3-layer WTe₂ is derived from its monolayer by removing the topmost Te atoms. b Upper panel: large-scale STM image of graphene-covered 1 T′-WTe₂ before applying a STM tip pulse. STM parameters: V_s = 1 V (bias voltage with respect to the sample), I = 100 pA (tunneling current). Inset: atomically resolved STM image (V_s = 1 V, I = 100 pA). Bottom panel: height profile recorded along the yellow arrow. c–e Upper panels: STM images after applying a STM tip pulse of 3 V, 4 V, and 8 V for 10 ms duration, respectively (V_s = 1 V, I = 100 pA). Bottom panels: height profiles recorded along the yellow arrows. f The depth of the pit as a function of its lateral size. Inset: Top and side views of the atomic structure of 2/3-layer WTe₂ embedded into 1 T′-WTe₂. The depth and the lateral size of the pit are marked.

Fig. 2. STM verification for realizing fractional-layer 1 T′-WTe₂.

a STM image of graphene-covered 1 T′-WTe₂ after applying a STM tip pulse of 6 V for 10 ms duration. The pit shows an irregular hexagonal outline (V_s = 1 V, I = 100 pA). b Typical STM image after applying an additional STM tip pulse of 4 V for 70 ms duration (V_s = 1 V, I = 100 pA). The pit becomes a regular hexagon. c, d Schematic of the formation process of 2/3-layer WTe₂. The arrow in panel (c) symbolically shows that the marked atoms are released. The outlines of the pits in panels (a–d) are marked by the dashed lines.

Fig. 3. Atomically resolved STM images around the tip-induced pit.

a, d Typical STM images of graphene-covered 1 T′-WTe₂ after applying a tip pulse of 8 V for 10 ms duration under the sample biases of 0.3 V and 1.0 V (I = 100 pA). The sizes of the images are 16 × 16 nm² and the resolutions are 320 × 320 pixels. The regions of graphene-covered 1 T′-WTe₂, 2/3-layer WTe₂, and β-Te are marked by the yellow, red, and green dashed boxes, respectively. b, e, Fourier transforms of panels a and d by using the square root normalization. The bright spots marked by the white, yellow, and red circles indicate the reciprocal lattices of graphene, 1 T′-WTe₂, and 2/3-layer WTe₂. c Model of atomically reconstructed 2/3-layer WTe₂, where a₂ = a₁ + b₁, b₂ = −3a₁ + b₁. f DFT calculation of the atomic structure and the corresponding STM simulation of the relaxed 2/3-layer WTe₂. g–i High-resolution STM images of graphene-covered 1 T′-WTe₂, 2/3-layer WTe₂, and β-Te recorded at the regions marked in panel a by the yellow, red, and green dashed boxes (V_s = 1 V, I = 100 pA). The sizes of the images are 4 × 4 nm², 6 × 6 nm², 3 × 3 nm², and the resolutions are 320 × 320 pixels. The measured superlattice constants of 2/3-layer WTe₂ are a₂ = 8.2 Å, b₂ = 13.7 Å with the intersection angle of 86°, and the rotated angle between a₂ and a₁ is 63°. j–l Fourier transforms of panels g-i by using the square root normalization.

Fig. 4. Electronic properties of 2/3-layer WTe₂.

a STM image of graphene-covered 1 T′-WTe₂ after applying a STM tip pulse of 8 V for 10 ms duration (V_s = 1 V, I = 100 pA). b, c Site-dependent scanning tunneling spectroscopy (STS) spectra recorded along the yellow and green arrows marked in panel a. The edges of the 2/3-layer WTe₂ are marked by the black dashed lines. d STS spectra acquired at the locations marked in panel a. The blue and red lines correspond to the STS spectra recorded on graphene-covered 1 T′-WTe₂ and 2/3-layer WTe₂, respectively. e–h Spectroscopic maps acquired at the same location as panel a under the sample bias of −0.6, −0.2, 0.5, and 1.0 eV, respectively.