The intrinsic defect structure of exfoliated MoS2 single layers revealed by Scanning Tunneling Microscopy

MoS2 single layers have recently emerged as strong competitors of graphene in electronic and optoelectronic device applications due to their intrinsic direct bandgap. However, transport measurements reveal the crucial role of defect-induced electronic states, pointing out the fundamental importance of characterizing their intrinsic defect structure. Transmission Electron Microscopy (TEM) is able to image atomic scale defects in MoS2 single layers, but the imaged defect structure is far from the one probed in the electronic devices, as the defect density and distribution are substantially altered during the TEM imaging. Here, we report that under special imaging conditions, STM measurements can fully resolve the native atomic scale defect structure of MoS2 single layers. Our STM investigations clearly resolve a high intrinsic concentration of individual sulfur atom vacancies, and experimentally identify the nature of the defect induced electronic mid-gap states, by combining topographic STM images with ab intio calculations. Experimental data on the intrinsic defect structure and the associated defect-bound electronic states that can be directly used for the interpretation of transport measurements are essential to fully understand the operation, reliability and performance limitations of realistic electronic devices based on MoS2 single layers.


I. Sample preparation
The MoS 2 single layers have been exfoliated onto Au (111) substrates using a novel mechanical exfoliation technique developed by us, and described elsewhere [1], yielding single layers of several hundreds of microns lateral size. To ensure the high structural quality of the investigated samples we used synthetic bulk MoS 2 crystals (2DSemiconductors), which we found of higher quality than naturally occurring crystals investigated by us. The exfoliation process involves only a mild sonication step of a few seconds, which is not expected to induce any additional defect sites as samples sonicated for longer times did not display a higher defect concentration. The single layer nature of such flakes has been confirmed by a combination of optical microscopy, Raman spectroscopy as well as STM measurements of step height relative to the gold substrate near the MoS 2 flake edges.

II. Scanning Tunneling Microscopy and Spectroscopy characterization
STM investigations have been performed on a Nanoscope E instrument, at room temperature, under ambient and in pure N 2 atmosphere, using an atmospheric hood, in order to protect the samples from oxidation during scanning with higher bias voltages (>1V). Landing the STM tip on a single layer MoS 2 flake has been achieved by guiding the tip under an optical microscope. This had been made possible by the large (>100 microns) lateral size of the exfoliated single layer flakes. Typical imaging parameters for atomic resolution were 5 -50 mV, and 1 -3 nA. To clearly resolve the atomic structure of point defects, ideal tip conditions and the fine tuning of the imaging parameters was needed. In contrast, to the apparent resolution of the hexagonal lattice of the top S atoms, the high quality images clearly resolving individual point defects could not be easily achieved.  Reliable tunneling spectroscopy over a single point defect could not be achieved due to the thermal drift at the room temperature. However, reproducible tunneling spectra could be acquired near the defect sites. A typical spectrum is shown in Fig S2.a.

Fig. S2. Tunneling spectra acquired on MoS 2 single layer deposited on Au (111) substrate.
The tunneling spectra display a band gap of about 1.8 eV in excellent agreement with previous results. However, the density of states within the gap has a small but finite value, which we attribute to the presence of the Au substrate. It is also apparent that the Fermi level (vertical blue line) is close to the valence band maximum While the spectrum in Fig S2 a, has been predominantly observed, in a few areas spectra with significantly shifted Fermi level position have also been observed (Fig. S2 b), evidencing the spatial inhomogeneity of the doping, which we attribute to the variation of the interaction strength between the MoS 2 layer and the Au (111) substrate, due to impurity intercalation in restricted areas.

III. Density Functional Calculations
The DFT calculations were performed within the framework of local density approximation (LDA) using the Vienna ab initio simulation package (VASP) [2,3]. Projector augmented wave (PAW) pseudo-potentials [4,5] were used and the kinetic energy cut-off for the plane wave expansion was 350 eV. We applied 8×8 surface unit cell (192 host atoms) where the inplane lattice constant (distance between neighbor S atoms) was 3.124 Å from our previous LDA calculations [6] and the vacuum thickness was 15 Å. The atomic structures were relaxed until the Hellmann-Feynman forces are less than 0.02 eV/Å. The Γ-point was used for the Brillouin-zone summation. STM images were simulated with the simple Tersoff-Hamann approximation [7] using the calculated local density of states.

IV. Sulfur Divacancies
The atomic structures of V S and V 2S are compared in Fig. S3 a and