Imaging of dopants in surface and sub-surface layers of the transition metal dichalcogenides WS 2 and WSe 2 by scanning tunneling microscopy

Abstract

Abstract. The van-der-Waals surfaces (0001) of the layered structure semiconductors WS 2 and WSe 2 are known to be free of intrinsic surface states. Therefore, they provide an ideal system for investigations of the influence of individual dopants on the local electronic properties, which can be measured by scanning tunneling microscopy (STM). Individual dopant sites were resolved as topographic depressions superimposed on the atomically resolved lattice. The apparent depth of these depressions showed a discrete statistical distribution and was attributed to the spatial depth of the dopant site. Using an STM-induced electrochemical process, we could locally expose the first and second sub-surface layer to correlate the previously recorded topographic contrast to the location of buried dopants. To our knowledge this is the first direct proof of the capability of STM to detect individual sub-surface dopants. An interpretation of the contrast mechanism is given in terms of tip-induced band-bending effects and current transport mechanisms involving minority charge carrier injection and majority charge carrier extraction.

Surface Defects on Natural MoS2

“…The difference between the intensity in the outlined area in panels a and b of Figure 5 is correlated with dopant localization in two different subsurface layers. 49,50 Similar observations were generated by defects in AsGa, InP, GaP, and InSb semiconductor surfaces. 49 MoS 2 Electronic Structure: Scanning Tunneling Spectroscopy Studies.…”

“…49 Such a depression could also be induced by a vacancy located at the subsurface layer. 50 Likewise, the bright contrast observed in Figure 5b could be caused by impurities located beneath or atop the topmost surface (donor atom). 49 Topographically, the local contrast changes the overall rms roughness slightly by an amount 0.02−0.05 nm.…”

Scanning tunneling microscopy investigation of nanostructures produced by Ar+ and He+ bombardment of MoS2 surfaces

“…In this case the dopant states induced by Ar + sputtering are p type or acceptor states as judged by their contrast dependence on STM imaging bias, 20,23 whereas the natural background defects are n type since they have the opposite contrast relationship in STM images ͓Figs. These peak shifts cannot be explained by local changes in the chemical environment for Mo-S bonding ͑changes of formal oxidation state͒ since sulfur cannot be reduced further than an oxidation state of −2.…”

“…[5][6][7][8][9][10][11] Basal plane defects ͑sulfur vacancies͒ produced at high temperatures during HDS have been also proposed to be active catalytic sites. [17][18][19][20][21][22][23][24] Secondary ion mass spectrometry of natural MoS 2 crystals was used by Permana, Lee, and Simon Ng to show that natural defects could be caused by intercalated vanadium or fluoride atoms. [14][15][16] The easy cleavage of MoS 2 and other layered transition metal dichalcogenides ͑TMDs͒ ͑MoSe 2 , WS 2 , WSe 2 ...͒ can produce large, stable, and atomically flat areas, making these materials ideal substrates for scanning tunneling microscopy ͑STM͒ investigations.…”

“…Various natural defects such as ringshaped a structures, topological hillocks, and depressions have been observed on the MoS 2 crystal surfaces with STM. 20 The effect of impurity atoms on STM image imperfections has been investigated by doping MoSe 2 with Re and depositing alkali metal atoms onto MoS 2 . These impurities have been imaged with STM as protrusions and ring structures depending on their ionic charges.…”