Intra-domain periodic defects in monolayer MoS2

Abstract

We present an ultra-high vacuum scanning tunneling microscopy (STM) study of structural defects in molybdenum disulfide thin films grown on silicon substrates by chemical vapor deposition. A distinctive type of grain boundary periodically arranged inside an isolated triangular domain, along with other inter-domain grain boundaries of various types, is observed. These periodic defects, about 50 nm apart and a few nanometers in width, remain hidden in optical or low-resolution microscopy studies. We report a complex growth mechanism that produces 2D nucleation and spiral growth features that can explain the topography in our films.The many incredible properties of graphene including high carrier mobility (200,000 cm 2 V −1 s −1 ) 1 have made it a very special material both from fundamental science and an engineering point of view. However, the lack of a band-gap in graphene causes high leakage current which makes it unsuitable for many optoelectronic purposes and logic-based devices and circuits. In contrast, transition metal dichalcogenides (TMDs) with the general chemical formula MX2 (M = Mo, W; X = S, Se, Te) provide a large family of two-dimensional (2D) crystals that vary greatly in physical and chemical properties 2 , ranging from metallic to semiconducting to insulators. Of all the TMDs, molybdenum sulfide (MoS2), with its indirect-to-direct band gap transition as a function of layer thickness, has been of particular interest for digital and optoelectronic applications. MoS2 has already been used to fabricate functional electronic circuit elements 3-6 , as well as used for optoelectronics 7-9 , valleytronics, spintronics 10, 11 and coupled electro-mechanics 12 .Most of the MoS2 material characterization and device demonstrations so far have been on exfoliated samples which suffer from low yield, and cannot be scaled up for practical applications. In order to address these problems, significant work has been done to introduce different growth techniques. Processes including liquid exfoliation 13 and direct sulfurization of molybdenum thin films 14 have been achieved to synthesize large MoS2 monolayers. However, the overall simplicity and the high quality of films obtained using the sulfurization of MoO3 has made it one of the most widely used methods of synthesizing large area monolayer MoS2 15-17 .Just like different synthesis techniques, various analytical techniques have been introduced. In addition to the commonly used techniques like scanning electron microscopy (SEM) and atomic force microscopy (AFM), techniques like Raman and photoluminescence (PL) spectroscopy have become common to ascertain the number of layers of these 2D materials. However, because of the resolution limit, these techniques only reveal a partial picture. SEM and AFM together show us the topographical and structural information. The spectroscopic techniques ascertain the energy levels to a certain degree. Recent techniques like microwave impedance microscopy (MIM) 18 have been used to map the dielectric constant of these films. Howev...

Two-Step Growth of Uniform Monolayer MoS2 Nanosheets by Metal–Organic Chemical Vapor Deposition

“…Following the postgrowth sulfurization process at 850 °C for 30 min, mostly isolated triangular MoS 2 domains of ∼30 μm in size are formed [as shown in the optical and SEM images in Figure 2 h,i, respectively], with a few occurrences of the domains merging to form different types of grain boundaries [as shown in Figure S4 ], as also observed in MoS 2 grown by the APCVD method. 41 − 43 Raman [ Figure 2 j] and PL [ Figure 2 k] spectroscopies closely match with that of APCVD-grown MoS 2 and the reported literature. 41 A slight shift in PL peak position in the case of MOCVD sulfurized domains may be attributed to the strain associated with the high-temperature annealing process.…”

“…This strain is relaxed upon transfer of the film from the growth substrate. 41 SEM images taken from six different spots of the MOCVD film following sulfurization [as marked in Figure 2 g] are shown in Figure 2 l. Raman spectra from different spots across different MoS 2 domains are shown in Figure S5 which does not show any significant variation. These observations confirm that unlike APCVD, this method produces triangular monolayer domains distributed homogeneously across the entire substrate.…”

“…Shift in the PL peak position upon transfer indicates relaxation of strain that develops during the high-temperature growth and annealing cycles. 41 Suitable MoS 2 domains were identified using a combination of optical contrast, Raman spectroscopy, and AFM images. Next, drain/source metal contacts were patterned using e-beam lithography and, subsequently, contact metals (Ni/Au 20 nm/30 nm) were deposited using e-beam evaporation followed by liftoff.…”

Progress in Contact, Doping and Mobility Engineering of MoS2: An Atomically Thin 2D Semiconductor

“…Moreover, synthetic growth techniques can also result in MoS 2 surface contamination and substrate property modification (giving rise to charged impurities and/or interface traps) and can also cause growth-induced strain in the MoS 2 films [313]. Furthermore, in addition to the various "inter-domain" dislocations and GBs, another distinctive type of narrowly spaced (~50 nm apart) "intra-domain" GBs or periodic defects, arranged in the form of concentric triangles, have also been reported in CVD-grown MoS 2 films by Roy et al (Figure 15e shows an STM image revealing these intra-domain periodic defects in CVD MoS 2 films) [314]. While several of the defect-passivation techniques described earlier in this section can also be applied to these synthetic MoS 2 films, it is highly necessary to optimize the synthetic growth process itself to achieve defect-free, single-crystalline and pure MoS 2 films over commercial wafer-scale substrates that will enable the integration of large-scale 2D MoS 2 -based devices and circuits.…”

Environmental Effects on the Electrical Characteristics of Back-Gated WSe2 Field-Effect Transistors

“…These graphene-like materials offer the advantages of sizeable and non-zero bandgap, high on/off ratio and quasi-ideal subthreshold swing, mechanical flexibility, and thermal and chemical stability. Similar to graphene, their electronic transport properties are strongly influenced by the choice of the metal contacts [ 8 , 9 , 10 ], by interface traps and impurities [ 11 , 12 ], as well as by structural defects and environmental exposure [ 13 , 14 , 15 , 16 , 17 ]. These effects need to be understood and controlled for technological applications.…”