Electronic structures of defects and magnetic impurities in MoS2 monolayers

Lu and Leburton Nanoscale Research Letters 2014, 9:676 http://www.nanoscalereslett.com/content/9/1/676 NANO EXPRESS Open Access Electronic structures of defects and magnetic impurities in MoS2 monolayers Shang-Chun Lu and Jean-Pierre Leburton* Abstract We provide a systematic and theoretical study of the electronic properties of a large number of impurities, vacancies, and adatoms in monolayer MoS2, including groups III and IV dopants, as well as magnetic transition metal atoms such as Mn, Fe, Co, V, Nb, and Ta. By using density functional theory over a 5 × 5 atomic cell, we identify the most promising element candidates for p-doping of MoS2. Specifically, we found VB group impurity elements, such as Ta, substituting Mo to achieve negative formation energy values with impurity states all sitting at less than 0.1 eV from the valence band maximum (VBM), making them the optimal p-type dopant candidates. Moreover, our 5 × 5 cell model shows that B, a group III element, can induce impurity states very close to the VBM with a low formation energy around 0.2 eV, which has not been reported previously. Among the magnetic impurities such as Mn, Fe, and Co with 1, 2, and 3 magnetic moments/atom, respectively, Mn has the lowest formation energy, the most localized spin distribution, and the nearest impurity level to the conduction band among those elements. Additionally, impurity levels and Fermi level for the above three elements are closer to the conduction band than the previous work (PCCP 16:8990-8996, 2014) which shows the possibility of n-type doping by Mn, thanks to our 5 × 5 cell model. Keywords: DFT; MoS2; P-type dopants; Magnetic impurities; Density-of-states; Formation energy PACS: 73.20. -r; 73.20.At; 73.20.Hb Background Recently, two-dimensional (2D) materials have attracted intensive attentions due not only to the rich and fundamental physics brought by them but also to their potential for nanoscale device applications [1]. Graphene [2-5] is the most well-known member in the family of 2D materials, but its gapless band structure has been deemed as a considerable drawback for realizing switching operation, which is essential for digital logic devices. Even though the bandgap of graphene can be engineered by depositing on particular substrates [6] or fabricating nanoribbons [7,8], it deteriorates the mobility. For this reason, researchers have turned to other kinds of 2D materials called transition metal dichalcogenides (TMDCs) [9]. These materials can also be exfoliated into 2D layers from their stacked crystal structure by using the same method as for graphene production [10]. Most intriguingly, their band structures are layer-thickness-dependent despite the weak * Correspondence: Department of Electrical and Computer Engineering, and Beckman Institute, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801, USA interlayer van de Waals forces, which indicates they are electronically tunable via thickness control. As the number of layer reduces from bulk value to monolayer, the bandgap of several TMDC materials changes from indirect to direct [11]. Among those and the most widely studied materials, monolayer MoS2 has emerged as a semiconducting alternative to graphene because of its large intrinsic direct bandgap of approximately 1.8 eV [12], which makes it suitable for optoelectronic and nanoelectronic applications. In addition, easy fabrication using exfoliation method, absence of interface dangling bonds, and superb electrostatic behavior are the main reasons why MoS2 is the subject of nanotechnology research and is a competitive candidate for logic devices of the next generation [10]. Recent experimental works have shown transistors made of singlelayer or few-layer unintentionally doped MoS2 exhibiting very high on/off ratios, exceeding 1 × 108, close-to-ideal SS (approximately 70 mV/dec), ultralow standby power, and mobility of at least 100 cm2/Vs [10] or even up to 700 cm2/Vs when high-k dielectrics are applied [13], which is competitively comparable to those of current Si- © 2014 Lu and Leburton; licensee Springer. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly credited. Lu and Leburton Nanoscale Research Letters 2014, 9:676 http://www.nanoscalereslett.com/content/9/1/676 based CMOS technology. On the other hand, good performances were also observed in unintentionally doped multi-layer MoS2 transistors [14]. Many unintentionally doped MoS2 transistors reported in the literature show either n-type [10] or p-type [15] behaviors, with their related defects or impurities altering the transport properties. Although simulations of the electronic and transport properties of MoS2 containing various dopants [16,17] have been reported, to the best of our knowledge, there is still lack of comprehensive and coherent understanding of the groups III and IV dopants on MoS2, even in the most recent reports [16]. For magnetic impurities, although there are a number of theoretical reports on Mn, Fe, and Co [17,18], our model on a 5 × 5 computational cell shows impurity levels and the Fermi levels located closer to the conduction band by about 0.1 ~ 0.2 eV and ~0.2 eV, respectively. In this paper, we provide a systematic study on the properties of various p-type dopants, vacancies, and magnetic impurities in monolayer MoS2 including group III dopants, i.e., B, Al, and Ga and group IV dopants, i.e., C, Si, and Ge, as well as magnetic transition metal elements such as V, Nb, and Ta. For the first time, the electronic properties of groups III and IV elements are studied comprehensively in a 5 × 5 simulation supercell of single-layer MoS2 in addition to the investigation of a large number of magnetic transition metal elements on their electronic states, formation energies, and spin properties. Moreover, Mo as an adsorbate in MoS2 is considered. Our objective is to identify the most promising candidates of p-type dopants and magnetic impurities for MoS2. The information will come in handy when fabricating doped devices such as field-effect transistors (FETs), optoelectronic devices, and all those requiring p-n junctions. In addition, the study of spin distribution around magnetic elements provides the basic knowledge for future applications of MoS2 spintronics, which is one of the possible scenarios in the beyond-CMOS technology. Methods In this work, density functional theory (DFT) is employed to perform ab initio calculations on the electronic and magnetic properties of monolayer MoS2 doped with impurities using the Quantum ESPRESSO software package [19]. Ultrasoft pseudopotentials are chosen for our simulations, which are carried out for a 5 × 5 × 1 hexagonal supercell (lateral dimension fixed to 15.83 × 15.83 Å2) with 25 Mo atoms and 50 S a (...truncated)