Defect-Tolerant Monolayer Transition Metal Dichalcogenides

Abstract

Localized electronic states formed inside the band gap of a semiconductor due to crystal defects can be detrimental to the material's opto-electronic properties. Semiconductors with lower tendency to form defect induced deep gap states are termed defect tolerant. Here we provide a systematic first-principles investigation of defect tolerance in 29 monolayer transition metal dichalcogenides (TMDs) of interest for nano-scale optoelectronics. We find that the TMDs based on group VI and X metals form deep gap states upon creation of a chalcogen (S, Se, Te) vacancy while the TMDs based on group IV metals form only shallow defect levels and are thus predicted to be defect tolerant. Interestingly, all the defect sensitive TMDs have valence and conduction bands with very similar orbital composition. This indicates a bonding/anti-bonding nature of the gap which in turn suggests that dangling bonds will fall inside the gap. These ideas are made quantitative by introducing a descriptor that measures the degree of similarity of the conduction and valence band manifolds. Finally, the study is generalized to non-polar nanoribbons of the TMDs where we find that only the defect sensitive materials form edge states within the band gap. * thygesen@fysik.dtu.dk 1 arXiv:1604.03232v1 [cond-mat.mtrl-sci]

Anomalous photoluminescence thermal quenching of sandwiched single layer MoS_2

“…Such defects are expected for the case of thermodynamically non-equilibrium growth processes such as MBE and CVD which create shallow states having low activation energies close to the band edges [12,32]. In contrast, V s and N 2 defects can establish deep level density of states spreading in energy up to 150-300 meV causing bound exciton peak as reported by Tongay et al [11] Thus, SL-MoS 2 can have both deep and shallow level defects [33]. Consequently, the enhancement of the µ-PL intensity of the sandwiched SL-MoS 2 with increasing temperatures is due to the raising radiative recombination rate of the excess delocalized carriers created from the carrier hopping from the shallow defect states to the band edges.…”

Computational identification of promising thermoelectric materials among known quasi-2D binary compounds

“…The bulk modulus (B) is the material property that inuences predictions of both m 0 and k L , as can be seen from eqn (4) and (5). Keeping all other parameters constant, a larger bulk modulus leads to higher m 0 as well as larger k L .…”

Experimental evidence of exciton capture by mid-gap defects in CVD grown monolayer MoSe2

“…In low-temperature grown (LTG) traditional semiconductors, where abundant mid-gap point defects are created intentionally, such as LTG-GaAs, 37,38 LTGInP, 39 and LTG-InGaP, 40 a sign change in the differential transmission or reflection signals is well considered as a signature of transition from initial absorption bleaching to defect-capturinginduced absorption. CVD grown monolayer TMDs are known to contain many mid-gap defects, 10 such as chalcogen atom vacancy, 13,14 antisite defect Mo S and Mo S2 , 12 and nitrogen/ oxygen impurities. 15 The existence of the mid-gap defects in our CVD MoSe 2 sample is manifested by a broad peak around 1000 nm (1.25 eV) in the PL data, which is measured under ambient condition (Fig.…”

“…11 There are a number of defect types in TMDs, such as chalcogen atom vacancies, impurities, interstitials, antisite defects, and dislocations. 10 Some of the defects can induce mid-gap states, for example, the antisite defect, 12 chalcogen atom vacancy 13,14 and nitrogen/oxygen impurities. 15 The mid-gap defects are believed to serve as either effective recombination centers or effective carrier traps, depending on whether the defects possess a small or large difference in capture rates of electrons and holes, respectively.…”