2.2 Fundamentals of 2D Materials
29
can be further utilised to manipulate the absorption and emission behaviour is given
in [156] (also see Figs. 2.13, 5.4 and 7.6).
Disorder in 2D Materials
It shall be noted that recent works in the literature have in detail addressed the role of
disorder in vdWs monolayers and heterostructures of 2D materials which can obscure
intrinsic properties (see [7]): Among the common sources of intrinsic disorder are
vacancies, anti-sites, substitutions, edges and grain boundaries. Typically, quality
differences exist between mechanically exfoliated 2D material from crystalline bulk
material and physical-vapour-deposition (PVD) or CVD grown samples, as the comparison of common point defects in MoS 2 or nanoscopic analysis of graphene indicated. For certain applications, point defects are rather a feature than a bug, when they
are optically-active (see for instance [157]) and can be deterministically generated
(e.g. by nano-precise ion bombardment [158]).
Also, sources of extrinsic disorder such as strain
7 (recognisable through Raman
mode shifts [160], band gap changes [161] or second-harmonic intensity profiles
[162]), adsorbates, surface roughness, charged impurities on surfaces and oxidation
of the 2D material have to be taken into account. It has been even shown that the
band gap of a TMDC can vary locally on the nanoscale between direct and indirect
type [7], motivating suspension of monolayers as a remedy.
Moreover, a proven method to improve the monolayer quality for various experiments is given by (few- to multi-layer) hBN encapsulation, as it improves the
brightness and linewidth [40, 58], as well as the emission lifetime [40], suppresses
adsorbate-related emission features [57, 59] and environmental disorder [42], reduces
strain, doping levels [58], as well as exciton–phonon coupling [40] and the effective
mass [52]. Furthermore, hBN encapsulation enabled the observation of various exciton complexes and the study of the grey exciton’s radiation profile (Fig. 2.7, see [47]
and references therein). On the other hand, hBN encapsulation reduces the binding
energy and band gap energy compared to suspended monolayers (see [78, 88], also
cf. [60]).
The interested reader is referred to [7] for further details on the discussion of
disorder.
In conclusion, the extraordinary properties of these materials [5, 56, 111] have
already encouraged worldwide endeavours to harness the 2D-materials’ features for
all kind of nanodevices (see [9, 13, 16] and references therein), but also for “valleytronics” (see [17, 75] and references therein), with graphene and hBN naturally
serving as ideal 2D contacts and insulators, respectively.
7 Strain can indeed affect excitonic modes, as a stretched lattice with increased mean particle distances in the plane can exhibit a band-gap energy reduction (cf. [159], also see comparison WSe 2
grown by CVD with its exfoliated counterpart both on a sapphire substrate and strain-related discussion in [87]).
29
can be further utilised to manipulate the absorption and emission behaviour is given
in [156] (also see Figs. 2.13, 5.4 and 7.6).
Disorder in 2D Materials
It shall be noted that recent works in the literature have in detail addressed the role of
disorder in vdWs monolayers and heterostructures of 2D materials which can obscure
intrinsic properties (see [7]): Among the common sources of intrinsic disorder are
vacancies, anti-sites, substitutions, edges and grain boundaries. Typically, quality
differences exist between mechanically exfoliated 2D material from crystalline bulk
material and physical-vapour-deposition (PVD) or CVD grown samples, as the comparison of common point defects in MoS 2 or nanoscopic analysis of graphene indicated. For certain applications, point defects are rather a feature than a bug, when they
are optically-active (see for instance [157]) and can be deterministically generated
(e.g. by nano-precise ion bombardment [158]).
Also, sources of extrinsic disorder such as strain
7 (recognisable through Raman
mode shifts [160], band gap changes [161] or second-harmonic intensity profiles
[162]), adsorbates, surface roughness, charged impurities on surfaces and oxidation
of the 2D material have to be taken into account. It has been even shown that the
band gap of a TMDC can vary locally on the nanoscale between direct and indirect
type [7], motivating suspension of monolayers as a remedy.
Moreover, a proven method to improve the monolayer quality for various experiments is given by (few- to multi-layer) hBN encapsulation, as it improves the
brightness and linewidth [40, 58], as well as the emission lifetime [40], suppresses
adsorbate-related emission features [57, 59] and environmental disorder [42], reduces
strain, doping levels [58], as well as exciton–phonon coupling [40] and the effective
mass [52]. Furthermore, hBN encapsulation enabled the observation of various exciton complexes and the study of the grey exciton’s radiation profile (Fig. 2.7, see [47]
and references therein). On the other hand, hBN encapsulation reduces the binding
energy and band gap energy compared to suspended monolayers (see [78, 88], also
cf. [60]).
The interested reader is referred to [7] for further details on the discussion of
disorder.
In conclusion, the extraordinary properties of these materials [5, 56, 111] have
already encouraged worldwide endeavours to harness the 2D-materials’ features for
all kind of nanodevices (see [9, 13, 16] and references therein), but also for “valleytronics” (see [17, 75] and references therein), with graphene and hBN naturally
serving as ideal 2D contacts and insulators, respectively.
7 Strain can indeed affect excitonic modes, as a stretched lattice with increased mean particle distances in the plane can exhibit a band-gap energy reduction (cf. [159], also see comparison WSe 2
grown by CVD with its exfoliated counterpart both on a sapphire substrate and strain-related discussion in [87]).