Rigid Band Shifts in Two-Dimensional Semiconductors through External Dielectric Screening

Abstract

We investigate the effects of environmental dielectric screening on the electronic dispersion and the band gap in the atomically-thin, quasi two-dimensional (2D) semiconductor WS2 using correlative angle-resolved photoemission and optical spectroscopies, along with first-principles calculations. We find the main effect of increased environmental screening to be a reduction of the band gap, with little change to the electronic dispersion of the band structure. These essentially rigid shifts of the bands results from the special spatial structure of the changes in the Coulomb potential induced by the dielectric environment in the 2D limit. Our results suggest dielectric engineering as a noninvasive method of tailoring the band structure of 2D semiconductors and provide guidance for understanding the electronic properties of 2D materials embedded in multilayer heterostructures.In monolayers of atomically-thin, quasi twodimensional (2D) semiconductors, the intrinsic screening of Coulomb interactions is reduced compared to their bulk crystals, since electric field lines between charges extend significantly outside the material. As a result, exciton binding energies are enhanced, reaching values of several hundreds of meV in the transition metal dichalcogenides (TMDCs) [1][2][3][4][5][6][7]. For the same reason, materials in close proximity to the monolayers enhance the effective screening of charge carrier interactions. By embedding atomically-thin materials in different dielectric environments, their band gaps, as well as exciton binding energies, can therefore be modified on an energy scale of the exciton binding energies themselves [8][9][10]. This sensitivity becomes particularly important in vertical heterostructures of 2D materials and enables a non-invasive way of designing nanoscale functionality, such as lateral heterojunctions, through the spatial control of substrate dielectrics [8,10,11].To exploit the full potential of tailoring Coulomb interactions through control of the dielectric environment, it is critical to understand its impact not only on the band gap but also on the valence and conduction band dispersions. The dispersion determines such basic properties as the effective masses of the carriers and the energy differences between different valleys within the Brillouin zone, and also affects the relative alignment between the valence and conduction bands of a homogeneous monolayer with spatially varying external dielectric screening. To date, experimental studies of dielectric engineering have mainly focused on optical spectroscopy or electronic transport measurements of TMDC monolay-ers, which only probe a small fraction of the full Brillouin zone. In general, however, perturbations to a material do not have the same effect on electronic states of different orbital character and can be expected to modify the band structure in different parts of the Brillouin zone differently.Here, through a combination of experiment and theory, we provide a generalized picture of the consequences of dielectric scree...

Evidence of the direct-to-indirect band gap transition in strained two-dimensional WS2 , MoS2 , and WSe2Blundo1, Felici2, Yildirim3 et al. 2020Phys. Rev. Research1246800View full textAdd to dashboardCite

“…The displayed fits yield A = 45 +1 −2 meV/ε%, E I (0) = 2.13 +0.07 −0.04 eV and I = 92 +17 −9 meV/ε% [E A (0) is fixed to 2.00 eV, the strain-free ML exciton energy]. These data compare rather favorably with the experimental [35,45], and theoretical [17,18,[20][21][22][24][25][26][27]46] ones as reported in Table I. However, we notice a large discrepancy between the E I (0)-E A (0) value derived in this work (130 meV) and those reported in other works.…”

“…1.8 ± 0.7 0.1 [5,17,[19][20][21][23][24][25]27,32] data embed exciton effects that are quite different for the A and I transitions: A heavier hole effective mass is reported in Ref. [26] at (m h = 2.45m 0 , m 0 is the electron mass in vacuum) compared to K (m h K = 0.48m 0 ) that results in a difference between the indirect and direct exciton binding energy [47] equal to +144 meV [48]. In turn, this brings our 130 meV exciton value to a 274 meV electronic value, in close agreement with the other works' reported in Table I.…”

“…The negative value of ε z all over the surface is caused by the membrane thinning following the in-plane tensile strain. The strain field across the dome is expected to induce remarkable changes in the electronic properties of the curved TMD membrane [5,[17][18][19][20][21][22][23][24][25][26][27], giving rise to the peculiar phenomenology displayed in Fig. 1(c).…”

Control of electron-electron interaction in graphene by proximity screening

“…lectrostatic screening by conducting gates has previously been employed to suppress charge inhomogeneity in graphene [1][2][3] , alter its plasmon spectra 4,5 , and renormalize electronic spectra of monolayer semiconductors 6,7 . Elementary electrostatics tells us that the electron charge e placed at the distance d from a bulk conductor leads to a dipole potential evolving as 2ed 2 =r 3 at large in-plane distances r ) d, which is much weaker than the original, unscreened Coulomb potential, e=r.…”

Low-Dimensional Semiconductors in Artificial Photosynthesis: An Outlook for the Interactions between Particles/Quasiparticles

“…For instance, given that Coulomb interactions between these charged species are related to dielectric screening, band gap modulation would be proceeded by extrinsically or intrinsically engineering the dielectric properties. 26 , 51 − 54 In this respect, Raja et al reported band gap modulation of two-dimensional TMDC nanosheets by tuning the surrounding dielectric environment. 52 They constructed heterojunctions between TMDCs and graphene/h-BN nanosheets for achieving dielectric environment regulation.…”