Chemistry Reference
In-Depth Information
14.6
Conclusions
In this Chapter we have reviewed the electrical response of two representative layered
materials for future devices-based on graphene and MoS
2
. We have focused on the
interplay between electric fields and screening properties of few-layer structures.
Density functional theory was the main tool used to compute the properties of the
analyzed systems. We have used simple models to understand the observed trends.
In particular, we find that the effective dielectric constant of graphene and MoS
2
is electrically tunable, with the layer thickness playing an important role in the
enhancement of the effect. The thicker the structure is, the stronger the modulation
with electric fields. The driving force for such behavior is due to the linear dependence
of the electrical polarization of the layers on the external field. The response field
computed from the polarization charge does not screen completely the external bias,
which generate higher interlayer fields at thinner structures. Differences due to semi-
metallic and semiconducting electronic character of the layers are observed in terms
of the field damping inside of the compounds: graphene tends to screen the external
field at the outermost layers of system, while MoS
2
the field penetrates deeper in the
layers. These results are in sound agreement with recent experiments performed for
both materials.
We have also explored the possibility to control the layer exfoliation using electric
fields. We have found that the induced interlayer charge imbalance generated by the
bias can drive the system to an unstable state where the layers can be separated from
each other. The interlayer equilibrium position is modified as a function of the field
magnitude, which induces a reduction of the van der Waals barrier that keeps the
layers together. As a result, there are variations of the interlayer separations even at
low-fields. This investigation is highly relevant in the interpretation of experimental
results underway since the field of 2D-materials is just in its beginning where several
techniques and effects are still to be developed and explored.
References
Bao W, Cai X, Kim D, Sridhara K, Fuhrer MS (2013) High mobility ambipolar MoS
2
field-effect
transistors: substrate and dielectric effects. Appl Phys Lett 102:042104-042108
Beal AR, Hughes HP (1979) Kramers-Kronig analysis of the reflectivity spectra of 2H-MoS
2
,
2H-MoSe
2
and 2H-MoTe
2
. J Phys C Solid State Phys 12:881-890
Bell MG, Liang WY (1976) Electron energy loss studies in solids: the transition metal dichalco-
genides. Adv Phys 1976:53-86
Bostwick A, Speck F, Seyller T, Horn K, Polini M, Asgari R, MacDonald AH, Rotenberg E (2010)
Observation of plasmarons in quasi-freestanding doped graphene. Science 328:999-1002
Castellanos-Gomez A, Cappelluti E, Roldán R, Agrait N, Guinea F, Rubio-Bollinger G (2013)
Electric-field screening in atomically thin layers of MoS
2
: the role of interlayer coupling. Adv
Mater 25:899-903
Castro EV, Novoselov KS, Morozov SV, Peres NMR, Lopes dos Santos JMB, Nilsson J, Guinea F,
Geim AK, Castro-Neto AH (2007) Biased bilayer graphene: semiconductor with a gap tunable
by the electric field effect. Phys Rev Lett 99:216802-216806
