ELFT HÍRADÓ

[Kádár György <kadargy AT mfa.kfki.hu>]

MFA Szeminárium

Arkady V. KRASHENINNIKOV (Aalto University, Finland):

2D Transition-Metal Dichalcogenides:
Doping, Alloying and Electronic Structure Engineering


2014. február 26.-én, szerdán 11:00 órakor
az MTA TTK MFA Műszaki Fizikai és
Anyagtudományi Intézet Tanácstermében,
KFKI 26. épület I. emelet

Minden érdeklődőt szívesen várunk
Kádár György (MTA TTK MFA)
kadar.gyorgy AT ttk.mta.hu

Abstract:
Following isolation of a single sheet of graphene, many other 2D systems 
were manufactured. Among them, single transition metal dichalcogenides 
(TMD) sheets have received a particular attention, as these materials 
exhibit intriguing electronic, optical and mechanical properties which 
can controlled be by varying material composition. Moreover, the 
properties can further be tuned by introduction of defects and impurities.
In this talk, I will present the results of our first-principles 
theoretical studies of the response of 2D TMDs to electron irradiation 
obtained in collaboration with several experimental groups [1-3]. We 
showed that vacancies produced by the electron beam agglomerate and form 
line structures, which can be used for engineering material properties 
[2]. We also assessed the radiation hardness of 2D TMD systems [1]. We 
further showed that TMDs can be doped by filling the vacancies with 
impurity atoms or introducing impurities during the growth stage [3]. We 
also studied the stability and electronic properties of single layers of 
mixed TMDs, such as MoS2x Se2(1−x), which can be referred to as 2D 
random alloys [4]. We demonstrated that 2D mixed ternary 
MoS2/MoSe2/MoTe2 compounds are thermodynamically stable at room 
temperature, so that such materials can be manufactured by CVD or 
exfoliation techniques as confirmed now by several experimental groups. 
By applying the effective band theory approach we showed that the direct 
gap in these material can continuously be tuned. Using GW 
first-principles calculations for few-layer and bulk MoS2, we 
investigated [5] the effects of quantum confinement on the electronic 
and optical structure of this layered material and effects of the 
environment [6]. By solving the Bethe-Salpeter equation, we evaluated 
the exciton energy in these systems which dramatically increases from 
0.1 eV in the bulk MoS2 to about 1 eV in the monolayer. The fundamental 
band gap increases as well, so that the optical transition energies 
remain nearly constant.
I will finally touch upon defects in bilayer 2D silica [7] and show that 
defects are strikingly similar to those in graphene with their 
morphology governed by the hexagonal symmetry of the lattice [8-9].
1. H-P. Komsa, J. Kotakoski, S. Kurasch, O. Lehtinen, U. Kaiser, and A. 
V. Krasheninnikov, Phys. Rev. Lett. 109 (2012) 035503.
2. H.-P. Komsa, S. Kurasch, O. Lehtinen, U. Kaiser, and A. V. 
Krasheninnikov, Phys. Rev. B 88 (2013) 035301.
3. Y.-C. Lin, D.O. Dumcenco, H.-P. Komsa, Y. Niimi, A.V. Krasheninnikov, 
Y.-S. Huang, and K. Suenaga, Advanced Materials (2014) in press.
4. H.-P. Komsa and A. V. Krasheninnikov, J. Phys. Chem. Lett. 3 (2012) 3652.
5. H.-P. Komsa and A. V. Krasheninnikov, Phys. Rev. B (Rapid Comm.) 86 
(2012) 241201.
6. H.-P. Komsa and A. V. Krasheninnikov, Phys. Rev. B 88 (2013) 085318.
7. P. Y. Huang, et al., Nano Letters 12 (2012) 1081.
8. T. Björkman, S. Kurasch, O. Lehtinen, J. Kotakoski, O.Yazyev, A. 
Srivastava, V. Skakalova, J. Smet, U. Kaiser, and A.V. Krasheninnikov, 
Scientific Reports 3 (2013) 3482.
9. F. Ben Romdhane, T. Bjorkman, J.A. Rodrıguez-Manzo, O. Cretu, A. V. 
Krasheninnikov, and Florian Banhart, ACS Nano 7 (2013) 5175.