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Epitaxial growth and study of 2D Se-based ultrathin films: Bi 2 Se 3, MoSe 2, HfSe 2, ZrSe 2 Aretouli E. Kleopatra 20/2/15 NCSR DEMOKRITOS, Athens, Greece.

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Presentation on theme: "Epitaxial growth and study of 2D Se-based ultrathin films: Bi 2 Se 3, MoSe 2, HfSe 2, ZrSe 2 Aretouli E. Kleopatra 20/2/15 NCSR DEMOKRITOS, Athens, Greece."— Presentation transcript:

1 Epitaxial growth and study of 2D Se-based ultrathin films: Bi 2 Se 3, MoSe 2, HfSe 2, ZrSe 2 Aretouli E. Kleopatra 20/2/15 NCSR DEMOKRITOS, Athens, Greece

2 Outline Topological insulators: Bi 2 Se 3 MBE growth and structural characterization Semiconducting Transition metal dichalcogenides (TMDs) MoSe 2 HfSe 2 ZrSe 2 Conclusions/ Future work Topological insulators: Bi 2 Se 3 MBE growth and structural characterization Semiconducting Transition metal dichalcogenides (TMDs) MoSe 2 HfSe 2 ZrSe 2 Conclusions/ Future work

3 3D Topological Insulators Bi 2 Se 3, Bi 2 Te 3, Bi 1-x Sb x Y. Xia et al., Nat. Phys. 5, 398 (2009)  Topologically protected Spin locked to orbital momentum  Backscattering is suppressed  Novel switching mechanisms/functionalities  Backscattering is suppressed  Novel switching mechanisms/functionalities k -k k spin e-e- Non-magnetic impurity “ insulating” bulk Gapless metallic surface states -k relativistic movement of e- : light-like particles Spin polarized (helical) Dirac cones

4 Ultra high vacuum champers for growth and structural characterization STM ARPES MBE XPS RHEED

5 HRTEM and XPS 1 QL ~ 1 nm Thick ~ 20 Quintuple Layers (QL) Se Bi 1 QL High epitaxial quality and “clean” crystalline interfaces [11-20] Bi 2 Se 3 /AlN No reaction –sharp crystalline interfaces Bi 2 Se 3 /AlN No reaction –sharp crystalline interfaces 3QL epitaxial Bi 2 Se 3 * P. Tsipas et al., ACS Nano, 8 (7), 6614 (2014) Substrates : 200 nm AlN(0001) /200 mm Si (111)

6 k //,y (Å -1 ) ΓΜΜ 0.47 eV 2 nd derivative ΓΜ Μ CB VB Gapless surface states 3 QL Bi 2 Se 3 /AlN(0001) 5 QL Bi 2 Se 3 /AlN(0001) EFEF E B (eV) EFEF Gapless surface states in ultrathin Bi 2 Se 3 3QL: Thinnest Bi 2 Se 3 with gapless surface states (Dirac cone) ever reported experimentally ! Reduce surface to volume ratio - applications in nanoelectronics 3QL: Thinnest Bi 2 Se 3 with gapless surface states (Dirac cone) ever reported experimentally ! Reduce surface to volume ratio - applications in nanoelectronics In-situ ARPES Gapless surface states Ultrathin films: Hybridization-gap opening Thick films (>6QL exp.): Non-interacting

7 Heterostructures with Chemically compatible semiconductors Two Layer MoSe 2 on Bi 2 Se 3 Template 2ML MoSe 2 at 300 o C 5QL Bi 2 Se 3 at 300 o C AlN [11-20] azimuth 5 QL a Bi2Se3 =4.14Å a MoSe2 =3.299 Å mismatch of ~20% a Bi2Se3 =4.14Å a MoSe2 =3.299 Å mismatch of ~20% 3QL Bi 2 Se 3 /2ML MoSe 2 /3QL Bi 2 Se 3 2ML MoSe 2 /5QL Bi 2 Se 3 RHEED a AlN =3.11Å a Bi2Se3 =4.14Å mismatch of~ 33% a AlN =3.11Å a Bi2Se3 =4.14Å mismatch of~ 33% Perfect alignment of the 2 hexagonal lattices No rotated domains-single crystal [11-20] MoSe2 //[ 11-20] Bi2Se3 // [11-20] AlN

8 Semiconducting 2D Transition Metals Dichalcogenides (TMDs)  anisotropic mechanical optical and electrical properties  Sizable band gap in the visible and NIR region of the solar spectrum x y z 6.5Å x y z Mo Se Layered TMDs crystals of the composition MX 2 : M: transition metal (VIB: Mo, W and IVB: Zr,Hf ) X: Chalcogen species (S, Se, Te) Applications in Optoelectronic devices(energy conversion systems) and Field Effect Transistors/ low power logic devices 2H structure 1T structure Indirect to direct band gap crossover when thickness reduces to a single layer Indirect band gap very close to Si Honeycomb like structures superior properties to those of graphene ??? MoSe 2 : E. Xenogiannopoulou et al. submitted 2014 HfSe 2 : K.E. Aretouli et al. submitted 2015 Se Hf

9 d b f MoSe 2 350°C MoSe 2 690°C AlN Bi 2 Se 3 300°C MoSe 2 300°C RHEED, TEM, STM of 3ML MoSe 2 /AlN(0001) STM image: honeycomb structure d e AlN MoSe 2 350°C MoSe 2 690°C estimated distance of 3.3 Å between Se-Se atoms ~ a MoSe2 =3.299 Å vdW gap 2 1 Line 1 3.3Å Se Mo 2Å Line 2 Se Mo Two step growth process AlN MoSe 2 350°C MoSe 2 690°C MoSe 2 350°C

10 He I He II 1 st Brillouin zone MoSe 2 K A Γ L M H Valence Band Imaging Binding energy (eV) RT measurements  Shift of VB at Γ-point to higher binding energy  Indirect to direct band gap transition in the 1ML limit ΓK MoSe2 =1.274 Å -1 EFEF k //,y (Å -1 ) E. Xenogiannopoulou et al. submitted ML MoSe 2 3 ML EFEF 6ML ΓΚΓ/ΑΚ/ΗΓ/ΑΚ/Η (a) (b) (c) He I 0 0 k //,y (Å -1 ) He II (d) (e) (f) Binding energy (eV) k //,y (Å -1 )

11 Raman and PL characterization of MoSe 2 films at ML-limit on AlN(0001) The direct band gap in single layers results in intense room temperature photoluminescence (PL) Applications from optoelectronics to energy conversion Active modes of MoSe 2 : A 1g at cm -1 E 2g at cm -1 B 2g at 352 cm -1 in few layer material Active modes of MoSe 2 : A 1g at cm -1 E 2g at cm -1 B 2g at 352 cm -1 in few layer material 190 meV ΓΜΚ A B

12 AlN 1ML HfSe 2 6ML HfSe 2 3ML MoSe 2 [11-20] azimuth ΚΓ Μ EfEf Energy (eV) K/H Μ/L Γ/ΑΓM Binding Energy E B (eV) k x (Å -1 ) (a) (b) HfSe 2 and MoSe 2 / HfSe 2 films on AlN(0001) Absence of strain aHfSe 2 =3.78Å v.d. Waals heteroepitaxy 6ML HfSe 2 mismatch of ~15% mismatch of ~6% HfSe 2 deposition at 570 o C Annealing at 810 o C DFT calculations DFT calculations XPS

13 Conclusions Thinnest Bi 2 Se 3 (3QL) with gapless surface states (Dirac cone) ever reported experimentally High structural quality MoSe 2 and HfSe 2 on AlN/Si substrates MoSe 2 /Bi 2 Se 3 and MoSe 2 /HfSe 2 multilayers can be produced Future work Exploring the semiconductors HfSe 2, ZrSe 2 Electrical characterization of Bi 2 Se 3, MoSe 2 HfSe 2, ZrSe 2 and their heterostructures Magnetoresistance measurements/ Hall effect measurements Conclusions Thinnest Bi 2 Se 3 (3QL) with gapless surface states (Dirac cone) ever reported experimentally High structural quality MoSe 2 and HfSe 2 on AlN/Si substrates MoSe 2 /Bi 2 Se 3 and MoSe 2 /HfSe 2 multilayers can be produced Future work Exploring the semiconductors HfSe 2, ZrSe 2 Electrical characterization of Bi 2 Se 3, MoSe 2 HfSe 2, ZrSe 2 and their heterostructures Magnetoresistance measurements/ Hall effect measurements

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