Aretouli E. Kleopatra 20/2/15 NCSR DEMOKRITOS, Athens, Greece Epitaxial growth and study of 2D Se-based ultrathin films: Bi2Se3, MoSe2, HfSe2 , ZrSe2 Aretouli E. Kleopatra 20/2/15 NCSR DEMOKRITOS, Athens, Greece

Outline Topological insulators: Bi2Se3 MBE growth and structural characterization Semiconducting Transition metal dichalcogenides (TMDs) MoSe2 HfSe2 ZrSe2 Conclusions/ Future work

3D Topological Insulators Bi2Se3 , Bi2Te3, Bi1-xSbx Spin locked to orbital momentum Gapless metallic surface states Spin polarized (helical) Dirac cones “insulating” bulk relativistic movement of e- : light-like particles Topologically protected spin k -k k Non-magnetic impurity e- -k -k Backscattering is suppressed Novel switching mechanisms/functionalities Y. Xia et al., Nat. Phys. 5, 398 (2009)

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

HRTEM and XPS [11-20] Substrates : 200 nm AlN(0001) /200 mm Si (111) 3QL epitaxial Bi2Se3 Substrates : 200 nm AlN(0001) /200 mm Si (111) 1 QL ~ 1 nm Thick ~ 20 Quintuple Layers (QL) Bi2Se3/AlN No reaction –sharp crystalline interfaces 1 QL Se Bi [11-20] *P. Tsipas et al., ACS Nano, 8 (7), 6614 (2014) High epitaxial quality and “clean” crystalline interfaces

Gapless surface states in ultrathin Bi2Se3 In-situ ARPES Ultrathin films: Hybridization-gap opening Thick films (>6QL exp.): Non-interacting k//,y (Å-1) k //,y (Å-1) Γ Μ 0.47 eV 2nd derivative CB VB Gapless surface states 3 QL Bi2Se3/AlN(0001) 5 QL Bi2Se3/AlN(0001) EF EB (eV) EF Gapless surface states 3QL: Thinnest Bi2Se3 with gapless surface states (Dirac cone) ever reported experimentally ! Reduce surface to volume ratio - applications in nanoelectronics

Heterostructures with Chemically compatible semiconductors Two Layer MoSe2 on Bi2Se3Template RHEED 2ML MoSe2/5QL Bi2Se3 2ML MoSe2 at 300 oC 5QL Bi2Se3 at 300 oC AlN [11-20] azimuth 3QL Bi2Se3/2ML MoSe2/3QL Bi2Se3 aAlN=3.11Å aBi2Se3=4.14Å mismatch of~ 33% 5 QL aBi2Se3=4.14Å aMoSe2=3.299 Å mismatch of ~20% Perfect alignment of the 2 hexagonal lattices No rotated domains-single crystal [11-20] MoSe2 //[11-20] Bi2Se3 // [11-20] AlN

Semiconducting 2D Transition Metals Dichalcogenides (TMDs) Layered TMDs crystals of the composition MX2 : M: transition metal (VIB: Mo, W and IVB: Zr,Hf ) X: Chalcogen species (S, Se, Te) Honeycomb like structures superior properties to those of graphene ??? 2H structure 1T structure Se x y z Mo x y z 6.5Å Se Hf Indirect to direct band gap crossover when thickness reduces to a single layer Indirect band gap very close to Si MoSe2 : E. Xenogiannopoulou et al. submitted 2014 HfSe2 : K.E. Aretouli et al. submitted 2015 anisotropic mechanical optical and electrical properties Sizable band gap in the visible and NIR region of the solar spectrum Applications in Optoelectronic devices(energy conversion systems) and Field Effect Transistors/ low power logic devices

RHEED, TEM, STM of 3ML MoSe2/AlN(0001) Two step growth process d AlN AlN MoSe2 350°C 690°C d AlN vdW gap e b MoSe2 350°C Bi2Se3 300°C MoSe2 350°C MoSe2 350°C MoSe2 690°C f MoSe2 300°C MoSe2 690°C STM image: honeycomb structure Line 1 3.3Å Se 2 1 Se Mo 2Å Line 2 Se Mo estimated distance of 3.3 Å between Se-Se atoms ~ aMoSe2=3.299 Å

Valence Band Imaging He I 1st Brillouin zone MoSe2 He II 1ML MoSe2 3 ML EF 6ML Γ Κ Γ/Α Κ/Η (a) (b) (c) He I Binding energy (eV) K A Γ L M H ΓKMoSe2 =1.274 Å-1 EF He II k// ,y(Å-1) He II (d) (e) (f) Binding energy (eV) k//,y (Å-1) Shift of VB at Γ-point to higher binding energy Indirect to direct band gap transition in the 1ML limit k//,y (Å-1) RT measurements E. Xenogiannopoulou et al. submitted 2014

Raman and PL characterization of MoSe2 films at ML-limit on AlN(0001) Active modes of MoSe2: A1g at 240.8 cm-1 E2g at 288.5 cm-1 B2g at 352 cm-1 in few layer material 190 meV Γ Μ Κ A B The direct band gap in single layers results in intense room temperature photoluminescence (PL) Applications from optoelectronics to energy conversion

HfSe2 and MoSe2 / HfSe2 films on AlN(0001) HfSe2 deposition at 570 oC Annealing at 810 oC 6ML HfSe2 Κ Γ Μ Ef 2 -1 1 -2 -3 -4 -5 Energy (eV) K/H Μ/L Γ/Α M Binding Energy EB (eV) kx (Å-1) (a) (b) AlN 1ML HfSe2 6ML HfSe2 3ML MoSe2 [11-20] azimuth XPS mismatch of ~6% DFT calculations mismatch of ~15% Absence of strain aHfSe2=3.78Å v.d. Waals heteroepitaxy

Conclusions Thinnest Bi2Se3 (3QL) with gapless surface states (Dirac cone) ever reported experimentally High structural quality MoSe2 and HfSe2 on AlN/Si substrates MoSe2/Bi2Se3 and MoSe2/HfSe2 multilayers can be produced Future work Exploring the semiconductors HfSe2, ZrSe2 Electrical characterization of Bi2Se3, MoSe2 HfSe2 , ZrSe2 and their heterostructures Magnetoresistance measurements/ Hall effect measurements

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