1. 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

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

3. 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)

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

5. 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

6. 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

7. 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

8. 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

9. 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

10. 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

11. Raman and PL characterization of MoSe2 films at ML-limit on AlN(0001)
Active modes of MoSe2:
A1g at cm-1
E2g at 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

12. 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

13. 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

14. Thank you for your attention