of 31
1
Field
-
effect transistors made from solution
-
grown two
-
dimensional tellurene
Yixiu Wang
1*
,
Gang Qiu
2
,
3
*
,
Ruoxing Wang
1
*
,
Shouyuan Huang
4
,
Qingxiao Wang
5
,
Yuanyue Liu
6
,
7
,
8
,
Yuchen Du
2,
3
,
William A. Goddard
III
6
,
Moon J. Kim
4
,
Xianfan Xu
3, 4
,
Peide
D.
Ye
2, 3
,
Wenzhuo
Wu
1
, 3
1
School of
Industrial
Engineering,
Purdue University
,
West Lafayette
,
Indiana 47907
, USA
2
School of
Electrical and Computer
Engineering,
Purdue University
,
West
Lafayette
,
Indiana
47907
, USA
3
Birck Nanotechnology Center, Purdue University, West Lafayette, Indiana 47907, United States
4
School of
Mechanical
Engineering,
Purdue University
,
West Lafayette
,
Indiana 47907
, USA
5
Department of
Materials Science and
Engineering,
University
of Texas at Dallas, Richardson,
Texas
75080, USA
6
The Resnick Sustainability Institute, California Institute of Technology, Pasadena, California
91125, United States
7
Materials and Process Simulation Center
,
California Institute of
Technology, Pasadena,
California 91125, United States
8
Texas Materials Institute,
D
epartment of Mechanical Engineering, The University of Texas at
Austin, Texas 78712, United States
e
-
mail:
Correspondence and requests for materials should
be addres
sed
to W
.
Z
. W
(
w
enzhuowu@purdue.edu
)
or
P
.
D
.
Y.
(
yep@purdue.edu
)
*
These authors contributed equally to this work.
2
Abstract
The reliable production of two
-
dimensional crystals is essential for
the development of
new technologies based on 2D materials. However, current synthesis methods suffer from a
variety of drawbacks, including limitations in crystal size and stability. Here, we report the
fabrication of large
-
area, high
-
quality 2D tellurium
(tellurene) using a substrate
-
free solution
process. Our approach can create crystals with a process
-
tunable
thickness, from monolayer
to tens of
nanomet
r
e
s
, and with lateral sizes of up to 100 μm. The chiral
-
chain van der Waals
structure of tellurene
gives rise to strong in
-
plane anisotropic properties and large thickness
-
dependent shifts in Raman vibrational modes, which is not observed in other 2D layered
materials. We also fabricate tellurene field
-
effect transistors, which exhibit air
-
stable
perfor
mance at room temperature for over two months, on/off ratios on the order of 10
6
and
field
-
effect mobilities of around 700 cm
2
/Vs. Furthermore, by scaling
down the channel
length and integrating with high
-
k dielectrics, transistors with a significant on
-
st
ate current
density of 1 A/mm are demonstrated
.
Main
The continuing development of two
-
dimensional materials, be it the exploration of new
science
1
-
3
or the implementation of new technologies
4
-
8
, requires reliable methods of
synthesising 2D crystals. Whether current approaches can be scaled up though remains
uncertain
9,10
and are restricted by factors such as growth substrates and conditions
11
-
13
, small
crystal sizes
14
and the instability of the synthesized material
s
11,15,16
.
3
Group VI
t
ellurium
(Te)
has
a unique chiral
-
chain
crystal lattice
where
individual
helical
chain
s
of Te atoms
are stacked together by van der Waals
(vdW)
type
bonds
and
spiral around
axes parallel to the
[
0001
]
direction
at the
centre
and corners of the hexagonal elementary
cell
17
(Fig. 1a).
Each
tellurium atom
is covalently bonded
with
its
two near
est
neighbours
on the
same chain
.
Earlier studies
revealed
bulk Te has
small effective
mass
es
and
high
hole
mobi
lit
ies
due to
spin
-
orbit
coupling
18
.
The lone
-
pair and anti
-
bonding orbitals give rise to a
slightly
indirect bandgap in the infrared r
egime (~0.35 eV) in bulk Te
19
,
which has a
conduction band
minimum (CBM)
located at the H
-
point
of the Brillouin zone
,
and a
valence band maximum
(VBM)
that
is slightly shifted from the H
-
point along the chain
direction, giv
ing rise to hole
pockets near H
-
point
20
.
When
the
thickness
is reduced
, the
indirect
feature becomes more
prominent, as shown by our first
-
principles calculations (see Methods for computation details).
For example, the VBM of 4
-
layer Te is further shifted to (0.43, 0.34) (in the unit of
the
surface
reciprocal
cell), while CBM remains at (
1/2, 1/3) (Fig. 1b
inset
). Accompanied by the shift of
VBM, the band gap also increases (Methods, Supplementary Fig. 1), due to the quantum
confinement effect, and eventually reaches ~ 1 eV for monolayer Te
21
.
T
e
has
other
appealing
properties,
e.g.
,
p
hotoconductivity
22
,
t
hermoelectric
ity
20
,
and
piezoelectricity
23
,
for
applications
in
sensors
,
optoelectronics
,
and energy devices.
A wealth of
synthetic
methods
ha
s
been
developed to
derive
Te
nanostructures
24
-
26
,
which
favour
the
1D form due to the
inherent
structural
anisotropy
in
Te.
Much
less
is known
about the
2D form
of Te and
related
properties.
Synthesis
and Structural
Characterization
of 2D Tellurene
In this article,
we
report
a substrate
-
free solution process
for synthesizing
large
-
area, high
-
quality 2D
Te
crystal
s
(termed tellurene) with
a
thickness
of
a monolayer to tens of
nanometres
4
and a
unique chiral
-
chain vdW structure
which is
fundamentally different from the layered vdW
materials.
We use the term
X
-
ene
to describe
2D
form
s
of elemental materials without
considering the specific bonding
21,27
.
The
samples are grown through the
reduction of sodium
tellurite (Na
2
TeO
3
)
by hydrazine hydrate (N
2
H
4
)
in an
alkaline
solution
at temperatures from
160
-
200
o
C
,
with the presence of
crystal
-
face
-
blocking ligand
polyvinylpyrrolidone
(PVP) (see
Methods).
Fig.
1c
inset
shows the optical image of a typical tellurene solution dispersion after
reactions at 180
o
C for 20 hours when
the
Na
2
TeO
3
:
PVP
mole ratio is
52.4:
1 (Methods).
The 2D
Te flakes can be transferred and assembled at large scale into a single layer continuous
thin film
through a Langmuir
-
Blodgett
(LB)
process
28
or a networked continuous thin film
8,29,30
through
ink
-
jet printing (Methods), onto various substrates (
Supplementary
Fig. 2) for future
characterization and device integration.
It should
be noted
that the curr
ent LB approaches still
lack the sort of desired capabilities
in terms of
the film continuity and uniformity, compared to
other methods such as ink
-
jet printing
8,29,30
, for assembling
solution
-
derived 2D materials into
high
-
performance
device. These preliminary results (Supplementary Fig. 2) show potential and
warrant more systematic work for future large
-
scale assembly and applications of solution
-
derived 2D function
al materials.
Individual
2D flakes
have edge lengths ranging from 50 to 100
m, and thicknesses from
10
to
100
nm (
Fig
s
. 1
d
and
Supplementary
Fig. 3
).
The structure, composition, and quality of these
tellurene crystals have been analyzed by high angle
annular dark field
scanning
transmission
electron microscopy (HAADF
-
STEM),
high
-
resolution transmission electron microscopy
(HRTEM),
energy dispersive X
-
ray spectroscopy (EDS), and X
-
ray diffraction (XRD)
(
Fig. 1
e
,
f
, and
Supplementary
Fig. 4
)
.
Fig. 1e
shows
a typical
atomically
-
resolved
HAADF
-
STEM image
of
5
tellurene
flake
(see Methods).
The helical
chains
and
a
threefold screw symmetry along <0001>
are visible
(
Fig. 1
e
)
.
The interplanar spacings are 2.2
Å
and 6.0 Å
, corresponding to
Te
(
1
2
1
0
)
and
(
0001
) planes
31
, respectively
. F
ig. 1
f
shows
the
selective area
electron diffraction
(SAED)
pattern along the [10
1
0] zone axis
, which is perpendicular to the top surface of the flake.
No
point defects or dislocations were observed over a large area within single crystals
(
Supplementary
Fig.
4
).
EDS
result
confirmed
the
chemical composition
of Te (
Supplementary
Fig.
4
).
Similar characterizations and analyses of dozens of
2D Te
flakes with different sizes
indicate that all samples
grow
laterally
along
the
<0001>
and
<1
2
1
0> directions
, with
the
vertical stacking along the <
10
1
0> directions (Fig. 1
g
).
Synthesis Mechanism
and
Geometric
Control
for 2D Tellurene
The
con
trolled
PVP
concentration
is the key
for
obtaining
2D
telluren
e
. Fig. 2a shows the
productivity (see
Methods
) of tellurene
grown at 180
o
C
with time for a
broad
range of
Na
2
TeO
3
/PVP mole ratios.
When a smaller amount of PVP
is
used
, the first
2D structures
occur
after a shorter reaction time (
Fig. 2a,
Supplementary
Fig
.
5
).
A closer
examination
of
reactions
with different PVP concentrations
reveals
an
intriguing
morphology evolution
in growth
products
with time
.
For each PVP concentration, the
initial
growth products a
re dominantly 1D
nanostructures
(
Fig. 2b,
Supplementary
Fig.
5
)
, similar to
previous reports
24
-
26
.
After a
certain
period
of reaction
,
structures
possessing
both 1D and 2D
characteristics start to emerge (Fig.
2b,
Supplementary
Fig.
5
).
TEM characterizations
indicate that
the long axes
(showing 1D
characteristics) of these
flakes
are
<0001>
oriented
,
and
the
lateral protruding
regions (showing
2D characteristics) grow along the <1
2
1
0
> directions,
with
the {10
1
0} facets
as the top/bottom
surfaces (
Supplementary
Fig.
6
).
The
2D
regions
are enclosed
by
edges
with atomic level step
6
roughness
(
Supplementary
Fig.
6
)
.
T
hese high energy edges
are
not specific to certain planes
during the intermediate states.
These structures also have
more uneven surfaces
compared to
2D tellurene (
Supplementary
Fig.
6
)
, further
manifesting
their
intermediate
nature.
Finally,
the
ratio of 2D tellurene flakes which have a straight {
1
2
1
0
}
edge
increases
with
a reduction
in
1D
and
intermediate
structures (
Supplementary
Fig.
5
)
and reaches a plateau after an extended
growth,
e.g.
~30 hours (Fig. 2a,
Supplementary
Fig. 5
).
The
growth
with a lower level of PVP has
a smaller
final productivity (Fig. 2a,
Supplementary
Fig. 5
)
.
The
observed
morphology evolution
suggests that
the balance between the kinetic and thermodynamic growth
dictates the
transformation from 1D structures to 2D
forms
(Fig. 2b)
.
In the initial growth, PVP
is
preferentially adsorbed
on the {10
1
0} surfaces of the
nucleated
seeds
26
, which
promotes
the
kinetic
-
driven 1D
gro
wth
(
Supplementary
Fig. 5
)
.
When the
reaction continues
,
{10
1
0} surfaces
of the formed structures
would
become partially covered due to the
insufficient PVP capping
.
Since {10
1
0}
surfaces have the lowest free energy
in tellurium
32
,
the
growth
of
{
10
1
0
}
surfaces
along the <1
2
1
0
> direction
significantly increases
through the
thermodynamic
-
driven
assembly,
giving rise to the
observed
intermediate structures.
The enhanced growth along the <1
2
1
0
>
directions
together with the continued
<0001>
growth
(
Supplementary
Fig. 6)
lead
s
to the
formation of 2D tellurene
(Supplementary
Fig. 5
, Fig. 2b
)
.
The
sizes and thicknesses of tellurene can
also
be
effectively
modulated
by
controlling
the
ratio between
sodium
tellurite
and PVP
(Fig. 2c
,
Supplementary
Fig. 7
)
.
The width of tellurene
monotonically decreases with the
reduction
of PVP level
;
the thickness
is
minimized
when
a
medium level of PVP
is used
(
e.g.
,
Na
2
TeO
3
/PVP ratio
~
52.4/1
, Group #12
in Fig. 2c
and
Supplementary
Fig. 7
)
, and increases with both the increase and decrease of PVP
(Fig. 2c
).
W
ith
7
a
small amount of PVP,
the solution
is supersaturated
with Te
source
,
and
homogeneous
nucleation
of Te
can occur
in large scale, consuming
re
source
for
subsequent
growth
. As a
result, the
Ostwald
ripening of
Te
nuclei
is shortened
, and the final
tellurene crystals
have
smaller
size
s
compared to samples grown
at
high
er
PVP
c
oncentration
s
.
The low PVP level
also
leads to more significant growth
along
thickness
directions.
On the other hand,
when the PVP
level is high,
the fewer nucleation events allow the
sufficient supply of Te source for
subsequent growth, leading to
the increased width and thickness.
Also
, t
he
productivity
of
tellurene
increases with the reaction temperature from 160
o
C to 180
o
C (
Supplementary
Fig.
8
).
This
is likely
because
higher temperature
promotes
the
forward reaction rate in the
half
reaction of endothermic hydrazine oxidation (see Supplementary Notes).
However, w
hen
temperature increases from
180
°C
to
200
°C
, the
possible
breaking of
the van der Waals bonds
between Te chains
by the excessive energy
could lead to the
saturated
productivity.
T
ellurene crystals with
a
thickness
smaller
than 10 nm
to ultimately monolayer
structure
can
be
further
derived
through a
solvent
-
assisted post
-
growth thinning
process
(see Methods
).
The thickness of tellurene decreases with time after acetone
is introduced
into the growth
solution
(
Supplementary
Fig.
9
).
After 6
hour
s
,
the average thickness of tellurene is reduced to ~
10 nm, with the
thinnest flake down to
4 nm
thick
(~
10
layers) (
Supplementary
Fig. 9
).
Due to
the
poor solubility in acetone, PVP molecules
tend
to desorb from the
tellurene
and
undergo
aggregation
33
, giving
rise to
the sediment of
tellurene
over the time
in acetone
(
Supplementary
Fig. 9
).
Lacking the protection of PVP
, the
tellurene
surface
s
get
exposed
and react with
the
alkaline
growth solution
(
pH ~
11.5
)
34
,
leading to the reduced thickness.
We have also
performed control experiments
using
other types of solvents
in the growth solution
8
(
Supplementary
Fig.
10
)
, the results of which
suggest that
PVP solubility i
n the solvent
significantly affects the
above
process.
L
arge
-
area (
up
to 100
-
m
in
lateral
dimensions
)
tellurene crystals with monolayer, bi
-
layer, tri
-
layer
and few
-
layer
thicknesses
can be
further
obtained
(Fig. 2d,
Supplementary
Fig. 11
)
,
by controlling
the pH value
s
of the tellurene
dispersion solution in the above post
-
growth thinning process
(see Methods, Fig. 2d,
Supplementary
Fig. 12)
.
Thickness
-
and Angle
-
dependent Raman Spectra
These high
-
quality ultrathin
tellurene
crystals with
controlled
thick
nesses provide an ideal
system to explore the
ir
intrinsic properties in the 2D limit.
We first characterize
d the optical
properties
of as
-
synthesized tellurene
with a wide range of thickness
(
from
a
monolayer
to
37.4 nm
)
by
angle
-
resolved
polarized
Raman spectroscopy at
room temperature (see
Methods).
The incident light comes in along the [
10
1
0
] direction and
is polarized
into the
[0001] helical chain direction of the tellurene.
T
he Raman spectra
of
tellurene samples with
different thickness
(Fig. 3a)
exhibits
three main Raman
-
active modes,
with
one
A
-
mode
and two
E
-
modes
which correspond
to
the
chain expansion in basal plane, bond
-
bending around [
1
2
10
]
direction and asymmetric stretching mainly along [0001] helical chain
35
, respectively
(
Supplementary
Fig. 1
3
).
For the 2D Te samples
thicker
than
20.5 nm,
t
hree Raman
-
active
modes locating at 92 cm
-
1
(E
1
transverse (TO) phonon
mode), 121 cm
-
1
(
A
1
-
mode) and
143
cm
-
1
(E
2
-
mode) were identified (Fig.
3a
),
which agrees well with previous observations in bulk
and
nanostructured
tellurium
36
-
38
,
indicating that although
these thicker crystals possess
2D
morphology
, the
ir
symmetric properties
can still be characterized as
bulk.
The
appreciable
effective dynamic charge
induced for
the
E
1
mode
in tellurium
leads to
a
split of E
1
doublets at
9
92
cm
-
1
and
105
cm
-
1
for transverse (TO) or longitudinal (LO) phonons
, respectively
37
.
The
absence of E
1
(
L
O
)
mode
in
our observed results for
2D Te thicker than
20.5 nm
, similar to
previous
reports
on
bulk and nanostructured tellurium
36
-
38
,
may be attributed to
the
different
signs in the deformation potential and electro
-
opti
c contribution to the Raman scattering
tensor,
which gives rise
to
the
cancellation if both contributions have the same magnitude
39
.
As
the thickness
decreases
from 20.5 nm to 9.1 nm (Fig.
3a
), the deformation potential
in tellurene
lattice
increase
s
while the electro
-
optic effect weaken
s
40
, leading to the appearance of
E
1
(
LO
)
mode
in
the
Raman spectra
for
2D Te
crystals
with
intermediate
thickness
.
When
the 2D Te’s
thickness
further
reduces
(smaller than 9.1 nm in Fig. 3a)
, the degeneracy in
the
E
1
TO and LO
modes
occurs with peak broadening, possibly due to the intra
-
chain atomic disp
lacement, the
electronic band structure
changes
and the symmetry assignments for each band
41
, all of which
are affected by the sample thickness
.
When
tellurene’s
thickness
decreases
, there are
significant
blue
-
shift
s
for both A
1
(shift
to
136
cm
-
1
for monolayer)
and E
2
modes
(shift to
149
cm
-
1
for monolayer)
(Fig. 3a)
. The
hardened
in
-
plane
E
2
vibration
mode
in
thinner
tellurene
,
similar to
reported observations for
black
phosphorus
42
and MoS
2
43
,
,
may be
attributed to the
enhanced
interlayer long
-
range Coulombic
interactions
when thinned down
.
The
observed
blue
-
shift for
the
A
1
mode
in
2D Te
,
in
strong
contrast to 2D layered
vdW
materials
which
usually witness red
-
shift for the
out
-
of
-
plane
vibration mode when thinned down
15,42,43
,
is thought to be closely related to
the
unique
chiral
-
chain
vdW
structure of
tellurene
.
When thinned down, t
he lattice deformation
within the 2D
plane
g
ave
rise to the
attenuated inter
-
chain vdW interaction
s
and
en
hanced intra
-
chain
covalent
interactions
in
the
individual
tellurene
layer
, leading
to
more effective
restoring forces
10
on
tellurium
atoms
and hence hardened
out
-
of
-
plane
A
1
vibration mode
(
Supplementary
Fig.
13)
.
Such unique
structure
of
tellurene
also
results in
the
giant
thickness
-
dependent
shift
in
Raman vibrational modes,
which is unseen in
2D
layered
vdW materials
42
-
44
.
The
interaction
between
the
substrate
(
SiO
2
/Si
)
and 2D Te flakes
could
also contribute to the
hardened
A
1
and
E
2
modes
36
.
The
stiffening
of vibrational
modes in monolayer tellurene (Fig. 3a) is consistent
with
its
structure reconstruction
where
extra
bonds
are
formed
between
neighbouring
chains
in
the single layer tellurium
21,27,45
.
Reduced in
-
plane symmetry in
the
chiral
-
chain
vdW
structure of tellurene indicates a
strong
in
-
plane anisotropy for
its
material properties
.
We further
characte
rized the anisotropic
optical properties of as
-
synthesized tellurene with three
different
thickness
es
(
28.5 nm, 13.5
nm
,
and 9.7 nm
)
by angle
-
resolved
polarized
Raman spectroscopy at
room temperature (see
Methods).
By
rotating the tellurene flake
s
in
steps of
15
o
, we
observe
d
the changes in the
angle
-
r
esolved Raman peak intensities (Fig.
3b
,
Supplementary
Fig
s
. 14a, 15a
).
We extracted
the peak intensities of different
modes
by fitting with
Lorentz
function and plotted them into
the corresponding polar figures (Figs.
3c
-
f
,
Supplementary
Figs. 14b
-
e
, 15b
-
d
)
(see Methods)
.
While all the modes change in intensity with the polarization angle, we find that the peak
for
the
A
1
mode
in all samples exhibits the largest sensitivity
to the relative orientation between
the [0001] direction and the polarization of the excitation laser (middle panel
s
of
Fig. 3b,
Supplementary
Figs. 14a, 15a
).
It is worth to note that the direction
of
maximum intensity in
A
1
polar plot changes
with the sample
thickness. More specifically,
A
1
polar plots for
the 13.5
nm and 28.5 nm sample
s
show
the maximum intensity at 90
°
and 270
° along
the
[1
2
1
0
]
direction
(
Fig.
3e
,
Supplementary
Fig.
15c
)
. However, when the thickness decreases to 9.7 nm,
11
the
maximum intensity
direction
switches to 0
°
and
180
° along
the
[0001] direction
(
Supplementary
Figs. 14
d
)
.
A s
imilar
phenomenon
also occurs
for E
1
-
L
O
mode
s in
the 13.5 nm
and 9.7 nm samples (
Fig.
3d,
Supplementary
Figs. 14
c
)
.
Such
thickness
-
dependent
anisotropic
Raman scattering could
be attributed
to the different absorption spectral range in [0001]
and
[1
2
1
0
]
directions
46
and anisotropic interference effect
41
.
The angle
-
resolved Raman
results
also
confirm that the helical Te atom chains in the as
-
synthesized tellurene are oriented
along the growth direction of the tellurene flake, which
matches the TEM results (Fig. 1e
).
Device Implementation of
Tellurene Field
-
effect Transistors
Finally, w
e
explor
ed
the electrical performance of
tellurene
field
-
effect
transistors
(FETs)
to
demonstrate its
great
potential for logic electronics application
.
Back
-
gate devices
were
fabricated
on high
-
k
dielectric
substrates
and s
ource
/
drain regions were patterned by
electron beam lithography
with the channel parallel to the
[0001]
direction of tellurene
(details
see Methods)
.
We chose Pd/Au (50/50 nm) as metal contacts since Pd has
relatively high work
function that can
reduce the contact resistance in
p
-
type
transistors
47
-
49
.
L
ong channel devices
were first
examined
(channel l
ength 3
m) where the
contact
resistance
is negligible, and
the
transistor behavior
is dominated by
intrinsic electrical properties of channel material.
Fig.
4a
show
s
the
transfer
curve of a typical 7.5
-
nm
-
thick
2D
Te
FET
measured at room temperature.
The device exhibits
p
-
type characteristics with
slight
ambipolar transport behavior due to its
narrow bandgap na
ture, with large drain current over
300
mA/mm
(see Supplementary
Fig.
23)
and high on/off ratio
on the order of
~
10
5
.
The
p
-
type behavior originates from the high level of
Te
valance band edge, as shown by our first
-
principles calculations (
Supplementary
Fig. 16).
Meanwhile, the process
-
tunable thickness of tellurene allows the modulation of electrical
12
performance in tellurene transistors.
Overall,
important
metrics of tellurene
-
based transistors
such as on/off ratio, mobility, and on
-
state current
level
are superior or comparable to
transistors based on other 2D materials
11,15,16,50
.
We further
investigate
d
the thickness
dependence of two key metrics of
device
performance,
namely
on/off ratios and field
-
effect
mobilities
,
for more than 50 2D Te long channel devices with flake thicknesses ranging from
over 35 nm down to
a
monolayer
(~0.5 nm
)
, to elucidate the transport mechanism of
2D Te
FETs
(Fig. 4b)
. The linear behavior of
the output curves in the low V
ds
region
(Supplementary
Fig.
23)
suggests that the contact resistance is low
(see Supplementary Notes and
Supplementary Fig. 19 for the extracted contact resistance)
which
ensures the sound
ness
of
the
field
-
effect mobility
extraction
from the slope of the linear region of the transfer curves (see
Methods).
The f
ield
-
effect mobilities
of 2D Te transistors peak
with ~700 cm
2
/Vs at room
temperature at around 16 nm thickness and decreases gradually with the further increase of
t
he thickness.
The transfer curves of devices with
thin
thickness of 2.8 nm (~ 6 layers), 1.7 nm (~
4 layers), 1.0 nm (bi
-
layer) and 0.5 nm (monolayer)
are included
in
Supplementary Fig. 24.
A
benchmark comparison with black phosphorus, which is also a narr
ow bandgap p
-
type 2D
material, shows that solution
-
synthesized 2D Te has ~ 2
-
3 times higher mobility than black
phosphorus when the same device structure, geometry, and mobility extraction method are
adopted
15
(Supplementary Fig. 21).
This
thickness
-
dependence
is similar to
other
layered
materials that experience screening and interlayer coupling
15,16
(Supplementary Notes and
Supplementary Fig. 20). The field
-
effect mobility is also affecte
d by the contribution
of the
carriers
from layers
near the semiconductor
-
oxide
interface
. Thinner samples are more
susceptible to the charge impurities at the interface and surface scattering, which explains the
13
decrease in the mobility of the
few
-
layer
te
llurene transistors.
We expect to be able to improve
the mobility of tellurene through approaches such as improving interfac
e quality with high
-
k
dielectric
51
or h
-
BN
en
capsulation to reduce the substrate phonon scattering and charge
impurity
.
For bi
-
layered tellurene transistors, external field
-
effect mobility
is dropped
to
~
1
cm
2
/Vs
.
This
is
due to
bandga
p increasing in few
-
layer tellurene
to form
much
higher Schottky
barrier, which may
introduce
large
deviation in extracting mobility due to the drastically
increase
d
contact resistance
.
Because of the reduced gate electrostatic control in thicker flakes,
the thickness
-
dependent on/off ratio
s
(
see Supplementary Notes
)
steeply decrease from
10
6
to less than
10
once the crystal thickness approaches the maximum depletion width of the
films, with a trend similar to other reported narro
w bandgap depletion
-
mode 2D FETs
15,16
.
The in
-
plane anisotropic electrical
transport properties
were also studied
at room
temperature.
To
minimize flake
-
to
-
flake variation and geometric non
-
ideality, we applied dry
-
etching method (see Methods) to trim two identical rectangles from the same 2D Te flake. One
of the rectangles
was aligned
along the 1D atomic chain
[0001]
direction and the othe
r along
[1
2
1
0
] direction (Fig.
4c
inset). Long channel FETs (channel length 8
m) were fabricated to
minimize contact influence and manifest the intrinsic material properties. The extracted field
-
effect mobilities along these two primary directions from se
ven 2D Te samples exhibit an
average anisotropic mobility ratio of 1.43±0.10 (Fig. 4
c
). A typical set of results measured from
a 22
-
nm
-
thick
sample
is shown
in
Supplementary Fig. 22. This
anisotropic ratio in mobility is
slightly lower than that reported f
or bulk tellurium
52
, possibly due to the enhanced surface
scattering in our ultrathin Te samples
.
Our first
-
principle calculations show a similar degree of
14
anisotropy in the
effective masses along these two orthogonal directions (Fig. 1b, 0.32 m
0
perpendicular to the chain and 0.30 m
0
along the chain, see Methods).
Great air
-
stability
was also demonstrated
in tellurene transistors with different flake
thickness. Electrical pe
rformance of a 15
-
nm
-
thick transistor
was monitored
after being
exposed
in
air for two months without any encapsulation, as shown in
Fig.
4d. No significant
degradation was observed in the same device during
two
-
month
period, except slight
threshold
voltag
e shift probably coming from sequential measurement variation
.
We further
demonstrated that such good air
-
stability is valid for almost
the entire
thickness range from
thick flakes down to 3 nm (See Supplementary Fig.
25
). For even thinner flakes, the thin films
are no longer conducting after
first
couple of days.
More strikingly, by
scaling down the channel length
and
integrating with
our ALD
-
grown
high
-
k dielectric, we achieved record high
drive
current
of
over 1 A/mm
at relatively low V
ds
=1.4
V.
Fig.
5a and 5b represent I
-
V curves of a short channel device
with channel length of 300 nm
fabricated on a
n
11
-
nm
-
thick Te flake. The on/off ratio at small drain bias (V
ds
=
-
0.05
V) is over
10
3
, which is still a decent value
,
c
onsidering its narrow bandgap of
~0.4
eV
(see Supplementary
Fig. 1)
.
T
he o
ff
-
state performance
is
slightly
deteriorated at large drain voltage (V
ds
=
-
1
V
,
pink
circles in
Fig.
5b
) due to the
short channel effect
. Large drain voltage reduces the barrier
height
for electron branch
and
electron current
i
s
unhindered
, which is also reflected in the upswing of
drain current at large V
ds
in output curve
(
Fig.
5a
)
.
Such effect is common in narrow
-
bandgap
short channel devi
ces
53,54
and can
be mediated
through proper contact engineering
54
.
Fig.
5c
shows
the
relationship between
two transistor key parameters
,
on/off ratio and maximum
drain current
,
of
over 30
devices with different channel thickness
. Generally speaking, a short
15
channel d
evice with flake thickness around
7
-
8
nm offers the best performance with on/off
ratio ~10
4
and maximum drain current >
6
00 mA/mm. It is also worth mentioning that the
maximum drain current
we achieved is
1.06 A/mm
, w
ith
several devices
exceeding
1 A/mm,
w
hich is so far the highest
value
among all the
two
dimensional
material transistors to our best
knowledge
53,55,56
. This number is also co
mparable to that of conventional s
emicon
ducto
r
devices.
Conclusion
s
We have
developed a
simple, low
-
cost
,
solution
-
based
approach for
the scalable synthesis
and assembly of 2D
Te
cryst
als.
These high
-
quality
2D
Te
crystals
have high carrier mobility and
are air
-
stable
(measured up to two months)
.
Our prototypical 2D Te device shows
a
good
all
-
around figure of merits
(
Supplementary Fig. 2
6
, Supplementary Table 1
)
compared to existing
2D materials
and record
-
high on
-
state current capacity
.
Our
approach has the potential
to
produce stable, high
-
quality, ultrathin semiconductors with
a
good
control of composition,
structure
,
and dimensions
,
opening up opportunities for
applications in electronics,
optoelectronics, energy
conversion
,
and energy storage.
2D
Tellurene, as a
chiral
-
chain
van der
Waals
solid
,
add
s
a new
class
of
nano
materials
to the large family of 2D
crystals
.
16
Methods
Synthesis of 2D tellurene crystals
In a typical procedure, analytical grade Na
2
TeO
3
(0.00045 mol) and
a certain amount of
poly(
-
vinyl pyrrolidone)
was put
into double distilled water (33 ml) at room temperature under
magnetic stirring to form a homogeneous solution. The resulting solution
was poured
into a
Teflon
-
lined stainless
-
steel autoclave, which
was then filled
with an
aqueous ammonia solution
(25%, w/w%) and hydrazine hydrate (80 %, w/w%). The autoclave was
sealed
and maintained at
the reaction temperature for a
designed
time. Then the autoclave was cooled to room
temperature naturally. The resulting silver
-
gray, solid
products were precipitated by centrifuge
at 5000 rpm for 5 minutes and washed
3
times with distilled water (to remove any ions
remaining in the final product).
Langmuir
-
Blodgett (LB)
transfer of tellurene
The hydrophilic 2D Te nanoflake monolayers can
be
transferred
to various substrates by the
Langmuir
-
Blodgett (LB) technique. The washed nanoflakes
were suspended
in a mixture solvent
made of
N, N
-
dimethylformamide
(DMF) and CHCl
3
(
e.g.,
in the ratio of 1.3:1). Then, the mixture
solvent
was dropped
into th
e deionized water. Too much DMF will result in the falling of 2D Te
in the water. It
is difficult to mix the
DMF, CHCl
3
and 2D Te when CHCl
3
is too much. After the
evaporation of the solvent, a monolayer assembly of 2D Te flakes
was observed
at the air/wat
er
interface. Then we can transfer the monolayer assembly of 2D Te onto the substrates.
First
-
Principles Calculations
Density Functional Theory calculations were performed using the Vienna Ab
-
initio Simulation
Package (VASP)
57
with projector augmented wave (PAW) pseudopotentials
58
. We used 500 eV for
17
the plane
-
wave cutoff,
5x5x1 Monkhorst
-
Pack sampling, and
fully
relaxed the systems until the
final
force on each atom was less than 0.01 eV/Å . The PBE exchange
-
correlation functional
is used
for relaxation of the
system
, and the HSE functional is employed to calculate the band
gaps (Fig.
S1) and the band edge levels (Fig. S10). We find a significant structural reconstruction for
monolayer Te, in agreement with reported result
21
. While for bilayer and thicker Te, the structure
is similar to that of bulk Te. Our calculations show a lattice parameter of 4.5 Å and 6.0 Å for
multilayers, in agreement with experiments. The adsorption of O on bilayer Te and P is
modeled
by
using 4x3 cell (see
Supplementary Fig.
12).
Structural characterization
The morphology of the ultrathin tellurene crystals
was identified
by optical microscopy
(Olympus BX
-
60). The thickness
was determined
by AFM (Keysight 5500).
High
-
resolution
STEM/TEM imaging and SAED has been performed using a probe
-
corrected JEM
-
ARM 200F
(JEOL USA, Inc.) operated at 200 kV and EDS has
been collected
by an X
-
MaxN100TLE detector
(Oxford Instruments). In HAADF
-
STEM mode, the convergence semi
-
angle of electron p
robe is
24
mrad,
and the collection angle for ADF detector
was set
to 90
-
370
mrad
.
Determination of tellurene productivity
To quantify the ratio of 2D tellurene flake in the products, we measured all the products in
the same process as follows: the freshly
prepared 2D tellurene solution (1 mL) was centrifuged
at 5000 rpm for 5 minutes after adding acetone (2 mL) and washed with alcohol and double
distilled water twice.
Then the 2D tellurene flakes were dispersed into
3
mL double distilled water.
After that,
we dropped 100 μL dispersed solution onto the
1×1
cm
2
SiO
2
/Si substrate. After the
water evaporation, we used the optical microscope
to record several images randomly covering
18
the 5×5 mm
2
area
. In the end, we analyze the
areas
covered by 2D tellurene by I
mageJ,
public
domain, Java
-
based image processing program developed at the National Institutes of Health. In
our case, we define the productivity as the ratio of the
2D
tellurene area in the
entire
image.
Solvent
-
assisted post
-
growth thinning process
For thinning process using the
alkaline
growth solution, the as
-
synthesized 2D tellurene
solution (1 mL)
was mixed
with acetone (3 mL)
at
the room temperature. After
a specific
time
(
e.g.,
6 hours), the thin 2D tellurene can be obtained by centrifuge at 50
00 rpm for 5 minutes.
After doing the LB process, the 2D tellurene can
be transferred
onto the substrate.
For thinning process using tellurene solution with controlled pH values, the suspension of as
-
synthesized 2D Te (1 mL) was centrifuged one time with t
he addition of 3 mL DI water. Then, the
2D Te
was dispersed
into a solution of 1 mL of NaOH and
3
mL of acetone. The concentration of
NaOH was varied to control the pH value of the above
4
mL solution. After that, the above
solution
was kept
at
room temper
ature for 2
-
10 hours. Finally, the thinned tellurene samples
were precipitated by centrifuge.
FET
Device fabrication and characterization
High
-
k dielectric stack consisting of
2
0 nm hafnium zirconium oxide (Hf
0.5
Zr
0.5
O
2
) and 2 nm
Al
2
O
3
was first deposited by atomic layer deposition (ALD) onto heavily doped n++ silicon wafer.
Upon transferring the tellurene flakes onto the substrate, source and drain
regions were
patterned by electron beam lithography (EBL)
. We chose 50/50 nm Pd/Au
for
c
ontact metal
since Pd has relatively high work function
which
benefits the p
-
type transistors by reducing
Schottky contact resistance. The
electrical
measurements were performed using Keithley 4200A
semiconductor characterization system.
19
By plugging numbe
rs into the
formula
:
퐹퐸
=
표푥
푑푠
, where
,
,
and
표푥
are
transconductance, channel length, channel width and gate oxide capacitance, we can derive the
field
-
effect mobilities for the tellurene transistors.
Field
-
effect mobilities
extracted from devices
fabricated on tellurene crystals with various thicknesses are displayed in
Fig. 4b
.
Devices for
anisotropic transport measurement were first patterned into two perpendicular rectangles along
the two principle in
-
plane directions of
tellurene with EBL and dry
-
etched into desired shapes
with BCl
3
and argon plasma. The rest of fabrication process follows the same route as before.
Raman Spectra
Angle
-
resolved Raman Spectra
were measured
at room temperature. The crystal symmetry
of Te ren
ders one A
1
mode, one A
2
mode (Raman
-
inactive), and two
doublet
E modes at Γ point
of Brillouin zone. Raman signal was excited by 633nm He
-
Ne laser. The incident light comes in
along [
-
1010] direction which is perpendicular to the Te flake surface and
was
polarized
into
[0001] direction, which is parallel to spiral atom chains and we denote this configuration as 0°. A
linear polarizer
was placed
in front of the
spectrometer
to
polarize reflected light
into
the same
direction with incident light. Polarized R
aman can eliminate all the other superimposed Raman
signals and manifests a clear trace of angle
-
dependent Raman spectrum evolution. Then by
rotating the Te
flake,
we observed angle
-
resolved Raman peak intensity change, as shown in Fig.
3a. We extracted th
e peak intensities of different
modes
by fitting with Lorentz function and
plotted them into polar figures (Fig. 3b
-
f). These angle dependent behaviors
were then fitted
by
calculating matrix multiplication:
×
×
,
where
and
are unit vectors of incident the
and refl
ected light direction and
is the Raman tensor of corresponding Raman modes
2
. The
20
angle
-
resolved Raman results confirm that the helical Te atom chain is indeed along the long axis
of the Te flake, which matches ou
r previous TEM results.
Data Availability Statement
The data that support the plots within this paper and other findings of this study are
available from the corresponding author upon reasonable request.
21
References
1
Castro Neto, A. H., Guinea, F., Peres, N. M. R., Novoselov, K. S. & Geim, A. K. The electronic
properties of graphene.
Reviews of Modern Physics
81
, 109
-
162 (2009).
2
Qian, X., Liu, J., Fu, L. & Li, J. Quantum spin Hall effect in two
-
dimensional transition
metal
dichalcogenides.
Science
346
, 1344
-
1347, (2014).
3
Chhowalla, M.
et al.
The chemistry of two
-
dimensional layered transition metal
dichalcogenide nanosheets.
Nat Chem
5
, 263
-
275 (2013).
4
Fiori, G.
et al.
Electronics based on two
-
dimensional material
s.
Nat Nano
9
, 768
-
779,
(2014).
5
Wang, Q. H., Kalantar
-
Zadeh, K., Kis, A., Coleman, J. N. & Strano, M. S. Electronics and
optoelectronics of two
-
dimensional transition metal dichalcogenides.
Nat Nano
7
, 699
-
712 (2012).
6
Wu, W.
et al.
Piezoelectricity of
single
-
atomic
-
layer MoS
2
for energy conversion and
piezotronics.
Nature
514
, 470
-
474, (2014).
7
Smith, R. J.
et al.
Large
-
Scale Exfoliation of Inorganic Layered Compounds in Aqueous
Surfactant Solutions.
Advanced Materials
23
, 3944
-
3948, (2011).
8
Bonaccor
so, F., Bartolotta, A., Coleman, J. N. & Backes, C. 2D
-
Crystal
-
Based Functional
Inks.
Advanced Materials
28
, 6136
-
6166, (2016).
9
Hao, Y.
et al.
The Role of Surface Oxygen in the Growth of Large Single
-
Crystal Graphene
on Copper.
Science
342
, 720 (2013).
10
Najmaei, S.
et al.
Vapour phase growth and grain boundary structure of molybdenum
disulphide atomic layers.
Nat Mater
12
, 754
-
759, (2013).
11
Tao, L.
et al.
Silicene field
-
effect transistors operating at room temperature.
Nat Nano
10
,
227
-
231, (2015).
1
2
Mannix, A. J.
et al.
Synthesis of borophenes: Anisotropic, two
-
dimensional boron
polymorphs.
Science
350
, 1513 (2015).
13
Zhu, F.
-
f.
et al.
Epitaxial growth of two
-
dimensional stanene.
Nat Mater
14
, 1020
-
1025,
(2015).
14
Coleman, J. N.
et al.
Two
-
Dimensi
onal Nanosheets Produced by Liquid Exfoliation of
Layered Materials.
Science
331
, 568
-
571, (2011).
15
Liu, H.
et al.
Phosphorene: An Unexplored 2D Semiconductor with a High Hole Mobility.
ACS Nano
8
, 4033
-
4041, (2014).
16
Li, L.
et al.
Black phosphorus fie
ld
-
effect transistors.
Nat Nano
9
, 372
-
377, (2014).
17
von Hippel, A. Structure and Conductivity in the VIb Group of the Periodic System.
The
Journal of Chemical Physics
16
, 372
-
380, (1948).
18
Doi, T., Nakao, K. & Kamimura, H. The Valence Band Structure o
f Tellurium. I. The k·p
Perturbation Method.
Journal of the Physical Society of Japan
28
, 36
-
43, (1970).
19
Coker, A., Lee, T. & Das, T. P. Investigation of the electronic
properties of tellurium
energy
-
band structure.
Physical Review B
22
, 2968
-
2975 (1980
).
20
Peng, H., Kioussis, N. & Snyder, G. J. E
lemental tellurium as a chiral
p
-
type thermoelectric
material.
Physical Review B
89
, 195206 (2014).
21
Zhu, Z.
et al.
Telluren
e
-
a monolayer of tellurium from
first
-
principles prediction.
arXiv:1605.03253
(2016).
22
22
Liu, J.
-
W., Zhu, J.
-
H., Zhang, C.
-
L., Liang, H.
-
W. & Yu, S.
-
H. Mesostructured Assemblies of
Ultrathin Superlong Tellurium Nanowires and Their Photoconductivity.
Journal of the
American Chemical Society
132
, 8945
-
8952, (2010).
23
Lee, T. I.
et al
.
High
-
Power Density Piezoelectric Energy Harvesting Using Radially Strained
Ultrathin Trigonal Tellurium Nanowire Assembly.
Advanced Materials
25
, 2920
-
2925,
(2013).
24
Mo, M.
et al.
Controlled Hydrothermal Synthesis of Thin Single
-
Crystal Tellurium
Nanob
elts and Nanotubes.
Advanced Materials
14
, 1658
-
1662,
(2002).
25
Mayers, B. & Xia, Y. One
-
dimensional nanostructures of trigonal tellurium with various
morphologies can be synthesized using a solution
-
phase approach.
Journal of Materials
Chemistry
12
, 1875
-
1881, (2002).
26
Qian, H.
-
S., Yu, S.
-
H., Gong, J.
-
Y., Luo, L.
-
B. & Fei, L.
-
f. High
-
Quality Luminescent Tellurium
Nanowires of Several Nanometers in Diameter and High Aspect Ratio Synthesized by a
Poly (Vinyl Pyrrolidone)
-
Assisted Hydrothermal Process.
Lan
gmuir
22
, 3830
-
3835, (2006).
27
Xian, L., Paz, A. P., Bianco, E., Ajayan, P. M. & Rubio, A. Square selenene and tellurene:
novel group VI elemental 2D semi
-
Dirac materials and topological insulators. (2016).
28
Zasadzinski, J. A., Viswanathan, R., Madsen,
L., Garnaes, J. & Schwartz, D. K. Langmuir
-
Blodgett films.
Science
263
, 1726 (1994).
29
Hu, G.
et al.
Black phosphorus ink formulation for inkjet printing of optoelectronics and
photonics.
Nature Communications
8
, 278, (2017).
30
Kelly, A. G.
et al.
All
-
pr
inted thin
-
film transistors from networks of liquid
-
exfoliated
nanosheets.
Science
356
, 69 (2017).
31
Cherin, P. & Unger, P. Two
-
dimensional refinement of the crystal structure of tellurium.
Acta Crystallographica
23
, 670
-
671, (1967).
32
Tran, R.
et al.
Su
rface energies of elemental crystals.
Scientific Data
3
, 160080, (2016).
33
Lan, W.
-
J., Yu, S.
-
H., Qian, H.
-
S. & Wan, Y. Dispersibility, Stabilization, and Chemical
Stability of Ultrathin Tellurium Nanowires in Acetone: Morphology Change,
Crystallization,
and Transformation into TeO2 in Different Solvents.
Langmuir
23
, 3409
-
3417, (2007).
34
Liu, J.
-
W., Wang, J.
-
L., Wang, Z.
-
H., Huang, W.
-
R. & Yu, S.
-
H. Manipulating Nanowire
Assembly for Flexible Transparent Electrodes.
Angewandte Chemie International Editi
on
53
, 13477
-
13482,
(2014).
35
Martin, R. M., Lucovsky, G. & Helliwell, K. Intermolecular bonding and lattice dynamics of
Se and Te.
Physical Review B
13
, 1383 (1976).
36
Du, Y.
et al.
One
-
Dimensional van der Waals Material Tellurium: Raman Spectroscopy
under Strain and Magneto
-
Transport.
Nano Letters
17
, 3965
-
3973
(2017).
37
Pine, A. & Dresselhaus, G. Raman spectra and lattice dynamics of tellurium.
Physical
Review B
4
, 356 (1971).
38
Wang, Q.
et al.
Van der Waals Epitaxy and Photoresponse of Hexagonal Tellurium
Nanoplates on Flexible Mica Sheets.
ACS Nano
8
, 7497
-
7505, (2014).
39
Richter, W. Extraordinary phonon Raman scattering and resonance enhancement in
tellurium.
Journal of Physic
s and Chemistry of Solids
33
, 2123
-
2128 (1972).
40
Qiu, J. & Jiang, Q. Film thickness dependence of electro
-
op
tic effects in epitaxial
Ba
0.7
Sr
0.3
TiO
3
thin films.
Journal of Applied Physics
102
, 074101 (2007).
23
41
Ling, X.
et al.
Anisotropic Electron
-
Photon and Electron
-
Phonon Interactions in Black
Phosphorus.
Nano Letters
16
, 2260
-
2267, (2016).
42
Wang, X.
et al.
Highly anisotropic and robust excitons in monolayer black phosphorus.
Nature nanotechnology
10
, 517
-
521 (2015).
43
Lee
, C.
et al.
Anomalous lattice vibrations of single
-
and few
-
layer MoS
2
.
ACS nano
4
, 2695
-
2700 (2010).
44
Ferrari, A. C.
et al.
Raman Spectrum of Graphene and Graphene Layers.
Physical Review
Letters
97
, 187401 (2006).
45
Huang, X.
et al.
Epitaxial growth an
d band structure of Te film on graphene.
Nano Letters
17
, 4619
-
4623 (2017).
46
Isomäki, H. & von Boehm, J. Optical absorption of tellurium.
Physica Scripta
25
, 801 (1982).
47
Deng, Y.
et al.
Towards high
-
performance two
-
dimensional black phosphorus
optoelectronic devices: the role of metal contacts.
2014 IEEE International Electron
Devices Meeting
doi: 10.1109/IEDM.2014.7046987
(2015).
48
Liu, Y., Xiao, H. & Goddard, W. A. Schottky
-
Barrier
-
Fre
e Contacts with Two
-
Dimensional
Semiconductors by Surface
-
Engineered MXenes.
Journal of the American Chemical
Society
138
, 15853
-
15856,
(2016).
49
Liu, Y., Stradins, P. & Wei, S.
-
H. Van der Waals metal
-
semiconductor junction: Weak Fermi
level pinning enabl
es effective tuning of Schottky barrier.
Science Advances
2
, e1600069
(2016).
50
RadisavljevicB, RadenovicA, BrivioJ, GiacomettiV & KisA. Single
-
layer MoS
2
transistors.
Nat Nano
6
, 147
-
150, (2011).
51
Jena, D. & Konar, A. Enhancement of Carrier Mobility in Semiconductor Nanostructures
by Dielectric Engineering.
Physical Review Letters
98
, 136805 (2007).
52
Rothkirch, L., Link, R., Sauer, W. & Manglus, F. Anisotropy of the Electric Conductivity of
Tellur
ium Single Crystals.
physica status solidi (b)
31
, 147
-
155, (1969).
53
Si, M., Yang, L., Du, Y. & Ye, P. D.
Black phosphorus field
-
effect transistor with record drain
current exceeding 1 A/mm.
2017 75th Annual Device Resea
rch Conference (DRC)
doi:
10.1109/
DRC.2017.7999395
(2017)
.
54
Yang, L.
et al.
How Important Is the Metal
Semiconductor Contact for Schottky Barrier
Transistors: A Case Study on Few
-
Layer Black Phosphorus?
ACS Omega
2
, 4173
-
4179,
(2017).
55
McClellan, C. J., Yalon, E., Smithe, K. K. H., Su
ryavanshi, S. V. & Pop, E.
Effective n
-
type
doping of monolayer MoS
2
by AlO
x
.
2017 75th Annual D
evice Research Conference (DRC)
doi:
10.1109/DRC.2017.7999392 (2017)
.
56
Liu, Y.
et al.
Pushing the Performance Limit of Sub
-
100 nm Molybdenum Disulfide
Transistors.
Nano Letters
16
, 6337
-
6342, (2016).
57
Kresse, G. & Furthmüller, J. Efficient iterative schemes for
ab initio
total
-
energy
calculations using a plane
-
wave basis set.
Phys. Rev.
B
54
, 11169
-
11186 (1996).
58
Kresse, G. & Joubert, D. From ultrasoft pseudopotentials to the projector augmented
-
wave method.
Phys. Rev. B
59
, 1758
-
1775 (1999).
24
Online Content
Supplementary notes and
Extended Data display items are available in the
online version of the paper; references unique to these sections appear only in the online
paper.
Acknowledgements
W. Z. W.
acknowledge
the College of Engineering and School of
Industrial
Engineering at Pu
rdue University for the startup support
.
W. Z. W
.
was
partially supported
by a
grant from the Oak Ridge Associated Universities (ORAU)
Junior Faculty Enhancement Award
Program.
Part of the solution synthesis work was supported by
the National Science
Foundation
under grant no.
CMMI
-
1663214
.
P.
D. Y.
was supported
by NSF/AFOSR 2DARE
Program,
ARO
,
and SRC
.
Q. W. and M. J. K.
were supported
by the
Center for Low Energy
Systems Technology (LEAST) and Center for South West Academy of Nanoelectronics (SWAN).
Y. L. thanks support from Resnick Prize Post
doctoral Fellowship at Caltech,
and
the startup
support from UT Austin
.
Y. L. and W. A. G.
were
supported
as part of the Computational
Materials Sciences Program funded by the U.S. Department of Energy, Office of Science, Basic
Energy Sciences, under Award Number DE
-
SC00014607.
This work used computational
resources of NREL (sponsored by DOE EERE), X
SEDE (NSF ACI
-
1053575), NERSC (DOE DE
-
AC02
-
05CH11231)
,
and the Texas Advanced Computing Center (TACC) at UT Austin
.
W
e would like to
thank
Dr.
Fengru Fan
for the helpful discussions.
Author Contribution Statement
W. Z. W.
and P. D. Y.
conceived
and supervised the project.
W.
Z. W.,
P. D. Y., Y. X. W., and G. Q.
designed the experiments.
Y. X. W. and R. X. W.
synthesized
the material.
G. Q.
and
Y. X. W.
fabricated the devices.
G. Q.
and Y. C. D.
performed the
electrical and optical
characterizatio
n.
S. Y.
H. and
Y. X. W. performed the
Raman measurement
under the supervision of X. F. X.
and W. Z. W.
Q. W.
and M. J. K.
performed the TEM
25
characterization.
Y. L. carried out the first
-
principles calculations under the supervision of W. A.
G.
Y. X. W.
and G. Q.
conducted the experiments. W. Z. W.,
P. D. Y., Y. X. W.
,
G. Q.
, and R. X. W.
analyzed the data
. W. Z. W.
and P. D. Y.
wrote
the manuscript.
Y. X. W.
,
G. Q.
, and R. X. W.
contributed equally to this work.
All authors have discussed the results and
commented on the
paper.
Competing financial interests
The authors declare no competing financial interests.
Author Information
Supplementary information is available in the online version of the paper.
Reprints and
permissions information is available online at
www.nature.com/reprints
.
Readers
are welcome to comment on the online
version
of the
paper
. Correspondence and requests for
materials should
be addressed
to
W. Z.
W.
(
wenzhuowu@purdue.edu
)
or
P. D. Y.
(
yep@purdue.edu
)
.
26
Figures
Figure 1
Solution
-
grown large
-
area 2D Te and material characterization
.
a
,
Atomic structure of
tellurium
.
b
,
The b
and
structure of 4
-
layer
Te, calculated by using PBE functional. Valence bands
27
are shown
in
blue
,
and
conduction bands
are in red
.
The i
nset
shows the local band structure
near the VBM.
Γ: (0,0); X: (0.5,
0); Y: (0, 0.5); S: (0.5,
0.5);
all in the units of the surf
ace reciprocal
lattice vectors.
S
ee
Supplementary Information
for further
information
of
the electronic structure
.
c
,
Optical image of solution
-
grown tellurene flakes
.
Inset:
Optical image of the tellurene solution
dispersion. The scale bar is 20
m.
d
,
AFM image of a 10
-
nm 2D Te flake. The scale bar is 30
m.
e
,
(HAADF)
-
STEM image
of
tellurene
. The false
-
colored (in blue) atoms
are superimposed
to the
original STEM image for
highlighting the helical structure.
f
,
Diffraction
pattern of tellurene.
g
,
3D
illustration of tellurene’s structure.