Physics in 2D Materials - Université Paris-Saclay · Today’s Topics Lecture 5 (final):h-BN/Black Phosphorus/Xene 5.1 hexagonal Boron-Nitride 5.2 Black Phosphorus 5.3 Xene

Physics in 2D Materials

Taro WAKAMURA (Université Paris-Saclay)

Lecture 5

Today’s Topics

Lecture 5 (final):h-BN/Black Phosphorus/Xene

5.1 hexagonal Boron-Nitride

5.2 Black Phosphorus

5.3 Xene

Hexagonal boron nitride

Hexagonal Boron-Nitride (h-BN)

Hexagonal boron nitride (h-BN) as a substrate for graphene

Hexagonal boron nitride

Two dimensional van-der Waals insulator

(Large band gap ~ 6 eV)

Atomically flat, less charge traps, small lattice mismatch with graphene

Good candidate as a substrate for graphene!

Easy to exfoliate

Disorders that reduce mobility also come

from resist residues

Graphene protected from external

environment should have better mobility

Graphene encapsulated by h-BNs

L. Wang, Science 342, 614 (2013).

Hexagonal boron-nitride (h-BN) is an ideal

material as a substrate for graphene: flat,

flee from charge inhomogeneity

Graphene encapsulated from two h-

BNs should be flee from resist residues,

charged impurities.


Transport measurements of graphene on h-BN

Additional peaks are observed in Rxx

Signature of the secondary Dirac points away

from the original Dirac point

Sign changes of Rxy around the secondary

Dirac points

Switch between electron & hole nature of mass-

less fermions around the secondary Dirac points

Moiré pattern is clearly observed by AFM images

M. Yankowitz et al., Nat. Phys. 8, 382 (2012).



Report on growth of high quality h-BN

High-quality h-BN single crystals were successfully

grown around 1600℃ and 5 GPa

Strong cathodoluminescence signal at 215 nm

= 5.765 eV (ultraviolet)

K. Watanabe et al., Nat. Mater. 3, 404 (2004).

More than 1000 times stronger than indirect

free exciton luminescence

h-BN has a direct bandgap

谷口尚他, 高圧力の科学と技術 15, 4 (2005).


Nature Materials

2004

Nature Photonics

2016

Hexagonal Boron Nitride

Black phosphorus

Introduction to phosphorus

Xene: graphene “like” 2D materials

Silicene

Germanene

Stanene

Arsenene

Antimonene

Bismuthene

Phospherene

Plumbene

Borophene

Black phosphorus vs graphene

Graphene: hexagonal structure & flat

Black Phosphorus: two fold symmetry & puckered

Stronger electronic coupling between layers

More difficult to exfoliate

F. Xia et al., Nat. Rev. Phys. 1, 306 (2019).

Physical properties of black phosphorus

Black phosphorus: Semiconductor

Band gap: 2 eV (monolayer = phosphorene)

0.2 eV (bulk)

Band gap (~0.2 eV) for 5-nm-thick device

L. Li et al., Nat. Nanotech 9, 372 (2014).

Thickness-dependent properties

Difference between monolayer and bulk

Bulk (crystal): Indirect band-gap semiconductor

Monolayer: Direct band-gap semiconductor

Band gap is located at the K (K’) point.

Slight difference of the lattice constant

(bulk 3.135 A, monolayer 3.193 A)

H. Terrones et al., Sci. Rep. 3, 1549 (2013).

Transition metal dichalcogenides (TMDs)

Similar to graphene with Dirac cones

at K (K’) points


Band structure as a function of # of layers

BP is always direct band-gap semiconductor

Semiconducting TMDCs

A. Carvalho et al., Nat. Rev. Mat. 1, 1 (2016).


Electronic properties


High on-off ratio (105 current modulation) 4 orders of magnitude larger than

conventional Si-based transistor

Cu

rrent

Gate voltage

Steeper increase of current

with Vg


“Bipolar” current

Hall

coeffic

ient

Gate voltage

Hole carrier

Electron carrier

Both carrier types are accessible

Mobility can reach up to 1000 cm2V-1s-1



Bandgap tuning by double gating

Device with 4 nm thick BP + top & Bottom gate

F. Xia et al., Nat. Rev. Phys. 1, 306 (2019).


BP pn-junction

6-7 nm VP on the local gates

Gate-defined pn junctions are possible

For global gating, strong modulation of Ids is observed

M. Buscema et al., Nat. Commun. 5, 4651 (2014).


BP pn-junction

Depending on the combination of electron or

hole-doped gating between the two local gates,

NP or PN junctions are possible

Clear diode effect is observed for NP or PN

junctions

M. Buscema et al., Nat. Commun. 5, 4651 (2014).


pn-junction generates finite photocurrent (ISC) with zero voltage bias

pn-junction generates finite photovoltage (VOC) with open circuit condition

Photocurrent/voltage under illumination M. Buscema et al., Nat. Commun. 5, 4651 (2014).


I-V characteristic under illumination: Increasing zero-bias I & open-circuit V

Zero-bias I & open-circuit V (photocurrent & voltage) are observed at l in the near

infrared range. M. Buscema et al., Nat. Commun. 5, 4651 (2014).

Brief summary

Hexagonal Boron-Nitride (h-BN) is an insulator with a large gap (~6

eV) and good for encapsulating other 2D materials

Black Phosphorus (BP) is a direct gap semiconductor, independent of

the thickness. The bandgap decreases with increasing thickness.

BP has a gap and relatively high mobility (~103 cm2V-1s-1), therefore

a good candidate for FET

BP has a puckered structure and is not flat. It is also anisotropic in 2D.

Xene

Introduction to xene


Silicene

Germanene

Stanene

Arsenene

Antimonene

Bismuthene

Phospherene

Plumbene

Borophene


sp2

sp3

Larger lattice constant prevents p-bonding

p-bondingXene is not flat due to a mixed sp2-sp3 character

of bonding

Free-standing xene is usually not flat

Silicene

Silicene: Si counterpart of graphene

Slightly buckled structure is the most stable

Semimetal & Dirac cone exists at K point

M. Houssa et al., J. Phys. Cond. Mat. 27, 253002 (2015).

Silicene

The most conventional substrate for the growth of Silicene: Ag(111)

Similar lattice constant

4x4 buckled structure is formed

Dirac cone like spectrum is observed by ARPES measurements

M. Houssa et al., J. Phys. Cond. Mat. 27, 253002 (2015).

Silicene Silicene FET

Silicene: Usually grown on a metallic substrate (e.g. Ag(111))

Transport measurements are difficult because of current shunting

Silicene grown on Ag(111) and capped by Al2O3 can be delaminated by a blade

Ag layer can be used as electrodes after chemical etching with KI

L. Tao et al., Nat. Nanotech. 10, 227 (2015).

Silicene

Linear I-V character: Ohmic contact between silicene and Ag

Dirac peak like graphene is clearly observed

L. Tao et al., Nat. Nanotech. 10, 227 (2015).

Germanene

Most stable state: Buckled honeycomb structure

Regardless of the buckling, the Dirac cone exists at K

points Similar to graphene

Smaller p-bonding results in smaller splitting between

bonding and antibonding states

A. Acun et al., J. Phys. Cond. Mat. 27, 443002 (2015).

Germanene

Bilayer graphene: AB stacking is naturally stable

AB stacking AA stacking

Bilayer germanene: AA stacking is naturally stable, similar bonding strength for

inter- and intra-layer bonding Difficult to exfoliate


Germanene

Germanene can be grown by MBE on metallic substrates

e.g. Pt(111), Au(111), Al(111)

Germanene on Pt(111) Germanene on Al(111)


Topological properties of silicene and germanene

Planar Silicene Low-buckled Silicene

Effective Hamiltonian for planar silicene

Same as graphene

Buckling enhances p-s coupling

Increasing effective SOI

C. -C. Liu et al., Phys. Rev. Lett. 107, 076802 (2011).


Topological gap engineering via buckling or strain

More than one order of magnitude enhancement



Planar Germanene Low-buckled Germanene

Similar enhancement of the topological gap is possible

Gap can be as large as 23.9 meV Nearly RT quantum spin Hall effect!




Silicene

Germanene

Stanene

Arsenene

Antimonene

Bismuthene

Phospherene

Plumbene

Borophene

Spin-orbit interaction depends on atomic number

The biggest advantage of graphene for spin transport

Atomic Number

Compared to silicene and germanene

(based on Si and Ge), stanine (based on Sn)

should be a better candidate as a 2D TI

Topological properties of stanene

Stanene: Topologically nontrivial (2D TI)

Topological gap can be enhanced by chemical functionalization

Functionalized stanenes (except by –H) exhibit enhanced topological gaps

Y. Xu et al., Phys. Rev. Lett. 111, 136804 (2013).


Stanene: Band inversion occurs at K point

Fulorinated Stanene: The bands are gapped at K, and the band inversion occurs at

G point

Stanane (with hydrogen): The bands are gapped at K and no band inversion at G

Topologically nontrivial

Y. Xu et al., Phys. Rev. Lett. 111, 136804 (2013).


Band inversion occurs between bonding state of p-orbitals and anti-bonding state

of s-orbital of Sn

Strongly depends on strain Y. Xu et al., Phys. Rev. Lett. 111, 136804 (2013).

Epitaxial growth of stanene

Bi2Te3 (111) Similar lattice constant to

that of stanene

Good candidate for the growth of stanene

STM images Modeled structure

F. -f. Zhu et al., Nat. Mater. 14, 1020 (2015).

Epitaxial growth of stanene

Band structure of stanine on Bi2Te3(111)

Stanene on Bi2Te3(111) Compressive strain

Compressive strain makes stanine metallic No signatures of QSH state

DFT results (Red: Stanene bands)

F. -f. Zhu et al., Nat. Mater. 14, 1020 (2015).

Superconducting stanene

Two states of bulk Sn: a-Sn & b-Sn

a-Sn: Stable in thin limit, but semimetallic & non-superconductive

b-Sn: Superconductive in bulk, but unstable in thin limit

Stanene on PbTe: a-phase

M. Liao et al., Nat. Phys. 14, 344 (2018).


Tc of stanene strongly depends on the number of the layer

Tc of stanene also depends on the number of the layer of PbTe



Superconductivity induced by electron-doping from PbTe substrates

More surface vacancies for thicker PbTe

Electron pocket as # of PbTe layer increases


Topological stanene

Stanene: Usually buckled structure

Stanene on Cu(111): Ultraflat stanine, honeycomb lattice like graphene

J. Deng et al., Nat. Mater. 17, 1081 (2018).

Topological stanene

A remarkable gap-opening at G (gap size ~ 300 meV)

Ultraflat stanene is owing to stretching, thereby it can

gain adsorption energy onto Cu

ARPES measurements


Topological stanene

Scanning tunneling spectroscopy (STS) measurements (Conductance measurements)

Enhanced conductance at the edge in the energy window between

-1.2 eV and -1.45 eV

Coincides with ARPES measurements, a signature of topological edge states




Silicene

Germanene

Stanene

Arsenene

Antimonene

Bismuthene

Phospherene

Plumbene

Borophene

Borophene

Borophene: “The lightest metal”

Unique structures based on a triangular unit

Stable on Ag, Cu, Ni Stable on Ag(111)

Many metastable structures are theoretically predicted

A. J. Mannix et al., Nat. Nanotech. 13, 444 (2018).

Borophene

Borophene: “The lightest metal”

Unique structures based on a triangular unit

Stable on Ag, Cu, Ni Stable on Ag(111)

Many metastable structures are theoretically predicted


Borophene

Successful growth of borophene on Ag(111)

1D stripe phase (c & e) and rhombohedral (d & f) are observed

STM SimulationStripe phase=n1/6 phase

Rhombohedral phase=n1/5 phase


Borophene

Borophene on Ag(111) substrate Moiré pattern due to interface interaction

No modulation3x1 onsite potential

modulation

b12 phase

Borophene

Dirac cones are split around X (or M) points in the BZ

There are three equivalent domains in the Brillouin zone (BZ) due to the symmetry

Borophene

Observation of split Dirac cones by ARPES

Dirac cone and saddle point are observed from different

cut images

Clear signatures of Dirac fermions in borophene

Plumbene

Experimental report in March 2019

Graphene’s “latest cousin”

Buckled honeycomb lattice structure for “free-

standing” plumbene

J. Yuhara et al., Adv. Mater. 1901017 (2019).

Plumbene

Graphene’s “latest cousin”

Buckled honeycomb lattice structure

Quantum spin Hall state is predicted for the

monolayer plumbene nanoribbon

Plumbene

Growth of plumbene on Pd(111)

Flat structure due to stretching by the substrate

Band structures are not observed yet

J. Yuhara et al., Adv. Mater. 1901017 (2019).



Silicene

Germanene

Stanene

Arsenene

Antimonene

Bismuthene

Phospherene

Plumbene

Borophene

Antimonene

Antimonene: Monolayer of antimone (Sb)

Puckered honeycomb structure similar to BP, QSHE may be induced by strain

Monolayer growth by MBE on WTe2

Z. -Q. Shi et al., Adv. Mater. 31, 1806130 (2018).

Antimonene


Scanning tunneling spectroscopy (STS) measurements:

Finite differential conductance around the zero voltage

Metallic nature

Antimonene

Metallic nature for thicker films: decreasing resistance when 10 or 20 layers of

antimonene is demosited on WTe2

Thicker antimonene is also possible


Antimonene

Stability in air

Exposure to O2 for 20 minBefore exposure Exposure to air for 12 h

Except some adsorbates, multi-layer antimonene seems stable in air


Antimonene

QSHE by strain

Antimonene Buckled structure as a free-standing form

By tensile strain, antimonene can become 2D TI (possible on e.g. h-BN)

M. Zhao et al., Sci. Rep. 5, 16108 (2015).

Introduction of topological insulators

Bilayer

[111]

Bilayer bismuth on (111) surface

Viewing from the top, it looks like a honeycomb lattice

with a bucked structure.

In [111] direction Bismuth crystal can be considered as a

stack of such a bilayer structure. Ph. Hofmann, Prog. Sci. Surf. 81, 191 (2006).

Bismuthene

Bismuthene: Bi allotrope

Unbuckled due to the large lattice constant of SiC(0001)

Lattice is stretched due to the substrate to form a flat hexagonal lattice

F. Reis et al., Science 357, 287 (2017).

Bismuthene

Without SOI: Dirac cone at K

Without intrinsic SOI, Dirac cone is gapped, and with intrinsic SOI+Rashba,

band edges are spin-splitARPES


Bismuthene

px & py orbitals play a major role for the low energy states

Strong onsite SOI drives the system into QSH state

Topological edge states are expected both for zigzag & armchair edges!


Bismuthene

STM measurements at bismuthene/SiC(0001) steps

Clear bulk band gap & edge conductance are observed

Signatures of the QSHE F. Reis et al., Science 357, 287 (2017).

Topological strainics for xene

Strain from the substrate plays important roles for topological properties

Compressive or tensile strain can switch the system between topologically

trivial and non-trivial states (topological strainics)

A. Molle et al., Nat. Mater. 16, 163 (2017).

To be conitinued...

Arsenene

Monolayer Te

Summary for Xene

Xene is a family 2D materials that do not exist naturally

For most of the xene mechanical exfoliation is difficult, and growth by molecular-

beam epitaxy (MBE) on a substrate is needed

Due to the interaction with the substrate, xene is often more buckled or stretched

than its free-standing form, which changes electronic properties

There are many xene which may become 2D topological insulators, and their

topological nature is strongly modulated by strain

Since xene is often grown on a metallic substrate, transfer of xene to an insulating

substrate is an issue to overcome

Documents

Physics in 2D Materials - Université Paris-Saclay · Today’s Topics Lecture 5 (final):h-BN/Black Phosphorus/Xene 5.1 hexagonal Boron-Nitride 5.2 Black Phosphorus 5.3 Xene