数理物質融合科学センター光量子計測器開発推進室発足会議

分光分析による表面物理化学

近藤剛弘
筑波大学 数理物質系 物質工学域
筑波大学 数理物質融合科学センター (CiRfSE)
物質変換材料部門

共同研究者
筑波大学：中村潤児教授、Guo研究員
中村近藤研究室のメンバーと卒業生
岡田晋教授

Web掲載用のためイントロダクションのスライドを一部割愛いたします極低温走査トンネル顕微鏡

Scanning tunneling microscope (STM)

Low temperature STM

STM・STS計測では固体表面の物理化学的な知見
Scanning tunneling spectroscopy (STS)
特に物性や化学反応過程などを原子分解能での顕微鏡観察とMeasurement
chamber
局所分光計測でOccupiedState明らかにすることが可能
STM (USM1500)
Bias voltage

Sample

EfTip

Ef

Liquid He

Local density of states(LDOS) can be measured

Background and motivation
Graphitic materialGraphene shows specific physicaland chemical properties among thegraphitic materials due to its uniqueelectronic structure

Graphite (3D)
Zero-gap semiconductor

Linear dispersion
Dirac Point

Graphene (2D)

Massless Dirac fermions
Graphene band structure
Carbon nanotube (1D)Graphene shows specific Landau levels under magnetic fields
Landau levels in 2D electron gas
EB

n = +3n = +2n = +1n = -1n = -2n = -3

Schematic image of Landau level
Landau levels in graphene

STS of Graphene on graphite
under magnetic fieldLi, et al. Phys Rev Lett 102 (2009) 176804

EB Landau level energy is proportionaln = +2to(n+1/2)B : two-dimensional electronn = +1 gas
nB : Graphene (massless Dirac fermion)
n=0
Novoseloval., Science,(massiveetDiracfermion)315 (2007) 1379
B n(n  1) : Bilayer Graphenen = -1(n is the landauLandauindex)
Large energy difference of graphenelevels isn = -2
G. Li, E. Y. Andrei, Nature Physics, 3 (2007) 623.
Schematic
image
Landau level
known
to leadto ofQuantum
Halleffects at room temperature

Graphene Landau level appeared at B = 0 TNano-bubble of grapheneon Pt(111)StrainRidge of graphene at HOPG

InequivalentNano-bubblehoppingof graphene
Nano-bubble ofon Ru(0001)
grapheneon Ru(0001)

Non-uniform shear strain
Inequivalent hoppingE nE  Ando,n et al., Phys. Rev. B 65 (2002) 235412.
Neto et al., Rev. Mod. Phys. 81 (2009) 109.

Gauge field
N. Levy et al.,Science, 329 (2010) 544.

E n

H. Yan et al., Phys. Geim,
Rev.etBal.,Nature Phys.6
J. Lu(2010)
et al.,
Nat. Comm,
3085 (2012)
035422.
Pseudo-magnetic
field 3 (2012) 823.
 Bs    A

Landau levels appearance are ascribed to thestrain-induced pseudo-magnetic fields

We have also observed Landau levels at B = 0T
Potassium (K) partially intercalated graphiteSTSK

B=0T

1 nm

E nBs  250 T
50 nm

K-free domains (Nanovalleys)
D. Guo, T. Kondo, J. Nakamura, et al., Nature Communications 3 (2012) 1068

Our proposed “domain model” for pseudo magnetic fields
On-site potential of carbon calculated by DFTeV

0.60

1 nm

KPotential gradient is formed

0.00

50 nm

Equipotential contourPotential gradientEquivalent hopping in
the contour direction

Inequivalent hopping along
the gradient direction

Gradient of on-site potentials results in inequivalent hoppingRecent progress
To prove the domain-model as anotherorigin for the pseudo-magnetic field

We have observed Landau levels ofbilayer graphene at the atomically flatarea of nitrogen-doped graphite at B = 0 T

Experiment
Low temperature STM

1. Cleaning of HOPG surface(1) Cleaving HOPG at atmosphere(2) 940K annealing (30 min.) in UHV

2. Nitrogen doping(1) Nitrogen ion bombardment(N/C : <0.04, 0.04, 1.9 at %)(2) 940 K annealing (30 min.)Nitrogen ion bombardment
Measurement
chamber

Ei = 200 eV, i = 0oUltrahigh vacuum

STM300 K
Liquid He

Highly oriented pyrolytic graphite（HOPG）

N1s XPS spectrum of nitrogen doped graphite

Photoelectron intensity [cps]N-HOPG (1.9 at%) after 940 K annealing

N1s
60

Graphitic nitrogen

Positively charged N
40

Experimental Data
Graphitic-N
Pyridinic-N
Sum NN

Pyridinic nitrogen

20

Negatively charged N0405

400395
Binding energy [eV]390Graphitic nitrogen (positively charged N) is dominant

STM image of nitrogen doped graphite- 109 mV
N-HOPG (<0.04 at%) after 940 K annealing+ 100 mVGraphiticnitrogen
Scan Size 10× 10 nm2 , It = 100 pA , Vs = -109 mV

Dark

brightScan Size 50 × 50 nm2
It = 100 pA , Vs = -109 mV

Pyridinic
nitrogen
Scan Size 10× 10 nm2 , It = 100 pA , Vs = -109 mV

Two types of defect are observed !
T. Kondo J. Nakamura et al., Phys. Rev. B 86 (2012) 035436

Graphiticnitrogen

Sample bias = +0.5 V

Negatively charged carbon
0.6ExperimentdI/dV (arb.units)

0.5

Positively
chargednitrogen

Experiment

0.4
0.3
0.2
0.1
0.0-600

60

N1s

40

200405

405

400
395
Binding energy [eV]400395390

390

Binding energy (eV)

-300

0300

600

Sample bias (mV)

TheoryExperimental Data
Graphitic-N
Pyridinic-NSum

δ- NN
δ+δ-

δ-

Simulated STS intensity

Photoelectron intensity [cps]Intensity(arb. units)

1 nm

TheoryCarbon atoms around graphitic nitrogen
Energy (mV)
may act as Lewis acidCarbon atoms around graphitic nitrogen has localized states-600 -300

at occupied region !

0300

600

T. Kondo J. Nakamura et al., Phys. Rev. B 86 (2012) 035436

Pyridinic
nitrogenSample bias = -0.1 V

Positively charged carbon

ExperimentdI/dV (arb.units)

1.2

Negatively
chargednitrogen

1.0Experiment

0.8
0.6
0.4
0.2-600 -400 -200

60N1s

Experimental Data
Graphitic-N
Pyridinic-N
Sum401 nm

..

0405

405

400
395
Binding energy [eV]400395

390

..

390

Binding energy (eV)

..

..

..

..

.. ..
..
..
..
N..
.. .. ..
δ..+ - δ+..
δ..

200 400 600

Sample bias (mV)

Theory200

Simulated STS intensity

Photoelectron intensity [cps]Intensity(arb. units)

0.0

TheoryCarbon atoms around pyridinic nitrogen
Energy (mV)may
act as nitrogen
Lewis baseCarbon atoms aroundpyridinic
has localized statesat occupied region !

-600 -400 -200

0200 400 600

electron lone pair

T. Kondo J. Nakamura et al., Phys. Rev. B 86 (2012) 035436

STS spectra on nitrogen doped graphite
N-HOPG (0.04 at%) after 940 K annealing

B=0T

1220.30

3413450.25

dI/dV [arb.unit]

678910115670.20

890.1510111213120.10

14151314

160.0515
162 , 2800 mV, 100 pA
50×nmnm
200×50200, 300 mV, 59 pA

Ts = 5 K, B = 0 T

0.00-800 -400

Many peaks appear in STS spectrum

0400

800

sample bias [mV]

STM and STS on nitrogen doped graphite at B = 0T-3
dI/dV (arb.units)

A-1-2

B

1032ABCDC

1 nm

DHeight(pm)Height (pm)

12080

BACD

4000246Distance(nm)Distance (nm)8STS peak energy (meV)-1000 -750 -500 -250

0250

500

750 1000

Sample bias (mV)
1000

Ts = 5 K, B = 0 T

5000A
BCD-500-1000-4dEn
m*d n ( n 1)

-202

4

10 [ eV ]0.039.10910 [ kg ] 255.32
 66.17![T ]Pseudo-magnetic
filedofcanbilayerbe estimatedas B STS
peaks
corresponds
tographene
Landaulevelsofbilayer
Landaulevels
appearat[eVFLAT
6.58210s ]1.60210graphene[ Asarea]e 31-3

16

19

Why LL-like peaks appear in STS at B = 0 T
STM
imageof N-graphite
Positivecharge
ofthe graphitic-N
Schematic
image
of potential
causes the potential gradientExperimental Data
Graphitic-N
Pyridinic-N
Sum

Graphitic nitrogen N1s

60

40

N+

200405-

N+400395
Binding energy [eV]390

N+-e-Onsite potential
ApparentheightPhotoelectron intensity [cps]Nitrogen doped-graphite-Graphitic nitrogen

dI/dV [arb.unit]

Nitrogen is positively charged

Equipotential domain

0.09Inequivalent hopping along
the potential gradient

0.06
0.03
0.00-1000

-500

0500

Sample bias [mV]1000

LLs formation at B = 0 T

Gradient of on-site potentials around graphitic nitrogen
DFT calculation of nitrogen dope bilayer graphene
NabA1

2
eV

4
5

Relative on-site
potential (eV)c6N 780-20.1AA’
9 10N

 10-4-6-0.4

1112-8-10
1234

5678

9 10 11 12

Atomindex
Atomindex

On-site potential of carbon next to nitrogen is
1.5 eV lower than carbon far from nitrogen!

On-site potential of carbon

3A’

There is a
gradient of onsite potentials

Summary••

Landau level peaks of bilayer graphene are observedin STS at the atomically flat area of nitrogen doped
HOPG at B = 0 T.
Domain model can explain the LL appearance.
(difference in the on-site potential)

StrainK-intercalated

Nitrogen0.3-2.0 nm
1 nm

50 nm

Science, 329, 544 (2010)

Nanobubble on Pt

Our work
Nature comm., 3, 1068 (2012)

K-free domains (Nanovalleys)

Our work
Scientific Reports 5, 16412 (2015)

N doped-graphite