Hexagonal Nanopits with the Zigzag Edge State on Graphite Surfaces

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C: Physical Processes in Nanomaterials and Nanostructures

Hexagonal Nanopits with the Zigzag Edge State on Graphite Surfaces Synthesized by Hydrogen-Plasma Etching Tomohiro Matsui, Hideki Sato, Kazuma Kita, André E. B. Amend, and Hiroshi Fukuyama J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b06885 • Publication Date (Web): 20 Aug 2019 Downloaded from pubs.acs.org on August 28, 2019

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The Journal of Physical Chemistry

Hexagonal Nanopits with the Zigzag Edge State on Graphite Surfaces Synthesized by Hydrogen-Plasma Etching ∗,†

Tomohiro Matsui,

Hideki Sato,

†

Kazuma Kita,

†

André E. B. Amend,

†

and

∗,†,‡

Hiroshi Fukuyama

†Department

of Physics, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan

‡Cryogenic

Research Center, The University of Tokyo, 2-11-16 Yayoi, Bunkyo-ku, Tokyo 113-0032, Japan

E-mail: [email protected]; [email protected] Phone: +81 (0)3 5841 8364; +81 (0)3 5841 4193. Fax: +81 (0)3 5841 8364; +81 (0)3 5804 4528

Abstract

spin polarization in zigzag nanoribbons which are promising key elements for future graphene

We studied, by scanning tunneling microscopy, the

morphology

of

nanopits

of

nanoelectronics.

monolayer

depth created at graphite surfaces by hydrogen

INTRODUCTION

plasma etching under various conditions such as H2 pressure, temperature, etching time, and RF power of the plasma generation.

Reecting

In addi-

the

honeycomb

lattice

structure,

tion to the known pressure-induced transition

graphene has two types of edges.

of the nanopit morphology, we found a sharp

the zigzag edge and the other the armchair

temperature-induced

one.

transition

small rather round nanopits of

from

many

∼150 nm size to

One is

Among various novel physical properties

of graphene,

1

the peculiar electronic state lo-

few large hexagonal ones of 300600 nm within

calized at the zigzag edge, the so called zigzag

a narrow temperature range. The remote and

edge state or simply edge state, is one of the

direct

most attractive ones.

plasma

modes

switching

mechanism,

Such a localized state

which was proposed to explain the pressure-

emerges due to symmetry breaking of the two

induced transition, is not directly applicable to

equivalent sublattices in the honeycomb struc-

this newly found transition. Scanning tunneling

ture at the zigzag edge. Inspired by the predic-

spectroscopy (STS) measurements of edges of

tion by Fujita and co-workers,

the hexagonal nanopits fabricated at graphite

was experimentally found by scanning tun-

surfaces by this method show clear signatures

neling microscopy and spectroscopy (STM/S)

of the peculiar electronic state localized at the

measurements at naturally existing monatomic

zigzag edge (the edge state), indicating that

step-edges at highly oriented pyrolytic graphite

the hexagonal nanopits consist of a high den-

(HOPG) surfaces

sity of zigzag edges.

in nanographites synthesized on HOPG.

The present study will

4,5

2,3

the edge state

and at similar step-edges

6

The

pave the way for microscopic understanding

decay length of the edge state was measured as

of the anisotropic etching mechanism and of

1.2

±

0.2 nm.

5

In principle, the zigzag edge state can be

ACS Paragon Plus Environment 1

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Page 2 of 15

spin polarized even under innitesimally small

icts among previous experimental results on

electron-electron interaction due to its at band

monolayer graphene exfoliated on

nature located near the Fermi energy (EF ).

et al.

2

In

39

SiO2 .

Yang

reported successful creation of hexago-

graphene nanoribbons with two parallel zigzag

nal nanopits of uniform size, while Diankov et

edges (zGNRs), the edge spins are predicted to

al.

order ferromagnetically (FM) along the edge

For a zGNR unzipped from a carbon nanotube,

and antiferromagnetically (AF) between the

no nanopit creation by H-plasma etching has

edges.

been found.

2,79

By applying a magnetic eld

in-plane electric eld,

11

10

or an

the AF conguration

will ip to the FM one.

40

and Hug et al.

41

reported circular ones.

32

Recently, a new insight on H-plasma etching

This can be applied

of graphite has been brought by Hug et al.

41

to carbon-based switching nanodevices because

They found that anisotropic etching, that grows

the system is expected to be semiconducting

hexagonal nanopits, proceeds when a graphite

with a band gap in the AF conguration while

sample is located outside of a glow regime of

it is metallic in the FM one.

the plasma; the remote plasma mode.

10,11

On

In order to experimentally test the theoretical

the other hand, when a sample is located inside

predictions, it is crucial to realize nearly pure

the glow, defect creation is promoted, resulting

zigzag edges, because the spin polarized states

in multi-layer terraced nanopits with irregularly

become less stable against the non-magnetic

shaped edges; the direct plasma mode. They

state under edge disorders.

After the rst

demonstrated that the direct/remote mode can

numerous tri-

be switched by tuning the gas pressure. How-

als have been made to shape graphene edges

ever, the role of etching temperature and other

on an atomic scale in order to fabricate zGNRs

parameters are not known.

1215

observations of the edge state,

with

the

spin-polarized

46

edge

include reactive ion etching,

state.

1618

They

In this article, we report on comprehensive

anisotropic

studies of the morphology of nanopits created

etching using metallic nanoparticles as cat-

at graphite surfaces by H-plasma etching. The

alysts,

nano-

shape, depth, and growth rate of the nanopits

force

are examined on an atomic scale with STM for

1924

carbothermal

lithography

with

STM

28

microscope (AFM) tips, bon nanotubes,

3034

29

etching, and

2527

atomic

unzipping of car-

samples etched under various conditions.

We

and bottom-up fabrica-

found that the morphology suddenly changes

However, non of them has succeeded

from many small nanopits to few large hexago-

in synthesizing zigzag edges which satisfy all the necessary conditions: (i) quality or purity

nal nanopits when the reaction temperature is ◦ increased from 450 to 500 C. Such a tran-

(low concentration of the armchair edge), (ii)

sition has not been reported before.

known edge termination, and (iii) scalability or

also found that the morphology changes from

reproducibility.

many irregularly-shaped multi-layer terraced

tions.

3538

Among the previous attempts,

It is

the hydro-

nanopits to few large hexagonal nanopits on

gen (H) plasma etching has apparent advan-

increasing the hydrogen gas pressure from 13

tages from the view point of the requirements

to 60 Pa. This pressure variation is consistent

(ii) and (iii).

with the previous report.

It is reported that this method

41

Most importantly,

can create hexagonal nanopits on graphite sur-

our STS measurements have veried that the

faces

with edges in parallel to the zigzag

edges of the hexagonal nanopits prepared by

3941

direction.

In addition, the nanopits can be

this method are zigzag edges of high purity with

created in arbitrary spatial arrangements, with

the most pronounced edge-state features ever

which one can synthesize zGNR at high den-

reported.

sities.

studied by STS are also discussed.

39,4244

However, so far, no atomic-scale

spectroscopy has been applied on the edges prepared by this method. Thus it is not clear if it meets the requirement (i) so as to be used in edge-state devices. In addition, there are con-


Spatial variations of the edge state

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EXPERIMENTAL

The morphology of etched sample surfaces is examined by two dierent STMs (UNISOK

H-plasma etching was performed with a home

Co., Ltd.) operated by a commercial controller

made apparatus illustrated in Figure 1. Freshly 3 cleaved graphite samples (∼ 5 × 2 × 0.3 mm )

(model SPM100, RHK Technology, Inc.).

topographic images are obtained in the con-

are placed on a Grafoil plate (Grafoil, Graftech 2 Inc.; 42×18 mm with 3 mm high rims at

stant current mode (I

= 1.0

nA,

V = 500

mV)

in atmospheric conditions. STS data are taken

three edges) and inserted in a quartz tube

at 4.7 K in ultra-high vacuum using a home

(42 mm i.d., 1.2 m long), at the center of a muf-

made STM/S system which can work in multi-

e furnace. The radial location is 2 mm from the tube inner edge.

All

extreme environments.

We examined graphite

45

supplied from three dierent companies (ZYA

RESULTS

grade HOPG, Advanced Ceramic Corp.; PGC, Toshiba Mitsubishi-Electric Industrial Systems

Pressure Dependence

als Corp.), and found no essential dierence in the etching results. H2 gas (7N purity) ows at

P

at the sample

Let us rst show the H2 gas pressure (P ) de-

location constant by evacuating with a rotary

pendence of H-plasma etching, where we ob-

vacuum pump with pumping speed 200 L/min through a needle valve.

P

tained results that agree very well with those

was estimated from

by Hug et al.

pressure values measured at both ends of the

41

Figures 2a-h are typical topo-

graphic STM images of graphite surfaces etched

quartz tube by pirani gauges and the calculated conductance of the system.

DISCUSS-

SION

Corp.; Kish graphite type-B, Covalent Materi-

a constant rate, while keeping

AND

at several dierent

H-plasma is gen-

P

of 13(a)(b), 60(c), 110(d),

150(e), 190(f ), 350(g), and 590(h) Pa, where

erated at 13.56 MHz with a copper coil wound

etching temperature (T ), etching time (t), and

around the quartz tube and an RF power source

RF power to generate plasma (WRF ) are xed ◦ at 500 C, 40 min., and 20 W, respectively.

(model TX03-9001-00, ADTEC plasma Technology co., Ltd.).

At the lowest pressure of 13 Pa, the surface

After stabilizing the furnace temperature and

is fully covered by irregularly-shaped nanopits

the H2 pressure, the H-plasma etching starts by

with many steps down to the 10th layer from

turning the RF power on. After etching for a

the top as shown in Figures 2a and b, where

certain time duration, the power supplies both

2b is a magnied image of a region in 2a. The

to the plasma generator and the furnace are

cross-sectional prole along the line indicated in

turned o at the same time.

It takes about ◦ 4 hours for the furnace to cool down to 200 C ◦ from 500 C while continuing to vacuum pump

Figure 2b is shown in Figure 2i, and each terrace height is replotted as a function of the step number in Figure 2j. From this, the averaged

the quartz tube.

step height is

0.369 ± 0.003

nm, which is 10 %

larger than the interlayer distance of graphite Pirani gauge

RF shielded box Copper coil

(0.335 nm). Possible intercalation of atomic H,

Furnace

Flow meter

one of the components of H plasma during the

Pirani gauge

H2 gas

etching process, and its subsequent deintercala-

Matching box RF power

Sample on graphite plate

tion in vacuum may expand the interlayer dis-

Rotary pump

tance.

Clearly, the defect formation is very

active here.

Figure 1: Schematic diagram of the experimental apparatus for H-plasma etching.

46,47

As

Graphite

P

increases to 60 Pa, the surface mor-

phology suddenly changes, i.e., the number of

samples are located in a quartz tube and a muf-

steps decreases rapidly, and the nanopit shape

e furnace under a continuous downstream of

changes closer to hexagonal (Figure 2c). At a

the H-plasma.

slightly higher


P (= 110

Pa), all the nanopits

The Journal of Physical Chemistry (c) 60 Pa

(b)

100 nm

(d) 110 Pa

(h) 590 Pa

Height (nm) m)

(i)

2 1

0 1 10

102

4

5

6

7

20 2000

Dmax (nm)

Area fraction (%) %)

20

3

Step number; n

No nanopit formation

40

2

Step edge formation (l)

60

1

Lateral distance (nm)

100 80

200 nm

∆h = 0.369 㼼 0.003 nm

2

0 1

10 20 30 40 50 60 70

200 nm

Step edge formation (k)

200 nm

(j) n=1 2 3 4 5 6

0 0

200 nm

(f) 190 Pa

200 nm

100 nm

50 nm

(g) 350 Pa

(e) 150 Pa

Height (nm) m)

(a) P = 13 Pa

No nanopit formation

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Page 4 of 15

1500 1000

Sn≥4 S3 S2

500

S1

0 1 10

103

102

P (Pa)

103

P (Pa)

Figure 2: STM images of graphite surfaces etched by H-plasma at dierent

P

of (a)(b) 13, (c) 60, T = 500 ◦ C,

(d) 110, (e) 150, (f ) 190, (g) 350, and (h) 590 Pa. The other parameters are xed at

t = 40

min., and

WRF = 20

W. (b) Magnied image of the region indicated by the square in (a).

(a) and (b) are represented in dierent contrasts from other STM images to clarify the multilayer structure. (i) Cross sectional prole along the solid line in (b), and (j) relation between the terrace height and the step number showing that the nanopit steps are of monolayer height. Note that the heights of the linear steps in (g) and (h) are much larger, i.e., 10 and 4 layers height, respectively. (k)

Sn

and (l)

Dmax

are plotted as a functions of

P.

It is clear that the etching character changes

rapidly between 13 and 60 Pa from irregular to anisotropic as

P

increases, which is consistent with

the observation in Ref. 41. However, in our case, the size distribution of the nanopits is much wider and

Dmax

is much larger.

become hexagonal and larger in size (Figure 2d)

h, respectively.

indicating that the defect formation is sup-

varies randomly from monatomic to 40 layers,

pressed and the anisotropic lateral etching be-

and their crystallographic direction is also ran-

comes dominant. Such tendencies steadily pro-

domly oriented with respect to the honeycomb

ceed up to

P = 190

Pa (Figures 2e-f ) where

lattice of graphite.

only few very large hexagonal nanopits (500 600 nm) grow.

We analyzed these STM images quantita-

It is not easy to judge if the

same tendencies still continue at

P ≥ 150

The height of such step-edges

tively by evaluating relative terrace areas of

Pa,

dierent layers. The results are shown in Fig-

Sn is an areal fraction of the Dmax is the diam-

partly because we cannot exclude the possibil-

ures 2k and l where

ity that the top most layer has already been

n-th

etched away, and partly because a new struc-

eter, for irregularly-shaped nanopits, or the di-

ture, which consists of deep linear-step-edges,

agonal length, for hexagonal ones, of the largest

appears as can be seen by the dark regions

nanopit.

on the left and right sides of Figures 2g and

substantially overlap,

layer from the top and


In cases where hexagonal nanopits

Dmax

was estimated from

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the length of one of six sides of the hexagon,

nanopits of uniform size suggesting that the

assuming its regular shape. Here, 0.22, 3.8, 12, 2 5.0, 17, 20 and 5 µm areas were surveyed for

nanopits grow only from preexisting defects.

samples treated at

P =

This dierence may come from a slightly larger

13, 60, 110, 150, 190,

oxygen contamination in our gas ow system

350 and 590 Pa, respectively. In these gures,

because oxygen is known to be strongly active

it is clear that the etching nature changes from

in creating surface defects on graphite.

irregular to anisotropic with increasing pressure This agrees very well with the result by Hug et

Temperature Dependence

al.

Next,

within a narrow range between 60 and 110 Pa.

41

who observed a similar change of etching

50

we examined the temperature depen-

character between 40 and 70 Pa and attributed

dence of the H-plasma etching with xed pa-

it to the direct/remote plasma mode change.

rameters of

plasma

scenario

to

the

Pa,

t = 40

min., and

WRF = 20 W. Surprisingly, we found an abrupt

We have checked the applicability of the direct/remote

P = 110

change of the etching character within a narrow ◦ temperature range between 450 and 500 C, as

present

data. With our experimental setup, when the the furnace,

will be described below. Figures 3a-f show STM

it is not possible to measure a distance (lg ) be-

images of graphite surfaces etched at dierent T from 200 to 700 ◦ C. Here, the number of sur-

plasma glow edge is located

inside

tween the glow edge and the end of the RF coil along the quartz tube.

Thus,

lg

face steps remains less than three layers at any

in this

T,

regime was estimated from the known relation,

√ lg ∝ 1/ P , 41

since the etching conditions belong to the

whose proportionality constant

remote plasma regime in which heavy etching

P and lg when outside the furnace

like Figure 2a never occurs. ◦ At T = 200 C, only few small nanopits with

on the downstream side. The estimation shows

nm are created as shown in Fig◦ ure 3a. At T = 300 C, many small nanopits

was determined by measuring the glow edge is located

that lg reaches the sample position (ls when

P ≥ 60

Dmax ≈ 40

= 0.36 m)

with rather round shape (Dmax

Pa, which is consistent with the

≈ 110

nm)

are created (Figure 3b). At 400 (Figure 3c) ◦ and 450 C (not shown), the size distribution

direct/remote scenario. The glowing region should roughly represent + + the spatial extension of H-ions such as H1 , H2 , + and H3 . The life time of the H-ions shortens with increasing gas pressure because of

becomes wider and the shape becomes more hexagonal.

Dmax

is less than 150 nm here. ◦ On the other hand, at 500 C (Figure 3d), the

more frequent collisions, resulting in a shorter

situation changes drastically where fewer large

lg .

hexagonal nanopits are found (Dmax

On the other hand, H-radicals created by

≈ 300

or

ionization and recombination processes survive

600 nm, depending on the measurement series

beyond the glowing region.

one

as described below). Thus the etching charac-

can expect that the plasma species responsible

ter changes drastically from isotropic (or ran-

for the defect creation and the isotropic etch-

dom) to anisotropic (or hexagonal) in such a ◦ narrow T -range between 450 and 500 C. As ◦ T increases further to 600 C, the shape of

Therefore,

ing (anisotropic etching) in the direct (remote) regime is H-ions (H-radicals).

41

According to

the measurement of H-plasma components as + 48,49 a function of pressure, H3 seems to be the main contributor over the other H-ions.

the nanopits is still hexagonal but the size de◦ creases (Figure 3e). At T = 700 C, only tiny

It should be noted, however, that there is a

at a very low density at the surface layer (Fig-

hexagonal nanopits (Dmax

nm) are found

ure 3f ).

quantitative dierence between our result and

41

≈ 50

We found a

The lateral etching eciency regardless of

large nanopit size distribution even in the re-

the anisotropy has a continuous dome-shaped

the previous one by Hug et al. mote plasma regime at

P ≥ 110 Pa

T -dependence

suggesting

sketched by the dashed line in

that the defect-creation is active over a wide

Figure 3g without showing any abrupt change

P

in the relative ratio

range, while they reported only hexagonal


S1

:

S2

:

S3 (≈ 30% :

The Journal of Physical Chemistry (a) T = 200 Υ

(c) 400 Υ

(b) 300 Υ

200 nm

(d) 500 Υ

(e) 600 Υ

200 nm

200 nm

200nm

(f) 700 Υ

200 nm

200 nm

(h) 700

(g) 100

600

80

Dmax (nm)

Areal fraction (%)

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Page 6 of 15

60 Sn≥4

40

S3

20

S2

500

Series I Series II Yang et al.

400 300 200 100

S1

0 100 200 300 400 500 600 700 800

0 100 200 300 400 500 600 700 800

T (°C)

T (°C)

Figure 3: STM images of graphite surfaces etched by H-plasma at dierent T of (a) 200, (b) 300, 700 ◦ C. The other etching parameters are xed at P = 110 Pa,

(c) 400, (d) 500, (e) 600, and (f )

t = 40 min., and WRF = 20 W.

(g)

in (h) is the result by Yang et al.

39

drastically change between 450 and

Sn

and (h)

Dmax

are plotted as a function of

T.

The dashed line

obtained with 30 Pa, 40 min., and 20 W. Etching characteristics 500 ◦ C from the more isotropic lateral etching with active defect

formation to the anisotropic lateral etching with reduced defect formation.

55% : 15%). Nevertheless, as shown in Figure 3h, Dmax jumps by a factor of two to three across the transition temperature between 450 ◦ and 500 C due to the change in the etching

Dmax at the same Tc . However, Dmax T > Tc are two times larger in Series I than II. Also, in Series II, S3 and S4 are larger than in Series I where no S4 is seen. The defect

nature.

creation rate in Series II seems to be slightly

jump in

values at

Apparently, the defect formation is ◦ severely suppressed at T ≥ 500 C, while it is active at lower Previous

T.

researchers

dome-shaped

enhanced. One possible explanation for this is a change of oxygen contamination in the gas ow

39,40

reported

similar

system. To take the data shown in Figures 3g 2 and h, we surveyed a 1030 µm surface area

T -dependences of the etching rate

but no characteristic change in the nanopit shape (see the dashed line in Figure 3h).

for each condition.

In

In order to test the applicability of the di-

general, etching reactions are expected to be suppressed at suciently low

T

rect/remote mechanism to the

lack of thermal energy. They will also be suppressed at suciently high instability of CH4 ,

51

T

by varying temperature.

which is expected to be

the nal product of the reaction,

furnace on the downstream side at

or due to

and

lg

thermal desorption of H before etching the sur-

53

The glow edge loca-

tion was again visually observed outside the

either due to the

52

T -driven mecha-

nism found here, we have examined if lg changes

because of the

WRF = 20

P = 13

Pa

W. Eventually, we found that

changes but in the opposite direction to that

T

expected from the model at rst glance. Specif-

We checked the reproducibility of the sharp

ically, lg increases by 30 mm with increasing T ◦ from 300 to 600 C. Thus some unknown mech-

face.

These are probable reasons for such a

dependence. transition at

Tc

by carrying out an independent

anism may control anisotropic etching indepen-

series of measurements (Series II) after the rst

dently from the direct/remote one.

series (I). As seen in Figure 3h, the two data

this cannot explain why Hug et al.

sets agree with each other in terms of the abrupt

anisotropic etching even at


41

T (= 400

However, found the ◦ C) ≤ Tc .

Page 7 of 15 Alternatively, increasing

T

(a) t = 20 min.

may aect the com-

(c) 40 min.

(b) 30 min.

position of the H-species in the sample region. For example, the fraction of H-radicals near the sample position may increase, which promotes anisotropic etching and elongates the glowing

200 nm

200 nm

200 nm

length.

(e) 60 min.

(d) 50 min.

In any case, to clarify the mechanism behind the the

T -driven transitional change of the etch-

ing character we found, direct measurements of the H-plasma components in the sample region and theoretical investigations of roles of the temperature and H-radical are highly desired.

(g) 700

60 40

S3 S2 S1

20

We studied the etching time dependence as well.

0 20

30

of graphite surfaces etched for 20, 30, 40, 50, ◦ and 60 min., respectively, at T = 600 C,

Figure 4:

50

400 300

toffset

200 100 0 0

60

10

20

30

WRF = 20 W. S1 decreases linearly with increasing t as expected from a constant etching rate, while S3 and S4 remain very small, even when S1 ≈ 50% at t = 60 min.

40

50

60

70

t (min.)

STM images of graphite surfaces

etched by H-plasma for dierent

and

t of (a) 20, (b)

30, (c) 40, (d) 50, and (e) 60 min. The other pa◦ rameters are xed at T = 600 C, P = 110 Pa and

WRF = 20

W. (f )

plotted as functions of

(see Figure 4f ), because the defect formation is

Sn t.

Dmax

and (g)

are

There seems to ex-

ist a nite oset time of 10-15 min. before the

rather suppressed at this temperature.

Dmax

40

500

t (min.)

Figures 4a, b, c, d, and e show STM images

Although

Series I Series II

600

80

Dmax (nm)

Time Dependence

P = 30 Pa,

200 nm

200 nm

(f) 100

Areal fraction (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


anisotropic etching starts to proceed.

data are widely scattered,

there seems to exist a nite time oset of the for (c)(d). These parameters belong to the re-

order of 10-15 min. before the nanopits start to grow (Figure 4g).

mote plasma mode.

Nanopits in the third layer

As expected, when

WRF

also start to appear after a longer time oset

increases from 10 to 25 W, the change in the

of 30-40 min. The nite time osets observed

hexagonal nanopit formation is more or less the

clearly in this inactive defect formation condi-

same as the change caused by increasing

tion can give some insights into the kinematics

Figures 4a-e.

of the surface defect formation. Recent molec-

S2

ular dynamics simulations

oset of 1015 W as shown in the

5355

demonstrated

and

S3

In this range,

S1

t

in

decreases and

increase monotonically with a nite

Sn

WRF

depen-

that nite uence is necessary for H ions to be

dence of

(Figure 5g) and etching rate (Fig-

adsorbed, break C-C bonds, and dissociate C

ure 5h) which is

atoms away on a graphene or graphite surface.

teresting result was obtained at the highest RF

Dmax

divided by

t.

An in-

power (= 50 W, Figures 5e,f ). Here, the surface

Plasma Power Dependence

morphology completely changes to many multilayer terraced nanopits with irregularly shaped

Figures 5a, b, c, d, and e show STM images of graphite surfaces etched at various

WRF

10, 15, 20, 25, and 50 W, respectively.

edges just like those in Figures 2a,b.

of

Again, we have checked a

WRF

dependence

of lg when the glow edge is located outside the

Fig-

WRF

ure 5f is a magnied image of the same surface

furnace, and found that when

as that for Figure 5e. Other parameters are ◦ xed at T = 600 C and P = 110 Pa except

from 25 to 50 W, lg increases by 4050 mm ◦ at T = 600 C and P = 40 Pa and at T = ◦ 23 C and P = 14 Pa. Although we don't know

that

t = 60

min. for (a)(b)(e)(f ) and 40 min.


increases


Scanning Tunneling Spectroscopy of Zigzag Edge State at Hexagonal Nanopit Edges

exactly what is happening inside the furnace, it is reasonable to speculate that the observed change in surface morphology is caused when the plasma glow edge moves across the sample region with increasing

WRF

from 25 to 50 W.

It is possible to judge if the linear edges of hexagonal nanopits are zigzag or armchair type

(a) WRF = 10 W, t = 60 min. (b) 15 W, 60 min.

(c) 20 W, 40 min.

by measuring relative angles (θ ) between one of atomic rows of the B-site carbon atoms, which are selectively visible with STM, and the edges.

200 nm

200 nm

(d) 25 W, 40 min.

The data indicate that all the

edges of hexagonal nanopits synthesized by the

200 nm

present H-plasma etching technique are exclu◦ ◦ ◦ sively of zigzag type (θ = 0 , 60 , or 120 ). It

(f)

(e) 50 W, 60 min.

46,56

is, however, technically very dicult to identify exact arrangements of the endmost carbon atoms along the edges even with the STM tech-

100 nm

(g) 100 80 60 40

(h) 15

the edge state would be the most direct and sensitive detection of the edge quality or purity

S3 S2 S1

20 0 15

20

nique. Instead, at present, STS observation of

200 nm

Etching rate (nm/min.)

200 nm

Areal fraction (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 8 of 15

10

of zigzag edge structure.

5 0 0

edge state is known to exist in the close vicin10

20

30

40

ity of only the zigzag edge and not the arm-

WRF (W)

chair one.

25

WRF (W)

Figure 5:

and 78 K in ultra high vacuum. Figure 6a is an STM image of one of corners of

WRF of (a) 10, (b) 15, (c) 20, (d) 25, and 50 W. The etching time t is (a)(b)(e)(f )

powers

60 min.

and (c)(d) 40 min.

P = 110

Pa. (f ) Magnied image of the same

a hexagonal nanopit with a diagonal length of 160 nm created on an HOPG surface. Etching

The other etch◦ ing parameters are kept at T = 600 C and

conditions applied here are the same as those ◦ for the sample shown in Figure 4d (600 C, 110 Pa, 50 min., 20 W). A typical tunneling

(e) and (f ) are repre-

spectrum taken in the vicinity of the long ver-

sented in dierent contrasts from other STM

tical edge in Figure 6a is shown by the orange

images to clarify the multilayer structure. (g)

Sn

circles in Figure 6b, while those taken far from

and (h) Etching rate are plotted as func-

tions of

the edge are shown by the black dots (on the up-

WRF , there seems to exist a nite oset

per terrace) and the open circles (on the lower

power of 1015 W until the anisotropic etching starts.

We have examined many dier-

perature STM/S apparatus operated at 4.7 K

STM images of graphite surfaces

surface as that for (e).

2,46

ent edges of monolayer depth with a low tem-

etched by H-plasma generated by dierent RF (e)(f )

This is because the

terrace).

The etching nature seems be changed

The locations where the three spec-

tra were taken are indicated by the orange dot

between 25 and 50 W from the anisotropic lat-

(near edge), the black dotted line (upper ter-

eral etching with reduced defect formation to

race), and the white dotted line (lower terrace)

the isotropic lateral etching with active defect

in Figure 6a. Here, dI /dV , the dierential tun-

formation.

nel conductance which is proportional to the local density of states (LDOS) of the graphite surface, is plotted as a function of the bias voltage (V ) between the STM tip and the sample surface.

V corresponds to an energy EF . In Figure 6b, only in

from the

dierence the spec-

trum near the edge a prominent peak is found


Page 9 of 15

(b) 0.8

B E

x

F

A

C

0.4

0

D

0.6

20 nm

(e)

(f)

A B C

0.2

䖃䖃

Carbon Hydrogen

STM tip

0 -200

-100

0

100

200

1

0.8 0.6 0.4 0.2 0 -200

-100

0

D Niimi et al. 100 200

dI/dV (arb. unit)

0

(c)

1

Distance along the edge; y (nm)

y

dI/dV (arb. unit)

(a)

dI/dV (arb. unit)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


10

10

5

5

0 -200

-100

0

100

0 200 -200

Bias voltage (mV)

Bias voltage (mV)

1.6

-100

0

(d)

100

200

Bias voltage (mV)

Figure 6: (a) STM image of an HOPG surface etched by H-plasma. The right dark side shows the corner of a hexagonal nanopit with zigzag edges of monolayer height. (b) Tunnel spectra obtained at three characteristically dierent positions that are near the edge (orange dots), on the upper terrace (black dots), and on the lower terrace (open circles) of the nanopit shown in (a). The specic

4 ≤ y ≤ 7 nm), the black dotted line (x = 23 nm, −19 ≤ y ≤ 12.5 nm), and the white dotted line (x = −16 nm, −19 ≤ y ≤ 12.5 nm), respectively. Only near the edge, a prominent peak at V ≈ 0 and adjacent dI /dV suppressions are found, which correspond to the zigzag edge state. (c)(d) Color plots of dI /dV as a function of y taken in the dotted rectangles in (a), which are at distances x = 10 (c) and 1 nm (d) from the positions are indicated in (a) by the orange dot (x

=1

nm,

edge, respectively. The edge state exists only in (d) with a spatial variation along the edge (domain structure; see main text). (e) Tunnel spectrum taken at a region of where

Vpeak

x=1

−3 ≤ y ≤ 0 nm T = 4.7 K. The small

nm and

appears at a slightly positive value. All the data were taken at

dots are the spectrum near a zigzag step edge of monoatomic height at an exfoliated graphite surface from Ref. 4. (f ) Cartoon of hexagonal nanopits synthesized on a graphite surface by Hplasma etching and an STM tip for STS observation. The edge carbon atoms are expected to be terminated by single hydrogen atoms.

at

V ≈ 0,

which is characteristic of the edge

details.

−11 mV.

Next we discuss the spatial variation of the

In addition to that, the peak is accompanied by

edge state studied by the STS technique par-

clear LDOS suppressions on both sides of the

ticularly along the edge.

peak. Such clear suppressions have never been

ordinate axes with the origin

reported before.

Figure 6a. Figures 6c and d are color plots of

state. The peak voltage (Vpeak ) here is

As we reported recently,

57

O

dI /dV as a function of

imbalance

taken at the two dierent distances

indicating the high purity of the

zigzag edge synthesized in this work.

xy

co-

as indicated in

they should be indicative of a large sublattice

58,59

y

Let us dene

(parallel to the edge)

x = 10

(c)

This is

and 1 nm (d) from the edge. The scanned areas

the rst spectroscopic evidence for the graphene

for these STS data are indicated by the dotted

zigzag edge state created by H-plasma etching.

rectangles in Figure 6a, in which the dI /dV

It is noted that the tunnel spectral results are

spectra were averaged over 2 nm widths in the

consistent with Raman spectroscopy measure-

x

ments of the D band peak which we made sepa-

decay length of only 12 nm,

rately for multilayer graphene samples with and

tect an LDOS enhancement near

without the successive anisotropic etching (LP

x = 10

etching) after the initial isotropic etching (HP

in the close vicinity of the edge (x

etching). See Supporting Information for more

ure 6d), the edge state exists almost all along

direction.

5,57

one cannot de-

V = 0 at all at

nm (see Figure 6c). On the other hand,


Since the edge state has a short

= 1 nm, Fig-


Page 10 of 15

the measurement range except at several posi-

here are always aligned to the zigzag direction

y = 2, 5.5, and 9 nm where the LDOS

and not the armchair one. Moreover, character-

peak is less clear or almost disappears. In other

istic LDOS features of the zigzag edge state, an

words, the edge state is divided spatially into

electronic state localized at the zigzag edge, are

∼ 3 nm long domains.

Vpeak is unchanged

clearly observed in STS measurements. This is

within each domain, it varies randomly between

spectroscopic evidence of the high purity zigzag

tions near

domains within

While

−20 ≤ Vpeak ≤ 20

mV. For

edge.

example, the spectrum averaged over the domain located at

+14

−3 ≤ y ≤ 0

Thus the anisotropic H-plasma etch-

ing is a promising route for the fabrication of

nm has a peak at

zGNR, one of the key device elements for fu-

mV (the green large circles in Figure 6e).

ture graphene nanoelectronics. The STS obser-

We observed similar domain structures for al-

vations also show a spatial distribution of the

most all edges we examined, and they may stem

edge state of the order of 23 nm along the edge.

from their geometrical imperfections.

In any

This demonstrates the high sensitivity of this

case, the edges must be highly zigzag-rich and

tool to evaluate the edge quality on an atomic

presumably of the best quality ever achieved

scale.

judging from the observed prominent LDOS

late theoretical works to elucidate the roles of

peak and clear adjacent suppressions. This can

the sample temperature and H-radicals in the

be seen by comparing it with the previously

anisotropic H-plasma etching of graphite sur-

known edge state spectra.

face and graphene.

4,5

One of them is

shown by the small dots in Figure 6e.

4

The present work will hopefully stimu-

Fig-

ure 6f shows a cartoon of a STS measurement

Supporting Information Avail-

of hexagonal nanopits with perfect edges.

able CONCLUSIONS

The

In summary, by use of STM, we have investi-

ing several key parameters such as temperature,

H2 pressure, etching time, and plasma gener-

T -induced

We found a

cially supported by Grant-in-Aid for Young Sci-

tran-

entists (B) (Grant No. 25800191), Scientic Research (C) (Grant No. 15K05159), and Scien-

raced nanopits to few large hexagonal nanopits,

tic Research (B) (Grant No. 18H01170) from

which is consistent with a recent report by other

JSPS. Authors acknowledge the free-to-use soft-

Our own measurement of the

ware, WSxM.

pressure dependence of the glowing plasma edge

We also appreciate the Cryo-

of the University of Tokyo for their supply of liq-

eral etching proceeds isotropically (anisotropi-

uid helium and for allowing us to use the laser

cally) when the graphite sample is located in-

Raman microscope, respectively.

side (outside) of the glowing regime. However,

T -induced

60

genic Research Center and the MERIT program

location supports their proposal, i.e., the lat-

the

We thank Y. Ohba and

and STS data analyses. This work was nan-

sition from irregularly-shaped multi-layer ter-

researchers.

spec-

struction of the H-plasma etching apparatus

round shape to few large hexagonal nanopits ◦ within a narrow range between 450 and 500 C.

41

Raman

Y. L. Liu for their technical assistances in con-

morphology from many small nanopits of rather

P -induced

which

hex_nanopit_supporting_information.pdf

Acknowledgement

transition of the surface

In addition, we observed a

in

free of charge.

at graphite surfaces by H-plasma etching vary-

new sharp

le,

etched by H-plasma are discussed, is available

gated nanopits with monolayer depth created

ation RF power, independently.

following

troscopy measurements of multilayer graphene

transition we found here cannot

be explained straightforwardly by this model. Edges of the hexagonal nanopits synthesized


Page 11 of 15 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


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Graphical TOC Entry 䖃

STM tip

䖃

Carbon Hydrogen

200 nm


Hexagonal Nanopits with the Zigzag Edge State on Graphite Surfaces

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