unoccupied electronic structure of ball-milled graphite

Unoccupied electronic structure of ball-milled graphite

Adriyan S. Milev,*aNguyen H. Tran,

aG. S. Kamali Kannangara

aand

Michael A. Wilsonab

Received 14th December 2009, Accepted 10th March 2010

First published as an Advance Article on the web 23rd April 2010

DOI: 10.1039/b926345d

Changes in electronic and vibrational structure of well characterised macrocrystalline graphite

milled by a planetary ball-mill are investigated by Raman spectroscopy and Near Edge X-ray

Absorption Fine structure (NEXAFS) measurements at the C K-edge. The electronic structure

changes at the surface and in the sub-surface of the particles are examined by comparing

two-different NEXAFS detection modes: total fluorescence yield (TFY) and partial electron yield

(PEY) respectively. When the in-plane crystallite sizes of graphite are decreased to nanosized

(from B160 nm to B9 nm), a new spectral structure appears in TFY at 284.1 eV which is not

present in the macrocrystalline graphite. This feature is assigned to electronic states associated

with zigzag edges. Further the TFY shows a shift of the main graphite p* band from 285.5 to

285.9 eV, attributed to breaking the conjugation and hence the electron localization effect during

milling, The TFY spectra also show strong spectral features at 287.5 and 288.6 eV, which suggest

that the local environment of carbon atoms changes from sp2 to more sp3 due to physical damage

of the graphite sheets and formation of structures other than aromatic hexagons. Complementary

Raman spectroscopic measurements demonstrate an up-shift of the graphite G band from

1575 to 1583 cm�1 en route to nanosize. The changes in TFY NEXAFS and Raman spectra are

attributed to modification of the sub-surface electronic structure due to the presence of defects in

the graphite crystal produced during milling. The discovery of the strong spectral feature at

284.1 eV in nanographite and the 0.4 eV up-shift of the p* band may open up possibilities to

influence the electronic transport properties of graphite by manipulation of defects during the

preparation of the nanographite.

Introduction

The element carbon can exist without bonding with other

elements in sp, sp2 or sp3 hybridised allotropes. Graphite is one

of the most common and interesting allotropic forms which

can have various degrees of order in its crystal structure. In

principle, pure graphite is a 3D layered material formed by

stacking of 2D hexagonal sheets where the sheets are stacked

usually in an alternating (ABAB. . .) sequence giving rise to a

unit cell that consists of four atoms (Fig. 1a and b).1

The bonding within the sheets is due to the overlap of sp2

hybridized 2s, 2px and 2py atomic orbitals of carbon atoms

giving rise to three s-bonds that form the hexagonal network

(s states).

The bonding between the sheets is derived from the overlap

between the non-hybridized out-of-plane 2pz orbitals from the

adjacent sheets (p-states).2 This electronic configuration gives

rise to stronger in-plane bonds (0.142 nm) and weaker out-of-plane

bonds (0.335 nm). At the edges the graphite sheets are

terminated by zigzag or armchair structures which may be

described to have dangling bonds.

A variety of graphitic forms are known in which there are

different degrees of disorder. This could be due to the presence

of small amounts of other elements giving rise to structures

known as anthracites, due to internal packing order sometimes

called amorphous regions or due to the disorder that occurs at

surfaces where the effective forces in the sub-surface are

Fig. 1 (a) 3D graphite lattice. The unit cell is given by dashed lines.

The stacking is called ABAB. . ., where A and B refer to two families of

planes shifted on one another. (b) Top view of 3D graphite lattice

showing the structural difference between the A-type and B-type

carbons.

a School of Natural Sciences, University of Western Sydney,Penrith South, DC NSW 1797, Australia.E-mail: [email protected]

bCSIRO Earth Sciences and Resource Engineering RiversideCorporate Park, 11 Julius Avenue, North Ryde, NSW 2113,Australia

This journal is �c the Owner Societies 2010 Phys. Chem. Chem. Phys., 2010, 12, 6685–6691 | 6685

PAPER www.rsc.org/pccp | Physical Chemistry Chemical Physics

challenged by surface forces of structure termination. One area

of topical interest is when the surface forces of graphite are

large enough to become substantial and begin to challenge

those present in the sub-surface. This is an interesting

phenomenon when particles become nanosized. Not

surprisingly, the properties of nanographite strongly depend

on the method of preparation because small changes in size

can influence the ratio of surface to sub-surface and they may

also affect the amount of defects in the graphite structure.3–5

Raman spectroscopy and Near Edge X-ray Absorption

Fine Structure Spectroscopy (NEXAFS) are very useful for

studying such changes.3–10 Raman spectra of graphites consist

of a G band peak at B1575 cm�1 and peaks at around

1345 cm�1 and at B1610 cm�1, corresponding to disorder

induced features (called D and D0 bands). The relative

intensities of the G and D peaks called the ID/IG ratio can

be used to establish degree of order. The full widths at half

maximum (fwhm) of the bands can also be measured and

interpreted.

The complementary NEXAFS spectroscopy can give

specific information on the atomic environments and can

discriminate between sp3 and sp2 configurations.9,10 The

carbon K-edge NEXAFS measurements are usually performed

in partial electron yield mode (PEY) which probes the top

1–5 nm layer or total electron yield (TEY) mode that probes

the 1–10 nm of the material. A third detection technique called

total fluorescence yield (TFY) mode, allows details of carbon

X-ray absorption in the sub-surface region 1–100 nm.11–13

The X-ray absorption of the carbon K edge is due to 1s - 2p

electronic transitions and usually consists of two energy

regions associated with 1s - p* transitions at B284–288 eV

and 1s - s* transitions at 4289 eV.9 It thus probes local

order. For all highly ordered graphites, these transitions

give rise to peaks at 285.4 and 291.7 eV, respectively.7,14,15

However, one or two additional resonances in the energy

range of 286 to 289 eV are sometimes observed. The origin

of these peaks is not universally agreed.7,9,16–22 Some

investigators have attributed a peak at B287–290 eV to

atom impurities such as C–O bonds18,19,21 or C–H bonds.9,17

However, experiments by Reihl et al.,22 demonstrated that the

feature at 287.5 eV is not affected by the exposure to activated

hydrogen or activated oxygen or small amounts of water and

independently, Atamny et al.,16 could find no C K-edge

spectral differences between the graphite samples exposed to

oxygen at 1000 K and pristine graphite surfaces. These results

suggest that the alternative assignment by Posternak et al.,20

and Fisher et al.,7 of a peak at B287.5 eV to an additional

three-dimensional interlayer states, may be correct. Recent

theoretical work by Coleman et al.,23 suggests that the peaks in

the range 287–288 eV can be attributed to the presence of

defects such as atomic vacancies in graphite. The interpretation

is that this is due to some sp3 bonded carbon atoms near

defects and can be backed by other evidence as discussed

further below in the bulk of the paper. It is probably

circumspect however, given the variable nature of graphite,

not to exclude C–O and C–H assignments for some graphites.

Our view is that these bonds will give rise to characteristic

vibrations which could be observed using infra red and/or

Raman spectroscopy.

We note that because of the variability of graphite

structures as discussed above, other less ordered or more

ordered structures may behave differently and are worth of

investigation. Elsewhere, we have reported Raman, preliminary

NEXAFS investigations and detailed X-ray diffraction analysis

on the crystallite sizes and their distributions of the graphite

used here as precursor.24–26 In this paper, we first discuss

NEXAFS results for nanographite prepared by ball milling.

Then we compare the surface and sub-surface electronic

structures of the products by measuring electron yield (PEY)

and total fluorescence yield (TFY) simultaneously and relate

them to Raman spectra of the same materials. It will be

demonstrated that the electronic structure of these nano-

graphites is unique due to the accumulation of lattice defects.

Experimental

Sample preparation

The precursor graphite powder (499% purity) was obtained

from Fluka. Graphite nanoparticles were prepared by ball-

milling of 10 g polycrystalline graphite powder in a Rietsch

Planetary Ball mill PM 100. Each sample was loaded into a

stainless-steel container (250 ml) together with 6 stainless-steel

balls (diameter 25 mm). The container was evacuated, then

purged with argon (499.999 purity, rate B3 l min�1 for 5 min)

and later the pressure was increased to about 200 kPa. The

milling was carried out for 3, 10 and 30 h at 400 rpm and the

samples were code named as G3, G10, and G30, respectively.

The precursor graphite was code named as G0. The

milling container was opened in a glove box under nitrogen

(499.99 purity) and the milled powders were transferred into

containers. The containers with the samples were kept sealed

until the NEXAFS measurements.

Transmission electron microscopy

Graphite samples were examined by a Philips Biofilter-120

transmission electron microscope (TEM) operating at 120 kV.

Samples for TEM observation were prepared by mixing

sample powder with acetone and dispersing the sample

aggregates by ultrasound treatment. A drop of the suspension

was transferred onto lacey carbon foils supported on copper

grids for examination.

Raman

The Raman spectra were acquired on a Renishaw inVia Reflex

spectrometer, with a 2400 grooves per millimetre grating,

equipped with a Peltier cooled CCD detector. The measurements

were carried out under a microscope (�50 objective) with the

collection optics based on a Leica DMLM confocal

microscope. The instrument was calibrated against the Stokes

Raman signal of pure Si at 520 cm�1. The 514.5 nm (2.41 eV)

line of an argon laser with power at the sample surface limited

to o1.6 mW was used to excite the samples. This relatively

low laser power was used to minimise the likelihood of sample

heating and thus to prevent downshifts of the Raman peak

position.27,28 Ten scans with acquisition time of 30 s were

co-added over the scan range of 1100–1800 cm�1, where.

All spectra were normalized to the intensity of the G band

6686 | Phys. Chem. Chem. Phys., 2010, 12, 6685–6691 This journal is �c the Owner Societies 2010

peak at B1575 cm�1. Peak positions, peak intensities, full

widths at half maximum (fwhm) were determined by software,

OPUS version 6.5 (Bruker). The peak positions and their full

widths at half maximum (fwhm) and the ID/IG integrated

intensity ratio (D and G peak area ratio) obtained from the

fitting to each spectrum are employed to quantify the changes

which occurred in the samples upon milling. The integrated

peak intensity ratio of the D and G bands (ID/IG) are used to

quantify the changes in-plane crystallite sizes (La).

NEXAFS

Carbon K-edge spectra were acquired using the wide range

beam-line (BL24A, energy approximately 10–1500 eV) at the

National Synchrotron Radiation Research Centre (NSRRC)

in Hsinchu, Taiwan. The carbon K-edge NEXAFS spectra

were acquired in the energy region from 275 to 322.5 eV.

Oxygen K-edge spectra in the energy region of 525–550 eV

were also recorded. To degas the absorbed species during

sample mounting, all samples were baked out at B200 1C in

the ultra high vacuum analysis chamber (B10�9 mbar) for 24 h.

The spectra were acquired at room temperature with the

surface sensitive, partial electron yield (about 1–10 nm

sampling depth for energies o1000 eV) and sub-surface

sensitive, fluorescence yield (sampling depth about 1–100 nm

at 200–500 eV energy range).12 The electron detector operated

in a partial yield mode, with a retarding potential of �70 V at

the entrance of the detector, which was used to repel low

energy electrons originating mainly from the sub-surface.

Spectra calibration

Normally, the calibration is referred to either p* or s*resonances, which are presumed independent of the sample

preparative conditions. The optics of the NEXAFS end-station

used here had a minor potassium contamination which in-fact

provided useful spectral features as an external reference

during calibration.29 First, the photon energy was calibrated

using the precursor graphite sample G0 by taking the value

of the C1s p*-transition to be at 285.5 eV and then the

energy position of the reference, potassium L-edge 2p3/2, was

recorded at 297.6 eV. All the samples, G3–G30, were

calibrated against the value of 297.6 eV. Thus, it was possible

to determine the shift in the position of all graphite-related

peaks for each sample without forcing them at certain energies

as per usual calibration method.19 Calibration, background

subtraction and pre- and post- C K-edge normalisation

procedures were applied on the spectra using an algorithm

developed by Newville et al.,30 and implemented in the

software package ATHENA.31 This removes any ambiguities

in the identification of the background and peak position and

thus provides a reliable criterion for a consistent normalisation

of all spectra.29

Results and discussion

Changes in Raman spectra on ball milling

The main spectral positions of all samples (G0–G30) were

similar to sp2 bonded graphitic materials already reported in

the literature (Fig. 2).32–38 However, some variation in the

relative intensities were observed.

The main features observed in the Raman spectra of our

samples as per the usual are a peak at 1575 cm�1, corresponding

to ordered graphite structure (graphite, G band) and peaks at

around 1345 cm�1 and at B1610 cm�1, corresponding to

disorder induced features (called D and D0 bands).33,34 The

main effects on Raman spectral features after milling of

graphite are; (i) the G peak moves from 1575 to 1583, and

its fwhm increases (ii) D and D0 -peaks intensities and fwhm

increase but no significant peak shifts are observed (Table 1).

The increased intensity of the D band in the Raman spectra of

graphites has been interpreted as decreased in-plane crystallite

sizes.35–38 On an atomic scale, this may be regarded as an

increase of the graphite edges because of the smaller size of the

crystallites, a feature which is expected on milling.

The ratio of the integrated intensities of the G band to D

band is proportional to the average in-plane crystallite size

(La) by the equation; La (nm) = (2.4 � 10�10)l4laser (ID/IG)�1,

where llaser is the frequency of the exciting laser in nm.39 Upon

milling, the average La decreases fromB160 nm (G0) toB9 nm

(G30). This indicates that the number of laterally broken

graphene sheets increases and therefore the amount of boundary

regions along the fracture lines also increases en route to the

nanocrystalline state.

The changes in G band spectral position and the broadening

of the G and D band features are more interesting. The G

band of graphite is characteristic of all sp2 sites, including

alkenic CQC sites and not just those in aromatic rings.4

It always lies in the range 1500–1630 cm�1, as it does in

Fig. 2 Comparison of the Raman spectra for the G0–G30 samples.

The spectra are normalised to the intensity of the G band peak set at 2.

Table 1 Normalized Raman spectra fitted with Lorentzian shapefunction. The in-plane crystallite sizes are also given. Peak intensityareas (Ia) are given

Sample

G D La

cm�1 Ia fwhm cm�1 Ia fwhm nm

G0 1575 62.6 19.8 1345 6.5 34.0 160G3 1575 72.2 22.8 1345 36.9 39.0 33G10 1581 84.2 29.5 1346 136.4 51.5 10G30 1583 99.7 39.7 1347 176.2 60.1 9


spectra of other aromatic and alkenic solids. Nevertheless

differences can be delineated. The uncongugated alkenic

CQC bonds are shorter than aromatic bonds, so they

have higher vibration frequencies. Thus, during gradual trans-

formation from sp2 bonded conjugated hexagonal rings to sp2

bonded chains, a shift in the G band to higher wave numbers is

expected. Moreover, it is well established that the width of the

G peak is proportional to the bond-angle or bond-length

disorder at the sp2 sites.4 On the other hand, the D band

intensity is proportional to the carbons only in the aromatic

rings in clusters with small sizes, whereas the D band broadening

is proportional to the distribution of clusters containing

hexagonal aromatic rings with different orders and dimensions.

In other words, the D band intensity is proportional to the

number of hexagonal aromatic rings in the cluster whereas

carbons in non-aromatic bonds do not contribute to the

intensity of the D band.4 That is, if there is any change in

total disorder where there would be no change in the intensity

of the D, but if the distribution of disordered regions changes,

this would still be reflected in D line broadening. For the

samples prepared here, we note that the D-peak intensity and

fwhm increase with increasing milling time. This suggests that

not only more disordered clusters are formed but also they

become randomly distributed. In addition, these data confirm

the presence of more edges due to the presence of smaller

crystallites and the introduction of other forms of disorder in

the crystallites during the ball milling of the graphite.

Changes in NEXAFS spectra on ball milling

Fig. 3a compares the normalized C 1s NEXAFS Partial

Electron Yield (PEY) spectra obtained for the precursor,

G0 and nanocrystalline samples, G3–G30. To minimise the

contribution from electrons from the sub-surface regions, we

adopted the procedure reported previously.9 That is, the

entrance of the electron detector was biased by a negative

voltage to repel the low-energy electrons. The spectrum for

G0 is in agreement with those previously reported.7

The first sharp feature at 285.5 eV corresponds to a

transition from the C 1s initial states to a final p* states which

have pz symmetry polarized along the out-of-plane direction.7

The strong features at 291.7 eV and 292.8 eV are attributed

to excitations from the C 1s initial states to unoccupied

s* orbitals which have pxy symmetry polarized along the

in-plane direction.7,15

Theoretical investigations by McCulloch and Brydson13

demonstrated that the s* peaks at 291.7 eV and 292.8 eV

(called s1* and s2* peaks) are due to partial overlap of the

pxy orbitals with pz orbitals along c-axis of structurally

non-equivalent carbon atoms that belong to two-different

sub-lattices (A- and B-type carbons in Fig. 1). However, in

PEY spectra, these are evident only in those graphite samples

which are highly crystalline where good alignment between the

graphene planes exists.13 Our spectral data show that s1* and

s2* peaks are present in the initial graphite sample (G0) but

change rapidly in their intensity with milling time. The

reduction in intensity of these features is particularly evident

in the spectra for samples G10 and G30 where the s* transitionsare broad and the s1* s2* peaks are no more resolvable.

The latter can be explained by poor ABAB. . . stacking order

of the graphene sheets due to milling. The PEY spectra of all

samples also show two relatively weak peaks at 287.5 eV and

at 288.6 eV which will be discussed in detail below.

Origin of 287.5 and 288.6 eV transitions

The PEY spectra of all samples show two relatively weak

peaks at 287.5 eV and at 288.6 eV. However, in the Total

Fluorescence Yield (TFY) spectra of the milled graphite these

two peaks are the strongest features (Fig. 3 b). We ruled out

the presence of impurities containing oxygen and hydrogen by

observing the near-edge features at energies 4530 eV in the

O K-edge NEXAFS spectra acquired in TFY mode (insert in

Fig. 3b) and analysing the Raman and IR spectral data for

each of the samples.

Fig. 3 Comparison of C K-edge NEXAFS spectra of ball-milled

graphite powders. (a) Partial Electron Yield spectra (b) Total Fluorescence

Yield spectra. X-ray absorption features of potassium L-edge used as

an internal calibrant are marked by the asterisk (K 2p3/2 is at 297.6 eV).

The K 2p1/2 of potassium gives rise to a second peak at 300.2 eV. The

insert in Fig. 3b shows O K edge NEXAFS spectra collected in

fluorescence yield mode. Raw spectra without processing or normalization

are presented. The spectra are compared to the O K-edge spectrum

from the Au electrode. No significant difference between the oxygen

contents of the unmilled and milled samples and the synchrotron

optics is observed.


Recent theoretical work by Coleman et al.,23 suggests that

vacancies in the lattice structure of graphite give rise to peaks in

the range 287–288 eV. For a carbon atom that is far from a

defect, the px and py states will coincide and form the in-plane

s bond as expected in a highly crystalline graphite. However,

close to a defect site, the px and py orbitals will no longer

degenerate due to decreased symmetry and an additional

NEXAFS peak appears for atoms close to a defect.23 The new

peak corresponds to a bond with an s-like character similar to

that found in sp3 bonded carbons such as cyclohexane40 and

diamond.41,42 Therefore, the presence of the peaks at 287.5 and

288.6 eV in our milled samples suggests that the hybridization of

the CQC bonds changes from sp2 to that with more sp3 character

due to physical damage of the graphite sheets and formation of

structures other than aromatic hexagons.

A newly discovered 284.1 eV transition and energy shift of the

transition at 285.5 eV

Fig. 3b shows the NEXAFS spectra acquired in total

fluorescence yield (TFY) mode. The TFY spectra of the

precursor graphite sample is quite similar to data in the

literature,11 but differ considerably for all milled samples,

especially in the energy region below 286 eV. That is, a new

strong feature at 284.1 eV and a shift of the p* transition from

285.5 to 285.9 eV is observed. Because of the different probe

depth of the PEY and TFY detection modes we assign these

new features to electronic states that are present in sub-surface

only. As far as we are aware these electronic states have not

been observed previously in any graphitic structures.

Theoretically, when an atomic vacancy is created in the

A (or B) sub-lattice, new localized p states at the lattice sites

around the zigzag edges appear while no edge states appear for

armchair edges.43–46 These states lie just above the Fermi level,

which for graphite, is set at 284.0 eV.15,47 We assign the new

spectral feature at 284.1 eV to zigzag edge states. If there are

heteroatoms present these new states may not be observed

because the vacancy is quenched by the heteroatom.48 Taking

into account the diminishing sizes of the graphite upon

milling, the enhancement in the density of states at 284.1 eV

is thus suggestive of an increase development of the zigzag

edge states as a function of the milling time.

Of course nanographite sheet has also armchair structures

at the edges. Heggie et al.,49 have suggested that, unlike the

zigzag edges, the armchair edges prefer to hybridize into

sp structures with triple bond character. This suggestion is

supported by our spectra where the position of the peak at

285.9 eV is in excellent agreement with the p* resonance of

triple bonded linear hydrocarbons such as acetylene.50 The

0.4 eV shift suggests that during milling the carbon–carbon

binding energy increases by re-hybridizing towards sp from sp2,

and these carbon pairs can achieve shorter bond lengths closer

to the triple-bond of acetylene (B0.129 nm). This increases the

carbon 1s binding energy due to the p-bond localization and

implies a greater net charge per carbon atom around defects. In

other words, breaking the conjugation which results in electron

localization effect seems to be the mechanism capable of

up-shifting the G Raman peak from 1575 to 1583 cm�1 and

the NEXAFS p* band from 285.5 to 285.9 eV.

It is worth noting that in Raman spectroscopy, the appearance

of theD band is explained by a double resonance (DR) scattering

mechanism due to electron-optical phonon and electron-electron

interactions.33,51 According to Pimenta et al.,5 and Cancado

et al.,52 the DR process can only be fulfilled at an armchair edge

(strong D band), while for a zigzag edge, the resonance process is

forbidden (weak D band). Therefore, the NEXAFS peak at

284.1 eV and the Raman D band atB1345 cm�1 have dissimilar

origin since the electronic transition at B284.1 eV is related to

zigzag edges while the Raman D band is related to armchair

edges. Thus, the electronic and vibrational properties of nano-

graphite are sensitive to particle size and shape particularly in

relation to the peripheral regions.

Physical description of ball milled graphite as the in plane

crystallite size changes

Under prolonged milling, the shear and impact forces

continuously bend, fracture and displace the graphite sheets

relative to each other and the ABAB...stacking sequence is no

longer maintained. Transmission electron micrograph

comparison of the G0 and G30 samples shows that the

graphite particles became progressively disordered, curled

and corrugated along the edges (Fig. 4a and b). The phase

diagram of carbon53 indicates that the graphite can transform

into diamond at high pressures and temperatures. Below room

temperature, the pressure for phase transformation from

graphite to diamond is of several GPa,53 which is in the

same order of magnitude achieved by high energy ball-mills

(up to 4 GPa).54 Although it is difficult to estimate the impact

pressure and temperature fluctuations in graphite during

ball-milling in the current investigation, our data show that

the pressure induced locally changes the bond from sp2 to

sp- and sp3-like character.

Based on the current investigation, a model is proposed

incorporating the structural change to microcrystalline

graphite en route to nanocrystalline graphite via ball-milling

(Fig. 5). The in-plane fragmentation leads to a chemical

transition from aromatic to non-aromatic and the p states

become increasingly localised, i.e. some of the hexagonal rings

change gradually to chains. This result in a less rigid structure,

made of fragmentised graphite sheets bonded laterally via

in-plane sp links (Fig. 5a). The strain energy of sp bond can

be minimised by bond rotation.

When an atom is removed from a graphite sheet, the

structure undergoes bond reconstruction near the vacancy

defect and becomes sp3 like. The planar arrangement around

a vacancy would actually be unstable, and the atoms

surrounding the vacancy are more likely to be displaced

out-of-plane. This increases the probability of inter-sheet inter-

action via sp3 C–C bridges when defect sites from neighbouring

sheets are in close proximity (Fig. 5b) in accord with

calculations by Heggie et al.,49 and simulations by Telling

et al.55 The formation of inter-sheet sp3 bridges serves to relieve

the strain and thereby to stabilise nanographite as a new

metastable 3D structure. The development of the 3D structure

can explain why NEXAFS transitions at 287.5 and 288.6 eV are

intense whereas no detectable levels of heteroatoms are present

in the sub-surface in the nanocrystalline graphite.


Conclusions

It is concluded that high-energy milling under an inert

atmosphere damages the graphite sheets resulting in the

formation of structural defects, such as displaced atoms,

vacancies, and inter-plane sp3 bonds. The nanosized domains

contacting sp2-bonded hexagonal carbons are connected to

each other by a random network of sp and sp3 bonded

carbons. Under prolonged milling, the interaction of

these defects with each other and surrounding lattice causes

formation of new 3D structure.

There is considerable difference in the C 1s absorption

spectra at the surface and in the sub-surface of nanocrystalline

samples. The NEXAFS spectra acquired in fluorescence yield

mode demonstrate that nanocrystalline graphite develops new

p* states that give rise to a sharp peak at 284.1 eV from the

sample sub-surface. No such peak is observed from the surface

of the nanocrystalline graphite. The intensity of the new

p* electronic states is strongly influenced by the in-plane

crystallite size of the nanographite particles.

The localization of the p electrons in the nanosized domains

leads to a strengthening of the in plane CQC bonds, thus

leading to the 8 cm�1 up-shift of the G Raman peak and 0.4 eV

up-shift of the main graphite p* band from 285.5 to 285.9 eV.

Because of the close link between the electric conductivity and

the p/p* states, the discovery of this 0.4 eV energy shift may

open up possibilities to engineer the transport properties of

graphite, via the controlled concentration of defects. During

modification of peripheral regions such edges can give rise to a

variety of special practical applications for such nanographites

such as enhanced mechanical strength, electrical conductivity,

energy storage capacity for batteries and super-capacitors, if

they can be harnessed as such.

Acknowledgements

This work was produced as part of the activities of the ARC

Centre of Excellence for Functional Nanomaterials funded by

the Australian Research Council under the ARC Centre of

Excellence Program and by a University of Western Sydney

(UWS) internal grant (#17312) for 2009. The authors would

like to thank Dr L. Fan and Dr Y. Yang from NSRRC,

Hsinchu Taiwan for NEXAFS support and the travel grant

under Access to Major Research Facilities Programme

(ANSTO).

References

1 J. C. Charlier, X. Gonze and J. P. Michenaud, Carbon, 1994, 32,289–299.

2 J. Robertson, Adv. Phys., 1986, 35, 317–374.3 A. C. Ferrari, A. Libassi, B. K. Tanner, V. Stolojan, J. Yuan,L. M. Brown, S. E. Rodil, B. Kleinsorge and J. Robertson,Phys. Rev. B: Condens. Matter Mater. Phys., 2000, 62, 11089–11103.

4 A. C. Ferrari and J. Robertson, Phys. Rev. B: Condens. MatterMater. Phys., 2000, 61, 14095–14107.

5 M. A. Pimenta, G. Dresselhaus, M. S. Dresselhaus,L. G. Cancado, A. Jorio and R. Saito, Phys. Chem. Chem. Phys.,2007, 9, 1276–1291.

6 A. C. Ferrari, Solid State Commun., 2007, 143, 47–57.

Fig. 4 Transmission Electron micrographs of graphite (a) Precursor

graphite, G0 and (b) after 30 h of milling, G30. The milled specimen is

broken into pieces with jagged edges implying a disappearance of the

initial layered structure of graphite along the c-axis but in some areas

still preserves the initial layered structure.

Fig. 5 (a) Schematic representation of the formation of sp bridge as a

result of a missing atom or vacancy. (b) Inter-planar bond-formation

via sp3 bonded carbons. The atoms at the upper edge and those at the

lower edge belong to different sheets.


7 D. A. Fischer, R. M.Wentzcovitch, R. G. Carr, A. Continenza andA. J. Freeman, Phys. Rev. B: Condens. Matter, 1991, 44,1427–1429.

8 S. R. Sails, D. J. Gardiner, M. Bowden, J. Savage and D. Rodway,Diamond Relat. Mater., 1996, 5, 589–591.

9 J. Stoehr, Springer Series in Surface Sciences, Vol. 25, NEXAFSSpectroscopy, 1992.

10 N. H. Tran, M. A. Wilson, A. S. Milev, J. R. Bartlett, R. N. Lamb,D. Martin and G. S. K. Kannangara, Adv. Colloid Interface Sci.,2009, 145, 23–41.

11 D. Arvanitis, U. Dobler, L. Wenzel, K. Baberschke and J. Stoehr,Journal de Physique, Colloque, 1986, C8C8/173–C178/178.

12 E. G. Rightor, A. P. Hitchcock, H. Ade, R. D. Leapman,S. G. Urquhart, A. P. Smith, G. Mitchell, D. Fischer, H. J. Shinand T. Warwick, J. Phys. Chem. B, 1997, 101, 1950–1960.

13 D. G. McCulloch and R. Byrdson, J. Phys.: Condens. Matter,1996, 8, 3835–3841.

14 R. Ahuja, P. A. Bruehwiler, J. M. Wills, B. Johansson,N. Maartensson and O. Eriksson, Phys. Rev. B: Condens. Matter,1996, 54, 14396–14404.

15 P. E. Batson, Phys. Rev. B: Condens. Matter, 1993, 48, 2608–2610.16 F. Atamny, J. Bloecker, B. Henschke, R. Schloegl, T. Schedel-

Niedrig, M. Keil and A. M. Bradshaw, J. Phys. Chem., 1992, 96,4522–4526.

17 N. Benchikh, F. Garrelie, C. Donnet, B. Bouchet-Fabre,K. Wolski, F. Rogemond, A. S. Loir and J. L. Subtil, Thin SolidFilms, 2005, 482, 287–292.

18 A. Gutierrez, J. Diaz and M. F. Lopez, Appl. Phys. A: Mater. Sci.Process., 1995, 61, 111–114.

19 C. Lenardi, M. Marino, E. Barborini, P. Piseri and P. Milani,European Physical Journal B: Condensed Matter Physics, 2005, 46,441–447.

20 M. Posternak, A. Baldereschi, A. J. Freeman, E. Wimmer andM. Weinert, Phys. Rev. Lett., 1983, 50, 761–764.

21 L. Ravagnan, G. Bongiorno, D. Bandiera, E. Salis, P. Piseri,P. Milani, C. Lenardi, M. Coreno, M. de Simone andK. C. Prince, Carbon, 2006, 44, 1518–1524.

22 B. Reihl, J. K. Gimzewski, J. M. Nicholls and E. Tosatti,Phys. Rev. B: Condens. Matter, 1986, 33, 5770–5773.

23 V. A. Coleman, R. Knut, O. Karis, H. Grennberg, U. Jansson,R. Quinlan, B. C. Holloway, B. Sanyal and O. Eriksson, J. Phys.D: Appl. Phys., 2008, 41, 062001/062001–062001/062004.

24 A. Milev, N. Tran, G. S. K. Kannangara and M. Wilson, Sci.Technol. Adv. Mater., 2006, 7, 834–838.

25 A. Milev, M. Wilson, G. S. K. Kannangara and N. Tran, Mater.Chem. Phys., 2008, 111, 346–350.

26 D. E. Smeulders, A. S. Milev, G. S. K. Kannangara andM. A. Wilson, J. Mater. Sci., 2005, 40, 655–662.

27 P. Tan, Y. Deng, Q. Zhao and W. Cheng, Appl. Phys. Lett., 1999,74, 1818–1820.

28 L. G. Cancado, M. A. Pimenta, B. R. A. Neves, G. Medeiros-Ribeiro, T. Enoki, Y. Kobayashi, K. Takai, K.-i. Fukui,M. S. Dresselhaus, R. Saito and A. Jorio, Phys. Rev. Lett., 2004,93, 047403/047401–047403/047404.

29 B. Watts, L. Thomsen and P. C. Dastoor, J. Electron Spectrosc.Relat. Phenom., 2006, 151, 105–120.

30 M. Newville, P. Livins, Y. Yacoby, J. J. Rehr and E. A. Stern,Phys. Rev. B: Condens. Matter, 1993, 47, 14126–14131.

31 B. Ravel and M. Newville, J. Synchrotron Radiat., 2005, 12,537–541.

32 L. M. Malard, M. A. Pimenta, G. Dresselhaus andM. S. Dresselhaus, Phys. Rep., 2009, 473, 51–87.

33 C. Thomsen and S. Reich, Phys. Rev. Lett., 2000, 85, 5214–5217.34 R. Saito, A. Jorio, A. G. Souza Filho, G. Dresselhaus,

M. S. Dresselhaus and M. A. Pimenta, Phys. Rev. Lett., 2002,88, 027401/027401–027401/027404.

35 M. Nakamizo, R. Kammereck and J. P. L. Walker, Carbon, 1974,12, 259–267.

36 K. Nakamura, M. Fujitsuka and M. Kitajima, Chem. Phys. Lett.,1990, 172, 205–208.

37 F. Tuinstra and J. L. Koenig, J. Chem. Phys., 1970, 53, 1126–1130.38 H. Wakayama, J. Mizuno, Y. Fukushima, K. Nagano,

T. Fukunaga and U. Mizutani, Carbon, 1999, 37, 947–952.39 L. G. Cancado, K. Takai, T. Enoki, M. Endo, Y. A. Kim,

H. Mizusaki, A. Jorio, L. N. Coelho, R. Magalhaes-Paniagoand M. A. Pimenta, Appl. Phys. Lett., 2006, 88, 163106/163101–163106/163103.

40 A. P. Hitchcock, D. C. Newbury, I. Ishii, J. Stohr, J. A. Horsley,R. D. Redwing, A. L. Johnson and F. Sette, J. Chem. Phys., 1986,85, 4849–4862.

41 C. Ziethen, O. Schmidt, G. K. L. Marx, G. Schonhense,R. Fromter, J. Gilles, J. Kirschner, C. M. Schneider andO. Groning, J. Electron Spectrosc. Relat. Phenom., 2000, 107,261–271.

42 F. L. Coffman, R. Cao, P. A. Pianetta, S. Kapoor, M. Kelly andL. J. Terminello, Appl. Phys. Lett., 1996, 69, 568–570.

43 K. Nakada, M. Fujita, G. Dresselhaus and M. S. Dresselhaus,Phys. Rev. B: Condens. Matter, 1996, 54, 17954–17961.

44 K. Nakada, M. Igami, K. Wakabayashi andM. Fujita,Mol. Cryst.Liq. Cryst., 1998, 310, 225–230.

45 M. Fujita, M. Igami, K. Wakabayashi and K. Nakada,Mol. Cryst.Liq. Cryst., 1998, 310, 173–178.

46 M. Fujita, K. Wakabayashi, K. Nakada and K. Kusakabe,J. Phys. Soc. Jpn., 1996, 65, 1920–1923.

47 E. J. Mele and J. J. Ritsko, Phys. Rev. Lett., 1979, 43, 68–71.48 M. A. H. Vozmediano, F. Guinea and M. P. Lopez-Sancho,

J. Phys. Chem. Solids, 2006, 67, 562–566.49 I. Suarez-Martinez, G. Savini, G. Haffenden, J. M. Campanera

and M. I. Heggie, Phys. Status Solidi C, 2007, 4, 2958–2962.50 A. P. Hitchcock and C. E. Brion, J. Electron Spectrosc. Relat.

Phenom., 1977, 10, 317–330.51 S. Piscanec, M. Lazzeri, F. Mauri, A. C. Ferrari and J. Robertson,

Phys. Rev. Lett., 2004, 93, 185503–185504.52 L. G. Cancado, M. A. Pimenta, B. R. A. Neves, M. S. S. Dantas

and A. Jorio, Phys. Rev. Lett., 2004, 93, 247401/247401–247401/247404.

53 F. P. Bundy, J. Chem. Phys., 1963, 38, 618–630.54 D. R. Maurice and T. H. Courtney, Metall. Trans. A, 1990, 21,

289–303.55 R. H. Telling, C. P. Ewels, A. A. El-Barbary and M. I. Heggie,

Nat. Mater., 2003, 2, 333–337.


unoccupied electronic structure of ball-milled graphite

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