Graphite and Graphite Intercalation
Compounds (GICs)
Julian Michalowsky
Hauptseminar Experimentalphysik - First Institute of Physics, University of
Stuttgart
February 25, 2014
J. Michalowsky (HS-PI1)
Graphite
February 25, 2014
1 / 27
Content
Carbon
Bonds and Hybridization
Graphite
Crystallographic Structure
General Properties
Electronic Properties
Graphite Intercalation Compounds (GICs)
J. Michalowsky (HS-PI1)
Graphite
February 25, 2014
2 / 27
Carbon
J. Michalowsky (HS-PI1)
Graphite
February 25, 2014
3 / 27
Carbon
natural isotopes: 12 C (98.93 %),
13 C (1.07 %), 14 C (trace amounts)
III IV
V
VI VII VIII
4 valence electrons
natural occurence e.g. as
diamond, graphite
applications:
solid lubricant
(powdered graphite)
gemstones (diamond)
drinking water filter
(activated carbon)
electrodes (glassy carbon)
J. Michalowsky (HS-PI1)
Graphite
taken from [web14a]
February 25, 2014
4 / 27
Carbon: Bonds
possible bonds
single (e.g. ethane)
double (e.g. ethene, benzene)
triple (e.g. ethyne)
geometric equivalence of orbitals
achieved by hybridization
sp3 (ethane)
sp2 (ethene, benzene)
sp (ethyne)
J. Michalowsky (HS-PI1)
Graphite
February 25, 2014
5 / 27
Carbon: sp2 -Hybridization
electron configuration:
[He] 2s2 2p2
process of hybridization:
1
2
3
one s-electron is lifted into
pz -orbital (energy gain)
px -, py -orbitals drop,
s-orbital raises
form 3 degenerate
sp2 -hybrid orbitals
J. Michalowsky (HS-PI1)
Graphite
February 25, 2014
6 / 27
Carbon: sp2 -Hybridization
electron configuration:
[He] 2s2 2p2
process of hybridization:
1
2
3
one s-electron is lifted into
pz -orbital (energy gain)
px -, py -orbitals drop,
s-orbital raises
form 3 degenerate
sp2 -hybrid orbitals
J. Michalowsky (HS-PI1)
Graphite
February 25, 2014
6 / 27
Carbon: sp2 -Hybridization
electron configuration:
[He] 2s2 2p2
process of hybridization:
1
2
3
one s-electron is lifted into
pz -orbital (energy gain)
px -, py -orbitals drop,
s-orbital raises
form 3 degenerate
sp2 -hybrid orbitals
J. Michalowsky (HS-PI1)
Graphite
February 25, 2014
6 / 27
Carbon: sp2 -Hybridization
electron configuration:
[He] 2s2 2p2
process of hybridization:
1
2
3
one s-electron is lifted into
pz -orbital (energy gain)
px -, py -orbitals drop,
s-orbital raises
form 3 degenerate
sp2 -hybrid orbitals
J. Michalowsky (HS-PI1)
Graphite
February 25, 2014
6 / 27
Carbon: sp2 -Hybridization
electron configuration:
[He] 2s2 2p2
process of hybridization:
1
2
3
one s-electron is lifted into
pz -orbital (energy gain)
px -, py -orbitals drop,
s-orbital raises
form 3 degenerate
sp2 -hybrid orbitals
J. Michalowsky (HS-PI1)
Graphite
February 25, 2014
6 / 27
Carbon: sp2 -Hybridization
electron configuration:
[He] 2s2 2p2
process of hybridization:
1
2
3
one s-electron is lifted into
pz -orbital (energy gain)
px -, py -orbitals drop,
s-orbital raises
form 3 degenerate
sp2 -hybrid orbitals
J. Michalowsky (HS-PI1)
Graphite
February 25, 2014
6 / 27
Graphite
J. Michalowsky (HS-PI1)
Graphite
February 25, 2014
7 / 27
Graphite
thermodynamically
most stable form
of elemental carbon
chemically inert
used as e.g.
pencil lead
refractory material
solid lubricant
taken from [web14b]
J. Michalowsky (HS-PI1)
Graphite
February 25, 2014
8 / 27
Graphite: Structure (1)
[
[
formed of hexagonal C rings
rings similar to benzene
ring formed of
6 sp2 -hybridized C atoms
strong, fixed σ bonds
between C atoms
π-electrons delocalized
over ring
J. Michalowsky (HS-PI1)
Graphite
February 25, 2014
9 / 27
Graphite: Structure (1)
[
[
formed of hexagonal C rings
rings similar to benzene
ring formed of
6 sp2 -hybridized C atoms
strong, fixed σ bonds
between C atoms
π-electrons delocalized
over ring
J. Michalowsky (HS-PI1)
Graphite
February 25, 2014
9 / 27
Graphite: Structure (2)
infinite planar continuation
of rings ⇒ graphene
hexagonal rings produce
honeycomb structure
strong covalent σ-bonds
between C atoms
π-electrons delocalized
over whole structure
J. Michalowsky (HS-PI1)
Graphite
February 25, 2014
10 / 27
Graphite: Structure (3)
parallel stacking of graphene
⇒ graphite
z
no covalent interlayer bonds
van der Waals interaction
induced by π-electrons
interlayer bonding
∼ 75 times weaker
than intralayer covalent σ-bonds
x
B
A
y
all data from [Pie94], chapter 3
J. Michalowsky (HS-PI1)
Graphite
February 25, 2014
11 / 27
Graphite: Structure (3)
ABAB stacking pattern
α C atoms (direct neighbours
in adjacent layers)
β C atoms
(no direct neighbours)
stacking of 2-dim. layers
120 ◦ isotropic
with respect to z-axis
anisotropic with respect
to x- and y-axis
highly anisotropic crystal
A
B
A
all data from [Pie94], chapter 3
J. Michalowsky (HS-PI1)
Graphite
February 25, 2014
11 / 27
Graphite: Structure (3)
graphite unit cell
contains 4 atoms
2 α type C atoms
2 β type C atoms
σ-bond length dσ = 0.141 nm
kJ
σ-bond strength Eσ = 524 mol
layer distance dl = 0.335 nm
van der Waals-bond strength
kJ
EvdW = 7 mol
A
B
A
all data from [Pie94], chapter 3
J. Michalowsky (HS-PI1)
Graphite
February 25, 2014
11 / 27
Graphite: Polycristalline Graphite
closest to ideal crystal:
synthetic HOPG
real crystals show defects:
vacancies
disinclination of
graphene sheets
stacking faults (ABC stacking)
impurities (intercalation)
J. Michalowsky (HS-PI1)
Graphite
C
B
A
February 25, 2014
12 / 27
Graphite: Polycristalline Graphite
crystallite imperfection
decreases anisotropy
forms of isotropic "graphite"
exist (e.g. glassy carbon)
consist of graphene sheets
but: sheets distributed
isotropically
these species do not classify
as graphite
("non-graphitized carbon")
J. Michalowsky (HS-PI1)
Graphite
C
B
A
February 25, 2014
12 / 27
Graphite: General Properties
remember: Eintra ' 75Einter
easy shearing
between basal planes
graphite powder used as
solid lubricant
J. Michalowsky (HS-PI1)
Graphite
February 25, 2014
13 / 27
Graphite: General Properties
anisotropic crystal with
graphene interfaces
poor general transport
properties in z-direction
(scattering)
anisotropic heat conduction
properties
z
W
in-plane: Kxy = 390 mK
,
comparable to
high conductivity metals
x
y
W
off-plane: Kz = 2.0 mK
,
good thermal insulator
anisotropy ∼ 200
all data from [Pie94], chapter 3
J. Michalowsky (HS-PI1)
Graphite
February 25, 2014
13 / 27
Graphite: General Properties
heat resistant, no melting point below 100 bar
high sublimation point Tsub ' 3900 K
solid at temperatures above highest melting points of
metals (e.g. tungsten, Tm = 3695 K)
J. Michalowsky (HS-PI1)
Graphite
February 25, 2014
13 / 27
Graphite: General Properties
chemically inert towards acids, alkalies, corrosive gases
but: easily oxidized at T > 350 ◦ C
no protective oxide layer (gaseous oxides CO, CO2 )
oxidation faster at defect sites or sheet edges
resistance to oxidation depends on crystallite perfection
J. Michalowsky (HS-PI1)
Graphite
February 25, 2014
13 / 27
Graphite: General Properties
graphite is diamagnetic (susceptibility χ < 0)
but: highly anisotropic χk ≥ 50χ⊥ (with respect to z-axis)
all data from [DD02], pp. 133
J. Michalowsky (HS-PI1)
Graphite
February 25, 2014
13 / 27
Graphite: Electronic Properties
anisotropy affects transport
(scattering)
π-electrons can move freely
in basal planes
high in-plane carrier mobility
2
μxy = 13000 cm
Vs
(Cu: μ = 35
z
x
y
cm2
)
Vs
all data from [DD02], pp. 86
J. Michalowsky (HS-PI1)
Graphite
February 25, 2014
14 / 27
Graphite: Electronic Properties
in-plane conductivity
1
σxy = 2.5 · 104 Ωcm
(Cu: σ = 5.8 · 105
1
)
Ωcm
off-plane conductivity
1
σz = 8.3 Ωcm
z
high conductivity anisotropy
(∼ 3000)
x
y
but: conductivity varies widely
with crystallite perfection
all data from [DD02], pp. 86
J. Michalowsky (HS-PI1)
Graphite
February 25, 2014
14 / 27
Graphite: Electronic Properties
low conductivity compared to
mobility
reason: low free carrier
concentration
1
c = 1.137 · 1019 cm
3
at room temperature
1
(Cu: c = 8.483 · 1022 cm
3)
z
x
y
all data from [DD02], pp. 86
J. Michalowsky (HS-PI1)
Graphite
February 25, 2014
14 / 27
overlap between valence/
conduction band
∆ovlp E = 36 meV
π-electrons form partially filled
conduction band
Energy [eV]
Graphite: Electronic Properties
no comparable mechanism in
z-direction
taken from [web14c]
J. Michalowsky (HS-PI1)
Graphite
February 25, 2014
14 / 27
Graphite: Electronic Properties
in-plane resistivity not
monotonous with temperature
first: decrease (more electrons in
conduction band)
then: increase (lower mean free
path of electrons)
off-plane resistivity decreases
with increase in temperature
graphite: in-plane semi-metal,
off-plane insulator
J. Michalowsky (HS-PI1)
Graphite
taken from [Pie94]
February 25, 2014
14 / 27
Graphite Intercalation
Compounds
(GICs)
J. Michalowsky (HS-PI1)
Graphite
February 25, 2014
15 / 27
Graphite Intercalation Compounds (GICs)
intercalation: injection of
intercalant particles between
crystal layers
intercalant: particles of a
different chemical species
intercalation requires:
anisotropic media
strong intraplanar, weak
interplanar binding forces
often highly unstable
J. Michalowsky (HS-PI1)
Graphite
February 25, 2014
16 / 27
Graphite Intercalation Compounds (GICs)
1
nomenclature:
1
2
3
A
B
A
B
B
A
B
A
A
B
intercalate layer
graphite bounding layer
graphite interior layer
2
3
J. Michalowsky (HS-PI1)
Graphite
February 25, 2014
16 / 27
Graphite Intercalation Compounds (GICs)
...BA|AB|BA... stacking
increased layer distance
greater layer distance dl with
larger intercalate
smallest: alkali metal GICs
(dl ' 0.5 nm)
biggest: organic ternary
compounds (up to dl = 2.5 nm)
J. Michalowsky (HS-PI1)
Graphite
A
B
A
B
B
A
B
A
A
B
February 25, 2014
16 / 27
Graphite Intercalation Compounds (GICs)
taken from [ESE03], p.43
J. Michalowsky (HS-PI1)
Graphite
February 25, 2014
17 / 27
GICs: Donor and Acceptor Compounds
+ + + +
-
- - - - -
donor
-
acceptor
+
Graphite
+
+
+
- - - ++ +
+
+
-+ - +- +- ++
++ +
+
-+ +- +- ++ + +
+
+
J. Michalowsky (HS-PI1)
+
+ + + +
donors (e.g. alkali metals):
provide electron
acceptors (e.g. halogens):
provide hole
-
intercalants provide free carriers via
charge transfer
-
-
+ + + +
+
February 25, 2014
+
18 / 27
GICs: Donor and Acceptor Compounds
increased free carrier concentration
compounds are metallic
alters electronic, thermal,
magnetic properties
provides a means of "tuning" the host
in-plane resistivity of
ICl GICs. taken from
[ESE03], p.193
J. Michalowsky (HS-PI1)
Graphite
February 25, 2014
18 / 27
GICs: Staging Phenomenon
GIC effect (stronger than in other
intercalation compounds)
staging: intercalate layers
equidistantly distributed
accurate staging even in dilute
compounds
phase transitions between
different stages
all data and information from [DD02], pp.31; [DCB+ 13]
J. Michalowsky (HS-PI1)
Graphite
February 25, 2014
19 / 27
GICs: Staging Phenomenon
intercalate electrostatics
screening length: ∼ 1 layer
non-electrostatic effect
assumed: long range lattice
strain interaction
macroscopic single stage
samples possible
(∼ 100 nm or ∼ 300 layers)
all data and information from [DD02], pp.31; [DCB+ 13]
J. Michalowsky (HS-PI1)
Graphite
February 25, 2014
19 / 27
GICs: Synthesis (Vapor-Phase Reaction)
popular for alkali metal GICs
2 chambers in glass tube in
2-zone oven
1st: graphite (TG )
2nd: alkali metal (TI )
TI determines vapor pressure
TG − TI determines stage
taken from [ESE03], p.10
taken from [ESE03], p.10
J. Michalowsky (HS-PI1)
Graphite
February 25, 2014
20 / 27
GICs: CaC6 (Example)
stage 1 donor compound
intercalate layer:
no random distribution
particles placed in rings
donate each up to 2 electrons
all data and information from [ESE03]
J. Michalowsky (HS-PI1)
Graphite
February 25, 2014
21 / 27
GICs: CaC6 (Example)
+-
-
+
free carrier concentration
increases
in-plane conductivity increases
exhibits superconductivity at
Tc = 11.5 K
Tc = 15.1 K at p = 8 GPa
all data and information from [ESE03]
J. Michalowsky (HS-PI1)
Graphite
-+
+
-
+
+
-
-
-+
+
+
- -+ - ++
+- +
+
- - -+ +
+
-+ +
+
+
+
-+
+
+
+
+
+
- - +
February 25, 2014
21 / 27
GICs: CaC6 (Example)
initial layer distance:
dl = 0.335 nm
intercalated layer distance:
dl = 0.460 nm
cystal thickens by ∼ 35%
all data and information from [ESE03]
J. Michalowsky (HS-PI1)
Graphite
February 25, 2014
21 / 27
GICs: CX AsF5 (Example)
acceptor compound
sandwich thickness dl = 0.810 nm
room temperature in-plane conductivity σxy = 6.2 · 105
better than copper (σ = 5.8 · 105
1
)
Ωcm
1
Ωcm
at 1/4 its weight
extreme anisotropy in conductivity (∼ 106 )
all data from [DD02], p.86; [ESE03], p.37
J. Michalowsky (HS-PI1)
Graphite
February 25, 2014
22 / 27
GICs: Electronic Properties
additional free carriers
high in-plane mobility of graphite
higher in-plane conductivity
donor: increased off-plane conductivity
acceptor: increased off-plane resistivity
J. Michalowsky (HS-PI1)
Graphite
February 25, 2014
23 / 27
GICs: Electronic Properties
Conductivity and anisotropy of various compounds ([DD02], p.89)
J. Michalowsky (HS-PI1)
Graphite
February 25, 2014
23 / 27
GICs: Magnetic Properties
intercalation affects magnetic susceptibility
reduces χk by at least one order of magnitude
donor compounds: tend to be paramagnetic
acceptor compounds: tend to be diamagnetic
magnitude of χ is stage dependent (see [DD02], p.133 for
more detail)
J. Michalowsky (HS-PI1)
Graphite
February 25, 2014
24 / 27
GICs: Superconducting Properties
only donor GICs exhibit
superconductivity
neither host nor intercalant are
superconducting
critical temperatures (at normal
pressure)
below 1 K (most alkali metal
GICs)
up to 11.5 K (CaC6 )
dependent on crystal purity and
atomic ratio
J. Michalowsky (HS-PI1)
Graphite
taken from [ESE03], p.220
February 25, 2014
25 / 27
GICs: Superconducting Properties
only donor GICs exhibit
superconductivity
neither host nor intercalant are
superconducting
critical temperatures (at normal
pressure)
below 1 K (most alkali metal
GICs)
up to 11.5 K (CaC6 )
dependent on crystal purity and
atomic ratio
J. Michalowsky (HS-PI1)
Graphite
taken from [ESE03], p.220
February 25, 2014
25 / 27
GICs: Superconducting Properties
superconducting properties
anisotropic
exhibits both type 1 and type 2
superconductivity
depending on angle θ between
field and z-axis
type 1: 0 ◦ ≤ θ ≤ 65 ◦
type 2: 65 ◦ ≤ θ ≤ 90 ◦
taken from [ESE03], p.221
J. Michalowsky (HS-PI1)
Graphite
February 25, 2014
25 / 27
GICs: Properties (Summary)
intercalation...
improves in-plane transport properties
can dope off-plane transport properties
can dope anisotropy in graphite host
can dope magnetic properties
can produce superconductors
can dope other properties depending on electronic mobility
intercalation is a powerful means of controlling physical
properties
J. Michalowsky (HS-PI1)
Graphite
February 25, 2014
26 / 27
Thank you
for your attention!
J. Michalowsky (HS-PI1)
Graphite
February 25, 2014
27 / 27
A.M. Dimiev, G. Ceriotti, N. Behabtu, D. Zakhidov,
M. Pasquali, R. Saito, and J.M. Tour.
Direct real-time monitoring of stage transitions in graphite
intercalation compounds.
ACS Nano, 7(3):2773–2780, 2013.
M.S. Dresselhaus and G. Dresselhaus.
Intercalation compounds of graphite.
Adv. Phys., 51:1–186, 2002.
T. Enoki, M. Suzuki, and M. Endo.
Graphite Intercalation Compounds and Applications.
Oxford University Press, Inc., 2003.
H.O. Pierson.
Handbook of Carbon, Graphite, Diamonds and Fullerenes.
Elsevier Inc., 1994.
February 2014.
J. Michalowsky (HS-PI1)
Graphite
February 25, 2014
27 / 27
http://oxford-labs.com/wpcontent/uploads/2009/04/periodic-table.jpg.
February 2014.
http://lh4.ggpht.com/-4eClqrJSW_4/URZ951h64I/AAAAAAAAFZI/WLCdRI9jY5s/s720/Graphite-schist.jpg.
February 2014.
http://ruby.chemie.unifreiburg.de/Vorlesung/Gif_bilder/Strukturchemie/c_graphit_bw
J. Michalowsky (HS-PI1)
Graphite
February 25, 2014
27 / 27